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