60 MATRIX MATERIALS Energy generated from the system to form the single bonds: (3.446 moles)370kJ/mole)=1275.02kJ Net energy generated: 1275.221172.32=102.7kJ b.Temperature increase: Heat stored in the material: Q=mncAT or△T=Q/mc) For the mass of the material,apart from the polyester,there is also the styrene.Each unit of the polyester corresponds to each unit of the styrene for complete crosslinking. The chemical formula for styrene is shown in the following: H H CH CH CH CH CH The mass of a styrene molecule is therefore: 8C+8H=96+8=104g/mole Mass of styrene corresponding to 122.4 g of polyester is: (122.4/142)104=89.65g Total mass of polyester and styrene: 122.4+89.65=212.05g The temperature increase can now be calculated using the above equation: AT=- (102.7kJ1cal1418J) (212.05g)(0.25cal/gC) =463℃ Note that the temperature increase is very high.The reason for this is because of the assumption of no heat loss.In reality,there is heat loss to the surrounding environ- ment.Also not all the heat is generated instantly.The process of bonding takes time. The curing process may take hours and even days,depending on the surrounding tem- perature.As such in real applications the temperature increase will be a lot less than the number above
60 MATRIX MATERIALS Energy generated from the system to form the single bonds: (3.446 moles)(370 kJ/mole) = 1275.02 kJ Net energy generated: 1275.2?1172.32 = 102.7 kJ b. Temperature increase: Heat stored in the material: Q = mc∆T or ∆T = Q/(mc) For the mass of the material, apart from the polyester, there is also the styrene. Each unit of the polyester corresponds to each unit of the styrene for complete crosslinking. The chemical formula for styrene is shown in the following: The mass of a styrene molecule is therefore: 8C + 8H = 96 + 8 = 104 g/mole Mass of styrene corresponding to 122.4 g of polyester is: (122.4/142)104 = 89.65 g Total mass of polyester and styrene: 122.4 + 89.65 = 212.05 g The temperature increase can now be calculated using the above equation: ∆T cal = ( . )( / . ) ( . )( ) 102 7 1 418 212 05 kJ J g 0.25cal / g C = 463°C Note that the temperature increase is very high. The reason for this is because of the assumption of no heat loss. In reality, there is heat loss to the surrounding environment. Also not all the heat is generated instantly. The process of bonding takes time. The curing process may take hours and even days, depending on the surrounding temperature. As such in real applications the temperature increase will be a lot less than the number above.
Thermoset Matrix Materials 61 TABLE 2.3 Energy Associated with the Atomic Bonds. Bond Energy (kJ/mole) C-C 370 C=C 680 C-H 435 O-H 500 N-H 430 C-0 360 C=0 535 On the atomic level,there is an amount of energy associated with the breaking or forming of each of the bonds.The energy associated with some of the bonds is shown in Table 2.3. 3.1.5.Photo-Activated Curing Systems The initiators for curing described above(such as MEKP)are readily activated agents.They start acting as soon as they are mixed together with the resin systems.There are also initiators that are activated by ultra- violet rays.These photo-initiators can be mixed in the resin system but they are not activated until photon energy provided by ultraviolet light is introduced.Once activated they start reactions similar to the case of con- ventional initiators.The use of photoinitiators allows some control over the time for the reaction to start but requires that the resin be translucent enough for light to be transmitted. 3.1.6.Polyester Properties One of the advantages of polyester resins is that the reactants,which are called monomers(Table 2.4),can be chosen from a wide assortment of diacids and diols to meet specific physical and chemical properties de- sired.Apart from maleic acids,one can also use maleic anhydrides, orthophthalic acids,isophthalic acids or orthophthalic anhydride.The difference between orthophthalic acids and isophthalic acids is that due to the location of the bonds on the benzene ring,the molecule of the isophthalic acids is straighter.Straighter molecules provide better pack- ing of the material,and this in turn gives higher density,better strength, stiffness and environmental resistance.The anhydride on the other hand has a close ring which must be broken by the application of more energy (heat)before further reaction can occur.As a rule of thumb,when a mole- cule contains benzene rings rather than just straight C-C bonds it dis-
On the atomic level, there is an amount of energy associated with the breaking or forming of each of the bonds. The energy associated with some of the bonds is shown in Table 2.3. 3.1.5. Photo-Activated Curing Systems The initiators for curing described above (such as MEKP) are readily activated agents. They start acting as soon as they are mixed together with the resin systems. There are also initiators that are activated by ultraviolet rays. These photo-initiators can be mixed in the resin system but they are not activated until photon energy provided by ultraviolet light is introduced. Once activated they start reactions similar to the case of conventional initiators. The use of photoinitiators allows some control over the time for the reaction to start but requires that the resin be translucent enough for light to be transmitted. 3.1.6. Polyester Properties One of the advantages of polyester resins is that the reactants, which are called monomers (Table 2.4), can be chosen from a wide assortment of diacids and diols to meet specific physical and chemical properties desired. Apart from maleic acids, one can also use maleic anhydrides, orthophthalic acids, isophthalic acids or orthophthalic anhydride. The difference between orthophthalic acids and isophthalic acids is that due to the location of the bonds on the benzene ring, the molecule of the isophthalic acids is straighter. Straighter molecules provide better packing of the material, and this in turn gives higher density, better strength, stiffness and environmental resistance. The anhydride on the other hand has a close ring which must be broken by the application of more energy (heat) before further reaction can occur. As a rule of thumb, when a molecule contains benzene rings rather than just straight C–C bonds it disThermoset Matrix Materials 61 TABLE 2.3 Energy Associated with the Atomic Bonds. Bond Energy (kJ/mole) C–C 370 C=C 680 C–H 435 O–H 500 N–H 430 C–O 360 C=O 535
TABLE 2.4 Choices of Reactants for Polyester Resins [3]. Reactants Chemical Structure Advantages Ethylene Glycol HO-CH,CH OH Basic reactant This molecule is aliphatic since it contains mainly linear chains. HO-CH-CH OH More compatible Propylene Glycol CH with styrene than This molecule is aliphatic and branched. ethylene glycol 0 Maleic Acid(also 9 Flexible,Low cost Fumaric Acid) HO-C一CH=CH-C-OH This molecule is aliphatic. CH=CH 0=C c=0 0 Maleic Anhydride The anhydride has a ring which needs to be opened before reac- tion can occur. 0 C-OH Orthophthalic Acid Rigid C-OH (Ortho) Relatively low cost This molecule is aromatic since it contains the benzene ring. 0 Orthophthalic Anhydride Rigid 0 This aromatic molecule has the ring which needs to be opened be- fore reaction can occur. 0 C-OH Isophthalic Acid C=0 Resilient(tough) (SO) Thermal stability OH The isophthalic molecule gets its name from the relative position of the connection between the COOH group with the benzene ring. 62
TABLE 2.4 Choices of Reactants for Polyester Resins [3]. Reactants Chemical Structure Advantages Ethylene Glycol This molecule is aliphatic since it contains mainly linear chains. Basic reactant Propylene Glycol This molecule is aliphatic and branched. More compatible with styrene than ethylene glycol Maleic Acid (also Fumaric Acid) This molecule is aliphatic. Flexible, Low cost Maleic Anhydride The anhydride has a ring which needs to be opened before reaction can occur. — Orthophthalic Acid (Ortho) This molecule is aromatic since it contains the benzene ring. Rigid Relatively low cost Orthophthalic Anhydride This aromatic molecule has the ring which needs to be opened before reaction can occur. Rigid Isophthalic Acid (ISO) The isophthalic molecule gets its name from the relative position of the connection between the COOH group with the benzene ring. Resilient (tough) Thermal stability 62