POLYACETAL RESIN COMPOSITION

Abstract: A polyacetal resin composition comprises a polyacetal and a potassium titanate fiber, being suitable for moulding a slidable part or article.

Description: The present invention relates to a polyacetal resin composition having superior friction charactistics, rigidity and the like suitable for producing sliding members, and in particular for mutually co-operating switch parts which slide on one another (and where one or both of the sliding members may be of the resin composition), used in electric circuits or parts such as various kinds of gear. Polyacetal resins are superior in mechanical and physical properties and therefore have been widely used in various fields. Although they are superior also in friction-abrasion characteristics per se and can be used for sliding members and the like even without containing any fillers, high rigidity and strength are required as well as superior friction-abrasion characteristics with the expansion of the uses of engineering plastics. It has been a general method of meeting such a requirement to add glass fibers to resins. However, the addition of glass fibers leads to the deterioration of the friction-abrasion characteristics. That is to say, since the addition of glass fibers leads to a deterioration of the surface properties, integers arranged to slide over as against parts made from said resins, (for example, such integers may be made of metals or resins) may become abraded or worn, and also the resins parts themselves maybe worn, in particular by powders or other particulates produced in the above described sliding action causing abrasion of the co-operating sliding surfaces whether metals or resins. Accordingly, when polacetal resins reinforced with glass fibers are used for sliding members such as switch parts, there are such problems that surfaces of both resins and metals, or of resins sliding over against co-operating resin surfaces are subject to wear and a malfunction, of e.g. an electric circuit, is induced due to powders produced in the above-described abrasion of metals or resins. In particular, since it is required that the abrasive wear is to be as little as possible when polyacetal resins reinforced with glass fibers are used for precision parts and the like, it is necessary to impart well-balanced high rigidity, high strength and superior friction-abrasion characteristics to resins used for precision parts and the like. Other attempts have been made to give the required improvements, for example as described in US―A―4 506 053, GB―A―1 398 290 and JP―A―49256/1984. The present inventors have found from their repeated investigations aiming at the development of a polyacetal resin having properties meeting the above-mentioned requirements that a particularly effective result is achieved by the addition to a polyacetal resin of an abrasion resisting agent such as fluororesins and silicone together with a potassium titanate fibre. Thus the present invention provides a specific polyacetal resin composition wherein a potassium titanate fiber is added to a polyacetal resin, and at least one abrasion resisting agent selected from the group consisting of fluororesins and silicone, is added according to the desired uses thereof. According to the invention there is provided a polyacetal resin composition which comprises a polyacetal, 7 to 30% by weight (based on the total composition) of a potassium titanate fiber and 0.5 to 30% by weight (based on the total composition) of at least one abrasion resisting agent selected from fluororesins and silicone. The invention further provides a process for producing the said composition by extrusion of a mixture of the said components The present invention will now be described in more detail. A polyacetal resin used in the present invention includes a polyoxymethylene homopolymer or a copolymer of which main chain chiefly consists of an oxymethylene chain. Potassium titanate fiber used in the present invention is a white acicular monocrystalline fiber having an average length of 5 to 120 µm, preferably 10 to 20 µm, and an average diameter of 0.1 to 1.0 µm, preferably 0.2 to 0.5 µm. In order to increase the adhesion of said potassium titanate fiber to the resin, the surface of said fiber may be treated with known surface treating agents. Said agents include those based on an aminosilane, an epoxysilane and a titanate, among which the one based on an aminosilane is particularly preferable. According to the present invention, at least one abrasion resisting agent selected from fluoroesins and silicone can be added to polyacetal resin composition containing a potassium titanate fiber and optionally lubricants therein. Potassium titanate fiber has a plastic reinforcing performance similar to a glass fiber and the resulting composition containing the potassium titanate fiber is superior to those containing a glass fiber in surface flatness and abrasion resistance. Therefore, when the polyacetal resin containing the potassium titanate fiber is used as a sliding member, the abrasion is reduced in both the sliding member and the part relative to which it slides in comparison with the polyacetal resin containing a glass fiber. Thus the composition containing only the potassium titanate fiber may also be thought to be suitable. However, a slight abrasion still occurs in the sliding member and a maloperation due to powders formed by sliding may take place occasionally when the composition containing only a potassium titanate fiber is used as sliding parts such as switches and other electronic parts, electric parts and precision parts such as in cameras or watches. As a result of repeated investigations aiming at the improvement in the above described points, the present inventors succeeded in obtaining a polyacetal resin composition having still more improved friction-abrasion characteristics by adding at least one abrasion resisting agent selected from fluororesins, silicone and lubricants to a polyacetal resin together with a potassium titanate fiber. According to the present invention, the abrasion resisting agent is in general added in an amount of 0.5 to 30% by weight based on the total composition. Referring to the abrasion resisting agents, the fluororesins are those obtained by polymerizing a monomer in which hydrogen atoms of ethylene are substituted by at least one fluorine atom. Tetrafluoroethylene resin or difluoroethylene resins in usually used. Also copolymers of these fluororesins or fluororesins containing other elements in a part thereof can be used. Of these, the tetrafluoroethylene resin is preferably used. When fluororesins are used as abrasion resisting agents, they are preferably used in an amount of 2 to 10% by weight based on the total composition. The addition of too little of said abrassion resisting agents leads to an insufficient improvement of the abrasion resistance while an excessive addition is not cost effective. It is desirable that the ratio of the potasium titanate fiber to the fluororesin is within the range of 40:1 to 1:1 by weight. In particular, if this ratio is within the range of 3 : 1 to 1 : 1 by weight, good results can be obtained, though other ratios may also be used according to uses. The silicone is called generically organo-polysiloxane and includes silicone oils, silicone rubbers and silicone resins. In particular, silicone oils comprising dimethylpolysiloxane of a considerably lower degree of polymerization and the like and/or silicone greases comprising silicone oils, and optionally lubricants such as metal soaps and the like are preferably used. The lubricants include those based on hydrocarbons, such as low molecular polyethylene wax, fatty acids, fatty acid amides, esters, metal soaps, fatty alcohols, polyhydric alcohols, polyglycols, polyglycerols, esters of fatty acids with polyhydric alcohols, esters of fatty acids with polyglycols or polyglycerols. The composition according to the present invention can be produced by various methods. For example, pellets obtained by extruding a preblend consisting of a resin ingredient, potassium titanate fiber and fluororesin in an extruder are molded in a molding machine. Known additives such as various kinds of organic high-molecular substance or inorganic fillers may further be added to the polyacetal resin composition according to the present invention according to the uses thereof. Said organic high-molecular substances include polyurethanes, vinylic compounds and copolymers thereof such as ethylene/vinyl acetate copolymer, ethylene/alkyl acrylate copolymer, styrene/butadiene/acrylonitrile copolymer and styrene/acrylonitrile copolymer; multi-phase graft copolymers comprising polyacrylate resins; and segmented thermoplastic copolyesters. Said inorganic fillers include, carbon fibers, mica, talc, calcium carbonate, metal fibers, and metal foils. In addition, antistatic agents or electrical conductivity improving agents such as electrically conductive carbon black (Ketjen Black, a product of Lion Akzo) coloring agents, releasing agents, nucleating agents, stabilizers and surface active agents may be added to the composition according to the present invention to give necessary properties thereto. These additives may be added when the potassium titanate fiber is blended with the resin, or may be added to the resin in the form of a mixture thereof with the potassium titanate fiber. Alternatively, they may be added together with a surface treating agent in the above-mentioned surface treatment process of the potassium titanate fiber, or maybe adhered to the potassium titanate fiber by means of a spreading agent. Further, they may be strongly bonded to the potassium titanate fiber by chemical reactions. The obtained composition according to the present invention shows not only remarkably superior friction-abrasion characteristics but also high rigidity and strength. Accordingly, they can be suitably used in the production of electronic parts, such as switches, and gears. The effects of the present invention will now be described in more detail with reference to examples and comparative examples. Examples 1 to 4: Comparative Examples 1 to 9 A potassium titanate fiber, a fluororesin (tetrafluoroethylene resin) powders, silicone oil and a lubricant are added to a polyacetal resin (Duracon®, a product of Polyplastics Co., Ltd.) in proportions as shown in the Table. The resulting blend is extruded in an extruder and then molded in an injection molding machine to prepare test pieces for specific abrasion, bending strength and bending modulus. The results are shown in the same Table. Specific abrasion under "thrust" was measured on a test piece at a pressure of 10 kg/cm² a linear sliding velocity of 30 cm/sec, a contact area of 2.0 cm² and a travelling distance of 6.5 km. Bending strength and bending modulus were measured in accordance with ASTM D 790. Separately, polyacetal resin compositions containing no additives and containing only glass fibers were tested in the same manner as above and the results are shown in the same Table as Comparative Examples. As is obvious from these results, the composition according to the present invention shows not only superior friction-abrasion characteristics but also high rigidity and strength. Accordingly, the composition according to the present invention is very usefully applied in the uses where not only superior sliding characteristics but also high rigidity and strength are required.

Claims: 1. A polyacetal resin composition which comprises a polyacetal, 7 to 30% by weight (based on the total composition) of a potassium titanate fiber and 0.5 to 30% by weight (based on the total composition) of at least one abrasion resisting agent selected from fluororesins and silicone. 2. A polyacetal resin composition as set forth in claim 1, wherein the abrasion resisting agent is a fluororesin. 3. A polyacetal resin composition as set forth in claim 2, wherein the ratio of the potassium titanate fiber to the fluororesin is within the range of 40:1 to 1:1 by weight. 4. A polyacetal resin composition as set forth in claim 3, wherein the ratio of the potassium titanate fiber to the fluororesin is within the range of 3:1 to 1:1 by weight. 5. A polyacetal resin composition as set forth in claim 1, wherein the abrasion resisting agent is a silicone selected from silicone oils comprising dimethyl polysiloxane having a low degree of polymerization and silicone greases comprising silicone oils. 6. A process for producing a polyacetal resin composition wherein a mixture of polyacetal resin, potassium titanate fibre and an abrasion resistant agent selected from fluororesins and silicones is extruded to form a composition containing the polyacetal resin, 1 to 30% by weight (based on the total composition) of the potassium titanate fibre and 0.5 to 30% by weight (based on the total composition) of the abrasion resisting agent.

Inventors: OKADA, TSUNEYOSHI, YANASE, YUKIMASA
Applicants: POLYPLASTICS CO. LTD.
Application year: 1985
Main classification: C08K7
IPC codes: C08K7, C08L23, C08L27, C08L59, C08L101, C08L1