Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-10-23
2001-04-24
Short, Patricia A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S404000, C524S512000, C525S154000, C525S155000
Reexamination Certificate
active
06221946
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a novel polyoxymethylene resin composition having slidability at high temperatures. More particularly, it relates to a polyoxymethylene resin composition having slidability at high temperatures which is excellent especially in slidability with respect to noncrystalline resins under a high load and at a high temperature and is suitable as materials for sliding parts in the fields of precision apparatuses, office automation machines and automobiles.
Hitherto, polyoxymethylene resins have been widely used for office automation appliances as well as various mechanical parts as engineering plastics having well-balanced mechanical properties and excellent abrasion resistance.
However, the polyoxymethylene resins do not necessarily satisfy the requirements as sliding parts only by the abrasion resistance which they inherently possess, and especially under high load and high temperature, friction coefficient against noncrystalline resins increases, resulting in increase of abrasion wear.
The demand for polyoxymethylene resins to have a slidability with respect to noncrystalline resins has been brought about because the metals used in CD-ROM chassis, cartridge gearboxes of printers, facsimile chassis, floppy disk chassis, etc. are gradually substituted with noncrystalline resins and there occur problems in slidability between the noncrystalline resin parts and the polyoxymethylene resin parts (gear, cam, lever, etc.) which are used in contact with the noncrystalline resin parts.
If the sliding part between the polyoxymethylene resin and the noncrystalline resin is coated with greases as usually conducted when sliding is not smooth, the noncrystalline resin is deteriorated. Therefore, development of improved polyoxymethylene resin materials is desired without delay.
BACKGROUND ART
As methods for improving slidability of polyoxymethylene resins, the following are known, namely, a method of adding esters of 1,4-butanediol with fatty acids to polyoxymethylene resins (JP-B-55-23304), a method of adding borates of glycerin monofatty acid esters (JP-B-57-15616), a method of adding polyolefins having an active hydrogen, polyethylene and isocyanates (JP-A-4-126758 or U.S. Pat. No. 5,344,875), a method of adding copolymers of &agr;-olefins and ethylene-vinyl monomers (JP-A-49-40346), a method of adding powders of ultra-high-molecular-weight polyethylene (EP-A-148743), etc. However, according to these methods, the slidability at room temperature has been improved to some extent, but the problems of increase of friction coefficient against noncrystalline resins and increase in abrasion wear under high load and at high temperature have not been solved at all.
Furthermore, JP-A-2-66365 discloses use of a composition obtained by adding a thermoplastic elastomer and a lubricant to a polyoxymethylene resin for low-noise gears. However, this reference makes no mention of the specific lubricant as used in the present invention and the method disclosed in this reference cannot give the slidability with respect to non-crystalline resins under high load and high temperature as aimed at by the present invention. JP-A-4-36341 discloses a method of adding a thermoplastic elastomer and an adduct of alcohol with alkylene oxide to a block copolymer containing a polyacetal, but this reference makes no mention of the specific lubricant as used in the present invention and the method disclosed in this reference cannot give the slidability with respect to noncrystalline resins under high load and high temperature as aimed at by the present invention.
On the other hand, there are known a method of blending polyoxymethylene with a polyolefin (JP-B-42-19498), a method of adding a liquid ethylene-&agr;-olefin random copolymer (JP-A-4-239566), a method of adding a modified &agr;-olefin polymer (JP-A-59-204652), and a method of adding a polyether copolymer having an average molecular weight of at least 500 (JP-A-5-70678). However, these known compositions have made substantially no improvement of the slidability with respect to noncrystalline resins under high load and high temperature as aimed at by the present invention.
The object of the present invention is to provide a polyoxymethylene resin composition having low friction coefficient and excellent abrasion resistance against noncrystalline resins under high load and high temperature (60-100° C.), particularly in point-contact reciprocation sliding test.
DISCLOSURE OF THE INVENTION
As a result of intensive research conducted by the inventors to attain the above object, it has been found that a polyoxymethylene resin composition comprising the following components can attain the above object:
(A) 70-99.89% by weight of a polyoxymethylene resin,
(B) 0.1-20% by weight of at least one ethylene-substituted vinyl copolymer having a melting point of 40-80° C. and prepared by the polymerization using a radical polymerization catalyst, a chromium-containing catalyst or a Ziegler catalyst, and
(C) 0.01-10% by weight of at least one liquid ethylene-&agr;-olefin random copolymer.
Thus, the present invention has been accomplished.
The present invention will be explained in detail below.
BEST MODE FOR CARRYING OUT THE INVENTION
The polyoxymethylene resin used as the component (A) in the present invention includes, for example, homopolymers obtained by the polymerization of formaldehyde monomer or cyclic oligomers thereof such as trimer (trioxan) and tetramer (tetraoxan) and blocking both the terminals of the resulting polymer with ether or ester bonds, oxymethylene copolymers containing 0.1-20% by weight of an oxyalkylene unit having 2-8 carbon atoms which are obtained by the copolymerization of the above-mentioned trioxane or tetraoxan with a cyclic ether such as ethylene oxide, propylene oxide, 1,3-dioxolan, formal of glycol or formal of diglycol, the above oxymethylene copolymers further having branched molecular chain, and oxymethylene block copolymers containing 50% by weight or more of a segment comprising oxymethylene unit and less than 50% by weight of a different component segment.
As for the flowability of the polyoxymethylene resins used, they have a melt index (MI; measured under the conditions of ASTM D1238-57 E) of preferably 10-50 g/10 min, more preferably 20-40 g/10 min.
Preferred polyoxymethylene resins are polyoxymethylene homopolymers, both the terminals of which are blocked with acetyl groups, from the point of slidability at high temperatures.
More preferred polyoxymethylene resins are polyoxymethylene block copolymers, one terminal of which is blocked with a residue of an adduct of alcohol with alkylene oxide from the viewpoint of low friction coefficient at high temperatures.
Especially preferred polyoxymethylene resins are blends of a polyoxymethylene homopolymer, both the terminals of which are blocked with acetyl groups and a polyoxymethylene block copolymer, one terminal of which is blocked with a residue of an adduct of alcohol with alkylene oxide from the viewpoint of low friction coefficient at high temperatures.
The polyoxymethylene block copolymer, one terminal of which is blocked with a residue of an adduct of alcohol with alkylene oxide can be produced by the process disclosed in JP-B-2-24307.
Preferred alcohols which constitute the residue of adduct of alcohol with alkylene oxide are aliphatic alcohols of 12-22 carbon atoms and preferred alkylene oxides are propylene oxide and tetramethylene oxide. The mol number of the alkylene oxide added to the alcohol in the adduct is 1-1,000, preferably 1-50.
The preferred number-average molecular weight of the portion of the block copolymer excluding the terminal group is 10,000-500,000.
The preferred blending ratio of the polyoxymethylene homopolymer, both the terminals of which are blocked with acetyl groups and the polyoxymethylene block copolymer, one terminal of which is blocked with a residue of an adduct of alcohol with alkylene oxide is 5-90% by weight, preferably 50-80% by weight of the block copolymer in 100 parts by weight of the total polyoxymethy
Ibe Sadao
Niino Masahiko
Asahi Kasei Kogyo Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Short Patricia A.
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