Chemical Crosslinking Of Soybean Protein And Its Effects On The Properties Of Soybean Protein-based Wood Adhesives

The sustainable development is obtaining more and more attentions with the ever-increasing consumption of non-renewable nature resources and more and more environmental pollution. Because the traditional UF, MF and PF wood adhesives rely on petroleum resource, and the toxic release components will be released from these adhesives during the production, storage and utilization, the R&D and application of environmentally friendly bio-adhesives become a focus in recent years. Soybean protein obtained from abundant renewable soy resource can be used to prepare wood adhesive, however, its poor water resistance and technological applicability prevent soy protein adhesive from wide application in the wood industry.In order to improve technological applicability of soy protein adhesive, a degraded soy protein (DSP) with high solid content and controllable viscosity was prepared via thermal-alkali degradation. The results indicated that the thermal-caustic degradation could break the molecular chains and globular structures of soy protein, by which increase its solubility and improve the reactivity and technological applicability of soy protein adhesive. Due to the fact that larger-molecular-weight soybean proten molecular chains were seriously cut down, the water resistance of soybean protein adhesive became worse. Therefore, it is necessary to re-build their larger molecular weight and crossliking structures for improving the water resistance of the soybean-based wood adhesive.As a result, a melamine-urea-formaldehyde resin and a sodium bisulfite-blocked polyisocyanate (PAPI) were developed as the chemical crosslinking agent for degraded soybean protein. The effect of MUF resin and its content on the properties of soybean protein adhesive was investigated, by which a soybean protein adhesive met the requirement of Type-II plywood according to Chinese Standard GB 9846-2004 was developed. Test results showed that the chemical reaction between DSP and MUF could form crosslinked structure and therefore improve the adhesion strength of soybean adhesive. The DSP could reduce the formaldehyde release of soyprotein adhesive-bonded plywood. The blocking technology of polyisocyanate and its influences on the properties of soybean protein adhesive were also investigated, by which another environmentally friendly soybean protein adhesive was developed. Test results indicated that the blocking agent NaHSO3 could effectively block PAPI and therefore prolonged its work life (or shelf life) of the adhesive. During the hot pressing, the blocked PAPI could be re-activated and then react with DSP to form a net-work structure, by which improve the water resistance of soybean protein adhesive with the wet bonding strength of plywood reached 0.8-1.0MPa. The obtained plywood met the requirement of type Ⅱ panel according to Chinese Standard GB 9846-2004.In general, both MUF and blocked polyisosyanate could improve the water resistance of their modified soybean protein adhesives. However, the soybean protein adhesive modified by MUF would release formaldehyde that was beyond the requirement for grade Eo plywood, while and the soybean protein adhesive modified by blocked polyisosyanate had shorter working life (for about 2-5h). This study gave the new way out for improving the water resistance of soybean protein adhesive, especially the blocked polyisocyanate- modified soybean protein adhesive showed good application potential for its environmental safe.