Effects of flax lignin addition on enzymatic oxidation of poly(ethylene adipate) urethanes

Lignin use in polymer industry has recently become very attractive from both economical and environmental reasons. In the specific case of blending, the addition of low lignin amounts was found to improve the properties and extends the application field of other natural or synthetic polymers, but the effects of oxidative enzymes on resulting blends are widely unknown. Thereby, the current study was carried out to determine some potential effects of small flax lignin concentrations (4.2 and 9.3 wt%) on the enzymatic degradation of a poly(ethylene adipate) urethane (PU). Thin cast films of PU and its lignin blends were incubated for 3 days at 30 °C with buffered solutions of fungal peroxidase and laccase extracted from Aspergillus sp., and compared with the untreated ones. Changes in surface structure and morphology were investigated by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR) and scanning electron microscopy (SEM), while the impact on bulk was assessed from tensile tests and thermogravimetry analysis (TGA). Although the addition of flax lignin reduces the surface structural modifications after enzyme treatment, the morphology, tensile and thermo-oxidative characteristics are still affected, with laccase showing the higher degradative efficiency. The lignin concentration, its high impact on the resulting blends morphology and relative low resistance to laccase and peroxidase degradation was the most important factors proved to driven the enzymatic oxidation.

► PU blends containing up to 10 wt% and preferably 4–5 wt% flax lignin presents enhanced mechanical properties and high susceptibility to enzymatic oxidation.
► Lignin influence the enzymatic oxidation by changing the blends morphology, and by its high degradation rates and relative numerous functional groups available for both physical associations and crosslinking reactions.
► Fungal laccase was more effective than peroxidase at degradation of PU and its corresponding blends.
► This work represents one of the first studies on oxidative degradation of both PU and lignin blends under the action of fungal enzymes.