Chapter 11 Plastics and composites 11.1 Utilization of polymeric materials 11.1.1 Introduction In Chapter 2 the basic chemistry and structural arrangements of long-chain molecules were described and it was shown how polymers can be broadly classified as thermoplastics, elastomers and thermosets. Certain aspects of their practical utilization will now be examined, with special attention to processing; this stage plays a decisive role in deciding if a particular polymeric material can be produced as a marketable commodity. The final section (11.3) will concern composites, extending from the well-known glass-reinforced polymers to those based upon ceramic and metallic matrices. 11.1.2 Mechanical aspects of T g As indicated in Chapter 2, it is customary to quote a glass-transition temperature T g for a polymer because it separates two very different r ´ egimes of mechanical behaviour. (The value of T g is nominal, being subject to the physical method and procedure used in its deter- mination). Below T g , the mass of entangled molecules is rigid. Above T g , viscoelastic effects come into play and it is therefore the lower temperature limit for pro- cessing thermoplastics. The structural effect of raising the temperature of a glassy polymer is to provide an input of thermal energy and to increase the vibrations of constituent atoms and molecules. Molecular mobilty increases significantly as T g is approached: rotation about C–C bonds in the chain molecules begins, the free volume of the structure increases and intermolecu- lar forces weaken. It becomes easier for applied forces to deform the structure and elastic moduli to fall. The mechanical properties of polymers are highly dependent upon time and temperature, the response to stress being partly viscous and partly elastic. For instance, ‘natural’ time periods are associated with the various molecular relaxation processes associated with the glass transition. In linear viscoelastic behaviour, total strain comprises a linear elastic (Hookean) com- ponent and a linear viscous (Newtonian) component. The stress–strain ratios depend upon time alone. In the more complex non-linear case, which usually applies to polymers, strain is a function of time and stress because molecular movements are involved. The phenomenon of stress relaxation can be used to chart the way in which the behaviour of a given polymer changes from glassy to rubbery. Figure 11.1 shows the non-linear response of a polymer that is subjected to constant strain ε 0 .Stress relaxes with time t. The relaxation modulus E r at time t is given by the expression: E r D  t /ε 0 11.1 Thus E r , which is represented by the slope of dotted join lines, decreases with time. This variation is shown more precisely by a plot of log E r versus log time t, as in Figure 11.2a. The thermoplastic Figure 11.1 Stress relaxation at constant strain. 352 Modern Physical Metallurgy and Materials Engineering Figure 11.2 Time–temperature dependence of elastic modulus in thermoplastic polymeric solid: (a) change in relaxation modulus E r t as function of time; (b) change in tensile modulus as function of temperature (from Hertzberg, 1989; by permission of John Wiley and Sons). polymer changes in character from a glassy solid, where the relaxation modulus is a maximum, to a rubbery solid. In the complementary Figure 11.2b, data from standard tensile tests on the same polymer at different temperatures are used to provide values of elastic moduli E. The similarity of profiles in Figures 11.2a and 11.2b illustrates the equivalence of time and temperature. (Theoretically, the modulus for a short time and a high temperature may be taken to equate to that for a combination of a long time and a low temperature; this concept is used in the preparation of relaxation modulus versus time graphs.) The glass transition temperature T g has been superimposed upon Figures 11.2a and 11.2b. Although single values of T g are usually quoted, the process of molecular rearrangement is complex and minor transitions are sometimes detectable. Thus, for PVC, the main glass transition occurs at tem- peratures above 80 ° C but there is a minor transition at 40 ° C. Consequently, at room temperature, PVC exhibits some rigidity yet can elongate slightly before fracture. Addition of a plasticizer liquid, which has a very low T g , lowers the T g value of a polymer. Sim- ilarly, T g for a copolymer lies between the T g values of the original monomers; its value will depend upon monomer proportions. In elastomeric structures, as the temperature is increased, the relatively few crosslinks begin to vibrate vigorously at T g and the elastomer becomes increas- ingly rubbery. As one would anticipate, T g values for rubbers lie well below room temperature. Increas- ing the degree of crosslinking in a given polymer has the effect of raising the entire level of the lower ‘rubbery’ plateau of the modulus versus temperature plot upwards as the polymer becomes more glassy in nature. The thermosets PMMA (Perspex, Lucite) and PS have T g values of 105 ° C and 81 ° C, respec- tively, and are accordingly hard and brittle at room temperature. 11.1.3 The role of additives Industrially, the term ‘plastic’ is applied to a polymer to which one or more property-modifying agents have been added. Numerous types of additive are used by manufacturers and fabricators; in fact, virgin polymers are rarely used. An additive has a specific function. Typical functions are to provide (1) protection from the service environment (anti-oxidants, anti-ozonants, anti-static agents, flame-retardants, ultraviolet radia- tion absorbers), (2) identification (dyes, pigments), (3) easier processability (plasticizers), (4) toughness, and (5) filler. In many instances the required amount of additive ranges from 0.1% to a few per cent. Although ultra- violet (UV) components of sunlight can structurally alter and degrade polymers, the effect is particularly marked in electric light fittings (e.g. yellowing). Stabi- lizing additives are advisable as some artificial light sources emit considerable amounts of UV radiation with wavelengths in the range 280–400 nm. For any polymer, there is a critical wavelength which will have the most damaging effect. For instance, a wavelength of 318.5 nm will degrade PS, which is a common choice of material for diffusers and refractors, by either causing cross-linking or by producing free radicals that react with oxygen. The action of a plasticizer (3) is to weaken inter- molecular bonding by increasing the separation of the chain molecules. The plasticizer may take the form of a liquid phase that is added after polymerization and before processing. Additions of a particulate toughener (4) such as rubber may approach 50% and the material is then normally regarded as a composite. A wide variety of fillers (5) is used for polymers. In the case of thermosets, substances such as mica, glass fibre and fine sawdust are used to improve engineering properties and to reduce the cost of moulded prod- ucts. PTFE has been used as a filler (15%) to improve the wear resistance of nylon components. Although usually electrically non-conductive, polymers can be made conductive by loading them with an appropriate filler (e.g. electromagnetic shielding, specimen mounts in SEM analysis). Fillers and other additives play an important role in the production of vulcanized rubbers. Inert fillers facilitate handling of the material before vulcanization (e.g. clay, barium sulphate). Reinforcing Plastics and composites 353 Figure 11.3 (a) World consumption of plastics, (b) plastics consumption by market sector, Western Europe, and (c) destination of post-consumer plastic waste, Western Europe (courtesy of Shell Briefing Services, London). fillers restrict the movement of segments between the branching points shown in Figure 2.24. For instance, carbon black has long been used as a filler for car tyres, giving substantial improvements in shear modulus, tear strength, hardness and resistance to abrasion by road surfaces. Tyres are subject to fluctuating stresses dur- ing their working life and it has been found useful to express their stress–strain behaviour in terms of a dynamic shear modulus (determined under cyclic stressing conditions at a specified frequency). Time is required for molecular rearrangements to take place in an elastomer; for this reason the dynamic modu- lus increases with the frequency used in the modulus test. The dynamic modulus at a given frequency is significantly enhanced by the crosslinking action of vulcanization and by the presence of carbon black. These carbon particles are extremely small, typically 20–50 nm diameter. Small amounts of anti-oxidants 1 and anti-ozonants are often beneficial. The principal agents of degradation under service conditions are extreme temperatures, oxygen and ozone, various liq- uids. In the last case a particular liquid may penetrate between the chains and cause swelling. 11.1.4 Some applications of important plastics Figures 11.3a and 11.3b summarize 1990 data on world and West European consumption of plastics. The survey included high-volume, low-price commodity plastics as well as engineering and advanced plastics. Thermoplastics dominate the market (i.e. PE, PVC and PP). Development of an entirely new type of plastic is extremely expensive and research in this direction is 1 In the 1930s, concern with oxidation led the Continental Rubber Works, Hannover, to experiment with nitrogen-inflation of tyres intended for use on Mercedes ‘Silver Arrows’, Grand Prix racing cars capable of 300 km/h. This practice was not adopted by Mercedes-Benz for track events. limited. Research is mainly concerned with improv- ing and reducing the cost of established materials (e.g. improved polymerization catalysts, composites, thermoplastic rubbers, waste recycling). The low- and high-density forms of polyethylene, LDPE and HDPE, were developed in the 1940s and 1950s, respectively. Extruded LDPE is widely used as thin films and coatings (e.g. packaging). HDPE is used for blow-moulded containers, injection-moulded crates and extruded pipes. An intermediate form of adjustable density known as linear low-density polyethylene, LLDPE, became available in the 1980s. Although more difficult to process than LDPE and HDPE, film- extruded LLDPE is now used widely in agriculture, horticulture and the construction industry (e.g. heavy- duty sacks, silage sheets, tunnel houses, cloches, damp- proof membranes, reservoir linings). Its tear strength and toughness have enabled the gauge of PE film to be reduced. A further variant of PE is ultra-high molecu- lar weight polyethylene, UHMPE, which is virtually devoid of residual traces of catalyst from polymer- ization. Because of its high molecular mass, it needs to processed by sintering. UHMPE provides the wear resistance and toughness required in artificial joints of surgical prostheses. Polyvinyl chloride (PVC) is the dominant plastic in the building and construction industries and has effec- tively replaced many traditional materials such as steel, cast iron, copper, lead and ceramics. For example, the unplasticized version (UPVC) is used for win- dow frames and external cladding panels because of its stiffness, hardness, low thermal conductivity and weather resistance. PVC is the standard material for piping in underground distribution systems for potable water (blue) and natural gas (yellow), being corrosion- resistant and offering small resistance to fluid flow. Although sizes of PVC pipes tend to be restricted, PVC linings are used to protect the bore of large-diameter pipes (e.g. concrete). The relatively low softening tem- perature of PVC has stimulated interest in alternative piping materials for underfloor heating systems. Poly- butylene (PB) has been used for this application; being 354 Modern Physical Metallurgy and Materials Engineering supported, it can operate continuously at a temperature of 80 ° C and can tolerate occasional excursions to 110 ° C. However, hot water with a high chlorine con- tent can cause failure. Other important building plastics are PP, ABS and polycarbonates. Transparent roofing sheets of twin-walled polycarbonate or PVC provide thermal insulation and diffuse illumination: both mate- rials need to be stabilized with additives to prevent UV degradation. The thermoplastic polypropylene, PP (Propathene) became available in the early 1960s. Its stiffness, tough- ness at low temperatures and resistance to chemicals, heat and creep T m D 165–170 ° C are exceptional. PP has been of particular interest to car designers in their quest for weight-saving and fuel efficiency. In a typi- cal modern saloon car, at least 10% of the total weight is plastic (i.e. approximately 100 kg). PP, the lowest- density thermoplastic (approximately 900 kg m 3 ), is increasingly used for interior and exterior automotive components (e.g. heating and ventilation ducts, radiator fans, body panels, bumpers (fenders)). It is amenable to filament-reinforcement, electroplating, blending as a copolymer (with ethylene or nylon), and can be pro- duced as mouldings (injection- or blow-), films and filaments. Polystyrene (PS) is intrinsically brittle. Engineering polymers such as PS, PP, nylon and polycarbonates are toughened by dispersing small rubber spheroids throughout the polymeric matrix; these particles con- centrate applied stresses and act as energy-absorbing sites of crazing. The toughened high-impact form of polystyrene is referred to as HIPS. PS, like PP, is nat- urally transparent but easily coloured. When supplied as expandable beads charged with a blowing agent, such as pentane, PS can be produced as a rigid heat- insulating foam. Some of the elastomers introduced in Chapter 2 will now be considered. Polyisoprene is natural rubber; unfortunately, because of reactive C–C links in the chains it does not have a high resistance to chemical attack and is prone to surface cracking and degradation (‘perishing’). Styrene-butadiene rubber (SBR), intro- duced in 1930, is still one of the principal synthetic rubbers (e.g. car tyres). 1 In this copolymer, repeat units of butadiene 2 are combined randomly with those of styrene. Polychloroprene (Neoprene), introduced in 1932, is noted for its resistance to oil and heat and is used for automotive components (e.g. seals, water circuit pipes). As indicated previously, additives play a vital role in rubber technology. The availability of a large family of 1 Synthetic (methyl) rubber was first produced in Germany during World War I as a result of the materials blockade; when used for tyres, vehicles had to be jacked-up overnight to prevent flat areas developing where they contacted the ground. 2 Butadiene, or but-2-ene, is an unsaturated derivative of butane C 4 H 10 ; the central digit indicates that the original butene monomer C 4 H 8 contains two double bonds. rubbers has encouraged innovative engineering design (e.g. motorway bridge bearings, mounts for oil-rigs and earthquake-proof buildings, vehicle suspension systems). Silicone rubbers may be regarded as being inter- mediate in character to polymers and ceramics. From Table 2.7 it can be seen that the long chains con- sist of alternating silicon and oxygen atoms. Although weaker than organic polymers based upon carbon chains, they retain important engineering properties, such as resilience, chemical stability and electrical insulation, over the very useful temperature range of 100 ° C to 300 ° C. These outstanding characteristics, combined with their cost-effectiveness, have led to the adoption of silicone rubbers by virtually every industry (e.g. medical implants, gaskets, seals, coatings). 11.1.5 Management of waste plastics Concern for the world’s environment and future energy supplies has focused attention on the fate of waste plastics, particularly those from the packaging, car and electrical/electronics sectors. Although recovery of values from metallic wastes has long been prac- tised, the diversity and often complex chemical nature of plastics raise some difficult problems. Nevertheless, despite the difficulties of re-use and recycling, it must be recognized that plastics offer remarkable properties and are frequently more cost- and energy-effective than traditional alternatives such as metals, ceramics and glasses. Worldwide, production of plastics accounts for about 4% of the demand for oil: transport accounts for about 54%. Enlightened designers now consider the whole life-cycle and environmental impact of a polymeric product, from manufacture to disposal, and endeavour to economize on mass (e.g. thinner thick- nesses for PE film and PET containers (‘lightweight- ing’)). Resort to plastics that ultimately decompose in sunlight (photodegradation) or by microbial action (biodegradation) represents a loss of material resource as they cannot be recycled; accordingly, their use tends to be restricted to specialized markets (e.g. agriculture, medicine). Figure 11.3c portrays the general pattern of plas- tics disposal in Western Europe. Landfilling is the main method but sites are being rapidly exhausted in some countries. The principal routes of waste man- agement are material recycling, energy recycling and chemical recycling. The first opportunity for material recycling occurs during manufacture, when uncontam- inated waste may be re-used. However, as in the case of recycled paper, there is a limit to the number of times that this is possible. Recycling of post-consumer waste is costly, involving problems of contamination, collection, identification and separation. 3 . Co-extruded 3 German legislation requires that, by 1995, 80% of all packaging (including plastics) must be collected separately from other waste and 64% of total waste recycled as material. Plastics and composites 355 blow-moulded containers are being produced with a three-layer wall in which recycled material is sand- wiched between layers of virgin polymer. In the Ger- man car industry efforts are being made to recycle flexible polyurethane foam, ABS and polyamides. New ABS radiator grilles can incorporate 30% from old recycled grilles. Plastics have a high content of carbon and hydrogen and can be regarded as fuels of useful calorific value. Incinerating furnaces act as energy-recycling devices, converting the chemical energy of plastics into ther- mal/electrical energy and recovering part of the energy originally expended in manufacture. Noxious fumes and vapours can be evolved (e.g. halogens); control and cleaning of flue gases are essential. Chemical recycling is of special interest because direct material recycling is not possible with some wastes. Furthermore, according to some estimates, only 20–30% of plastic waste can be re-used after material recycling. Chemical treatment, which is an indirect material recycling route, recovers monomers and polymer-based products that can be passed on as feedstocks to chemical and petrochemical industries. Hydrogenation of waste shows promise and is used to produce synthetic oil. 11.2 Behaviour of plastics during processing 11.2.1 Cold-drawing and crazing A polymeric structure is often envisaged as an entan- gled mass of chain molecules. As the T g values for many commercial polymers are fairly low, one assumes that thermal agitation causes molecules to wriggle at ambient temperatures. Raising the tempera- ture increases the violence of molecular agitation and, under the action of stress, molecules become more likely to slide past each other, uncoil as they rotate about their carbon–carbon bonds, and extend in length. We will first concentrate upon mechanistic aspects of two important modes of deformation; namely, the development of highly-preferred molecular orienta- tions in semi-crystalline polymers by cold-drawing and the occurrence of crazing in glassy polymers. Cold-drawing can be observed when a semi- crystalline structure containing spherulites is subjected to a tensile test at room temperature. A neck appears in the central portion of the test-piece. As the test-piece extends, this neck remains constant in cross-section but increases considerably in length. This process forms a necked length that is stronger and stiffer than material beyond the neck. At first, the effect of applied tensile stress is to produce relative movement between the crystalline lamellae and the interlamellar regions of disordered molecules. Lamellae that are normal to the direction of principal stress rotate in a manner reminiscent of slip-plane rotation in metallic single crystals, and break down into smaller blocks. These blocks are then drawn into tandem sequences known as microfibrils (Figure 11.4). The individual blocks retain their chain-folding conformation and are linked together by the numerous tie molecules which form as the original lamellae unfold. A bundle of these highly-oriented microfibrils forms a fibril (small fibre). The microfibrils in a bundle are separated by amorphous material and are joined by surviving interlamellar tie molecules. The pronounced molecular orientation of this type of fibrous structure maximizes the contribution of strong covalent bonds to strength and stiffness while minimizing the effect of weak intermolecular forces. Industrially, cold-drawing techniques which take advantage of the anisotropic nature of polymer crys- tallites are widely used in the production of synthetic fibres and filaments (e.g. Terylene). (Similarly, biax- ial stretching is used to induce exceptional strength in film and sheet and bottles e.g. Melinex). Crystalliza- tion in certain polymers can be very protracted. For instance, because nylon 6,6 has a T g value slightly below ambient temperature, it can continue to crystal- lize and densify over a long period of time during nor- mal service, causing undesirable after-shrinkage. This metastability is obviated by ‘annealing’ nylon briefly at a temperature of 120 ° C, which is below T m : less per- fect crystals melt while the more stable crystals grow. Stretching nylon 6,6 at room temperature during the actual freezing process also encourages crystallization and develops a strengthening preferred orientation of crystallites. Let us now turn from the bulk effect of cold-drawing to a form of localized inhomogeneous deformation, or yielding. In crazing, thin bands of expanded material form in the polymer at a stress much lower than the bulk yield stress for the polymer. Crazes are usually associated with glassy polymers (PMMA and PS) but may occur in semi-crystalline polymers (PP). They are Figure 11.4 Persistence of crystalline block structure in three microfibrils during deformation. 356 Modern Physical Metallurgy and Materials Engineering several microns wide and fairly constant in width: they can scatter incident light and are visible to the unaided eye (e.g. transparent glassy polymers). As in the stress- corrosion of metals, crazing of regions in tension may be induced by a chemical agent (e.g. ethanol on PMMA). The plane of a craze is always at right angles to the principal tensile stress. Structurally, each craze consists of interconnected microvoids, 10–20 nm in size, and is bridged by large numbers of molecule- orientated fibrils, 10–40 nm in diameter. The voidage is about 40–50%. As a craze widens, bridging fibrils extend by drawing in molecules from the side walls. Unlike the type of craze found in glazes on pottery, it is not a true crack, being capable of sustaining some load. Nevertheless, it is a zone of weakness and can initiate brittle fracture. Each craze has a stress-intensifying tip which can propagate through the bulk polymer. Crazing can take a variety of forms and may even be beneficial. For instance, when impact causes crazes to form around rubber globules in ABS polymers, the myriad newly-created surfaces absorb energy and toughen the material. Various theoretical models of craze formation have been proposed. One suggestion is that triaxial stresses effectively lower T g and, when tensile strain has exceeded a critical value, induce a glass-to-rubber transition in the vicinity of a flaw, or similar heterogeneity. Hydrostatic stresses then cause microcavities to nucleate within this rubbery zone. As Figure 11.5 shows, it is possible to portray the strength/temperature relations for a polymeric material on a deformation map. This diagram refers to PMMA and shows the fields for cold-drawing, crazing, viscous flow and brittle fracture, together with superimposed contours of strain rate over a range of 10 6 to1s 1 . 11.2.2 Processing methods for thermoplastics Processing technology has a special place in the remarkable history of the polymer industry: polymer Figure 11.5 Deformation map for PMMA showing deformation regions as a function of normalized stress versus normalized temperature (from Ashby and Jones, 1986; permission of Elsevier Science Ltd, UK). chemistry decides the character of individual molecules but it is the processing stage which enables them to be arranged to maximum advantage. Despite the variety of methods available for converting feedstock pow- ders and granules of thermoplastics into useful shapes, these methods usually share up to four common stages of production; that is, (1) mixing, melting and homog- enization, (2) transport and shaping of a melt, (3) drawing or blowing, and (4) finishing. Processing brings about physical, and often chemi- cal, changes. In comparison with energy requirements for processing other materials, those for polymers are relatively low. Temperature control is vital because it decides melt fluidity. There is also a risk of ther- mal degradation because, in addition to having limited thermal stability, polymers have a low thermal con- ductivity and readily overheat. Processing is usually rapid, involving high rates of shear. The main methods that will be used to illustrate technological aspects of processing thermoplastics are depicted in Figure 11.6. Injection-moulding of thermoplastics, such as PE and PS, is broadly similar in principle to the pressure die-casting of light metals, being capable of produc- ing mouldings of engineering components rapidly with repeatable precision (Figure 11.6a). In each cycle, a metered amount (shot) of polymer melt is forcibly injected by the reciprocating screw into a ‘cold’ cavity (cooled by oil or water channels). When solidifica- tion is complete, the two-part mould opens and the moulded shape is ejected. Cooling rates are faster than with parison moulds in blow-moulding because heat is removed from two surfaces. The capital out- lay for injection-moulding tends to be high because of the high pressures involved and machining costs for multi-impression moulding dies. In die design, special attention is given to the location of weld lines, where different flows coalesce, and of feeding gates. Com- puter modelling can be used to simulate the melt flow and distributions of temperature and pressure within the mould cavity. This prior simulation helps to lessen dependence upon traditional moulding trials, which are costly. Microprocessors are used to monitor and con- trol pressure and feed rates continuously during the moulding process; for example, the flow rates into a complex cavity can be rapid initially and then reduced to ensure that flow-dividing obstructions do not pro- duce weakening weld lines. Extrusion is widely used to shape thermoplastics into continuous lengths of sheet, tube, bar, filament, etc. with a constant and exact cross-sectional profile (Figure 11.6b). A long Archimedean screw (auger) rotates and conveys feedstock through carefully pro- portioned feed, compression and metering sections. The polymer is electrically heated in each of the three barrel sections and frictionally heated as it is ‘shear- thinned’ by the screw. Finally, it is forced through a die orifice. Microprocessor control systems are avail- able to measure pressure at the die inlet and to keep it constant by ‘trimming’ the rotational speed of the screw. Dimensional control of the product benefits Plastics and composites 357 Figure 11.6 (a) Injection-moulding machine, (b) production of plastic pipe by extrusion and (c) thermoforming of plastic sheet (from Mills, 1986; by permission of Edward Arnold). from this device. On leaving the die, the continuously- formed extrudate enters cooling and haul-off sections. Frequently, the extrudate provides the preform for a second operation. For example, in a continuous melt- inflation technique, tubular sheet of LDPE or HDPE from an annular die is drawn upwards and inflated with air to form thin film: stretching and thinning cease when crystallization is complete at about 120 ° C. Similarly, in the blow-moulding of bottles and air- ducting, etc., tubular extrudate (parison) moves ver- tically downwards into an open split-mould. As the mould closes, the parison is inflated with air at a pres- sure of about 5 atmospheres and assumes the shape of the cooled mould surfaces. Relatively inexpensive alu- minium moulds can be used because stresses are low. Thermoforming (Figure 11.6c) is another secondary method for processing extruded thermoplastic sheet, being particularly suitable for large thin-walled hol- low shapes such as baths, boat hulls and car bodies (e.g. ABS, PS, PVC, PMMA). In the basic version of the thermoforming, a frame-held sheet is located above the mould, heated by infrared radiation until rubbery and then drawn by vacuum into close contact with the mould surface. The hot sheet is deformed and thinned by biaxial stresses. In a high-pressure version of ther- moforming, air at a pressure of several atmospheres acts on the opposite side of the sheet to the vacuum and improves the ability of the sheet to register fine mould detail. The draw ratio, which is the ratio of mould depth to mould width, is a useful control param- eter. For a given polymer, it is possible to construct a plot of draw ratio versus temperature which can be used as a ‘map’ to show various regions where there are risks of incomplete corner filling, bursts and pin- holes. Unfortunately, thinning is most pronounced at vulnerable corners. Thermoforming offers an econom- ical alternative to moulding but cycle times are rather long and the final shape needs trimming. 11.2.3 Production of thermosets Development of methods for shaping thermosetting materials is restricted by the need to accommodate a curing reaction and the absence of a stable viscoelas- tic state. Until fairly recently, these restrictions tended to limit the size of thermoset products. Compres- sion moulding of a thermosetting P-F resin (Bakelite) within a simple cylindrical steel mould is a well-known laboratory method for mounting metallurgical samples. Resin granules, sometimes mixed with hardening or electrically-conducting additives, are loaded into the mould, then heated and compressed until crosslinking reactions are complete. In transfer moulding, which can produce more intricate shapes, resin is melted in a primary chamber and then transferred to a vented moulding chamber for final curing. In the car indus- try, body panels with good bending stiffness are pro- duced from thermosetting sheet-moulding compounds (SMC). A composite sheet is prepared by laying down layers of randomly-oriented, chopped glass fibres, cal- cium carbonate powder and polyester resin. The sheet is placed in a moulding press and subjected to heat and pressure. Energy requirements are attractively low. Greater exploitation of thermosets for large car parts has been made possible by reaction injection-moulding 358 Modern Physical Metallurgy and Materials Engineering (RIM). In this process, polymerization takes place dur- ing forming. Two or more streams of very fluid chemi- cal reactants are pumped at high velocity into a mixing chamber. The mixture bottom-feeds a closed chamber where polymerization is completed and a solid forms. Mouldings intended for high-temperature service are stabilized, or post-cured, by heating at a temperature of 100 ° C for about 30 min. The reactive system in RIM can be polyurethyane-, nylon- or polyurea-forming. The basic chemical criterion is that polymerization in the mould should be virtually complete after about 30 s. Foaming agents can be used to form compo- nents with a dense skin and a cellular core. When glass fibres are added to one of the reactants, the process is called reinforced reaction injection-moulding (RRIM). RIM now competes with the injection-moulding of thermoplastics. Capital costs, energy requirements and moulding pressures are lower and, unlike injection- mouldings, thick sections are not subject to shrinkage problems (‘sinks’ and voids). Cycle times for RIM- thermosets are becoming comparable with those for injection-moulded thermoplastics and mouldings of SMC. Stringent control is necessary during the RIM process. Temperature, composition and viscosity are rapidly changing in the fluid stream and there is a chal- lenging need to develop appropriate dynamic models of mass transport and reaction kinetics. 11.2.4 Viscous aspects of melt behaviour Melts of thermoplastic polymers behave in a highly viscous manner when subjected to stress during pro- cessing. Flow through die orifices and mould chan- nels is streamline (laminar), rather than turbulent, with shear conditions usually predominating. Let us now adopt a fluid mechanics approach and consider the effects of shear stress, temperature and hydro- static pressure on melt behaviour. Typical rates of strain (shear rates) range from 10–10 3 s 1 (extrusion) to 10 3 –10 5 s 1 (injection-moulding). When a melt is being forced through a die, the shear rate at the die wall is calculable as a function of the volumetric flow rate and the geometry of the orifice. At the necessarily high levels of stress required, the classic Newtonian relation between shear stress and shear (strain) rate is not obeyed: an increase in shear stress produces a disproportionately large increase in shear rate. In other words, the shear stress/shear rate ratio, which is now referred to as the ‘apparent shear viscosity’, falls. Terms such as ‘pseudo-plastic’ and ‘shear-thinning’ are applied to this non-Newtonian flow behaviour. 1 The general working range of apparent shear viscosity for extrusion, injection-moulding, etc. is 10–10 4 Ns m 2 . (Shear viscosities at low and high stress levels are measured by cone-and-plate and capillary extrusion techniques, respectively.) 1 In thixotropic behaviour, viscosity decreases with increase in the duration of shear (rather than the shear rate). Figure 11.7 shows the typical fall in apparent shear viscosity which occurs as the shear stress is increased. If Newtonian flow prevailed, the plotted line would be horizontal. This type of diagram is plotted for fixed values of temperature and hydrostatic pressure. A change in either of these two conditions will displace the flow curve significantly. Thus, either raising the temperature or decreasing the hydrostatic (bulk) pres- sure will lower the apparent shear viscosity. The latter increases with average molecular mass. For instance, fluidity at a low stress, as determined by the standard melt flow index (MFI) test, 1 is inversely proportional to molecular mass. At low stress and for a given molecular mass, a polymer with a broad distribution of molecular mass tends to become more pseudo-plastic than one with a narrow distribution. However, at high stress, a reverse tendency is possible and the version with a broader distribution may be less pseudo-plastic. Figure 11.8 provides a comparison of the flow behaviour of five different thermoplastics and is useful for comparing the suitability of different processes. It indicates that acrylics are relatively difficult to extrude Figure 11.7 Typical plot of apparent shear viscosity versus shear stress for LDPE at 210 ° C and atmospheric pressure: effects of increasing temperature T and hydrostatic pressure P shown (after Powell, 1974; courtesy of Plastics Division, Imperial Chemical Industries Plc). 1 This important test, which originated in ICI laboratories during the development of PE, is used for most thermoplastics by polymer manufacturers and processors. The MFI is the mass of melt extruded through a standard cylindrical die in a prescribed period under conditions of constant temperature and compression load. Plastics and composites 359 Figure 11.8 Typical curves of apparent shear viscosity versus shear stress for five thermoplastics at atmospheric pressure. A Extrusion-grade LDPE at 170 ° C;B extrusion-grade PP at 230 ° C; C moulding-grade acrylic at 230 ° C; D moulding-grade acetal copolymer at 200 ° C;E moulding-grade nylon at 285 ° C (after Powell, 1974; courtesy of Plastics Division, Imperial Chemical Industries Plc.). and that PP is suited to the much faster deformation process of injection-moulding. In all cases, Newtonian flow is evident at relatively low levels of shear stress. The following type of power law equation has been found to provide a reasonable fit with practical data and has enabled pseudo-plastic behaviour to be quantified in a convenient form:  D C n 11.2 where C and n are constants. Now  D Á, hence the viscosity Á D C n1 . The characteristic term n 1 can be derived from the line gradient of a graphical plot of log viscosity versus log shear rate. In practice, the power law index n ranges from unity (Newtonian flow) to <0.2, depending upon the polymer. This index decreases in magnitude as the shear rate increases and the thermoplastic melt behaves in an increasingly pseudo-plastic manner. So far, attention has been concentrated on the vis- cous aspects of melt behaviour during extrusion and injection-moulding, with emphasis on shear processes. In forming operations such as blow-moulding and filament-drawing, extensional flow predominates and tensile stresses become crucial; for these conditions, it is appropriate to define tensile viscosity, the coun- terpart of shear viscosity, as the ratio of tensile stress to tensile strain rate. At low stresses, tensile viscosity is independent of tensile stress. As the level of tensile stress rises, tensile viscosity either remains constant (nylon 6,6), rises (LDPE) or falls (PP, HDPE). This characteristic is relevant to the stability of dimensions and form. For example, during blow-moulding, thin- ning walls should have a tolerance for local weak spots or stress concentrations. PP and HDPE lack this tol- erance and there is a risk that ‘tension-thinning’ will lead to rupture. On the other hand, the tensile viscos- ity of LDPE rises with tensile stress and failure during wall-thinning is less likely. 11.2.5 Elastic aspects of melt behaviour While being deformed and forced through an extru- sion die, the melt stores elastic strain energy. As extrudate emerges from the die, stresses are released, some elastic recovery takes place and the extrudate swells. Dimensionally, the degree of swell is typically expressed by the ratio of extrudate diameter to die diameter; the elastic implications of the shear process are expressed by the following modulus:  D / R 11.3 where  is the elastic shear modulus,  is the shear stress at die wall, and  R is the recoverable shear strain. The magnitude of modulus  depends upon the poly- mer, molecular mass distribution and the level of shear stress. (Unlike viscosity, dependency of elasticity upon temperature, hydrostatic pressure and average molec- ular mass is slight.) If the molecular mass distribution is wide, the elastic shear modulus is low and elastic recovery is appreciable but slow. For a narrow distribu- tion, with its greater similarities in molecular lengths, recovery is less but faster. With regard to stress level, the modulus remains constant at low shear stresses but usually increases at the high stresses used commer- cially, giving appreciable recovery. The balance between elastic to viscous behaviour during deformation can be gauged by comparing the deformation time with the relaxation time or ‘natural time’ () of the polymer.  is the ratio of apparent viscosity to elastic shear modulus Á/, and derives from the Maxwell model of deformation. The term vis- coelasticity originated from the development of such models (e.g. Maxwell, Voigt, standard linear solid (SLS)). The Maxwell model is a mechanical analogue that provides a useful, albeit imperfect, simulation of viscoelasticity and stress relaxation in linear polymers above T g (Figure 11.9). It is based upon conditions of constant strain. A viscously damped ‘Newtonian’ dashpot, representing the viscous component of defor- mation, and a spring, representing the elastic compo- nent, are combined in series. At time t, the stress  is exponentially related to the initial stress  0 , as follows:  D  0 expt/ 11.4 where  is the relaxation time. When t × ,there is sufficient time for viscous movement of chain 360 Modern Physical Metallurgy and Materials Engineering Figure 11.9 Representation of stress relaxation under constant strain conditions (Maxwell model). molecules to take place and stress will fall rapidly. When t − , elastic behaviour predominates. The magnitude of  ranges from infinity, at the start of rubbery behaviour, to zero at the start of viscous behaviour. A real polymer contains different lengths of molecules and therefore features a spectrum of relax- ation times. Nevertheless, although best suited to poly- mers of low molecular mass, the Maxwell model offers a reasonable first approximation for melts. Let us now apply the relaxation time concept to an injection-moulding process in which a thermo- plastic acrylic at a temperature of 230 ° C is sheared rapidly at a rate of 10 5 s 1 . Assume that the injection (deformation) time is 2 s. For the shear rate given, Figure 11.8 indicates that the apparent shear viscosity is 9 Ns m 2 and the corresponding maximum shear stress is 0.9MNm 2 . At this shear stress, the elastic shear modulus for acrylic is 0.21 MN m 2 . The value of D Á/ is 43 µs, which is very small compared to the injection time of 2 s, hence viscous behaviour will predominate. A similar procedure can be applied to deformation by extrusion. For instance, PP with a relaxation time of 0.5 s might pass though the extru- sion die in 20 s. The time difference is smaller than the previous example of injection-moulding, indicating that although deformation is mainly viscous, elastic- ity will play a greater part than in injection-moulding. The previously-mentioned phenomenon of die swell then becomes understandable. (Swelling is equivalent to the spring action in the Maxwell model.) Although the degree of elastic behaviour may be relatively small during injection-moulding and extrusion, it can, nevertheless, sometimes cause serious flow defects. Relaxation times for extensional flow, as employed in blow-moulding, can be derived from the ratio of apparent tensile viscosity to elastic tensile modulus E D /ε R . Suppose that a PP parison at a temper- ature of 230 ° C hangs for 5 s before inflation with air. If the tensile viscosity and tensile modulus are 36 kN s m 2 and 4.6kNm 2 , respectively, the relax- ation time is roughly 8 s. Hence sagging of the parison under its own weight will be predominantly elastic. 11.2.6 Flow defects The complex nature of possible flow defects under- lines the need for careful product design (sections, shapes, tools) and close control of raw materials and operational variables (temperatures, shear rates, cool- ing arrangements). The quality of processing makes a vital contribution to the engineering performance of a polymer. Ideally, melt flow should be streamlined throughout the shaping process. If the entry angle of an extru- sion die causes an abrupt change in flow direction, the melt assumes a natural angle as it converges upon the die entry and a relatively stagnant ‘dead zone’ is cre- ated at the back of the die. In this region, the melt will have a different thermal history. In addition to its dominant shear component, the convergent flow con- tains an extensional component that increases rapidly during convergence. If the extensional stress reaches a critical value, localized ‘melt fracture’ will occur at a frequency depending upon conditions. The fragments produced recover some of the extensional strain. The effect upon the emerging extrudate can range from a matt finish to gross helical distortions. The associated flow condition is often termed ‘non-laminar’ despite the fact that the calculated value of the dimensionless Reynolds number is very low. The choice of entry angle for the die is crucial and depends partly upon the polymer. As a melt passes through the die, velocity gradients develop, with the melt near the die surface moving slower than the central melt. Upon leaving the die, the outer layers of extrudate accelerate, eliminating the velocity gradient. Above a critical velocity, the resul- tant stresses rupture the surface to give a ‘sharkskin’ effect which can range in severity from a matt finish to regular ridging perpendicular to the extrusion direc- tion. ‘Sharkskin’ is most likely when the polymer has a high average molecular mass (i.e. highly viscous) and a narrow molecular mass distribution (i.e. highly elastic); these factors cause surface stress to build up rapidly and to relax slowly. Fast extrusion at a low temperature favours this defect. Heating of the tip of the die lowers viscosity and reduces its likelihood. An inhomogeneous melt will produce a non-uniform recovery of elastic strain at the cooling surface and influence its final texture. Thorough mixing before shaping is essential. However, inhomogeneity may exist on a molecular scale. For instance, in both injection-moulding and extrusion, a broad distribution of molecular mass gives a more matt finish than a nar- row distribution. Thus, extrusion of a polymer with a narrow mass distribution at a rate slow enough to prevent the development of ‘sharkskin’ will favour a high-gloss finish. [...]... composites concept Metals and Materials, May, 273–278, Institute of Materials Harris, B (1986) Engineering Composite Materials Institute of Metals, London Hertzberg, R W (1989) Deformation and Fracture Mechanics of Engineering Materials, 3rd edn Wiley, Chichester Hughes, J D H (1986) Metals and Materials, June, pp 365–368, Institute of Materials Hull, D (1981) An Introduction to Composite Materials Cambridge... laser mirrors), and graphite/glassceramic (bearings, seals and brakes) The SiC fibre/lithium aluminosilicate (LAS) glass composite is a candidate material for new types of heat-engine 11.3.2.4 In-situ composites and nanocomposites Alloy microstructures, such as eutectics and eutectoids, containing a dispersion of fibres or lamellae within a 374 Modern Physical Metallurgy and Materials Engineering matrix... the preparation of mechanical testpieces 372 Modern Physical Metallurgy and Materials Engineering Despite these formidable problems, the basic idea of reinforcing a strong deformable matrix with elastic fibres retains its appeal to the aerospace, defence and automotive industries and active research on MMCs continues worldwide There is a need to expand and consolidate the database for properties of... isotropic sequences are 0/ C 45/ 45/ 45/ C 45/0 and 366 Modern Physical Metallurgy and Materials Engineering 0/ C 60/ 60/ 60/ C 60/0 The stacking of plies is symmetrical about the mid-plane of the laminate in order to prevent distortion and to ensure a uniform response to working stresses Randomly-oriented short fibres of glass are commonly used in sheets and in three-dimensional mouldings In fact, fibre... particles are 1 Patented by Osprey Metals, adopted under licence and further developed by Alcan International Ltd, Banbury 370 Modern Physical Metallurgy and Materials Engineering injected into this stream Fine-grained MMC deposits of SiC/aluminium alloy and Al2 O3 /steel can be built as plate, tube, billets for hot-working, cladding, etc The pathogenic risks associated with certain types of fine particles,... Advances in engineering plastics manufacture Metals and Materials, May, 281–284, Institute of Materials Chapter 12 Corrosion and surface engineering 12.1 The engineering importance of surfaces The general truth of the engineering maxim that ‘most problems are surface problems’ is immediately apparent when one considers the nature of metallic corrosion and wear, the fatigue-cracking of metals and the effect... Thermoplastics: Properties and Design, (ed R M Ogorkiewicz), Chap 11 by P C Powell, Wiley, Chichester Kelly, A (1986) Strong Solids Clarendon Press, Oxford King, J E (1989) Metals and Materials, 720–6 Institute of Materials Lemkey, F D (1984) Advanced in situ composites In Chap 14, Industrial Materials Science and Engineering (ed L E Murr) Marcel Dekker Mascia, L (1989) Thermoplastics: Materials Engineering 2nd... introduced the concept of ‘critical fibre length’ 364 Modern Physical Metallurgy and Materials Engineering Figure 11 .13 Distribution of tensile stress in a short fibre f between tensile force and interfacial shear force is: υ 2 d /4 D d.υl 11.10 Hence the gradient υ /υl for the build-up of tensile stress is 4 /d For the critical condition (Figure 11.13b), the gradient is f divided by the critical transfer... some elevated temperature This is known as the standard dissociation temperature when the oxide is in equilibrium with the pure element and oxygen at 1 atm pressure In the case of gold, the oxide is not stable at room temperature, for silver 378 Modern Physical Metallurgy and Materials Engineering Ag2 O dissociates when gently heated to about 200° C, and the oxides of the Pt group of metals around 1000°... solubility and fracture in nanoscale structures are often tentative Further reading Ashbee, K H G (1993) Fundamental Principles of Fiber Reinforced Composites, 2nd edn Technomic Publ Co Inc., Lancaster, USA Ashby, M F and Jones, D R H (1986) Engineering Materials, 2, Elsevier Science, Oxford Brook, R J and MacKenzie, R A D (1993) Nanocomposite materials Materials World, January, 27–30, Institute of Materials . microfibrils during deformation. 356 Modern Physical Metallurgy and Materials Engineering several microns wide and fairly constant in width: they can scatter incident light and are visible to the unaided eye. was proposed for wharf and off-shore oil platform construction in Norwegian waters. 362 Modern Physical Metallurgy and Materials Engineering Figure 11.10 ‘Parallel’ (a) and ‘series’ (b) models. when 368 Modern Physical Metallurgy and Materials Engineering Figure 11.16 Structure of (a) carbon fibre and (b) aramid fibre (from Hughes, June 1986, pp. 365–8; by permission of the Institute of Materials) . high-modulus