Effect of lipases on the surface properties of wheat straw

Abstract

Surface characteristics of untreated and lipase-treated wheat straw (WS) have been investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and SEM–Energy dispersive X-ray spectroscopy (SEM–EDX). Static contact angle of WS surface was determined by the pendant drop method. The WS thermal stability was also investigated using the thermogravimetric analysis (TGA). It was shown that the lipases from Candida cylindracea insignificantly changed the chemical group, microscopic morphology, and wettability of the WS inner surface. However, FTIR showed the lipases could effectively remove the hydrophobic lipophilic extractives and silica from the outer surface of untreated WS, and increased the hydroxyl group content in the outer surface. SEM images also exhibited the lipases stripped off the dense hydrophobic layer on the WS outer surface with the help of shear force. SEM–EDX analysis showed that the lipase treatment reduced Si content on the WS outer surface from 12.44 to 1.33%. The water contact angle of the WS outer surface decreased from 92.7° to 65.2° after lipase treatment, which indicated that the wettability of the lipase-treated WS outer surface was equivalent to that of the inner surface with a water contact angle of about 63°. Lipase-treated and untreated WS had a similar thermal stability. Therefore, the lipase treatment was one of the potential methods to improve the wettability of natural fibers in composite material processing. Comparative testing of particleboards produced from the whole WS untreated and treated by the lipases is under way to evaluate the overall effects of the lipase treatment on the improvement in their mechanic performances.