Morphology And Mechanical Properties Of Water Assisted Co-Injection Polypropylene/Polyamide 6 Blend

Water-assisted co-injection molding(WACIM)is an innovative injection molding technology that can be used to produce hollow part with multilayer structure.It combines the advantages of water-assisted co-injection molding(WAIM)and co-injection molding(CIM).The blending polymers are used in WACIM technology to improve product performance and suitability.However,there is no research on the WACIM process of blending polymers at home and abroad.Therefore,the WACIM process of the Polypropylene(PP)/Polyamide 6(PA6)blends as the outer layer and PP as the inner layer was research object.In order to better understand the WACIM process of blending polymers,combining experimental and theoretical methods,the effects of processing parameters and the content of dispersed phase on the residual wall thickness(RWT),mechanical property,and micromorphology were studied.The main content and conclusion of this research were summarized as follow:(1)The forming principle,process and theoretical research of WAIM,CIM,WACIM and the application of blending polymers were stated briefly.The main subjects of this research thesis were derived from the reviews.(2)The PP/PA6 blends with 10,20,30,and 40 wt% of PA6 were prepared by a blending extrusion granulator.And melt index was measured.It was found that with the increase of temperature,the melt index of blending polymers increased.And with PA6 content increasing,the melt index of blending polymers decreased.(3)The orthogonal experiment was carried out to investigate the influence of processing parameters on the RWT of WACIM process of PP/PA6 blends with the 30wt% of PA6.It was found that with the increase of outer melt temperature,total residual wall thickness(T-RWT)and outer layer residual wall thickness(OL-RWT)decreased.However it nearly has no effect on inner layer residual wall thickness(IL-RWT).With the increase of the melt switching delay time,increased the T-RWT and OL-RWT.It nearly has no effect on IL-RWT above 3s.With the increase of water injection delay time,T-RWT,IL-RWT and OL-RWT increased.With the increase of water injection pressure,T-RWT and IL-RWT decreased.It nearly has no effect on OL-RWT above 7MPa.With the increase of inner melt injection pressure,IL-RWT increased and OL-RWT decreased.And it nearly has no effect on T-RWT.With the increase of mold temperature,T-RWT,IL-RWT and OL-RWT decreased.The single factor experiment method was carried out to investigate the influence of PA6 content on the RWT of WACIM process of PP/PA6 blends.The process conditions were constant.When the content of PA6 was 10 wt%,the OL-RWT was slightly thinner.When the content of PA6 was from 20 wt% to 40 wt%,the OL-RWT was slightly thicker.(4)The tensile strength experiment were carried out to investigate the influence of processing parameters and PA6 content on tensile strength of WACIM pipes.It was found that with the increase of outer melt temperature,tensile strength of WACIM pipes decreased.With the increase of melt switching delay time and water injection delay time,increased the tensile strength of WACIM pipes.And the mold temperature nearly has no effect on tensile strength of WACIM pipes.With PA6 content increasing,tensile load and tensile strength of sample maximum increased.When the content of PA6 was from 10% to 20%,increased the elongation at break.When the content of PA6 was from 20% to 40%,decreased the elongation at break.(5)In order to study the relationship of microstructure,processing parameters and mechanical properties,polarized optical microscopy(POM)and differential scanning calorimetry(DSC)were applied to observe the crystal morphology and crystal crystallinity.It was found that crystals on the near the mold wall,the outer near interface,the inner near interface and the near water were small and distributed densely,while the outer intermediate layer and inner intermediate layer had relatively bigger crystals.Crystals of the sample No.22 with the lowest tensile strength was distributed densely than the sample No.5 with the highest tensile strength.On the outside layer of the pipe,the crystallinity of PP in the WACIM pipe fittings with different mass proportions was decreased from near the mold wall to near the interface.The variation of the crystallinity of PP of the PA6 content percentage of 10% was small and the crystallinity of PP of the PA6 content percentage of 40% decreased significantly.Except for the PA6 content percentage of 30%,the other three proportions show that with the increase of PA6 content,the crystallinity of PP increased.On the inside layer of the pipe,the crystallinity of PP is irregular.On the outside layer of the pipe,the crystallinity of PA6 of the PA6 content percentage of 30% was decreased from near the mold wall to inner near interface.Except for the PA6 content percentage of 30%,the other three proportions show that the crystallinity of PA6 was highest on the outer intermediate layer and the crystallinity of PA6 on the near mold wall and near the inner layer interface was lower than the outer intermediate layer.The crystallinity of PA6 of the PA6 content percentage of 40% were similar on the middle and sides.On the inside layer of the sample No.22 and the sample No.5,PA6 on the inner near interface and near water layer had lower crystallinity than inner intermediate layer.On the outside layer of the pipe,the crystallinity of PP of the sample No.5 was lower from the outside to the inside.And the crystallinity of PP of the sample No.22 was highest on the inner layer interface.On the outside layer of the pipe,the crystals of PA6 of the sample No.22 was bigger than the sample No.5.So with the increase of crystallinity of PA6,the tensile strength of WACIM pipes increased.(6)The simulation of the filling process of WACIM was achieved by means of ANSYS-Fluent software platform and user-defined-function(UDF).It was found that with the penetration of the inner melt,the velocity profile of the cross section of the outer melt in front of the leading edge was close to the plunger flow.The outer melt had a smaller drag flow under the flow of the inner melt after penetrating the leading edge.With the penetration of the inner melt front,the outer melt speed decreased rapidly.With the penetration of the inner layer,the peak of the melt rate of the outer layer near the solidified wall surface decreased rapidly.With the penetration of the inner frontier,the strain rate of the inner melt in the inner penetration interface increased rapidly.With the penetration of the inner melt,the strain rate of the outer melt near the penetration interface increased slowly and then decreased rapidly.With the penetration of water,the outer melt was not affected and the outer inner melt was not dragged.With the penetration of the water front,the velocity of the inner melt near the penetration interface decreased rapidly.With the penetration of water,the peak strain rate of the inner melt near the outer melt quickly dropped to zero.With the penetration of the water front,the strain rate of the inner melt near the water penetration interface increased and then dropped to zero.The reason for the difference in crystal morphology and crystallinity was explained through simulation.The molding mechanism and molding process of WACIM of PP/PA6 blends were revealed by combining the results of experiment and observation.