Composites and size coated glass fibers used therein

Glass fiber reinforced composite having high flexural strength to density ratio and resistance to strength loss incident to exposure to water, said composite comprising cured unsaturated thermosetting resin reinforced by glass fibers having thereon a size coating comprising maleic anhydride adduct of 1,2-polybutadiene having a number average molecular weight of more than 10,000 and a crystallinity ranging from 0% to 50%, said adduct being soluble in an organic solvent, the amount of said adduct forming the size coating ranging from about 0.1 grams to about 6 grams per 100 grams of glass fibers, said size coated glass fibers being present in an amount ranging from about 20% to about 70% by weight of said composite.