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A comparative study of UHMWPE fibers prepared by flash-spinning and gel-spinning
Materials Letters 147 (2015) 79–81
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A comparative study of UHMWPE fibers prepared by flash-spinning and gel-spinning Lei Xia a,b,n, Peng Xi b,c, Bowen Cheng a,b,c a
School of Textile, Tianjin Polytechnic University, Tianjin 300160, China Key Laboratory of Advanced Textile Composites, Ministry of Education of China, Tianjin 300160, China c Tianjin Multicipal Kay Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, Tianjin 300160, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 December 2014 Accepted 10 February 2015 Available online 18 February 2015
Ultrahigh molecular weight polyethylene fibers were firstly fabricated by flash-spinning and were compared with UHMWPE commercial fibers prepared using a traditional gel-spinning method. The difference of their molecular structure transformation was discussed, and the crystallinity of gelspinning fiber was higher than the flash-spinning's. The fiber prepared by flash-spinning was a bunch of thinner fibers with a diameter of several micrometers rather than a single one fabricated by gelspinning. The flash-spinning fiber had relatively rough surface while the gel-spinning fiber had the smooth appearance. The thinner diameter and the rougher surface result in the relatively higher specific surface area of the flash-spinning fiber. The thermal and structure of these two fibers were characterized by DSC and X-ray diffraction. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polymers Fiber technology Microstructure Thermal analysis
1. Introduction Ultrahigh molecular weight polyethylene (UHMWPE) fiber is a new kind of high performance fiber developed in the recent two decades. Its distinctive properties are low friction coefficient, high wear resistance, high mechanical properties [1,2], especially high strength can be reached by the UHMWPE fiber due to orientation of its very long polymeric chains. However, the long polymeric chains cause the UHMWPE difficult to be processed by injection molding and conventional screw extrusion, and there are less reports about nano/micro-fibers of UHMWPE. There is a popular belief that several desirable characteristics such as high surface area to volume ratio, flexibility in surface functionality, and superior mechanical properties are achieved when the diameters of the fibers are reduced to nano/ micrometer [3]. Surely, the UHMWPE with these outstanding properties will expand the product applications. Electrospinning has been recognized as an effective and versatile method to process solution or melt, mainly of polymers, into continuous nano/micron fibers in high electricfields [4,5]. Unfortunately, the UHMWPE is very difficult to electrospin due to its high viscosity and low solubility in general organic solvents [6]. Flash-spinning is a novel method for preparing plexifilamentary film-fibril strands from fiber-forming polymers [7]. In this
n Corresponding author at: School of Textile, Tianjin Polytechnic University, Tianjin 300160, China. Tel.: þ 86 022 83955353; fax: þ86 022 83955287. E-mail address: [email protected] (L. Xia).
http://dx.doi.org/10.1016/j.matlet.2015.02.046 0167-577X/& 2015 Elsevier B.V. All rights reserved.
process, the polymer was dissolved in a solvent, which was a nonsolvent for the polymer at or below its normal boiling point, at a temperature above the normal boiling point of the solvent. Then the solution was extruded into a region of substantially lower pressure and temperature. The superheating temperature caused the solvent to vaporize and thereby the extrudate was cooled to form a plexifilamentary film-fibril strand of the polymer [8,9]. Preferred polymers include crystalline polyhydrocarbons such as polyethylene and polypropylene [10]. In our study, the UHMWPE fibers were fabricated via flash-spinning and a comparative study with the fibers prepared using the gel-spinning method was investigated.
2. Experimental The UHMWPE powder was supplied by Sinopec Qilu Company with an average molecular weight of 1 106 g/mol and a density of 0.935 g/cm3. 1,2-Dichloroethane, cyclopentane and Antioxidant Irganox 1010 were purchased from Tianjin Kermel Chemical Reagent (China) and used without any further purification. In the flash-spinning experiment, a certain amount of UHMWPE was dispersed in a mixture of 1,2-dichloroethane and cyclopentane inside an autoclave. Next, the autoclave was filled with carbon dioxide to a pressure of 5 Mpa after expelling the oxygen. The mixture was heated to 120 1C at 2 1C/min for 1 h with mechanical agitation, the temperature was then elevated to 180 1C at 2 1C/min and maintained
80
L. Xia et al. / Materials Letters 147 (2015) 79–81
for 0.5 h to obtain a homogeneous solution. Then the solution was passed through a steel nozzle whose inner diameter was 1 mm and length was 60 mm, the solvents soon vaporized and the fibers emerged from the solution. The fibers collected from the flashspinning were coded as FS-F; The UHMWPE fibers prepared by gelspinning were purchased from Beijing Tongyizhong Specialty Fiber Technology & Development CO., Ltd. and coded as GS-F for comparison (stretch ratio:30). The morphology of the fibers was observed by a Field Scanning Electron Microscopy (FE-SEM) (S-4800, Hitachi Co., Japan). The Brunauer–Emmett–Teller (BET) surface area was measured by using a surface area analyzer (SAA: Sorptomatic 1990, ThermoFinnigan Co.). Differential scanning calorimetry (DSC) test was recorded using NETZSCH DSC 200 F3 in the ranging from room temperature to 400 1C. X-ray diffraction pattern (XRD) was performed on an X-ray diffractometer (D/MAX-2500, Rigaku), the diffraction angle ranged from 51to 401. The mechanical properties were tested by a universal testing machine (STA-1150, Orientec, Japan).
3. Results and discussion In the flash-spinning process, the temperature of the solution and the spinning pressure were two factors mostly affecting the morphology of the products. Only certain special combination of temperature and pressure for the solvent–polymer system would produce the microcellular products [7]. The polymer could not fully dissolve in the solvent if the temperature was low, and the polymer may suffer thermal degradation at high temperature. Similarly, the fibers could not be prepared when the pressure was not high enough due to the poor stretch force while the very high pressure proposed a higher request of the equipment. So, the temperature ranging from 150 1C to 210 1C and the spinning pressure between 8 MPa and 20 MPa were suitable for the flash-spinning. Furthermore, the temperature of the nozzle and the concentration of the UHMWPE were fixed at 100 1C and 5%, respectively. Fig. 1 was the structure transformation of FS-F and GS-F from the dissolution in a proper solvent to a strong and stiff fiber. The FS-F was prepared by one step and the structure change of molecule chains was shown in Fig. 1(a). The molecular chains, which were random coil in the solution, were rapidly cooled and stretched in a very short time. Parts of the chains were basically aligned along with the fiber, while parts of the chains were still crystallized or amorphous due to the less powerful gas-drawing. The GS-F was obtained mainly after two steps and the structure change of GS-F was represented schematically in Fig. 1(b). In the
first step, the polymer was crystallized to form lamellar, foldedchain single crystals and then the continued array of fibrous shishkebabs crystals when a low draw ratio was applied. The second and most useful step was the hot drawing, the structure changed from the fibrous crystal to a highly oriented, fully extended chain crystals. Before hot drawing, the molecules were folded, hot drawing caused the molecular chains to unfold and orientate along the fiber length direction [6]. The morphology of the FS-F and the GS-F was observed in Fig. 2. The SEM image of FS-F was shown in Fig. 2(a), it can be seen that the fiber prepared by flash-spinning was not a single one but a bunch of thinner fibers, which was just like a yarn without any twist. The majority of the thinner fibers were bonded to each other with diameter ranging from 0.2 μm to 5 μm and had relatively rough surface (inset of Fig. 2(a)). As shown in Fig. 2(b), the GS-F with smooth surface, round shape and some crevices could be observed and the diameter was about 40 μm. Contrast to the GS-F, the hierarchical structures of the FS-F significantly enhanced the surface-to-volume ratio (shown in Table 1). Fig. 3(a) shows the DSC thermograms of FS-F and GS-F and the raw material powder. The melting point of the raw material powder was 142.0 1C. For the FS-F and GS-F, the melting points were both slightly shifted to lower temperature region, which were 131 1C and 136 1C, respectively. The reason might be attributed to molecular weight degradation during the high temperature spinning process, and the degree of degradation of FS-F was higher than the GS-F's due to the high pressure. The X-ray diffraction patterns of the two fibers are shown in Fig. 3(b), the prominent peaks located at 19.41, 21.51, and 23.91were corresponding to (100), (110) and (200) crystal lattice planes of UHMWPE, respectively. And the GS-F showed sharper peaks than the FS-F's. The reasons lying in the hot drawing process of the GS-F were drived by mechanical force and the draw ratio was higher, while the FS-F was soon cooled in the stretching process and the gas flow was not strong enough to stretch the fiber in low-temperature environment. The GS-F has more ordered molecular chains and higher crystallinity than the FS-F's. The mechanic properties of the fibers are crucial for the industrial application. The data of breaking strength, Young's modulus, and breaking elongation of FS-F and GS-F were shown in Table 1. It can be concluded that the breaking strength and Young's modulus of FS-F were significantly lower while the breaking elongation of FS-F was higher than the GS-F's. The differences in mechanical properties of FSF and GS-F appeared to be related to the degree of crystallinity. So, it is obvious to conclude that the degrees of crystallinity and orientation of the GS-F were not yet achieved in the one step flash-spinning process. The mechanic data also validated the analysis in demonstrating the molecular transformation of these two kinds of fibers.
Fig. 1. Structural transformation of (a) FS-F and (b) GS-F.
L. Xia et al. / Materials Letters 147 (2015) 79–81
81
Fig. 2. SEM of (a) FS-F and (b) GS-F.
Table 1 Preparation parameters and properties of FS-F and GS-F. Sample code
Concentration (%)
Fiber diameter (mm)
Specific surface area (m2/g)
Tensile strength (cN/dtex)
Young's modulus (cN/dtex)
Elongation (%)
FS-F GS-F
5% 5%
37 2 407 5
12.361 0.265
7.45 23.52
77.52 852.37
31.6 2.8
Fig. 3. DSC and XRD of FS-F and GS-F.
4. Conclusions The UHMWPE microfiber was successfully fabricated by flashspinning using 1,2-Dichloroethane and cyclopentane as solvent. Compared with the commercial GS-F, the degrees of crystallinity and orientation of the FS-F were not yet achieved in the flashspinning process. The FS-F had lower strength and modulus, which was still higher than most of the other polymer fibers, but higher specific surface area and breaking elongation than the GS-F's. The exactly intrinsic characteristic proved the FS-F to be suitable for various potential applications such as filter, microsensors, protective clothing and so on. Acknowledgments The work is supported by National Natural Science Foundation of China under Grant nos. 52103112 (Xia), 51373118 (Xi) for their financial support. References [1] Dangsheng X. Friction and wear properties of UHMWPE composites reinforced with carbon fiber. Mater Lett 2005;159:175–9.
[2] Suiyi L, Dagang L. Electrically conductive charcoal powder/ultrahigh molecular weight polyethylene composites. Mater Lett 2014;137:409–12. [3] Zhengming H, Zhang YZ, Kotaki M. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos Sci Technol 2003;63:2223–53. [4] Sun B, Long YZ, Zhang HD, Li MM, Duvail JL, Jiang XY, et al. Advances in threedimensional nanofibrous macrostructures via electrospinning. Prog Polym Sci 2014;39:862–90. [5] Xiuyan L, Wang Z, Jiaona W. Preparation and properties of TPU micro/nanofibers by a laser melt-electrospinning system. Polym Eng Sci 2014;54:1412–7. [6] Rein DM, Shavit-Hadar L, Khalfin RL, Cohen Y, Shuster K, Zussman E. Electrospinning of ultrahigh-molecular-weight polyethylene nanofibers. J Polym Sci Poly Phys 2007;45:766–73. [7] Kim SY, Purnama P, Kim SH. Fabrication of poly(l-lactide) fibers/sheets using supercritical fluid through flash-spinning process. Macromol Res 2010;18:1233–6. [8] Shin H, Siemionko RK. Inventor. E. I. du Pont de Nemours and Company, assignee. Flash spinning process. United States Patent US 5672307; Sep 2 1997. [9] Shin H, Siemionko RK. Inventor. E. I. du Pont de Nemours and Company, assignee. Flash-spinning solution. United States Patent US 5977237; Nov 2 1999. [10] Blades H, White JR. Inventor. E. I. du Pont de Nemours and Company, assignee. Process for producing molecularly oriented structures by extrusion of a polymer solution. United States Patent US 3227784; Jan 4 1966.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A comparative study of UHMWPE fibers prepared by flash-spinning and gel-spinning Lei Xia a,b,n, Peng Xi b,c, Bowen Cheng a,b,c a
School of Textile, Tianjin Polytechnic University, Tianjin 300160, China Key Laboratory of Advanced Textile Composites, Ministry of Education of China, Tianjin 300160, China c Tianjin Multicipal Kay Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, Tianjin 300160, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 December 2014 Accepted 10 February 2015 Available online 18 February 2015
Ultrahigh molecular weight polyethylene fibers were firstly fabricated by flash-spinning and were compared with UHMWPE commercial fibers prepared using a traditional gel-spinning method. The difference of their molecular structure transformation was discussed, and the crystallinity of gelspinning fiber was higher than the flash-spinning's. The fiber prepared by flash-spinning was a bunch of thinner fibers with a diameter of several micrometers rather than a single one fabricated by gelspinning. The flash-spinning fiber had relatively rough surface while the gel-spinning fiber had the smooth appearance. The thinner diameter and the rougher surface result in the relatively higher specific surface area of the flash-spinning fiber. The thermal and structure of these two fibers were characterized by DSC and X-ray diffraction. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polymers Fiber technology Microstructure Thermal analysis
1. Introduction Ultrahigh molecular weight polyethylene (UHMWPE) fiber is a new kind of high performance fiber developed in the recent two decades. Its distinctive properties are low friction coefficient, high wear resistance, high mechanical properties [1,2], especially high strength can be reached by the UHMWPE fiber due to orientation of its very long polymeric chains. However, the long polymeric chains cause the UHMWPE difficult to be processed by injection molding and conventional screw extrusion, and there are less reports about nano/micro-fibers of UHMWPE. There is a popular belief that several desirable characteristics such as high surface area to volume ratio, flexibility in surface functionality, and superior mechanical properties are achieved when the diameters of the fibers are reduced to nano/ micrometer [3]. Surely, the UHMWPE with these outstanding properties will expand the product applications. Electrospinning has been recognized as an effective and versatile method to process solution or melt, mainly of polymers, into continuous nano/micron fibers in high electricfields [4,5]. Unfortunately, the UHMWPE is very difficult to electrospin due to its high viscosity and low solubility in general organic solvents [6]. Flash-spinning is a novel method for preparing plexifilamentary film-fibril strands from fiber-forming polymers [7]. In this
n Corresponding author at: School of Textile, Tianjin Polytechnic University, Tianjin 300160, China. Tel.: þ 86 022 83955353; fax: þ86 022 83955287. E-mail address: [email protected] (L. Xia).
http://dx.doi.org/10.1016/j.matlet.2015.02.046 0167-577X/& 2015 Elsevier B.V. All rights reserved.
process, the polymer was dissolved in a solvent, which was a nonsolvent for the polymer at or below its normal boiling point, at a temperature above the normal boiling point of the solvent. Then the solution was extruded into a region of substantially lower pressure and temperature. The superheating temperature caused the solvent to vaporize and thereby the extrudate was cooled to form a plexifilamentary film-fibril strand of the polymer [8,9]. Preferred polymers include crystalline polyhydrocarbons such as polyethylene and polypropylene [10]. In our study, the UHMWPE fibers were fabricated via flash-spinning and a comparative study with the fibers prepared using the gel-spinning method was investigated.
2. Experimental The UHMWPE powder was supplied by Sinopec Qilu Company with an average molecular weight of 1 106 g/mol and a density of 0.935 g/cm3. 1,2-Dichloroethane, cyclopentane and Antioxidant Irganox 1010 were purchased from Tianjin Kermel Chemical Reagent (China) and used without any further purification. In the flash-spinning experiment, a certain amount of UHMWPE was dispersed in a mixture of 1,2-dichloroethane and cyclopentane inside an autoclave. Next, the autoclave was filled with carbon dioxide to a pressure of 5 Mpa after expelling the oxygen. The mixture was heated to 120 1C at 2 1C/min for 1 h with mechanical agitation, the temperature was then elevated to 180 1C at 2 1C/min and maintained
80
L. Xia et al. / Materials Letters 147 (2015) 79–81
for 0.5 h to obtain a homogeneous solution. Then the solution was passed through a steel nozzle whose inner diameter was 1 mm and length was 60 mm, the solvents soon vaporized and the fibers emerged from the solution. The fibers collected from the flashspinning were coded as FS-F; The UHMWPE fibers prepared by gelspinning were purchased from Beijing Tongyizhong Specialty Fiber Technology & Development CO., Ltd. and coded as GS-F for comparison (stretch ratio:30). The morphology of the fibers was observed by a Field Scanning Electron Microscopy (FE-SEM) (S-4800, Hitachi Co., Japan). The Brunauer–Emmett–Teller (BET) surface area was measured by using a surface area analyzer (SAA: Sorptomatic 1990, ThermoFinnigan Co.). Differential scanning calorimetry (DSC) test was recorded using NETZSCH DSC 200 F3 in the ranging from room temperature to 400 1C. X-ray diffraction pattern (XRD) was performed on an X-ray diffractometer (D/MAX-2500, Rigaku), the diffraction angle ranged from 51to 401. The mechanical properties were tested by a universal testing machine (STA-1150, Orientec, Japan).
3. Results and discussion In the flash-spinning process, the temperature of the solution and the spinning pressure were two factors mostly affecting the morphology of the products. Only certain special combination of temperature and pressure for the solvent–polymer system would produce the microcellular products [7]. The polymer could not fully dissolve in the solvent if the temperature was low, and the polymer may suffer thermal degradation at high temperature. Similarly, the fibers could not be prepared when the pressure was not high enough due to the poor stretch force while the very high pressure proposed a higher request of the equipment. So, the temperature ranging from 150 1C to 210 1C and the spinning pressure between 8 MPa and 20 MPa were suitable for the flash-spinning. Furthermore, the temperature of the nozzle and the concentration of the UHMWPE were fixed at 100 1C and 5%, respectively. Fig. 1 was the structure transformation of FS-F and GS-F from the dissolution in a proper solvent to a strong and stiff fiber. The FS-F was prepared by one step and the structure change of molecule chains was shown in Fig. 1(a). The molecular chains, which were random coil in the solution, were rapidly cooled and stretched in a very short time. Parts of the chains were basically aligned along with the fiber, while parts of the chains were still crystallized or amorphous due to the less powerful gas-drawing. The GS-F was obtained mainly after two steps and the structure change of GS-F was represented schematically in Fig. 1(b). In the
first step, the polymer was crystallized to form lamellar, foldedchain single crystals and then the continued array of fibrous shishkebabs crystals when a low draw ratio was applied. The second and most useful step was the hot drawing, the structure changed from the fibrous crystal to a highly oriented, fully extended chain crystals. Before hot drawing, the molecules were folded, hot drawing caused the molecular chains to unfold and orientate along the fiber length direction [6]. The morphology of the FS-F and the GS-F was observed in Fig. 2. The SEM image of FS-F was shown in Fig. 2(a), it can be seen that the fiber prepared by flash-spinning was not a single one but a bunch of thinner fibers, which was just like a yarn without any twist. The majority of the thinner fibers were bonded to each other with diameter ranging from 0.2 μm to 5 μm and had relatively rough surface (inset of Fig. 2(a)). As shown in Fig. 2(b), the GS-F with smooth surface, round shape and some crevices could be observed and the diameter was about 40 μm. Contrast to the GS-F, the hierarchical structures of the FS-F significantly enhanced the surface-to-volume ratio (shown in Table 1). Fig. 3(a) shows the DSC thermograms of FS-F and GS-F and the raw material powder. The melting point of the raw material powder was 142.0 1C. For the FS-F and GS-F, the melting points were both slightly shifted to lower temperature region, which were 131 1C and 136 1C, respectively. The reason might be attributed to molecular weight degradation during the high temperature spinning process, and the degree of degradation of FS-F was higher than the GS-F's due to the high pressure. The X-ray diffraction patterns of the two fibers are shown in Fig. 3(b), the prominent peaks located at 19.41, 21.51, and 23.91were corresponding to (100), (110) and (200) crystal lattice planes of UHMWPE, respectively. And the GS-F showed sharper peaks than the FS-F's. The reasons lying in the hot drawing process of the GS-F were drived by mechanical force and the draw ratio was higher, while the FS-F was soon cooled in the stretching process and the gas flow was not strong enough to stretch the fiber in low-temperature environment. The GS-F has more ordered molecular chains and higher crystallinity than the FS-F's. The mechanic properties of the fibers are crucial for the industrial application. The data of breaking strength, Young's modulus, and breaking elongation of FS-F and GS-F were shown in Table 1. It can be concluded that the breaking strength and Young's modulus of FS-F were significantly lower while the breaking elongation of FS-F was higher than the GS-F's. The differences in mechanical properties of FSF and GS-F appeared to be related to the degree of crystallinity. So, it is obvious to conclude that the degrees of crystallinity and orientation of the GS-F were not yet achieved in the one step flash-spinning process. The mechanic data also validated the analysis in demonstrating the molecular transformation of these two kinds of fibers.
Fig. 1. Structural transformation of (a) FS-F and (b) GS-F.
L. Xia et al. / Materials Letters 147 (2015) 79–81
81
Fig. 2. SEM of (a) FS-F and (b) GS-F.
Table 1 Preparation parameters and properties of FS-F and GS-F. Sample code
Concentration (%)
Fiber diameter (mm)
Specific surface area (m2/g)
Tensile strength (cN/dtex)
Young's modulus (cN/dtex)
Elongation (%)
FS-F GS-F
5% 5%
37 2 407 5
12.361 0.265
7.45 23.52
77.52 852.37
31.6 2.8
Fig. 3. DSC and XRD of FS-F and GS-F.
4. Conclusions The UHMWPE microfiber was successfully fabricated by flashspinning using 1,2-Dichloroethane and cyclopentane as solvent. Compared with the commercial GS-F, the degrees of crystallinity and orientation of the FS-F were not yet achieved in the flashspinning process. The FS-F had lower strength and modulus, which was still higher than most of the other polymer fibers, but higher specific surface area and breaking elongation than the GS-F's. The exactly intrinsic characteristic proved the FS-F to be suitable for various potential applications such as filter, microsensors, protective clothing and so on. Acknowledgments The work is supported by National Natural Science Foundation of China under Grant nos. 52103112 (Xia), 51373118 (Xi) for their financial support. References [1] Dangsheng X. Friction and wear properties of UHMWPE composites reinforced with carbon fiber. Mater Lett 2005;159:175–9.
[2] Suiyi L, Dagang L. Electrically conductive charcoal powder/ultrahigh molecular weight polyethylene composites. Mater Lett 2014;137:409–12. [3] Zhengming H, Zhang YZ, Kotaki M. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos Sci Technol 2003;63:2223–53. [4] Sun B, Long YZ, Zhang HD, Li MM, Duvail JL, Jiang XY, et al. Advances in threedimensional nanofibrous macrostructures via electrospinning. Prog Polym Sci 2014;39:862–90. [5] Xiuyan L, Wang Z, Jiaona W. Preparation and properties of TPU micro/nanofibers by a laser melt-electrospinning system. Polym Eng Sci 2014;54:1412–7. [6] Rein DM, Shavit-Hadar L, Khalfin RL, Cohen Y, Shuster K, Zussman E. Electrospinning of ultrahigh-molecular-weight polyethylene nanofibers. J Polym Sci Poly Phys 2007;45:766–73. [7] Kim SY, Purnama P, Kim SH. Fabrication of poly(l-lactide) fibers/sheets using supercritical fluid through flash-spinning process. Macromol Res 2010;18:1233–6. [8] Shin H, Siemionko RK. Inventor. E. I. du Pont de Nemours and Company, assignee. Flash spinning process. United States Patent US 5672307; Sep 2 1997. [9] Shin H, Siemionko RK. Inventor. E. I. du Pont de Nemours and Company, assignee. Flash-spinning solution. United States Patent US 5977237; Nov 2 1999. [10] Blades H, White JR. Inventor. E. I. du Pont de Nemours and Company, assignee. Process for producing molecularly oriented structures by extrusion of a polymer solution. United States Patent US 3227784; Jan 4 1966.