Preparation And Properties Of Formaldehyde-Free Adhesive For Wood-processing

So far, Urea–formaldehyde(UF) and phenol–formaldehyde(PF) resins are still dominant in the wood-adhesive market. Wooden products where UF and PF are used will inevitably release formaldehyde, so producing the negative effects on human health and environment. Therefore, it is of great significance in theory and practice to design and prepare environmental-friendly formaldehyde-free adhesives with excellent properties via the chemical modification of lignin as an important renewable biomass resource.Inspired by the molecular structure of mussel adhesive protein having excellent wet bonding strength, a modified lignin having catechol moiety, M-Lignin, were prepared using renewable lignin from wood as a raw material, which is cheap and easily available. Two groups of formaldehyde-free adhesives M-Lignin/PEI and M-Lignin/PU were designed and prepared by choosing polyethylenimine(PEI) and polyurethane(PU) as another component of adhesives, respectively. The effects of the phenolic hydroxyl content in M-Lignin molecular chains and the adhesive compositions on the bonding properties and thermal property of the prepared adhesives were systematically investigated, and then the design and preparation principles of the formaldehyde-free lignin adhesives with catechol moiety were revealed. Specific research contents and results are as follows.According to the mechanism of the nucleophilic substitution reaction, enzyme-hydrolyzed lignin, EH-Lignin, was first demethylated in the presence of a nucleophile C12H25 SH and a catalyst CH3 ONa, thus preparing a series of M-Lignin samples with different phenolic hydroxyl contents. The effects of the amounts of C12H25 SH and CH3 ONa in the reaction system on the demethylation of lignin were analyzed by measuring the phenolic hydroxyl content to confirm optimal reactant system for preparing M-Lignin with high phenolic hydroxyl content. The effects of the phenolic hydroxyl content on the structure, polarity and thermal stability of M-Lignin were studied. The results indicate that in the optimal reactant system for preparing M-Lignin, the mass ratio of EH-Lignin to C12H25 SH is 1:2 and the mole ratio of C12H25 SH to CH3 ONa is 1:2.5. The as-prepared M-Lignin can contain 7.24 wt % of phenolic hydroxyl, which is 2.23 times as high as that of EH-Lignin, and its weight average molecular is 830, which is decreased by 32.90 % compared with that of EH-Lignin. Besides, M-Lignin possesses stronger polarity than that of EH-Lignin, and its thermal stability does not change significantly.Based on the above results, a series of M-Lignin/PEI and M-Lignin/PU adhesives, where the phenolic hydroxyl content of M-Lignin or the M-Lignin content was different, were prepared through the combination of M-Lignin with PEI containing abundant amino groups and having high reactivity or PU having strong designability of maxromolecular structure. The dry bonding strength(?dry), water-soaking bonding strength(?WST), hot-water bonding strength(?HWT) and boiling-water bonding strength(?BWT) of the prepared adhesives were measured by referring to the national standard concerned. The effects of the phenolic hydroxyl content of M-Lignin and the M-Lignin content on the bonding properties and thermal property of the adhesives were investigated. Based on these studies, a change in the molecular structure of the cured M-Lignin/PEI was characterized by FTIR, and the bonding mechanism of M-Lignin/PEI adhesives was analyzed.The results indicate that the M-Lignin/PEI sample P-7 prepared using 75 wt % M-Lignin having 7.24 wt % phenolic hydroxyl content can exhibit an optimal water resistance, whose ?dry, ?WST, ?HWT and ?BWT are 4.52 MPa, 3.18 MPa, 1.97 MPa and 1.46 MPa, respectively; and its thermal stability declines to some extent. The M-Lignin/PU sample PU-5 prepared using 33 wt % M-Lignin having 7.24 wt % phenolic hydroxyl group can possess the optimal bonding property and thermal stability, whose ?dry, ?WST, ?HWT and ?BWT are 3.99 MPa, 2.75 MPa, 1.94 MPa and 0.98 MPa respectively, and it exhibits an optimal thermal stability, whose Td,max is raised from 368.9 ?C(PU-0) to 392.4 ?C. The ?BWT values of M-Lignin/PEI and M-Lignin/PU adhesives prepared in this work are higher than 0.7 MPa, which exceeds the requirement for Class I plywood prescribed by the national standard.