Terephthalic acid and ethylene glycol were used to prepare poly(ethylene terephthalate)-co-amide salts with adipate pentamethylenediamine (AP salts) as the modified monomer, improving the hygroscopicity and softness of poly(ethylene terephthalate) fibers. One of the facets of concern is that the yellowing index of poly(ethylene terephthalate)-co-amide salts increases from 3.8 to 13.8 than poly(ethylene terephthalate) and increases from 2.9 to 11.7 for fibers, limiting its acceptance in textile applications. The convention underpinning the yellowing originated from thermal oxidation and Schiff base reaction in the blend of polyester/polyamide, generating yellowing substances with conjugated imine structure. However, in this paper, an unconventional yellowing mechanism was discovered, attributed to the reaction of AP salts and acetaldehyde (a by-product of poly(ethylene terephthalate)). The structure of yellowing substances was characterized and confirmed the role of conjugated diketoamine (O=C–CH2–CONH–) and conjugated enamines (O=C–C=C–NH–). It is difficult for fiber applications to overcome the yellowing issue. Nevertheless, an inhibiting method for the yellowing phenomenon was provided, end-capping technology, by a clearer understanding of the yellowing processes. Thus, poly(ethylene terephthalate)/APA was prepared with the end-capping product carboxyl-terminated diamide (APA) as the modified monomer, which was synthesized by the reaction of AP salts and adipic acid. The yellowing index decreased from 13.8 to 8.5 than poly(ethylene terephthalate)-co-amide salts and decreased from 11.7 to 6.9 for fibers. This significantly improves the yellowing problem and provides technical support for colorless copolymer fibers.
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