High-density polyethylene pipe with high resistance to slow crack growth prepared via rotation extrusion

Slow crack growth (SCG) is one failure principal mode in polyethylene (PE) pressure pipe applications. In the conventional extrusion process, the molecular chains in the plastic pipes are oriented along the axial direction, which are disadvantageous to their resistance to SCG. In order to change the orientation direction of molecules in the plastic pipe, a new rotation extrusion processing system was designed to extrude high-density polyethylene (HDPE) pipes, and a thorough research was done on the effect of the rotation speed on its microstructure and resistance to SCG during the rotation extrusion. The experimental results showed that when the die rotated during the extrusion process of PE pipes, the hoop stress exerted on the polymer melt could make the molecular orientation deviate from the axial direction, and therefore the consequent multi-axial orientation of molecular chains could be obtained. As a result, the PE pipe with better resistance to SCG was prepared. Compared to the PE pipe produced by the conventional extrusion, the crack initiation time of the PE pipe manufactured by the novel method increased from 27 to 57 h.     

复合应力场下挤出HDPE增强管材性能的研究

用自行研制的能产生先剪切后拉伸的复合应力场挤出成型装置，挤出高密度聚乙烯（HDPE）管材，对管材周向、轴向力学性能进行了初步的研究。与一般牌号为DGDA6098的HDPE比，在HDPE中添加高相对分子质量高密度聚乙烯（HMWHDPE）后，发现HMWHDPE能够诱导HDPE沿应力场方向产生大分子取向和结晶。利用差示扫描量热仪（DSC）和扫描电镜（SEM）检测手段对试样的凝聚态结构进行分析，证实了复合应力场下制备的自增强管材双向力学性能都提高了。     

Synergistic effect of self‐assembling nucleating agent and crystallization promoter on polypropylene random copolymer pipes via rotation extrusion

In this study, high hoop tensile strength and toughness polypropylene random copolymer (PPR) pipes were successfully prepared through rotation extrusion and synergistic effect of self‐assembling nucleating agent (TMB‐5) and crystallization promoter (isotactic polypropylene, iPP). The result indicated low temperature toughness of PPR pipes could be improved by incorporating TMB‐5 and iPP, as the result of highly improved PPR crystallization capability and abundant β‐form crystal production. Both molecular chains and anisotropic crystallites deviated off the axial direction due to the hoop stress generated by rotation extrusion, leading to increased hoop orientation and pronouncing enhancement in hoop strength. Accordingly, the hoop tensile strength and impact strength of the modified PPR pipe reached 28.9MPa and 5.7kJ/m², increased by 126% and 43% compared to the convention‐extruded PPR pipe. POLYM. ENG. SCI., 56:866–873, 2016. © 2016 Society of Plastics Engineers     

Effect of rotation extrusion and molecular weight on ductile failure of polyethylene pipes under hydrostatic pressure

ABSTRACT

In this study, two kinds of polyethylene (PE) with different molecular weight were processed into the pipes via rotation extrusion and the failure behaviours under hydrostatic pressure as well as molecular relaxation were investigated. The experimental results showed that the high molecular weight PE exhibited slower relaxation behaviour with higher relaxation activation energy to facilitate the formation of shish-kebab under flow field, while for the low molecular weight one, the stretching molecules easily relaxed back coil state and spherulites were prone to form. Therefore, the low molecular weight PE pipes via convention and rotation extrusions had similar isotropic spherulite morphology and short failure times. In the case of high molecular weight PE, during the convention extrusion, polymer melts flowed along the axial direction to induce the alignment of shish-kebab accordingly, which went against to the hoop stress in hydrostatic pressure. With the rotation of mandrel and die, the direction deviated from the axis so that PE pipe exhibited better resistance to the hoop stress. The failure time was 182?h, 264% longer than the convention-extruded one. Accordingly, a new strategy to prepare high hydrostatic pressure PE pipe under cooperative effects of rotation extrusion and long molecular relaxation time was proposed.     

The influence of hoop shear field on the structure and performances of glass fiber reinforced three-layer polypropylene random copolymer pipe

Three-layer pipe has many advantages over single layer one, especially for the pipe of glass fiber (GF) reinforced materials. But the hoop strength of the pipe produced via convention extrusion is poor because GFs orient along axial direction. In this work, a self-designed rotation extrusion system was adopted to extrude GF reinforced three-layer polypropylene random copolymer (PPR) pipe, in which a hoop shear field was applied to the polymer matrix and fibers in the middle layer. The structure and performance of pipes were investigated via scanning electronic microscope (SEM) and synchrotron two-dimensional wide-angel X-ray diffraction (2D-WAXD). Due to the hoop shear field, the orientation of GFs in middle layer deviated from axial direction. As a result, PPR pipes with enhanced hoop tensile strength were obtained. Because of the three-layer structure and the production process, the molecular chains of middle layer did not emerge distinct orientation after rotation shear, as shown in 2D-WAXD and SEM experimental results. This three-layer pipe rotation extrusion system offers a novel method for the modification of pipes in manufacture industry. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46985.     

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     

复合应力场下制备自增强的HDPE管材

利用自行研制的先剪切后拉伸的挤管口模,对3种不同配比的HDPE6100M/HMWHDPE共混物挤出管材的周向、轴向强度进行研究,实验中发现少量的HMWHDPE有利于HDPE管材在复合引力场挤出中实现力学性能的双向自增强,预示了少量的HMWHDPE有利于诱导HDPE6100M分子沿应力场方向取向、结晶。     

Morphology and property of polyethylene pipe extruded at the low mandrel rotation

During the rotation extrusion of polyethylene (PE) pipes, with the rotating mandrel, compressed air as a cooling medium was introduced through their interior to achieve the quick cooling of the inner wall. The experimental results showed that the hoop stress exerted by mandrel rotation could promote the molecular orientation in the hoop direction; moreover, the introduction of compressed air could quicken its inner wall's cooling rate so as to slow down the relaxation of the oriented molecule and to reserve the orientation structure. Therefore, the hoop orientation degree increased with the increasing inner wall's cooling rate. As a result, the performance of the PE pipe was greatly enhanced. The hoop tensile strength of the PE pipe produced by the novel extrusion method increased from original 24.1 MPa up to 35 MPa; the pipe's crack initiation time increased from 27 to 60 h and the crack growth rate slowed down. POLYM. ENG. SCI., 50:1743–1750, 2010. © 2010 Society of Plastics Engineers     

Control of rotation extrusion over shish‐kebab crystal alignment in polyethylene pipe and its effect on the pipe's crack resistance

A self‐designed rotation extrusion system was adopted to extrude polyethylene (PE) pipe. The experimental results showed that when the mandrel and die rotated in the same directions during the PE pipe extrusion, apart from the axial stress, all polymer melts in PE pipes were also subjected to the hoop stress so that the formed shish‐kebab crystal in the whole pipe deviated from the axial direction greatly, which was further fixed by the double cooling on both inner and outer walls. As a result, the PE pipe with better resistance to slow crack growth was prepared. As compared to the PE pipe produced by the convention extrusion, the crack initiation time of the PE pipe manufactured by the novel method increased from 27 to 174 h, by 544%. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Control over crystalline form in polypropylene pipe via mandrel rotation extrusion

In this paper, isotactic polypropylene pipes were prepared via mandrel rotation extrusion and the effects of mandrel rotation speed on crystalline form of Polypropylene (PP) pipes were investigated. The results indicated that properly high mandrel rotation speed could promote the growth of β crystal markedly in the inner surface of PP pipes, while too high mandrel rotation speed could induce the formation of α column crystal and suppress the formation of β crystal. However, only similar α spherulites appeared in the outer layer of PP pipes prepared by different mandrel rotation speed. This implied there should be different key factors impacting the crystalline form along thickness direction of PP pipes: for external layer, its “cooling rate controlled” and for internal layer, its “mandrel rotation speed controlled”. As a result, gradient crystalline structure was obtained by the compounding effects of cooling rate and mandrel rotation speed.     

Continuously enhanced hoop strength of rotation-extruded polypropylene pipe via self-assembly β nucleating agent with different aspect ratio

Self-assembling β nucleating agents (TMB-5) and self-designed rotation extrusion device were applied to optimize and control over hoop strength of polypropylene (PP) pipes. By adjusting final heating temperature, TMB-5 efficiently self-assembled into fibrous morphology with controllable aspect ratio, and acted as an oriented template to direct the epitaxial crystallization of PP, into β-crystals with various lamellae-stacking patterns. The obtained structural information clearly demonstrated that enlarging the aspect ratio of TMB-5 could increase the ordering index and packing intensity of lamellae, moreover, impose them transforming from random arrangement to axial-orientation and then to orthogonal orientation. Among them, the anisotropic ones were further guided to align off the axial-direction of PP pipes via rotation extrusion, where the hoop drag flow caused by the mandrel rotation was superposed on the axial flow. As a result, the hoop tensile strength was enhanced monotonously from 21.3 MPa to 30 MPa for the rotation extruded PP pipes with increasing the aspect ratio of TMB-5.     

Joint effects of rotational extrusion and TiO2 on performance and antimicrobial properties of extruded polypropylene copolymer pipes

In this study, effects of titanium dioxide (TiO₂) and rotation extrusion on structures and properties of polypropylene random copolymer (PPR) pipes were investigated. The experimental results showed that with the presence of TiO₂, not only the antibacterial ability of PPR pipe was improved significantly but also the toughness was enhanced since a large number of PP chains were promoted to crystallize into β‐form crystals. Furthermore, when rotation extrusion was introduced into the process of PPR pipe, the drag hoop flow caused by mandrel and die rotation was superposed on the axial flow, so the polymer melts in the annulus underwent a helical flow and its flow direction deviated from the axis to drive the molecular orientation off the axial direction, bringing out the increased hoop strength. As a result, PPR pipe with excellent performance was prepared under the combined effect of rotation extrusion and TiO₂. The antibacterial activity was 99.2%, the hoop tensile strength reached 27.5 MPa, 67.7% higher than that of the convention‐extruded PPR pipe with TiO₂, and the impact strength was 10.9 kJ/m², increased by 81.6% compared to that of the rotation‐extruded pure PPR pipe. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42410.     

单向拉伸流场中挤出双向自增强透明HDPE管材

采用普通的工业用挤出设备，通过对加工参数的特殊控制，在单向拉伸流场中制得透明双向增强高密度聚乙烯管材，其轴向拉伸强度比普通高密度聚乙烯管材提高了3．2倍，周向拉伸强度提高了1．4倍。利用SEM，DSC等测试手段对管材的微观结构进行了表征，揭示了管材双向自增强的原因是形成了串晶互锁结构，管材透明的原因是晶粒尺寸比可见光的波长小。     

Effect of the inner wall cooling rate on the structure and properties of a polyethylene pipe extruded at a high rotation speed

A novel rotation extrusion processing system was self‐designed to prepare high‐performance polyethylene (PE) pipes. In this study, during the extrusion of the PE pipes at a high mandrel rotation speed, compressed air, as a cooling medium, was introduced through their interior to achieve the quick cooling of the inner wall and the effects of the inner wall cooling rate on the microstructure and mechanical properties of the obtained PE pipes were investigated. The experimental results showed that in contrast to conventional extrusion, the molecular orientation deviated from the axial direction under a high mandrel rotation speed and was fixed by the inner wall cooling; with increasing cooling rate, the orientation degree also increased. On the other hand, cooling promoted the augmentation of spherulites. So when the cooling rate reached a certain high point, the effect of cooling on the formation of spherulites was stronger than that on the fixation of the orientation. A much higher cooling rate decreased the orientation degree, which was closely related to the performance of the PE pipe. As a result, there was an optimal cooling rate of the inner wall during the rotation extrusion for better performance of the PE pipe. When the cooling rate was 1.5°C/s, the hoop strength of the PE pipe produced by the novel extrusion method increased from the original 24.1 MPa up to 37.1 MPa without a decrease in the axial strength, and the pipe's crack initiation time increased from 27 to 70 h. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Morphological and mechanical property studies of polymer extrudates obtained by the rotational extrusion process

The orientation of the reinforcing fibers in glass fiber filled polypropylene tubular extrudates has been controlled effectively by the superposition of the linear flow in a melt extruder with torsional flow generated by rotating the capillary portion of the extrusion die of the extruder. The so produced extrudates have mechanical properties which can be balanced along the hoop and extrusion direction by adjusting the extrusion rate and the rotational speed of the die for example, the breaking load of 75N along the hoop direction increases by ～40% when the capillary of the die was rotated at 80 rpm. At the same time the Young's modulus in the extrusion direction decreased from 1100 MPa under conventional extrusion conditions to 800 MPa.     

Effect of annular expansion pipe extruder head on mechanical properties of pipes

With polymer pipes being used more commonly, performance requirements are increasing. Studies on the enhancement of mechanical properties of polymer pipes are particularly important. In this study, a self-designed annular expansion pipe extruder head was used to enhance the mechanical properties of HDPE pipes. Different morphologies of the HDPE pipes were produced under different processing conditions. When the extrusion angle was 30° (P30), the best mechanical properties were obtained. The hoop tensile strength and axial tensile strength were 14.5% and 41.0% higher, respectively, compared with the specimen without expansion (P0). This improvement of mechanical properties can be attributed to several reasons. First, the processing parameters of P30 reached the threshold shear rate and strain for shish-kebab formation, as shown by scanning electron microscopy. Second, P30 has the highest orientation parameter and crystallinity of 0.679 and 67.27%, respectively, from 2D wide-angle diffraction (WAXD). Polarized FTIR shows the same trend as 2D-WAXD. Third, the outer bamboo-like self-reinforced structure is formed inside the pipe at 30° expansion angle while the core layer has a well-formed crystal structure; the special structure improves the overall performance of HDPE pipe. This method can be utilized in large-scale industrial production.     

Lamellar crystalline structure of hard elastic HDPE films and its influence on microporous membrane formation

The development of hard elastic high-density polyethylene (HDPE) precursor films and its influence on the microporous membrane formation have been investigated. As a first step, the HDPE precursor films with ‘row-nucleated lamellar crystalline’ structure were prepared by applying elongation stress to the HDPE melt during T-die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long-period lamellar spacing, crystallite size, and degree of crystallinity. The processing (melt extension and annealing temperature)-structure (lamellar crystalline structure)-property (hard elasticity) relationship of HDPE precursor films was also investigated. The uniaxial stretching of hard elastic HDPE precursor films induces the bending of crystalline lamellae, which leads to the formation of micropores between them. The observation of morphology and air permeability for the HDPE microporous membranes have revealed that the well-developed porous structures characterized by superior air permeability were established preferably from the precursor films prepared by the high stress levels and the high annealing temperatures. Finally, the relationship between the hard elasticity of HDPE precursor films and the air permeability of corresponding microporous membranes was discussed.     

复合应力场下挤出HDPE管材自增强的研究

用自行研制的能产生剪切和拉伸复合应力场的特殊挤出成型装置挤出HDPE管材,对管材周向、轴向力学性能进行了初步的研究。在通用级聚乙烯(DGDA6098)中添加高分子量HDPE后,发现该HDPE能够诱导DGDA6098沿应力场方向产生大分子取向和结晶,从而实现了复合应力场下管材力学性能的双向自增强。     

A mandrel-rotating die to produce high-hoop-strength HDPE pipe by self-reinforcement

This article provides a method to obtain high-hoop-strength high-density polyethylene (HDPE) pipe by mandrel-rotating extrusion. With properly selected processing temperature, pressure, and mandrel-rotating speed, the hoop strength of 90 Mpa has been got. Differential scanning calorimetry and X-ray scattering found the great strength enhancement was owning to high degree of macromolecular orientation in circumferential direction and the shish-kebab structure. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 323–328, 1998     

Effects of melt‐extension and annealing on row‐nucleated lamellar crystalline structure of HDPE films

The row‐nucleated lamellar crystalline structure of high‐density polyethylene (HDPE) films was prepared by applying elongation stress to HDPE melt during T‐die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long‐period lamellar spacing, crystallite size, and degree of crystallinity. The contribution of melt‐extension represented by draw‐down‐ratio (DDR) to the overall orientation was found to be most noticeable than other processing variables. Meanwhile, the long‐period lamellar spacing, the crystallite size, and the degree of crystallinity were influenced predominantly by the annealing temperature. Finally, the processing (melt extension and annealing temperature) – structure (lamellar crystalline structure) – property (hard elasticity) relationship of HDPE films was investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3326–3333, 2007