Research On Preparation Of Non-formaldehyde Adhesive By Modified Brown-rotted Wood

The emission of formaldehyde during the production of and the use of wood composites bonded with resins pollutes the working and our living environment. China has lots of deserted wood decayed by brown-rot fungi because of Chinese medicinal materials—Poria Cocos planting, and brown-rot fungi decompose wood, leaving a modified, brown colored lignin residue, which provides a potential material for use, so it has the great future for making non-formaldehyde wood adhesive by deserted brown-rot wood. In this paper, brown-rot Pinus massoniana decayed by Poria Cocos is taken as the object of study. Firstly, the main chemical componets and the structural characterization of milled wood lignins (MWLs) in sound and brown-rot Pinus massoniana are described. Then brow-rot lignin is extracted by 1% NaOH and activated by NaBH4, Raney-Ni and tetralin. Thirdly, non-formaldehyde wood adhesive is prepared by modified brown-rotted wood with polyethylenimine, and adhesion properties are also tested. To investigate the mechanism of modification, scuffling and curing, samples are made to conduct FTIR, UV, 1H NMR, GPC, ESR spectroscopy, and chemical analyses methods. The main research results were summarized as following:(1) The Total phenol content in the brown-rotted wood is eight times the total phenol content in sound wood, but the methoxyl group content in brown-rot wood is half of that in sound. Compared with sound wood, the contents of holocellulose, pentosan cellulose, hemicellulose and relative crystallinity reduce 63.87%, 35.92%, 65.00%, 61.81% and 51.86%; but the contents of acid-insoluble lignin, alcohol-benzene solubility and 1% NaOH extractive rise 82.20%,507.14% and 383.24%.(2) Milled wood lignins (MWLs) are isolated from sound and naturally brown-rotted Pinus massoniana by Bj?rkman and Lundquist techniques. The total acidic group, carbonyl, carboxyl, and phenol hydroxyl contents in the B-MWL are near two times higher than those in the S-MWL. Based on their methoxyl, elemental analyses and 1H NMR, the C9 formulas are calculated for sound and brown-rotted lignin as C9H8.78O2.85 (OCH3)0.83 and C9H8.63O3.01 (OCH3)0.71. Both lignins are mainly guaiacyl type lignin, andβ-O-4 structures constituted the main intermonomeric connections. The molecular weights of two lignins are mainly distributed between 3000 and 10000, and the MWLs of brown-rot wood have relatively lower molecular weights than those in sound wood. The observed spectral parameters in both lignins indicated that the spectrum could be assigned to the phenoxy radicals, and the overall concentration of stable free radicals in B-MWL is slightly less than that in S-MWL.(3) After modification by NaBH4, at the condition of 2% of NaBH4 dosage(based on dry weight of lignin), reaction time 6 h and room temperature, the contents of total hydroxyl, alcoholic hydroxyl and phenolic hydroxyl in lignin are 21.09%,17.26%,3.83%. The aliphatic hydroxyl content and total hydroxyl content in brown-rotted lignin increase 89.46% and 59.05%, but phenol hydroxyl content dose not increase much. Based on FTIR, UV and 1H NMR, NaBH4 reduces the carbonyl in side chain of lignin into hydroxyl with polymerization and demethylation, moreover, the molecular weights in lignin after reduced by NaBH4 increase to some extent.(4) Brown-rotted wood is also activated using Raney-Ni as catalyst and tetralin as hydrogen-donor. This method activates the wood and increases the hydroxyl content in lignin. Under the same condition, the content of total hydroxyl in 280℃is maximum, 17.40%, and the content of phenol hydroxyl in 260℃is maximum, 4.18%. After activated by Raney-Ni and tetralin, the contents of three kinds of hydroxyl increase, which means Raney-Ni and tetralin have activation effect to lignin. However, more trials are needed to optimize the technology of extracting and separating the activated lignin.(5) The adhesive is prepared by modified brown-rotted wood with polyethylenimine. The result shows that under the same conditions, the bond strength (0.80MPa, wood failure 72%) of the plywood produced under 0.2% NaBH4 dosage(based on dry weight of lignin), and the bond strength (0.86MPa, wood failure 29%)of plywood made under molecular weight 70000 PEI, both meet the requirement of the Chinese National standard, for type II general purpose plywood. In addition, the free formaldehyde content is 0.028g/kg according to GB18583-2001, which is substantially below the standard. Based on FTIR and TG-DTA analysis, the reaction mechanism of modified brown-rotted wood with polyethylenimine is determined. The partial carbonyls in modified brown-rotted lignin bond to the primary and secondary amines in polyethylenimine, which results in the formation of a highly cross-linked water-insoluble lignin-PEI network Copolymer. And the lignin-PEI adhesive first penetrates wood and then solidifies during hot-pressing.(6) The method of spreading the materials separately is studied. Brown-rot lignin extracted by 1% NaOH and polyethylenimin, acetylated brown-rot lignin and polyethylenimin are separately spreaded on the veneer and made into plywood. Under the same condition, the bond strength (0.81MPa, wood failure 42%) of plywood made by acetylated brown-rot lignin and polyethylenimin meets the requirement of the Chinese National standard, for type II general purpose plywood.