Developing The Wood Adhesive From Defatted Soy Flour

The main disadvantage of soybean-based adhesive is its poor water resistance.This paper attempted to develop an environmental friendly wood adhesive with relatively strong water resistance from defatted soy flour (SF).Two modification methods were designed to improve the water resistance. In the first program, soy flour was denatured by sodium dodecyl sulfate (SDS) first and then glutaraldehyde (GA) was used as a cross-linking reagent. NaOH was applied in another plan as a denaturing reagent, and then enzyme (Microbial transglutaminase, MTGase) was introduced to form covalent bond.The results of the first program showed that the water resistance of SDS-denatured SF was improved significantly when pH was 5.0. The optimum combination was pH=5.0,SDS usage=0.5wt.% (based on the mass of defatted soy flour),reaction temperature=30.0℃and reaction time=4.0h. GA played an important role in enhancing the bonding property. When the pH of SDS-denatured SF solution was adjusted to 12.0, the shearing strength of the resulted adhesive was improved up to 0.68 MPa (According to GB/T 9846.1～■9846.8-2004《Plywood》standardⅡ).When reaction time was controlled within 1.0h, water resistance increased with the increasing of GA usage. Excessively longer reaction time would result in high viscosity, leading to poor bonding strength. Viscosity of modified adhesive increased as storage time extended and shear-thinning phenomena was observed after mechanically mixing, however, a good bonding strength was still kept(above 0.65MPa).The result of Thermo gravimetry(TG)analysis illuminated that the resulting product did not thermolysis until 220℃.The analysis of the Fourier transform infrared spectroscopy (FT-IR) and Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that chemical crosslinkage have been formed among uncoiled soy protein molecules.In the second method, the result of a Lg(34) experiment showed that reaction environment (pH,treatment time and treatment temperature) had an important effect on soybean protein denaturation. pH value palyed a significant role in SF denaturation, and the optimized formula of alkali denaturation was pH=9.5,reaction temperature=50.0℃and reaction time=0.5h. On the basis of determined denatured condition, another L9(34) orthogonal experiment was performed to investigate the effects of MTGase usage, pH and reaction time on the water resistance of modified SF adhesive. The results revealed that the MTGase addition was an essential factor. The highest shearing strength (0.54MPa) was gained when MTGase dosage 2.0U/g was used, and the viscosity would growed fastly with the increasing of MTGase. Super bonding strength obtained when pH and reactin time were adjusted to 8.0 and 75.0 min, respectively. After storaging for several hours, the modified SF adhesive became sticky and could not flow any more. Then viscosity could decrease via rapid mechanical agitation, and the adhesive samples remained desirable bonding performance (above 0.50MPa) with 63.0℃water treatment. The result of TG exhibited that heat degradation reaction did not occurred before 220℃.SDS-PAGE and FT-IR analysis indicated that the relative molecular mass of modified soybean protein mocules increased, which illustrated that chemical cross-linking bond have formed intermolecularly.