Process for modifying glycol terephthalate polyesters to contain CH.sub.2 - - .sub.2 --N linkages

In the process of preparing a linear glycol terephthalate polyester and fabricating the polyester into filaments of 15 to 26 relative viscosity for preparing staple fiber yarns for knitting into fabric, the method of improving pilling resistance in the knitted fabric which comprises modifying the polyester to contain --CH.sub.2 --S--CH.sub.2 -- or ##EQU1## linking units in molecular chains of the polyester, each of the indicated nitrogen bonds being attached to a hydrocarbon group, fabricating the modified polyester into filaments wherein the mole percent sulfur or nitrogen times the ratio of the number of CH.sub.2 --S or CH.sub.2 --N links in the molecular chain to the total number of CH.sub.2 --S or CH.sub.2 --N links has a value within limits defined as follows:

• 0. 3 to 1.7 for 15 relative viscosity
• 0.6 to 2.0 for 17.5 relative viscosity
• 0.7 to 2.2 for 20 relative viscosity
• 1.0 to 2.5 for 26 relative viscosity,
• and subsequently treating the modified polyester with hypochlorous acid to reduce the relative viscosity to a value within the range of 8 to 12.5 said relative viscosity is the ratio of the viscosity of a solution of 0.8 grams of polyester, dissolved at room temperature in 10 milliliters of hexafluoroisopropanol containing 80 parts per million of H.sub.2 SO.sub.4, to the viscosity of the H.sub.2 S0.sub.4 --containing hexafluoroisopropanol itself, both measured at 25.degree. C. in a capillary viscometer and expressed in the same units.