Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-08-16
2002-04-02
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S251000
Reexamination Certificate
active
06365655
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for stabilizing an oxymethylene copolymer having unstable terminal groups (such an oxymethylene copolymer which is hereinafter, frequently referred to simply as a “crude oxymethylene copolymer”) and a stabilizer reagent for use in stabilizing such a crude oxymethylene copolymer. More particularly, the present invention is concerned with a method for stabilizing a crude oxymethylene copolymer, which comprises subjecting a crude oxymethylene copolymer to heat treatment in the presence of a specific quaternary ammonium compound, and is also concerned with a stabilizer reagent for use in stabilizing a crude oxymethylene copolymer by heat treatment, which mainly comprises the above-mentioned specific quaternary ammonium compound. In the method of the present invention, even by the addition of a very small amount of the quaternary ammonium compound, the decomposition rate of the unstable terminal groups of the crude oxymethylene copolymer can be remarkably increased, thereby facilitating the stabilization of the crude oxymethylene copolymer. Therefore, by the method of the present invention, it has become possible to easily produce an excellent oxymethylene copolymer within a short period of time, which has substantially no unstable terminal groups and, therefore, is excellent with respect to both thermal stability and color tone. Further, the stabilizer reagent of the present invention, when used in the above-mentioned heat treatment, can remarkably increase the decomposition rate of the unstable terminal groups of the crude oxymethylene copolymer, thereby facilitating the stabilization of the crude oxymethylene copolymer.
BACKGROUND OF THE INVENTION
Conventionally, oxymethylene copolymers have been widely used as materials for automobile parts, electronic parts, electrical parts, etc., since the oxymethylene copolymers have not only a good balance of mechanical properties, but also excellent fatigue resistance properties. The oxymethylene copolymer is produced by copolymerizing formaldehyde or a cyclic acetal (such as trioxane which is a cyclic trimer of formaldehyde) with a cyclic ether and/or a cyclic formal. However, the thus obtained oxymethylene copolymer as such cannot be put into practical use for the following reason. With respect to such an oxymethylene copolymer, some of the copolymer chains thereof have terminal —(OCH
2
)
n
—OH groups. The above-mentioned terminal groups are thermally unstable and, hence, easily decompose by, for example, heating at the time of the molding of the oxymethylene copolymer, thereby generating a large amount of formaldehyde. The generation of a large amount of formaldehyde is disadvantageous, since the oxymethylene copolymer is likely to suffer from foaming during the molding thereof and traces of escape of gaseous formaldehyde are left in the surface of the resultant shaped article to thereby cause the surface appearance to be poor. Further, the generated formaldehyde is oxidized by oxygen present in the molding machine to thereby form formic acid, and the formic acid by-product promotes the decomposition of the main chain of the oxymethylene copolymer.
Conventional methods for stabilizing an oxymethylene copolymer having thermally unstable terminal —(OCH
2
)
n
—OH groups (i.e., crude oxymethylene copolymer) include a method in which the terminal groups of the crude copolymer are acetylated, etherified or urethanized, and a method in which the unstable terminal groups of the crude copolymer are decomposed. Of these methods, the method in which the unstable terminal groups are decomposed is advantageously used for stabilizing the crude oxymethylene copolymer. Specific known methods for decomposing the unstable terminal groups of the crude oxymethylene copolymer include a method in which the crude oxymethylene copolymer is heated in water or an organic solvent (optionally, in the presence of a basic substance capable of decomposing the unstable terminal groups) (heat treatment method using water or an organic solvent), and a method in which the stabilization of a crude oxymethylene copolymer is conducted by heat-melting the copolymer (melt method). The heat treatment method using water or an organic solvent requires various steps, such as the steps of separation (filtration), recovery and washing of the treated copolymer, whereas, the melt method is commercially advantageous, since a stabilized copolymer can be directly obtained.
As an example of a conventionally known heat treatment method using water or an organic solvent, there is a method in which-a crude oxymethylene copolymer is subjected to heat treatment in a medium (for example, water, or a mixture of water and methanol) which is incapable of completely dissolving therein the crude oxymethylene copolymer (that is, the heat treatment of the copolymer is conducted in a heterogenous system) to thereby decompose the unstable terminal units of the copolymer {see, for example Examined Japanese Patent Application Publication No. 40-10435 and Examined Japanese Patent Application Publication No. 43-7553 (corresponding to U.S. Pat. No. 3,337,504)}However, in this method, the heat treatment needs to be conducted at a temperature near the melting point of the oxymethylene copolymer in order to increase the decomposition rate of the unstable terminal groups. In addition, the heat treatment needs to be conducted for a long period of time in order to decompose the unstable terminal groups as much as possible. In this method, even by the heat treatment under the above-mentioned stringent conditions, the decomposition of the unstable terminal groups is unsatisfactory. Further, this method also has a problem in that the obtained oxymethylene copolymer is likely to suffer discoloration due to the heat treatment conducted at a high temperature for a long period time. Examined Japanese Patent Application Publication No. 40-11627 discloses a method in which a crude oxymethylene copolymer is exposed to a saturated vapor mixture (formed from a volatile organic solvent, a volatile base and water) at 100° C. or more under atmospheric or higher pressure, thereby decomposing the unstable terminal groups of the copolymer. However, also in this method, the decomposition of unstable terminal groups is unsatisfactory.
With respect to the conventionally known melt methods in which a crude oxymethylene copolymer is heated to thereby melt the copolymer, and the unstable terminal groups of the copolymer are decomposed while maintaining the copolymer in a molten state. For example, Examined Japanese Patent Application Publication No. 39-8071 discloses a method in which a molten copolymer is kneaded using a roll mill for a predetermined period of time; Examined Japanese Patent Application Publication No. 43-1875 and Examined Japanese Patent Application Publication No.44-11907 (corresponding to U.S. Pat. No. 3,418,280) disclose a method in which a crude oxymethylene copolymer is heat-melted in an extruder in the presence of water and an alcohol, and optionally an alkaline substance; Examined Japanese Patent Application Publication No. 58-11450 (corresponding to U.S. Pat. No. 4,366,305) discloses a method in which the decomposition of the unstable terminal groups of a crude oxymethylene copolymer is conducted by treating the crude oxymethylene copolymer in a molten form under reduced pressure using a special surface-renewing mixer; Unexamined Japanese Patent Application Laid-Open Specification No. 58-152012 (corresponding to European Patent No. 0 088 541) discloses a method in which the stabilization of a crude oxymethylene copolymer is conducted using a system comprising a single screw extruder for melting a crude oxymethylene copolymer, a static mixer having a reaction zone for decomposing the unstable terminal groups of the crude copolymer by mixing, in accordance with the principle of flow division and rearrangement, the crude copolymer and a reactant comprising water and a compound capable of producing a hydroxide in the presence of water, and
Nagahara Hajime
Ohtsuki Satoru
Tanaka Tadashi
Tanimura Noritaka
Asahi Kasei Kogyo Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Cain Edward J.
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