| Micron and nano-cellulose fibers have the advantages of a wide range of sources,low density,biodegradability and renewability of natural fibers,high modulus and fine structure,are good reinforcing material for the preparation of composites.However,micron/nanoscale cellulose fibers and thermoplastic polyolefins are difficult to be uniformly mixed and the compatibility between them are poor,which lead to the result that cellulose fibers does not realize their reinforcement capacity sufficiently.This is mainly due to the high specific surface area and the large amount of polar hydroxyl groups of micron/nano-cellulose fibers.The micron/nano-cellulose fibers tend to agglomerate during the drying process,which result in the difficulty of dispersing cellulose fibers in the nonpolar polyolefin matrix and failing to form strong interfacial bonding.In addition,most of dried micron/nano-cellulose fibers are light and fluffy,which differs greatly from polyolefin,leading to difficulties in blending during the preparation of composites.In order to solve these problems,micron/nano-cellulose reinforced high density polyethylene(HDPE)composites were prepared via wet mixing,wet feeding extrusion,pre-drying treatment and surfactant treatment these simple and easy-to-operate methods in this study.The mechanism of uniform dispersion of micron/nano-cellulose fibers and the improvement of properties of composites were investigated to lay the theoretical foundation for the development and application of micron/nano-cellulose reinforced thermoplastic polymer composites.The main research contents and results are as follows:(1)Research on the preparation of pulp fiber/HDPE composites via wet feeding extrusion.An internal mixer was used to pre-mix soaked pulp fibers with HDPE at 30?.The wet pulp fiber/HDPE mixtures were directly fed into twin-screw extruder and melt-blended.The composites prepared by compression molding.The experimental results show that with increasing the number of melt-blending times,the agglomeration area of pulp fibers in HDPE dramatically decreased,the dispersion of pulp fibers in the matrix and the interface between fiber and matrix improved,the density of the composites and the crystallinity of the HDPE increased,however,the colour of composites darkened and the length of pulp fiber decreased.The flexural strength and modulus,storage modulus,loss modulus and complex viscosity ?*of the composites fabricated by melt-blending twice reached the maximum value.The flexural strength and modulus of the composites were 42.0%and 225.3%higher,respectively,than that of the neat HDPE.(2)Research on the preparation of pulp fiber/HDPE composites via different drying methods.A beater was used to pre-mix pulp fibers with HDPE in water.The pulp/HDPE mixture after dewatering by using a spin-dryer was treated by three different methods:oven drying,pre-pressing followed oven drying and without drying.The pulp fiber/HDPE mixtures were fed into twin-screw extruder and melt-blended.The composites prepared by compression molding.The effects of the different drying methods and melt-blending twice on the pulp fiber dispersion and the properties of resulted composites were discussed.The experimental results show that both pre-drying and twin-screw extruder melt-blending twice are benefit to decrease the agglomeration area of pulp fiber in HDPE matrix and the interfacial bonding between pulp fibers and HDPE matrix.However,pre-drying and twin-screw extruder melt-blending twice lead to the decrement of the length of pulp fibers.The flexural strength and modulus,storage modulus,and complex viscosity ?*of the composite prepared by pre-pressing followed oven drying were the highest while the deformation and residual deformation of short-term creep test were minimal.The performances of softwood fiber reinforced composites were higher than that of hardwood fiber reinforced composites.The flexural strength and modulus of softwood pulp fiber reinforced composites prepared by pre-pressing followed oven drying and melt-blending once were the highest,which were 70.3%and 229.6%higher,respectively,than that of the neat HDPE.(3)Research on the preparation of microfibrillated cellulose(MFC)/HDPE composites via melt-blending.The MFC/HDPE composites prepared by mixing the plastic powder and MFC in water at high speed,vacuum filtration,drying,crushing,melt-blending,and finally compression molding.The effects of drying methods,maleic anhydride grafted polyethylene(MAPE)and the content of MFC on the properties of the composites were studied.The experimental results show that although MFC can be mixed well with HDPE in water,MFC still formed agglomeration in the process of drying.The melt-blending effectively improved the dispersion of MFC.With the increasing of MFC content,the flexural modulus,storage modulus and loss modulus of MFC/HDPE composites increased while the melt fluidity decreased?the flexural strength and modulus,storage modulus,loss modulus modulus and melt fluidity of composites increased after adding MAPE.With the aid of 5%MAPE,the tensile strength and modulus of composites increased by 34.6%and 122.2%,respectively,compared with that of neat HDPE after filled with 20%MFC.There was no significant difference of the properties composites between the oven drying(50?)and freeze-drying.The conventional drying method can also be used to prepare MFC/HDPE composites with good performance.(4)Research on the MFC/HDPE composites via surfactants as dispersion medium.The MFC was treated by using a mixture of nonionic surfactant sorbitan oleate(Span 80)and polyoxyethylene sorbitan monooleate(Tween 80).The treated MFC was dispersed in xylene and then the HDPE was dissolved in it.The MFC/HDPE mixture was obtained after the xylene evaporated.The surfactant in MFC/HDPE mixture was washed with ethanol.After drying,the MFC/HDPE mixture were melt-blended by using a twin-screw extruder.The composites prepared by compression molding.The experimental results show that the viscosity of MFC/surfactant emulsion was low and the Zata potential of it was the highest when the mass ratio of Span 80 to Tween 80 was 1:1.The surfactant treated MFC can be dispersed uniformly in xylene when the weight of surfactant mixture was 6 times higher than that of MFC.Scanning electron microscopy analysis showed that MFCs were uniformly dispersed in HDPE before and after alcohol washing.With the increasing of the number of alcohol washing,the surfactant content decreased.The crystallinity of the HDPE matrix,the tensile strength,modulus and thermal stability of the MFC/HDPE composites improved with the decreasing of surfactant content.The mechanical properties and pyrolysis performance of MFC/HDPE composites tend to be stable after alcohol washing over 3 times.(5)Research on the influence of preparation methods on the properties of MFC/HDPE composites.The MFC/HDPE composites were prepared by surfactant dispersion method of MFC in three different preparation ways:HDPE dissolved in the treated MFC-xylene suspension,the MFC/HDPE mixture was directly molded into MFC/HDPE composites after xylene volatilization,alcohol washing and oven dring;HDPE and MAPE dissolved in the treated MFC-xylene suspension,the MFC/HDPE/MAPE mixture was directly molded into MFC/HDPE composites after xylene volatilization,alcohol washing and oven dring;HDPE dissolved in the treated MFC-xylene suspension,the MFC/HDPE mixture was blended with MAPE after xylene volatilization,alcohol washing and oven dring,and then the MFC/HDPE/MAPE mixtures were molded into MFC/HDPE composites after twin-screw extruder melt-blending.The experimental results show that the tensile strength and modulus,storage modulus and loss modulus of composites were significantly increased while the interfacial compatibility between fibers and HDPE was improved after adding MAPE and melt-blending.With the aid of synergistic effect of MAPE and melt-blending,the tensile strength and modulus of composite filled with 20%MFC were the highest,which were 83.2%and 166.7%higher,respectively,than that of the neat HDPE. |
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