Study On The Main Physical And Mechanical Properties Of Cork From Quercus Variabilis

Cork is the bark of cork oak tree,which belongs to periderm in anatomy.Cork has many excellent properties,such as light weight,sound absorption,heat insulation,wear resistance,insulation,chemical stability and so on.Cork in China is mainly extracted from Quercus variabilis.The relationship between the physical and mechanical properties of cork and its processing and utilization is very close.Study on the the physical and mechanical properties of cork can provide basic data for its rational utilization,and also provide theoretical basis for improving the processing performance of cork and developing new cork materials.In this paper,the cork from Quercus variabilis in Qinba mountain area of Shaanxi Province was taken as the research object.By means of scanning electron microscope,infrared spectrum,X-ray energy spectrum,surface contact angle test,permeability test,dynamic mechanical analysis,sound absorption and thermal conductivity test,the structure and surface characteristics,moisture absorption and permeability,dynamic viscoelasticity,sound absorption and thermal conductivity of cork were studied.The main research results were as follows:(1)Compared with the virgin cork,the surface of the reproduction cork was flat and the cracks were shallow.Impurities such as sclereid and lenticel could be observed in reproduction cork.The sclereid was white speckled,and the lenticel was striped in the cross section,with random distribution.The strength of the characteristic peak of suberin in sclereid and lenticel was relatively weak compared with that in pure cork,indicating that the content of suberin in impurity was lower than that in pure cork.(2)There were 16 kinds of chemical elements in reproduction cork.The content of C element was the highest.The percentage of mass and atom of C were 64.04% and 72.37% respectively.The content of O element is next.The percentage of mass and atom of O were 26.30% and 22.32% respectively.Ca is the highest content of metal element.The percentage of mass and atom of Ca were 0.75% and 0.26% respectively.(3)The surface lightness(L*),red green index(a*)and yellow blue index(b*)of reproduction cork were 60.8,14.0 and 30.6 respectively.The surface of reproduction cork was hydrophobic.The initial contact angles of distilled water,formamide and diiomethane on the surface of reproduction cork were 112.0 °,85.1 ° and 75.2 ° respectively.After 10 s,the contact angles decreased to 102.1 °,30.7 ° and 72.1 ° respectively.(4)The moisture content of reproduction cork could reach to 4.28%,6.05% and 8.97% respectively after being dried to the absolute dry condition and kept at 45%,65% and 85% humidity for 30 days(20 ℃).The penetration rate of alcohol reagent and distilled water in cork slowed down gradually with the extension of time,and the penetration capacity of each reagent in cork was different.During the same time of penetration,the relationship between the volume penetration of reagent in the virgin cork was: methanol was the largest,followed by ethanol,n-propyl alcohol,isopropyl alcohol and distilled water.The relationship between the molar penetration was: methanol was the largest,distilled water and ethanol were the second,and n-propanol and isopropanol were the lowest.The relationship between the penetration of each reagent in reproduction cork was basically the same as that of virgin cork.With the increase of the number of carbon atoms,the permeability of alcohol reagents in cork decreased.The linear relationship between the molar permeability and the reciprocal of the viscosity of the reagents was good,and the permeation process of each reagent in cork conformed to the linear laminar flow.(5)The storage modulus of reproduction cork from Q.variabilis decreased with the increase of temperature under different test frequencies.There were three obvious peaks in loss factor curves.The three loss processes were caused by glass transition in the amorphous zone of suberin,melting of related components in cork and loss of adsorbed water,and thermal softening or degradation of cellulose and hemicellulose,respectively.When the reproduction cork from Q.variabilis was tested along the axial,radial and tangential directions respectively,the change trends of storage modulus,loss modulus and loss factor curves were basically the same.While at the same test temperature,the values of storage modulus were obviously different in three directions,and the loss modulus was also different.Cork from Q.variabilis and Q.surber had loss peaks at different temperatures under the experimental conditions.The loss peaks of virgin cork and reproduction cork from Q.variabilis appeared at-10 ℃ and-19 ℃,respectively,and the corresponding loss peaks of reproduction cork from Q.surber appeared at 14 ℃.Under the two deformation modes of tension and compression,the change trends of E’,E’’ and tan δ were basically the same,and the loss process inside the material was the same.(6)The average sound absorption coefficients of 3 mm,6 mm,8 mm and 10 mm reproduction cork were 0.034,0.0366,0.0537 and 0.0850 respectively.The sound absorption coefficient of reproduction cork is very small in the middle and low frequency range(50 Hz～3500 Hz),and the sound absorption performance is poor.There was an absorption peak in the high frequency range(3500 Hz～6400 Hz),and the sound absorption performance was good.With the increase of cork thickness,the propagation path and retention time of incident sound wave in the material increased.The average sound absorption coefficient and sound absorption performance of cork increased in the whole frequency band.(7)Cork from Quercus variabilis is a kind of thermal insulation material.The radial thermal conductivity of absolutely dry and air dry virgin cork measured in this test were 0.0607 W/(m·K)and 0.0687 W/(m·K)respectively.The radial thermal conductivity of absolutely dry and air dry reproduction cork were 0.0676 W/(m·K)and 0.0707 W/(m·K).Under the same moisture content,the thermal conductivity of reproduction cork is slightly higher than that of virgin cork.The thermal conductivity of water is higher than that of air.Affected by moisture content,the thermal conductivity of air dried cork was slightly higher than that of absolutely dried cork.