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
1997-02-12
2002-06-04
Hoke, Veronica P. (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...
Reexamination Certificate
active
06399699
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a polyacetal resin composition excellent in long-term thermal aging resistance, more particularly to a polyoxymethylene composition which has characteristics of polyoxymethylene such as mechanical properties, moldability, thermal stability and the like and is excellent in long-term thermal aging resistance under high-temperature circumstances.
2. Description of the Related Art
Polyacetal resins are known as engineering resins excellent in mechanical properties, rigidity, creep characteristics, solvent resistance and the like, and used in a wide field including, for example, mechanical moving parts and the like used in automobile, electrical appliances and the like. As the uses of resins are widened and diversified, the demand for quality has become severe. In particular, in the use thereof under high-temperature circumstances, long-term thermal aging resistance is required. However, no conventional polyacetal resins have aging resistance on a satisfactory level, and the use thereof is limited at present.
As a conventional method for preventing the polyacetal resin from being deteriorated under high-temperature circumstances, there has been known a method in which an additive such as a thermal stabilizer, an antioxidant or the like is compounded. For example, JP-B-62-58,387 and U.S. Pat. No. 4,342,680 propose improving the thermal stability with an additive composed of a ternary combination of an amine-substituted triazine, a sterically hindered phenol and a metal-containing compound. JP-A-2-209,944 proposes improving the long-term thermal resistance by the co-use of three kinds of antioxidants with a calcium salt of a fatty acid. Also, JP-B-55-22,508 and U.S. Pat. No. 3,743,614 disclose improving the stability against oxidation and thermal decomposition by use of a hindered phenolic compound together with an alkaline earth metal salt of a carboxylic acid having 10 to 20 carbon atoms and/or an alkaline earth metal hydroxide. Furthermore, JP-B-60-56,748 and UK-A-1,425,771 propose improving the stability against heat and oxygen by use of a hindered phenolic compound and an alkaline earth metal salt of an aliphatic carboxylic acid having 22 to 36 carbon atoms.
Though the long-term thermal aging resistance of a polyacetal resin can be improved by compounding therein an additive, the improvement is still insufficient. The use of such polyacetal resins over a long period of time under high temperature circumstance results in the reduction of mechanical properties and ultimately leads to degradation of the polyacetal resin.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the disadvantages of conventional polyacetal resins, to provide a polyacetal resin composition excellent in long-term thermal aging resistance, and to provide methods for making the same.
The present inventors have examined the long-term thermal aging resistance of polyacetal resins and have consequently found that a polyacetal resin composition comprising a particular polyacetal, an antioxidant and a basic substance in specific amounts achieves the above-mentioned object, based on which this invention has been completed.
According to this invention, there is provided a polyacetal resin composition which comprises 100 parts by weight of a polyacetal such that the residual fluorine concentration is not more than about 13 ppm and that when the resin is heated at about 230° C. for about 30 minutes in nitrogen the concentration of formaldehyde generated is not more than about 500 ppm, from about 0.01 to about 3 parts by weight of an antioxidant and from about 0.001 to about 5 parts by weight of a basic substance.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The polyacetal used in the resin composition of this invention is produced by, for example, the following method:
First of all, the main monomer used in the production of the polyacetal is trioxane which is a cyclic oligomer of formaldehyde and the comonomer used therewith is a cyclic ether represented by the following general formula (I):
wherein each of R
1
to R
4
independently represents a hydrogen atom, a C
1
-C
5
alkyl group which is unsubstituted or substituted with 1 to 3 halogen atoms, and each R
5
independently represents a methylene or oxymethylene group, which is unsubstituted or substituted with 1 or 2 C
1
-C
5
alkyl groups or 1 or 2 halogen atoms (in this case, p represents an integer of 0 to 3), or each R
5
independently represents a divalent group represented by the following formula (II) or (III):
—(CH
2
)
q
—O—CH
2
— (II)
—(OCH
2
CH
2
)
q
—O—CH
2
— (III)
in which p in the formula (I) is 1 and q in the formula (II) or (III) represents an integer of 1 to 4.
Typical examples of other comonomers include, for example, ethylene oxide, propylene oxide, 1,3-dioxorane, 1,3,5-trioxepane, 1,4-butanediolformal, epichlorohydrin diglycolformal and the like. The concentration of the comonomer is not critical; however, it is usually not less than 0.0005 mole but not more than 0.15 mole per mole of the trioxane.
As the polyacetal in this invention, there can be used either a homopolymer obtained by homopolymerizing the above trioxane or a copolymer obtained by polymerizing a mixture of the above trioxane and the above comonomer.
A suitable polymerization catalyst which can be used in the production of the polyacetal of the present invention includes, but is not limited to, at least one of boron trifluoride, boron trifluoride hydrate, a coordination complex compound of boron trifluoride with an organic compound containing an oxygen atom or a sulfur atom (e.g. an ether such as diethyl ether, di-n-butyl ether, a thioether such as ethylthioether). The polymerization catalyst is preferably a coordination complex compound of boron trifluoride with the above organic compound, and the coordination complex compound includes, but is not limited to, boron trifluoride diethyl ether, boron trifluoride dibutyl ether or mixtures thereof.
During the polyacetal production using the above polymerization catalyst, residual fluorine is accumulated. An important point of this invention is to use a polyacetal having a residual fluorine concentration of not more than about 13 ppm. It is preferable to use a polyacetal having a residual fluorine concentration of not more than about 8 ppm. When the residual fluorine concentration of the polyacetal exceeds about 13 ppm, the use of a polyacetal resin composition under high-temperature circumstances for a long period of time results in reduction of the long-term thermal aging resistance of the resin composition.
As a method for producing a polyacetal having a low residual fluorine concentration, it is effective to control the polymerization catalyst concentration to not more than a certain value during the polymerization. Specifically, it is preferable to control the polymerization catalyst concentration to not more than about 3.0×10
−5
mole per mole of trioxane or, if a comonomer is used, per mole of a total of trioxane and the comonomer(s). In particular, in order to obtain a polyacetal having a residual fluorine concentration of not more than about 8 ppm, it is preferable to control the polymerization catalyst concentration to not more than 1.5×10
−5
mole per mole of trioxane or, if a comonomer is used, per mole of a total of trioxane and the comonomer(s). When the polymerization catalyst concentration is high during the polymerization and the residual fluorine concentration of the polyacetal produced by the polymerization is more than about 13 ppm, it is possible to wash the polyacetal with a solvent to remove the polymerization catalyst therefrom to reduce the residual fluorine concentration of the polyacetal to not more than about 13 ppm. Specifically, washing methods, include but are not limited to, washing, with hot water, steam or a mixture of water and an organic solvent at a high temperature, a polyacetal obtained by deactivating the polymerization cataly
Hata Tadashige
Tanigawa Yukio
Asahi Kasei Kogyo Kabushiki Kaisha
Hoke Veronica P.
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