Laccase-catalyzed Graft Modification Of Chitosan And Its Implication In Preparing Hydrophobic Paper

Chitosan is the second naturally abundant polysaccharide next to cellulose,with the biodegradability,non-toxicity and film-forming property.It can be used as antioxidant,fruit and vegetable fresh-keeping agent in food industry,sizing agent and paper strengthening agent in papermaking industry.However,the chitosan has a very low antioxidative activity and is easy to absorb moisture.As a packaging material,the mechanical strength properties,printing property and dimensional stability of paper can be negatively affected because the paper fiber is also easy to absorb moisture.So,it is necessary to enhance the antioxidate activity or hydrophobicity of chitosan using enzymatic method and also apply it in the pulp fiber modification or paper coating process,which has a significant importance to develop the environment-friendly multifunctional chitosan-based packaging material and wrapping paper with improved antioxidation,hydrophobicity and strength properties.In this paper,chitosan was modified by laccase with 4-hexyloxyphenol(HP).The antioxidant capacity,hydrophobicity and other properties of modified chitosan were studied;the enzymatic process conditions of improving the antioxidation were optimized;the structure and surface morphology of modified chitosan were analyzed.The pulp fiber was treated by laccase/phenolic compound/chitosan to prepare the paper with increased hydrophobicity and wet strength.The modification process conditions were optimized.The modified chitosan were coated on the surface of paper to improve the antioxidation,hydrophobicity and the strength properties.The effects of coating concentration of modified chitosan on the hydrophobicity of paper were also discussed.The main conclusions in this thesis are as follows.The chitosan was treated by laccase with 4-hexyloxyphenol(HP).The results showed that the ABTS·+ and DPPH· inhibition effects of modified chitosan was improved significantly.Compared to the pure chitosan,the ABTS·+ inhibition effects of modified chitosan increased from 9.64% to 37.05% when the chitosan concentrate was 2.25 mg/ml and the DPPH·scavenging activity increased from 1.1% to 9.4% when the chitosan concentrate was 1.5 mg/ml.The solubility of the modified sample in acetic acid(pH4.5)was reduced by 58.64%;the swelling ratio in water was decreased by 14.30%;the opacity was increased by 0.3 Amm-1;the static contact angle increased nearly 20° and has a strong stability;the tensile strength and elongation at break were decreased by 18% and 26%,respectively.The color of modified sample varied a lot.The optimal conditions of enhancing antioxidant activity of modified chitosan were: the chitosan/HP ratio of 1:2,the chitosan/methonal solution ratio of 20:4(v:v),the chitosan concentration of 0.3%(w/v),the temperature of 45 oC,the laccase dosage of 200 U/g HP,reaction time of 14 h and pH of 4.0.The modified chitosan by laccase with HP was analyzed by UV,FT-IR,solid 13 C NMR spectrum,SEM and scanning probe microscopy(SPM).The results showed that 4-hexyloxyphenol was successfully grafted onto chitosan.The elemental analysis showed that the content of carbon increased significantly and the content of nitrogen decreased obviously compared to the untreated chitosan.A great deal ofgranular substance was observed on the surface of modified chitosan.The unbleached kraft pulp was treated by laccase,phenolic compound and chitosan.The results showed that the hydrophobic properties and wet tensile strength of the paper modified by laccase/HP/chitosan were significantly enhanced.Compared to the untreated sample,the sizing degree of modified pulp paper increased from 1.0 s to 227.1 s;the capillary absorption height decreased from 57.0 mm to 0.4 mm;the kappa number increased from 26.8 to 29.3;the wet/dry tensile strength increased from 3.2% to 9.7%;the static contact angle increased from75.0° to 97.5°.The water retention value of pulp decreased from 176.8% to171.3%;the Cobb value decreased from 117.3 g/m2 to 103.2 g/m2.The optimal process conditions of enhancing the hydrophobicity of unbleached kraft pulp were: the laccase dosage of 60 U/g,the HP dosage of 1%,the reaction time of 2h,the temperature of 50 oC,the concentration of the pulp 5%,the pH of 5.5.Under the optimal conditions,the sizing degree of paper increased from 0 s to 198 s.the capillary absorption height decreased from 11.2 mm to 0.4 mm.The Cobb value decreased from 91.0 g/m2 to 48.8 g/m2.The APMP,OCC and KP papers were coated by modified chitosan with laccase and different phenolic compounds.The results showed that the hydrophobicity and tensile strength of paper coated by HP-grafted or LG-grafted chitosan were improved.Especially,the HP-grafted chitosan provided the paper a highest sizing degree.The sizing degree of the resulted paper was enhanced with the increase of chitosan concentration.