Thermal Curing And Bonding Mechanism Of Lignin Based Adhesive

In order to clarify the curing mechanism of lignin-based adhesive during hot-pressing process and the bonding mechanism between lignin-based adhesive and wood veneer,in this study,lignin was isolated from wood with polyethylene glycol-400 as solvent and used as adhesive.The chemical structure and viscosity of lignin-based adhesive were investigated comprehensively.The lignin-based adhesive was utilized to prepare plywood without further modification.The effects of hot-pressing parameters on the mechanical properties and water-proofing properties of plywood were examined.Then the mechanical properties and formaldehyde release of plywood prepared with hexamethylenetetraamine(hexamine)were discussed.Futhermore,the curing mechanism of lignin-based adhesive and bonding mechanism between adhesive and veneer were also explored.The conclusion are as follows:1.The lignin was successfully isolated from wood with polyethylene glycol as solvent,and was utilized as adhesive without any modification to prepare plywood which reached the requirments of ?type plywood of Chinese national standard.2.The bonding strength,MOE and MOR of lignin-based adhesive plywood increased and then slightly decreased with the increasing hot-pressing pressure(When the hot-pressing pressure changes,the hot-pressing temperature and the hot-pressing time are 180? and 705s respectively)and hot-pressing time(When the hot-pressing time changes,the hot-pressing temperature and the hot-pressing pressure are 180? and 1.8MPa respectively),while the bonding strength increased with the increasing hot-pressing temperature(When the hot-pressing temperature changes,the hot-pressing pressure and the hot-pressing time are 1.8MPa and 705s respectively).With the increase of hot-pressing pressure and time and temperature,the water absorption and thickness swelling rate of water absorption of plywood showed the decreasing trend.3.During the hot-pressing processes,the decomposition of molecular structure of lignin-based adhesive occurred,promoting the entering of lignin-based adhesive into the porous structure existing on the surface of wood veneer.Lignin-based adhesive kept the curred status at the lowing temperature after hot-pressing process,which achieved the aim of bonding between adhesive and wood veneer.4.The bonding strength of plywood could sharply increased with the modification of hexamine and increased with the increasing of hexamine.Besides,the formaldehyde release increased with the increasing of hexamine(According to the lignin-based adhesive of ratios are 1%,2%,5%,10%,15%),and the lowest release could reach the E0 level.The bonding strength of plywood after modification of hexamine increased 91%compared with that without modification,and increased with the increasing of hot-pressing pressure(When the hot-pressing pressure changes,the hot-pressing temperature and the hot-pressing time are 190? and 705s respectively)and time(When the hot-pressing time changes,the hot-pressing temperature and the hot-pressing pressure are 190? and 2.0 MPa respectively)and temperature(When the hot-pressing temperature changes,the hot-pressing pressure and the hot-pressing time are 2.0MPa and 705s respectively).Furthermore,the MOE and MOR of plywood kept the increasing trend.5.The ether bonds of lignin-based adhesive decreased during hot-pressing process,and the methylene bonds were formed,resulting in the increase of cross-linkage structure and curring.Therefore,the modified lignin-based adhesive showed the deeper curring than that of unmodified ones,and also showed the higher bonding strength.6.The lignin-based adhesive in plywood interface under the hot-pressing pressure of 1.4 MPa was discontinuous,and obvious fault structure could be observed.The thickness of adhesive interface increased and no fault structure occurred when the hot-pressing pressure of 1.8 MPa was used.However,when the higher hot-pressing pressure of 2.2 MPa was used,the continuous adhesive in plywood was obtained but the thickness of adhesive decreased.Besides,the permeability of lignin-based adhesive on the cross-section and radial-section were improved.The flowability of adhesive after modification with hexamine was poorer than that of adhesive without modification.No obvious fault structure was observed and the thickness decreased after modification,which was caused by the lower flowability.7.The condensation reaction between the hydroxyl groups of wood and lignin-based adhesive occurred during hot-pressing process,leading to the appearance of new ether bonds and then the chemical bonding force increased.The results from Zeta electrical analysis showed that the electrical conductivity of lignin-based adhesive exhibited minus values before and after hot-pressing process.The much difference of electrical conductivity could be observed after hot-pressing process,which results in the bigger electrical repelling force between wood and adhesive.8.The bonding strength of plywood prepared with veneers treated by hydrothermal treatment,and firstly increased then decreased with the increasing of treatment temperatures.However,little change of bonding strength of plywood prepared with modified lignin-based adhesive with hexamine was obtained because of poor permeability of adhesive caused by the much cross-linkage in plywood.