Amorphous polymer 7g FIGURE 6-8 Specific volume versus temperature relationship for amorphous and semicrystallin polymers(Tg, glass transition temperature; Tm, melting temperature).(K.K.Chawla, Composite Material -Science& Engineering, Springer-Verlag, New York, NY, 1987, p 60) Glassy Fluid emperature FIGURE 6.9 Variation of modulus of an amorphous polymer with temperature.(K.K.Chawla Composite Material-Science& Engineering, Springer-Verlag, New York, NY, 1987, p. 63) bonding, and are highly cross-linked. In fact, they may have a higher Tg than polymers that have only covalent bonding and have less cross-linking In amorphous polymers, there is no apparent order among the molecules and the molecular chains are arranged in a random manner. When polymers precipitate from dilute solutions, small, platelike single-crystalline regions called lamellae or crystallites form. In the lamellae, long molecular chains are folded in a regular manner, and many lamellae group together to form spherulites much like grains in metal Most linear polymers soften and melt upon heating. These are called thermoplastics and re readily shaped using liquid forming techniques. Examples include low-and high-density polyethylene, polystyrene, and PMMA When the molecules in a polymer are cross-linked in the form of a network, they do not soften on heating, and are called thermosetting polymers. Common thermosetting polymers include phenolic, polyester, polyurethane, and silicone. Thermosetting polymers decompose on heating As noted, cross-linking makes sliding of molecules past one another difficult, thus making the 412 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
E >o r Q. CO Amorphous j J I I I Semicrystalline I polymer I I I I l v Tg T m Temperature FIGURE 6-8 Specific volume versus temperature relationship for amorphous and semicrystalline polymers (Tg, glass transition temperature; Tin, melting temperature). (K. K. Chawla, Composite Material- Science & Engineering, Springer-Verlag, New York, NY, 1987, p. 60). UJ 0 _J ' ! | ' _ J, , Tg __ 91a__ssz__ Tf. Fluid ~, I I I Temperature FIGURE 6-9 Variation of modulus of an amorphous polymer with temperature. (K. K. Chawla, Composite Material- Science & Engineering, Springer-Verlag, New York, NY, 1987, p. 63). bonding, and are highly cross-linked. In fact, they may have a higher Tg than polymers that have only covalent bonding and have less cross-linking. In amorphous polymers, there is no apparent order among the molecules and the molecular chains are arranged in a random manner. When polymers precipitate from dilute solutions, small, platelike single-crystalline regions called lamellae or crystallites form. In the lamellae, long molecular chains are folded in a regular manner, and many lamellae group together to form spherulites much like grains in metals. Most linear polymers soften and melt upon heating. These are called thermoplastics and are readily shaped using liquid forming techniques. Examples include low- and high-density polyethylene, polystyrene, and PMMA. When the molecules in a polymer are cross-linked in the form of a network, they do not soften on heating, and are called thermosetting polymers. Common thermosetting polymers include phenolic, polyester, polyurethane, and silicone. Thermosetting polymers decompose on heating. As noted, cross-linking makes sliding of molecules past one another difficult, thus making the 412 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
polymer strong and rigid. A typical example is that of rubber cross-linked with sulfur, that is, vulcanized rubber. Vulcanized rubber has 10 times the strength of natural rubber of polymers based on the type of rep If one type of repeating unit forms a polymer chain, it is called a homopolymer. In contrast, polymer chains having two different monomers form co-polymers. If the two different monomers are distributed randomly along the chain, then the polymer is called a regular or random co polymer. If, however, a long sequence of one monomer is followed by a long sequence of another monomer, the polymer is called a block co-polymer. If a chain of one type of monomer has branches of another type, then a graft co-polymer is said to form Figure 6-10 schematically illustrates these various polymer structures Molecular weight is a very important parameter that determines the properties of poly mers. Many mechanical properties increase with increasing molecular weight(although polymer processing also becomes more difficult with increasing molecular weight). Another important parameter is degree of polymerization(DP), which is a measure of the number of basic units (mers) in a polymer. The molecular weight (Mw) and the DP are related by mW=dP X MW)u, where(MW)u is the molecular weight of the repeating unit. Because polymers contain various types of molecules, each of which has a different Mw and DP, the molecular weight of the polymer is characterized by a distribution function. A narrower distribution indicates a omogeneous polymer with many repeating identical units whereas a broad distribution indi- cates that the polymer is composed of a large number of different species. In general, therefore, it is convenient to specify an average molecular weight or degree of polymerization. Unlike many common substances of low molecular weight, polymers can have very large molecular weights, For example, the molecular weights of natural rubber and polyethylene(a synthetic polymer)can be greater than 10 and 10, respectively. In addition, the molecular diameters of high-molecular-weight polymers can be over two orders of magnitude larger than ordinary ubstances such as water that have low molecular weights Polymers can be amorphous or partially crystalline, and the amount of crystallinity in a polymer can vary from 30 to 90%. The elastic modulus and strength of polymers increase with increasing crystallinity, although fully crystalline polymers are unrealistic. The long and complex molecular chains in a polymer are easily tangled, resulting in large segments of molecular chains hat may remain trapped between crystalline regions and never reorganize themselves into an ordered molecular assembly characteristic of a fully crystalline state. The molecular arrangement a polymer also influences the extent of crystallization; for example, linear molecules with small side groups can readily crystallize whereas branched-chain molecules with bulky side Random Block FIGURE 6-10 Schematic representation of random, block, and graft copolymers (K K. Chawla, Composite Material -Science& Engineering, Springer-Verlag, New York, NY, 1987, p. 61) Composite Materials 413
polymer strong and rigid. A typical example is that of rubber cross-linked with sulfur, that is, vulcanized rubber. Vulcanized rubber has 10 times the strength of natural rubber. There is another type of classification of polymers based on the type of repeating unit. If one type of repeating unit forms a polymer chain, it is called a homopolymer. In contrast, polymer chains having two different monomers form co-polymers. If the two different monomers are distributed randomly along the chain, then the polymer is called a regular or random copolymer. If, however, a long sequence of one monomer is followed by a long sequence of another monomer, the polymer is called a block co-polymer. If a chain of one type of monomer has branches of another type, then a graft co-polymer is said to form. Figure 6-10 schematically illustrates these various polymer structures. Molecular weight is a very important parameter that determines the properties of polymers. Many mechanical properties increase with increasing molecular weight (although polymer processing also becomes more difficult with increasing molecular weight). Another important parameter is degree of polymerization (DP), which is a measure of the number of basic units (mers) in a polymer. The molecular weight (MW) and the DP are related by MW = DP x (MW)u, where (MW)u is the molecular weight of the repeating unit. Because polymers contain various types of molecules, each of which has a different MW and DP, the molecular weight of the polymer is characterized by a distribution function. A narrower distribution indicates a homogeneous polymer with many repeating identical units whereas a broad distribution indicates that the polymer is composed of a large number of different species. In general, therefore, it is convenient to specify an average molecular weight or degree of polymerization. Unlike many common substances of low molecular weight, polymers can have very large molecular weights. For example, the molecular weights of natural rubber and polyethylene (a synthetic polymer) can be greater than 106 and 105 , respectively. In addition, the molecular diameters of high-molecular-weight polymers can be over two orders of magnitude larger than ordinary substances such as water that have low molecular weights. Polymers can be amorphous or partially crystalline, and the amount of crystallinity in a polymer can vary from 30 to 90%. The elastic modulus and strength of polymers increase with increasing crystallinity, although fully crystalline polymers are unrealistic. The long and complex molecular chains in a polymer are easily tangled, resulting in large segments of molecular chains that may remain trapped between crystalline regions and never reorganize themselves into an ordered molecular assembly characteristic of a fully crystalline state. The molecular arrangement in a polymer also influences the extent of crystallization; for example, linear molecules with small side groups can readily crystallize whereas branched-chain molecules with bulky side Random Block FIGURE 6-10 Schematic representation of random, block, and graft copolymers. (K. K. Chawla, Composite Material- Science & Engineering, Springer-Verlag, New York, NY, 1987, p. 61) Composite Materials 413
:时0 Properties of Polymeric Matrices Glassy polymers follow Hooke 's law and exhibit a linear elastic response to applied stress.The elastic strain in glassy polymers is less than 1%. In contrast, elastomers(rubbery polymers) show a nonlinear elastic behavior with a large elastic range(a few percent strain) as shown in Figure 6-6. The large elastic strain in elastomers is caused by a redistribution of the tangled molecular chains under an applied stress Highly cross-linked thermosetting resins such as polyesters, epoxies, and polyimides have high modulus and strength, but are also extremely brittle. The fracture energy(100-200 J m-2) of thermosetting resins is only slightly better than that of inorganic glasses(10-30 J m-2).In contrast,thermoplastic resins such as polymethylmetacrylate(PMMa)have fracture energies on the order of 1000 J. m-2 because their large free volume facilitates absorption of the energy associated with crack propagation. The fracture toughness of hard and brittle thermosets such a epoxies and polyester resins is improved by distributing small (a few micrometers in size)and soft rubbery inclusions in the brittle matrix. The most common method is simple mechanical blending of the soft, elastomer particles and the thermoset resin, or copolymerization of a mixture of the two Polymers show a significant temperature dependence of their elastic modulus as shown in Figure 6-8. Below the glass transition temperature Tg, the polymer behaves as a hard and rigid solid, with an elastic modulus of about 5-7 GPa. Above Tg, the modulus drops significantly and the polymer exhibits a rubbery behavior. Upon heating the polymer to above its meltin temperature Tf at which the polymer becomes fluid, the modulus drops abruptly. The glass transition temperature, melting temperature, and selected mechanical and thermal properties of common polymeric materials are listed in Table 6-2. The thermal expansion of polymers, in particular thermoplastics, is strongly temperature- dependent, exhibits a non-linear increase with temperature, and is generally an order of magnitude or two greater than that of metals and ceramics. Therefore, in polymer composites the thermal expansion mismatch between inorganic fibers and polymer matrix is large, which may cause thermal stresses and cracking. Both thermosets and thermoplastics are used as matrix materials for polymer composites. For xample, polyesters, epoxies and polyimides are commonly used matrices in fiber-reinforced composites. Polyesters have fair resistance to water and various chemicals, as well as to aging ut they shrink between 4 and 8%on In contrast, thermosetting epoxy resins have better moisture resistance, lower shrinkage on curing(about 3%), a higher maximum-use temperature, and good adhesion to glass fibers. Polyimides have a relatively high service-temperature range 250-300oC but, they are brittle and have low fracture energies of 15-70 J.m. Polymers degrade at high temperature and by moisture absorption, which causes swelling and a reduction in Tg. If the polymer is reinforced with fibers bonded to the matrix, then moisture absorption may cause severe intemal stresses in the composite Polymer composites are fabricated using pultrusion and filament winding. In pultrusion (Figure 6-11), continuous fiber tows (i. e, fiber bundle with parallel strands of fibers)are impreg- nated with a thermoset resin and drawn through a die that forms the final component shape (e.g, tubes, rods, etc. ) Hollow parts are made by pultruding the composite feedstock around a 414 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
groups do not crystallize easily. Thus, linear high-density polyethylene can attain up to 90% crystallization whereas branched polyethylene can attain only about 65% crystallization. Properties of Polymeric Matrices Glassy polymers follow Hooke's law and exhibit a linear elastic response to applied stress. The elastic strain in glassy polymers is less than 1%. In contrast, elastomers (rubbery polymers) show a nonlinear elastic behavior with a large elastic range (a few percent strain) as shown in Figure 6-6. The large elastic strain in elastomers is caused by a redistribution of the tangled molecular chains under an applied stress. Highly cross-linked thermosetting resins such as polyesters, epoxies, and polyimides have high modulus and strength, but are also extremely brittle. The fracture energy (100-200 J.m -2) of thermosetting resins is only slightly better than that of inorganic glasses (10-30 J.m-2). In contrast, thermoplastic resins such as polymethylmetacrylate (PMMA) have fracture energies on the order of 1000 J.m -2 because their large free volume facilitates absorption of the energy associated with crack propagation. The fracture toughness of hard and brittle thermosets such as epoxies and polyester resins is improved by distributing small (a few micrometers in size) and soft rubbery inclusions in the brittle matrix. The most common method is simple mechanical blending of the soft, elastomer particles and the thermoset resin, or copolymerization of a mixture of the two. Polymers show a significant temperature dependence of their elastic modulus as shown in Figure 6-8. Below the glass transition temperature Tg, the polymer behaves as a hard and rigid solid, with an elastic modulus of about 5-7 GPa. Above Tg, the modulus drops significantly and the polymer exhibits a rubbery behavior. Upon heating the polymer to above its melting temperature Tf at which the polymer becomes fluid, the modulus drops abruptly. The glass transition temperature, melting temperature, and selected mechanical and thermal properties of common polymeric materials are listed in Table 6-2. The thermal expansion of polymers, in particular thermoplastics, is strongly temperaturedependent, exhibits a non-linear increase with temperature, and is generally an order of magnitude or two greater than that of metals and ceramics. Therefore, in polymer composites the thermal expansion mismatch between inorganic fibers and polymer matrix is large, which may cause thermal stresses and cracking. Both thermosets and thermoplastics are used as matrix materials for polymer composites. For example, polyesters, epoxies and polyimides are commonly used matrices in fiber-reinforced composites. Polyesters have fair resistance to water and various chemicals, as well as to aging, but they shrink between 4 and 8% on curing. In contrast, thermosetting epoxy resins have better moisture resistance, lower shrinkage on curing (about 3%), a higher maximum-use temperature, and good adhesion to glass fibers. Polyimides have a relatively high service-temperature range, 250-300~ but, they are brittle and have low fracture energies of 15-70 J.m -2. Polymers degrade at high temperature and by moisture absorption, which causes swelling and a reduction in Tg. If the polymer is reinforced with fibers bonded to the matrix, then moisture absorption may cause severe internal stresses in the composite. Polymer Composites Polymer composites are fabricated using pultrusion and filament winding. In pultrusion (Figure 6-11), continuous fiber tows (i.e., fiber bundle with parallel strands of fibers) are impregnated with a thermoset resin and drawn through a die that forms the final component shape (e.g., tubes, rods, etc.). Hollow parts are made by pultruding the composite feedstock around a 414 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
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forming Curing Pullers FIGURE 6-11 Schematic diagram of the pultrusion process of making polymer composites D Callister, Jr, Materials Science and Engineering: An Introduction, 6th ed, Wiley, New York, 2003,p.555) core or mandrel. The drawing rate and fiber volume fraction are controlled. The component is chen cured in a preheated precision die to obtain the final shape and size. Glass-, carbon-, and aramid fiber-reinforced polyesters, vinyl esters, and epoxy resin matrix composites containing relatively large(40% or higher) volume fraction of aligned continuous fibers are produced via lis method In filament winding, fiber strands or tows are first coated with a resin by passing them through a resin bath, and resin-coated fibers are automatically and continuously wound onto a mandrel for subsequent curing. The fiber volume fraction is controlled by the spacing between fiber strands and by the number of fiber layers wound on the mandrel. The mechanical properties of the composite are influenced not only by the fiber volume fraction but also by the winding pattern(helical, circumferential, etc. After the required number of fiber layers have been wound on the mandrel, the composite is cured in an oven and the mandrel removed to obtain a hollow composite object. Rocket motor castings, pipes, and pressure vessels are made using filament Another widely used method to make polymer-matrix composite consists of stacking lay ers of partially-cured thin(<1 mm)sheets of resin-impregnate and directionally aligned fibers called prepregs either manually(hand layup)or automatically in a three-dimensional sand- wich structure Prepregs are covered with a backing paper that is removed prior to lamination Prepregs are made by sandwiching fiber tows between sheets of carrier paper that is coated witl the resin matrix. On pressing the paper over fiber tows using heated rollers(a process called calendaring"), the resin melts and impregnates the fibers, thus forming a prepreg. A prepreg may be cut to various angles relative to the fiber axis to give prepregs of different orientations For example, a prepreg with fibers parallel to the long dimension is a zero-degree lamina or ply, and a prepreg that is cut with fibers perpendicular to the long dimension is a 90-degree ply (intermediate angles are also used). Figure 6-12 shows stacking of prepregs in a 01=45/90 orientation. The composite properties can be predicted from the theory of composite mechanics for a given stacking sequence and fiber orientation, which permits premeditated design of the composite for properties. For example, the coefficient of thermal expansion(CTE)of boron- epoxy and carbon-epoxy composites can be systematically varied between-5x10- and 30x 10-6oK-, by controlling the ply angle as shown in Figure 6-13. After stacking the plies in the desired orientation, the final curing of the component is done under heat and pressure For polymer composite with thermoplastic matrices, liquid-phase fabrication methods, such as 416 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
Fiber rovings Preforming , die impregnation tank Curing die Pullers ki..~, iiii FIGURE 6-11 Schematic diagram of the pultrusion process of making polymer composites. (W. D. Callister, Jr., Materials Science and Engineering: An Introduction, 6th ed., Wiley, New York, 2003, p. 555). core or mandrel. The drawing rate and fiber volume fraction are controlled. The component is then cured in a preheated precision die to obtain the final shape and size. Glass-, carbon-, and aramid fiber-reinforced polyesters, vinyl esters, and epoxy resin matrix composites containing relatively large (40% or higher) volume fraction of aligned continuous fibers are produced via this method. In filament winding, fiber strands or tows are first coated with a resin by passing them through a resin bath, and resin-coated fibers are automatically and continuously wound onto a mandrel for subsequent curing. The fiber volume fraction is controlled by the spacing between fiber strands and by the number of fiber layers wound on the mandrel. The mechanical properties of the composite are influenced not only by the fiber volume fraction but also by the winding pattern (helical, circumferential, etc.). After the required number of fiber layers have been wound on the mandrel, the composite is cured in an oven and the mandrel removed to obtain a hollow composite object. Rocket motor castings, pipes, and pressure vessels are made using filament winding. Another widely used method to make polymer-matrix composite consists of stacking layers of partially-cured thin (<1 mm) sheets of resin-impregnate and directionally aligned fibers called prepregs either manually (hand layup) or automatically in a three-dimensional sandwich structure. Prepregs are covered with a backing paper that is removed prior to lamination. Prepregs are made by sandwiching fiber tows between sheets of carder paper that is coated with the resin matrix. On pressing the paper over fiber tows using heated rollers (a process called "calendaring"), the resin melts and impregnates the fibers, thus forming a prepreg. A prepreg may be cut to various angles relative to the fiber axis to give prepregs of different orientations. For example, a prepreg with fibers parallel to the long dimension is a zero-degree lamina or ply, and a prepreg that is cut with fibers perpendicular to the long dimension is a 90-degree ply (intermediate angles are also used). Figure 6-12 shows stacking of prepregs in a 0 ~176 ~ orientation. The composite properties can be predicted from the theory of composite mechanics for a given stacking sequence and fiber orientation, which permits premeditated design of the composite for properties. For example, the coefficient of thermal expansion (CTE) of boronepoxy and carbon-epoxy composites can be systematically varied between -5 x ~ and 30 x 10 -6 oK-l, by controlling the ply angle as shown in Figure 6-13. After stacking the plies in the desired orientation, the final curing of the component is done under heat and pressure. For polymer composite with thermoplastic matrices, liquid-phase fabrication methods, such as 416 MATERIALS PROCESSING AND MANUFACTURING SCIENCE