Influence of fillers on the stiffness and strength of a polyester resin

Stiffness of a dough moulding compound containing a typical amount of calcium carbonate and glass fibers was found to be little higher than that of the polyester resin filled with the same amount, by weight, only of calcium carbonate but considerably lower than that of the resin containing the same quantity of glass fibers. Ratios of the moduli of the filled to the unfilled resin were as high in shear as in bending and the creep in bending indicated that the effectiveness of the fillers in stiffening the resin was maintained or even increased with time under load. Inclusion of calcium carbonate alone in the resin significantly reduced the stresses and even more the strains at which it failed, in contrast to the fibers, but in combination with the fibers resulted in the highest work to failure.