Investigation Of The Structure And Property Of Twice Melt Injection Molded Isotactic Polypropylene Products

The isotactic polypropylene(iPP)is one of the most used commercial polymer materials because of its low manufacturing cost and versatility.For polymer products,a variety of processing processes determine the structure of the product,the structure determines the performance of the product.For semi-crystalline polymers and other gauge polypropylene,the arrangement of molecular chains and their aggregation structure depend to a large extent on the flow field and shear field in molding process.The crystallization and structural regulation of the equal-gauge polypropylene have always been the focus and hotspot in the field of polymer research.In this work,the relevant experimental samples were prepared by TMIM(twice melt injection molding)with isotactic polypropylene(iPP).iPP/MWCNTs composites and iPP/β-NA composites are chosen as raw materials.Through structural morphology(polarizing microscope,2D X-ray diffraction)and mechanical properties characterization(creep and recovery behavior research,dynamic mechanical analysis,tensile properties test),the flow shear field,microstructure,crystallization behavior,macroscopic mechanical properties,rheological properties and steady state of the material during processing were studied.To determine the relationship between shear field-crystal structure-mechanical properties.The main experimental conclusions of this thesis are as follows:(1)In this work,to study the creep and recovery behavior of different morphologies,twice melt injection molding was used to produce iPP samples with different skin-core structure.We obtained a controlled skin-core structure of iPP samples via adjusting time intervals.The various skin-core structures had been confirmed by POM.Compared with the M1 samples,M2 samples presented better resistivity to creep behavior,because of the influence of shear layer thicknesses.The deformation and compliance of the M2 samples were much less than M1 samples at all the applied stress levels and temperatures.As the time interval increases beyond 9 s,the M2 samples show higher creep level and higher unrecoverable strain than those with lower time intervals.In addition,the creep behavior of the M1 samples and M2 samples can be satisfactorily described by the Burger’s model and the Findley power law model.Furthermore,the recovery behavior can be predicted quite well by the Weibull distribution equation.The calculated parameters confirm that the highly oriented structure take the resistivity to the creep and recovery behavior.(2)The TMIM samples of iPP/MWCNTs composites were studied.For the iPP/MWCNTs composites system,the rigidity and hardness of the whole composite system are increased by the addition of nano-filler MWCNTs,and the tensile strength of the nanometer packing is obviously increased.The difference between M1-CNTs and M2-CNTs skin-core structure can be observed by POM.Because the interface between MWCNTs and polypropylene matrix inhibits the entanglement of the entangled macromolecular chain in the polypropylene matrix,the polymer becomes easy to break,which results in the elongation at break from 616.42%of the M1 sample to 54.49%of the M1-CNTs sample.This not only makes the thicker orientation structure in the injection molding process difficult to generate,but also causes the whole system to be less affected by the TMIM process.(3)For the iPP/β-NA composite system,the addition ofβnucleating agent(β-NA)causes a large number ofβ-crystal nuclei in the composite system,and the overall crystallinity is significantly improved.The high shear force of the melt in the cavity causes the tensile orientation of theβ-crystal attached to the nuclei,and the shear force hinders the natural growth of theβ-crystal.The X_β of the M2-0.2 sample cortex and core layer were smaller than that of M1-0.2 sample X_β.A large number ofβ-crystal nuclei are introduced,andβ-crystal attach to the growth of nuclei.In the process of TMIM,the tensile orientation ofβ-crystal attached to nuclei hinders the natural growth ofβ-crystal,and the growth ofα-crystal is hindered by the growth ofβ-crystal,which leads to the improvement of the crystallinity of the whole composite system X_c.