260 LIQUID COMPOSITE MOLDING 2.3.1.Polyester and Vinyl Ester In common with the hand-laminating industry,RTM has been domi- nated (in tonnage)by the use of polyester resins.Unsaturated polyesters are produced via a condensation reaction of organic acids(maleic and phthalic anhydride)with ethylene or propylene glycol to produce esters (Chapter 2).The styrene content of an unsaturated polyester is important in that it controls the resin viscosity and thereby the impregnation pro- cess.Increasing styrene content will decrease the viscosity but will in- crease the heat of reaction and peak exotherms at the expense of the final mechanical properties.Excessive styrene content may be detrimental to product quality since any residual monomer following curing or post-curing may continue to be lost in service with dimensional changes in the finished part. 2.3.1.1.Shrinkage Control Additives (Low Profile Additives or LPA) Polyesters and vinyl esters exhibit a significant amount of shrinkage FIGURE 7.10 Preform made by braiding.2.3.1. Polyester and Vinyl Ester In common with the hand-laminating industry, RTM has been domi￾nated (in tonnage) by the use of polyester resins. Unsaturated polyesters are produced via a condensation reaction of organic acids (maleic and phthalic anhydride) with ethylene or propylene glycol to produce esters (Chapter 2). The styrene content of an unsaturated polyester is important in that it controls the resin viscosity and thereby the impregnation pro￾cess. Increasing styrene content will decrease the viscosity but will in￾crease the heat of reaction and peak exotherms at the expense of the final mechanical properties. Excessive styrene content may be detrimental to product quality since any residual monomer following curing or post-curing may continue to be lost in service with dimensional changes in the finished part. 2.3.1.1. Shrinkage Control Additives (Low Profile Additives or LPA) Polyesters and vinyl esters exhibit a significant amount of shrinkage 260 LIQUID COMPOSITE MOLDING FIGURE 7.10 Preform made by braiding