POLYACETAL RESIN COMPOSITION AND SLIDING PART MADE THEREFROM

Abstract: A polyacetal resin composition comprising a polyacetal, a polyolefin, and carbon black dispersed in a special morphology, which is excellent in thermal conductivity, sliding property, moldability, and molding apearance. The composition provides a sliding part for electrical equipments, e.g., gears, bearings, cams, etc., which has excellent sliding properties, makes low noise, and exhibits high abrasion resistance.

Description: FIELD OF THE INVENTION This invention relates to a polyacetal resin composition comprising a polyacetal, a high-density polyethylene, and carbon black dispersed in a special morphology, which is excellent in thermal conductivity, sliding property, moldability, and molding appearance. BACKGROUND OF THE INVENTION In recent years, plastics have been widespread in the fields of various domestic appliances, office automation equipments, etc. with the improvement of accuracy and properties due to the development of molding techniques, new plastic materials, and the like. Upon a particular consideration of gears that are sliding parts of these equipments, plastics are widely employed chiefly as gears of small transmission torque or small-sized gears from the standpoint of decreased cost resulted from mass production, corrosion resistance, reduced noise, and the like in spite of their inferiority in stiffness or strength. Polyacetal resins have been generally used for the particular use as precise gear of small size because of their relatively well-balanced characteristics as required for gear materials, such as molding accuracy, fatigue strength, etc. Polyacetal itself is an engineering resin excellent in sliding properties, and studies have been directed to broadening of utility of polyacetal by incorporating carbon black thereinto to thereby impart thermal conductivity. However, a polyacetal composition having incorporated therein carbon black in an amount sufficient to obtain required properties has suffered many problems, such as decomposition of polyacetal during kneading, serious reduction in moldability, extreme deterioration in molding appearance, and the like. In addition, the sliding part (e.g., gears) comprising a polyacetal resin makes a disturbing noise when used under severe conditions, i.e., a high load and a high speed. With the strong demand for reduction of noise caused by various domestic applicances, office automation equipments, etc., it has been keenly demanded to improve molding accuracy, abrasion resistance, and sliding properties of sliding parts, e.g., gears, bearings, cams, etc. In order to meet these requirements, it has been proposed to add an amide compound and polyethylene wax to polyacetal containing conductive carbon black as disclosed in U.S. Patent 4,555,357. This proposal achieves some improvements on the above-described problems but still has another disadvantage that sliding properties inherently possessed by polyacetal are deteriorated. DE-A-28 08 675 discloses compositions including polyoxymethylene, electrically conductive soot and polyethylene with a density of .91 to .94 g/cm³ and with a melt index of .1 to 2000. SUMMARY OF THE INVENTION In the light of these circumstances, the present inventors have conducted intensive and extensive investigations. As a result, it has now been found that a polyacetal resin composition in which polyacetal, carbon black, and a high-density polyethylene are dispersed under a special morphological control is excellent in thermal conductivity, moldability, and molding appearance and superior to polyacetal itself in sliding properties. That is, the present invention provides a polyacetal resin composition comprising (a) from 60 to 94 parts by weight of polyacetal, (b) from 4 to 30 parts by weight of a high-density polyethylene and (c) from 2 to 10 parts by weight of carbon black, each per 100 parts by weight of the total amount of the components (a), (b), and (c), wherein a melt flow rate of said high-density polyethylene as component (b) is from 0.08 to 8 g /10min. and a ratio of a melt flow rate of said polyacetal as component (a) to that of component (b) is from 0.2 to 25, and the density of said high-density polyethylene as component (b) is from 0.945 to 0.98 g/cm³. In a preferred embodiment of the aforesaid polyacetal resin composition, the component (c) is substantially included in a disperse phase of the component (b). The present invention further provides a strikingly low-noise sliding part comprising the aforesaid polyacetal resin composition. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 illustrates a noise determination system. Figure 2 illustrates a sliding property determination system. Figure 3 is a graph showing the results of measurements of abrasion resistance. DETAILED DESCRIPTION OF THE INVENTION The term "sliding part" as used herein means parts of electrical equipments undergoing sliding on driving and includes gears, bearings, cams, levers, etc. The polyacetal which can be used as component (a) in the present invention is a polyoxymethylene homopolymer or a copolymer whose main chain comprises an oxymethylene unit. Polyoxymethylene homopolymer is a thermoplastic linear high polymer having a recurring unit structure represented by formula: (R·CHO) n wherein R represents a hydrogen atom or a methyl group, and includes an acetal homopolymer that is a uniform polymer of an aldehyde. The acetal copolymer includes a polymer comprising an oxymethylene chain in which an oxyalkylene unit represented by formula: wherein R¹ and R², which may be the same or different, each represents a hydrogen atom, an alkyl group having from 1 to 5 carbon atoms, preferably 1 to 3 carbon atoms, or an aryl group; and m represents an integer of from 2 to 8, is incorporated at random. The proportion of the oxyalkylene unit in the acetal copolymer generally ranges from 0.05 to 50 mols, and preferably from 0.1 to 20 mols, per 100 mols of the oxymethylene unit. Specific examples of the oxyalkylene unit are an oxyethylene unit, an oxypropylene unit, an oxytrimethylene unit, an oxytetramethylene unit, an oxybutylene unit, and an oxyphenylethylene unit. Of these preferred are oxyethylene and oxytetramethylene units from the standpoint of improving properties of the polyacetal resin composition. The above-described acetal copolymer can be prepared by copolymerizing or reacting formaldehyde, trioxane or polyoxymethylene with a cyclic ether or a cyclic formal. In the present invention, the acetal homo- or copolymer produced by polymerization reaction is used as compounding component after its terminals are converted to stable groups. The high-density polyethylene which can be used as component (b) is an ethylene homopolymer; a random or block copolymer of ethylene and one or more of other α-olefins; and a random, block or graft copolymer of ethylene and a vinyl monomer, e.g., acrylic acid, maleic anhydride, etc. The copolymer of ethylene and comonomer(s) should contain at least 60% by weight of an ethylene unit. Specific examples of the high-density polyethylene as the component (b) include ethylene homopolymers, e.g., medium to low-pressure-processed high-density polyethylene, etc.; and copolymers of an ethylene and other monomer(s), e.g., a random or block ethylene-propylene copolymer, a ethylene-butene-1 copolymer, a maleic anhydride-grafted polyethylene, etc. Preferred of these high-density polyethylene are an ethylene homopolymer and a copolymer of ethylene and other α-olefin or α-olefins, which have a density between 0.945 g/cm³ and 0.98 g/cm³. The comonomers copolymerizable with ethylene include olefins, e.g., propylene, butene, hexene, octene, etc., and vinyl compounds, e.g., vinyl acetate, (meth)acrylic acid, vinyl (meth)acrylate, etc. The carbon black which can be used as component (c) is conventional and includes furnace black, acetylene black, thermal black, channel black, etc. It is preferable to use electrically conductive carbon black. Particularly advantageous is electrically conductive carbon black having a BET specific surface area (N₂ adsorption) of 40 m²/g or more, preferably 400 m²/g or more, and more preferably 850 m²/g or more, e.g., Ketjen Black Produced by KBIC A.G. The components (a), (b), and (c) are compounded at such ratios that the resulting composition contains from 60 to 94 parts, and preferably from 65 to 85 parts, by weight of the component (a), from 4 to 30 parts, and preferably from 10 to 25 parts, by weight of the component (b), and from 2 to 10 parts, and preferably from 4 to 8 parts, by weight of the component (c), each per 100 parts by weight of the total of the components (a), (b), and (c). If the compounding ratio of the component (c) is less than 2 parts by weight, the resulting composition hardly shows thermal conductivity and enjoys only a poor effect of improving sliding properties. On the other hand, if it exceeds 10 parts by weight, moldability of the composition is deteriorated. If the compounding ratio of the component (b) is less than 4 parts by weight, sliding properties cannot be improved sufficiently. If it exceeds 30 parts by weight, sliding properties are rather reduced. When considering a dispersed system of the composition according to the present invention, it is particularly preferable that the component (c) is substantially included in a domain of the component (b). The phase "the component (c) is substantially included in a domain of the component (b)" as used herein means a morphological structure in which the component (c) is substantially embraced in the dispersed phase (domain) of the component (b) which substantially forms a continuous circuit through the composition. Formation of such morphology makes it possible to minutely control dispersion of carbon black and to markedly improve moldability and molding appearance. The substantial inclusion of carbon black in the continuous high-density polyethylene circuit produces an additional effect that the resulting polyacetal resin composition exhibits improved electrical conductivity over the conventional polyacetal resin composition containing the same carbon black. In order to form the above-mentioned special morphology, it is effective to properly control the melt viscosities of the compounds (a) and (b). More specifically, the morphology can be achieved by controlling a ratio of the melt flow rate (MFR) of the component (a) (X) to that of the component (b) (Y) [X/Y] within a range of from 0.2 to 25, preferably from 0.4 to 20, more preferably from 0.5 to 15, the MFR being determined at 190°C under a load of 2.16 kg (ASTM-D 1238170). In this connection, it is preferable to use polyacetal, as component (a), having an MFR (X) of from 2 to 30 g/10min., and more preferably from 5 to 20 g/10min., and a polyolefin, as component (b), having an MFR (Y) of from 2 to 8 g/10min. While the mode of blending the components (a), (b), and (c) is not particularly restricted, it is desirable that the components (b) and (c) are previously melt-kneaded and then the component (a) is melt-kneaded therewith. The component (b) having dispersed therein the component (c) may be dispersed in the matrix component (a) in the form of spheres, or preferably in a fiber-like extended form or in the form of a network structure comprising fibrous particles connected among themselves. The resin composition according to the present invention can contain various auxiliary components in addition to the essential components (a), (b), and (c). Examples of such auxiliary components include phenol type antioxidants, e.g., 2,6-di-t-butyl-4-methylphenol, 1,1,3-tri-(2-methyl-4-hydroxy-5-t-butylphenyl)butane, tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamic ester)]methane, n-octadecyl-β-(4'-hydroxy-3',5'-di-t-butylphenyl)propionate, etc.; sulfur type antioxidants, e.g., dilauryl thiodipropionate, distearyl thiodipropionate, laurylstearyl thiodipropionate, tetrakis(methylene-3-dodecyl thiopropionate)methane, etc.; phosphorus type antioxidants, e.g., di(dinonylphenyl)-mono-(p-nonylphenyl) phosphite, etc.; higher fatty acid metal salts, e.g., calcium stearate, magnesium stearate, zinc oleate, etc.; lubricants, e.g., stearamide, etc.; ultraviolet absorbents; antistatics; copper resistance agents; flame retardants; and the like. Addition of these additive components are effective to improve various properties, such as a physical property balance, surface properties of a molding (e.g., scratch resistance, gloss, weld appearance, silver streaking, flow marks, etc.), printability, coating properties, adhesiveness, plating properties, moldability, durability, and the like. The above-enumerated auxiliary components may be used either individually or in combinations thereof. If desired, the composition of the present invention can further contain inorganic fillers, e.g., calcium carbonate, talc, barium sulfate, mica, wollastonite, potassium titanate, carbon fiber, magnesium hydroxide, etc. The resin composition of the present invention can be obtained by kneading the above-described components by the use of a batch mixer, e.g., a Banbury mixer, a roll, a Brabender Plastograph, etc., a single screw extruder, or a twin screw extruder. The thus obtained polyacetal resin composition according to the present invention is markedly excellent in electrical conductivity, thermal conductivity, and sliding properties and also exhibits abrasion resistance and low noise making properties ascribed to the excellent sliding properties. The morphology of the resin composition of the present invention was observed as follows. The composition in the form of pellets were injection molded to prepare a 2 mm thick sheet specimen. The sheet was cut into a thin slice across the resin flow direction by the use of a microtome, and a projected image of the sliced film was observed under a biological microscope. Further, a slice across the resin flow direction was cut out of the same specimen as prepared above by the use of a ultramicrotome and, after ion etching, the slice was scanned by means of a scanning electron microscope to observe a secondary electron image. As described above, the polyacetal resin composition of the present invention is remarkably superior is electrical and thermal conductivity, apearance of moldings thereof, and sliding properties. Accordingly, the composition provides on molding sliding parts for appliances, such as gears, bearings, cams, etc., which have excellent sliding properties, make low noise, and have high abrasion resistance. The present invention is now illustrated in greater detail with reference to Examples and Comparative Examples shown below, but it should be understood that the present invention is not limited thereto. In these examples, various properties were determined according to the following test methods. 1) Specific Volume Resistance: Standard: According to SRIS 2301 of Nippon Rubber Institute (Wheatstone bridge method) Specimen: Since the standard specimen is not limited in shape, an injection molding article of 2 cm in width, 8 cm in length, and 2 cm in thickness was used. For determination of resistance, a 5 mm wide area from the both ends (shorter edges) was coated with a conductive coating "Dotite type RA3", a trade name of a synthetic resin enamel coating produced by Fujikura Kasei Co., Ltd.; toluene content: 90 to 100%. Measurement Condition: Temperature: 21 to 23°C Humidity: 55 to 60% RH Measurement was conducted after 24 hours from the application of the conductive coating. 2) Thermal Conductivity: Thermal conductivity was determined according to JIS C-2141 (laser flash method). That is, a specimen of 10 mm in diameter and 1 to 2 mm in thickness was irradiated with a laser beam, and an increase in temperature of the surface and back sides of the specimen and an absorbed energy on the surface of the specimen were measured. A thermal conductivity was calculated from equations (1) to (3): Thermal Conductivity = a·c·d (3) wherein ΔTm is the maximum increase temperature (K) of a back side of specimen, t ½ is called a half·time, which is the time (sec.) until the temperature of a back side of specimen is increased by one second of ΔTm, Qi is an absorbed energy; d is a density; and L is a thickness of a specimen. 3) Sliding Properties (limit of PV): As shown in Figure 2, a cylinder obtained from the resin composition of the present invention (OD: 25.6 mm; ID: 20.0 mm; area of each end: 2.0 cm²) was put on an ABS plate of 30 mm in width, 30 mm in length, and 2 mm in thickness, and the cylinder was rotated at an increasing peripheral speed starting at 17.3 cm/sec while being pressed onto the ABS plate under a constant contact pressure of 1 kg/cm². The contact surface between the ABS plate and the cyclinder was melted by a heat of friction to cause an abrupt increase in torque. The PV value at which the torque began to increase, i.e., a limit of PV (kg/cm²·cm/sec=kg/cm·sec) was measured. EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 5 Polyacetal having an MFR of 9.0 g/10 min as component (a), high-density polyethylene A having a density of 0.963 g/m³ and an MFR of 5.0 g/10 min or high-density polyethylene B having a density of 0.954 g/cm³ and an MFR of 0.3 g/10 min as component (b), and carbon black having a specific surface area of 1050 m²/g (Ketjen Black) or carbon black having a specific surface area of 80 m²/g (acetylene black) as component (b) were mixed at a mixing ratio as shown in Table 1, and the mixture was kneaded in a twin screw extruder to obtain pellets. The X/Y ratio in the case of using the high-density polyethylene A was 1.8, and that in the case of using the high-density polyethylene B was 30. The pellets were molded in an injection molding machine to prepare test specimens for determination of various physical properties. The results of the determination are shown in Table 1. The results of Table 1 prove that the compositions according to the present invention are markedly superior to the comparative compositions in terms of electrical and thermal conductivity, sliding properties, moldability, and appearance. EXAMPLE 5 Pellets of the resin composition were prepared in the same manner as in Example 3. The pellets were injection molded to obtain a precision gear having a diameter of 30 mm. Morphological analysis on the dispersed system of the molded gear revealed that the component (c) was substantially included in the domain of the component (b). The resulting gear was tested for noise making property according to the method described below in comparison with a metal gear (made of "S45C") or polyacetal gear of the same size. Gearwheels A and B as indicated in Table 2 were engaged with each other as illustrated in Figure 1 and driven at a given rotation (700 rpm) and under a given load (torque: 20 kg·cm). The noise produced from the sliding gearwheels was determined by means of a microphone 1, a noise meter 2, and an FFT analyzer 3 to determine a nose level (dB). The results obtained are shown Table 2. Combination of Gearwheels Noise Level (dB) Remark Wheel A Wheel B Gear of Invention Metal gear 83 Invention " Polyacetal gear 83 " " Gear of Invention 83 " Metal gear Metal gear 93 Comparison Polyacetal gear " 88 " " Polyacetal gear 88 " As can be seen from Table 2, the special plastic gears according to the present invention make significantly reduced noise as compared with the metal gear or polyacetal gear. Further, the pellets were molded into a plate. A rotating ring made of a steel was brought into contact with each of the above plate and a polyacetal plate under a prescribed contact pressure. The wear each of the plate of the present invention (▪) and the polyacetal plate (□) on the sliding surface after 1 km running was measured. The results obtained are shown in the graph of Figure 3.

Claims: A polyacetal resin composition comprising (a) from 60 to 94 parts by weight of polyacetal, (b) from 4 to 30 parts by weight of high-density polyethylene, and (c) from 2 to 10 parts by weight of carbon black, each per 100 parts by weight of the total amount of the components (a), (b) and (c), wherein a melt flow rate of said high-density polyethylene as component (b) is from .08 to 8 g/10 min. and the ratio of the melt flow rate of said polyacetal as component (a) to that of component (b) is from .2 to 25, and the density of said high-density polyethylene as component (b) is from .945 to .98 g/cm³. A polyacetal resin composition as claimed in claim 1, wherein the component (c) is substantially included in a disperse phase of the component (b). A polyacetal resin composition as claimed in claim 1, wherein said high-density polyethylene as component (b) is selected from an ethylene homopolymer and a copolymer of ethylene and other α-olefin or α-olefins. A polyacetal resin composition as claimed in claim 1, wherein said melt flow rate of high-density polyethylene as component (b) is from 2 to 8 g/10 min. A polyacetal resin composition as claimed in claim 1, wherein said carbon black as component (c) has a specific surface area of 40 m²/g or more as measured according to a BET method by nitrogen adsorption. A polyacetal resin composition as claimed in claim 5, wherein said specific surface area is 400 m²/g or more. A polyacetal resin composition as claimed in claim 5, wherein said specific surface area is 850 m²/g or more. A polyacetal resin composition as claimed in claim 1, wherein said components (a), (b), and (c) are present in an amount of from 65 to 85 parts by weight, from 10 to 25 parts by weight, and from 4 to 8 parts by weight, respectively, per 100 parts by weight of the total amount of the components (a), (b), and (c). A polyacetal resin composition as claimed in claim 1, wherein said ratio of melt flow rate is from 0.4 to 20. A polyacetal resin composition as claimed in claim 9, wherein said ratio is from 0.5 to 15. A polyacetal resin composition as claimed in claim 1, wherein the component (b) is dispersed in the component (a) in the form of spheres, in a fiber-like extended form or in the form of a network structure comprising fibrous particles connected among themselves. A sliding part for electrical equipments comprising a molding of a polyacetal resin composition according to any of the claims 1-11.

Inventors: IKEDA, MASAKAZU, MATSUO, NORIO, OKAMURA, MICHIYA, YUI, HIROSHI
Applicants: MITSUBISHI PETROCHEMICAL CO., LTD.
Application year: 1988
Main classification: C08L59
IPC codes: C08L59, C08K3, C08L23