Bonding Quality of Chemically-Modified Soybean Protein Concentrate-Based Adhesives in Particleboards from Rice Husks

The aim of the present study was to upgrade the bonding quality and water resistance of medium-density particleboards based on rice husks (RH) as a wood substitute and soybean protein concentrate (SPC) as the binder via chemical modification of SPC. Alkali (A), citric acid (CA) and boric acid (BA) were used to modify proteins and the carbohydrate complex in SPC. The effect of chemical treatment performed on SPC was followed by Fourier transform infrared, differential scanning calorimetry, thermo-gravimetric analysis and initial apparent viscosity measurements. Board properties were evaluated in terms of internal bond (IB) and physical properties. Results revealed that boards bonded with SPC treated with boric acid, exhibited the highest IB and the lowest water absorption and thickness swelling at 2 and 24 h, due to cross-linking reactions with exposed OH-groups in the amorphous region of cellulose of RH. Results demonstrate that boric acid-modified–SPC-bonded boards met the requirements of IB recommended by the US Standard ANSI A208.1-2009 for M1, MS, M2 and M3-grade medium-density particleboards but failed to pass the thickness swelling required. This issue of BSPC-RH boards is compensated for by the benefit of being formaldehyde-free which makes them suitable for indoor applications.