Solving the plastic dilemma: the fungal and bacterial biodegradability of polyurethanes

Abstract

Polyurethane (PU) is a plastic polymer which, due to its various desirable characteristics, has been applied extensively in domestic, industrial and medical fields for the past 50 years. Subsequently, an increasing amount of PU waste is generated annually. PU, like many other plastics, is highly resistant to degradation and is a substantial threat to our environment. Currently PU wastes are handled through conventional disposal techniques such as landfill, incineration and recycling. Due to the many drawbacks of these techniques, a 'greener' alternative is necessary, and biodegradation appears to be the most promising option. Biodegradation has the potential to completely mineralise plastic waste or recover the input materials and better enable recycling. There are hurdles to overcome however, primarily the efficiency of the process and the presence of waste plastics with inherently different chemical structures. This review will focus on polyurethanes and their biodegradation, outlining the difficulty of degrading different versions of the same material and strategies for achieving more efficient biodegradation.

Keywords: Biodegradable polymers; Biodegradation; Plastic disposal; Polyurethane; Recycling.

Fig. 1

Schematics of typical polyurethane syntheses. A prepolymer is first produced by reaction between a diol and diisocyanate, forming regular urethane (NC(O)O) linkages. Chain extension is then performed on the addition of either additional diol or diamine. Modified from (Heath and Cooper 2013)

Fig. 2

PU degradation in the natural environment. Abiotic factors are major initiators of the degradation process (left); oxidation and/or hydrolysis lead to fragmentation and formation of microplastics. Microbes can adhere to the polymer at any point (left, centre) and further facilitate degradation reactions using enzymes (right). If degradation proceeds to the point of liberation of monomers or sufficiently small oligomers, microbes may internalise said fragments and metabolise them

Fig. 3

Possible hydrolysis of some types of bonds in PU. Heath and Cooper (2013)