Characterization And Regulation Of Stress Relaxation Time For Natural Biomass Fiber Reinforced Polypropylene Composites

Natural biomass fiber reinforced thermoplastic composites have obtained wide applications in packaging and related industry for its excellent mechanical performance,lower price and low carbon features,providing an effective solution for plastic white pollution,shortage of petrochemical resources and pollution of residues from forestry and agriculture.The stress relaxation behavior of the composite significantly influences rheological and mechanical responding properties,dimensional accuracy and production efficiency of products as well as mold structure design.However,the relaxation mechanism as well as mathematical and analytical analysis concerning stress relaxation behavior during processing of such composite have not been investigated in depth in the literature.On the other hand,the processing and molding of such composite has been restricted to extrusion and injection,which cannot satisfy the requirements of high filled composites and products of irregular structure.Taking natural biomass fiber reinforced plastic composite as the research object,the physical model of stress relaxation for polymer melts and the composite filled with natural biomass fibers was established.The mathematical expression of key parameters in stress relaxation of the composite under different working conditions have been deduced.The key parameters in stress relaxation were later characterized by traditional torque rheometer and self-made thermal mechanical analyzer under shear,compression and bending loading.The conformity between experimental and analytical results reveals the accuracy and reliability of the derived theoretical model.An online extrusion compression molding technique was designed and introduced to manufacture packaging boxes of irregular structure with such composite for the poor flowability.Based on the stress relaxation mechanism and theoretical model the stress relaxation features and influencing factors during melt stock online extrusion measuring and compressing process were discussed in details.The relationship between above mentioned key parameters and melt stock measuring accuracy,stability and efficiency,buckling deformation,physical and mechanical performance as well as percent of pass were investigated.The research contents and conclusions can be drawn as follows.(1)A physical and mathematical model for describing the stress relaxation behavior of natural biomass fiber reinforced plastic composite was established.The micro stress relaxation mechanism of the composite was explored from the theoretical point of view.The results show that shear relaxation time of the composite melt was strongly related to the viscosity of the composite.It increased with the weight average molecular weight but decreased with molecular weight distribution index.And a higher profound effect of molecular weight distribution index on shear relaxation time of the composite could be observed for the composite with a higher weight average molecular weight.(2)A traditional torque rheometer and exponential decay model was used to characterize the shear stress relaxation time of natural biomass fiber reinforced polypropylene composite during melting and mixing process.The experimental results obtained was compared with that calculated from analytical solutions as a function of formula and processing parameters.A good agreement could be observed between theoretical and experimental results.The experimental data verifies the accuracy and reliability of predictions for stress relaxation time.The results obtained show that natural biomass fiber content could influence the shear stress relaxation time in an upward parabolic manner with the peak volume fraction of 20 %.The higher aspect ratio could bring about longer shear relaxation time.The intensity of the interface between natural biomass fiber and polypropylene chains could promote the stress transfer and lengthen the shear relaxation time with the coupling agent threshold content of 3 wt%.(3)Compression and bending stress relaxation time of natural biomass fiber reinforced polypropylene composite was measured with self-made thermal mechanical analyzer under methyl-silicone oil bath heating.The measured experimental results show that the compression and bending stress relaxation behavior shows significant difference compared with that obtained from torque rheometer.A more obvious stress overshot could be observed during the test and two relaxation process with dramatic different relaxation rate could be separated from the whole stress relaxation process.The relaxation time in phase I is two orders of magnitude lower than that in phase II.The stress decay amplitude in phase I was about 3.5 times as much as that in phase II.The stress decay amplitude in phase I increases with the natural biomass fiber content,but the stress decay amplitude in phase II decreases with the natural biomass fiber content.(4)An online extrusion compression molding technique was designed and introduced to manufacture highly filled natural biomass fiber plastic composite packaging boxes of irregular structure.Based on the theoretical analysis of stress relaxation model,the measuring accuracy,stability and efficiency of the composite during online measuring process for the composite melt stock was investigated experimentally.The relationship between stress relaxation rate and measuring accuracy as well as stability was analyzed.The results show that the standard deviation of the composite melt mass decreased with melt flow index of polypropylene and a ‘U' trend of that could be observed with the increase of natural biomass fiber content with a valley natural biomass fiber content of around 25 wt%.The standard deviation of the composite melt mass could be reduced by increasing the backpressure of the piston in the cylinder but too high backpressure up to1.5 MPa could result in instability during switching process of the double position measuring device.A negative correlation could be found between the measuring efficiency of composite melt stock and standard deviation of measured composite melt mass.(5)Online extrusion compression molding technique was used to fabricate natural biomass fiber reinforced polypropylene composite packaging boxes.The effect of relaxation time on the buckling deformation and production efficiency of compression molded packaging boxes was investigated.The differences in stress relaxation parameters,mechanical performance,buckling deformation and production efficiency in online extrusion compression molding were compared with those in traditional two step injection molding.The results show that the buckling deformation of packaging boxes made by online extrusion compression molding technique was 32.4 % lower than those made by injection molding and the dimensional standard deviation declined by 49.6 %.The tensile strength,elongation at break and impact strength of the packaging boxes made by online extrusion compression molding was superior to those made by injection.Physical and mathematical model of stress relaxation for natural biomass fiber reinforced plastic composite was established from the perspectives of molecular structure,movement modes of polymer chains,and interfacial interaction between polymer chains and natural biomass fibers.An online extrusion compression molding technique was designed especially for such composite with poor flowability to manufacture packaging box of irregular structure.The stress relaxation behavior and its influence on the indicative variables in the composite melt stock measuring and compression process were investigated.The theoretical model proposed can be used to analyze and predict stress relaxation parameters and further to control the external controllable variables for regulating stress relaxation and related physical and mechanical performance during forming process.It was of great guiding significance to improve the processing quality and production efficiency for the manufacturing of natural biomass fiber reinforced plastic composite.