Thermal Effect And Mechanism Of Eucalyptus Vetlita Wood Under Vaccum Heat Treatment

Heat treatment is an imporant method for improving dimensional stability of wood.Compared with other ways, vacuum heat-treatment is better in retainning wood strength andcolor. However, research about steaming heat hreatment and vaccum drying were commonwhile vacuum heat treatment was rarely reported. Taken small pieces of Eucalyptus pellitawood, which is a usual fast-growing tree in south china, as research object, treated it withvaccum heat treatment, heat transfer law during vaccum heat treatment and change of woodproperties after vaccum heat treatment werestudied in this reach, mechanism of woodproperties chang was discussed. Main conclusions were as follows:1Time-temperature curve during vaccum heat treatment could be fitted well by hill1function, in which R2was above0.99, so hill1function could be used as time-temperaturecurve fitting model. Except independent variable time (t), dependent variable temperature (T),there were for parameter in the function: Tstart, Tend, k, and n. Tstartwas defined as theenvironment temperature, and Tendas the heat treatment temperature, both of the two could beviewed as constant. Close connection existed between parameter k&n and thickness of testpiece, the relationship could be described by upper second-degree parabolic function; Timeneeded of test piece climbing up to a given temperature was related with thickness of test piece,but also with the stage of the temperature lied in. In prophase (T <(Tstart+Tend)/2), as thetestpiece thickness augmenting, the time needed extended, in anaphase(T<(Tstart+Tend)/2), both kand n changed, which had different affects on time needed, so, the time needed could not bejudged under this condition.2Taken heat absorption per unit mass (Qm), max rate of temperature increase(Vmax), meanrate of temperature increase(Vmean) as detecting index, heat treatment temperature(Taim),thickness of test piece(ts), distence between test point to test piece top(lth), distence betweentest point to side of test piece(wth) as experimental parameters, adopted analysis of varianceand mulitiple comparions to analysis the impact factors and influence degree of heat transfer uniformity during heat treatment, results shows that: influence of Taimto Qm, Vmaxand Vmeanhad significant difference at0.05level, influence of tsto Qm, Vmeanhad significant difference at0.05level, influence of lthto Vmaxhad significant difference at0.05level, while influence ofwthto Qm, Vmaxand Vmeanhad no significant difference at0.05level. On length and width oftest piece, except the front end, heat treasfer ranged uniformly, heat transfer diversity shouldonly be considered in thickness. If measured by Qm, wood treated by120℃~240℃wouldsatify the need of multiplity of thermal effectiveness.3After treated under vaccum heat treatment, strength was enchanced firstly then reduced,dimensional stability and weight loss ratio were augmented, oven-dry density was decreased asheat treatment temperature increasing. Rangeability of weight loss ratio and oven-dry densitywere similar when the heat treatment was under200℃, while over200℃, change rate ofweight loss ratio was larger than oven-dry density, which might caused by cell crimpling underthermal. Improvemeng degree of dimensional stability showed a trend of growing slow at firstthen get fast. Volume shrinkage ratio from wet to dry and volume swelling ratio from dry towet could be reduced about10%under160℃, while reached about65%under240℃, ifcontinue to increase heat treatment temperature after240℃, improvement of dimensionalstability would be faint.Heat treatment during200℃~240℃would be suitable for dimensionalstability. Bending strength got top value at200℃, improved by about25%, while modulus ofelasticity got top value at160℃, improved by6.5%, so heat treatment during160℃~200℃would be suitable considering from wood strength.4Reasons for change of oven-dry density and weight loss ratio might be as follows: astemperature increasing, wood component decomposing deepened, weight loss ratio increased,oven-dry density reduced, as wood cell crimple under thermal, volume reduced to a certaindegree, which caused oven-dry density changed slower than weight loss ratio.Reasons for dimensional stability improvement:①as heat hreatment temperatureincreasing, hemicellulose was easy to be decomposed as its poor heat tolerance, which washydrophility component in wood, hygroscopicity of wood was reduced consequently;② during heat treatment, association of hydrogen bond was formed between O-H bond, whichreduced O-H bond quantity in wood, and improved its dimensional stability(it could be provedby the absorption peak lied in3423cm-1moved to low frequency in FTIR);③amorphousregion turned into crystalline region in cellulose.Reasons for wood strength changing:①as wood cell crimpling under heat treatmentmade cell shape changed, stress distribution was some different between treated and untreatedwood when wood suffered with force, which caused wood strength change;②when heattreatment temperature was under200℃, as heat treatment temperature increasing, amorphousregion turned into crystalline region in cellulose, intermolecular force boost up, strength ofwood largened; if continuing to increase heat treatment temperature(above240℃), associationof hydrogen bond in cellulose reduced because of the thermal motion on molecular chain,intermolecular force decrescence, strength of wood reduced;③nearby the heat treatmenttemperature200℃, lignin became soft and ropy as glass transition took place, which couldplay a cementation part for wood and strengthen the cellular construction, and then improvewood strength, but when heat treatment above240℃, decomposition of hemicellulose andcellulose was so severe that lignin strengthen to wood could be negligible, wood strengthreduced seriously.