A process for preparing a polyethylene filament

Abstract: An optical fiber cord or cable comprising at least one optical fiber and at least one tensile member for reinforcing said optical fiber. said tensile member comprising at least one polyethylene filament having a viscosity average mole­ cular weight of not less than 200,000, a tenacity of not less than 20 g/d and a tensile modulus of not less than 600 g/d.

Description: The present invention relates to a process for preparing a polyethylene filament having high tenacity and high tensile modulus and being useful as a reinforcing material for optical fiber cords or cables. In order to understand the invention, the following about tensile members is presented herein as technical background An optical fiber cord or cable comprises an optical fiber as the essential part and a tensile member as the reinforcing material. Namely, a tensile member is used for preventing an optical fiber from its breakage which will be caused by application of excessive tension thereto. Such tensile member is usually made of a material which is excellent in tensile modulus and bending rigidity. As the tensile member for optical fiber cords or cables, metallic materials (e.g. steel wire) have heretofore been used and it was also tried to use plastic materials (e.g. polyethylene fiber, polypropylene fiber, polyamide fiber, polyester fiber). In general, metallic materials have a sufficiently high tenacity and high tensile modulus suitable for such use. However, they have a high density, and therefore the resulting tensile member is very heavy in weight. Further, the problem of electromagnetic interference due to strike by lightning arises. Plastic materials do not have drawbacks as recognized for metallic materials; i.e. they are of light weight and do not have such problem as electromagnetic interference due to strike by lightning. However, their tenacity and tensile modulus are usually lower than those of metallic materials. Recently, there has been provided an aromatic polyamide fiber of high tensile modulus ("Kevlar" manufactured by Du Pont). In comparison with steel wire, it is of light weight. As shown in Table 1, for instance, the weight of the aromatic polyamide fiber showing a nearly equal tenacity to that of steel wire is from 0.28 to 0.6 when the weight of steel wire is taken as 1 (this being called as "weight ratio"). Thus, the aromatic polyamide fiber can provide a tensile member of high tensile modulus and light weight. In order to attain a nearly equal tenacity, however, the aromatic polyamide fiber is required to have larger denier or diameter values than steel wire. For instance, the diameter ratio of the aromatic polyamide fiber to steel wire is 1.24 - 1.8 : 1. Nevertheless, the aromatic polyamide fiber having large denier or diameter is hardly obtainable under the present technique, and in fact, the filament of the aromatic polyamide fiber now available on the market has less than 2 denier 0.222 tex or less than 0.014 mm in diameter. Therefore, it is practically essential for achievement of the same level of tenacity with the aromatic polyamide fiber as steel wire to use a multiple number of filaments, i.e. a bundle of filaments. Unfortunately, however, the tensile member using a bundle of filaments of the aromatic polyamide fiber is extremely low in bending rigidity. Although the bending rigidity can be improved by bonding said bundle of filaments together by the aid of a thermosetting plastic resin, the resultant tensile member is unfavorably increased in volume, weight ratio and diameter ratio. In addition, the tenacity and tensile modulus of such tensile member are considerably decreased in comparison with those before bonding. Besides, the aromatic polyamide fiber is still not enough suitable with respect to abrasion resistance and resistance to fatigue from flexing. EP-A-0 091 253 discloses a method for the production of a thermotropic polymer, as well as the use of such polymer as reinforcing material in optical cables. EP-A-0 064 167 discloses a process for the preparation of tensile members, said process comprises spinning polyethylene having a viscosity average molecular weight of not less than 200 000 in a solution state, drawing the resulting gel-like filament, whereby said drawing may also be effected with at least two gel-like filaments as bundled to give the tensile member as an integral body and/or while heating. The filament thus obtained has a tenacity of not less than 20 g/d, a tensile modulus of not less than 600 g/d and a section area of not less than 0,018 mm². In "Journal of Materials Science", No. 15, 1980, pages 505-514 a process for the production of polyethylene filaments is disclosed which comprises dissolving the polymer in decalin at 150 °C and pumping the highly viscous polymer solution at 130 °C through capillaries. The as-spun filament is then quenched in cold water to form a gel-like fiber, which is subsequently drawn in a hot-air oven at 120 °C to yield a solvent free highly oriented polyethylene structure. The filament thus obtained has also a high tensile modulus and a high tenacity. "Plastics and Rubber International", Vol. 5, No. 4, 1980, pages 145-149 discloses the use of highly oriented polyethylene filaments as reinforcing members in optical cables. As a result of the extensive study, it has now been found that the process according to the invention leads to a filament of ultra high molecular weight polyethylene and having a certain tenacity and a certain tensile modulus, which can provide a tensile member particularly suitable as a reinforcing material for an optical fiber to make an optical fiber cord or cable. In detail the present invention relates to a process for preparing a polyethylene filament having a viscosity average molecular weight of not less than 200,000, a tenacity of no less than 20 g/d, a tensile modulus of not less than 600 g/d, and a cross-sectional area of not less than 0.018 mm², said process comprising: compressing a gel-like sheet prepared from a solution of polyethylene having a viscosity average molecular weight of not less than 200,000 in a solvent at a temperature lower than the dissolving temperature of the gel-like sheet, thereby removing a portion of the solvent contained therein, folding or rolling said compressed sheet, and drawing said folded or rolled sheet at a draw ratio of not less than 20, whereby said filament is obtained. Conventional polyethylene fibers are usually prepared by a melt spinning procedure without using any solvent. With the increase of the degree of polymerization, the melt viscosity is markedly enhanced so that the spinning operation becomes difficult. Also, the increase of the degree of polymerization makes the molecular chain of polyethylene longer, and the entanglement among the molecular chains is thus increased. As the result, the orientation of the molecular chains in a certain direction by drawing after the spinning operation becomes difficult. Because of this reason, the viscosity average molecular weight of polyethylene to be melt spun is restricted to be less than 200,000. In the present invention, polyethylene to be used has a viscosity average molecular weight of not less than 200,000 and is formed into filaments by a certain specific procedure as hereinafter explained so that the resultant filaments can have high tenacity and high tensile modulus. As stated above, the polyethylene to be used in this invention should have a viscosity average molecular weight of not less than 200,000, preferably not less than 300,000, more preferably not less than 500,000, the most preferably not less than 1,000,000. A larger viscosity average molecular weight is better, because higher tenacity and higher tensile modulus can be attained. Thus, no upper limit is required on the viscosity average molecular weight. Usually, however, it may be not more than 10,000,000, particularly not more than 8,000,000, more particularly not more than 6,000,000. For technical use, the polyethylene filament should have a tenacity of not less than 20 g/d (17.65 cN/dtex), preferably not less than 23 g/d (20.3 cN/dtex), more preferably not less than 25 g/d (22.07 cN/dtex), and a tensile modulus of not less than 600 g/d (529.6 cN/dtex), preferably not less than 700 g/d (617.9 cN/dtex), more preferably not less than 800 g/d (706.14 cN/dtex). When these physical constants are satisfied, the polyethylene filament can provide the tensile member with performances sufficient to use as an industrial tensile member, particularly a tensile member for optical fiber cord or cable which is required to have a tensile modulus of not less than 5,000 - 7,000 kg/mm². When the tenacity is less than 20 g/d (17.65 cN/dtex), a tensile modulus of not less than 600 g/d (529.6 cN/dtex) can hardly be attained. In case of the tensile modulus being less than 600 g/d (529.6 cN/dtex), the desired bending rigidity is not obtainable so that the performances as required for an industrial tensile member can not be achieved. As understood from the above descriptions, higher tenacity and higher tensile modulus are better. From the practical viewpoint, however, the maximum values for tenacity and tensile modulus are ordinarily 60 g/d (52.96 cN/dtex) and 2,000 g/d (1765.4 cN/dtex), respectively. In addition, the polyethylene filament is favorable to have a long period spacing of not less than 200 Å (20 nm), particularly of not less than 300 Å (30 nm), although it is not essential. With respect to the pure bending of an elastic bar onto which no shear force acts, the following relationship is given: wherein M represents the bending moment, E represents the tensile modulus (Young's modulus), I represents the secondary moment at section, R represents the curvature radius at the mediate layer of the bar and D is the diameter of the bar in case of having a round section. Since the degree of bending rigidity corresponds to the bending moment, it is proportional to E and I of the product. I is determined by the geometrical shape of the section of the bar. The above relationship is substantially applicable to the bending rigidity of a fiber or filament without any great error; i.e. the bending rigidity of a fiber or filament is proportional to the tensile modulus (E) and proportional to the fourth power of the diameter of the fiber or filament at section. Accordingly, the bending rigidity of a multifilament (multiple filaments) is small in comparison with that of a mono-filament (single filament) when they have a nearly equal denier. When the denier of each filment of a multi-filament is smaller and the number of the filaments is increased, the multi-filament will be bent easier. When the denier of each filament is larger and the number of filaments is decreased, the multi-filament will be bent more difficultly. In general, a tensile member having a higher bending moment, i.e. M in the formula (1), is more favorable. Thus, the tensile modulus (E), which is decided on the physical property of its material, is better to be larger. The secondary moment at section (I) may be decided depending upon the purpose and use. When, for instance, the bending moment is the same, the secondary moment at section (I) is smaller with larger tensile modulus (E). In case of the material having a small density, there is obtainable a tensile member having a small section area, a light weight and a good handling property. The filament obtained a section area of not less than 0.018 mm², preferably not less than 0.030 mm², more preferably not less than 0.05 mm². No particular upper limit is present, but it may be usually not more than 180 mm², practically not more than 8.0 mm², more practically not more than 1.8 mm². The tensile member comprising at least one polyethylene filament which is obtained according to the process of the invention is characteristically small in variation of the dynamic modulus with temperature. For instance, it affords a E' 80°C /E' 20°C value (i.e. the variation rate of the dynamic modulus at 80°C to that at 20°C) of not less than 60 %, preferably not less than 70 %, more preferably not less than 80 %. In other words, the tensile member comprising at least one polyethylene filament which is obtained according to the process of the invention shows only little depression in modulus even under such a high temperature as 80°C. This is quite advantageous for the practical use. The performances required for the tensile member are varied with the purpose and use. Usually, a tensile modulus of not less than 600 g/d (529.6 cN/dtex) may assure that the tensile member will be practically usable for various uses. When a tensile member is to be used for optical fiber cord or cable, the tensile modulus may be not less than 5,000 - 7,000 kg/mm². As understood from the formula (1), a tensile member having a too small section area is not suitable because the desired bending rigidity is hardly obtained. The tensile member comprising at least one polyethylene filament which is obtained according to the process of the invention satisfies said performances required for industrial tensile members and is particularly suitable for optical fiber cord or cable. The filament has usually a section area of not less than 0.018 mm², preferably not less than 0.030 mm², more preferably not less than 0.05 mm². In comparison with conventional metallic tensile members, it is advantageous in that it has a much lighter weight and is not influenced by electromagnetic interference due to strike by lightning. According to the process of the present invention the filament may be prepared by dissolving ultra high molecular weight polyethylene in a solvent and cooling the resultant solution to produce gel-like particles. A gel-like sheet composed of gel-like particles is compressed to remove a portion of the solvent therefrom and then folded or rolled, and drawn, whereby a filament is obtained. Explaining this procedure more in detail, ultra high molecular weight polyethylene is dissolved in a solvent (e.g. decalin, liquid paraffin) while heating to make a polyethylene concentration of 0.5 to 15 % by weight. The resulting solution is cooled so that gel-like particles or their collective blocks are formed. When desired, the gel-like particles or their collective blocks are broken to make them finer. The resultant dispersion is filtered, for instance, by the use of a filtration apparatus such as a paper machine to make a gel-like sheet. The gel-like sheet is squeezed, for instance, by passing through the gap between two rollers, which are disposed so as to apply a certain load thereto, to remove a portion of the solvent therefrom. A typical example of the squeezing manner is shown in Fig. 1 wherein R and R' are rotating rollers, S is a gel-like sheet and O indicates the direction to which the gel-like sheet proceeds. Namely, the gel-like sheet (S) is passed through the gap between two rotating rollers (R, R'), whereby the gel-like sheet (S) is compressed and simultaneously a portion of the solvent is squeezed out. The thus compressed sheet is then folded or rolled, and then drawn at a ratio of not less than 20. Upon drawing, the use of a die with a hole having an appropriate shape at section can afford the filamant of desired section shape such as round. The die as usable is provided preferably with a hole having a section area smaller than that of the compressed sheet as folded or rolled to be supplied thereto and a conical introducing port which makes drawing easier. For die drawing, the compressed sheet as folded or rolled is supplied to the die and protruded through the die so heated as to carry out the drawing at an appropriate temperature (e.g. 90 to 130°C) lower than the melting point of the supplied sheet. A typical example of the die is shown in Fig. 2. This Figure shows the enlarged schematic view of the section of the die, and O indicates the direction to which the compressed sheet as folded or rolled proceeds, T is the compressed sheet, Q is the die and d is the diameter of the hole of the die. The compressed sheet (T) containing the solvent is folded and supplied to the draw die (Q) having a conical introduction port and passed therethrough, whereby a shaped product having a round section is obtained. For obtaining the filament of sufficiently high tensile modulus, the product as once drawn may be further subjected to die drawing through a die having a hole of smaller section area than that of the hole in the die as previously drawn. Alternatively, the once drawn product may be subjected to heat drawing without using a die. The thus obtained filament has a section corresponding to the section of the hole of the die used. No crack is observed at the section. Thus, it is compact and of even quality and has a high tenacity of 20 g/d (17.65 cN/dtex) or more and a high tensile modulus of 600 g/d (529.6 cN/dtex) or more. Further, it can have a great section area. As stated above, the filament is useful as a tensile member and particularly suitable as a reinforcing material for an optical fiber to make an optical fiber cord or cable. For this purpose, one or more filaments may be used. The tensile member comprising at least one polyethylene filament which is obtained according to the process of the present invention has high tenacity and high tensile modulus. It has a somewhat larger diameter ratio (e.g. 1.20 - 1.86) than steel wire and a much smaller weight ratio (0.18 to 0.42) than steel wire. Compared with the aromatic polyamide fiber, it is excellent in abrasion resistance and resistance to fatigue from flexing. In comparison with steel wire and the aromatic polyamide fiber, it is excellent in chemical resistance, particularly resistance to acidic substances. Therefore, those conventional tensile members are apt to be deteriorated during their application in the air or under the ground, while no material deterioration is observed on the tensile member of the invention. It is particularly notable that the tensile member comprising at least one polyethylene filament which is obtained according to the process of the present invention can be made into one having a large section area with high tenacity and high tensile modulus. Thus, the bending rigidity is excellent. In the following Examples part(s) and % are by weight unless otherwise indicated. Measurement of various physical constants of the polyethylene filaments was carried out in the following manner: 1) Viscosity average molecular weight ( M v):- The viscosity was measured in the decalin solution at 135°C according to the method as described in ASTM D2857 to determine the intrinsic viscosity [η], which was then introduced into the following equation to calculate the viscosity average molecular weight ( M v): M v = 3.64 x 10⁴ x [η] 1.39 2) Tenacity:- Measured by the constant speed elongation method as described in JIS (Japan Industrial Standard) L1013 (1981). 3) Tensile modulus:- Measured by the initial resistance to stretching method as described in JIS L1013 (1981). 4) Diameter ratio and weight ratio of tensile member:- The diameter ratio and weight ratio of the tensile member were determined by taking the diameter and weight of the steel wire having a nearly equal tensile modulus (20,000 kg/mm²) to that of the tensile member respectively as 1. 5) Density:- Measured by the gradient density tube method as described in JIS L1013 (1981), 7.14.2. 6) Dynamic modulus E' (dyne/cm):- The dynamic modulus E' was measured from 20 to about 160°C while heating with a driving frequency of about 110 Hz and a temperature elevation rate of about 1°C/mm² by the use of a direct reading dynamic visco-elastic tester "VIBRON" Model DDV-II or "RHEOVIBRON" Model DDV-III (manufactured by Toyo Sokki) to obtain the dynamic modulus-temperature characteristics. The dynamic modulus at 80°C (E' 80°C ) and at 20°C (E' 20°C ) as determined above were introduced into the following formula to obtain the E' 80°C /E' 20°C rate (%): (E' 80°C /E 20°C ) x 100 7) Long period spacing of fiber:- Using an X ray diffraction apparatus "Rotar Flex" (manufactured by Rigaku Denki), the small angle X ray scattering strength curve was measured under the following conditions, and the long period spacing was calculated from the position of the peak: Detector: PSPC (proportional counter probe manufactured by Rigaku Denki); Camera radius: 510 mm; PSPC resolving power: 0.007°/ch Tube voltage of X ray producing apparatus: 45 Kv Tube current of X ray producing apparatus: 50 mA First pinhole slit: 0.15 mm (diameter) Second pinhole slit: 0.15 mm (diameter) Size of beam stopper: 1.7 mm wide Measurement time: 5 minutes Example 1 Polyethylene ( M v, 2 x 10⁶) was dissolved in decalin at 160°C to make a solution having a polyethylene concentration of 3 %. The resultant solution was allowed to cool at room temperature for 10 hours to make a gel-like material, followed by pulverization by the aid of a homo-mixer. The thus prepared material comprised gel-like particles and a solvent, the average particle size of the particles being 80 µm. After removal of the solvent by filtration, the particles were allowed to stand on a filter cloth, whereby a gel-like sheet of 4 mm in thickness was obtained. The gel-like sheet still contained the solvent, in which the polyethylene concentration being 16 %, and showed a melting point of 92°C. The gel-like sheet on the filter cloth was permitted to go through between two metal rolls rotating at even speed under compression to eliminate the solvent, whereby a compressed sheet was obtained. The diameter of the rolls was 150 mm, each rolls being spaced by a distance of 0.7 mm, and a rotating speed was 7 times/minute. The compression was carried out at room temperature (ca. 27°C). The compressed sheet contained polyethylene in a concentration of 48 %, had a thickness of 0.6 mm and a width of 200 mm and showed a high strength as not breaking even at bending. The sheet was supplied to a draw die while plaiting it in a width of 15 mm for drawing and taken up at a speed of 0.5 m/minute. The draw die was provided with a round sectional hole, of which the diameter and the length were respectively 4 mm and 5 mm, and a cone-shaped conduit, the half angle of the cone being 10°, of 40 mm in length and kept at 110°C. The compressed sheet as plaited passed through the draw die was drawn 9.4 times in lengthwise direction and the filament obtained had a round section. Scarce crack on the section could be found so that it could hardly be imagined that the obtained filament was made from a sheet-like material. The thus obtained drawn filament was further permitted to go through between a hot-air bath of 135°C and subjected to a conventional drawing method under heating by the aid of two rolls having different rotating speed so as to obtain a desired hard drawn monofilament, which had a round section of 1.1 mm. The characteristic values of the monofilament are shown in Table 1. Values for other fibers are shown for comparison. Although the monofilament of this Example has a yarn size of 8,322 denier (0.924 tex), it exhibits a high tenacity as well as a high tensile modulus. The dynamic modulus at 80°C is 87.2 % of that at 20°C and the long period spacing is 450 Å (45 nm). Reference Example 1 Polyethylene ( M v, 6.5 x 10⁴) was dissolved in decalin at 160°C to make a spinning liquid having a polyethylene concentration of 50 %. The spinning liquid kept at 130°C was extruded through a nozzle having round holes, each hole having a diameter of 2 mm, into the air of room temperature, followed by cooling to make gel-like filaments. A bundle of gel-like filaments was drawn in four steps at four different temperatures under which the gel-like filaments were not cut. Namely, drawing was carried out under the following conditions to give a total draw ratio of 56: Step Temperature (°C) Draw ratio 1st 80 8 2nd 100 3 3rd 120 1.8 4th 130 1.3 The drawn filaments had a sectional area of 0.0267 mm² (corresponding to a circle having a diameter of 0.184 mm), a long axis of 0.20 mm and a short axis of 0.17 mm. According to the denier method, it may be expressed as 230 denier (25.3 tex). The characteristic values of the monofilament are shown in Table 1. The average molecular weight of the monofilament of this Example is 6.5 x 10⁴ and said monofilament is inferior in tenacity and tensile modulus. The dynamic modulus at 80°C is 56.6 % of that at 20°C and the heat resistance of the filaments is not so good. The long period spacing is so short as 180 Å (18 nm).

Claims: A process for preparing a polyethylene filament having a viscosity average molecular weight of not less than 200,000, a tenacity of no less than 20 g/d, a tensile modulus of not less than 600 g/d, and a cross-sectional area of not less than 0.018 mm², said process comprising: compressing a gel-like sheet prepared from a solution of polyethylene having a viscosity average molecular weight of not less than 200,000 in a solvent at a temperature lower than the dissolving temperature of the gel-like sheet, thereby removing a portion of the solvent contained therein, folding or rolling said compressed sheet, and drawing said folded or rolled sheet at a draw ratio of not less than 20, whereby said filament is obtained.

Inventors: HAYASHI MASAHIRO, OHTA TOSHIHIKO, OKADA FUJIO, HAYASHI, MASAHIRO, OHTA, TOSHIHIKO, OKADA, FUJIO
Applicants: TOYO BOSEKI, TOYO BOSEKI KABUSHIKI KAISHA
Application year: 1984
Main classification: D01F6
IPC codes: H01B7, D07B1, G02B6, D01F6