The Effect Of Cell Wall Compositions On The Rheologic Properties Of Poplar Wood/HDPE Blend

The wood-plastic composites (WPC) can be processed by extrusion or injection, but it has difficulties in processing when increasing wood loading. To improve the thermalplasticity of wood, the molding problem of high wood content could be resolved to some extent. Because of the interaction between the main compositions-cellulose, hemicellulose and lignin in the wood cell wall, it can not be thermally melt. Plasticizing wood with ionic liquid has been proposed during the extrusion process, which may improve the processability of the composite. In this study, rheology of wood and its major components with high density polyethylene (HDPE) mixture was investigated, and aimimg at providing information for a choosing or synthetising a suitable ionic liquid in the future. NaC102, NaOH and CH3COOH were respectively used to prepare holocellulose, hemicelluloses extracted component and alpha-cellulose from poplar wood powder. Laser particle size analyzer, Fourier transform infrared spectroscope, X-ray diffraction and thermal gravimetric analyzer were used to characterize wood flour and three samples separated from cell walls. Torque rheometer, MiniLab micro blend rheometer, rotational rheometer and the high-pressure capillary rheometer were used to evaluate the dynamic and steady-state rheological properties of four mixtures (Holocellulose-HDPE, WF-HDPE, Hemi extracted WF-HDPE and a-cellulose-HDPE).The results of FTIR proved that the separation and purification method used can effectively remove the specific composition, and did not destroy other compositions. XRD analysis showed that the pattern of cellulose in wood flour and holocellulose are cellulose I, however, it is cellulose Ⅱ in hemi-extracted and a-cellulose after NaOH treatment. Crystallinity of the four wooden samples analysis showed that the alpha-cellulose was the highest, and crystallinity of wood was the lowest. Hemicellulose had the lowest thermal stability in three wood compositions, and the temperature of weight losses interval was narrow. Thermal stability of lignin was better, and the decomposition runs through the text temperature period.Torque, temperature, pressure, and the defference pressure of four mixtures had been measured through using torque rheometer and MiniLab micro blend rheometer, respectively. At the initial stage, torque of four mixtures increased with the time, followed by a decrease gradualus to an equilibrium after the samples were completely added into the rheometer. The temperature declined rapidly when added the mixture and increased. At the time that all the mixture was added into the chamber, the temperature reached a lowest value. An stable temperature can be obtained there after the pressure showed an increase at the intial stage and a decrease to an equilibrium value with the time. A peak appeared at the time that all the mixture was added in the rheometer. The pressure difference also showed the same trendency. Change of pressure characterized the difference of samples’viscosity in the melting process. The curves of four mixtures had the same order:α-cellulose-HDPE> Hemi extracted WF-HDPE> WF-HDPE> Holocellulose-HDPE, indicating that α-cellulose is the most unfavorable to the extrusion processing of WPC in the three main cell wall compositions Hemicellulose has the minimum effect in extrusion processing. Lignin’s effect is between the two compositions.Using a rotary rheometer, the linear viscoelastic regions of four mixtures were determined according to dynamic strain sweep. Comparing G’to G" at different frequencies, each sample exhibited both the elastic and viscous characteristics with increasing frequency. The sample showed more elastic behavior than the viscous behavior, which reveals the poor processability in WPC processing. The complex viscosity n*of four mixtures decreased gradually with increasing frequency ω, showing a typical nature of pseudoplastic fluid. Tanδ of each sample was less than1, indicating a significant solid elastic characteristic. At the same frequency the G\G", η*and tanδ of four mixtures exhibited an order in α-Cellulose-HDPE> Hemi extracted WF-HDPE> WF-HDPE> holocellulose-HDPE. This showed that the α-cellulose composition in cell wall is a key factor influencing WPC extrusion processing; the hemicellulose had little effect on the extrusion processing, and lignin’s effect is between these two components.Capillary rheology showed, the actual shear stress of four mixtures almost linearly increased with apparent shear rate. Apparent shear viscosity reduced as the actual shear rate, showing the characteristic of "shear thinning". At the same shear rate, shear stress and shear viscosity presetted a descending order as a-Cellulose-HDPE> Hemi extracted WF-HDPE> WF-HDPE> holocellulose-HDPE. From the point of view of steady shear rheology, it confirms again that rigid cellulose cause difficulties in WPC extrusion processing, and the hemicellulose can enhanced WPC extrudability. Lignin’s extrudability is between the cellulose and hemicellulose.