| Cork,the secondary protection organization of oak trees,belongs to the periderm in anatomy.Quercus variabilis is the main species of cork source in China.Boiling treatment is one step of pre-treatment of raw material in cork industry.Study on the boiling treatment of cork from Quercus variabilis,has a great significance of improving processing performances,enlarging application scope and increasing economic benefits,and can also provides basic data and theoretical supports for subsequent processing.Cork samples from Quercus variabilis were collected from Qinba mountains in Shaanxi province.Based on relevant national and industrial standards and literatures,the basic properties of cork from Quercus variabilis were measured,the influences of boiling and reagent treatments on its main physical properties,cellular structure and the mechanical properties of cell wall were studied and the results showed that:(1)Virgin and reproduction cork were a homogeneous tissue of thin-walled cells,respectively.The cells,as an irregular polyhedral packed in columns parallel to the radial direction of the tree,were structurally arranged without intercellular voids.The corrugation was obvious on the virgin and reproduction cork cell walls.On average,in the transverse section,the cell wall thickness of earlycork of virgin and reproduction cork were 1.10?m and1.02?m,and of latecork were 1.52?m and 1.99?m,respectively.Within the scope of test,the thickness ranges of annual ring of the virgin and reproduction cork were 0.31~2.21 mm and0.42~1.59 mm,the density ranges were 0.197~0.355 g.cm-3 and 0.198~0.344 g.cm-3,respectively.On average,the average radial compression resilience rate of virgin and reproduction cork after pressure relief 24 h were 88.42% and 90.82%,respectively.(2)The chemical composition of virgin and reproduction cork were 36.34% and 40.41%suberin,20.12% and 18.40% acid-insoluble lignin,18.29% and 21.75% holocellulose,12.79% and 11.52% extractives,2.44% and 1.08% ash,respectively,and suberin was the main component of the Quercus variabilis cork cell wall.The difference in content of chemical composition was one of the main reasons of virgin and reproduction cork with differentphysical performances.(3)Treatment under different temperature conditions for 1h and boiling for different processing durations led to volume expansion,density,hardness,and surface lightness reduction of the cork from Quercus variabilis.After treated,the dimension change rate in the radial of cork was significantly higher than that in non-radial.After boiled for 1h,the volume expansion rate of virgin and reproduction cork were 17.1% and 26.8%.Density reduction rate were 15% and 21%,respectively.Hardness reduction rate were 19.6% and 27.6%,respectively.Surface lightness reduction were 15.51 and 13.59,respectively.(4)Among cork samples from Quercus variabilis with NaOH,KOH,hydrogen peroxide,alkaline hydrogen perxide,and ammonium hydroxide treatments for 1h under 75?,only the samples treated with hydrogen peroxide had increases of surface lightness and improvements of physical properties.Within the scope of test,after treated with 3%,5%,7% hydrogen peroxide,range of the surface comprehensive color difference ?E* compared with untreated reproduction cork was 6.56~10.50.Volume expansion rate range was 20.8%~29.2%,and surface hardness reduction rate range was 29.1%~30.6%,respectively.Within the scope of test,the radial compression resilience rate of samples treated with 7% hydrogen peroxide maintained at a high level,the average value after pressure relief 24 h was 87.46%.The compositions of filtrate from different reagent treatments were also analyzed.The results showed that hydrogen peroxide treatment extracted only minute quantity of hemicelluloses and none water-soluble lignin from cork.Other reagent treatments all separated part of hemicelluloses and water-soluble lignin of cork out.(5)Nanoindentation testing showed that,average values of elastic modulus of virgin and reproduction untreated cork cell walls were 10.20 and 11.47 GPa.Average values of hardness were 0.58 and 0.54 GPa,respectively.Compared with untreated samples,the average elastic modulus of reproduction cork after boiled for 3h increased to 11.81 GPa and hardness reduced to 0.51 GPa,therefore boiling treatment improved the mechanical properties of cork cell wall. |
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