Study on the Mechanisms of Glucose Activation and Wood Cell Wall Modification with Activated Glucose
Author:Guo Wen Jun
Supervisor:xie yan jun xiao ze fang holger militz
The extensively studied chemical modification methods,including acetylation,furfurylation,and impregnation with low-molecular-weight resins,can considerably improve the dimensional stability and durability of wood.Several methods have been commercially introduced to the wood market at varying scales.However,most of the modifying agents used are derived from fossil resources,therefore,they are unsustainable.Furthermore,the release of volatiles,such as formaldehyde,free phenols or acetic acid,during the treating process and the service life of the products arose environment and health concerns.This study explores to develop an eco-friendly wood modification method based on the utilization of natural compound monosaccharide.The monosaccharide was activated with Fenton’s reagent to improve its fixation ability in wood cell wall by converting the hydroxyl groups into aldehyde and carboxyl groups.The activated monosaccharide could therefore fix in the cell wall by esterification and etherification with the cell wall polymers and by self-condensation.As a consequence of the cell wall modification,the density and dimensional stability of wood,as well as the biodegradation durability,were improved.The optimum activation method was explored.The wood was modified with the activated glucose(AG)and the physical and mechanical properties of the modified wood,as well as the biodegradation durability,were comprehensively characterized.The mechanisms of the glucose activation and wood cell wall modification were analyzed.The main contents and results are as follows:(1)The relationship between the molecular structure of the sugar and the activation efficiency is studied by comparing the activation efficiency of glucose,fructose,sucrose and maltose.Glucose was chosen to be the target sugar considering of its highest fixation ratio and moderate activation rate as well as its low cost among the four sugar types.The effect of the activation parameters on the activation efficiency of glucose was determined.The results showed that the H2O2 concentration controlled the activation ratio of glucose.Additionally,the Fe(Ⅱ)concentration and activation temperature mainly determined the oxidation reaction rate.The Fenton reaction in an acidic solution resulted in higher activation efficacy of glucose and better fixation in the filter paper,compared to the reaction in an alkaline solution.There was a slow activation stage and a rapid activation stage in the activation process of glucose.And the main conversion took place in the late stage of activation.The glucose cannot be fixed in the filter paper,but the AG exhibited a fixation ratio of up to 48.2%due to formation of active groups,such as carboxyl and aldehyde,as evidenced by Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry.(2)To achieve an efficient fixation of AG in wood,the effects of treatment parameters on the fixation ratio and mechanical strength were studied.The established optimum method of modifying wood with AG was:the concentration of activation glucose lower than 20%,the curing time of 24 h,the curing temperature of 120℃ and not to adjust the pH of the impregnation solution.Using a combinative agent of citric acid(CA)and AG can further improve the fixation ratio.The addition of sodium hypophosphite(SHP)didn’t improve the fixation ratio.However,the SHP can effectively inhibit the loss of mechanical strength.The optimum process of the combinative treatment was:AG concentration 18%,CA concentration 8%,curing temperature 120℃,curing time 24 h,with SHP as catalyst.(3)The physical-mechanical properties of the wood treated with AG alone or in combination with CA were characterized.The results showed that the density of wood was positively correlated with the concentration of treating solution and was improved by up to 28%due to the treatment.The weight percent gain(WPG)of wood varied from 6 to 40%.The leaching ratio of treating agents from wood was reduced from 98%to 70%by treatment with AG solely and it was further reduced to 50%with the aid of CA.The bulking coefficient of wood was linearly correlated with the WPG and varied from 2 to 9%.The dimensional stability of wood was improved and the anti-swelling efficiency increased up to 43%.Meanwhile,the modulus of elasticity and the modulus of rupture were comparable for both the AG-and CA-treated woods.The combined treatments with AG and CA could compensate for the brittleness of the CA-treated wood,resulting from reduced crosslinking by the CA within the wood matrix.(4)The resistance to fungal infection of wood modified with AG solely and in combination with CA was studied.The results showed that the efficacy of the treatment with AG alone against the fungi improved with an increasing AG concentration.The combinative treatment with AG and CA could further enhance the resistance of wood to brown rot and blue stain.These results demonstrated that modification of wood with AG/CA can considerably improve the durability of wood.The modified wood has the potential for outdoor application.The antifungal infection mechanism of the modified wood was summarized as follows:1)The treated wood was in a water-saturated state due to the high hygroscopicity of the deposited chemicals,as a result,the oxygen content in the wood is lower than that required by the aerobic fungi;2)the deposition of the AG in the cell wall micropores blocked the channels and pathways for the degradation agents and enzymes secreted by the fungi;3)The chemical structure of the wood cell wall polymers was altered due to the modification and became unrecognized by the enzymes.(5)The modification mechanism was studied form the aspects of physical and chemical structures of modified wood,respectively.The results showed that:1)In terms of physical structure,the modification agents can penetrate into the cell wall and cause bulking of the cell wall.The shrinkage of wood was reduced in this way.The crosslinking between the cell wall matrix was enhanced at the AG concentration below 9%and restrained the swelling of cell wall.The effects of cross-linking and destruction of the cell wall was in equilibrium at the AG concentration of 18%.The bulking and cross-linking of the cell wall due to the treatment with AG is the intrinsic mechanism for the improved dimensional stability.In the case of treatment with AG and CA,the damage to the cell wall is to a greater extent than the cross-linking effect,but the bulking of the cell wall due to the chemicals still contributed to an improved dimensional stability of wood.The crystallinity degree of the cell wall was not influenced by the treatment;2)In terms of chemical structure,the AG can undergo self-polymerization to form macromolecular substance,such as polyester et al.,and can block the cell wall micropores.The ester bond formed between the AG and cell wall polymers was also verified.Theoretically,the formation of acetal bond between cell wall polymers and the aldehydes in AG should also take place but could not be proved in this study.The blockage of the cell wall micropores and the change in the chemical structure of cell wall polymers accounted for the improved durability of wood to some extent.This study proposed the concept of "modifying wood with the component unit of wood cell wall".The dimensional stability and biological durability of wood was enhanced while the natural characteristics of the cell wall was maintained.The exploration provides new ideas for developing novel wood modification technologies.