Morphologies and mechanical properties of HDPE induced by small amount of high‐molecular‐weight polyolefin and shear stress produced by dynamic packing injection molding

To better understand the effect of a small amount of high‐molecular‐weight polyethylene (HMWPE) on the mechanical properties and crystal morphology under the shear stress field, the dynamic packing injection molding (DPIM) was used to prepare the oriented pure polyethylene and its blends with 4% HMWPE. The experiment substantiated that the further improvement of tensile strength along the flow direction (MD) of high‐density polyethylene (HDPE)/HMWPE samples was achieved, whereas the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional molding. Tensile strength in both flow and TDs were highly enhanced, with improvements from 23 to 76 MPa in MD and from 23 to 31 MPa in TD, besides the toughness was highly improved. So, the samples of HDPE/HMWPE transformed from high strength and brittleness to high strength and toughness. The obtained samples were characterized via SEM and TEM. For HDPE/HMWPE, the lamellae of the one shish‐kebab in the oriented region may be stretched into other shish‐kebab structures, and one lamella enjoys two shish or even more. This unique crystal morphology could lead to no yielding and necking phenomena in the stress–strain curves of HDPE/HMWPE samples by DPIM. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Biaxially self‐reinforced high‐density polyethylene prepared by dynamic packing injection molding. I. Processing parameters and mechanical properties

Uniaxial oscillating stress field by dynamic packing injection molding (DPIM) is well established as a means of producing uniaxially self‐reinforced polyethylene and polypropylene. Here, the effects on the mechanical properties of high‐density polyethylene (HDPE) in both flow direction (MD) and transverse direction (TD) of packing modules and processing parameters in DPIM are described. Both biaxially and uniaxially self‐reinforced HDPE samples are obtained by uniaxial shear injection molding. The most remarkable biaxially self‐reinforced HDPE specimens show a 42% increase of the tensile strength in both MD and TD. The difference of stress–strain behavior and impact strength between MD and TD for the DPIM moldings indicates the asymmetry of microstructure in the two directions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1584–1590, 2004     

Mechanical Properties and Crystal Structure of HDPE Induced by Small Amount of High-molecular-weight and Low-molecular-weight Polyethylene under Shear Stress Produced by Dynamic Packing Injection Molding

In order to better understand the effect of small amount of both high-molecular-weight polyethylene (HMWPE) and low-molecular-weight polyethylene (LMWPE) on the mechanical properties and crystal morphology under the shear stress field, the dynamic packing injection molding (DPIM) was used to prepare the oriented pure polyethylene samples and its blends ones with different contents of HMWPE and LMWPE. The experiment substantiated that the further improvement of tensile strength and impact stength along the flow direction (MD) of HDPE/HMWPE/LMWPE samples was achieved, while the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional molding. When the contents of HMWPE and LMWPE were respectively 8% in blends, the tensile strength in both flow and transverse directions of the samples were highly enhanced, with improvements from 27.75 MPa to 115.43 MPa (about 316%), in MD and from 23MPa to 32.74 MPa (about 42.34%), in TD; besides the impact strength was improved from 21.55 KJ/m² to 72.6 KJ/m² (about 236.89%), in MD but decreased from 17 KJ/m² to 6.92 KJ/m² in TD. The obtained samples were characterized via DSC, WAXD and SEM. For HDPE/HMWPE/LMWPE, the shish-kebab structure which is composed of stretched chains (shish) and lamellae (kebab) was seen in the oriented region of DPIM samples and the spherulites existed in the oriented region of SPIM samples. Furthermore, the appropriate amount HMWPE and LMWPE (about 8%, respectively) blended into mixture can improve the thickness and the length of lamellae, and the degree of crystallinity in shear region by DPIM which were approved by DSC and SEM, at the same time, it can also enhance the intensity of orientation of lamellae in shear region confirmed by SEM and WAXD. The reason of improvement of mechanical properties is the existence of these thicker, longer and more orientated lamellae in shear region.     

Self‐reinforcement of high‐density polyethylene/low‐density polyethylene prepared by oscillating packing injection molding under low pressure

This article reports the toughness improvement of high‐density polyethylene (HDPE) by low‐density polyethylene (LDPE) in oscillating packing injection molding, whereas tensile strength and modulus are greatly enhanced by oscillating packing at the same time. Compared with self‐reinforced pure HDPE, the tensile strength of HDPE/LDPE (80/20 wt %) keeps at the same level, and toughness increases. Multilayer structure on the fracture surface of self‐reinforced HDPE/LDPE specimens can be observed by scanning electron microscope. The central layer of the fracture surface breaks in a ductile manner, whereas the break of shear layer is somewhat brittle. The strength and modulus increase is due to the high orientation of macromolecules along the flow direction, refined crystallization, and shish‐kebab crystals. Differential scanning calorimetry and wide‐angle X‐ray diffraction find cocrystallization occurs between HDPE and LDPE. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 799–804, 1999     

Studies on blends of high‐density polyethylene and polypropylene produced by oscillating shear stress field

Tensile strength and morphology of blends of high‐density polyethylene (HDPE) and polypropylene (PP) obtained by oscillating packing injection molding were investigated via Universal Testing Machine, DSC, and SAXS. Tensile strength is greatly enhanced from 24.5 MPa to more than 90 MPa for pure HDPE and for blends with PP content less than 10 wt %. There exists a sharp decrease of tensile strength when PP content is more than 10 wt %. The shear‐induced morphologies with core in the center, oriented zone surrounding the core and skin layer are observed in the cross‐section areas of the samples. Interestingly, a sharp decrease of oriented zone is seen when PP content is more than 10 wt %, associated with the sharp decrease of tensile strength. DSC result shows double melting peaks with a high‐temperature melting peak that is not present in the endotherm obtained from the central core and obtained from the samples by static packing injection molding, which indicates the existence of shish‐kebab structure in the oriented zone. However, there is no difference of crystallinity between the samples by oscillating and by static packing injection molding. SAXS was used to analyze the complicated morphologies induced by shear stress, and results show that the crystal thickness could be greatly increased under shear stress. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 58–63, 2002     

Insert injection molding of high‐density polyethylene single‐polymer composites

A process for making high‐density polyethylene (HDPE) single‐polymer composites (SPCs) by insert injection molding was investigated. HDPE SPCs with relatively good tensile and interfacial properties were prepared within a short cycle time within a temperature range of 40°C. Melt‐spun HDPE fibers were made from the same resin as the matrix. The fibers were heat treated in silicone oil, with and without tension, to study the changes of fiber properties upon exposure to high temperature. HDPE SPCs containing about 30 wt% lab‐made HDPE fabric achieved a tensile strength of 50 MPa, 2.8 times that of neat HDPE. The peel strength of HDPE SPCs increased with increasing injection temperature and achieved a maximum value of 16.7 N/cm. Optical micrographs of polished transverse cross‐sections of the SPC samples showed that higher injection temperature is beneficial to the wetting and permeation properties of the matrix. Scanning electronic microscope photographs suggested good bonding and compatibility between the fibers and the matrix. POLYM. ENG. SCI., 55:2448–2456, 2015. © 2015 Society of Plastics Engineers     

HDPE在少量HMWPE和剪切应力双重诱导作用下结构与性能的关系

将少量高分子量聚乙烯(HMWPE)和动态剪切应力场同时引入到高密度聚乙烯(HDPE)的注射成型过程中,在双重诱导的作用下制备了性能良好的聚合物材料,其拉伸强度和缺口冲击强度都有大幅度提高;同时通过形态表征(差示扫描量热、广角X射线衍射、扫描电子显微镜分析)对增强增韧的机理进行了探讨。     

Orientation effects on the deformation and fracture properties of high‐density polyethylene/ethylene vinyl acetate (HDPE/EVA) blends

In this paper, the tensile deformation and fracture toughness of high‐density polyethylene (HDPE)/ethylene vinyl acetate (EVA) blends, obtained by dynamic packing injection moulding, have been comprehensively investigated in different directions of rectangle samples, including longitudinal, latitudinal and oblique directions relative to the flow direction. Two kinds of EVA were used with VA content 16 wt% (16EVA) and 33 wt% (33EVA) to control the interfacial interactions. The results indicate that molecular orientation and interfacial interaction play very important roles to determine the tensile behaviour and fracture toughness. Biaxial‐reinforcement of tensile strength was seen for HDPE/16EVA blends but only uniaxial‐reinforcement was observed for HDPE/33EVA blends. The difference is caused by the different interfacial interactions as highlighted by the peel test, scanning electron microscopy (SEM) observation as well as theoretical evaluation. Very high impact strength, decreasing with increasing EVA content, was observed when the fracture propagation is perpendicular to the shear flow direction, while a low impact strength, increasing slightly increasing with EVA content, was seen when the fracture propagation is parallel to the shear flow. The fracture of oblique samples is always along the flow direction instead of along the impact direction or tensile direction. The tensile behaviour and fracture toughness are discussed on the basis of the formation of transcrystalline zones, orientation of EVA particles and matrix toughness of HDPE in different directions. Copyright © 2004 Society of Chemical Industry     

Biaxially self‐reinforced high‐density polyethylene prepared by dynamic packing injection molding. II. Microstructure investigation

Differential scanning calorimetry, wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, and transmission electron microscope are employed to study the microstructure of biaxially self‐reinforced high‐density polyethylene (HDPE) prepared in uniaxial oscillating stress field by dynamic packing injection molding. The results indicate that the biaxial self‐reinforcement of HDPE is mainly due to the existence of interlocking shish‐kebab morphology (i.e., zip fastener structure), along with the orientation of lamellae and molecular chains and the enhanced crystallinity. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1591–1596, 2004     

Study of the effect of organic clay on the shear‐induced crystallization of high‐density polyethylene through dynamic‐packing injection molding

Organic nanoparticles as heterogeneous nucleators have a great effect on the crystallization of polymer matrices in nanocomposite systems, and the effect will be enhanced under shear flow. A home‐made dynamic‐packing injection molding (DPIM) device was developed to explore the effect of organic clay on the shear‐induced crystallization of high‐density polyethylene (HDPE). Differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and scanning electron microscopy (SEM) were used to characterize the flow‐induced crystalline structure of HDPE/clay nanocomposite injection moldings. It was found that higher crystallinity and thicker crystal planes which contribute to the improvement of mechanical properties were achieved in HDPE/clay nanocomposite samples prepared by DPIM. DSC results clearly showed that an increase of about 16% in crystallinity was achieved in dynamic HDPE/clay nanocomposite samples compared with traditional unfilled HDPE samples. WAXD confirmed that dynamic HDPE/clay nanocomposite samples had maximum crystal sizes at the (110) and (200) planes of 335 and 305 Å, respectively. SEM images indicated that the arrangement of crystalline structures in dynamic HDPE/clay samples was altered slightly compared with unfilled HDPE samples prepared using the same processing parameters. The results showed that organic clay was beneficial for increasing crystallinity and crystal size in the HDPE/clay nanocomposite system under shear flow. Meanwhile the arrangement of crystalline structures was insignificantly affected by the organic clay, and the preferred regular arrangement of lamellae could still be formed in the dynamic HDPE/clay nanocomposite system. Copyright © 2010 Society of Chemical Industry     

Biaxial self‐reinforcement of isotactic polypropylene prepared in uniaxial oscillating stress field by injection molding. I. Processing conditions and mechanical properties

This research explores the longitudinal and latitudinal mechanical properties of injection‐molded isotactic polypropylene (iPP) prepared in a uniaxial oscillating stress field by oscillating packing injection molding (OPIM). The methods, processing conditions, and mechanical test results for iPP by conventional injection molding (CIM) and OPIM are described. The mechanical properties in the flow direction (MD) and transverse direction (TD) of the OPIM moldings indicate three types of self‐reinforced iPP moldings. The pronounced biaxially self‐reinforced iPP specimens exhibit a 55–70% increase of the tensile strength and more than a fourfold increase of the impact strength in the MD, together with more than a 40% increase of the tensile strength and a 30–40% increase of the impact strength in the TD. The OPIM moldings show different stress–strain behavior in the MD and TD. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1906–1910, 2000     

Mechanical properties,stress‐relaxation,and orientation of double bubble biaxially oriented polyethylene films

Biaxially oriented linear low density polyethylene (LLDPE) films were produced using the double bubble process with different machine direction (MD) orientation levels and the same transverse direction (TD) blow‐up ratio. Their mechanical behavior was characterized in terms of the tensile strength and tear resistance. The viscoelastic behavior of oriented films was studied using dynamic‐mechanical thermal analysis (DMTA). The microstructure and orientation were characterized using microscopy, X‐ray diffraction pole figures, and birefringence. The results indicate that MD ultimate tensile strength increases and the TD one decreases with MD stretching ratio. Tear propagation resistance, in general, remained mainly constant in TD and decreased in MD, as the draw ratio was increased. The morphology analyses exhibit a typical biaxial lamellar structure for all samples with different lamellar dimensions. Orientation of c‐axis in crystalline phase, molecular chain in amorphous phase along MD increased with draw ratio. In most crystals, a‐axis was located in the normal direction (ND) and the b‐axis in the ND–TD plane. A good correlation was observed between c‐axis orientation factor and MD mechanical properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3545–3553, 2006     

Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom‐made electromagnetic dynamic plasticating extruder was adopted to extrude high‐density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high‐circumferential‐strength pipes. RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial self‐reinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide‐angle X‐ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology. Copyright © 2008 Society of Chemical Industry     

Super polyolefin blends achieved via dynamic packing injection molding: the morphology and mechanical properties of HDPE/EVA blends

As a part of long-term project aimed at super polyolefin blends, in this work, we report the mechanical reinforcement and phase morphology of the blends of high-density polyethylene (HDPE) and ethylene vinyl acetate (EVA) achieved by dynamic packing injection molding. The shear stress (achieved by dynamic packing injection molding) and interfacial interaction (obtained by using EVA with different VA content) have a great effect on phase morphology and thus mechanical properties. The super HDPE/EVA blends having high modulus (1.9–2.2 GPa), high tensile strength (100–120 MPa) and high impact strength (six times as that of pure HDPE) have been prepared by controlling the phase separation, molecular orientation and crystal morphology of the blends. The phase inversion was also found to shift towards lower EVA content under shear stress. The enhancement of tensile strength and modulus originates from the formation of oriented layer, while the high impact strength is related to shear induced phase morphology. DSC studies indicated that the shish kebab crystal structure that also contributes to the enhancement of tensile strength is formed in the oriented layer. The dramatic improvement of impact strength may result from the formation of microfibers and elongated EVA particles along the flow direction. Wu's toughening theory was found non-applicable for the elongated and oriented rubber particles, and a brittle–ductile–brittle transition was observed with increasing EVA content.     

在单方向往复低剪切力场中生成双向自增强HDPE试样的研究——(Ⅰ)试样制备及工艺条件对力学性能的影响

介绍在动态保压注塑成型技术提供的单方向往复低剪切应力场作用下来制备双向自增强试样。作者设计并制造了成型装置，初步研究了其成型原理、成型工艺、探讨了自增强效果与各工艺条件之间的关系。结果表明，采用本文所述的动态保压注塑成型技术显著提高了ＨＤＰＥ试样的力学性能——流动方向和垂直流动方向的拉伸强度均从２５ＭＰａ提高到３６ＭＰａ以上，达到了双向自增强的效果。自增强ＨＤＰＥ试样的拉伸强度强烈依赖于熔体的流动条件：流动方向的拉伸强度随液压站输出压力的提高而提高，垂直流动方向的拉伸强度则有一个对应最大拉伸强度的液压站输出压力；模具温度对拉伸强度的影响与压力对拉伸强度的影响相类似；熔体温度的提高有利于两个方向拉伸强度的提高；保压周期太长或太短均会使拉伸强度下降。     

Mechanical and thermal properties of high‐density polyethylene toughened with glass beads

Glass beads were used to improve the mechanical and thermal properties of high‐density polyethylene (HDPE). HDPE/glass‐bead blends were prepared in a Brabender‐like apparatus, and this was followed by press molding. Static tensile measurements showed that the modulus of the HDPE/glass‐bead blends increased considerably with increasing glass‐bead content, whereas the yield stress remained roughly unchanged at first and then decreased slowly with increasing glass‐bead content. Izod impact tests at room temperature revealed that the impact strength changed very slowly with increasing glass‐bead content up to a critical value; thereafter, it increased sharply with increasing glass‐bead content. That is, the Izod impact strength of the blends underwent a sharp transition with increasing glass‐bead content. It was calculated that the critical interparticle distance for the HDPE/glass‐bead blends at room temperature (25°C) was 2.5 μm. Scanning electron microscopy observations indicated that the high impact strength of the HDPE/glass‐bead blends resulted from the deformation of the HDPE matrix. Dynamic mechanical analyses and thermogravimetric measurements implied that the heat resistance and heat stability of the blends tended to increase considerably with increasing glass‐bead content. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2102–2107, 2003     

Wood‐thermoplastic composites from wood flour and high‐density polyethylene

High‐density polyethylene/wood flour (HDPE/WF) composites were prepared by a twin‐screw extruder. The effects of WF, silane coupling agents, polymer compatibilizers, and their content on the comprehensive properties of the WF/HDPE composites have been studied in detail, including the mechanical, thermal, and rheological properties and microstructure. The results showed that both silane coupling agents and polymer compatibilizers could improve the interfacial adhesion between WF and HDPE, and further improve the properties of WF/HDPE composites, especially with AX8900 as a compatibilizer giving higher impact strength, and with HDPE‐g‐MAH as a compatibilizer giving the best tensile and flexural properties. The resultant composite has higher strength (tensile strength = 51.03 MPa) and better heat deflection temperature (63.1°C). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Study on the Improvement of Crystallization in HDPE Induced by High–Molecular-Weight Polyethylene Through Dynamic Packing Injection Molding

The effect of high–molecular-weight polyethylene (HMWPE) on crystal morphology was investigated for high-density polyethylene (HDPE) through dynamic packing injection molding (DPIM). With the aid of differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), and scanning electron microscopy (SEM) measurements, a typical web-like shish kebab morphology, which markedly increases stiffness and toughness, was found in HMWPE-induced samples through DPIM. The SEM results show that the much better web-like shish kebab structure, in which most of the lamellae connect different columns, compared with conventional shish kebab, was formed in HDPE blends with 4% HMWPE content (B4) through DPIM. The WAXD studies indicate that orientation degrees of crystallographic planes (110) and (200) in the B4 samples were much higher than those of samples molded by static packing injection molding and B0 samples molded by DPIM. A combination of the higher degree of crystal orientation and the formation of web-like shish kebab led to simultaneous great increments of stiffness and toughness, which overcomes the traditional limitation that stiffness and toughness cannot be greatly enhanced simultaneously. All these results show that HWMPE favored for great improvement of crystal structures in HDPE when its content is appropriate through DPIM.     

Influence of mold design on the mechanical properties of high-pressure injection—molded polyethylene

High-pressure injection molding (nominal pressure 500 MPa) is known to substantially improve the mechanical properties of high-density polyethylene of a high molecular weight (HMWPE). This work shows that if the mold is equipped with an exit cavity, the tensile modulus and strength of HMWPE-bars molded is further improved at high pressure levels. The maximum values of the stiffness and strength (thin bars, 1 mm) obtained with the exit chamber is about 12 GPa and 260 MPa, respectively. The improvement due to the exit cavity is of the order of 30 percent for the tensile strength for thicknesses lower than 4mm, while the modulus increases about 1 to 1.5 GPa for bars with thicknesses between 1 and 6 mm. The orientation of the melt during the filling of the mold was also found to have an influence on the mechanical properties of the HMWPE bars.     

Morphological and physicomechanical analysis of high‐density polyethylene filled with Salago fiber

The environmental issues associated with the mass discarding of waste plastics in the Philippines have significantly raised for the past decade. However, this country is a home to many natural fibers which necessitates the development of ecofriendly materials to diminish the environmental footprint of polymers. High‐density polyethylene (HDPE) was filled with floured untreated and 5 wt % alkaline‐treated Salago fiber via melt compounding. The physical and mechanical characteristics of both types of composites were measured and compared. The composite filled with 30 wt % untreated fiber became very brittle, showing tensile strength and impact resistance of 15.8 MPa and 4.9 kJ/m², respectively. Alkaline treatment improved the mechanical properties of untreated composites, but not above the value of virgin HDPE. Nevertheless, the flexural strength of treated composites exceeded that of the virgin HDPE. Untreated composites absorbed water twice as the treated ones. Finally, morphological and fractography inspection on tensile and flexural test specimens showed improvement made by treatment on the interfacial adhesion between fiber and thermoplastic, corroborating the results from mechanical properties test. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46479.