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.     

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     

Highly endurable hydrostatic pressure polyethylene pipe prepared by the combination of rotation extrusion and lightly cross-linked polyethylene

In this study, lightly cross-linked polyethylene (XPE) with adequate crosslinking networks and high fluidity was prepared by optimized 5 kGy electron beam irradiation, and further added into polyethylene (PE) resins to suppress the molecules relaxation. Based on the relaxation time and the time of shish-kebab fixed by crystallization, it demonstrated successfully that with the addition of 5% XPE, the thermal movement was suppressed to prolong the relaxation time from 0.79 s to 1.15 s, longer than the fixation time, providing insurance for the formation of shish-kebab. Further, rotation extrusion was adopted to manipulate the flow pattern. With the rotation of mandrel and die, the flow of polymer melt changed from axial movement to helical forward, directing the molecular orientation and shish-kebab to deviate from the axis. Accordingly, under coupled effects of XPE and rotation extrusion, massive off-axis shish-kebabs were generated, endowing the prepared pipe with excellent resistance to hydrostatic pressure, as evidenced by that the failure time was 297 h, 213.7 times longer than that of the convention-extruded pure one.     

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.     

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     

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 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     

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.     

The Effect of Rotational Extrusion on the Structure and Properties of HDPE Pipes

High density polyethylene (HDPE) pipes were prepared using a novel rotational extrusion processing system. The experimental results showed that the hoop stress exerted by either mandrel rotation or die rotation could have the macromolecular chains oriented in hoop direction and alter the crystallization behavior of HDPE during rotational extrusion, resulting in forming transcrystals with larger crystalline size, thicker lamellae, higher Tm. Therefore, the mechanical properties of HDPE pipes were greatly improved in hoop direction, which was attributed to the changes of the crystalline morphology and the molecular orientation under the action of the hoop stress field.     

PPR管材专用树脂的结构与加工性能

通过核磁共振分析和相对分子质量分布测试研究了无规共聚聚丙烯(PPR)管材专用树脂的结构与性能,确定了试样的最佳挤出实验条件,完成了优化条件下挤出管材的静液压试验和爆破试验.PPR管材专用树脂的乙烯摩尔分数为3％～4％,重均分子量为55×104左右,挤出加工温度为230℃时制品性能最佳;管材制品规格为φ32 mm×2.9 mm时,爆破压力超过4.200 MPa,并通过了3.8 MPa,95℃,165 h的静液压测试.     

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     

The effect of a polypropylene skin on the structure and properties of PE/PP dual layer pressure pipe

A new and growing family of polyethylene (PE)‐based pressure pipes have a polypropylene (PP) skin. The effect of the PP skin on the structure and properties of the core PE pipe was investigated by comparing the skinned pipe with an uncoated pipe made from the same PE material and with the same dimensions. The annealing effect introduced by the skin changed the PE core pipe density profile across the wall thickness, increasing density in the PE core pipe near to its outer surface. The density at the bore of the coated and the uncoated pipe was similar. The melting temperature and enthalpy of melting data from DSC agreed with the density profile results. The melting temperature of PE core pipe material close to the PP skin increased with increasing skin thickness. Residual stress assessment indicated that, as the PP skin thickness increased, the PE core pipe had a lower level of overall residual stress in the hoop direction. Long‐term hydrostatic strength (LTHS) tests were carried out and showed a higher strength for the coated pipe than the uncoated one. The observed structural changes have been used to explain the relative strength of these two PE pipes. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Fracture prediction in tough polyethylene pipes using measured craze strength

In this study, an empirical model is developed that predicts the time to failure for PE pipes under combined pressure and deflection loads. The time‐dependent craze strength of different PE materials is measured using the circumferentially deep‐notched tensile (CDNT) test. In agreement with previous research, results indicate that bimodal materials with comonomer side‐chain densities biased toward high‐molecular‐weight PE molecules exhibit significantly higher long‐term craze strengths. A comparison of currently available PE materials with CDNT samples taken from a PE pipe that failed by slow crack growth in service clearly indicates the superior performance of new‐generation materials. Using measured craze strength data from the CDNT test, a simplified model for predicting failure in buried PE pipes is developed. Extending previous research, the reference stress concept is used to calculate an equivalent craze stress for a pipe subjected to combined internal pressure and deflection loads. Good agreement is obtained between the model predictions and observed failure times in an experimental test‐bed study of pipes under in‐service loading conditions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Multi‐Relaxation Test to Characterize PE Pipe Performance

A new concept is proposed, which uses results from a multi‐relaxation test to characterize transition of deformation mechanisms in polyethylene (PE) pipes, for plastic deformation from the amorphous phase only to the involvement of the crystalline phase. The former mechanism is believed to lead to brittle fracture, while the latter to ductile fracture. This phenomenon is believed to be related to the transition from ductile to brittle (DB) fracture that has been observed in creep tests of PE pipes by reducing the applied stress below a critical level. This paper presents results from 6 PE pipes of different density and molecular weight distribution. The results suggest that high‐density PE pipes require a higher deformation level for the DB transition than the medium‐density PE pipes. The results also suggest that the trend of change in the critical stress level for the DB transition is close to the trend of change in the hydrostatic design base, but the former takes less than two weeks to complete, while the latter more than 1 year. Therefore, the multi‐relaxation test can be used as an alternative method to characterize PE pipe performance, as a means for preliminary screening or in‐service monitoring of pipe performance.     

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。     

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。     

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.     

含少量高分子量组分的HDPE共混物在复合应力场中成型管材的性能研究

将少量相对高分子量的高密度聚乙烯(HDPE)加入到相对低分子量的通用级HDPE中,在复合应力场作用下挤出制成管材。通过力学性能测试、SEM、WAXD及DSC分析对制品的性能及结构进行了表征,结果表明:在剪切或拉伸应力场中,高分子量HDPE的大分子链会成为初级晶核,促进诱导体系生成大量倾斜排列的串晶和串晶互锁结构,明显改善了管材试样周轴两向的力学性能。     

Effects of high molecular weight component on crystallization of polyethylene under shear flow

Crystallization of polyethylene (PE) blends of low and high molecular weight components under shear flow was studied using time-resolved depolarized light scattering (DPLS), focusing on effects of the high molecular weight component on the shish-kebab structure formation. Anisotropic two-dimensional scattering pattern due to shish-like structure formation was observed above a certain concentration of the high molecular weight PE. The threshold was about 2.5-3 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. On the basis of these results a gel-spinning-like mechanism for the shish-like structure formation has been proposed. The DPLS results also implied that the shish-like structure was mainly formed from the high molecular weight PE. This was confirmed by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements on an elongated PE blend of low molecular weight deuterated PE and high molecular weight hydrogenated PE (3 wt%).     

Thermal-oxidative aging performance and life prediction of polyethylene pipe under cyclic and constant internal pressure

To study the aging characteristic of the polyethylene (PE) 100 pipe under constant and cyclic internal pressure, a new accelerated thermal-oxidative aging test equipment was developed. Thermal-oxidative aging behaviors of PE pipe under constant pressure and cyclic pressure were studied in various temperatures (80, 95, and 110 °C) by accelerated aging tests. The tensile test result shows that the fracture strength of PE pipe decreases as the aging time prolongs. Meanwhile, the pipe aged in the higher temperature behaves faster decreasing rate. A faster decreasing rate is also observed under the condition of cyclic pressure, compared to that under constant pressure. The thermal stability of the pipe gradually reduces and the reducing rate under cyclic pressure is higher than that under constant pressure. The result of infrared spectroscopy test suggests that oxygen-containing groups (i.e., CO) are formed on the surface of PE pipes, indicating that oxidative degradation phenomenon of PE pipes occurs during the aging process. Furthermore, based on the measured result of fracture strength, lifetime prediction models of PE 100 pipe under constant pressure and cyclic pressure are proposed. The result shows that the lifetime of PE 100 pipes obviously decreased under cyclic pressure, and it is 27.96% shorter than that under constant pressure. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47766.