METHOD FOR PREPARING MOLDED ARTICLES OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE

Abstract: The present invention provides a method for preparing a molded article of an ultra-high molecular weight polyethy­ lene which is characterized by compression-molding, or heating and pressurizing a porous sheet of said ultra-high molecular weight polyethylene or the porous sheet super­ posed with a substrate by the use of a mold in order to den­ sify said sheet.

Description: BACKGROUND OF THE INVENTION (i) Field of the Invention The present invention relates to a method for preparing molded articles of an ultra-high molecular weight polyethylene which has a high melt viscosity, an excellent wear resistance and impact resistance, and a small friction coefficient. (ii) Description of the Prior Art Since an ultra-high molecular weight polyethylene (hereinafter referred to as UHMWPE) is excellent in a wear resistance and has a small friction coefficient to a variety of materials, its application to sliding parts is now desired. For example, when an artificial joint is made from a metal or a ceramic, it cannot be used without any additional treatment, because of its great friction coefficient between sliding surfaces. Therefore, it is necessary that a lubricating material is interposed between the sliding surfaces. Some properties which the lubricating material must possess are (1) to be nontoxic and odorless, (2) to be of a sufficient lubricating function over a long period of time, (3) to render the surfaces of parts smooth and to permit a smooth movement, and (4) to bring about no chemical reaction. Taking into account these conditions, it will be easily understood that an ultra-high molecular weight polyethylene, UHMWPE, is suitable for lubrication. The above mentioned lubricating material may be used in the form of plane parts on occasion, but it may usually be taken in hemispherical and more intricate forms. Further, the UHMWPE material is eligible for a transfer pipe for a slurry and a powder which will cause a severe abrasion on metallic materials, an elbow in which the flow direction of a fluid to be transferred therethrough changes, an agitated vessel, a sliding part and the like. In addition, since the UHMWPE is excellent in an impact resistance, particularly the resistance to a repeated impact, it is strongly desired that the UHMWPE material is utilized as an impact-absorbing stopper, or a hollow molded article such as a bellows-shaped article or the like in various industrial machines. Articles, sheets and plates of the UHMWPE are formed by various methods, for example, sintering, ram extrusion using a plunger pump, forging and compression molding. However, it is very difficult to directly produce thin sheets or films (hereinafter referred to simply as "sheets") of less than 3 mm in thickness from the crude powder of the UHMWPE. According to the present industrial practice, thin sheets of this kind are produced by applying a postforming such as skiving to a cylinder obtained by one of the above-mentioned methods. With this method, however, the processing cost is high and continuous production of thin sheets on a large scale is quite difficult. EP-A-0158116 discloses a process for preparing coverings for antennas which are prepared by heating and pressing fabrics of fiber materials and thermoplastic materials, preferably of ultra-high molecular weight polyethylene, in which process the thermoplastic polyethylene material is used in form of a film or foil According to page 4, lines 17 to 19, preferably the thermoplastic material is used in form of foils, which are cut into the desired shape or, alternatively, are deep-drawn parts. Any teaching with respect to porous sheets of ultra-high molecular weight polyethylene, wherein the UHMWPE particles are loosely bound, which sheets are used according to the present invention, cannot be derived from this document. In the paragraph bridging pages 8 and 9 of the present description, it is described that such sheets are prepared as the first step of the present process. The difficulty in processing the UHMWPE arises from the fact that is has quite a high melt viscosity and has no good melt flow properties, and therefore, the conventionally employed blown-film extrusion method and T-die method in which the pressure drop is large, are not suitable. In consequence, it is strongly desired on an industrial scale to manufacture the lubricating parts directly from the crude powder of the UHMWPE, if possible. In general, if an attempt is made to thermally mold the lubricating parts from the UHMWPE crude powder, the latter will be concentrated on the bottom of a mold, and if compression molding is carried out in the state of such an inclined distribution, the resulting article will have a bottom portion locally thickened. In particular, it is very difficult or actually impossible that a thin-wall article having a non-planar form such as a cylindrical, a hemispherical, a spherical or a corugate shape is molded with a high thickness accuracy by the compression molding. This reason is as follows: In order to mold the article having an accurate thickness, it is necessary that the UHMWPE crude powder is distributed in a predetermined thickness in the mold, but the UHMWPE crude powder which freely flows at room temperature cannot be set, against the gravity, in the predetermined thickness in the mold. Moreover, the molded article having an arbitrary hollow shape such as a pipe, an elbow, a tee, a vessel, or a profile cannot be easily manufactured from the crude UHMWPE powder. It is also difficult to manufacture composite molded articles from the UHMWPE material. For example, for the purpose of giving slipping properties and a wear resistance to the surface of a rubber, while rebound or non-rebound properties of the rubber are maintained, it is desirable to superpose a thin UHMWPE layer having a thickness of about 0.5 mm on the rubber. However, when the usual compression molding method is employed, the UHMWPE crude powder cannot be set uniformly and thinly on the surface of the rubber. Thus, the UHMWPE layer is first molded and then skived to prepare a desired thin sheet. Afterward, the thus prepared thin sheet is superposed on the rubber. It is natural, therefore, that the article manufactured in this way is expensive. For the improvement on a poor stiffness of the UHMWPE layer, it is desirable to superpose the said layer on a metallic plate, a curable resin, a synthetic resin, a cloth or the like having a less elongation. However, a composite sheet having the thin UHMWPE layer cannot be made from the UHMWPE crude powder by the compression molding method, because the UHMWPE crude powder cannot be set in the mold with an accurate thickness. The UHMWPE cannot be, therefore, applied to a fender or a door panel for an automobile for which a thinness, a high stiffness and a high impact resistance are necessary. OBJECTS OF THE INVENTION In view of the above described state of the prior art, the inventors of the present application have carried out extensive studies in order to solve these problems, and as a result, the present invention has been accomplished. A first object of the present invention is to provide a method for molding, with a good thickness accuracy, a thin-wall UHMWPE porous sheet into a thin-wall molded article having a special shape such as a cylindrical, a hemispherical, a spherical or a corrugate shape into which it is very difficult or impossible to mold a UHMWPE crude powder conventionally. A second object of the present invention is to provide a method for easily preparing an arbitrary hollow molded article such as a pipe, a corrugate pipe, an elbow, a tee, a bellows or a vessel, or a profile article from the aforesaid UHMWPE porous sheet. A third object of the present invention is to provide a method for easily preparing a composite UHMWPE article by superposing a UHMWPE porous sheet on at least one substrate selected from a thermoplastic resin, a curable resin, a metallic sheet, a metallic foil, a rubber, a cloth, a paper and a ceramic. SUMMARY OF THE INVENTION The present invention is directed to a method for preparing a molded article of the UHMWPE which is characterized by compression-molding, or by heating and pressurizing a UHMWPE porous sheet or a UHMWPE porous sheet superposed with a substrate by the use of a mold, to densify said sheet. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic section of a deformable member in which an elbow-like steel pipe having through holes is coated with a silicone tube; Fig. 2 is a side view of the deformable member around which a preformed UHMWPE sheet is wound; Fig. 3 is a perspective view of an elbow mold; Fig. 4 is a schematic section of the elbow mold in which the deformable member wound with a preformed UHMWPE sheet is set; Fig. 5 is a perspective view of a cylindrical deformable member around which a preformed UHMEPE sheet is wound; Fig. 6 is a perspective view of a bellows mold; Fig. 7 is a schematic section of the bellows mold in which a deformable member wound with a preformed UHMWPE sheet is set; Fig. 8 is a perspective view of a cylindrical deformable member around which a preformed UHMWPE sheet is wound; Fig. 9 is a front view of an end portion of the member shown in Fig. 8, the end portion being folded; Fig. 10 is a perspective view of a cylinder mold; Fig. 11 is a schematic section of the cylinder mold in which the deformable member and a preformed UHMWPE sheet are set; Fig. 12 is a perspective view of a hemispherical deformable member which is covered with a preformed UHMWPE sheet; Fig. 13 is a perspective view of a hemisphere mold; Fig. 14 is a schematic section of the hemisphere mold in which the deformable member and a preformed UHMWPE sheet are set; Fig. 15 is a schematic section of a deformable member regarding one embodiment of the present invention in which a rubber material is employed; Fig. 16 is a perspective view of the deformable member around which a preformed UHMWPE sheet is wound in the form of spiral; Fig. 17 is a schematic perspective view of a cylinder mold; Fig. 18 is a schematic section of the cylinder mold in which the deformable member wound with a preformed UHMWPE sheet is set; and Fig. 19 is a perspective view showing another manner of winding the preformed UHMWPE sheet. DETAILED DESCRIPTION OF THE INVENTION The UHMWPE referred to in the present invention means the polyethylene resins having a very high molecular weight as represented by an intrinsic viscosity of 8 dℓ/g or higher in a decalin solution at 135°C. A porous sheet used in the present invention means a porous sheet prepared by compounding extractable material to the UHMWPE, molding and removing solvent-soluble components by extraction therefrom, alternatively by feeding the UHMWPE powder to between at least two heated rolls rotating in mutually reverse directions, and melting it into a sheet at a molding temperature more than 140°C. In the latter type of the porous sheet, the UHMWPE particles are loosely bound, and this sheet is more excellent in thickness accuracy and processability and can be manufactured continuously and inexpensively, though being poorer in a tensile strength, an elongation at break and the like, as compared with the completely densified sheet. Therefore, the aforesaid porous sheet can be preferably utilized as for a fabrication and will be referred to as preformed sheet. The thermoplastic resins that are used as the substrates are exemplified by polyolefin resins including homopolymers such as high density or low density polyethylene, polypropylene, polybutene-1 and poly-4-methyl-pentene-1; copolymers of one of α-olefins such as ethylene or propylene as major components with other polar or nonpolar monomers, e.g., copolymers of ethylene or propylene with other α-olefins, ethylene-vinyl acetate copolymer or its saponification product, and copolymers of ethylene with unsaturated carboxylic acids or their derivatives; polystyrene resin; polyacrylic resins; polyvinyl chloride resin; polyvinylidene chloride resin; polyamide resins such as Nylon-6 (trademark of E.I. du Pont), Nylon-6,6, Nylon-11, Nylon-12 and aromatic polyamides; polycarbonate resin; cellulose resins; polyester resins such as polyethylene terephthalate and polybutylene terephthalate; fluorocarbon resins such as polytetrafluoroethylene; polyimide resins; polyamide imide resins; polyether-ether ketone resins; polyether sulfone resins; polysulfone resins; polyphenylene oxide resins; polyphenylene sulfide resins; and polyacetal resins. The curable resins that are used as the substrates are exemplified by epoxy resins, phenol resins, urethane resins, urea resins, melamine resins, diallyl phthalate resins, and unsaturated polyester resins. The rubbers as the substrates are exemplified by synthetic rubbers such as polybutadiene, polyisoprene, neoprene rubber, ethylene-propylene copolymer, ethylene-propylene-diene copolymer, styrene-butadiene copolymer, styrene-acrylonitrile copolymer, styrene-acrylonitrile-butadiene copolymer, butyl rubber, polyurethane rubber, chlorosulfonated polyethylene, chlorinated polyethylene, fluororubber, and Thiokol (trademark of Thiokol Chemical Corp.); and natural rubber. The metals as the substrates are exemplified by metal foils, metal sheets, wire nets, and punched plates of metals such as iron, aluminum, copper, zinc, brass, and nickel. The cloths as the substrates are exemplified by woven or non-woven fabrics and nets of fibrous materials such as tape yarns or monofilaments of the foregoing thermoplastic resins; thinly and finely divided or fibrillated polymeric film; woven fabrics of synthetic resin fibers such as polyvinyl alcohol fibers and polyethylene terephthalate fibers, and natural fibers such as cotton, silk and linen; and woven fabrics, mats and felts of inorganic fibers, e.g., glass fibers, carbon fibers and metal fibers. The papers as the substrates are not limited to any particular kinds, but are exemplified by packaging papers such as kraft paper and machine glazed paper; thin paper such as glassine paper; boards and other machine-made paper; Japanese paper such as sliding door paper (shoji paper); any synthetic papers such as styrene paper and polyvinyl alcohol paper. Furthermore, it is to be noted that the foregoing thermoplastic resin sheets containing fillers can also be used in the method of the present invention. The above fillers are exemplified by calcium carbonate, magnesium carbonate, calcium sulfate, calcium sulfite, calcium silicate, clay, diatomaceous earth, talc, alumina, siliceous sand, glass powder, iron oxide, metal powders, antimony trioxide, silicon carbide, silicon nitride, silica, boron nitride, aluminum nitride, wood flour, carbon black, mica, glass flakes, sericite, pyrophyllite, aluminum flakes, graphite, Shirasu balloons (made in Japan), metal balloons, glass balloons, pumice powder, glass fiber, carbon fiber, graphite fiber, whisker, metal fiber, asbestos, and organic fibers. The compounding ratio of the above exemplified fillers varies according to the purposes and uses of the laminated sheets; however, the amount is preferably in the range of 5 to 1000 parts by weight of a filler to 100 parts by weight of a thermoplastic resin. Moreover, foamed materials and cross-linked substances derived from the foregoing thermoplastic resins can also be employed as the substrates in the method of the invention. A first invention of the present application is directed to a method for preparing a molded article of the UHMWPE porous sheet or a laminate of the UHMWPE porous sheet and a substrate by compression-molding, or heating and pressurizing the UHMWPE porous sheet or the said porous sheet superposed with the substrate, and melting the said sheet to densify it. The very important matter in the present invention is that molding is carried out by the use of the preformed sheet of the UHMWPE. The above mentioned compression molding may be carried out by setting, in a mold, the preformed UHMWPE sheet and the substrate which have been heated up to a level of room temperature to 300°C, preferably 130 to 250°C, applying a pressure of 0.1 kg/cm² (0.098 bar) or more, preferably 1 kg/cm² (0.98 bar) or more for a period of several seconds to several hours, cooling and taking out the resulting article from the mold, or alternatively, instead of heating the preformed sheet and the substrate, there may be taken a procedure of setting the preformed sheet and the substrate in the mold heated up to 140 to 300°C, applying pressure and cooling the resulting article. Further, both the mold and the preformed UHMWPE sheet may be heated. In these manners, each of the preformed sheet and the substrate is not limited to only one, and the two or more preformed sheets and substrates may be used superposing them so as to give a necessary thickness to the finished articles. The higher the pressure to be applied is, the better, but as the pressure is getting higher, the strength of the mold to be used must be increased. Therefore, it is undesirable to increase the pressure too high, though not being limited particularly. In the present invention, the laminate of the UHMWPE sheet and the substrate can be prepared without interposing any adhesive therebetween, but preferably an adhesive is used therebetween. In the case of using no adhesive, it is preferred that the preformed UHMWPE sheet and/or the substrate is subjected to a surface treatment such as a flame treatment, a corona discharge treatment or a plasma treatment. The usable adhesives are not limited especially. Examples of the adhesives arbitrarily used in the present invention include usual epoxy series, polyurethane series, modified acrylic series, cyanoacrylate series, anaerobic adhesives, various hot melt series, various emulsion series, urea resin series, melamine series, phenol series and rubber series adhesives, as well as copolymers of ethylene and unsaturated carboxylic acids or their derivatives such as vinyl acetate, acrylic acid, methacrylic acid, maleic acid and maleic anhydride, copolymers of ethylene and with glycidyl derivatives, ionomers, and polyolefins graft-modified with the aforesaid unsaturated carboxylic acids. In the present invention, the constitution consisting of the preformed UHMWPE sheet and the substrate is not restricted particularly, but if a high stiffness is required, it will be preferable that the substrate layer having a high stiffness is disposed at a position as near to the surface of the article as possible. If a high stiffness, an excellent wear resistance and low frictional properties are desired, it will be preferred that an additional UHMWPE sheet is further put on the substrate having the high stiffness. A second invention of the present application is directed to a method for preparing a hollow article such as a hollow pipe, an elbow or a tee, or a profile of the UHMWPE which is characterized by coating a deformable member consisting of a cylindrical form, a profile or the like with the above mentioned porous sheet or the said porous sheet superposed with the substrate, setting the member in a deformable mold, heating the mold to expand or deform the deformable member and to thereby allow the sheet to adhere closely to the mold under pressure, and melting the sheet for its densification. The deformable member used in the present invention is composed of a rubbery material, a plastic alloy, a shape memory alloy or the like, and when the UHMWPE porous sheet is wound around the deformable member, a non-adhesive material such as Teflon or silicone is preferably interposed between the sheet and the easy-deforming member. The expansion of the deformable member is suitably carried out by applying an internal presssure to the interior of the deformable member with the aid of a gas or a liquid, and this internal pressure can be supplied by the utilization of the thermal expansion of the gas or the liquid through heating, or by means of a compressor. In the case that the above mentioned deformable member is composed of the rubbery material, it is recommendable to insert a pipe-like support having holes into the rubbery material, in winding the UHMWPE sheet around the deformable member, whereby the deformable member will not be prevented from being deformed during the winding operation and thus this operation will be easily carried out. In the present invention, the deformable member wound with the UHMWPE sheet is then set in a predetermined mold, and the latter is heated up to a level of 140 to 300°C so as to expand the easy-deforming member, so that the sheet is allowed to adhere closely to the mold under pressure and is melted for its densification. In this case, a means for heating the mold is not limited particularly, and electricity, a gas, an oil bath, steam or the like may be used as the heating means. Example 1 A UHMWPE powder was fed to between two rolls rotating in mutually reverse directions under conditions that a clearance between two rolls was 0.2 mm and a molding temperature was 154°C, thereby preparing porous sheets each having a thickness of 0.35 mm. These porous sheets were superposed to prepare laminates composed of 2 sheets (0.7 mm thick) and 4 sheets (1.4 mm thick), respectively, and each laminate was set in a hemisphere mold (draw ratio = 0.2) consisting of a pair of upper and lower portions which had been heated up to 180°C, and molding was carried out by means of a hydraulic press in order to prepare molded articles of 0.5 mm and 1 mm thick. With regard to the molded articles of 0.5 mm and 1 mm thick, mean deviations of thickness were as small as 0.05 mm and 0.06 mm, respectively, which fact indicated that these molded articles had a uniform thickness. Each mean deviation just described was obtained by measuring the thickness at 6 points at an interval of 1 cm in a radial direction on a hemispherical molded article having a diameter of 10 cm and a depth of 2 cm. Comparative Example 1 The UHMWPE powder used in Example 1 was set in a mold, and heating was then carried out up to 180°C, followed by press molding in order to prepare molded articles of 0.5 mm and 1 mm thick, as in Example 1. For each molded article, a mean deviation of thickness was measured. In the case of this Comparative Example 1, the mean deviations of the molded products of 0.5 mm and 1 mm thick were as large as 0.16 mm and 0.25 mm, respectively, which fact indicated that the procedure of Comparative Example 1 could not provide any molded articles having a uniform thickness. Example 2 A UHMWPE powder (trade name Hizex Million 240 (S); Mitsui Petrochemical Industries, Ltd.) was fed to between two rolls rotating in mutually reverse directions under conditions that the roll gap was 0.2 mm, a molding temperature was 154°C and the roll peripheral speed was 2 m/min, thereby melting the powder and preparing preformed sheets each having a thickness of 0.34 mm and a void ratio of 11%. Next, an elbow-like steel pipe 2 having a 1 mm diameter holes, as shown in Fig. 1, was covered with a silicon rubber tube 3 having a thickness of 3 mm, and the above preformed sheet 1 was then wound around the rubber tube 3 in a spiral state so as to overlap the edge portions of the sheet 1, as shown in Fig. 2. The resulting deformable member around which the preformed sheet had been wound was placed in an openable elbow mold 6 heated up to 180°C which was a little larger than the deformable member, as shown in Fig. 3. Then, the mold was closed, and the silicone rubber tube 3 was fastened on the end portion thereof by a ring 5 (Fig. 4). Fig. 4 shows a section in which the formable member was set in the mold. An air pressure of 12 kg/cm² (11.76 bar) was applied through a pressurizing hole 4 to expand the silicone rubber tube 3 and to thereby press the preformed sheet 1 against the mold 6. The heating and pressurizing operation was carried out for 5 minutes in this way, whereby the sheet was melted and densified. Next, the mold 6 was cooled to a level of room temperature, remaining pressurized, and the air was drawn off through the pressurizing hole 4. The mold was then opened, and at this time, the elbow of the UHMWPE which was completely densified could be obtained. With regard to the thus obtained elbow-like molded article, outer diameters at its both end portions was 55 mm, and an outer diameter at its intermediate portion was 50 mm. In its outer peripheral portion of the bend, a radius was 100 mm and a wall thickness was 0.3 mm, and in its inner peripheral portion, a radius was 50 mm and a wall thickness was 0.6 mm. Further, a test piece was prepared by cutting off a certain portion of the elbow-like molded article and was subjected to a physical properties measuring test. The results are set forth here. Example 3 A sheet preformed in the same manner as in Example 2 was applied onto a silicone rubber tube 3 as the deformable member with which an elbow-like steel pipe 2 had been covered and an additional sheet was further wound in a spiral state over the former one in order to increase the wall thickness of its preformed sheet. Except for this step, the same procedure as in Example 2 was carried out to prepare an elbow, the outer and inner peripheral portions of the bend having a similar wall thickness of 0.6 mm. Example 4 A preformed sheet 1 having a width of 12 cm which had been prepared in the same manner as in Example 2 was wound 10 times around a silicone rubber tube 3 as the deformable member with which a cylindrical steel member 2 having holes was covered (see a perspective view shown in Fig. 5), and the whole assembly was then placed in a bellows mold 6 shown in Fig. 6 and Fig. 7. Except for these steps, the same procedure as in Example 2 was carried out. As a result, a bellows was obtained, the size of the molded article being a length = 120 mm, a pitch number = 9, a maximum outer diameter = 80 mm, a minimum outer diameter = 76 mm, a maximum inner diameter = 73 mm, a minimum inner diameter = 71 mm, a wall thickness at top of thread = 3.5 mm and a wall thickness at bottom of thread = 2.5 mm. Example 5 There was used a steel cylindrical member 2 having holes which was covered with a silicone rubber tube 3 as a deformable member, and the same preformed sheet 1 having a width of 18 cm as in Example 2 was wound 5 times around the member 2 (Fig. 8). The end portion of the preformed sheet protruded from one end of the cylindrical member was folded (Fig. 9) to form a circular shape. The deformable member was then placed in a heated mold 6 which was a little larger than the member therein (Figs. 10 and 11). Next, air was blown through the pressurizing hole 4, and the same procedure as in Example 2 was repeated to obtain a cylindrical hollow molded article, the size of the latter being a height = 140 mm, an outer diameter = 80 mm, a wall thickness of the side = 1.5 mm and a wall thickness of the bottom = 1.5 mm. Example 6 A steel male mold 2 having holes which was hemispherical at the top and was covered with a silicone rubber tube 3 was covered on the hemispherical portion thereof with a hemispherical preformed sheet 1 having a diameter of 65 mm (Fig. 12), and a peripheral portion of the sheet 1 was folded to form a perfect hemispherical shape. Afterward, the whole assembly was placed in a mold 6 shown in Figs. 13 and 14. In the same procedure as in Example 2, molding was carried out to obtain a hemispherical molded article, the size of the latter being a diameter = 40 mm, a wall thickness at the central portion = 0.3 mm and a wall thickness at the peripheral portion = 0.6 mm. Example 7 A cylindrical steel pipe 2 having 1 mm diameter holes, as shown in Fig. 15, was covered with a silicone rubber tube 3 having a thickness of 3 mm, and a preformed sheet 1 was then wound around the rubber tube 3 in a spiral state so as to overlap the edge portions of the sheet 1, as shown in Fig. 16. The resulting easy-deforming member around which the preformed sheet had been wound was placed in an openable cylindrical mold 6 heated up to 180°C which was a little larger than the easy-deforming member, as shown in Fig. 17. Then, the mold was closed, and the silicone rubber tube 3 was fastened on the end portion thereof by a ring 5 (Fig. 18). Fig. 18 shows a section in which the easy-forming member was set in the mold. An air pressure of 12 kg/cm²(11.76 bar) was applied through a pressurizing hole 4 to expand the silicone rubber tube 3 and to thereby press the preformed sheet 1 against the mold 6. The heating and pressurizing operation was carried out for 5 minutes in this way, whereby the sheet was melted and densified. Then, the mold 6 was cooled to a level of room temperature, remaining pressurized, and the air was drawn off through the pressurizing hole 4. The mold was then opened, and at this time, a UHMWPE pipe which was completely densified was obtained. This pipe had an outer diameter of 50 mm and a wall thickness of 0.3 mm. Further, a test piece was prepared by cutting off a certain portion of the obtained pipe and was subjected to a physical properties measuring test. The results are set forth here. Example 8 The same procedure as in Example 7 was carried out with the exception that three preformed sheets were superposed, and a pipe was obtained having an outer diameter of 50 mm and a wall thickness of 0.9 mm. Physical properties were evaluated. The results are as follows: Example 9 The same procedure as in Example 7 was carried out with the exception that a preformed sheet was wound in a spiral state as shown in Fig. 19 without overlapping the edge portions of the sheet as shown in Fig. 16 and was further wound in a crossing direction so as to overlap and cross the already wound sheet, so that a pipe was obtained having an outer diameter of 50 mm and a wall thickness of 0.6 mm. Physical properties are as follows: Example 10 An unsaturated polyester resin consisting of propylene glycol, maleic anhydride, phthalic anhydride, adipic acid and styrene was blended with a hardening accelerator, and a glass cloth was impregnated with the resulting blend to prepare a precursor comprising an unsaturated polyester resin composition. Next, between a UHMWPE sheet preformed in the same manner as in Example 1 and the aforesaid precursor, a film of an ethylene-vinyl acetate-glycidyl methacrylate copolymer was interposed as an adhesive, and compression molding was then carried out. The results are set forth in Table 1. It can be understood from the results in Table 1 that the laminate of the unsaturated polyester and the UHMWPE sheet was improved remarkably in elastic modulus in flexure. Further, it is apparent that the laminate was also improved noticeably in the resistance to a repeated impact, when a thickness of the UHMWPE was 0.4 mm or more. In particular, when the outermost layer of the laminate was the UHMWPE sheet, the improvement in the impact strength was great. Example 11 An urethane binder (trade name BOND KU10; Konishi Co., Ltd.) was applied onto the surface of a non-rebounding rubber (trade name HANENITE; Naigai Rubber Co., Ltd.) having a thickness of 5 mm, and a UHMWPE sheet preformed in the same procedure as in Example 1 was superposed on the surface coated with the binder. Compression molding was then carried out at 170°C under a pressure of 4 kg/cm²(≈ bar) for 20 minutes and was afterward cooled still in the pressurized state, in order to obtain a laminate of the UHMWPE sheet and the rubber. The results of its physical properties are set forth in Table 2. Comparative Example 2 The same procedure as in Example 11 was repeated with the exception that a UHMWPE crude powder was substituted for the preformed UHMWPE sheet, in order to obtain a laminate of the UHMWPE and the rubber. The results of its physical properties are set forth in Table 2. The results in Table 2 indicate that when the thickness of the preformed sheet was in excess of 1 mm, properties of the non-bound rubber were impaired largely. Further, when the UHMWPE crude powder was used, the thickness of the UHMWPE layer was noticeably uneven, and thus any uniform products were not manufactured. In addition, in order to prepare the UHMWPE layer having a thickness of 1 mm or less, a very great labor was required, and for this reason, the manufacture of such a UHMWPE layer was impossible in fact. Tensile tensile strength at yeild point ...200 kg/cm Example 12 A UHMWPE sheet preformed by the same procedure as in Example 1 was superposed on an aluminum plate (a substrate) with the interposition of a polyethylene (an adhesive) modified with maleic anhydride, and pressure molding was carried out at a temperature of 180°C. Additional experiments were performed varying the substrates and the adhesives, and all the resluts are set forth in Table 3. The UHMWPE is poor in processability, and thus is difficult to mold this by means of a usual extrusion or injection molding. Therefore, with regard to thin-wall molded articles, hollow molded articles such as a pipe, a bellows, an elbow and a tee, and special form articles, their molding is very hard. For example, a short pipe may be manufactured by cutting operation, but the manufacture of a long pipe is impossible in such a way. Moreover, the cutting operation tends to increase a cost noticeably. According to the present invention, however, the long pipes can be manufactured inexpensively and simply. In addition, such a long pipe has a remarkable effect in delivering a slurry, a powder or the like therethrough, and the special form articles such as a stirrer, a vessel, a bellows and the like regarding the present invention can be utilized as tools for civil engineering and construction and as parts for various industrial machines.

Claims: A method for preparing hollow articles or molded articles comprising an ultra-high molecular weight polyethylene which method comprises the steps of feeding an ultra-high molecular weight polyethylene powder to between at least two heated rollers rotating in mutually reverse directions, melting said powder at a melting temperature of more than 140° C in order to obtain a preformed porous sheet, and compression-molding, or heating and pressurizing said preformed porous sheet of said ultra-high molecular weight polyethylene or said preformed porous sheet superposed with a substrate by the use of a mold in order to densify said preformed porous sheet. A method according to claim 1 wherein at least one of the molds consists of a deformable member and said mold and/or said sheet is heated to expand or deform said deformable member and to thereby cause said sheet to adhere closely to said mold, whereby said sheet is melted and densified. A method according to any of claims 1 and 2, wherein said ultra-high molecular weight polyethylene has an intrinsic viscosity of 8 dℓ/g or more in a decalin solution at 135° C. A method according to any of claims 1 to 3, wherein said substrate is composed of at least one material selected from a thermoplastic resin, a curable resin, a rubber, a metallic foil, a metallic plate, a wire mesh, a cloth and a paper.

Inventors: AOYAMA, TAKEO, HATTORI, JUNICHI, IMAI, TERUO, UEHARA, MIKIO
Applicants: NIPPON PETROCHEMICALS CO., LTD.
Application year: 1986
Main classification: B29C67
IPC codes: B29C43, B29C67