vinyl chloride resins , particularly poly and chlorinated poly are reinforced with glass fibers which have been sized with a particular aminosilane coupling agent and a particularly characterized basic film former . at the processing temperature of vc resin , containing a stabilizer against degradation during thermoforming , for example in an extruder , or on a mixing mill used to blend the resin with sized glass fibers , a reaction occurs between an aminosilane coupling agent and the vc resin which reaction involves c ═ c bonds in the homopolymer , provided the polymer film former in the size is more basic than poly . the film former catalyzes the thermal dehydrohalogenation of the vc homopolymer at the fiber - resin interface so as to generate allylic cl moieties in chains of the homopolymer , which moieties react with the amine groups of the aminosilane . the size is most conveniently coated on the fibers from a sizing solution , dispersion or emulsion containing the coupling agent and film former in which allylic cl moieties in vc resin chains become covalently bonded to an aminosilane , so that even upon thorough extraction of a gfr vc composite with tetrahydrofuran , sufficient bound vc resin remains so that x - ray photoelectron spectroscopic examination indicates that the ratio of total chlorine to total carbon remaining on the surface of the glass fibers , i . e . the ratio of the areas of the &# 34 ; cl / c &# 34 ; peaks , is at least 0 . 91 .