The resin matrix: Its influence on sheet molding compound mechanical properties

Abstract A series of sheet molding compound (SMC) materials have been prepared in which the only variable was the polyester resin used as the matrix. The polyester resins were all synthesized from maleic anhydride, propylene glycol and 1,2 propane diol. Systematic variations were made in the molar ratio of maleic of to phthalic anhydride used, and also in the hydroxyl values of the polyesters. The resins were also separately crosslinked with styrene, and the fracture toughness of the cast sheets measured. The toughness of the SMC was assumed to be measurable by linear elastic fracture mechanics (LEFM) methods. Results indicated that the resin matrix toughness was inversely related to polyester unsaturation but the SMC toughness was directly related to it. The hydroxyl values of the resins were determined by nuclear magnetic resonance (NMR) of the silyated derivatives. The tensile properties of the SMC were investigated. Modulus increased steadily with resin hydroxyl value, but tensile strength showed a maximum at 50 mg. These observations indicate the importance of hydroxyl value in SMC resin formulation, arising from its effect on fiber‐resin bond strength.