reinforcement, as the name implies, strengthens or adds muscle to a com- posite. 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 dic- tated by the property to be im- parted to the composite.
Reinforcement materials may be either non-fibrous or fibrous in nature.
The spectrum of non-fibrous reinforcements in modern com- posites includes calcium car- bonate, calcium silicate, oxides of silicon, aluminium or titanium and sulphates of calcium and barium. Carbon black, mica, fabri- cated spheres of glass and ceramics are also good non- fibrous reinforcements. These reinforcements help impart cer- tain desirable characteristics to composites. The plasticised polyvinyl chloride (PVC) and inert fillers like calcium car- bonate, 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-
22 HARDY COMPOSITES
~YPES OF FILLERS IN COMPOSITES
0
Organic 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 pack- ing food items. These have to be of very high grade and meet stringent safety requirements.
ADDING MUSCLE 23
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 effec- tive 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 mul- ticoloured, 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 be- stow 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-
24 HARDY COMPOSITES
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. Al- most 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,
ADDING MUSCLE 25 -
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 convention- al 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 ad- vantages. It is lightweight, has high tensile strength and high heat resistance. It is virtually fireproof, inexpensi~e and avail- able 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 im- parts 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 solu- tions.
Glass fibre based composite materials are not suitable for very high performance applications. This is because these
26 HARDY COMPOSITES
~
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, replace- ments for glass fibres become necessary and carbon fibres often replace them. Carbon fibres have strength vastly supe- rior to that of glass fibres and have superior heat stability as
ADDING MUSCLE 27
Elasticity or strength?
well. Chemical composition plays an important part in deter- mining 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.
28 HARDY COMPOSITES
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 rang- ing temperature differences or thermal cycles, they find ap- plication in electronics instrumentation and space vehicles.
GRAPHIL is a PAN-based fibre especially designed for ap- plications 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 ex- pensive 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 ap- parent. One such use is their role as reinforcements in com- posites.
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,
ADDING MUSCLE 29
The world of polymers
30 HARDY COMPOSITES
brakes and several other friction applica- tions. In recent years, applications in high performance struc- tures for aircraft, space systems, automobiles, sea-faring vessels, solid-rocket missile cases and sporting goods have been spurred by the development of im- proved aramid fibres, the class to which Kev- lar 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 reinforce- ments. Terephthalate and nylon fibres are used in advanced composites in fields as diverse as automatives, food packag- ing and construction application. High density polyethylene fibres are used in helmets, conveyor belts, hoses, artificial limbs and joints. Sports goods such as bicycles, ski equip- ment, 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 rac- quets 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.
ADDING MUSCLE 31
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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 inor- ganic 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.