Biodegradability and Thermostability of Renewable Waterborne Polyurethane Synthesized from Trihydroxyl poly(L-malic acid)/glycol Based Polyols and Polycaprolactone Diol

Abstract

A biodegradable composite polyester-based waterborne polyurethane (L-AWPU) was synthesized by coupling L-malic acid with polyethylene glycol, resulting in poly (L-malic acid ethylene glycol ester, PL-A), with polycaprolactone (PCL) being used as a raw material. The primary objective of this research was to enhance biodegradability while maintaining thermal stability and mechanical properties. To achieve this, the content of PL-A in the polymer system was meticulously controlled, leading to the formulation of L-AWPU with outstanding biodegradability and thermal stability. Compared to the traditional PCL-WPU, the incorporation of PL-A not only introduced naturally degradable bio-based molecular chains into WPU but also increased the branching degree of the polyurethane molecular chains. This was attributed to the trifunctional hydroxyl group present in malic acid, which formed intramolecular cross-linking structures. When the PL-A content reached 20 mol%, the temperature of thermal weight loss (T5%) and maximum weight loss rate (T_max) of the polyurethane coating rose from 237.93 to 259.91 °C to 333.86 and 343.13 °C, respectively. This indicated that the establishment of intramolecular cross-linking structures and the increase in molecular weight had effectively enhanced the thermal stability of the polyurethane. At a PL-A content of 20 mol%, there was no significant decrease in the tensile strength and elongation at break of L-AWPU when compared to PCL-WPU.

Graphical Abstract