Morphology And Mechanical Properties Of Water Assisted Injection Molded Polypropylene/Polyamide6 Blends

Water assisted injection molding(WAIM)can be used to produce hollow part by utilizing high pressure water to penetrate in the core molten plastic.The WAIM process of polymer blends can lighten product as well as shorten molding cycle and save energy,so it has a great potential for development and a bright prospect.However,the research on the WAIM process of polymer blends has rarely reported.Therefore,a study on the WAIM of polypropylene(PP)/polyamide6(PA6)blends were carried out based on our lab-developed WAIM experimental platform.The experimental and theoretical methods were combined to investigate the the influence of the content of dispersion phase and processing parameters on residual wall thickness(RWT),surface quality and mechanical property.And the relationship among processing parameters,dispersion phase,micromorphology and mechanical property were explored via observing the micromorphology,including phase morphology,crystal morphology,and crystallinity.In addition,the processing mechanisim of the WAIM process of PP/PA6 blends was revealed by the flow field evolution during the process which is obtained from numerical simulation.The main content of this research were summarized as follow:1)The background,relative meris and technical principle of blending polymer and WAIM technic were briefly introduced in this paper,and the research direction was also identified.2)The PP/PA6 blends with 10,20,30,and 40 wt% of PA6 were prepared by a twin-screw extruder.Their related rheological and thermal properties were measured.3)The orthogonal experiment and single factor experiment were carried out to investigate the influence of PA6 content and processing parametes,like melt temperature,mould temperature,melt injection pressure,water injection pressure and water injection delay time,on the RWT and surface quality of WAIM pipe of PP/PA6 blends.It was found that the influence of parameters on RWT from large to small was water injection delay time,mould tenperature,water injection pressure,melt injection pressure.With the increase of melt temperature,mould temperature and water injection pressure,RWT decreased gradually.With the increase of water injection delay time,RWT increased.Increasing the content of PA6 decreased the RWT,while an excessive PA6 content reaching up to 40 wt %,increased the RWT.4)The influence of processing parameters and PA6 content on tensile strength of WAIM pipes of PP/PA6 blends were studied.The results showed that melt temperature had the biggest influence on tensile strength,followed by mould temperature,then water injection pressure and delay time,however melt injection pressure nearly has no effect on tensile strength.With PA6 content increasing,sample tensile strength and breaking elongation of PP/PA6 WAIM pipe decreased gradually.But when PA6 content reached 40wt% the tensile strength and breaked elongation increased on the contrary.5)In order to study the influence of processing parameters and PA6 content on the morphology of WAIM pipes of PP/PA6 blends and the relationship between microstructure and mechanical properties,scanning electron microscopy(SEM),polarized optical microscopy(POM)and differential scanning calorimetry(DSC)were applied to observe the phase morphology,crystal morphology and distribution and crystallinity of the samples with the maximum and minimum tensile strengthes The phase abservation result showed that the dispersion phase of core layer and outside layer had striped or sheet morphology which were influenced by shearing field,while middle layer had axiolitic dispersion phase morphology.Besides,the dispersion phase at outside layer had larger slenderness ratio than core layer.With PA6 content increasing,the size of dispersion phase getting larger as well as slenderness ratio.Striped dispersion phase was beneficial to enhance tensile strength while sheet dispersion phase weaken the tensile strength for breaking the integrality of PP base.In addition,the dispersion phase tended to grow bigger even became continuous phase with higher melt temperature and PA6 content.The crystal morphology abservation results showed that crystals at core layer and outside layer were small and distributed densely while the middle layer had relatively bigger crystal.And crystal at core layer and outside layer tended to distribute linearly along melt flow direction.Further more,with PA6 content reasing crystal tended to grow bigger but an excessive PA6 content reaching up to 30wt% crystal stoped growing.The crystallinity test results show: PP only had ? crystal form while PA6 had ? and ? crystal form in the process of WAIM.And the DSC curves showed that the melt peak of ? slowly moved to ? melt peak along with PA6 content increasing,and merged as one peak when PA6 content reached up to 40wt%.Besides,PP at core layer and ouside layer had higher crystallinity than middle layer.PA6 crystallinity at outsied layer was higher than both core layer and middle layer,and with PA6 content increasing crystallinity of outside layer and middle layer increased slowly while core layer increased greatly.Moreover the results also showed that low PA6 content may caused difficulty in crystallization for PA6.And the changing of processing parameters can change the temperature field and shearing field which could influence the nucleating and growing of crystal.The biggest difference of crystallinity between the samples with maximum and minimum tensile strengthes were that the sample with maximum tensile strength had bigger crystallinity at ouside layer which was good for intensifying tensile strength.6)CFD models were built and UDF functions were compiled to get the molding flow field of different PA6 content and different processing parameters from Fluent simulation platform.The reliability of numerical simulation was confirmed by comparing 30wt% of PA6 content of PP/PA6 WAIM simulation results with experiment results.Temperature field,velocity field and pressure field were obtained to analyse the formation mechanism of phase morphology,crystal morphology and crystallinity under different processing parameters and different PA6 content,which revealed the molding mechanism of WAIM of PP/PA 6blends.