Tertiary recycling of poly(ethylene terephthalate) wastes for production of polyurethane–polyisocyanurate foams

The present work aims at tertiary recycling of poly(ethylene terephthalate) (PET) wastes to derive raw materials for production of polyurethane–polyisocyanurate foams. The first step involves glycolytic depolymerization of PET in the presence of diethylene glycol (DEG), under the influence of microwave irradiation with an aim to reduce the energy intensiveness of the process. Experimental conditions like PET:DEG ratio and reaction time were optimized to maximize the extent of PET conversion. Multiple extrusion of PET led to reduction in its molecular weight, which increased its reactivity towards glycolysis. The time required for glycolytic depolymerization of PET could be significantly reduced by employing microwave irradiation instead of conventional thermal process. The glycolysate was subsequently reacted with two different diacids: adipic acid (AA) and sebacic acid (SA) to obtain aromatic oligoesters. For the purpose of comparison, analogous aliphatic oligoesters were also prepared by reacting AA and SA with DEG under similar reaction conditions. The oligoesters were subsequently employed as a raw material for the preparation of polyurethane foams by reacting with diphenylmethane diisocyanate. The hydroxyl:isocyanate ratio in the formulation was decreased to introduce isocyanurate linkages, which resulted in the formation of polyisocyanurate foams with high compressive strength. Various characterization techniques were employed to determine the effect of the aromatic phenyl group and chain length of the diacid on the mechanical and structural properties of the resultant foams.