Preparation And Characterization Of High Strength And Toughness HDPE-based Composites By Oscillatory Push-pull Molding Process

High-density polyethylene?HDPE?materials have attracted the widespread attention from both the industry and academia due to the advantages of low cost,easy processing,good acid and alkali resistance,and good mechanical properties.In addition,the high flexibility and regularity of HDPE macromolecular chains makes its conformational change extremely sensitive to external action fields?such as temperature field,flow field,pressure field,etc.?.Therefore,the crystal morphology of HDPE is easier to control.Synchronously reinforced and toughened HDPE composite materials have become the focus of research in order to broaden the application of HDPE.However,the traditional HDPE blending modifications are not ideal for its balance of strength and toughness.The macroscopic properties of polymer materials are determined by their internal microstructures,and the formation of these microstructures is strongly dependent on the processing method.Herence,this paper will focus on the optimization of the aggregation structure of HDPE,and carry out a series of research work with novel oscillatory push-pull molding methods to explore the strengthen and toughen mechanism of HDPE-based cmoposites.The specific research contents and main conclusions are as follows:?1?Deeply understand the orientation mechanism of HDPE under the different oscillatory push-pull shear flow fields intensity.HDPE samples with different crystals were prepared under a series of gradient oscillatory push-pull shear flow field.And then,the DSC,SEM,2D-SAXS,and 2D-WAXD were used to study the thermodynamic properties,crystal morphology and crystal structure of each sample in detail.The results show that within a certain intensity range of oscillatory push-pull shear flow field,the HDPE crystal morphology gradually developes from spherulite to oriented shish-kebab crystals as the the flow field intensity increases.However,when the intensity of this special flow field is too high,the crystal regularity is decreases,and various defects appear in the shish-kebab crystals.Molecular dynamics simulation results also show that there is an optimal shear strength for the crystallization of HDPE.Under this optimal condition,the crystals have the best regularity and the highest orientation degree.Molecular dynamics simulation results have a good correspondence with the experimental results.The mechanical performance showed that the tensile strength,Young's modulus,bending strength,impact strength,etc.increased first and then decreased with the increase of the oscillation push-pull shear flow field intensity.By adjusting the intensity of the oscillatory push-pull shear flow field,the tensile strength,Young's modulus,bending strength,bending modulus and impact toughness of the HDPE specimen under the optimal conditions?LOPPM-M?were increased by 176.2%,124.5%,129.2%,243.6%and 207.8%,respectively,achieving the original intention of the self-reinforcing.Based on the analysis of the experimental and molecular dynamics simulation results,the corresponding relationship between"molding process-microstructure-macroscopic properties"of HDPE is established,which provides theoretical guidance for the customized processing of semi-crystalline polymers.?2?Based on an in-depth understanding of the response behavior of HDPE to the oscillatory push-pull shear flow field,PTFE was selected and added into the HDPE matrix,and the PTFE nanofibers were formed in situ under the intense shear flow field,and the nanohybrid shish-kebab crystals were constructed finally.The introduction of PTFE greatly increases the density of the shish-kebab crystals,and the hybrid shish-kebab crystals interlocked with each other to form a crystal network.More importantly,the length of shish unit in the nano-hybrid shish-kebab crystals is obviously prolongated due to the guidance of PTFE nanofibers.And the reduced spacing between shish also promotes the interlocking effect of kebab lamellae.A large number of nano-hybrid shish-kebab crystals are induced by the in-situ PTFE nanofibers.The limitation of small number of shish-kebab crystals in a single polymer product is solved,so that the mechanical properties is greatly improved.In particular,when 5%PTFE is added,the tensile strength,Young's modulus and impact toughness reache 133.4MPa,4388.1MPa and69.1k J/m²,respectively,which are 503.6%,496.4%and 209.9%higher than the injection molded pure HDPE sample.?3?Exploring the application of general plastics in engineering field is an important research direction in the plastics processing industry.By adding the long-chain UHMWPE component to HDPE matrix,and preparing it under oscillatory push-pull shear flow field,the potential value of HDPE in engineering applications was explored.Benefiting from the advantages of long-chain structure in suppressing relaxation and inducing shish structure,denser shish-kebab crystals with micro-shish-kebab crystals and macro-shish-kebab crystals alternately formed in the sample.Owing to the inherent advantages of shish-kebab crystals,the mechanical properties and friction properties have been significantly improved.Compared with the conventional injection molded pure HDPE,tensile strength,Young's modulus and impact toughness were increased by 375.4%,587.4%and 677.4%when 30%UHMWPE was added,reaching 110.3MPa,5054.1 MPa and 48.2k J/m².The mass wear rate was decreased from18.6mg/MC of injection molded pure HDPE to 4.2mg/MC of LOPPM-30UH sample,and it was better than the compression molded pure UHMWPE of 7.5mg/MC.Thus,the HDPE/UHMWPE composites with dense shish-kebab crystals are expected to be applied in the fields of engineering plastic replacement and joint replacement.?4?The human load-bearing bone is a typical anisotropic structure,which gives it high strength and toughness.However,most of the currently load-bearing bone substitute materials are isotropic structure,which makes it difficult to meet the replacement requirements due to the poor mechanical properties.By analyzing the microstructure of natural load-bearing bones,the structural design and bionic preparation was carried out using the oscillatory push-pull molding method.As a result,an anisotropic bone-like structure was successfully constructed in the samples,its strength and modulus meet the requirements of human load-bearing bone replacement,and superior to the commercially available HAPEX^TM products.Combining the in vitro cell culture and in vivo bone defect transplantation experiment analysis,the HDPE/HA load-bearing bone substitute material with shish-kebab crystals exhibits good cell proliferation and adhesion,and forms a strong and stable interface layer with the host bone,which exhibit great potential in clinical applications.A dense shish-kebab crystal is constructured under the intense oscillatory push-pull shear flow field,which realizes the self-reinforcement and self-toughening of several HDPE-based composites,and promotes the application of HDPE in the high-end field.