《纺织复合材料》课程参考文献（Principles of the Manufacturing of Composite Materials）CHAPTER 3 Reinforcements-Fibers

Copyrighted Materials Copyright 2009 DEStech Publications Retrieved from www.knovel.com CHAPTER 3 Reinforcements-Fibers 1. GENERAL Reinforcements for composite materials can be in the form of fibers, particles, or flakes. Each has its own unique application, although fibers are the most common in composites and have the most influence on prop- erties. The reasons for this, discussed in Chapter 1, are as follows: . The large aspect ratio (length over diameter) of the fiber, which gives rise to effective shear stress transfer between the matrix and the reinforcement . The ability for fibers to be bent to sharp radius, allowing them to fit into sharp radius of curvature of the parts. . Numerous techniques to manufacture fibers such as spinning, chemical vapor deposition, or oxidation. The fibers used in composite materials appear at different scales. The manufacturing of fiber composites involves the use of fibers from the mi- crometer level up to the centimeter level. Figure 3.1 shows the different scales of fiber forms. At the smaller scale (level a) are the individual fila- ments with diameters of about 10 microns. These are usually bundled to- gether to form tows consisting of thousands of individual filaments (level b). There are tows of 3 k (3000) filaments, 6 k, 12 k etc. These tows can then be combined either with or without the addition of resin for adhe- siveness. If resin is used, the tows can be combined to form tapes. When resin is not used, the tows can be woven together to make dry woven fab- rics, or the tows can be braided or knitted together to make dry fiber pre- forms (level c). The final composite part is made by placing many of 99

CHAPTER 3 1. GENERAL Reinforcements for composite materials can be in the form of fibers, particles, or flakes. Each has its own unique application, although fibers are the most common in composites and have the most influence on prop￾erties. The reasons for this, discussed in Chapter 1, are as follows: • The large aspect ratio (length over diameter) of the fiber, which gives rise to effective shear stress transfer between the matrix and the reinforcement • The ability for fibers to be bent to sharp radius, allowing them to fit into sharp radius of curvature of the parts. • Numerous techniques to manufacture fibers such as spinning, chemical vapor deposition, or oxidation. The fibers used in composite materials appear at different scales. The manufacturing of fiber composites involves the use of fibers from the mi￾crometer level up to the centimeter level. Figure 3.1 shows the different scales of fiber forms. At the smaller scale (level a) are the individual fila￾ments with diameters of about 10 microns. These are usually bundled to￾gether to form tows consisting of thousands of individual filaments (level b). There are tows of 3 k (3000) filaments, 6 k, 12 k etc. These tows can then be combined either with or without the addition of resin for adhe￾siveness. If resin is used, the tows can be combined to form tapes. When resin is not used, the tows can be woven together to make dry woven fab￾rics, or the tows can be braided or knitted together to make dry fiber pre￾forms (level c). The final composite part is made by placing many of 99

100 REINFORCEMENTS-FIBERS O Tape Single filament d≈104m (a) Woven fabric Braid Many fibers,tow (3K,6KI2K) Stack of many (b) Knit layers (d) Mat (c) FIGURE 3.I Fiber forms at different scales. these layers on each other.Pressure is usually applied to press these fiber layers together(level d).Note that the load applied in this case is along the thickness direction (z direction)of the fiber bed.This is different from the situation where the load is applied within the plane of the fiber bed (such as along the fiber direction or transverse to the fiber direction as in normal operating conditions for load bearing composite structures). The load during the manufacturing stage is different from the loads dur- ing the operation stage and it is important to distinguish this. The discussion on the fibers will be presented in the sequence of in- creasing scale level,beginning with the individual filaments. 2.INDIVIDUAL FILAMENTS Individual filaments are normally produced by drawing from a melt of the material (such as glass)or by drawing from thermoplastic molecules and then chopping away secondary atoms from the main backbone of the fiber(such as carbon fibers).The three common fibers used for making composites are glass,carbon,Kevlar and thermoplastic fibers

these layers on each other. Pressure is usually applied to press these fiber layers together (level d). Note that the load applied in this case is along the thickness direction (z direction) of the fiber bed. This is different from the situation where the load is applied within the plane of the fiber bed (such as along the fiber direction or transverse to the fiber direction as in normal operating conditions for load bearing composite structures). The load during the manufacturing stage is different from the loads dur￾ing the operation stage and it is important to distinguish this. The discussion on the fibers will be presented in the sequence of in￾creasing scale level, beginning with the individual filaments. 2. INDIVIDUAL FILAMENTS Individual filaments are normally produced by drawing from a melt of the material (such as glass) or by drawing from thermoplastic molecules and then chopping away secondary atoms from the main backbone of the fiber (such as carbon fibers). The three common fibers used for making composites are glass, carbon, Kevlar and thermoplastic fibers. 100 REINFORCEMENTS—FIBERS FIGURE 3.1 Fiber forms at different scales

Individual Filaments 101 2.1.Glass Fibers 2.1.1.Glass Fiber Manufacturing Process The raw materials for glass fibers are silica sand,boric acid,and other minor ingredients (e.g.,clay,coal,and fluorospar).These are dry mixed in a high temperature refractory furnace.The temperature of this melt varies for each glass composition,but is generally about 2300F (1260°C). The process used for the manufacture of glass fibers from the molten glass mixture is illustrated schematically in Figure 3.2.First the mixture of silica sand and ingredients is put in a batch silo.They are then mixed and weighed.The mixture is transported to a batch charging unit where the mixture is fed into the furnace.The temperature in the furnace varies from 1540C at the melting stage to 1425C at the refining stage.The molten glass flows to the forehearth stage where the temperature is re- duced to 1260C.At the bottom of the forehearth station,there is a plate made of platinum which contains many tiny holes.The molten glass flows through these tiny holes and forms filaments.These filaments are then pulled mechanically to make smaller filaments.The diameter of the Sand Metal oxides Batch Furnace Refiner Forehearth 1540℃ 14250 1371℃ Molten glass 1260C 签1370℃2 1340*0 ,e Platinum plate with tiny holes Forehearth Bushing Water spray Filament □Applicator Gathering shoe FIGURE 3.2 Fiberglass manufacturing process [4]

2.1. Glass Fibers 2.1.1. Glass Fiber Manufacturing Process The raw materials for glass fibers are silica sand, boric acid, and other minor ingredients (e.g., clay, coal, and fluorospar). These are dry mixed in a high temperature refractory furnace. The temperature of this melt varies for each glass composition, but is generally about 2300°F (1260°C). The process used for the manufacture of glass fibers from the molten glass mixture is illustrated schematically in Figure 3.2. First the mixture of silica sand and ingredients is put in a batch silo. They are then mixed and weighed. The mixture is transported to a batch charging unit where the mixture is fed into the furnace. The temperature in the furnace varies from 1540°C at the melting stage to 1425°C at the refining stage. The molten glass flows to the forehearth stage where the temperature is re￾duced to 1260°C. At the bottom of the forehearth station, there is a plate made of platinum which contains many tiny holes. The molten glass flows through these tiny holes and forms filaments. These filaments are then pulled mechanically to make smaller filaments. The diameter of the Individual Filaments 101 FIGURE 3.2 Fiberglass manufacturing process [4]

102 REINFORCEMENTS-FIBERS filaments depends on the speed of drawing.A chemical called sizing is applied on the surface of the fibers at this stage.The sizing is used to re- duce the friction between the fibers to prevent breakage.A finish can also be applied to the fiber.Finish is a type of chemical used to protect the sur- face of the fibers and to provide good bonding with the matrix material when the composite is made.Many filaments are bundled together to form tows or strands.These tows or strands are then wound onto a creel for shipping purpose,or they can be cut into short fibers. 2.1.2.Types of Glass Fibers Glass is an amorphous material that consists of a silica (SiO,)back- bone with various oxide components to give specific compositions and properties.Several types of glass fibers are manufactured but only three are used often in composites:E glass,S glass (and its variation S,),and C glass.Table 3.1 shows the composition of the different types of glasses. E glass(E for electrical grade)fibers have a composition of calcium aluminoborosilicate and calcium oxide,used when strength and electri- cal resistivity are required.The most common fiberglass used in compos- ites,E glass is inexpensive in comparison with other types.Eglass fibers are used as short fiber reinforcements for engineering thermoplastics;as fibers used with polyester or vinyl ester matrix for automotive composite components;and for fiber reinforced rods used for civil applications such as boats,seats,or trays.S glass(S for strength)is approximately TABLE 3.1 Composition of Glasses Used in Composite Materials [3]. EGlass S Glass C Glass Range Range Range (%) (%) (%) Silicon oxide 52-56 65 64-68 Calcium oxide 16-25 11-15 Aluminum oxide 12-16 25 3-5 Boric oxide 5-10 4-6 Magnesium oxide 0-5 10 2-4 Sodium oxide and 0-2 一 7-10 potassium oxide Titanium oxide 0-15 一 Iron 0-1 一 二 Iron oxide 0-0.8 二 0-0.8 Barium oxide 0-1

filaments depends on the speed of drawing. A chemical called sizing is applied on the surface of the fibers at this stage. The sizing is used to re￾duce the friction between the fibers to prevent breakage. A finish can also be applied to the fiber. Finish is a type of chemical used to protect the sur￾face of the fibers and to provide good bonding with the matrix material when the composite is made. Many filaments are bundled together to form tows or strands. These tows or strands are then wound onto a creel for shipping purpose, or they can be cut into short fibers. 2.1.2. Types of Glass Fibers Glass is an amorphous material that consists of a silica (SiO2) back￾bone with various oxide components to give specific compositions and properties. Several types of glass fibers are manufactured but only three are used often in composites: E glass, S glass (and its variation S2), and C glass. Table 3.1 shows the composition of the different types of glasses. E glass (E for electrical grade) fibers have a composition of calcium aluminoborosilicate and calcium oxide, used when strength and electri￾cal resistivity are required. The most common fiberglass used in compos￾ites, E glass is inexpensive in comparison with other types. E glass fibers are used as short fiber reinforcements for engineering thermoplastics; as fibers used with polyester or vinyl ester matrix for automotive composite components; and for fiber reinforced rods used for civil applications such as boats, seats, or trays. S glass (S for strength) is approximately 102 REINFORCEMENTS—FIBERS TABLE 3.1 Composition of Glasses Used in Composite Materials [3]. E Glass Range (%) S Glass Range (%) C Glass Range (%) Silicon oxide 52–56 65 64–68 Calcium oxide 16–25 — 11–15 Aluminum oxide 12–16 25 3–5 Boric oxide 5–10 — 4–6 Magnesium oxide 0–5 10 2–4 Sodium oxide and potassium oxide 0–2 — 7–10 Titanium oxide 0–15 — — Iron 0–1 — — Iron oxide 0–0.8 — 0–0.8 Barium oxide — — 0–1

Individual Filaments 103 40%higher in strength than E glass and offers better retention of proper- ties at elevated temperatures.S glass is often used in advanced compos- ites when strength is a premium.C glass(C for corrosion)is used in corrosive environments because of the chemical stability of its soda lime borosilicate composition. Table 3.2 gives the physical,mechanical,thermal,electrical,and opti- cal properties of glass fibers.Some properties,such as tensile strength and tensile modulus,are measured on the fibers directly.Other physical properties are measured on glass that has been formed into a patty or block sample and then annealed to relieve the forming stress.Note that the modulus of glass (about 70 GPa)is about the same as that of alumi- num;however the strength of glass is much larger than that of aluminum. Glass fibers normally have many defects on their surfaces.This is due to the abrasion between the fibers.The longer the fiber,the more defects there are.As such,the tensile strength of the fibers depends on their length.Figure 3.3 shows the effect of length on the tensile strengths of fibers. Moisture has a detrimental effect on glass strength.The decrease in strength with increasing temperature is more pronounced in E glass than in S glass.However,the modulus decreases at about the same rate with increasing temperature for both E glass and S glass.The decrease is due to the rearrangement of the molecules into a less compact and hence more flexible configuration. 10 S GLASS E GLASS 10 100 FIGURE 3.3 Effect of fiber length on tensile strength of fiber

40% higher in strength than E glass and offers better retention of proper￾ties at elevated temperatures. S glass is often used in advanced compos￾ites when strength is a premium. C glass (C for corrosion) is used in corrosive environments because of the chemical stability of its soda lime borosilicate composition. Table 3.2 gives the physical, mechanical, thermal, electrical, and opti￾cal properties of glass fibers. Some properties, such as tensile strength and tensile modulus, are measured on the fibers directly. Other physical properties are measured on glass that has been formed into a patty or block sample and then annealed to relieve the forming stress. Note that the modulus of glass (about 70 GPa) is about the same as that of alumi￾num; however the strength of glass is much larger than that of aluminum. Glass fibers normally have many defects on their surfaces. This is due to the abrasion between the fibers. The longer the fiber, the more defects there are. As such, the tensile strength of the fibers depends on their length. Figure 3.3 shows the effect of length on the tensile strengths of fibers. Moisture has a detrimental effect on glass strength. The decrease in strength with increasing temperature is more pronounced in E glass than in S glass. However, the modulus decreases at about the same rate with increasing temperature for both E glass and S glass. The decrease is due to the rearrangement of the molecules into a less compact and hence more flexible configuration. Individual Filaments 103 FIGURE 3.3 Effect of fiber length on tensile strength of fiber

104 REINFORCEMENTS-FIBERS TABLE 3.2 Properties of Glasses [1]. Type of Glass Property C E s Density(g/cm3) 2.49-2.50 2.54-2.62 2.48-2.50 Tensile strength(MPa) @22C 3006-3280 3417 4544 @371C 2597 3724-4408 @538C 1708 2392 Tensile modulus(GPa) @22C 68.3 71.8 84.7 @538C 80.6 88.2 Elongation 0.03 0.035 0.04 Coefficient of thermal 7.2 5.0 5.6 expansion(10-6m/m/C) Heat Capacity (J/kg.C)@22C) 800 800 800 Softening point,C 749-750 841-846 970 2.1.3.Surface Treatment [1] Glass fibers are extremely fragile and abrade easily during processing. The problem is especially evident in processes such as weaving,al- though almost any handling or moving process will cause abrasion of the glass fibers. To guard against loss of strength,which depends strongly on surface defects that may be caused during handling,a chemical sizing (or coat- ing)is applied to the fibers.This sizing protects the fibers during han- dling,and also holds the individual fibers together. Usually sizing is temporary and after it is removed a finish is added. However in other cases the sizing also acts as the finish.The finish im- proves the compatibility of the fiber with the matrix.Typical finishes would be polyvinyl acetate modified with chromic chloride complex and/or organosilane coupling agents. Coupling agents,molecules which are compatible at one end with the silane structure of the glass and at the other with the matrix,can be thought of as bridges connecting the reinforcement and the matrix.Fig- ure 3.4 shows the bridging nature of the coupling agent between the glass fiber and the polymer matrix material.The coupling agents can combine with both the glass fiber and the polymer matrix material to form a sepa- rate phase called the interphase.This interphase may have different properties from either the glass or the polymer material.Figure 3.5 shows a schematic of the interphase

2.1.3. Surface Treatment [1] Glass fibers are extremely fragile and abrade easily during processing. The problem is especially evident in processes such as weaving, al￾though almost any handling or moving process will cause abrasion of the glass fibers. To guard against loss of strength, which depends strongly on surface defects that may be caused during handling, a chemical sizing (or coat￾ing) is applied to the fibers. This sizing protects the fibers during han￾dling, and also holds the individual fibers together. Usually sizing is temporary and after it is removed a finish is added. However in other cases the sizing also acts as the finish. The finish im￾proves the compatibility of the fiber with the matrix. Typical finishes would be polyvinyl acetate modified with chromic chloride complex and/or organosilane coupling agents. Coupling agents, molecules which are compatible at one end with the silane structure of the glass and at the other with the matrix, can be thought of as bridges connecting the reinforcement and the matrix. Fig￾ure 3.4 shows the bridging nature of the coupling agent between the glass fiber and the polymer matrix material. The coupling agents can combine with both the glass fiber and the polymer matrix material to form a sepa￾rate phase called the interphase. This interphase may have different properties from either the glass or the polymer material. Figure 3.5 shows a schematic of the interphase. 104 REINFORCEMENTS—FIBERS TABLE 3.2 Properties of Glasses [1]. Property Type of Glass CE S Density (g/cm3) 2.49–2.50 2.54–2.62 2.48–2.50 Tensile strength (MPa) @22°C 3006–3280 3417 4544 @371°C — 2597 3724–4408 @538°C — 1708 2392 Tensile modulus (GPa) @22°C 68.3 71.8 84.7 @538°C — 80.6 88.2 Elongation 0.03 0.035 0.04 Coefficient of thermal expansion (10−6m/m/°C) 7.2 5.0 5.6 Heat Capacity (J/kg.C)@22°C) 800 800 800 Softening point, °C 749–750 841–846 970

Individual Filaments 105 Matrix polymer R R R Si Si Si 99999 0 999 Si SiSiSiSiSi SiSi Si Glass FIGURE 3.4 Idealized coupling of matrix and glass by organofunctional silane. The use of a coupling agent can have significant effect on the mechani- cal properties of a composite.Changes of over 100%of the composites tensile,flexural or compressive strength with different choices of cou- pling agents are not uncommon for dry specimens.Because glass fibers are somewhat sensitive to moisture,the proper bonding of the glass with the matrix can also improve the mechanical properties in adverse envi- ronments.Therefore,both the application of the part and the matrix to be used should be known before specifying the fiberglass and the finish. When glass fibers are used in pultrusion or filament winding,the strands must have high integrity,thorough wetting by the resin,and uni- form processability under constant applied strain. Matrix polymer ( 平然菜然世然# Polymer network Interphase 芯 Glass FIGURE 3.5 Silane and matrix interphase polymer network

The use of a coupling agent can have significant effect on the mechani￾cal properties of a composite. Changes of over 100% of the composites tensile, flexural or compressive strength with different choices of cou￾pling agents are not uncommon for dry specimens. Because glass fibers are somewhat sensitive to moisture, the proper bonding of the glass with the matrix can also improve the mechanical properties in adverse envi￾ronments. Therefore, both the application of the part and the matrix to be used should be known before specifying the fiberglass and the finish. When glass fibers are used in pultrusion or filament winding, the strands must have high integrity, thorough wetting by the resin, and uni￾form processability under constant applied strain. Individual Filaments 105 FIGURE 3.4 Idealized coupling of matrix and glass by organofunctional silane. FIGURE 3.5 Silane and matrix interphase polymer network

106 REINFORCEMENTS-FIBERS 2.2.Carbon/Graphite Fibers [1] Carbon/graphite fibers are used extensively in making composites for aerospace applications.Carbon and graphite are both based on layered structures of hexagonal rings of carbon.Structures of this type are called grapheme and are related to true graphite,although some differences exist in the structure. While the terms are often used interchangeably,carbon and graphite fibers are different,at least in theory.Graphite fibers are those carbon fi- bers that have been subjected to heat treatment in excess of 3000F (1650C),possess 3-D ordering of the atoms,have carbon content in ex- cess of 99%(although the graphite structure is still less than 75%)and have tensile modulus on the order or 344 GPa(50 Msi). 2.2.1.Fabrication Process for Carbon/Graphite Fibers Carbon fibers are made using a raw material called the precursor.The- oretically there are three types of precursors.These are polyacrylonitrile (PAN),pitch and cellulose.However the PAN based and Pitch based pre- cursors are more common and are discussed below. 2.2.1.1.PAN Based Precursor [1] The principle of making carbon fiber made from the PAN precursor is outlined in Figure 3.6.In this process,one begins with the polyacrylonitrile (PAN)molecules.PAN molecules are thermoplastic polymers made by addition polymerization(first row,left,in Figure 3.6). With the application of heat,the triple bond between the carbon and the nitrogen atoms is broken.This is replaced by the double bond between the carbon and nitrogen,and a single bond between the nitrogen atom and another carbon atom,forming a ring structure (first row,right).Upon heating to between 400C-600C,the process of dehydrogenation(re- moval of hydrogen atoms)takes place in which many of the hydrogen at- oms are removed.The structure becomes two dimensional with many hexagons formed(second row in Figure 3.6).Upon further heating to be- tween 600C-1300C,the process of denitrogenation takes place, wherein the nitrogen atoms are removed,leaving a structure consisting mainly of carbon atoms (third and fourth row in Figure 3.6).Sheets of many hexagons are stacked against each other. A microscopic cross section of a carbon fiber is shown in Figure 3.7. This shows about one-quarter of a carbon fiber,where the vertical direc- tion coincides with the axis of the fiber.Note that the fiber consists of

2.2. Carbon/Graphite Fibers [1] Carbon/graphite fibers are used extensively in making composites for aerospace applications. Carbon and graphite are both based on layered structures of hexagonal rings of carbon. Structures of this type are called grapheme and are related to true graphite, although some differences exist in the structure. While the terms are often used interchangeably, carbon and graphite fibers are different, at least in theory. Graphite fibers are those carbon fi￾bers that have been subjected to heat treatment in excess of 3000°F (1650°C), possess 3-D ordering of the atoms, have carbon content in ex￾cess of 99% (although the graphite structure is still less than 75%) and have tensile modulus on the order or 344 GPa (50 Msi). 2.2.1. Fabrication Process for Carbon/Graphite Fibers Carbon fibers are made using a raw material called the precursor. The￾oretically there are three types of precursors. These are polyacrylonitrile (PAN), pitch and cellulose. However the PAN based and Pitch based pre￾cursors are more common and are discussed below. 2.2.1.1. PAN Based Precursor [1] The principle of making carbon fiber made from the PAN precursor is outlined in Figure 3.6. In this process, one begins with the polyacrylonitrile (PAN) molecules. PAN molecules are thermoplastic polymers made by addition polymerization (first row, left, in Figure 3.6). With the application of heat, the triple bond between the carbon and the nitrogen atoms is broken. This is replaced by the double bond between the carbon and nitrogen, and a single bond between the nitrogen atom and another carbon atom, forming a ring structure (first row, right). Upon heating to between 400°C–600°C, the process of dehydrogenation (re￾moval of hydrogen atoms) takes place in which many of the hydrogen at￾oms are removed. The structure becomes two dimensional with many hexagons formed (second row in Figure 3.6). Upon further heating to be￾tween 600°C–1300°C, the process of denitrogenation takes place, wherein the nitrogen atoms are removed, leaving a structure consisting mainly of carbon atoms (third and fourth row in Figure 3.6). Sheets of many hexagons are stacked against each other. A microscopic cross section of a carbon fiber is shown in Figure 3.7. This shows about one-quarter of a carbon fiber, where the vertical direc￾tion coincides with the axis of the fiber. Note that the fiber consists of 106 REINFORCEMENTS—FIBERS

Individual Filaments 107 many sheets containing the hexagonal arrangement.The considerable strength and stiffness within the plane of this sheet are based on the strength and stiffness of the carbon-carbon bond,which is the same as the bond in diamond,except that diamond has bonds in three directions and the fiber only in two.The strength transverse to the sheet (normal to the axis of the fiber)is low.This is similar to onion layers,where one can peel them from each other rather easily.Carbon (or graphite)fibers,in which the strength and stiffness along the third direction is much weaker than CH, CH, HEAT In the above.polyacrolynitrile molecule,which is a thermoplastic,is subjected to heat. This breaks the triple bond between C and N atoms and forms double bond between C and N atoms.In addition,rings are formed. H N 738990F400600C1 DEHYROGENATION In the above,application of higher temperature removes the hydrogen atoms (dehydrogenation process) 738-990℉(00-600C1 DEHYROGENATION 9082350F600-1300℃ DENITRDGENATION In the above,increasing the temperature to more than 600C removes the nitrogen (denitrogenation)and links between hexagons of carbon atoms are formed. In the above,most nitrogen atoms are removed.What remains are sheets of hexagons of carbon atoms. FIGURE 3.6 Formation of the carbon fibers from PAN [1]

many sheets containing the hexagonal arrangement. The considerable strength and stiffness within the plane of this sheet are based on the strength and stiffness of the carbon-carbon bond, which is the same as the bond in diamond, except that diamond has bonds in three directions and the fiber only in two. The strength transverse to the sheet (normal to the axis of the fiber) is low. This is similar to onion layers, where one can peel them from each other rather easily. Carbon (or graphite) fibers, in which the strength and stiffness along the third direction is much weaker than Individual Filaments 107 FIGURE 3.6 Formation of the carbon fibers from PAN [1]

108 REINFORCEMENTS-FIBERS FIGURE 3.7 Configuration of a microscopic cross section of a carbon fiber(Repro- duced from S.C.Bennet,D.L.Johnson,and W.Johnson,Journal of Materials Science, 18,1983,p.3337,with kind permission from Springer). those in the in-plane directions,are therefore anisotropic,their properties depending upon direction. The different steps in the manufacturing of carbon fibers from PAN is shown in Figure 3.8.The PAN-based precursor is first stabilized by ther- mosetting (crosslinking)so that the polymers do not melt in subsequent processing steps.This thermosetting step,which requires moderate heat, must be accompanied by a stretching of the fibers(or,more appropri- ately,a holding of the fibers at constant length against their inherent shrinkage as they become stabilized). The fibers are then carbonized or,in other words,pyrolyzed,until they are essentially transformed into all-carbon fibers.It is during this stage that the high mechanical property levels are developed.The rapid evolu- tion of gases up to 1800F(982C)requires that the heatup rate be quite slow to avoided forming voids and other defects.At about 1800F (982C),PAN-based fibers are approximately 94%carbon and 6%nitro- gen,with further reductions in the nitrogen content until approximately 2300F (1260C),when the carbon content is over 99.7%. Graphitization is carried out at temperatures in excess of 3200F (1760C)to improve the tensile modulus of the fiber by improving the

those in the in-plane directions, are therefore anisotropic, their properties depending upon direction. The different steps in the manufacturing of carbon fibers from PAN is shown in Figure 3.8. The PAN-based precursor is first stabilized by ther￾mosetting (crosslinking) so that the polymers do not melt in subsequent processing steps. This thermosetting step, which requires moderate heat, must be accompanied by a stretching of the fibers (or, more appropri￾ately, a holding of the fibers at constant length against their inherent shrinkage as they become stabilized). The fibers are then carbonized or, in other words, pyrolyzed, until they are essentially transformed into all-carbon fibers. It is during this stage that the high mechanical property levels are developed. The rapid evolu￾tion of gases up to 1800°F (982°C) requires that the heatup rate be quite slow to avoided forming voids and other defects. At about 1800°F (982°C), PAN-based fibers are approximately 94% carbon and 6% nitro￾gen, with further reductions in the nitrogen content until approximately 2300°F (1260°C), when the carbon content is over 99.7%. Graphitization is carried out at temperatures in excess of 3200°F (1760°C) to improve the tensile modulus of the fiber by improving the 108 REINFORCEMENTS—FIBERS FIGURE 3.7 Configuration of a microscopic cross section of a carbon fiber (Repro￾duced from S.C. Bennet, D.L. Johnson, and W. Johnson, Journal of Materials Science, 18, 1983, p. 3337, with kind permission from Springer)