Glutaraldehyde-crosslinked soy protein–wood biocomposites: chemical interactions and property optimization

In this work, ternary wood bioplastic composites (WBPCs) were prepared by mixing refined wood sawdust (SD), soy protein isolate (SPI) and glutaraldehyde (GTA). The physicochemical and mechanical properties of these composites were determined by flexural, hardness, and environmental resistance tests. Optimal mechanical strength and hardness were observed for GTA/SPI ratios between 0.19 and 0.38. High resistance to biological degradation was obtained for GTA/SPI ratios greater than 0.5. The WBPCs, along with binary SPI-GTA, SD-GTA and SD-SPI samples, were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG). The objective was to establish the chemical interactions between the SPI and GTA, and eventually with the lignocellulosic compounds, to relate them to the changes in properties as the GTA/SPI ratio varied. It was established that chemical interactions occur between GTA carbonyl groups and SPI primary amine groups, since evidence of imine (C=N) formation was found. Thus, crosslinking reactions between GTA and protein chains can take place to obtain a network structure, which would explain the improved mechanical properties of WBPCs for GTA/SPI < 0.4. For samples with GTA/SPI > 0.5, free GTA was detected and branched chains may have formed instead of a network structure. In both cases, GTA in excess led to a composite with reduced mechanical properties but higher resistance to biological degradation and deterioration. In addition, it was determined there were chemical interactions between GTA carbonyl groups and OH groups of cellulose and hemicellulose, which explained the increase in the hydrophobicity of sawdust when GTA was added to the composites.