JP7833320B2 - Polyoxymethylene resin composition - Google Patents

Description

This invention relates to a polyoxymethylene resin composition.

Polyoxymethylene resin compositions are engineering plastics used in a wide range of fields, including electrical and electronic materials, automotive, and various industrial materials, due to their excellent mechanical properties, moldability, and sliding properties. They are particularly often used in automotive fuel-related components, such as fuel pump modules, valves, gasoline tanks, and gasoline tank flanges, where organic solvent gas permeability is considered important.

Polyoxymethylene resin compositions are generally known for their high crystallinity and resistance to organic solvent gases. However, with the increasing use of resin components due to automobile weight reduction, further improvements in gas permeability are desired. Furthermore, gas permeability has been studied in the past, and for example, methods for producing polyoxymethylene copolymers obtained by copolymerizing 1,3-dioxolane containing 200 ppm or less of 1,4-dioxane in a range of 0.1 to 2.0% by weight relative to trioxane, and technologies for molded articles produced from polyoxymethylene copolymers having a melting point of 167°C to 173°C, and in which the low molecular weight polyoxymethylene copolymer extracted with chloroform contained in the polyoxymethylene copolymer is 5000 ppm or less (see, for example, Patent Documents 1 and 2).

Patent No. 5371897 Japanese Patent Publication No. 2001-11196

However, the technologies disclosed in Patent Documents 1 and 2 concern polyoxymethylene copolymers and cannot be applied to polyoxymethylene homopolymers with higher crystallinity and synthesis. Furthermore, even with polyoxymethylene copolymers, there is a need for further improvement in gas permeability resistance.

Therefore, the present invention aims to provide a polyoxymethylene resin composition with excellent gas permeability resistance.

The inventors of this invention, after diligently studying the above-mentioned problems, discovered that the above objectives can be achieved by using a polyoxymethylene polymer having acetylated polymer ends and a tungsten compound, and by setting the mass ratio of tungsten within a specific range. This led to the completion of the present invention.

In other words, the present invention is as follows:
[1]
(a) a polyoxymethylene polymer and (b) a tungsten compound,
(a) At least a portion of the polymer ends of the polyoxymethylene polymer are acetylated,
The (b) tungsten compound is contained in an amount of 0.01 to 100 ppm by mass of tungsten element relative to the total mass of the polyoxymethylene resin composition.
A polyoxymethylene resin composition characterized by the following features.
[2]
The polyoxymethylene resin composition according to [1], wherein the (b) tungsten compound is at least one selected from the group consisting of phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicic tungstic acid, silicic molybdotungstovanadic acid, and acidic salts thereof.
[3]
The polyoxymethylene resin composition according to [1] or [2], wherein the (a) polyoxymethylene polymer comprises 50% by mass or more of a polyoxymethylene homopolymer.
[4]
The polyoxymethylene resin composition according to any one of [1] to [3], wherein the content of acetylated polymer ends is 1.0 × ^10⁻⁴ (units) or more per unit of oxymethylene (-O- _CH₂- ), which is the main chain repeating unit of the polyoxymethylene polymer.

The polyoxymethylene resin composition of the present invention exhibits excellent gas permeability resistance.

The embodiments for carrying out the present invention (hereinafter referred to as "this embodiment") will be described in detail below. The present invention is not limited to the following embodiments, and can be implemented in various modifications within the scope of its gist.

The polyoxymethylene resin composition of this embodiment comprises at least (a) a polyoxymethylene polymer and (b) a tungsten compound, wherein (a) is a polymer in which at least a portion of the polymer ends are acetylated, and (b) is included in a proportion such that the mass of tungsten element is 0.01 to 100 ppm by mass in 100% by mass of the polyoxymethylene resin composition.
The mass percentage of (a) polyoxymethylene polymer in 100% by mass of the polyoxymethylene resin composition of this embodiment is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and may also be less than 100% by mass, and may be 99% by mass or less.
The polyoxymethylene resin composition of this embodiment may consist only of (a) a polyoxymethylene polymer and (b) a tungsten compound, or it may contain other components other than (a) a polyoxymethylene polymer and (b) a tungsten compound (for example, additives described later).
In this specification, (a) polyoxymethylene polymers may be simply referred to as "(a)". Also, (b) tungsten compounds may be simply referred to as "(b)".

<(a) Polyoxymethylene polymer>
Polyoxymethylene polymers are a general term for polymers having an oxymethylene ( _-CH₂O- ) structure as their unit structure. Polyoxymethylene polymers can be broadly divided into two types: homopolymers, in which the polymer main chain consists only of oxymethylene units, and copolymers, in which the polymer main chain contains arbitrary units (for example, oxyalkylene units) in addition to oxymethylene units. The above polyoxymethylene polymers are also called polyacetals, acetal resins, or polyacetal resins. Examples of the above polyoxymethylene polymers include polyoxymethylene homopolymers (hereinafter simply referred to as homopolymers) and polyoxymethylene copolymers (hereinafter simply referred to as copolymers), and known types may also be used.

Examples of raw materials for homopolymers include formaldehyde and cyclic oligomers of formaldehyde (trioxane, tetraoxane). Homopolymerization of these raw materials is a typical method for producing homopolymers.

As raw materials for copolymers, in addition to the homopolymer raw materials mentioned above, examples include ethylene oxide, propylene oxide, epichlorohydrin, 1,3-dioxolane, and cyclic formal glycols or diglycols such as 1,4-butanediol formal. Copolymers can be obtained by copolymerizing these. Furthermore, branched polyoxymethylene copolymers obtained by copolymerizing a formaldehyde monomer and/or a cyclic oligomer of formaldehyde with a monofunctional glycidyl ether, and crosslinked polyoxymethylene copolymers obtained by copolymerizing with a polyfunctional glycidyl ether can also be used as copolymers.

Furthermore, the polyoxymethylene polymer may be a polyoxymethylene homopolymer having a block component obtained by polymerizing a formaldehyde monomer or a cyclic oligomer of formaldehyde in the presence of a compound having a functional group such as a hydroxyl group at both or one of its ends, such as a polyalkylene glycol.

Similarly, the polyoxymethylene polymer may be a polyoxymethylene copolymer having a block component obtained by copolymerizing a cyclic oligomer of formaldehyde, such as a monomer of formaldehyde or its trimer (trioxane) and tetramer (tetraoxane), with a cyclic ether or cyclic formal in the presence of a compound having a functional group such as a hydroxyl group at both or one of its ends, for example, hydrogenated polybutadiene glycol.

Furthermore, these polyoxymethylene polymers may be used individually or in combination of two or more types.
When combining two or more polyoxymethylene polymers, the polyoxymethylene polymer is preferably one that contains 50% by mass or more of polyoxymethylene homopolymer from the viewpoint of rigidity and gas permeability, more preferably one that contains 80% by mass or more, even more preferably one that contains 95% by mass or more, and especially preferably one in which substantially all (at least 99% by mass or more) is homopolymer. The percentages here are based on the total amount of polyoxymethylene polymer being 100% by mass.

Polyoxymethylene can be obtained by methods such as anionic polymerization and cationic polymerization. Specifically, polyoxymethylene homopolymers can be obtained by anionic polymerization or cationic polymerization. Polyoxymethylene copolymers can be obtained by cationic polymerization. Details of each polymerization method will be described later.

<Tungsten Compounds>
The polyoxymethylene resin composition of this embodiment contains a tungsten compound.
The tungsten compound is preferably at least one tungsten compound selected from the group consisting of heteropoly acids, isopoly acids, and their acidic salts. Specifically, it is preferably at least one selected from the group consisting of phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicic acid, silicic molybdotungstovanadic acid, and their acidic salts. Among these, phosphotungstic acid is particularly preferred.

In this embodiment, the mass ratio of tungsten element to 100% by mass of the polyoxymethylene resin composition is preferably 0.01 to 100 ppm by mass, more preferably 0.1 to 50 ppm by mass, and even more preferably 1 to 10 ppm by mass. This range provides an excellent balance between suppression of decomposition gases during melting and resistance to gas permeability.

Tungsten compounds may be added during end-stabilization and pelletizing of polyoxymethylene polymers, or they may be included in the polyoxymethylene resin composition as a residue of the polymerization catalyst.

<Amount of acetyl ring>
The above polyoxymethylene polymer (preferably a polyoxymethylene homopolymer) preferably has acetyl functional groups at at least some of its polymer ends (preferably both ends or one end). Since the end groups of the crude polyoxymethylene polymer are thermally unstable, for practical use, the crude polyoxymethylene polymer includes a step of adding a specific organic acid anhydride to the crude polyoxymethylene polymer to perform a stabilization reaction at least some of the polymer ends (end acetylation step). In this embodiment, the organic acid anhydride used in the polymer end stabilization reaction is not particularly limited, but examples include carboxylic acid anhydrides such as benzoic anhydride, succinic anhydride, maleic anhydride, glutaric anhydride, phthalic anhydride, propionic anhydride, and acetic anhydride, with acetic anhydride being preferred. These organic acid anhydrides may be used individually or in combination of two or more.
In this specification, the polyoxymethylene polymer before acetylation may be referred to as the crude polyoxymethylene polymer.

The content of acetylated polymer ends in the above polyoxymethylene polymer is preferably 1.0 × 10⁻⁴ units or more, and more preferably 5.0 × ^10⁻⁴ units or more, per unit of the [ _-OCH₂- ], which is the main chain repeating unit of the polyoxymethylene polymer. There is no need to limit the upper limit, but in practice it is preferably 20.0 × ^10⁻⁴ (units) or less, and preferably 10.0 × ^10⁻⁴ (units) or less. Good gas permeability resistance can be obtained by staying within ^the above range.

<Anionic polymerization of polyoxymethylene homopolymers>
An example of a method for obtaining polyoxymethylene homopolymers by anionic polymerization is described below.

For example, polymerization can be carried out by a slurry method using purified formaldehyde gas. It can be produced by feeding formaldehyde (the monomer), a chain transfer agent (molecular weight modifier), and a polymerization catalyst into a polymerization reactor containing a hydrocarbon polymerization solvent, and polymerizing them by slurry polymerization. Furthermore, it is preferable to use formaldehyde gas that contains as few impurities as possible that cause polymerization termination and chain transfer during the polymerization reaction, such as water, methanol, and formic acid. If these impurities are present in excess, unexpected chain transfer reactions will occur, preventing the acquisition of the desired molecular weight product. In particular, the amount of water is preferably 100 ppm by mass or less, and even more preferably 50 ppm by mass or less, relative to 100% by mass of formaldehyde gas. The polymerization method is not limited to the above, and polymerization can also be carried out by known methods.

The molecular weight of polyoxymethylene homopolymers can be adjusted by chain transfer using molecular weight modifiers such as carboxylic acid anhydride or other carboxylic acids. Propionic anhydride and acetic anhydride are particularly preferred as molecular weight modifiers, with acetic anhydride being more preferred.

The amount of molecular weight modifier introduced may be adjusted and determined according to the properties of the target polyoxymethylene homopolymer (particularly the melt flow rate). For example, it is preferable that the melt flow rate (MFR value (according to ISO 1133)) of the polyoxymethylene homopolymer be in the range of 0.1 to 100 g/10 min, and more preferably in the range of 1.0 to 70 g/10 min. By setting the MFR value of the polyoxymethylene homopolymer within the above range, a polyoxymethylene homopolymer with excellent mechanical strength can be obtained.

As the polymerization catalyst, anionic polymerization catalysts are preferred, and onium salt polymerization catalysts represented by the following general formula (I) are more preferred.
[R ₁ R ₂ R ₃ R ₄ M] ⁺ X ^- (I)
(In formula (I), _R1 , _R2 , _R3 , and _R4 each independently represent an alkyl group, M represents an element with a lone pair of electrons, and X represents a nucleophilic group.)
Polymerization catalysts may be used individually or in combination of two or more types.

Among onium salt polymerization catalysts, quaternary phosphonium salt compounds such as tetraethylphosphonium iodide and tributylethylphosphonium iodide, and quaternary ammonium salt compounds such as tetramethylammonium bromide and dimethyldistearylammonium acetate are preferred.

The amount of onium salt polymerization catalysts, such as quaternary phosphonium salt compounds and quaternary ammonium salt compounds, added is preferably 0.00001 to 0.01 mol per 1 mol of formaldehyde, more preferably 0.00003 to 0.005 mol, and even more preferably 0.00005 to 0.003 mol.

The hydrocarbon polymerization solvent can be any solvent that does not react with formaldehyde, and is not particularly limited. Examples include pentane, isopentane, hexane, cyclohexane, heptane, octane, nonane, decane, and benzene, with hexane being particularly preferred. These hydrocarbon solvents may be used individually or in mixtures of two or more.

When obtaining polyoxymethylene homopolymers by anionic polymerization, tungsten compounds can be added during subsequent processes such as end stabilization and pelletizing to achieve good gas permeability resistance.

<Cational polymerization of polyoxymethylene homopolymers>
An example of a method for obtaining polyoxymethylene homopolymers by cationic polymerization is described below.

For example, polymerization can be carried out using a bulk method with trioxane or cyclic tetramers, which are cyclic trimers of formaldehyde. While there are no particular limitations on the shape (structure) of the polymerization reactor used, generally, a twin-screw paddle or screw-type agitated mixing polymerization reactor capable of passing a heat transfer medium through the jacket is preferable.

As for polymerization methods, for example, a method in which trioxane, a cationic catalyst, and an optional chain transfer agent are supplied to a polymerization reactor and polymerization is carried out. The polymerization reaction temperature is preferably maintained in the range of 63 to 135°C, more preferably in the range of 70 to 120°C, and even more preferably in the range of 70 to 100°C. The residence (reaction) time in the polymerization reactor is preferably 0.1 to 30 minutes, more preferably 0.1 to 25 minutes, and even more preferably 0.1 to 20 minutes. By adjusting the polymerization reaction temperature and residence time in the polymerization reactor to the above ranges, the thermal decomposition of the polyoxymethylene polymer can be more effectively suppressed, and a more thermally stable polyoxymethylene polymer can be produced.

Preferred cationic catalysts include Lewis acids, protic acids and their esters or anhydrides. Examples of Lewis acids, though not limited to those listed below, include halides of boric acid, tin, titanium, phosphorus, arsenic, and antimony. More specifically, examples include boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentafluoride, phosphorus pentachloride, antimony pentafluoride, and their complex compounds or salts.
Furthermore, examples of protonic acids and their esters or anhydrides include, but are not limited to, perchloroic acid, trifluoromethanesulfonic acid, tertiary butyl perchloroate, acetyl perchloroate, trimethyloxonium hexafluorophosphate, heteropoly acids, isopoly acids, acidic salts of heteropoly acids, and acidic salts of isopoly acids.
The amount of cationic catalyst introduced is 1 × 10⁻⁹ mol to 5 × 10⁻² mol per mol of formaldehyde (for example, the cyclic trimer or cyclic tetramer of formaldehyde described above), preferably 2 × 10⁻⁹ mol to 1 × 10⁻² mol, and more preferably 5 × 10⁻⁹ mol to 1 × 10⁻³ mol. Using a cationic catalyst within the above range tends to result in superior polymerization rates and a greater suppression of thermal decomposition of the polyoxymethylene polymer by the catalyst remaining in the polyoxymethylene polymer.
Furthermore, it is desirable to dilute the above-mentioned cationic catalyst with an inert diluent solvent that does not adversely affect the polymerization reaction. By diluting the cationic catalyst, the polymerization reaction can be carried out more uniformly, and it tends to be possible to produce polyoxymethylene polymers with less variation in physical properties.

When obtaining polyoxymethylene homopolymers by cationic polymerization, tungsten compounds (preferably the tungsten compounds described above) may be used as polymerization catalysts. Furthermore, good gas permeability resistance can be obtained by adding them during subsequent processes such as end stabilization and pelletizing.

When polymerization is carried out using a cationic catalyst, a low molecular weight acetal represented by the following general formula can also be used as a chain transfer agent.
R-(CH ₂ -O) _n -R
(In the formula, R represents one selected from the group consisting of hydrogen, branched or linear alkyl groups, branched or linear alkoxy groups, and hydroxyl groups. n represents an integer between 1 and 20.) In particular, by using acetals with a molecular weight of 200 or less, preferably 60 to 170, the molecular weight of the final polyoxymethylene polymer can be well adjusted. The low molecular weight acetals represented by the above general formula are not limited to the following, but examples include methylal, methoxymethylal, dimethoxymethylal, trimethoxymethylal, etc. These may be used individually or in combination of two or more.
From the viewpoint of controlling the molecular weight of the target polyoxymethylene polymer to a suitable range, the amount of low molecular weight acetal represented by the general formula added is preferably in the range of 0.1 × 10⁻⁵ to 0.2 × 10⁻² mol, more preferably in the range of 0.1 × 10⁻⁵ to 0.2 × 10⁻³ mol, and even more preferably in the range of 0.1 × 10⁻⁵ to 0.1 × 10⁻³ mol, per 1 mol of formaldehyde (for example, the cyclic trimer or cyclic tetramer of formaldehyde mentioned above).

<Cational polymerization of polyoxymethylene copolymers>
The method for obtaining polyoxymethylene copolymers by cationic polymerization is almost equivalent to the method for obtaining polyoxymethylene homopolymers by cationic polymerization described above, except for the addition of copolymer components (comonomers).

The comonomer is a component copolymerizable with the above-mentioned trioxane, and examples include ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, styrene oxide, oxatan, 1,3-dioxolane, ethylene glycol formal, propylene glycol formal, diethylene glycol formal, triethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal, and 1,6-hexanediol formal. Among these, 1,3-dioxolane and 1,4-butanediol formal are preferred as the cyclic ether and/or cyclic formal. These may be used individually or in combination of two or more.

The amount of comonomer added is generally preferably 0.1 to 60 mol%, more preferably 0.1 to 20 mol%, and even more preferably 0.13 to 10 mol% per 100 mol of trioxane.
Furthermore, when the above polyoxymethylene is obtained using a tetramer of formaldehyde (tetraoxane), the amount of comonomer added is preferably 0.13 to 90 mol%, more preferably 0.14 to 30 mol%, and even more preferably 0.16 to 13 mol%, per 100 mol of tetraoxane.

When obtaining polyoxymethylene copolymers by cationic polymerization, tungsten compounds can be used as polymerization catalysts, and good gas permeability resistance can also be obtained by adding them during the subsequent pelletizing process.

<Terminal acetylation>
Crude polyoxymethylene homopolymers and copolymers obtained by polymerization have thermally unstable end groups. Therefore, after catalyst deactivation, it is preferable to stabilize these unstable end groups by reacting them with an esterifying agent or etherifying agent in the liquid or gas phase, or, in the case of crude polyoxymethylene copolymers, to stabilize them by decomposing and removing the unstable end groups as described later, in order to suppress the decomposition of polyoxymethylene during melt processing.

Terminal stabilization can be achieved using a single method or a combination of two or more methods. Among these, esterification, which allows for the simultaneous introduction of terminal acetyl groups, is preferred. When combining methods, it is preferable to select esterification as one of the stabilization methods.

The stabilization of the end groups of crude polyoxymethylene homopolymers by esterification can be carried out, for example, by introducing the crude polyoxymethylene homopolymer, an esterifying agent, and an esterification catalyst into an end-stabilization reactor containing an optional hydrocarbon solvent, and allowing the reaction to proceed. The reaction temperature and time are preferably, for example, 130 to 165°C and 1 to 100 minutes; more preferably 135 to 160°C and 5 to 100 minutes; and even more preferably 140 to 160°C and 10 to 100 minutes.

As an esterifying agent to sequester and stabilize the terminal groups of the crude polyoxymethylene homopolymer described above, an acid anhydride represented by the following general formula (II) can be used.
R ₅ COOCOR ₆ ...(II)
(In formula (II), _R5 and _R6 each independently represent an alkyl group. _R5 and _R6 may be the same or different from each other. ^R5 and ^R6 may be linked to each other to form a cyclic structure.)

The esterifying agent is not limited to the following, but examples include benzoic anhydride, succinic anhydride, maleic anhydride, glutaric anhydride, phthalic anhydride, propionic anhydride, and acetic anhydride, with acetic anhydride being preferred. These esterifying agents may be used individually or in combination of two or more.

Examples of the esterification catalyst include tungsten compounds comprising alkali metal salts of carboxylic acids having 1 to 18 carbon atoms, heteropoly acids, isopoly acids, and their acidic salts. Alkali metal salts of carboxylic acids having 1 to 18 carbon atoms are preferred, and the amount added can be appropriately selected in the range of 1 to 1000 ppm by mass relative to the mass of the polyoxymethylene homopolymer.

The alkali metal salts of carboxylic acids having 1 to 18 carbon atoms are not limited to those listed below, but examples include alkali metal salts of carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caprylic acid, enanthic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, and stearic acid. Examples of alkali metals include lithium, sodium, potassium, rubidium, and cesium. Among these alkali metal salts of carboxylic acids, lithium acetate, sodium acetate, and potassium acetate are preferred.

Specifically, tungsten compounds include phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicic acid, silicic molybdotungstovanadic acid, and their acidic salts. Among these, phosphotungstic acid and silicic acid are preferred.

As an etherifying agent for stabilizing the end groups of the crude polyoxymethylene homopolymer described above, an orthoester of an aliphatic or aromatic acid and an aliphatic, alicyclic, or aromatic alcohol can be selected, such as methyl orthoformate or ethyl orthoformate, methyl orthoacetate or ethyl orthoacetate, methyl orthobenzoate or ethyl orthobenzoate, and orthocarbonate, specifically ethyl orthocarbonate. This can be stabilized using Lewis acid-type catalysts such as moderate-strength organic acids like p-toluenesulfonic acid, acetic acid, and oxalic acid, or moderate-strength mineral acids like dimethyl sulfate and diethyl sulfate.

When stabilizing the terminal groups of crude polyoxymethylene homopolymers by etherification, the solvent used in the etherification reaction is not limited to the following, but examples include low-boiling aliphatic organic solvents such as pentane, hexane, cyclohexane, and benzene; alicyclic and aromatic hydrocarbon organic solvents; and halogenated lower aliphatic organic solvents such as methylene chloride, chloroform, and carbon tetrachloride.

<Methods for terminal stabilization other than acetylation>
In addition to the terminal acetylation step described above, a terminal stabilizer may be optionally added. The terminal stabilizer is not particularly limited and can be an aliphatic amine compound such as ammonia, triethylamine, or tributylamine; an inorganic weak acid salt of alkali metals or alkaline earth metals such as hydroxides, carbonates, phosphates, silicates, and borates of alkali metals or alkaline earth metals such as sodium, potassium, magnesium, calcium, or barium; or an organic acid salt of alkali metals or alkaline earth metals such as formate, acetate, stearate, palmitate, propionate, and oxalate. Among these, aliphatic amine compounds are preferred, and triethylamine is even more preferred.

The method for decomposing and removing unstable end portions is not particularly limited. For example, one method involves heat-treating the polyoxymethylene copolymer in a molten state at a temperature above its melting point and below 260°C in the presence of an end-stabilizing agent such as triethylamine. Examples of heat-treating methods include using a single-screw or twin-screw extruder equipped with a vent vacuum device, with a twin-screw extruder being preferred.

<Additives>
The polyoxymethylene resin composition of this embodiment may optionally contain various known additives to improve its physical properties. Examples of additives include antioxidants, acid scavengers, formaldehyde scavengers, weathering agents, mold release agents, conductive agents, plasticizers, nucleating agents, basicity auxiliaries, pigments, dyes, thermoplastic elastomers and other resins, inorganic fillers, organic fillers, etc. However, the above additives do not include (a) polyoxymethylene polymers and (b) tungsten compounds.

The present invention will be specifically described below with reference to examples and comparative examples. The present invention is not limited in any way by these examples.

The measurement methods used in the examples and comparative examples are shown below.
<Tungsten elemental amount evaluation>
0.1 g of polyoxymethylene resin composition was accurately weighed into a fluororesin decomposition vessel, and sulfuric acid (Kanto Chemical Co., Ltd., Ultra-high purity sulfuric acid Ultrapur) and nitric acid (Kanto Chemical Co., Ltd., Ultra-high purity nitric acid EL grade) were added. Pressurized acid decomposition was then performed using a microwave decomposition apparatus (Milestone General Co., Ltd., ETHOS One). The decomposition solution was diluted to a final volume of 50 mL and measured by ICP-MS (Agilent Technologies Co., Ltd., Agilent 7500CX, multi-point measurement method) to quantify the mass (mass ppm) of tungsten element in 100% by mass of the polyoxymethylene resin composition.

<Acetyl group quantity evaluation>
Approximately 1 g of polyoxymethylene resin composition was pressed at 5 MPa for 5 seconds using a hot press heated to 205°C to form a sheet. 15 mg of the sheet-like polyoxymethylene resin composition and 1.2 g of HFIP-d2 (containing 0.4% TFA-Na) were weighed and dissolved by permeation stirring at 50°C for 2 hours. The solution was filtered using a disposable filter (0.45 μm) and then subjected to 1H-NMR measurement (JEOL ECZ500, 512 scans). The peaks at 4.9–5.25 ppm were identified as peaks derived from the polyoxymethylene main chain _-CH₂O- , and the peak at 2.15 ppm was identified as peaks derived from the polyoxymethylene terminal acetyl group -OC(O) _CH₃. The number of acetyl groups per oxymethylene group (-O- _CH₂- ), which is the repeating main chain unit of the polyoxymethylene polymer, was quantified from the ratio of their integral values.

<Gas Permeability Resistance Evaluation>
Polyoxymethylene resin composition pellets were molded into strip-shaped molded bodies measuring 130 mm x 13 mm x 2 mm using an injection molding machine (Toshiba IS-80A) at a cylinder temperature of 200°C, an injection pressure of 60 kgf/ ^cm² , an injection time of 15 seconds, a cooling time of 25 seconds, and a mold temperature of 70°C. These molded bodies were cut into 38 mm diameter discs and fitted snugly into the opening of a 38 mm inner diameter stainless steel cylindrical container containing 50 mL of organic solvent (gasoline, 15 vol% methanol gasoline, or methanol), and then the lid was closed. After heating the cylindrical container with the test piece in a constant temperature bath at 60°C for 750 hours, the amount of organic solvent lost (g) was measured and converted to the amount of gas permeating a molded body with an area of 1 ^m² and a thickness of 1 mm per day (g/[mm・day・^m² ]). A smaller value indicates superior gas permeability resistance.

[Examples 1-6]
Purified formaldehyde, dimethyldistearylammonium acetate as a polymerization catalyst, and acetic anhydride as a chain transfer agent were added to a 60°C n-hexane solution and polymerization was carried out. The amount of polymerization catalyst added is shown in Table 1. The amount of acetic anhydride added was 0.5 × ^10⁻³ mol per 1 mol of monomer (formaldehyde in this example). The granular polyoxymethylene slurry of the polymer was filtered through a center with a filter cloth and dried under a nitrogen atmosphere at 60°C for 10 hours to obtain a crude polyoxymethylene polymer.
To 5.0 kg of the crude polyoxymethylene polymer obtained as described above, 8.1 kg of acetic anhydride was added as a terminal stabilizer, 32.4 kg of n-hexane as an inert solvent, and a tungsten compound (see Table 1) as a terminal stabilization catalyst. Furthermore, 1.25 kg of acetic anhydride was added, and the mixture was stirred at 160°C for 1 hour under a nitrogen atmosphere to stabilize the terminals. The obtained polyoxymethylene polymer was then mixed with Irganox 245 in the amounts shown in Table 1 and supplied at a rate of 3 kg/hr to an extruder (L/D = 44, L: distance from the raw material feed port to the discharge port of the twin-screw extruder (m), D: inner diameter of the twin-screw extruder (m), BT-30 extruder manufactured by Plastics Engineering Laboratory Co., Ltd.) for pelletizing. The obtained pellets were used to evaluate the tungsten content, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Example 7]
Polymerization was carried out by feeding 2.0 kg/hr of trioxane, methylal as a chain transfer agent, and the amount of phosphotungstic acid solution pre-dissolved in diethylene glycol dimethyl ether as shown in Table 1 into a twin-screw paddle-type continuous polymerization reactor (manufactured by Kurimoto Iron Works Co., Ltd., diameter 2B, L/D = 14.8) rotating in the same direction, set to 80°C. The crude polyoxymethylene slurry discharged from the polymerization reactor was added to water and stirred at room temperature for 1 hour to remove unreacted trioxane, etc. Then, it was filtered using a centrifuge and dried at 100°C for 10 hours to obtain a crude polyoxymethylene polymer.
To 5.0 kg of the crude polyoxymethylene polymer obtained as described above, 40.5 kg of acetic anhydride was added as a terminal stabilizer, and potassium acetate (60 ppm relative to the weight of acetic anhydride) was added as a terminal stabilization catalyst. The mixture was stirred at 150°C for 1 hour in a sealed container under a nitrogen atmosphere to stabilize the terminals. The slurry was then filtered, washed three times with acetone, filtered again, and dried at 140°C for 3 hours under a nitrogen atmosphere. The obtained polyoxymethylene polymer was then mixed with Irganox 245 in the amounts shown in Table 1, fed into an extruder at a rate of 3 kg/hr, and pelletized. The resulting pellets were used to evaluate the tungsten content, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Example 8]
Pellets were obtained in the same manner as in Examples 1 to 6, except that phosphotungstic acid was added in the amounts listed in Table 1 as a terminal stabilizing catalyst. The obtained pellets were used to evaluate the tungsten content, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Comparative Example 1]
Pellets were obtained in the same manner as in Examples 1 to 6, except that potassium acetate was added as a terminal stabilizing catalyst instead of the tungsten compound at a concentration of 60 ppm relative to the total weight of acetic anhydride and n-hexane. The obtained pellets were used to evaluate the amount of tungsten element, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Comparative Example 2]
Pellets were obtained in the same manner as in Examples 1 to 6, except that phosphotungstic acid was added in the amounts listed in Table 1 as a terminal stabilizing catalyst. The obtained pellets were used to evaluate the tungsten content, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Comparative Example 3]
Crude polyoxymethylene polymers were obtained by the method described in Example 7, except that 1,3-dioxolane was fed in the amount shown in Table 1 during polymerization. Irganox 245 was added to the polyoxymethylene polymer in the amount shown in Table 1, and the mixture was supplied to an extruder at a rate of 3 kg/hr to obtain pellets. The obtained pellets were used to evaluate the tungsten content, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Comparative Example 4]
Crude polyoxymethylene polymers were obtained in the same manner as in Example 7, except that the polymerization catalyst was changed to a solution of boron trifluoride dibutyl ether pre-dissolved in cyclohexane. These polymers were then end-stabilized and pelletized. The resulting pellets were used to evaluate the tungsten content, acetyl group content, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[Comparative Example 5]
In the method described in Example 7, crude polyoxymethylene polymer was obtained in the same manner as described above, except that 1,3-dioxolane was fed during polymerization in the amount shown in Table 1, and the polymerization catalyst was changed to a solution of boron trifluoride dibutyl ether pre-dissolved in cyclohexane. The polymer was then pelletized. The obtained pellets were used to evaluate the amount of tungsten, acetyl groups, and gas permeability resistance described above. The evaluation results are shown in Table 1.

The results from Examples 1-8 and Comparative Examples 1-5 demonstrate that polyoxymethylene resin compositions with acetylated polymer ends and tungsten compounds within specific concentrations exhibit excellent gas permeability resistance.

The polyoxymethylene resin composition of the present invention is industrially useful as a polyoxymethylene resin composition with excellent gas permeability resistance.

Claims (4)

(a) a polyoxymethylene polymer and (b) a tungsten compound,
(a) At least a portion of the polymer ends of the polyoxymethylene polymer are acetylated,
The (b) tungsten compound is contained in an amount of 0.01 to 100 ppm by mass of tungsten element relative to the total mass of the polyoxymethylene resin composition.
A polyoxymethylene resin composition characterized by the following features. The polyoxymethylene resin composition according to claim 1, wherein the tungsten compound (b) is at least one selected from the group consisting of phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicic tungstic acid, silicic molybdotungstovanadic acid, and their acidic salts. The polyoxymethylene resin composition according to claim 1 or 2, wherein the (a) polyoxymethylene polymer contains 50% by mass or more of polyoxymethylene homopolymer. The polyoxymethylene resin composition according to any one of claims 1 to 3, wherein the content of acetylated polymer ends is 1.0 × ^10⁻⁴ (units) or more per unit of oxymethylene (-O- _CH₂- ), which is the main chain repeating unit of the polyoxymethylene polymer.