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
50 MATRIX MATERIALS permeability of the fiber networks,and also on the viscosity of the resin. At reasonably low temperatures (less than 100C),the viscosity of thermoset matrix is much lower than that of thermoplastic matrix.Table 2.1 shows the viscosity of a few thermoset and thermoplastic matrix ma- terials.Due to their low viscosity,thermoset matrix materials can flow to the surface of fibers more easily as compared to thermoplastic matrix. The difference in viscosity between thermoset matrix and thermoplastic matrix is the main reason for the predominance of thermoset matrix com- posites as compared to thermoplastic matrix composites. 3.THERMOSET MATRIX MATERIALS The presentation on thermoset matrix materials will concentrate on two materials:polyester and epoxy,with some brief presentation on other types of materials.This is because the mechanism of operation of these two materials is representative for other materials. 3.1.Polyester Resins 3.1.1.General Compared with epoxy resins,polyester resins are lower in cost but lim- ited in use due to less adaptable properties.Polyesters have been used mainly with glass fibers (normally E glass)to make many commercial products such as pipes,boats,corrosion resistant equipment,automotive components,and fiber reinforced rods for concrete reinforcement. 3.1.2.Polyester Chemical Structure and Polymer Formation Polyesters are formed by the condensation polymerization of a diacid and a dialcohol (a diacid means two organic acid groups are present in a molecule,and a dialcohol,sometimes called a diol,has two alcohol groups in the molecule).A typical reaction is shown in Figure 2.3,in which maleic acid is made to react with ethylene glycol to form polyes- ter.In this reaction,the acid group(O=C-OH)on one end of the diacid reacts with the alcohol group(CH2OH)on one end of the diol to form a bond linking the two molecules and to give out water as a byproduct.The linking group which is formed is called an ester(C-O-C=O).This stepis called a condensation reaction. The resulting product still has another acid group on one end and an- other alcohol group on the other.Both of these ends are still capable of
permeability of the fiber networks, and also on the viscosity of the resin. At reasonably low temperatures (less than 100°C), the viscosity of thermoset matrix is much lower than that of thermoplastic matrix. Table 2.1 shows the viscosity of a few thermoset and thermoplastic matrix materials. Due to their low viscosity, thermoset matrix materials can flow to the surface of fibers more easily as compared to thermoplastic matrix. The difference in viscosity between thermoset matrix and thermoplastic matrix is the main reason for the predominance of thermoset matrix composites as compared to thermoplastic matrix composites. 3. THERMOSET MATRIX MATERIALS The presentation on thermoset matrix materials will concentrate on two materials: polyester and epoxy, with some brief presentation on other types of materials. This is because the mechanism of operation of these two materials is representative for other materials. 3.1. Polyester Resins 3.1.1. General Compared with epoxy resins, polyester resins are lower in cost but limited in use due to less adaptable properties. Polyesters have been used mainly with glass fibers (normally E glass) to make many commercial products such as pipes, boats, corrosion resistant equipment, automotive components, and fiber reinforced rods for concrete reinforcement. 3.1.2. Polyester Chemical Structure and Polymer Formation Polyesters are formed by the condensation polymerization of a diacid and a dialcohol (a diacid means two organic acid groups are present in a molecule, and a dialcohol, sometimes called a diol, has two alcohol groups in the molecule). A typical reaction is shown in Figure 2.3, in which maleic acid is made to react with ethylene glycol to form polyester. In this reaction, the acid group (O=C–OH) on one end of the diacid reacts with the alcohol group (CH2OH) on one end of the diol to form a bond linking the two molecules and to give out water as a byproduct. The linking group which is formed is called an ester(C–O–C=O). This step is called a condensation reaction. The resulting product still has another acid group on one end and another alcohol group on the other. Both of these ends are still capable of 50 MATRIX MATERIALS
Thermoset Matrix Materials 51 undergoing further condensation reactions and then to repeat again and again.Therefore,with sufficient reactant materials,chains of alternating acid and alcohol groups will form and will have regularly repeated units as shown in the polymerization step of Figure 2.3.One unit of the repeat- ing units is shown within the bracket at the bottom of the figure.Sub- script n represents the number of repeated units.A large n value indicates a longer(or larger)molecule.Many polymers would have an n of several hundred,although some polymers exist with n of less than 20. REACTANTS H O-C/CHCH CO-HHO CH CH O-H Acid Group Makes Water Alcohol Group MALEIC ACID ETHYLENE GLYCOL (A Diacid) (A Dialcohol) In the above,the -0-H end of the maleic acid molecule is reaction with the -H end of the glycol molecule. FIRST CONDENSATION REACTION PRODUCTS H O-C-CH-CHC Q/CHCH O-H +H,O AN ESTER (REMOVED) ESTER LNKAGE In the above,combination of-O-H and-H forms water H2O.The remaining parts of the two types of molecules connects together to form an ester linkage. 。。CH-CH&00 4 cHo+Ho8cH=0HgOh oa-or 8o 8 on-aon In the above,similar reaction at other ends of the acid molecule and the glycol molecule can take place.The result is an ester molecule with two carboxylic ends (COOH). POLYMERIZATION O CH,CH2 O d cn-cn o c n)cn-c REPEATING UNIT When many units of ester connects together due to the reactions,polyester molecules will be formed.The part in the bracket shows one unit of the repeating units. FIGURE 2.3 Condensation polymerization of a polyester
undergoing further condensation reactions and then to repeat again and again. Therefore, with sufficient reactant materials, chains of alternating acid and alcohol groups will form and will have regularly repeated units as shown in the polymerization step of Figure 2.3. One unit of the repeating units is shown within the bracket at the bottom of the figure. Subscript n represents the number of repeated units. A large n value indicates a longer (or larger) molecule. Many polymers would have an n of several hundred, although some polymers exist with n of less than 20. Thermoset Matrix Materials 51 FIGURE 2.3 Condensation polymerization of a polyester
52 MATRIX MATERIALS Polyester Polyester Resin FIGURE 2.4 Glass container with liquid polyester. These chains are called polymers(from the word for many parts).Be- cause the linking group formed by acids and alcohols are esters,the term given to this type of resulting polymer is polyester.A quantity of these polyester polymer chains is collectively called polyester resin.(Poly- mers,in general,in the uncured state,are called resins.)For a certain amount of resin,there is a distribution of sizes of molecules,i.e.the resin is an ensemble of molecules of different lengths.This is because the for- mation of the molecules is due to the availability of the reactants.The molecular weight of the material is the average value of all molecules in the material.At room temperature,polyester appears as a liquid.Figure 2.4 shows a sample of polymer in a glass container. Example 2.1 It is desired to make a polyester resin using 100 g of maleic acid and a corresponding amount of ethylene glycol.A stoichiometric amount of ethylene glycol is used.After the condensate is removed,how many grams of polyester are obtained? Mass of different atoms: C=12 g/mol,H=1 g/mol,O=16 g/mol,N 14g/mol
These chains are called polymers (from the word for many parts). Because the linking group formed by acids and alcohols are esters, the term given to this type of resulting polymer is polyester. A quantity of these polyester polymer chains is collectively called polyester resin. (Polymers, in general, in the uncured state, are called resins.) For a certain amount of resin, there is a distribution of sizes of molecules, i.e. the resin is an ensemble of molecules of different lengths. This is because the formation of the molecules is due to the availability of the reactants. The molecular weight of the material is the average value of all molecules in the material. At room temperature, polyester appears as a liquid. Figure 2.4 shows a sample of polymer in a glass container. 52 MATRIX MATERIALS FIGURE 2.4 Glass container with liquid polyester. Example 2.1 It is desired to make a polyester resin using 100 g of maleic acid and a corresponding amount of ethylene glycol. A stoichiometric amount of ethylene glycol is used. After the condensate is removed, how many grams of polyester are obtained? Mass of different atoms: C = 12 g/mol, H = 1 g/mol, O = 16 g/mol, N = 14g/mol
Thermoset Matrix Materials 53 Solution The molecular structures of maleic acid and ofethylene glycol are shown below.From this,the molecular mass of each material is calculated as: Maleic acid 0 HO-C-CH=CH-C-OH Ethylene glycol HO-CH CH OH Maleic acid:4C+4H+40=4(12+1 +16)=116 g/mole Ethylene glycol:2C 6H+20=24+6+32 =62 g/mole The reaction takes place as shown in the following: It can be seen that one molecule of maleic acid reacts with one molecule of ethylene glycol to make a unit of ester and two water molecules. Q and c-o o c REPEATING UNIT Mass of the water molecule:2H+O=18 g/mole. Let M be the mass of the polyester made using 100 g of maleic acid,we have: (M/100)=(116+62-36)/116=1.224 Mp=122.4g 3.1.3.Polyester Crosslinking The polymers formed in the reaction illustrated in Figure 2.3 are not crosslinked since no chemical bond has been formed between the various chains.(The chains are often mechanically intertwined,but that is not crosslinking.)However,the diacid chosen in this case(maleic acid)con-
3.1.3. Polyester Crosslinking The polymers formed in the reaction illustrated in Figure 2.3 are not crosslinked since no chemical bond has been formed between the various chains. (The chains are often mechanically intertwined, but that is not crosslinking.) However, the diacid chosen in this case (maleic acid) conThermoset Matrix Materials 53 Solution The molecular structures of maleic acid and of ethylene glycol are shown below. From this, the molecular mass of each material is calculated as: Maleic acid Ethylene glycol Maleic acid: 4C + 4H + 4O = 4 (12 + 1 + 16) =116 g/mole Ethylene glycol: 2C + 6H + 2O = 24 + 6 + 32 = 62 g/mole The reaction takes place as shown in the following: It can be seen that one molecule of maleic acid reacts with one molecule of ethylene glycol to make a unit of ester and two water molecules. and Mass of the water molecule: 2H + O = 18 g/mole. Let Mp be the mass of the polyester made using 100 g of maleic acid, we have: (Mp/100) = (116 + 62 − 36)/116 = 1.224 Mp = 122.4 g
54 MATRIX MATERIALS TABLE 2.2 Structures of Commercial Organic Peroxides [2]. Name of Peroxide Chemical Structure Hydrogen peroxide H-O0-H Hydroperoxides R-O0-H Dialkyl peroxides R-OO-R Diacyl peroxides R-C(O)-00-C(O)-R Peroxyesters R-C(O)-00-R Peroxy acids R-C(O)-00-H Peroxy ketals R2C-00-R2 Peroxy dicarbonates R-OC(O)-00-C(O)O-R tained a carbon-carbon double bond which survived the polymerization reaction and is contained in every repeating unit of the polymer.(When a reactant or polymer contains a carbon-carbon double bond,the term un- saturated is often applied to it.Therefore maleic acid is an unsaturated diacid and the resulting polymer is an unsaturated polyester.)This unsaturation is critical since the carbon-carbon double bond is the location where crosslinking occurs. The crosslinking occurs by the addition polymerization reaction as shown in Figure 2.5.In this figure,the RO-tends to react with another active site from another molecule [a dash (-)on the right hand side of the oxygen atom O indicates that it is reactive].This reaction utilizes a crosslinking agent(styrene,in the example)that reacts with the polyester polymer chains to provide the crosslinks.(The styrene also lowers the initial viscosity to improve processing.) Normally the styrene molecules are mixed together with the polyester molecules and shipped in a container.Under normal conditions [room temperature for a limited time (months)],the styrene molecules do not react with the polyester molecules.Initiators are usually required to start the reaction.The reaction steps shown in Figure 2.5 are explained below. 3.1.3.1.Initiation Step The crosslinking reaction is initiated by a molecule that readily pro- duces free radicals (a chemical species with unpaired electrons).The most common group of such molecules is the organic peroxides (for ex- ample,methyl ethyl ketone peroxide-MEKP). Organic peroxides are useful as initiators or crosslinking agents be- cause of the thermally unstable O-O bond which decomposes to form free radicals.Organic peroxides may be viewed as derivatives of hydro- gen peroxides in which one or both hydrogens are replaced by organic
tained a carbon-carbon double bond which survived the polymerization reaction and is contained in every repeating unit of the polymer. (When a reactant or polymer contains a carbon-carbon double bond, the term unsaturated is often applied to it. Therefore maleic acid is an unsaturated diacid and the resulting polymer is an unsaturated polyester.) This unsaturation is critical since the carbon-carbon double bond is the location where crosslinking occurs. The crosslinking occurs by the addition polymerization reaction as shown in Figure 2.5. In this figure, the RO– tends to react with another active site from another molecule [a dash (–) on the right hand side of the oxygen atom O indicates that it is reactive]. This reaction utilizes a crosslinking agent (styrene, in the example) that reacts with the polyester polymer chains to provide the crosslinks. (The styrene also lowers the initial viscosity to improve processing.) Normally the styrene molecules are mixed together with the polyester molecules and shipped in a container. Under normal conditions [room temperature for a limited time (months)], the styrene molecules do not react with the polyester molecules. Initiators are usually required to start the reaction. The reaction steps shown in Figure 2.5 are explained below. 3.1.3.1. Initiation Step The crosslinking reaction is initiated by a molecule that readily produces free radicals (a chemical species with unpaired electrons). The most common group of such molecules is the organic peroxides (for example, methyl ethyl ketone peroxide—MEKP). Organic peroxides are useful as initiators or crosslinking agents because of the thermally unstable O–O bond which decomposes to form free radicals. Organic peroxides may be viewed as derivatives of hydrogen peroxides in which one or both hydrogens are replaced by organic 54 MATRIX MATERIALS TABLE 2.2 Structures of Commercial Organic Peroxides [2]. Name of Peroxide Chemical Structure Hydrogen peroxide H–OO–H Hydroperoxides R–OO–H Dialkyl peroxides R–OO–R Diacyl peroxides R–C(O)–OO–C(O)–R Peroxyesters R–C(O)–OO–R Peroxy acids R–C(O)–OO–H Peroxy ketals R2–C–OO–R2 Peroxy dicarbonates R–OC(O)–OO–C(O)O–R
