Polyoxymethylene copolymer and molded article thereof

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From aldehyde or derivative thereof as reactant

Reexamination Certificate

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C528S233000, C528S250000, C528S425000

Reexamination Certificate

active

06617416

ABSTRACT:

DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to a polyoxymethylene copolymer that is excellent in heat stability, excellent in bending fatigue resistance and remarkably free of deposition on a mold during molding thereof, and a molded article thereof.
2. Prior Art
A polyoxymethylene copolymer is excellent in mechanical and thermal performances and is widely used as a typical engineering plastic in the fields of machinery, electric/electronic appliances and automobiles.
Generally, a polyoxymethylene copolymer is obtained by copolymerizing formaldehyde or its cyclic oligomer such as trioxane or tetraoxane with a comonomer copolymerizable therewith. However, it is known that the polymer easily undergoes decomposition at its terminal.
For example, it is described in U.S. Pat. Nos. 25 4,681,927, 4,814,424, 5,688,897 and 4,751,272 that trioxane and 1 to 5 mol %, based on the trioxane, of a comonomer such as 1,3-dioxolane are copolymerized to obtain a polyoxymethylene copolymer.
For obtaining a stabilized polyoxymethylene copolymer, therefore, it is conventional practice to treat terminal molecules of the polyoxymethylene copolymer in various ways and add thereto additives such as an antioxidant, a heat stabilizer, and the like.
For example, for improving a polyoxymethylene copolymer in heat stability, it is disclosed, for example, in Japanese Patent Publication No. 40-21148 that a triazine derivative typified by melamine, a so-called formaldehyde capturer, is added.
However, the triazine derivative is an additive effective for improving a polyoxymethylene copolymer in heat stability on one hand, but is poor in compatibility with a polyoxymethylene copolymer on the other hand. When an oxymethylene copolymer containing a large amount of the triazine derivative is continuously molded, therefore, there is caused a problem that the triazine derivative adheres to a mold (degradation in mold deposit resistance).
Further, JP-A-8-59767 discloses that an oxymethylene copolymer produced from trioxane and 3 to 7 mol %, based on the trioxane, of 1,3-dioxolane by a conventional method has fewer portions that cause heat unstability than a copolymer obtained using ethylene oxide as a comonomer. However, the amount of 1,3-dioxolane in the above production method is insufficient for producing an effect on improvement of heat stability.
Meanwhile, the polymerization yield in prior art is improved by a method in which the amount of a catalyst is increased. However, it is known that the mere increase in the catalyst amount undesirably promotes the formation of unstable portions.
PROBLEMS TO BE SOLVED BY THE INVENTION
It is a first object of the present invention to provide a polyoxymethylene copolymer having excellent heat stability and excellent properties against bending fatigue (to be referred to as “bending durability” hereinafter).
It is a second object of the present invention to provide a polyoxymethylene copolymer which forms remarkably few deposits adhering to a mold during molding, that is, which is excellent in moldability.
It is a third object of the present invention to provide a polyoxymethylene copolymer which accomplishes a high polymerization yield and attains a low weight loss under heat.
It is another object of the present invention to provide a polyoxymethylene copolymer molded article having excellent properties against bending fatigue.
It is still another object of the present invention to provide a method for producing a polyoxymethylene copolymer having the above-described excellent properties.
MEANS TO SOLVE THE PROBLEMS
According to studies made by the present inventors, it has been found that the above objects of the present invention are achieved by providing a polyoxymethylene copolymer which is a polyoxymethylene copolymer obtained by copolymerizing trioxane and 8 to 20 mol, per 100 mol of the trioxane, of 1,3-dioxolane and which (i) has a crystallization time period of 10 to 2,000 seconds at 143° C., (ii) withstands 30 to 1,000 cycles of a bending durability test and (iii) has residence heat stability lasting for 40 minutes or more.
According to the present invention, there can be obtained a polyoxymethylene copolymer excellent in bending durability and also excellent in heat stability. According to the present invention, further, there can be obtained a polyoxymethylene copolymer which remarkably causes few deposits adhering to a mold during molding and is therefore excellent in molding productivity.
The polyoxymethylene copolymer of the present invention will be explained further in detail hereinafter.
The polyoxymethylene copolymer of the present invention is obtained from trioxane as a main monomer and 8 to 20 mol, per 100 mol of the trioxane, of 1,3-dioxolane as a comonomer. In this case, a cation-active catalyst is used as a catalyst.
The polymerization method of the polyoxymethylene copolymer includes a bulk polymerization method and a melt polymerization method. As a polymerization method, for example, preferred is a bulk polymerization method using substantially no solvent or a quasi-bulk polymerization method using 20% by weight or less, based on the monomers, of a solvent. These bulk polymerization method is a method in which monomers in a molten state are polymerized and a solid polymer in a bulk or powdered state is obtained as the polymerization proceeds.
The monomer as a raw material is trioxane that is a cyclic trimer of formaldehyde, and as a comonomer, 1,3-dioxolane is used. The amount of 1,3-dioxolane per 100 mol of trioxane is in the range of from 8 to 20 mol, preferably in the range of from 8.5 to 18 mol, particularly preferably in the range of from 9 to 15 mol. When the amount of 1,3-dioxolane is greater than the above upper limit, the polymerization yield is low. When it is smaller than the above lower limit, the heat stability is low.
The polymerization catalyst is selected from cation-active catalysts. These cation-active catalysts include Lewis acids typified by halides of boron, tin, titanium, phosphorus, arsenic and antimony, specifically, compounds such as boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride, antimony pentafluoride and complexes or salts of these; protonic acids such as trifluoromethanesulfonic acid, perchloric acid, esters of protonic acids typified by an ester of perchloric acid and a lower aliphatic alcohol and anhydrides of protonic acids typified by mixed anhydrides of perchloric acid and a lower aliphatic carboxylic acid; triethyloxonium hexafluorophosphate, triphenylmethylhexafluoroarsenate, acetylhexafluoroborate, heteropolyacid or acidic salt thereof, and isopolyacid or acidic salt thereof. Of these, a compound containing boron trifluoride, a hydrate of boron trifluoride or a coordination complex compound thereof is particularly suitable, and boron trifluoride diethyl etherate and boron trifluoride dibutyl etherate that are coordination complexes with ethers are particularly preferred.
The amount of the above catalyst per mole of trioxane is generally 1×10
−7
to 1×10
−3
mol, preferably 1×10
−7
to 1×10
−4
mol. When the amount of the catalyst is greater than the above upper limit, the heat stability is low, and when it is smaller than the above lower limit, the polymerization yield is low.
For adjusting the molecular weight of the polyoxymethylene copolymer, the above polymerization method may use a proper molecular weight adjusting agent as required. The molecular weight adjusting agent includes a carboxylic acid, a carboxylic acid anhydride, an ester, an amide, an imide, phenols and an acetal compound. Phenol, 2,6-dimethylphenol, methylal and polyoxymethylene dimethoxide are particularly preferred, and methylal is the most preferred. The molecular weight adjusting agent is used alone or in the form of a solution. When it is used in the form of a solution, the solvent therefor includes aliphatic hydrocarbons such as hexane, heptane and cy

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