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

Abstract

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 acidmodified-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.

Thermal modification of soy proteins in the vacuum chamber and wood adhesion

“…Since pH adjustment enables the adhesive penetration, the improvements in adhesive bonding strength with pH adjustment could be expected. In addition, previous researches showed that denaturation of soy proteins with NaOH increased their adhesive bonding strength and water resistance Sun and Bian 1999;Mo et al 2004;Ciannamea et al 2012). Alkali can break internal hydrogen bonds in the coiled protein molecule, which in turn unfolds and exposes many polar groups (hydroxyl and carboxyl side-chain groups), able to bond with exposed hydroxyl groups from the ligno-cellulosic substrate (Ciannamea et al 2010).…”

“…However, the effectiveness of natural adhesives is often inferior compared to the synthetic ones. Therefore, the development of adhesives based on natural materials has focused on improving the mechanical properties and water resistance of the adhesive bonds with different processes, namely physical, chemical and enzymatic modifications, and extraction of materials in a form containing a high proportion of protein such as soy protein isolate (SPI), glycinin or conglycinin extracted from soy flour Kalapathy et al 1995Kalapathy et al , 1997Sun and Bian 1999;Huang and Sun 2000a, b;Zhong et al 2002;Kumar et al 2004;Mo et al 2004Mo et al , 2006Zhang and Hua 2007;Sun 2008, 2010;Li et al 2009;Ciannamea et al 2010Ciannamea et al , 2012Liu et al 2010;Qi et al 2012). Soy proteins have been recently considered as a petroleum polymer alternative in the manufacture of adhesives, plastics and various binders (Zhong and Sun 2000).…”

“…Tanford (1968) described denaturation mechanisms of soy proteins with different chemicals such as NaOH, urea, guanidine hydrochloride (GuHCl) and detergents. Later, researchers investigated the influences of different denaturants on the adhesive properties of soy protein adhesives Kalapathy et al 1995Kalapathy et al , 1997Sun and Bian 1999;Huang and Sun 2000a, b;Zhong et al 2002;Kumar et al 2004;Mo et al 2004;Zhang and Hua 2007;Liu et al 2010;Ciannamea et al 2012;Qi et al 2012). Physical modification such as heating of dispersion during preparation, adding chemical modifiers such as NaOH, urea, guanidine hydrochloride, citric and boric acid, anionic and cationic detergents, sodium sulfite and sodium bisulfite, calcium carbonate, and adding enzymatic modifiers such as papain, urease and trypsin, have been shown to enhance adhesive properties of soy proteins.…”

“…The bonding strength of a protein adhesive also depends on the solid content and the viscosity of the dispersion. There are three factors, partially dependent on the viscosity that an adhesive needs to fulfill to form a proper bond: wet the surface, flow over and penetrate into the substrate without losing the adhesiveness between particles (Ciannamea et al 2012). Adhesive penetration into the substrate plays an especially important role in wood adhesion, because wood is a porous material.…”

Bonding of THM modified Moso bamboo (Phyllostachys pubescens Mazel) using modified soybean protein isolate (SPI) based adhesives

“…The fact that the bond strength of the non-porous THM bamboo was lower suggests that mechanical interlocking is also a strong contributor to bond strength. During the adhesive preparation stage the addition of alkali (pH adjustment to 10) causes partial denaturation of the tightly coiled protein molecules by breaking down hydrogen bonds Mo et al 2004;Ciannamea et al 2010Ciannamea et al , 2012. This increases the contact area of protein molecules and opens up many more polar groups for bonding with polar groups on the wood substrate (Ciannamea et al 2010).…”