4 Polymeric Matrix Materials 4.1 Introduction The matrix,as discussed in the Chapter 1,serves the following functions:1) transfers load into and out from the fibers,2)separates the fibers to prevent failure of adjacent fibers when one fails,3)protects the fiber from environmental damage,supports the fibers in the shape of the component. The mechanical properties of the composite that are significantly affected by the properties of the polymeric matrix (and fiber/matrix bond strength)include 1)longitudinal compression strength,2)transverse tensile strength,and 3)interlaminar shear strength.These are generally called matrix-dominated properties. To be suitable as matrices,polymers must also have resistance to aircraft solvents such as fuel,hydraulic fluid,and paint stripper and to service temperatures typically up to around 80C for civil and 150C for military applications;however,a capability to over 200C may be required in some applications. In the production of advanced composites suitable for aerospace applications, it is important that the method of matrix incorporation does not damage the reinforcement fibers or inadvertently change their orientation.One suitable method is to infiltrate an aligned fiber bed with a low-viscosity liquid that is then converted,by chemical reaction or simply by cooling,to form a continuous solid matrix with the desired properties. 4.1.1 Background on Polymeric Materials Polymers consist of very long chain molecules,generally with a backbone consisting of covalently bonded carbon atoms.In the simplest type of polymer, each carbon atom is joined to two others to form a linear polymer.The other two available bonds not used in the chain are linked to side groups (Fig.4.1).If, however,the carbon atoms link with carbon atoms that are not in simple groups, then a three-dimensional network or cross-linked polymer results. Figure 4.2 is a highly simplified schematiccomparing the polymer backbones of linear and cross-linked polymer chain configurations.There are,however, several intermediate forms,including branched and ladder polymers,shown in 81
4 Polymeric Matrix Materials 4.1 Introduction The matrix, as discussed in the Chapter 1, serves the following functions: 1) transfers load into and out from the fibers, 2) separates the fibers to prevent failure of adjacent fibers when one fails, 3) protects the fiber from environmental damage, supports the fibers in the shape of the component. The mechanical properties of the composite that are significantly affected by the properties of the polymeric matrix (and fiber/matrix bond strength) include 1) longitudinal compression strength, 2) transverse tensile strength, and 3) interlaminar shear strength. These are generally called matrix-dominated properties. To be suitable as matrices, polymers must also have resistance to aircraft solvents such as fuel, hydraulic fluid, and paint stripper and to service temperatures typically up to around 80°C for civil and 150°C for military applications; however, a capability to over 200°C may be required in some applications. In the production of advanced composites suitable for aerospace applications, it is important that the method of matrix incorporation does not damage the reinforcement fibers or inadvertently change their orientation. One suitable method is to infiltrate an aligned fiber bed with a low-viscosity liquid that is then converted, by chemical reaction or simply by cooling, to form a continuous solid matrix with the desired properties. 4.1.1 Background on Polymeric Materials Polymers consist of very long chain molecules, 1 generally with a backbone consisting of covalently bonded carbon atoms. In the simplest type of polymer, each carbon atom is joined to two others to form a linear polymer. The other two available bonds not used in the chain are linked to side groups (Fig. 4.1). If, however, the carbon atoms link with carbon atoms that are not in simple groups, then a three-dimensional network or cross-linked polymer results. Figure 4.2 is a highly simplified schematic 2 comparing the polymer backbones of linear and cross-linked polymer chain configurations. There are, however, several intermediate forms, including branched and ladder polymers, shown in 81
82 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES CH H C一C一C CH3 H CH H Fig.4.1 Example of the polymer chain arrangement of a simple linear polymer. Figure 4.2.Polymer branching can occur with linear or cross-linked polymers; ladder polymers are made of two linear polymers joined by regular linkages.As may be expected,ladder polymers are more rigid than simple linear polymers. Linear molecules can be characterized in terms of the molecular weight,which is an indication of the average length of the molecular chain;however,this has no meaning for cross-linked polymers because these do not form as discrete molecules.The formula for a linear polymer is(M),where M is the repeating unit and n the degree of polymerization.For example,in a sample of the polymer with an average n of 5000,the range will typically be in the range 1000-10,000. If there is only one type of repeating unit,the polymer is called a homopolymer.When,however,there are two types of repeating unit based on two types of monomer,the resulting polymer is called a copolymer.In a block copolymer,each repeating unit has a regular distribution of long sequences.If, however,the sequence is random,the resulting polymer is called a regular or random copolymer.If there is a chain of one type of polymer with branches of another type,the resulting polymer is called a graft copolymer. c d) Fig.4.2 Schematic illustration of the molecular configuration of a)linear, b)branched,c)cross-linked,and d)ladder polymers
82 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Fig. 4.1 3 H 3 H I I C I I I I CH 3 H CH 3 H Example of the polymer chain arrangement of a simple linear polymer. Figure 4.2. Polymer branching can occur with linear or cross-linked polymers; ladder polymers are made of two linear polymers joined by regular linkages. As may be expected, ladder polymers are more rigid than simple linear polymers. Linear molecules can be characterized in terms of the molecular weight, which is an indication of the average length of the molecular chain; however, this has no meaning for cross-linked polymers because these do not form as discrete molecules. The formula for a linear polymer is (M)n where M is the repeating unit and n the degree of polymerization. For example, in a sample of the polymer with an average n of 5000, the range will typically be in the range 1000-10,000. If there is only one type of repeating unit, the polymer is called a homopolymer. When, however, there are two types of repeating unit based on two types of monomer, the resulting polymer is called a copolymer. In a block copolymer, each repeating unit has a regular distribution of long sequences. If, however, the sequence is random, the resulting polymer is called a regular or random copolymer. If there is a chain of one type of polymer with branches of another type, the resulting polymer is called a graft copolymer. a) b) c) Fig. 4.2 Schematic illustration of the molecular b) branched, c) cross-linked, and d) ladder polymers. d) configuration of a) linear
POLYMERIC MATRIX MATERIALS 83 The process of polymerization generally involves the linking of carbon atoms in short-chain organic compounds called monomers,through reaction or by catalysis.The two principal types of polymerization are called condensation polymerization and addition polymerization. In condensation polymerization,the monomers link together (or cure) producing water or other small molecules as a by-product,while in addi- tion polymerization,the monomers link without producing any by-product. The production of small volatile molecules during matrix formation is highly undesirable,as it can lead to extensive voiding.However,voiding can be minimized in these systems if polymerization occurs under high-pressure conditions. Polymers with a three-dimensional network structure are called thermosetting polymers,and the process of network formation is called curing.The precursor materials used to form the network are monomer or oligomer (several monomers joined together)mixtures called resins.These,depending on formulation and temperature,can range in viscosity from free-flowing liquids,similar to a light oil (100,000 centipoise).Curing is brought about by the reaction of the resin with a curing agent(which may be another resin)or a catalyst,often at elevated temperature.Thermosets,once cured,become solids that cannot be melted and reformed.Thermoplastics are higher molecular weight linear polymers that undergo no permanent chemical change on heating (below the decomposition temperature).They flow upon heating so that they can be reformed. Finally,a common chemical theme among polymeric materials used for aerospace matrices is that they contain rigid rings (aromatic rings)in their structure.This provides the required chemical resistance and mechanical properties at elevated temperature. 4.1.2 Structure and Mechanical Properties Thermosetting and thermoplastic polymers differ in many respects.One important difference is that some degree of crystallinity is possible with thermoplastics,whereas thermosets are amorphous.The degree of crystallinity in thermoplastic polymers depends on many parameters,particularly those that allow or inhibit easy alignment of the polymer chains,for example the size and regularity of the side groups.Depending on the temperature,the molecular chains are in a constant state of motion relative to one another.At modest temperatures,depen- ding on the polymer chain,islands of crystallinity exist in an amorphous matrix. The crystalline regions consist of regions of aligned chains,generally produced by folding of a single chain(Fig.4.3).In some cases,order is further increased by groups of crystals forming ordered regions known as spherulites because of their spherical geometry.The important point is that the density of the polymer is much higher in the crystalline regions,but its random molecular
POLYMERIC MATRIX MATERIALS 83 The process of polymerization generally involves the linking of carbon atoms in short-chain organic compounds called monomers, through reaction or by catalysis. The two principal types of polymerization are called condensation polymerization and addition polymerization. In condensation polymerization, the monomers link together (or cure) producing water or other small molecules as a by-product, while in addition polymerization, the monomers link without producing any by-product. The production of small volatile molecules during matrix formation is highly undesirable, as it can lead to extensive voiding. However, voiding can be minimized in these systems if polymerization occurs under high-pressure conditions. Polymers with a three-dimensional network structure are called thermosetting polymers, and the process of network formation is called curing. The precursor materials used to form the network are monomer or oligomer (several monomers joined together) mixtures called resins. These, depending on formulation and temperature, can range in viscosity from free-flowing liquids, similar to a light oil ( 100,000 centipoise). Curing is brought about by the reaction of the resin with a curing agent (which may be another resin) or a catalyst, often at elevated temperature. Thermosets, once cured, become solids that cannot be melted and reformed. Thermoplastics are higher molecular weight linear polymers that undergo no permanent chemical change on heating (below the decomposition temperature). They flow upon heating so that they can be reformed. Finally, a common chemical theme among polymeric materials used for aerospace matrices is that they contain rigid rings (aromatic rings) in their structure. This provides the required chemical resistance and mechanical properties at elevated temperature. 4.1.2 Structure and Mechanical Properties Thermosetting and thermoplastic polymers differ in many respects. One important difference is that some degree of crystallinity is possible with thermoplastics, whereas thermosets are amorphous. The degree of crystallinity in thermoplastic polymers depends on many parameters, particularly those that allow or inhibit easy alignment of the polymer chains, for example the size and regularity of the side groups. Depending on the temperature, the molecular chains are in a constant state of motion relative to one another. At modest temperatures, depending on the polymer chain, islands of crystallinity exist in an amorphous matrix. The crystalline regions consist of regions of aligned chains, generally produced by folding of a single chain (Fig. 4.3). In some cases, order is further increased by groups of crystals forming ordered regions known as spherulites because of their spherical geometry. The important point is that the density of the polymer is much higher in the crystalline regions, but its random molecular
84 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Fig.4.3 Schematic representation of a linear polymer showing amorphous and local areas of crystallization where the polymer chains are aligned. motion is reduced,resulting in increases in mechanical properties such as stiffness and chemical properties such as solvent resistance.This degree of order is changed at elevated temperature as the crystalline order is reduced,resulting in a marked but gradual change in,for example,stiffness.Unlike simple solids where a definite transition from a crystal to a liquid occurs at a specific temperature Tm,in polymers T is a range and melting results in an amorphous semi-solid material (Fig.4.4).In contrast,since there are no regions of crystallinity,thermosets and amorphous thermoplastics show no sharp melting point,but a gradual reduction in stiffness over a range of temperatures. At low temperatures,thermoplastics form a solid that may be partially crystalline and partially amorphous.The degree of crystallinity depends on the polymer structure and the cooling rate.With rapid cooling or with polymers having bulky side chains,the structure could be largely amorphous. Below a certain temperature called the glass-transition temperature Ta,the random molecular motion drops to a very low level,which is particularly marked in the amorphous regions.The chains thus become set in their random patterns, and the material becomes rigid and glass-like.Above Ts,polymers exhibit a low stiffness and rubbery behavior.This behavior above and below Te also occurs for similar reasons in thermosets,but is not so marked as the movement of the polymer chains is restricted by the cross-links. Figure 4.5 schematically illustrates this behavior for 1)a crystalline thermoplastic,2)an amorphous thermoplastic,3)a thermoset,and 4)a rubber. Because the crystalline regions inhibit slippage of the polymer chains even above T,the drop in stiffness is dramatic when crystalline melting occurs
84 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Fig. 4.3 Schematic representation of a linear polymer showing amorphous and local areas of crystallization where the polymer chains are aligned. motion is reduced, resulting in increases in mechanical properties such as stiffness and chemical properties such as solvent resistance. This degree of order is changed at elevated temperature as the crystalline order is reduced, resulting in a marked but gradual change in, for example, stiffness. Unlike simple solids where a definite transition from a crystal to a liquid occurs at a specific temperature Tm, in polymers Tm is a range and melting results in an amorphous semi-solid material (Fig. 4.4). In contrast, since there are no regions of crystallinity, thermosets and amorphous thermoplastics show no sharp melting point, but a gradual reduction in stiffness over a range of temperatures. At low temperatures, thermoplastics form a solid that may be partially crystalline and partially amorphous. The degree of crystallinity depends on the polymer structure and the cooling rate. With rapid cooling or with polymers having bulky side chains, the structure could be largely amorphous. Below a certain temperature called the glass-transition temperature Tg, the random molecular motion drops to a very low level, which is particularly marked in the amorphous regions. The chains thus become set in their random patterns, and the material becomes rigid and glass-like. Above Tg, polymers exhibit a low stiffness and rubbery behavior. This behavior above and below Tg also occurs for similar reasons in thermosets, but is not so marked as the movement of the polymer chains is restricted by the cross-links. Figure 4.5 schematically illustrates this behavior for 1) a crystalline thermoplastic, 2) an amorphous thermoplastic, 3) a thermoset, and 4) a rubber. Because the crystalline regions inhibit slippage of the polymer chains even above Tg, the drop in stiffness is dramatic when crystalline melting occurs
POLYMERIC MATRIX MATERIALS 85 Liquid Specific Crystalline LOW MOLECULAR volume solid WEIGHT MATERIAL m +-Temperature Amorphous Specific volume POLYMERIC Crystalline MATERIAL Tm Temperature Fig.4.4 Schematic plot of the variation of specific volume with temperature for a low molecular weight material,such as a metal,and for a linear polymeric material. The temperature determined for Te is generally a function of the method of measurement.Most practical determinations of Te involve stressing the sample and determining the effect of temperature.The speed of application of the stress and the rate of change of temperature has a pronounced effect on the observed temperature of the glass transition.Modern dynamic mechanical (thermal) analysis equipment is able to simultaneously carry out such stress testing at a Thermosetting plastic Young's modulus Crystalline thermoplastic Rubber Amorphous thermoplastic Temperature Fig.4.5 Schematic plot of variation of Young's modulus with temperature for various types of polymer
POLYMERIC MATRIX MATERIALS 85 Specific volume Specific volume J _ Tm h Temperature Amorphou~..-..- .~ /i I ~Tm I , I .... '. .... --- Temperature Fig. 4.4 Schematic plot of the variation of specific volume with temperature for a low molecular weight material, such as a metal, and for a linear polymeric material. The temperature determined for Tg is generally a function of the method of measurement. Most practical determinations of Tg involve stressing the sample and determining the effect of temperature. The speed of application of the stress and the rate of change of temperature has a pronounced effect on the observed temperature of the glass transition. Modem dynamic mechanical (thermal) analysis equipment is able to simultaneously carry out such stress testing at a Young's modulus ~ermosetting plastic Cryst2/l~/estic . Amorphous thermoplastic ~ Temperature Fig. 4.5 Schematic plot of variation of Young's modulus with temperature for various types of polymer
86 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES range of frequencies.The mechanical failure mode of particular composite items made with these resins could therefore be different at different temperatures as well as under different testing regimens.Many rubbery materials will fail in a "glassy manner"if tested at a high enough speed. 4.2 Thermoset and Thermoplastic Polymer Matrix Materials As discussed earlier,polymers fall into the two major categories: thermosetting and thermoplastic.Thermosets have the great advantage that they allow fabrication of composites at relatively low temperatures and pressures because they go through a low-viscosity stage (sometimes very low)before polymerization and cross-linking. Based on Ref.3,the relative properties of thermosets and thermoplastics,and their advantages and disadvantages,are summarized in Table 4.1. Table 4.1 Thermoset Matrices and Thermoplastic Matrices Thermoset Thermoplastic Main Characteristics Undergoes chemical change when cured Non-reacting,no cure required Low strain to failure High strain to failure ·Low fracture energy High fracture energy Processing is irreversible Very high viscosity ● Very low viscosity possible Processing is reversible ● Absorbs moisture ● Absorbs little moisture Highly resistant to solvents Limited resistance to organic solvents,in some cases Advantages Relatively low processing temperature Short processing times possible Good fiber wetting Reusable scrap Formable into complex shapes Post-formable can be reprocessed Liquid-resin manufacturing feasible Rapid processing Resistant to creep Unlimited shelf life without refrigeration High delamination resistance Disadvantages Long processing time Lower resistance to solvents Long (~1-2 h)cure Requires high temperature (300- Restricted storage life 400C)and pressure processing (requires refrigeration) Can be prone to creep Very poor drapability and tack
86 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES range of frequencies. The mechanical failure mode of particular composite items made with these resins could therefore be different at different temperatures as well as under different testing regimens. Many rubbery materials will fail in a "glassy manner" if tested at a high enough speed. 4.2 Thermoset and Thermoplastic Polymer Matrix Materials As discussed earlier, polymers fall into the two major categories: thermosetting and thermoplastic. Thermosets have the great advantage that they allow fabrication of composites at relatively low temperatures and pressures because they go through a low-viscosity stage (sometimes very low) before polymerization and cross-linking. Based on Ref. 3, the relative properties of thermosets and thermoplastics, and their advantages and disadvantages, are summarized in Table 4.1. Table 4.1 Thermoset Matrices and Thermoplastic Matrices Thermoset Thermoplastic Main Characteristics • Undergoes chemical change when cured • Non-reacting, no cure required • Low strain to failure • High strain to failure • Low fracture energy • High fracture energy • Processing is irreversible • Very high viscosity • Very low viscosity possible • Processing is reversible • Absorbs moisture • Absorbs little moisture • Highly resistant to solvents • Limited resistance to organic solvents, in some cases Advantages • Relatively low processing temperature • • Good fiber wetting • • Formable into complex shapes • • Liquid-resin manufacturing feasible • • Resistant to creep * • Long processing time • Long (~ 1-2 h) cure • Restricted storage life (requires refrigeration) Disadvantages Short processing times possible Reusable scrap Post-formable can be reprocessed Rapid processing Unlimited shelf life without refrigeration High delamination resistance Lower resistance to solvents Requires high temperature (300- 400 ° C) and pressure processing Can be prone to creep Very poor drapability and tack
POLYMERIC MATRIX MATERIALS 87 Of all thermosetting resins,epoxy resins are the most widely used in aircraft structures.Epoxies have excellent chemical and mechanical properties,have low shrinkage,and adhere adequately to most types of fiber.Importantly,they go through a low-viscosity stage during cure and so allow for the use of liquid resin- forming techniques such as resin-transfer molding(RTM).In general,the glass transition temperature Te of epoxy resins increases with increasing temperature of cure.Thereby,epoxy systems cured at 120C and 180C have upper(dry) service temperatures of 100-130C and 150C,respectively (note a significant margin on Ts is mandated in designs;see Chapter 12).It is important to note that Te is reduced significantly by absorbed moisture,as discussed later. Bismaleimide resins (BMIs)have similar excellent processibility and comparable mechanical properties to epoxies.Importantly,they can operate at higher temperatures;however,as with epoxies,Te is markedly reduced by absorbed moisture.Generally,BMI resins cured at around 200C have upper service temperatures above 180C.The higher cost of BMI resins limits their use to applications where the operating temperatures exceed the capability of epoxies. If even higher operating temperatures are required,composites based on polyimide resin matrices may be the only option.These high-temperature thermosetting resins typically cure around 270C and allow operating tem- peratures of up to 300C.However,there are penalties of a higher cost than BMIs and much more difficult processing. Thermosetting resins have relatively low fracture strains and fracture toughness as inelastic deformation is limited by the highly cross-linked structure. This translates into poor fracture resistance in the composite.These systems also absorb atmospheric moisture,in some cases over 3%,resulting in reduced matrix- dominated properties in the composite,such as elevated temperature shear and compressive strength. Thermoplastics suitable for use as matrices for high-performance composites include polymers such as polyetheretherketone(PEEK),for applications up to approximately 120C;polyetherketone (PEK)for up to 145C;and polyimide (thermoplastic type)for up to 270C.Fabrication of thermoplastic composites involves melting and forming steps.Because these materials are already fully polymerized,their viscosity,even when melted,is generally much higher than that of most thermosetting resins.They are thus not well suited to conventional liquid resin techniques such as RTM.Fabrication techniques based on resin- film infusion (RFI)or pre-preging (pre-coating the fibers by dissolving the polymer in an appropriate solvent)and then hot-pressing are more appropriate (See Chapter 5). An advantage of thermoplastic composites is their higher retained hot/wet properties as they absorb less moisture (typically around 0.2%)than thermosetting resin composites.These polymers also have a much higher strain to failure because they can undergo plastic deformation,resulting in significantly improved impact resistance
POLYMERIC MATRIX MATERIALS 87 Of all thermosetting resins, epoxy resins are the most widely used in aircraft structures. Epoxies have excellent chemical and mechanical properties, have low shrinkage, and adhere adequately to most types of fiber. Importantly, they go through a low-viscosity stage during cure and so allow for the use of liquid resinforming techniques such as resin-transfer molding (RTM). In general, the glass transition temperature Tg of epoxy resins increases with increasing temperature of cure. Thereby, epoxy systems cured at 120°C and 180°C have upper (dry) service temperatures of 100-130 °C and 150 °C, respectively (note a significant margin on Tg is mandated in designs; see Chapter 12). It is important to note that Tg is reduced significantly by absorbed moisture, as discussed later. Bismaleimide resins (BMIs) have similar excellent processibility and comparable mechanical properties to epoxies. Importantly, they can operate at higher temperatures; however, as with epoxies, Tg is markedly reduced by absorbed moisture. Generally, BMI resins cured at around 200°C have upper service temperatures above 180 °C. The higher cost of BMI resins limits their use to applications where the operating temperatures exceed the capability of epoxies. If even higher operating temperatures are required, composites based on polyimide resin matrices may be the only option. These high-temperature thermosetting resins typically cure around 270°C and allow operating temperatures of up to 300°C. However, there are penalties of a higher cost than BMIs and much more difficult processing. Thermosetting resins have relatively low fracture strains and fracture toughness as inelastic deformation is limited by the highly cross-linked structure. This translates into poor fracture resistance in the composite. These systems also absorb atmospheric moisture, in some cases over 3%, resulting in reduced matrixdominated properties in the composite, such as elevated temperature shear and compressive strength. Thermoplastics suitable for use as matrices for high-performance composites include polymers such as polyetheretherketone (PEEK), for applications up to approximately 120°C; polyetherketone (PEK) for up to 145°C; and polyimide (thermoplastic type) for up to 270°C. Fabrication of thermoplastic composites involves melting and forming steps. 4 Because these materials are already fully polymerized, their viscosity, even when melted, is generally much higher than that of most thermosetting resins. They are thus not well suited to conventional liquid resin techniques such as RTM. Fabrication techniques based on resinfilm infusion (RFI) or pre-preging (pre-coating the fibers by dissolving the polymer in an appropriate solvent) and then hot-pressing are more appropriate (See Chapter 5). An advantage of thermoplastic composites is their higher retained hot/wet properties as they absorb less moisture (typically around 0.2%) than thermosetting resin composites. These polymers also have a much higher strain to failure because they can undergo plastic deformation, resulting in significantly improved impact resistance
88 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Aerospace-grade thermoplastics are generally more expensive than thermo- sets and are more costly to fabricate,due to the high temperatures and pressures involved.In addition,with continual research and development improvements in thermosets,even the toughness advantage of thermoplastic composites is being eroded.There is little doubt that thermoplastics will be used extensively in the future-especially in niche areas in which high resistance to impact or edge damage (for example,in the case of doors)justifies the higher cost. The comparative physical properties of the standard matrix resins are discussed in Ref.5 and 6,and Ref.7 provides a good overview of the chemistry of the various systems. The following sections provide more details on the various polymer matrix materials. 4.3 Thermosetting Resin Systems Table 4.2 and 4.3 list some of the relevant attributes of some of the thermosetting thermoplastic matrices,and Table 4.4 provides details on some of the important properties,including fracture energy. 4.3.1 Epoxy Resins Epoxy resinss are a class of compounds containing two or more epoxide groups per molecule.Figure 4.6 depicts the structures of the major epoxy systems used in aerospace composite matrices.The epoxide is the three-membered ring formed by the oxygen and the two carbons.It is also called an oxirane ring,or the glycidyl group. Epoxies are formed by reacting polyphenols or other active hydrogen compounds with epichlorohydrin under basic conditions.The most common phenol used is bisphenol A(Diphenylolpropane).It provides the basis of a whole family of aerospace epoxy resins having the general structure shown in Figure 4.6a.These are usually complex mixtures of molecules with various values of n.The lower the value of n or the more complex the mixture,the lower the resin viscosity but the more brittle the final cured resin.Trade names of these resins include such materials as Epikote or Epon 828,Dow DER 331,and Araldite F. There are many other epoxy resins manufactured for special purposes,but of particular importance in advanced aerospace materials are the tetraglycidyl derivative of diaminodiphenylmethane(TGDDM)(Fig.4.6b)and the triglycidyl derivative of p-aminophenol (TGAP)(Fig.4.6c).The high functionality of these materials makes for higher resin reactivity and greater cross-linking,which translates into higher composite stiffness and glass transition temperatures. In contrast,use is often made of small amounts of reactive diluent epoxy resins such as the bis epoxy from butane diol (Fig.4.6d)to improve the flow
88 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Aerospace-grade thermoplastics are generally more expensive than thermosets and are more costly to fabricate, due to the high temperatures and pressures involved. In addition, with continual research and development improvements in thermosets, even the toughness advantage of thermoplastic composites is being eroded. There is little doubt that thermoplastics will be used extensively in the future--especially in niche areas in which high resistance to impact or edge damage (for example, in the case of doors) justifies the higher cost. The comparative physical properties of the standard matrix resins are discussed in Ref. 5 and 6, and Ref. 7 provides a good overview of the chemistry of the various systems. The following sections provide more details on the various polymer matrix materials. 4.3 Thermosetting Resin Systems Table 4.2 and 4.3 list some of the relevant attributes of some of the thermosetting thermoplastic matrices, and Table 4.4 provides details on some of the important properties, including fracture energy. 4.3.1 Epoxy Resins Epoxy resins 8 are a class of compounds containing two or more epoxide groups per molecule. Figure 4.6 depicts the structures of the major epoxy systems used in aerospace composite matrices. The epoxide is the three-membered ring formed by the oxygen and the two carbons. It is also called an oxirane ring, or the glycidyl group. Epoxies are formed by reacting polyphenols or other active hydrogen compounds with epichlorohydrin under basic conditions. The most common phenol used is bisphenol A (Diphenylolpropane). It provides the basis of a whole family of aerospace epoxy resins having the general structure shown in Figure 4.6a. These are usually complex mixtures of molecules with various values of n. The lower the value of n or the more complex the mixture, the lower the resin viscosity but the more brittle the final cured resin. Trade names of these resins include such materials as Epikote or Epon 828, Dow DER 331, and Araldite F. There are many other epoxy resins manufactured for special purposes, but of particular importance in advanced aerospace materials are the tetraglycidyl derivative of diaminodiphenylmethane (TGDDM) (Fig. 4.6b) and the triglycidyl derivative of p-aminophenol (TGAP) (Fig. 4.6c). The high functionality of these materials makes for higher resin reactivity and greater cross-linking, which translates into higher composite stiffness and glass transition temperatures. In contrast, use is often made of small amounts of reactive diluent epoxy resins such as the bis epoxy from butane diol (Fig. 4.6d) to improve the flow
~Cure Max (wet) Comments on Performance Matrix Examples C/KPa Capability C as Matrix Epoxy Hexcel 920 120/700 80+ Best properties all around. Epoxy Hercules 3501-6 180/700 100+ Excellent adhesion to fibers. Fiberite 934 Easy to process,wide viscosity range,good wetting, Narmco 5208 and large process window. Not prone to voiding,low volatile emission. Excellent water and other chemical resistance. Large database for aerospace application. Fairly low toughness,composites sensitive to impact damage. Limited temperature capability. Absorbs moisture,reducing elevated temperature. Mechanical properties. Sensitive to UV exposure. YMERIC Epoxy Hercules 3502 180/700 100+ All of the above,plus: Toughened Fiberite 977-2 Improved tolerance to damage. Increased moisture sensitivity. BMI Hexcel F560 180/700+ 230 Exceeds epoxy temperature capability. postcure Relatively easy to process. 200 Even less tough than epoxies Undergoes shrinkage during cure. MATRIX MATERIALS Prone to microcrack with thermal cycling. BMI Cytec 5250-4 180/700 180 All of the characteristics listed for BMI,plus: Toughened More damage resistant. Lower temperature capability than untoughned. Polyimide PMR-15 300/4100 320 Resistant to oxidation. Condensation Similar properties to epoxy-matrix composites possible but overextended temperature range. Low toughness. Prone to severe voiding. Difficult to process. 8
U X o X q~ POLYMERIC MATRIX MATERIALS 6 0 0 .~ ~e~ ~-~ .~ ~0~ -~ ~ ~ '~ =~ ~ g~='~7 .~ o~o.~ ~ ~ ~ = o = ~ + t'N 8 I ;>., X X 0 0 89 °~ M ~m ~M oo oo I 0 © ~ o 0 C
8 Table 4.3 Some Details on Selected Thermoplastic Matrix Materials Used in Aerospace Composites(continued) Process Polymer Examples Temp~C TsC Comments on Performance as a Matrix PEEK, Victrex 400 145 Excellent mechanical properties,including COMPOSITE Polyether- toughness.For application at temperatures up to etherketone 120C.Highly resistant to damage by aircraft fluid.Excellent fire resistance.Most widely used MATE for high-performance composites. PPS, Ryton 340 90 Good strength,stiffness,and temperature RIALS Polyphenylene- capability.Some grades have low viscosity, sulfide aiding composite manufacture.Resistant to most FOR PSF, Udel 400 190 aircraft fluids,but attacked by some solvents, Polysulfone including paint stripper.Good fire resistance.Low AIR PAS, Radel 400 220 impact resistance. Polyarylsulfone RAFT PES, Vitrex 400 230 Polyether- sulfone PL,Polyimide Kapton 390 320 Selected for highest-temperature applications. Highly viscous.Difficult to process. STRUCTURES PEI, Ultem 370 215 Selected for high-temperature applications. Polyetherimide Lower cost than PI
90 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES I E ~J ~J O ,,,... E [.. O