Sizing glass fibers for thermoplastic resin reinforcement

An aqueous dispersion suitable for use as a sizing composition for glass fibers comprising a polyurethane cationomer obtained from aliphatic and/or cycloaliphatic diisocyanates and phthalic ester groups containing polyhydroxy compounds as a film-forming agent, said polyurethane cationomer being characterized by

• (a) an average particle size of about 0.05.mu. to 0.5.mu.,
• (b) an ionic group content of about 5 to 30 milliequivalents per 100 g, and
• (c) forming a film when dried, this film having a tensile strength of al least about 50 kp/cm.sup.2, an elongation at break of about 100-600%, a Shore-A-hardness of about 50-90, a swelling in water of less than about 30%, and being insoluble in 80% aqueous tetrahydrofuran after heating to 140.degree. C.