Role of Ultrahigh Molecular Weight Polyethylene during Rotation

Research Note pubs.acs.org/IECR

Role of Ultrahigh Molecular Weight Polyethylene during Rotation Extrusion of Polyethylene Pipe Lin Pi, Xiaoyang Hu, Min Nie,* and Qi Wang State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China ABSTRACT: A small amount of ultrahigh molecular weight polyethylene (UHMWPE) was added to a PE matrix that was then processed into a pipe by a self-designed rotation extrusion system. The results showed that during the rotation extrusion of the PE pipe, the polymer melts helically moved forward in an off-axis direction to induce molecular orientation and shish-kebab alignment deviate from the axial direction. However, because of sufficient relaxation of oriented molecules at high temperature, numerous spherulites arose in the pure PE pipe. With the addition of UHMWPE, the flow character of the PE matrix was tailored so the molecular relaxation was suppressed and the flow effect on formation of shish-kebab was amplified, bringing about denser shish-kebabs off the axial direction in the PE pipe. As a result, a PE pipe with excellent resistance to slow crack growth was prepared.

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system.8 During the rotation extrusion of PE pipes, with the superposition of the axial flow and the hoop drag flow caused by the mandrel rotation, the polymer melts in the annulus underwent a helical flow which deviated from the axial direction to induce the molecular orientation of PE off the axial direction. Therefore, the “bamboo-like crazing” was avoided for the finished PE pipes and there was an increase in the resistance to the craze propagation along the pipe’s axial direction;9 however, the molecular orientation results from the competition between flow-induced molecular stretching and subsequent relaxation.10,11 During rotation extrusion, the oriented molecules easily relaxed into a coiled state due to thermal motion and thus there was only a small amount of oriented molecules in the final PE pipe prepared by rotation extrusion. It is extremely challenging to increase the content of oriented molecules off the axis in the PE pipe. Generally, slowing down the molecular relaxation will favor the stability of the flow-induced orientation state. Keller showed that the relaxation time of the polymers was a function of their molecular weight.12 High molecular weight polymers have a long relaxation time resulting from the existence of many entanglement points. Under a given flow field, their oriented precursors induced by flow can survive a longer time than polymers with low molecular weight.13,14 It is clear that high molecular weight components play an important role in promotion of the molecular orientation. Regarding the role of molecular weight, this paper introduced a small amount of ultrahigh molecular weight polyethylene (UHMWPE) into the commercially available PE pipe resin in combination with rotation extrusion to tailor the relaxation character of the melts and control the morphology in the PE pipe so as to prepare a PE pipe with excellent resistance to SCG.

INTRODUCTION Since first introduced into pipe production over 50 years ago, polyethylene (PE) pipes have been superior to pipes made from other materials (e.g., steel, copper, ductile iron) because they are lightweight, corrosion resistant, and are easily processed and welded. They are widely used in gas transportation, water supply, agricultural irrigation, and other fields.1 Resistance to slow crack growth (SCG) which determines the operating lifetime and reliability of PE pipe, is the most important property for its application.2,3 Therefore, preparation of PE pipes with excellent resistance to SCG is of great significance. The performances of polymer products is relative to the morphologies decided by the thermo-mechanical history that polymer melts experience.4,5 During the processing, the polymers are under complex and intense flow fields which in turn influence the subsequent crystallization as well as the morphology and final properties of the product. It is evident that the energy required by cracks propagating perpendicular to the molecular orientation is higher than that along the orientation; that is, the samples with the orientation in line with the stress can exhibit better resistance to SCG.6 In the application of PE pipes, they mostly are subjected to a hoop stress twice as high as the axial stress. Obviously, it is important to improve the molecular hoop orientation. However, in the conventional extrusion of PE pipes, since the polymer melts flow along the axial direction, the molecules are oriented along the axial direction and their axial performance is superior to the hoop one, resulting in that cracks often propagate parallel to the axis.7 This orientation state goes against the pipe application. It is well-known that for a natural pipe, such as bamboo, the fibers align along the bamboo axial direction so axial cracks, rather than transverse cracks, easily occur. The topological structure of bamboo leads to the expectation that the molecular orientation off the axial direction will enhance the performances of PE pipes. Accordingly, novel rotation extrusion equipment has been designed and developed by our group, in which the mandrel can rotate by adjusting the motor and gear reduction © XXXX American Chemical Society

Received: June 18, 2014 Revised: August 21, 2014 Accepted: August 22, 2014

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dx.doi.org/10.1021/ie502452u | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Research Note

EXPERIMENTAL SECTION Materials. The material used in our experiment was a commercially available PE pipe resin, provided by MaoMing Petrochemical Co., Ltd. (Guangdong, China). Its melt index was 0.28 g/10 min, measured at 190 °C under 2.16 kg and the weight-average molecular weight was 1.86 × 105 g/mol. Ultrahigh molecular weight polyethylene (UHMWPE) was supplied by Beijing Dongfang Petrochemical Co., Ltd. (Beijing, China). Its weight-average molecular weight was 3.5 × 106 g/ mol. Arkopal OP-10 was purchased from Chengdu Kelong Chemical Agents Co., Ltd. (Chengdu, China). Its function was to accelerate a slow crack growth process, where Arkopal OP-10 could diffuse into the amorphous regions of polymer to cause the expansion and disentanglement of the molecular network.15 Sample Preparation. PE containing 1 wt % UHMWPE was premixed in a high-speed mixer and subsequently extruded in a twin-screw extruder. Then the obtained pellets were extruded into PE pipe by using the self-designed rotation extrusion equipment. This equipment has a rotatable mandrel driven by a motor and a gear reduction system. During extrusion of a polymer pipe, hoop drag flow caused by mandrel rotation can be superposed on the axial traction flow so the melts undergo a helical flow deviated from the axial direction, which changes the molecular orientation. Its schematic diagram is shown in Figure 1, and more details on this equipment were

out on the BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. The samples were cut along the axial direction and placed with their axial direction perpendicular to the beam and vertical in the patterns. The wavelength generated by the synchrotron light source was 0.124 nm and the distance between the sample and the detector was 205 mm. Slow Crack Growth Experiment. According to ISO 13480:1997, a cone experiment was adopted to evaluate the PE pipe’s resistance to slow crack growth. A PE pipe of 15 cm length was cut from the prepared pipe and inserted by a metallic cone at a constant speed. Then, a single 10 mm long notch was cut by a fresh razor blade along the axial direction and the PE pipe was put into a 5 wt % aqueous solution of Arkopal OP-10 at 80 °C. At regular time intervals, the crack length increment was measured. Rheological Measurements. Rheological measurements were performed on a AR2000EX rotational rheometer (TA Instruments, USA) with a parallel plate geometry of 25 mm in diameter and a gap of 0.9 mm at constant temperatures in a nitrogen atmosphere. The sample was heated to 190 °C and kept for 5 min. Then dynamic frequency sweep measurements were carried out from frequency of 100 to 0.01 rad/s.

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RESULTS AND DISCUSSION Effect of UHMWPE on PE Pipe Morphology during Rotation Extrusion. Figure 2 showed the crystalline

Figure 2. SEM photos of (a) pure PE pipe; (b and c) PE containing 1 wt % UHMWPE pipe prepared via mandrel rotation extrusion. The direction of arrow showed the axial direction.

morphologies for the mandrel-rotation-extruded pure PE pipe and PE pipe containing UHMWPE. Clearly, both of the PE pipes turned out to be of the anisotropic structures, which were composed of some cluster-like crystallites and parallel stacked folded chain lamellae perpendicular to this cluster, consistent with the features of the typical “shish-kebab” morphology described in the literature.17,18 Moreover, their alignment direction was not parallel to but at an angle relative to the axial direction, so as compared with the shish-kebab parallel to the axial in the conventionally extruded PE pipe,7 this structure facilitated the performance refinement of PE pipe. However, for pure PE pipe, the surface was covered by numerous spherulites and a small amount of the oriented lamellae. It demonstrated that less oriented molecules arose in the pure PE pipe during the rotation extrusion. On the contrary, the addition of UHMWPE induced the crystalline morphological evolution from isotropic spherulite to anisotropic shish-kebab and promoted the formation of dense and compact shish-kebab, as shown in Figure 2b,c. This indicated that the introduction of UHMWPE increased the molecular orientation degrees. Good evidence came from 2D-WAXD patterns in Figure 3. Clearly, for both of the PE pipes, there were strong reflections of the (200) plane and the direction was

Figure 1. Schematic diagram of polymer pipe’s rotation extrusion equipment.

described elsewhere.8,16 In this paper, during the extrusion process of a PE pipe, when the mandrel rotated at the speed of 4 rpm, a PE pipe with the diameter of 32 mm and wall thickness of ∼2 mm was prepared. For comparison, rotationextruded PE pipe without UHMWPE was also prepared at the same processing conditions. Characterization. Scanning Electron Microscope (SEM) Observation. The morphologies of the prepared PE pipes were observed by an Inspect F(FEI) SEM instrument (FEI Co., Netherlands) at 0.5 Torr and 20 kV. Prior to the observation, the SEM samples were cut along the axial direction of the pipes and then put into the 50 °C permanganic etchant for 3 h to remove the amorphous phase. The etched samples were carefully washed by dilute sulfuric acid, hydrogen peroxide, and distilled water, and then were gold-sputtered for observation. Two-Dimensional Wide-Angle X-ray Diffraction (2DWAXD). The synchrotron 2D-WAXD experiments were carried B



Research Note

Figure 3. 2D-WAXD patterns of (a) pure PE pipe; (b) PE containing 1 wt % UHMWPE pipe prepared via mandrel rotation extrusion.

off the axial direction, indicating that the preferential orientation deviated from the axial direction. This was in agreement with the results of the SEM observations in Figure 2. And the (200) intensity distribution along the azimuthal angle between 0 and 360° was integrated and the orientation degree of the sample was quantitatively evaluated according to Hermans orientation equation:19 f=

3⟨cos2 φ⟩−1 2

⟨cos2 φ⟩ =

∫0

π /2

Figure 4. Crack length increment during cone testing for the rotationextruded PE pipe and PE pipe containing 1 wt % UHMWPE.

(1)

conventional extrusion of the PE pipes, shish-kebab crystals are along the axial direction, which is adverse to their application. It is well-known that flow can orient the molecular chains along the flow direction.20 In the subsequent crystallization, the oriented molecules come together to form the fibrillar bundles, which, as the templates, trigger the epitaxial crystallization of the polymer chains perpendicular to the fiber-like bundles, and form shish-kebab.21 Obviously, the flow direction could determine the direction of molecular orientation and the shish-kebab alignment. Conventionally, the polymer melts flowed along the axial direction so that the molecular chains and the alignment of the shish-kebab crystals were oriented along the axial direction. 7 During rotation extrusion, the hoop drag flow caused by mandrel rotation was superposed on the axial flow so the polymer melts in the annular channel moved forward helically in an axis-deviating direction. Therefore, the formed shish-kebab deviated from the axial direction. Additionally, the molecular orientation depends on the competition between flow-induced chain stretching and subsequent relaxation.22,23 During the rotation extrusion of PE pipe, the processing temperature was high enough to induce more molecular relaxation due to thermal motion of the polymer chains. As a result, many spherulites could form instead of shish-kebab structures in the pure pipe. Obviously, to slow down the relaxation was a way to retain the oriented structures. It was evident that the relaxation time of a polymer was scaled with its molecular weight and could be calculated based on the tube model proposed by Doi and Edwards.24

2

I(ϕ) sin ϕ cos ϕ dϕ

∫0

π /2

I(ϕ) sin ϕ dϕ

(2)

where φ is the angle between the normal of the (200) crystal plane and flow direction, and I is the intensity. For an isotropic sample f = 0 while a perfectly parallel oriented sample gives f = 1. The result showed that compared to pure PE pipe, the addition of UHMWPE increased the orientation degree from 0.26 to 0.58. The shish-kebab was composed of the stretched chain fibrils (shish) and the oriented chain-folded lamellae (kebabs) orthonormally. Obviously, it was anisotropic and the formation could improve the orientation degree. Therefore, the higher orientation degree with the addition of UHMWPE was in accordance with denser shish-kebabs shown in the SEM photos. This further confirmed that UHMWPE could facilitate the formation of shish-kebabs off the axial direction during the rotation extrusion. Effect of UHMWPE on PE Pipe Resistance to SCG. Figure 4 depicted the crack length increment versus loading time in the cone test for the rotation-extruded PE pipe and PE pipe with UHMWPE. It was seen that the SCG process contained two stages: crack initiation and crack growth. In Figure 4, the intersection was defined as the crack initiation time and the slope characterized the crack growth rate. For pure PE pipes, the crack initiation time was only 43 h and the crack growth rate was 0.16 mm/h. Clearly, with the presence of UHMWPE, the PE pipe’s resistance to SCG became better. Its crack initiation time and crack growth rate was 92 h and 0.11 mm/h, respectively. Relationship between Morphology and Performance of PE Pipe. Anisotropic shish-kebab morphology can endow materials excellent resistance to SCG along the orientation direction. On the basis of stress analysis on the application of PE pipes, the maximum imposed stress is in the hoop direction. Therefore, hoop alignment of the shish-kebab is a key to improve the performance of PE pipes. However, since the polymer melts flow along the axial direction during the

τs = τe × (M /Me)2

(3)

where M is the average molecular weight of polymer, Me is the average molecular weight between entanglements and τe is the entanglement equilibration time. For PE, Me = 828 g/mol, τe = 7 × 10−9 s at 190 °C.25 The higher the molecular weight of the polymer is, the longer the relaxation time is. Therefore, the molecular weight of the polymer plays an important role in the formation of a shishkebab. Hsiao and Matsuba etc. have confirmed that there exists a critical molecular weight M* for shish-kebabs, and only the polymers with molecular weights higher than M* could remain C



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in the orientation state induced by flow to form shish-kebabs more easily, while polymers of lower molecular weight would relax back to the randomly coiled state and form spherulites.13,26,27 Therefore, the high molecular weight component facilitates flow-induced shish-kebabs. In this study, the molecular weight of UHMWPE introduced into PE matrix was 3.5 × 106 g/mol while that of the PE matrix was 1.86 × 105 g/mol. According to eq 3, the relaxation time of UHMWPE and PE were 125 ms and 0.35 ms, respectively. Obviously, The longer relaxation time of UHMWPE was beneficial in retaining the orientation state. Moreover, UHMWPE had long molecular chains and easily entangled with the neighboring chains to increase entanglement points in the PE matrix.28 The chain entanglement points acted as physical cross-linking points to limit the chain movement so as to enhance the viscosity, as shown in Figure 5. Under the high

Research Note

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-28-85402465. Tel: +86-28-85405133. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China [Grant No. 51127003, 51303114 and 51121001]. 2D-WAXD measurements were performed by Shanghai Synchrotron Radiation Facility.

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REFERENCES

(1) Frank, A.; Pinter, G.; Lang, R. W. Prediction of the remaining lifetime of polyethylene pipes after up to 30 years in use. Polym. Test. 2009, 28, 737−745. (2) García, R. A.; Carrero, A.; Aroca, M.; Prieto, O.; Domínguez, C. Slow crack growth resistance in resin blends of chromium and metallocene catalyzed ethylene−hexene copolymers for pipe applications. Polym. Eng. Sci. 2008, 48, 925−933. (3) Hutař, P.; Ševčík, M.; Náhlík, L.; Pinter, G.; Frank, A.; Mitev, I. A numerical methodology for lifetime estimation of HDPE pressure pipes. Eng. Frac. Mech. 2011, 78, 3049−3058. (4) Li, W.; Guan, C.; Xu, J.; Chen, Z. R.; Jiang, B.; Wang, J.; Yang, Y. Bimodal/broad polyethylene prepared in a disentangled state. Ind. Eng. Chem. Res. 2013, 53, 1088−1096. (5) Pérez-González, J.; Denn, M. M. Flow enhancement in the continuous extrusion of linear low-density polyethylene. Ind. Eng. Chem. Res. 2001, 40, 4309−4316. (6) Lu, X.; Zhou, Z.; Brown, N. The anisotropy of slow crack growth in polyethylene pipes. Polym. Eng. Sci. 1994, 34, 109−115. (7) Nie, M.; Wang, Q.; Bai, S. B.; Li, Z.; Huang, A. The formation and evolution of the hierarchical structure of polyethylene pipe during extrusion processing. J. Macromol. Sci. Part B: Polym. Phys. 2013, 53, 205−216. (8) Wang, Q.; Zhang, J.; Guo, Y.; Bai, S. B.; Hua, Z. K. Preparation method and device for high-performance polymer pipe. China Patent 101337425, 2010. (9) Nie, M.; Wang, Q.; Bai, S. Morphology and property of polyethylene pipe extruded at the low mandrel rotation. Polym. Eng. Sci. 2010, 50, 1743−1750. (10) Kornfield, J. A.; Kumaraswamy, G.; Issaian, A. M. Recent advances in understanding flow effects on polymer crystallization. Ind. Eng. Chem. Res. 2002, 41, 6383−6392. (11) Phillips, A. W.; Bhatia, A.; Zhu, P. W.; Edward, G. Shish formation and relaxation in sheared isotactic polypropylene containing nucleating particles. Macromolecules 2011, 44, 3517−3528. (12) Narh, K. A.; Odell, J. A.; Keller, A. Temperature dependence of the conformational relaxation time of polymer molecules in elongational flow: Invariance of the molecular weight exponent. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 335−340. (13) Dikovsky, D.; Marom, G.; Avila-Orta, C. A.; Somani, R. H.; Hsiao, B. S. Shear-induced crystallization in isotactic polypropylene containing ultra-high molecular weight polyethylene oriented precursor domains. Polymer 2005, 46, 3096−3104. (14) Keum, J. K.; Zuo, F.; Hsiao, B. S. Formation and stability of shear-induced shish-kebab structure in highly entangled melts of UHMWPE/HDPE blends. Macromolecules 2008, 41, 4766−4776. (15) Pinter, G.; Haager, M.; Lang, R. W. Influence of nonylphenolpolyglycol-ether environments on the results of the full notch creep test. Polym. Test. 2007, 26, 700−710. (16) Liu, W.; Wang, Q.; Nie, M. Structure and performance of polybutene-1 pipes produced via mandrel rotation extrusion. J. Polym. Eng. 2014, 43, 15−22. (17) Kimata, S.; Sakurai, T.; Nozue, Y.; Kasahara, T.; Yamaguchi, N.; Karino, T.; Shibayama, M.; Kornfield, J. A. Molecular basis of the

Figure 5. Relationship of complex viscosity and frequency for pure PE and PE containing 1 wt % UHMWPE.

viscosity, the molecules moved with more difficultly, which further suppressed the molecular relaxation. As a result, with the addition of UHMWPE, more oriented molecules were retained to a large extent and subsequently acted as the shish nucleus to induce the crystallization of the folded-chains lamellae on their surface, bringing about a higher orientation degree and denser shish-kebabs. Hence, compared to the pure PE pipe, the rotation-extruded PE pipe with UHMWPE had superior resistance to slow crack growth.

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CONCLUSION This paper mainly focused on the effects of UHMWPE on crystalline morphology and the SCG performance of PE pipes prepared via mandrel rotation extrusion. Experimental results showed that during the mandrel rotation extrusion of PE pipe, the polymer melts flowed forward helically, so the flow direction deviated from the axial direction, which induced the alignment of shish-kebab crystals off the axial direction. However, the addition of UHMWPE restrained chain relaxation and facilitated the formation of shish-kebabs. As a result, the rotation-extruded PE pipe exhibited dense and compact shishkebabs off the axial direction and thus had excellent resistance to SCG compared to pure PE pipe. D



Research Note

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Role of Ultrahigh Molecular Weight Polyethylene during Rotation

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