Self-lubricated polyacetal molding compositions and molded articles formed thereof

Abstract: Self-lubricating polyacetal resin compositions and molded articles (eg "wear parts") are provided which comprise a polyacetal base resin and a silica powder in which a silicone polymer (e.g. silicone oil) is absorbed. The silica powder acts as a carrier for the silicone polymer so as to improve the dispersibility thereof in the polyacetal base resin as well as to allow greater silicone polymer "loading" of the polyacetal base resin. As a result, improved lubricant dispersibility and sliding characteristics are achieved.

Description: The present invention generally relates to self-lubricated polyacetal molding compositions which exhibit anti-friction and wear-resistant properties, to processes for making such compositions, and to molded sliding members (e.g. wear parts or bearings) formed of the same. The self-lubricated polyacetal resin compositions is comprised of a blend of a polyacetal base resin, and a self-lubricating effective amount of a silica powder in which a silicone polymer (eg silicone oil) is absorbed. The invention is related to the invention described and claimed in the copending and commonly owned European patent application No. 0 420 564. Polyacetal resins are well known engineering resins due to their balanced mechanical properties. In this regard, polyacetal resins usually exhibit desirable physical properties in terms of their frictional and abrasive characteristics, electrical properties and chemical and heat-resistance properties. Certain properties of the polyacetal resin are, however, required to be continually improved due to special environments of use. The sliding characteristics of polyacetal resins are examples of properties which sometimes need improvement. In this regard, polyacetal resins have recently been employed as guide members for travelling magnetic tapes of audio and video machines, as well as for guide posts of tape cassettes. Other representative examples of sliding parts where strict requirements for sliding characteristics must be satisfied include guide members, such as guide rollers and guide posts for use in the tape travel system of VTRs, 8mm VTRs and the like. In general, guide members used in the aforementioned applications have been made of machined metallic materials. When using metallic materials, however, it is necessary that precautions be observed in order to prevent damage to the tape due to sliding contact with the guide member during operation. In this regard, it is thus usually necessary to reduce the frictional coefficient of metallic guide posts by subjecting the surface which contacts the tape to a surface-smoothing treatment, such as plating, polishing, lapping and the like. Such surface treatments are economically disadvantageous (i.e. they increase the cost of the guide members) and necessarily require separately handling and/or special fabrication techniques in order to prevent the guide post from being damaged prior to use. For these reasons, proposals have been made to incorporate various lubricants in resin guide members so as to provide alternative materials for conventional metals used in such end-use applications. Polyacetal resins having excellent mechanical properties, electrical properties and the like have specifically been proposed for tape guide posts as an alternative to conventional metallic materials by incorporating various lubricants in the polyacetal base resins. Examples of lubricants that have been proposed include fluororesins, polyolefin resins and silicone resins. In addition, solid lubricants, such as graphite and molybdenum disulphide, and lubricating oils such as paraffin oils, spindle oils and silicone oils, have been proposed. In particular, various proposals have been advanced regarding incorporating silicone oil (which exhibits excellent heat-resistance properties, low-temperature characteristics, and chemical and physical stability) into polyacetal base resins. In general, most lubricants (including silicone oil) typically exhibit very poor compatibility characteristics with respect to their affinity to polyacetal resins. This incompatibility between polyacetal resins on the one hand and lubricants (including silicone oil) on the other hand, makes it very difficult to incorporate such lubricants into the polyacetal resins in a homogenous manner. Furthermore, even when compositions are successfully prepared, lubricating oils usually tend to bleed from the surface of the resin during molding so that relative sliding occurs between the individual resin particles and/or between the resin particles and the screw of a conventional screw extruder. This in turn causes the resin to experience poor "bite" by the screw and thus insufficient resin plasticization of the resin ensues. In extreme cases, such incompatability between lubricating oils and polyacetal base resin makes it impossible to conduct molding operations. As noted above, however, even if such molding is successfully performed, the lubricating oil usually "bleeds" from the surface of the molded article and thus transfers and/or adheres to the tape surface in contact therewith. As a result, the lubricant-contaminated tape usually is discolored. Furthermore, lubricating oils disadvantageously lower the surface hardness of the molded article. As mentioned briefly above, several proposals have been made regarding improving the self-lubricity characteristics of polyacetal base resin including the addition of lubricating oils, particularly silicone oils. More specifically, it has been proposed in the past to introduce silicone oils into polyacetal base resins together with a carrier substance for the silicone oil so as to alleviate the above-described problems relating to dispersibility and/or compatibility of the silicone oil with respect to the polyacetal base resin. Examples of inorganic carriers that have been proposed in the past include active carbon and graphite, while examples of organic carriers include high molecular weight polyethylene, ethylene copolymers, and silicone rubber. Active carbon and graphite, however, are incapable of satisfactorily absorbing and holding silicone oil. High molecular weight polyethylene and ethylene copolymers and the like similarly exhibit poor affinity to silicone oils and therefore are less than satisfactory. Silicone rubber, on the other hand, although exhibiting satisfactory compatibility with silicone oil (and thus being able to improve considerably with the processing and fabrication characteristics of the resin) is incapable of satisfactorily retaining the silicone oil under sliding conditions. Furthermore, silicone rubber flows when subjected to elevated temperatures, thereby causing rubber to collect on the surface of molded resin articles. As a result, a thin film is formed on the surface of the molded article which easily peels off when subjected to external stress - i.e. as may be encountered by contact with a running audio/visual tape. Thus, the silicone oil carriers that have been proposed in the past do not alleviate the "bleeding" problems that have been encountered with respect to silicone oils. Although the addition of fluororesins, olefin resins and the like serves to improve the sliding characteristics of polyacetal resin to some extent, such resins in and of themselves typically exhibit poor compatibility with the polyacetal resin. Thus, surface-peeling of molded articles and/or formation of mold deposits during fabrication of such molded articles usually ensues. Furthermore, addition of such solid lubricants can cause the sliding properties to degrade gradually over time due to a powder film forming on the surface of the wear part resulting from the abrasion of the tape in contact with the surface of the molded article. As is evident from the above discussion, the known prior art techniques for providing self-lubricity and/or anti-frictional characteristics to polyacetal resins have yet to produce satisfactory results for short-term and long-term sliding characteristics. Therefore, further improvements in this regard have been needed. It is towards satisfying such a need that the present invention is directed. According to the present invention, improved self-lubricated polyacetal molding compositions may be obtained by the steps of absorbing within a silica powder carrier having a mean particle diameter of between 5 and 300 µm, a silicone oil in an amount between 20 and 80% (based on the weight of the silica powder), and then blending with a polyacetal base resin a self-lubricating effective amount of between 0.5 and 25 wt% based on the total composition weight of the silica powder carrier with absorbed silicone oil therein. The silica powder, which is highly compatible with the polyacetal base resin, is easily dispersed throughout the polyacetal base resin so that the lubricating properties of the silicone oil can be fully realized. Moreover, the silica powder satisfactorily retains the silicone oil in composition. As a result, problems associated with "bleeding" of the silicone oil onto surfaces of molded parts formed of the compositions of the present invention are alleviated. Thus the present invention provides a self-lubricating polyacetal resin composition comprising a blend of a polyacetal base resin, and a self-lubricating effective amount of a silica powder in which a silicone polymer is absorbed, the silicone-absorbing silica powder being present in an amount between 0.5 and 25 wt% based on the total composition weight and having a mean particle diameter of between 5 and 300µm, and the silica powder having between 20 and 80% (based on the weight of the silica powder) of silicone polymer absorbed therein. The present invention further provides a self-lubricated molded article comprising a self-lubricating polyacetal resin molding composition as herein defined. Molded articles, such as guide members or magnetic tape travel systems (particularly guide posts for tape cassettes) will thus exhibit satisfactory self-lubricity properties. The compositions of the present invention include up to 99.5wt% of a polyacetal base resin; the balance is silica powder in which between 20 and 80 wt%, preferably between 30 and 80 wt%, (based on the weight of the silica powder) of a silicone polymer is absorbed, and any optional additive such as will be further referred to later in this specification. Reference will hereinafter be made to the accompanying drawings wherein like references numerals throughout the various figures denote like structural elements, and wherein; Figure 1A is an end elevation view of a molded cylindrical tape guide post according to the present invention; Figure 1B is a cross-sectional elevational view of the guide post shown in Figure 1A, as taken along line 1B-1B therein; and Figure 2 is a schematic perspective view of the testing machine used in the following Examples to determine the coefficient of friction for the molded test pieces. The polyacetal base resins that may be used satisfactorily in the compositions of the present invention may be any polyacetal homopolymer or polyacetal copolymer having a main chain principally composed of repeating oxymethylene (-CH 2 O-) units. Furthermore, polyacetal resins modified by cross-linking or graft copolymerization according to known techniques may also be used as the base resins according to the compositions of the present invention. The degree of polymerization for the polyacetal base resin is not particularly limited, provided that the moldability of the resin is not affected. As noted previously, the compositions of the present invention are especially characterized by incorporating a silica powder in which a silicone polymer has been absorbed. The silicone-absorbing silica powder that is satisfactorily used in the compositions of the present invention may be prepared by blending finely divided silica with between 20 and 80 wt % of a silicone polymer, and then forming a powder of the resulting silicone-absorbed silica. Specific examples of silicone-absorbent silica powder usable in the compositions of this invention include finely divided silicic anhydride prepared by dry processing, natural silicic acid and silicates and hydrous silicic acid prepared by wet processing. Among these, finely divided silicic anhydride is preferred. The silicone polymer that is absorbed into the silica is preferably selected from silicone oil, silicone rubber and/or silicone resin, with silicone oil being especially preferred. Silicone oils that may be used include unsubstituted dimethylpolysiloxane, or dimethylpolysiloxane wherein some of the methyl groups are substituted by one or more substituents selected from hydrogen, phenyl groups, halogenated phenyl groups, halogenated alkyl groups, and fluoroester groups. Although there is no particular limitation on the viscosity of the silicone oil that may be used, it is preferably between 500 and 200,000 cSt (at 25°C) in consideration of several factors, including its absorption into the silica, production of the silica powder, dispersibility of the silica-absorbed silicone product into the resin, workability of the resin during melt-kneading and molding, improvements in the sliding property, and the longevity of such improvements. The above-described silicone-absorbing silica powder has a mean particle diameter of between 5 and 300 µm to ensure that it is dispersible into the polyacetal base resin, and the resin composition exhibits satisfactory workability during melt-kneading processing. Preferably, the mean particle diameter of the silicone-absorbing silica powder is 200 µm or less. The silicone-absorbing silica powder is blended into the polyacetal base resin in an amount between 0.5 and 25% by weight (based on the total composition weight). When the amount of the silica powder with absorbed silicone is less than 0.5% by weight, no significant improvements in the self-lubricity properties of the polyacetal composition are obtained. On the other hand, when the amount of the silicone-absorbing silica powder exceeds 25% by weight, then the other desirable properties of the polyacetal base resin deteriorate. The silicone-absorbing silica powder is most preferably blended with the polyacetal base resin in an amount between 2 and 20% % by weight, and especially between 3 and 15% by weight. As noted previously, the present invention is especially characterized by the addition of a silicone polymer-absorbing silica powder to the polyacetal resin. In this regard, it is known that silicone polymers, particularly silicone oils, may be added alone or in combination of two or more of the same with an inorganic powder during extrusion. In such a situation, however, the inorganic powder does not include any pre-absorbed silicone oil therein. Nonetheless, separately adding silicone oil and inorganic powders during extrusion typically results in improvements to the sliding properties of the resulting resin composition as compared to the use of other known lubricants. However, when lubricants are needed to be added in relatively large amounts so as to achieve even further improvements in lubrication, the lubricant is non-uniformly dispersed within the base resin (i.e. due to its poor compatibility therewith). This, in turn, causes various problems during manufacture (e.g. during extrusion and/or molding). Furthermore, even when silicone oil is incorporated into base resins, there is a possibility that it will "bleed" onto the surfaces of molded articles thereby making it impractical for use as a component for tape travel guide members, for example, since the silicone oil may detrimentally affect the tape in contact therewith. In contrast, however, the addition of a silica powder having silicone polymer (e.g. silicone oil) absorbed therein according to the present invention provides a material that is especially adaptable for use as a tape cassette guidepost since the above-noted disadvantages associated with the separate addition of inorganic powders and silicone oils have been alleviated. In addition, the use of silica powder in which silicone oil has been absorbed impacts excellent effects with respect to the sliding characteristics of the resulting resin composition that are necessary for tape guide members. It should also be noted that the addition of an inorganic rigid material has typically been thought to be detrimental to sliding properties of resin compositions, such as wear and abrasion resistance properties. However, according to the present invention, the addition of silica powder (itself an inorganic rigid material) having a silicone polymer absorbed therein is surprisingly free from the disadvantages typically associated with inorganic rigid materials in terms of detrimentally affecting the resin compositions' sliding characteristics. In addition to the necessary components described above, the resin compositions of the present invention may include known stabilizers for the purpose of enhancing the weather/light stability of the resin composition. Furthermore, other various additives typically incorporated in engineering plastics may be added to the resin compositions of the present invention as may be desired to achieve specific physical properties. Examples of such additives include colorants, such as dyes and pigments, lubricants, mold release agents, plasticizers, crystallization promoters, nucleating agents, anti-static agents, surfactants, impact-modifying polymers and organic improvers. In addition, fibrous, flaky or particulate inorganic, organic and/or metallic fillers may be used either alone or in the form of a mixture of two or more thereof. In this regard, the amount of the fillers incorporated into the resin compositions of the present invention should not be excessive so as to detract from the sliding characteristics of the resin composition which is achieved by means of the present invention. When the composition of the present invention is used to form tape travel guide members, electrostatic charges may build up over time and thereby cause some problems. In this respect, it is thus particularly preferred to incorporate an anti-static agent into the compositions of the present invention so as to counteract this tendency in this specific end-use application. The compositions according to the present invention can easily be prepared by known techniques which have been generally used in preparing conventional resin compositions. For example, the ingredients may first be mixed and then kneaded and extruded in a uniaxial or biaxial extruder to prepare pellets. Pellets may then be molded to form molded sliding members. Alternatively, pellets having different compositions may be prepared as a master batch with a specific amount of such pellets being mixed with other components (diluted) and then molded to form sliding members. In the preparation of the compositions according to the present invention, it is most desirable to improve the dispersibility of the additives whereby at least part of the polyacetal base resin is ground into a powder which is then mixed with the other components. The thus obtained mixture may then be extruded to form pellets and/or a final molded article. As should now be more apparent, the resin compositions of the present invention are especially adapted to being formed into components whereby self-lubricating properties are important, such as for use as tape guide members for audio and video magnetic tape equipment (e.g. video tape recorders, audio cassette players/recorders and the like). The compositions of the present invention are, moreover, suitable for use as an alternative for various sintered alloy bearings (e.g. motor bearings, end bearings, capstan bearings, and the like) or as mechanical parts for certain equipment outside the video and audio field, e.g. as components for clocks, facsimile machines, computers and automotive components. Suffice it to say that, due to the compatibility of the silicone polymer in the polyacetal resin that is promoted by means of the present invention (e.g. the lack of "surface bleeding"), the compositions of the present invention may be satisfactorily employed as self-lubricating "wear parts" for a variety of end-use applications. EXAMPLES The present invention is further illustrated by the following non-limiting examples. Test pieces were evaluated to obtain the data noted in the accompanying Tables. The evaluations were conducted according to the following techniques: (1) Extrudability (observation during pelletization of the composition): Extrusion was conducted with a twin-screw extruder equipped with a vent having an inner diameter of 30mm to observe the state of extrusion. The bite of the screw, vent-up, strand-foaming, surging phenomenon, and the like were visually observed and collectively evaluated according to the following five qualitative ranks. (2) Moldability Plasticization time was measured with a screw-type injection molding machine operating with a cylinder temperature of 190°C, and screw speed of 120 RPM. A large value of plasticization time indicates the moldability of the resin is poor because of "slippage" of the pellet on the screw (i.e. poor "bite"). (3) Surface Bleeding: A cylindrical molded article (guide post) shown in Figures 1A and 1B was produced. A VTR tape was wound around the post and allowed to stand in a thermo-hygrostat (80°C, 95% RH) for 150 hr. The bleeding, transfer and adhesion of oil on the molded article and onto the magnetic tape were visually observed and evaluated according to the following five qualitative ranks. (4) Coefficient of friction A cylindrical molded article as shown in Figures 1A and 1B was mounted on part A of a testing machine shown in Figure 2. A VTR tape having a length of 200 mm was fixed to part B, and a load of 100 g was applied to part C. A moveable part D was reciprocated to bring the molded article into sliding contact with the tape, and the coefficient of friction (µ) was determined by means of a U gauge E and evaluated. The following conditions were maintained during the evaluation: Environmental conditions: 23°C, 50%RH Number of reciprocation runs: 100 Tape speed: 20 mm/sec (5) Surface roughness The surface roughness in the axial direction of a cylindrical molded article as shown in Figure 1 was measured by means of a surface roughness and configuration measuring machine manufactured by Tokyo Seimitus Co, Ltd (Surfcom S54A). (6) Surface hardness (Vickers hardness) The surface hardness of a cylindrical molded article as shown in Figures 1A and 1B was measured by means of a microvickers hardness meter manufactured by Matsuzawa Seiki Co, Ltd. (MHT-ILSD). The larger the value, the harder the molded article. (7) Service test (travelling test) A metallic guide post was removed from a commercially available VHS tape cassette (Scotch EG120; a product of Sumitomo 3M), and a cylindrical molded article having the same dimension was inserted into the same place to prepare a test cassette. The following tests were conducted by making use of this test cassette. (a) Travelling time (FF and REW times) The above-described cassette was mounted on a commercially available VHS tape deck, and FF (fast-forward) and REW (rewind) operations were conducted to measure the tape travelling time per reciprocation. The results were compared with those of a commercially available VHS tape cassette provided with a metallic guide post. A larger value and scattering of values indicate poor tape sliding characteristics. (b) Tape flaws after travelling test: FF and REW operations of the test cassette were repeated 50 times in the VHS tape deck. The contact surface of the tape of the test cassette was then observed under a metallographical microscope. Flaws in the tape surface were evaluated using the five following qualitative ranks; Examples 1 to 6 and Comparative Examples 1 to 11 A polyacetal resin (Duracon M90; a product of Polyplastics Co Ltd) was blended with a silica powder having various silicone oils absorbed therein in proportions specified in Table 1. The blend was melt-kneaded with a twin-screw extruder to pelletize the composition. A test piece was then evaluated in the manner described above. For comparison purposes, test pieces formed from compositions that lacked a silica powder and compositions separately containing a silicone oil and/or silica powder were also evaluated as shown in Table 2. Comparative Example 12 Service tests (travelling tests) as described above were conducted using a commercially available tape cassettes. The results are given in Table 2.

Claims: A self-lubricating polyacetal resin composition comprising a blend of a polyacetal base resin, and a self-lubricating effective amount of a silica powder in which a silicone polymer is absorbed, the silica powder with absorbed silicone being present in an amount between 0.5 and 25 wt% based on the total composition weight and having a mean particle diameter of between 5 and 300µm, and the silica powder having between 20 and 80% (based on the weight of the silica powder) of silicone polymer absorbed therein. A self-lubricating polyacetal resin composition as claimed in any preceding claim 1, wherein the silicone polymer is a silicone oil. A self-lubricated molded article comprising a polyacetal composition according to claim 1 or 2. A self-lubricated molded article as claimed in claim 3, which is in the form of a magnetic tape guide member. A self-lubricated guide post for magnetic tape comprising a polyacetal composition according to claim 1 or 2. A method of making a self-lubricated polyacetal molding composition comprising the steps of absorbing within a silica powder carrier having a mean particle diameter of between 5 and 300 µm, a silicone oil in an amount between 20 and 80% (based on the weight of the silica powder), and then blending with a polyacetal base resin a self-lubricating effective amount of between 0.5 and 25 wt% based on the total composition weight of the silica powder carrier with absorbed silicone oil therein. A method as claimed in claim 6, wherein the said method includes mixing a silicone oil with the silica powder, allowing the silica powder to absorb the silicone oil in an amount between 20 and 80% by weight of the silica powder, and then forming a powder of the resulting absorbed mixture. The invention as claimed in any preceding claim, in which the silicone oil has a viscosity of between 500 and 200,000 cSt (measured at 25°C). The invention as claimed in any preceding claim, in which the silicone polymer is absorbed in the silica powder in an amount between 30 and 80 wt%, (based on the weight of the silica powder). The invention as claimed in any preceding claim, in which the silica powder is finely divided silicic anhydride. The invention as claimed in any preceding claim, in which the silica powder has a mean particle diameter of 200 µm or less. The invention as claimed in any preceding claim, in which the silica powder with absorbed silicone polymer is present in an amount between 2 and 20 wt% based on the total composition.

Inventors: ENDO TOSHIHIKO, KANOTO OSAMU, MATSUNAGA NOBUYUKI, TAKAYAMA KATSUNORI, ENDO, TOSHIHIKO, KANOTO, OSAMU, TAKAYAMA, KATSUNORI
Applicants: POLYPLASTICS CO, POLYPLASTICS CO. LTD.
Application year: 1990
Main classification: C08K3
IPC codes: C08L59, C08L83, C08K3, C08K9, G11B23