Treatment of polyester filaments with aromatic isocyanate mixture

In the process, including a dry heat drawing step, of making filaments of synthetic linear condensation polyester useful as reinforcement for rubber articles wherein an organic isocyanate is applied to such filaments to improve the adhesion thereof to rubber, the improvement comprising applying, as the organic isocyanate, a mixture of 50 to 90% di-functional aromatic isocyanates, and the remainder higher than di-functional aromatic isocyanates, and then drawing said polyester filaments to 4 to 7 times their undrawn length after applying said mixture of aromatic isocyanates, said drawing step being dry heat drawing, performed by heating the filament in air in contact with a heated surface.