Study On Improving Cross-linking Between Plant Fibers By Oxidative Activation Of Lignin

Wood supply shortage in China has restricted the development of production fiber products (fiberboard, paper, etc.). However, China is rich in agricultural residues. Research and development on non-wood fiber material is a effective way to solve the promlem between the supply and requirement of wood. On the other hand, chemical additives which can cross-link with fiber, such as urea-formaldehyde resin (UF) and melamine formaldehyde resin (MF), are usually used to improve adhesion between fibers in the production of fiberboard and paperboard. However, because release of free formaldehyde during use will bring harm to people's health, biological methods is a promising way to enhance the strength of fiber products effectively without polution. In this thesis, laccase and Fenton reagent activated lignin of fiber surface and plus lignin to improve adhesion between fibers, with a view to increase the fiber products strength.The results showed:(1) After laccase activated fiber surface lignin and acelosolv lignin, internal bond of fiberboard increased 89.5% as compared with the blank sample. Thickness swelling reduced 69.4%. The optimal conditions was as follows: laccase dosage 30 U/g, pH value 4.5-5.5, acelosolv lignin dosage 8%, 45℃, 2 h. Under this conditions, TS, MOR, MOE and IB reached 18.5%, 27.8 MPa, 3.2 GPa and 0.84 MPa, respectively. Laccase combined with acelosolv lignin could improve the wet and dry strength of paperboard, in particular the wet strength. Wet tensile index increased by 2.2 times and wet burst index increased by 2.2 times. The optimal enzymatic treatment conditions of improving wet strength was as follows: laccase dosage 20 U/g dry pulp, pH value 5.0, acelosolv lignin dosage 1.5%. Under the optimal condition, mechanical properties of corrugated medium of wheat straw CMP pulp could meet A grade standard of GB13023-91.(2) Laccase combined lignin from the effluent in secondary settling tank lignin could improve the mechanical properties of fiberboard obviously. Especially, modulus of rupture and modulus of elasticity. Modulus of rupture and modulus of elasticity were increased by 28.72% and 41.18% as compared with the blank samples.In the laccase system, adding a small amount of the lignin from the effluent could improve dry and wet strength of paperboard. Particularly, wet strength increases significantly. Wet tensile index increased 11.94% as compared with the sample treated by laccase only and increased by 54.11% as compared with the blank sample. The wet burst index increased by 53.3% as compared with the sample treated by laccase only. Through the catalysis of the laccase, the precipitated lignin from enzymatic treatment of wastewater has good enhancement effect on fiberboard. The internal bond increased by 71.05%. Thickness swelling decreased by 75.9% as compared with the blank sample.(3) The enhancement of fiberboard is more obvious after Cd²⁺or Cu²⁺ was added to the laccase system. Effect of Cd²⁺ on the internal bond is most significant. Internal bond of fiberboard using Cd²⁺ increased by 68.4% as compared with the blank sample and increased by 36.2% as compared with laccase treatment sample. When the Cu²⁺ (the activator of laccase) concentration was 3 mmol/L, the internal bond reached maximum. The internal bond of fiberboard using Cu²⁺ increased by 27.7% as compared with the blank sample.In several common metal ions, effect of Cd²⁺ and Cu²⁺ on wet strength of the paperboard are obvious. When Cd²⁺ was applied, the wet tensile index was increased by 41.10% as compared with the blank sample and increased 19.77% as compared with the sample treated by laccase only. The wet tensile index of the sample using Cu²⁺ increased 35.62% compared with the blank sample, increased by 15.12% compared with the sample treated by laccase only.(4) It was found that the Michael Addition reaction occured between amino acids and lignin. The content of amino and carboxyl groups of fiber increase. Therefore, the number of hydrogen bonds increased. Gelatin and laccase also promote the cross-linking between the fibers.In the representative amino acids, tyrosine had greatest effect on the internal bond of the enzymatic fiberboard. The internal bond increased by 92.1% compared with the blank sample, reaching 0.73 MPa. Histidine has the most obvious effect on modulus of rupture and modulus of elasticity of fiberboard,which were increased by 100.0% and 110.3% compared with the blank sample, respectively. Change of water resistance characteristic of fiberboard treated with amino acids and laccase was not obvious. Laccase combined with gelatin could improve the strength of the paper. The effect of laccase treatment condition and the dosage of gelatin on paper strength was explored. The optimal treatment condition was as follows: gelatin dosage 2% (based on dry pulp), pH 6, dosage of laccase 50 U/g dry pulp.(5) Mechanical properties and water resistance characteristics of the fiberboard treated by Fenton reagent were significantly improved. Performance of fiberboard treated by Fenton reagent combined with Kraft lignin had a further increase. Internal bond reached 0.86 MPa. Thickness swelling decreased to 25.3%.Unbleached softwood Kraft pulp with high lignin content was treated by Fenton reagent. The crystalline of fiber was decreased by 8.4%. Viscosity of cellulose was reduced by 25.7%. It was showed that Fenton reagent degraded carbohydrates. Wet and dry strength of paper treated by Fenton reagent increased to some extent compared with untreated paper, except tear index. The whiteness of paper treated by Fenton reagent was reduced by 6.5%.