Preparation of ultrahigh-molecular-weight polyethylene fibers by combination of melt-spinning and melt-drawing

Preparation of ultrahigh-molecular-weight polyethylene fibers by combination of melt-spinning and melt-drawing

Materials Today Communications 23 (2020) 100864 Contents lists available at ScienceDirect Materials Today Communications journal homepage: www.elsev...

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Materials Today Communications 23 (2020) 100864

Contents lists available at ScienceDirect

Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm

Preparation of ultrahigh-molecular-weight polyethylene fibers by combination of melt-spinning and melt-drawing

T

Masaki Kakiagea,b,*,1, Dai Fukagawac a

Institute for Fiber Engineering, Shinshu University (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan b Division of Molecular Science, Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan c Department of Textile Science and Technology, Graduate School of Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultrahigh-molecular-weight polyethylene (UHMW-PE) fiber Melt-spinning Melt-drawing

Ultrahigh-molecular-weight polyethylene (UHMW-PE) fibers were prepared by a combination of melt-spinning and melt-drawing. An as-spun fiber without melt fracture was continuously obtained by melt-spinning from UHMW-PE powder. The as-spun fiber was melt-drawn at 145 and 150 °C. The fiber diameter was reduced by the melt-drawing process. The melt-drawn fiber consisted of extended- and folded-chain crystals. The chain orientation with the development of the extended-chain crystals was accelerated by the melt-drawing at 145 °C under higher strain rates, resulting in higher tensile strength. Consequently, a UHMW-PE fiber with a diameter of approximately 150 μm and a high tensile strength of approximately 1.1 GPa was prepared by the combination of melt-spinning and melt-drawing.

1. Introduction Ultrahigh-molecular-weight polyethylene (UHMW-PE) with an MW of over 106 is an important high-performance polymer material. UHMW-PE fibers are widely applied in the production of ropes and textiles. Molten UHMW-PE has a high melt viscosity, which leads to poor melt-processability. Hence, a gel-spinning technique is used for the industrial production of high-performance UHMW-PE fibers [1–4]. Unfortunately, however, this process requires a large amount of an organic solvent since gel-spinning uses a dilute UHMW-PE solution and needs an extraction process. Therefore, it is necessary to develop a solvent-free melt-spinning technique (green processing) for producing high-performance UHMW-PE fibers. The preparation of UHMW-PE fibers by melt-spinning has not been reported. The main issue in achieving the melt-spinning of UHMW-PE is the melt fracture phenomenon [5]. The high melt viscosity of molten UHMW-PE induces melt fracture on the surface of fibers (extrudates) prepared by melt-spinning (melt-extrusion). On the other hand, the high melt viscosity of molten UHMW-PE may be advantageous if tensile drawing can be induced in the molten state. A higher viscosity means a longer relaxation time [6]; thus, the chain orientation will be retained when the deformation time scale exceeds the chain relaxation during drawing from the melt. Namely, UHMW-PE can be drawn even from the



Corresponding author. E-mail address: [email protected] (M. Kakiage). 1 Present address (M. Kakiage): Gunma University. https://doi.org/10.1016/j.mtcomm.2019.100864 Received 15 December 2019; Accepted 19 December 2019 Available online 23 December 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.

molten state owing to its high melt viscosity. This melt-drawing technique for a UHMW-PE film has been developed hitherto [7–22]. Molten UHMW-PE exhibits elastic deformability, which is homogeneously transmitted within the entire sample through entanglements, resulting in oriented crystallization. This melt-drawing process has been applied to other polymers with high melt viscosity such as poly(tetrafluoroethylene) [23,24] and poly(tetramethyl-p-silphenylenesiloxane) [25]. In this study, we attempt to produce high-performance UHMW-PE fibers by melt-processing, i.e., a combination of melt-spinning and meltdrawing. First, an as-spun fiber without melt fracture was prepared by melt-spinning from UHMW-PE powder. The optimum conditions for melt-spinning to form an as-spun UHMW-PE fiber without melt fracture were investigated. Following this melt-spinning process, the obtained as-spun fiber was melt-drawn to achieve oriented crystallization and reduce the fiber diameter. The melt-drawing of UHMW-PE forms a shish-kebab structure consisting of extended-chain crystals (ECCs) and folded-chain crystals (FCCs) [7–10,15,16,21]. The ECCs are formed by oriented crystallization with the disentanglement of molecular chains during the melt-drawing [8–13,15,17,21], and the development of ECCs improves the mechanical properties [9,10]. The relationship between the melt-drawing conditions, temperature and strain rate, and the crystalline structure and mechanical properties of the melt-drawn

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UHMW-PE fibers were investigated. 2. Experimental section 2.1. Materials UHMW-PE reactor powder having a viscosity-average MW of 1.15 × 106 (HI-ZEX MILLION 145 M, Mitsui Chemicals, Inc., Japan) was used. Before melt-spinning, the UHMW-PE powder was mixed in an acetone solution of two antioxidants, 0.5 wt% (based on polymer) of both octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (ADK STAB AO-50, ADEKA Corporation, Japan) and 2,2′-methylenebis(4,6di-tert-butylphenyl) 2-ethylhexyl phosphite (ADK STAB HP-10, ADEKA Corporation, Japan), and dried at room temperature (RT). 2.2. Melt-spinning Melt-spinning of UHMW-PE powder was conducted using a DSM Xplore MC15 twin-screw microcompounder with a nozzle. The nozzle diameter and length/diameter ratio (L/D) were 1.0 mm and 5, respectively. The barrel temperature (Tb) and screw speed were 160−200 °C and 5 rpm, respectively. The UHMW-PE powder was continuously supplied to the barrel of the microcompounder and extruded under a constant force (approximately 6500 N). The nozzle was heated to 160−200 °C (Tn). The extruded sample was taken up at 280 mm/min at RT, and an as-spun fiber was obtained. 2.3. Melt-drawing The obtained as-spun fiber was melt-drawn at 145 and 150 °C, considerably above the sample melting temperature, in a silicone oil bath. Before drawing, the sample was held at the drawing temperature for 5 min for temperature equilibration. The strain rate of drawing was 1–40/min and the draw ratio (DR) was 20. 2.4. Characterization A Rigaku Thermo Plus DSC8230 was used for differential scanning calorimetry (DSC) measurements. Heating scans were performed up to 180 °C at a rate of 5 °C/min under a nitrogen gas flow (50 mL/min). The crystallinity for the as-spun fiber was calculated from heat of fusion (ΔHf), assuming the ΔHf of perfect PE crystals to be 290 J/g [26]. Wideangle X-ray diffraction (WAXD) measurements were performed using an X-ray generator (Rigaku RA-Micro7) equipped with an imaging plate (Rigaku R-AXIS IV++). The X-ray source was the CuKα line (0.15418 nm) generated at 40 kV and 20 mA. The exposure time was set to 1 min and the camera length was 150 mm. Microbeam small-angle X-ray scattering (SAXS) measurements of melt-drawn fibers were performed using a synchrotron radiation source at the beamline BL40XU of SPring8 (Japan Synchrotron Radiation Research Institute (JASRI), Japan). The wavelength of the synchrotron beam was 0.155 nm and the beam size was 12 μm. The exposure time was set to 400 ms and the camera length was 2626 mm. The SAXS patterns were recorded on an ORCA-Flash4.0 Digital CMOS camera (C11440, Hamamatsu Photonics) with an image intensifier (V7739 P/ION, Hamamatsu Photonics). Gel permeation chromatography (GPC) curves were recorded in a solvent of 1,2,4-trichlorobenzene at 140 °C. Digital microscopy observations were conducted using a KEYENCE VHX-2000 digital microscope with a VHZ100UR lens. Field-emission scanning electron microscopy (FE-SEM) observations of melt-drawn fibers after the removal of the amorphous component by etching with fuming nitric acid were conducted with a Hitachi S-5000 field-emission scanning electron microscope operated at 5.0 kV. The fuming nitric acid etching of melt-drawn fibers was performed at 60 °C for 12 h. The samples were coated with Pt-Pd before the observations. Tensile testing was conducted using a tensile tester (TENSILON RTC-1250A, A&D Co., Ltd.). The gauge length and tensile

Fig. 1. Digital microscopic images of melt-extruded samples prepared at (left) different Tb values of 170−200 °C without nozzle heater and (right) different (Tb, Tn) values of (160, 160), (170, 170), (180, 180), (190, 190), and (200, 200) °C.

speed were set to 20 mm and 20 mm/min, respectively.

3. Results and discussion 3.1. Preparation of as-spun UHMW-PE fiber by melt-spinning Melt-spinning using a nozzle (1.0 mm diameter) thicker than a usual nozzle for melt-spinning (approximately 0.1-0.3 mm diameter) is promising for increasing the processability of molten UHMW-PE. To prepare an as-spun fiber by melt-spinning, different combinations of the temperatures of the barrel (Tb) and nozzle (Tn) were investigated. Fig. 1 (left) shows the digital microscopic images of the melt-extruded samples prepared at different Tb values without the nozzle heater. No extruded sample was obtained by preparation at Tb =160 °C owing to the high extrusion pressure above a critical force. Extruded samples were obtained at 170 °C and above. An extruded sample with a relatively 2

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smooth surface was obtained at 170 °C, although continuous extrusion was not possible. An extruded sample was continuously obtained at 180 °C but melt fracture was observed. Decreasing the melt viscosity enables continuous extrusion but increases the shear stress, inducing melt fracture. Furthermore, tears were observed in the extruded samples prepared at 190 and 200 °C. Thus, the nozzle heater was attached to improve the thermal state around the nozzle. Fig. 1 (right) shows the digital microscopic images of the melt-extruded samples prepared at different (Tb, Tn) values. Tn was set to be the same as Tb to evaluate the effect of the nozzle heater. An extruded sample was obtained at (Tb, Tn) = (160, 160) °C, even though no extruded sample was obtained at Tb =160 °C without the nozzle heater. This indicates that the heat retention around the nozzle increases the fluidity of the sample in the nozzle, enabling the preparation of an extruded sample. The extruded sample had a smooth surface without melt fracture since the extrusion at the lower temperature of 160 °C reduces the shear stress. However, the extrusion rate was low compared with the take-up speed and the extruded sample was hard; thus, continuous taking up was impossible. Melt fracture was observed for the extruded sample prepared at (Tb, Tn) = (170, 170) °C and above owing to the increase in the shear stress with increasing fluidity. These results suggest that higher fluidity of the molten sample is required for continuous extrusion, and appropriate solidification is required for the preparation of an extruded sample without melt fracture. Therefore, the extrusion was carried out using a combination of Tb =190 °C, which provides higher fluidity for the extrusion, and Tn =160 °C, which provides appropriate solidification for the nozzle. Fig. 2 shows a digital microscopic image of the melt-spun sample prepared at (Tb, Tn) of (190, 160) °C. A homogeneous extruded sample without melt fracture was continuously obtained and was taken up. The diameter of the obtained taken-up fiber was approximately 850 μm. Consequently, an as-spun fiber without melt fracture was obtained by melt-spinning from UHMW-PE powder in this study. Melt-spinning using a thicker nozzle at optimal values of Tb and Tn resulted in the continuous preparation of an as-spun fiber without melt fracture from the molten UHMW-PE with high viscosity. GPC analyses of the raw UHMW-PE powder and as-spun fiber revealed that the melt-extrusion in this study did not cause a significant decrease in MW (Fig. 3). Fig. 4 shows the DSC heating thermogram of the as-spun fiber. A single melting endotherm was observed at 132 °C. The crystallinity for the as-spun fiber evaluated by the DSC measurement was 49 %. Fig. 5 shows the WAXD pattern and azimuthal profile extracted along the azimuthal direction at the orthorhombic (110) reflection for the as-spun fiber. A broad arcshaped (110) reflection was observed on the equator [Fig. 5(a)], indicating the inclination of the FCC to the taken-up direction. Three peaks at azimuthal angles (β) of 90° (central peak) and 70 and 110°

Fig. 3. GPC curves for raw UHMW-PE powder and as-spun fiber.

Fig. 4. DSC heating thermogram for as-spun fiber.

(split peaks) were observed in the azimuthal profile [Fig. 5(b)]. For a detailed analysis of the chain orientation, the full width at half maximum (FWHM) of the peaks in the azimuthal profile was evaluated from the peak separation using the pseudo-Voigt and Gaussian functions. The degree of crystalline orientation (Dc) was evaluated using the following equation.

Dc (%) =

180 − Σ FWHM(deg.) × 100 180

(1)

Dc for the as-spun fiber was 56.8 %. The results of the DSC and WAXD measurements indicate that the as-spun fiber consists of FCCs roughly oriented to the take-up direction. 3.2. Preparation of UHMW-PE fibers by melt-drawing The as-spun fiber could be melt-drawn at 145 and 150 °C. On the other hand, the as-spun fiber was broken during holding at 155 °C; thus, melt-drawing was not possible at 155 °C. Fig. 6 shows the change in the

Fig. 2. Digital microscopic image of melt-spun sample prepared at (Tb, Tn) of (190, 160) oC (as-spun fiber). 3

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Fig. 7. Digital microscopic image of melt-drawn fiber prepared at 145 °C with DR of 20 under strain rate of 40/min.

fiber diameter with DR for the melt-drawn samples prepared at 145 °C under a strain rate of 40/min. The diameter decreased with increasing DR. The rate of diameter reduction coincided with that when assuming no volume change during melt-drawing, implying the rubber elasticity of molten UHMW-PE. No dependence of the fiber diameter on the drawing temperature or the strain rate was observed. Fig. 7 shows a digital microscopic image of the melt-drawn fiber prepared at 145 °C with a DR of 20 under a strain rate of 40/min. A homogeneous meltdrawn fiber with a diameter of approximately 150 μm was observed. Consequently, the diameter of the UHMW-PE fiber was successfully reduced by the melt-drawing process. Fig. 8 shows the DSC heating thermograms for the melt-drawn fibers prepared at 145 and 150 °C with a constant DR of 20 under different strain rates. Two endotherms, at approximately 130 °C and above 140 °C, were observed for the melt-drawn fibers, although a single melting endotherm was observed for the as-spun fiber (Fig. 4). Previous reports on the melt-drawing of UHMW-PE [7–10,16,21] revealed that the lower endotherm is due to the melting of FCCs and the higher endotherm is due to the melting of ECCs and the orthorhombic-hexagonal phase transition/melting of the hexagonal phase attributed to ECCs [16,18,27,28]. The ECCs are formed by the oriented crystallization during melt-drawing [8–13,15,17,21]. In the melt-drawing process of UHMW-PE, deep entanglements, which have a lower chain mobility [17], effectively transmit the applied stress [8,10,13,15,17,21], resulting in ECC growth by the oriented crystallization. The higher endotherm clearly appeared at a strain rate of 3/min for the melt-drawn fibers prepared at 145 °C and became predominant with increasing strain rate [Fig. 8(a)]. This indicates the development of ECCs with increasing strain rate owing to the increase in the applied stress. Two endotherms were also observed for the melt-drawn fibers prepared at 150 °C, but the variation in the endotherms was small [Fig. 8(b)]. The higher endotherm was unclear below 6/min, and two endotherms coexisted at a strain rate of 40/min. For the melt-drawing at 150 °C, the number of deep entanglements is decreased by chain relaxation, resulting in lower applied stress and less ECC development. In contrast, increasing the strain rate promotes the development of ECCs. This behavior can be explained by the temperature-strain rate relationship for melt-drawing [12]. Melt-drawing at a higher strain rate is comparable to that at a lower temperature. In fact, the DSC heating thermogram for the melt-drawn fiber prepared at 150 °C under a strain rate of 40/min [Fig. 8(b)] coincided with that for the melt-drawn fiber prepared at 145 °C under a strain rate of 6/min [Fig. 8(a)]. Fig. 9 shows the WAXD patterns for the melt-drawn fibers prepared at 145 and 150 °C with a constant DR of 20 under different strain rates. A (110) reflection consisting of both spot-shaped and arc-shaped reflections was observed on the equator, although only a broad arc-

Fig. 5. (a) WAXD pattern and (b) azimuthal profile extracted along azimuthal direction at orthorhombic (110) reflection of WAXD pattern for as-spun fiber.

Fig. 6. Change in fiber diameter with DR for melt-drawn samples prepared at 145 °C under strain rate of 40/min.

4

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Fig. 8. DSC heating thermograms for melt-drawn fibers with a constant DR of 20 prepared at (a) 145 and (b) 150 °C under different strain rates.

orientation for the split peaks for the melt-drawn fibers prepared at 145 °C was higher than that for the fibers prepared at 150 °C. FCCs, corresponding to the split peaks, are mainly formed during cooling after melt-drawing [8–10,15,21]. The decrease in the number of well-stresstransferred components during the melt-drawing at 150 °C causes the increase in the amount of FCC with lower chain orientation. Fig. 14 shows the changes in the crystallinity (Xc) for the melt-drawn fibers. Xc was calculated from the 2θ profile as the ratio of the area of the crystalline reflection peaks (orthorhombic (110) [A(110)] and (200) [A(200)] reflections) to the total area of the diffraction profile (∑A) using the following equation.

shaped reflection was observed for the as-spun fiber [Fig. 5(a)], indicating the improved chain orientation in the drawing direction. This combination of spot- and arc-shaped reflections is typical for the shishkebab structure of a melt-drawn UHMW-PE [7–10]. The shish crystals (ECCs) formed during drawing from the melt exhibit the spot-shaped reflection. The arc-shaped reflection transformed to the spot-shaped reflection with increasing strain rate, indicating that the chain orientation is accelerated with increasing strain rate. The transformation of the (110) reflection from an arc-shaped to spot-shaped reflection was more gradual at 150 °C than that at 145 °C. Changes in the chain orientation for the melt-drawn fibers were quantitatively analyzed by comparing the azimuthal profiles extracted along the azimuthal direction at the (110) reflection, as shown in Fig. 10. A narrower central peak (β = 90°) with high chain orientation was observed for the meltdrawn fibers prepared at 145 °C [Fig. 10(a)]. On the other hand, a central peak and split peaks (β = 70 and 110°) coexisted at the lower strain rates for the melt-drawn fibers prepared at 150 °C [Fig. 10(b)]. The relaxation of deep entanglements reduces the stress transfer during melt-drawing at lower strain rates at 150 °C, resulting in lower chain orientation. Increasing the strain rate inhibits the relaxation of deep entanglements, resulting in higher chain orientation even for the meltdrawing at 150 °C. Fig. 11 shows the changes in Dc for the melt-drawn fibers evaluated using Eq. (1). Dc increased with increasing strain rate for the melt-drawn fibers prepared at 145 °C, and Dc was 90.6 % at the strain rate of 40/min. The increase in Dc was gradual for the meltdrawn fibers prepared at 150 °C, and Dc was 74.1 % at the strain rate of 40/min. This value of Dc was similar to that for the melt-drawn fiber prepared at 145 °C and 6/min (73.0 %), corresponding to the result of the DSC measurements (Fig. 8), reflecting the temperature-strain rate relationship for melt-drawing. For detailed analysis, the area fraction and the degree of orientation for the central peak and the split peaks were estimated [29,30], as shown in Fig. 12. Note that the trend of the variations of the area fraction of the central peak and split peaks [Fig. 12(a)] is similar to that of the higher and lower peaks in the DSC measurements (Fig. 13). This indicates that the central peak in the azimuthal profile corresponds to ECCs with the higher melting temperature and the split peaks correspond to FCCs with the lower melting temperature. The increase in the area fraction of the central peak was more rapid for the melt-drawn fibers prepared at 145 °C than for the fibers prepared at 150 °C, even though the degree of orientation for the central peak was comparable. This indicates that the amount of ECC increases with decreasing melt-drawing temperature, whereas the crystalline orientation does not significantly change. The area fraction of the split peaks for the melt-drawn fibers prepared at 145 °C was lower than that for the fibers prepared at 150 °C, and the degree of

Xc (%) =

A(100) + A(200) × 100 ΣA

(2)

The 2θ profile was obtained by the summation over the azimuth angles from 0 to 180° for the WAXD pattern, and was resolved into amorphous scattering and (110) and (200) reflection peaks using the pseudo-Voigt function. Xc increased with increasing strain rate for both drawing temperatures. The increase in Xc was greater for the meltdrawing at 145 °C, in which the behavior was similar to the changes in the area fractions of the central peak in the WAXD measurements [Fig. 12(a)] and the higher peak in the DSC measurements (Fig. 13). This implies that the increase in the crystallinity for the melt-drawn fiber originates from the oriented crystallization into ECCs during the melt-drawing, i.e., the disentanglement of molecular chains. The crystalline morphologies of the obtained melt-drawn fibers were investigated. Fig. 15 shows the SAXS patterns for the melt-drawn fibers prepared at 145 °C with a DR of 20 under strain rates of 1 and 40/ min. The pattern for the melt-drawn fiber prepared under a strain rate of 1/min [Fig. 15(a)] exhibited an equatorial streak attributed to the ECCs and meridional scattering maxima attributed to the FCCs, indicating the formation of a shish-kebab structure [31,32]. On the other hand, the pattern for the melt-drawn fiber prepared under a strain rate of 40/min [Fig. 15(b)] exhibited almost only the equatorial streak, indicating that ECCs developed by the melt-drawing under higher strain rates. For detailed analyses of the crystalline morphology, we applied a fuming nitric acid etching to the melt-drawn fibers. The amorphous component was selectively removed by etching, thus emphasizing the crystalline structure [9,10]. Fig. 16 shows the FE-SEM images of the melt-drawn fibers prepared at 145 °C with a DR of 20 under strain rates of 1 and 40/min after etching with fuming nitric acid. A shish-kebab structure consisting of ECCs and FCCs [9,10] was clearly observed for both melt-drawn fibers. ECCs were dominantly formed for the meltdrawn fiber prepared under a strain rate of 40/min [Fig. 16(b)], whereas FCCs were dominantly formed for the melt-drawn fiber 5

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Fig. 9. WAXD patterns for melt-drawn fibers with a constant DR of 20 prepared at (top) 145 and (bottom) 150 °C under different strain rates.

tensile modulus increased with decreasing drawing temperature and increasing strain rate. This behavior can be explained from the crystalline structure development of the melt-drawn fiber revealed by the results of the DSC, WAXD, and SAXS measurements and FE-SEM observation. ECCs developed in the melt-drawing at the lower temperature of 145 °C and under a higher strain rate [Figs. 12(a) and 13], resulting in higher tensile strength (Fig. 17). The correlation between the DSC result and the mechanical properties is also recognized for gel-spun UHMW-PE fibers [27,33]. The tensile strength for the melt-drawn fiber prepared at 145 °C with a DR of 20 under the strain rate of 40/min, which exhibits the most enhanced ECC formation and the highest Dc, was approximately 1.1 GPa. Consequently, a UHMW-PE fiber with high

prepared under a strain rate of 1/min [Fig. 16(a)]. The molecules with more stress transfer during the melt-drawing form ECCs. On the other hand, the molecules with less stress transfer during the melt-drawing form FCCs that crystallize during cooling. The high strain rate inhibits the chain relaxation during the melt-drawing, resulting in the dominant formation of ECCs in the obtained melt-drawn fiber. These morphologies of the melt-drawn fibers are consistent with the results of DSC [Figs. 8(a) and 13] and WAXD [Figs. 9 and 12(a)] measurements. Table 1 shows the tensile strength, tensile modulus, and elongation at break for the as-spun fiber and melt-drawn fibers, and Fig. 17 shows the changes in the tensile strength with the strain rate for the meltdrawn fibers prepared at 145 and 150 °C. The tensile strength and 6

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Fig. 10. Azimuthal profiles extracted along azimuthal direction at (110) reflection of WAXD patterns for melt-drawn fibers prepared at (a) 145 and (b) 150 °C.

Fig. 11. Changes in Dc with strain rate for melt-drawn fibers prepared at 145 and 150 °C.

Fig. 13. Changes in area fraction of lower and higher peaks with strain rate for melt-drawn fibers prepared at 145 and 150 °C in DSC measurements shown in Fig. 8.

Fig. 12. Changes in (a) area fraction and (b) degree of orientation of central peak and split peaks with strain rate for melt-drawn fibers prepared at 145 and 150 °C. 7

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Table 1 Mechanical properties of as-spun fiber and melt-drawn fibers.

as-spun 145 °C-1/min 145 °C-3/min 145 °C-6/min 145 °C-10/min 145 °C-20/min 145 °C-40/min 150 °C-1/min 150 °C-3/min 150 °C-6/min 150 °C-10/min 150 °C-20/min 150 °C-40/min

Strength (GPa)

Modulus (GPa)

Elongation at break (%)

0.08 0.40 0.60 0.71 0.89 1.03 1.06 0.14 0.36 0.52 0.52 0.57 0.68

1.0 13.2 21.1 27.6 25.7 26.0 28.2 8.9 12.7 20.4 21.6 21.8 27.9

113.3 5.0 5.4 5.7 5.6 5.2 4.4 2.4 4.8 4.5 4.0 3.9 4.0

Fig. 14. Changes in Xc with strain rate for melt-drawn fibers prepared at 145 and 150 °C.

Fig. 15. SAXS patterns for melt-drawn fibers prepared at 145 °C with DR of 20 under strain rates of (a) 1 and (b) 40/min. Fig. 17. Changes in tensile strength with strain rate for melt-drawn fibers prepared at 145 and 150 °C.

tensile strength was prepared by the combination of melt-spinning and melt-drawing. This approach is a promising methodology for the green processing of high-performance UHMW-PE fibers.

850 μm. The as-spun fiber was melt-drawn at 145 and 150 °C. The fiber diameter was reduced by the melt-drawing process, and the diameter of the obtained melt-drawn fiber with a DR of 20 was approximately 150 μm. The melt-drawn fiber consisted of ECCs and FCCs. The chain orientation with the development of the ECCs was accelerated by meltdrawing at 145 °C under higher strain rates. The tensile strength of the melt-drawn fibers increased with decreasing drawing temperature and increasing strain rate, reflecting the enhanced ECC formation and

4. Conclusions UHMW-PE fibers were prepared by a combination of melt-spinning and melt-drawing in this study. An as-spun fiber was prepared by meltspinning from UHMW-PE powder. A homogeneous extruded sample without melt fracture was continuously obtained at (Tb, Tn) of (190, 160) °C. The diameter of the obtained as-spun fiber was approximately

Fig. 16. FE-SEM images of melt-drawn fibers prepared at 145 °C with DR of 20 under strain rates of (a) 1 and (b) 40/min after etching with fuming nitric acid. The fiber axis is horizontal. 8

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higher chain orientation. The tensile strength for the melt-drawn fiber prepared at 145 °C with a DR of 20 under a strain rate of 40/min, which exhibited the most enhanced ECC formation and the highest Dc, was approximately 1.1 GPa. Consequently, a UHMW-PE fiber with high tensile strength was prepared by the combination of melt-spinning and melt-drawing.

[14]

[15]

Declaration of Competing Interest

[16]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[17]

[18]

Acknowledgements [19]

This work was partly supported by The Ogasawara Foundation for the Promotion of Science & Engineering. Microbeam SAXS measurements using synchrotron radiation were performed at the BL40XU of SPring-8 with the approval of JASRI (Proposal No. 2019B1347). We appreciate the cooperation of Drs. Kouki Aoyama and Hiroyasu Masunaga (JASRI).

[20]

[21]

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