Free radical polymerisation polyester resin

Chain-Growth Polymerisation with Termination A major exponent of this class of polymerisations are free radical polymerisations in the presence of a radical initiator. A classic example is the crosslinking (co)polymerisation of unsaturated polyester resins with styrene, initiated by the decomposition of a peroxide initiator. Some important reaction steps involved in free radical polymerisations are sketched in Reaction scheme 4. 

For a free radical polymerisation system, an unsaturated polyester resin, an auto-acceleration was observed close to the onset of vitrification. To model the curing kinetics for these systems, including the mobility-controlled regions, a specific diffusion control model will need to be incorporated in a mechanistic reaction model. The heat capacity and the mobility factor can still give information about how vitrification is occurring, and how it is related to the auto-acceleration effect. 

Figure 1.2a Isothermal reaction rate versus time plot of a free radical-polymerised unsaturated polyester resin. Reprinted with permission from S.V. Muzumdar and L.J. Lee, Polymer Engineering and Science, 1996, 36, 7, 943 1996, John Wiley...

Polyesters are macromolecules made by reacting a diacid or dianhydride with a dihydroxy compound (diols). To make unsaturated polyesters, maleic anhydride or fumeric acid is used in addition to a saturated acid, which provides unsaturation in the structure. The most commonly used anhydrides are maleic anhydride (unsaturated) and phthalic anhydride (saturated). The commonest diols are ethylene glycol or propylene glycol. Use of an unsaturated anhydride is very critical to provide unsaturation in the structure, which is utilised to cure the resin by free-radical polymerisation. The chemical reaction for the synthesis of UPE is shown in Figure 2.13. 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

Vinyl ester resins are similar to unsaturated polyester resins in that they are cured by a free radical initiated polymerisation. However, they differ from the polyesters in that the unsaturation is at the ends of the molecule rather than along the polymer chain. Unlike polyesters, vinyl esters show a greater resistance to hydrolysis as well as lower peak exotherm temperatures and less shrinkage upon cure. Cured vinyl ester resins exhibit excellent resistance to acids, bases and solvents. They also show improved strain to failure, toughness and glass transition temperatures over polyesters. They can be used in filament winding, pultrusion, resin injection, vacuum moulding and conventional hand lay-up. 

As discussed in earlier sections, UPE resins are a mixture of an unsaturated polyester, styrene and an inhibitor. When the resin is mixed with a peroxide initiator and activator (cobalt octoate/napthenate), free radicals are formed. At the initial stage, all or most of the free radicals generated are consumed by the inhibitor. The driving force for the preferable reaction of free radicals with the inhibitor is the higher stability of inhibitor radicals. Once the inhibitor molecules are depleted, free radicals, produced from the initiator, initiate polymerisation of the polyesters. Styrene serves as an agent to link the adjacent polyester molecules. The curing of UPE resin (polyester + styrene) involves different types of reactions ... 

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