| Resin impregnation,as one of the suitable methods for modification of fast-growing wood,gives us a series of desired performance change with respect to the physical-mechanical properties.However,the change of “wood-water” behavior and wood microstructure after resin impregnation retards the moisture transfer during drying.In addition,it is impossible to render the resin in wood cured completely after conventional drying.Consequently,the popularization and application of resin modification for wood face many challenges.Therefore,the influence of different pretreatment methods before drying combined with drying methods on the change of melamine-urea-formaldehyde（MUF）resin solution within wood was studied firstly.After that,the moisture transfer and MUF resin curing characteristic within MUF resin impreg during drying,as well as the interaction relationship between moisture transfer and resin curing was systematically investigated.Moreover,the influence of curing temperature on physical and mechanical properties of MUF resin impreg was also investigated.This study could provide theoretical guidance for the drying schedule optimizing of resin impregnated wood and facilitates the popularization and application of resin modification for wood.The major achievements of this study were summarized as follows:1.The different pretreatment methods before drying combined with drying methods affected the process of moisture transfer and phase-change of resin in MUF resin impreg.As a result,the resin content and distribution within wood were distinct after each treatment,causing differential modifying effect.After MUF resin impregnation,storing the impreg in high humidity condition or placing it into the resin solution resulted in more resin penetrating into the cell wall compared with that of air drying pretreatment,leading to a higher bulking ratio and anti-swelling efficiency.After the same pretreatment,wet drying process favored more resin penetrating through the cell wall and fixing into wood compared with that of direct drying,resulting in a higher weight percent gain,bulking ratio and anti-swelling efficiency.2.The peak number and peak attribution in the T2 distribution curve for MUF resin impreg were same as that of untreated green wood sample.While the T2 value of the water in the wood ray and wood fiber lumens was increased,and the T2 value of the bound water was decreased.There were three peaks in T2 distribution curves,which corresponded to the water in and on the cell wall（donated as peak I）,the water in the wood ray and wood fiber lumens（peak II）,and the water in the vessel lumens（peak III）,respectively.At moisture content（MC）above fiber saturation point（FSP）,the T2 value of peak II for green wood sample was 58.05 ms.However,it reduced to 45.53 ms and increased to 80.26 ms for water-saturated wood sample and MUF resin impreg,respectively.In addition,the position of peak I and peak III was almost unchanged for water-saturated wood sample and MUF resin impreg compared to that of green wood sample.At MC below FSP,the T2 value of peak I at low MC stage was shorter for MUF resin impreg compared with that of the green wood sample and the water-saturated wood sample.3.The moisture transfer rate was reduced after resin impregnation,which causing an obvious moisture gradient in wood.At MC above FSP,the average drying rate of free water in vessel lumens（6.0%/d）,wood ray and wood fiber lumens（7.5%/d）was obviously decreased for MUF resin impreg,which was half of the water-saturated wood sample in corresponding cell lumens.At MC below FSP,the diffusion of the bound water from the inner layer to the surface layer took longer.During the whole drying process,the moisture gradient during drying for MUF resin impreg was more obvious compared with that of the green wood sample and the water-saturated wood sample.4.Dynamic mechanical analysis（DMA）for MUF resin impreg indicated that the water evaporation accompanied the resin polycondensation during drying.During drying,the normalized storage modulus of MUF resin impreg increased quickly and reached a maximum value.After that,it increased slowly and approached equilibrium.At the same time,the normalized loss modulus increased fast and reached a maximum value,forming an obvious mechanical loss peak.Then,it decreased gradually and approached equilibrium.5.The LF-NMR and DMA results indicated that the moisture transfer and resin polycondensation took place simultaneously and influenced each other during drying.At MC above FSP,the extra free water acted as diluent and energy barrier,which retarded the polycondensation of the resin.On the other hand,the transfer and evaporation of the free water were conducive to the resin polycondensation.At MC below FSP,the influence of bound water on resin polycondensation remained unclear.On the other hand,the continuous phase-change of the resin during drying blocked the cell wall and cell cavities and sorption sites at some degree,reducing the free water and bound water transfer efficiency in consequence.The interaction between moisture transfer and resin curing varied from the inner layer to the surface layer of the wood sample.The moisture transfer and phase-change of the resin solution within the inner layer were influenced by that of the surface layer.6.The cured resin deposited in vessel and wood fiber lumens in hemispherical and spherical particle form.The radial/tangential surface of vessel lumens and the pit membrane of the vessel were largely coated with resin particles.The lumens of wood fiber were slightly covered by scattered resin particle.The lumens of wood ray were obviously filled with resin.The maximum height of the hemispherical particle or the diameter of the spherical particle in vessel lumens and wood ray lumens was less than 5 μm and 2 μm,respectively.7.The water vapor adsorption property of MUF resin impreg was affected by curing treatment after drying.After curing treatment at 60 °C,80 °C,100 °C,120 °C,140 °C,the corresponding equilibrium moisture content of MUF resin impreg under 95% relative humidity was 30.03%,20.47%,14.94%,14.49%,and 13.99%,respectively,while the equilibrium moisture content of control group treated at 100 °C was 21.21%.The mechanical properties of MUF resin impreg,as in the case with vapor adsorption properties,was also significant influenced by curing temperature at α=0.01 level.The bending strength,bend elastic modulus and the surface hardness of tangential section of MUF resin impreg increased at first and then decreased with the increasing curing temperature.The surface hardness of transverse section increased with the increase of curing temperature,while the surface hardness of radial section fluctuated with increasing curing temperature.FTIR and 13 C CP/MAS NMR analysis showed that the content of hydroxymethyl and active hydrogen in the amino group decreased with increasing curing temperature,which indicated the polycondensation of resin was more complete.Hence,it brought about higher cohesive strength and lower hygroscopic property. |
