Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-05-07
2001-04-03
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S275000, C524S377000, C524S378000, C524S384000, C524S394000, C524S400000
Reexamination Certificate
active
06211268
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to a polyoxymethylene resin composition for obtaining a molded product having extremely low shrink anisotropy when it is left for long time after molding or in a high-temperature atmosphere and excellent thermal stability.
2. Prior Art
A polyoxymethylene resin is used in a wide variety of application fields such as mechanical, electric and electronic, automobile, construction material and household goods fields as a typical engineering plastic due to its excellent mechanical properties, sliding properties, chemical resistance, fatigue resistance and the like.
It is known that the polyoxymethylene resin has poor thermal stability due to its molecular structure and readily decomposes due to the break of a main chain caused by depolymerization from the terminal of the polymer or a thermal oxidization decomposition reaction. Further, since formic acid formed by the oxidation reaction of formaldehyde produced by the decomposition promotes the thermal oxidation decomposition reaction of the polyoxymethylene resin, the thermal stability of the polyoxymethylene resin is greatly impaired with the result that the practical applicability of the resin is lost. Therefore, the addition of an amine-substituted triazine compound typified by melamine, so-called “formaldehyde scavenger”, is essential to the improvement of the thermal stability of the polyoxymethylene resin.
However, the addition of the amine-substituted triazine compound which is an essential ingredient for the improvement of the thermal stability of the polyoxymethylene resin increases the shrink anisotropy of a molded product, and a molded product of the polyoxymethylene resin has a large molding shrinkage factor because the polyoxymethylene resin has high crystallinity. Therefore, the application of the polyoxymethylene resin in precision parts which require high dimensional stability is limited in most cases and improvement on the shrink anisotropy of the resin has been desired.
As one of the methods for improving the shrink anisotropy of a molded product of this polyoxymethylene resin, attempts are being made to add an inorganic filler such as talc. This method can reduce shrink anisotropy to a certain degree but it involves such a problem that the characteristic properties of the polyoxymethylene resin are impaired, that is, physical properties such as impact resistance deteriorate.
Meanwhile, a technology for blending various resins to improve shrink anisotropy has been proposed as another method for improving shrink anisotropy. For example, JP-A 4-108848 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes polyoxymethylene homopolymer and copolymer compositions comprising different polyoxymethylenes, which involves such molding problems as a reduction in thermal stability and the difficulty of uniform plasticization. JP-A 64-38463 proposes a composition comprising a specific high-viscosity polystyrene resin, JP-A 4-214756 proposes a composition comprising a polystyrene-based resin and acryl-based resin, JP-A 6-248163 proposes a composition comprising a polycarbonate-based resin, a phenol-based polymer compound and a filler, JP-A 6-299046 proposes a composition comprising a styrene-based resin, a phenol-based polymer compound and a filler, and JP-A 7-292187 proposes a composition comprising a polystyrene-based resin having a hydroxyl group, a copolymer of a polyacrylic acid ester and styrene, and a polyfunctional isocyanate. However, all of these compositions have such a defect that the characteristic features of the polyoxymethylene resin are greatly impaired as exemplified by reductions in physical properties, deterioration in the surface state of a molded product, a reduction in thermal stability and the like caused by the occurrence of a delamination or layer separation phenomenon, a rise in viscosity and poor dispersibility.
In view of the above situation, it is an object of the present invention to provide a polyoxymethylene resin composition which can give a molded product having extremely low shrink anisotropy when it is left for a long time after molding or in a high-temperature atmosphere and excellent thermal stability without impairing the characteristic properties of a polyoxymethylene resin. It is another object of the present invention to provide a polyoxymethylene resin composition which can give a molded product required to have high dimensional stability, such as a precision part.
JP-B 37-8816 (the term “JP-B” as used herein means an “examined Japanese patent publication”) discloses a method for improving the flowability of a polyoxymethylene resin at the time of molding by adding polyethylene glycol and JP-A 56-163144 discloses a method for improving the hot water resistance of a polyoxymethylene resin by adding polyethylene glycol. Surprisingly, the inventors of the present invention have found that a molded product having extremely low shrink anisotropy when it is left for a long time or in a high-temperature atmosphere is obtained by selecting polyethylene glycol having a molecular weight larger than a specific value and adding it to a polyoxymethylene copolymer together with an amine-substituted triazine compound. The present invention has been accomplished based on this finding.
Meanwhile, JP-A 59-51937 and JP-A 60-86155 disclose a method for improving the dispersibility of carbon black by adding polyolefin wax to a polyoxymethylene resin. Further, JP-A 3-70764 and JP-A 4-224856 disclose a method for improving the abrasion resistance of a polyoxymethylene resin by adding polyolefin wax. Further, JP-A 8-3236 discloses a method for improving the self-lubrication of a polyoxymethylene resin by adding polyolefin wax. Surprisingly, the present inventors have found that a molded product having extremely low shrink anisotropy when it is left for a long time or in a high-temperature atmosphere is obtained by selecting polyolefin wax having an acid value higher than a specific value and adding it to a polyoxymethylene copolymer together with an amine-substituted triazine compound. The present invention has been accomplished based on this finding.
That is, the present invention is a polyoxymethylene resin composition which substantially comprises (A) 100 parts by weight of a polyoxymethylene copolymer, (B) 0.01 to 7 parts by weight of an amine-substituted triazine compound, and (C) 0.01 to 5 parts by weight of (C-1) polyethylene glycol having an average molecular weight of 10,000 or more and/or (C-2) modified polyolefin wax having an acidic group with an acid value of 0.5 to 60 mg-KOH/g.
The present invention will be described in detail hereinunder.
The polyoxymethylene copolymer (A) used in the present invention is generally a copolymer containing 0.4 to 40 mol %, preferably 0.4 to 10 mol % of oxyalkylene units in the main chain of oxymethylene. The copolymer is obtained by polymerizing formaldehyde and/or a cyclic oligomer thereof (for example, trioxan or tetraoxan) as a main monomer/s and a cyclic ether as a copolymerizable component in the presence of a polymerization catalyst.
The cyclic ether used as a copolymerizable component is preferably a compound represented by the following general formula (1).
wherein R
1
, R
2
, R
3
and R
4
are the same or different and each a hydrogen atom or alkyl group having 1 to 5 carbon atoms, and R
5
is a methylene group, oxymethylene group, or methylene group or oxymethylene group substituted by an alkyl group (n is an integer of 0 to 3), or a divalent group represented by the following general formula (2) or (3) (n is 1, and m is an integer of 1 to 4).
—(CH
2
)
m
—O—CH
2
— (2)
—(O—CH
2
—CH
2
)
m
—O—CH
2
— (3)
Specific examples of the cyclic ether include ethylene oxide, propylene oxide, 1,3-dioxolan, 1,3-dioxepan, 1,3,5-trioxepan, 1,3,6-trioxocan. Out of these, 1,3-dioxolan is particularly preferred from the view point of the thermal stability of the obtained resin composition.
The polymerization cat
Kurashige Kazuo
Matsumura Takatoshi
Mimura Hiroshi
Nishizawa Chiharu
Yada Hiroshi
Mitsubishi Gas Chemical Co Inc
Short Patricia A.
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