Moisture and heat resistant coating for glass fibers

In a composite comprising a resin matrix reinforced with glass fibers embedded therein, the improvement which comprises a mositure and heat resistant finish coating on the surface of said glass fibers bonded to said fibers and said resin consisting essentially of approximately, by weight, 40-98 percent of a polyimide film former selected from the group consisting of (1) a fully imidized and polymerized polyimide containing the repeating unit ##STR29## wherein R.sub.1 is a tetravalent radical selected from the group consisting of ##STR30## wherein X = O, S, SO.sub.2, CH.sub.2, ##STR31## and R.sub.2 is a divalent aromatic or aliphatic radical selected from the group consisting of ##STR32## and R.sub.3 is an aromatic divalent radical selected from the group consisting of ##STR33## where X = CH.sub.2, O, S or ##STR34## (2) an addition type resin intermediate derived from the low molecular weight polyimide monomer ##STR35## wherein R.sub.1 is an unsaturated divalent system such as ##STR36## and (3) an hydroxylated aromatic polyimide in the amic acid stage having the repeating unit ##STR37## wherein n is an integer of sufficient magnitude, e.g., at least approximately 5, 2-60 percent of a silane coupling agent characterized by the formula

• R -- Si -- X.sub.3
• wherein R is an organic group containing amino, epoxy, aminehydrochloride or carboxy functionality such as ##STR38## and X is an organic group which contains ether or ester functionality such as --OCH.sub.3, --OCH.sub.2 CH.sub.3, ##STR39## or --OCH.sub.2 CH.sub.2 OCH.sub.3 and up to 30 percent of a vinylpyrrolidone polymer wetting and lubricating agent, said coating having a thickness of approximately 0.02-25 microns.