ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE SOLUTION

Abstract: The present application provides an ultra-high-­ molecular-weight polyethylene solution which is produced by the steps of (1) subjecting a kerosene fraction having a boiling point of 160 to 300°C to a nucleus hydrogenation treatment at a pressure of 30 to 100 bars (kg/cm²) at a temperature of 100 to 300°C in the presence of a metallic catalyst for nucleus hydrogenation of aromatic nuclei (2); separating and removing at least a part of n-paraffins present in the kerosene therefrom by the use of a molecular sieve made up of a synthesized zeolite in order to obtain a residual oil (3); fractionating the latter by a rectification apparatus to obtain a hydrocarbon mixture fraction which has a boiling point of 180 to 250°C and which is substantially free from naphthalene and biphenyl (4); and dissolving, in the hydrocarbon mixture fraction, an ultra-high-molecular-weight poly­ ethylene having an intrinsic viscosity of 8 or more in decalin at 135°C.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a solution for the preparation of a spinning original dope which is used to manufacture polyethylene fibers and films having a high strength and a high modulus of elasticity. In particular, it relates to a concentrated and uniform solution of an ultra-high-molecular-weight polyethylene. (2) Description of the Prior Art A so-called ultra-high-molecular-weight polyethylene having an intrinsic viscosity (η) of 800 cm³/g (8 dl/g) or more in decalin at 135°C and having a noticeably high molecular weight of one million or more is utilized as engineering plastics which can be characterized by excellent shock resistance and wear resistance as well as high self-lubricating properties. Therefore, the ultra-high-molecular-weight polyethylene is now used in the forms of hoppers, silos, various gears, lining materials and ski backing materials, in many fields of food machines, civil engineering machines, chemical machines, agriculture, mining and sports/leisure articles. The ultra-high-molecular-weight polyethylene has a higher molecular weight as compared with prevalent or conventional polyethylenes. In consequence, if a high orientation of the ultra-high-molecular-weight polyethylene is achieved, a stretched product having a higher strength and elasticity than ever can be procured. That is the reason why much attention is now paid to a variety of investigations on the technique of the above-mentioned high orientation. However, the ultra-high-molecular-weight polyethylene has a much higher melting viscosity than prevalent polyethylenes, and therefore it can scarcely be melt-extruded in a usual manner in a molding process and it also is impossible at present to achieve the high orientation by stretching. On the other hand, Pole Smith, Pieta Yang Lemstra et al have suggested a method for preparing fibers having a high strength and a high modulus of elasticity by stretching, in a high magnification, a gel which has been obtained from a decalin dope of an ultra-high-molecular-weight polyethylene (U.S. Patent Nos. 4422993 and 4430383). In this dope, a polymer of a 1.5 × 10⁶ weight-average molecular weight is present at a concentration of 3 wt% and that of a 4 × 10⁶ weight-average molecular weight is present at an extremely low concentration of 1 wt%. In the practice of the suggested technique, therefore, there is the problem that a great amount of a solvent must be treated, and there are also economical problems. For this reason, a highly concentrated solution of the ultra-high-molecular-weight polyethylene is desired and demanded. As solvents which are useful for the preparation of the ultra-high-molecular-weight polyethylene solution, there have been suggested xylene, decalin, o-dichlorobenzene and trichlorobenzene as disclosed in U.S. Patent Nos. 4413110, 4422993, 4430383 and 4536536 and European Patent No. 139141, but they are not sufficient. Further, in order to prepare a more concentrated solution, a method of irradiating using ultrasonic waves and a method of concentrating a dilute solution have been suggested, but in both of such methods, molecular chains tend to be cut. It is fair to say that the aforesaid methods are not preferable for the purpose that properties inherent in the ultra-high-molecular-weight polyethylene are elicited therefrom by the subsequent molding process. As is clear from the above, it is never easy to prepare the uniform dope to be molded by dissolving the ultra-high-molecular-weight polyethylene in a solvent, which fact prevents the application of the above suggested process on an industrial scale. In addition thereto, there is a method in Japanese Patent Provisional Publication No. 136820/1983 in which a double cylinder is employed to prepare fibers from an ultra-high-molecular-weight polyethylene solution. In this disclosed method, the ultra-high-molecular-weight polyethylene solution is placed in the double cylinder, and an inside cylinder is rotated at a high speed, while the solution is maintained at such a high temperature as not to crystallize by itself. In this case, when a fine yarn is brought into contact with the surface of the inside cylinder, the yarn functions as a seed for crystallization, so that a fibrous ultra-high-molecular-weight polyethylene crystal grows under the rotation of the cylinder. When the seed is drawn out from the system in compliance with the growth of the polyethylene crystal, a continuous fiber can be manufactured. Also in this method, it is crucial for its success to select a suitable solvent for dissolving the ultra-high-molecular-weight polyethylene therein. SUMMARY OF THE INVENTION In view of the above situation, the inventors of the present application have researched intensively to solve these problems, and as a result, they have found that if a solution of an ultra-high-molecular-weight polyethylene in a specific solvent is used, a product solution will be obtained which can be processed with extreme ease and which has excellent physical properties. In consequence, the present invention has been completed. That is, according to the present invention, there is provided an ultra-high-molecular-weight polyethylene solution which is produced by the steps of subjecting a kerosene fraction having a boiling point of 160 to 300°C to a nucleus hydrogenation treatment at a pressure of 3 - 10 × 10⁶ Pa (30 to 100 kg/cm²) at a temperature of 100 to 300°C in the presence of a metal catalyst for nucleus hydrogenation of aromatic nuclei; separating and removing at least a part of n-paraffins present in the kerosene therefrom by the use of a molecular sieve made up of a synthetic zeolite in order to obtain a residual oil; fractionating the latter by a rectificaticn apparatus to obtain a hydrocarbon mixture fraction which has a boiling point of 180 to 230°C and which is substantially free from naphthalene and biphenyl; and dissolving, in the hydrocarbon mixture fraction, an ultra-high-molecular-weight polyethylene having an intrinsic viscosity of 800 cm³/g (8 dl/g) or more in decalin at 135°C. Additionally, a method for producing a shaped article is provided by the present invention, said shaped article being obtained from the above ultra-high-molecular-weight polyethylene solution by way of gel fiber or gel film formation, by treating the gel fiber or gel film so as to remove the solvent, drying the gel and, optionally, subjecting it to a stretching process. Now, the present invention will be described in detail, In the present invention, a kerosene fraction having a boiling point of 160 to 300°C is hydrogenated with the aid of a metal catalyst for nucleus hydrogenation of an aromatic nuclei. As this kind of catalyst, any of known metallic catalysts for the hydrogenation of the aromatic nuclei can be preferably used, particularly nickel, copper, cobalt, tungsten, molybdenum, platinum, rhodium, palladium and ruthenium. Examples of the catalysts which can be preferably used in the present invention include nickel, nickel oxide, nickel/diatomaceous earth, Raney nickel, nickel/copper, platinum, platinum oxide, platinum/activated carbon, platinum/rhodium, platinum/alumina, platinum/lithium/alumina, rhodium/activated carbon, palladium, cobalt, Raney cobalt, ruthenium, tungsten sulfide/nickel sulfide/alumina, tungsten, copper, molybdenium, rhodium and cobalt/molybdenum. The pressure used in the present invention is within the range of 3 - 10 x 10⁶ Pa {30 to 100 bars (kg/cm²)} and the temperature is within the range of 100 to 300°C. When the pressure is lower than 3 × 10⁶ Pa {30 bars (kg/cm²)}, or when the temperature is lower than 100°C, the hydrogenation does not make progress sufficiently. Inversely, both a higher pressure then 10⁷ Pa {100 bars (kg/cm²)} and a higher temperature than 300°C are unsuitable, because a secondary reaction such as a decomposition predominates. A suitable reaction time for the hydrogenation can be chosen. For example, LHSV (i.e. Liquid hourly space velocity) is usually within the range of 0.01 to 100. With regard to a reaction system, either a batch system and a continuous/flow system can be taken. The aromatic hydrocarbon is nucleus-hydrogenated to naphthenes by means of the hydrogenation treatment, but the degree of the hydrogenation can be optionally chosen in compliance with the above-mentioned catalyst and the reaction conditions Further, the hydrogenation treatment is often accompanied by secondary reactions such as decomposition, isomerization, so that products formed through these secondary reactions are also inevitably contained in the kerosene fraction. Subsequently to the above-mentioned hydrogenation treatment, a molecular sieve made up of a synthesized zeolite having an average 5 x 10⁻¹⁰ m (5Å) pore size is used to separate and remove at least a part of n-paraffins in the kerosene fraction therefrom, thereby obtaining a residual oil. The method for separating and obtaining n-paraffins from a hydrocarbon mixture by repeating adsorption and desorption in a gaseous or a liquid phase with the aid of a molecular sieve has been heretofore used industrially extensively as a manufacturing method of n-paraffins. That is, there are the Molex process comprising the steps of using, as a fixed bed, a molecular sieve made up of a synthesized zeolite having many pores of 5 x 10⁻¹⁰ m (5Å) and carrying out adsorption and desorption of n-paraffins alternately in a liquid phase, washing the molecular sieve, on which the n-paraffins are adsorbed, with a low-molecular paraffin for desorption in order to desorb the n-paraffins, and separating the joined low-molecular paraffin therefrom by distillation and recycling it, the TSF process (Texaco Selective Finishing Process) comprising the steps of adsorbing n-paraffins in a gaseous phase by the utilization of adsorption/desorption with the aid of a molecular sieve made up of a synthesized zeolite having similar 5 x 10⁻¹⁰ m (5Å) pores, and carrying out the desorption of the n-paraffins by washing them out with a low-molecular paraffin; the Isosieve process in which a molecular sieve made up of a similar synthesized zeolite having 5 x 10⁻¹⁰ m (5Å) pores is used but in which adsorption/desorption of n-paraffins onto the molecular sieve is carried out by repeating application and release of a pressure alternately; and the Esso process involving a vapor phase/liquid bed process, the Esso process comprising the steps of adsorbing n-paraffins continuously by a molecular sieve of a synthesized zeolite having 5 x 10⁻¹⁰ m (5Å) pores on a liquid bed in an adsorbing apparatus, desorbing the n-paraffins therefrom at a higher temperature than that of the adsorption step in a recovery device, and returning the recovered molecular sieve from the recovery device to an adsorption device so as to reuse the molecular sieve. The n-paraffins can be separated by any of these methods using the molecular sieve which is made up of a synthesized zeolite having pores of an average size of 5 x 10⁻¹⁰ m (5Å). In the processes for separating n-paraffins, the n-paraffins alone ought to be separated theoretically, but in a chosen process, hydrocarbons other than the n-paraffins are separated together with the latter, so that the content of components other than the n-paraffins in the obtained residual oil is liable to change. Taking this point into consideration, a urea adduct process using crystalline urea is also effective as a method for separating n-paraffins on an industrial scale. With regard to the present invention, however, it is proper to make use of the molecular sieve made up of the synthesized zeolite. In such a manner as described above, the n-paraffins are separated from the hydrogenated kerosene fraction in order to obtain a residual oil. Next, the residual oil is subjected to rectification. In the latter process, for example, a rectification apparatus comprising two or more distillation columns is used, and light hydrocarbons are removed from the top of a first column of the apparatus and a desired fraction is taken out from the top of a second column or from the tops of the subsequent columns. Of course, if a separation efficiency based on the number of distillation steps therein is proper, the light and heavy hydrocarbons are removed respectively from top and bottom of the rectification apparatus comprising one distillation column, and the desired fraction is taken out from the middle portion of the column. The above-mentioned rectification apparatus permits obtaining the hydrocarbon mixture fraction having a boiling point of 180 to 250°C. In the fraction of the present invention, naphthenes and isoparaffins are present as main components, but there are scarcely contained benzene, toluene, xylene, naphthalene and biphenyl. Therefore, the thus obtained fraction is to be prefered from the viewpoint of toxicity, safety and dissolving power for an ultra-high-molecular-weight polyethylene, and the isoparaffins present in fee fraction contribute to repressing a peculiar smell of the naphthenes. The ultra-high-molecular-weight polyethylene has an extraordinarily greater molecular weight than a usual molecular weight polyethylene, and therefore a twining or entangling state of its molecular chains is also incomparably more intricate than that of the usual molecular weight polyethylene. In a molding method of an ultra-high-molecular-weight polyethylene solution, it is considered to be an essential process that the twining degree of the molecule is lowered by liquefaction. With regard to the hydrocarbon mixture fraction of the present invention, the twined ultra-high-molecular-weight polyethylene is disentangled suitably by interactions of the respective components of the fraction which exist in proper contents, though its reason is not elucidated yet in detail. As a result, the concentrated ultra-high-molecular weight polyethylene solution is prepared, and articles molded therefrom also are excellent in physical properties. In view of the fact that a synergistic effect of the respective components in the hydrocarbon mixture fraction of the present invention can be expected, it is necessary that the boiling point of the fraction of the present invention is within the above-mentioned range. The deviation of the boiling point from the range means, that the existent components and their contents are unbalanced unpreferably. When the melting point is lower than 180°C, a vapor pressure of the fraction is too high. In such a case, a pressure-resistant container is required, and at the time of dissolving, the vapor (gas) of the fraction tends to go into the melted polyethylene, which fact will be the cause of an unpreferable yarn cutting phenomenon in the stretching step because of voids formed in the yarns. To the contrary, when the melting point is higher than 250°C, a drying step takes a longer time and requires a heating operation, and it is difficult to remove the solvent. In order to prepare an ultra-high-molecular-weight polyethylene solution having an intrinsic viscosity of 800 cm³/g (8 dl/g) or more in decalin at 135°C by the use of the above-mentioned hydrocarbon mixture fraction, a known conventional heating technique can be employed. For example, there is a method in which a polyethylene is merely dissolved by heating up to a temperature higher than its melting point, a method similar to the above method but involving the irradiation of ultrasonic wave, and a method of concentrating a previously prepared diluted solution to a more concentrated solution. In addition, another method has been suggested in which for the control of swelling, grains of an ultra-high-molecular-weight polyethylene are subjected to a wetting treatment by the use of a small amount of a low-boiling solvent and the same kind of solvent which has been heated up to a high temperature with moderate stirring is then added thereto at a time. Such a method can provide the solution in which the concentration of the ultra-high-molecular weight polyethylene is as high as 50 wt%. Judging from the fact that the solution of the present invention will be molded, the present invention does not intend to prepare an extremely highly concentrated solution. In reality, the concentration of the polymer in the solution should be chosen by taking into consideration the moldability of the solution and the performance of fibers and films which will be made therefrom. Consequently, in the present invention, an appropriate concentration of the polymer in the solution is within the range of 0.001 wt% to 50 wt%, preferably 0.01 wt% to 30 wt%. The molding of this solution may be carried out by first causing it to gel or by using a double cylinder as mentioned above, and then fibers or films of the ultra-high-molecular-weight polyethylene may be obtained. In the case of molding the solution of the present invention, a method is suitable in which the prepared solution is first cooled to gel. When cooled, the polyethylene solution can solidify and gel without any precipitation of the polyethylene and without any phase separation in the macroscopic meaning, with the volume of the solution maintained. However, this phenomenon can be achieved only by properly selecting a kind of solvent and its concentration. However, in the present invention, there are always formed physical linking points in the gel resulting from the formation of fine crystals by partial crystallization in molecular chains of the ultra-high-molecular weight polyethylene because of good interactions between the respective components in the fraction and polyethylene molecules, whereby the solution using the fraction of the present invention can be changed into the gel having a good three-dimensional network on the level of the molecular size. When gel films are desired, they may be prepared by casting the solution in a suitable manner, or alternatively by extruding the solution through a slit of a T-die, followed by cooling. On the other hand, gel fibers may be manufactured by a so-called wet spinning process. That is, a suitable anti-oxidant, if necessary, may be added to the solution, and the latter may be then forced to go through nozzles, each having a suitable diameter, by the use of a usual gear pump, and the solution may be guided into water so as to cool, so that the desired gel fibers can be obtained. The thus obtained gel is stable in so far as it is preserved in a medium such as water, and therefore it can be prevented that the gel will crystallize and hence extrude a solvent therefrom. The fiber gel or the film gel is then subjected to a solvent removal treatment, a drying treatment and a stretching treatment. The stretching before solvent removal may be possible. The latter stretching treatment may be carried out at a suitably elevated temperature. Therefore, instead of a sequence of the above-mentioned treatments, the solvent removal and drying treatments may be carried out simultaneously together with the stretching treatment in the stretching process. In the latter process, a temperature level, a take-up rate, a stretching degree may be chosen suitably. When the solution of the present invention and the gel prepared therefrom are used, the following effects can be obtained: (a) Since the solution of the present invention is excellent in dissolving power to an ultra-high-molecular weight polyethylene, fibers and films having a high strength and a high modulus of elasticity can be manufactured very stably by stretching its gel. (b) The high dissolving power to the ultra-high-molecular weight polyethylene permits producing the uniform and highly concentrated solution in which molecular chains of the ultra-high-molecular weight polyethylene are sufficiently disentangled, and therefore fibers and films can be manufactured at an extremely low cost by stretching its gel. (c) The present invention can provide a highly concentrated and uniform ultra-high-molecular weight polyethylene solution and its gel without using any ad hoc dissolver and any dissolving technique, and fibers and films having a high strength and a high modulus of elasticity can be stably and economically manufactured by stretching the gel. EXAMPLES Now, the present invention will be described in detail in accordance with examples, but its modifications and alterations should not be limited to these examples. In this specification, temperatures are all represented by centigrade, and parts and percents are all on the basis of weight, unless otherwise specified. Solvent Preparation 1 A kerosene fraction (which was composed of 65 wt% of paraffins, 25 wt% of naphthenes and 10 wt% of aromatic hydrocarbons) having a boiling point of 150 to 280°C which had been obtained by the distillation of a crude oil was hydrogenated under conditions of 280°C and 9x 10⁶ Pa (90 kg/cm²) by the use of a nickel-tungsten catalyst for hydrogenation of aromatic nuclei. The resulting product was then fed into a separating column packed with a molecular sieve made up of a synthesized zeolite and having average pores of 5 x 10⁻¹⁰ m (5Å), and n-paraffins were removed out therefrom in accordance with the Molex process. The fraction in which the n-paraffins had been diminished was subjected to rectification in a two-tower rectification apparatus in order to obtain a hydrocarbon mixture fraction, i.e, a solvent having a boiling point of 185 to 226°C. The composition and properties of this solvent are set forth in Table 1. Solvent Preparation 2 The same procedure as in Example 1 was repeated with the exception that a kerosene fraction having different properties than in Example 1 was used, in order to obtain a solvent. The composition and properties of the solvent are set forth in Table 1. Example 1 A small amount of an antioxidant was added to the fraction of Solvent Preparation 1, and a polyethylene having an intrinsic viscosity of 1600 cm³/kg (16 dl/g) in decalin at 135°C was further added thereto and then dissolved therein by stirring at 140°C for 3 hours in order to obtain a 10 wt% ultra-high-molecular-weight polyethylene solution. In this solution, any undissolved and cohered particles were not observed, and anything did not adhere onto the side walls and the bottom of a vessel. Next, gel fibers containing a solvent were spun out of this solution at 130°C by the use of a spinning die, each spinning nozzle of which had a diameter of 1.0 mm. During this period of time, neither yarn cutting nor yearn irregularity did occur, which fact was indicative that the solution in this example was a uniform spinning original dope. Afterward, the gel fibers were passed through a pipe-like oven heated to 120°C in order to stretch them 30-fold. The modulus of elasticity and the strength of the fibers are set forth in Table 2. Comparative Example 1 The same procedure as in Example 1 was repeated with the exception that decalin was substituted for the mixed solvent of the present invention. In the resulting 10 wt% decalin solution, large gel particles were present, and therefore a uniform solution could not be prepared. When a spinning test was made for this solution, year cutting often occurred, and therefore spinning was impossible. Comparative Example 2 Following the same procedure as in Comparative Example 1, a 5 wt% decalin solution was prepared. A spinning test was performed, but spinning was unstable and yarn irregularities occurred. Eventually, a modulus of elasticity and a strength were at low levels, as shown in Table 2. Example 2 A spinning test was made in the same manner as in Example 1 with the exception that the mixed solvent of Solvent Preparation 2 was substituted. Spinning could be stably accomplished. The modulus of elasticity and the strength are shown in Table 2. Boiling Point of Solvent °C Conc. of Polymer in Sol. (wt%) Modulus of Elasticity (g/d) Strength (g/d) Example 1 185 to 226 10 987 30.1 Example 2 216 to 241 10 979 30.0 Comp. Ex. 1 187 to 196 10 * * Comp. Ex. 2 187 to 196 5 200 14.1 * It could not be measured.

Claims: An ultra-high-molecular-weight polyethylene solution which is produced by the steps of subjecting a kerosene fraction having a boiling point of 160 to 300°C to a nucleus hydrogenation treatment at a pressure of 3 - 10 × 10⁶ Pa {30 to 100 bars (kg/cm²)} at a temperature of 100 to 300°C in the presence of a metal catalyst for nucleus hydrogenation of aromatic nuclei; separating and removing at least a part of n-paraffins present in said kerosene therefrom by the use of a molecular sieve comprising a synthetic zeolite having an average pore size of 5 × 10⁻¹⁰ m (5 Å) in order to obtain a residual oil; fractionating the latter by a rectification apparatus to obtain a hydrocarbon mixture fraction which has a boiling point of 180 to 250°C and which is substantially free from naphthalene and biphenyl; and dissolving, in said hydrocarbon mixture fraction, an ultra-high-molecular-weight polyethylene having an intrinsic viscosity of 800 cm³/g (8 dl/g) or more in decalin at 135°C. An ultra-high-molecular-weight polyethylene solution according to Claim 1 wherein said ultra-high-molecular-weight polyethylene is dissolved at a concentration of 50 wt% or less. An ultra-high-molecular-weight polyethylene solution according to Claim 1 wherein said metals are selected from the group consisting of nickel, copper, cobalt, tungsten, molybdenum, platinum, rhodium, palladium and ruthenium. An ultra-high-molecular-weight polyethylene gel which is produced from the Ultra-high-molecular-weight polyethylene solution according to Claim 1. The gel of Claim 4 in the form of a shaped article. The gel of Claim 4 in the form of a fiber or a film. Method for producing a shaped article which is obtained from the ultra-high-molecular-weight polyethylene gel according to claim 6 by treating the gel fiber or gel film so as to remove the solvent, drying the gel and, optionally, subjecting it to a stretching process.

Inventors: MATSUURA, KAZUO, SANO, AKIRA, TORII, MICHIAKI, YOKOYAMA, SHIGEKI
Applicants: NIPPON PETROCHEMICALS CO., LTD.
Application year: 1987
Main classification: C08J3
IPC codes: D01F6, C10G67, C08J3, C08L23, C08L91