Chemical recycling of PET using neopentyl glycol: Reaction kinetics and preparation of polyurethane coatings

Abstract

Recycling of post-consumer poly(ethylene terephthalate) (PET) is a worldwide concern due to its environmental impact and large increasing volume of these materials produced by society. Current work aims at effective recycling of waste PET from soft drink bottles by glycolysis method and thereby utilization of the recycled product for coating application.In the investigation, we report neopentyl glycol (NPG), which till date is not studied much for chemical recycling of PET, as an effective diol for glycolysis reaction. NPG was used in a molar ratio of 1:6 (PET:NPG) and the glycolysis reaction was carried out at 200–220°C in presence of 0.5% zinc acetate as a trans-esterification catalyst. The progress of glycolysis reaction was studied by means of molecular weight of residual PET and hydroxyl value of the glycolyzed oligomer. The glycolyzed oligomer was also characterized using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) to study its molecular weight distribution and the thermal characteristics respectively. The glycolyzed monomeric product was purified and characterized by conventional method of hydroxyl and saponification number, Fourier transform infrared spectroscopy (FTIR), Proton NMR (1H NMR) spectroscopy, DSC, X-ray diffraction (XRD) and gas chromatography–mass spectroscopy (GC–MS). Further polyester (PE) polyol was successfully synthesized by polyesterification of glycolyzed monomeric product, adipic acid, isophthalic acid and trimethylol propane. Polyurethane coatings were prepared from the polyester polyol and different commercial polyisocyanate curing agents and applied on mild steel panels. The coated films were evaluated for their optical, mechanical and chemical properties. Thermal properties of coatings were investigated by DSC and thermogravimetric analysis (TGA).