Polyurethanes is the fifth most demanded synthetic polymers in Europe with an estimated demand of 3.5 billion tons in 2015. Polyurethanes are mainly used in insulation materials, refrigerators, freezers, furniture and bedding. They are formed mostly by polymerization of polyisocyanates and polyols together with additives such as catalysts, cross linkers and chain extenders, among others. Polyols can form ether or ester bonds, resulting in polyether or polyester polyurethanes. On the other hand, Polyisocyanate compounds can have an aliphatic, polycyclic or aromatic nature. Most commonly used polyisocynates for PU synthesis are 4,4′-methylene diphenyl diisocyanate (MDI) and toluene-2,4-diisocyanate (TDI) and their precursors 4,4′-diaminodiphenylmethane (MDA) and 2,4-diaminotoluene (2,4-TDA), respectively. Both of these amines, MDA and TDA have been proposed in the European Chemicals Agency to be identified as “Substances of Very High Concern,” specifically in the category of “Carcinogenic, Mutagenic or toxic to Reproduction.
Recycling of Polyurethanes, biological or chemical is a difficult process due to the diverse composition and complexity of Polyurethanes. Biodegradation of Polyester based PU have been reported by some researchers using fungal, bacterial and enzymatic hydrolysis. The biodegradation of synthetic polymers in general is a two-step process. The first step involves enzymatic degradation of polymers into monomer units which are mineralized in the cell.
“The fate of plastic waste and a sustainable use of synthetic polymers is one of the major challenges of the twenty-first century. Waste valorization strategies can contribute to the solution of this problem. Besides chemical recycling, biological degradation could be a promising tool. Among the high diversity of synthetic polymers, polyurethanes are widely used as foams and insulation materials. In order to examine bacterial biodegradability of polyurethanes, a soil bacterium was isolated from a site rich in brittle plastic waste”, writes the researchers.
The researchers isolated the strain from a soil samples taken from a site rich in brittle plastic waste. They identified the species as Pseudomonas sp. by 16S rRNA gene sequencing (Illumina) and membrane fatty acid profile (FAME). The strain was able to grow on a PU-diol solution, a polyurethane oligomer as the sole source of carbon and energy. In addition, the strain was able to use 2,4-diaminotoluene, a common precursor and putative degradation intermediate of polyurethanes as sole source of energy, carbon, and nitrogen.
The whole genome sequencing of the strain also revealed the presence of numerous catabolic genes for aromatic compounds. They found mono and di-oxygenases as main enzymes responsible for degradation. The researchers have deposited the genome sequence to GENBANK and named the strain as Pseudomonas sp. TDA1
Reference: Toward Biorecycling: Isolation of a Soil Bacterium That Grows on a Polyurethane Oligomer and Monomer. Frontiers in Microbiology.