Hardy Composites

CSIR GOLDEN JUBILEE SERIES-

HARDY "COMPOSITES

N S K PRASAD

N.S.K. PRASAD

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Publications & Information Directorate (CSIR) Dr K.S. Krishnan Marg New Delhi 110012 India

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Hardy Composites N.5.K. Prasad • © Publications & Information Directorate First Edition : November 1992 Second Edition Third Edition

: February 1994 : January 1996

ISBN: 81-7236-049-5

CSIR Golden Jubilee Series Publication No. 10 Series Editor Volume Editors: Cover Design Illustrations Production

: Dr. Bal Phondke S.K. Nag and Sukanya Datta : Pradip Banerjee : Mohan Singh, ML Mehta, Neeru Sharma, Sushila Vohra and Malkhan Singh : Radhe Sham, Vinod Sharma, K.B. Nagpal and s.c. Mamgain

Designed, Printed and Published by Publications & Information Directorate (CSIR) Dr K.S. Krishnan Marg, New Delhi 110012, India

For sale in India only.

Price: Rs. 30/-

The Council of Scientific & Industrial Research (CSIR), established in 1942, is committed to the advancement of scientific knowledge, and economic and industrial development of the country. Over the years CSIR has created a base for scientific capability and excellence spanning a wide spectrum of areas enabling it to carry out research and development as well as provide national standards, testing and certification facilities. It has also been training researchers, popularizing science and helping in the inculcation of scientific temper in the country. The CSIR today is a well-knit and action-oriented network of 41 laboratories spread throughout the country with activities ranging from molecular biology to mining, medicinal plants to mechanical engineering, mathematical modelling to metrology, chemicals to coal and so on. While discharging its mandate, CSIR has not lost sight of the necessity to remain at the cutting edge of science in order to be in a position to acquire and generate expertise in frontier areas of technology. CSIR's contributions to high-tech and emerging areas of science and technology are recognized among others for precocious flowering of tissue cultured bamboo, DNA fingerprinting, development of non-noble metal zeolite catalysts, mining of polymetallic nodules from the Indian Ocean bed, building an all-composite light research aircraft, high temperature superconductivity, to mention only a few. Being acutely aware that the pace of scientific and technological development cannot be maintained without a steady influx of bright young scientists, CSIR has undertaken a vigorous programme of human resource development which includes, inter alia, collaborative efforts with the University Grants Commission aimed ZItnurturing the budding careers of fresh science and technology graduates. However, all these would not yield the desired results in the absence of an atmosphere appreciative of advances in science

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and technology. If the people at large remain in awe of science and consider it as something which is far removed from their realms, scientific culture cannot take root. CSIR has been alive to this problem and has been active in taking science to the people, particularly through the print medium. It has an active programme aimed at popularization of science, its concepts, achievements and utility, by bringing it to the doorsteps of the masses through both print and electronic media. This is expected to serve a dual purpose. First, it would create awareness and interest among the intelligent layman and, secondly, it would help youngsters at the point of choosing an academic career in getting a broad-based knowledge about science in general and its frontier areas in particular. Such familiarity would not only kindle in them deep and abiding interest in matters scientific but would also be instrumental in helping them to choose the scientific or technological education that is best suited to them according to their own interests and aptitudes. There would be no groping in the dark for them. However, this is one field where enough is never enough. This was the driving consideration when it was decided to bring out in this 50th anniversary year of CSIR a series of profusely illustrated and specially written popular monographs on a judicious mix of scientific and technological subjects varying from the outer space to the inner space. Some of the important subjects covered are astronomy, meteorology, oceanography, new materials, immunology and biotechnology. It is hoped that this series of monographs would be able to whet the varied appetites of a wide cross-section of the target readership and spur them on to gathering further knowledge on the subjects of their choice and liking. An exciting sojourn through the wonderland of science, we hope, awaits the reader. We can only wish him Bon voyage and say, happy hunting.

Human endeavour towards continuous improvement in the quality of life has resulted in a host of new materials and new technologies. Material surfaces get exposed to various environments and also come in contact with other surfaces, similar or dissimilar. Tailoring of material surfaces to suit critical areas of technology has become an art. With the emergence of new fibres of plastics, glass and carbon steel is no longer the strongest material. An imaginative blending of materials with tailored surfaces and incorporation of reinforcement materials in fibrous and nO:l-fibrous plans have created 'Hardy Composite3'. The composite systems are made up of metal, polymer or a ceramic matrix, a particulate or fibrous filler acting as reinforcement and a compatibilizer which tailors the surface and acts as an effective interface between the matrix and the reinforcement. The inherent strength of composites and their ability to withstand critical environment have revolutionized our transportation and communication systems, engineering goods, chemical equipment and packaging materials. Human organ prostheses have also become versatile by the availability of these new materials. The hardy composites, apart from their impact on our present lifestyles, breathe the message that 'unity is strength' and that diverse people can intermingle effectively resulting in composite cultures.

Dedicated to

Science Lovers

Bare Facts In Adding Muscle Glossary Giving Shapely Details At Action Play Shape Joining Hands

... 21 37 1 12 61 69 44 32

wo and two do not always make four. They can be juxtaposed to form the number twentytwo. This just emphasizes the fact that the whole is often better than the sum of its parts. It is the practical manifestation of this phenomenon which has given birth to a very broad and versatile class of man-made materials: composites.

Bare Facts

A composite, as its name suggests, is made by combining two or more dissimilar materials in such a way that the resultant material is endowed with properties superior to any of its parental ones. The ancients knew that when they made alloys of metals they often got good results. By melting together copper and tin they made bronze, the earliest known alloy. Bronze had qualities superior to its parent metals neither of which could be identified once the alloy was formed. To cite a chemical analogy, when sodium and chlorine, each individually harmful if ingested, react chemically, they form the harmless sodium chloride or common salt. Thus, both sodium and chlorine lose their individual identity to create a totally new compound with its own unique characteristics.

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Unlike an alloy or a chemically synthesized compound, however, the components of a composite neither take part in a chemical reaction nor do they dissolve or completely merge with one another. Nevertheless they remain strongly bonded together while maintaining an interface between one another and act in concert to give a much improved performance. Composites are not new. The first composite was probably born in Biblical times when man added chopped straw to clay to make stronger bricks. The trend continued unabated. The iron-rod reinforced concrete, widely used in modern buildings, is also an example of composites. This, however, is just the tip of the iceberg. As man's understanding of nature increased and he dug deeper into the treasure trove that nature offered, a host of new materials became available to him. But, with the inexorable march of civilization man felt that more novel materials were required. Necessity, it is said, is the mother of invention. So, to meet his ever increasing and diversifying needs, man started fabricating new materials from a judicious combination or manipulation of the old. A quantum leap in science and technology brought about the Industrial Revolution in the 19th century. As this revolution progressed and encompassed every aspect of human life, be it travel, work or play, an increasing need was felt for materials robust in nature and capable of resisting fatigue, environmental corrosion, pressure, stress and exposure to chemicals. These also had to be adaptable for use under extreme temperature variations. Newer and more versatile composite materials evolved as an answer to this need. Their emergence has had a tremendous impact in several fields like transportation, marine engineering, chemical equipment and machinery, construction, electrical and electronic equipment, sports goods and medical engineering. The aerospace and defence industries have also benefited greatly from the lightweight yet extremely hard composites that have evolved of late.

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BARE FAGS

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Others Automobiles

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Major marlrets for advanced composite materials in 1995

These alternatives traditional materials

have taken the industry by storm. Composites are one of the fastest growing ind us tries with USA being the major consumer of composite materials. The global consumption of composites is now around two million tonnes annually and growing at the rate of 10 per cent every year.

Composites are materials based on the controlled distribution of one (or more) material(s), termed reinforcement, in a continuous phase of a second material called the matrix. The reinforcement is added to provide strength and stiffness to the composite. The matrix is also known as the binder material. Its function is to make the composite resistant to degradation. There is another, optional class of constituents in composites. This comprises fillers, additives and auxiliary chemicals. It is a matter of choice, dictated by the qualities a composite must possess, whether this class of compounds be br<;mght into use at all.If the insert is added not to boost the strength of the composite but to make it cost-effective or to change a property other than mechanical properties, it is called a filler. Additives are especially chosen and added to improve some specific property. These are usually added in very small quantities. Auxiliary chemicals are usually added to assist or facilitate the processmg of composites. The addition of fillers, additives or auxiliary chemicals does not mean that qualities of composites are being compromised at the

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A compatibilizer is a go-between

altar of economic gains, as is the case when foodstuffs are adulterated. Rather, the addition of fillers, additives and auxiliary chemicals is a matter of deliberate choice l;Jywhich the qualities of composites are enhanced. The ultimate performance of acomposite depends not only on the matrix and reinforcement but also on the matrix-reinforcement interface. The interface is a critical part of composite technology and fabrication techniques. It is controlled by a third material called the coupling agent or compatibilizer. It serves as a go-between for the matrix and the reinforcement. The coupling agent can overcome the weak interaction between the matrix and the reinforcement. The matrix and reinforcement differ in their chemical nature and surface characteristics but the addition of the coupling agent

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results in their closer association, leading to improved strength of the resultant composite. With the giant strides taken by composites 'of late there have been parallel developments in the field of coupling agents too. This new class of compounds has evolved in the last two decades to cater to divergent matrices, reinforcements and composite fabrication techniques. Composite materials may be broadly classified into natural and synthetic. Our bones are excellent examples of natural composites, being made up of two basic kinds of materials. The organic component consists largely of carbohydrates, fats and proteins while calcium phosphate is the inorganic component. The organic components make bones pliable while the inorganic component confers rigidity and strength. Syn- .

The organic components make bones pliable while the inorganic component confers rigidity and strength

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thetic composite materials are generally prepared by taking the individual constituents and physically combining them by different techniques. The composite matrix can be a plastic (resin), a metal or a ceramic. It is responsible for the inte-grity of the composite compound. Plastics matrix based composites constitute more than 95 per cent of composite materials in use today. Plastics are of two main types: thermoplastics and thermosets. Thermoplastics melt or soften when heated but become rigid and hard when cooled. THus, they can be melted down to be reprocessed many times. Thermoset plastics, however, do not become soft on heating and do not melt once they are set. Very high temperatures cause them to decompose. They are tougher and harder than thermoplastics. Both thermoset and thermoplastics are basically long chains of carbon atoms. The main difference is that two chains of thermoplastics are capable of sliding over each other without interference from the other chains. In thermosets, the chains are entangled wi th one another and hence no chain can slide over any other chain. B0th types of plastics have their uses. If a relatively soft and flexible plastic is required, thermoplastics are chosen while thermosets are ideal if good mechanical strength is required. Since the matrix resin determines the properties of composite, the choice of a particular resin too is determined by the ultimate properties that the composite must have. In metal matrix composites, the choice of the metal is governed by factors such as lightweight and high temperature resistance. The aerospace industry has always stressed its need for lightweight metals, metal alloys or substitutes which could be used to make aircraft parts. These increase the fuel efficiency and maneuverability of an aircraft without jeopardizing safety. Boron reinforced aluminium is very popular for aircraft applica tions. Ceramic matrices are chosen for their toughness.

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Normally, composites -are made up of two dissimilar materials. But in certain cases there are exceptions too. Ceramic-ceramic composites are one of them. In these composites ceramic matrices are reinforced with ceramic fibres. These are considered composites even though both the matrix and reinforcement are ceramic because the two are in different forms. The matrix maybe a sheet or fluid and the reinforcement present as fibre. Ceramic matrices reinforced with ceramic fibres are in demand for making biomedical

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All the kiT)g's horses and all the king's men Could not pull the composite apart again

implants and aerospace vehicles. Ceramic composites are used to make protective heat-shields for missiles to protect them from friction induced high temperatures generated by the very fast movement of the missiles through air. These composites are also used by the defense industry to provide a tough armour to tanks. While matrix and reinforcement are no doubt the two main items that determine how well the composite will work, the overall performance of a composite also depends on the nature of the bond between the matrix and the reinforcement. This is because this pond or interface governs the transfer of stresses and strains from the matrix to the reinforcement, thus controlling the mechanical performance of the composite. The matrix and the reinforcement differ in their chemical nature and surface characteristics and there is thus bound to be a gap in the two phases of a composite no matter how closely they may be bonded. Coupling agents or Compatibilizers come into play at this junction. They are, as their names indicate, a "go-between" for the matrix and the reinforcement. The use of a compatibilizer forms a closer association between the matrix-and the reinforcement. Thus, there is continuity and uniformity of properties throughout the composite. This even distribution means that the ultimate

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strength of the composite is enhanced. This also prevents environmental moisture, gases and chemicals that adversely affect composite performance from permeating it. The choice of the coupling agent depends on the function of the matrix and the nature of the reinforcement. The quantity of coupling agent that is to be used is governed by the desired mechanical properties of the interface. The coupling agents themselves are also a new class of compounds that have evolved in the last two decades to cater to divergent matrices, reinforcements and composite fabrication techniques. The interface control is regarded as the most critical area and is usually eva,luated by commercial experience and closely guarded as secret data by the manufacturers. The use of different types of matrices and reinforcements have resulted in the adaptation of different techniques for the fabrication of the composite. The fabrication method has to suit the properties and physical nature of the particular type of resin and reinforcement used. Thus, the fabrication technique has also evolved greatly in the past few years for it is also a significant factor in determining the ul timate strength of the composite. Composites have become a part of our everyday life, but so quiet and efficient has the take over been that most of us are unaware of the role that they play in making the objects around us long lasting and aesthetically attractive.

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he matrix is an important component of a composite. Not only does it give shape to the composite but also makes it resistant to harsh environmental conditions.

Shapely Details

Spectacular strides in chemistry have spewed out a bewilderingly wide array of suitable matrices. These could be metal, ceramics or, as is usually the case, polymers. A polymer is a giant organic molecule formed by the union of simple molecules called monomers. The meaning of these words become self-explanatory when broken down; the word 'poly' means many, 'monos' mean single and 'meros' means parts. A 'polymer' thus means compound of many parts or many units. Just as thousands of beads can be linked together to form a necklace, so too can monomer molecules form a polymer. Monomer molecules vary for different plastics. When two or more different types of monomers are involved, the product is called a copolymer. Plastics are by far the most commonly used polymers today. Thermoset plastics, both in the monomeric and polymeric stages are suitable matrices. Their 'chemical structure is such that

SHAPELY

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~B'

A homopolymer is made up of the same monomers, whereas a copolymer can have different monomers

they have a strong cross-linked network which provides -rigidity_ The structure can be visualized by imagining lines of monkeys holding hands. Not only do the monkeys hold the

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hands of their adjacent partners at either side but some of them also hold on to others in the next line by means of their tails. Such an arrangement means rigidity because the crosslinkages hold the lines apart and these cannot reorien t or slide past one another. This strong, stiff, cross-linked network stands the industry in good stead when rigidity is a prime criterion. Thermosets also have low viscosity or thickness and thus allow ready impregnatiion of fillers. Thermoset resins include organic compounds like alkyls, polyesters, epoxies, phenolics, silicones and urethanes.

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The general purpose polyester resins are the cheapest resins suitable for making fibre reinforced plastics. Epoxy resins score over the others in that they are amenable to processing by a variety of techniques. They also accept a wide range of fillers and pigments. In addition, they possess desirable chemical properties and toughness, coupled with good electrical properties. The novel vinyl ester resins, originally manufactured and marketed by the American company Dow Chemicals under the name Derakane, are essentiallya compromise between epoxies and polyesters. They are basically epoxy resins with special chemical groups that allow them to behave like polyesters. This marries the chemical resistance, toughness, ductility and affordable price of the epoxies with the malleability and fabrication convenience of polyesters. Phenolics are the third most important composite matrices or binders. Phenol-formaldehyde resins have corne to the fore whenever fire resistance and low smoke evolution have been the criteria of choice. Glass fabric-phenolic composites are excellent for underground train panels where smoke is not desirable. They are also used in high performance cars and in cabin and cargo furnishings of aircrafts. Similarly, the outstanding electrical properties of silicone-fibreglass laminates have helped them to carve out their own industrial niche. Silicones are later additions to the family of plastic. These are based on a polymeric chain where silicon and oxygen atoms alternately form a sort of an inorganic skeleton. Silicones are expensive but their outstanding non-inflammable and heat- resistant nature warrant their use in many places. Heat shields and solar panels use silicones. The range of thermoplastics is particularly impressive. This includes the commonly used and familiar compounds like polyethylene or polythene, as it is better known, polyvinyl chloride (PVC), polypropylene (PP) and polyethyleneterephthalate (PET) as well as advanced materials

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such as nylon (pol yamide), vespel (polymide), torlon (pol y am i de-i mide) and arbel, aryl on or durel (polyacrylate). Polythene is the most widely used plastic in the world. It is manufactured from the monomer eth ylene, which is a combustible gas made up of carbon and hydrogen, and obtained from crude petroleum. These are two types of poly thene, low density polyethylene (LOPE) and high density polyethylene (HOPE).

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Ethylene molecules join to form a single, giant polyethylene mol(?(2ule

rye has a structure similar to polythene except tha t there are chlorine a toms a ttached to al terna te carbon atoms instead of hydrogen atoms. It has remarkable resistance to water, grease, oil, petrol and many corrosi ve chemicals. rye coated on woven fabrics prod uce leathercloth or rexin.

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Polypropylene made its debut as the 'space-age polymer'. It is a polymer of propylene - a fact from which it derived its name. It is perhaps the lightest known industrial polymer, although its strength to weight ratio is very high when compared to other polymers. It is hard, tough and can resist high temperatures. Polypropylene has an almost unique nonvulnerability to bending. Polypropylene sheets do not break even when bent alternately in opposite directions thousands of times. PET is a polyester made from ethylene glycol and terephthalic acid. The transparent bottles in which cooking oil is sold are made of this plastic. These are totally impervious to air. Nylon is the first truly man-made fibre. It belongs to a group of polymers called polyamides and was invented in the 1930's by Wallace Hume CAROTHERS (1896-1937). The delicate looking nylon thread can equal if not surpass the strength of steel. It has only one-seventh the weight of a steel wire of the same diameter and length and hence its tensile strength per unit weight is superior to steel. Polyimides like vespal are not new. This polymer was patented in 1969 even though it took two more years for it to be commercially produced as films. These films are particularly successful from the commercial point of view because of their comparative cheapn~ss. Polyamide-imides are again man's variation on a theme. Here the basic polyimide structure was modified by incorporating amide linkages into it. This led to improved processibility, but it also made the compounds less resistant to high temperatures. Temperature sensitivity is thus their greatest drawback, although the parental polyimides are well recognized as having superior properties especially suited for extreme environments. Metal matrix composites form another group of extremely promising materials. The metals that have excited interest are aluminium, magnesium and metals with special electrical

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Model of a part of a nylon polymer; formation of a nylon chain (inset)

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DETAILS

Vertical Shear Webs, Stabilizer Engine....•••.••. '" Thrust Structure

Rudder

,~'-...: Landing Gear ~ Doors Cargo Doors and Radiator Panels Fuselage Panels

A space shuttle orbiter configuration showing composite applications under study

properties, like copper and lead. Titanium and nickel attract interest because of their great strength. Metal matrix composites are markedly superior to their unreinforced metal counterparts, particularly with respect to strength and stability at very high temperatures. In comparison to resin matrix composites, metal/matrix composites are easier to form and weld. They are also more resistant to both heat and cold. Thus, they can also resist hazardous environmental conditions better. This makes them prime candidates for use as structural materials in demanding environments. However, metal matrix composites are more expensive to produce than resin matrix composites. But, depending on the field of specialization, metal matrices are often the best choice for composites. Reinforced copper, for example, replaces steel in making the Gatling gun. The new composite gun is lightweight and this allows it swifter move-

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ment and more maneuverability, which gives it a decisive advantage over old fashioned heavy guns. Aluminium reinforced with boron is a favourite of the aerospace industry. Cri tical aircraft and space shuttle designs need superior materials. Boron aluminium composites have the strength and rigidity of steel while being lighter than aluminium. It is not surprising, therefore, that it is the only material which has been fully qualified for critical structures like those in space shuttle orbiters. Aluminium-titanium alloys reinforced with graphite are used when stability against heat is the criterion. Copper, reinforced suitably, enjoys widespread use in diverse fields like ordnance, space, missiles and electronics. Lastly, there are the ceramic matrices. Ceramic matrix composites evolved when naturally occurring particles like clay, zircon, graphite and mica incorporated. into aluminium or its alloys gave good results. This ushered in a whole new class of matrices which are now gaining worldwide popularity. A ceramic matrix composite valve in a high pressure slurry pump can increase its service life 45 times over that of conventional stainless steel ones. These matrices are exceedingly versatile and find use in areas that require the ultimate in resistance to abrasion, erosion and the ability to withstand chemical treatment as well as heat shock. But matrices tell just half the tale. Inconspicuous yet integral support is provided by reinforcements which add another dimension to the multifaceted story of composites.

reinforcement, as the name implies, strengthens or adds muscle to a composite. The structural properties of composites are primarily governed by reinforcements or fillers. There exists a large number of reinforcement materials to choose from and the choice is dictated by the property to be imparted to the composite. Reinforcement materials may be either non-fibrous or fibrous in nature.

Adding Muscle

The spectrum of non-fibrous reinforcements in modern composites includes calcium carbonate, calcium silicate, oxides of silicon, aluminium or titanium and sulphates of calcium and barium. Carbon black, mica, fabricated spheres of glass and ceramics are also good nonfibrous reinforcements. These reinforcements help impart certain desirable characteristics to composites. The plasticised polyvinyl chloride (PVC) and inert fillers like calcium carbonate, china clay, talc or barium sulphate increase hardness and improve electrical insulation properties. They also reduce tackiness and make the product look nice. Over and above all this, addition of these fillers consider-

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~YPES OF FILLERS IN COMPOSITES

Organic

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Inorganic

ably reduces production cost. The mineral wollastonite, on the other hand, is the reinforcement of choice for providing excellent stiffness and strength. It is also the only material suited for liquid crystal polymers used as matrix resins. Kaolin provides increased impact strength and is extensively used with nylons. Talc is used as an antiblocking agent for low density polyolefins which are specifically used for packing food items. These have to be of very high grade and meet stringent safety requirements.

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Lightweight yet strong furniture are ideal for public places

Mica is widely used to improve appearance and as a barrier material in polypropylene. When coated with zinc, copper, aluminium or stainless ste~l, mica is more cost effective than metal flakes and, as such, is used for conductive and decorative effects. Public places like auditoriums and airport lounges have colourful and very modern furniture these days. These multicoloured, strong yet lightweight and extremely attractive furniture are made of polypropylene. These have a very smooth and warm feel, so that the user feels comfortable. Talc and calcium carbonate of controlled particle size bestow stiffness and ceramic-like feel to polypropylene outdoor furniture. Calcium and barium sulphates are used as fillers in microwave cookware compounds, since they add weight to the product and impart chemical resistance to it. High purity silica is used for reinforcing cast polyurethane compounds, epoxy castings, and wire and cable compounds for high temperature insulation. Thus, by judicious selection of rein-

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Hollow spheres help stop propagation of cracks

forcements, an entire range of qualities may be introduced in a composite. Hollow spheres are especially created reinforcements. Almost 98 per cent of these are made of vitreous and borosilicate glass. A special type called 'cenosphere' is made from fly ash, alumina and carbon. Hollow spheres find application in buoyancy products, acoustic panels, moulds and tooling fillers. They contribute to the stability of the product, provide impact or shock-resistant properties and add rigidity and stress resistance to the composite. This may sound like a paradox. For how can fragile glass beads provide impact insulation? The answer lies not so much in the property of the beads as in the way spheres behave when packed together. These hollow spheres behave in the same way as marbles do when packed in a box. There is enough space left between individual marbles even when tightly packed. In contrast,

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packed cubes of a similar size do not leave so much space. Due to this structural behavior of composites, the impact of a shock wave is drastically reduced. This is so, because a stressing force acting upon a composite is used up in moving the hollow spheres, the shifting of which broadens the tip of a crack. Therefore, the effect of a shock wave is lessened as it spreads over a large area, thus preventing the composite from cracking. Hollow spheres are generally used with epoxies, polyesters, vinyls, and polyurethanes. Silica-alumina glass beads improve impact strength in polyesters, polyurethanes, thermosets and exhibit high compression strength and heat stability. Fibrous reinforcements have been the key factor in the rapid advancement of composite technology into conventional areas as well as specialized, hi-tech ones. There is now a wide spectrum of reinforcements to choose from. Glass fibre is the most widely used reinforcement, making up almost 90 per cent (in terms of volume) of all fibrous reinforcements used. This is because it has several advantages. It is lightweight, has high tensile strength and high heat resistance. It is virtually fireproof, inexpensi~e and available in various forms: yarns, cords, tapes, braids and mats to name but a few. There are quite a few grades of glass fibre to choose from. The common grades are E-glass fibre, C-glass fibre, S-glass fibre, Z-glass fibre, M-glass fibre and D-glass fibre. Each grade has a specific property. For example, E-glass fibre has excellent corrosion and environmental resistance. It also imparts a high level of electrical resistivity. C-glass fibre offers higher acid-resistant properties than E-glass while Z-glass fibre produces exellent corrosion resistance to alkaline solutions. Glass fibre based composite materials are not suitable for very high performance applications. This is because these

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fibres are self-abrasive and cannot tolerate much stress. They have relatively low fatigue resistance. They also have poor adhesion to matrix resins which means that they do not impregnate the matrix uniformly and well. Thus, replacements for glass fibres become necessary and carbon fibres often replace them. Carbon fibres have strength vastly superior to that of glass fibres and have superior heat stability as

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Elasticity or strength?

well. Chemical composition plays an important part in determining the properties of carbon fibres. This depends on the source material used to make these fibres. Generally, the higher the carbon content of the fibre, the greater the elastic characteristic and the higher the nou-carbon content, the more the strength. Thus, depending on whether the emphasis is on elasticity or on strength, the user can choose his own carbon fibre. The usual precursor material used are pitch, polyacrylonitrile (PAN) and staple rayon fibres.

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All objects expand on heating and contract on cooling and many exposed structures show a daily as well as seasonal cycle of expansion and contraction depending on the ambient temperature. Engineers routinely make allowances for this when making bridges or laying railway tracks. But as carbon fibres show no expansion or contraction across widely ranging temperature differences or thermal cycles, they find application in electronics instrumentation and space vehicles. GRAPHIL is a PAN-based fibre especially designed for applications calling for exceptional structural stability of the product. Because carbon fibres are heat stable, they can be used to reinforce ceramics, metals and plastics, thus yielding a plethora of composites. Boron fibres have low density coupled with high tensile strength and are extremely hard. Boron composites are expensive but since they are inherently tough they possess the greatest potential for use in aircrafts. Polymers touch every moment of our lives. The nylon toothbrush, the plastic bucket, the polyester dress material or the polystyrene umbrella handle are all polymers. Knowingly or unknowingly every individual today relies on polymers to meet his needs. Apart from their direct uses, polymers also meet many other demands which may not be quickly apparent. One such use is their role as reinforcements in composites. Polymer fibres have for long been used as reinforcements in automobile tyres, large balloons, parachutes, fuel tanks, storage tanks of various types and in rubber coated fabrics. Such use was initiated by the American company Du Pont which introduced 'Kevlar . Kevler is a plastic which is five times stronger than steel on a weight to weight basis. It is a product derived from a reaction between para-phenylene diamine and terephthaloyl chloride. It has high strength, high temperature durability, impact resistance and can buffer vibrations too. It is not surprising, therefore, that it has replaced asbestos and found wide application in clutches,

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The world of polymers

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brakes and several other friction applications. In recent years, applications in high performance structures for aircraft, space systems, automobiles, sea-faring vessels, solid-rocket missile cases and sporting goods have been spurred by the development of improved aramid fibres, the class to which Kevlar belongs. The success of aramid fibres has also spawned a variety of other polymer fibres based on nylon, terephthalates, polyethylene, polyetherketone and polyphenylene sulphide, all of which are used as reinforcements. Terephthalate and nylon fibres are used in advanced composites in fields as diverse as automatives, food packaging and construction application. High density polyethylene fibres are used in helmets, conveyor belts, hoses, artificial limbs and joints. Sports goods such as bicycles, ski equipment, fishing rods and tennis racquets are also made using polyethylene reinforcements. So good are the tennis racquets made of composite materials that Bjorn BORG had to switch over to them when he recently found that the wooden racquets that had given him so many Grand slam victories including those at Wimbledon in the past were at a definite disadvantage at keeping pace. Jahangir KHAN the squash player par excellence plays with composite racquets.

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Polymer fibres have also been used in composites for grill opening panels, door support beams, highway guard rails and chemical plant hand rails. Ultrahigh molecular weight polyethylene (UHMWPE), which on a weight to weight basis is claimed to be the strongest fibre available today, is used to make hulls of sail boats, tilt rotor aircraft and racing cars. Natural fibres include both organic fibres as well as inorganic fibres. Organic fibres are the traditionally used fibres of jute or coir while inorganic fibres include asbestos which may be incorporated in many forms. Asbestos is a variety of fibrous minerals derived from silica. Asbestos fibres provide thermal protection. They are resistant to heat, flame, many chemicals and moisture. Asbestos reinforced plastics carry the advantages of corrosion and erosion resistance with tolerance to high temperature, low cost and good machinability. Asbestos fibres have another use apart from their direct use as a reinforcement. They are used to better the bond between resin and glass fibres, thus making glass fibre reinforced plastic structures cost effective and economically feasible. However, the overall performance of a composite depends not only on the correct match of matrix and appropriate reinforcement. It also depends on the bond or interface forged between the two phases. Interface coupling agents not only dictate how well a composite will eventually perform but in themselves provide a fascinating study of the concept that two dissimilar materials could be held together by a third intermediate.

ow/

hat two dissimilar materials can be held together as a functional whole by means of an intermediate coupling agent is not a new idea. The concept of the universe as held by the ancient Greek philosopher PLATO was nothing but that of a composite. He visualized the universe as being made up of four distinct elements: earth, air, fire and water.

Joining Hands

It was not till the contemporary times, however, that composites entered the world of hi-tech. With the advance in composite technology in recent times, the interface or bond between the matrix and the reinforcement came under scrutiny. It was realized that the overall performance of a composite is largely dependent on the interface. This is because the interface governs the transfer of stresses or strains, also called impact energies, from matrix to reinforcement. This, ultimately, controls the mechanical performance of a composite. The gap between matrix and reinforcement must be minimized. Consequently, theimpregnation of the reinforcement by the matrix must simultaneously be maximized. This ensures continuity and uniformity of proper-

••••

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33

JOINING HANDS

\

ties throughout the composite which, in turn, prevents environmental moisture, gases or chemicals from permeating the composite and adversely affecting its performance. Coupling agents have come into play, as far as composites are concerned, in a big way. They have an important role in improving the bonding of the matrix and the reinforcement at the interface. It is necessary to have coupling agents, especially when an inorganic moiety is used as reinforcement in organic resin matrices. The first commercially successful material to be used as a coupling agent was a methacrylate chromic chloride (Volan). This was designed for use with polyester epoxy and phenolic resins. As the need for better coupling agents multiplied and technology advanced in parallel, a new class of coupling agents emerged. These were the organic derivatives of inorganic elements which functioned or behaved as organic compounds. Such organo-functional derivatives derived their

HARDY COMPOSITES

34

names from those of their parent elements. For example, silanes, titanates, zirconates and aluminates took their names from silicon, titanium, zirconium and aluminium respectively. As most of the fillers used in composites are siliceous in nature, coupling agents derived froIP silicon are the most widely used. For non-siliceous fillers like carbon or calcium carbonate, the coupling agents derived from titanium and zirconium are better suited. Silicon based coupling agents are of the general formula R-Si (CHz)nXl where R is an organic radical and X is a group which can undergo hydrolysis. The organic radical in the coupling agent can be vinyl, chloropropyl, epoxy, methacrylate, primary amine, diamine, mercapto or any other group desired for compatibility with a particular resin system. The hydrolysable group may be either a hydroxy, chloro, alkoxy, acetate or dialkyl amino group. Polymeric silanes have also been developed for application as coupling agents. Silane coupling in composites have a big role to play in the treatment of fibre glass, the most common reinforcement. Adhesion between the coupling agent and glass requires not only a reactive silane but also reactive sites called 'silanols' on the glass surface. Silane coupling agents react with the surface silanols through their hydrolysable groups. The silane is pre-hydrolysed to silanol. This reacts with the glass through a silanol condensation reaction. R SHOCHJh

3HzO >R-Si(OHh R

R

R

o-si-o-si-o-si-O

6 6

3

RSi(O::+H Inorganic glass surface

.~ Polymerised silane on glass surface

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JOINING HANDS

For best bonding between the silanized inorganic surface and the resin the functional group (R) attached to silicon should readily react with the resin. Generally, 0.2 -1.0 per cent of the silane on the surface of a filler is sufficient to improve dispersion and reinforcement of the polymer matrix. Just like silanes, the corresponding organo-functional derivatives of titanium, or organotitanates, have a general formula:

C

(RO)m - Ti - (0 - X - R' - Y)n

In the above formula, m and n can vary from 1 to 6. According to the increasing values of m and n, the organotitanates are classified as monoalkoxy, chelate, coordinate, quart, neoalkoxy and cyclohetero atom types. The titanate use level varies from very low or minute quantities to 10 per cent of composite formulations. In general, they are used in the range 0.1 - 0.5 per cent. Organotitanate coupling agents have gained c':>mmercial importance as they show several desirable characteristics. They encourage adhesion between reinforcement and matrix, and make a composite resistant to water and corrosion. They also give some protection against fire by making the composite flame retardant. Zirconate coupling agents are similar to titanates. Aluminate and zirco-aluminate coupling agents are under development. The development of new plastics is oriented not only towards novel types of plastic materials but also towards the creation of polymer alloys and blends. The objective is to combine two well known plastics to achieve a product with a balance of the best properties of the parent plastics and also to simultaneously achieve cost reductions. Polymer systems like polypropylene-nylon, polystyrene-ole fins, polyethylene and terephthalate-olefins have thus evolved. The combination of two dissimilar plastics to produce a suitable blend also requires the use of compatibilizers or coupling agents. Such polymer-polymer blending has been achieved by the use of several compatibilizers such as maleic anhydrid-e-, styrene

36

HARDY COMPOSITES

AREAS OF APPLICATION OF ORGANO- FUNCTION AL SILANES

Alkyl

Silazane Amino Diamino epoxy phenyl Modified Carboxyamide Epoxy Methacryl Mercapto

Silica treatment for silicone elasSilane type Hydrophobic Applications treatment Epoxies, Polyesters, urethane, rubber, acrylic, polyolefins, polysulPolyimide, photoresist adhesion High temperature thermoplastics Nylon, Rubber epoxy, reinforcements, phenolics, nitrile, melamines, phenolics, polysultemperature polymers and thermosets melamines, other thermosets and thermoplastics phide composites, coatings, circui t boards sealants acrylics, polysulphide hesion tomers, promoter Novalac photoresist adnylon, PVC, urethanes, acrylics

maleic anhydride and reactive polystyrene. The new polymer systems have enhanced the potential of forming new composites.

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he best matrix, reinforcement and compatilizer will not give the best composite material unless the correct production technology is adopted to give it shape. The use of various types of matrix resins and reinforcements and the different properties desired in the final cornposi te has thus necessitated the utilization of many fabrication techniques. Each technique has its own advantages and this is selectively made use of by the industry.

Giving Shape

There are basically two approaches for the processing of composite materials. In the first, the ingredients are taken separately and combined at about the time of fabrication. The second technique relies on a semi-processed mixture of fibre and matrix. This is usually used for bulk production. However, there are almost as many variations of these two themes as there are types of composites. Hand lay up: This is the simplest composite fabrication method. It involves placing of fibre-reinforcement mats or fabric in an open mould. The resin may be then applied by pouring, brushing or spraying. Mats pre-impregnated with resins may also be used. Hand lay up offers the max-

HARDY COMPOSITES

38

Fabrication tcchniques

Vacuum bag moulding Pressure bag moulding Autoclave moulding

Vacuum

impregnation moulding

Resin

transfcr moulding

Reinforced reaction jnjec~

lion moulding

Processing techniques of composite materials

Reinforccment

Hand layup

imum in design flexibility with the minimum in equipment investment. But it does involve a lot of manual labour. A closely related fabrication method, tape laying which uses pre-impregnated ribbons has also become an important composite fabrication method. This, unlike the hand lay up is an automated process.

GIVING SHAPE

39

Spray-up: In spray-up, bundles of filaments used as reinforcement, and which are called roving are fed through a chopper into a resin stream and then sprayed directly into an open mould. Spraying-up equipment is usually inexpensive and portable and roving is the least expensive form of reinforcement.

Vacuum-bag

Spray-up

Vacuum-bag: Closely related to spray-up a.nd hand lay up, vacuum- bag moulding involves the placement of a flexible film over the resin and fibres in the mould. The joints around it are sealed and a vacuum is created. This results in better adhesion between layers and hence in a better composite.

Pressure-bag: This is analogous to vacuum-bag molding except that air pressure is applied directly to the rubber sheet. Pressurized steam may be used to bring about a change in the property of the resin, a process called curing. Both vacuum-bag and pressure-bag moulding are more labour-in tens-ivetRaIl hand lay-up or spray-up.

I Mould

Laminate

Pressure-bag

Autoclave moulding: Additional heat and pressure may be applied with autoclave equipment. The equipment which is similar in principle to a domestic pressure cooker is generally expensive. But autoclaves do allow higher reinforcement levels resulting in faster cures and superior properties in the

40

HARDY COMPOSITES

composites. Exotic, high-temperature resins can be used by this technique to make novel composites. Reinforced reaction injection moulding ( RRIM): It involves the mixing together of reactive resin components and injecting them into a mould while they are still liquid. Inside the mould they polymerize to their final shape. Polyurethanes, high temperature polymers, polyesters, nylon and epoxy are all available as commercial RRIM systems materials. Filament winding: Providing the highest strength-toweight ratio, filament winding consists of feeding reinforcement filament or roving through Laminat,,a resin batch and winding it on a Continuous strand mandrel A. mandrel is a core roving around which paper, fabric or resin im pregna ted glass is ~ ~ wound to form pipes, tubes or ~ vessels. Special winding machinery lays down the impregnated roving in predeterFilament winding mied patterns, giving maximum strength where required. After the appropriate layers are applied, the wound mandrel is cured and the molded part removed from the mandrel. Filament winding provides the greatest control over orientation and uniformity. Pultrusion: This is a continuous method for moulding parts with a constant cross-section. It is perfect for moulding pipes, beams and fishing rods. Pultrusion involves passing continuous roving through a resin bath and then drawing the resinim pregna ted reinforcemen t material through a steel die. The Resin applicator die controls the shape and is usual1y heated to initiate cure. Pultrusion But the final cure is carried out in

41

GIVING SHAPE

an oven or heating chamber through which the stock is drawn. The process yields continuous lengths of material with high unidirectional strength as well as high reinforcement to resin ratio. Pultrusion and filament winding are two of the fastest growing composite fabrication methods. Injection moulding: It is the fastest growing method of fabricating composites. Injection moulding can produce comple}t, highly detaIled parts ranging in size from small precision components to automobile bumpers. In this process, pellets, compound concentrates or resin-reinforcement blends are heated until they are fluid. They are then injected under high pressure into a cold, closed mould. Thermoplastic matrices are perfect for injection mouldinK- Injection moulding of Injection moulding thermosets is also undergoing rapid growth, for which the process usually involves injection of cool reactants into a mould that is heated to carry out curing. Resin transfer moulding: In this process, fibre mats are placed in the desired orientation in a mould which is closed and filled with low viscosity resin. The mould is then heated to cure reactive resins. But retaining fibre orientation during resin transfer moulding is sometimes difficult and placement of fibre mats is often labour intensive. Cold stamping: It is a high speed process, which involves preheating reinforced thermoplastic sheet blanks and stamping them in to the desired configuration on metal stamping process. The ready integration into current production lines and

'Reinforced

thennoplastics sheet ~

~

R

\\ 1//

Heat

source

Cold stamping

HARDY COMPOSITES

42

I l

The puppy keeps its cool

the high mechanical properties available with a rapidly growing family of engineering resin matrices and continuous reinforcements promise to make cold stamping a key process in the industrial composites area. The thermoplastic advanced composites, promise significant advantages over thermoset systems. Their damage tolerance is 10 times as high. They have improved microcrack

43

GIVING SHAPE

resistance and negligible moisture absorption. They also have superior flame and radiation resistance. They require no refrigeration and do not undergo change in properties even after extended storage. Parts may be reheated and reformed which means that the scrap can be recycled. Furthermore, no toxic emission is produced during processing. Despite these advantages, the use of thermoplastic reinforced composites is still small and thermoset matrices command around 90 per cent of the advanced composite market. Part of this dominance is due to the early development of thermosets that could be used with the reinforcements, be fabricated into light weight complex parts and tolerate the heat as well as mechanical and fatigue loads placed upon them. Nevertheless, a wide and growing family of new materials and their novel combinations promise more thermoplastic advanced composites in the future. However, just making a composite is not the end of the matter for the manufacturers. The newly formed composite has to be tested for its inherent qualities. Also strict quality control measures have to be taken to ensure that there is uniformity in the quali ties of the composites produced in each batch. There are many parameters which must be assesed before a composite is released to the market. There are standard methods for testing most of the properties. But certain inherent defects in composities cannot be directly assessed by these methods. Defects such as wrinkles, presence of moisture, unintended inclusions, damaged reinforcements, matrix cracking and reinforcement misalignment cannot be assessed by conventional methods. Specialized testing procedures have to be employed to assess the quality of the composite when these parameters have to be judged.

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In Action

he brave new world of composites is multipronged in action. There is practically no sphere of modern life where composites have not made their presence felt. The industries that have gained substantially by replacing conventional materials with composites are many. The transportation industry in particular has been revolutionized by the advantages derived from the use of composites. The electric and electronics industry, chemical and nuclear plants as well as the biomedical industry that makes artificial limbs have also greatly benefited from composites. Little wonder then, that the advanced composites industryis a multi-billion dollar one and growing by leaps and bounds every year. In peace time, in war, at work or at play, composites are replacing many of the conventional materials and doing the jobs more efficiently too. From the simple bicycle to the space shuttle, composites have reshaped the transportation ind ustry. Peugot, the famous French car manufacturers, have recently entered the field of bicycle manufacture. In keeping with the times, the Peugot bicycle relies heavily on composite

IN

ACTION

45

Cycling to victory with composites

materials. The frames of these hi-tech bicycles are made of glass / carbon-epoxy composites. This leads to a 30 per cent red uction in weight with the added ad vantage that the frames are eight per cent stronger than conventional alloys. The composite cycles have more than proved their worth. Greg Lemond, three-time winner of the Tour de France, the world's

46

HARDY COMPOSITES

most exacting and exciting bicycle race, used a composite f+am-€-bic-y:~in 1990 to pedal his way to his third victory.

Lightweight yet sturdy

Not far behind the bicycle ind ustry in using com posi tes are the motorcycle manufacturers. Heron Suzuki in collaboration with Ciba-Geigy Bonded Structures have designed and built

IN

ACTION

47

a 500 cc motorcycle with a carbon fibre composite chassis. It has a high stiffness to weight ratio which is a hallmark of composites. This eliminates many problems pre'viously encountered with steel frame racers. It also shows remarkable resistance to damage. An earlier model Heron Suzuki motorcycle emerged unscathed in a multi cartwheel crash at 225 krn per hour. The chassis is at present undergoing further assessment and evaluation at the Suzuki Motor Company in Japan. Motoforms of Brussieu, France, one of the very few European companies specializing in making motorcycles for major rallies, now uses composites too. The shell of the Motoforms motorcycles is a composite structure weighing only 6 kg. The parts are made from glass fibres like Lyvetex and Kelvar laid up with an Araldite matrix system. The entire motorcycle weighs only 130 kg, but is super tough and efficient. It took the battering and vibrations of a 13,000 krn rally over a tough African terrain without cracking under stress. Motorcycles thus modified by Motoforms have won laurels at the Rally des Pharaons in Egypt and Rallye del' Atlas in Morocco. Glass fibre-polyester or epoxy composites are the usual materials for the prod uction of automobiles and caravans and commercial vehicles. In India, the bodies of Dolphin, Mon-

48

HARDY COMPOSITES

Mikki, Eddy Electric and Miracle

IN

49

ACTION

GTP ZXTurbo

tana and Sipani Dl cars are made of glass fibre reinforced plastics. Even though most Indian cars have metal bodies, entrepreneurs planning to introduce new cars are opting for fibre glass. Mikki, Edd YElectric and Miracle, three low priced cars slated to hit the Indian roads in the near future, would have fibre glass bodies. In many European countries, the bodies of cars, trucks and buses are made of composite materials. For racing cars too, composites are often the material of choice. Tankers used for transporting milk, fruit juices, wbe or chemical products are also made of glass fibre reinforced plastics. Hummel, a high mobile multipurpose vehicle is the successor to the jeep. It has hoods, grill, doors and battery made of fibre reinforced plastics.

HARDY COMPOSITES

50

Passenger cars are also being incorporated with composites parts. Fibre reinforced epoxy composite shafts are used in automobiles because they are efficient and have a simple construction design as compared to conventional materials. They also have better noise and vibration control. High speed rally cars have extremely demanding requirements, which have" to be D}et in totality for. commef.1q.a1:M performance on the track. An Aralditematrix resin has Been chosen by Ford and Hamble Composite Systems of UK to prod uce the aramid / epbxy rear end for a new rally car named RS 200. In 1989, the NissanGTP Turbo came first in nine out of 15 events. The GTP ZX Turbo owes at least part of its performance to the advanced composites used to make its outer shell and some inner parts as well which reduced its overall weight by about 30 per cent. In 1991, a two- seater prototype with new technology and low-drag design was tested. This car, a Ren:mlt Laguna, can accelerate from zero to 100 km per hour in only six seconds and then reach a top speed of 250 km per hour. It also has a lightweight, corrosion resistant carbon fibre-Araldi te matrix composite bod y. G and V.Duqueine, the car manufacturers, specialize in the design

Renault Laguna

IN

ACTION

51

Composite belts enhance safety in tankers

and fabrication of composite structures and for several years had built all- composite shells for Formula 3 and Formula 3000 cars. Composite materials are also used to enhance safety in tankers carrying hazardous chemicals. AluminiuIlf tanks have to be reinforced to minimize the risk of damage and leakage in the event of accidents. The conventional response would be to increase wall thickness by welding an extra aluminium sheet on a tank. However, a continuous weld

52

HARDY COMPOSITES

extending the entire length of the tanks could significantly modify the properties of the original structure. The welding operation itself could raise serious explosion hazards in old tanks . .High performance composite materials were, therefore, pressed into service. Ararnid fibre fabrics laid wet with Araldite were used as a belt, which reinforced both the sides and the ends of tank. This cost effective method has been in use since 1988 and ensures crash safety of thousands of German tankers. Even railways are being transformed by composites. The British Railways Board has designed a high speed train with reinforced plastic foam body. Composites Aquitaine, a firm that specializes in composite structures has built coaches with glass phenolic parts for Taiwan's Metro.

Light Canard Research Aircraft (LCRA)

IN

ACTION

53

Advanced Light Helicopter (ALH)

The aircraft industry has always been interested in the use of composites. In jet liners, a reduction of a single kilogram in weight can mean $ 2000 in life time fuel economy for an aircraft. So, it is only natural that aircraft manufacturers have started using composites in jet engines. India has not lagged behind in using composites for aircrafts. In 1986, the National Aeronautical Laboratory, Bangalore, successfully test flew the first Indian aircraft to be made entirely out of composite materials instead of conventional aluminium alloys. This all-composite Light Canard Research Aircraft (LCRA) was made entirely out of rigid foam and fibre-glass composites. The technology used in making the LCRA is similar to that used by Rutan Aircraft in USA for making the Voyager, which created aviation history by flying

54

HARDY COMPOSITES

non-stop around the world. Not only are major components being made of composiotes but smaller parts like piston rings are also being replaced by composites. This has resulted in reduced engine wear and tear, and led to low maintenance costs. The share of composite parts and structures in air crafts is expected to touch 55 per cent soon. Carbon-carbon composites are well suited for aircraft brakes. These cost roughly twice as much as the conventional metal brakes, but are economical because of the weight saving they provide. These brakes also require less maintenance in terms of landings per overhaul. The first airliner to use carbon-carbon composites was Concorde, but now almost all military as well as civil aircrafts use this type of composites. Replacement of metals by polymer composites in helicopters has resulted in a more readily assembled unit with 9000 fewer components, weight reduction and improved reliability. The amphibious vehicle Sea Wind, which can 'land' on water as well as fly was made from reinforced vinyl ester resin. The Advanced Light Helicopter (ALH), the first prototype of which has been designed and developed by the Hindustan Aeronautics Ltd. is no exception. The singular feature of this helicopter is the extensive use of fibre composites. Fibre reinforced composites include glass, carbon or Kevlar in a matrix of epoxy resin. About 60 per cent of the helicopter's surface is made up of composites. Hindustan Aeronautics Ltd. is the first to use composites for stress bearing structures such as the four-bladed tail rotor. The four-bladed hingeless main motor also comes with a composite hub. Wide use of composites has drastically cut down the weight of an ALH. It weights just 2,500 kg. This apart, the use of composites instead of the -conventional metallic alloys makes an ALH difficult to detect by radar. This will be of advantage during war. This Indian helicopter prototype is also designed to perform equally well during peacetime when it would transport cargo, perform coast guard duties

IN

ACTION

55

and rescue operations. It could be pressed into, emerg~ncy medical services too. It was successfully test-flown' by Capt. Baljit Singh Chhoker on 31 August, 1992.

ULM C with composite fuselage

The Super Puma AS 332 MK 2 helicopter is built by Aerospatiale. This leading European manufacture has achieved a 20 per cent weight reduction by using composites instead of light alloys. Another ultralight aircraft that can even function as a glider when required is the ULM C. Manufactured in France for maintaining surveillance on forests, and oil and gas pipelines, it uses composites for its fuselage. Metal fatigue is a thorn in the flesh of the aviation industry. It is the failure caused by decay of mechanical properties of metallic parts due to repeated stress on them. Metal fatigue can and do lead to disasters. However, the problem can be solved by using a novel composite material comprising a laminar or sheet-like combination of aluminium and advanced glass/epoxy composite. This new composite is

56

HARDY COMPOSITES

marketed under the generic name Aerospace ARALL. A glass fibre reinforced variety of Aerospace ARALL has been intro-

Metal fatigue-no longer a problem

duced. It is called GLARE. It has extraordinary resistance to fatigue and do not allow microcracks to propagate and lead to bigger ones. GLARE is also very light and could save 25-30 per cent of fuselage weight in an aircraft. It is a cost effective replacement for aluminium. And since it is resistant to fatigue, it means a further cost reduction due to longer life and less frequent inspection. Even spacecrafts use com posites. There are many instances of the use of glass reinforced laminate construction in manned space vehicles. Glass was chosen to provide heat insulation as well as to minimize weight. The Apollo boost protective cover used glass composites. Food containers, equipment protective covers, numerous clips and brackets in

IN ACfION

57

the Apollo command module were fabricated from glasscloth reinforced polyimide-Iaminate materials. Heat shields, designed to protect the spacecraft from the heat generated by friction as it moves through air, were made from glass-cloth reinforced by phenolic resin.

Saturn S-11 composite applications

The module transporting astronauts from lunar orbit to the surface of the moon and back had glass filament reinforced silicone laminates in the crew compartment ceilings, side panels and electrical covers. The ladder used by the astronauts to descend from the craft on to the lunar surface was also made of composites. Glass fibre reinforced composites have been used in Saturn S-II booster used to launch Apolio vehicles. Boron epoxy composites have been used in Pioneer 10 spacecraft. Space shuttle orbiters use a variety of composites.

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HARDY COMPOSITES

. Outenegment l.magnetometer boom

Pioneer 10 spacecraft composite applications

Orbiters use boron-epoxy, graphite-epoxy, boron-polyimide, graphite-polyimide, boron-aluminium and Kevlar-epoxy composites to name but a few. The area where the use of composites has rewritten the rules of nature is the field of biomedical implants. The loss of a limb now no longer has the same crippling implications as it once did. Fitted with life-like composite limbs, amputees can not only lead normal lives but also participate in strenuous pastimes like bicycling, squash and skiing. Endolite, mainly a carbon-fibre composite system, is a high- technology prosthesis. It is an advanced lightweight system with ingenious knee, ankle and foot designs. Ther-

IN ACTION

59

Near normal life with composite prosthesis

moformed polyethylene and polyurethane foam are used to simulate flesh, which is covered with life-like silicone skin. Made by a British firm, Endolite composite prosthesis allows amputees to lead normal lives. At present about 65 per cent of the British amputees use the Endolite system. USA and Germany are also large scale users of this composite system. India too is experimenting with cornposites in the biomedical field. Composite calipers or braces have been jointly developed for the first time in India by the Department of

HARDY COMPOSITES

60

Aerospace Engineering, Indian Institute of Technology, Bombay and the SDM Hospital, Jaipur. The calipers are made of thermosetting plastic materialneinforced by carbon or glass fibre as well as pure polypropylene material. They are lighter and more comfortable to wear as the materials can be moulded to suit the contours of the human body. Clinical trials carried out on over 1,000 patients have confirmed their better acceptability and superiority over conventional calipers made of metal and leather, which are heavy and cumbersome. Biomaterial composites utilising bioceramics include bioceramic coatings on metals and polymers and combination of surface active glass ceramics and polylactic acid with metal, carbon or calcium/ phosphorus based glass fibres. Isotropic carbons produced at low temperatures have exceptional wear and fatigue properties associated wtih biocompatibilityand hence are best suited for clinical applications. Other fields where composites are also being used are electrical and nuclear industries. In the electrical industry, composites are increasingly being used in incandescent and fluorescent lamps. In the r.uclear industry, composites based on resin materials are used in equipment for radiation monitoring, radiation protection, high vacuum apparatus, control rods and cladding materials to name but a few. The use of composites is by no means restricted to only these industries. At work or at play the versatile composites have carved out their own niche and in doing so have greatly enriched the quality of modern life.

o~

he sports industry is on the threshold of a revolution made possible by composites. From fishing and pole vaulting rods to racing yachts and baseball bats, composites have pervaded every sphere of the sports industry.

At Play

Taiwan-based Kunann Enterprises has attracted the attention of materials suppliers and developers because of its concentration on polymer composite prod ucts. In 1982, Kunnan produced the world's first carbon fibre epoxy tennis racquet. The next year it introduced the first carbon fibre epoxy squash racquet. The latest introduction in the field is the carbon fibre/epoxy teardrop badminton racquet. Kunnan also has plans of moving into the manufacture of golf sets. After seven years of manufacturing carbon composite golf club shafts Kunnan has now launched the Pro Kennex IGS golf club. This uses a previously unavailable type of carbon fibre composite material which is cured at very high pressure. The process allows carbon fibres to be laminated more tightly, thus increasing the stiffness and strength ofthe composite. In tests, this new club has repeatedly achieved

HARDY COMPOSITES

62

drives in excess ot more than 300 m. The company aims to launch a complete $et of one piece all-composite golf clubs by 1993. The genesis of the modern vaulting pole is a story of the advances in athletics equipment made possible by composites. The bamboo pole of yesteryears made way for Swedish steel till Sergei Bubka of the erstwhile USSR vaulted to global fame using a pole made of aluminium and fibre reinforced plastics. Anglers too have not lagged behind in using composites. Hi-tech fishing rods made glassphenolic/polyester composites maintain their straightness for a long time.

Composite powerboat

It has been smooth sailing for composite powerboats and yachts. The 1990 classic Parker Enduro Formula 1 powerboat race on the Colorado river was won by a catamaran built with Araldite matrix system with a carbon fibre fabric. It achieved an average speed of 186 km/hour in America's toughest, powerboat race. Performance at various races has demonstrated the superiority of composite sailboats over those made of conventional materials. The former can withstand the massive stresses

AT

PLAY

63

Merit, the champion composite sailboat

of ocean racing without sustaining structural damage to the hulls, decks or interior structure. In 1989/90 a sail boat named Merit, with an all-composite hull and deck proved that composite material structure, together with the right crew and skipper, can result in the fastest sloop ever to sail around the world.

HARDY COMPOSITES

64

AGNES-2DD

Design Research Associates of France have constructed cruising catamarans that feature all-composite structures. The 25-metre, 40-tonne cruising catamaran is reportedly the largest all-composite craft of its type to have been constructed anywhere. Another French company, the Constructions Mecaniques de Normandie has designed AGNES-200, the largest air-cushion catamaran commissioned anywhere in the world. It is the only one to incorporate a helicopter landing pad and its 51-metre twin hull uses composite structures.

Bull's eye!

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PLAY

65

Sharp shooters too are opting for composite rifles. Manufacturers claim that they have perfected the art of gunsmithing with the advanced technology of epoxies to produce a synthetic rifle stock that is far superior to wood stock in performance, handling and durability. Most of the stock reinforcements include fibre glass, aramid and graphite fibres.

Playing sweet music

From guns to hi-tech harps that make sweet music is a jump composites have executed with consummate ease. Stratline, a French firm, has designed an all-composite structure for harps that look like the 19th century models, but are functionally state-of-the- art. These harps have been patented by the maker and musicians have reportedly found it ap-

HARDY COMPOSITES

66

preciably simpler to adjust. These are also quite strong as compared to the conventional harps with wooden frames. Guitars too are being fashioned using composite ~ystems. All composite concert guitars are being played by well known virtuosos such as Alvaro Pierri, Rod olfo Lahoz and Alain Carl Garcia. These all-composite guitars have been based on extensive studies and are characterized by exceptionally clear and resonant sound.

Horsing around

AT

67

PLAY

Composites show their mettle on stage too. Not long ago, a production by Pier Luigi Pizzi of Hector Berlioz's "The Trojans" at the Opera de Paris featured a two tonne horse, 12 metres high, 20 metres long and 2.8 metres wide. The enraptured audience did not know that this 16 piece structure, that could be assembled on stage, used a glass/ Araldite composite to achieve the desired strength to weight ratio.

The luxurious world of composites

Composites, therefore, are here to stay. At work and at play. But at La Tour Rose, a well known luxury hotel and gourmet restaurctnt in Lyon, France, one perhaps understands how all encompassing their reach is. At this hotel is a suite that is furnished in an all-composite fashion. The double bed, divan, bedside table, wardrobe, easy chairs, coffee table, TV console, chairs and desk are all made of composite materials. In comparison with the global composites markets the Indian industry is still in its infancy. Although fibre glass reinforced composites are in vogue in the country the volume

68

HARDY

COMPOSITES

for chemical and other industries is small. One of the main drawbacks has been the need to import at high cost the compatibilizers, whose role in the ultimate strength of the composites is crucial. But the recent development of carbon fibre production in the country augers well for the sports goods,' structural engineering parts and biomedical components in the years to come.

O~

Cladding: The covering of a fuel element in a nuclear reactor to prevent corrosion. It also prevents the escape of fission products. Cure: To modify or change the properties of a resin by chemical reaction. This is usually accomplished by the action of either heat or a catalyst, or both. The process may be carried out with or without pressure. Hydrolysis: A reaction between a compound and water. It is the che~ca1 decomposition of a substance by water. The water is also decomposed. The reaction can be expressed as AB + H20 = A(OH) + HB (where AB is the compound). Salts of weak acids, bases or both are partially hydrolyzed in solution. Polyolefins: Synthetic fibres made of long chain organic polymers. The polymers are formed by joining together a large number of molecules of unsaturated hydrocarbons like ethylene and propylene. Resistivity: Electrical resistance of a conductor of unit cross-sectional area and unit length. A characteristic property of each material, resistivity is useful in comparing various materials on the basis of their ability to conduct electric currents. High resistivity is characteristic of poor conductors and vice versa. Roving: I'tconsists of a collection of bundles of continuous filaments either as untwisted strands or as twisted yarn. Rovings may be lightly twisted, but for filament winding

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HARDY COMPOSITES

they are generally wound as tapes with very little twisting. Glass rovings are usually used in filament winding. Shockwave: A strong pressure wave in any medium such as air, water or a solid substance. Tensile strength: A measure of the tenacity of asubstance. It is the resistance offered by a material to stress generated by stretching. It is expressed as the stress, that is the force per unit cross-sectional area required to break the material.

FLAWLESS

in function, hardy by design and versatile in application, composites are peerless performers. Fashioned for toughness and the capability to withstand extremes of conditions, composites are made by bringing individual components together in such a manner that the whole is better than the sum of its parts. Composites have no doubt been used since biblical times. But today they are quietly and efficiently replacing conventional materials on the terra firma and up in the sky, in the inner space of human bodies and the uncharted regions of outer space. In the process, they form parts of as diverse objects as helicopters to tennis racquets and biomedical implants to spacecrafts. This attractive and lavishly illustrated book written especially for the non-specialist describes with consummate skill the many advantages of the synthetic era which is perhaps at its height today because of the hardy composites.

About the Author N.S.K. Prasad (b. 1930) did his M.Sc. (Chemistry) from Nagpur University, Diploma in chemical engineering from Indian Institute of Science, Bangalore, and was awarded Ph.D. in chemistry by Bombay University.

He served the

Bhabha Atomic Research Centre, Trombay, for 37 years, mostly in the area of speciality

chemicals. At present he is working as a consultant to a private

chemical company. Dr Prasad has contributed Economic

popular science articles to Science Today and

Times. He has also prepared global survey reports on some spe-

cial topics. Hardy Composites is his first popular science book.

ISBN: 81-7236-049-5