| As the increasing demand for lightweight materials with high-performance,the polymer foaming is the main method to realize this objective.Both the polymer modification,such as chain extending,long branching,crosslinking,nanocomposites addition,blending and varying foaming conditions are aimed to the rheological properties for better foam structure.It was worthy to thoroughly investigating the key generic technology between foaming and the in-situ rheological properties of polymer/CO2 solution.In this work,the measurement method for directly and accurately determing the rheological properties of polymer under high pressure was established and validated.Based on this method,the non-isothermal and isothermal crystallization behavior of polymer/CO2 were characterized and followed the investigation between the foaming and the crystallization.Considering that crystal is responsible for the melt strength improvement,the LLDPE/LDPE blends were attempted to tune the foaming by introducing the crystals into the matrix.The key factors were revealed by comprehensively analyze the experimental and numerical simulation results.The inert gas,helium(He),was selected as the pressurizing medium to study the effect of pressure on the rheological properties.The pressure shift factors were fitted based on the Barus equation.The rheological properties were sensitive to pressure increase.As the pressure increasing to a higher value,the rheology levelled off for HDPE whereas a positive correlation remained for LDPE.The combined effect of CO2 pressure and concentration on the rheology was investigated after gas saturation of more than 12 hours to ensure the thorough dissolution of the gas into the polymer.Compared the thickening effect of pressure,the plasticization effect of dissolved gas had a predominant influence on the polymer rheology.The storage modulus was more sensitive than the loss modulus to both the pressure and gas concentration.The effect of gas concentration was described by the Fugita-Kishimoto(F-K)model on the assumption that the coupled effect of pressure and CO2 concentration could be separated.A model based on the Arrehenius equation,Barus equation and F-K model that systematically describes the effect of temperature-pressure-gas content was proposed and validated.A modified high-pressure rheometer was used to characterize the crystallization behaviors of LDPE under compressed CO2 at the pressures ranging from 0.1 to 28 MPa.The depression magnitude of the crystallization onset temperature was just 4 ? even the saturation pressure was as high as 28 MPa.During the isothermal crystallization process characterized using high pressure rheometer,there were three regions including the starting crystallization where the modulus nearly invariable,the crystallization ongoing where the modulus increased rapidly and the crystallization ending where the modulus nearly stabilized at a higher value.Based on these findings,a batch foaming process was established to investigate the relative crystallinity(Xt)on the LDPE foaming behavior quantitatively.The bubble size reduced from 155 ?m to 135 ?m and expansion ratio increased from 28 to 32 as the Xt increased from 0 to 25%.Further improvement of the Xt lead to smaller bubble size while unsatisfied bubble structure.The rheological properties including small amplitude oscillation,relaxation time and extension rheology of LLDPE/LDPE blends were thoroughly investigated.The batch foaming of LLDPE and LLDPE/LDPE(95/5,wt/wt)blends were conducted with various crystallinity.The bubble coalescence and collapse was seriously though the complex viscosity and storage modulus were varied with different value by prolonging the isothermal crystallization time.In order to explore the key factors that affect the foaming process,a numerical simulation employing the cell model and the Considere construction were used to predict the bubble growth and the onset of bubble rupture.It was found that the higher viscosity just retarded the bubble growth while brought merely no effect on the bubble stability.The relaxation time improvement is beneficial to the bubble growth by postponing the bubble rupture.Therefore,the polymer modification should.be focused on the chains relaxation instead of just increasing the viscosity.In addition,it was found that both of the Henry constant and gas diffusivity mainly affect the bubble growth rate rather than the growth stability.The compatibility of LDPE/HDPE and LDPE/PP blends were investigated using thermal analysis and rheology methods.For LDPE/HDPE blends,the melting,crystallization process as well as the Cole-Cole plots and the weighted relaxation spectrum pointed to good compatibility following shear-induced mixing process used in the blend preparation.The distribution of PP phase in the blend was observed using POM.At low PP content,the blend resulted in a sea-island structure,which transitioned into a co-continuous structure as the level of PP was increase,especially the lower MI case.Extrusion foaming of the blends was conducted in a single extruder with gas injection.The incompatible interface was found to be of benefit for the cell nucleation as seen from the expansion angle;however such blends were not able to sustain adequate foaming due to potential gas loss,especially in the case of a co-continuous structure.In the case of a sea-island structure,the expansion ratio was higher and resulted in larger bubbles as well.For the co-continuous structure,both the cell size as well as the expansion ratio was smaller.The viscoelastic properties and diffusion coefficients of CO2 in the blends were accurately measured via rheometric and MSB measurements.The CO2 diffusion coefficient in the co-continuous structure was found to be higher than in the case of the compatible blends.It can therefore be concluded that the viscoelastic properties are not the primary driving factor affecting the foaming behavior of these immiscible blends.The poor foaming performance could possibly be linked to interfacial incompatibility.This is,especially more pronounced in the case of co-continuous phase structures where the interface is larger,thus leading to increased passage for interfacial gas escape. |
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