Enhancing flame retardancy and hydrolysis resistance of flame retardant copolyester fibers by reactive carbodiimide

Polyethylene terephthalate flame retardant copolyester (FRPET) copolymerized with 2-carboxyethyl (phenyl) phosphinic acid (CEPPA) is one of the major copolymerized commercial flame-retardant copolyesters. Similar to conventional PET fibers, the mechanical and flame retardant properties of FRPET fibers after alkaline water post-treatment are reduced. In this work, two kinds of carbodiimide-based masterbatches are added to FRPET fibers, the thermal processing stability, hydrolysis resistance and flame retardancy are significantly improved. The structural and physicochemical properties of the modified FRPET fibers were characterized. The processing thermal stability of FRPET is improved due to the reaction of the double bond structure in carbodiimide with the terminal carboxyl group of polyester to form an acyl urea structure. After melt extrusion, the viscosity reduction of the modified FRPET fiber is only 1.6%, which is about 10 times lower compared to that of FRPET (17.6%). After being treated with 50 g/L NaOH solution at 95°C for 1 h, the tensile strength retention rates of the modified FRPET fibers are over 85.0%, which is much higher than that of FRPET fibers (71.8%), showing excellent hydrolysis resistance. In addition, the vertical burning performance of the modified FRPET fibers is improved from V-2 to V-0, and the limiting oxygen index could still reach 25.6%. Besides that, the mechanism of the combustion process is further discussed.