| The shortage of natural forest and relative richness in plantation forest in China makes itespecially important to find a way of efficiently utilizing low-density fast-growing wood andreducing the vast demand for high quality wood. Viscoelasctic Thermal Compression（VTC）is a novel technology for modification of low-density fast-growing wood. This researchproposed the hot pressing and drying treatment prior to VTC processing，optimized theoperational parameters and looked into the mechanisms of VTC modification，and theninvestigated the bonding performance and mechanisms of VTC-treated wood. Fast-growingwestern hemlock（Tsuga heterophylla）was chosen as a test material. Main research contentsand results are summarized as follows:（1）Hot pressing and drying treatment of high moisture content wood veneers wasdeveloped, which smoothened surface of veneers and improved outturn percentage anddrying rate. Effects of drying temperature and time on dry-basis moisture content, dryingratio and drying rate were studied. Equation was established to predict the relationshipsbetween drying temperature, drying time and dry-basis moisture content using regressionanalysis. In considering of drying rate and darkening of wood color, compressing drgree of9.5%, drying temperature of170°C and drying time of90s were chosen as optimal dryingconditions.（2）Influences of VTC technological parameters including processing temperatureduring conditioning and compressing stages, conditioning time, venting time, compressingtime and target compressing degree on properties of VTC-treated wood were determined. Theresults showed that higher temperature caused increased mass loss and decreased equilibriummoisture content, and longer conditioning time was able to reduce final thickness andthickness swelling of VTC-treated wood, while longer venting time, on the contrast, wouldresult in higher thickness swelling, and finally, as compressing degree increased, VTC-treatedwood tended to have reduced thickness, increased density, modulus of elasticity and modulusof rupture, and increased thickness swelling.（3）Regression was used to create empirical equations to predict modulus of elasticity,modulus of rupture, and thickness swelling. For highest bending properties, the best VTC conditions were obtained with processing temperature of170oC, conditioning time of180s,venting time of10s, compression time of300s, and target compression degree of200%; Forlowest thickness swelling, the best VTC conditions were determined with processingtemperature of160°C, conditioning time of600s, venting time of10s, compression time of180s, and target compression degree of50%.（4）Mechanisms of VTC modification were explored through looking into changes inmicroscopic physical characteristics and wood chemistry. Micromorphology of westernhemlock before and after VTC treatment was observed under Scanning Electron Microscope（SEM）. Content of wood extractives and pH values of water-soluble extractives of woodbefore and after VTC treatment were determined. Several types of extractives and wood cellwall substance were isolated and analyzed with Fourier Transform Infrared Spectroscopy（FTIR）. The results concluded that after VTC treatment, volume of cell lumen was reducedbut structure of cell wall was maintained; The increased extractives content indicated thatpolymers were degraded under heat and steam, which reduced wood hygroscopicity andresulted in darker color of wood; VTC treatment caused degradation of polysaccharideespecially hemicelluloses but did not obviously affect α-cellulose and lignin and thusmaintained mechanical strength of wood. Degradation of hemicellulose would help reducehygroscopicity of wood.（5）Bonding performance of VTC-treated wood was assessed and effects of adhesivetype, spread amount and veneer density were studied. Optimal spread amounts of three typesof adhesive were determined. VTC-treated wood bonded with PF or PUR adhesive showedsignificantly higher bondline shear strength than untreated wood, while when PMDI was usedas adhesive, bondline shear strength of VTC-treated and untreated wood showed no obviousdifference. PMDI and PUR turned out to have better bonding performance than PF. Theappropriate spread amounts of PE, PMDI and PUR for bonding high density VTC-treatedwood were determined, which were82,34and15g/m2, respectively. Plywoods made fromVTC-treated wood had higher bonding strength and bond durability than untreated wood andexceeded the wood failure requirements for exterior plywood established by VoluntaryProduct Standard PS1-09.（6）Micromorphology of wood surface and bondline were observed through fluorescencemicroscope（FM）and SEM. Mechanisms of bonding including depth and type of adhesivepenetration in wood before and after VTC treatment were analyzed. FTIR was conducted todetect the chemical reaction between wood and adhesive during bonding. The results showedthat VTC treatment made veneer surface smoother, which contributed to easier wetting ofadhesives and formation of an uniform layer of adhesive, and thus increased bonding strength. FTIR spectra indicated that chemical reaction possibly occurred between adhesives and woodcell wall substance during hot-pressing. |
