Fabrication Of Polypropylene-based Conductive Composites Using Foam Injection Molding And Their Properties Research

Currently,conductive polymer composites(CPCs)are widely used in antistatic,electromagnetic interface shielding,sensors.However,the conductive fillers in the injection molded CPCs samples were highly oriented along the flow direction(I-P direction),resulting in a decrease of the thickness direction(T-P direction)conductivity.Foam injection molding can enhanced the interconnected fillers and promoted the formation of conductive network.In addition,the growth of cell resulted in conductive fillers oriented in the T-P direction,enhance the formation of conductive network in the T-P direction.In this paper,polypropylene(PP)was used as the matrix,carbon black(CB)and multi-walled carbon nanotubes(MWCNT)were used as conductive fillers.Foam injection molding effect on the microstructure of conductive network was studied.The main results are as follows:(1)The structure and performance of PP/CB samples by foam injection molding.The T-P electrical conductivity and SEM test results show that the introduction of cell structure can improve the T-P electrical conductivity of sample.In addition,the reduction of distance between CB particles were found in the SEM images of foam samples.The T-P electrical conductivity and microstructure of skin layer and core layer were investigated,the results show that the T-P electrical conductivity of skin layer is much lower than that of the core layer.This was attributed to the large shear effect on the skin layer of sample,which makes the CB aggregate of skin layer oriented along the I-P direction.In addition,the WAXD test results showed that the crystallinity of skin layer was low,which made the crystallization behavior have weaker effect on the volume exclusion of CB in the amorphous region.These two reasons resulted in the low T-P electrical conductivity of the skin layer.However,the gas released by the chemical foaming agent was dissolved in the matrix by foam injection molding,which decreased the viscosity of melt and shear stress during filling.Therefore,the T-P electrical conductivity of skin layer of foam samples was higher than that of the solid samples.Moreover,the T-P electrical conductivity of core layer of foam samples is higher than that of solid sample due to the large number of cell structure.The mechanical properties test results show that the higher CB content is not conducive to the improvement of mechanical properties,and cell structure will also weaken the mechanical properties of foam samples.(2)Process parameters effect on the cellular morphology and the T-P electrical conductivity.Foam samples with different cellular morphology were prepared by changing the process parameters,and then the cellular morphology and T-P electrical conductivity of samples were tested by SEM and insulation resistance tester.The results show that increasing cell density and thickness of foam layer can improve the T-P electrical conductivity of foam samples.But there is a critical value of cell density,when cell density is too large,the T-P electrical conductivity of foam sample decreases.(3)The structure and performance of PP/CB/MWCNT samples by foam injection molding.The hybrid fillers system with CB and MWCNT significantly improved the T-P electrical conductivity of sample compared to a system with only CB,and the grape-cluster-like conductive network formed by MWCNT and CB can be observed in the SEM images.We also found that the T-P electrical conductivity of solid samples at the near location was lower than that at the middle and end location.The thickness of shear layer at different location was tested by POM.It was found that the shear layer at the near location was thicker than the middle and the end location,which was the primary factor causing the inhomogeneity of T-P electrical conductivity of samples.Moreover,the cell density and thickness of skin layer at different positions also have obvious differences for foam samples,which leads to the inhomogeneity of T-P electrical conductivity at the different positions.This inhomogeneity of T-P electrical conductivity will limit the practical application of conductive polymer products.The SEM and T-P electrical conductivity of skin and core layer results show that MWCNT of skin layer are oriented and breakage along the flow direction due to a high shear stresses of skin layer,Therefore,the increase of MWCNT content has no obvious effect on the improvement of T-P electrical conductivity of skin layer.In addition,the addition of MWCNT can improve mechanical properties of samples,which can offset the damage to mechanical properties of foam samples due to the existence of cell structure.