Extended Chain Crystals of Ultra High Molecular Weight Polyethylene Fiber Prepared by Melt Spinning

Qiang Zhang, Qingzhao Wang, Yong Chen

Shandong University of Science and Technology, Qingdao, Shandong CHINA

Correspondence to: Qiang Zhang email: [email protected]

ABSTRACT The extended-chain crystal structure of the Subsequently, the fiber is hot drawn with the aim at UHMWPE fibers prepared by melt spinning method inducing orientation. This processing method was studied with WAXD, DSC and SEM. At draw involves significant quantities of organic solvents ratio (DR) values higher than 24, double melting requiring removal and recycling. In the case of peaks were observed from the DSC thermograms, spinning, this is 10 kg of solvent/kg of polymer [8]. the corresponding temperatures being 137.1-142°C In order to achieve the production of high strength and 142-145°C, respectively. With the increase of ultrahigh molecular weight polyethylene fiber DR values, the lengths in a and b axis of the unit without organic solvent，many alternative routes cell shortened, and the unit cell remained were developed to prepare high strength UHMWPE orthorhombic even if the DR values increased to 60. fibers without organic solvent, such as solid state At DR values lower than 24, the crystallinity and hot drawing [9, 10] , solid state extrusion [11,12], orientation degree of the UHMWPE fibers affected free growth 13], and surface growth [14] .but all the the tensile strength of fibers markedly. At DR methods mentioned above were not applied values were higher than 24, the abundantly successfully in industry. produced extended-chain crystals mainly dominated the improvement of tensile strength of the Melt spinning is an efficient, simple, and UHMWPE fibers. non-solvent method. However, UHMWPE melts in a highly elastic state even, at close to degradation Keywords: UHMWPE, Fibers, Melt spinning, temperature owning to the random entanglements Extended-chain crystals, Tensile strength between the extremely long chains. Therefore, UHMWPE fibers cannot be prepared by melt INTRODUCTION spinning. UHMWPE needs to be modified to Ultra-high molecular weight polyethylene enhance its fluidity and processing performance to (UHMWPE) fiber is the next highly oriented fiber employ the melt spinning method. In recent years, with extended-chain structure after Kevlar, in order to improve the processing performance of UHMWPE fibers have excellent resistance to UV UHMWPE, much effort has been directed towards radiation, chemical resistance, high specific energy modifying UHMWPE to obtain better fluidity. One absorption, low dielectric constant, low coefficient effective way to reduce the melt viscosity is to fill of friction, and anti-cutting performances. These UHWMPE with a small amount of conventional PE properties can prove particularly useful in a range of (HDPE, LDPE, and LLDPE), PEG and PP that applications, especially for aerospace and generally have a lower average molecular weight military[1-5]. At present, gel spinning is the only of [15-17] to improve the mobility of the polymer method used to produce ultrahigh molecular weight significantly. Recently, there have been patents polyethylene fiber. This method was first about UHMWPE fibers prepared by melt the discovered by Capaccio and Ward [6,7], then spinning method using high fluidity UHMWPE patented in Holland by DSM in the 1970s, and resin [18, 19]. In our laboratory ， using the successfully used in industry. This method involves modified high mobility UHMWPE, the the dissolving of UHMWPE in decalin at elevated middle-strength fiber with a tensile strength of temperatures, followed by recrystallization, drawing 1.6GPa and tensile modulus of 50 GPa was of the resulting unentangled gel, and drying. successfully prepared by the melt spinning method

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[20,21]. Obviously, the tensile strength of hot Silicone oil bath set at 90°C to get the total draw UHMWPE fibers prepared by a melt spinning ratios the specimens with 12,24,36,48 and 60. method is much higher than the common PE fibers, but lower than UHMWPE fibers obtained by the gel CHARACTERIZATION spinning method. UHMWPE fibers prepared by the WAXD Measurements gel-spun method were primarily applied in the Crystallinity aerospace and military fields due to high Wide-angle X-ray diffraction experiments were manufacturing costs. The middle strength carried out using a Rigaku D/Max2500 system with UHMWPE FIBERs prepared by melt the spun wavelength of 1.542 Å at 40 kV and 100 mA; the method have great potential in civil areas, such as WAXD data were measured covering a range of rope and nets. two theta from 10° to 30° with a scanning speed of 2°/min. The WAXD Spectrum was treated by As is well known, the properties of a fibers depend multipeak resolution method with Jade software to on its microstructure. In all microstructures, the get the crystallinity of the UHMWPE fiber extended chain crystals have a most important specimens. effect on the tensile strength of the fibers, the higher extended chain crystal fiber contents in the fiber, The Degree of Orientation the better its tensile strength. Extended chain The wide-angle X-ray diffraction experiments were crystals in the fiber come from folded chain crystals carried out using the Rigaku D/Max2500 system, during the drawing process. Many researchers have conditions as above; then 2θ was fixed on the carried out detailed studies on extended chain scanning curve maxima (21.4°), azimuth angle crystals of UHMWPE fibers prepared by the gel scanning was carried out in the range of -90° -270° spinning method [22-24], but there have been few with 15° /min. The degree of orientation in the studies of extended chain crystals of the UHMWPE crystalline phases of UHMWPE fibers can be fibers produced by the melt spinning process. In this obtained through Eq. (1): paper, the formation of extended chain crystals and its effect on tensile strength of the UHMWPE fibers prepared by melt spinning method during the (1) drawing process and at high drawn ratios is discussed. Where，∏ was orientation degree (%); H was the EXPERIMENTAL FWHM of the intensity distribution curve of Debye Material ring along the equatorial line. The material used in this study was high liquidly UHMWPE resin particles with MW 2*106 , MFR is Cell Parameters 0.5g/10min, produced by Beijing Beiqing Lianke The wide-angle X-ray diffraction experiments were nano Plastic Co., Ltd. Unfortunately, the processing carried out using the Rigaku D/Max2500 system, conditions were not made available by the conditions as above; the WAXD data were manufacturers due to commercial reasons. measured covering a range of 2θ from 10° to 60°. SiO2 was used to correct diffraction angle to Preparation of UHMWPE Fibers obtained the accurate 2θ value of (110), (200) and Melt spinning was performed on a single-screw (020) crystal face, thereby obtained accurate melt extruder with Φ35mm and L/D=25, the d-spacing value of each crystal face, the value of d spinneret contains one orifice of 1mm diameter. The obtained was used to compute the lattice parameters extruder was set with four different temperature a and b. zones of 160°C, 190°C, 250°C, and 240°C for zone 1, zone 2, zone 3, and die, respectively; Screw DSC Measurements speed was 2r/min; the as-prepared filaments were The differential scanning calorimetry (DSC) collected at a take-up speed of 10 m/min and drawn experiments were carried out using a DSC1 at room temperature with the draw ratios of 1 or 6 Differential Scanning Calorimeter. Typical sample on a three-positioned drawing platform, respectively. weights used were approximately 5 mg. A heating The as-prepared fiber with the 6 draw ratios (DR) rate of 10°C/min and a temperature range from was drawn with draw ratio of 2, 4, 6, 8, and 10 in a 90°C to 170°C were selected. The DSC Spectrum

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was treated by multipeak resolution method with and higher melting point owing to their huge inner STARe software to get the contents of extended stress. chain crystal in the fibers. Figure 2 shows the SEM of the inner structure of SEM Measurements the UHMWPE fiber drawn to 60 ratios, which The morphology of fiber was studied by using a presents the abundant microfiber structures KYKY2800B SEM with an acceleration voltage of composed of high-oriented extended-chain crystals 20 kV. in the high-drawing UHMWPE fiber. With the great proportion of folded-chain lamellae in the Tensile Measurements UHMWPE fiber, the first endothermic peak can still The tensile properties of fibers were measured in a be observed in Figure 1. However, with the tensile testing machine at a falling speed of 50 increases in the DR, the area of the second mm/min. At least 10 tests were averaged for each endothermic peak increased, whereas that of the sample first endothermic peak decreased.

RESULTS AND DISCUSSION Supposing that the crystal structure of the Extended Chain Crystals UHMWPE fiber is composed of folded-chain and As a flexible macromolecular chain, the PE extended-chain crystals, and then their contents are molecular chain generally tends to recover to the represented by the area of the two endothermic steady state with the lowest free energy. During peaks. Overlapping peaks were separated by using melt crystallization, PE macromolecular chain the DSC peaking separation method, which produces sphaerocrystal, which is composed of estimates the content of the extended-chain crystals chain-folded lamellae. However, under unique in the whole crystal structure of the UHMWPE fiber. conditions, such as high-pressure solid extrusion Figure 3 shows the results. The content of the and super draw, the folded-chain lamellae inside the extended-chain crystals increased with the increase PE crystal may be transformed into extended-chain in the DR and achieved continuous linear growth crystals under the effects of external forces. when the UHMWPE fiber was drawn from 36 to 60 ratios. Figure 1 shows the DSC thermograms of UHMWPE fiber with different draw ratios. As can be seen in Figure 1, Double endothermic peaks were observed from the DSC curve with draw ratio (DR) values higher than 24. With the increase of draw ratio values, the area of the second endothermic peak increased continuously. The temperature of both endothermic peaks also increased. The temperature of the first endothermic peak increased from 137.1 to 142°C, which indicated the destruction of folded-chain crystals in the UHMWPE fiber. The temperature of the second endothermic peak increased from 142 to 145°C, which indicated the destruction of more complete structures in the fiber. According to the analysis results of the unit cell part, no new crystal system was produced after the UHMWPE fiber was drawn to 24 ratios. Therefore, the melt spinning of UHMWPE fiber formed extended chain crystals with more complete structure after high drawing, whose destruction was manifested by the second endothermic peak. High drawing transformed the folded-chain lamella structure in the UHMWPE fiber into high-oriented extended -chain crystals. FIGURE 1. DSC thermograms of melt spun UHMWPE fiber Extended- chain crystals presented tighter stacking specimens with different DR values.

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To calculate PE cell parameters (a, b and c) through the WAXD patterns, the 2θ of six crystal faces, (110), (200), (210), (020), (011), and (310), has to be measured accurately. However, the 2θ of (210), (011), and (310) cannot be measured accurately with the existing methods because of their extremely weak diffraction peaks. Accordingly, we measured the 2θ of (110), (200), and (020) to calculate the d of these three crystal faces. Next, cell parameters (a and b) could be calculated through d; c had a value of 0.255 nm according to literature data [22].

Table I presents the analysis results. The data show FIGURE 2. Scanning electron micrograph of inner structure of no change in the crystal system after the UHMWPE UHMWPE fiber with 60 DR values. fiber was drawn to 60 ratios, which still belongs to the orthorhombic system. With the increase of DR values, a and b axes shortened slightly. Given the fixed length of c axis, the density of the crystalline area also increased. Such variations of cell parameters were caused by the development of extended-chain crystals. After the drawn fiber turned into extended-chain crystals from folded-chain crystals, a and b axes will surely shorten. This finding is consistent with the experimental results.

FIGURE 3. The extended chain crystals content of drawn UHMWPE fiber specimens with different DR values.

X-Ray Diffraction Pattern and Cell Parameters As can be seen in Figure 4, all of the sample fibers contained two typical characterized diffraction peaks of (110) and (200) crystal faces at 2θ = 21.4° and 24°, respectively. The melt spun UHMWPE fiber and gel spun ones both have the Orthorhombic crystals structure. The intensity and FWHM increased gradually with increasing DR values. Notably, the sample that was drawn 60 ratios presented relatively small diffraction peak corresponding to the monoclinic (001) plane at 2θ = 19.6°. These features were also observed in the results of the gel spun fiber.

FIGURE 4. WAXD patterns of melt spun UHMWPE fiber Specimens with different DR values and gel spun ones.

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TABLE I. The cell parameters of drawn UHMWPE fiber These results indicated that the improvement of the specimens with different DR values. UHMWPE fiber specimens tensile strength was

mainly dominated by the significantly increased 21 3 DR a(nm) b(nm) c(nm) d c (g/cm ) crystallinity and orientation degree of the drawn

1 0.74 0.493 0.255 1.008 UHMWPE fiber specimens as DR values were lower than 24, and by the abundantly produced 6 0.74 0.493 0.255 1.008 extended chain crystals as DR values were higher 12 0.74 0.493 0.255 1.008 than 24. The production of extended-chain crystals 24 0.74 0.492 0.255 1.012 caused high-parallel arrangement of fiber, more 36 0.739 0.489 0.255 1.02 compact structure, and enhanced Van Der Waals’ 48 0.736 0.488 0.255 1.022 force between macromolecular chains, and thus, preventing the slippage between these chains. 60 0.734 0.487 0.255 1.025 Meanwhile, the binding molecules between microcrystals increased significantly with the Extended Chain Crystals and Tensile Strength drawing of folded chains, which increased the The main PE chain is connected through covalent loading elements in the microfiber and the strength bonds. With strong covalent bonding force on the and modulus of drawn fiber, as well as effectively main chain and the relative weaker Van Der Waals’ improved the fiber tensile strength. force between two neighboring macromolecular chains, the slippage between macromolecular chains According to Table II, the tensile strength of the is the major failure during the tensile fracture of UHMWPE fiber of the Spectra series of DSM fiber. Figure 5 illustrates the tensile strength of Company (Netherlands) and that of the Dyneema drawn UHMWPE fiber specimens with different series of the U.S. Allied Signal Company increased DR values. As can be seen in Figure 5, slight with the increase of straight-chain crystal content. variation of tensile strength of drawn UHMWPE These two UHMWPE fibers have high content of fiber specimens was observed as DR values straight-chain crystals (about 90%, or even 98%) that increased from 1 to 6. During pre-drawing, the have tensile strength higher than 3 GPa. However, the as-prepared fiber with high temperature was still in UHMWPE fiber prepared by melt spinning method melting state when the drawing was mainly caused showed low content of extended-chain crystals (56%) by the slippage of the macromolecular chains. This and low tensile strength (1.8 GPa). These features phenomenon enabled the UHMWPE fiber may be the main reasons for its poorer tensile specimens to maintain similar tensile strength. strength compared with the gel spinning fiber. However, as DR values increased from 6 to 60, the tensile strength of the fibers showed a linear growth with the increase of DR values. Higher DR values results in higher tensile strength of fiber. When the fiber was drawn to 48 ratios, the curve varied slowly and deviated from the original straight line, but the tensile strength increased significantly as DR values increased from 48 to 60. According to the measuring results of drawn UHMWPE fiber specimens crystallinity (Figure 6) and orientation degree (Figure 7) with different DR values, both crystallinity and orientation degree increased significantly with the increase of DR values when DR values were lower than 24. However, when DR FIGURE 5. The tensile strength of drawn UHMWPE fiber values were higher than 24, the crystallinity and specimens with different DR values. orientation degree increased very slowly with the increase of DR values and tended to saturate. The content of extended-chain crystals, however, increased continuously with the increase of DR values when DR values were higher than 24 (Figure 3).

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CONCLUSION As the DR values were higher than 24, double melting peaks were observed from the DSC thermograms of the drawn UHMWPE fibers prepared by melt spinning method, with the increase of draw ratio values, the temperature of the first melting peak increased from 137.1 to 142 , which indicated the destruction of folded-chain crystals in the UHMWPE fibers. The temperature of the℃ second melting peak increased from 142° to 145 , which indicated the destruction of extended-chain crystals in the UHMWPE fibers. With the ℃increase of draw ratio values, the content of the extended-chain crystals FIGURE 6. The crystallinity values of drawn UHMWPE increased continuously; a and b axes of unit cells composite fiber specimens with different DR values. shortened slightly, the density of the crystalline area in the UHMWPE fibers also increased. The improvement of the fiber tensile strength was mainly dominated by the significantly increased crystallinity and orientation degree of the UHMWPE fibers when DR values were lower than 24, as DR values were higher than 24, crystallinity and orientation degree of the UHMWPE fibers tended to saturate, increasing of the content of extended-chain crystals in the UHMWPE fibers, which effectively improved the fibers tensile strength. The UHMWPE fibers made by melt spinning method showed low content of extended-chain crystals (56%), these features may be the main reasons for its poorer tensile strength FIGURE 7. The crystal orientation values of drawn UHMWPE fiber specimens with different DR values. compared with the gel spinning fibers. Low content of extended-chain crystals of melt spun UHMWPE TABLE II. Tensile Strength and Extended chain crystals content of fiber may be due to the low drawn ratios, as we all different UHMWPE fibers. know, the gel spun UHMWPE fibers were acquired through secondary drawing or tertiary drawing, and Tensile Young Extended- the total drawn ratios were more than 100. So by Sample Strength modulus chain Reference (GPa) (GPa) crystals (%) optimizing the drawing process parameters to Prepared in improve the effective drawing ratio is an effective 1.6±0.1 50±2 56±1 This work our lab way to improve the content of extended chain SPECTRA900 2.61 79 88.2 22 crystals and mechanical properties of melt spun SPECTRA UHMWPE fiber. 3.25 113 91.2 22 1000 SPECTRA REFERENCES 3.25 116 96.6 22 2000 [1] P. Smith, Lemstra PJ. Macromol Chem., DYNEEMA 1979, 180, 2983. 2.7 89 94.2 22 SK60 [2] Wanlin Pan; Zhao-feng Liu, Zu-ming Hu, DYNEEMA 3 95 96 22 SK65 Lei Chen, Jing Zhu, Yun-rong Yu. SFC, DYNEEMA 2006, 11, 6. 3.4 107 96.4 22 SK75 [3] Xiulan You, Panpan Hu, Zhaofeng, Liu Journal of Textile Research, 2010, 31,146. [4] Taekyeong Kim, Seonhee Jeon,Dongsup K wak,Yuri Chae, Fiber. Polym., 2012, 13, 212.

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[5] Taekyeong Kim, Seonhee Jeon, Fiber. Polym., 14, 105 (2013). [6] G. Capaccio, I.M. Ward. Polymer, 1974, 15, 233. [7] G. Capaccio, I.M. Ward. Polymer, 1975, 16, 239. [8] Jinggang Gai, Hulin Li. J Appl Polym Sci, 2007, 106, 3023. [9] J.M. Andrews, I.M. Ward, J. Mater. Sci., 1970, 5, 411. [10] Paul Smith, Henri. D, Chanzy, Bruno. P, Rotzinger, J. Mater. Sci., 1987, 22,523. [11] W.M. Perkins, N.J. Capiati, R.S. Porter. Polym. Eng. Sci, 1976, 16, 200. [12] T. Kanamoto, A. Tsuruta, K. Tanaka, M. Takeda, R.S. Porter. Macromolecules, 1988, 21, 470. [13] A. Zwijnenburg, A.J. Pennings. Colloid Polym. Sci, 1975, 253, 452. [14] A. Zwijnenburg, A.J. Pennings. J. Polym. Sci. Polym. Lett. Ed, 1976, 14, 339. [15] Kyu, T., Vadhar, P. J Appl Polym Sci, 1986, 32, 5575. [16] Xie, M. J., Liu, X. L., Li, H. L. J Appl Polym Sci, 2006,100, 1282. [17] Jinggang Gai, Huilin Li. J Appl Polym Sci, 2007,106, 3023. [18] SAKAMOTO, Godo. EP patent, B1, 1493851, 2012-01-04. [19] Yi REN. U.S. Patent, A1, 2012/0214946. 2012-08-23. [20] Wanqing Zhen, Qingzhao Wang; Jinxi Wu, Haimin Wang. J Synthetic Fiber in China,2011, 3, 5. [21] Qiang Zhang , Qingzhao Wang , Yong Chen, J Appl Polym Sci,2013,130,3930. [22] Ismail Karacan, J Appl Polym Sci, 2008, 101，1317. [23] Jen-Taut Yeh,Shui-Chuan Lin, Cheng-Wei Tu, J Mater Sci, 2008,43,4892. [24] P.M. Pakhmomv, V.P. Galitsyn, A.L. Krylov，Fiber Chemistry, 2005, 37, 319.

AUTHORS’ ADDRESSES Qiang Zhang Qingzhao Wang Yong Chen School of Materials Science and Engineering Shandong University of Science and Technology 579 Qianwangang Road Economic & Technical Development Zone Qingdao, Shandong 266590 CHINA

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