| Plant fibers are very important most abundant reproducible macromolecular resources for the subsistence of the mankind. Except for cotton fiber, most plant fiber consists of a matrix high molecular compounds of cellulose, hemicellulose and lignin, wich have no hot melting properties to meet the process of extrusion like plastics. Traditional wood processing is to apply intact cell wall with compound middle lamella (CML), which demands high quality wood material and very limited in products. The moden technologies based on destroy of compound middle lamella are much more popular in utilization of natural macromolecular material as plant fibers. By these processes, paper can be formed through hydrogen bonding or make fiberboards by association of free fibers and by wood adhesive Therefore, either hydrogen bonding or synthetic resin adhesive has the disadvantage of low physical strength and chemical stability as compared with the bonding way of CML in natural wood, and their inherent disadvantage limits the application fields.In the world, Urea-formaldehyde resin (UF), Phenol-formaldehyde resin (PF) and melamine formaldehyde resin (MF) have great demands, which are widely used to enhance the mechanical properties of fiberboard and paper (especially wet strength), but the source of these adhesives are nonrenewable petroleum products. Moreover, these adhesives are not environmental friendly, because UF, PF and MF will release free formaldehyde during the use of fiberboard [1], which is very harmful for human health as invisible killer, especially for indoor decoration. Formaldehyde can denature protein through combination with amino group in protein, and disrupt metabolism of cell, cause severe harm to human cells and cancer [20, 21]. In order to eliminate pollution of formaldehyde of fiberboard completely, development of fiberboard and paperboard with formaldehyde-free adhesive like lignin is promising. The progress in wood chemistry and enzyme engineering make it possible to realize the"drug-free"and"adhesives-free''in manufacture of wood-based panel industry and special paper[2].In this research, we simulated the lignification process in the initial stage of cell wall. Lignin in fibers, kraft lignin, lignin precursor (coniferin), p-hydroxycinnamic acid and carbohydrates were polymerized with fibers through catalyzer of lignin oxidase (laccase). Then, fiberboard and paperboard were produced. In order to understand the changes of chemical bonds between carbohydrate and additive mentioned above, carbon-13 isotope enrichment technique was applied to trace the crosslinking of lignin precursor (coniferin) between fibers. SEM and CP/MAS 13C-NMR etc. were used to determine the structure of lignin and polysaccharides with the treared fiberboard sample.In the present research, coniferin-[α-13C] was polymerized with fibers of Bio-chemimechanical pulp from wheat straw with catalyzer of lignin oxidase (laccase) andβ-glucosidase. The morphological change of lignin in the process of lignin deposit in cell wall and the obtained DHP-pectin complexes were analyzed by 13C-enrichment combined with CP/MAS 13C-NMR and SEM. Firstly, coniferin-[α-13C] was synthesized. In the presence of lignin peroxidase andβ-glucosidase, dehydrogenation polymre (DHP) and DHP-pectin complexes (DHP-PC) were obtained by dehydrogenation polymerization reaction together with pectin. The CP/MAS 13C-NMR and SEM were used to determine the morphology and chemcial structure of complexs and crosslinking between fibers after treatment. The subtance which was similar to lignin in middle lamella of plant cell (DHP-CC) was re-built between lignin and polysaccharide as pectin and xylan in fibers through benzyl ether linkages, ester bonds and minor acetal linkages mainly occurred atα-position of phenylpropane units. It is also found thatβ-O-4,β-βandβ-5 were main substructures in DHP-carbohydrate complexes (DHP-CC). Wheat straw Medium Density Fibreboard (MDF) could form strong chemical bond by cross-linking among free fibers. The ashesive subtance is similar to lignin in middle lamella of plant cell. Therefore, it can improve mechanical properties of MDF and reduce thickness expansion rate of water absorbing (TS) markedly.The p-hydroxycinnamic acid was polymerized with fibers of Bio-chemimechanical pulp from wheat straw in the existence of catalyzer of lignin oxidase (laccase) andβ-glucosidase. Its application in fiberboard and paperboard was also investigated. Firstly, the synthesis route of p-hydroxycinnamic acid and optimized the technological condition were studied. The p-hydroxycinnamic acid was synthesized from p-hydroxybenzaldehyde and malonic acid by Knoevenagel condensation reaction in toluene and pyridine using piperidine and aniline as the catalyst. As compared with conventional ways, reaction time was reduced and the yield was increased. Wheat-straw MDF were prepared from the treatment of p-hydroxycinnamic acid and pectin in the presence of laccase. When the amount of p-hydroxycinnamic acid was 4%, the results showed that mechanical properties of MDF prepared almost reached the First Grade Standard of GB/T 11718-1999. Corrugating medium from wheat straw was also prepared after the treatment of p-hydroxycinnamic acid in the presence of laccase. The results showed that breaking length, ring-crush strength and bursting index had an increasing trend with increases of p-hydroxycinnamic acid, these properties increased significanly by 48.1%, 67.3% and 44.7% respectively compared with the untreated control sample and increased by 37.2%, 60.2% and 33.9% respectively compared to the sample treated with laccase only. As a result, mechanical properties of corrugating medium of wheat straw prepared could meet A Grade Standard of GB13023-91.The Kraft lignin of wheat straw was polymerized together with fibers of Bio-chemimechanical pulp from wheat straw catalyzed by lignin oxidase (laccase). The results showed that wheat-straw MDF could be prepared after the treatment of the Kraft lignin of wheat straw in the presence of laccase. When the content of the Kraft lignin of wheat straw was 20%, the mechanical properties of MDF prepared could meet the First Grade Standard of GB/T 11718-1999. With the same way, corrugating medium of wheat straw were prepared from the treatment of the Kraft lignin of wheat straw in the presence of laccase, when the content of the Kraft lignin of wheat straw was 8%, breaking length, ring-crush strength and bursting index increased obviously by 31.0%, 44.9% and 22.9% respectively as compared with the untreated control sample, and increased about 21.4%, 39.2% and 13.8% respectively compared with the paperboard treated by laccase only. As a result, mechanical properties of corrugating medium of wheat straw prepared could meet A Grade Standard of GB13023-91. The corrugating medium of wheat straw was prepared from the treatment of the Kraft lignin of wheat straw with laccase only. When the content of the Kraft lignin of wheat straw was 2%, the properties increased about -0.5%,2.1% and -1.2% respectively as compared with the untreated control sample. The breaking length, ring-crush strength and bursting index of corrugating medium of wheat straw from treatment of the Kraft lignin of wheat straw in the presence of laccase increased about 31.7%,42.0% and 24.3% respectively as compared with the treatment with the Kraft lignin only. The research develops a new way for the application of the Kraft lignin as strength additive in fiberboard and paperboard.
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