Study On The Morphology And Properties Of Multi-Phase Polyolefin Blends Via High Speed Thin-Wall Injection Molding

Since industrial production of polyolefins in the 1930 s,they have been widely used in various fields of society due to their advantages of light weight,low price,large output,easy processing,and excellent performance.With the development of society,the performance requirements for polyolefin materials are getting higher and higher,which requires modification of polyolefins.How to control the phase morphology of blends during blends modification play a key role for obtaining the desired properties.Owing to the advantages of short processing cycle,automatic production,molding products with complex shape,and good product repeatability,more than 50% of polymer products were processed by injection molding.High-speed thin-wall injection molding is widely used for 3C products and automotive due to the advantages of shorter processing cycle,thinner mold,and more uniform internal stress.Main materials for 3C products and automotive are polyolefins.Compared with ordinary injection molding,high-speed thin-wall injection molding has more extreme dynamic conditions-high shear rate and melt flow rate which is caused by high injection speed and thin mold thickness,and confinement and extremely fast cooling rate which is caused by thin mold thickness.in previous work,it’s observed that the polypropylene(PP)/polyethylene(PE)blend can be used to fabricate an alternating multilayer structure via high speed thin-wall injection molding.Based on this find,the effects of injection conditions,viscosity ratio,melt elasticity,and interfacial tension on the microstructure of PP based blends during high speed thin-wall injection molding was studied.Although previous works have made some studies on the phase morphology evolution of PP based blends during high speed thin-wall injection molding,there are still many problems that have not yet been systematically researched.for example,the effect of injection conditions on the flow field,shear field and temperature field,and the effect of flow field,shear field and temperature field on the microstructure of the blends during the high speed thin-wall injection molding.Under the kinetic condition of high flow rate,high shear rate,and high cooling rate during high speed thin-wall injection molding,the influence of molecular chain structure of the polymer,alloy in the kettle,composition ratio,compatibilizer and filler on phase morphology evolution of the blend.Based on above problems,this thesis starts from high speed thin-wall injection molding products of polyolefin blends,and closely focuses on the influence of injection condition on flow field,shear field,and temperature field,and then the influence of flow field,shear field,and temperature field on the microstructure and properties of polyolefin blends,and the influence of molecular structure of polymer,alloy in the kettle,composition ratio,filler on the phase morphology and property during high speed thin-wall injection molding,deeply explore the "PolymerProcessing-Structure-Properties" relationship.The main findings are shown as follows:1)to explore how do injection conditions affect the flow field,shear field and temperature field during high speed thin-wall injection molding,and how do these field affect the structure and properties of the blends,the first part of this thesis uses finite element analysis software Moldflow to study the flow field,shear field and temperature field during high speed thin-wall injection molding with different gates,different injection speed and different mold thickness,and study the microstructure and mechanical properties of HSTWIM samples with the injection condition same as simulation.It’s observed that the L-shaped gate can make the more uniform flow field,shear field and temperature field during high speed thin-wall injection molding;the higher the injection speed,the faster the average flow rate and average shear rate during the filling step,and the higher the melt temperature after filling.The thicker the mold thickness,the lower the cooling rate during the high speed thin-wall injection molding,the lower the average flow rate and average shear rate during the filling step,and the more uniform the flow field,shear field,and temperature field.Uniform flow field,shear field and temperature field can well eliminate the appearance flaws such as flow marks and weld marks,and can also make the internal stress more uniform.The bigger the flow rate and shear rate during injection molding,the more regular the alternating multilayer structure.However,at least a filling time of 0.08 s is required to ensure full deformation of the dispersed phase.The bigger the cooling rate,the more regular the alternating layer structure.Some lamellae with a thickness of hundreds of nanometers,not the continuous alternating multilayer structure was not observed in high speed thin-wall injection molding samples with a mold thickness of 1 mm.The more regular the alternating multilayer structure of HSTWIM samples,the more obvious the anisotropic mechanical properties.The mechanical properties of HSTWIM samples with a mold thickness of 1 mm are almost isotropic.2)to explore how do the microstructure of the blends with different aggregated structures evolve during the high speed thin-wall injection molding,the second part of this thesis selects different high-density polyethylene(HDPE)/PP blends(PPs are alloy in the kettle and have different content of ethylene-propylene rubber(EPR))to study the effect of alloy in the kettle on the structure and properties of HDPE/PP blends under the extreme dynamic conditions(high shear,high melt flow rate and high cooling rate)during high speed thin-wall injection molding.It’s observed that an alternating multilayer structure and an epitaxial crystallization of PE molecular chains on PP crystals are observed in all HDPE/PP blends(maximum EPR content is 21%)of HSTWIM samples.Thanks to the phase-in-phase structure of PP,some EPR was observed on the interface of PP and PE of HSTWIM samples of HDPE/PP blends containing EPR.These EPR on the phase interface can effectively enhance the interfacial interaction.The multilayer structure,epitaxial crystallization and EPR on phase interface between PP and PE which is brought by high shear rate,high flow rate and extremely high cooling rate could well absorb and transfer energy during the deformation under external force.It’s why HSTWIM samples of HDPE/PP blends containing EPR have excellent toughness(the fracture energy of HDPE/ICP blends increase from 70 N/m2 for compression molding samples to 14440 N/m2 for HSTWIM samples.).It could provide some guidance for the preparation of functional materials with good toughness.3)to explore how do the microstructure of the blends with different composition ratios and fillers evolves during high speed thin-wall injection molding,the fourth part of this thesis selects HDPE/impact polypropylene(ICP)blends and HDPE-carbon nanotube(CNT)/ICP composites with different composition ratios to study the influence of composition ratio and CNT on the structure and properties of HDPE/ICP blends under high shear rate,high melt flow rate and extremely fast cooling rate during high speed thin-wall injection molding.It’s observed that when the contents of HDPE and ICP are close(60:40,50:50 and 40:60),alternating multilayer structures and epitaxial crystallization of PE molecular chain on PP crystal could be observed in HSTWIM samples of HDPE/ICP blends and HDPE-CNT/ICP composites.The EPR in ICP will migrate to the PE phase during HSTWIM,and some EPR would locate on the phase interface of PP and PE under the extreme kinetic conditions of HSTWIM.These EPR on phase interface can enhance the interfacial interaction between PP and PE.The EPR on phase interface,alternating multilayer structure and epitaxial crystallization are the reasons for the enhancement of the mechanical properties of PP/HDPE blend(the fracture energy of HDPE/ICP 50:50 increase about 572% than that of pure HDPE and 131% than that of pure ICP).Compared with HDPE,CNT makes it more difficult to deform for HDPE-CNT phase during HSTWIM.When HDPE is a dispersed phase in HDPE-CNT/ICP composites,it would change from lamella to ellipsoid.4)to explore how do the microstructure of polymer blends with different compatibilizer content and different composition ratios evolve during HSTWIM,the fifth part of this thesis selects h-i PP/low density polyethylene(LDPE)/ethylene-vinyl alcohol copolymer(EVOH)blends and h-i PP/LDPE/polyamide(PA)blends with different compatibilizer content and different composition ratios to study the effect of compatibilizer and composition ratios on the structure and properties of these threephase polyolefin blends under high melt flow rate,high shear rate and high cooling rate of HSTWIM.It’s observed that the EVOH and PA are both lamellae in the HSTWIM samples of these three-phase blends,and the size and aspect ratio of the lamellae increase with the increasing of EVOH and PA content.The size of the lamellae decreases with the increasing of the compatibilizer content.The aspect ratio of disperse size of HSTWIM and the size of the dispersed phase of CM sample decrease firstly and then increase with the increasing of the compatibilizer content,and reach the minimum value when the compatibilizer content is 2%.it shows that extreme kinetic conditions(high shear rate,high melt rate,and high cooling rate)during HSTWIM could effectively increase the compatibilization efficiency of compatibilizer when the compatibilizer content is more than 2%.Compatibilizers also increase the storage modulus and loss modulus of these blends.The effect of the hybrid compatibilizer on the interfacial tension,disperse phase size,and aspect ratio is the synergy result of two compatibilizers.The larger the aspect ratio and the longer the diameter,the better the barrier property of these material.The smaller the size of EVOH and PA,the better the toughness of the material,and the higher the strength and modulus of the material.5)to explore how the microstructure of polymer blends with different molecular structures(these ethylene-propylene copolymers have different ethylene content)evolves during high speed thin-wall injection molding,the third part of this thesis selects PP/TPE blends with different molecular structures to study the influence of the molecular structure of polyolefins on the phase morphology,crystallization behavior and luminousness of PP/TPE blends under high shear rate,high melt rate and high cooling rate.It’s observed that the higher the ethylene content in TPE molecular chain,the worse the compatibility between TPE and PP,the larger the size of the dispersed phase of the PP/TPE blend via compression molding.The high shear rate,high melt rate and high cooling rate during high speed thin-wall injection molding could effectively reduce the dispersed phase size,crystallinity and crystal size.For high speed thin-wall injection molded products,when the ethylene content difference between the two phases in the blend is greater than 15%,the size of the dispersed phase in the blend is micrometers;the ethylene content difference between the two phases of the blend is 12%～15%,the size of the dispersed phase of the blend is nanometers;and when the ethylene content difference between the two phases in the blend is less than12%,the blend has a homogeneous structure.The size of the dispersed phase with a micron level will greatly affect the luminousness of the material,so the homo-isotactic polypropylene(h-i PP)/TPE3 and h-i PP2/TPE3 with a micron-level dispersed phase have a much lower luminousness than other blends.