Chemical Components Effect On Mechanical Properties Of Wood Cell Wall

Wood cell wall is a load-bearing unit in trees. It can be regarded as laminated nano-composites that cellulose microfibril as a reinforcement embedded in matrix of hemicellulose and lignin. Wood cell wall mainly consists of cellulose, hemicellulose and lignin. The arrangement of chemical components, their interaction and their mechanical properties results in specific mechanical properties of wood cell wall, which finally affect the macroscopic properties of wood. A better understanding of these structure-property relationships is crucial for insighting into the origin of the nature of wood physical and mechanical properties, for guiding the genetic improvement of trees, for cell wall structural composite material stimulations, as well as for a substantial improvement of wood and paper products. In this study, the mature latewood tracheids of Chinese Fir (Cunninghamia lancolata (Lamb.) Hook) were selected, and targeted modification method, which is sodium chlorite (NaClO2) for delignification and sodium hydroxide (NaOH) at different concentrations for extraction of hemicellulose, was used. We applied wet chemical analysis, Infrared and Raman spectroscopy to monitor the changes of chemical components in cell wall. At the same time, we used X-ray diffraction to study the influence of chemical components on cell wall structure. And we used single-fiber-test and nanoindentation technology to study the effect of chemical components on mechanical properties of wood cell wall. Subsequently, we attempted to establish the quantitative relationship between chemical components and mechanical properties of wood cell wall, to clarify the mechanism behind. The main results are summarized as follows:(1) Spectra analysis showed that the degradation of lignin happened first after treatment with sodium chlorite, thereafter xylan degraded, and finally glucomannan degraded by treatments with sodium hydroxide at different concentrations. At the same time, cellulose I transformed into cellulose II after treatment with 10% NaOH. The crystallinity and cellulose crystallite size of different treatments were studied by X-ray diffraction. And the relative crystallinity increased. While the cellulose crystal structure transformed into cellulose II after treatment with 10% NaOH. Meanwhile the relative crystallinity decreased. The cellulose crystallite size increased after the delignification treatment. However,the cellulose crystallite size reduced after the hemicellulose-extracted treatment.(2) The tensile test and cyclic tensile tests on single fibers of the mature latewood of Chinese Fir showed that the change of chemical components did not affect single fibers tensile behavior. The tensile stress-strain curves of single fibers (MFA around 10°) showed only small plastic deformations before rupture. And the cyclic tensile curves of single fibers with targeted modification treatments also showed visco-elastic plastic characteristic.(3) Lignin as one of the matrix had little impact on the longitudinal tensile elastic modulus and indentation modulus of cell wall in dry condition. The loss rate of longitudinal tensile elastic modulus and indentation modulus was about 5.10% and 6.53% respectively after treatment for delignification (reduced 99%). While lignin affected the tensile strength, elongation and hardness, significantly. The tensile strength and elongation of cell wall increased with the reduction of lignin content. The hardness of cell wall reduced 16.98% after delignified treatment.(4) Hemicelluloses affected the longitudinal tensile elastic modulus and indentation modulus of cell wall, significantly. The loss rate of longitudinal tensile elastic modulus and indentation modulus was about 11.57% and 9.16%, respectively, after hemicellulose extracted treatments; and the loss rate of longitudinal tensile elastic modulus was 9.55% after xylan extracted; while after glucomannan extracted, the loss rate was only 2.24%. Hemicellulose (especially xylan) affected the tensile strength and elongation of cell wall, significantly. The tensile strength and elongation of cell wall increased with the degradation of hemicellulose. Compared to delignified cell wall, the tensile strength,elongation, hardness reduced 32.15%, 21.12%,0.87%, respectively after hemicellulose ectracted.(5) Single fiber tests and nanoindentation tests showed that the contribution of cellulose for longitudinal tensile elastic modulus up to 84%, for indentation modulus up to 85%, for the tensile strength up to 96%, for elongation up to 95% and for hardness up to 82%. (6) The fracture mode of mechanical isolated fiber, delignified fiber and hemicellulose-extracted fiber was different. The fracture shape of mechanical isolated fiber and delignified fiber presented oblique tooth profile and the fracture surface was rough. It showed obvious ductile brittle fracture performance. While the fracture shape of hemicellulose-extracted fiber, had low elongation, brittle surface, and smoother than the other fibers.(7) We established a structural model on the relationship between cellulose, hemicellulose and lignin in cell wall. It showed that cellulose which was the source of cell wall strength, was the main structural component in cell wall working as a framework substance. Hemicelluloses (xylan and glucomannan) connected with the highly ordered cellulose of the microfibrils and lignin, and most of xylan contacted with glucomannan and lignin. Glucomannan and cellulose in close contact within the cell wall, and the force between xylan and glucomannan was less than that between glucomannan and cellulose. Xylan acted as an interfacial coupling agent between highly ordered cellulose of the microfibrils and lignin, which was important to maintain the integrity of cell wall mechanics. Lignin linked to xylan, but the connection was easy to destroy. To a certain extent, its existence enhanced the mechanical properties of cell wall.