| Polypropylene(PP)has a wide range of applications in many fields due to its good impact resistance,electrical insulation,easy processing and chemical stability.Although the production capacity of PP has increased significantly,it still has some shortcomings,especially its low temperature impact properties that limit its universal application.The self-developed PP in China can only meet 70%to 75%of the domestic demand,and high performance PP for special applications is still dependent on imports.Therefore,in order to improve the lowtemperature brittleness of PP,obtain high value-added PP products and broaden the use of PP,it is necessary to conduct research on high performance polypropylene copolymer.Using the impact polypropylene copolymer(IPC)with high stiffness and high flow produced by a pilot PP plant at the PetroChina Lanzhou Chemical Research Center was used as the raw material,this research studied its molecular structure,mechanical properties,crystallisation behaviour and rheological behaviour.(1)The IPC with high rigidity and high flow was classified by single solvent classification method,and the chain structure of each grade was studied by infrared spectroscopy(FTIR)and gel permeation chromatography(GPC).In addition,the phase structure of the IPC samples was characterised and analysed using scanning electron microscopy(SEM),and the mechanical and processing properties of the samples were investigated using a pendulum impact tester,an electronic universal testing machine and a melt index meter.The results showed that the high stiffness and high flow IPC samples were mainly composed of homopolymerised polypropylene and ethylene propylene rubber(EPR),with a small amount of ethylene propylene block copolymer(EbP).The heavy average molecular weights of the five high stiffness and high flow IPC samples all ranged from 1.2×105 to 1.7×105,and the molecular weights did not vary greatly.As the ethylene content of the high stiffness and high flow IPC increased,the number of small sized pores increased,while the number of large sized pores decreased,and the size of the EPR phase gradually tended to be smaller.In this study,when the ethylene content was at 5.01%,the impact strength,low temperature impact strength,tensile stress and bending modulus of the high stiffness high flow IPC samples were better,as were the processability properties.(2)The crystalline shape,crystallinity,grain size and isothermal crystallisation process of the high stiffness and high flow IPC were analysed using wide angle X-ray diffraction(WAXD),polarised light microscopy(POM)and differential scanning calorimetry(DSC).In addition,the variation of parameters during the non-isothermal crystallisation of IPC was investigated using a non-isothermal crystallisation kinetic model.The results show that the IPC samples with high stiffness and high flow were mainly dominated by the typical α crystalline form.As the ethylene content and EPR phase content increased,the crystallinity of IPC with high stiffness and high flow decreased,the spherical crystal size is smaller,finer grains were obtained,the degree of randomness of the sample gradually increased,the melting range becomes wider and the lower the starting processing temperature was,which was beneficial to the processing of the material.As the cooling rate increased,the crystallisation onset temperature and crystallisation peak temperature of the high stiffness and high flow IPC samples gradually decreased,the half peak of crystallisation widened and the exothermic peak widened.In addition,the crystallisation rate decreased with the gradual introduction of ethylene monomer.The experimental data from the non-isothermal crystallisation process was analysed by DSC method and it was found that if the amount of ethylene monomer introduced into the high rigidity and high flow IPC was low,the branched chains produced were few and short and did not impede the crystallisation behaviour of the whole sample during the crystallisation process,whereas if the amount of ethylene monomer introduced was high,the branched chains produced were many and long and took longer to crystallise and the crystallisation rate was slower.The addition of ethylene monomer to high stiffness and high flow IPC reduced its molecular chain regularity and hindered the movement of molecular chain segments,increasing the activation energy of IPC crystal growth to a large extent,leading to an increase in the activation energy of crystallisation.(3)Using capillary rheometry,the rheological behaviour of IPC under different conditions was investigated to determine the fluid properties and processing of the material by studying the correlation between the molecular structure of IPC and its rheological behaviour.In addition,the effect of molecular structure on its energy storage modulus,loss modulus,complex viscosity number and Han-curve was also investigated using a rotational rheometer.The results showed that high stiffness and high flow IPC is a typical pseudoplastic fluid,its non-Newtonianity and consistency coefficient decreased with increasing temperature,and the shear rate was linearly related to the shear stress at different temperatures,which indicates that the rheological behaviour of the IPC melt between 190℃ and 210℃was in accordance with the power law equation.The more ethylene monomer was introduced,the earlier the second plateau appeared and the greater the degree of non-homogeneity of the high stiffness and high flow IPC samples.The increase in temperature also lead to an increase in the degree of nonhomogeneity of the high stiffness and high flow IPC samples.At the same test temperature,the introduction of ethylene monomer and EPR phases inhibits the movement of molecular chain segments of PP during deformation,thus increasing the energy storage modulus and loss modulus of the high stiffness and high flow IPC samples. |
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