Copyrighted Materials Copyright 2009 DEStech Publications Retrieved from www.knovel.con CHAPTER 2 Matrix Materials 1. INTRODUCTION As presented in Chapter 1, composite materials consist of three main parts: fibers, matrix and interface between fibers and matrix. The func- tions and importance of the matrix were described in Chapter 1. The principles involving the matrix in the manufacturing of composites are covered in this chapter. It is not the intention of this book to cover all matrices that are available for the manufacturing of composites. There are many excellent books that cover the topics and these should be consulted for the properties of these matrix materials. One example is the Handbook of Composites by Lubin [1]. The emphasis of this book is on the principles that govern the behavior of the matrix material during the manufacturing of composites. This in- volves an understanding of the basic chemical structure of the material. how this structure evolves during the manufacturing process, and how this evolution will influence the quality of the resulting composite struc- tures. Both main categories of polymer matrix materials will be dis- cussed: thermoset matrix composites and thermoplastic matrix composites. 2. DIFFERENT TYPES OF MATRIX MATERIALS AND THEIR PROMINENCE Matrix materials are generally polymers, metals, or ceramics. In a way 45
CHAPTER 2 1. INTRODUCTION As presented in Chapter 1, composite materials consist of three main parts: fibers, matrix and interface between fibers and matrix. The functions and importance of the matrix were described in Chapter 1. The principles involving the matrix in the manufacturing of composites are covered in this chapter. It is not the intention of this book to cover all matrices that are available for the manufacturing of composites. There are many excellent books that cover the topics and these should be consulted for the properties of these matrix materials. One example is the Handbook of Composites by Lubin [1]. The emphasis of this book is on the principles that govern the behavior of the matrix material during the manufacturing of composites. This involves an understanding of the basic chemical structure of the material, how this structure evolves during the manufacturing process, and how this evolution will influence the quality of the resulting composite structures. Both main categories of polymer matrix materials will be discussed: thermoset matrix composites and thermoplastic matrix composites. 2. DIFFERENT TYPES OF MATRIX MATERIALS AND THEIR PROMINENCE Matrix materials are generally polymers, metals, or ceramics. In a way 45
46 MATRIX MATERIALS this sentence says that any material can serve as a matrix material.In real- ity,however,the majority of composites that exist on the market are made of polymer matrix composites.Among these,thermoset matrix compos- ites are more predominant than thermoplastic composites. The reason why there are more polymer matrix composites than metal matrix composites and ceramic matrix composites is due to the second requirement in the interface discussion in Section 3.3 of Chapter 1:Com- patibility between the matrix and the fibers.It was shown in Chapter 1 that the surface energy of metals is on the order of 400-2000 dyne/cm while the surface energy of polymers is on the order of 30-45 dyne/cm. The surface energy of glass fibers is about 500 dyne/cm,that of graphite is about 50 dyne/cm and about 44 dyne/cm for Kevlar fibers.Thermody- namic requirements call for the surface tension of the matrix (in liquid form)to be less than that of the fibers to ensure bonding.Since the sur- face energy of liquid metals is much larger than that of the solid fibers,it is very difficult for liquid metals to bond onto the surface of solid fibers. As such,it is difficult to make metal matrix composites,in spite of the fact that metal matrix composites can offer desirable properties such as high temperature resistance.The same explanation can be used for ce- ramic matrix composites.This does not mean that metal matrix and ce- ramic matrix composites do not exist.They do,but only in rare cases of high temperature applications. Among the polymer matrix composites,there are thermoset matrix composites and thermoplastic matrix composites.The differences be- tween thermoset and thermoplastic resins are explained below. 2.1 Thermoset and Thermoplastic Matrix Materials The similarities and differences between thermoset and thermoplastic composites can be understood if one compares the processes by which these two types of materials are made. First consider a typical thermoset polymer such as epoxy.To make this material,one first starts with the epoxy molecules.The epoxy molecules are relatively small [on the order of about 20-30 carbon-carbon(C-C) links].This is relatively short as compared to the order of a few hundreds or thousands of C-C links for thermoplastic molecules.Since the length of the thermoset molecules is short,the material consisting of them usu- ally has low viscosity and appears in the form of liquid at room tempera- ture or moderately high temperature(about 100C).Figure 2.1(a)shows a schematic of the molecules in a thermoset resin.Since the material ap- pears in liquid form,in order to make a solid out of it,the molecules must be tied together with molecules of some other type.The tying molecules
this sentence says that any material can serve as a matrix material. In reality, however, the majority of composites that exist on the market are made of polymer matrix composites. Among these, thermoset matrix composites are more predominant than thermoplastic composites. The reason why there are more polymer matrix composites than metal matrix composites and ceramic matrix composites is due to the second requirement in the interface discussion in Section 3.3 of Chapter 1: Compatibility between the matrix and the fibers. It was shown in Chapter 1 that the surface energy of metals is on the order of 400–2000 dyne/cm while the surface energy of polymers is on the order of 30–45 dyne/cm. The surface energy of glass fibers is about 500 dyne/cm, that of graphite is about 50 dyne/cm and about 44 dyne/cm for Kevlar fibers. Thermodynamic requirements call for the surface tension of the matrix (in liquid form) to be less than that of the fibers to ensure bonding. Since the surface energy of liquid metals is much larger than that of the solid fibers, it is very difficult for liquid metals to bond onto the surface of solid fibers. As such, it is difficult to make metal matrix composites, in spite of the fact that metal matrix composites can offer desirable properties such as high temperature resistance. The same explanation can be used for ceramic matrix composites. This does not mean that metal matrix and ceramic matrix composites do not exist. They do, but only in rare cases of high temperature applications. Among the polymer matrix composites, there are thermoset matrix composites and thermoplastic matrix composites. The differences between thermoset and thermoplastic resins are explained below. 2.1 Thermoset and Thermoplastic Matrix Materials The similarities and differences between thermoset and thermoplastic composites can be understood if one compares the processes by which these two types of materials are made. First consider a typical thermoset polymer such as epoxy. To make this material, one first starts with the epoxy molecules. The epoxy molecules are relatively small [on the order of about 20–30 carbon-carbon (C–C) links]. This is relatively short as compared to the order of a few hundreds or thousands of C–C links for thermoplastic molecules. Since the length of the thermoset molecules is short, the material consisting of them usually has low viscosity and appears in the form of liquid at room temperature or moderately high temperature (about 100°C). Figure 2.1(a) shows a schematic of the molecules in a thermoset resin. Since the material appears in liquid form, in order to make a solid out of it, the molecules must be tied together with molecules of some other type. The tying molecules 46 MATRIX MATERIALS
Different Types of Matrix Materials and Their Prominence 47 are called the linkers or curing agents.Figure 2.1(b)shows a schematic of the linker molecules. In some cases (such as polyester)the linker molecules may not react easily with the resin molecules when they come into contact.For these cases,the linker molecules can be mixed together with the resin molecule in a container for shipping purposes [Figure 2.1(c)].When the linking is desired,one needs to add into the mixture an initiator(an unstable type of molecule)which will start the reaction. In other cases(such as epoxies)the linker molecules may react easily with the resin molecules.For these cases,the linker molecules can not be mixed together with the resin molecules until the time the manufacturer is ready to incorporate the resin systems together with the fibers. When the proper conditions for linking occur(discussed later in this chapter),the tying molecules will link the resin molecules together as shown in Figure 2.1(d).This 3-D linking network is a solid and it repre- sents the solid thermoset resin.Since the ties (links)are made by chemi- (b) (d) (e) FIGURE 2.I Schematic of (a)the molecules in a thermoset resin,(b)the linking mole- cules,(c)the resin molecules and the linking molecules in a container before linking re- actions,(d)the thermoset resin network after linking reactions,and (e)a partially linked network
are called the linkers or curing agents. Figure 2.1(b) shows a schematic of the linker molecules. In some cases (such as polyester) the linker molecules may not react easily with the resin molecules when they come into contact. For these cases, the linker molecules can be mixed together with the resin molecule in a container for shipping purposes [Figure 2.1(c)]. When the linking is desired, one needs to add into the mixture an initiator (an unstable type of molecule) which will start the reaction. In other cases (such as epoxies) the linker molecules may react easily with the resin molecules. For these cases, the linker molecules can not be mixed together with the resin molecules until the time the manufacturer is ready to incorporate the resin systems together with the fibers. When the proper conditions for linking occur (discussed later in this chapter), the tying molecules will link the resin molecules together as shown in Figure 2.1(d). This 3-D linking network is a solid and it represents the solid thermoset resin. Since the ties (links) are made by chemiDifferent Types of Matrix Materials and Their Prominence 47 FIGURE 2.1 Schematic of (a) the molecules in a thermoset resin, (b) the linking molecules, (c) the resin molecules and the linking molecules in a container before linking reactions, (d) the thermoset resin network after linking reactions, and (e) a partially linked network
48 MATRIX MATERIALS FIGURE 2.2 Schematic of the molecules in a thermoplastic resin. cal bonding,once set,the shape of a component made of thermoset resin cannot be changed by heating. The linking between the linker molecules and the resin molecules takes place whenever an active end of the resin molecule is in the vicinity of an active region of the linker molecule.All links(millions and mil- lions of them)need to be complete in order to create a solid 3D network. This process takes time(several hours,sometimes several days).One can intervene in the process by allowing only a portion of the links to be formed and retarding the remaining reactions.This can be done either by lowering the temperature or adding in retarding molecules(called inhibi- tors)somewhere during the process.The result of this is a partially linked network [Figure 2.1(e)]which exhibits itself as a viscous liquid (or flexi- ble solid)which can be handled like a liquid but remains tacky for bond- ing purposes.This is the process for making preimpregnated layers (prepregs). Next consider a typical thermoplastic resin for composite applications such as polyetheretherketone(PEEK).Thermoplastic molecules can be very long.Each molecule may contain up to several hundreds or thou- sands of C-C links.Figure 2.2 shows a schematic of these very large molecules.Due to high molecular length,it is difficult for these mole- cules to move around at room or moderate temperature.In order for these molecules to be able to move relative to each other,high temperature needs to be applied.The viscosity of these resins is large even at high
cal bonding, once set, the shape of a component made of thermoset resin cannot be changed by heating. The linking between the linker molecules and the resin molecules takes place whenever an active end of the resin molecule is in the vicinity of an active region of the linker molecule. All links (millions and millions of them) need to be complete in order to create a solid 3D network. This process takes time (several hours, sometimes several days). One can intervene in the process by allowing only a portion of the links to be formed and retarding the remaining reactions. This can be done either by lowering the temperature or adding in retarding molecules (called inhibitors) somewhere during the process. The result of this is a partially linked network [Figure 2.1(e)] which exhibits itself as a viscous liquid (or flexible solid) which can be handled like a liquid but remains tacky for bonding purposes. This is the process for making preimpregnated layers (prepregs). Next consider a typical thermoplastic resin for composite applications such as polyetheretherketone (PEEK). Thermoplastic molecules can be very long. Each molecule may contain up to several hundreds or thousands of C–C links. Figure 2.2 shows a schematic of these very large molecules. Due to high molecular length, it is difficult for these molecules to move around at room or moderate temperature. In order for these molecules to be able to move relative to each other, high temperature needs to be applied. The viscosity of these resins is large even at high 48 MATRIX MATERIALS FIGURE 2.2 Schematic of the molecules in a thermoplastic resin
Different Types of Matrix Materials and Their Prominence 49 TABLE 2.1 Viscosity(in centipoise)of a Few Thermoset and Thermoplastic Materials(1 Pa-sec 10 Poise 1000 centipoise). Material 20℃ 25C TOC Air 0.0187 Water 1 Polyester 100-300 Vinyl ester 100-300 #10 Motor oil 500 Golden syrup 2.500 Epoxy (Shell Epon 828-14 600 phrMPDA,15 phr BGE) Epoxy (Shell 826 16 phr MPDA, 750 10phr BGE) Epoxy (Dow 332-16 phr MPDA,10 500 phr BGE) Molasses 105 Epoxy 5208 100@177C BMI 1000@150C Ryton(thermoplastic) 107@313C PEEK(thermoplastic) 105@400°C Utem(thermoplastic) 108@305C Torlon(thermoplastic) 109@350°C temperature (Table 2.1).However when the material is cooled down,it becomes solid fairly quickly.The processing time therefore can be much shorter (on the order of minutes)as compared to thermoset resins(on the order of several hours or days)where time needs to be allowed for all the linking reactions to complete. There are more thermoset matrix composites than thermoplastic com- posites.The explanation can be referred to the first condition discussed in the interface section (3.3)in Chapter 1,i.e.,availability of the resin at the surface of the fibers.In order for the matrix to bond to the surface of the fibers,the resin has to be available at the surface of the fibers.This seems to be an obvious requirement but it has strong implications.For the resin to be available at the surface of the fibers,the manufacturer has to put it there.In the case of prepregs,the resin is already placed on the surface of the fibers and so this does not seem to be critical during the fab- rication of the part(it is critical to assure the availability of resin at the surface of the fiber during the fabrication of the prepregs).For a process such as resin transfer molding,however,resin needs to be pumped so that it can flow to the surface of the fibers.The flow of resin depends on the
temperature (Table 2.1). However when the material is cooled down, it becomes solid fairly quickly. The processing time therefore can be much shorter (on the order of minutes) as compared to thermoset resins (on the order of several hours or days) where time needs to be allowed for all the linking reactions to complete. There are more thermoset matrix composites than thermoplastic composites. The explanation can be referred to the first condition discussed in the interface section (3.3) in Chapter 1, i.e., availability of the resin at the surface of the fibers. In order for the matrix to bond to the surface of the fibers, the resin has to be available at the surface of the fibers. This seems to be an obvious requirement but it has strong implications. For the resin to be available at the surface of the fibers, the manufacturer has to put it there. In the case of prepregs, the resin is already placed on the surface of the fibers and so this does not seem to be critical during the fabrication of the part (it is critical to assure the availability of resin at the surface of the fiber during the fabrication of the prepregs). For a process such as resin transfer molding, however, resin needs to be pumped so that it can flow to the surface of the fibers. The flow of resin depends on the Different Types of Matrix Materials and Their Prominence 49 TABLE 2.1 Viscosity (in centipoise) of a Few Thermoset and Thermoplastic Materials (1 Pa-sec = 10 Poise = 1000 centipoise). Material 20°C 25°C T°C Air 0.0187 Water 1 Polyester 100–300 Vinyl ester 100–300 #10 Motor oil 500 Golden syrup 2,500 Epoxy (Shell Epon 828-14 phrMPDA, 15 phr BGE) 600 Epoxy (Shell 826 16 phr MPDA, 10phr BGE) 750 Epoxy (Dow 332-16 phr MPDA, 10 phr BGE) 500 Molasses 105 Epoxy 5208 100 @ 177°C BMI 1000 @ 150°C Ryton (thermoplastic) 107 @ 313°C PEEK (thermoplastic) 106 @ 400°C Utem (thermoplastic) 108 @ 305°C Torlon (thermoplastic) 109 @ 350°C