Update on Troubleshooting the PVC Extrusion Process

Update on Troubleshooting the PVC Extrusion Process Natami Subramanian Muralisrinivasan iSmithers – A Smithers Group C...

Author: Natamai Subramanian Muralisrinivasan

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Update on Troubleshooting the PVC Extrusion Process

Natami Subramanian Muralisrinivasan

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 2011 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: 978-184735-550-8 (Hardback) 978-184735-551-5 (ebook)

Typeset by Argil Services Printed and bound by Lightning Source Inc.

P

reface

The basic work for ‘Update on Troubleshooting in PVC Extrusion’ was laid out during my industrial visits to pipe, proﬁle, ﬁlm, and sheet industries. An obvious strength of this book is that its nine chapters capture most of the literature citations in this arena. The coverage is intentionally contemporary. Chapters 1–5 serve as starting points for PVC extrusion processing. Chapter 6 covers PVC composites and discusses progress in environmental protection. Recently, considerable pressure for more effective substitutes for wood has come from the interest garnered from applying PVC–wood composites in environmentally friendly products. Chapter 7 deals with problems and defects during processing. Chapter 8 is dedicated to the analysis of troubleshooting (particularly in processing and end products). This book addresses problems speciﬁc to extrusion such as PVC pipe, proﬁle, sheet and ﬁlm extrusion. It guides the user through the methods in a user-friendly manner. The solutions mentioned in this book are easy to incorporate. In summary, this book is a valuable addition to the repertoire of PVC processing as well as an excellent source of information for engineers, academics, processors, and students interested in processing methods and troubleshooting. Knowledge of the content will also provide the reader with a communication tool for discussing the subject with a PVC specialist. I would like to thank God for his support in helping me to write this book. In particular I also express appreciation to my wife and sons

iii

Update on Troubleshooting the PVC Extrusion Process

for their continuing support, and to my professors from the beginning of my career for their encouragement. Final thanks go to Frances Powers- Gardiner, Cal Parkinson, Eleanor Garmson and others at iSmithers Publishing for their help in my endeavour.

Muralisrinivasan Natamai Subramanian February 2011

iv

C

ontents

1

Introduction ............................................................. 1

2

Poly (Vinyl Chloride) ................................................ 5 2.1 Characteristics ................................................. 5 2.2 Poly (Vinyl Chloride) Structure ....................... 6 2.2.1 Crystallinity .......................................... 8 2.2.2 Thermal Stability of Poly (Vinyl Chloride) .................................... 8 2.3 Manufacture of Poly (Vinyl Chloride) Resin .. 10 2.3.1 Raw Material ..................................... 11 2.3.2 Energy Requirement ........................... 11 2.3.3 Process ................................................ 11 2.3.4 K-Value .............................................. 12 2.4 Modiﬁcation of Poly (Vinyl Chloride) ............ 12 2.4.1 By the Addition of Metal Compounds 12 2.4.2 Structural Modiﬁcation....................... 14 2.5 Regrind Poly (Vinyl Chloride) Material ........ 14 2.6 Advantages of Poly (Vinyl Chloride) .............. 15 2.7 Disadvantages ................................................ 16

3

Additives and Compounding .................................. 23 3.1 Poly (Vinyl Chloride) Formulation ................. 23

Update on Troubleshooting the PVC Extrusion Process

3.2 Role of Additives ........................................... 24 3.3 Classiﬁcation of Poly(Vinyl Chloride) Additives ........................................................ 26 3.3.1 Heat Stabilisers ................................... 27 3.3.1.1 Lead Stabilisers .................... 29 3.3.1.2 Secondary Heat Stabilisers ... 29 3.3.1.3 Tin Stabilisers ...................... 30 3.3.1.4 Calcium–zinc Stabilisers ....... 31 3.3.1.5 Other Heat Stabilisers .......... 32 3.3.1.6 One-pack Stabilisers ............ 32 3.3.2 Lubricants........................................... 33 3.3.3 Impact Modiﬁers ................................ 34 3.3.4 Plasticisers .......................................... 34 3.3.5 Fillers .................................................. 36 3.3.6 Flame Retardants ................................ 37 3.3.7 Blowing Agents ................................... 37 3.3.8 Pigments ............................................ 38 3.3.9 Coupling Agents ................................. 39 3.3.10 Smoke Suppressants ............................ 39 3.4 Migration of Additives ................................... 39 3.5 Compounding ................................................ 40 3.5.1 Technology ......................................... 41 4

Poly(Vinyl Chloride) Extrusion .............................. 49 4.1 Basic Requirement of Poly (Vinyl Chloride) Compounds ................................................... 49 4.1.1 Machine.............................................. 50

vi

Contents

4.3

4.4

4.4 4.5

4.6

4.7 4.8 4.9 5

4.1.2 Processing .......................................... 50 Extrusion ....................................................... 50 4.3.1 Extruder: Operating Conditions ......... 51 4.3.2 Screw .................................................. 52 4.3.3 Processing Technology ........................ 53 4.3.4 Processing Temperature ...................... 54 4.3.5 Power Consumption ........................... 54 Extrusion ....................................................... 55 4.4.1 Single-screw Extruder ......................... 56 4.4.1.1 Advantages/Disadvantages ... 57 4.4.2 Twin-screw Extrusion ......................... 57 4.4.2.1 Advantages .......................... 58 4.4.3 Die Design .......................................... 59 Calibration System ......................................... 60 Poly (Vinyl Chloride) Pipe Extrusion ............. 60 4.5.1 Pipe Dies ............................................. 62 4.5.2 Cooling System ................................... 62 Poly (Vinyl Chloride) Proﬁle Extrusion .......... 63 4.6.1 Calibrator ........................................... 63 4.6.2 Cooling System ................................... 65 Poly (Vinyl Chloride) Sheet Extrusion ............ 65 Poly (Vinyl Chloride) Films ............................ 67 Regrind .......................................................... 68

Degradation and Stabilisation of Poly(Vinyl Chloride) ............................................... 71 5.1 Degradation of Poly (Vinyl Chloride)............. 71

vii

Update on Troubleshooting the PVC Extrusion Process

5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6

Structural Defects .......................................... 72 Tacticity ......................................................... 73 Dehydrochlorination ...................................... 73 Stabilisers ....................................................... 74 Impurities ..................................................... 75 Photochemical Degradation ........................... 75 Mechanism of Degradation ............................ 76 Poly(Vinyl Chloride) Stabilisation .................. 78

Poly(Vinyl Chloride)–Wood Composites ................ 93 6.1 Additives ........................................................ 94 6.2 Properties of Wood-Poly(vinyl Chloride) Composites .................................................... 96 6.3 Processing ...................................................... 96 6.4 Advantages of Wood-Poly(Vinyl Chloride) Composites ................................................... 98

7

Poly(Vinyl Chloride) Extrusion: Problems and Defects ........................................... 103

8

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion ............................................................. 107 8.1 Problems and Troubleshooting in Pipe/Proﬁle Extrusion .................................. 108 8.1.1 Production Problems in the Extruder 108 8.1.1.1 Problem: Difﬁcult to String-up Melt during Startup ............................... 109

viii

Contents

8.1.1.2

Problem: Powder Pulled into a Vacuum.................... 109 8.1.1.3 Problem: High Bearing Throat at Back Pressure ..... 109 8.1.1.4 Problem: Motor Load is too High ............................ 110 8.1.1.5 Problem: Low Output ........ 113 8.1.2 Product Problems.............................. 114 8.1.2 Production Problems in Downstream Areas ................................................ 115 8.1.2.1 Problem: A Longitudinal Scratch in Pipe or Proﬁle is Found While Sizing ............ 115 8.1.2.2 Problem: Folding of Material in the Calibrator .. 115 8.1.2.3 Problem: Blowouts in the Cooling Zone ..................... 116 8.1.2.4 Problem: Uncontrolled Wall Thickness .................. 118 8.1.2.5 Problem: Poor Inner Surface .............................. 119 8.1.2.6 Problem: Regular Wavy Lumps ................................ 120 8.1.2.7 Problem: Irregular Lumps (Random With No Regular Pattern) ............................. 121 8.1.2.8 Problem: Dimples on the Product ............................. 122 8.1.2.9 Problem: Burning or Yellowing of the Extrudate 122

ix

Update on Troubleshooting the PVC Extrusion Process

8.1.2.10 Problem: Poor Overall Appearance ....................... 124 8.1.2.11 Problem: Dull Surface Appearance ....................... 125 8.1.2.12 Problem: Low Results in the Drop Weight Impact Test .. 125 8.1.2.13 Problem: Gauge Variation . 126 8.1.2.14 Problem: Degassing is Difﬁcult ............................ 128 8.1.2.15 Problem: Frictional Heat: Zone-4 Overheating .......... 129 8.1.2.16 Problem: Melt Fracture and Surface Roughness ..... 130 8.1.2.17 Problem: Lumpy, Cold or Oval (egg-shaped) mark Surfaces ............................ 131 8.1.2.18 Problem: Over-lubrication 132 8.1.2.19 Problem: Impact Failure .... 133 8.1.2.20 Problem: Black Specks ...... 134 8.1.2.21 Problem: Variation in Load or Amperage from Batch-to-Batch or Between Batches ............................. 135 8.1.3 Quality Problems in the End Product 136 8.1.3.1 Problem: Failure in the Methylene Chloride Test .... 136 8.1.3.1.1 Problem - Inside Portion Granular ............................ 136 8.1.3.1.2 Problem: Middle Portion is Mealy ................................ 138 x

Contents

8.1.3.1.3 Problem: Outside Portion is Mealy ................................ 139 8.1.3.2 Problem: Bubbles at the Oil Reversion Test at the Inner Surface ............................... 139 8.1.4 Quality Problems in Pipes ................. 141 8.2 Troubleshooting for Poly (Vinyl Chloride) Blown Film .................................................. 143 8.3 Troubleshooting for Poly (Vinyl Chloride) Sheets ........................................................... 145 9

Future Requirements: Developments in Poly(Vinyl Chloride) ............................................. 149 9.1 9.2 9.3 9.4 9.5

Poly(Vinyl Chloride) Formulation ................ 149 Wood – Poly(Vinyl Chloride) Composites .... 150 Medical Applications ................................... 150 Construction ................................................ 150 Biodegradation ............................................ 150

xi

1

Introduction

Poly(vinylchloride) (PVC) has a broad range of application. It has substituted many conventional materials (especially metals) in various applications. PVC has advantages over conventional materials due to its toughness and flexibility. PVC is also an easily processable and lowcost material. During processing, PVC requires comparatively less energy compared with the manufacture of, for example, paper and metal. Its physical and resistance properties make PVC a substitute for many applications. For some PVC products, mechanical properties such as strength and toughness are inadequate [1–6]. PVC can be made softer and more flexible by the addition of plasticizers. In applications such as building materials, pipes and plumbing products, PVC, as a hard thermoplastic, offers more strength and rigidity than most other plastics. It has outstanding response to functional additives, which permit its use in designed engineering applications [7, 8]. In the last four decades, PVC has become a major building material and leading synthetic polymer. Global vinyl production is >30 million tons per year [9–12], and has significant cost as well as processing advantages. A large part of the PVC produced worldwide is used for outdoor applications (e.g., house siding panels, waste-water tubes, window profiles) [13]. PVC can incorporate with additives to suit many applications. Although additives are required to meet various technical requirements, most PVC resin is used for building, furnishing, electronic and medical applications. Besides the cost of the raw materials, PVC has outstanding chemical resistance to a wide range of corrosive liquids, and it can last for a long time.

1

Update on Troubleshooting the PVC Extrusion Process

In general, PVC is an important commercial plastic with many rigid and flexible applications. It is used in building and construction, packaging, electrical applications, electronic applications, automotive applications, furniture, office equipment, healthcare and, to some extent, in clothing and footwear. The only continuous production of PVC is extrusion, and it is used to manufacture pipe, profile, sheet and film. PVC is an engineering polymer for the ‘average man’. Toughness is the key parameter that determines if PVC products can be used as engineering materials [14]. It delivers excellent durability, chemical resistance and other value-added properties.

References 1.

J. Fang and P. Fowler, Journal of the Science of Food and Agriculture, 2003, 1, 3/4, 82.

2.

D.N. Saheb and J. Jog, Advanced Polymer Technology, 1999, 18, 4, 351.

3.

B. Seymour Raymond, Polymer Chemistry: An Introduction, Marcel Dekker Inc., New York, NY, USA, 1971, p.268.

4.

Y. Orhan, J. Hrenovic and H. Buyukgungor, Acta Chimica Slovenica, 2004, 51, 3, 579.

5.

A.L. Andrady, S.H. Hamid, X. Hu and A. Torikai, Journal of Photochemistry and Photobiology B: Biology, 1998, 46, 1-3, 96.

6.

P. Meenakshi, S.E. Noorjahan, R. Rajini, U. Venkateswarlu, C. Rose and T.P. Sastry, Bulletin of Material Science, 2002, 25, 1, 25.

7.

L. Nass in Encyclopedia of PVC, 2nd Edition, Eds., L. Nass and C.A. Heiberger, Marcel Dekker, New York, NY, USA, 1985.

2

Introduction

8.

Plastic Piping Handbook, Eds., D. Willoughby, R. Dodge Woodson and R. Sutherland, McGraw-Hill, New York, NY, USA, 2002.

9.

M. Engelmann, Angewandte Makromolekulare Chemie, 1997, 244, 1, 1.

10. A. Bos and S.R. Tan in Proceedings of an Institute of Materials Conference - PVC 96, Brighton, UK, 1996, p.77. 11. Y. Saeki and T. Emura, Progress in Polymer Science, 2002, 27, 10, 2055. 12. D. Braun, Journal of Polymer Science: Polymer Chemistry Edition, 2004, 42, 3, 578. 13. A. Andreas in Plastics Additives Handbook, 3rd Edition, Eds., R. Gächter, H. Müller and P.P. Klemchuk, Hanser, Munich, Germany, 1990, p.271. 14. W. Jiang, L-J. An and B-Z. Jiang, Chinese Journal of Polymer Science, 2003, 21, 2, 129.

3

2

Poly (Vinyl Chloride)

Poly(vinylchloride) (PVC) is a polar polymer. It is interesting because of its backbone constructed from repeating chlorine atoms. PVC is one of the world’s leading synthetic polymers [1]. It is basically an amorphous material [2, 3] and also a vinyl polymer. It has the vinyl group (CH2=CH-). PVC is manufactured from vinyl chloride (VC) monomer (CH2=CHCl). It is a colourless gas possessing a faintly sweet odour which can cause anaesthesia at high concentrations. Through common usage, the word ‘vinyl’ generally refers to PVC and its copolymers, even though other examples of group members of the vinyl ‘family’ (e.g., polyethylene, polypropylene, polystyrene, polyvinylacetae, polymethylmethacrylate) are available [4]. Its amorphous characteristics confer application advantages [2, 3]. PVC has the ability to be compounded with many additives to produce a wide range of flexible and rigid products represents the major factor responsible for the success and versatility of PVC processing [5]. In the last four decades, PVC has become a major building material. Most of the global production of vinyl is directed to building applications, furnishings, and electronics. PVC enjoys significant cost as well as processing advantages. However, several additives are required to meet the various technical requirements.

2.1 Characteristics PVC has outstanding chemical resistance to wide range of corrosive

5

Update on Troubleshooting the PVC Extrusion Process

fluids. It offers more strength and rigidity than most other thermoplastics. Chlorine in PVC accounts for 56.8% of the total weight. Hence, PVC is less affected by the cost of petroleum and natural gas than other polymers. The price of petroleum and natural gas is volatile, and PVC is less expensive compared with other polymers [6]. Some of the important characteristics of PVC are shown in Table 2.1.

Table 2.1 Characteristics of PVC Properties Density Tensile stress Modulus of elasticity Decomposition temperature

Value

Unit

Reference

1380–1410

kg/m

[7]

40–60

MPa

[7]

2–7

GPa

[7]

210–360

°C

[8]

3

PVC is considered to be ~55% syndiotactic [2, 9, 10]. Since a growth of species with conversion destroy all memory and may only be regenerated and observed after subsequent processing, hence domain is not feature of PVC morphology in high conversion samples [10]. Decomposition occurs with release of 58% HCl in an auto-catalytic dehydrochlorination step through a degenerate branched chain freeradical reaction mechanism [12].

2.2 Poly (Vinyl Chloride) Structure The structure of PVC (Figure 2.1) is described as the SEM structure cryogenically fractured porous PVC particle and it’s interior [11].

6

Poly (Vinyl Chloride)

• The grain has an approximate size of 50–250 μm (average size, 130 μm). It originates from a visible constituent of free-flowing powder comprising more than one monomer droplet • Sub-grain agglomerates have an approximate size of 10–150 μm (average, 40 μm) made from polymerised monomer droplets. However, particles of approximate size 1–10 μm (average, 5 μm) are formed during the early stage with polymerisation by coalescence or of primary particles (1–2 μm) that grow upon conversion (Figure 2.1) • Primary particles grow from the domain with a particle size of 0.6–0.8 μm (average, 0.7 μm). They are formed at low conversion (i.e., <2%) by coalescence of micro-domains, and grow upon conversion to the sizes shown in Figure 2.1

Micro-domain (0.01µm) Domain (0.1µm)

a Grain (100µm)

Agglomerate (10µm)

5µm

Primary particle (1µm)

Figure 2.1 Scanning electron microscope (SEM) - porous structure of the original PVC particle and its interior. Reproduced with permission from M. Narkis, M. Shach-Caplan, Y. Haba and M.S. Silverstein, Journal of Vinyl and Additive Technology, 2004, 10, 3, 112. ©2004 Society of Plastics Engineers [11]

7

Update on Troubleshooting the PVC Extrusion Process

Domains originate from the nucleus of the primary particle, which has a particle size of 0.1–0.2 μm (average, 0.2 μm). Domains contain ~103 micro-domains. They are observed only at low conversion (<2%) or after mechanical working. They are used to describe only 0.1-μm species and become primary particles as soon as growth starts. Micro-domains have a particle size of 0.01–0.02 μm (average, 0.02 μm) originate from the smallest species and aggregate to polymer chains of ~50 in number.

2.2.1 Crystallinity PVC has crystallinity in the range of 5–10% as indicated by X-ray diffraction [9] for the unannealed polymer. The extent of crystallinity can also be determined using differential scanning calorimetry and thermogravimetry, and a maximum of 5.7% is obtained for an annealing temperature of 130 °C [13]. During processing, the thermal treatment of semicrystalline polymers is known to affect the crystallinity, particularly if this is carried out above the glass transition temperature [14]. Important chemical changes occur during the manufacturing and processing of PVC [15, 16].

2.2.2 Thermal Stability of Poly (Vinyl Chloride) The thermal stability of PVC is considered to be a serious problem. Thermal degradation of PVC results in an unacceptable discoloration of the polymer and a drastic change in its mechanical properties [17]. The instability of PVC is discussed below [18–21]. Unless it is stabilised, PVC will degrade while processing. Even below its melting point, PVC loses HCl and becomes discoloured [22–25]. Discoloration is associated with loss of some of the useful properties of the polymer. The poor stability of PVC requires the addition of heat stabilisers during processing. However, the mechanisms which occur during thermal degradation are incompletely understood.

8

Poly (Vinyl Chloride)

Unmodified unplasticised poly(vinyl chloride) (PVC-U) has the disadvantage of being prone to brittleness and is notch-sensitive. It has been observed that normally ductile materials often fail prematurely through brittle fracture. Under impact (high velocity) loading conditions, temperature and strain rate become more important due to the viscoelastic nature of polymers. Extensive research and development has therefore been carried out to formulate polymers with high impact resistance [1]. The nature of PVC thermal instability is associated with the possible structural defects listed below [18–21]: • Labile tertiary and internal allyl chlorides [22–25] • Tertiary hydrogen and chlorine atoms associated with branches [25] • Head-to-head structures • Labile chlorines in isotactic fragments [23, 26–29] • Relatively high isotactic content • Steric order of the monomer units in PVC [30–32] • Presence of double bonds as end groups [33] • Oxygen-containing groups [33–35] • Peroxide residues [36] As stated above, even below its melting point, PVC loses HCl and becomes discoloured [37–40]. The thermal stability of PVC is of great importance in industry, and the process involves zipper dehydrochlorination and the generation of polyene sequences. The latter readily form crosslinked, long polyene sequences of undesirable color in the material [26–29], as well as low-molecular-weight products such benzene, toluene,

9

Update on Troubleshooting the PVC Extrusion Process

styrene, naphthahlene or anthracene [26–32]. The results of such secondary reactions as well as the mechanical behaviour and color of formulations [29] may be modified. As stated above, the nature of PVC thermal instability is associated with its structural defects, namely: • Labile tertiary and internal allyl chlorides [33–36] as well as labile chlorines in isotactic fragments [34, 37–42] • Tertiary hydrogen and chlorine atoms associated with branches [43] • Head-to-head structures These defects arise from processes that occur at high temperature in free-radical polymerisation such as backbiting, chain transfer to monomer/polymer, and relatively high isotactic content. Under the action of strong nucleophiles (e.g., thiols, thiolates), labile chlorines are replaced and the thermal stability of PVC increased. Another approach is to use a low-temperature ‘living’ radical polymerisation technique that affords PVC which is virtually free of such structural defects and provides thermally stable PVC [44–46]. Control of the core-shell structure in PVC has been reported to improve thermal stability [47]. Copolymerisation of VC with imide monomers and further chlorination of PVC to increase the heat resistance of PVC has also been investigated [48–55].

2.3 Manufacture of Poly (Vinyl Chloride) Resin Ethylene has been a source of hydrocarbons for the manufacture of VC in recent times (even though acetylene derived from coal may be another source of hydrocarbons). The production of VC also needs chlorine, which is produced mainly from common salt (NaCl). Chlorine accounts for 56.8% of the total weight of PVC.

10

Poly (Vinyl Chloride)

2.3.1 Raw Material In the manufacture of PVC, liquid dialkyl peroxydicarbonates are used as initiators. Above 10 °C, most of these initiators undergo self-accelerated decomposition [56]. Even though initiators and other chemicals are used in large quantities during the manufacture of PVC resin, the defect sites created during manufacturing require stabilization of PVC during processing in the equipment.

2.3.2 Energy Requirement For PVC manufacture, electricity consumption of ~47 billion kWh per year is based on an attributable fraction of 40% of chlorine demand. The production of chlorine to make one tonne of PVC consumes ~1,800 kWh of electricity on a per mass basis, and is based on the fact that pure PVC is 59% chlorine by weight. The chemical synthesis of ethylenedichloride (EDC), vinylchloride monomer (VCM), and PVC, as well as the production of additives in the vinyl product, consumes additional energy [57]. An estimate of the total energy consumption required for the manufacture of PVC is beyond the scope of this book, but remains cheaper than for other polymeric materials.

2.3.3 Process PVC is one of the most versatile plastics. It is the second largest resin manufactured by volume worldwide. PVC production has been described as one of the most remarkable milestones in the history of polymer technology [9, 58]. Figure 2.2 is the flow diagram of the suspension polymerisation of PVC. It is manufactured from the polymerisation of VCM. An appropriate quantity of dispersant, demineralised water and VCM is stirred in a vessel. The resultant mixture is transferred to a slurry tank for the polymerisation process at a suitable temperature. The excess VCM along with other gaseous material is further purified. The pure VCM is transferred to the mixing tank. From the slurry tank, the polymerised product is

11

Update on Troubleshooting the PVC Extrusion Process

transferred to a drier, where the water is removed. Finally, the PVC produced is transferred to a silo for further packing of the material.

2.3.4 K-Value Many grades and types of PVC are available. This allows applications as diverse as flexible sheets, pressure pipes, transparent bottles, and medical products to be produced. Many different additives and stabiliser systems are used to get the suitable properties for these applications [59]. Moreover, during high-temperature processing and throughout the service life of the products, the polymer might be subjected to degradation [60, 61]. The K-value is a traditional unit of measurement used until now by manufacturers to describe the molecular weight of PVC materials. The K-value describes the molecular weight of PVC resin. The higher the K-value, the more difficult it is to process. The lower the K-value, the easier it is to process, but it will have poor mechanical and other properties. A K-value between 55 and 80 is suitable for different kinds of processing and various applications [62].

2.4 Modification of Poly (Vinyl Chloride) 2.4.1 By the Addition of Metal Compounds PVC has the lowest thermal stability of all carbon chain polymers. The main indicator of such degradation is the elimination of HCl, which is followed by colouration of the resin. It has been suggested that the process starts when structural defects are generated during the polymerisation reaction [63].

12

Recovery VCM Waste Gas xxxx

VCM Tank

Gas Holder Coarm Recovery VCM Tank

Reactor

Disporrant

Vaccum Pump

Demi. Water

Fine Recovery VCM Tank

Compressor Waste Water Treatment

Blow Down Drum

Exhaust

Centrifuge Product Silo

Fluid loyd bed dryer Waste Water Treatment

Hot Water xxxx Over flow

Product

13

Figure 2.2 Manufacture of PVC suspension resin. Reproduced with permission from Y. Saeki and T. Eumua, Progress in Polymer Science, 2002, 27, 10, 2055. ©2002, Elsevier [10]

Poly (Vinyl Chloride)

Slurry Tank

Update on Troubleshooting the PVC Extrusion Process

The thermal stability of PVC is improved by: • Replacement of labile chlorine by heavy metal compounds • The mixture of Ca/Zn carboxylates is one of the oldest systems used; it is becoming important again due to its lack of toxicity. Metal carboxylates were considered to be HCl scavengers until Frye and Horst [64] demonstrated the esterification reaction with the polymer, substituting allylic chlorides. It is well known that the Zn carboxylate is the most active and that the Ca carboxylate acts mainly as an HCl scavenger. According to some authors [65], the calcium soap reduces the rate of dehydrochlorination by avoiding the 1,3-rearrangements and controlling propagation of the HCl elimination reaction • The action of strong nucleophiles such as thiols or thiolates • Using a low-temperature living radical polymerisation technique that affords PVC that is free of structural defects [49–51] • Control of the core-shell structure [52]

2.4.2 Structural Modification Copolymerisation of VC with imide monomer and further chlorination increases heat resistance [53–55]. Specific application with improved properties from PVC has been initiated to make new materials such as copolymerisation of VC monomer, grafting, blending, and chemical modifications such as nucleophilic substitution [22, 66–69]. Nano composites such as PVC/clay enhance the thermal stability [70]. Control of core-shell structure in PVC has been reported to improve the thermal stability [65].

2.5 Regrind Poly (Vinyl Chloride) Material Use of PVC regrinds affect the processability of PVC and its properties. Several factors are involved before their use in processing: 14

Poly (Vinyl Chloride)

1. As the heat history of the PVC regrinds increase, the roughness and edge tear (melt fracture) of the extrudate increases 2. A higher molecular weight of regrind PVC does not have as much effect on smoothness and edge tear during processing 3. Low molecular weight regrind PVC allows a wide melt temperature opportunity for easy processing Incorporating regrinds with heat history with virgin PVC compounds at low concentrations have much less effect on extrusion quality. For reprocessing, high concentrations allow a wide temperature range for processing. Toughness is not affected by the heat history of regrinds, but is strongly dependent upon the extrusion melt temperature. Higher temperatures result in tougher products. Whether the regrind can be processed multiple times is dependent upon its thermal stability and product requirements.

2.6 Advantages of Poly (Vinyl Chloride) PVC has the following advantages: • Relatively cheap and versatile • Broad range of rigid and flexible applications • Easy to fabricate and long-lasting • Outstanding chemical resistance to a wide range of corrosive fluids • Offer strength and rigidity • Excellent mechanical properties • High compatibility with additives and good processability • Possible to manufacture rigid and flexible products

15

Update on Troubleshooting the PVC Extrusion Process

• Possible to manufacture designed engineering applications • Possible to to incorporate additives to suit many different applications

2.7 Disadvantages The main disadvantages of PVC are (i) the limited thermal stability which requires the addition of heat stabilisers to prevent dehydrochlorination, and (ii) discoloration during processing and application. Unmodified unplasticised PVC has the disadvantage of being prone to occasional brittleness and is notch-sensitive. It has been observed that normally ductile materials often fail prematurely through brittle fracture.

References 1.

A.Bos and S.R. Tan in the Proceedings of an Institute of Materials Conference - PVC 96, Brighton, UK, 1996, p.77.

2.

M. Beltan, J.C. Garcia and A. Marcilla, European Polymer Journal, 1997, 33, 4, 453.

3.

J. Leadbitter, Progress in Polymer Science, 2002, 27, 10, 2197.

4.

H. Sarvetnick in Polyvinyl Chloride, RE Krieger Publishing Comapany, Huntington, New York, USA, 1977.

5.

M. Engelmann, Angewandte Makromolekulare Chemie, 1997, 244, 1, 1.

6.

L. Nass in Encyclopedia of PVC, 2nd Edition, Eds., L. Nass and C.A. Heiberger, Marcel Dekker, New York, NY, USA, 1985.

16

Poly (Vinyl Chloride)

7.

Plastic Piping Handbook, Eds., D. Willoughby, R. Dodge Woodson and R. Sutherland, McGraw-Hill, New York, NY, USA, 2002.

8.

R. Miranda, H. Pakdel, C. Roy and C. Vasile, Polymer Degradation and Stability, 2000, 67, 2, 209.

9.

W. Wenig, Journal of Polymer Science, Part B: Polymer Physics Edition, 1978, 16, 9, 1635.

10. Y. Saeki and T. Eumua, Progress in Polymer Science, 2002, 27, 10, 2055. 11. M. Narkis, M. Shach-Caplan, Y. Haba and M.S. Silverstein, Journal of Vinyl and Additive Technology, 2004, 10, 3, 112. 12. B.B. Troitskii, L.S. Troitskaya, A.S. Yakhnov, M.A. Novikova, V.N. Denisova, V.K. Cherkasov and M.P. Bubnov, Polymer Degradation and Stability, 1997, 58, 1/2, 83. 13. S. Ohta, T. Kajiyama and M. Takayanagi, Polymer Engineering & Science, 1976, 7, 16, 465. 14. A. Gray and M. Gilbert, Polymer, 1976, 17, 1, 44. 15. K-H. Illers, Makromolekulare Chemie, 1969, 127, 1, 1. 16. B.D. Gupta and J. Verdu, Journal of Polymer Engineering, 1988, 8, 1. 17. B. Ivan, T.T. Nagy, T. Kelen, B. Turcsanyi and F. Tudos, Polymer Bulletin, 1980, 2, 1, 83. 18. R. Bacaloglu and M. Fisch, Polymer Degradation and Stability, 1994, 45, 3, 315. 19. I. McNeill, L. Memetea and W.J. Cole, Polymer Degradation and Stability, 1995, 49, 1, 181.

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Update on Troubleshooting the PVC Extrusion Process

20. P. Simon, Polymer Degradation and Stability, 1990, 29, 2, 155. 21. R. Simon, Polymer Degradation and Stability, 1992, 36, 1, 85. 22. W.H. Starnes, Jr., Progress in Polymer Science, 2002, 27, 10, 2133. 23. W.H. Starnes, Jr., Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 2451. 24. D. Braun, B. Boehringer, B. Ivan, T. Kelen and F. Tudos, European Polymer Journal, 1986, 22, 1, 1. 25. T. Hjertburg and E.M. Sorvik, Polymer, 1983, 24, 6, 685. 26. G. Martinez, C. Mijangos and C. Millan, Revista de Plasticos Modernos, 1982, 43, 312, 629. 27. G. Martinez, C. Mijangos and J. Millan, Journal of Macromolecular Science: Pure and Applied Chemistry, 1982, A17, 7, 1129. 28. G. Martinez and J.L. Millan, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2002, 40, 22, 3944. 29. G. Martinez and J.L. Millan, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 23, 6052. 30. C. Mijangos, G. Martinez, A. Michel, J. Millan and A. Guyot, European Polymer Materials, 1984, 20, 1, 1. 31. G. Martineze, J.M. Gomez-Elvira and T. Millan, Polymer Degradation and Stability, 1993, 40, 1, 1. 32. T. Radiotis and G.R. Brown, Journal of Macromolecular Science A, 1997, 34, 5, 743.

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33. J.L. Millan, G. Martineze, J.M. Gomez-Elvira, N. Guarrotxena and P. Tiemblo, Polymer, 1996, 37, 2, 219. 34. T. Hjertburg and E.M. Sorvik, Polymer, 1983, 24, 6, 673. 35. T. Hjetberg and E.M. Sorvik in Report to the IUPAC Working Party on PVC, Cleveland, OH, USA, 1980. 36. N. Bensemra, T.V. Hoang and A. Guyot, Polymer Degradation and Stability, 1990, 28, 2, 173. 37. J. Bauer and A. Sabel, Angewandte Makromolekulare Chemie, 1975, 47, 15. 38. F.E. Okieimen and O.C. Eromonsele, European Polymer Journal, 2000, 36, 3, 525. 39. N.A. Mohamed, M.W. Sabaa, K.D. Khalil and A.A. Yassin, Polymer Degradation and Stability, 2001, 72, 1, 53. 40. M.W. Sabaa, N.A. Mohamed, E.H. Oreby and A.A. Yassin, Polymer Degradation and Stability, 2002, 76, 3, 367. 41. B. Li, Polymer Degradation and Stability, 2000, 68, 2, 197. 42. R. Bacaloglu and M.H. Fisch in Plastics Additives Handbook, 5th Edition, Ed., H. Zweifel, Hanser, Munich, Germany, 2001, Chapter 3. 43. F. Tudos, T. Kelen, T.T. Nagy and B. Turcsanyi, Pure Applied Chemistry, 1974, 38, 1/2, 201. 44. T. Kelen, Journal of Macromolecular Science, 1978, 12, 3, 349. 45. R.P. Lattimer and W.J. Kroenke, Journal of Applied Polymer Science, 1980, 25, 1, 101. 46. V. Bellenger, L.B. Carette, E. Fontaine and J.A. Verdu, European Polymer Journal, 1982, 18, 4, 337. 19

Update on Troubleshooting the PVC Extrusion Process

47. R.P. Lattimer and W.J. Kroenke, Journal of Applied Polymer Science, 1982, 27, 4, 1355. 48. G. Montaudo and C. Puglisi, Polymer Degradation and Stability, 1991, 33, 2, 229. 49. V. Percec, A.V. Popov, E. Ramirez-Castillo, J.F.J. Coelho and L.A. Hinojosa-Falcon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 24, 6267. 50. V. Percec, A.V. Popov, E. Ramirez-Castillo, J.F.J. Coelho and L.A. Hinojosa-Falcon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 4, 779. 51. V. Percec, E. Ramirez-Castillo, L.A. Hinojosa-Falcon and A.V. Popov, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 10, 2185. 52. K. Endo, Progress in Polymer Science, 2002, 27, 10, 2021. 53. N.A. Mohamed and W.M. Al-Magribi, Polymer Degradation and Stability, 2003, 82, 3, 421. 54. D.D. Sotiropoulou, K.G. Gravalos and N.K. Kalfoglou, Journal of Applied Polymer Science, 1992, 45, 2, 273. 55. J.F. Maggioni, A. Eich, B.A. Wolf and S.P. Nunes, Polymer, 2000, 41, 12, 4743. 56. P. Frenkel and E. Pettijohn, Journal of Vinyl and Additive Technology, 1999, 5, 3, 165. 57. J. Thornton in Environmental Impacts of Polyvinyl Chloride Building Materials, Healthy Building Network, Washington, DC, 2002. 58. D. Braun, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 3, 578.

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59. Taschenbuch der Kunststoff—Additive, 3rd Edition, Eds., R. Gächter and H. Müller Hanser, Munich, Germany, 1989. 60. S.H. Hamid, M.B. Amin, A.G. Maadhah and A.M. AlJarallah in the Proceedings of the SPE Conference – ANTEC 1992 – Plastics: Shaping the Future, Detroit, MI, USA, 1992, 1, p.215. 61. D.W. Riley, Journal of Vinyl Technology, 1990, 12, 1, 20. 62. Kunststoff Handbuch: Polyvinylchlorid, Ed., K.H. Felger, Hanser, Munich, Germany, 1986, 2/1, p.73. 63. G. Allen and J.C. Bevington in Comprehensive Polymer Science, Volume 6, 1st Edition, Pergamon, Oxford, UK, 1989. 64. A.H. Frye and R.W. Horst, Journal of Polymer Science, 1959, 40, 419. 65. M.H. Fish and R. Bacaloglu, Journal of Vinyl Addititive Technology, 1999, 5, 4, 205. 66. T. Uma, T. Mahalingam and U. Stimming, Materials Chemistry and Physics, 2004, 85, 131. 67. R. Joseph, K.E. George and D.J. Francis, International Journal of Polymeric Materials, 1986, 11, 2, 95. 68. S. Marian and G. Levin, Journal of Applied Polymer Science, 1981, 26, 10 3295. 69. E. Beati and M. Pegoraro, Angewandte Makromolekulare Chemie, 1978, 73, 1, 35. 70. L. Van der Ven, M.L.M. Van Gemert, L.F. Batenburg, J.J. Keern, L.H. Gielgens, T.P.M. Koster and H.R. Fischer, Applied Clay Science, 2000, 17, 1/2, 25.

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3

Additives and Compounding

The market for plastics additives is expected to grow 4.5% per year through to 2010 [1]. The additives market in the USA is worth $32 billion, and usage of additives in poly(vinyl chloride) (PVC) accounts for 85% of the total market [2]. Additives are necessary to modify and improve the properties of plastics. To make the processability of PVC easier, it has been necessary to add heat stabilisers, lubricants, pigments, and fillers. In PVC, heat stabilisers and lubricants mitigate properties such as degradation and adhesion to equipment, which are undesirable features. From the beginning of the PVC industry, much has been learnt about processing difficulties. This has led to studies into the development of additives. Even though there is growing concern about the safety of additives such as phthalate plasticisers, halogenated flame retardants, and lead-based heat stabilisers used for PVC production, usage of these additives continues in many parts of the world.

3.1 Poly (Vinyl Chloride) Formulation PVC requires stabilisers along with certain types of lubrications to prevent PVC from adhering to the surface and altering the flowability of the melt. The lubrication used is dependent upon the application (pipe, profile, or sheet extrusion). Wax-based or stearic acid-based lubricants as used in all three systems (injection, extrusion, calendaring) are similar. However, they are used in different quantities and combinations according to the final application (pipe, profile, or

23

Update on Troubleshooting the PVC Extrusion Process

sheet extrusion). Pigmentation and ultraviolet (UV) stabilisation is provided in most cases by the highly abrasive material TiO2. Calcium carbonate has always been considered to be another important ingredient. The additives used in PVC formulations are mainly plasticisers, stabilisers, lubricants and fillers. Some additives can migrate to the surface during use, where they are lost by volatilisation or diffusion upon contact with other surfaces. Stabilisers for PVC have low mobility but can change their function by consumption or degradation. Fillers usually remain in their initial form and quantity. Thus, to determine if a used PVC product can be reprocessed, an essential step before reprocessing is the determination of the degree of deterioration of the chemical structure of the base polymer as well as the loss of additives and their functionality [3]. At the heart of a dynamically processed PVC component is a formulation containing lubricants and heat stabilisers. These key ingredients are the cornerstone of every PVC formulation [4]. However, in PVC formulations or products, additives alone cannot provide a balance of final product properties and processing characteristics. Tin-based stabilisers are used in the USA for PVC formulations. Tin-based stabilisers have a lower lubricating effect. Lead-based stabilisers are very common in Europe and Asia. They have fairly good heat-stabilising effects but few lubricating effects. Relatively non-toxic calcium–zinc stabilisers are used in many countries. Calcium–zinc stabilisers have poor thermal stability and there are concerns regarding their toxicity [5].

3.2 Role of Additives Additives should: • Achieve an optimum balance of ecological and economical benefits

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Additives and Compounding

• Enable tailor-made systems to be made • Be able to be used for product engineering • Have strong influences in applications • Perform in the polymer • Enhance the essential key properties of plastics • Ensure efficient processing without sacrificing physical properties • Provide higher performance • Provide the intended functions • Be thermodynamically constrained from free migration to the surface Additives belong to a broad category of essential components in PVC formulations. It is difficult to define all the functions of additives in PVC formulations. Additives are chemicals used in plastics with or without reactions. Inorganic as well as organic substances termed ‘PVC additives’ can be used to suppress the undesirable effects which coincide with plate-out phenomena [6]. Additives make PVC useful and versatile. In PVC products, inappropriate processing results in poor quality of the end product. However, additives are defined by different sources in different contexts [7]. Additives are required in minimum quantities during the processing of PVC and its end-product applications. Additives such as phthalate plasticisers may lead to leaching and can change surface properties. The additives used in PVC formulations are mainly stabilisers, plasticisers, lubricants, processing aids and fillers. Coupling agents and antimicrobial agents are used in PVC production if necessary. Additives do not change the PVC particles, but could change the volume of the external voids between them. This is attributable to the efficiency of each additive, and its structure and polarity. However,

25

Update on Troubleshooting the PVC Extrusion Process

the additives lead to denser packing of the PVC powder. This reduces its inter-particle interaction at the expense of interaction between the additive and PVC particle [8].

3.3 Classification of Poly (Vinyl Chloride) Additives Economic cost and processing by injection molding, extrusion and calendering make PVC a universal polymer with many applications [9]. Such applications include pipes, profiles, floor coverings, cable insulation, roofing sheets, packaging foils, bottles, and medical products. The PVC industry uses a large amount of additives. PVC is an important thermoplastic with low stability among carbon-chain polymers, and undergoes severe degradation with elimination of HCl below its melting temperature [10–12]. Therefore, in the PVC industry, the significance of additives increases as the formulations become more precise and sophisticated [13]. The amount of additive added in the formulation varies with respect to the processing technique used in the manufacture of products. Additives are usually employed in small quantities to improve the processing, performance, appearance and use. The major factor responsible is the ability to compound with many additives to a wide range of flexible and rigid products. The additives used in PVC can be classified as: • Heat stabilisers • Lubricants • Impact modifiers • Plasticisers • Fillers • Flame retardants

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Additives and Compounding

• Blowing agents • Pigments • Coupling agents • Smoke suppressants

3.3.1 Heat Stabilisers Pure PVC is a rigid polymer at room temperature with low thermal stability. Hence, PVC requires heat stabilisers during processing at high temperatures. The stability of PVC can be readily modified by using heat stabilisers. Many metallic compounds have been proposed and used as thermal stabilisers to protect PVC during processing and shaping. The main functions of stabilisers are to: • Prevent degradation during processing • React with HCl when it is liberated from PVC • Replace labile chlorine atoms (this may initiate the dehydrochlorination of more stable groups) • Enhance heat stability Thermal stabilisers based on compounds with lead, tin, barium, calcium, and zinc have been employed for decades to improve the stability of PVC during processing. Common thermal stabilisers in use for the stabilisation of PVC are usually basic lead salt, metallic soaps, as well as esters or mercaptides of dialkyltin. These stabilisers seem to be established in many parts of the world. These metal stabilisers are utilised in PVC compounding and processing because PVC catalyses its own decomposition. Heat stabilisers are used in PVC to construct and extend the life of the end-product. Addition of

27

Update on Troubleshooting the PVC Extrusion Process

a sufficient quantity of heat stabilisers prevents dehydrochlorination and discoloration during processing and application [14–16]. The poor stability of PVC requires heat stabilisers during processing. PVC has been used in the study of various aspects of stabilisation. Thermal stabilisers based on lead, tin, and calcium–zinc are established in certain parts of the world. Several organometallic compounds and inorganic salts have also been used as heat stabilizers for many years. After stabiliser addition, part of the stabiliser will be consumed during processing and sometimes during the application period. Currently, obtaining increased stability with low metalcontent stabilisation is the key research area. Stabilisers must (i) breakdown the polyene sequences which cause discoloration of degraded PVC, and (ii) inhibit dehydrochlorination [17]. Different kinds of stabilisers are used to inhibit the release of hydrogen chloride from PVC to ensure adequate processing. Thermal stability can be improved by: • Using highly specific material for particular applications • Understanding the degradation and stabilisation of PVC • Formulating with stabilisers Due to the requirement of heat stabilisers to stop the thermal degradation of PVC, different types of metal soap (e.g., stearates of lead, cadmium, barium, calcium, and zinc) are used. Metal soaps of dicarboxylic acids are fairly heat-stable and may be suitable as PVC stabilisers. Thermal stabilisers of lead and tin are the most effective. They can be substituted by non-toxic calcium–zinc stabilisers due to the toxicity of heavy metals. Mono- and di- alkyl compounds (e.g., maleates, carboxylates, mercaptides) of tin are also used as heat stabilisers during PVC processing [18]. These stabilisers retard the appearance of discoloration during processing by accepting liberated HCl from PVC [19–23]. The efficiency of the stabiliser decreases after compounding with PVC with the necessary additives. The residual

28

Additives and Compounding

stability of the PVC products is a useful feature before they are recycled [24]. Heat stabilisers of PVC include metal salts of organic acids [25–28], organometallic compounds, and inhibitors of radical chain reactions. Increased addition of heat stabilisers in the PVC formulation decreases the maximum concentration of HCl, and efficiency is increased as the calcium-to-chloride molar ratio increases. However, the efficiency is not proportional to the increase in the amount of stabiliser [29]. PVC catalyses its own decomposition, so metal stabilisers are added to vinyl for construction and other extended-life applications. Common PVC additives that are particularly hazardous are lead, cadmium, and organotins, with global consumption of each by vinyl estimated to be in the thousands of tonnes per year.

3.3.1.1 Lead Stabilisers In PVC, lead stabilisers have proved to be very successful. In the formulations, lead stabilisers containing lead octate, lead stearate, tribasic lead sulphate, dibasic lead phthalate, and dibasic lead phosphate are used for different applications. Lead stabilisers provide heat-ageing resistance, prevent discoloration, and steady process viscosity. Other non-lead additives have been suggested to supplement lead stabilisers [30]. Heavy metal stabilisers have become less popular due to environmental concerns [31]. Lead stabilisers have proved to be very successful in stopping HCl from being released during processing. Such stabilisers start from lead sulfates to the higherperformance phthalate and fumarate-based systems. In many PVC products, lead replacement has made significant inroads.

3.3.1.2 Secondary Heat Stabilisers In PVC formulations, along with lead compounds, secondary organic stabilisers such as epoxides, polyols, ß-diketones, and dihydropyridine

29

Update on Troubleshooting the PVC Extrusion Process

can be used [19, 20, 32–35]. In secondary stabilisers, non-metallic epoxy compounds enhance the effectiveness of metal soaps [36]. Highly basic calcium stearates can be superior to neutral or slightly basic grades of calcium stearate for use as secondary heat stabilisers for PVC. This allows for the use of lower levels of lead- or organotinbased heat stabilisers. This offers overall improved economics and weathering performance, along with retention of the processing characteristics and physical properties of rigid PVC compounds. The synergistic effects of highly basic calcium stearates with low levels of organotin stabilisers should allow for the cost-effective replacement of other stabilisers with more environmentally acceptable stabilisers [37]. We reported on the epoxidation of sunflower oil and the effects of epoxidised sunflower oil (ESO) on the thermal degradation and stabilisation of PVC in the presence of metal carboxylates (Ba/Cd and Ca/Zn stearates) [38]. ESO showed excellent properties as a secondary stabiliser for PVC. These marked effects of ESO were not observed in the absence of metal carboxylates. Further investigations on the thermal stabilisation of PVC by ESO in combination with Ca/ Zn stearates have also been reported. The influence of the amount of oxirane oxygen in the ESO and of the ratio of Zn and Ca stearates (1/1, 1/1 and 2/1) have also been considered. Several inorganic lead compounds and organic secondary stabilisers such epoxides, polyols, phosphites, b-diketones, and dihydropyridine are also used in industrial recipes [19, 20, 32–35]. Epoxy compounds are typical non-metallic stabilisers for PVC [36]. They are generally regarded as secondary stabilisers and used to enhance the effectiveness of metal soaps. They act as acceptors for the liberated HCl [21, 32] and retardants for the appearance of discoloration [22, 23].

3.3.1.3 Tin Stabilisers Tin stabilisers inhibit the degradation of PVC during processing by co-ordination with the labile chlorine sites of PVC. This is 30

Additives and Compounding

particularly true in the case of carboxylate or thiolate groups, or esters or mercaptides of dialkyltin [39–43]. Tin stabilisers form three types of bonds: SnC, SnO and SnS. This is in comparison with metal soaps, which form only MO (where M is the metal). The stability of the bond between tin and carbon is a critical factor because any further reaction with HCl leads to the formation of a Lewis acid, RSnCl3 or SnCl4. In practice, organotin stabilisers can decompose peroxides and exhibit retarding effects [44]. The higher the content of dibutyl tin dilaurate, the greater is the concentration of shorter polyene sequences [45].

3.3.1.4 Calcium–zinc Stabilisers Basic calcium- and zinc-based products have initial heat stability. However, they are susceptible to inferior long-term heat ageing along with water/moisture access over relatively short time periods. Hence the insulation properties of PVC are affected [46]. Mixtures of calcium/zinc carboxylates are becoming important heat stabilisers due to their lack of toxicity. These are some of the oldest stabiliser systems [47, 48]. In such mixtures, calcium carboxylate reduces the rate of elimination of HCl from PVC, and the calcium soap reduces the rate of dehydrochlorination by avoiding the 1,3-rearrangements and controlling the propagation of the HClelimination reaction. Such mixtures act mainly as HCl scavengers [50]. Calcium and zinc carboxylates can react with labile chlorine atoms in PVC. Mixtures of Ca/Zn carboxylates are becoming important again due to their lack of toxicity. Metal carboxylates were considered to be HCl scavengers until Frye and Horst [49] demonstrated the esterification reaction with the polymer by substituting with allylic chlorides. It is well known that the Zn carboxylate is the most active and that the Ca carboxylate acts mainly as an HCl scavenger. Zinc stearate has undesirable effects on stabilisation and

31

Update on Troubleshooting the PVC Extrusion Process

promotes a sudden dehydrochlorination of PVC. Increase in the concentration of zinc chloride as the reactive product induces sudden dehydrochlorination [51]. Hence, zinc chloride is inactivated and cannot catalyse the dehydrochlorination. However, calcium chloride does not promote the sudden dehydrochloination. Pentaerythirtol is used to hinder the detrimental effect of zinc chloride. Pentaerythritol is used widely to considerably delay the degradation time for PVC.

3.3.1.5 Other Heat Stabilisers Compounds of barium and cadmium are used in flexible and calendering applications. In Ba–Cd compounds, barium acts as a scavenger of HCl and inhibits further degradation, and cadmium is used to provide colour retention. However, cadmium-free products are currently in demand (particularly for PVC compounds made in the USA) [31]. Phosphate esters have an expanding role in the development of environmentally friendly vinyl stabiliser systems in the stabilisation of flexible PVC compounds [52]. Quinone tin polymers act as stabilisers through intervention in the radical process of degradation and through effective absorption of the degradation products [46]. Plasticised PVC compounds with various stabiliser systems show a colour change at 180 °C. In comparison with the Ba–Cd stearate system, the colour change occurs with various stearates and salts as well as with octyl tin mercaptide. The Ba–Cd stearate system has better stabilization, and the Ca–Ba–Zn system has a similar performance [53].

3.3.1.6 One-pack Stabilisers ‘One-pack stabiliser systems’ are simple to use as a concentrate. They may contain one or more other ingredient along with the stabiliser at a much higher concentration. As the number of chemicals involved in the heat stabilisation of PVC increases, compounding becomes more 32

Additives and Compounding

difficult. Hence, an alternate procedure with many heat-stabilising additives has been developed as a single composite compound called the one-pack system. Varieties of one-pack stabilisers are available which combine heat stabilisers, lubricants and other ingredients. The levels of addition of each ingredient in the formulation differ depending upon the manufacturer. The advantage of the one-pack system is that it is dust-free, pollution-free, and a composite. The main disadvantage is that each one-pack system requires a separate procedure to formulate and process in the equipment.

3.3.2 Lubricants Lubricants are used to prevent PVC materials from adhering on the barrel surface and thereby promoting degradation and altering the flowability of the melt. The nature and usage of lubricants is dependent upon the processing equipment. Wax (particularly polyethylene wax and paraffin wax) is used in PVC processing as an external lubricant. Wax is a non-polar material. Paraffin wax, polyethylene wax, and long-chain fatty esters promote forward movement of the material in the equipment due to film formation on the barrel and screw surfaces. Stearic acid is used as an internal lubricant in PVC products such as pipes, profiles, and sheets. In general, stearic acid or polar lubricants attract intermolecular particles and bring particles closer. Therefore, such lubricants retard forward flow of the material from the processing equipment. Quantity and combinations differ to accommodate the various applications. Excess lubricants in the formulation can create problems of output reduction during extrusion. Excess lubricants form a layer on the barrel surface and reduce the conveying efficiency of the PVC compound (and sometimes discolour or degrade the material). Lubricants are used to control fusion and reduce shear heating in the extrusion processing of rigid PVC compounds. Lubricant systems containing complex esters provide improved compound stability,

33

Update on Troubleshooting the PVC Extrusion Process

weatherability, and physical properties in comparison with ethylene bis stearamide and paraffin. The complex esters result in lower compound viscosity during processing [54]. Lubricants must be: compatible to reduce the tendency to plate-out; more efficient; allow faster extrusion speeds. PVC formulations require lubrications to prevent adhering of PVC to metal surfaces during extrusion. The lubricants used are dependent upon processing and the equipment. Wax and stearic acid are used as lubricants. They are used in different proportions to other ingredients.

3.3.3 Impact Modifiers The impact resistance of PVC is enhanced through the introduction of rubbery dispersed-phase material such as acrylonitrile-butadienestyrene and nitrile rubber. This has been commercially exploited on a large scale, and is one way to develop high-impact-strength PVC compounds [55, 56]. To improve the formulation of PVC for impact properties, certain features need to be considered [57]: • PVC is a ductile material • PVC often fails prematurely through brittle fracture • The viscoelastic nature of PVC • Parameters such as temperature and strain rate under impact loading with high velocity

3.3.4 Plasticisers To improve the flexibility and softness of PVC, plasticisers such as phthalates, phosphates, trimellitates, adipates, and citrates are employed. Global plasticiser demand was 4,647,000 tonnes in 2000,

34

Additives and Compounding

and shows an annual growth of 2.1%. In the past, this slow growth stagnation could have reflected the enduring argument over the use of phthalates in vinyl [58]. Plasticisers are considered to represent ~58% of the total additives market, most of which accounts for flexible PVC manufacturing [59]. The usage of plasticisers is dependent upon: • The type of PVC, its molecular weight and compatibility • The type and concentration of plasticiser, its molecular weight, branching and polarity • Homogeneity during compounding • Processing method Once PVC have been blended and processed with additives, these additives should remain in the end-products obtained. However, plasticisers can be released from flexible PVC. Phthalate plasticisers such as bis-(2-ethylhexyl) phthalate (DEHP) and di-(isononyl) phthalate (DINP) are used for medical applications due to their high compatibility with PVC as well as their softening ability with important increases in the flexibility of PVC formulations [60]. The adipate plasticisers start with di-2-ethylhexyl adipate and increase in molecular weight up to polymeric plasticisers. As the molecular weight of plasticisers increases, volatility and extraction by various media improves, and UV light stability increases [61]. Plasticised PVC with di-(isodecyl)diphthalate (DIDP) requires more energy than dihexylphthalate (DHP), and results in earlier fusion and exhibits a slightly higher flow rate [62]. Neopentyl glycol diesters with linear and C4–C12-branched saturated monocarboxylic acids show good plasticising properties in PVC compounds (especially at low temperatures) [63].

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Update on Troubleshooting the PVC Extrusion Process

3.3.5 Fillers PVC compounds often involve the careful specification and addition of mineral fillers. Rigidity is sensitive to the size and shape of the filler. Glass fibre is the most efficient filler. Talc is more efficient than calcium carbonate. The impact performance is very sensitive to the particle size, and precipitated calcium carbonate is the only filler to act as an impact modifier [64]. Compared with PVC, CaCO3 is a ground mineral and has no melt characteristics. Used in combination with PVC, it increases the modulus of elasticity, reduces the tensile strength, and reduces the cost of the formulation. This provides a resin price of PVC that is at a good ratio to the cost/density of calcium carbonate. At higher concentrations, it can add significantly to the wear of screws and barrels of standard extrusion systems. To keep the wear at a reasonable level with high-fill PVC formulations fillers (particularly calcium carbonate) they should possess the following properties: • Surface area should be ≥5.17 m2/g • Top cut should be >8 µm • The acid insoluble should be higher than 47% acid insoluble should be >47% • The median on the top cut should be <2.6 µm Calcium carbonate can be made hydrophobic by coating with stearic acid or calcium stearate. As a result of this action, agglomeration, the viscosity of the suspension in DOP, and absorption of plasticiser are reduced [65]. Rigid PVC with higher filler content with a size range of 0.07–3 µm and 0–8 phr acrylic impact modifier shows an increase in impact properties by increasing the concentration of impact modifier and the sub-micron level of calcium carbonate. The flexural modulus 36

Additives and Compounding

increases with increasing filler and decreasing impact modifier contents. Mechanical properties such as notched Izod and falling weight impact, low-temperature impact, and flexural modulus can be enhanced. Using ultrafine fillers, the level of addition of impact modifiers can be reduced [66]. Plasticised PVC compounds with lithium carbonate and various calcium carbonates act as HCl absorbers. The synergistic effect of the fillers on HCl uptake influences the mechanical properties and oxygen index of the plasticised PVC. The fillers (particularly in combination) are effective as HCl absorbers [67]. Precipitated silica is an effective additive for the reduction of plateout in PVC compounds. It is more effective than fumed silica with respect to plate-out reduction. It reduces the plate-out without loss in stress–strain and tear properties. The order of addition has a significant effect on the amount of plate-out [68].

3.3.6 Flame Retardants PVC is not considered to be particularly flammable. However, with flexible PVC, the products can often contribute to smoke hazards due to the aromatic volatiles produced. Char formation is considered to be smoke suppression. Even though crosslinking occurs during ignition, char formation is only part of the process. Alumina trihydrate in PVC compounds acts as a flame retardant as well as improving flammability. It also reduces smoke emission if degradation occurs [69].

3.3.7 Blowing Agents In PVC dry-blend formulations, blowing agents are used in the processing of foams. The higher the decomposition temperatures, the better is the blowing agent efficiency. Blowing agents such as azodicarbonamide (AZO), dinitrosopentamethylenetetramine

37

Update on Troubleshooting the PVC Extrusion Process

(DNPT), and sodium bicarbonate (NaHCO3) are usually employed. However, using liquid stabilisers, the stabiliser absorbed in the resin particle is not in intimate contact with the blowing agent. Hence, the blowing agent is not catalysed in the presence of liquid stabilisers [70]. The specific gravity of PVC–wood powder can be reduced using foaming agents. AZO is an odourless, non-toxic material that is chemically stable, easy to handle, and which releases a large volume of gas. Therefore, with AZO, the dry blend can be processed over a wide range of concentrations, oven temperatures and processing times. DNPT decomposes at a lower temperature than AZO. It has slight residual odour and is less chemically stable. The exothermal decomposition of DNPT can be utilised for the melting of PVC. At higher dosages or higher processing temperatures, PVC degrades due to the exothermic nature of DNPT. NaHCO3 is non-toxic and inexpensive. It is odourless, and releases water molecules as a decomposition byproduct during processing. It produces a good cell structure at optimum concentration. However, at higher concentration, the cell structure completely collapses, leading to a rough surface to the end-products.

3.3.8 Pigments Titanium dioxide (TiO2) is used as a pigment and UV stabiliser in many PVC formulations. TiO2 is available in two forms: rutile and anatase. The former has a specific shape and gives a better pigment effect, whereas anatase has an irregular shape but is less expensive. It is a highly abrasive material. It is used as much as 12 parts per 100 parts of PVC. TiO2 is preferably used in conjunction with calcium carbonate to enhance the opacity of the product.

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Additives and Compounding

3.3.9 Coupling Agents Organometallic coupling agents can bring productivity improvements in filled and unfilled as well as flexible and rigid PVC compounds. Organometallic chemistry is specific and sensitive to factors such as: heat stabilizers; dosage of coupling agent; surfaces of the mixing equipment; distributive mixing and dispersion; sequence of addition; and filler levels. The benefit from optimisation requires an understanding of the appropriate usage of coupling agents and addition to the PVC formulation [71]. Poly[methylene(polyphenyl isocyanate)] (PMPPIC), g-aminopropyltriethoxy-silane, and metallic copper complex have been proved to be effective coupling agents for PVC–wood powder composites. Wood flour with other fillers (e.g., mica, glass fibres) to form hybrid reinforcements can enhance the mechanical properties of composites. PVC compounds treated with coated xonotlite have shown improved tensile strength but with reduced elongation compared with untreated materials with little difference in impact strength. The coupling agent appeared to improve the adhesion between the filler and matrix. The treated material appeared to give rubber-like properties despite the decrease in elongation [72].

3.3.10 Smoke Suppressants Oxides of molybdenum, cerium, antimony and tin are used as smoke suppressants. Among the oxides, antimony trioxide shows significant synergistic activity along with moderate synergism even in combination with other oxides. The mechanism of smoke suppression is complex and does not conform with conventional usage of smoke suppressants [73].

3.4 Migration of Additives Change in weight or hardness indicates the migration of additives in 39

Update on Troubleshooting the PVC Extrusion Process

PVC products. The results are dependent upon the geometry of the system employed. In terms of engineering, the diffusion coefficient is an important parameter in utilisation of the migration process. Particularly in PVC, predicting theoretically the degree of migration is important. Heat stabilisers have low mobility but can change their function by consumption or degradation. Fillers remain unchanged in form and quantity below their decomposition temperature. To summarise, additives are present in PVC formulations. They can be stabilisers, plasticisers, lubricants, fillers, pigments, polymeric modifiers, or colourants. Heat stabilisers have low mobility, but can be utilised during processing. Additives with low melting points (e.g., lubricants) can migrate to the surface during processing or use. They may volatilise or diffuse in contact with other surfaces. Fillers usually remain in their initial form and quantity [3].

3.5 Compounding The compounding of PVC is the combination of appropriate additives with resin to regulate the behaviour of extrusion. It is one of the most important phases in PVC processing. PVC products are produced by mixing PVC powder with other additives aimed to improve and control the properties of the end-product. Low K-value-resins produce end products with poor physical properties. Suspension resins are generally less expensive and easier to process. Powder blend compounds are usually employed as suspension resins which possess specific particle characteristics, particle size, and molecular-weight distribution. Compounds produced with suspension resins are free-flowing powders, and therefore they do not have to meet viscosity requirements. Dry blends can be kept for indefinite periods (in simple fibre drums if necessary) without changes in consistency. It is common to produce PVC products from a powder blend or its compounds. Dry blend compounding is used in the extrusion of flexible and rigid PVC products such as pipes, profiles and film.

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Additives and Compounding

In principle, the procedures for compounding of resins, additives and fillers usually occur in hot -cool mixture equipment. PVC compounding is a difficult task involving mixing of heat stabilisers, lubricants, fillers and other additives. Mixing is a critical function in most PVC processing operations. The resultant compound after mixing is in powder form. PVC products from powder blend or dry blend are used in processing. The dry blend is used in the extrusion of shapes, blow moulding of bottles, and in the injection moulding of rigid PVC or plasticised PVC. With respect to mixing, powder is better than granulated or pelleted PVC compound. It rapidly becomes extremely difficult as the number of additives increases. Over time, mixing and compounding in PVC processing has changed from the addition of individual ingredients to a one-pack system which incorporates all the necessary chemicals. Fine particles may lead to conveying and air entrapment.

3.5.1 Technology Compounding involving resins along with pigments, fillers, and other random-size solids such as one-pack heat stabilisers should be pre-dispersed in a mixer. In PVC compounding, a high shear mixer (for the dispersion) and a jacketed low shear mixer (for cooling) is preferred. The capacity of the chamber of the high shear mixer is critical. The size of the mixer is relative to the blade size. The mixing cycle is 15–20 minutes. For compounding vinyl dry blend formulations, the equipment comprises a high shear mixer (for the dispersion) and a jacketed low shear mixer (for cooling). The capacity of the chamber of the high shear mixer is critical. For ultimate homogeneity, pigments, fillers and any other large- or random-size solids should be pre-dispersed and ground to uniform size in a plasticiser on suitable equipment (e.g., three-roll paint mill). Low or intermediate shear mixers require external heating and excess

41

Update on Troubleshooting the PVC Extrusion Process

time for compounding. This gives a compound with less homogeneity, which leads to variation in the extrusion process. Non-homogeneous compounds lead to surging during extrusion. Without pre-dispersion, more waste occurs due to particle agglomeration during feeding in the extrusion machinery [70]. Low or intermediate shear mixers can be employed but have several disadvantages: external heating is required; extended compounding times are required; poorer compound homogeneity results; more waste due to particle agglomeration; and solids must be pre-dispersed. There have been developments in PVC mix preparation with reference to system mixers [74]. The resin is charged to the shearing mixer and the cycle started at 1,500 rpm to 2,500 rpm. The plasticiser and pre-dispersed solids are then added. Mixing is continued until 110 °C. The compound is then cooled to ≥40 °C. The compound at this stage will be moist. Compactable material with some agglomeration involves varying the amount of plasticiser used. PVC and the stabiliser should be blended for ~30 seconds before the other ingredients are added to the mixture. The filler must be added early to achieve good dispersion from the shear effect. The drop temperature of the hot mixer should be ≥110 °C. Double-batching does not benefit any extrusion process. Even though it is cheap, it is an unqualified way to increase blending capacity and results in a reduction of thermal stability and flowability. In PVC compounding, double-batching permits a significant increase in throughput in the heating and cooling mixer, with simultaneous energy saving. The separation of the compound can be compensated by homogenisation effects during processing, and also provides highquality extruded products [75]. However, double-batching results in an expensive finished product.

References 1.

42

R. Coons, Chemical Week, 2009, 171, 21, 17.

Additives and Compounding

2.

D. Platt, Performance Chemicals Europe, 2001, 16, 5, 41.

3.

H. Herbst, K. Hoffman, R. Pfaendner and H. Zweifel in Frontiers in the Science and Technology of Polymer Recycling, Eds., G. Akovali, C.A. Bernardo, J. Leidner, A.U. Leszek and M. Xanthos, Kluwer, Dordrecht, The Netherlands, 1998, p.75.

4.

K.A. Mesch and J.L. Newberg in Property Enhancement with Modifiers and Additives, Retec Proceedings, New Brunswick, NJ, USA, 1994, p.161.

5.

L. SuPing, T. Grailer, A. Belu, J. Schley, T. Bartlett, C. Hobot, R. Sparer and D. Untereker , Polymer, 2007, 48, 20, 6049.

6.

D.S. Van Es, J. Steenwijk, G.E. Frissen, H.C. Van Der Kolk, J. Van Haveren, J.W. Geus and L.W. Jenneskens, Polymer Degradation and Stability, 2008, 93, 1, 50.

7.

A. Tamhankar, European Chemical News, 2002, 76, 1990, 16.

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E.N. Zilberman and F. Lerner, Journal of Vinyl Technology, 1994, 16, 4, 197.

9.

Kunststoff-Handbuch, Polyvinylchloride, Volume 2/1, Ed., K.H. Felger, Hanser, Munich, Germany, 1986.

10. M. Asahina and M. Onozuka, Journal of Polymer Science, 1994, A2, 3505. 11. K.B. Abbas and E.M. Sorvik, Journal of Applied Polymer Science, 1975, 19, 2991. 12. Z. Vymazal, E. Czako, K. Volka and J. Stepek, European Polymer Journal, 1985, 16, 149. 13. V. Wigotsky, Plastics Engineering, 1987, 43, 2, 21.

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Update on Troubleshooting the PVC Extrusion Process

14. J. Wypych in Polymer Science Library 3, Eds., A.D. Jenkins, Elsevier, Amsterdam, The Netherlands, 1985. 15. D. Braun in Developments in Polymer Degradation 3, Ed., N Grassie, Applied Science Publishers, London, UK, 1981. 16. T. Kelen in Polymer Degradation, Van Nostrand Reinhold Company, New York, NY, USA, 1983. 17. K.S. Minsker, S.V. Kolesov and G.E. Zaikov in Degradation and Stabilisation of Vinyl Chloride-Based Polymers, Pergamon, Oxford, UK, 1998, p.313. 18. M. Minagawa, Polymer Degradation and Stability, 1989, 25, 121. 19. R.D. Dworkin, Journal of Vinyl and Additive Technology, 1989, 11, 1, 15. 20. R. Benavides, M. Edge, N.S. Allen and M.M. Tellez, Journal of Applied Polymer Science, 1998, 68, 11. 21. F.E. Okieimen and C.E. Sogbaike, European Polymer Journal, 1996, 32, 12, 1457. 22. T. Iida, J. Kawato, K. Maruyama and K. Goto, Journal of Applied Polymer Science, 1987, 34, 2355. 23. D.F. Anderson and D.A. McKenzie, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1970, 1, 18, 2905. 24. D. Braun, Die Makromolekulare Chemie, Macromolecular Symposia, 1992, 57, 265. 25. M. Bartholin, N. Bensemra, T.V. Hoang and A. Guyot, Polymer Bulletin, 1990, 23, 425.

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26. G.Y. Levai, G.Y. Oeskey and Z.S. Nyitrai, Polymer Degradation and Stability, 1994, 43, 159. 27. N. Bensemra, T.V.Hoang and A. Guyot, Polymer Degradation and Stability, 1990, 29, 175. 28. H.I. Gokcel, O. Kose and J. Kokturk, European Polymer Journal, 1999, 35, 1501. 29. H.M. Zhu, X.G. Jiang, J.H. Yan, Y. Chi and K.F. Cen, Journal of Analytical and Applied Pyrolysis, 2008, 82, 1, 1. 30. P. Baker and R.F. Grossman in the Proceedings of the Vinyl Retec ‘94: PVC - Building Opportunities, Pittsburgh, PA, USA, 1994, Paper.13, p.10. 31. Modern Plastics International, 1992, 22, 9, 55. 32. J. Wypych, Journal of Applied Polymer Science, 1975, 19, 3387. 33. N. Bensemra, V.H. Tran, A. Guyot, M. Gay and L. Carette, Polymer Degradation and Stability, 1989, 24, 89. 34. N. Bensemra, V.H. Tran and A. Guyot, Polymer Degradation and Stability, 1990, 29, 175. 35. F.E. Okieimen and J.E. Ebhoaye, Die Angewandte Makromolekulare Chemie, 1993, 206, 11. 36. J. Stepek and H. Daoustm in Additives for Plastics, Springer, New York, NY, USA. 1983. 37. S. Kodali, W. Hood, T. Jennings and M. Fender, Polymer Preprints, Chicago, IL, USA, 2001, 42, 2. 38. M.T. Benaniba, N. Belhaneche-Bensemra and G. Gelbard, Polymer Degradation and Stability, 2001, 74, 501.

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Update on Troubleshooting the PVC Extrusion Process

39. X. Ruijianm Polymer Degradation and Stability, 1990, 28, 323. 40. C. Garrigues, A. Guyot and V.H. Tran, Polymer Degradation and Stability, 1994, 43, 299. 41. J. Wypych in Polyvinyl Chloride Stabilisation, Elsevier, Amsterdam, The Netherlands, 1986, p.87. 42. Encyclopedia of Polymer Science and Technology, Volume 12, Eds., H.F. Mark, N.G. Gaylord and N.M. Bikales, Wiley, New York, NY, USA, 1970, p.725. 43. V.H. Tran, T.P. Nguyen and P. Molinie, Polymer Degradation and Stability, 1996, 53, 279. 44. J. Wypych in PVC Stabilisation, Elsevier, New York, NY, USA, 1986. p.224. 45. X. Rujian, Z. Dafei and Z. Seren, Polymer Degradation and Stability, 1989, 27, 203. 46. A.A. Yassin, M.W. Sabaa and N.A. Mohamed, Polymer Degradation and Stability, 1985, 13, 255. 47. G.Y. Levai, G.Y. Ocskay and Z.S. Nyitrai, Polymer Degradation and Stability, 1989, 26, 11. 48. R.F. Grossman, Journal of Vinyl Technology, 1990, 12, 34. 49. A.H. Frye and R.W. Horst, Journal of Polymer Science, 1959, 40, 419. 50. M.H. Fish and R. Bacaloglu, Journal of Vinyl Technology, 1999, 5, 4, 205. 51. I.J. González-Ortiz, M. Arellano, M.J. Sánchez-Peña, E. Mendizábal, Polymer Degradation and Stability, 2006, 91, 4-6, 2715.

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52. M.R. Jakupca, M.E. Harr and D.R. Stevenson in the Proceedings of the Vinyltec, Huron, OH, SPE, Brookfield, CT, USA, 2003, Paper No.17, p.5. 53. B.S. Galle, Y.S. Soin, Y.V. Ovchinnikov, E.A. Sereda and G.A. Pishin, International Polymer Science and Technology, 1993, 20, 11, p.T/76. 54. J.A. Falter and K.S. Geick, Journal of Vinyl Technology, 1994, 16, 2, 112. 55. C.B. Bucknall in Toughened Plastics, Applied Science, London, UK, 1977. 56. C.B. Bucknall, Journal of Elastomers and Plastics, 1982, 12, 204. 57. A. Bos and S.R. Tan in the Proceedings of the PVC Pipes— Current Status and New Developments, Brighton, UK, 1996. 58. R. Colvin, Modern Plastics International, 2003, 33, 9, 40. 59. B.G. Sampat, Chemical Weekly, LII, 2007, p.37. 60. N. Burgos and A. Jiménez, Polymer Degradation and Stability, 2009, 94, 1473. 61. W.J. Eldridge, Journal of Vinyl Technology, 1994, 16, 1, 26. 62. A. Datta and D.G. Baird, International Polymer Processing, 1991, 6, 3, 199. 63. I. Nanu and R.F. Pape, Materiale Plastice, 1979, 16, 4, 218. 64. British Plastics and Rubber, November 2003, p.4. 65. K. Wieduwilt and P. Schimmel, Plaste und Kautschuk, 1974, 21, 8, 602.

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Update on Troubleshooting the PVC Extrusion Process

66. W.S. Bryant and H.E. Wiebking in the Proceedings of the 60th ANTEC SPE Annual Technical Conference, San Francisco, CA, SPE, Brookfield, CT, USA, 2002, Paper No.571. 67. S. Zhu, Y. Zhang and C. Zhang, Polymer Testing, 2003, 22, 5, 539. 68. J.W. Maisel, Journal of Vinyl Technology, 1986, 8, 3, 112. 69. J.E. Hartitz and R.A. Yount, Polymer Engineering and Science, 1978, 18, 7, 549. 70. J.V. Hartman, R.R. Kozlowski and T. Podnar, Jr., Journal of Cellular Plastics, 1966, 2, 214. 71. S.J. Monte and G. Sugerman, Plastics Compounding, 1989, 12, 7, 59. 72.

I. Souma, K. Nohara, Nippon Gomu Kyokaishi, 1980, 53, 7, 443.

73. A. Gonzalez, Revista de Plasticos Modernos, 1984, 47, 334, 394. 74. R. Honemeyer, Kunststoffberater, 1983, 28, 3, 23. 75. M. Grosse-Aschhoff in the Proceedings of the PVC 1999 Conference, Brighton, UK, 1999, p.302.

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4

Poly(Vinyl Chloride) Extrusion

The extrusion process was introduced towards the end of eighteenth century mainly to produce lead pipes. The extrusion process is currently used in poly(vinyl chloride) (PVC) production and has gained widespread application. It is used to produce pipes for drinking-water supply, profiles, and door frames. PVC has very high thermal conductivity and melts very slowly. The melting of PVC is dependent upon thermo-mechanical and rheological properties [1]. With improved mechanical and chemical properties PVC products can result in improvement in high-speed extrusion techniques. PVC extrusion involves forcing material into a screw-enclosed container to manufacture materials with different cross-sectional areas and dimensions. The cross-sectional area of the extruded product will conform to that of the die opening [2, 3]. PVC extrusion has become an increasingly important operation in the processing industry. Enormous growth is occurring in the PVC arena in the amount and type that is extruded and in the number of extrusion machines in operation. Current challenges are related to output rate and achieving longer run times by reducing plate out [4].

4.1 Basic Requirement of Poly (Vinyl Chloride) Compounds PVC is compatible with many additives such as plasticisers, impact modifiers, and heat stabilisers. Its mechanical properties can be

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Update on Troubleshooting the PVC Extrusion Process

modulated from rigid to flexible end products. PVC can be used to produce relatively low-cost materials [5]. The processing of PVC pipe and profiles start from blending of raw materials to semi-finished products passing through extrusion machines.

4.1.1 Machine The PVC compound to be extruded should have: • Constant material quality • Low plate-out in the die and calibrator • Low wear of processing unit, die and calibrator • Low material fluctuations

4.1.2 Processing PVC compounds in extrusion technology should provide: • High-quality products (even with lean formulations) • Higher output with better performance • A large processing window • Less wear

4.3 Extrusion Single- or twin-screw extrusion involves material conveying, melting, mixing, venting, and homogenising. Product quality is considerably affected by the extrusion technique. Mixing and homogenising in twin-screw extrusion is good, and end products have better quality than in single-screw extrusion.

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Poly(Vinyl Chloride) Extrusion

In the solid bed of resin based compounds, no grain destruction occurs from the original grain boundaries. After considerable intragrain fusion, both the solid bed of resin compounds and melt pool with development of elasticity at a high rate [6]. A good manufacturing process in PVC processing is based on: • Material flow rate inside the extruder • Output rate • Economical cost • Quality of end products

4.3.1 Extruder: Operating Conditions The extruder must have: • Good conduction and appropriate residence time for processing of the PVC formulation in the compression zone • Effective cooling of the screw in the metering zone • Low wear • Homogenous melt in the metering zone In the extrusion process, heat energy is usually supplied in the area between the feed opening and the compression zone. With the exception of shear energy, this heat energy is transferred from the barrel and screw into the material. Heat must be removed from the melt in the metering zone. The quantity of heat removed is directly dependent upon the angle sine of the screw bore. In the compression zone, the maximum pressure is before the vent, hence the greater wear in the compression zone than in the metering zone. The pressure in the metering zone is very much dependent

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Update on Troubleshooting the PVC Extrusion Process

upon the die connected to it. In the compression zone, apart from the contact pressures of the screw on the barrel, chemical factors are also present which also contribute to the wear. An extruder consists of a threaded shaft or screw rotating in a fixed cylinder or barrel in addition to a feed port and die. The screw cylinder is constantly changing its position relative to the barrel. Advance of the melt through the barrel of the extruder follows a helical path which is a mirror image of the helix on the screw. The operating conditions of an extruder are dependent upon: • Screw design • Process parameters (e.g., output, vacuum) • Melt temperature • Reproducible adjustment of the die and calibrator • Appropriate reading of the melt temperature and melt pressure • Constant properties of cooling water

4.3.2 Screw In PVC processing, the dry blend is forced through a plasticating extruder to get the desired product. The screw is responsible for the melting, homogenisation, and pumping of the material. The screw is key factor for a good processing operation. Screw design is the main factor in processing to achieve better-quality products or higher outputs. The screw should provide benefits to the material such as: • Stability under pressure • Good homogenisation

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Poly(Vinyl Chloride) Extrusion

• Long residence time • Uniform heating of the PVC compound • Large surface for easy processing • Wear protection coating • Controlled and optimum plasticising to allow high throughput • Low melt temperature • Good homogeneity through defined shear • Wide processing window To better understand extrusion requires knowledge of: • Heat transfer • Movement of solids in the feed section of the screw • Movement of solids before initial melting • Behaviour of molten polymers under high shear stresses for short periods of time [7] The thin film formed between the solid bed and the metal surface during extrusion generates a large amount of heat due to shearing. With increased screw speed, melting occurs primarily by the heat generated inside the melt film. However, the barrel usually removes the excess heat. PVC melts fast with higher heat generation with high viscosity.

4.3.3 Processing Technology In an extruder, the PVC compound (commonly known as the ‘dry blend’) enters at the feed zone and advances along the helical channel of the screw. The heater provides heat to the barrel. Mechanical

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Update on Troubleshooting the PVC Extrusion Process

energy input from the motor drive to the screw appears as heat and pressure energy. This heat and pressure energy melts the material. The pressure that has been built up forces the melt through the die. During processing, melting occurs primarily during heat generation. The melt film forms in dissipative melting. The melting rate and shear stress is expected to decrease the machine load with decreasing viscosity. Shear stress is dependent upon the viscosity in the PVC melt. The melting rate is dependent upon the formulation, compounding, screw speed and viscosity during processing. In PVC, viscosity and velocity in the melt are dependent upon the shear and temperature in a complex way.

4.3.4 Processing Temperature Melt temperature profiles influence the rheological properties of PVC. The quality of the final product is based on rheological properties. PVC at high temperature may lead to degradation or undesirable side reactions. Processing of PVC requires a narrow temperature profile and heat-stabilised compounds during processing [8]. The thermal stability of PVC is purely dependent upon temperature and residence time in an extruder. PVC compounds usually require a melt temperature of 190–195 °C. The melt temperature is needed to develop excellent physical properties in PVC products. Streamlined equipment provides a long run without interrupting the process. A small increase in temperature will provide better output and melt uniformity. Higher processing temperature without appropriate stability and lubricants can lead to earlier degradation.

4.3.5 Power Consumption Calculation of power consumption is very important in PVC extrusion. The PVC compound is fed as powder or granules. Most of the powder is consumed during shearing action of the screw and

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Poly(Vinyl Chloride) Extrusion

barrel to overcome the resistance of the viscous material during processing. Due to frictional heat, the temperature rise in the material is dependent upon the radial clearance between the screw and barrel. Shear heating and heat conduction have considerable effects on the change in melt temperature during the flow. Heat conduction is relatively greater around the barrel wall, and shear heating is relatively greater around the duct centre [9]. The processing temperature of the material has a large effect on the radial clearance. Too large a clearance leads to increased leakage and hence reduces the output and efficiency of the extruder.

4.4 Extrusion PVC extrusion involves: • Shaping the PVC compound by forcing it through a die • Using raw material in pellet form in case of single-screw extrusion and in the form of powder dry blend for twin-screw extrusion • A screw to convey and pack the solids • Mechanical work by rotation and external heating to melt the material • Pumping molten material from the barrel and passing it through a die The die is designed to give the desired cross-sectional shape. The extrudate cools and solidifies after leaving the die through the calibration system. The melt-string (extrudate) leaves the calibration system and cooling process. After leaving cooling system, the product is fed into a take up mechanism which is used to drawdown the product with applied pressure.

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Update on Troubleshooting the PVC Extrusion Process

4.4.1 Single-screw Extruder Compaction of the solid bed of PVC occurs during its feed in a single-screw extruder. This solid bed rotates with the screw. The PVC melts while rubbing on the hot barrel [10, 11]. Within the solid bed and the barrel, a thin melt film is formed. From the melt film, heat is generated. This melting mechanism is known as ‘dissipative mixing’. A single-screw extruder has traditionally been used due to its versatility and economical cost. It is possible to utilise the same machine to manufacture a wide range of sheets with different thicknesses if different cross-sections of die are used. In a single-screw extruder, there is no venting zone. Hence, entrapment of air and moisture during feeding can occur, and the problems can be replicated in the end product. Powdered material needs a special handling technique. Pelletised or powder compounds can be used with two-stage screws with an open vent or applied vacuum at the vent. However, two-stage screws require a careful balance of output rates, dies and formulations. A crammer feed is recommended to assure uniform feed of the powder. In single-screw extrusion, with lower extruder capacity, it is difficult to fill wide, thick dies during processing to maintain velocity and low melt temperature to prevent early degradation in the die. Even at the recommended processing temperature, rigid PVC typically has higher viscosity. Hence, it is important in processing to provide screw cooling and efficient barrel cooling. Generation of heat energy is lower with low viscosity. Low viscosity requires lower shear stress. Melting decreases with decreasing viscosity. PVC formulations behave as wall-slipping materials during fusion in a single-screw extruder. The melt collects near the passive flight of the screw. The internal fusion of grains and breakdown of the fused material is dependent upon the location of the material around the screw [12]. These challenges are related to increasing the output rate and achieving longer run times with reduction of plate-out.

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Poly(Vinyl Chloride) Extrusion

4.4.1.1 Advantages/Disadvantages The major advantages and disadvantages of single screw extruder are mentioned below. Advantages: • Low initial and maintenance costs • Better to run with granules or pellets • Size of the granules or regrind of PVC is not limited Disadvantages: • Lower output rates • Better formulation required with high heat stability

4.4.2 Twin-screw Extrusion Twin-screw extruders are often used in PVC extrusion due to their efficiency and degree of mixing during processing. The final products are dependent upon the efficiency of the extruder [13]. Even small changes in melt temperature affect the processing in twin-screw extrusion during processing [14]. In a twin-screw extruder, uniform heating of materials provide the basic need for a homogenous extrudate. An increased discharge zone provides higher stability under pressure. Increased residence time improves melt homogeneity. Twin screws rotating in opposite directions cause a pressure buildup in the C-shaped chamber below the screws. This pressure pushes the extruder screw against the barrel wall, resulting in frictional wear on screws and barrels. During processing, the PVC compound melts, hence the crystallites melt. Once the crystallites melt, the grains fuse together with generation of new crystallites. They form in the unoccupied grain boundaries, and hence the elasticity of the material is increased [15].

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Update on Troubleshooting the PVC Extrusion Process

In PVC extrusion, the vapours and gases produced during the process are vented. Initially the melting process starts and then vents gases and vapours. Finally, pressure is developed. Venting is done in a de-volatilisation section of the second stage through a port in the barrel to a low pressure or vacuum. Between the vent and extruder exit, pumping must develop pressure according to the load at the extruder exit. In twin screw extrusion, the melt transportation and accumulation of the melt build the pressure in the form of vacuum or low pressure in the de-volatilisation zone in terms of the required pressure at the exit. Twin-screw extruders run well with powdered compounds. Even with higher output rates, they deliver melt uniformity with low melt temperature. The uniformity and low melt temperature is due to the lower shear rate of the screws. PVC formulations require better lubrication due to the lubrication needs of the screw and die. The deeper channels of the screws are susceptible to backflow. Once shear heating increases, the output decreases faster with back pressure. Twin-screw extruders are more complex than single-screw extruders. Two types of twin-screw extruders are in use: parallel screws and conical screws. In conical twin screw extrusion, the screw needs increased oil temperature with low barrel temperatures. Parallelscrew extruders require a higher feed zone temperature. With rigid PVC (RPVC), the size limits heat conduction and the hardness forces the screws out against the barrel walls and causes higher barrel wear.

4.4.2.1 Advantages The major advantages and disadvantages of twin screw extruder are mentioned below. Advantages: • Better to run with powder compounds • High-volume applications such as PVC pipes and fitting

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Poly(Vinyl Chloride) Extrusion

• High output rates • Requires lower melt temperature because of the lower shear rates in the screws • Deliver melt uniformity even at higher output rates Disadvantages: • Higher initial and maintenance costs • Use of granules or regrind of rigid PVC cause higher barrel wear • Size of the granules or regrind of PVC is limited

4.4.3 Die Design In PVC processing, sudden changes in melt flow in the adaptor and die cause changes in the melt stream direction. There may be a chance of material hang-up and, ultimately, degradation. A flexible lip is used to adjust the minor imbalances in flow. The flow channel of the adaptor and die should be chrome-plated. The material spreads in the flow channel. The dimensions are dependent upon the PVC formulation, melt temperature, output rate, die width, and thickness of the end product. Inappropriate heaters or thermocouples can cause flow imbalance, and lead to earlier degradation. The material can leak from inappropriate seal surfaces of the adaptor and die. The leaked material may stagnate and degrade. The degraded material can cause corrosion and damage the die. The die is the significant determining factor in product quality. It takes the melt from the extruder and delivers it under the same conditions of temperature and pressure through the entire cross-section. It is critically important that the pipe die used is optimised to the relevant demands. From this arises two basic requirements for the die: 1. It must be suited to the specific characteristics of the material to be processed 59

Update on Troubleshooting the PVC Extrusion Process

2. It must be correctly designed with respect to the throughput and product size The die must be continuously optimised to meet the demands of higher outputs. High performance is utilised to optimise the rheological behaviour of the material.

4.4 Calibration System The calibration system is commonly known as the ‘sizer’, ‘vacuum die’ or ‘calibrator’. The purpose of the calibration system is to shape the product into the finished contour. Vacuum calibrators form the shape and cool the product. They are designed to dissipate the heat in the critical section of the calibration unit. Frictional forces are involved in the normal range. Extrusion speeds decide the wear in the calibration system. The surface quality of the product is predominantly based on the PVC formulation

4.5 Poly (Vinyl Chloride) Pipe Extrusion Processing of PVC pipes or profiles starts from the blending of raw materials to semi-finished products passing through the extruder. PVC processing is usually carried out at 170–200 °C [16]. The extrudate from the pipe or profile extrusion is twisted into small sizes as it is pulled through the cooling tanks. A piece of start-up pipe is used on larger sizes instead of stringing by hand. A PVC pipe extrusion line is illustrated in Figure 4.1.

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Hopper Barrel Die Calibrator Water spray Die head Cooling tank

Motor

Pipe stack

Figure 4.1 PVC pipe extrusion

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Controllers

Cutter

Update on Troubleshooting the PVC Extrusion Process

4.5.1 Pipe Dies Extruder accessories are described in the following sections. • Depend on the polymer to be processed • Properties of the melt such as clinging to, or sliding over, the wall • Thermal stability Spider dies are recommended for PVC pipe extrusion. The die consists of the material inlet, mandrel support and die gap as the functional zones. The precision of extrusion is very high. Extrusion downstream equipments consist of crosshead annular die, cooling trough, and take-off. The linear velocity at which the product is pulled during the extrusion process is dependent upon its dimension and extrusion conditions. The basis for the design of extrusion dies is the system for single-layer pipe extrusion. The important parts of a die are: • Spider • Lattice basket • Spiral mandrel

4.5.2 Cooling System Screw cooling is an important control system in the processing of pipes and profiles. Two methods of cooling systems are in use depending upon the machine. Screw cooling in a closed system in which water is filled. Due to various friction characteristics (or screw length), water turns into steam. The conversion of water to steam occurs in the screw due to heat supplied from the intake zone. It is cooled and frictional heat is removed mostly in the compression and metering zones. Screws with

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water cooling are maintenance-free and show low efficiency in terms of output and wear. The wear occurs between the screw and barrel. External control of the screw by means of pumping heated oil through the screw zone to remove heat from the metering zone and supply it on return to the intake zone of the extruder. External cooling systems require more maintenance and care. Frictional heat is reduced during processing, when the material is exposed to more oil cooling.

4.6 Poly (Vinyl Chloride) Profile Extrusion Profile extrusion is more difficult than any other type of processing. With material use, the design of the profile uses maximum possible haul-off speed. In profile systems, the dies are longer in length. Hence, the length of the parallel outlet zones is increased to avoid the memory effect. The inlet channel in the die is wider and reduces continuously in cross-section towards the outlet. Figure 4.2 illustrates a PVC profile extrusion line.

4.6.1 Calibrator For a high-speed extrusion, calibrators made from brass are favourable. Stainless-steel calibrators can be used for high haul-off speeds. Intensive cooling is required in profile extrusion. The number of vacuum slots used is dependent upon the cooling requirements. For a high-performance profile extrusion, a short calibrator with a long vacuum tank with exchangeable calibrating plates can be used. The cooling inside the tank runs as spray tanks or circulating water tanks. Both types of water spray system affect the profile. However, the cooling length is dependent upon the profile design. Calibration tables are different and are dependent upon the tool systems.

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Barrel Die Die head Calibration table

Motor

Water spray Cooling tank

Controllers

Haul-off Cutter

Profile stack

Figure 4.2 PVC profile extrusion

Update on Troubleshooting the PVC Extrusion Process

64 Hopper

Poly(Vinyl Chloride) Extrusion

4.6.2 Cooling System For the appropriate design of cooling equipment and its dimensions in profile extrusion, knowing the temperature fields inside the profile can be very useful. The profile cooling during extrusion is dependent upon estimation of the heat transfer coefficient. There is a direct relationship between the temperature profile and profile deformation. The knowledge gained from simple profiles can be used to better understand the behaviour of the material during the cooling process.

4.7 Poly (Vinyl Chloride) Sheet Extrusion In sheet extrusion, polished stack rolls with chromed steel are used. Embossed rolls with chromed steel, rubber or silicone may be used to impart patterns onto the PVC sheet. The extrudate is initially introduced in the nip roll between the centre and top roll. The meltstring follows the center roll is then allowed to flow in the bottom roll and finally to the back up roll for cooling purpose. The sheet is allowed to cool so, to release internal stresses, the sheet-to-puller speed is adjusted. Figure 4.3 illustrates the complete line of PVC sheet production. Maintaining the uniformity of the thickness of the sheet is very important. Sheet thickness is controlled by adjusting the flow from the die or adjusting the gap between the rolls. The temperature of the rolls must be balanced to have the best surface and fast cooling. Differences in roll temperature lead to warping or curling as well as residual stress. The roll temperatures used are 50–90 °C. In PVC processing, screen packs are recommended. Screen packs eliminate contamination and prevent ‘dip lips’, sticking, surface imperfections or gel formation. Screen packs increase back pressure, thereby improving melt mixing. With melt uniformity, the output rate can often be increased. Screen packs usually have shorter run lengths due to contamination accumulation (particularly in regrinds).

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Barrel Motor

Coat hanger Calender die Mell rolls temperature

Cooling zone

Sheet cutter

Die head

+ + Controllers Sheets

Figure 4.3 PVC Sheet extrusion

Update on Troubleshooting the PVC Extrusion Process

66 Hopper

Poly(Vinyl Chloride) Extrusion

4.8 Poly (Vinyl Chloride) Films The film-blowing process is an economic method for manufacturing plastic films. In this process, films are thin and oriented. Film orientation can alter film strength. PVC films are used in various applications in the packaging industry. Rubbish bags, carrier bags, and thin sheets are often manufactured using the film-blowing process.

Take off Winder Bubble Hopper

Air

Barrel Motor

Die head

Controllers

Figure 4.4 PVC film plant

In this process, PVC material is extruded from an annular die. It is inflated along the circumferential direction and simultaneously stretched along the axial direction. Inflation of bubbles is actuated by forcing air into the bubble. The volume is increased and stretched along the axial direction. Stretching is achieved by controlling the

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velocity of the nip rollers and the mass flow rate at the die. A bubble is formed between the die, and nip rollers are supported by an additional structure on which the guide rollers and nip rollers are mounted. A complete line of a PVC film plant is illustrated in Figure 4.4.

4.9 Regrind Regrind can be used and has finite thermal stability. Every time during processing, the material’s life is used from the earlier thermal stability of material. With little or no problems, regrind can be used twice of thrice. It may be possible to utilise 100% regrind. To a certain extent, regrind can be mixed with virgin PVC at a consistent rate. Normal thermoplastics cling to the wall, and plasticising takes place directly on the cylinder wall under the influence of feed pressure and the shear rate. Melt collects in a melt reservoir in front of the leading edge, whereas the remaining solid part forms a bed of solid material behind the trailing edge. The melt reservoir increases in size towards the front of the screw, whereas the proportion of solid material decreases. The plasticising capacity of a screw varies in proportion to the width of the solid bed. Hence, with constant flight depth, it decreases along the extruder channel. This effect can be reduced by decreasing the flight depth because the height of the solid bed decreases faster than its width. If the decrease in flight depth is too rapid or the screw plasticising capacity is too low, the solid bed may break up. The solid-particle mechanism is therefore no longer effective. The solid parts can now be melted only by heat conduction from the melt. With conventional screws it is therefore possible that nonmasticated particles can reach the screw tip. To prevent this action, lots of shearing and mixing elements are placed before the screw tip to maintain the remaining plasticising and homogenising actions.

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References 1.

S.H. Kuo and C.I. Chung, Polymer Engineering and Science, 1989, 29, 7, 448.

2.

S. Kalpakjian and S. Schmid in Manufacturing Engineering and Technology, 4th Edition, Ed., S. Kalpakjian, Prentice Hall, Upper Saddle River, NJ, USA, 2001.

3.

L. Donati and L. Tomesani, Journal of Materials Processing Technology, 2004, 153/154, 366.

4.

R.W. Decker and J.A. Falter in the Proceedings of the SPE Conference – Vinyltec ‘99, Ontario, Canada, 1999, p.48.

5.

W. Huber, B. Grasl-Kraupp and R. Schulte-Hermann, Critical Reviews in Toxicology, 1996, 26, 4, 365.

6.

T.E. Fahey, Journal of Macromolecular Science B: Physics, 1981, 20, 3, 415.

7.

C.H. Jepson, Industrial and Engineering Chemistry, 1953, 45, 5, 992.

8.

C. Miaw, A. Hasson and G. Balch in the Proceedings of the 43rd SPE ANTEC Conference, Washington, DC, USA, 1985, p.76.

9.

N. Sombatsompop and M. Panapoy, Journal of Materials Science, 2000, 35, 24, 6131.

10. N. Sombatsompop and A.K. Wood in the Proceedings of the 56th SPE ANTEC Conference, Atlanta, GA, USA, 1998, p.482. 11. C.I. Chung in Extrusion of Polymers: Theory and Practice, Hanser Publishers, Munich, Germany, 2000.

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12. J.A. Covas and M. Gilbert, Polymer Engineering and Science, 1992, 32, 11, 743. 13. T. Nietsch, P. Cassagnau and A. Michel, International Polymer Processing, 1997, 12, 4, 307. 14. N. Sombatsompop and M. Panapoy, Polymer Testing, 2001, 20, 2, 217. 15. S.H. Hookanson, D.L. Smith and J.L. Irvine, Journal of Vinyl Technology, 1986, 8, 1, 11. 16. B.D. Gupta and J. Verdu, Journal of Polymer Engineering, 1988, 8, 1, 1.

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Degradation and Stabilisation of Poly(Vinyl Chloride)

Several million tonnes of poly(vinyl chloride) (PVC) products produced every year worldwide are consumed for outdoor applications such as house siding panels, waste water tubes, and window profiles. PVC products have a lifetime of ~10 years. Internal factors such as the tacticity and morphology of PVC, stabilisers, lubricants, and impact modifiers affect the properties of such products. With respect to synthetic polymers, PVC is one of the major leading polymers [1]. However among thermoplastics, PVC requires elaborate protection from thermal degradation during processing. There is a big difference between academic and industrial work with respect to overcoming the problem of thermal degradation. In academic work, instrumental methods and established techniques have been used to study the degradation and stabilisation processes. The thermal and photochemical degradation of PVC has been studied for a long time [2–4].

5.1 Degradation of Poly (Vinyl Chloride) PVC during exposure to higher temperatures results in degradation of the material with an unacceptable change in color and a drastic change in its mechanical properties [5]. The main disadvantage of PVC is the rather limited thermal stability. This requires addition of heat stabilisers to prevent dehydrochlorination and discoloration during processing and application. This degradation and stabilisation of PVC [6–10] is due to various defect sites in the polymer chain.

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Thermal degradation of PVC is due to: • Structural defects • Temperature • Insufficient stabilising effect of heat stabilisers • Presence of impurities in PVC and the additives used during compounding and processing • Byproducts (mostly metal chlorides accumulated with PVC products) • Ultraviolet (UV) irradiation

5.2 Structural Defects Thermal degradation of PVC is, in general, considered to be initiated at unstable sites within the polymer [11–16] as well as defects generated during the polymerisation reaction [17]. Various defect sites in the polymer chain are thought to be responsible for the degradation [6, 9, 10, 17–19] such as unsaturated chain ends, headto-tail structures, oxidised structures and branches. PVC defects arise from processes that occur at high temperature in free-radical polymerisation such as backbiting, chain transfer to monomer/ polymer, and relatively high isotactic content [12, 20, 21]. Thermal degradation is considered to be a serious problem. PVC releases hydrogen chloride (HCl) even below its melting point and becomes discoloured [6, 9, 19, 20, 22, 23]. PVC must be formulated with heat stabilisers and lubricants so that it can be processed. Possible defect structures in PVC resin particles include: • Labile tertiary and allylic chlorine atoms [24–27] • Labile chlorine atoms in isotactic fragments [28–32]

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Degradation and Stabilisation of Poly(Vinyl Chloride)

• Tertiary hydrogen atom- and chlorine atom-associated branches [33] • Terminal end groups such as double bonds [34] • Oxygen-containing groups [33–35] • Peroxide residues [33, 36] • Head-to-head structures during PVC manufacture [33–36] Labile chlorine atoms may initiate the dehydrochlorination of more stable groups. The steric order of monomer units in the polymer chain (tacticity) may have some influence on degradation.

5.3 Tacticity In PVC, tacticity may have some influence on degradation [37–39]. Defects such as backbiting, chain transfer to monomer/polymers, and relatively high isotactic content occurs during processing. Therefore, it is essential to stabilise PVC against degradation during its processing and it use at high temperatures.

5.4 Dehydrochlorination Low stability is an inherent property of PVC. The main indicator of such degradation is the elimination of HCl, which is followed by coloration of the resin. Dehydrochlorination is the release of HCl during PVC processing at high temperatures. In certain formulations, sudden blackening occurs due to subsequent dehydrochlorination [21, 38]. It is known that this polymer undergoes severe degradation via zip-elimination of HCl at relatively low temperatures [39–41]. Degraded PVC is characterised by the development of intense discoloration resulting from the formation of conjugated polyene structures [42–44].

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Chemical reactions from PVC production result in the formation of carotinoid polyenes from the intrinsic defects by elimination of hydrochloride and their oxidation via allyl peroxy radicals, thereby breaking C-C bonds [45–48]. During the evolution of HCl, doublebond sequences are formed in the polymer chains. Such polyenes are known to be responsible for the coloration of the resin, and supposed to be a labile site for oxidation. It has been shown that polyene sequences are formed at a very low percentage of dehydrochlorination [49] and increase only their concentration (not their length) with degradation time unless a different formulation or conditions are used. The appearance of long polyene sequences in the first steps of dehydrochlorination [50] seems to be stabilised against the action of oxygen (at least in the presence of UV radiation). Zipper dehydrochlorination and the generation of polyene sequences occur in the polymer chains after degradation of the polymer at high temperatures [51]. The polyenes are highly reactive and form crosslinked, long polyene sequences that are discoloured (or have an undesirable color in the material) [19–21, 54–57]. The results of such reactions may lead to significant change in mechanical behaviour and colour formation. Dehydrochlorinated PVC contains blocks of conjugated double bonds [56]. During thermal degradation, polyenes are known to be responsible and a labile site for oxidation. Oxidation reactions are recognised by the growth of infrared (IR) absorption bands for >C=O at 1720 cm–1. Ultraviolet–visible (UV–VIS) spectroscopy shows polyenes resulting from elimination reactions absorbing below a wavelength of 600–700 nm [57].

5.5 Stabilisers Processing without stabilisers and lubricants leads to PVC degradation. The degradation occurs with liberation of HCl due to the cracking of PVC. Even processing PVC with an insufficient quantity of stabiliser in the formulation leads to degradation. This

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results in colour changes from white to yellow, from yellow to brown, and finally to black, which leads to disaster. Not only an insufficient quantity of stabiliser but also an insufficient quantity of lubricant results in flow hindrance in the processing equipment, and leads to colour changes. However, irrespective of the heat stabilising capacity of the heat stabiliser, the efficiencies suffer from the deleterious effect of the byproducts produced mostly from many metal chlorides accumulated during the reaction of the heat stabilisers with PVC material. These metal chlorides are considered to be strong catalysts for the subsequent dehydrochlorination process. They are also responsible for the sudden blackening of certain formulations, and may present a serious environmental problem [37–39]. This has recently, led to the extensive use of organic stabilisers for the thermal stabilisation of PVC [57–59].

5.6 Impurities Various aspects of degradation in relation to PVC stabilisation must be understood. Contaminants present in the PVC material and in the additives act as powerful promoters of degradation and cause considerable distortion. Hence, the effectiveness of the stabiliser system is significantly decreased due to such unknown contamination [37].

5.7 Photochemical Degradation External factors such as solar UV radiation, moisture, temperature, and pollutants promote the degradation of properties [60, 61]. These factors affect the structure of PVC and the quantities of the additives present within it. Products are slowly degraded under the influence of the environment. This is indicated by yellowing (or other discoloration) or the loss of brightness with filler or pigments in PVC, progressive destruction of the surface layer with gloss deterioration,

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and loss of mechanical strength. PVC in outdoor uses requires protection from mechanical deterioration, as well as attack by heat and light (normal and UV). Out of the variety of PVC compounds, transparent rigid material is the best for long service [62]. PVC weathering causes a photochemical degradation that produces chain rupture, formation of oxidised structures, and HCl generation [15, 63, 64]. These chemical changes subsequently produce a loss in mechanical properties and formation of a yellow colour due to polyene propagation. UV stabilisers only slow down these reactions and do not stop degradation. However, these products degrade slowly under environmental influences as indicated by yellowing (or other discoloration or loss of brightness with fillers and colour pigments in PVC) and progressive destruction of surface layers (deteriorating gloss and eventually decrease in mechanical strength). Hence, much work has been devoted to developing an understanding of and to control PVC degradation. Accordingly, PVC may degrade thermally as well as by photochemical means. Solar irradiation (UV light and heating effect) and oxygen from the atmosphere may trigger both mechanisms. In contrast to other polymeric materials, moisture is not an important factor. The underlying chemical reactions are the formation of carotinoid polyenes from intrinsic defects by the elimination of HCl, and their oxidation (via allyl peroxy radicals), leading to the breaking of residual C-C bonds [46–49].

5.8 Mechanism of Degradation Defects at high temperature that occur in free-radical polymerisation are backbiting, chain transfer to monomer and polymers, and self-initiated chain reactions; relatively high isotactic content is also important. The mechanisms that underlie degradation are incompletely understood. The type of reaction is also dependent upon the conditions (e.g., temperature, presence of oxygen) during the decomposition. The main labile sites for dehydrochlorination

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are the allylic and tertiary chlorines. Radical, ionic and molecular mechanisms have been proposed [65], including a mechanism consisting of an initiation reaction, cis–trans isomerisations, and 1,3 rearrangements and propagation [66-67]. PVC may degrade by thermal as well as photochemical reactions. The effect of UV light and the heating effect from solar irradiation and oxygen from the atmosphere may trigger both mechanisms. Moisture is not an important factor with PVC. In principle, the chemical reactions are the formation of carotinoid polyenes from intrinsic defects by the elimination of HCl and their oxidation via allyl peroxy radicals to break residual C-C bonds [46–49]. The thermal degradation mechanism is similar to the photochemical degradation mechanism. However, photodegradation generates more HCl molecules [47, 64]. The elimination of HCl at relatively low temperatures (~100 °C) or under the influence of light is one of the fundamental aspects of PVC decomposition. Figure 5.1 illustrates the first stage: this reaction leads to the formation of double bonds followed by a rapid ‘zipper-like’ splitting off of further HCl molecules to give polyene sequences. These sequences (with an average length of 6–14 conjugated double bonds) cause the polymer to turn yellow, brown, and eventually black [2–4].

CI CI CI

CI CI - HCI

CI

CI

- HCI n

Figure 5.1 PVC dehydrochlorination (schematic). Reproduced with permission from D. Braun, Progress in Polymer Science, 2002, 27, 2171. ©2002, Elsevier [57]

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Elimination reactions can be followed by UV–VIS spectroscopy because the resulting polyenes absorb below a wavelength of 600–700 nm, and oxidation reactions are recognised by the growth of IR absorption bands for >C=O at 1720 cm–1. Due to the slow diffusion of oxygen into PVC, oxidation products develop only in the upper 200 mm of a plate, whereas polyenes are generated up to 400 mm below [58]. These, however, act as intrinsic UV absorbers and shield the polymer bulk.

5.9 Poly(Vinyl Chloride) Stabilisation Stabilisation of PVC against thermal degradation is essential for processing and use at high temperatures. Thermal stabilisers intervene in the dehydrochlorination process and react with the double bonds created on the backbone chains as a result of HCl loss during degradation. Thermal stabilisers are known to function as PVC stabilisers by replacing labile atoms. The inhibitor suppresses the elimination of HCl. It interrupts the formation of a conjugated polyene sequence in the polymer [5]. The main function of heat stabilisers is to prevent degradation during processing. Thermal stabilisers replace labile chlorine atoms to enhance heat stability. Stabilisation of PVC involves substituting the labile chlorine atoms or a reaction with the HCl generated during processing [53, 68, 69]. Thermal stabilisers can block or retard thermal oxidative degradation. Moreover, the stabiliser may disrupt conjugated systems, thereby reducing polymer discoloration. Mercaptans are typical examples of these stabilisers [70]. A part of the stabiliser will be consumed during processing and sometimes during the application period. Hence, the efficiency of the stabiliser system is considerably reduced after compounding PVC with the necessary additives [70]. Thermal stabilisers can react with the evolved HCl gas to retard the deleterious catalytic action of the eliminated HCl [71, 72].

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Under the action of strong nucleophiles (e.g., thiols, thiolates), labile chlorines are replaced and the thermal stability of PVC increased. Another approach is to use a low temperature ‘living’ radical polymerisation technique that enables PVC to be almost free of such structural defects and provides thermal stability [73–76]. Control of the core-shell structure in PVC has been reported to improve thermal stability [75]. Stabilisation mechanisms are controversial because PVC is the plastic with probably the highest variety of stabilizers. Indeed, PVC cannot be used as a plastic material without stabilisers. Thermal stabilisers such as organometallic compounds and inorganic salts are particularly effective. Mercaptans are organic stabilisers [70]. Esters or mercaptides of dialkyltin [76–78] can exchange the labile chlorines in the backbone chains for more stable ester or mercaptide groups derived from the stabiliser. Moreover, quinone–tin polymers act as stabilisers through intervention in the radical process of degradation and through effective absorption of the degradation products [79]. Environmental problems have recently led to the extensive use of organic stabilisers for the thermal stabilisation of PVC [60, 61]. Mixtures of calcium–zinc carboxylates are one of the oldest systems used. They are becoming important again due to their lack of toxicity. Metal carboxylates are considered to be HCl scavengers and involve substitution of allylic chloride through an esterification reaction with PVC [80]. It is well known that the zinc carboxylate is the most active and that the calcium carboxylate acts mainly as an HCl scavenger. Calcium soap [81] reduces the rate of dehydrochlorination by avoiding the 1,3-rearrangements and controlling propagation of the HCl elimination reaction. Formation of a coordinated complex between both carboxylates was proposed in 1967 [81], and has been used regularly to explain certain types of unknown behaviour [80, 81]. The main function of metal soaps is the interaction with growing polyenes by the formation of a coordinated complex between both carboxylates [82–84].

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Using mixtures of zinc and calcium carboxylates reveals that calcium carboxylates act as ester exchangers with zinc chloride. A sufficient quantity of calcium stearate prevents the formation of zinc chloride [85–89]. Zinc carboxylates have are strong Lewis acids and scavenge HCl. They react with the allylic chlorine atom, resulting in the formation of zinc chloride, which promotes PVC dehydrochlorination [90, 91]. Pre-heated stearates have effects on material color during initial processing and after processing. They can be useful for industrial purposes when designing the formulation. They affect the formulation composition with respect to initial mechanical behaviour and release of HCl during post-processing. However, the additional cost of preheating can be justified upon achieving satisfactory movement on the thermal stability of formulations. Pre-heated calcium stearate mixed with zinc stearate leads to a reduction in heat stability. During processing, calcium stearate may have decomposed and further does not participate in the stabilisation and zinc stearate is the only heat stabilizer available for further heat stabilisation. Pre-heated zinc stearate alone shows significant changes in the thermal stability of PVC. However, if calcium and zinc stearates are mixed and pre-heated, a slight improvement in thermal stability occurs. Pre-heated zinc stearate mixed with calcium stearate shows moderate improvement in heat stabilisation. There is controversy in the literature about the effect of pre-heated stearates on the thermal stability of unplasticised PVC formulations, and there are no reports about such effects on industrial-grade plasticised PVC compounds [91]. Common thermal stabilisers in use for the stabilisation of PVC are usually basic lead salts [92], metallic soaps [93–96] and esters or mercaptides of dialkyltin [78, 80, 97]. The most important stabilisers of PVC are different metal soaps such as lead, cadmium, barium, calcium and zinc carboxylates and some di- and mono-alkyltin compounds (e.g., maleates, carboxylates, mercaptides) [80]. Several inorganic lead compounds and organic secondary stabilisers such as epoxides, polyols, phosphites, b-diketones, and

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Degradation and Stabilisation of Poly(Vinyl Chloride)

dihydropyridine are also used in industrial recipes [94, 98–103]. They act as acceptors for the liberated HCl [99, 104] and retardants for the appearance of discoloration [105–107]. Metal salts of organic acids, organometallic compounds, and inhibitors of radical chain reactions are used in the stabilisation of PVC. PVC stabilisers are usually basic salts [73] which can react with the evolved HCl gas to retard the deleterious catalytic action of the eliminated HCl [75, 93] or metallic soaps [73, 93–95]. Thermo-oxidative degradation of PVC can be blocked or retarded by metal soaps, epoxy compounds, phosphites, aliphatic and aromatic compounds and organotin compounds. Organotin stabilisers are mainly used in the production of sheets, profiles, general purpose and potable water pipes, sidings, films, foils, bottles and articles for paper packaging. Organic thermal stabilisers for the thermal stabilisation of PVC have recently been extensively studied [60, 95]. Improving the thermal stability of PVC requires the use of stabilisers in processing. Additives that have found practical application as thermal stabilisers for PVC include metal salts of organic acids, organometallic compounds, and inhibitors of radical-chain reactions. Copolymerisation of vinyl chloride monomer, grafting, blending, and chemical modifications such as nucleophilic substitution [61, 108–110] and copolymerisation with imide monomers and further chlorination of PVC to increase the heat resistance of PVC has been studied [113–115]. Alternatively, thermal stability can be enhanced by the preparation of PVC/clay nanocomposites [116]. Several studies have been initiated to improve the various properties of PVC for making new materials for specific applications. Research on PVC materials to modify their properties for specific applications is underway [110, 113–115]. This includes copolymerisation of vnyl chloride monomer, grafting, blending and chemical modifications [110, 112, 113, 117]. Using stabilisers with the action of nucleophilic

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reactions, these defects are replaced and, ultimately, thermal stability is increased.

References 1.

A. Bos and S.R. Tan, PVC Pipes—Current Status and New Developments, Conference Papers PVC’96, Brighton, UK, 1996.

2.

D. Braun in Developments in Polymer Degradation 3, Ed., N. Grassie, Applied Science Publishers, London, UK, 1981.

3.

T. Kelen in Polymer Degradation, Van Nostrand Reinhold Company, New York, NY, USA, 1983.

4.

J. Wypych in Polymer Science Library 3, Ed., A.D. Jenkins, Elsevier, Amsterdam, The Netherlands, 1985.

5.

B. Ivan, T.T. Nagy, T. Kelen, B. Turcsanyi and F. Tudos, Polymer Bulletin, 1980, 2, 83.

6.

F.E. Okieimen and O.C. Eromonsele, European Polymer Journal, 2000, 36, 525.

7.

N.A. Mohamed, M.W. Sabaa, E.H. Oreby and A.A. Yassin, European Polymer Journal, 2002, 76, 367.

8.

Kh.D Khalil and A.A. Yassin, Polymer Degradation and Stability, 2001, 72, 53.

9.

B. Li, Polymer Degradation and Stability, 2000, 68, 197.

10. N.A. Mohamed, W. Mohamed and A. Magrhi, Polymer Degradation and Stability, 2003, 80, 275. 11. R. Bacaloghi and M. Fisch, Polymer Degradation and Stability, 1994, 45, 315.

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12. P. Simon, Polymer Degradation and Stability, 1992, 36, 85. 13. I. McNeill, I. Memetea and W.J. Cole, Polymer Degradation and Stability, 1995, 49, 181. 14. P. Simon, Polymer Degradation and Stability, 1990, 29, 155. 15. V. Percec, E. Ramirez-Castillo, L.A. Hinojosa-Falcon and A.V. Popov, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 2185. 16. E.D. Owen in Degradation Stabilisation of PVC, Elsevier Applied Publishers, London, UK, 1984. 17. G. Allen and J.C. Bevington in Comprehensive Polymer Science, Volume 6, 1st Edition, Pergamon Press, Oxford, UK, 1989. 18. N.A. Mohamed, M.W. Sabaa, Kh.D. Khalil and A.A. Yassin, Polymer Degradation and Stability, 2001, 72, 53. 19. M.W. Sabaa, N.A. Mohamed, E.H. Oreby and A.A. Yassin, Polymer Degradation and Stability, 2002, 76, 367. 20. V. Percec, A.V. Popov, E. Ramirez-Castillo, J.F.J. Coelho and L.A. Hinojosa-Falcon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 6267. 21. V. Percec, A.V. Popov, E. Ramirez-Castillo, J.F.J. Coelho and L.A. Hinojosa-Falcon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 779. 22. T. Millan, G. Martineze, J.M. Gomez-Elvira, N. Guarrotxena and P. Tumblo, Polymer, 1996, 37, 219. 23. N.A. Mohamed, W. Mohamed and A. Magrhi, Polymer, 2003, 80, 275.

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24. D. Braun, B. Bohcinger, B. Ivan, T. Kelen and F. Tudos, European Polymer Journal, 1986, 22, 1. 25. T. Hjertburg and E.M. Sorvik, Polymer, 1983, 24, 685. 26. W.H. Starnes, Progress in Polymer Science, 2002, 27, 2133. 27. W. H. Starnes, Journal of Polymer Science: Polymer Chemistry, 2005, 43, 2451. 28. G. Martinez, C. Mijangos and J. Millan, Revista de Plasticos Modernos, 1982, 43, 629. 29. G. Martinez, C. Mijangos and J. Millan, Journal of Macromolecular Science Chemistry A, 1982, 17, 1129. 30. G. Martinez and J.L. Millan, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2002, 40, 3944. 31. G. Martinez and J.L. Millan, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 6052. 32. C. Mijangos, G. Martinez, A. Michel, J. Millan and A. Guyot, European Polymer Materials, 1984, 20, 1. 33. T. Hjertburg and E.M. Sorvik, Polymer Degradation and Stability, 1983, 24, 673. 34. T. Hjetberg and E.M. Sorvik, Report IUPAC Working Party on PVC, Cleveland, OH, USA, 1980. 35. N. Bensemra, T.V. Hoang and A. Guyot, Polymer Degradation and Stability, 1990, 28, 173. 36. J. Bauer and A. Sabel, Die Angewandte Makromolekulare Chemie, 1975, 47, 15. 37. Sbarski, E. Kosior and S.N. Bhattacharya, International Polymer Proceedings, 1997, 12, 341.

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38. G. Martineze, J.M. Gomez-Elvira and T. Millan, Polymer Degradation and Stability, 1993, 40, 1. 39. T. Radiotis and G.R. Brown, JMS-Pure Applied Chemistry, 1997, A34, 743. 40. Z. Ahmed and W.J. Mazoor, Thermal Analysis, 1992, 38, 2349. 41. W. Khan and Z. Ahmed, Polymer Degradation and Stability, 1996, 53, 243. 42. M. Asahina and M. Onozuka, Journal of Polymer Science, 1994, A2, 3505. 43. K.B. Abbas and E.M. Sorvik, Journal of Applied Polymer Science, 1975, 19, 2991. 44. Z. Vymazad, E. Czako, K. Volka and J. Stepek, European Polymer Journal, 1985, 16, 149. 45. V.D. Daniels and H.H. Rees, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1974, 12, 2115. 46. D. Braun and D. Sonderhof, Polymer Bulletin, 1985, 14, 39. 47. Benavides R, Edge M, Allen NS, Tellez MM, Polymer Bulletin, 1999, 42, 273. 48. Skowronski TA, Rabek JF, Ranby B. Polymer Degradation and Stability, 1984, 8, 37. 49. S. Gaumet and J-L. Gardette, Polymer Degradation and Stability, 1991, 33, 17. 50. J-L. Gardette and J. Lemaire, Polymer Degradation and Stability, 1991, 34, 135. 51. J-L. Gardette and J. Lemaire, Journal of Vinyl Additive Technology, 1997, 3, 107. 85

Update on Troubleshooting the PVC Extrusion Process

52. M. Veronelli, M. Mauro and S. Bresadola, Polymer Degradation and Stability, 1999, 66, 349. 53. R. Benavides, M. Edge, N.S. Allen, M. Shah and M.M. Tellez, Polymer Degradation and Stability, 1995, 48, 377. 54. I.J. González-Ortiz, M. Arellano, M.J. Sánchez-Peña and E. Mendizábal, Polymer Degradation and Stability, 2006, 91, 2715. 55. R. Bacaloglu and M.H. Fisch in Plastics Additives Handbook, 5th Edition, Ed., H. Zweifel, Hanser, Munich, Germany, 2001, Chapter 3. 56. F. Tudos, T. Kelen, T.T. Nagy and B. Turcsanyi, Pure and Applied Chemistry, 1974, 38, 201. 57. D. Braun, Progress in Polymer Science, 2002, 27, 2171. 58. T. Kelen, Journal of Macromolecular Science Chemistry, 1978, A12, 349. 59. R.P. Lattimer and W.J. Kroenke, Journal of Applied Polymer Science, 1980, 25, 101. 60. K.S. Minsker, S.V. Kolesov and G.E. Zaikov in Degradation and Stabilisation of Vinyl Chloride-Based Polymers, Pergamon, Oxford, UK, 1998, p.76. 61. a) M. Beltra´n and A. Marcilla, Polymer Degradation and Stability, 1995, 48, 219. 62. C. Anton-Prinet, G. Mur, M. Gay, L. Audouin and J. Verdu, Polymer Degradation and Stability, 1998, 61, 211. 63. N.A. Mohamed, A.A. Yassin, Kh.D. Khalil and M.W. Sabaa, Polymer Degradation and Stability, 2000, 70, 5.

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64. M.W. Sabaa, N.A. Mohamed, Kh.D. Khalil and A.A. Yassin, Polymer Degradation and Stability, 2000, 70, 121. 65. A. Andreas in Plastics Additives Handbook, 3rd Edition, Eds., R. Gaèchter, H. MuÈ ller and P.P. Klemchuk, Hanser Publishers, Munich, Germany, 1990, p.271. 66. A. Maier, Polymer Engineering & Science, 1996, 36, 1502. 67. V. Chiriac, M. Chiriac, D. Arion, D., Pavel, R. Antonie, E. Grosu, L. Pop, M. Burlacel, M. Gutiu, Materiale Plastice, 2000, 37, 2, 88. 68. L. Jian, Z. Dafei and Z. Deren, Polymer Degradation and Stability, 1991, 31, 1. 69. F. Castillo, G. Martinez, R. Sastre and J. Millan, Revista de Plasticos Modernos, 1987, 367, 86. 70. K. Patel, A. Velazquez, H.S. Calderon and G.R. Brown, Journal of Applied Polymer Science, 1992, 46, 179. 71. R. Bacaloglu and M. Fish, Polymer Degradation and Stability, 1994, 45, 301. 72. E. Ureta and M.E. Cantú, Journal of Applied Polymer Science, 2000, 77, 2603. 73. Encyclopedia of Polymer Science and Technology, Volume 12, Eds., H.F. Mark, N.G. Gaylord and Bikales Wiley, New York, NY, USA, 1970, p.725. 74. D. Braun, Die Makromolekulare Chemie, Macromolecular Symposia, 1992, 57, 265. 75. R.J. Meier and B.J. Kip, Polymer Degradation and Stability, 1992, 38, 69.

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76. P. Simon and L. Valko, Polymer Degradation and Stability, 1992, 35, 249. 77. V. Percec, A.V. Popov, E. Ramirez-Castillo, J.F.J. Coelho and L.A. Hinojosa-Falcon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 6267. 78. V. Percec, A.V. Popov, E. Ramirez-Castillo, J.F.J. Coelho and L.A. Hinojosa-Falcon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 779. 79. V. Percec, E. Ramirez-Castillo, L.A. Hinojosa-Falcon, A.V. Popov, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 2185. 80. K. Endo, Progress in Polymer Science, 2002, 27, 2021. 81. V.H. Tran, T.P. Nguyen and P. Molinie, Polymer Degradation and Stability, 1996, 53, 279. 82. X. Ruijian, Polymer Degradation and Stability, 1990, 28, 323. 83. C. Garrigues, A. Guyot and V.H. Tran, Polymer Degradation and Stability, 1994, 43, 299. 84. A.A. Yassin, M.W. Sabaa and N.A. Mohamed, Polymer Degradation and Stability, 1985, 13, 255. 85. A.H. Frye and R.W. Horst, Journal of Polymer Science, 1959, 40, 419. 86. M.H. Fish and R. Bacaloglu, Journal of Vinyl Additive Technology, 1999, 5, 4, 205. 87. G.Y. Levai, G.Y. Ocskay and Z.S. Nyitrai, Polymer Degradation and Stability, 1989, 26, 11.

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88. R. Benavides, M. Edge and N.S. Allen, Polymer Degradation and Stability, 1994, 44, 375. 89. R.F. Grossman, Journal of Vinyl Technology, 1990, 12, 34. 90. B. Ivan, B. Turcsanyi, T. Kelen and F. Tudos, Journal of Vinyl Technology, 1990, 12, 126. 91. D. Balköse, H.I. Gökcel and S.E. Göktepe, European Polymer Journal, 2001, 37, 1191. 92. K.B. Abbas and E. Sorvik, Journal of Vinyl Additive Technology, 1980, 2, 87. 93. M. Onozuka, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1967, 5, 2229. 94. N.L. Thomas, Plastics and Rubber Processing and Applications, 1993, 19, 263. 95. G. Ayrey, B.C. Head and R.C. Poller, Journal of Polymer Science, Macromolecular Reviews, 1974, 8, 1. 96. H.I. Gökçel and D. Balköse, Advances in Polymer Technology, 1998, 17, 63. 97. L.J. Gonzalez-Ortiz, M. Arellano, C.F. Jasso, E. Mendizabal and J.M. Sanchez- Pena, Polymer Degradation and Stability, 2005, 90, 154. 98. R.D.J. Dworkin, Vinyl Technology, 1989, 11, 15. 99. M. Bartholin, N. Bensemra, T.V. Hoang and A. Guyot, Polymer Bulletin, 1990, 23, 425. 100. G.Y. Levai, G.Y. Oeskey and Z.S. Nyitrai, Polymer Degradation and Stability, 1994, 43, 159. 101. N. Bensemra, T.V. Hoang and A. Guyot, Polymer Degradation and Stability, 1990, 29, 175. 89

Update on Troubleshooting the PVC Extrusion Process

102. H.I. Gokcel, O. Balkose and U. Kokturk, European Polymer Journal, 1999, 35, 1501. 103. M. Minagawa, Polymer Degradation and Stability, 1989, 25, 121. 104. J. Wypych, Journal of Applied Polymer Science, 1975, 19, 3387. 105. N. Bensemra, V.H. Tran, A. Guyot, M. Gay and L. Carette, Polymer Degradation and Stability, 1989, 24, 89. 106. N. Bensemra, V.H. Tran and A. Guyot, Polymer Degradation and Stability, 1990, 29, 175. 107. F.E. Okieimen and J.E. Ebhoaye, Angewandte Makromolekulare Chemie, 1993, 206, 11. 108. R. Benavides, M. Edge, N.S. Allen and M.M. Tellez, Journal of Applied Polymer Science, 1998, 68, 11. 109. F.E. Okieimen and C.E. Sogbaike, European Polymer Journal, 1996, 32, 12, 1457. 110. D.F. Anderson and D.A. McKenzie, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1970, 1, 18, 2905. 111. T. Iida, J. Kawato, K. Maruyama and K. Goto, Journal of Applied Polymer Science, 1987, 34, 2355. 112. T. Uma, T. Mahalingam and U. Stimming, Materials Chemistry and Physics, 2004, 85, 131. 113. R. Joseph, K.E. George and D.J. Francis, International Journal of Polymer Materials, 1986, 11, 95. 114. S. Marian and G. Levin, Journal of Applied Polymer Science, 1981, 26, 3295.

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115. E. Beati and M. Pegoraro, Die Angewandte Makromolekulare Chemie, 1978, 73, 35. 116. N.A. Mohamed and Al-Magribi, Polymer Degradation and Stability, 2003, 82, 421. 117. D.D. Sotiropoulou, K.G. Gravalos and N.K. Kalfoglou, Journal of Applied Polymer Science, 1992, 45, 273. 118. J.F. Maggioni, A. Eich, B.A. Wolf and S.P. Nunes, Polymer, 2000, 41, 4743.

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Poly(Vinyl Chloride)–Wood Composites

Poly(vinyl chloride) (PVC) has been used as a polymer matrix for wood–plastic composites. The growth in wood–PVC composite (WPC) products has been slowly increasing since 2006. The outdoor durability of WPC products is more like that of PVC made from mixtures of wood filler (typically of 40–80% by weight) and plastics. However, the deployed loading level is >60 wt%. This results in serous disadvantages: the high density and lack of flexibility of the end products, the low mechanical properties, and problematic compounding and processing. Extrusion is one of the techniques used in the manufacture of wood– plastic composites. The manufacture of wood–plastic composites started in the 1980s, but dramatic growth has recently been experienced [1]. The market for WPC has been very active, with growth of 200% from 2002 to 2010 among wood–plastic composites [2]. Composites with UV resistance and dimensional stabilities are better than those of solid wood. Wood–polymer composites and wood–plastic composites are entirely different [3–11]. The main application of wood–polymer composites is flooring. In WPC composites, the advantage of wood includes low density, low equipment abrasiveness, relatively low cost, and good biodegradability; PVC provides good moisture and decay resistance [1, 9, 10]. In addition, various surface optical effects can be obtained by adding different wood species and coloured pigments [12]. WPC composites are used in many applications because they: are easy to process; have high productivity; have low economical cost; and have good versatility. However, for certain specific uses, mechanical

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properties such as strength and toughness of materials are inadequate. Composites have better specific properties and find applications in diverse fields, ranging from domestic appliances to spacecraft [13]. The use of wood in thermoplastics has been plagued by the thermalstability limitation of wood, as well as the difficulties in obtaining good filler dispersion and strong interfacial adhesion [14–20].

6.1 Additives In the composites industry, additives are commonly used to modify behaviour, appearance, mechanical properties and processability [21]. Additives are still needed for manufacturing products with WPC composites to maintain the properties. In addition to the wood component, several classes of PVC additives (i.e., impact modifiers), processing aids and lubricants are used to improve processing. The correct selection and use of these additives can predict an overall formulation designed to produce a composite with optimum properties [22]. The processing of wood-filled wood composite WCP is challenging because of the high temperature increase due to shear heating, high melt viscosity, and low melt strength that leads to poor extrusion quality. To overcome these difficulties, very-high-molecular-weight process aids are beneficial. However, it has been shown that their chemical composition is a critical parameter [23]. The use of antimicrobial agents is dependent upon the type and content of wood fibre, microbe types to the application, environment, and humidity. Aminosilane has been promoted for the interaction between wood fibre and PVC as an adhesion promoter [24]. It has been observed to be a suitable for WPC composites, significantly improving the tensile strength of the composites. Other treatments (dichlorodiethylsilane, phthalic anhydride, maleated polypropylene) were found to be ineffective [25].

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Phosphate esters are often used as flame retardants. However, they have negative effects on the environment and a limited effect on flame retardancy and smoke suppression. Flame retardants such as antimony trioxide and halogenated organic compounds as well as smoke suppressants such as molybdenum trioxide and ammonium octamolybdate have been shown to be effective. Antimony trioxide promotes the dehydrochlorination of PVC. Thus, there are reservations about the general use of this retardant. Recently, inorganic flame retardants and fillers such as alumina trihydrate and magnesium hydroxide have been used for the purpose. The use of layered silicate polymer nanocomposites has been proposed as a totally new and promising approach for the fire retardancy and smoke suppression of polymers. They also produce remarkable improvements in certain material properties (e.g., mechanical strength, optical properties, electric properties, fire retardancy). A low loading of nano-montmorillonite particles (5–10 wt% content) can reduce the maximum rate of heat release by 70% [26–28]. Coupling agents such as copper amine, silanes, maleic anhydride and their grafting polymers improve composite properties [12, 29–31]. However, unlike polyolefin–wood fibre composites, the conversion of the hydrophilic surface of wood to a hydrophobic surface is not sufficiently effective for enhancing the adhesion of PVC to wood fibre. The Lewis acid–base interactions for decomposition are significant for enhancing interfacial adhesion. The other reason is that the induced acid–base reaction favours fire retardancy and smoke suppression [9, 32]. WPC composites must resist fire and suppress smoke. Pure PVC has high chlorine content (56.8%), which gives good flame retardancy. However, the high content of wood fibre and low-molecular-weight additives (as well as subsequent thermal degradation) reduces the flame retardancy of the composites.

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PVC generates a lot of black smoke during forced burning. Flame retardants and smoke suppressants are added to prevent smoke evolution, reduce heat release, and to lower the extent of burning. Flame retardants and smoke suppressants are often incorporated with WPC composites.

6.2 Properties of Wood-Poly(vinyl Chloride) Composites In structural applications, WPC products are viable substitutes for solid woods and PVC. WPC products are strongly influenced by wood content [33]. An increase in wood fraction increases the notched impact energy, flexural strength, and elastic modulus while decreasing the melt index, tensile strength and tensile elongation at break. Moreover, increasing the fibre size causes the melt index, flexural and tensile moduli, as well as tensile elongation at break to increase.

6.3 Processing PVC–wood flour composites have attracted attention due to their combination of good mechanical properties, chemical stability and water resistance. PVC–wood flour composites are used for decking, siding, and indoor building materials [34]. However, the processing of PVC–wood flour composites is associated with poor interfacial compatibility, poor fire resistance and weak impact strength. In addition, these composites burn readily, so the application may be faced with more danger from fire. Among wood–plastic composites, wood–PVC thermoplastic composites are manufactured by incorporating and dispersing wood fibres or wood flour into molten plastics to form composite materials. The advantages of plastics and wood are therefore employed. The poor interfacial attraction between the hydrophilic wood and the hydrophobic plastic matrix results in poor adhesion. Hence, stress is

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transferred from the matrix to the wood flour fibre, thereby reducing mechanical strength and ductility. Extruded PVC–wood flour composites are being developing rapidly thanks to recent developments in formulation technology. Potential modifications with additives may lead to improvements in key properties [35]. Wood filler with moisture content of 6–9% by weight and PVC is compounded into pellets. Wood filler is pelletising at a high aspect ratio to reinforce the structure. Downstream feeding may be the preferred processing configuration [36]. Figure 6.1 shows a conical intermeshed twin-screw extruder. A kneading zone is an important part in screw design where the wood and PVC will be mixed and pumped to the die. Mixing of PVC and wood creates high-viscosity flow. Hence, care must be taken during compounding with appropriate addition of additives such as heat stabilisers and lubricants.

Precompression zone Metering zone

Compression zone

Vent zone

Feed zone

Kneading zone

Figure 6.1 A conical intermeshed twin-screw extruder (schematic). Reproduced with permission from [37]. ©2004, Society of Plastics Engineers

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6.4 Advantages of Wood-Poly(Vinyl Chloride) Composites The major advantages of WPC composites are: 1. WPC composites provide low outdoor maintenance costs. 2. Products can also be extruded without expensive wood-working operations. 3. WPC composites have superior mechanical properties with respect to resistance to weathering and flame-retardancy. 4. WPC made out of PVC can be cut, sawed, nailed, screwed, and processed by conventional wood-working equipment [12, 38]. 5. WPC composites are used in window/door profiles, decking, railing and siding; interior and marine applications are under development.

References 1.

C. Clemons, Forest Products Journal, 2002, 52, 6, 10.

2.

H. Jiang and D.P. Kamdem, Journal of Vinyl Additive Technology, 2004, 10, 2, 59.

3.

M.H. Schnieder, Wood and Fiber Science, 1994, 26, 142.

4.

M.H. Schnieder, Wood Science and Technology, 1995, 29, 121.

5.

J.A. Meyer, Forest Products Journal, 1995, 15, 362.

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U.P. Wang, Journal of the Chinese Chemical Society, 1975, 22, 77.

7.

A. Hebeish and J.T. Guthrie, Polymer/Properties and Applications, Springer-Verlag, New York, NY, USA, 1981.

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8.

A.E. Witt, Radiation Physics and Chemistry, 1981, 18, 67.

9.

J.A. Meyer, Wood Science and Technology, 1981, 14, 49.

10. J.A. Meyer, Forest Products Journal, 1982, 32, 24. 11. J.A. Meyer, Technologist, 1987, 1, 3, 4. 12. A.K. Bledzki, S. Reihmane and J. Gassan, Polymer Plastics Technology and Engineering, 1998, 37, 4, 451. 13. D.N. Saheb and J. Jog, Advanced Polymer Technology, 1999, 18, 4, 351. 14. L.M. Matuana, J.J. Balatineez, and C.B. Park, Polymer Engineering and Science, 1998, 38, 765. 15. D. Maidas and B.V. Kokta, Journal of Vinyl Technology, 1993, 15, 38. 16. L.M. Matuana, C.B. Park, and J.J. Balatineez, Polymer Engineering and Science, 1998, 38, 1862. 17. L.M. Matuana, C.B. Park and J.J. Balatineez, Polymer Engineering and Science, 200, 80, 1943. 18. L.M. Matuana and F. Mengeloglu, Journal of Vinyl Additive Technology, 2001, 7, 67. 19. J.Z. Lu, Q. Wu, and I.I. Negulescu, Wood and Fiber Science, 2002, 34, 434. 20. H. Jiang, D.P.Kamdem, B. Bezubic, and P. Ruede, Journal of Vinyl Additive Technology, 2003, 9, 138. 21. M.R. Snyder, Composites Technology, 2008, 14, 1, 40. 22. F. Sim in Proceedings of an SPE Conference - Vinyltec 2004, Iselin, NJ, 2004, Paper 21.

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23. B. Azimipour and P. Schipper in Proceedings of an SPE Conference - Vinyltec, 2002, p.337. 24. L.M. Matuana and F. Mengeloqlu, Journal of Vinyl Additive Technology, 2002, 8, 4, 264. 25. L.M. Matuana, R.T. Woodhams, J.J. Balatinecz and C.B. Park, Polymer Composites, 1998, 19, 4, 446. 26. G. Beyer, Plastics, Additives and Compounding, 2005, 7, 5, 32. 27. M. Bartholmai and B. Schartel, Polymers for Advanced Technologies, 2004, 15, 355. 28. T.J. Pinnavaia, L. Tie, P.D. Kaviratna and M.S. Wang, Journal of Engineering and Applied Sciences, 1994, 346, 81. 29. B.L. Shah and L.M. Matuana, Journal of Vinyl Additive Technology, 2005, 11, 160. 30. H.H. Jiang and D.P. Kamdem, Journal of Vinyl Additive Technology, 2004, 10, 2, 70. 31. A.K. Bledzki, M. Letman, A. Viksne and L. Rence, Composites Part A: Applied Science and Manufacturing, 2005, 36, 6, 789. 32. S.M. Lai, F.C. Yeh, Y. Wang, H.C. Chan and H.F. Shen, Journal of Applied Polymer Science, 2003, 87, 3, 487. 33. N. Stark and M. Berger in the Proceedings of a Forest Products Society Conference - Functional Fillers for Thermoplastics and Thermosets, Madison, WI, USA, 1997, p.119. 34. H.H. Jiang and D.P. Kamdem, Journal of Vinyl Additive Technology, 2004, 2, 10.

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35. P. Kroushl in Proceedings of an SPE Conference - Vinyltec 2002, Itasca, IL,USA, 2002, Session 4, Paper 4, p.463. 36. R. Cutillo and S. Jackson in Proceedings of an SPE Conference - Vinyltec 2005, Philadelphia, PA, USA, 2005, Paper 13. 37. H. Jiang and D.P. Kamdem, Journal of Vinyl & Additive Technology, 2004, 10, 2, 59. 38. A.K. Bledzki, J. Gassan and S. Theis, Mechanics of Composite Materials, 1998, 34, 563.

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7

Poly(Vinyl Chloride) Extrusion: Problems and Defects

Poly(vinyl chloride) (PVC) is used in the production of hundreds of products that are very important in construction, electronics, healthcare, and other applications. In processing, PVC is used in >90% of all extrusions in Europe [1]. Like other group members such as polyethylene, polypropylene, polystyrene, polyvinylacetate, polymethylmethacrylate and polyvinylidene chloride, PVC has the vinyl group (CH2=CH-). However, ‘vinyl’ generally refers to PVC and its copolymer. Examination of problems and defects is very important in PVC extrusion. PVC extrusion involves blending between resins and additives in high-speed mixtures. The defects are related to PVC compounds and processing. The formulation of PVC decides the processing and product performance. The useful service lifetimes of PVC products is estimated by subjecting the products to constant testing. In PVC extrusion, process variations are one among several problems that have an effect on temperature. The melt temperature is important to ensure constant processing. Thermal degradation is primarily an initiated break in the polymer chain, and does not have a direct bearing on the decrease in molecular weight. It is a function of temperature, rheological parameters and molecular weight. Degradation is a too severe a problem and requires cleaning of the extruder. The effects of degradation are: a reduction in physical properties; surface defects; process instability; wear; and increased quality-control costs [2]. There are many defects and problems that are specific to a particular

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type of extrusion operation. In extrusion, uneven cooling can cause bending or collapse of the product. It is important to know the relative temperature distribution during cooling of an extruded product. Similarly, problems with surging are due to solids conveying or a melting problem, changes must be based on this assumption [3]. Selecting PVC for industrial applications (e.g., automotive, domestic appliances, garden) is strongly dependent upon PVC composition, part design and processing conditions. Fillers, pigments, and other additives may have a greater influence, and there may be a need to characterise material selection for the required application. Extrusion problems must first be diagnosed to determine the solutions. Shortterm implementation of the solution is important in PVC processing. Particularly in a continuous process such as extrusion, it is important to have solutions to solve problems and defects. ‘Visible defects’ due to running a process too fast can result in, for example, shark-skin finishes, melt fracture, failure to hold dimensional tolerances, air occlusion, and visible discoloration of products. ‘Hidden defects’ are often more damaging because a large quantity of off-grade material may have been produced before the realization that a mistake has been made. Such hidden defects include: loss of needed residual heat stability and light stability; subsequent warpage due to built-in strains; low sag temperature; and low heat distortion temperature. Incipient degradation causing loss of residual stability may often be detected by checking parts for fluorescence under ultraviolet (UV) light, which reveals the presence of short sequences of double bonds in degrading PVC molecules [4]. Flow velocity and stress discontinuity is responsible for surface defects. Such discontinuities may induce a high flow stretching rate or cohesive failure of the polymer melt, consequently causing the onset of shark-skin finishes [5–8]. With respect to problems related to extrusion (particularly problems that occur inside the extruder), it is important to have good

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instrumentation. Melting, mixing and metering of PVC is totally obscured by the extruder and is dependent upon instrumentation to determine the events occurring within the extruder. The seriousness of the problem can be assessed only by instrumentation. Problems and defects are based on the part morphology of PVC. Part morphology is defined by crystal content, morphology and orientation, non-crystalline content and orientation as well as all important interconnections between these morphological units. The levels of these properties are determined by the orientation of the molecular chains as well as chain packaging. The crystallinity and strained chain conformation impact property retention with respect to the glass transition temperature and the melting point. The extruded PVC products are essentially identical chemically. PVC processing in the future will centre on the management of chain entanglements, structural defects, and process control. Several aspects of molecular-structure control will influence solid-phase processing of polymers with respect to the networks, initial morphology in crystalline polymers, and thermally activated processes. PVC extrusion offers several advantages such as continuous processing, easy processability and relatively economical cost. However, differences in the rheology of the PVC compound can lead to the formation of several defects and problems depending on other parameters such as process settings and the die. Defects and problems can be detrimental in terms of the mechanical strength and appearance of the products. Problems regarding diffusion during PVC processing can be neglected because PVC is compounded with other ingredients. Understanding the role of various parameters such as rheology, geometric details, and process parameters are useful for the prediction and prevention of problems and defects.

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M. Paci and F.P. La Mantia, Polymer Degradation and Stability, 1999, 63, 1, 11.

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E.M. Mount, III., in the Proceedings of the 61st Annual SPE Conference - ANTEC 2003, Nashville, TN, USA, 2003, 1, 251.

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A.W. Coaker in Applied Polymer Science: 21st Century, Eds., C. Craver and C. Carraher, Elsevier, Oxford, UK, 2000, p.107.

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Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Studies on poly(vinyl chloride) (PVC) problem-solving have traditionally used deterministic tasks that require execution of a systematic series of steps to reach a rational and optimal solution. Most of the problems are characterised by uncertainty, the need to consider an enormous number of variables (e.g., raw material, processing method, machinery, product) as well as the need to optimise the solution to multiple interacting constraints. It is necessary to have a strategy to identify the problem. It is important to have multiple strategies to solve the problem. Once an action plan is ready, several factors influence the formation and selection of the strategy. Assessment of the problem allows for the development of more efficient problem-solving strategies in PVC processing. Each problem requires more information related to the raw material, formulation, processing technique, machinery, and end product. Problems are likely to fit into a given plan or one needs to select the most appropriate plan. Troubleshooting goes to show: • There is typically more than one way to solve a problem • Ability and experience affect the strategy chosen In troubleshooting of PVC extrusion, solving the problem and optimising the process is paramount. This involves finding the shortest route through a set of points and returning to the raw material used in the processing. Diagnosing problems involves examining the properties and settings of the extrusion process to control the

107

Update on Troubleshooting the PVC Extrusion Process

performance of a particular extrusion problem. This is followed by examining the outcome of the change and interpretation of the results [1]. Selecting the optimum solutions to the problems is one of the most crucial challenges in PVC processing. Without troubleshooting methods, many extrusion problems would be unsolved or solved incorrectly, which can lead to huge wastage. The challenges are: • Limitation of human capabilities to compare or to decide. The challenges become more intricate if the comparison is made on the basis of multiple solutions • The possible solution in comparison with the early data may not match due to variable parameters

8.1 Problems and Troubleshooting in Pipe/Profile Extrusion To achieve better extrusion, materials should appear in the vent with a ‘cheesy’ appearance instead of a melted appearance. In the cheesy condition, the powder can be appropriately degassed. The screw flight in the feed section should be properly filled with powder so the open feed throat is effectively sealed off.

8.1.1 Production Problems in the Extruder Problems in pipe and profile extrusion are common. They can be divided into three areas: extrusion, calibration and product. During extrusion processing or troubleshooting, four points must be kept in mind: 1. Do not allow physical damage to the equipment 2. The product must meet the quality requirement 108

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

3. Increase temperature in steps of 5 °C but, at the same time, too much of an increase in temperature may lead to burning 4. In the adaptor and spider portion of the die, care must be taken to control the temperature because cooling of these zones is very difficult

8.1.1.1 Problem: Difficult to String-up Melt during Startup This problem may be due to a lack of strength. During processing, the extrudate (string-up melt) is hot and breaks easily when leading through the cooling tank to the puller. Figure 8.1 illustrates the troubleshooting process to solve the problem of difficult to stringup melt material during startup. The material after extrusion, at high temperature, (the extrudate) can exhibit reddish or black lines which may lead to charring. This is commonly known as burning in PVC extrusion.

8.1.1.2 Problem: Powder Pulled into a Vacuum This problem may be due to a loss of vacuum of vent or excessive clogging in vacuum traps or filters. Figure 8.2 illustrates the troubleshooting process to solve the problem of a product pulled into a vacuum. Reduce production rate by reducing machine’s screw rotation per minute (RPM) and appropriately adjust take-off or puller RPM.

8.1.1.3 Problem: High Bearing Throat at Back Pressure This problem can be indicated in the thrust indicator of the back pressure gauge. Figure 8.3 illustrates the troubleshooting process to solve a high bearing throat at back pressure.

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Update on Troubleshooting the PVC Extrusion Process

8.1.1.4 Problem: Motor Load is too High This problem may be due to poorly plasticised material in the degassing zone. Screws should be filled and the motor speed must match with the feed dosing. Figure 8.4 illustrates the troubleshooting process to solve the problem of a too-high motor load (i.e., ampere or power reading reaches the maximum to the motor). If the restrictor is wire mesh or filters, the size of the restrictor is increased or it is removed.

Note: Indicates Increase

Reduce temperature Motor

Indicates Decrease Bz 1 Bz 2

Bz 3

Bz 4

Barrel zones Controllers

If “No”

Increase Temperature Aaptor

Increase Temperature Aaptor

Spider

Die

Die

Pipe Die

Profile Die

Figure 8.1 Problem: difficult to string-up melt during startup

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Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Increase Vaccum Motor

Bz 1 Bz 2 Bz 3 Bz 4 Barrel zones Controllers

Note: Indicates Increase If “No” Indicates Decrease

Motor

Bz 1 Bz 2 Bz 3 Bz 4 Bz 5 Barrel zones Controllers

If “No”

Increase Temperature

Motor

Oil hot-cool unit

Increase Temperature Pump Bz 1 Bz 2 Bz 3 Bz 4

If “No” Barrel zones Controllers

Figure 8.2 Problem: powder pulled into a vacuum

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Update on Troubleshooting the PVC Extrusion Process

Increase Extruder RPM Barrel temperature

Motor

Bz 1

Bz 2

Bz 3

Bz 4

Note: IndicatesIncrease IndicatesDecrease Controllers

If “No’

FeederRPM

Motor

Bz 1

Bz 2

Bz 3

Bz 4 If “No’

Reduce tempeerature Controllers

Oil hot-cool unit If “No’

Aaptor

Increase Temperature

Spider

Die Pipe Die

(OR)

Pump

Increase Temperature

Aaptor

Die Profile Die

Figure 8.3 Problem: high bearing throat at back pressure

112

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion Feeder RPM Note:

Motor

IndicatesIncrease IndicatesDecrease

Bz 1

Bz 2

Bz 3

Bz 4

If “No’

Controllers

Increase Temperature

Motor

Bz 1

Bz 2

Bz 3

Bz 4 Barrel zones

If “No’

Aaptor Aaptor

Increase Tempature

Spider

Die

Aaptor

(OR)

Pipe Die

Controllers

Increase Tempature

Die Profile Die Try different formutatio n

If “No’

PVC compounding Hot-coolmixr

Figure 8.4 Problem: Motor load is too high

8.1.1.5 Problem: Low Output A machine that is not reaching the expected production may be due to caking of the material in the hopper or due to high temperature. For given product dimensions, the production is below the normal output and RPM of the machine. Figure 8.5 illustrates the troubleshooting process to solve the problem of low output.

113

Update on Troubleshooting the PVC Extrusion Process Note:

Check Extruder RPM

Bz 1

IndicatesIncrease

Bz 4

Bz 3

Bz 2

IndicatesDecrease

Check Extruder RPM

Barrel zones

Controllers

Motor If “No’

Bz 1

DecreaseTemperature

Bz 2

Bz 3

Bz 4

Motor

Bz 1

Bz 4

Bz 3

Bz 2

If “No’

Controllers

Barrel zones Try diftrent Controllers

If “No’

Increase Extruder RPM

Bz 1

If “No’

formulatio n

PVC compounding Hot- coolmixer

Bz 2

Bz 3

Bz 4

Barrel zones

If “No’

CheckWear and tear Controllers

Figure 8.5 Problem: low output

8.1.2 Product Problems During processing in the production process, the product must be checked for appearance, melt fracture, lumpy, and cool or oval (eggshaped) mark surfaces. Lubricants and process aids decrease the melt viscosity and from the sticking of PVC in the extruder. High levels 114

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

of fillers decrease properties and a higher temperature is required for processing. A sufficient level of stabiliser must be used to ensure appropriate stability during processing. Heat stability can be adversely affected by incompatibility of the additives with PVC. Stabilisers should be highly compatible with PVC. The use of solid barium/cadmium stabilisers could adversely affect post-extrusion processes. The use of high levels of lubricants with high PVC incompatibility may cause surface blooming or exudation after extrusion. This will affect the quality of the product.

8.1.2 Production Problems in Downstream Areas The troubleshooting flowcharts shown below explain the way to solve problems related to downstream areas once the extrudate emerges from the extruder.

8.1.2.1 Problem: A Longitudinal Scratch in Pipe or Profile is Found While Sizing Scratching is a production and quality problem. Figure 8.6 illustrates the troubleshooting process to solve a longitudinal scratch while sizing.

8.1.2.2 Problem: Folding of Material in the Calibrator The folding of material in the calibrator may be due to filler or delustrant build-up or plate-out. In such cases, kerosene or lubrication oil must be placed at periodical intervals to remove the plate. Figure 8.7 illustrates the troubleshooting process to solve the folding of material in the calibrator.

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Update on Troubleshooting the PVC Extrusion Process

8.1.2.3 Problem: Blowouts in the Cooling Zone Blowouts in the cooling zone may be due to lumps or holes present in the material. Figure 8.8 illustrates the troubleshooting process to solve the problem of blowouts in the cooling zone. In the case of pipe products, air pressure must be decreased within the pipe if using a floating pressure plug.

Note:

Change PVC formulation

Indicates Increase Indicates Decrease Mandrel Burr or plate-out

If “No”

may be present

PVC compounding Hot-cool mixer

Pipe Burr or plate-out may be present Profile

Figure 8.6 Problem: a longitudinal scratch in a pipe or profile found while sizing

116

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion Note: Indicates Increase

plate-out or white deposition

Indicates Decrease

Pipe Calibrator Change PVC formulation

If “No”

Profile calibrator plate-out or white deposition

PVC compounding Hot-cool mixer

Put kerosene or lubricating oil

Mandrel Burr or plate-out If “No” may be present

Pipe Burr or plate-out may be present Profile

Figure 8.7 Problem: folding of material in the calibrator

117

Update on Troubleshooting the PVC Extrusion Process Increase water velocity

Die

Note:

May be lumps or hole

Indicates Increase

Cooling tank

Indicates Decrease plate-out or white deposition

Pipe Calibrator

If “No”

Water Inlet Increase Temperature

Aaptor

Spider Pipe Die

plate-out or white deposition

Increase Temperature

Aaptor

Profile calibrator

If “No”

Put kerosene or lubricating oil

Die

If “No”

Profile Die

Mandrel Burr or plate-out

Die

may be present

Reduce the water temperature

May be lumps or hole

Cooling tank

If “No” Pipe Burr or plate-out may be present

Water Inlet

Profile

Figure 8.8 Problem: blowouts in the cooling zone

8.1.2.4 Problem: Uncontrolled Wall Thickness Thin and thick spots on the product may be due to uneven viscosity in the melt reaching the die. The variation does not respond to die adjustment. Figure 8.9 illustrates the troubleshooting process to solve the problem of uncontrolled wall thickness.

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Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Reduce temperature Motor

Note: Indicates Increase Bz 1 Bz 2 Bz 3 Bz 4

Indicates Decrease Barrel zones Controllers

If “No” Increase Temperature

Increase Temperature

Aaptor

Aaptor

Spider

Die

(OR)

Pipe Die

Die Profile Die

Figure 8.9 Problem: uncontrolled wall thickness

8.1.2.5 Problem: Poor Inner Surface If small cracks are visible on the inside of the product and can be seen as small swellings on the outside of the product, this may be due to poor fusion of the material. Figure 8.10 illustrates the troubleshooting process to solve the problem of a poor inner surface. Die land length has to be changed or increased, if possible.

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Update on Troubleshooting the PVC Extrusion Process

Note: Motor

Indicates Increase

Reduce temperature

Indicates Decrease

If “No”

Bz 1 Bz 2 Bz 3 Bz 4

Increase Temperature Motor Controllers Bz 1 Bz 2 Bz 3 Bz 4

If “No”

Barrel zones Controllers Spider Aaptor

Decrease Temperature

Die

Decrease Temperature

Aaptor

If “No”

(OR) Die

Pipe Die

Profile Die Increase Temperature Oil hot-cool unit

Pump

Figure 8.10 Problem: poor inner surface

8.1.2.6 Problem: Regular Wavy Lumps For this particular problem, the die system must be changed to increase the land length of the die system and the compression ratio between the spider and annular opening of the die system. Figure 8.11 illustrates the troubleshooting process to solve the problem of regular wavy lumps.

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Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Note: Indicates Increase If “No”

Decrease Temperature

Indicates Decrease

Decrease Temperature Motor

Oil hot-cool unit

Bz 1 Bz 2 Bz 3 Bz 4

If “No”

Barrel zones Controllers

Spider Aaptor

Decrease Temperature

Die Pipe Die

Aaptor

Decrease Temperature

If “No”

(OR) Die Profile Die

Reduce extruder RPM Motor Bz 1 Bz 2 Bz 3 Bz 4

Controllers

Figure 8.11 Problem: regular wavy lumps

8.1.2.7 Problem: Irregular Lumps (Random With No Regular Pattern) Figure 8.12 illustrates the troubleshooting process to solve the problem of irregular lumps which are random with no regular pattern. Using an adaptor with a smaller hole in the die system will give better mixing.

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Update on Troubleshooting the PVC Extrusion Process

Barrel temperature

Screw temperature

Bz 1 Bz 2 Bz 3 Bz 4

Note: Indicates Increase Indicates Decrease

Figure 8.12 Problem: irregular lumps (random with no regular pattern)

8.1.2.8 Problem: Dimples on the Product Figure 8.13 illustrates the troubleshooting process to solve the problem of dimples on the product. The pipe should be examined for voids. Collapse of voids in the pipe may give dimples on the surface.

8.1.2.9 Problem: Burning or Yellowing of the Extrudate During extrusion, the problem occurs due to black specks or discoloration such as yellowing or burning of melt string. Any temperature zone that approaches >195 °C should be suspected. Figure 8.14 illustrates the troubleshooting process to solve the problem of burning or yellowing of the extrudate. The adaptor and spider are often the causes of burned material. Check for mismatched metal parts, dechromed surfaces, or other areas where material could stick and burn.

122

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion Increase water velocity

Die

Note:

Cooling tank

Indicates Increase Indicates Decrease

Check Vaccum

Water Inlet

Bz 1 Bz 2 Bz 3 Bz 4 Barrel zones Controllers

Figure 8.13 Problem: dimples on the product Reduce temperature

Motor

Note: Indicates Increase

Bz 1 Bz 2 Bz 3 Bz 4

Indicates Decrease

Barrel zones

If “No”

Controllers

Reduce Temperature

Check Burnt material Oil hot-cool unit

Spider Aaptor

Pump

Decrease Temperature

Die

(OR)

Pipe Die

Die Profile Die

Spider Aaptor

Aaptor

Decrease Temperature

Die Pipe Die

Aaptor

Decrease Temperature

(OR) Die Profile Die

Figure 8.14 Problem: burning or yellowing of the extrudate 123

Update on Troubleshooting the PVC Extrusion Process

If the die is removed, burning shows in the adaptor area. Remove the barrel or screw, and use a purge compound to clear the material sticking to the screws. Because of the large mass of material in the head area, changes will not be rapid. In some cases, a temperature change may take as long as 1 hour to take effect. Check the vents for burned compounds that may be falling back.

8.1.2.10 Problem: Poor Overall Appearance Figure 8.15 illustrates the troubleshooting process to solve the problem of poor overall appearance. The adaptor and spider are often causes of burned material.

Troubleshooting process – Pipe/Profile extrusion Flow chart – 8.1.2.10 Problem – Overall appearance bad

Check rubber gaskets in cooling tanks to see if they are too tight

Black specks or yellowing or discolored or burning

Suspect any temperature zone approach 195 ºC

Checks sizer for alignment and vibration

Overall appearance bad Check puller for slipping being too tight, uneven pulling or vibration

Check spider and adaptor

Check for mismatched metal parts, dechromed surfaces, or other areas where material could stick and burn

Figure 8.15 Problem: poor overall appearance

124

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.2.11 Problem: Dull Surface Appearance Figure 8.16 illustrates the troubleshooting process to solve the problem of dull surface appearance of the products.

Surface appearance dull

Surface dull, increase temperature

Surface glaze, decrease temperature

Figure 8.16 Problem: dull surface appearance

8.1.2.12 Problem: Low Results in the Drop Weight Impact Test Figure 8.17 illustrates the troubleshooting process to solve the problem of low results in the drop weight impact test. Poor impact sometimes results in the die system; the shear must be increased through the use of a large spider.

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Update on Troubleshooting the PVC Extrusion Process

Increase average heat by increasing barrel and screw

Check vent for volatiles removing

Low results in drop Weight impact test

Check for powder has not plugged screen to vacuum pipe or lines

Check material for moisture or volatiles

Figure 8.17 Problem: low results in the drop weight impact test

8.1.2.13 Problem: Gauge Variation Figure 8.18 illustrates the troubleshooting process to solve the problem of gauge variation. The basic extrusion process converts thermoplastic materials into a continuous melt stream. Extrusion dies are then employed for spreading the melt to a given width and establishing a uniform cross-section. Sometime gauge variations occur in wall thickness with time. It may not be a requirement to make any die adjustments.

126

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Gauge variation

Yes Gauge variation Yes Check exlruder for surging No Check die melt flow No Check temperature contoller and thermocouple function Yes Finish No Check previous data

Figure 8.18 Problem: gauge variation

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Update on Troubleshooting the PVC Extrusion Process

8.1.2.14 Problem: Degassing is Difficult Figure 8.19 illustrates the troubleshooting process to solve the problem of difficult degassing. In the case of a fully plasticised material in the degassing zone, decrease the temperature in zones 1 and 2.

Degassing difficult

Yes Increase screw as much as screw allows No Increase barrel zone 1 & 2 temperature not beyond 180 ºC No Increase screw core thermo-regulation Yes Finish No Check previous data

Figure 8.19 Problem: degassing is difficult

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Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.2.15 Problem: Frictional Heat: Zone-4 Overheating Figure 8.20 illustrates the troubleshooting process to solve the problem of overheating due to the development of frictional heat.

Frictional heat

Yes Check controllers are working

No

Check cooling agent flow

No Decrease thermo-core regulation Yes Finish No Check previous data

Figure 8.20 Problem: frictional heat: zone-4 overheating

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Update on Troubleshooting the PVC Extrusion Process

8.1.2.16 Problem: Melt Fracture and Surface Roughness Figure 8.21 illustrates the troubleshooting process to solve the problem of melt fracture and surface roughness. Melt fracture occurs due to low fusion of the material. In this case, all die surfaces are clean and smooth, and a melt fracture or surface roughness occurs anyway. If surface roughness increases, more calcium stearate or polyethylene wax may be necessary.

Melt fracture and surface roughness

Yes Low temperature profile - increase temperature profile No

Increase lubrication level in formulation

No Material vent out - slow down the screw speed Yes Finish No Check previous data

Figure 8.21 Problem: melt fracture and surface roughness

130

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.2.17 Problem: Lumpy, Cold or Oval (egg-shaped) mark Surfaces Figure 8.22 illustrates the troubleshooting process to solve the problem of lumpy, cold or oval surfaces. Over-lubrication or an inappropriate temperature profile can result in lumpy, cold or oval surfaces.

Lump, cold or ovary surfaces

Yes Adjust temperature profile

No

Adjust oil hotcool temperature

No Adjust or reduce lubrication level Yes Finish No Check previous data

Figure 8.22 Problem: lumpy, cold or ovary surfaces

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Update on Troubleshooting the PVC Extrusion Process

8.1.2.18 Problem: Over-lubrication Figure 8.23 illustrates the troubleshooting process to solve the problem of over-lubrication. Worn-out barrels and screws will cause an appearance of over-lubrication due to decreased levels of shear and material transport in the extruder. Slow reduction in load or amperage and output under identical operating conditions is also a sign of worn-out barrels and screws. A decrease in external lubricants will temporarily alleviate this problem, as will the addition of ≤10% of a pelletised form of the formulation. However, these are, at best, temporary solutions, and product properties may suffer.

Over lubrication

Yes Worn out screw and barrel. No Indicates reduction in amperage or output No Feed the material in pellets or granular form Yes Finish No Check previous data

Figure 8.23 Problem: over-lubrication 132

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.2.19 Problem: Impact Failure Figure 8.24 illustrates the troubleshooting process to solve the problem of impact failure. Inconsistent impact along the profile can be due to trapped gas. Impact-modified formulations tend to collect moisture. Over-lubrication can reduce shear in the feed and transition to an extent that poor dispersion of the impact modifier and inconsistent impact will result.

Impact failure

Yes Adjust screw and barrel temperature No

Check material flowing properly in feed zone

No Decrease lubrication level Yes Finish No Check previous data

Figure 8.24 Problem: impact failure

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Update on Troubleshooting the PVC Extrusion Process

8.1.2.20 Problem: Black Specks Figure 8.25 illustrates the troubleshooting process to solve the problem of black specks. A prolonged extrusion run can sometimes result in black specks in the extrudate due to plate out. High loading of filler can cause exudation or plate out.

Black specks

Yes Adjust the formulation No

Choose better stabilizer and lubricants

No Check barrel and die temperatures Yes Finish No Check previous data

Figure 8.25 Problem: black specks

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Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.2.21 Problem: Variation in Load or Amperage from Batch-to-Batch or Between Batches Figure 8.26 illustrates the troubleshooting process to solve the problem of variation in load or amperage from batch-to-batch or between batches. The quality of the raw material and compounding must be checked. Inherent viscosity (K value) and the particle-size distribution of the PVC resin will affect bulk density, fusion rate, and output. A good quality-control programme for incoming raw materials and blended compound will maintain product consistency and productivity levels.

Variation in load or amperage

Yes Check raw material quality No

Check compounding

No Check feeding in the extruder Yes Finish No Check previous data

Figure 8.26 Problem: variation in load or amperage from batch-tobatch or between batches 135

Update on Troubleshooting the PVC Extrusion Process

8.1.3 Quality Problems in the End Product After production, the product may have defects and problems which may be related to the extrusion process. The flowcharts below illustrate the troubleshooting method used to solve the quality-quality problems in the end product using the extrusion process.

8.1.3.1 Problem: Failure in the Methylene Chloride Test Figure 8.27-8.29 illustrates the troubleshooting process to solve the problem of failure in the methylene chloride test. One of the methods to check the quality of the end product is the methylene chloride test.

8.1.3.1.1 Problem - Inside Portion Granular

136

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Failure in methylene chloride test

Yes

Increase Temperature Oil hot-cool unit

Pump

Motor

No

Increase Temperature

Bz 1 Bz 2 Bz 3 Bz 4 Barrel zones Controllers No

Increase Temperature Aaptor

Aaptor Spider

Die Pipe Die

Increase Temperature

Die Profile Die

Figure 8.27 Problem - Inside portion mealy

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Update on Troubleshooting the PVC Extrusion Process

8.1.3.1.2 Problem: Middle Portion is Mealy

Failure in methylene chloride test

Yes

Increase Temperature Oil hot-cool unit

Pump

Motor

No

Increase Temperature

Bz 1 Bz 2 Bz 3 Bz 4 Barrel zones Controllers

Figure 8.28 Problem: Middle portion is mealy

138

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.3.1.3 Problem: Outside Portion is Mealy

Failure in methylene chloride test

Yes Increase Temperature Aaptor

Aaptor Spider

Die Pipe Die

Increase Temperature

Die Profile Die

Figure 8.29 Problem: outside portion is mealy

8.1.3.2 Problem: Bubbles at the Oil Reversion Test at the Inner Surface Figure 8.30 illustrates troubleshooting for bubbles at the oil reversion test at the inner surface. It is due to the die, which is not suitable for the size required.

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Update on Troubleshooting the PVC Extrusion Process

Bubble at oil reversion testinner surface Yes

Motor

Increase Temperature

Bz 1 Bz 2 Bz 3 Bz 4 Barrel zones Controllers No

Increase Vaccum Motor

Bz 1 Bz 2 Bz 3 Bz 4 Barrel zones Controllers

Figure 8.30 Problem: bubble at the oil reversion test at the inner surface

140

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.1.4 Quality Problems in Pipes Table 8.1 shows the problems related to pipe quality and their probable solution.

Table 8.1 Troubleshooting problems of pipe products with probable extrusion solutions No.

Problem

Probable cause

Troubleshooting method

Mean outer diameter minimum

Insufficient air pressure or vacuum

Increase air pressure or vacuum

Mean outer diameter - over the maximum

Excess air pressure

Reduce air pressure

Outer surface is porous

Surface looks porous due to low temperature

Increase die temperature Also increase other temperatures

Over surface glossy roughness

Excessive temperature leads to a loss of glossy surface

Reduce temperature of the barrel and die

Burned particles on outer surface

Excessive temperature in barrel and die

Decrease barrel temperature in zones 1–3 Possibly increase the speed of the extruder

Burned streaks or streaks on the outer surface

Excessive temperature in the barrel and die

Decrease the die temperature Decrease the overall temperature of the barrel and die

Longitudinal streaks and grooves on the outer surface

Excessive barrel and die temperature

Decrease die temperature Decrease overall temperature of the barrel and die

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Update on Troubleshooting the PVC Extrusion Process

142

Inner surface uneven (rugged) due to use of regrind

Excessive temperature in the barrel zone and mandrel

Increase barrel zone 4 Reduce Mandrel temperature

Inner surface glossy and wavy (overheated)

Excessive temperature in the barrel zone, mandrel and screw

Decrease the temperatures of barrel zones 1–4 Reduce Mandrel temperature Reduce Screw oil cool unit temperature

Longitudinal streaks and grooves on the inner surface

Excessive temperature in the mandrel

Decrease mandrel temperature

Increased porosity in the middle of the wall (thick wall pipes)

Low temperature in the barrel zone and low vacuum

Increase vacuum in the vent zone Increase temperature of barrel zones 1–4

Maximum and minimum tolerances encoded in places located near each other

Excessive temperature in the die and mandrel

Decrease die and mandrel temperatures If not possible to dismantle, clear and reassemble

Thinnest section of the wall located horizontally

Low screw and barrel temperature

Increase screw oil heat cool temperature Increase temperature of barrel zone 4

Vacuum pump drawing raw material

Low temperature in the barrel zone and an excessive vacuum

Decrease vacuum in the vent zone Increase the temperature of barrel zones 1–3

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.2 Troubleshooting for Poly (Vinyl Chloride) Blown Film Table 8.2 illustrates the troubleshooting solution for PVC blown films. Raw material has to be checked thoroughly in blown film to minimize the problems.

Table 8.2 Troubleshooting problems of PVC blown film with probable extrusion solutions No.

Problem

Probable cause

Troubleshooting method

Blocking

Due to poor cooling process Moisture content too high

Reduce resin temperature and increase cooling time Lower humidity of the surrounding air Addition of anti-blocking agents such as silicate powder or slip agent into the air blower

Poor clarity

Mainly due to unsuitable raw material, processing temperature being too low, and a poor cooling process

Change raw materials or adjust formulation Increase the processing temperature Lower the cooling temperature further for better cooling Increase the blown ratio

Sagging

May be due to curtain Slipping of film guide Non-uniformity of blown air

Uneven thickness of film; adjust slit thickness of the die lip Use a longer guide with smaller angle Adjust the direction and rate of blown air Lower the processing temperature

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Update on Troubleshooting the PVC Extrusion Process

144

Scratch mark drawing raw material

May be due to poor surface quality of the guide plate Poor rotation of the guide roll

Polish guide pin Adjustment of the guide roll

Fish eye

May be due to contamination by foreign matter Inferior quality of raw materials Poor dispersion of filler and moisture absorption

Cleaning die, screw/barrel of extruder Switching raw material Substitute with filler of better dispersibility Provide ample drying for raw materials

Spider mark

Mainly due to poor die condition

Cleaning or repairing of die Increase resin temperature

Uneven width of sheet/poor dimensional stability

Due to poor cooling process

Adjustment of cooling conditions

Sticking of melt resin around the die lip

Sticking of resin on the die orifice

Clean the die orifice

Decomposed material

Because of burn-up or hand-up in the die or extruder

Adjust formulation and replace unsuitable materials Clean the die and extruder

Pin hole

Due to processing temperature being too low

Increase setting temperature of the cylinder

Poor fusion

Because of unsuitable raw materials or formulation Poor dispersion of raw materials and low setting temperature

Change raw materials or adjust formulation Replace with raw materials of better dispersion Increase processing temperature

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

8.3 Troubleshooting for Poly (Vinyl Chloride) Sheets In PVC sheet production, material should appear through a coathanger die. There are a lot of possibilities to have problems due to raw materials, compounding and processing. There may be possibility of using the Table 8.3 on troubleshooting problems of PVC sheet to minimize the wastage of the material.

Table 8.3 Troubleshooting problems of PVC sheets with probable extrusion solutions No.

Problem

Probable cause

Troubleshooting method

Fish eye

Presence of high-molecularweight resin

Check all resins and additives for oversized particles, dust or foreign matter Prevent partial gelation to increase shear rate

Oil stain

Reduce moisture content of compound

Replace with materials of better compatibility Use vacuum-type hoppers and a vent-type pre-mixing machine Remove oil film on the final calender roll Prevent adhesion of volatile matter on the take-up roll; clean off oil materials

(observable only under high humidity)

145

Update on Troubleshooting the PVC Extrusion Process

146

Poor ink adhesion and printability

Due to bleeding or oil stains

Use additives which have high compatibility with PVC Pay special attention to the migrating lubricants Reduce the moisture content of the compound Replace with materials of better compatibility Use vacuum-type hoppers and a vent-type pre-mixing machine Remove oil film on the final calender roll Prevent adhesion of volatile matter on the take-up roll; clean off oil materials

Unequal tension on the cooling roll and take-off roll

Increase external lubricant to prevent cohesion

Prevent cohesion of melt on the calender roll Adjust the take-off to give equal tension Gradual cooling instead of shock cooling Do not slack the sheet from the cooling roll to the take-off roll Due to mixing of cold resins in the sheet during processing Ensure appropriate mixing of the compound Maintain stock temperature and constant compound feeding rate Increase temperature of rolls (to soften the resin on the roll bank) Maintain a suitable amount of resin on the bank (smaller bank diameter)

Pin hole

Foreign matter, burnt resin or filler coming off from the sheet at the cooling rate and takeoff rolls

Prevent contamination by foreign matter Check the powder particle size of all solid materials Remove foreign matter on the roll (in this case, pin holes generated in the same cycle)

Troubleshooting Problems in Poly(Vinyl Chloride) Extrusion

Blooming (white powder appears on sheet surface)

Poor compatibility among components present in the compound

Migration of additives from the compound to the contacted plastics Wrinkles in transverse direction

Cold mark

Replace with materials of higher compatibility (e.g., changing of fillers in opaque PVC can sometimes prevent blooming and bleeding)

Reduce concentration of additives such as lubricants and plasticisers; avoid unsaturated materials in the formulation Remove volatile matter in the raw materials and compounds Due to the excellent compatibility between the additives and materials of contacted objects, but replace with materials of lower compatibility with the contacted objects

Blocking (sticking of rolled and piled sheets)

Oozing of lowmelting-point materials to the sheet surface

Presence of vacuum cavities between sheets; replace with materials of higher melting point Apply anti-blocking agents or silica in formulations Apply anti-static agent and allow ample cooling Material: increase lubricant to prevent sticking Adjust formulation to prevent plate out Ensure adequate gelation during mixing; ensure adequate dispersion of additives Calendering: remove stain on the calendar roll surface Prevent plate out Improve smoothness of the roll surface by plating or grinding

Electromagnetic charges

147

Update on Troubleshooting the PVC Extrusion Process

Bubble

Presence of volatile matter, moisture or air in the sheet

Reduce volatile matter and moisture in raw materials and additives: optimum lubrication Pre-heat the system for the compound (e.g., vacuum hopper, vent-type extruder); supply stock constantly Avoid supplying cold stock; maintain consistent and smooth rolling back (prevent chapped surface of rolls) Prevent sheet from coming up the rolls; try to obtain smooth take off; uneven thickness of sheet or machine direction Examine formulation to reduce peeling; improve heat stability and increase the amount of lubricant Ensure good mixing in the blender kneader Calendering: clean the roll surfaces, equalise the temperature of the rolls; adjust roll clearance by crossing and bending; check for burns at the edge of the sheet and check lubrication on the calender rolls Take off-try to achieve smooth take-off

Troubleshooting results show substantial agreement on several basic aspects of performance. However, there is one fundamental issue on which they diverge: whether or not performance differs reliably across individual problems.

References 1.

148

E.M. Mount, III., in the Proceedings of the 61st Annual SPE Conference - ANTEC 2003, Nashville, TN, USA, 2003, p.251.

9

Future Requirements: Developments in Poly(Vinyl Chloride)

Extruded products such as pipes and profiles are the usual applications of poly(vinyl chloride) (PVC). The requirement of homogeneity of structure and properties along the length of the extruded products is of high importance in technical products. The properties along the length of the extruded PVC products may be determined by the measurement of mechanical properties [1–4]. In PVC, inclusion of nitrile rubber improves the oil and solvent resistance, low-temperature flexibility, abrasion resistance, flex resistance, tear resistance, and ageing characteristics [5].

9.1 Poly(Vinyl Chloride) Formulation In PVC formulation, many areas of rigid PVC have not seen dramatic changes in technology over the past 10–15 years. There are areas where the technology is developing rapidly. Rigid PVC formulations have garnered much interest in three areas, in the extrusion of: • PVC foam • PVC–wood composites The most recent formulation technology in each area indicates enhancement by additives on the processability, appearance and performance of the final product. Potential applications of each formulation lead to improvements in certain key properties [6].

149

Update on Troubleshooting the PVC Extrusion Process

9.2 Wood – Poly(Vinyl Chloride) Composites There are environmental reasons for replacing part of the plastic with wood. Challenges for wood–plastic composites include improving the toughness, reducing the weight, and improving long-term properties. There has been a lot of research over the past decades on different types of coupling agents to improve the adhesion between wood and plastic. Wood–plastic composites (particularly PVC–wood composites) are used to replace impregnated wood in many outdoor applications because of recent regulations regarding forest preservation.

9.3 Medical Applications PVC material is very useful for the manufacture of blood bags and other medical applications. PVC is flame-retardant and easy to degrade due to degradation of chlorine present in the polymer, which leads to colour changes. Many flexible products are very useful in medical applications. However, plasticiser leaching in the case of phthalate plasticisers proved to be very harmful. Hence, flexible PVC requires better plasticisers and no oozing.

9.4 Construction PVC is useful in the construction industry. Pipes and profiles are made out of different techniques from other construction products. Flame retardancy is the important property required in the building industry.

9.5 Biodegradation PVC is an unstable polymer. As the chlorine in PVC is freely available, degradation occurs continuously and leads to yellowness or burned material. PVC is cheaper compared with other materials, so mixing of biodegradable materials is not worthwhile.

150

Future Requirements: Developments in Poly(Vinyl Chloride)

In indoor pollution (e.g., chemical, biological, irritants), PVC–wood composites are well-suited and replace traditional materials in building and construction sectors. They require less maintenance and cleaning, which lowers the level of indoor pollution [7]. Furthermore, improvements in long-term properties such as durability during outdoor exposure and long-term load performance, are necessary. Exposure to ultraviolet radiation and moisture during outdoor use is of particular concern for wood–plastic composites [8].

References 1.

B. Terselius, J.F. Jansson and J.J. Bystedt, Journal of Macromolecular Science, Part B: Physics, 1981, 20, 3, 403.

2.

M. Gilbert, Plastics and Rubber International, 1985, 10, 3, 16.

3.

O.P. Obande and M. Gilbert, Plastics and Rubber Processing and Applications, 1988, 10, 4, 231.

4.

K. Bortel and P. Szewczyk, Polimery, 1996, 41, 11/12, 643.

5.

P. Giudici and P.W. Milner in the Proceedings of the 2nd PRI European Conference, Brussels, Belgium, 1976, p.C1.

6.

P. Kroushl in the Proceedings of the SPE Conference Vinyltec 2000, Itasca, IL, USA, p.463.

7.

F.E. Borrelli, Journal of Vinyl and Additive Technology, 2007, 13, 3, 138.

8.

N.M. Stark and L.M. Matuana, Journal of Applied Polymer Science, 2004, 94, 6, 2263.

151

A

bbreviations

ABS

Acrylonitrile-butadiene-styrene

AZO

Azodicarbonamide

DNPT

Dinitrosopentamethylenetetramine

ESO

Epoxidised sunflower oil

HCl

Hydrogen chloride

IR

Infrared

NaCl

Salt

PVC

Poly(vinyl chloride)

RPM

Rotation per minute

RPVC

Rigid poly(vinyl chloride)

TiO2

Titanium dioxide

UPVC

Unplasticised-poly(vinyl chloride)

UV

Ultraviolet

UV-VIS

Ultraviolet–visible

VC

Vinyl chloride

VCM

Vinyl chloride monomer

153

Update on Troubleshooting the PVC Extrusion Process

WPC

154

Wood–poly(vinyl chloride)

INDEX

Index Terms

Links

A Acrylonitrile-butadiene-styrene

34

Additives

94

Agglomerates, sub-grain

104

148

7

Aminosilane

94

Antimony trioxide

95

B Backbiting

76

Barium-cadmium stearate system

32

Black specks

134

Blending

14

81

Blocking

143

147

Blooming

147

Blow moulding

41

Blowing agents

27

Azodicarbonamide

37

Dinitrosopentamethylenetetramine

38

Sodium bicarbonate

38

Blowouts

116

37

118

C Calcium-barium-zinc system

32

Calcium carboxylates

80

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Index Terms Calcium soap

Links 79

Calcium-zinc Carboxylates

79

Stabilisers

24

non-toxic

31

28

Calendering

23

26

Calibration system

55

60

Sizer

60

Calibrator

60

Carotionoid polyenes

76

Charring Chlorine, labile

63

109 14

Cold mark

147

Colorants

40

Compression zone

51

62

Copolymerisation

10

14

Coupling agents

27

γ-Aminopropyltriethoxy-silane

39

Metallic copper complex

39

Organometallic

39

Poly[methylene(polyphenyl isocyanate)]

39

Crammer feed Crystallinity Curling

147

81

56 8 65

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Index Terms

Links

D Degassing zone Dehydrochlorination

128 14

16

28

31

71

73

80

95

Degradation, photochemical

71

75

De-volatilisation zone

58

Dialkyltin, mercaptides of

79

Di-(isodecyl)diphthalate

35

Die design

65

28

71

Die lip

144

Die opening

49

Die system

120

Dies, surface roughness

130

Dihexylphthalate Dimples Discoloration

35 122 8

Dissipative mixing

56

Double-batching

42

Drop weight impact test Dry blend compounding Dull surface appearance

59

16

125 53 40 125

E Epoxidised sunflower oil

30

Esters

79

Ethylene dichloride

11

Extrudate

109

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Index Terms

Links

Extruded polyvinyl chloride - wood flour composites Extruder

97 52

Parallel-screw

58

Single screw

57

Extrusion

54

23

26

34

40

42

49

51

55

93

104

122

136

141 Bubble

148

Hidden defects

104

High-speed

63

Irregular lumps

121

Over-lubrication in

131

Pin holes

144

Regular wavy lumps

120

Scratching

115

146

Single-screw

50

55

Twin-screw

50

55

57

24

26

36

95

104

F Feed dosing Fillers

110

Calcium carbonate

36

Glass fibre

36

Talc

36

Ultrafine

37

41

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Index Terms Film-blowing process Bubble Fish eye Flame retardants

Links 67 68 144 26

Alumina trihydrate

37

Char formation

37

Smoke suppression

37

Floating pressure plug

116

Flow velocity

104

Foaming agents

38

Free-radical polymerisation

10

Frictional heat

37

95

39

95

72

76

150

129

G Gauge variation Grafting

126 14

81

H Heat conduction

55

Heat stabilisers

24

26

40

49

71

75

78

97

Lead

23

28

One-pack

41

Heavy metal stabilisers

29

Hot-cool mixture equipment

41

Hydrogen chloride absorbers

37

scavengers

14

31

79

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Index Terms

Links

I Impact failure

133

Impact modifier

26

36

49

Injection moulding

23

26

41

Initiators, self-accelerated decomposition Inorganic salts

11 79

K Kneading zone

97

K-values

12

135

L Lattice basket

62

Lead salts

80

Lewis acid-base interactions

95

Liquid stabilisers

38

Long-chain fatty esters

33

Low or intermediate shear mixers

42

Lubricants

24

26

33

40

72

74

97

114

M Magnesium hydroxide Melt fracture

95 130

Melt string

55

Melt temperature

54

Mercaptans

78

103

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Index Terms

Links

Metal carboxylates

31

Metallic soaps

80

Metering zone

51

Methylene chloride test

136

Migration of additives

39

Mixer, high shear

41

Mixer, jacketed low shear

41

79 62

N Neopentyl glycol diesters

35

Nip rollers

68

Nitrile rubber

34

Nucleophiles

14

79

Nucleophilic substitution

14

81

O Oil reversion test Bubble Oil stain Organometallic compounds

139 139 145 29

79

81

P Paraffin wax

33

Particle aggolomeration

42

Pellets

97

Phosphate esters

32

Pigmentation

24

95

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Index Terms Pigments Titanium dioxide

Links 27

38

40

38

Pipe die

59

Pipe extrusion, single-layer

62

Pipe/profile extrusion

23

108

124

Plasticisers

24

34

40

Adipates

34

Citrates

34

Leaching of

150

Phosphates

34

Phthalates

34

Bis-(2-ethylhexyl)phthalate

35

Di-(isononyl) phthalate

35

Trimellitates

96

Plate-out reduction

37

Polyene propagation

76

Polyethylene wax

33 7

Living radical

79

Low-temperature living radical

10

Suspension

11

Polymers, synthetic

5

Polyolefin-wood fibre composites

62

49

150

34

Plastic matrix, hydrophobic

Polymerisation

104

11

72

14

95

Polyvinyl chloride Additives Blown films spider mark

25 143 144

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

Core-shell structure of

14

Extrusion

58

Foam

24

Morphology

6

Pipe extrusion

60

Plasticised compounds

37

Production

11

Regrinds

15

Resin

11

Rigid

36

23 72

135

145

Stabilisers

78

Tacticity of

71

Unmodified and unplasticised

16

Weathering

76

Wood composites

107

149

Formulation

Sheet production

103

149

Wood flour composites

96

Wood powder

38

Powder blend compounds

40

Profile extrusion

63

Short calibrator

63

Quinone tin polymers

32

73

151

Q 79

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Index Terms

Links

R Regrind

68

Resins, low K-value

40

Retardants

81

S Sagging Scanning electron microscope

143 7

Screen packs

65

Screw cooling

62

Screw design

52

Screw speed

53

Shear heating

55

Shear stress

54

Sheet extrusion

23

Warping of

65

Silica, precipitated

37

Smoke suppressants

27

Solar irradiation

76

Solid-particle mechanism

68

Spider dies

62

Spiral mandrel

62

Stabilisation, ultraviolet

24

Stabilisers

24

β-diketones

29

Dihydropyridine

29

Epoxides

29

Organic

75

65

96

40

74

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

β-diketones (Cont.) Organic thermal

81

Organotin

31

Polyols

29

Secondary heat

30

Secondary organic

29

Solid barium/cadmium

81

115

Tin

24

Ultraviolet

76

Stabiliser systems, one-pack

32

Stainless-steel calibrators

63

Stearic acid

33

Stearic acid-based lubricants

23

Stress discontinuity

104

String-up melt

109

Surging

42

Swelling

119

30

T Thermal degradation Thermal instability

8

28

30

71

74

77

95

103

9

Thermal stabilisation

75

81

Thermal stabilisers

27

78

9

12

14

16

28

54

68

80

Thermal stability Thermo-oxidative degradation

81

Thermoplastics

68

This page has been reformatted by Knovel to provide easier navigation.

Index Terms Toxicity

Links 14

24

31

79

56

118

U Ultraviolet irradiation Ultraviolet radiation Ultraviolet-visible spectroscopy

72 151 74

78

V Vacuum calibrators

60

Vacuum die

60

Venting

58

zone

56

Vinylchloride monomer

11

Viscosity

40

Inherent

135

Melt

114

Visible defects

54

104

W Water spray system

63

Wood filler

93

Wood-filled wood composite

94

Wood flour

39

Wood, hydrophilic

96

Wood - plastic composites

93

Wood - polyvinyl chloride composites

97

150

150

composites

94

products

93

96

98

This page has been reformatted by Knovel to provide easier navigation.

Index Terms Wood - polymer composites

Links 93

Wood - polyvinyl chloride thermoplastic composites

96

Zinc carboxylates

80

Z Zipper dehydrochlorination Zipper-like splitting

9

74

77

This page has been reformatted by Knovel to provide easier navigation.