Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Bob Goss

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-139-5 (hardback) 978-1-84735-138-8 (softback) 978-1-84735-140-1 (ebook)

Typeset by Integra Software Services Pvt. Ltd. Printed and bound by Lightning Source Inc.

C

ontents

Preface ................................................................................................................. vii 1.

Introduction to Adhesives.............................................................................. 1 1.1

Cyanoacrylates .................................................................................... 1 1.1.1

1.2

UV-curing Adhesives ............................................................................ 9 1.2.1

The Curing Process ................................................................. 9

1.2.2

Health and Safety with UV ................................................... 13

1.2.3

The Curing Equipment .......................................................... 13

1.2.4

Curing Adhesive Tack-free .................................................... 14

1.2.5

Types of UV Adhesives .......................................................... 16

1.2.6

Benefits of UV Adhesives ....................................................... 17

1.3

Two-part Acrylics .............................................................................. 17

1.4

Epoxies .............................................................................................. 19 1.4.1

1.5

1.6

Advantages and Disadvantages of Epoxies ............................ 21

Flexible Adhesive Sealants ................................................................. 21 1.5.1

Silicone Adhesive Sealants ..................................................... 22

1.5.2

Polyurethane Adhesive Sealants ............................................ 23

1.5.3

Modified Silane Adhesive Sealants ........................................ 24

Hot Melt Adhesives ........................................................................... 25 1.6.1

2.

Types of Cyanoacrylate ........................................................... 3

Reactive Hot Melts ............................................................... 25

Engineering Thermoplastics......................................................................... 27 2.1

Introduction ...................................................................................... 27

2.2

Amorphous Thermoplastics ............................................................... 27

2.3

Semi-crystalline Polymers .................................................................. 29

2.4

Adhesive Performance on Thermoplastics .......................................... 31

i

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts 2.4.1

ABS ....................................................................................... 32

2.4.2

LCP....................................................................................... 32

2.4.3

Polyamide ............................................................................. 33

2.4.4

PBT ....................................................................................... 34

2.4.5

PC ......................................................................................... 36

2.4.6

PEEK .................................................................................... 37

2.4.7

PES and PUS ......................................................................... 38

2.4.8

PE ......................................................................................... 39

2.4.9

PET ....................................................................................... 40

2.4.10 PMMA.................................................................................. 40 2.4.11 POM ..................................................................................... 41 2.4.12 PPO ...................................................................................... 42 2.4.13 PPS........................................................................................ 43 2.4.14 PP ......................................................................................... 43 2.4.15 PS.......................................................................................... 45 2.4.16 PTFE ..................................................................................... 46 2.4.17 PVC ...................................................................................... 47 2.5 3.

Engineering Thermoset Plastics ................................................................... 51 3.1

Introduction ...................................................................................... 51

3.2

Adhesive Performance on Thermoset Plastics..................................... 53

3.3 4.

ii

General Comments on Adhesive Bonding of Thermoplastics ............. 47

3.2.1

Diallyl Phthalate (DAP) ......................................................... 54

3.2.2

Epoxies ................................................................................. 55

3.2.3

Phenolics ............................................................................... 55

3.2.4

Polyester (Thermoset) ........................................................... 56

3.2.5

Polyurethanes........................................................................ 57

3.2.6

Polyimides............................................................................. 58

General Comments on Adhesive Bonding of Thermoset Plastics ........ 59

Elastomers and Thermoplastic Elastomers (TPE)......................................... 61 4.1

Introduction ...................................................................................... 61

4.2

Adhesive Performance on Elastomers................................................. 61 4.2.1

Butyl Rubber......................................................................... 62

4.2.2

Copolyester TPE ................................................................... 63

Contents 4.2.3

Ethylene Acrylic (EEA) Rubber ............................................. 64

4.2.4

Ethylene Propylene Diene Monomer Rubber (EPDM) .......... 64

4.2.5

Ethylene-Vinyl Acetate Co-polymer (EVA) ............................ 65

4.2.6

Fluorosilicone Rubber ........................................................... 66

4.2.7

Natural Rubber ..................................................................... 66

4.2.8

Nitrile Rubber ....................................................................... 67

4.2.9

Neoprene Rubber .................................................................. 68

4.2.10 Polyisoprene.......................................................................... 68 4.2.11 Polyolefin Elastomers ............................................................ 68 4.2.12 Silicone Rubber ..................................................................... 70 4.2.13 Styrene-Butadiene Rubber (SBR) ........................................... 70 4.2.14 Styrenic TPE ......................................................................... 70 4.2.15 Thermoplastic Vulcanisates (TPV)......................................... 72 4.3 5.

6.

General Comments on Bonding of Elastomers ................................... 72

Joint Design................................................................................................. 75 5.1

Introduction ...................................................................................... 75

5.2

Lap Joint ........................................................................................... 75 5.2.1

Joint Width versus Joint Overlap .......................................... 76

5.2.2

Optimising Joints to Minimise Stress .................................... 78

5.3

Double Lap Joint (Tongue and Groove) ............................................. 80

5.4

Cylindrical Joints ............................................................................... 81 5.4.1

Design Details ....................................................................... 82

5.4.2

Cross Holes........................................................................... 84

5.4.3

Blind Holes ........................................................................... 85

5.5

Butt Joint ........................................................................................... 86

5.6

Bond Line Thickness .......................................................................... 88

5.7

Thermal Effects ................................................................................. 89

5.8

Selecting the Viscosity of the Adhesive ............................................... 89

5.9

Surface Preparation ........................................................................... 91

Bonding of Low-energy Plastics and Rubbers .............................................. 93 6.1

Surface Wetting.................................................................................. 93

6.2

Measuring Surface Energy ................................................................. 97

6.3

Surface Treatments ............................................................................ 97 iii

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

6.4 7.

6.3.1

Abrasion ............................................................................... 98

6.3.2

Corona Discharge ................................................................. 98

6.3.3

Plasma Treatment.................................................................. 98

6.3.4

Flame Treatment ................................................................... 98

6.3.5

Use of Primers ....................................................................... 99

Two-part Acrylics .............................................................................. 99

Selecting the Adhesive ............................................................................... 101 7.1

Introduction .................................................................................... 101

7.2

Factors for Consideration ................................................................ 102 7.2.1

Selection of Materials .......................................................... 102

7.2.2

Adhesive Performance ......................................................... 103

7.2.3

Durability and Long Term Performance and Temperature Resistance....................................................... 104

7.2.4

Surface Preparation ............................................................. 104

7.2.5

Ease of Application ............................................................. 105

7.2.6

Joint Design ........................................................................ 105

7.2.7

Viscosity.............................................................................. 106

7.2.8

Cure Speed .......................................................................... 107

7.2.9

Gap-filling Capability .......................................................... 107

7.2.10 Sealing Capability ............................................................... 107 7.2.11 Health and Safety (H&S) .................................................... 108 7.2.12 Approvals ........................................................................... 111 7.2.13 Recycling Adhesives ............................................................ 111 7.3 8.

Dispensing Adhesives in Production .......................................................... 115 8.1

iv

A Summary for Adhesive Selection .................................................. 112

Basic Principles ................................................................................ 115 8.1.1

Single- or Two-part Adhesive .............................................. 115

8.1.2

Viscosity (see Section 5.8) ................................................... 116

8.1.3

Cycle Time .......................................................................... 118

8.1.4

Cure Method ...................................................................... 118

8.1.5

Dispense Quantity ............................................................... 118

8.1.6

Open Time .......................................................................... 120

8.1.7

Health and Safety ................................................................ 120

Contents 8.1.8 8.2

8.3 9.

Cost .................................................................................... 121

Dispensing Systems .......................................................................... 121 8.2.1

Manual Units ...................................................................... 122

8.2.2

Semi-automatic Dispensers .................................................. 123

8.2.3

Syringe Dispensing .............................................................. 123

8.2.4

Pressure Pot Dispensing ...................................................... 123

Automatic Systems .......................................................................... 125

Durability and Environmental Testing ....................................................... 127 9.1

Introduction .................................................................................... 127 9.1.1

Surface Finish and Surface Preparation ............................... 127

9.1.2

Joint Design ........................................................................ 128

9.1.3

Substrate Bonded ................................................................ 129

9.2

Effect of Humidity and Water Absorption ....................................... 130

9.3

Durability of Cyanoacrylates ........................................................... 130 9.3.1

9.4

Cyanoacrylates for Medical Applications ............................ 132

Durability of UV-curing Adhesives................................................... 132 9.4.1

UV Adhesives for Medical Applications .............................. 133

9.5

Durability of Two-Part Acrylics ....................................................... 135

9.6

Durability of Epoxies ....................................................................... 135

9.7

Environmental Testing ..................................................................... 135

10. Troubleshooting ........................................................................................ 139 10.1 ‘No Glue’ – Inspecting for the Presence of Adhesive ........................ 139 10.1.1 ‘No Glue’ – Verifying the Adhesive Has Been Dispensed ..... 140 10.1.2 ‘No Glue’ – Air Bubbles and Voids...................................... 141 10.1.3 ‘No Glue’– Destructive and Non-destructive Methods ........ 142 10.1.4 ‘No Glue’ – Other Factors ................................................... 142 10.2 ‘No Cure’ ........................................................................................ 143 10.2.1 ‘No Cure’ – Odour .............................................................. 143 10.2.2 ‘No Cure’ – Factors Inhibiting Cure .................................... 143 10.2.3 ‘No Cure’ – Disturbing Partially Cured Adhesive ................ 144 10.2.4 ‘No Cure’ – Differential Scanning Calorimetry (DSC) ......... 144 10.2.5 ‘No Cure’ – Adhesive Curing Problems ............................... 145

v

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts 10.2.6 Blooming of Cyanoacrylates ............................................... 145 10.3 ‘No Stick’ ........................................................................................ 148 10.3.1 Theories of Adhesion .......................................................... 148 10.3.2 Cohesive ............................................................................. 149 10.3.3 Adhesive Failure .................................................................. 150 10.3.4 Substrate Failure ................................................................. 151 10.4 No Performance .............................................................................. 152 10.4.1 Surface Analysis .................................................................. 153 10.4.2 Defining the Failure Mode .................................................. 153 Abbreviations .................................................................................................... 155 Author Index..................................................................................................... 157 Subject Index..................................................................................................... 159

vi

P

reface

Plastics are an integral part of everyday life. There are huge numbers of manufacturing processes that use plastics either for the complete build or for sub-components within the assembly. Even components nominally composed entirely of metals may well include a polymeric material to coat, seal or adhesively bond the various components. There are countless different grades available and new trade names are introduced every year. Plastic manufacturers are able to formulate grades to meet the performance needs for almost any application. Not only is there a limitless number of plastics available but also there is a vast number of adhesives available. You only have to type the word ‘adhesives’ into an internet search engine and you will receive a cascade of data on adhesives of all types. This guide is written to help designers of plastic engineering components select an adhesive that may be suitable for their intended application. It is not intended for the packaging industry or for the bonding of composite materials and deliberately no mention is given to mechanical clips, ultrasonic welding or other plastic joining methods as these would be outside the scope of this guide. The adhesives discussed in this guide are aimed primarily at ‘small part’ bonding – parts typically (but not exclusively) smaller than an A4 pad. There are chapters on the bonding of ‘difficult’ plastics, joint design and dispensing systems but an adhesive cannot be selected for an application solely on the information given in this guide as it is not possible to provide environmental data for every combination of adhesive, plastic grade and joint configuration and so it is always recommended that adhesive manufacturers are consulted and trials are conducted. The guide discusses 30 of the most commonly used generic families of both thermoplastics and thermoset plastics and also includes a number of commonly used rubbers and elastomers. The final chapter discusses methods of troubleshooting possible reasons for an adhesive failure. Bob Goss

vii

1

Introduction to Adhesives

Choosing the best adhesive grade for a production application can be an exacting design task. This is especially true when joining dissimilar materials and when bonding certain engineering plastics. Adhesives can provide the optimum – indeed often the only – assembly method. However, it is all too often that the adhesive is not fully considered at the design stage. This can result in much time and trouble for engineers to get the prototypes into production. Four main types of adhesives are discussed in this book: •

Cyanoacrylates,

•

UV-curing acrylics,

•

Two-part acrylics, and

•

Epoxies.

There are of course many other types of adhesives suitable for bonding plastics and mention of some of these is given in Section 1.5.

1.1 Cyanoacrylates Cyanoacrylates are one of the most widely used adhesives for bonding small plastic parts. In liquid form cyanoacrylates or ‘super glues’ are stored in high-density polyethylene bottles to minimise the ingress of moisture into the liquid. These adhesives are generally available as colourless liquids and grades vary in viscosity range from 3 milliPascal seconds (mPa-s) (i.e., a thin liquid) through to a thixotropic gel for application to vertical surfaces or for highly porous materials. Cyanoacrylate technology has been available since the early 1960s and is extensively used by both industry and consumer alike. Considerable advancements in this technology have been made over the years and new grades and versions are not uncommon in the ‘New Product’ announcements in the Technical Press. 1

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Monomer

Acidic Stabiliser

Moisture

Figure 1.1 Curing of cyanoacrylates Cyanoacrylate adhesives cure readily on most surfaces where there are traces of moisture present. The moisture neutralises the stabiliser to initiate curing within a few seconds. In Figure 1.1 above, the large spheres represent the adhesive monomer and the smaller spheres represent the acidic stabiliser with the dark spheres representing the surface moisture. As the cyanoacrylate comes into contact with the surface moisture, the acidic stabiliser is neutralised and chains of adhesive molecules build up on the surfaces and inter-weave to bind the surfaces together and polymerise the adhesive. The cure speed of a cyanoacrylate, if left open on a surface (as on the left-hand side of Figure 1.2), will be relatively slow (several hours) because there is insufficient moisture (although the cyanoacrylate will cure at the surface interface). When the adhesive is between two close fitting surfaces (right-hand side of Figure 1.2), there is moisture on both surfaces and the cyanoacrylate will cure rapidly.

FAST CURE

SLOW CURE

Water Vapour

Monomer

Surface Moisture

Acidic Stabiliser

Monomer

Figure 1.2 Closing the joint The two major factors affecting cure speed are the percentage relative humidity and the gap. The optimum cure condition for cyanoacrylates is when the relative humidity (RH) is between 40% RH and 60% RH. Lower relative humidities, i.e., 20% RH, will result in a slower cure, and high RH (80% RH) results in a faster cure. High relative humidity can be detrimental as the cyanoacrylate sometimes cures so fast 2

Introduction to Adhesives that the adhesive polymerises before it has properly adhered to the surface and the resulting bond is poor. 100 90

% Strength

80 70 60 50 40 30 20 10

0

5 sec

30 sec

1 min 10 mins

1 hr

4 hrs 12–24 hrs

Figure 1.3 Cure speed of a cyanoacrylate The gap between parts should ideally be less than 0.1 mm and the thinner the gap the faster the cure; generally thin gaps will produce the strongest joints. Some grades of cyanoacrylate will fill gaps up to 0.5 mm and the ultraviolet (UV) curing grades are capable of curing through gaps up to 5 or 6 mm. Figure 1.3 above shows how the strength of a cyanoacrylate develops with time. Generally, cyanoacrylates will gain handling strength within the first minute or so but they continue to cure over the next 24 hours and can in some circumstances increase in strength two-fold in this time [1]. Data sheets for cyanoacrylates will quote ‘fixture speed’ or ‘handling strength’ and this will vary between different substrates for the same grade of cyanoacrylate adhesive. In a production situation the time to gain fixture strength is often a key factor in the cycle time and the ‘fixture strength’ will depend on the actual application but is generally regarded as the time when parts can be picked up gently without the adhesive joint failing. The ISO test for fixture strength is 0.1 N/mm2. The cyanoacrylate should not be disturbed during the critical time whilst it is polymerising, as the adhesive may never subsequently gain its full strength.

1.1.1 Types of Cyanoacrylate There are many different types of cyanoacrylate and so when designers are contemplating the use of an adhesive for specific project, they have to consider which type is most 3

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts appropriate – especially when it comes to production considerations and performance criteria. The following types of cyanoacrylate will be discussed in this section: •

Ethyl,

•

Methyl,

•

Alkoxy ethyl,

•

Surface insensitive,

•

Toughened,

•

Thermally resistant,

•

Flexible, and

•

UV-curing grades.

1.1.1.1 Ethyl Cyanoacrylates The ethyl cyanoacrylates are probably the most common of all the standard cyanoacrylates and the most widely used. The ethyl cyanoacrylates are best suited for bonding most plastics and elastomers to themselves and have excellent adhesion to polycarbonate, acrylonitrile butadiene styrene, poly(vinyl chloride) (PVC) and butyl rubber amongst many.

1.1.1.2 Methyl Cyanoacrylates With a smaller molecule size the methyl cyanoacrylates have better affinity to metals and can sometimes offer better resistance to solvents. The methyl grades of cyanoacrylate would often be specified for bonding plastic to metal or rubber to metal applications. The basic monomer is a very thin (low viscosity) liquid typically around 3–5 mPa-s. These methyl cyanoacrylate adhesives can be used to ‘wick-in’ to pre-assembled items such as for locking small adjustment screws or for increasing the strength of two press-fitted components.

1.1.1.3 Alkoxy Ethyl Cyanoacrylates Ethyl and methyl cyanoacrylates have a sharp odour and are eye and nasal irritants, especially in enclosed spaces or where large volumes of cyanoacrylate are present.

4

Introduction to Adhesives This high volatility of cyanoacrylates can lead to the formation of a white ‘bloom’ adjacent to the bond line (see Section 10.2.6). Alkoxy ethyl cyanoacrylates with a high molecular weight are available that virtually eliminate the odour and minimise the blooming. However, these high-molecular-weight products are slower curing and do not always offer the same adhesion performance as the standard ethyl grades. In some applications a slower curing product is useful as it can give the operator more time to align component parts during the assembly process.

1.1.1.4 Surface Insensitive Cyanoacrylates (For Acidic and Porous Substrates) Some substrates, for example paper, cork, cardboard, leather, dichromated metals and some fabrics, can be slightly acidic and this surface acidity will result in a very slow cure or in some circumstances inhibit curing completely. For these acidic substrates, a surface insensitive grade of cyanoacrylate must be used to ensure proper cure. The surface insensitive grades include special additives that can convert surface contamination into an activating species towards cyanoacrylates and then allow the adhesive to continue curing in the usual way. Such ‘surface insensitive’ cyanoacrylates are good general-purpose bonders – making them particularly suited to applications where the condition of the substrate is not well defined. This additive is now used in some consumer grades of ‘super glue’ because it reduces the dependence on surface moisture to cure and ultimately leads to good bond strength on many substrates.

1.1.1.5 Toughened Cyanoacrylates Cyanoacrylates can be toughened by the addition of rubber particles. The standard methyl or ethyl cyanoacrylates can have poor impact strength and low peel strength but modified ethyl cyanoacrylates were developed in the mid 1980s that show superior peel strength and often high humidity resistance. The principle of toughening the adhesive is that the rubber particles minimise the propagation of cracks as the adhesive is subjected to high peel or cleavage loads. Figure 1.4 shows a high peel load situation. The diagram on the left shows a crack propagating through the adhesive bond line on a standard ethyl cyanoacrylate. On the righthand side, the adhesive is toughened by the addition of rubber particles and so the crack only gets as far as the rubber particle and the stress is then dissipated and the crack has to reform only to meet another ‘crack arrester’. The resulting adhesive bond line is therefore considerably more resistant to peel and cleavage loads than

5

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts a conventional cyanoacrylate. However, the toughened cyanoacrylates are slower curing than standard grades.

Non Toughened

Toughened

Figure 1.4 Toughened cyanoacrylates offer better resistance to peel loads due to the presence of rubber particles in the adhesive matrix

Figure 1.5 Using a toughened cyanoacrylate to bond the rubber grip on to the handlebar of a trials motorcycle The rubber toughened cyanoacrylates are particularly well suited for rubber-to-metal bonding applications. In Figure 1.5 below, a toughened cyanoacrylate is used to bond the rubber grip onto the handlebars of a motorcycle. In this application, the benefit of using the rubber-toughened grade is that it is slower curing than a standard ethyl grade as, with a long engagement length and close fitting parts such as this, a standard cyanoacrylate would be too fast and it would not be possible to assemble the parts.

1.1.1.6 Thermally Resistant Cyanoacrylates The typical maximum operating temperature of a standard ethyl cyanoacrylate is 85 °C to 100 °C and the bond strength can fall rapidly after 100 °C (Figure 1.6). 6

Introduction to Adhesives

% RT Strength

100 75 50 25 0

0

150

50 100 Temperature, °C

Figure 1.6 Hot strength of a standard ethyl cyanoacrylate – measured on steel lap shears Allyl based cyanoacrylates were developed to improve the hot strength but these products required a secondary heat cure and parts to be clamped during the heat cycle to allow the allyl group to fully crosslink. Other cyanoacrylates with additives (such as phthalic anhydrides) have been formulated and these products do provide improved long-term strength at 120 °C.

% Initial Strength, measured at RT

For the cyanoacrylate with the high-temperature additives, the data are presented slightly differently as the adhesive gains hot strength as it is aged at temperature [1]. However, it can be seen (Figure 1.7) that there is still a 50% improvement in the performance. 30 °C

100

100 °C

76 121 °C

60 26

0

600

1000

1600

2000

Hours

Figure 1.7 Heat ageing strength curve for a cyanoacrylate with high-temperature additives

1.1.1.7 Flexible Cyanoacrylates Flexible cyanoacrylates were introduced in 2003 and are based on a modified ethyl monomer with considerably improved flexibility over the standard product. These 7

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts cyanoacrylates were developed for the loudspeaker industry (Figure 1.8), where a degree of flexibility in the coil to suspension bonded joint is desirable. The flexible grades of cyanoacrylate are also surface insensitive as they are used for the bonding of leather goods and fabrics and other similar applications where a flexible adhesive joint is required together with the ability to bond to these acidic substrates. Flexible cyanoacrylates have a slightly lower high-temperature resistance than standard ethyl grade products (maximum 75 °C).

Figure 1.8 A flexible adhesive is used to bond the suspension to the coil in a loudspeaker

1.1.1.8 UV-curing Cyanoacrylates One of the more recent introductions to cyanoacrylate technology are UV-curable grades. These are modified ethyl-based products with added photoinitiators absorbing UV light in the UVA region (around 365 nm). Light-curing cyanoacrylates cure almost immediately to a hard, tack-free finish in less than 3 seconds when exposed to highintensity UV light. Whereas most cyanoacrylates require a thin bond line (<0.2 mm), the UV-curing cyanoacrylates will cure through depths up to 5 mm or 6 mm when exposed to the appropriate light source. The adhesive that has been applied in areas not exposed to light then cures by the residual moisture mechanism found in regular cyanoacrylates. Light-curing cyanoacrylate adhesives provide processing and performance benefits for high-speed assembly processes. They offer high bond strength to a wide variety of substrates and achieve high performance with guaranteed complete cure.

8

Introduction to Adhesives

1.2 UV-curing Adhesives The ‘cure-on-demand’ properties of UV adhesives, together with rapid polymerisation, has led to their use in high-volume assembly processes where fast throughput and reduced work-in-progress levels offer real cost savings [2]. UV adhesives are generally single-part products allowing easy application. UV light must penetrate through to the bond line for the adhesive to cure, thus limiting at least one substrate to be clear or translucent to allow sufficient UV light to be transmitted. A UV-curing adhesive requires investment in cure equipment but does offer the benefits of curing on demand and optical clarity.

1.2.1 The Curing Process Light energy is defined by its wavelength and intensity. The wavelength is measured in nanometres (10–9 metres) and the intensity is measured in milliwatts/cm2 (mW/cm2). In order to cure a UV adhesive it is important to give the adhesive the correct wavelength (to match the photoinitiator) and the correct dosage of UV light. The total exposure (dosage) is measured in millijoules/cm2 (mJ/cm2) [watts × seconds = joules]. UV light is part of the electromagnetic spectrum and the wavelength is between 100 nm (nanometres) and 700 nm (Figure 1.9). Traditionally UV light is split into three categories: •

UVA (320–390 nm) UVA is part of natural sunlight and most ordinary light bulbs will produce a small amount of UVA. Human skin will tan under UVA but at low intensities UVA is not harmful.

•

UVB (280–320 nm) UVB is also found in sunlight and this has a shorter wavelength and a higher energy and thus is more aggressive than UVA. Many suntan lotions will filter out UVB.

•

UVC (200–280 nm) UVC has the highest energy and is therefore the most harmful of the UV wavelengths and can produce skin cancer. Lamps producing this wavelength should be properly shielded to protect operators.

9

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts The Electromagnetic Spectrum

104

100

1 UV

Gamma Rays

10–2

10–1

Visible Light

101

10–5

1/100 Infra-Red

103

Microwaves 104

eV Radiation Energy Radiowaves

105 nm Wavelength

VUV

100

UVC

200

UVB

300

UV

Visible Light

UVA

Blue

400

Green

500

Yellow

600

Red

700 nm

Figure 1.9 The electromagnetic spectrum

In addition to the three traditional UV categories, there are two further categories within the electromagnetic spectrum: •

Vacuum UV, which exists only in a vacuum between 100 and 200 nm.

•

UVV (UV Visible) (390–470 nm) This is at the visible end of the UV spectrum (the human eye can see light in the range 400–700 nm) and is not as harmful as UVA except at high intensities.

A UV adhesive contains photoinitiators and it is important that the adhesive is provided with sufficient UV energy to split the photoinitiator, thus creating a free radical which initiates the polymerisation of the adhesive. Figures 1.10–1.12 give a diagrammatic representation of the cure process.

Figure 1.10 The UV adhesive in liquid form 10

Introduction to Adhesives

Figure 1.11 UV light splits the photoinitiator

Figure 1.12 The adhesive polymerises Figures 1.10–1.12: Photoinitiator Reactive intermediate Monomer

Note that it is important to measure the UV intensity at the bond line and not at the UV source as the intensity can be significantly reduced away from the light source and especially as it passes through the substrate (Figure 1.13). Most UV adhesives require a minimum intensity at the bond line of 5 mW/cm2 but for most production applications the recommended intensity should not be less than 50 mW/cm2. Some light guide units will produce even higher intensities (>5000 mW/cm2) and, in general, the higher the intensity, the faster the cure. As a general rule, the amount of energy required to cure a UV acrylic adhesive is 1500 mJ/cm2. So if the intensity of the UV light (at the bond line) is 100 mW/cm2 then the cure time should be 15 seconds. 11

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 1.13 The intensity of UV light reduces as the distance and thickness of substrate increase Many of the original UV products (developed for glass bonding) cured at a wavelength of 365 nm (UVA) and this was ideal for glass bonding as the UVA wavelength is transmitted by most grades of glass. Many plastics, however (e.g., polycarbonate), can act as a UV filter (especially to UVA and to shorter wavelengths) and thus the UV adhesives with UVA photoinitiators would not cure under polycarbonate (see Figure 1.14). The wide use of polycarbonate and flexible PVC in the medical industry resulted in the development of a range of adhesives which were not only considerably more flexible but also cured at the visible end of the UV spectrum (UVV) 390 – 470 nm, thus overcoming the problem of absorbance at shorter wavelengths. Whilst these products are described as ‘visible light curing’ adhesives, it should be noted that an investment in UV equipment is still required for rapid curing.

420 (nm)

420 (nm)

420 (nm)

420 (nm)

365 (nm)

365 (nm)

365 (nm)

365 (nm)

220 (nm)

220 (nm)

Glass

Clear Plastic

Figure 1.14 Glass will filter out all wavelengths <300 nm and many plastics will filter out <400 nm 12

Introduction to Adhesives

1.2.2 Health and Safety with UV UV radiation can damage eyes and skin and UV equipment should be shielded to minimise any stray light. Use safety glasses and gloves and place dark or nonreflecting materials adjacent to the radiation equipment. Many UV bulbs will contain mercury and a proper disposal/absorption kit is required for damaged or broken bulbs.

1.2.3 The Curing Equipment It is important that the UV adhesive not only sees the correct frequency of UV but also has sufficient intensity. The UV lamp is an essential part of the process and many different UV lamps are available to suit a wide range of applications and budgets. Light sources can range from a simple bench-top open unit through to a fully automated conveyor system with several lamps, incorporating special fixturing to hold the components in place during the cure cycle. UV flood-light sources can produce relatively large quantities of infrared radiation [2], which can result in significant heating of the components if long cure times are required. In this situation care must be taken when positioning the components and in the design of fixtures. Special reflectors can also be used to focus the UV light for maximum efficiency and to reduce the infrared heat. UV light guides (or ‘wand’ systems as shown in Figure 1.15) are often specified for small components as these units produce high-intensity light over a diameter of about 10 mm.

Figure 1.15 A UV wand system 13

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Most UV lamps do not respond well to being switched on/off repeatedly as this can reduce the life of the UV bulb, but light-emitting-diode (LED) curing systems are now available and these can offer a much longer bulb life. The LED lamps (Figure 1.16) emit a relatively narrow spectral output at high intensity (Figure 1.17) so it is essential that the LED lamp matches the UV adhesive photoinitiator closely.

Reflector

Plastic lens Connecting wire

Cathode

Anode

LED chip

Figure 1.16 Basic diagram of an LED lamp 100 90

Radiant intensity (%)

80 70 60 50 40 30 20 10 0 300

350 400 Wavelength (nm)

450

Figure 1.17 LED spectral output

1.2.4 Curing Adhesive Tack-free If a UV adhesive is fully enclosed within two surfaces (as in the application shown in Figure 1.18) there is no adhesive outside the joint and so there is no requirement for a tack-free joint. 14

Introduction to Adhesives

Figure 1.18 Blood collection units are bonded together with a UV-curing acrylic There is no adhesive outside the joint and so no requirement for a tack free finish Where the adhesive may be exposed outside the joint (Figure 1.19), it is expected and it may be essential that the surface of the adhesive should be ‘tack-free’. Some UV adhesives (especially some of the earlier versions) are slightly anaerobic – that is they are inhibited from fully curing by the presence of oxygen in the surrounding atmosphere at the surface. This sticky or partially uncured adhesive layer is only a few microns thick but can attract dust and in some circumstances can contaminate adjacent components.

Figure 1.19 A UV acrylic is used to bond and seal this street lamp sensor and a tack-free surface is required on the exposed surface of the adhesive In Figure 1.20 the lamp is producing only UVA wavelength light and so the surface of the adhesive will be sticky. In order to overcome this oxygen inhibition effect,

15

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

365 nm

Figure 1.20 The surface of some UV adhesives will remain slightly tacky if the UV lamp produces only UVA a lamp that produces higher energy and shorter wavelength (UVC) is required (Figure 1.21) to ensure that a tack-free finish is achieved. The degree of tack-free cure is to some extent subjective and many of the latergeneration UV adhesives will be less susceptible to the oxygen inhibition effect. UV cyanoacrylates and UV epoxies, for example, will give excellent tack-free finishes even under relatively low-intensity UVA lamps.

1.2.5 Types of UV Adhesives UV acrylic formulations cure to form thermoset resins and can provide a variety of properties from extremely flexible (>250% elongation) to very hard and rigid with a high modulus. The UV adhesives are offered in a selection of viscosities from thin liquids (less than 100 mPa-s) to highly thixotropic gel consistency with a viscosity greater than 50,000 mPa-s – making them suitable for a wide range of applications.

254 nm

Figure 1.21 A UV lamp producing the UVC wavelength is required for a tack-free cure 16

Introduction to Adhesives Some UV adhesives have a secondary cure feature so that if the adhesive is ‘shadowed’ from the UV light it will cure by another cure mechanism. The principal types of adhesives available with these characteristics are: •

UV-curing cyanoacrylates – these products cure by surface moisture.

•

UV-curing anaerobics – although not widely used for plastics as anaerobic adhesives can stress crack many thermoplastics, these adhesives can be used on thermoset plastics and will cure due to absence of oxygen and metal part activity. The use of activators is sometimes required, especially if there are no metal parts present.

•

UV-curing silicones – these room temperature vulcanising (RTV) products will release a by-product and cure due to the presence of atmospheric moisture.

•

UV + heat cure – some UV adhesives can be heat cured – usually at temperatures >120 °C for around 15 minutes.

1.2.6 Benefits of UV Adhesives UV adhesives offer the following advantages to the user: •

Speed of cure – usually less than 30 seconds thus allowing increased productivity,

•

Cure on demand – the adhesive only cures when exposed to light thus allowing time for the alignment of component parts,

•

Optical clarity – most of the UV adhesives are clear and this is ideal for aesthetic or optical applications,

•

Single part – there is no need to mix adhesive or apply primer, and

•

Solvent free – UV adhesives are 100% solids and contain no hazardous solvents.

1.3 Two-part Acrylics Two-part acrylics are available either as two-part resins or as a resin for Part A with Part B as a liquid activator. The two-part resin products are formulated with Part A and Part B of similar viscosities and designed to be dispensed as bead-on-bead or mix-in-the-nozzle. Two-part acrylics (also referred to as methyl methacrylates or methylmethacrylates (MMA)) can offer benefits over cyanoacrylates with improved gap fill and typically 17

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts improved durability on metals. Many different grades are available and some are more suited for metals than plastics although they do not give good performance on elastomers, fluoropolymers or polyolefins. There is also a risk that they may stress crack certain thermoplastics (especially the liquid activators) and care is required to ensure that the solvent-based liquid activator is applied to the least susceptible surface. The MMA products are differentiated from other two-part adhesive systems like epoxies or polyurethanes by the fact that the cure is relatively insensitive to the mix ratio. They are comparatively inexpensive and although earlier versions were brittle when cured, the more recent developments are significantly tougher and more flexible generally give improved peel strength over cyanoacrylates. The mix-in-the-nozzle grades can cure in less than 60 seconds although more typically the cure time is 5–15 minutes to handling strength and 12–24 hours to full strength. Some grades of MMA give outstanding performance on unprepared or oily steel with fixture times of about 10 minutes. They will also withstand a paint bake cycle (and benefit from the heat cure) and so are used in some sheet metal industries to replace rivets. Some toughened acrylic grades can be difficult to dispense due to the ‘stringiness’ of the additives in the adhesive matrix and so difficult to manage on small component parts. They often have a high odour (although low-odour versions are available) and some acrylics are flammable. The MMA are usually toughened with rubber dissolved in the monomer (Figure 1.22) and it is these rubber particles that provide the high peel strength and toughness. MMA have proved to be extremely useful in the bonding of larger plastic components such as bonding automotive bumpers, fibreglass components and assembling boats. A more recent development in MMA technology are products that will bond polyolefin plastics successfully (see Section 6.4).

Non Toughened

Toughened

Figure 1.22 MMA are toughened with rubber particles 18

Introduction to Adhesives

1.4 Epoxies Epoxies are thermoset products normally prepared by mixing a resin and a hardener or curing agent. The mixing can be achieved by mixing the correct quantities by hand on a small pallet (Figure 1.23) or (more usually for production applications) by dispensing directly from the original containers via a helix nozzle (Figure 1.24). Precise measuring of the resin and hardener is essential to ensure the epoxy reaction is completed and thus the correct properties of the final adhesive are achieved. This critical mix ratio is perhaps a disadvantage of epoxies over other two-part adhesive systems. The helix nozzle ensures good mixing, provided there are sufficient mixing segments in the nozzle (Figure 1.25).

Figure 1.23 Mixing epoxy on a pallet

Figure 1.24 Mixing epoxy in a helix nozzle 19

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 1.25 An epoxy nozzle

Epoxies exotherm during the cure process and so when a large amount of mixed epoxy is dispensed onto a component part, the temperature will increase thus accelerating the cure time. Correspondingly if only small quantities of epoxy are dispensed, the cure speed may well be longer than quoted on the data sheet as there is insufficient mass of adhesive present to help drive the cure along. Epoxies can be heat cured and as a good rule of thumb the cure time will be halved for every 10 °C rise in temperature. Single-part epoxies are available (where the resin and hardener are pre-mixed) but these usually require refrigerated storage and a heat cure. Epoxies are extremely versatile adhesives due to the large number of combinations of epoxy resins and hardeners that are available, each of which can give a different molecular structure in the resulting cured polymer [3]. Additives include fillers, rubber tougheners, plasticisers and other modifiers to customise adhesives for specific applications. A range of electrically conductive (silver-filled) epoxies are available and these are becoming popular in electronics bonding and sealing applications. An electrically conductive adhesive will never have the same performance in terms 20

Introduction to Adhesives of conductivity or durability as a soldered joint (where a true inter-metallic layer is achieved) but they can offer high resistance to humidity and continuous service temperatures up to 140 °C [4]. Epoxies generally have excellent adhesion to metals, ceramics and glass, although on most amorphous thermoplastics epoxies will usually be outperformed by MMA, UV adhesives or cyanoacrylates. Epoxies will bond well to thermoset plastics and are widely used for bonding sheet moulding compound door and body panels in the transportation industries. Epoxies do not adhere well to elastomers, fluoropolymers or polyolefin plastics. They will fill very large gaps and have excellent thermal and environmental resistance and so are widely used for the potting and encapsulation of electronic components and printed circuit boards. Epoxies can be modified with various additives, fillers, plasticisers, rubber tougheners and other polymers and so there are a huge number of epoxy adhesives available and numerous specialist grades.

1.4.1 Advantages and Disadvantages of Epoxies Advantages include the high bond strength, adhesion to a wide range of materials and excellent solvent and water resistance. Epoxies will fill large gaps, which is a distinct advantage over cyanoacrylates and reactive acrylics. Epoxies are widely used and generally lower cost than UV or cyanoacrylates. They can often be the most durable and toughest of the adhesive technologies. Epoxies, however, are not always the best adhesive for bonding small engineering plastic components as the dispensing of the two-part versions can be problematic (see Section 8.1) and they are relatively slow to cure compared to most adhesive technologies (although the cure speed can be accelerated by heat). Some hardeners can be toxic and have a high odour and so this can cause health and safety issues in the workplace or at the dispensing station. Very-low-viscosity versions are generally not possible because the base resins often have a high molecular weight.

1.5 Flexible Adhesive Sealants Somewhere in the whole catalogue of adhesive families, the product is classified as a ‘sealant’ and not necessarily as an ‘adhesive’. The application itself will tend to define the difference as much as the generic adhesive type. Very often, in order for a sealant 21

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts to successfully fulfil its function, it must offer a good degree of adhesion against the surface it is sealing otherwise the media will permeate along the joint line between the sealant and the substrate (Figure 1.26). One possible method of distinguishing sealants and adhesives is to compare the typical cohesive strength of the products with flexibility (% extension). Figure 1.27 shows how some of the most common adhesives and sealants for the bonding of plastic materials might be classified. This graph is not entirely correct as there will always be exceptions to the categories but it does show that there is no distinct definition between an adhesive and a sealant.

A Figure 1.26 If the sealant does not adhere, a leak path is possible (arrow A)

Epoxies

Bonding UV Acrylics

CA

Elastic Bonding MMA

Elastic Sealing

PU Adhesives

elt

tm Ho

Cohesive Strength [N/mm2]

High performance bonding

PU, MS Elastic Bonding Silicones PU, MS Elastic Sealing

So lv Ba ent sed

Butyl tapes

Elongation [%]

Figure 1.27 Adhesives versus sealants CA = Cyanoacrylate adhesives, MS = modified silane adhesives, PU = polyurethane adhesives, MMA = methylmethacrylate adhesives, UV = ultraviolet cure adhesives

1.5.1 Silicone Adhesive Sealants Single-component silicone adhesives are usually RTV based and so cure slowly in the presence of atmospheric moisture. The RTV will release a by-product during the cure 22

Introduction to Adhesives cycle and the most common of these is acetic acid. These ‘acetoxy’ curing silicones will skin over in about 15–20 minutes and achieve a full depth of cure of 4–6 mm over 24–48 hours depending on the % RH at the time of cure (Figure 1.28).

Depth of Cure, mm

5 4 3 2 1 0 0

6

12

18

24

Cure Time, hours

Figure 1.28 Cure speed curve for an RTV acetoxy silicone (50% RH, 23 °C)

The acetic acid by-product can be corrosive, particularly to copper terminals on printed circuit boards and other electrical items. For this reason other RTV silicone sealants have been developed with oxime- or alkoxy-based by-products. These can be slightly slower to cure than the acetoxy-based products but often the adhesion to plastics is considerably improved. Two-component systems can also be formulated with no by-products and which cure very quickly. One of the main benefits of silicone technology is its outstanding temperature resistance (typically >250 °C) but a disadvantage is that the products cannot be over-painted after cure. In many industrial premises where there is a paint shop, RTV silicones are prohibited due to the contamination from the by-products.

1.5.2 Polyurethane Adhesive Sealants Polyurethane adhesives for plastics are available primarily in both single-component and two-component forms. They are very tough, flexible products with excellent peel strength and are also extremely versatile for the bonding of larger plastic components. Many grades of polyurethanes are certainly adhesives and not just sealants and most automotive windscreens are now bonded in with a polyurethane-based adhesive. The excellent adhesion, durability and good flexibility are key properties. Note that most 23

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts automotive manufacturers will include a black strip at the periphery of the windshield and this strip is not only to improve the aesthetics of the bond where the black polyurethane might have extruded outside the joint but it also serves as a UV protection to the adhesive as polyurethanes generally have poor resistance to ultraviolet light. Single-part polyurethanes rely on an atmospheric moisture cure and will release isocyanates during the cure cycle. Cure times are relatively slow with a skin-over time of about 20–30 minutes and complete cure in 3–7 days depending on the grade. The isocyanates can lead to health and safety concerns in the workplace as they would be classified as irritants and in some cases toxic. Two-part polyurethanes still release isocyanates but the cure time is considerably improved. Two-component polyurethanes are the most common type used for plastics bonding, although primers are sometimes required to improve adhesion.

1.5.3 Modified Silane Adhesive Sealants Modified silanes (MS) are odourless, non-corrosive and do not contain isocyanates, solvents, silicones or PVC. They are normally available as single-component adhesive sealants (although two-part systems are also available) and require atmospheric moisture during the cure cycle. When fully cured they are a flexible elastomeric product with excellent adhesion properties to a wide range of materials and are compatible with paint systems. MS products demonstrate good UV resistance and can therefore be used for interior and exterior applications. They do not usually have the cohesive strength of a polyurethane or the temperature resistance of a silicone but high- and low-modulus versions are available. Modified silane products bond well to most metals, glass and ceramics and depending on the grade will bond to PVC, polyamide and most polyesters [5]. Trials are recommended and roughening the surface will generally result in an increase in adhesion. Modified silane products are used for seam sealing in a wide variety of industries including the elastic bonding of metals and plastics used in side panelling and roof-skin bonding in vehicle and caravan manufacture; elastic interior and/or exterior seam and joint sealing in vehicle body construction, railway carriage construction, the air conditioning and ventilation industry, the electrical industry and for shower doors. The slow cure cycle of MS adhesive sealants has been addressed by some manufacturers and two-part (mix-in-the-nozzle) versions are increasingly being made available. The cure time on these products is typically less than 15 minutes.

24

Introduction to Adhesives

1.6 Hot Melt Adhesives Hot melts were introduced into the market many years ago and are widely used in the packaging, labelling, woodworking and bookbinding industries but are also used for bonding plastics. Hot melts are 100% solid single-component adhesives that are applied in a molten state and solidify by cooling. They do not contain any carrier material (e.g., water or solvent) and so can offer extremely fast setting times and excellent gap-filling capabilities [6]. Hot melts are available in a variety of forms including tapes, films, pellets, cylinders, cubes and blocks. For small part bonding, they are usually dispensed in ‘stick’ form via a heated hand-gun or in bulk from semiautomatic or automatic application systems. The two most important characteristics of hot melts are their open time and green strength. The open time is the time that the adhesive remains fluid enough after dispensing the adhesive before it will no longer wet the mating surface and thus create an effective bond. The green strength is a measure of the rate of cure and is often also referred to as the handling strength. These two factors are inexorably related as the longer the adhesive is left ‘open’ on a surface, the thicker it gets and so its ability to wet the surface reduces, thus leading to a drop in bond strength. Hot melts are easy-to-use, medium-strength adhesives and can be made rigid or flexible. They are thermoplastic materials and so typically have a maximum operating temperature of 90 °C but higher temperature grades are available (see Section 1.6.1). Hot melts are often used where the bond area is large as they can be difficult to dispense onto small components where the width of the bond is less than 5 mm, as the rheology of these products is such that they have a tendency to ‘string’ during the dispensing cycle. On larger parts, however, where there is a bigger bonded area and more scope for accommodating less precise dispensing criteria, they invariably show faster curing times than cyanoacrylates and certainly considerably improved gap-filling capabilities. One concern for hot melts is the relatively high application temperature (160–180 °C) as this can damage heat sensitive components and incur health and safety issues.

1.6.1 Reactive Hot Melts Reactive hot melts are single-component adhesives that are applied in the molten form (like conventional hot-melt adhesives) but have an additional secondary cure with atmospheric moisture to give a crosslinked polymer. Reactive hot melts will give 25

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts almost instant fixture strength as the hot melt solidifies and over the next few days will increase in strength as the secondary cure urethane element is completed. The reactive hot melts bond well to many substrates, and have good high-temperature performance and considerably improved cold flexibility over conventional hot melts. They also have excellent water and vapour resistance and improved solvent resistance and are used for major structural applications such as the assembly of large panels in insulated garage doors and for the lamination of PVC sections in window construction.

References 1.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2000, p.264 and p.273.

2.

D.E. Packham, Handbook of Adhesion, Longman Scientific & Technical, Harlow, UK, 1992, 369.

3.

D.J. Dunn, Engineering and Structural Adhesives, Rapra Review Report No.169, Rapra Technology Ltd, Shrewsbury, UK, 2004, p.6.

4.

Henkel Technical Data Sheet Loctite 3880, 2003.

5.

Henkel Technical Information Sheet Terostat MS 939, 2006, March.

6.

D.J. Dunn, Adhesives and Sealants – Technology, Applications and Markets, Rapra Market Report, Rapra Technology Ltd, Shrewsbury UK, 2003, p.34.

26

2

Engineering Thermoplastics

2.1 Introduction The annual consumption of engineering plastics continues to grow [1] and the commodity plastics (polyethylene, polypropylene, poly(vinyl chloride) (PVC) and polystyrene) make up approximately 75% of the total consumption. These commodity plastics are widely used in the packaging industry and in the building/ construction industries (water pipes, vessels, guttering, road cones). Engineering plastics is a term loosely used to describe thermoplastics and thermoset plastics which offer a range of engineering properties (stiffness, strength, toughness and heat resistance) and are used in sectors such as automotive, household appliances and medical devices, etc., and this adhesive guide focuses on these engineering materials as this is where most detailed design effort for the manufacture of small parts is concentrated. The automotive sector is perhaps the largest consumer of engineering and high performance plastics but other sectors (medical device, electrical and electronics, etc.,) are also considerable users. In the automotive sector, engineering polymers such as polybutylene terephthalate (PBT) are replacing polyamide in some car electrical components because of their improved dimensional stability. Also, with the ever increasing temperature requirements in under-bonnet applications, liquid crystal polymer (LCP) and polyphenylene sulfide (PPS) have also been replacing polyamide [1]. There are four main polymer classes (thermoplastics, thermosets, elastomers and thermoplastic elastomers) but thermoplastics fall into two distinct classes as amorphous thermoplastics and semi-crystalline thermoplastics (Figure 2.1).

2.2 Amorphous Thermoplastics Plastics that can be repeatedly solidified from the melt state without significant crystallisation are described as amorphous thermoplastics. Amorphous thermoplastics 27

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts have long macromolecule chains which are highly entangled with large side chains (Figure 2.2).

Polymer Classes

Thermoplastics

Amorphous

Thermoplastic Elastomers

Thermosets

SemiCrystalline

Elastomers

Figure 2.1 Polymer classes Amorphous thermoplastics are frequently used in applications where clarity is important and in applications where they may be thermo-processed (e.g., ultrasonic welding). They can be moulded to a high-dimensional accuracy and stability, and have a good appearance and mechanical properties (depending on the grade). At elevated temperatures or at high stress levels, the polymer chains can uncoil and thus alter the properties of the polymer. Generally, amorphous thermoplastics are less chemically resistant than semi-crystalline thermoplastics and can be subject to stress cracking and can swell or dissolve in some solvents.

Figure 2.2 Amorphous thermoplastic

28

Engineering Thermoplastics Some typical examples of amorphous thermoplastics are acrylonitrile-butadiene-styrene (ABS), polymethylmethacrylate (PMMA), polystyrene (PS) and polycarbonate (PC). Applications for amorphous thermoplastics vary from car bumpers to Lego® bricks, and from motorcycle helmets to ski boots.

2.3 Semi-crystalline Polymers Semi-crystalline polymers are plastics that contain areas of both crystalline molecular structure and amorphous regions as well (Figure 2.3). The degree of crystallisation depends on the chemical structure (generic type), the rate of cooling and the thermal history in the post-moulded state. Very rapid cooling in thin sections can result in insignificant levels of crystallisation and this is used to great effect in the production of transparent film (e.g., polyethylene (PE), polyethylene terephthalate and polypropylene (PP)). Increased crystallisation increases and improves hardness, modulus, strength, abrasion resistance, shrinkage, density and opacity. Semi-crystalline thermoplastics are generally used in applications where high fatigue resistance or repeated cyclic loading is involved (bearings, linkages, etc.). They are often reinforced with glass fibres and this considerably improves the high-temperature stiffness and the strength. However, the shrinkage can be variable and difficult to predict. Semi-crystalline thermoplastics are noted for very good electrical properties as well as the ability to withstand both high heat and severe chemical environments. Examples of semi-crystalline thermoplastics include polyamide, polyarylamide, LCP and PBT [1].

Figure 2.3 Semi-crystalline plastic 29

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts The amorphous thermoplastics are generally easier to adhesively bond than the semicrystalline thermoplastics, although some nylons will bond quite well (Table 2.1).

Table 2.1 Some examples of amorphous and semi-crystalline thermoplastics Amorphous Semi-crystalline Polyphenylene oxide (PPO) (Noryl) Polyetheretherketone (PEEK) PC Polytetrafluoroethylene (PTFE) Polyethersulfone (PES) PBT PS Polyamide (PA) Polysulfone (PUS) PPS Polyamide (amorphous) (Nylon) Polyethylene terephthalate (PET) PMMA, Acrylic Polyoxymethylene (POM) (Acetal) PVC PP ABS PE Ionomers LCP The glass transition temperature (Tg) is an important property of thermoplastics and is the temperature at which the plastic will change in its mechanical properties (depending on the degree of crystallinity) due to the onset of the breakdown of the physical bonds within its structure (see Figure 2.4). The glass transition

10 102 103 104

B

10−2

10−1

1

A

10−3

Relaxation Modulus (MPa)

Glass transition region

50

100 Tg

150

200 Tm

Temperature °C

Figure 2.4 Influence of temperature on relaxation modulus for thermoplastics ((A) amorphous, and (B) semi-crystalline) 30

Engineering Thermoplastics temperature is not a sharp temperature point but the average value of the transition range. Thermoplastics are elastic and flexible above a glass transition temperature Tg, specific for each one, and in some cases a low Tg is useful and in other applications a high Tg is required. A polyethylene ice box container has a very low Tg as it is essential that it remains flexible at temperatures below −20 °C. A blood filter must retain all of its structural integrity at temperatures >37 °C and therefore polycarbonate is often the selected plastic as it has a high Tg (amongst other properties).

2.4 Adhesive Performance on Thermoplastics In this section, a guide is given to indicate the performance of several different adhesives for a selection of thermoplastics. The bulk of this information was taken from ‘The Loctite Design Guide for Bonding Plastics’ [2] issued by Henkel Ltd and for more detailed information the reader should refer to this guide. An adhesive cannot be selected solely on the basis of bond strength information as other factors such as cure speed, environmental resistance and dispensing method will all influence the final choice. The values given for the shear strength are given to provide a general idea of how each of the nine adhesives performed on the particular material. Different performances will be achieved depending on the fillers, lubricants, colourants and anti-static additives. The nine different adhesives selected for these charts were as follows: •

Three cyanoacrylates (standard ethyl, toughened grade and ethyl + primer),

•

Two acrylics (a standard methylmethacrylate (MMA) and a polyolefin bonder),

•

Two epoxies (a 5-minute epoxy and a longer cure grade),

•

One room-temperature-vulcanising silicone (alkoxy cure), and

•

One ultraviolet (UV) acrylic (cure at 420 nm).

A brief description of each adhesive is given, together with a few general comments on the overall performance of the adhesives. The alkoxy silicone often showed a strength of 1.4 N/mm2 for many of the plastics tested but in this case the adhesive has failed cohesively. A silicone adhesive is often used where sealing might be the main priority of the adhesive rather than structural strength.

31

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

2.4.1 ABS ABS is a generic name for a family of amorphous thermoplastics produced by combining three monomers: acrylonitrile, butadiene and styrene. Different ratios of these monomers provide variations in strength, stiffness, impact resistance and surface appearance and so there are many different versions of ABS, each with their own particular properties. Many adhesives are stronger than the substrate itself when tested in tensile shear (Table 2.2) and ABS would typically be regarded as a relatively easy-to-bond material. However, ABS can sometimes be stress cracked by some adhesives.

2.4.2 LCP Liquid crystal polymers (also called aromatic copolyesters) have outstanding mechanical properties at both ambient and extreme temperatures. They are also highly resistant to many chemicals and so are used for many automotive under-bonnet and aerospace applications. A number of filled and reinforced grades are available and this material can be processed with very short cycle times.

Table 2.2 Adhesive shear strengths (ABS) Adhesive type Description Shear strength (N/mm2) Standard ethyl >24.1* Toughened >16.6* Cyanoacrylate Ethyl + primer >23.1* MMA 11.7 Two-part acrylic Polyolefin bonder 13.8 ‘5-minute’ epoxy 3.1 Epoxy Standard epoxy 12.4 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure >24.1* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

32

Engineering Thermoplastics

Table 2.3 Adhesive shear strengths (LCP) Adhesive type Description Shear strength (N/mm2) Standard ethyl 5.2 Cyanoacrylate Toughened 3.5 Ethyl + primer 2.8 MMA 3.8 Two-part acrylic Polyolefin bonder 3.1 ‘5-minute’ epoxy 4.1 Epoxy Standard epoxy 6.9 Silicone Alkoxy silicone 1.0 UV acrylic Visible light cure 4.5 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

LCP is flame resistant and so is often used in the electrical industry for stator insulation and connectors, etc. Surface roughening of LCP typically improves adhesion and this plastic can be bonded with most adhesives with epoxies generally giving the highest tensile shear strengths (Table 2.3).

2.4.3 Polyamide Polyamide Nylon was one of the earliest plastics and has been commercially available since the late 1930s. The standard grades are PA6, PA66 and PA12 but there area also many speciality grades available including lubricated, plasticised, flame retardant and, perhaps most common of all, glass filled. Polyamide is tough, inexpensive and has good resistance to heat and chemicals but polyamides can sometimes have poor dimensional stability due to water absorption. The main advantage of PA6 over PA66 is that it is easier to process and produces lower mould shrinkage but PA66 has better low-temperature toughness and good fatigue resistance.

33

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts PA12 has lower water absorption and improved UV resistance but has a lower temperature resistance than the other polyamide grades. Polyamides are used in a wide range of applications including under-bonnet applications such as the air intake manifold, and the air and cooling system peripherals. It is also used to manufacture electrical components such as switches, connectors and contactors. Applications are as diverse as castor wheels for furniture to aerosol valves. Polyamides can usually be bonded quite easily (Table 2.4) but long-term durability can be an issue in some applications due to water absorption. The alkoxy silicone product shows good strength and durability on polyamide and the result of 1.7 N/mm2 is a measure of the cohesive strength of the silicone rather than the adhesive strength to the base material.

2.4.4 PBT The most notable properties of PBT are its chemical resistance and mechanical properties. It is also noted for its high stiffness and strength, low water absorption and high dimensional stability.

Table 2.4 Adhesive shear strengths (polyamide) Adhesive type Description Shear strength (N/mm2) Standard ethyl >25.0* Toughened 16.9 Cyanoacrylate Ethyl + primer 11.0 MMA 6.6 Two-part acrylic Polyolefin bonder 3.8 ‘5-minute’ epoxy 2.8 Epoxy Standard epoxy 5.5 Silicone Alkoxy silicone 1.7 UV acrylic Visible light cure 1.4 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

34

Engineering Thermoplastics There are a number of different grades and types of PBT, including glass reinforced grades. It can be used for single-use medical devices but not for devices that might be subjected to repeated autoclaving as it does not have sufficient temperature resistance. PBT polymer blends combine the properties of partially crystalline PBT with those of amorphous thermoplastics such as polycarbonate or ABS. The amorphous partner improves the impact strength and warpage behaviour, while the PBT ensures the chemical resistance of the blend. PBT is used in the electrical lighting industry for its electrical insulation properties, dimensional stability and colour stability. It is also used in this industry for printedcircuit-board connectors and sockets, junction boxes and housings for switches and capacitors. PBT would typically be used for car interior or exterior components rather than underbonnet applications, including ash trays, windscreen wiper arms and door handles, or where a high-quality, weather-resistant surface is a key requirement. In these trials [2], (Table 2.5) the cyanoacrylate with a primer showed excellent results but bonding PBT with engineering adhesives is often variable as it can depend on the

Adhesive type

Table 2.5 Adhesive shear strengths (PBT) Description Shear strength (N/mm2) Standard ethyl 1.7

Cyanoacrylate

Toughened Ethyl + primer

0.7 >21.9*

MMA

2.4

Polyolefin bonder

7.6

‘5-minute’ epoxy

3.8

Standard epoxy

4.8

Silicone

Alkoxy silicone

1.4

UV acrylic

Visible light cure

1.4

Two-part acrylic Epoxy

*

Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

35

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts particular grade and in some applications a surface pre-treatment may be necessary. Roughening of the surface generally shows an improvement in bond strength.

2.4.5 PC There is a wide range of polycarbonate resins and compounds available and they are often blended with ABS for greater versatility. Polycarbonate is often selected for its strength and clarity and its ability to be moulded into almost any shape and so is used as a replacement for glass for safety goggles, lenses, electrical switch panels and business machine housings. PC/ABS is used in the automotive sector for instrument panels and loudspeaker chassis and grilles. Polycarbonate is also widely used in the medical industry for items such as blood centrifuge bowls, safety syringes and intravenous connectors as it is generally biocompatible. Polycarbonate is usually easy to bond with many adhesives giving higher strengths than the substrate material. However, some adhesives will stress crack polycarbonate. The use of cyanoacrylate primer showed a statistical decrease in strength in these trials [2], but substrate failure was achieved with both a standard cyanoacrylate and a UV acrylic (Table 2.6). The epoxy also showed excellent strengths on this material.

Table 2.6 Adhesive shear strengths (polycarbonate) Adhesive type Description Shear strength (N/mm2) Standard ethyl >26.6* Toughened 5.2 Cyanoacrylate Ethyl + primer 13.8 MMA 7.8 Two-part acrylic Polyolefin bonder 5.9 ‘5-minute’ epoxy 6.2 Epoxy Standard epoxy 18.3 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure >25.5* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

36

Engineering Thermoplastics

2.4.6 PEEK PEEK is a high-performance thermoplastic which is well suited for high-temperature environments. It is often one of the highest-priced engineering plastics but does offer a unique combination of properties. The high-temperature performance (continuous service temperature up to 200 °C) and excellent mechanical and electrical properties make it one of the leading contenders as a replacement for metal parts. It also has outstanding radiation resistance (for medical applications) and very good sunlight and weathering resistance. PEEK can be processed on all conventional processing technologies and so this material is often used in demanding and niche high-temperature engineering plastic applications. It has been used in a wide variety of applications including chemical pumps, coffee machines and microchips. PEEK is also easy to machine and so is often used in industry for the production of high-precision plastic components in small batches. PEEK is difficult to bond with engineering adhesives due to its low surface energy and usually requires some form of surface treatment (e.g., plasma) to give a good bond (Table 2.7).

Table 2.7 Adhesive shear strengths (PEEK) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.7 Cyanoacrylate Toughened 1.0 Ethyl + primer 1.7 MMA 2.1 Two-part acrylic Polyolefin bonder 2.1 ‘5-minute’ epoxy 1.7 Epoxy Standard epoxy 3.5 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 3.6 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

37

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

2.4.7 PES and PUS PES is a high-temperature amorphous thermoplastic and is used in applications where high stiffness and high continuous operating temperatures are a pre-requisite. PES can, however, be degraded by UV light and so has a low resistance to weathering. PES can be sterilised repeatedly (autoclaved) and so is used for some medical devices including instrument trays and infusion equipment. The major disadvantage to adhesively joining PES (and polysulfone) is that they are extremely prone to stress cracking by uncured adhesives and so any excess uncured adhesive should be either cured or removed from the surface immediately. The UV acrylic gave excellent results on the grade of polyethersulfone tested [2] (Table 2.8) but good results were also achieved with the specialist ‘polyolefin bonder’, a 10:1 two-part acrylic.

Table 2.8 Adhesive shear strengths (PES) Adhesive type Description Shear strength (N/mm2) (PES) Standard ethyl 11.0 Cyanoacrylate Toughened 4.5 Ethyl + primer 1.0 MMA 6.9 Two-part acrylic Polyolefin bonder 13.8 ‘5-minute’ epoxy 3.1 Epoxy Standard epoxy 4.5 Silicone Alkoxy silicone 1.0 UV acrylic Visible light cure 21.0 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

38

Engineering Thermoplastics

2.4.8 PE Polyethylene is a lightweight, semi-crystalline thermoplastic and is available in the following major physical grades: •

Low-density polyethylene,

•

Medium-density polyethylene,

•

High-density polyethylene, and

•

Ultra-high-molecular-weight polyethylene.

Polyethylene is not generally regarded as a high-performance engineering plastic but it is extremely versatile and inexpensive and so is one of the most popular of all plastics. It offers good or excellent resistance to many chemicals and can be processed by all conventional methods including rotational moulding. Polyethylene has a low surface energy (31 mN/m) and so will generally require surface preparation prior to bonding. The two-part acrylic ‘polyolefin bonder’ showed good strengths in these trials [2] (Table 2.9).

Table 2.9 Adhesive shear strengths (polyethylene) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.0 Cyanoacrylate Toughened <0.3 Ethyl + primer 3.5 MMA 1.0 Two-part acrylic Polyolefin bonder >9.7* ‘5-minute’ epoxy 1.4 Epoxy Standard epoxy 1.0 Silicone Alkoxy silicone <0.3 UV acrylic Visible light cure 2.4 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

39

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

2.4.9 PET The clarity, toughness and barrier properties make PET best suited for its primary market – blow-moulded carbonated soft drinks containers. Speciality grades include flame retardant, impact modified and glass, mineral, carbon, PTFE and mica filled. Reinforced PET is probably the stiffest of all engineering thermoplastics and has outstanding weatherability and heat resistance. Outside of the drinks industry, PET is often used for photographic film, recording tape, flexible printed circuit boards, transformer insulation and transparent stationery. Bonding PET with cyanoacrylates often created bonds that were stronger than the PET substrate but some epoxies offered significantly lower strengths (Table 2.10).

Table 2.10 Adhesive shear strengths (PET) Adhesive type Description Shear strength (N/mm2) Standard ethyl >22.1* Toughened 3.8 Cyanoacrylate Ethyl + primer >12.1* MMA 2.4 Two-part acrylic Polyolefin bonder 3.1 ‘5-minute’ epoxy 2.1 Epoxy Standard epoxy 3.1 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 7.9 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

2.4.10 PMMA PMMA is the most common member of the acrylic family and widely known as the ICI trade name of ‘Perspex’. Acrylic has excellent transparency and no appreciable yellowing under sunlight. They are therefore used in the construction industry and in the electrical industry for lighting systems and lenses.

40

Engineering Thermoplastics

Table 2.11 Adhesive shear strengths (PMMA) Adhesive type Description Shear strength (N/mm2) Standard ethyl >27.2* Toughened 4.1 Cyanoacrylate Ethyl + primer 1.7 MMA 6.6 Two-part acrylic Polyolefin bonder 12.1 ‘5-minute’ epoxy 2.1 Epoxy Standard epoxy 6.9 Silicone Alkoxy silicone 0.1 UV acrylic Visible light cure 12.1 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

Acrylics are not usually recommended for high-temperature applications but they do offer good chemical resistance. Acrylics can be bonded with most adhesives and in these trials [2] cyanoacrylates (without primer) and UV-curing acrylics gave best results (Table 2.11). The silicone adhesive did not show good strengths.

2.4.11 POM POM is widely known as ‘acetal’ and a well-known trade name is ‘Delrin’, manufactured by DuPont. This is a popular engineering plastic to replace parts that might otherwise be manufactured from metals. It is generally easy to machine and therefore makes for a good engineering plastic for small batch production or prototype applications. Acetal is a highly crystalline polymer with good abrasion resistance, low water absorption and favourable frictional and wear properties. It exhibits high physical strength as well as excellent creep and impact resistance. Applications include bearings, gears and conveyor belt links but also chemical pumps and, due to its resilience and recovery, it is also used for clips and snap-fit connections. 41

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 2.12 Adhesive shear strengths (acetal) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.4 Cyanoacrylate Toughened 0.7 Ethyl + primer 11.7 MMA 1.4 Two-part acrylic Polyolefin bonder 2.4 ‘5-minute’ epoxy 1.7 Epoxy Standard epoxy 2.1 Silicone Alkoxy silicone 0.3 UV acrylic Visible light cure 1.7 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

Like PEEK, this material is also difficult to bond (Table 2.12) with engineering adhesives and usually requires some form of surface treatment to give a good bond.

2.4.12 PPO PPO is an engineering thermoplastic known for its excellent radiation resistance, oxidation resistance, thermal stability and electrical properties. Typical applications include television cabinets, car spoilers and laptop computer outer shells. PPO has one of the lowest water absorption rates of any of the engineering thermoplastics and has excellent flame retardance, electrical properties and impact strength. One of the best-known trade names for this material is Noryl (GE Plastics) and this is not only widely used in the automotive industry, but also in the building and construction sector and for switchgear, etc. PPO is relatively easy to bond but can be stress cracked by uncured cyanoacrylates, UV acrylics and solvent-based activators and primers. Best results were achieved in these trials [2] with a standard cyanoacrylate and the use of primer only produced a marginal increase in strength (Table 2.13). All the other adhesives tested gave medium to lower strengths (including the toughened cyanoacrylate).

42

Engineering Thermoplastics

Table 2.13 Adhesive shear strengths (PPO) Adhesive type Description Shear strength (N/mm2) Standard ethyl 11.0 Cyanoacrylate Toughened 3.5 Ethyl + primer 12.1 MMA 2.1 Two-part acrylic Polyolefin bonder 9.3 ‘5-minute’ epoxy 1.4 Epoxy Standard epoxy 5.9 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 6.6 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

2.4.13 PPS PPS is a crystalline polymer with a relatively high melt temperature (285 °C). It can sometimes be difficult to process but it does offer an excellent combination of longterm thermal stability and superior chemical resistance below 200 °C. Its inherent flame resistance and good mechanical properties make this material a popular choice for high-quality moulded parts that are uniform and reproducible. PPS is widely used in the automotive industry in fuel systems, coolant systems, brake systems and many other under-bonnet applications. Outside of the automotive industry it is used for various components including pumps, impellors and valves, motor relays and some thermostat components. Although shear strengths achieved in these trials [2] were relatively low, roughening the PPS generally gives a significant increase in bond strengths (Table 2.14).

2.4.14 PP Polypropylene is one of the lightest engineering thermoplastics (SG = 0.90) and it has excellent moisture resistance. One of the major disadvantages of polypropylene is its poor impact strength at low temperatures. It does, however, offer excellent fatigue resistance and it is widely used for luggage, packaging, toys and storage battery cases.

43

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 2.14 Adhesive shear strengths (PPS) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.0 Cyanoacrylate Toughened 0.7 Ethyl + primer 2.8 MMA 2.1 Two-part acrylic Polyolefin bonder 4.1 ‘5-minute’ epoxy 1.0 Epoxy Standard epoxy 5.5 Silicone Alkoxy silicone 0.7 UV acrylic Visible light cure 3.8 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

Table 2.15 Adhesive shear strengths (polypropylene) Adhesive type Description Shear strength (N/mm2) Standard ethyl 0.3 Cyanoacrylate Toughened 0.3 Ethyl + primer >13.5* MMA <0.3 Two-part acrylic Polyolefin bonder 12.4 ‘5-minute’ epoxy 0.6 Epoxy Standard epoxy 0.6 Silicone Alkoxy silicone <0.3 UV acrylic Visible light cure 0.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

44

Engineering Thermoplastics Unfilled PP is flammable and degraded by UV light; however, flame retardant and UV-stabilised grades are available. Polypropylene is used in a very wide range of applications from pipes for chemical plant to automotive cooling fans and tool handles. PP has a very low surface energy (29 mN/m) and so adhesives do not readily wet the surface. In addition, polypropylene is a very non-polar polymer consisting entirely of carbon and hydrogen atoms, whereas most adhesives contain oxygen, nitrogen and other electron-rich atoms and are polar materials. The carbon and hydrogen atoms in polyolefins are very unreactive towards many chemicals, thus precluding adhesion through chemical reactions. Polypropylene, therefore, generally requires pre-treatment prior to bonding although the polyolefin bonder did give excellent strengths in these trials [2] (Table 2.15).

2.4.15 PS Polystyrene is a low-cost commodity plastic and is available in three main categories: •

Crystal polystyrene,

•

High-impact polystyrene (HIPS), and

•

Expanded polystyrene (EPS).

Crystal polystyrene is an amorphous polymer with superior clarity but generally lower impact strength than polycarbonate. HIPS is a toughened grade with reduced optical clarity. EPS is a low-density foam and so is very widely used as a packaging material or as an insulating material. Crystal polystyrene and HIPS can usually be bonded with most engineering adhesives although polystyrene can be susceptible to stress cracking by some activators, primers and accelerators. EPS is not easy to bond and is attacked by many adhesives although hot melts can show good results. In these trials [2], the cyanoacrylate primer showed an improvement in bond strength but in many applications HIPS can be bonded with cyanoacrylates satisfactorily

45

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 2.16 Adhesive shear strengths (HIPS) Adhesive type Description Shear strength (N/mm2) (HIPS) Standard ethyl 9.3 Cyanoacrylate Toughened 3.1 Ethyl + primer 12.1 MMA 4.8 Two-part acrylic Polyolefin bonder 6.2 ‘5-minute’ epoxy 2.4 Epoxy Standard epoxy 3.5 Silicone Alkoxy silicone 0.7 UV acrylic Visible light cure 9.3 (Crystal) Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

without the need for primer (Table 2.16). HIPS is often opaque and so UV acrylics may not be suitable if this is the case.

2.4.16 PTFE The highly crystalline fluoropolymers include fluorinated ethylene polyethylene, perfluoroalkoxyethylene, polychlorotrifluoroethylene and ethylene-tetrafluoroethylene co-polymer but PTFE is perhaps the most widely used and certainly best known as ‘Teflon’ in non-stick cookware. The coefficient of friction of PTFE is lower than almost any other material and it has excellent temperature and mechanical properties although it does have a tendency to creep. The outstanding chemical resistance and electrical properties mean that it is often used in applications that require long-term performance in extreme service environments. Like PEEK and POM, this material has a very low surface energy and so will require some form of surface treatment prior to bonding with engineering adhesives (Table 2.17). Cyanoacrylate with primer shows some adhesion but etching kits for the fluoropolymers are also available.

46

Engineering Thermoplastics

Table 2.17 Adhesive shear strengths (PTFE) Adhesive type Description Shear strength (N/mm2) Standard ethyl 2.4 Cyanoacrylate Toughened 1.4 Ethyl + primer 7.2 MMA 0.3 Two-part acrylic Polyolefin bonder 3.1 ‘5-minute’ epoxy 0.7 Epoxy Standard epoxy 0.3 Silicone Alkoxy silicone <0.3 UV acrylic Visible light cure 1.0 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

2.4.17 PVC PVC is the most widely used of all the vinyls and is available in many forms, from soft flexible vinyl (used for car seat covers, medical devices and dinghies) through to rigid vinyl (used in toy manufacture, plumbing and pipework fittings and window construction). PVC is readily bonded by most engineering adhesives (Table 2.18) although it can be stress cracked by some slower curing cyanoacrylates. Cyanoacrylates would not be recommended for the repair or manufacture of rubber dinghies and fishponds and so on, as the combined effects of water immersion and flexibility are likely to be detrimental to this family of adhesives. For flexible clear PVC medical device tubing, the UV acrylics are often used.

2.5 General Comments on Adhesive Bonding of Thermoplastics Structural adhesives for plastics are more often than not epoxies, two-part acrylics, cyanoacrylates or UV-cure adhesives. There are hundreds of different epoxy grades available and, whilst many of these are far more suited for bonding metals than thermoplastics, epoxies are used for some

47

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 2.18 Adhesive shear strengths (PVC) Adhesive type Description Shear strength (N/mm2) Standard ethyl >25.2* Toughened >11.0* Cyanoacrylate Ethyl + primer >19.7* MMA 16.2 Two-part acrylic Polyolefin bonder 10.0 ‘5-minute’ epoxy 2.8 Epoxy Standard epoxy 9.3 Silicone Alkoxy silicone 1.0 UV acrylic Visible light cure >17.6* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

thermoplastic applications, especially if the gaps are relatively large or if the adhesive is to be used as a potting or encapsulation material as well as an adhesive. Epoxies will not bond to the non-polar plastics (polyethylene, polypropylene or the fluoropolymers but they do show good adhesion to ABS, polyamide, polycarbonate and other similar widely used materials. As a ‘rule of thumb’, the faster-curing (5-minute) epoxies often show lower strengths on plastics than the longer curing grades. Two-part acrylics (or MMA) bond exceptionally well to many thermoplastics and these products are often used for the assembly of point-of-sale plastic assemblies where ease of application, strength and aesthetics are important. Cyanoacrylates are probably the most versatile adhesives for the bonding of thermoplastics [3] as they are single-part, fast-curing adhesives with good strength and can offer excellent durability if the joint design and plastic grades are chosen carefully at the onset (see Section 7.2.2). Cyanoacrylates can be toughened and are particularly suitable for bonding rubber to plastics if over-moulding is not possible. Cyanoacrylates are typically suitable for temperatures up to 100 °C, although some speciality cyanoacrylates are available for higher operating temperatures. One of the major benefits of cyanoacrylates is that they can be used (with a primer) to bond the non-polar plastics. More details about these primers are given in Section 6.2

48

Engineering Thermoplastics and, although this primer makes the adhesive system two-part, it can be a lower cost method for production than an investment in other surface treatments such as plasma or corona treatment. Cyanoacrylates are best suited where the gaps are small (<0.15 mm) and although some grades will fill bigger gaps there is always a risk of ‘blooming’ (see Section 10.2.6). The three major benefits of UV-curing adhesives are the speed of cure, the optical clarity and the ‘cure on demand’. Although an investment in UV-curing equipment is required, these adhesives are successfully used on transparent substrates like polycarbonate and PVC [4].

References 1.

D. Platt, Engineering and High Performance Plastics, Rapra Technology Ltd, Shrewsbury, UK, 2003.

2.

The Loctite Guide to Bonding Plastics, Henkel Ltd, Hatfield, UK, 2006, p.4.

3.

D.J. Dunn, Engineering and Structural Adhesives, Rapra Review Report No. 169, Rapra Technology Ltd, Shrewsbury, UK, 2004.

4.

B. Goss in Proceedings of the 2nd Rapra Technology Conference on Joining Plastics, London, UK, 2006, Paper No. 4.

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3

Engineering Thermoset Plastics

3.1 Introduction There are now so many thermoset plastics available that it can be difficult to differentiate between the various types. Thermoset resins are used for the manufacture of a wide range of parts from canoes to wind turbine blades and from polymer concrete to epoxy adhesives and so there is only a brief discussion of thermoset plastics included in this guide as it is essentially aimed at the bonding of small parts. Thermosetting plastics (thermosets) are polymer materials that irreversibly cure, to a chemically crosslinked stronger form [1]. Once formed and cooled they cannot be reprocessed and will decompose before they can melt. Thermoset materials are generally stronger than thermoplastic materials due to their rigid three-dimensional network of bonds and are also better suited to high-temperature applications up to the decomposition temperature of the material, see Figure 3.1.

Figure 3.1 Thermoset plastics 51

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Therefore, a thermoset material cannot be melted and re-shaped after it is cured and so cannot be recycled, except as filler material [2]. Some examples of thermosets are: •

Vulcanised rubber,

•

Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plasticware),

•

Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard),

•

Melamine resin (used on worktop surfaces),

•

Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic), and

•

Polyimide (used in printed circuit boards and in body parts of modern airplanes).

Some methods of moulding thermosets are: •

Reactive injection moulding (used for objects like milk bottle crates),

•

Extrusion moulding (used for making pipes, threads of fabric and insulation for electrical cables),

•

Compression moulding (used to shape most thermosetting plastics), and

•

Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines and emblems as well as production and replacement parts).

Thermoset plastics differ from thermoplastics in that they are crosslinked when they are polymerised and so do not melt prior to thermal degradation. Some of the advantages of thermosets include: •

Generally improved temperature resistance over thermoplastics,

•

Lower creep properties and stress relaxation than thermoplastics,

•

Thermosets are often harder and more scratch resistant than thermoplastics, and

•

Improved solvent and UV resistance over thermoplastics.

52

Engineering Thermoset Plastics The main disadvantages are: •

Thermosets can not be recycled,

•

Low strain at break,

•

Slower production rates compared with hot melt thermoplastic processes, and

•

The resins and catalysts are often more hazardous to health.

3.2 Adhesive Performance on Thermoset Plastics In this section, a guide is given to indicate the performance of several different adhesives for a selection of six thermoset plastics. The bulk of this information was taken from ‘The Loctite Design Guide for Bonding Plastics’ issued by Henkel Ltd [3] and for more detailed information the reader should refer to this guide. An adhesive cannot be selected solely on the basis of bond strength information as other factors such as cure speed, environmental resistance and dispensing method will all influence the final choice. The values given for the shear strength are given to provide a general idea of how each of the nine adhesives performed on the particular material. Different performances will be achieved depending on the fillers, lubricants, colourants and anti-static additives within the grade of plastic. The nine different adhesives selected for these charts were as follows: •

Three cyanoacrylates (standard ethyl, toughened grade and ethyl + primer),

•

Two acrylics (a standard methylmethacrylate (MMA) and a polyolefin bonder),

•

Two epoxies (a 5-minute epoxy and a longer cure grade),

•

One room temperature vulcanised silicone (alkoxy cure), and

•

One ultraviolet (UV) acrylic (cure at 420 nm).

A brief description of each adhesive is given together with a few general comments on the overall performance of the adhesives. The alkoxy silicone often showed a strength of 1.4 N/mm2 for many of the plastics tested but in this case the adhesive has failed cohesively. A silicone adhesive is often 53

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts used where sealing might be the main priority of the adhesive rather than structural strength. In the case of UV adhesives where the material was invariably opaque, the plastic was bonded to a polycarbonate test specimen.

3.2.1 Diallyl Phthalate (DAP) DAP is the most commonly used of the allylic esters, which are a branch of the polyester family. These thermoset products are typically selected for outstanding dimensional stability, ease of moulding and excellent electrical properties. DAP can be bonded with most engineering adhesives and good strengths [3] are achieved for many different adhesive technologies (Table 3.1). The cyanoacrylates (without primer) showed excellent results in these trials (Table 3.1) but DAP are often used in moist environments and so care must be taken if selecting a cyanoacrylate as a structural adhesive.

Table 3.1 Adhesive shear strengths (DAP) Adhesive type Description Shear strength (N/mm2) Standard ethyl >21.7* Toughened >13.5* Cyanoacrylate Ethyl + primer 1.0 MMA 5.2 Two-part acrylic Polyolefin bonder 4.1 ‘5-minute’ epoxy 3.1 Epoxy Standard epoxy 12.0 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 2.4 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset

54

Engineering Thermoset Plastics

3.2.2 Epoxies Epoxy resins are usually supplied as one-part frozen pre-mixes or two-part systems. There are numerous formulations; each has its own particular characteristic and can vary in viscosity from a thin liquid to a gel consistency. Epoxies are used as adhesives, coatings and binding resins and have excellent abrasion resistance and chemical resistance. Due to the vast array of fillers and different types of epoxy resin, properties vary substantially. Epoxies are generally strong, and also heat, chemical and abrasion resistant. Many printed circuit boards are fibreglass-filled epoxy. Other applications range from marine coatings and encapsulations to adhesives and floorings. The Araldite® series of epoxy resins, manufactured by Huntsman, are perhaps the most well-known, typically two-part systems and these epoxy adhesives would generally be expected to bond well to epoxy thermoset plastics. Cyanoacrylates showed good results in these trials [3], but would not be expected to offer the same durability as epoxy-based adhesives. Note that there are so many different grades of epoxy materials that it is impossible to give specific data (Table 3.2) is intended as a guideline and tests would always be necessary to confirm adhesion performance.

Table 3.2 Adhesive shear strengths (epoxy resins) Adhesive type Description Shear strength (N/mm2) Standard ethyl 13.1 Cyanoacrylate Toughened 17.1 Ethyl + primer 1.7 MMA 11.7 Two-part acrylic Polyolefin bonder 12.1 ‘5-minute’ epoxy 17.9 Epoxy Standard epoxy 20.6 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 10.3 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of epoxy, fillers, surface finish, etc. Recycling information = thermoset

3.2.3 Phenolics Phenolics are one of the earliest bonding agents and are typically heat cured to form a highly crystalline thermosetting polymer. Phenolics have moderate strength compared 55

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts to other plastics but have higher hardness and greater rigidity then most thermoset and thermoplastics. They are made up from a reaction between phenol and formaldehyde so are often referred to as phenol formaldehyde or melamine formaldehyde. Typical applications include handles, electrical terminals and switches. There are many different variants and many have excellent flame resistance and so are used in many public sector applications (mass transit, marine, etc). Epoxy-based and two-part acrylic engineering adhesives generally bond well to phenolics and roughening the surface was found to be beneficial for the UV-curing acrylics (Table 3.3).

3.2.4 Polyester (Thermoset) ‘Fibreglass’ (an unsaturated polyester resin) is one of the best known commercial thermosetting resins for composites and is widely used for large structural applications such as boat hulls and aerospace applications. Polyesters are known for their excellent electrical properties and are widely used in home electrical appliances that require high temperature stability. Specialty grades available include flame retardant, glass filled and magnetisable ferrite filled grades.

Table 3.3 Adhesive shear strengths (phenolics) Adhesive type Description Shear strength (N/mm2) Standard ethyl 4.1 Cyanoacrylate Toughened 11.0 Ethyl + primer 1.0 MMA 9.0 Two-part acrylic Polyolefin bonder 13.1 ‘5-minute’ epoxy 7.6 Epoxy Standard epoxy 17.6 Silicone Alkoxy silicone 1.7 UV acrylic Visible light cure 7.6 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset plastic

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Engineering Thermoset Plastics

Table 3.4 Adhesive shear strengths (thermoset polyesters) Adhesive type Description Shear strength (N/mm2) Standard ethyl >9.3* Toughened >9.3* Cyanoacrylate Ethyl + primer 2.4 MMA 7.2 Two-part acrylic Polyolefin bonder 5.5 ‘5-minute’ epoxy >8.6* Epoxy Standard epoxy 5.9 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 4.1 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset plastic

Most engineering adhesives will bond readily to thermoset polyesters – often achieving substrate failure on test pieces (Table 3.4). Roughening the surface is usually beneficial, especially with the faster-curing cyanoacrylate adhesives. With the exception of the UV-curing acrylic, good strengths were achieved on polyester with all the different adhesives tested [3]. The alkoxy silicone showed a strength of 1.4 N/mm2 but in this case the adhesive has failed cohesively. A silicone adhesive is often used where sealing might be the main priority of the adhesive rather than structural strength.

3.2.5 Polyurethanes Polyurethanes can have the physical structure of a solid casting, a flexible elastomer or soft or rigid foams. It can be either a thermoplastic or more usually it is a thermoset material. The isocyanates in polyurethanes often result in health and safety implications during cure or processing and so must be carefully controlled. Polyurethane plastics can sometimes be difficult to bond and some surface treatment may be necessary. With the wide variety of grades and end-products available, it is 57

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts difficult to give specific data. The data given in Table 3.5 is for guidelines only. Some polyurethanes can be stress cracked by cyanoacrylate adhesives and by some solventbased activators or primers.

Table 3.5 Adhesive shear strengths (polyurethanes) Adhesive type Description Shear strength (N/mm2) Standard ethyl 2.4 Cyanoacrylate Toughened 1.4 Ethyl + primer 9.7 MMA 4.8 Two-part acrylic Polyolefin bonder 4.8 ‘5-minute’ epoxy 2.8 Epoxy Standard epoxy 5.2 Silicone Alkoxy silicone 0.3 UV acrylic Visible light cure 7.9 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset plastic

3.2.6 Polyimides Available as both thermoplastic and thermoset resins, polyimides (PI) are a family of some of the most heat- and fire-resistant polymers known. Polyimide is said to possess a greater resistance to heat than any other unfilled organic material. Unlike most plastics, PI are available in laminates and shapes, moulded parts and stock shapes. Polyimide parts are fabricated by techniques ranging from powder-metallurgy methods to conventional injection, transfer and compression moulding and extrusion. In general, mouldings and laminates are based on thermoset resins, although some are made from thermoplastic grades. Laminates are based on continuous reinforcements where moulding resins contain chopped (short) fibre reinforcements. ‘Kapton®’ is one of the best-known trade names. Polyimides can be bonded with cyanoacrylates (Table 3.6) but the rigidity of these adhesives often results in a high stress concentration at the periphery of the joint and this induces premature failure of the material (see Section 10.3.4).

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Engineering Thermoset Plastics

Table 3.6 Adhesive shear strength (polyimides) Adhesive type Description Shear strength (N/mm2) Standard ethyl >5.5* Toughened >5.5* Cyanoacrylate Ethyl + primer >4.5* MMA 6.6 Two-part acrylic Polyolefin bonder 3.5 ‘5-minute’ epoxy 6.6 Epoxy Standard epoxy >11.7* Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure >5.5* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

3.3 General Comments on Adhesive Bonding of Thermoset Plastics Structural adhesives for thermoset plastics are more often than not epoxies, two-part acrylics and sometimes cyanoacrylates [4]. UV-cure adhesives are not widely used as many thermoset plastics are not clear. Epoxies will bond to most thermoset plastics but as mentioned in Section 1.4 there are many different epoxy adhesive grades available and some epoxies are far more suited for bonding metals than plastics. Two-part acrylics (or MMA) bond well to many thermoset plastics and these products are often used for the assembly of larger thermoset plastic components. They generally cure faster than epoxies and the mix ratio of two-part MMA acrylics is not as critical as it can be for epoxy adhesives. However, many MMA have a high odour and have a tendency to string and many grades are flammable. Standard ethyl-based cyanoacrylates will bond thermoset plastics but the modified toughened grades are often specified for thermosets due to their improved impact resistance.

59

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Cyanoacrylates are best suited where the gaps are small (<0.15 mm) and although some grades will fill bigger gaps there is always a risk of ‘blooming’ (see Section 10.2.6) if the cyanoacrylate is slow to cure.

References 1.

K. Forsdyke and T.F. Starr, Thermoset Resins, Rapra Technology Ltd, Shrewsbury, UK, 2002, p.7.

2.

V. Goodship, Introduction to Plastics Recycling, 2nd Edition, Rapra Technology Ltd, Shrewsbury, UK, 2007, p.110.

3.

The Loctite Guide to Bonding Plastics, Henkel Limited, Hatfield, UK, 2006, p.4.

4.

D.J. Dunn, Engineering and Structural Adhesives, Rapra Review Report No.169, Rapra Technology Ltd, Shrewsbury, UK, 2004, p.13.

60

4

Elastomers and Thermoplastic Elastomers (TPE)

4.1 Introduction Due to the virtually limitless combination of elastomer types, fillers and additives that can be compounded, a huge variety of elastomeric materials are available for almost any application requiring elastomeric properties. For this reason, it is very unlikely that there will be bond strength data for every adhesive/elastomer combination. This chapter is designed to give some general guidelines for 20 different families of elastomers using four different generic adhesive types. This chapter focuses entirely on bonding vulcanised rubber and makes no mention of bonding agents used during the vulcanisation process. The ‘Handbook of Rubber Bonding’ [1] is recommended for further reading on this subject.

4.2 Adhesive Performance on Elastomers In this section, a guide is given to indicate the performance of several different adhesives for a selection of elastomers and thermoplastic elastomers. Most of the performance data was taken from ‘The Loctite Design Guide for Bonding Rubber’ issued by Henkel Ltd [2] and for more detailed information the reader should refer to this guide. An adhesive cannot be selected solely on the basis of bond strength information as other factors such as cure speed, environmental resistance and dispensing method will all influence the final choice. The values given for the shear strength are given to provide a general idea of how each of the nine adhesives performed on the particular material. Different performances will be achieved depending on the fillers, lubricants, colourants and anti-static additives within the grade of elastomer. The nine different adhesives selected for these charts were as follows: •

Three cyanoacrylates (standard ethyl, toughened grade and ethyl + primer),

•

One two-part acrylic (a standard methylmethacrylate (MMA)), 61

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts •

One room temperature vulcanising (RTV) silicone (alkoxy cure), and

•

One ultraviolet (UV) acrylic (cure at 420 nm).

The shear strengths given in the tables below are given as a guideline only as invariably substrate failure of the elastomer was achieved, thus making it difficult to make performance comparisons. Using a standard lap shear test method, the elastomers deform all too easily thus introducing peel and cleavage forces into the joint. In addition, there are also many types of additives and fillers produced by many different companies and some will significantly influence the adhesion to the elastomer in question and so it is recommended that trials are conducted before selecting the most appropriate adhesive for the application. A brief description of each adhesive is given together with a few general comments on the overall performance of the adhesives. The alkoxy silicone often showed a strength of 0.4 N/mm2 for many of the elastomers tested but in this case the adhesive has failed cohesively. A silicone adhesive is often used where sealing might be the main priority of the adhesive rather than structural strength. In the case of the UV adhesive, the elastomer material (invariably opaque) was bonded to polycarbonate and the adhesive cured through the polycarbonate.

4.2.1 Butyl Rubber Butyl rubber is one of the most widely used thermoset elastomers and is typically used for inner tubes and other industrial gas bladders. Butyl rubber is also available in a halogenated form with either bromine or chlorine and this often increases its thermal performance while retaining the low gas permeability to gas and moisture. Butyl rubbers can be bonded with most engineering adhesives and cyanoacrylates show high strength with the substrate failing before the adhesive bond (Table 4.1). Some solvent-based adhesives show good strength on butyl rubber, especially if the rubber is cleaned with a chlorinated solvent prior to bonding.

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Elastomers and Thermoplastic Elastomers (TPE)

Table 4.1 Adhesive shear strength (butyl rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >0.8* Toughened >0.7* Cyanoacrylate Ethyl + primer >1.0* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.6 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information [3] = rubber

4.2.2 Copolyester TPE Copolyester TPE is a high-cost elastomer but has above-average performance. Plasticisers are not used when forming copolyesters. This makes them purer than most TPE, which consequently makes them especially well suited for medical and food applications. Typical applications include fuel tanks, drive belts and cables. Like most elastomers, copolyesters can be bonded with cyanoacrylates and in these trials [2] no improvement was gained by using the primer (Table 4.2).

Table 4.2 Adhesive shear strength (copolyester TPE) Adhesive type Description Shear strength (N/mm2) Standard ethyl 10.8 Cyanoacrylate Toughened 3.5 Ethyl + primer 10.4 Two-part acrylic MMA 2.4 Silicone Alkoxy silicone 1.2 UV acrylic Visible light cure 8.4 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

4.2.3 Ethylene Acrylic (EEA) Rubber EEA rubbers have better heat resistance and low-temperature flexibility than polyacrylate rubbers and they also offer excellent resistance to water. This, coupled with its resistance to UV and ozone, gives it excellent weathering resistance. EEA rubbers offer poor resistance to non-mineral oil brake fluid, esters and ketones. They do, however, offer excellent resistance to diesel fuel, kerosene, ethylene glycol and water. Table 4.3 gives typical shear strengths for this material.

4.2.4 Ethylene Propylene Diene Monomer Rubber (EPDM) EPDM is known for its superior resistance to ozone and oxidation as well as its low cost. Typical engineering applications include seals, hoses, belts, cable covers and weather-strips. EPDM can be difficult to bond (Table 4.4) and many cyanoacrylates will not show particularly good adhesion to EPDM. However, some speciality grades of cyanoacrylate will show good adhesion to EPDM without the use of primer. The silicones, UV acrylics and two-part acrylics all show relatively poor performance on EPDM.

Table 4.3 Adhesive shear strength (EEA rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >2.3* Toughened 0.9 Cyanoacrylate Ethyl + primer >2.9* Two-part acrylic MMA 1.2 Silicone Alkoxy silicone 0.8 UV acrylic Visible light cure 1.3 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Elastomers and Thermoplastic Elastomers (TPE)

Table 4.4 Adhesive shear strength (EPDM) Adhesive type Description Shear strength (N/mm2) Special ethyl >4.7* Toughened 1.6 Cyanoacrylate Ethyl + primer >4.0* Two-part acrylic MMA 0.9 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

4.2.5 Ethylene-Vinyl Acetate Co-polymer (EVA) EVA is thermoplastic and it is available in various forms depending on the level of co-polymer in the vinyl acetate. This affects the elasticity of the material and also the performance of adhesives. EVA is used for disposable gloves, hoses, tubes and anaesthesia face masks. Table 4.5 gives some typical adhesive shear strengths for this versatile material.

Table 4.5 Adhesive shear strength (EVA) Adhesive type Description Shear strength (N/mm2) Standard ethyl 3.8 Cyanoacrylate Toughened 3.6 Ethyl + primer >5.7* Two-part acrylic MMA 1.5 Silicone Alkoxy silicone 0.07 UV acrylic Visible light cure 4.6 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

4.2.6 Fluorosilicone Rubber Fluorosilicones are renowned for their fuel resistance and utility in extreme-temperature service environments. The siloxane backbone results in a polymer with excellent UV, ozone and thermal resistance. The maximum recommended service temperature is in excess of 200 °C (392 °F) for most grades with brittle points as low as −65 °C. Fluorosilicones are used for ‘O’ rings and for wire and cable insulation. Whilst some adhesive shear strengths are given in Table 4.6 fluorosilicones can often be recommended for high-temperature applications and so if the operating temperature is >100 °C, it is unlikely that cyanoacrylates could be used as suitable adhesives.

4.2.7 Natural Rubber Natural rubber is created by processing the latex of a plant, Hevea brasiliensis, which is indigenous to the Amazon valley and is the only known plant to produce highmolecular-weight linear polymer. The latex is ‘tapped’ from the tree then collected and treated with a stabiliser and brought to a processing centre. There are many different types and grades of natural rubber varying with colour, cleanliness and uniformity of appearance. As with most elastomers, natural rubber can be readily bonded with cyanoacrylates although in these trials [2] the adhesion achieved with the toughened cyanoacrylates was relatively low (Table 4.7).

Table 4.6 Adhesive shear strengths (fluorosilicone rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >1.4* Toughened 0.5 Cyanoacrylate Ethyl + primer >1.7* Two-part acrylic MMA 0.6 Silicone Alkoxy silicone 1.6 UV acrylic Visible light cure 0.8 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

66

Elastomers and Thermoplastic Elastomers (TPE)

Table 4.7 Adhesive shear strengths (natural rubber) Adhesive type

Description Standard ethyl

Shear strength (N/mm2) >2.1*

Toughened

Cyanoacrylate

Ethyl + primer

0.9 >1.8*

Two-part acrylic

MMA

0.3

Silicone

Alkoxy silicone

0.3

UV acrylic

Visible light cure

1.6

*

Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

4.2.8 Nitrile Rubber Nitrile rubber is one of the most widely used elastomers and is known for its superior high- and low-temperature performance and its exceptional oil, petrol and solvent resistance. Typical applications include hoses, shoes and flooring. Nitrile rubber is generally easy to bond with cyanoacrylates (Table 4.8) and the use of a primer is not normally necessary. Of the other adhesives tested, the two-part acrylics and the UV acrylics showed promising adhesion.

Table 4.8 Adhesive shear strengths (nitrile rubber) Adhesive type

Description Standard ethyl

Cyanoacrylate

Toughened Ethyl + primer

Shear strength (N/mm2) >2.0* 1.8 >2.0*

Two-part acrylic

MMA

1.7

Silicone

Alkoxy silicone

0.9

UV acrylic

Visible light cure

1.6

*

Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

4.2.9 Neoprene Rubber Neoprene rubber is a thermoset elastomer used for window seals, hoses, cable insulation and for gaskets. There are various forms of neoprene available but they will all bond well with cyanoacrylates and a primer is not usually required (Table 4.9).

4.2.10 Polyisoprene Polyisoprene is a thermoset elastomer and has high tensile properties and its main advantage over natural rubber is that it is generally easier to process. It is used for the manufacture of rubber bands, baby milk bottle teats, sporting goods and engine mounts. Cyanoacrylates show good adhesion to polyisoprene and primers are not usually required (Table 4.10).

4.2.11 Polyolefin Elastomers This TPE is used for handles on power and hand tools but also as grips on everything from pens to golf clubs. Manufacturers use two-shot mouldings to form these grips but sometimes adhesives are required for prototype parts or for supplementing

Table 4.9 Adhesive shear strength (neoprene rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >2.1* Toughened >1.8* Cyanoacrylate Ethyl + primer >1.8* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.3 UV acrylic Visible light cure 1.4 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = thermoset rubber

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Elastomers and Thermoplastic Elastomers (TPE)

Table 4.10 Adhesive shear strength (polyisoprene) Adhesive type Description Shear strength (N/mm2) Standard ethyl >1.7* Toughened >1.7* Cyanoacrylate Ethyl + primer >2.0* Two-part acrylic MMA 0.3 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

mechanical fits or small production runs where the tooling for a two-shot moulding is prohibitively expensive. Thermoplastic elastomers can be quite difficult to adhesively bond (Table 4.11) and invariably a primer or some other form of surface treatment (see Section 6.3) is necessary.

Table 4.11 Adhesive shear strengths (polyolefin elastomers) Adhesive type Description Shear strength (N/mm2) Standard ethyl 2.7 Cyanoacrylate Toughened 1.9 Ethyl + primer >3.4* Two-part acrylic MMA 1.1 Silicone Alkoxy silicone 1.3 UV acrylic Visible light cure 1.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 4.12 Adhesive shear strengths (silicone rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl 0.5 Cyanoacrylate Toughened <0.1 Ethyl + primer 3.5 Two-part acrylic MMA <0.1 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 0.2 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

4.2.12 Silicone Rubber Silicone rubber is often used where operating temperatures are high as they retain much of their tensile strength and compression set resistance at temperatures up to 200 °C. Typical applications include oven door gaskets, high-temperature hoses and implantable devices. Due to the high operating temperatures of silicone rubber, cyanoacrylates are not always suitable and RTV silicones may be the only adhesive option if the operating temperature is >150 °C. Silicones have a low surface energy and so will benefit hugely from surface treatment prior to bonding (Table 4.12).

4.2.13 Styrene-Butadiene Rubber (SBR) SBR offers excellent abrasion resistance so is used mainly for automotive tyres. Other applications include shoe soles, waterproof materials and asphalt. SBR generally bonds well with cyanoacrylates and a surface primer is not usually required (Table 4.13).

4.2.14 Styrenic TPE Styrenic TPE are probably the most widely used thermoplastic elastomers. There are three distinctly different main types:

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Elastomers and Thermoplastic Elastomers (TPE)

Table 4.13 Adhesive shear strengths (SBR) Adhesive type Description Shear strength (N/mm2) Standard ethyl >1.8* Toughened >1.3* Cyanoacrylate Ethyl + primer >1.8* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.8 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber •

Styrene-butadiene-styrene block co-polymers

•

Styrene-isoprene-styrene block co-polymers

•

Styrene-ethylene-butylene-styrene block co-polymers [4]

From an adhesive viewpoint, these materials will often require a primer or some other kind of surface preparation for an effective bond (Table 4.14). However, there are many different blends and additives for styrenic TPE and so trials are recommended to verify the optimum adhesive/primer combination.

Table 4.14 Adhesive shear strengths (styrenic TPE) Adhesive type Description Shear strength (N/mm2) Standard ethyl >3.0* Toughened 2.5 Cyanoacrylate Ethyl + primer >2.8* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.9 UV acrylic Visible light cure 1.2 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

71

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Again cyanoacrylates are amongst the first choice adhesives for these materials and would be suitable for many applications up to 80 °C operating temperatures.

4.2.15 Thermoplastic Vulcanisates (TPV) One of the most widely used TPV is sold under the trade name of Santoprene™ and this material has a low surface energy, thus making it difficult to bond with engineering adhesives. Whilst cyanoacrylates with a surface primer give good results (Table 4.15), the failure modes are invariably at the adhesive-Santoprene™ interface. On some grades of Santoprene™ (e.g., 103-50), adhesive shear strengths up to 8 N/mm2 can be achieved with an ethyl-based cyanoacrylate and primer but with other grades (e.g., Santoprene™ 201-55), only 1.5 N/mm2 is achieved.

4.3 General Comments on Bonding of Elastomers Polychloroprene, nitrile, natural rubber, styrene butadiene rubber and butyl rubber can all be readily bonded with cyanoacrylates. EPDM and fluoroelastomers (such as Viton) can also be bonded, although only with specific grades of cyanoacrylate. The silicone rubbers and thermoplastic elastomers will usually require a primer but will also bond with cyanoacrylates. Cyanoacrylates are relatively rigid adhesives and so may not be suitable in some applications where a high degree of flexibility is required. The toughened cyanoacrylates do show improved flexibility but are a little slower curing and sometimes

Table 4.15 Adhesive shear strengths (TPV) Adhesive type Description Shear strength (N/mm2) Standard ethyl 0.5 Cyanoacrylate Toughened 0.6 Ethyl + primer 1.5 Two-part acrylic MMA 0.3 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.8 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

72

Elastomers and Thermoplastic Elastomers (TPE) show slightly lower shear strengths than a standard ethyl grade. However, where an elastomer is to be bonded to a metal, then a rubber-toughened cyanoacrylate might well prove the best adhesive. Note that cyanoacrylates are best suited for non-structural applications and are best suited for temperatures up to 100 °C. Modified flexible cyanoacrylates (see Section 1.1.1.7) also show improved flexibility (Figure 4.1) but the glass transition temperature of these adhesives is lower and so the hot strength and heat aging properties of these products is limited to around 70 °C.

Figure 4.1 Nitrile rubber bonded with a flexible cyanoacrylate

If, therefore, a high degree of flexibility is required at higher temperatures then a two-part acrylic- or silicone-based product may be more suitable.

References 1.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2000.

2.

The Loctite Guide to Bonding Rubber and Elastomers, Volume 2, Henkel Ltd, Hatfield, UK, 2006.

3.

V. Goodship, Introduction to Plastics Recycling, 2nd Edition, Rapra Technology Ltd, Shrewsbury, UK, 2007, 52.

4.

P. Dufton, Thermoplastic Elastomers Industry Analysis Report, Rapra Technology Ltd, Shrewsbury, UK, 2001, 8 to p.8.

73

5

Joint Design

5.1 Introduction This chapter discusses the overall concepts for designing adhesively bonded joints and some guidelines for some of the more common joint designs are discussed. The specific types of joints that will be considered include: •

Lap joint,

•

Double lap joint (tongue and groove),

•

Cylindrical joint, and

•

Butt joint.

5.2 Lap Joint This is the most standard adhesive joint and the one widely used as the standard for testing the performance of adhesives. When a lap joint (Figure 5.1) is loaded as shown in the direction of the arrows, the adhesive in the joint is subjected to primarily shear loads but also with an element of tensile loading, especially if the substrates show some flexibility (Figure 5.2).

Figure 5.1 Lap shear joint

75

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.2 Tensile shear loading (exaggerated to show effect)

5.2.1 Joint Width versus Joint Overlap In a standard lap shear joint, if the overlap length is increased, the strength of the joint does not necessarily increase linearly for the same standard conditions (i.e., same adhesive, same substrate, same surface preparation, and so on). In Figure 5.3 below the bond area has been increased from 1000 mm2 to 2000 mm2 but the strength will only increase by a small percentage. The reason for this can be seen in the stress distribution curves in Figure 5.4. Similarly, in Figure 5.5, both these joints have the same bonded area (1000 mm2) but the joint on the right will be the stronger joint due to the reduced stress concentrations at the end of the joint. From the shear stress distribution curve in Figure 5.4 we can see that the highest stress is at the ends of the bond and by increasing the joint overlap there is no significant change in the strength of the bond.

40 mm 25 mm

80 mm 25 mm

25 mm × 80 mm

Figure 5.3 Extending the overlap on a lap shear joint 76

Joint Design

2

3

Failure load/N

1

2

3

1

Overlapping length/mm

Figure 5.4 The joint strength does not increase linearly with joint overlap due to the stress concentrations at the ends of the joint

25 mm

40 mm 50 mm 20 mm

25 mm × 40 mm

50 mm × 20 mm

Figure 5.5 Which joint is the strongest?

This is because the joint starts to break at the stress peak at the end of the overlap where the adhesion or cohesive strength of the adhesive is exceeded. By increasing the width of the joint, the shear stress distribution is not changed and so the failure load of lap joints increases in the same proportion as the joint width increases (Figure 5.6). 77

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Bond Strength

Increasing Width

Increasing Overlap

Bond Area

Figure 5.6 Increasing joint width will increase the bond strength

Shear forces can cause mechanical deformations in the substrates and the appearance of peak stress points; this is of particular concern when the components being assembled have a thin cross section and when the materials have a low modulus of elasticity. Failure is most likely to occur at the ends where maximum stress is present. The ‘nominal shear strength’ of adhesives (as indicated by manufacturers) is measured under controlled experimental conditions. Lower shear strengths must be expected to take into account the specific characteristics of an application. If long-lasting forces of varying nature acting on the bonded joints appear under actual operating conditions, the value used for calculations should be less than the nominal shear strength of the adhesive. When the surfaces of the components being bonded are poorly fitting, the contact between such surfaces must be good and ensured by the assembly technique. When clamping pressure is removed after the adhesive is cured, elastic recovery of the components may introduce pre-service stresses in the assembly; these stresses may be in addition to the forces experienced under operating conditions and should be allowed for.

5.2.2 Optimising Joints to Minimise Stress A fillet of adhesive is almost invariably formed outside the joint (Figure 5.7) and this can be beneficial in that it reduces the stress concentrations at the end of the joint. For this reason the excess adhesive should only be removed for aesthetic purposes.

78

Joint Design

Figure 5.7 A small fillet of adhesive outside the joint can reduce the stress concentrations

The lap shear joint has been the subject of much academic study, particularly with metal substrates but due to the wide variety of plastics available there seems to be little published data when it comes to bonding plastics. In some plastic engineering applications the load on the joint is very low, perhaps only the weight of the bonded parts, and so it will not be necessary to pay too much attention to joint design and the shape most easily fabricated can be selected. For higher load joints the joint design will require more careful consideration and testing prior to manufacture. A few simple suggestions for improving the stress distribution across a lap shear joint are shown in Figure 5.8 but the application and manufacturing process will define the options available. Rigid adhesives (e.g., standard ethyl cyanoacrylates) are used for bonding assemblies when the joints can be designed to be subjected only to shear and normal static forces. If the application is subjected to peel stresses or impact loads, a toughened adhesive (e.g., epoxy, two-part acrylic or toughened cyanoacrylate) would be more suitable.

Figure 5.8 Some options for a lap shear joint [1] 79

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

5.3 Double Lap Joint (Tongue and Groove) A tongue and groove (or double lap shear) joint (Figure 5.9) can be an ideal joint design for many applications, especially for bonding a lid onto a box. The adhesive can be dispensed into the groove, thus ensuring positive location of the liquid adhesive and when the joint is closed and the adhesive cured, the adhesive is essentially under shear loading and so will have high strengths. Note, however, even with this design which would appear to allow symmetrical loading, under high load the substrates may distort and thus subject the adhesive to a cleavage/peel load [2]. The wall thickness of the substrate will define whether there is sufficient width to include a double lap shear joint and if the thickness is less than 3 mm, it may well be difficult to achieve the tolerances required. One of the issues that can occur with a tongue and groove joint is allowing for where the adhesive will flow to when the joint is closed. In Figure 5.9, when the joint is closed, excess adhesive has nowhere to flow to and may therefore spill out and impair the aesthetics of the assembly.

Figure 5.9 Tongue and groove joint (with no allowance for excess adhesive) 80

Joint Design

Figure 5.10 A modified tongue and an offset ensures the adhesive finds the easiest path to the larger gap and thus minimises excess adhesive outside the joint

In Figure 5.10, the design has been modified slightly to allow for the adhesive to flow into a slightly larger gap, thus improving the aesthetics of the joint.

5.4 Cylindrical Joints A common application for adhesives is where co-axial (cylindrical parts) require to be joined. Figure 5.11 shows a typical application where the black polypropylene spigot is to be bonded and sealed into the red elastomeric (thermoplastic elastomers) housing. In this case, both these plastics are low-surface-energy materials and so will require either a surface primer or some form of surface treatment to increase the surface wetting (see Chapter 6). In this application a medium viscosity ethyl cyanoacrylate with a primer was selected. The length to diameter (L/D) ratio and the diametral clearances are both important factors in terms of the final strength of the bond. In this application the L/D ratio is 1.2 and so ideally suited for an adhesive. 81

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.11 Joining dissimilar plastics (cylindrical joint) Thin bond lines where the diametral clearance is less than 0.1 mm are usually best for most industrial applications. Figure 5.12 shows a much higher L/D ratio and in this application a cyanoacrylate might not be the most appropriate adhesive as it may cure before the parts can be fully assembled. For long engagement lengths such as these and where the outer (female) substrate is opaque (and a ultraviolet (UV) adhesive cannot be used) then the adhesive can be wicked into the joint after assembly [3]. Note the small chamfer in Figure 5.12 to allow for this capillary action. In this case a diametral clearance of 0.04 mm has been selected as the low viscosity cyanoacrylates have limited gap-filling capabilities. If the gaps were larger (0.2 mm) then a two-part acrylic or epoxy may be the most appropriate adhesive. If the outer (female) substrate had been transparent in Figure 5.12, it may have been possible to use an ultraviolet-curing adhesive. The benefit of UV adhesives is that the product will only cure ‘on demand’ and so there will be plenty of time to rotate the component parts to ensure full joint coverage. Where the assembly process is horizontal, the chamfer is not always necessary but the male part may require to be rotated to ensure full joint coverage.

5.4.1 Design Details In many cylindrical bonding applications involving plastics, it is common practice to dispense the adhesive onto the inner (male) substrate and then close the joint with a rotating action. This in principle is fine but invariably excess adhesive is applied and the excess will be forced out of the joint as shown in Figure 5.13.

82

Joint Design

d

D

D – d = 0.04 mm

L

Figure 5.12 Tube bonding with long engagement lengths

Figure 5.13 Applying adhesive to male component 83

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.14 Applying adhesive to the female component

Perhaps the easiest option is to control the dispensing carefully to minimise excess adhesive (see Section 8.1.5). However, one option to overcome excess adhesive being pushed outside the joint is to apply the adhesive to the inside of the female component (Figure 5.14). In this case (Figure 5.14) the adhesive tends to get pushed along the component and will end up on the inside. This may in its turn cause issues with the possibility that the adhesive may block or interfere with other fluids passing through the connector. A third option for cylindrical component parts is to design in a small recess so that the adhesive has somewhere to escape to when the joint is closed (Figure 5.15).

5.4.2 Cross Holes Another method of applying adhesive to cylindrical parts is to introduce the adhesive via a cross hole (Figure 5.16), although this does involve additional detailed mould design.

84

Joint Design

Recess

Figure 5.15 Parts designed with a recess to minimise excess adhesive

Figure 5.16 Dispensing the adhesive via cross holes

5.4.3 Blind Holes Where a ‘blind’ hole exists if the adhesive is applied to the male part and the joint closed, it is likely that the air trapped in the base of the hole will push all the adhesive out of the hole and thus starve the joint of adhesive (Figure 5.17). The best practice is to apply the adhesive into the base of the hole and then assemble the parts (Figure 5.18). Note, however, that in small-diameter holes it is entirely possible that ‘hydraulic locking’ can occur and so a small bleed hole to allow the air to escape may also be necessary.

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Figure 5.17 Air trapped in the base of a blind hole will force the adhesive out of the joint

5.5 Butt Joint The butt joint can be in various configurations (Figure 5.19) and this is almost certainly the easiest and lowest-cost joint. The success of this joint will depend on the materials bonded, the loads on the joint and the thicknesses of the substrates involved. If the adherends are rigid and a moment or offset load is applied to this joint then the adhesive will be subjected to quite a severe cleavage or peel load (Figure 5.20) and as such the butt joint is generally regarded as a poor joint design. In some applications the butt joint may be the only possible method of assembling the component parts and, if aesthetics are important, the small fillet of excess adhesive outside the joint (which adds to the strength of the joint) may also be undesirable. One example of this in the plastics industry is the assembly of shelving where acrylic or polycarbonate sheets are bonded together to form ‘points of sale’ display equipment. 86

Joint Design

Figure 5.18 For blind holes, apply the adhesive to the base of the hole

Figure 5.19 Various forms of a butt joint 87

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.20 An offset tensile load or a moment can create high cleavage or peel loads in the joint line

Small chamfer

Figure 5.21 A small chamfer will increase the strength of the joint

In these cases a small internal chamfer can help to improve the integrity and aesthetics of the joint and it allows for some tolerance on the dispensed quantity of adhesive (Figure 5.21) as it can be quite difficult to dispense the exact quantity of adhesive so that it stops exactly at the edge of the joint. The chamfer creates a small gap to allow for the adhesive flow. In these applications a variety of adhesives are used including UV cure, cyanoacrylates and two-part adhesives (epoxies or acrylics) and the adhesive is selected for its clarity, cost, speed of cure and ease of use.

5.6 Bond Line Thickness The gap between the parts and therefore the thickness of the adhesive film has an important bearing on the characteristics of the joint [4]. A thick bond line (>0.25 mm) will generally be a weakening feature for cyanoacrylates as the mechanical strength of the cured cyanoacrylate film is likely to be less than the plastic or other substrate 88

Joint Design bonded. For most applications where cyanoacrylates are involved, thin films (<0.05 mm) often give the best results. UV adhesives will fill gaps of several millimetres and this can be beneficial if parts are poorly fitting or if wide tolerances exist, however a thin joint line (<0.1 mm) will usually give the strongest joint. Where high peel or cleavage loads exist, a thicker bond line with a flexible or toughened adhesive can reduce the higher stress concentrations and thus increase the peel load. Epoxies, polyurethanes, two-part acrylics and the adhesive/sealant products (silicones, and modified silanes) all have excellent gap-filling capabilities and some of these products will offer excellent resistance to impact loading and peel loads. Assemblies with varying joint gaps should be avoided as much as possible. This practice will avoid uneven stress distributions, irregular pressures in the assembly and internal curing stresses.

5.7 Thermal Effects All adhesives display changes in their bulk properties when the temperature changes. This change is particularly marked (as with many engineering plastics) when the glass transition temperature (Tg) is passed. The Tg of a cyanoacrylate is of the order of 120 °C whereas a UV acrylic generally has a lower Tg (50–80 °C). These changes in adhesive properties above and below Tg will affect the stress distribution pattern in the adhesive joint. This can be important if the adhesive is to be used to transmit power directly through shear loading. When materials with different coefficients of thermal expansion (CTE) are joined, shear stresses result when the assembly is heated or cooled. Many engineering plastics have a CTE value in the range 80–100 × 10–6 mm/mm/°C but sometimes differences can occur. For example, liquid crystal polymer has a CTE of 10 × 10–6 mm/mm/°C, whereas acrylic has a CTE of 80 × 10–6 mm/mm/°C, and if these two substrates were to be bonded with a cyanoacrylate (CTE = 80 × 10–6 mm/mm/°C [5]) then the adhesive could be subjected to some quite severe stresses at the extreme operating temperature range. In this case a thicker bond line and more compliant or flexible adhesive (e.g., a flexible UV acrylic) may reduce problems.

5.8 Selecting the Viscosity of the Adhesive Viscosity is a product property associated with all engineering adhesives and with most sealants. The rheology of an adhesive is the key to determining its potential field of application (gap-filling capability, strength before curing, penetrability, orientation

89

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts of parts receiving adhesive, etc.) and the dispensing systems to be used. Viscosity is defined as a measure of the resistance of a fluid to flow or mathematically as the ratio of a shear stress to a velocity gradient. A measure of this fluid ‘thickness’ is traditionally expressed in centipoise (cP) values, although the SI units are milliPascal seconds (mPa-s). Fortunately, 1 cP = 1 mPa-s and so most engineers still refer to viscosity in centipoises. Viscosity has to be considered on a logarithmic scale and the higher the number, the thicker the product. Water has a viscosity at 22 °C of 1 mPa-s and some common fluids are listed in Table 5.1 for reference purposes. Viscosity decreases as temperature increases. This is due to the fact that an increase in temperature results in an increase in the kinetic energy of the fluid molecules that weakens the intermolecular attraction and encourages molecules to separate in the presence of shearing forces. Rule of thumb: a 10 °C rise in temperature will half the viscosity of the liquid. Vice versa, a 10 °C drop will double the viscosity (see Section 8.1.2). Viscous behaviour could be described approximately by Newton’s law: ‘The resistance resulting from the lack of slipping by different layers within a liquid, other factors being equal, is proportional to the rate at which these layers are separated from each other.’ In other words, the force applied is proportional to the rate of deformation. For the majority of liquids the viscosity is independent of the velocity gradient and so they exhibit Newtonian behaviour. Many engineering adhesives, however, have nonlinear behaviour and are therefore termed non-Newtonian. A thixotropic adhesive is one which will reduce in viscosity as it is sheared (or moved). When the shearing

Table 5.1 Viscosities of some common fluids Product Approximate viscosity (mPa-s) Water 1 Anti-freeze (water glycol) 10–20 Engine oil 100–300 Gear oil 2000–3000 Thick syrup 10,000–30,000 Peanut butter 150,000–250,000 Bath sealant 1,000,000–3,000,000

90

Joint Design force is removed from the adhesive, the viscosity will regain its higher value and this may be time-dependent on the history of the shearing force. The lower viscosity adhesives (2–20 mPa-s) are often used where the adhesive needs to be post-applied as these products will tend to have good wicking and capillary characteristics. The higher viscosity adhesives (>10,000 mPa-s) are less flowable and would be selected where the adhesive has to fill a larger gap or is required to be dispensed onto a narrow ledge and must not migrate away from the bond line (see Section 8.1). Some adhesives – often the higher viscosity epoxies, two-part acrylics and certainly the silicone and modified silane sealants – have good ‘wet’ strength and this property can be useful for applications where an initial ‘grab strength’ is useful.

5.9 Surface Preparation Good wetting of the substrate surface is essential for developing reliable bonds. Adhesives that do not wet the surface will not spread out and fill substrate surface irregularities. Wetting occurs when the surface tension of the liquid adhesive is lower than the critical surface tension of the substrates being bonded (see Section 6.1). In many applications one of the benefits of using adhesives is that surface treatments are not normally required on engineering plastics and indeed in many applications parts are bonded ‘as received’. Nevertheless, a well-defined surface finish will optimise bonding and ensure the repetition of bonding characteristics on large assembly lines by maintaining the designed quality levels. Slightly roughened surfaces will be beneficial [6] and in certain applications this can be achieved by spark eroding the surface of the mould to provide a key for the adhesive. Solvent cleaning with solvents such as isopropyl alcohol is generally a very acceptable cleaning method although it should be noted that sometimes these solvents can stress crack or craze some amorphous thermoplastics [7]. They can also remove all traces of moisture from the surface and this can slow down or even inhibit the cure of cyanoacrylates. Correct surface preparation also results in improved durability [8] (see Section 9.1.1). No single adhesive will satisfy all needs. Designers and engineers must balance a variety of adhesive properties to obtain the required bond strength and ease of use in production. 91

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References 1.

J. Shields, Adhesives Handbook, 3rd Edition, Butterworths & Co, London, UK, 1984, p.13.

2.

Adhesives and the Engineer: A Review of the role of Modern Adhesives in the Structural and Mechanical Engineering Industries, Ed., W.A. Lees, Mechanical Engineering Publications Ltd, London, UK, 1989, p.36.

3.

B. Goss in Proceedings of the 2nd Rapra Technology Conference on Joining Plastics, London, UK, 2006, Paper No.4.

4.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2001, p.268.

5.

Technical Data Sheet, Loctite 401, Henkel Ltd, Hatfield, UK, 2009.

6.

Handbook of Adhesion, Ed., D.E. Packham, Longman Scientific & Technical, Harlow, UK, 1992, p.350.

7.

The Loctite Design Guide for Bonding Plastics, Henkel Ltd, Hatfield, UK, 2006, p.4.

8.

Industrial Adhesion Problems, Eds., D.M. Brewis and D. Briggs, Orbital Press Oxford, 1985, p.4.

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6

Bonding of Low-energy Plastics and Rubbers

6.1 Surface Wetting Silicone rubber, polytetrafluoroethylene (PTFE), Acetal and the polyolefin plastics (polypropylene, polyethylene) are always a challenge to the adhesive engineer due to the low surface energy of these materials. Whilst the detailed consideration of surface tension is more in the province of the physicist than the engineer, wetting (the establishment of contact) plays a significant role in adhesion. Surface tension causes many liquids to behave as an elastic sheet and allows insects, such as the water boatman, to walk on water (Figure 6.1). It also allows small objects, even metal ones such as needles and razor blades, to float on the surface of water and it is the cause of capillary action. For good wetting and therefore good adhesion, the adhesive must be capable of spreading over the solid surface displacing air and any other surface contaminants that may be present. The scientific study of interfacial properties has developed measurement and analytical techniques that give a detailed analysis of components that determine wetting equilibria and surface/interfacial energy.

Figure 6.1 The surface tension allows the water boatman to ‘walk’ on water

93

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Surface tension has the dimension of force per unit length or of energy per unit area. The two are equivalent – but when referring to energy per unit of area most engineers use the term surface energy, which is a more general term in the sense that it applies also to solids and not just liquids. For many years surface tension was measured in dynes/cm and many engineers still use this unit today. The modern SI unit however is mN/m (milli-Newtons per metre) and is the same as dynes/cm. Since no liquid can exist in a perfect vacuum for very long, the surface of any liquid is an interface between that liquid and some other medium. The top surface of a pond, for example, is an interface between the pond water and the air. Surface tension, then, is not a property of the liquid alone, but a property of the liquid’s interface with another medium. Young [1] developed the relationship between the contact angle and the three interfacial tension points that describe a sessile drop, Equation 6.1. γsv = γsl + γlv cosθ

(6.1)

where γsv is the solid/vapour point, γsl is the solid/liquid point, γlv is the liquid/vapour point and θ is the contact angle (Figure 6.2).

θθ

Low surface energy

θ

High surface energy

Figure 6.2 Low contact angles favour better wetting 94

Bonding of Low-energy Plastics and Rubbers For the common sessile or pendant drop shape the Laplace equation describes the relationship of the two radii of the elliptical sessile drop with the pressure across the surface and the surface tension, Equation 6.2.

(

1 1 + ___ ΔP = σ ___ R1 R2

)

(6.2)

where ΔP is the pressure, σ is the surface tension and R1 and R2 are the principal radii of curvature. For an adhesive to ‘wet’ a surface, it requires a lower surface tension than the surface energy of the solid. If this condition is not met, the liquid does not spread across the surface but forms spherical droplets on the surface. Water has a relatively high surface tension (70 mN/m) and so on a highly polished car bonnet, the water will form droplets (Figure 6.3) because the waxed surface of the metal bonnet will have a lower surface energy than the water and so prevents wetting. Wetting of plastic surfaces is much more complex than wetting clean metal surfaces. Plastics and adhesives are both polymeric materials and thus have similar physical properties, including wetting tensions. Plastic-bonded joints do not have the large difference between the critical surface tension of the substrate and that of the adhesive,

Figure 6.3 Water forming droplets on a polished car bonnet [2] 95

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts which ensures wetting for metals. In addition, many plastics have notoriously low critical wetting tensions. Polyethylene (PE) and polypropylene (PP), with critical surface tensions of 31 and 29 mN/m respectively, present serious wetting challenges for most adhesives. The surface tension for an ethyl cyanoacrylate is 33 mN/m and so the surface energy of the solid must be greater than 33 mN/m to achieve good wetting. Other plastics such as polystyrene and polyvinyl chloride (PVC) have higher critical surface tensions and present less of a problem. Table 6.1 shows some surface energy values for a range of materials and it can be seen that PTFE has a surface energy of 18 mN/m and therefore cannot be bonded without surface pre-treatment. PVC, however, has a surface energy of about 38 mN/m and can therefore be bonded. Most industrial adhesives (e.g., cyanoacrylate, epoxy, polyurethane, room-temperaturevulcanising silicone and most acrylic adhesives) do not adhere to PP and PE. Indeed these adhesives are often packaged in PP or PE bottles so that the adhesive itself can be dispensed without sticking to the bottle.

Table 6.1 Surface tension values for some plastics Material Surface tension (mN/m) PTFE 18 Acetal 22 Polypropylene 29 Polyethylene 31 Polystyrene 35–37 Polymethylmethacrylate (acrylic) 39 PVC 39 Polyethylene terephthalate 41–47 Polycarbonate 46 Nylon 6 46

Polyolefins and fluoropolymers are also difficult to bond for other reasons: •

Low porosity – there is no opportunity for the adhesive to penetrate into the plastic and give mechanical interlocking.

•

No functional groups – polyolefins are comprised entirely of carbon and hydrogen atoms and are very non-polar polymers. Most adhesives contain oxygen, nitrogen

96

Bonding of Low-energy Plastics and Rubbers and other electron-rich atoms and are polar materials, and if (like polyolefins) the carbon and hydrogen bonds are very unreactive, there is no opportunity for the adhesive to form chemical bonds. •

Surface weaknesses – Some plastics have weak boundary layers due to the low tensile strengths between some of the molecules at the surface of the plastic.

•

Mould release agents can also be the cause of low adhesion if they are silicone or PTFE based and are transferred across from the mould tool.

6.2 Measuring Surface Energy When a designer is selecting an adhesive for a specific application, the engineering properties of the individual plastic will be considered carefully. All too often, however, the data supplied by the plastic manufacturer will include melting point, mould shrinkage, tensile modulus, hardness, dielectric properties, water absorption, density and thermal conductivity but almost never the surface energy of the plastic, which is one of the key properties required for the adhesive application engineer. The use of surface-tension pens is a simple technique to measure surface energy. Each pen contains ink of a known surface tension and if the ink ‘globulates’ or breaks up, the surface energy of the tested surface is lower than the ink and if the pen is seen to write without the ink breaking into smaller particles, the surface energy of the tested surface is higher than the ink. The use of pens with different inks therefore provides a reasonably accurate measurement of the wetting properties of the tested surface. Most engineering adhesives have a surface tension of approximately 33 mN/m and the plastic needs only to be just above this for the adhesive to wet the surface and therefore bond. The degree of adhesion may well depend on other factors such as surface finish, the gaps between the mating parts and the type of plastic, but once the adhesive starts to wet the surface some degree of adhesion should be obtained. Unlike metals, plastics and elastomers do not have the large difference between the critical surface tension of the substrate and that of the adhesive and so when poor wetting occurs, there are methods to treat the surface for better bonding.

6.3 Surface Treatments Several techniques are in use within the plastics industry, including corona discharge, plasma etching, flame treating and the use of chemical primers to enhance surface energy.

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6.3.1 Abrasion One of the easiest forms of surface preparation is simply cleaning and abrading the surface. The most common procedure is a solvent wipe, followed by abrasion and then a final solvent wipe. The solvent selected should not craze or soften the plastic. Grit blasting is the most effective abrasion method, although using aluminium oxide cloth also works well. The final solvent rinse removes residue from abrasion. Using cleaning and abrasion first ensures that wetting problems are not caused by surface contamination. Another potential benefit is that removing the surface layer of plastic may expose material with better wetting characteristics due to a different crystalline microstructure.

6.3.2 Corona Discharge The corona discharge technique consists of having the polymer film pass over a metal electrode coated with a dielectric material which receives a high voltage from a high-frequency generator (10–20 kHz). Normally the voltage increases cyclically until the gas ionises, generating a plasma at atmospheric pressure that is known as ‘corona discharge’. This is a highly effective treatment for polyolefins that creates adhesion-enhancing carbonyl groups on the surface and raises the surface energy of the polymer [3].

6.3.3 Plasma Treatment Plasma surface treatment increases the surface energy of a substrate by bombarding the substrate surface with ions of a gas such as argon. Plasma treatment can be performed at atmospheric conditions or in a sealed chamber under extremely low pressures. By selecting appropriate gases and exposure conditions, the surface can be cleaned, etched or chemically activated. The results typically show up to a two- or three-fold increase in surface wetting [3].

6.3.4 Flame Treatment Flame treatment is often used to change the surface characteristics of plastics. It involves passing the surface of the plastic through the oxidising portion of a natural gas flame. The surface is rapidly melted and quenched by the process; some oxidation of the surface may occur at the same time. Exposure to the flame is only a few seconds. Flame treatment is widely used for PE and PP, but has also been applied to other

98

Bonding of Low-energy Plastics and Rubbers plastics, including thermoplastic polyester, polyacetal and polyphenylene sulfide. Specially designed gas burners are available for this process, but butane torches can be used for laboratory trials.

6.3.5 Use of Primers PTFE and other fluoropolymers have been treated using a solution of sodium in liquid ammonia and other etching solutions [3]. This method dramatically improves surface-wetting characteristics, and the plastic can then readily be bonded using a wide range of adhesives. In the late 1980s primers were introduced that considerably enhance the adhesion of cyanoacrylates to polyolefins. The primer changes the surface condition of the plastic, creating bond sites for the cyanoacrylate adhesive. The effect of a polyolefin primer when used with a cyanoacrylate on polypropylene should not be underestimated. Bond strengths are often 25 to 40 times higher than those achieved when using the same adhesive without primer (Figure 6.4). Note that these polyolefin primers are only suitable for cyanoacrylate adhesives and are not compatible with other technology adhesives.

6.4 Two-part Acrylics The introduction within the last few years of two-part acrylics for the bonding of polyolefins has given the design engineer another option for the bonding of the polyolefin plastics.

Bond Strengths (MPa)

Bonding with Cyanoacrylates 10 8 6 4 2 0 PVC or PC

Polypropylene (unprimed)

Polypropylene (primed)

Figure 6.4 Typical adhesive shear strengths on a selection of materials [4]

99

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts These 10:1 mix ratio acrylics show excellent adhesion to polyethylene and polypropylene with handling strengths in less than 10 minutes. The products contain glass beads or fillers to control the bond-line thickness to 0.2 mm or 0.25 mm and so the joint should be designed to accommodate these fillers. These two-part acrylics do not require any pre-treatment of the joint surfaces or any surface primer and will bond polyethylene, polypropylene and ethylene copolymers with shear strengths in the range 4–8 N/mm2. They can be used on many other substrates and so can be used as a general-purpose adhesive, although they are not recommended for bonding PTFE or the fluoropolymers. The resistance to water and high humidity environments is good but the mix ratio is critical to avoid unpredictable results.

References 1.

F. Bashforth and J.C. Adams, An Attempt to Test the Theory of Capillary Action, Cambridge University Press, Cambridge, UK, 1883.

2.

Henkel Media On-line, The Henkel Brand Database, 2010.

3.

Industrial Adhesion Problems, Eds., D.M. Brewis and D. Briggs, Orbital Press, Oxford, UK, 1985.

4.

The Loctite Design Guide for Bonding Plastics, Volume 4, Henkel Ltd, Hatfield, UK, 2006.

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7

Selecting the Adhesive

7.1 Introduction You only have to type the words ‘adhesives for plastics’ into an internet search engine and you are rewarded with thousands of websites, each one of which is trying to tempt you to visit their site for one reason or another. There is a bewildering range of adhesives available to the end user and this chapter will by no means attempt to answer all the questions or even give all the options available. Hopefully, however, it will provide the reader with some guidelines as to the factors that should be considered when selecting an adhesive for the bonding of engineering plastics and elastomers. Adhesives and sealants are a multi-billion dollar industry that serves many applications and end-markets. Adhesives are used in a huge variety of industries, from automotive to medical and from electrical switchgear to sports goods. Adhesives are primarily designed to bond parts together and sealants prevent leakage of fluids or gases or seal against the ingress of atmospheric components into an assembly. The benefits of adhesives for the joining of plastics include: •

They distribute loads across the entire joint area,

•

They have the ability to bond dissimilar materials,

•

They can attenuate mechanical vibrations and sound,

•

They can offer the ability to joint thinner materials,

•

They give unobtrusive bond lines and thus improve the aesthetics of the joint,

•

They can act as both adhesive and sealant, and

•

The capital equipment investment is typically considerably less than it might be for ultrasonic welding or similar processes. 101

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7.2 Factors for Consideration Factors that may need to be considered for the selection of an appropriate adhesive include the following: •

Selection of materials,

•

Adhesive performance, °

Tensile strength,

°

Peel strength,

°

Impact loading,

•

Durability and long-term performance,

•

Temperature resistance,

•

Surface preparation,

•

Ease of application,

•

Joint design,

•

Cure speed and curing equipment,

•

Gap-filling capability,

•

Sealing capability,

•

Health and safety issues,

•

Approvals, and

•

Recycling.

7.2.1 Selection of Materials Many engineering plastics are selected on the basis of other properties (mouldability, stiffness, temperature performance, etc.) and the suitability for adhesive bonding is unlikely to be considered in the first instance. Polyetheretherketone (PEEK), for example, is a very good engineering plastic with some excellent properties but, for the adhesive engineer, this is one of the most difficult materials to bond and will 102

Selecting the Adhesive almost certainly require some pre-treatment. In many applications a plastic such as polyamide or polycarbonate may well suffice and thus be much easier to join. The selection of the adhesive is all too often left until the ‘last minute’ and this can result in serious and costly delays to the product launch. Just as there is no universal plastic for every application, there is no universal adhesive and so the bonding process should be treated as an integral part of the entire design and assembly operation. Plastics and elastomers that have a low surface energy and are therefore more difficult to bond are listed below: •

Polyethylene,

•

Polypropylene,

•

Polytetrafluoroethylene (Teflon) and most of the fluoropolymers,

•

Acetal (Delrin),

•

PEEK,

•

Silicone rubber, and

•

Some thermoplastic elastomers.

The bonding of these low-surface-energy plastics is discussed in more detail in Section 6.3.

7.2.2 Adhesive Performance Perhaps one of the most fundamental and key properties associated with the selection of the adhesive is to make sure that it bonds well to the two materials selected. Cyanoacrylates generally have good affinity to plastics and a good number of ultraviolet (UV) adhesive grades are also particularly suited for plastics, whilst other UV adhesives (with silane additives) are more suitable for glass – the product data sheet should be studied carefully to ensure compatibility with both adherends. Epoxies are good general-purpose products and can show excellent adhesion but are slower cure. Some guidelines as to the tensile shear performance on plastics and elastomers with a number of different adhesives are given in Sections 2.2, 3.2 and 4.2 but adhesive strengths are very dependent on the joint design and the thicknesses of the adherends.

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts For example, polyethylene terephthalate (PET) plastic can be bonded with many different types of adhesives, including cyanoacrylates, UV acrylics and epoxies, and good strengths can be obtained on standard lap shear parts (1.6 mm thick). However, if thin films (<0.75 mm thick) of PET are to be bonded then it will be much easier to subject the adhesive to a peel load and the measured strength of the adhesive on the same grade of PET will be considerably lower. The toughened adhesives will show improved peel strength (see Section 1.1.1.5) but note that the peel strength of an adhesive will generally be less than 10% of the nominal shear strength and could be as low as 1%. Impact resistance is often a key requirement for components operating under dynamic stresses but this property is not usually quoted by adhesive manufacturers as the joint design, materials bonded and type of loading will all affect the impact strength performance. Toughened or flexible adhesives would generally be expected to show improved impact resistance and the highly crosslinked toughened epoxybased adhesives are often the material of choice for applications where high impact loads are present.

7.2.3 Durability and Long Term Performance and Temperature Resistance Most adhesives will operate within the range −40 °C to 120 °C and speciality grades will operate to higher temperatures. A room-temperature-vulcanising silicone will often be suitable up to >250 °C. On the other hand a standard ethyl based cyanoacrylate will only operate to about 90 °C. A significant point to consider when selecting an adhesive is its moisture or humidity resistance as water will invariably reduce the strength of adhesives. For this reason, the technical data of an adhesive frequently lists the durability of bonds in humid environments after prolonged time periods. Normally charts are included that list the percentage of initial resistance retained by the adhesive bond over time under known humidity conditions. Durability of adhesives is discussed in more detail in Section 9.1.

7.2.4 Surface Preparation Many plastics can be bonded ‘as received’ and this will be a significant benefit for the selection of the adhesive. Solvent wiping with chlorinated solvents or alcohols can remove dirt or mould release agents; however, some plastics have a low surface energy (see Section 6.1) and will require pre-treatment [1].

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Selecting the Adhesive The surface finish of the selected plastic can have an important bearing on the final adhesion. Slightly roughened surfaces by light abrasion will usually be beneficial for adhesion and the durability of the adhesive and in certain applications this can be achieved by spark eroding the surface of the mould. A surface roughness of 1–2 Ra will assist the mechanical keying of the adhesive into the substrate.

7.2.5 Ease of Application The suitability of an application procedure will be an important consideration in the selection of an adhesive. It is not sufficient that an adhesive can provide the necessary performance. It must also be possible to apply the adhesive at the required location and at an acceptable cost. Single-part, fast-cure adhesives such as cyanoacrylates are ideal for high production rates and can be dispensed using pressure-time or other relatively low-cost equipment systems. The two-part adhesives such as acrylics and epoxies offer the benefits of higher peel strengths and good gap-filling capability but have to be dispensed through a mixer nozzle or applied as ‘bead-on-bead’. The UV adhesives are one of the easiest adhesives to dispense but do require an investment in curing equipment. Dispensing of the adhesives is discussed in more detail in Section 8.1.

7.2.6 Joint Design The joint design is fundamental to the integrity and efficacy of the performance of the adhesive. Some detailed notes on joint design are given in Chapter 5, but it is important to try and understand all the forces that the adhesive is being subjected to. In practice, this means that the nature and magnitude of the stresses expected during the working life of the assembly must be understood before deciding on the type of adhesive that should be used. If the force acts in one direction over a long period of time, it can result in the phenomenon of creep. This leads to a slow, but constant, relative displacement of the bonded components. This displacement progresses until the adhesive bond fails. The creep represents a material behaviour exhibited not only by plastic materials and adhesives, but also by metals (although less noticeably). The crosslinked thermosetting adhesives (e.g., epoxies) generally have a higher resistance to creep than the thermoplastic adhesives such as cyanoacrylates. Correct design of the assembly can prevent this phenomenon from appearing or at least reduce it to allowable limits.

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Figure 7.1 Peel forces should be avoided

From a design viewpoint, every application will be different and so considered independently but some important guidelines are as follows: •

Always use the largest possible area and align the joints such that stresses can be absorbed in the direction of greatest bond strength.

•

Maximise shear forces and minimise peel and cleavage forces (Figure 7.1).

•

For most applications, the gap between the parts should be thin (<0.15 mm) but sometimes a thicker bond line can be beneficial for impact loading or where the adhesive is required to absorb stresses due to differing coefficients of thermal expansion.

•

Slightly rougher surfaces are usually beneficial and oils, mould release agents and other surface contaminants should be avoided.

7.2.7 Viscosity The viscosity of the adhesive can also be a significant factor in adhesive selection. Lowviscosity adhesives (<50 mPa-s) should be used where gaps are small and the adhesive is required to flow or wick into parts that have been pre-assembled. A mediumviscosity adhesive (up to 1000 mPa-s) may be more appropriate for the bonding of, say, cylindrical parts where the diametral gap is of the order of 0.1 mm. High-viscosity adhesives (>10,000 mPa-s) would be specified for larger gaps or where the adhesive is required not to flow away from the joint prior to assembly and cure. 106

Selecting the Adhesive

7.2.8 Cure Speed On some production lines a very fast curing adhesive is necessary to minimise workin-progress. But in other applications where alignment of the parts is difficult a longer cure may be necessary and this may be achieved by using a ‘cure-on-demand’ UV-curing adhesive. The cure speed of two-part acrylics is typically in the range of 5–50 minutes depending on the grade and epoxies can be much slower than this with some grades taking greater than four hours to achieve handling strength. Epoxies can be heat cured to accelerate the cure and the time to achieve handling strength will depend on the mass of epoxy present and the temperature applied. Cyanoacrylates should be cured quite quickly otherwise this may result in ‘blooming’ (see Section 10.2.6). It is important that no movement takes place in the joint line during the curing process as this can sometimes lead to failure (see Section 10.2.3). Appropriate jigs or fixtures may be necessary to allow the adhesive to gain sufficient strength prior to the next process operation.

7.2.9 Gap-filling Capability One of the benefits of adhesives is that they will fill gaps between poorly fitting parts or parts with large manufacturing tolerances. The cure of epoxies is not affected by the gap between the parts but cyanoacrylates are very much driven by this gap and are limited to gaps of less than 0.2 mm. The gap therefore has an important bearing on the family of adhesives that will be best suited for the application. If the gap between two cylindrical parts is very small (<0.05 mm) then it is unlikely that an epoxy will be suitable as it will probably be too high in viscosity and will be pushed out of the joint as the parts are assembled, resulting in joint starvation. In this case a low-viscosity cyanoacrylate or UV adhesive that will capillary down into the interstitial spaces may be better suited. The optimum gap for most adhesive applications for engineering plastics is between 0.05 mm and 0.2 mm but sometimes a thicker joint with a flexible adhesive is the best solution, especially if there are high peel loads or the adhesive is acting primarily as a sealant and not as an adhesive (see Section 1.5).

7.2.10 Sealing Capability The distinction between an adhesive and a sealant is not well defined as adhesives are often required to act as sealants. In the same way, a sealant has to show some 107

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts adhesion properties to a substrate to ensure that it has made intimate contact and thus prevents the fluid or gas escaping in the boundary layer between the sealant and the substrate. The silicone adhesive family often provides the desired properties for many applications where a seal is required. These products are suitable where there are severe movements between substrates caused by changes in temperature, pressure and vibration. However, aggressive chemicals such as jet fuel, strong acids or alkalis, organic chemicals, can cause swelling and debonding of silicone sealants and thus affect the permeability and integrity of the seal.

7.2.11 Health and Safety (H&S) This section does not attempt to discuss the health and safety of engineering adhesives in full detail but merely gives an overview of some of the regulations and major precautions that may need to be considered when selecting adhesives for the bonding of engineering plastics or elastomers. The H&S issue is quite rightly increasingly becoming more prevalent in the workplace and this gives adhesive chemists ever greater challenges. The Health and Safety Executive (HSE) website (www.hse.gov.uk) [2] gives very thorough notes and guidance on all aspects of health and safety in the workplace. The suppliers of all adhesives and chemicals in the EC (and indeed in most countries) are required to provide an Material Safety Data Sheet (MSDS) for each product they sell. The MSDS (Section 15) will also include risk phrases (R) and Safty phrases (S) and many of these will also be included on the product packaging. Examples of ‘Risk phrases’ in use prior to April 2009: •

R20

Harmful by inhalation,

•

R21

Harmful in contact with skin.

Examples of ‘Safety phrases’ in use prior to April 2009: •

S16

Keep away from sources of ignition,

•

S36

Wear suitable protective clothing.

However, in April 2009, the MSDS sheets were no longer covered by the Chemicals Hazard Information and Packaging for Supply (CHIP) Regulations and have been transferred to the European Registration, Evaluation, Authorisation and restriction 108

Selecting the Adhesive of Chemicals (REACH) Regulation. Under the REACH Directive, new harmonised warning and precautionary statements for labels are replacing the existing risk and safety phrases: Examples of new Hazard Statements for labels: •

H240 – Heating may cause an explosion,

•

H320 – Causes eye irritation, and

•

H401 – Toxic to aquatic life.

Examples of new Precautionary Statements for labels: •

P102 – Keep out of reach of children,

•

P271 – Use only outdoors or in well-ventilated area, and

•

P410 – Protect from sunlight.

Examples of some of the original symbols (pre-April 2009) are given in Table 7.1.

Table 7.1 A selection of standard warning symbols and their meanings Symbol Abbreviation Hazard Description of hazard F+ Extremely Chemicals that have an extremely low flammable flash point and boiling point, and gases that catch fire in contact with air T Toxic Chemicals that at low levels cause damage to health C

Corrosive

Chemicals that may destroy living tissue on contact

Xi

Irritant

Chemicals that may cause inflammation to the skin or other mucous membranes

Xn

Harmful

Chemicals that may cause damage to health

N

Dangerous for the environment

Chemicals that may present an immediate or delayed danger to one or more components of the environment 109

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts The European Union (EU) have tried to establish a standard hazard classification for all dangerous chemicals and these are listed in an Annex to the EU Directive. This annex is kept up to date via an Amendment to Technical Progress (ATP), which reviews the classification of all substances. These changes in regulations sometimes mean that an adhesive purchased in 2002 and then again in 2009 may have changed its warning label from, say, ‘Irritant’ to ‘Harmful’. This will be because (in the light of new test data) one of the ingredients of that adhesive will have been reclassified even although the adhesive formulation itself and the concentration of that chemical has not changed. For example, the 25th ATP changed cyanoacrylate monomer to an ‘Irritant’ classification resulting in St Andrew’s cross and new labelling for all cyanoacrylates. By 2015, international Globally Harmonised System of Classification and Labelling of Chemicals (GHS) symbols will replace the European symbols (Table 7.2) and so the cyanoacrylate label will be changed again to comply with the regulations utilising the GHS labelling system.

Table 7.2 A selection of the GHS warning symbols and their meanings Symbol Hazard Description of Hazard Corrosive Chemicals that may destroy living tissue on contact

Extremely flammable

Chemicals that have an extremely low flash point and boiling point, and gases that catch fire in contact with air

Toxic

Chemicals that at low levels cause damage to health

Irritant

Chemicals that may cause inflammation to the skin or other mucous membranes

Harmful

Chemicals that may cause damage to health

Control of Substances Hazardous to Health (COSHH) is the law that requires employers to control substances hazardous to health. Where adhesives are used in the workplace a COSHH assessment could include, for example: •

Provision of training for employees,

•

Ensuring adequate ventilation,

110

Selecting the Adhesive •

The provision of adhesive dispensers to minimise risk of spillage/overapplication,

•

Providing appropriate protective gloves, UV glasses, etc., depending on the application.

Every industrial application will be different and the HSE website gives some very useful guidelines and worked examples of COSHH assessments.

7.2.12 Approvals The MSDS only refers to the adhesive in its liquid state and once the adhesive is cured it is often a requirement that the adhesive has to meet certain approvals before it can be used for the desired application. There are many different approvals and standard test methods for adhesives but some of the more common ones for a variety of different industries are listed: •

Commercial – Rolls Royce, Ford, nuclear industry, rail industry, etc.,

•

ISO 10993 – medical industry,

•

WRAS Approval – drinking water,

•

Gas approval,

•

Food and Drug Approval (USA), and

•

Ministry of Defence Standards.

7.2.13 Recycling Adhesives Adhesives are mostly thermoset plastics when cured (although cyanoacrylates are thermoplastic) and in any case are a highly complex polymer. In most engineering applications, the mass of adhesive within an assembly or sub-assembly is negligible and only a few percent of the total mass of the component and it will be difficult (and very expensive) to extract just the adhesive from the component at the end of life of the component. Nevertheless under the End of Life Vehicles Directive 200/53/EC (2005), adhesive suppliers are now required to provide statements regarding the composition to

111

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts automotive manufacturers in order to ensure that the vehicle can be safely recycled at its end of life [3] and [4].

7.3 A Summary for Adhesive Selection Each application will be different and the selection of the adhesive will depend on all of the factors given above and probably many more. Engineers will have their own ‘favourites’ based on their own experiences and this small section is intended only as a very general overview of five different adhesive types. For clear plastics where optical clarity and speed of cure are essential, the UV adhesives are an obvious first choice. However, these products are amongst the higher cost adhesives and curing equipment is required. Epoxies will fill large gaps and can show excellent durability but dispensing of small quantities (<0.1 g) can be difficult if the volume dispensed is not sufficient to displace the quantity pre-mixed in the dispensing nozzle during the cure time of the adhesive. Cyanoacrylates are often the first choice for the bonding of elastomers, especially where the application is essentially non-structural. For the bonding of small, colouredplastic assemblies, cyanoacrylates also show excellent adhesion (Figure 7.2). Two-part acrylics will bond many different materials and can tolerate a wider variation in mix ratios than epoxies. They can show good toughness and clarity but often have a higher odour than many of the other adhesive families.

Figure 7.2 An ophthalmic blade is bonded to a plastic handle using a cyanoacrylate 112

Selecting the Adhesive The low-energy plastics always represent a bonding challenge and these are discussed in Section 6.3. For optimum flexibility and high-temperature performance the silicone adhesives are outstanding but they are slow curing and generally relatively high in viscosity. Recent introductions include two-part silicones that can overcome the slow cure issue.

References 1.

D.J. Dunn, Engineering and Structural Adhesives, Rapra Review Report No.169, Rapra Technology Ltd, Shrewsbury, UK, 2004, p.13.

2.

www.hse.gov.uk, Accessed May 2009.

3.

www.recycle-more.co.uk, Accessed January 2009.

4.

V. Goodship, Introduction to Plastics Recycling, 2nd Edition, Rapra Technology Ltd, Shrewsbury, UK, 2007, p.140.

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8

Dispensing Adhesives in Production

8.1 Basic Principles Adhesives can be applied as drops or beads, roll coated, sprayed or screen printed. The application equipment ranges from simple hand guns, bottles and tubes to highly sophisticated six-axis robots with automated parts handling. There are a number of factors that need to be considered before selecting a dispense system: •

Single-part or two-part adhesive,

•

Viscosity of the adhesive,

•

Dispense quantity,

•

Cycle time to achieve production,

•

Cure method,

•

Open time of the adhesive,

•

Health and safety considerations, and

•

Cost.

8.1.1 Single- or Two-part Adhesive Single-part adhesives will usually offer benefits (from a dispensing viewpoint) over two-part adhesives as no mixing or dual application is necessary. Most epoxies are supplied as two-part adhesives and typically these would be supplied in twin cartridges or syringes and then mixed in a helix nozzle (Figure 8.1).

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Temperature/Viscosity curve for a UV Acrylic

Viscosity (mPa-s)

30000 25000 20000 15000 10000 5000 0 0

10

20

30 40 Temperature °C

50

60

Figure 8.1 The viscosity of the adhesive can double for only a 10 °C fall in temperature

In some industries, particularly where only small quantities are required, the epoxy adhesive can be pre-mixed in a small container and then decanted into a syringe where it can be dispensed via a syringe system (see Section 8.3). The risk here is that the viscosity of the epoxy will gradually increase as the adhesive begins to cure in the syringe thus making it more difficult to control the quantity dispensed. Some two-part adhesives can be dispensed as a liquid activator (accelerator) and an adhesive. In this case, the activator is applied to one surface and the adhesive to the other and cure will commence when the joint is closed. In other two-part acrylic applications, the adhesive is applied as ‘bead-on-bead’ and so a mixer nozzle is not necessary. This bead-on-bead method is not normally suitable for epoxies as the mix ratio of epoxies is quite critical and thorough mixing cannot always be guaranteed with a bead-on-bead process. Two-part acrylics, however, are far more tolerant of mix ratio and so bead-on-bead systems can work well in a highspeed production environment, although care is required to ensure that the nozzle tips do not become contaminated. Single part adhesives will nearly always be the most convenient to dispense, especially for the bonding of small component parts.

8.1.2 Viscosity (see Section 5.8) The viscosity of the adhesive is one of the most important factors in deciding the choice of dispense system. Adhesives of a viscosity lower than about 30,000 mPa-s (also known as centipoise) are usually packaged in bottles and above 30,000 mPa-s they 116

Dispensing Adhesives in Production will be packaged in syringes or cartridges. However, the syringe pack is widely used for all viscosity adhesives. For most adhesive-dispensing applications the flow will be laminar (with the Reynolds Number Re <2000) and so the following equation applies: D4 ΔP t

M ~ ____ µ,L

(8.1)

where M = mass of adhesive dispensed (grams), µ = viscosity of the adhesive (mPa-s), D = diameter of the pipe or nozzle (mm), t = dispense time (seconds), L = length of the feed tube (mm), ΔP = pressure applied (Pa), [1]. The fixed variables in most dispensing systems will be the diameter of the tubing (D), the length of the tubing (L) and the viscosity of the adhesive (µ). The function variables are the driving pressure (ΔP) and the dispense time (t). For this reason many adhesive-dispensing units are known as ‘pressure-time’ systems. This equation shows that small changes to the diameter of the pipe or nozzle will have the most significant effect on the mass (or quantity) of adhesive dispensed. As with most fluids, the viscosity of the adhesive will change with temperature and Figure 8.2 shows a typical viscosity/temperature curve for an ultraviolet (UV) acrylic. At 25 °C, the viscosity of this adhesive is 5000 mPa-s but at 15 °C, the viscosity is 10,000 mPa-s and correspondingly at 35 °C the viscosity has decreased to 2500 mPa-s. These changes in viscosity will therefore give corresponding inverse changes to the dispense quantity and so a good control of temperature is required to minimise errors in pressure-time dispensing (see Section 8.2.4). Some adhesives are solvent based and as the solvent evaporates, the percentage of solids in the adhesive will increase thus increasing the viscosity. This also will have an effect on the quantity dispensed. The rheology of some adhesives can be quite complex and these ‘non-Newtonian’ fluids may require special attention, particularly if they have a tendency to separate in a dispense valve causing blockage and thus significantly affecting the quantity dispensed.

Figure 8.2 A dispensed bead of adhesive can often exhibit this ‘dog bone’ effect due to pressure surges as the valve opens and shuts 117

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

8.1.3 Cycle Time The cycle time has an important bearing on the investment required to automate the system and the number of operators or workstations that may be necessary. Whether manual, semi-automatic or automatic, the dispensing equipment system specified will depend on the type of application and workplace conditions, such as dispense rate, parts per hour, operator availability and materials handling. Adhesive dispense systems are available for most production lines varying from bench-top applicators to fully automated robotic units with dispense times of <50 ms.

8.1.4 Cure Method The cure method of the adhesive very often dictates the type of dispensing equipment to be used. Cyanoacrylates are extremely sensitive to moisture and so a clean dry air supply to the pressure pot is essential to ensure the adhesive does not thicken in the pot. UV adhesives will require black or opaque containers and tubing to prevent the adhesive curing from stray UV light. Some adhesives (anaerobics) are metal-part active and so inert plastic parts should be used throughout the dispensing line. In some automated adhesive dispense systems for two-part adhesives (e.g., epoxies and acrylics) a ‘spit cycle’ is incorporated into the machine such that if the mixer nozzle is left idle for a pre-determined time, a small quantity of adhesive is dispensed into a container to prevent the adhesive curing in the mixer nozzle.

8.1.5 Dispense Quantity In a precision medical application the quantity to be dispensed might be less than a milligram of adhesive but for a seal around a solar panel, the quantity could be several grams and it is unlikely that the same piece of dispense equipment will be suitable for both applications. The positional accuracy of the adhesive may also be a significant factor when selecting an adhesive dispensing system. In these situations, an investment in a three-axis, four-axis or six-axis robot may be necessary but sometimes well-engineered jigs can significantly assist the operator at the dispensing station. One of the issues typically encountered when dispensing a bead of high viscosity adhesive is the ‘dog bone’ effect, whereby the bead of adhesive is not consistent along its length but often has a higher ‘blob’ at each end (Figure 8.2).

118

Dispensing Adhesives in Production The reason for this is when an adhesive is dispensed at high pressure, there is a pressure surge as the valve opens causing excess adhesive to flow. This quickly disappears as the flow rate steadies but at the end of the bead as the valve closes some adhesive can be pushed forward. Careful valve design and robot programming can minimise these effects but it is always difficult to join a bead to create a continuous seal and totally eliminate the ‘knit’ point in the bead (Figure 8.3). One possible method for minimising the ‘knit joint’ is to design the groove in the component to allow for a small surge of adhesive and include a run-in or run-out point (Figures 8.4 and 8.5). In Figure 8.5, the design allows for the ‘start-stop’ point of the adhesive bead thus minimising the effect of the ‘knit’ joint.

Figure 8.3 Minimising the effect of the ‘knit’ joint in a closed bead can be difficult to achieve even on a robot

Figure 8.4 Standard groove

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 8.5 Groove has been altered to allow for dispensing of adhesive

8.1.6 Open Time The ‘open time’ of an adhesive is the time it can remain on an adherend before the joint needs to be closed. For epoxies this would typically be about 50% of the fixture time as any longer time could result in the adhesive not wetting and thus not adhering properly to the second substrate. UV adhesives have an almost infinite ‘open time’ although exposure to workshop light and daylight can initiate slow polymerisation. Cyanoacrylates would normally have a relatively short open time as they are very sensitive to surface moisture and to a lesser extent atmospheric moisture. In most cyanoacrylate applications, the joint should be closed within a few seconds and certainly not more than a few minutes. Room-temperature-vulcanising silicone adhesives will ‘skin over’ in about 10 minutes and so the joint should be closed within a five-minute period to ensure that full wetting and therefore adhesion takes place. For fully automatic dispensing systems, this means that some consideration must be made to ensure that the joint is closed within the specified time.

8.1.7 Health and Safety The use of dispensing equipment can help to minimise the risk of operators coming into direct contact with the adhesives and thus improve the health and safety at the workstations. Correctly specified equipment will ensure that the optimum quantity of adhesive is dispensed on to the component parts thus improving the overall quality and minimising the possibility of excess adhesive on the part. Most dispensing equipment will be required to meet the latest European standards including the mark. The letters ‘CE’ are the abbreviation of French phrase

120

Dispensing Adhesives in Production ‘Conformité Européene’, which literally means ‘European Conformity’. The term initially used was ‘EC Mark’ and it was officially replaced by ‘CE Marking’ in the Directive 93/68/EEC in 1993. ‘CE Marking’ is now used in all European Union official documents. CE Marking essentially means that the product is safe under normal or reasonably foreseeable conditions of use, including duration, and does not present any risk or only the minimum risks compatible with the product’s use. The adhesive itself is unlikely to have CE Marking because the final use of the adhesive is outside the control of the adhesive manufacturer.

8.1.8 Cost Since the dispensing equipment can be set to apply the optimum quantity, there will be no wastage of adhesive and so costs are reduced and in many applications this can provide a pay-back towards the capital cost of the equipment. The cost of the equipment will vary widely with its complexity and degree of automation. The characteristics of the dispensing unit chosen will always depend on the benefit obtained. These benefits could include improved aesthetics or quality, faster throughput, less rework and improved health and safety. Naturally, more sophisticated component parts will require higher investments in dispensing equipment, particularly when a standard unit cannot be used and one must be designed to meet the particular characteristics of the application.

8.2 Dispensing Systems Based on a very general classification that includes the degree of automation of the system, three major groups can be described: •

Manual units: all operations require handling by an operator. Common examples are the well-known manual extrusion guns, manually operated peristaltic pumps, hand-held pinch valves and the hot-melt guns.

•

Semi-automatic units: require the control of an operator but use a mechanical, pneumatic or electrical system for dispensing and hence allow a controlled application volume. Semi-automatic units may also include other features that facilitate adhesive handling and dispensing.

•

Automatic units: can be inserted in an assembly line and controlled automatically by a programmable logic controller or personal computer. 121

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

8.2.1 Manual Units For many applications, adhesive dispensing from the original package is suitable and the adhesive can be dispensed by squeezing the tube or bottle. Many of the adhesive bottles and tubes have a moulded nozzle to help control the quantity dispensed and in some applications additional control can be achieved by fitting a small nozzle onto the bottle (Figure 8.6). Many of the two-part acrylic or epoxy-based adhesives are packaged in a twin syringe and a small hand gun is employed to mix the adhesive in the nozzle and apply to the component part (Figure 8.7).

Figure 8.6 In this application a small, pink nozzle has been fitted onto the adhesive bottle to control the quantity applied

Figure 8.7 Applying an epoxy to secure plastic buoyancy spacers to a Kevlar rope on a marine seismic cable

122

Dispensing Adhesives in Production

8.2.2 Semi-automatic Dispensers One of the most commonly used semi-automatic dispense systems is the pressuretime method and this is extensively used for a wide range of production engineering applications. The adhesive is stored in a pressurised vessel and pressure is applied to force the adhesive to the dispensing tip. There are two distinct variations of the pressure-time method: •

Syringe dispensing,

•

Pressure pot dispensing.

8.2.3 Syringe Dispensing The syringe dispense system is a very popular system and ideal for adhesives and sealants packaged in standard syringes. The principle is that a timed pulse of air is applied to the back of the syringe and this then forces the adhesive through the nozzle tip. The quantity dispensed is varied by the pressure applied, the nozzle diameter and the time of the air pulse [2]. Figure 8.8 shows a typical syringe dispenser. An anti-drip feature is often incorporated using a vacuum to hold back the adhesive whilst the system is idle. Other features include digital pressure control and low-level monitoring of the fluid in the syringe.

8.2.4 Pressure Pot Dispensing Normally a dispensing system consists of a tank, a dispensing valve and a control console. They require only a pneumatic line or nitrogen bottle to provide the

Figure 8.8 A typical syringe dispenser

123

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts necessary pressure for system operation and an electrical outlet. The valves and respective peripherals are controlled and operated by means of electric and electronic components. The type of pressurised tank is determined by the adhesive viscosity. The following types of tanks can be found, among others: •

Gravity-operated,

•

Pressurised vessel,

•

Cartridge holder pneumatic plunger, and

•

Tank equipped with powered follower plate.

For very-low-viscosity adhesives and for some activators or primers a gravity-fed system is often used. Low- and medium-viscosity products up to about 30,000 mPa-s are typically packaged in bottles and require a pressurised tank. Normally, pressure is applied and kept inside the tank which holds the adhesive bottle. It is connected to the outside by a tube. When the system is pressurised, the product is forced through the connecting tube to a dispense valve (see Figure 8.9) [3]. Higher viscosity products are typically packaged in cartridges to facilitate dispensing with hand guns and for semi-automatic dispense systems, pressure is applied to the Product Feedline

Shut-off Value

Cover

Air Supply

Product Reservoir

Figure 8.9 Principle of pressure pot dispensing 124

Dispensing Adhesives in Production rear of the cartridge by means of a pneumatic plunger thus forcing the piston to move and extrude the product through the nozzle (Figure 8.10). In many of these semi-automatic cartridge systems a valve may not be required and flow of adhesive is controlled by the pressure and vacuum suck-back alone.

Air

Su

ppl

y

Figure 8.10 A pressurised cartridge for dispensing high-viscosity adhesives However, for optimum control and accuracy a dispense valve is fitted and there are many different types available including pinch valves, poppet valves and diaphragm valves. Valve control can be manually triggered from a foot switch or automatically activated via a proximity switch as the component is offered up to the nozzle tip. With these pressure-time systems, any variation in adhesive viscosity will lead to irregular dispensing. One option to minimise this is to fit heater jackets to the adhesive dispense valves so that the temperature is controlled above ambient temperature (e.g., 35 °C). The shelf life of the adhesive at the higher temperature may have to be considered if this method is adopted. A second option to overcome this is to use volumetric or positive displacement systems where a cavity of known volume is filled then discharged to dispense perfectly reproducible quantities of adhesive, irrespective of the viscosity of the adhesive. Peristaltic pump systems rely on squeezing a small diameter tube around a set of rollers and so are also a positive displacement system and, with a variable speed drive, very small, precise quantities of adhesive can be dispensed.

8.3 Automatic Systems Many of the automatic systems are based on the semi-automatic units but include a number of additional features for improved control. 125

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Detectors can be fitted to indicate the quantity of product remaining in the pressure pot and thus predict when the product will be completely used up. For some systems an automatic change-over valve can be installed to switch supply from a second pressure pot thus maintaining supply of adhesive to the production line. The dispense valve can be mounted on robots, for tracing complex profiles, particularly when dispensing adhesives on high-volume production lines. These valves can be fitted with pressure sensors and, depending on sensitivity, can detect breaks in the adhesive supply due to entrainment of air bubbles (see Section 10.1.1). Another option is to detect the adhesive on the component part. This can be more complex and often relies on the adhesive itself either showing good contrast against the substrate or the adhesive fluorescing so that it can be visually picked up under a suitable camera detector.

References 1.

B.S. Massey, Mechanics of Fluids, 2nd Edition. Van Nostrand Reinhold Company Ltd, London, 1972, p.142.

2.

Loctite Worldwide Design Handbook, 2nd Edition, Henkel Ltd, Hatfield, UK, 1999, 370.

3.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2001, p.275.

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9

Durability and Environmental Testing

9.1 Introduction The durability of adhesives for the structural bonding of metals is well documented and reference [1] is one of many papers on this subject. Much of the data that exist are based on lap shears using toughened epoxy-based adhesives and epoxies are widely used for the bonding of steel or aluminium parts. Due to the huge variety of plastics and elastomers available and the wide range and types of adhesives on the market, there are quite understandably relatively few environmental test data available for the adhesive bonding of plastics. However, many of the rules that apply for the improved durability of the adhesive bonding of metals can also be applied to the bonding of plastics. Some of the factors associated with the durability of adhesives on plastics are: •

Surface finish and surface preparation,

•

Joint design, and

•

Substrate bonded.

9.1.1 Surface Finish and Surface Preparation Surface finish and surface preparation are both key factors in the success of an adhesively bonded joint and, in many applications, roughening the plastic surface can be beneficial to the overall bond strength and the durability. If the adhesive is injection moulded, it is often possible to spark erode the mould tool to give a slightly rougher surface finish at the bond line thus improving the mechanical keying of the adhesive to the surface. A surface finish of between 1 and 2 Ra will invariably improve the adhesion performance of cyanoacrylates to thermoplastics.

127

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts In many non-structural applications, adhesives are used to bond engineering plastics without the need for any surface preparation and indeed this brings out the benefit of using the adhesive, especially when bonding dissimilar materials (where ultrasonic welding or other joining methods may not be possible). However, surface contaminants such as oil and grease or the presence of mouldrelease agents will inhibit the substrate adhesion and thus compromise the long-term performance of the bonded joint. Internal and external mould-release agents are used to guarantee easy release of moulded plastic or rubber parts and can sometimes be transferred from the mould tool onto the surface of the plastic. The most common method of cleaning is a solvent wipe but the solvent selected should not craze or soften the plastic [2]. Mould-release agents are described as internal if they are already mixed with the granules and take effect during the processing of the plastic or rubber. These mould release agents may be distributed throughout the whole material so that even mechanical abrasion may not be effective. Some plastics with a low surface energy will require a surface treatment process prior to bonding and this is covered in more detail in Section 6.3.

9.1.2 Joint Design Joint design is discussed in more detail in Chapter 5 and can have an important influence on the durability of the joint so a few general points relating to durability are discussed here. Adhesive bond lines will always be susceptible to environmental attack and a thick bond line (>0.2 mm) offers a ready path for access by moisture or other solvents. Thinner bond line gaps therefore are invariably beneficial. Increasing the bonded area will usually decrease the overall load on the joint and therefore improve the reliability and durability as there is a lower overall stress on the joint. The adhesive will always be stronger in compressive, tensile and shear loads than in peel or cleavage loads and typically the peel load of an adhesive will be less than 5% of the nominal shear strength so ensuring that the bonded joint cannot be subjected to peel or cleavage loads will improve the overall integrity of the joint. Designing the joint to avoid water or moisture traps wherever practical is logical and sensible as water is usually the enemy of adhesive joints. Many industries now require resistance after prolonged exposure to high temperature and humidity levels (85% RH, 85 °C) and in these circumstances close-tolerance components and using plastics that are not hydroscopic are essential.

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9.1.3 Substrate Bonded Plastics with a low surface energy such a polyetheretherketone, acetal polyethylene, polypropylene and silicone rubber are more difficult to bond and do require surface treatment. Whilst some of these plastics offer very good engineering properties, the low surface energy results in lower adhesion and therefore the resulting durability may be poor. Selecting a higher-surface-energy plastic at the design stage can significantly improve the chances of achieving a durable joint. In the loudspeaker application shown in Figure 9.1, adhesives are used for almost every joint during the assembly process. The magnet is bonded to the pole piece and the top plate using two-part acrylic adhesives and parts are bonded without any special surface preparation (other than ensuring that there is no excessive oil or grease or other deposits on the two substrates to be bonded). Although the metal chassis is staked to the top plate, a low-modulus adhesive/sealant is applied between the joint to eliminate any buzzing that might occur. The coil, suspension, cone and surround joints are all bonded with adhesives (typically two-part acrylics or cyanoacrylates but also water and solvent-based adhesives) and in the automotive industry this fully assembled loudspeaker will be expected to

Figure 9.1 Adhesives are widely used in the assembly of a loudspeaker

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts withstand some severe environmental test program before it can be approved for use in a motor vehicle. The selection of the adhesive grades for all of the joints above will depend on the power rating of the speaker, the environmental test specification and the substrates used for the various component parts. Other factors that will influence the selection of the adhesive will be ease of dispensing, cure time of the adhesive, cured properties of the adhesive (flexibility, modulus, etc.) and the cost of the adhesive. For example, ultraviolet (UV)-curing adhesives may often be a higher price than other technology adhesives but the fast cure and ease of dispensing benefits of the single-part UV product may outweigh any work-in-progress costs of other longer-cure, lower-cost, two-part adhesives.

9.2 Effect of Humidity and Water Absorption A significant point to consider when selecting an adhesive is its moisture resistance. For this reason, the technical data of an adhesive frequently lists the durability of bonds in humid environments after prolonged time periods. Normally charts are included that list the percentage of initial resistance retained by the adhesive bond over time under known humidity conditions. Many of the test standards used by adhesive manufacturers are for lap shear data and invariably grit-blasted mild steel is used as the substrate and in the case of metal-to-metal bonds, the only way moisture can enter is through the adhesive. Once separation has begun, a weak cohesion layer is formed that withstands little or no force and the joint will fail. When plastics are the substrates, however, and the water reaches the bond interface, the adhesive and the substrate compete for absorption and the overall effect is that the complete assembly can show good humidity resistance. Although it depends on the chemistry of each adhesive family, thermosets can generally be said to be more durable when exposed to water than thermoplastics because they have a more closed molecular structure in which the water ingress progresses more slowly.

9.3 Durability of Cyanoacrylates Assemblies bonded with cyanoacrylate have shown entirely satisfactory long-term performance in a variety of applications particularly where at least one of the substrates is an amorphous thermoplastic or an elastomer. Typically, accelerated testing is done at high temperatures and extreme conditions to predict whether long-term bond performance will be acceptable.

130

Durability and Environmental Testing Testing has shown that the resistance of a cyanoacrylate-bonded joint to long-term exposure to humidity or immersion in water is dramatically affected by the type of substrate [3]. Cyanoacrylates may well show poor durability on metal substrates (even the rubber-toughened grades), which often show excellent initial strength on metals. The standard test for adhesives is to use lap shears as shown in Figure 9.2. The nominal overlap length is 12.5 mm (½") and, in the case of metals, the surfaces will be grit blasted prior to bonding.

12.5 mm 25 mm

Figure 9.2 Standard lap shear test coupon

Figure 9.3 shows the effect of immersion in water for rubber-toughened cyanoacrylates on aluminium and steel [4].

Durability of Cyanoacrylates on Metals Strength (N/mm2)

30 25 20

Steel

15

Aluminium

10 5 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Days Immersion in water

Figure 9.3 Effect of water on metal lap shears bonded with rubber-toughened cyanoacrylates

Cyanoacrylates are best suited where the bond line gap is less than 0.05 mm but, even under these conditions, the trends are clear. On steel, there was a decline in bond strength over time; on aluminium, the strength dropped off more abruptly, after just 2 weeks of immersion. On amorphous thermoplastics, however, such as acrylonitrile butadiene styrene (ABS) and polycarbonate, standard ethyl cyanoacrylates showed excellent bond-strength 131

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts retention after 4 weeks of immersion in water (Figure 9.4). Similar results have also been seen with neoprene rubber.

Durability of Cyanoacrylates on Thermoplastics Strength (N/mm2)

30 25 20

ABS

15

PC

10 5 0 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 Days Immersion in water

Figure 9.4 Effect of water on plastic lap shears bonded with ethyl cyanoacrylates

The ability of cyanoacrylates to resist attack from moisture when bonded to polymeric substrates can be most drastically tested by subjecting bonded assemblies to autoclaving. The autoclaving process combines the environmental stresses of high temperature, high-pressure and humidity. As such, it provides a good indicator of the ability of adhesives to withstand exposure to moisture.

9.3.1 Cyanoacrylates for Medical Applications Cyanoacrylates are quite widely used for the bonding of medical devices (especially small component parts, see Figure 9.5) as these adhesives show excellent adhesion to many of the plastics used in this industry. In this industry component parts are sterilised and, whilst cyanoacrylates will withstand gamma sterilisation and ethylene oxide (EtO) sterilisation processes, they would not generally be recommended for applications where the component assembly will be steam autoclaved.

9.4 Durability of UV-curing Adhesives The durability of UV-curing acrylic adhesives will again depend very much on the substrates bonded, the joint design and the gap between the parts. Some UV adhesives with silane additives are specifically formulated for the bonding of glass, whereas other UV-curing adhesives are best suited for plastics. UV adhesives generally will show good adhesion and thus good durability on PVC, polycarbonate, ABS and polysulfone but are typically not the most appropriate adhesive for bonding to elastomers. 132

Durability and Environmental Testing

Figure 9.5 Bonding polyurethane tubing to poly(vinyl chloride) (PVC) connector (this part is gamma sterilised) UV adhesives are sometimes used for the bonding of ‘point of sale’ display stands as the UV adhesive can give strong, clear bonds with fast cure. UV adhesives have also been used for the bonding of automotive sub-components, especially lamp housings and for the potting/encapsulation of small electronic parts. High temperatures (>150 °C) may discolour UV acrylics and for these higher temperature applications a UV-curing silicone should be considered.

9.4.1 UV Adhesives for Medical Applications Typical application areas for UV adhesives in the medical industry include the assembly of blood collection devices, endoscopes, hearing aids, IV sets, infusion pimps, catheters and diagnostic imaging equipment. The UV adhesive is often used to bond a stainless steel guide wire inside a polycarbonate luer or similar moulded part. Other applications include cannula/hub bonding, blood filters, tracheal tubes, colostomy bags and appliances, cuff and tube assemblies and inflators. Most of these devices are single-use and so will be sterilised by gamma radiation or by EtO and this will not usually affect the performance of UV adhesives. Some grades of UV adhesives will withstand repeated autoclave cycling (steam at 135 °C) but many grades will deteriorate after a number of cycles and so tests should always be conducted.

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Figure 9.6 Bonding tracheotomy cuffs [5]

Figure 9.6 shows the bonding of tracheotomy cuffs and balloon catheters. In both applications the UV adhesive allows the parts to be correctly positioned prior to curing. UV adhesives are more flexible than cyanoacrylate adhesives and are therefore well suited to applications where some flexibility is required. For applications where a high degree of flexibility is required and where repeated autoclaving may be necessary, the UV-curing silicone products are used (Figure 9.7). These products are immobilised with UV light and then continue to fully cure by absorbing moisture from the atmosphere. The products are also referred to as UV-curing room-temperature-vulcanising silicones.

Figure 9.7 UV silicone adhesives are extremely flexible

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Durability and Environmental Testing Without UV, the products will skin over in about 10–15 minutes but will cure to a depth of up to 6 mm in 72 hours. The cure process is characterised by the release of by-product, which often carries a distinctive odour (e.g., vinegar). Neutral cure versions (alkoxy silicones) are also available.

9.5 Durability of Two-Part Acrylics Two-part acrylics will often show slightly improved durability over cyanoacrylates, particularly where the gaps are bigger and one of the substrates is not a plastic. However, two-part acrylics do not bond well to elastomers and here cyanoacrylates will show much better durability. For impact resistance and peel strength, the tougheners in two-part acrylics will be of major benefit and, since these adhesives will typically be of lower cost than UV adhesives or cyanoacrylates, they are often used for the larger applications. As with all of these adhesives, the durability will depend on many factors and it will always be necessary to chose an adhesive system (i.e., pre-treatment/cure speed/ application technique, etc.).

9.6 Durability of Epoxies Epoxies cure to become thermoset adhesives and so show excellent humidity resistance, especially if one of the substrates is metal. For long-term, durable adhesively bonded metal-to-metal joints, toughened epoxies are hard to beat [6] but where plastics are involved, some of the other technology adhesives invariably offer benefits in terms of ease of application and cure speed whilst still retaining satisfactory environmental resistance. Indeed epoxies do not often show good adhesion to elastomers and for the thermoplastic elastomers cyanoacrylates will generally show the best environmental resistance. Epoxies will show quite good adhesion (and hence durability) to thermoset plastics but many thermoplastics may require careful surface preparation if this type of adhesive is to be used.

9.7 Environmental Testing There is usually very little accurate durability data available for the bonding of engineering plastics and it has to be admitted that the specialist adhesive engineers 135

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Environmental resistance

100

% Strength

80 A

60

B

40

C

20 0 100

200

300

400

500

600

700

800

900 1000

Time (hours)

Figure 9.8 Three possible results from an environmental test

can usually only offer the design engineer the benefit of experience rather than all the data they yearn for. The data supplied on the adhesive technical data sheet can only be used as an initial starting point and it can be very difficult to cross-refer the data to the actual application for the end user. Every adhesive application is different and much of the data have to be gained by laboratory testing. It is often the case that a bonded part will show some signs of failure within the first 1000 hours in a severe environment. Figure 9.8 demonstrates the interpretation of the various outputs of an environmental test. Figure 9.8 is a schematic diagram showing three possible outcomes from a series of environmental tests. The ideal result would be ‘A’, i.e., there has been no degradation in strength (or performance) of the adhesive bond after environmental cycling. Result ‘B’ may be acceptable in as much as that whilst there has been some deterioration in strength (or performance) of the adhesive bond it essentially has stabilised and the final strength may well still be acceptable for the intended application. Result ‘C’ is clearly unacceptable as the adhesive bond performance has continued to fall.

References 1.

136

L.F.M. Silva and R.D. Adams in Proceedings of the IOM 6th European Adhesion Conference - Adhesion 02, Glasgow, UK, 2002, p.31.

Durability and Environmental Testing 2.

B. Goss in Proceedings of the Rapra Technology Conference on Joining of Plastics, London, UK, 2006, Paper No.4.

3.

Technical Data Sheet Loctite 406, Henkel UK Ltd, Hatfield, UK, 2008.

4

P.J. Courtney and C. Verosky, Advances in Cyanoacrylate Technology for Device Assembly, Medical Device and Diagnostic Industry Magazine, 1999, September.

5.

B. Goss and H. Handwerker, in Proceedings of the MEDTEC Conference, Stuttgart, Germany, 2003.

6.

Industrial Adhesion Problems, Eds., D.M. Brewis and D. Briggs, Orbital Press, Oxford, UK, 1985, p.143.

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10

Troubleshooting

In this chapter the failure modes of adhesives are discussed and some practical hints and tips are included to help identify and rectify the reason for the failure mode. There are a whole host of reasons as to why an adhesive might fail for a particular joint and the list below is not necessarily complete and in many applications it may be a combination of factors that lead to a failure. The failures can often be categorised into one of four main areas: •

‘No glue’,

•

‘No cure’,

•

‘No stick’, and

•

‘No performance’.

10.1 ‘No Glue’ – Inspecting for the Presence of Adhesive Whilst this might seem obvious, it is essential that there is sufficient adhesive to fill the joint. Joint starvation can often be the reason for poor joint strength. In some applications air voids and cavities can often mean the difference between a ‘good’ or ‘bad’ joint, especially if the adhesive is required to act as a sealant as well as an adhesive. In clear plastic components, the inspection technique should be relatively straightforward but with opaque plastics it can be difficult to ascertain whether the adhesive has fully filled the joint. Visual inspection of a failed component is usually the first step to determine the quantity of adhesive in the joint. Some adhesives will fluoresce under ultraviolet (UV) and this will aid the inspection process but if the adhesive is a very similar colour or texture to the substrate then a more careful or complex inspection technique may be required. One such method is to use Fourier transform infrared (FTIR). FTIR methods can identify individual elements of both the substrate and the adhesive and 139

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts therefore define exactly what is present on the surface but the assistance of the adhesive manufacturer may be required to disclose the formulation of the adhesive polymer. For clear joints, adhesives with a fluorescent agent can be used to simplify the inspection technique and in the medical industry this property is widely utilised. Adhesives are used to bond the stainless steel cannulae to the plastic hubs (Figure 10.1) and in this high-volume application (up to ten assemblies per second) it is essential that every needle assembly is bonded. The adhesive used (typically either a UV acrylic or a single-part epoxy) fluoresces at about 450 nm and so the presence of adhesive on the cannula can be verified automatically using inspection cameras and if it does not ‘see’ the adhesive fillet, it will eject the part off the line.

The adhesive at the ‘dome’ fluoresces under UV light and so can be inspected using an automatic camera system.

Figure 10.1 Bonding a cannula to a hub With opaque plastics, a fluorescent adhesive can still be used but the camera would be set to inspect the component parts before the joint is closed.

10.1.1 ‘No Glue’ – Verifying the Adhesive Has Been Dispensed A method sometimes used is to check the pressure-time response of the adhesive dispense valve. In a pressure-time dispensing system (see Section 8.3), the adhesive is forced under pressure through a valve so that the desired quantity is dispensed onto the component part. As the dispense valve opens and closes, the pressure-time response curve can be monitored just upstream of the nozzle tip. Figure 10.2 shows a typical pressure-time output as the valve operates. There is a brief surge in pressure as the valve opens and then this levels out fairly quickly until the end of the elapsed dispense time. If there is a bubble in the adhesive or a blocked nozzle the pressure-time curve will be changed (Figure 10.3) and by integrating the area under the curve, the software controller [1] can quickly determine whether the dispensed quantity was within preset tolerances. 140

Pressure in valve

Troubleshooting 10 9 8 7 6 5 4 3 2 1 0 0

5

10

15

20

Time (seconds)

Pressure in valve

Figure 10.2 The area under the curve represents the total volume of adhesive dispensed 10 9 8 7 6 5 4 3 2 1 0 0

5

10

15

20

Time (seconds)

Figure 10.3 A blocked nozzle or bubble will show up as a different pressure-time curve

10.1.2 ‘No Glue’ – Air Bubbles and Voids Small air bubbles in the joint may not affect the overall strength of the joint but can cause leaks or in some cases be an aesthetic issue. These bubbles may originate from the adhesive packaging but can also be due to shrinkage of the adhesive as it cures. Most of the acrylic-based adhesives will shrink on cure slightly whilst epoxies tend to show lower shrinkage. Applying smaller volumes of adhesive will minimise the shrinkage voids and so in some applications it may be necessary to apply the adhesive in two ‘lots’. The dispensing system may also be the cause of bubbles: any liquid will absorb air in solution when it is pressurised and this air is released in the form of bubbles when the liquid adhesive returns to atmospheric pressure. The best example of this is a bottle of lemonade. The lemonade appears perfectly clear until such time when the top is unscrewed and the pressure is released and immediately bubbles will appear 141

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts in the liquid. The same effect can happen to a bottle of adhesive in a pressurised vessel. Inside the pressure pot, the adhesive will slowly absorb air and so when the adhesive is dispensed down the tubing, this air will be released in the form of bubbles. These bubbles in turn can cause problems in terms of voids in the joint or ‘run-on’ at the nozzle tip due to the build-up of air in the dispense valve. Reducing the adhesive pressure and using gravity-fed systems will minimise the effect of bubbles.

10.1.3 ‘No Glue’– Destructive and Non-destructive Methods Destructive testing usually involves uplifting samples from the production line and determining the performance of the adhesive bond by strength testing against a predetermined standard. This may not always be possible, especially if the adhesive has extended cure times but with fast-curing adhesives (cyanoacrylates or UV-curing products) the application of force to determine the failure load will verify the adhesive performance. Non-destructive testing can be as simple as a visual inspection as a trained eye will detect a surprising number of faulty joints, including close inspection of the adhesive bond line at the joint periphery, misaligned parts or uneven adhesive bond lines. Another method of non-destructive testing is to apply a proof load to the parts. Care must be taken to design the proof test so that it does not overstress the part and thus cause damage that will reduce its service life. A typical proof load might be 20% of the ultimate failure load. There are also a number of more sophisticated non-destructive techniques (NDT) which measure the frequency response of the bonded component part and can thus identify the presence (or absence) of adhesive in the joint. These NDT methods use ultrasonic testing, whereby high-frequency, highly directional sound waves analyse the adherends and find hidden internal flaws [2]. Perhaps a simpler method of determining the quantity – particularly for medium- to high-volume applications – is to weigh the quantity of adhesive dispensed from the application equipment or weigh component parts before and after adhesive has been dispensed.

10.1.4 ‘No Glue’ – Other Factors Has the correct grade of adhesive been used? It is not unknown for the wrong grade of adhesive to be inadvertently used on the production line and if the adhesive container is situated in a pressure pot or similar 142

Troubleshooting dispenser, it will be out of sight and therefore it may not be immediately obvious that the incorrect adhesive is being used. It is also entirely possible that the incorrect grade of adhesive was specified for the application and another adhesive may be better suited for the application. Is the adhesive still in shelf life? Most adhesives will have a shelf life and invariably if the adhesive is used beyond its nominal life, the viscosity, the cure speed or the final cured properties of that product may well be out of specification. Was the adhesive stored correctly or has it been used straight from the fridge and not had time to return to ambient temperature? The optimum storage conditions for many adhesives is in a refrigerator but if the adhesive is used directly from the fridge at 2 °C to 8 °C, not only will the viscosity be higher thus affecting the dispensing characteristics, but also the cure speed also may be prolonged.

10.2 ‘No Cure’ Having checked for the presence of adhesive (‘no glue’), the next step is to check whether the adhesive has cured. This again should be relatively straightforward in as much as that uncured adhesive will be liquid and this should be relatively easy to check when components are separated.

10.2.1 ‘No Cure’ – Odour The adhesive must be fully cured for an effective and durable joint to be formed. Most engineering adhesives will display an odour when in the uncured state and, once cured, the adhesive is essentially a thermoset or thermoplastic and so inert and usually without odour. One key inspection technique therefore when trying to establish the reason for the failure of an adhesively bonded joint is simply to check for any odour in the bond line and compare this to a bottle of the original adhesive. Any sign of odour usually suggests that the adhesive has not fully cured.

10.2.2 ‘No Cure’ – Factors Inhibiting Cure Lack of cure could be due to a number of factors and this may well depend on the type of adhesive being used. 143

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Cyanoacrylates are very dependent on the presence of small amounts of moisture on the surface and if the relative humidity in the working area is less than 25% RH, the cyanoacrylate will be slow to cure. Surface acidity can also interfere with the curing of cyanoacrylates (see Section 1.2). Epoxies on the other hand are very independent of the surface condition and, providing the epoxy has been mixed properly (or in the case of single-part epoxies had sufficient heat cure), then the epoxy will cure. UV acrylics can be inhibited from cure if there is insufficient UV energy to split the photoinitiator and this may occur if the plastics to be bonded are heavily coloured or UV opaque or if there was insufficient intensity from the UV source to initiate cure. The mix ratio of two-part acrylics is not as critical as the mix ratio of epoxies but the activator part does need to be present for the adhesive to cure. Excessive activator can sometimes result in aggressive curing conditions and thus a weaker final cured polymer. The presence of nitrites on metal surfaces can also inhibit the cure of acrylics.

10.2.3 ‘No Cure’ – Disturbing Partially Cured Adhesive Adhesives do not like to be disturbed whilst they are in the critical phase of changing from a liquid to a cured adhesive. The polymer chains will be beginning to form and if broken at an early stage the chains will not always repair and then go on to create the crucial links that make up the adhesive polymer. This can be difficult to detect when inspecting a failed adhesive joint and it is probably much easier to review the assembly process to check that the adhesive is not excessively loaded during the initial curing cycle.

10.2.4 ‘No Cure’ – Differential Scanning Calorimetry (DSC) There are more sophisticated techniques to determine the extent of cure but these do require the use of specialised (and expensive) equipment. DSC is a thermo-analytical technique [2] in which the difference in the amount of heat required to increase the temperature of the sample of suspect uncured adhesive and a fully cured reference are measured as a function of temperature. Both the sample and reference are maintained at the same temperature throughout the experiment and DSC will detect any energy changes or heat capacity changes with great sensitivity and thus determine whether the test sample has fully cured. 144

Troubleshooting

10.2.5 ‘No Cure’ – Adhesive Curing Problems Sometimes cyanoacrylates will form a white powder adjacent to the bond line. This is known as ‘blooming’ or sometimes ‘frosting’. This section explains the reason for blooming and suggests various options to eliminate it from the production line. Also included are general guidelines for troubleshooting an application.

10.2.6 Blooming of Cyanoacrylates Blooming occurs when cyanoacrylate molecules escape from the main body of the adhesive and react with water vapour in the surrounding air. The molecules of cyanoacrylate cure and then fall to the adjacent surface as a white powder (Figure 10.4). Even with rubber-toughened cyanoacrylates (which are often black in colour), it is the monomer which escapes and the deposit will still be white. An example of blooming is shown in Figure 10.5 where excess adhesive was applied to the end cover of a shelf support unit. There are three primary causes of blooming [3]: 1. Excess adhesive, 2. Slow cure, and 3. Low relative humidity.

Water Vapour

Monomer

Surface Moisture

Acidic Stabiliser

Figure 10.4 When a cyanoacrylate ‘blooms’, the adhesive monomer escapes from the bond line and cures on an adjacent surface 145

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 10.5 On the left-hand plastic extrusion, the cyanoacrylate has bloomed

10.2.6.1 Excess Adhesive When excess adhesive has been applied, the surface-to-volume-of-adhesive ratio is too low and the moisture on the surface will be insufficient to neutralise the stabiliser in the adhesive. The cyanoacrylate vapour will escape and fuming will occur. Reduce the quantity of the adhesive by using fine-bore application nozzles and/or dispensing equipment.

10.2.6.2 Slow Cure In the same way as excess adhesive can cause blooming, a slow cure may give a similar result. The cyanoacrylate at the periphery of the joint will search for available moisture from the surrounding air and may then cure as a white powder on the adjacent surface. A slow cure may be the result of excess adhesive but may also be caused by acidic deposits on the substrate. These acidic deposits can cancel out the neutralising effect of the initiators (moisture) and result in very slow polymerisation or in some cases inhibition of cure completely. Slow cure can also be overcome by using an activator (or accelerator). The activators increase the level of initiators on the surface to negate the stabiliser and thus increase the speed of polymerisation. UV-curing cyanoacrylates have also been used in applications to accelerate the cure speed and thus eliminate the possibility of blooming. Slow cure may also be due to a thick bond line (>0.2 mm). Cyanoacrylates are most suited to applications where the bond line is less than 0.1 mm thick, although a deeper cure depth is possible using activators (or through the use of UV-cure grades). Plasma treatment of a plastic surface can remove all the surface moisture and parts may need to be left to re-acclimatise prior to bonding with cyanoacrylates.

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Troubleshooting

10.2.6.3 Low Relative Humidity Low relative humidities (less than 20% RH) will also encourage the formation of blooming and irritant fumes because there is less moisture on the surface to initiate the cure. Best results are obtained when the relative humidity is between 40% and 60% RH. Higher humidity will accelerate the cure process but could affect the final bond strength and in some cases increase the risk of blooming. The use of activators can considerably assist the cure but placing a container of water adjacent to the workstation has been known to increase the local relative humidity. Blooming does not always occur during the first few seconds; indeed it is more likely that parts will bloom some hours after assembly (up to 24–48 hours later). A bonding application is often one of the last operations in a production cycle and care is required to ensure that parts are not put straight into a sealed (or semisealed) container immediately after cyanoacrylate bonding as there is a risk that the cyanoacrylate will bloom in the box, resulting in a poor appearance to the end-product. Blooming does not affect the strength of the bonded assembly and is usually only aesthetic. However, in some applications, e.g., the bonding of a rubber ring seal near an infrared sensor housing, blooming may occur on the surface of the sensor thus rendering it inoperative. In this case the use of a low-bloom product should eliminate the possibility of blooming. In summary therefore there are three methods of overcoming blooming: •

Avoid excess adhesive The use of dispensing equipment and ensuring that the minimum quantity of adhesive required to fill the joint is applied will give best results.

•

Ensure fast cure The longer the cyanoacrylate remains liquid the greater the risk of blooming. Designing assemblies with close-fitting parts and therefore achieving thin bond lines will increase the speed of polymerisation and will decrease the risk of blooming. The use of activators or a UV-curing grade is recommended for the fast cure of exposed fillets of cyanoacrylates after assembly.

•

Use a heavy-molecular-weight cyanoacrylate (low-bloom product) The heavy-molecular-weight cyanoacrylates are ideal for applications where bonds must be cosmetically perfect or for delicate electrical and electronic assemblies. The

147

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts additional low-odour characteristic of these cyanoacrylates is ideal where operators are required to work in confined unventilated spaces. The low-bloom cyanoacrylates are also slower curing than standard ethyl or methyl grades, which means that more time is available to assemble parts where careful alignment is required.

10.3 ‘No Stick’ The third (and probably the most common) failure mode is when the adhesive does not adhere properly.

10.3.1 Theories of Adhesion It is now generally recognised that adhesives stick to surfaces by one of (or a combination of) four different methods (Figure 10.6) and these methods are discussed briefly here but more detail can be found in references [2] and [4].

Mechanical

Diffusion

Adsorption/ Electrostatic

Figure 10.6 Four methods by which an adhesive sticks to a surface

•

Mechanical keying Perhaps the most widely used of all the adhesion theories is that the adhesive fills any interstitial spaces between the two surfaces and mechanically keys into the surface. The adhesive must not only wet the substrate, but also have the right rheological properties to penetrate any pores and irregularities before the adhesive starts to cure. Slightly rougher surface finishes are nearly always beneficial in any

148

Troubleshooting adhesive bonding application and can often improve the strength and durability of the bonded joint. •

Diffusion Some adhesives will ‘diffuse’ into the surface. The polymer chains of the adhesive interact locally with the polymer chains of the substrate and the two ‘fuse’ together. Solvent-based adhesives are probably the best example of this adhesion theory. Cyclohexanone and similar solvents are widely used in the medical industry for bonding tubular poly(vinyl chloride) (and other plastic) medical devices. The cyclohexanone is usually applied to the male part by dipping the tube into a container. The solvent evaporates quite quickly but there is sufficient time to assemble the components and as the solvent evaporates, it bonds the two parts together to give a very effective bond.

•

Adsorption The adsorption theory [4] essentially attributes adhesive strength to the forces which act between molecules and atoms in the structure of matter and are known as ‘valence forces’. These can include physical and chemical bonds. The chemical bonds include covalent and ionic bonding between atoms and electrons and the physical bonds include the presence of dipole (or polar) interactions (also known as van der Waals forces).

•

Electrostatic This theory is based on the presence of an electrical double layer between the adhesive and the substrate. It has been found difficult to quantify the electrostatic forces [4] but they can undoubtedly contribute to the total adhesive strength.

There are three modes of failure of an adhesive joint: •

Cohesive,

•

Adhesive, and

•

Substrate.

10.3.2 Cohesive Cohesive failure is when the failure mode is through the adhesive bond line. Inspection of the failed adhesive will show adhesive remaining on both surfaces (Figure 10.7).

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Figure 10.7 Cohesive failure of the adhesive

If this is the failure mode then the adhesive itself should be investigated. Thinner bond lines are often stronger (especially under shear or tensile loading) but it may be that in some applications a lower modulus adhesive may prove to be better suited if shock or impact resistance is required as these can help to absorb and distribute the high stresses associated with high impact or peel loading. Alternatively a stronger adhesive will be necessary to increase the strength of the bond.

10.3.3 Adhesive Failure In Figure 10.8, the adhesive has remained predominantly on the lower surface and this suggests some surface contamination or other factor impeding the adhesion to that surface. Mould-release agents are sometimes used to help release plastics from the mould tool, especially for the first batch of parts off the tool when a silicone- or polytetrafluoroethylene-based release spray might be used. These release agents do not always remain on the tool but get transferred across to the component and, since they are low-surface-energy liquids, they prevent the adhesive from bonding to the component. If a region of low strength exists at the substrate surface, it may originate from weak boundary layers such as dust, grease and metal oxides of relatively low strength. Oxide layers on brass and aluminium can often be pulled away by an adhesive.

Figure 10.8 Adhesive failure

150

Troubleshooting Low-surface-energy plastics such as the polyolefin family will always be a challenge to the adhesive application engineer, especially if the adhesive bond line is to be subjected to peel loading. Bonding to ‘difficult’ plastics and the wetting of adhesives is discussed in more detail in Section 6.1. Another reason for adhesive failure might be excessively fast cure. Cyanoacrylates will sometimes cure so rapidly on an alkaline surface that they polymerise before they have a chance to properly adhere to the surface. A glazed or glossy appearance to the failed cyanoacrylate is often an indication that the adhesive has cured too quickly. Plated metals sometimes have traces of alkalinity remaining on the surface and washing with an aqueous cleaner can rectify the situation.

10.3.4 Substrate Failure In most situations, if ‘substrate failure’ is achieved (Figure 10.9) then no more could be asked of the adhesive as the adhesive bond is stronger than the plastic or elastomer bonded. Sometimes, however, the adhesive will change the mechanical properties of the complete assembly, especially if the adhesive creates highly stressed regions at the periphery of the bonded joint. With an elastic modulus of 1–2 GPa, a typical plastic (e.g., polycarbonate) is over 100 times more flexible than steel for identical shaped components. In a lap shear joint (Figure 10.10), this flexibility means that more bending and differential shearing (compared to steel) will occur in the bonded joint as the assembly is placed under load.

Figure 10.9 Substrate failure

Figure 10.10 Bonded lap shear joint 151

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Stress

This flexibility leads to increased stress concentrations near the ends of the overlap (Figure 10.11) and these can sometimes lead to substrate failure at relatively low loads. These high-stress concentrations can be reduced in many cases effectively by careful selection of joint design parameters. Most important among these are the elastic modulus of the adhesive, length of the overlap and thickness of the bond line between the two substrates. A lower modulus adhesive reduces the stress concentrations by accommodating the relative motion of the two substrates with greater shear compliance. The extreme case is when a rubbery adhesive is used. Such an adhesive is so rubbery that shear deformations can be accommodated without creating significant stress concentrations. But an adhesive this flexible may not be able to accommodate the structural loads on an actual assembly without excessive deformation.

Bond location

Figure 10.11 Stress distribution across a lap shear joint Adhesives can also stress crack or ‘craze’ certain plastics and this can also induce premature failure of the substrate. This generally happens whilst the adhesive is uncured and the plastic part is pre-stressed due to abrupt changes of section. The liquid adhesive softens and weakens the plastic leading to the formation of cracks and the liquid adhesive then penetrates these cracks causing further damage. With fast-curing adhesives, this is less likely to occur as once the adhesive has cured it is essentially a thermoset plastic (or thermoplastic) itself and is therefore inert. Amorphous thermoplastics are more prone to stress cracking than others and so it is important therefore to ensure that the adhesive is compatible with the substrate.

10.4 No Performance One of the most important requirements of an adhesive joint is the ability to retain a significant proportion of its properties under the wide variety of environmental conditions that are likely to be encountered during its service life. One of the most aggressive and hostile environments for any adhesive is the combination of high humidity and high temperature and many automotive customers now demand performance after 1000 hours at 85 °C and 85% RH.

152

Troubleshooting Durability of adhesives is also discussed in Sections 9.3 to 9.6 and this is perhaps the most difficult area to troubleshoot as, by its nature, it is likely to be some time after the component parts were assembled that a troubleshoot is required. It will almost certainly be necessary to include in any investigation into the loss of performance the three factors described above (no glue, no cure, no stick). The next step is to examine the adhesive bond line.

10.4.1 Surface Analysis Any discolouration of the adhesive in the joint may well suggest that the assembly has been subjected to environmental attack. Excessive temperatures may burn or discolour the adhesive and water ingress can cause some adhesives to discolour or lose their hardness. Thicker bond lines are often more prone to environmental attack and joints that are subjected to peel or cleavage loads are also more likely to suffer from harsh environments. Analysing the failed surface often provides clues as to the root cause of failure. There are also some sophisticated surface analytical techniques available including X-ray photoelectron spectroscopy, Secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy [4]. These surface-analysis techniques are beyond the scope of this guide but all three are particularly useful for identifying surface contaminants to a depth of a few nanometres or even less with SIMS. Adhesion problems, however, are usually complex and they are unlikely, in general, to be solved by the application of a single analytical technique, although these can provide very good directives as to the source of the problem.

10.4.2 Defining the Failure Mode Sometimes the problem of durability is not clearly defined and the statement that ‘the adhesive is failing’ is not particularly helpful but ‘5% of the parts fail after 6 weeks in a hot, humid country’ is far more useful information to the engineer. A similar question that might be relevant is: ‘Have the failures occurred with this batch of adhesive/components or do the failures occur on multiple batches?’ It is these kinds of questions that will help to identify the failure. One technique is to try and replicate the failure mode. If the failures can be replicated under controlled conditions then it is likely that a better understanding of the failure mode can be obtained.

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts The lack of performance in an adhesive joint could be due to a number or a combination of factors although the most common environmental problem for adhesive joints is water or high temperature with high humidity.

References 1.

Loctite Equipment Sourcebook, Published literature, Henkel Ltd, Hatfield, UK, 2007.

2.

D.E. Packham, Handbook of Adhesion, Longman Scientific & Technical, Harlow, Essex, UK, 1992.

3.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2001, p.280.

4.

Industrial Adhesion Problems, Eds., D.M. Brewis and D. Briggs, Orbital Press Oxford, 1985.

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A

bbreviations

ABS

Acrylonitrile-butadiene-styrene

ATP

Amendment to Technical Progress

CA

Cyanoacrylate adhesive(s)

CE

Conformité Européene

CHIP

Chemicals Hazard Information and Packaging for Supply

COSHH Control of Substances Hazardous to Health CTE

Coefficients of thermal expansion

DAP

Diallyl phthalate(s)

DSC

Differential scanning calorimetry

EEA

Ethylene acrylic

EC

European Conformity

EPS

Expanded polystyrene

EPDM

Ethylene-propylene-diene terpolymer

EtO

Ethylene oxide

EU

European Union

EVA

Ethylene-vinyl acetate co-polymer

GHS

Globally Harmonised System of Classification and Labelling of Chemicals

H&S

Health and Safety

HIPS

High-impact polystyrene

HSE

The Health and Safety Executive

ISO

International Organisation for Standardisation

LCP

Liquid crystal polymer

L/D

Length to diameter

LED

Light-emitting diode

MMA

Methylmethacrylate(s)

MS

Modified silane(s)

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Material Safety Data Sheet

NDT

Non-destructive techniques

PA

Polyamide

PBT

Polybutylene terephthalate

PC

Polycarbonate

PE

Polyethylene

PEEK

Polyetheretherketone

PES

Polyethersulfone

PET

Polyethylene terephthalate

PI

Polyimide(s)

PMMA

Polymethylmethacrylate

POM

Polyoxymethylene

PP

Polypropylene

PPO

Polyphenylene oxide

PPS

Polyphenylene sulfide

PS

Polystyrene

PTFE

Polytetrafluoroethylene

PU

Polyurethane(s)

PUS

Polysulfone

PVC

Poly(vinyl chloride)

REACH Registration, Evaluation, Authorisation and Restriction of CHemicals RH

Relative humidity

RTV

Room temperature vulcanising

SBR

Styrene-butadiene rubber

SG

Specific gravity

SIMS

Secondary ion mass spectrometry

Tg

Glass transition temperature

TPE

Thermoplastic elastomer(s)

TPV

Thermoplastic vulcanisate(s)

UV

Ultraviolet

UVA

Ultraviolet-A

UVB

Ultraviolet-B

UVC

Ultraviolet-C

UVV

Ultraviolet-visible

WRAS

Water Regulation Advisory Service

156

A

uthor Index

A Adams, J.C. 94 Adams, R.D. 127

B Bashforth, F. 94 Brewis, D.M. 91, 98–99, 135, 148–149, 153 Briggs, D. 91, 98–99, 135, 148–149, 153

C Courtney, P. J. 131 Crowther, B. 3, 7, 61, 88, 124, 145

D Dufton, P. 20, 25, 48, 59, 71, 104 Dunn, D.J. 20, 25, 48, 59, 104

F Forsdyke, K. 51

G Goodship, V. 52, 63, 112 Goss, B. 49, 82, 128, 134

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H Handwerker, H. 134

L Lees, W. A. 80

M Massey, B.S. 117

P Packham, D.E. 9, 13, 91, 142, 144, 148 Platt, D. 27, 29

S Starr, T.F. 51 Shields, J. 79 Silva, L.F.M. 127

V Verosky, C. 131

158

S

ubject Index

Note: The letters ‘f’ and ‘t’ following the locators refer to figures and tables, respectively.

A Abrasion grit blasting 98 mechanical 128 resistance 29, 41, 55, 70 surface treatments 98 ABS see Acrylonitrile butadiene styrene Acetal 41, 93, 103 adhesive shear strengths 41–42 Acetal polyethylene 129 ‘Acetoxy’ curing silicones 23 Acrylics 18, 21, 31, 41, 53, 88, 100, 105, 144 -based adhesives 141 methylmethacrylate 59 two-part 17–18, 48, 59, 99–100, 135 ultraviolet 47, 144 Acrylonitrile butadiene styrene 29, 32, 35–36, 48, 131 Adhesion, theories of 148–149 adhesive sticking, methods 148f adsorption 149 bonded joint, strength of 149 cohesive failure 149, 150f diffusion 149 dipole 149 electrostatic forces 149 lower modulus adhesive 150 polymer chains 149 solvent-based adhesives 149 ‘valence forces’ 149 Adhesives, introduction to 1–26 bonding plastics 1 cyanoacrylates 1–8 159

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts epoxies 19–21 flexible adhesive sealants 21–24 hot melt adhesives 25–26 low-viscosity 106, 124 open time of 120 selecting viscosity 89–91 two-part acrylics 17–18 ultraviolet-curing adhesives 9–17 Adhesives, dispensing systems 121–125 automatic units 121 dispensers, semi-automatic 121, 123 dispensing syringe 123 extrusion guns, manual 121 hot-melt guns 121 manual units 121, 122 peristaltic pumps, manually operated 121 pinch valves, hand-held 121 pneumatic system 121 pressure pot dispensing 123–125 programmable logic controller 121 Amorphous thermoplastics 21, 27–29, 28f, 29, 30, 32, 35, 91, 131, 152 acrylonitrile-butadiene-styrene 29 chemically resistant 28 macromolecule chains 28 polymer classes 28 polystyrene 29 Araldite 55 Auger electron spectroscopy 153 Automatic systems 125–126 dispense valve 126 for complex profiles 126 semi-automatic units 125

B Bakelite 52 ‘Bead-on-bead’ application 17, 105, 116 Blooming, 49, 60, 107, 145-148 Bonding of elastomers 72–73 adhesive shear strengths 72t glass transition temperature 73 modified flexible cyanoacrylates 73 160

Subject Index nitrile rubber bonded with flexible cyanoacrylate 73f rubber-toughened cyanoacrylate 73 Bonding of low-energy plastics 93–100 surface energy, measurement of 97 surface treatments 97–99 abrasion 98 corona discharge 98 flame treatment 98–99 plasma treatment 98 use of primers 99 surface wetting 93–97 two-part acrylics 99–100 Bonding of thermoplastics 47–49 adhesive shear strengths 48t blooming 49 cyanoacrylates 48 epoxy grades 47 transparent substrates 49 two-part acrylics 48 Bonding of thermoset plastics 59–60 blooming 60 cyanoacrylates, ethyl-based 59 methylmethacrylate acrylics 59 two-part acrylics 59 Bond line thickness 88–89 cleavage loads 89 cured cyanoacrylate film 88 epoxies or acrylics 88 internal chamfer 88 joint gaps 86, 89 peel load 89 polyurethanes 89 silicones and modified silanes 89 Butt joint 86–88 adherends 86 adhesive 88 blind holes, applying adhesives to 87f forms of butt joint 87f increasing strength of joint by small chamfer 88f offset load 86 peel load 86 in joint line 88f

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Butyl rubber 4, 62–63, 72 adhesive shear strength of 63 gas permeability of 62 halogenated form 62 solvent-based adhesives 62 thermal performance 62

C Chemicals Hazard Information and Packaging for Supply Regulations 108 Control of Substances Hazardous to Health 110–111 Copolyester thermoplastic elastomer 63 adhesive shear strength 63t cyanoacrylates 63 performance of 63 Corona discharge 97, 98 COSHH see Control of Substances Hazardous to Health Cure method 118 ‘Cure-on-demand’ UV-curing adhesive 9, 107 Curing, systems, light-emitting-diode 14 spectral output 14f Cyanoacrylates 1–8, 31, 61, 103 alkoxy ethyl 4–5 blooming of 145-148 acidic deposits 146f adhesive monomer, escaping of 145f bonding application 147 ethyl/methyl grades 147 excess adhesive 146 heavy-molecular-weight 147 low relative humidity 147–148 plasma treatment 146 plastic extrusion 146f polymerisation 146 rubber-toughened cyanoacrylates 145 ultraviolet-curing activator 146 vapour 146 water vapour 145 cure speed of 3f fixture strength 3 ultraviolet 3 curing of 2f acidic stabilizer for 2f 162

Subject Index adhesive monomer for 2f durability of 130-132 ethyl 4 flexible 7–8 methyl 4 ophthalmic blade bonded 112 surface insensitive 5 thermally resistant 6–7 thixotropic gel 1 toughened 5–6, 6f crack arrester 5 ethyl cyanoacrylates 5 resistance to peel loads 6f types of 1–8 ultraviolet-curing 8 Cyanoacrylates, alkoxy ethyl 4–5 ‘bloom’ 5 molecular weight 5 volatility 5 Cyanoacrylates, allyl-based 7 Cyanoacrylates, ethyl 4 acrylonitrile butadiene styrene 4 butyl rubber 4 polycarbonate 4 poly(vinyl chloride) 4 Cyanoacrylates, gamma sterilisation of 132 Cyanoacrylates, surface insensitive 5 acidic substrates 5 porous substrates 5 super glue 5 Cyanoacrylate, ultraviolet-curing 4, 147 Cycle time 118 Cyclohexanone 149 Cylindrical joints 81-86 air trapping 86f blind holes 85–86 cross holes 84–85 design details 82–84 ethyl cyanoacrylate 81 low viscosity 82 hydraulic locking 85 inner (male) substrate 82

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts joining dissimilar plastics 82 length to diameter ratio 81 outer (female) substrate 81, 82 surface wetting 81 tube bonding 83f ultraviolet-curing adhesive 82

D Diallyl phthalate 54 adhesive shear strengths 54t Differential scanning calorimetry 144 Adhesives, dispensing of 115–126 automatic systems 125–126 basic principles 115–121 cost 121 cure method 118 cycle time 118 dispensing systems 121–125 dispense quantity 118–120 health and safety 120–121 open time 120 single- or two-part adhesive 115–116 viscosity 116–117 ‘dog bone’ effect 118 knit joint 119 in closed bead, minimising effect of 119f positional accuracy 118 robot programming 119 standard/altered groove 119–120f valve design 119 Dog bone effect 117–118 Double lap joint 80–81 liquid adhesive 80 modified tongue and offset 81f Double lap shear joint 80 DSC see Differential scanning calorimetry Durability 127–136 cyanoacrylates 130–132 effect of water on ethyl 132f effect of water on metal lap shears 131f for medical applications 132 polymeric substrates 132 164

Subject Index standard lap shear test coupon 131f effect of humidity 130 effect of water absorption 130 environmental testing of 135–136 epoxies 135 based adhesives 127 humidity resistance 135 humidity and water absorption 130 grit-blasted mild steel 130 moisture resistance 130 thermoplastics 130 joint design 128 substrate bonded 127, 129–130 surface finish and preparation 127–128 surface preparation 127-128 two-part acrylics 135 resistance and peel strength 135 UV-curing adhesives 132–135 bonding polyurethane tubing 133f for medical applications 133–135 joint design 132 potting/encapsulation 133 silane additives 132 silicone 133

E Elastomeric housing 81 Elastomers and thermoplastic elastomers 61–73 adhesive performance 61–72 alkoxy silicone 62 anti-static additives 61 butyl rubber 62–63 copolyester tpe 63 cure speed 61–72 ethylene-propylene diene monomer 64–65 ethylene-vinyl acetate copolymer 65 fluorosilicone rubber 66 natural rubber 66–67 Neoprene rubber 68 nitrile rubber 67 polycarbonate 62 polyisoprene 68 165

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts polyolefin 68–70 resistance 61 styrene-butadiene-rubber 70 silicone rubber 70 styrenic thermoplastic vulcanisates 70–72 thermoplastic vulcanisates 72 bonding vulcanised rubber 61 bond strength 61 End of Life Vehicles Directive 111 Environmental testing 135–136 degradation in strength 136 resistance 136f see also Durability EPDM see Ethylene propylene diene monomer rubber Epoxies 19–21, 31, 53, 55, 103 adhesive shear strengths 55t advantages and disadvantages of 21 amorphous thermoplastics 20, 21 curing agent 19 durability see Durability of epoxies elastomers 21 fibreglass-filled 55 fluoropolymers 21 high bond strength 21 resins 52 silver-filled 19, 20 single-part 20 solvent resistence 21 two-part systems 55 viscosity of 55 water resistance of 21 EPS see Expanded polystyrene Ethylene acrylic rubber 64 adhesive shear strength of 64t Ethylene oxide sterilisation 132 Ethylene propylene diene monomer rubber 64–65 adhesive shear strength of 65t Ethylene-vinyl acetate co-polymer 65 adhesive shear strength of 65t European Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Regulation 108–109 EVA see Ethylene-Vinyl Acetate Co-polymer Expanded polystyrene 45 166

Subject Index

F Failure, adhesive 150–152 fast cure 151 joint flexibility 151-152 mould-release agents 150 peel loading 151 stress cracking 152 substrate 151-152 Fibreglass 56 Flame treatment 98–99 butane torches 98 oxidation 98 polyacetal 99 polyphenylene sulfide 99 thermoplastic polyester 99 Flexible adhesive sealants 21–24 cohesive strength of 22 cure speed curve 23f modified silane adhesive sealants 24 modified silanes 24 polyamide 24 polyurethane adhesive sealants 23–24 silicone adhesive sealants 22–23 Flexible cyanoacrylates 7–8 loudspeaker applications 8f Fluoropolymers 18, 21, 46, 48, 96, 99, 100, 103 functional groups 96 low porosity 96 mould release agents 97 surface weakness 97 Fluorosilicone rubber 66 adhesive shear strengths 66t ‘O’ rings 66 service temperature of 66 siloxane backbone 66 Fourier transform infrared spectroscopy 139

G Glass transition temperature 30–31, 73, 89 Globally Harmonised System of Classification and Labelling of Chemicals symbols 110, 116 warning symbols 110t 167

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Grit blasting 98

H Health and safety 120–121 CE marking 120 Health and Safety Executive 108, 111 Hevea brasiliensis 66 Hot melt adhesives 25–26 automatic application systems 25 bonding of plastics 25 crosslinked polymer 25 dispensing cycle 25 green strength 25 reactive hot melts 25–26 urethane 26 Hydraulic locking of 85

J Jet fuel 108 Joint design 75–91 bond line thickness 88–89 butt joint 86–88 cylindrical joints 81–86 double lap joint (tongue and groove) 80–81 durability of 128 high temperature/humidity levels 128 moisture traps 128 peel load 128 lap joint 75–79 surface preparation 91 thermal effects 89 glass transition temperature 89 liquid crystal polymer 89 UV acrylic 89 Joint overlap 76-78 strength of joint 76 overlap length 76 overlap on lap shear joint 76f Joint width 76–78 bond area 76 elastic recovery of 78 increasing bond strength of 78f 168

Subject Index lap joints, failure load of 77 ‘nominal shear strength’ 78 peak stress point 78 strength comparison 77f stress distribution curves 76, 77f Joints, optimising of 78–79 adhesive, fillet of 79f joint load 79 lap shear joint, options for 79f rigid adhesives 79 stress concentration 78

K Kapton 58

L Labels, hazard statements for 109 Labels, precautionary statements for 109 Lap joint 80 adhesive 77 double lap joint 80 width versus overlap 76–78 lower/nominal shear strength 78 optimisation of 78–79 pre-service stresses 78 stress peak 77 tensile shear loading 76f Laplace equation 95 Lap shear joint 75f Lego bricks 29 Liquid crystal polymer 27, 32–33, 89 adhesive shear strengths 32–33t

M Manual units 122 applying an epoxy 122f epoxy-based adhesives 122 mixing adhesive 122f two-part acrylic 122 Material Safety Data Sheet 108, 111 Melamine resin 52 169

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Methyl cyanoacrylates 4 Methylmethacrylates see Methyl cyanoacrylates; Two-part acrylics Modified silanes 24 seam sealing, used for 24 slow cure cycle 24 see also Flexible adhesive sealants

N Natural rubber 66–67 adhesive shear strengths of 67t toughened cyanoacrylates 66 Neoprene rubber 68, 132 adhesive shear strength 68t cyanoacrylate bonding 68 Nitrile rubber 67 adhesive shear strength 67 high/low temperature performance 67 solvent resistance 67 No cure 145–148 blooming of cyanoacrylates 145–146 excess adhesive 146 low relative humidity 147 methods of overcoming 147–148 slow cure 146 curing problems 145 disturbing partially cured adhesive 144 curing cycle 144 failed adhesive joint 144 polymer chains 144 differential scanning calorimetry 144 heat capacity changes 144 thermo-analytical technique 144 factors inhibiting cure 143–144 cured polymer 144 cyanoacrylates 144 epoxies 144 mix ratio 144 nitrites 144 relative humidity 144 UV acrylics 144 odour 143 of bond line 143 170

Subject Index of thermoset 143 No glue 139-143 air bubbles and voids 141–142 acrylic-based adhesives 141 gravity-fed systems 142 shrinkage voids 141 destructive and non-destructive methods adhesive dispensed, quantity of 142 fast-curing adhesives 142 strength testing 142 ultrasonic testing 142 visual inspection 142 inspecting for presence 141–142 fluorescent agent 140 high-volume application 140 joint starvation 139 sealant 139 other factors 142–143 adhesive, incorrect grade of 143 ambient temperature 143 shelf life 143 Non-destructive techniques 142 No performance 152–154 defining the failure mode 153–154 surface analysis 153 Noryl 42 see also Polyphenylene oxide No stick 148–152 adhesive failure 150–151 cohesive 149–150 substrate failure 151–152 theories of adhesion 148–149

O ‘O’ rings 66

P PBT see Polybutylene terephthalate PE see Polyethylene PEEK see Polyetheretherketone ‘Perspex’ see Polymethylmethacrylate Polyethersulfone and polysulfone 38 171

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts adhesive shear strengths 38t low weathering resistance 38 polyolefin bonder 38 stress cracking 38 PET see Polyethylene terephthalate Phenolics 55–56 adhesive shear strength 56t epoxy-based 56 melamine formaldehyde 56 two-part acrylic engineering adhesives 56 Photoinitiator 8–12, 144 Plasma treatment 98 Plastic engineering applications 79 Plastics, low-surface-energy, bonding of 103, 151 PMMA see Polymethylmethacrylate Polyamide 33–34 adhesive shear strength 34t alkoxy silicone 34 dimensional stability 33 Nylon 33 under-bonnet applications 34 Polybutylene terephthalate 27, 34–36 adhesive shear strength 35t chemical resistance 34 electrical insulation properties 35 glass reinforced grades 35 Polycarbonate 4, 29, 36, 131 adhesive shear strength 36t blended with acrylonitrile-butadiene-styrene 36 medical industry 12 used with flexible poly(vinylchloride) 12 Polyester (thermoset) 56–57 adhesive shear strength 57t magnetisable ferrite filled grades 56 silicone adhesive 57 structural applications 56 with cyanoacrylate adhesives 57 Polyetheretherketone 102, 129 adhesive shear strength 37t radiation/weathering resistance 37 replacement for metal parts 37 surface treatment 37

172

Subject Index Polyethylene 29, 96, 103 adhesive shear strength 39t bottles 1 low/high/medium-density 39 low surface energy 39 ultra-high molecular weight 39 Polyethylene terephthalate 40 adhesive shear strength 40t soft drink containers 40 Polyimides 52, 58–59 adhesive shear strength 59t powder-metallurgy 58 Polyisoprene 68 adhesive shear strength 69t Polymethylmethacrylate 29, 40–41 adhesive shear strength 41t Polyolefin elastomers 68–70 surface treatment of 69 two-shot moulding 68 Polyoxymethylene 30, 41–42, 46 Polyphenylene oxide 30t, 42–43 adhesive shear strength 43t stress cracked 42 Polyphenylene sulfide 27, 43 bond strength 43 chemical resistance 43 thermal stability 43 Polypropylene 29, 96, 103, 129 adhesive shear strength 44t impact strength 43 low surface energy 45 moisture resistance 43 Polystyrene 45–46, 96 crystal polystyrene 45 EPS see Expanded polystyrene high-impact polystyrene 45, 46t stress cracking 45 Polytetrafluoroethylene 30t, 46–47, 93 adhesive shear strength 47t low surface energy 46 release spray 150 Polyurethane adhesive sealants 23–24

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts black polyurethane 24 isocyanates 24 single-component/two-component 23 Polyurethanes 18, 57–58 adhesive shear strength 58t isocyanates 57 solid casting 57 stress cracking 58 Poly(vinyl chloride) 4, 47, 96 medical device tubing 47 soft flexible/rigid vinyl 47 POM see Polyoxymethylene PP see Polypropylene PPO see Polyphenylene oxide Pressure-time 140-141 curve 141 dispensing system 140 response curve 140 Primers 99 cyanoacrylate adhesion 99 bond strengths 99 fluoropolymers 99 polyolefin 99 PTFE see Polytetrafluoroethylene PS see Polystyrene PVC see Poly(vinyl chloride)

R Relative humidity 2, 23, 128, 144, 147, 152 Resins, thermoset 16, 58 Rigid adhesives 79 Risk phrases 108 Robots 115, 118-119, 126

S Safety phrases 108 Santoprene 72 SBR see Styrene-butadiene rubber Sealants 21–24, 81, 91, 101, 107–108, 123 Secondary ion mass spectrometry 153 Adhesive, selection 101-113 adhesive performance 103–104 174

Subject Index cure speed 107 durability 104 ease of application 105 gap-filling capability 107 health and safety 108–111 joint design guidelines 105–106 label hazard statements 109 label, precautionary statements 109 long term performance 104 peel forces 106 recycling of adhesives 111–112 sealing capability 107–108 selection of materials 102–103 surface preparation 104–105 temperature resistance 104 viscosity 106 warning symbols 109 Semi-automatic dispensers 123 ambient temperature 125 cartridge holder pneumatic plunger 124 gravity-operated 124 heater jackets 125 peristaltic pump systems 125 pneumatic plunger 125 positive displacement systems 125 powered follower plate 124 pressure pot dispensing 123–125 pressure-time method 123 pressurised cartridge for dispensing high-viscosity adhesives 125f pressurised tank 124 semi-automatic dispense systems 124 syringe dispensing 123 typical syringe dispenser 123f Semi-crystalline polymers 29–31 amorphous and semi-crystalline thermoplastics 30t degree of crystallization 29 polyethylene terephthalate 29 repeated cyclic loading 29 semi-crystalline plastic 29f thermoplastics, temperature influence 30f Semi-crystalline thermoplastics 27–30 Silicone adhesive sealants 22–23, 108

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts ‘acetoxy’ 22, 23 cure speed curve 23f Silicone, temperature vulcanised 53 Silicone rubber 70, 103 adhesive shear strength 69t low surface energy 70 operating temperatures 70 Silicones, ultraviolet-curing 17, 134 Single- or two-part adhesives 115–116 bead-on-bead 116 epoxies 115 helix nozzle 115 liquid activator 116 mixer nozzle 116 syringe system 116 temperature/viscosity curve for UV acrylic 116f Standard lap shear test method 62 Stress distribution 76, 77, 79, 89, 152f Styrene-butadiene rubber 70 adhesive shear strength 71t Styrenic thermoplastic elastomer 70–72 adhesive shear strength 71t styrene-butadiene-styrene block co-polymers 71 Substrate bonded plastics 127,129-130 adhesives in loudspeaker 129f cure time 130 dispensing of adhesives 130 low-modulus adhesive/sealant 129 low surface energy 129 single-part UV product 130 Substrate failure 151–152, 151f adhesive bond 151 amorphous thermoplastics 151 bonded joint 151 bonded lap shear joint 151f formation of cracks 152 stress distribution across a lap shear joint 152f Super glues see Cyanoacrylates Surface analysis 153 adhesive, discolouration of 153 Surface energy 97 adhesive application engineer 97

176

Subject Index degree of adhesion 97 of plastic 97 surface-tension pens 97 Surface finish and preparation 127–128 bond line 128 injection moulding 127 mechanical abrasion 128 mould release agents 128 spark erosion 127 surface treatment process 128 use of solvent wipes 128 Surface preparation 91 amorphous thermoplastics 91 bond strength 91 durability 91 engineering plastics 91 isopropyl alcohol 91 solvent cleaning 91 solvent wiping 104 surface tension 91 wetting 91 Surface treatments 97-99 abrasion 98 corona discharge 98 flame treatment 98–99 plasma treatment 98 use of primers 99 Surface wetting 93–97 capillary action 93 contact angle 94 critical wetting tension 96 ethyl cyanoacrylate 96 interfacial tension points 94 Laplace equation 95 polyolefin plastics 93 polytetrafluoroethylene 96 silicone rubber 93 surface/interfacial energy 93–94 surface tension 93–94 values for some plastics 96t wetting equilibria/tensions 93–95

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T Teflon 46 see also Polytetrafluoroethylene Theories of adhesion see Adhesion, theories of Thermally resistant cyanoacrylates 6–7 ethyl cyanoacrylate 6 heat ageing strength curve 7f high-temperature additives 7 phthalic anhydrides 7 steel lap shears, hot strength of 7f Thermoplastics 27–49 adhesive performance of 31–47 acrylonitrile-butadiene-styrene 32 liquid crystal polymer 32–33 polyamide 33–34 polybutylene terephthalate 34–36 polycarbonate 36 polyetheretherketone 37 polyethersulfone 38 polyethylene 39 polyethylene terephthalate 40 polymethylmethacrylate 40–41 polyoxymethylene 41–42 polyphenylene oxide 42–43 polyphenylene sulfide 43 polypropylene 43–45 polystyrene 45–46 polysulfone 38 polytetrafluoroethylene 46–47 poly(vinylchloride) 47 amorphous 27–29 Lego bricks 29 semi-crystalline polymers 29–31 Thermoplastic vulcanisates 72 cyanoacrylate, ethyl-based 72 Santoprene 72 Thermoset plastics 27, 51f, 52 adhesive performance on 53–58 bond strength 53–58 diallyl phthalate 54 epoxies 53, 55

178

Subject Index phenolics 55–56 polyester (thermoset) 56–57 polyimides 58–59 polyurethanes 57–58 ultraviolet acrylic 53 advantages of 52 compression moulding 52 decomposition temperature 51 disadvantages 53 extrusion moulding 52 melt thermoplastic process 53 moulding methods of 52 reactive injection moulding 52 spin casting 52 thermoset resins 51 Thermoset polyesters adhesive shear strength 57t Thermoset resins 16, 51, 58 Thermosetting adhesives, crosslinked 105 Thixotropic gel 1, 16 Tongue and groove joint 80-81, 80f TPV see Thermoplastic vulcanisates Troubleshooting 139–154 ‘no cure’ see No cure ‘no glue’ see No glue no performance 152–154 defining the failure mode 153–154 surface analysis 153 ‘no stick’ see No stick Two-part acrylics 17–18, 99–100, 112 adhesive shear strengths 99f bonding automotive bumpers 18 bonding of polyolefin plastics 99 bond-line thickness 100 durability see Durability of two-part acrylics fibreglass 18 glass beads 100 liquid activator 17 methylmethacrylate, toughened with rubber particles 18f paint bake cycle 18 PP see Polypropylene

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

U Ultrasonic welding 28, 101, 128 Ultraviolet acrylic 31, 41, 53, 62, 89, 117 Ultraviolet adhesive 103 Ultraviolet adhesives for medical applications 133–135 alkoxy silicones 135 balloon catheters 134 cannula/hub bonding 133 curing of silicones 134 gamma radiation 133 polycarbonate luer 133 silicone adhesives 134f tracheotomy cuffs 134f Ultraviolet-curing adhesives 9–17 blood collection units 15f cure-on-demand 9 curing adhesive tack-free 14–16 curing equipment 13–14 light-emitting diode spectral output 14 ultraviolet flood-light/lamp 13 ultraviolet wand system 13f curing process 9–12 electromagnetic spectrum 9, 10f glass bonding 12 intensity of ultraviolet light 12 UVA/UVB/UVC 9 durability see Durability of Ultraviolet-curing adhesives health and safety 13 optical clarity 17 polymerisation 9 solvent free 17 speed of cure 17 ‘tack-free’ 14–15 types of ultraviolet adhesives 16–17 thermoset resins 16 UVC wavelength 16f viscosity 16 ultraviolet acrylic adhesive 15f Ultraviolet-curing anaerobic adhesives 17 Ultraviolet-curing cyanoacrylate adhesives 8 photoinitiators 8, 10 residual moisture 8 180

Subject Index Ultraviolet flood-light/lamp 13

V Van der Waals forces 149 Viscosity 116–117 ‘dog bone’ effect 117f kinetic energy 90 Newton’s law 90 ‘non-Newtonian’ fluids 117 pressure-time dispensing 117 solvent-based adhesives 117 thixotropic adhesive 90 viscosity/temperature curve 117 ‘wet’ strength 91 ‘Visible light curing’ adhesives see UV-curing adhesives Viton 72 Vulcanised rubber 52, 61

W Warning symbols 109 X X-ray photoelectron spectroscopy 153

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