| Lignocellulosic cell walls have high strength and rigidity due to their unique chemical composition and sophisticated structure. However, lignocellulosic material (LM) is not a good heat conductor due to its porous nature. During high temperature processing, the surface of LF degrades, while the inner cell wall remains intact. In its native state, LF is not readily soluble in common solvents and cannot be directly melt-processed through conventional extrusion or molding techniques. Conventional manufacture methods of wood with knife and abrasion milling processes generate numerous wastes. This study explores a new approach to effectively utilize lignocellulosic resources based on in-situ/dynamic plasticization. First, we analyzed mechanism of in-situ/dynamic plasticization of LM and then developed a preliminary theoretical system for plastic processing of LM. The main contents and results are as follows:(1) It is difficult to accurately measure the phase transition of fibriforms or powdered lignocellulosics using traditional differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques. Therefore, we used a novel model of DMA combining material pocket technique (MP-DMA) to understand the dynamic viscoelasticity of poplar sapwood fiber. We prepared four types of wood fiber with various compositions including native wood fiber (WF), hemicellulose-removed fiber (HR), holocellulose (HC), and a-cellulose (aC) and treated them with an ionic liquid,1-ethyl-3-methylimidazolium chloride ([Emim]Cl) to a weight percent gain of36%. We then tested the [Emim]Cl-plasticized wood fibers with MP-DMA and DSC. We also tested the [Emim]Cl-plasticized wood strips with traditional DMA for comparison.Results show that MP-DMA is more sensitive than DSC. The storage modulus of the fiber decreased as follows:αC>HR>WF>HC. The storage modulus of [Emim]Cl-plasticized wood fibers decreased significantly. Results show that [Emim]Cl can destroy the hydrogen-bonding system in crystal cellulose, amorphous cellulose, hemicellulose and lignin. Three distinct transitions were observed for [Emim]Cl-plasticized wood fibers. The peak at approximately30℃may be due to the relaxation of amorphous cellulose plasticized by the residual moisture and [Emim]Cl. The main peak at around90℃may be due to the relaxation of amorphous compositions of the cell wall plasticized by [Emim]Cl. The shoulder at140℃may be due to relaxation of crystalline cellulose dissolved by [Emim]Cl. The plasticization mechanism of wood fiber by [Emim]Cl was also revealed.(2)To reveal the mechanism of in-situ/dynamic plasticization for LM under the combined effects of ionic liquid (IL), pressure and high temperature, we conducted increasing temperature compression tests of IL-treated poplar wood under constant pressure (796kPa) with a rotational rheometer and measured the recovered strain of the compressed samples in moisture. Results show that both the IL type and IL concentration affect the plasticizing efficacy on wood cell walls. Increasing the concentration of ILs results in a lower softening temperature and a higher compression strain. Among the ILs examined,[Emim]C1exhibited the most beneficial plasticizing effects. Results suggest that the thermoplastic deformation of the IL-treated wood was not caused by the melting of ILs and the degradation of IL/wood. Instead, this was caused by the breakdown of inter-and intra-molecular hydrogen bonds between cell wall polymers through the combined effect of ILs, pressure, and high temperature. SEM shows that cell walls treated with higher concentrations of ILs showed permanent strain>0.5without fracture, and the recovered strain was smaller than3%. Decrystallization and recrystallization of cellulose in IL-plasticized wood occurred during compression, forming and conditioning. These results revealed the in-situ/dynamic plasticization of cell walls by IL. The thermoplasticity of cell walls improved significantly under the combined effect of IL, pressure and high temperature. The plasticizing efficacy (including solvation, replacement of hydrogen bond and decrystallization, etc.) decreased dramatically when cooled to room temperature, and the molecular motion of cell wall polymers slowed down upon cooling. During this process, cell walls without fracture regained their inherent mechanical properties, whereas the crystal structure, hydrogen-bonding system, aggregation structure and molecular motion changed.(3) To investigate the effects of removing cell wall composition on the thermoplasticity of LM, we melt-blended αC, HR, WF and HC with high density polyethylene (HDPE) with a twin-screw extruder and characterized the rheological properties of the blends with a Haake microcompounder, torque rheometer, capillary rheometer, and rotational rheometer. We used injection molding to create tensile and impact test samples. Results show that HC exhibited a highly porous and flexible structure upon removal of lignin. Rigidity of HR increased compared to WF, and aC showed the highest rigidity and aspect ratio. SEM shows that HC exhibited significant thermoplastic deformation during extrusion/injection molding processing. The torque, shear stress, viscosity and moduli of the blend melts indicated by the rheological properties obtained from the four methods of assessing rheological behavior are ranked as follows:aC/HDPE> HR/HDPE> WF/HDPE> HC/HDPE. aC/HDPE exhibited the highest tensile strength and impact toughness. Tensile strength, modulus and elongation at break of HC/HDPE increased slightly compared to those of WF/HDPE, while impact strength decreased slightly. Results show that the in-situ/dynamic plasticization of cell walls during extrusion at high temperatures can improve the processability of blend melts, and that the mechanical properties of composites at room temperature are not degraded.(4) We treated wood flour with IL and then melt-blended it with HDPE in order to examine the effects of in-situ/dynamic plasticization of cell wall plasticized by IL on the processability of wood flour/HDPE blends. Results from XRD, MP-DMA, SEM reveal that IL-plasticized wood flour exhibited significant thermoplastic deformation. However, the in-situ/dynamic plasticization of IL-plasticized wood flour did not improve the processability of wood flour/HDPE blends at a low shear rate. This is due to a significant increase in the surface polarity of wood flour, which facilitates filler-filler interaction, resulting in agglomeration and hindered dispersion of wood flour in the HDPE matrix. At high processing speeds, the shear stress and shear viscosity decreased or remained unchanged.[Hemim]Cl can broaden the processing window of WPCs with stable flow and smooth product surfaces.(5) To investigate the effects of surface polarity of wood flour on the processability of wood flour/HDPE blends, we treated wood flour with glutaraldehyde (GA) and1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) to decrease surface polarity to various levels. Results show that the surface polarity of wood flour can dramatically influence the processability of WPC. A small amount of GA appears to decrease the melt torque, shear stress, viscosity and moduli of wood flour/HDPE blends. This may be because GA treatment decreased the surface polarity of wood flour significantly, facilitating the dispersion of wood flour in HDPE melts and thereby improving the processability of WPC. However, DMDHEU treatment had no positive effect on the processability of WPC. |
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