Biodegradable thermoplastic polyurethanes

Abstract

The overall aim of this work was to investigate the biodegradability of a number of polyurethane elastomers synthesised by different methods and targeted for a specific agricultural purpose in which the polyurethane was required to be degradable in soil after its useful life. Polyurethanes were synthesised commercially using two different methods; a ‘one-shot’ method where all of the reactants were added simultaneously, and a ‘pre-polymer’ method, in which the isocyanate and polyol were reacted together before addition of the chain extender. The effect of the method of synthesis on the rate of degradation and biodegradation was investigated using accelerated alkaline hydrolysis, enzymatic hydrolysis and soil burial, where it was found that the polyurethane synthesised by the ‘pre-polymer’ method hydrolysed faster under alkaline conditions (21 days) than that synthesised by the ‘one-shot’ method (56 days). This was found to be due to differences in the polymer morphology, with an increase in microcrystalline domains occurring during the ‘one-shot’ process. The effect of the chemical constituents of the synthesised polyurethanes on the rate of degradation and biodegradation were also investigated. Comparison of polyurethanes synthesised with an aliphatic (H12MDI) and an aromatic isocyanate (MDI) resulted in an increase in the rate of alkaline hydrolysis with the use of H12MDI. This was found to be affected mainly by differences in the morphology, with an increase in microphase separation and a decrease in microcrystalline regions in the case of the use of H12MDI Polyurethanes were synthesised using different polyols; PEA, PCL, PEG and PCL/PEG (50:50) to investigate the effect of the polyol on the rate of biodegradation, where it was found that the polyurethane containing a combination of the two polyols, PCL/PEG (50:50), degraded under both accelerated hydrolysis conditions and soil burial. This was thought to be due to the combination of both hydrophilic (PEG) and hydrophobic (PCL) charactyers of the polyols, which had contributed to increasing the diffusion of water into the polymer matrix (hydrophilic PEG), and also to inducing the microbial degradation by hydrophobic interactions (PCL). The incorporation of the additives; iron stearate, cellulose and Cloisite 30B were examined as a means of increasing the degradation and biodegradation of the polyurethane polymers. Addition of iron stearate was found to decrease the thermal stability of the polyurethane, which resulted in an increase in polyurethane degradation under alkaline conditions at 45oC, and biodegradation under soil burial conditions at 50oC. The incorporation of cellulose into the polyurethane increased the rate of alkaline hydrolysis and biodegradation in soil. This polyurethane (PU CE) was also susceptible towards enzymatic degradation by Aspergillus niger. The incorporation of the organically-modified nanoclay Cloisite 30B has decreased the microcrystalline domain structure contained within the polyurethane, and this was found to decrease the rate of alkaline hydrolysis dramatically (degraded within 7 days).