Thermoset Matrix Materials 55 radicals.Table 2.2 lists the structures of several commercially available organic peroxides. The rate of free radical production is highly temperature dependent, therefore,the reaction to produce free radicals,which in turn initiates the crosslinking reaction,can be accelerated by increasing the temperature. REACTANTS -0-CH,CH,o叶n CH-CH, POLYESTER PEROXIDE CROSSLINKING AGENT INITIATOR (STYRENE) In the above reaction,a polyester material system consists of the polvester molecules,the peroxide initiafor molecules (such as MEKP)and the curing (or cross linking)agents are mired together inside a pot. INITIATION STEP HC0-CHCH2O叶n O-RL New FREE RADICAL Attachment of FREE RADICAL formed from DOUBLE BOND from INITIATOR In the above,the initiator (-O-R)is making a reaction with one of the carbon atoms that hud a double bond (=)After the reaction.the double bond in this carbon atom (and its parmner carbon atom)is broken and it is replaced by a single bond (-)The parmner carbon atom also has an active arm which is ready to make reaction with another adjacent molecule) BRIDGING STEP CHCH-C O-CH:CH BOND formed between POLYMER New FREE RADICAL and CROSSLINKING AGENT In the above.the active arm of the partner carbon atom makes a link with an adjacent styrene monomer.Due to this reaction,the double bond between the two carbon atoms in the sryrene monomer is broken and is replaced by a single bond berween the rwo carbon atoms.The carbon atom in the styrene monomer that is not connected to the ester molecule now hus an active arm that is ready to react with another adjacent molecule. CROSSLINKED POLYMERS 0 C-CH-CHC0-CHCH O叶n CH,OR BOND between CROSSLINKER and a SECOND POLYMER O-CH CH2 On SITE for other CROSSLINKS In the above,repeats of the reaction berween the polyester molecules and styrene molecules gives rise to a crosslinking network between polyester molecule and styrene molecule.This is the polyester resin system. FIGURE 2.5 Addition (or free-radical)cross-linking of polyester [3]
radicals. Table 2.2 lists the structures of several commercially available organic peroxides. The rate of free radical production is highly temperature dependent, therefore, the reaction to produce free radicals, which in turn initiates the crosslinking reaction, can be accelerated by increasing the temperature. Thermoset Matrix Materials 55 FIGURE 2.5 Addition (or free-radical) cross-linking of polyester [3]
56 MATRIX MATERIALS -50m FIGURE 2.6 Small container of MEKP. Since polymerization reactions take place at various temperatures,or- ganic peroxides have been developed with different decomposition rates and these can be conveniently expressed by half-lives (the time required to decompose 50%of the peroxide in a diluent solution at a given temper- ature).The half-lives of most peroxides at about 100C vary from 0.5 hours to 50 hours [2].Initiator compounds are sometimes called cata- lysts.Figure 2.6 shows a small container of MEKP.These free radicals attack the carbon-carbon double bonds and bond to one of the dou- ble-bond carbons,reducing the double bond to a single bond and produc- ing a new free radical on the other formerly double-bonded carbon.This new free radical is then free to react with any other carbon-carbon double bond.Since the styrene is more mobile than the polyester molecules,the most likely new double bond to react with would be in the styrene.Usu- ally only a small amount of initiators(about 1%)is added to the polyes- ter/styrene system to start the reaction.Adding large amounts of initiators is dangerous due to the simultaneous reactions of many links and this can cause a large amount of heat generated over a short duration of time. 3.1.3.2.Bridging Step At the end of the initiation step,many radicals (the free bonds from the carbon atom C-)are produced.The radicals are produced from the breaking of the double C=C bonds in either the styrene molecules or in the polyester molecules.When these free radicals are in the vicinity of each other,they connect.The results are the single C-C bonds between the radicals.This forms the link between the styrene molecules and the
Since polymerization reactions take place at various temperatures, organic peroxides have been developed with different decomposition rates and these can be conveniently expressed by half-lives (the time required to decompose 50% of the peroxide in a diluent solution at a given temperature). The half-lives of most peroxides at about 100°C vary from 0.5 hours to 50 hours [2]. Initiator compounds are sometimes called catalysts. Figure 2.6 shows a small container of MEKP. These free radicals attack the carbon-carbon double bonds and bond to one of the double-bond carbons, reducing the double bond to a single bond and producing a new free radical on the other formerly double-bonded carbon. This new free radical is then free to react with any other carbon-carbon double bond. Since the styrene is more mobile than the polyester molecules, the most likely new double bond to react with would be in the styrene. Usually only a small amount of initiators (about 1%) is added to the polyester/styrene system to start the reaction. Adding large amounts of initiators is dangerous due to the simultaneous reactions of many links and this can cause a large amount of heat generated over a short duration of time. 3.1.3.2. Bridging Step At the end of the initiation step, many radicals (the free bonds from the carbon atom C–) are produced. The radicals are produced from the breaking of the double C=C bonds in either the styrene molecules or in the polyester molecules. When these free radicals are in the vicinity of each other, they connect. The results are the single C–C bonds between the radicals. This forms the link between the styrene molecules and the 56 MATRIX MATERIALS FIGURE 2.6 Small container of MEKP
Thermoset Matrix Materials 57 polyester molecules.The formation of the single C-C bonds in turn cre- ates new free radicals.These free radicals are again available for reaction with other carbon-carbon double bonds,and reactions with other polyes- ter molecules would create further crosslinks.The reaction continues to propagate the crosslinks. 3.1.3.3.Crosslinking Step The net result is a large network of interconnected polymers in which styrene serves as the crosslink(bridge)between the polymers.Since the peroxide is generally present in small quantities(typically 1%)the final structure does not show the peroxide attached to the polystyrene mole- cule as this would generally not be present except on the endmost crosslink.This crosslinking mechanism is called addition or free-radical crosslinking.This entire crosslinking reaction process is called curing. Styrene is not the only crosslinker (sometimes crosslinkers are called curing systems)for polyester systems,although it is the most widely used. Other chemicals in use include vinyl toluene,chlorostyrene,methyl methacrylate,and diallyl phthalate.The physical and chemical properties of the crosslinked polymer are affected by the curing system.For instance, the use of chlorostyrene imparts flame resistance to the polymer. Example 2.2:Crosslinking of Polyester It is desired to make a polyester using 100 g of maleic acid and ethylene glycol.A stoichiometric amount of ethylene glycol is used.Cross linking is done using styrene. Assume that one styrene monomer corresponds to one oligoester(this assumption is to simplify the calculation to illustrate the principle;in reality the crosslink between oliester molecules can be in the range from 1 to 4 styren monomers).From the cross linking process,how many C=C bonds are broken,how many C-Cbonds are formed? Solution Continuing from the same problem in Example 2.1: Number of bonds broken and formed 0 H=H&0 -cH c o)+R0-+ CH=CH2 POLYESTER INITIATOR CROSSLINKING AGENT (STYRENE) Initiating Step c o-a O-R
polyester molecules. The formation of the single C–C bonds in turn creates new free radicals. These free radicals are again available for reaction with other carbon-carbon double bonds, and reactions with other polyester molecules would create further crosslinks. The reaction continues to propagate the crosslinks. 3.1.3.3. Crosslinking Step The net result is a large network of interconnected polymers in which styrene serves as the crosslink (bridge) between the polymers. Since the peroxide is generally present in small quantities (typically 1%) the final structure does not show the peroxide attached to the polystyrene molecule as this would generally not be present except on the endmost crosslink. This crosslinking mechanism is called addition or free-radical crosslinking. This entire crosslinking reaction process is called curing. Styrene is not the only crosslinker (sometimes crosslinkers are called curing systems) for polyester systems, although it is the most widely used. Other chemicals in use include vinyl toluene, chlorostyrene, methyl methacrylate, and diallyl phthalate. The physical and chemical properties of the crosslinked polymer are affected by the curing system. For instance, the use of chlorostyrene imparts flame resistance to the polymer. Thermoset Matrix Materials 57 Example 2.2: Crosslinking of Polyester It is desired to make a polyester using 100 g of maleic acid and ethylene glycol. A stoichiometric amount of ethylene glycol is used. Cross linking is done using styrene. Assume that one styrene monomer corresponds to one oligoester (this assumption is to simplify the calculation to illustrate the principle; in reality the crosslink between oliester molecules can be in the range from 1 to 4 styren monomers). From the cross linking process, how many C=C bonds are broken, how many C–C bonds are formed? Solution Continuing from the same problem in Example 2.1: Number of bonds broken and formed Initiating Step
58 MATRIX MATERIALS Bridging Step Crosslinking Step CH-CH,OR CcH-》 CH-CH The crosslinking process is illustrated above. For each pair of polyester units,it can be seen that four C=C bonds are broken and eight C-Cbonds are formed.For the C=C bonds,two are from the two polyester units, and the other two are within the structure of the two styrene molecules.For the eight C-C bonds,two are replacing the two C=C bonds within the ester units,the other two are replacing the two C=C bonds within the two styrene molecules,the other four are connecting the two styrene molecules with the two ester units. Therefore for each polyester unit,there are two C=C bonds broken and four C-C bonds formed. The chemical formula for a polyester unit is: The mass of a polyester unit is: 6C+6H+40=72+6+64=142 g/mole Since there are 122.4 g of polyester made,the number of bonds involved is: C=C bonds:(122.4/142)2 =1.724 mole or 1.038 x 1024 bonds.(Note that 1 mole 0.602×1024. C-C bonds:2 x 1.724 =3.446 moles or 2.076 x 1024 bonds
58 MATRIX MATERIALS Bridging Step Crosslinking Step The crosslinking process is illustrated above. For each pair of polyester units, it can be seen that four C=C bonds are broken and eight C–C bonds are formed. For the C=C bonds, two are from the two polyester units, and the other two are within the structure of the two styrene molecules. For the eight C–C bonds, two are replacing the two C=C bonds within the ester units, the other two are replacing the two C=C bonds within the two styrene molecules, the other four are connecting the two styrene molecules with the two ester units. Therefore for each polyester unit, there are two C=C bonds broken and four C–C bonds formed. The chemical formula for a polyester unit is: The mass of a polyester unit is: 6C + 6H + 4O = 72 + 6 + 64 = 142 g/mole Since there are 122.4 g of polyester made, the number of bonds involved is: C=C bonds: (122.4/142) 2 = 1.724 mole or 1.038 × 1024 bonds. (Note that 1 mole = 0.602 × 1024). C–C bonds: 2 × 1.724 = 3.446 moles or 2.076 × 1024 bonds
Thermoset Matrix Materials 59 3.1.4.Crosslinking (Curing)is an Exothermic Reaction (Heat is Generated) The crosslinking of polymer from individual molecules into a polymer 3-D network is an exothermic reaction (i.e.,heat is generated from the process).What this means is that the temperature of the material will in- crease as a result of curing.For the curing of thin laminates(about a few millimeters thick),this increase in temperature may not create a prob- lem;but for the curing of thicker laminates the heat generated from inside the laminate may not be conducted away easily,due to the fact that poly- mers such as polyesters(or epoxies)do not conduct heat.Over a short pe- riod of time,the temperature increase at a given location can be quite large,resulting in the burning or degradation of the matrix material. From the thermodynamic point of view,to break a bond,one needs to add energy into the system.For example,to break a piece of chalk with one's fingers,it has to be pulled apart or bent.Energy is added to the piece of chalk by the pulling or bending motion of the fingers.On the other hand,if the bonds between the broken pieces of chalk were to be bonded together again,energy would be generated from the bonds by the law of conservation of energy.This energy will increase the temperature of the chalk unless the heat is conducted away. Example 2.3:Heat Generation and Temperature Increase It is desired to make a polyester using 100 g of maleic acid and ethylene glycol.A stoichiometric amount ofethylene glycol is used.Crosslinking is done using styrene. Continuing from the same problem in Examples 2.1 and 2.2: a.The energy required to break a C=C bond is 680 kJ/mole and the energy created by forming aC-C bond is 370kJ/mole.How much energy is generated during the polymerization process?1 mole =0.602 x 10 b.The heat capacity of polyester is 0.25 cal/g/C.Assuming no heat loss,what is the increase in temperature of the polyester? Mass of carbon C=12 g/mole,H=1 g/mole,O=16 g/mole 1 calorie =4.18 Joule Solution a.Energy generated: Considering the energy in the bonds,one has: Energy inputted into the system to break the double bonds: (1.724mole)(680kJ/mole)=1172.32kJ
3.1.4. Crosslinking (Curing) is an Exothermic Reaction (Heat is Generated) The crosslinking of polymer from individual molecules into a polymer 3-D network is an exothermic reaction (i.e., heat is generated from the process). What this means is that the temperature of the material will increase as a result of curing. For the curing of thin laminates (about a few millimeters thick), this increase in temperature may not create a problem; but for the curing of thicker laminates the heat generated from inside the laminate may not be conducted away easily, due to the fact that polymers such as polyesters (or epoxies) do not conduct heat. Over a short period of time, the temperature increase at a given location can be quite large, resulting in the burning or degradation of the matrix material. From the thermodynamic point of view, to break a bond, one needs to add energy into the system. For example, to break a piece of chalk with one’s fingers, it has to be pulled apart or bent. Energy is added to the piece of chalk by the pulling or bending motion of the fingers. On the other hand, if the bonds between the broken pieces of chalk were to be bonded together again, energy would be generated from the bonds by the law of conservation of energy. This energy will increase the temperature of the chalk unless the heat is conducted away. Thermoset Matrix Materials 59 Example 2.3: Heat Generation and Temperature Increase It is desired to make a polyester using 100 g of maleic acid and ethylene glycol. A stoichiometric amount of ethylene glycol is used. Crosslinking is done using styrene. Continuing from the same problem in Examples 2.1 and 2.2: a. The energy required to break a C=C bond is 680 kJ/mole and the energy created by forming a C–C bond is 370 kJ/mole. How much energy is generated during the polymerization process? 1 mole = 0.602 × 1024 b. The heat capacity of polyester is 0.25 cal/g/°C. Assuming no heat loss, what is the increase in temperature of the polyester? Mass of carbon C= 12 g/mole, H = 1 g/mole, O = 16 g/mole 1 calorie = 4.18 Joule Solution a. Energy generated: Considering the energy in the bonds, one has: Energy inputted into the system to break the double bonds: (1.724 mole) (680 kJ/mole) = 1172.32 kJ