Condensation kinetics of polyphthalamides. II. Polyesters and diamines

Polyphthalamides are distinguished by outstanding properties. Only their price keeps them from replacing PA66 in many applications, in particular; in automobiles. This article deals with a simple, economically attractive process of upcycling postconsumer polyethylene terephthalate (PET) waste to polyphthalamides. Basic studies on the amidation of acids and esters were described in Part I. The amidation of esters is accompanied by a side reaction of amine alkylation, which, in the case of polyesters, leads to branching and crosslinking. But the crosslinking can be avoided in mixtures with excessive diamine. Therefore, a heterogeneous amidation process was developed in which solid PET granules are simultaneously dissolved in and amidated by a liquid diamine. As long as the PET diamine system is heterogeneous, it reacts under amino excess conditions. Later, when all PET is dissolved, it comes back to stoichiometric conditions again, so long-chained polyamides can be obtained. In terms of molar mass and viscosity, this amidation of solid PET proceeds very similarly to the amidation of terephthalic acid (TPA), which is so far the common route towards polyphthalamides. PET waste could replace TPA.