Effects Of Thermal Treatment On Characterizations And Properties Of Cork From Quercus Variabilis

Cork has many excellent properties such as low density,high hydrophobicity,resilience,thermal and sound insulation,and is widely used in lots of fields.Thermal treatment is one of the processes in the cork processing.Studying on the effect of high temperature thermal treatment on the characteristics and properties of cork from Quercus variabilis plays an important role in upgrading the technology of processing,improving the quality of products and enriching the research content of cork.The reproduction cork was obtained from Qinba mountain area,Shaanxi Province.The physical and mechanical characterizations,surface properties,energy properties,chemical composition and anatomical structures of thermal treated cork were characterized via DMA,DVS,profile density analysis,high heating value?HHV?analysis,proximate analysis,elemental analysis,SEM,CP/MAS ¹³C NMR and other analysis methods.The research conclusions were as follows:?1?The profile density and surface hardness of cork was decreased,and the hygroscopicity of cork was increased after thermal treated under air conditions.And the change was more obvious with the increasing temperatures.The surface hardness,hygroscopicity and hygroscopic hysteresis of cork was reduced by thermal treatment under nitrogen.The equilibrium moisture content of cork was decreased at the same humidity as the thermal treatment temperature increased.?2?Dynamic mechanical analysis showed that the storage modulus of untreated cork decreased gradually during the temperature increase from-100 to 300?,and the peaks of loss factor curve appeared in the corresponding temperature range.The storage modulus of cork was reduced and the positions and intensities of loss peaks were changed by thermal treatment under nitrogen or air conditions.?3?Compared to untreated cork,the lightness L*of thermal treated cork decreased,and total colour difference?E increased after thermal treatment under nitrogen or air conditions.A relationship model between?E and ML was established in the range of mass loss less than 40%for thermal treated cork under air conditions,which provided theoretical support for the quality control of products and the prediction of performance during high temperature thermal treatment.When the mass loss exceeded 40%,?E of the thermal treated cork was stable between 41.51-43.84 NBS.?4?Thermal treatment under nitrogen was proved to improve the energy properties of cork.The cork still retained 94.79%energy with a mass loss of 18.30%,and the HHV and fixed carbon content increased after thermal treated at 300?.Thermal treatment under nitrogen of cork at 200?resulted in degradation of carbohydrates,and lignin started degrading at 250-300?;suberin had high thermal resistance,but also commenced degrading at 300?.?5?After thermal treatment under nitrogen,cork cells swelled with enlarged cell lumen,smoother cell walls with less corrugation,and less sediments on the cell wall.The thickness of the cork cell wall decreased significantly when the temperature reached 350?.Compared with untreated cork,the average thickness of cell wall was reduced by 54.87%after thermal treatment at 400?for 60 min under nitrogen.