Efficient Formation Of Benzyl Ester Bonds Between Cell Wall Polysaccharide And DHP And Enhancement Of Paper Strengh

Hydrogen bonds between paper fibers,whose bond energy is low and easily destroyed by water molecules.The characteristics of hydrogen bonds greatly limits the range of applications of paper.Formaldehyde pollution will occur when the formaldehyde-containing adhesives are applied in the manufacture of conventional paper.The high mechanical strength of woody cell walls is thought to stem from the structure of lignin-carbohydrate complexes(LCC)in the plant.In this paper,efficient formation of benzyl ester bonds between cell wall polysaccharide and DHP and enhancement of paper strength were investigated,respectively,through the simulation of the process of biosynthesis of benzyl ester bonds in the structure of LCC.The effect of the pH of the reaction system on the addition reaction of glucuronic acid to quinone methides generated in the biosynthesis of dehydrogenation polymer(DHP)was first investigated.DHP and DHP-glucuronic acid complexes(Dehydrogenation Polymer,DHP)were formed with D-glucuronic acid and coniferin as precursor of lignin biosynthesis catalyzed by a mixture containing laccase,?-glucosidase,O2 at different pH values(pH=4.0,pH=5.0,pH=6.0,pH=7.0),respectively.The structure of the product and the number of benzyl ester bonds were analyzed by IR,13C-NMR,HPLC,ion chromatography and elemental analysis.The results showed pH values in the reaction system had an important influence on formation of the benzyl ester bonds.Acidic conditions favored the reaction of quinone methide intermediates with carboxyl groups from glucuronic acid to form benzyl ester bonds in the biosynthesis of DHP.However,slightly acidic conditions(pH=6)enhanced the reaction of quinone methide intermediates with glucose.In the neutral condition,the DHP-protein complex could be efficiently synthesized by the addition reaction of quinone methide intermediates with amino acids in protein.In order to further study the reactivity of plant cell wall polysaccharide with quinone methides generated in the biosynthesis of DHP.The ?-cellulose and glucomannan were selectively oxidized with TEMPO/NaClO/NaBr mediated oxidation system in order to increase content of carboxyl groups in carbohydrate fractions.Then,coniferin labeled at side chain as precursor of lignin biosynthesis were polymerized with ?-cellulose and glucomannan catalyzed by a mixture containing laccase,?-glucosidase,O2 under acid condition(pH=4.0)and neutral condition(pH=7.0),respectively,to obtain DHP-cellulose complexes and DHP-glucomannan complexes.The results showed that ?-cellulose and glucomannan were oxidized by TEMPO/NaClO/NaBr oxidation system.Acidic conditions(pH4.0)favored the reaction of quinone methides with carboxyl groups from ?-cellulose and glucomannan to form benzyl ester bonds in the biosynthesis of DHP?On the basis of the above research,unbleached softwood pulp were selectively oxidized with TEMPO/NaClO/NaBr mediated oxidation system in order to increase content of carboxyl groups in carbohydrate fractions.Formation of benzyl ester bonds between DHP and unbleached softwood pulp with large amount carboxyl groups and improvement of paper strength were investigated,respectively.It was found a large number of benzyl ester bonds were formed between DHP and carboxyl groups on the surface of the fibers.The benzyl ester bonds could improve the dry and wet strength of the paper,especially,wet strength was increased obviously.The dry tensile index of treated paper increased from 62.76 N·m/g to 65.01 N·m/g and wet tensile index increased from 2.75 N·m/g to 17.06 N·m/g,with an increase 6.2 times as compared to untreated paper under the coniferin dosage of 10% and fiber carboxyl group content of 263.3 mmol/kg.