| Low molecular weight resin impregnation modification is an important technical means to improve the dimensional stability of wood.The multi-scale pore structure and chemical composition of wood changed after resin impregnation and drying treatment,which affected the water absorption and moisture adsorption characteristics of impregnated wood.In this paper,populus tomentosa wood of fast-growing plantation was taken as the object,which was impregnated with 10%,20%concentration of low molecular weight melamine modified urea formaldehyde(MUF)resin and treated with 60℃～140℃ temperatures.To study the effect of resin impregnation modification on the pore structure and chemical characteristics of poplar wood.The moisture adsorption,water absorption,and water molecule relaxation behavior of resin impregnated wood samples was elucidated,and the relationship between the moisture absorption,water absorption characteristics,chemical characteristics,and the pore structure of MUF resin impregnated wood was revealed.The results were helpful to enrich the basic theory of moisture relationship of resin impregnated wood and benefited to invite the resin impregnated wood and drying treatment.The main conclusions were as follows:1.The MUF resin adhered to and filled the cell structure of poplar vessels and wood fiber with hemispherical or spherical protrusions,and the gas permeability decreased significantly.the macropore porosity,average macropore,mespore and micropore pore size of 10%resin impregnation,140℃ drying treated samples(RIW10-140℃)and 20%resin impregnation,140℃ drying treated samples(RIW20-140℃)were 19.20%,0.49 μm,3.43 nm,1.01 nm,and 9.85%,0.49 μm,3.07 nm,0.98 nm,respectively,which were all smaller than CK(40.2%,0.50 μm,3.85 nm,1.58 nm).2.Compared with the CK,the infrared characteristic absorption peak of the hygroscopic groups of the impregnated sample was decreased,and the content of C element connected with the carbonyl group in hemicellulose and cellulose was reduced,indicating that the number of hygroscopic groups in the cell wall was reduced.The increasing of the drying temperature was conducive to the polycondensation reaction between hydroxymethyl and amino,and the number of residual hygroscopic groups on the resin molecules was reduced.The cross-linking reaction between resin molecules and resin wood leaded to the increase of cell wall stiffness and the decrease of relaxation degree of freedom,which was not conducive to water absorption.The transverse relaxation time(T2)corresponding to cell wall relaxation peaks of CK,RIW10-140℃ and RIW20-140℃ samples were 0.017 ms,0.016 ms,and 0.015 ms,respectively.The peak areas were 19202,24823,and 25135(dimensionless),respectively.3.With the increase of resin solution concentration and drying temperature,the adsorbed water content of monolayer and multimolecular layer and the equilibrium moisture content of hygroscopic were decreased.The physical filling of wood cell wall pores by resin reduced the relaxation freedom of water absorption.During the whole hygroscopic process,the T2 values of water absorption at RIW10-140℃ and RIW20-140℃ were lower than those of raw materials,and hygroscopic completion(48 h)was achieved.The T2 of CK,RIW10-140℃,RIW20-140℃ were 6.8ms,1.48ms,1.05ms,respectively.4.Compared with the CK,with the increase of resin concentration and drying temperature,the water absorption rate of impregnated wood samples decreased by 48.6%at most,and the water absorption resistance rate increased by 85.1%at most.The physical filling of resin on vessels,wood fibers and other macropores reduced the freedom of free water relaxation,resulting in the whole water absorption process.The free water T2 of RIW10140℃ and RIW20-140℃ samples were smaller than that of CK.The resin mainly filled the cell lumen of vessels,causing the free water relaxation peaks in vessels to fuse with the free water relaxation peaks in the wood fibers and wood rays. |
