Synthesis, characterization and biodegradation studies of polyurethanes: Effect of unsaturation on biodegradability

Abstract

The presence of unsaturation in the main chain of the polymer promotes the biodegradation process. To elucidate this hypothesis, one unsaturated polyurethane (PUU) and another saturated polyurethane (PUS) were synthesized and then biodegraded, and evidence was found to support this hypothesis. The polyurethanes were synthesized by a polycondensation reaction with yields up to 97%. It is important to note that both polyurethanes were constituted only by the recalcitrant hard segment and showed low crystallinity and molecular weight. Spectroscopic, thermal, and chromatographic techniques were used for physical and structural characterization. Both polyurethanes were biodegraded by the BP8 microbial community and the Cladosporium tenuissimum A3.I.1 fungus during a two-month period. A postbiodegradation characterization revealed the detriment of properties in both materials, indicating successful biodegradation. As a general trend, more efficient biodegradation was observed by the Cladosporium tenuissimum fungus A3.I.1 than by the BP8 microbial community. Specifically, with the fungus, the infrared analysis showed a decrease in the characteristic bands as well as the appearance of new carboxylic acid signals (approximately 1701 cm^-1), suggesting the enzymatic cleavage of the urethane group. By comparison to polyurethanes, PUU showed superior biodegradation; using the fungus, a 51% decrease in molecular weight (M_w) for PUU was achieved, in contrast with 36% achieved for PUS. Despite the low crystallinity and molecular weight, the determining factor in biodegradation was the presence of unsaturations along the main chain. Thus, a more efficient oxidative attack is carried out by microorganisms on double bonds. The novel PUU showed similar biodegradation to the known polyester-type PU with highly hydrolysable groups. Consequently, PUU represents a green alternative to conventional polyurethanes and is a key material to achieve biorecycling.

Keywords: Biodegradation; Biorecycling; Environmental microorganisms; Environmentally friendly polyurethane; Polyurethane synthesis; Unsaturation effect.