Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams

Abstract

Polyurethane (PU) is widely used in many aspects of modern life because of its versatility and resistance. However, PU waste disposal generates large problems, since it is slowly degraded, there are limited recycling processes, and its destruction may generate toxic compounds. In this work, we isolated fungal strains able to grow in mineral medium with a polyester PU (PS-PU; Impranil DLN) or a polyether PU (PE-PU; Poly Lack) varnish as the only carbon source. Of the eight best Impranil-degrading strains, the six best degraders belonged to the Cladosporium cladosporioides complex, including the species C. pseudocladosporioides, C. tenuissimum, C. asperulatum, and C. montecillanum, and the two others were identified as Aspergillus fumigatus and Penicillium chrysogenum The best Impranil degrader, C. pseudocladosporioides strain T1.PL.1, degraded up to 87% after 14 days of incubation. Fourier transform infrared (FTIR) spectroscopy analysis of Impranil degradation by this strain showed a loss of carbonyl groups (1,729 cm(-1)) and N-H bonds (1,540 and 1,261 cm(-1)), and gas chromatography-mass spectrometry (GC-MS) analysis showed a decrease in ester compounds and increase in alcohols and hexane diisocyanate, indicating the hydrolysis of ester and urethane bonds. Extracellular esterase and low urease, but not protease activities were detected at 7 and 14 days of culture in Impranil. The best eight Impranil-degrading fungi were also able to degrade solid foams of the highly recalcitrant PE-PU type to different extents, with the highest levels generating up to 65% of dry-weight losses not previously reported. Scanning electron microscopy (SEM) analysis of fungus-treated foams showed melted and thinner cell wall structures than the non-fungus-treated ones, demonstrating fungal biodegradative action on PE-PU.

Importance: Polyurethane waste disposal has become a serious problem. In this work, fungal strains able to efficiently degrade different types of polyurethanes are reported, and their biodegradative activity was studied by different experimental approaches. Varnish biodegradation analyses showed that fungi were able to break down the polymer in some of their precursors, offering the possibility that they may be recovered and used for new polyurethane synthesis. Also, the levels of degradation of solid polyether polyurethane foams reported in this work have never been observed previously. Isolation of efficient polyurethane-degrading microorganisms and delving into the mechanisms they used to degrade the polymer provide the basis for the development of biotechnological processes for polyurethane biodegradation and recycling.

FIG 1

Growth and Impranil degradation by some of the selected fungal strains. (a) Growth of fungal strains showing different types of development in MM with Impranil. Left tube, control; right tube, fungus-inoculated medium. (b) Quantitative analysis of Impranil degradation by the 15 fungal isolates showing the highest degradation levels. All data were normalized to the negative control. The eight darker-shaded columns represent the strains that were selected for further analysis. n = 3. The error bars represent standard deviations. Statistically significant differences are indicated by a versus a′ and b versus b′.

FIG 2

Representative FTIR spectroscopy analysis of Impranil degradation by fungi. Spectra of MM with Impranil noninoculated and inoculated with Cladosporium pseudocladosporioides strain T1.PL.1 after 14 days of incubation. All the eight best Impranil-degrading fungi presented similar results. Changes reflecting Impranil degradation are observed in the signals at 1,729 cm⁻¹ (non-hydrogen-bonded urethane C=O stretch and ester from polyol fraction), 1,540 cm⁻¹, and 1,261 cm⁻¹ (urethane C—N—H bending and stretch, respectively). Signal assignments are based on references and .

FIG 3

Fungal growth of Cladosporium asperulatum BP8.I.3 over and inside PE-PU foam. Representative progression of fungal growth is shown by one of the best Impranil-degrading strains, from initial colonization (1 day) on the surface of foams to extensive invasion (12 days) on the whole piece of foam. The growth on foam B (without flame retardant) is shown.

FIG 4

Scanning electron micrographs of fungal growth inside PE-PU foam. Hyphal networks are observed in samples of foam pieces incubated for 7 days with three of the best Impranil-degrading fungi: Cladosporium tenuissimum strains A2.PP.5 and A3.I.1 and C. pseudocladosporioides strain T1.PL.1.

FIG 5

Biodegradation of PE-PU foams by the best Impranil-degrading fungi. (a) Effects on size and form of PU foams after fungal treatment. Foam A (upper row) and foam B (lower row) were incubated with the four best Impranil-degrading fungi for 21 days. Each pair of foam pieces presents the control foam (left) compared to the fungus-treated piece (right). (b) Quantitative analysis of PE-PU foam degradation (represented as weight loss) by the best Impranil-degrading-fungi. Each value was normalized by subtracting the weight loss of noninoculated controls. n = 3. The bars represent standard deviations. For foam A degradation, an asterisk denotes a statistically significant difference. For foam B degradation, statistically significant differences are indicated by a versus a′, b versus b′, and c versus c′.

FIG 6

Scanning electron micrographs of PE-PU foam ultrastructure after fungal growth. Foam pieces were incubated during 21 days with three of the best Impranil-degrading fungal strains: Cladosporium tenuissimum A2.PP.5 and A3.I.1 and C. pseudocladosporioides T1.PL.1; after that time, mycelia were destroyed to analyze their effect on PU structure.