Polyoxymethylene resin composition

Abstract

The present invention aims to provide a polyoxymethylene resin composition with excellent gas permeability resistance. [Solution] The polyoxymethylene resin composition of the present invention comprises (a) a polyoxymethylene polymer and (b) a tungsten compound, wherein at least a portion of the polymer ends of (a) are acetylated, and (b) contains 0.01 to 100 ppm by mass of tungsten elemental mass relative to the total mass of the polyoxymethylene resin composition.

Description

[Technical Field]

[0001] This invention relates to a polyoxymethylene resin composition. [Background technology]

[0002] 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.

[0003] Polyoxymethylene resin compositions are known for their high crystallinity and are generally resistant to the permeation of 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). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 5371897 [Patent Document 2] Japanese Patent Publication No. 2001-11196 [Overview of the project] [Problems that the invention aims to solve]

[0005] 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.

[0006] Therefore, the present invention aims to provide a polyoxymethylene resin composition that exhibits excellent gas permeability resistance. [Means for solving the problem]

[0007] As a result of diligent research into the above-mentioned problems, the inventors of the present invention have found that the above objective can be achieved by including a polyoxymethylene polymer having acetylated polymer ends and a tungsten compound, and by setting the mass ratio of the tungsten element within a specific range, thereby completing the present invention.

[0008] In other words, the present invention is as follows: [1] (a) Polyoxymethylene polymer and (b) comprising a tungsten compound, (a) At least a portion of the polymer ends of the polyoxymethylene polymer are acetylated, The tungsten compound (b) contains 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 content of acetylated polymer ends is 1.0 × 10⁻¹⁶ per unit of oxymethylene (-O-CH2-), which is the main chain repeating unit of the polyoxymethylene polymer. -4 A polyoxymethylene resin composition according to any one of [1] to [3], wherein the composition is (unit) or greater. [Effects of the Invention]

[0009] The polyoxymethylene resin composition of the present invention exhibits excellent gas permeability resistance. [Modes for carrying out the invention]

[0010] 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.

[0011] 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 addition, in this specification, (a) a polyoxymethylene polymer may be simply referred to as “(a)”. Also, (b) a tungsten compound may be simply referred to as “(b)”.

[0012] <(a) polyoxymethylene polymer> A polyoxymethylene polymer is a general term for polymers having an oxymethylene (-CH2O-) structure as a unit structure. When polyoxymethylene polymers are roughly classified, there are two types: a homopolymer in which the polymer main chain consists only of oxymethylene units, and a copolymer containing an arbitrary unit (for example, an oxyalkylene unit) in addition to the oxymethylene unit. The above polyoxymethylene polymer is also called polyacetal, acetal resin, or polyacetal resin. Examples of the above polyoxymethylene polymer include polyoxymethylene homopolymers (also simply referred to as homopolymers in this specification) and polyoxymethylene copolymers (also simply referred to as copolymers in this specification), and known ones may be used.

[0013] Examples of raw materials for homopolymers include formaldehyde and cyclic oligomers of formaldehyde (trioxane, tetraoxane). Bulk polymerization of these raw materials is a typical method for producing homopolymers.

[0014] Examples of raw materials for copolymers include, in addition to the raw materials for the above homopolymers, cyclic formals of glycols or diglycols such as ethylene oxide, propylene oxide, epichlorohydrin, 1,3-dioxolane, and 1,4-butanediol formal. Copolymers can be obtained by copolymerizing these. Also, as copolymers, branched polyoxymethylene copolymers obtained by copolymerizing a monomer of formaldehyde and / or a cyclic oligomer of formaldehyde with a monofunctional glycidyl ether, and polyoxymethylene copolymers having a crosslinked structure obtained by copolymerizing with a polyfunctional glycidyl ether can also be used.

[0015] Furthermore, the polyoxymethylene polymer may be a polyoxymethylene homopolymer having a block component obtained by polymerizing a monomer of formaldehyde or a cyclic oligomer of formaldehyde in the presence of a compound having a functional group such as a hydroxyl group at both ends or one end, for example, a polyalkylene glycol.

[0016] Similarly, the polyoxymethylene polymer may be a polyoxymethylene copolymer having a block component obtained by copolymerizing a monomer of formaldehyde or a cyclic oligomer of formaldehyde such as its trimer (trioxane) and tetramer (tetraoxane) with a cyclic ether or a cyclic formal in the presence of a compound having a functional group such as a hydroxyl group at both ends or one end, for example, hydrogenated polybutadiene glycol.

[0017] These polyoxymethylene polymers may be used alone or in combination of two or more. When combining two or more polyoxymethylene polymers, those containing 50% by mass or more of the polyoxymethylene homopolymer are preferred from the viewpoints of rigidity and gas barrier properties, those containing 80% by mass or more are more preferred, those containing 95% by mass or more are further preferred, and those substantially all (at least 99% by mass or more) being homopolymers are particularly preferred. Here, the percentages are based on the total amount of the polyoxymethylene polymer being 100% by mass.

[0018] As a method for obtaining polyoxymethylene, for example, it can be obtained by anionic polymerization or cationic polymerization. Specifically, to obtain a polyoxymethylene homopolymer, it can be obtained by anionic polymerization or cationic polymerization. Also, to obtain a polyoxymethylene copolymer, it can be obtained by cationic polymerization. Details of each polymerization method will be described later.

[0019] <Tungsten compound> 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.

[0020] 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 permeation.

[0021] Tungsten compounds may be added during terminal stabilization and pelletizing of polyoxymethylene polymers, or they may be included in the polyoxymethylene resin composition as a residue of the polymerization catalyst.

[0022] <Acetyl terminus amount> 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.

[0023] The content of acetylated polymer ends in the above polyoxymethylene polymer is 1.0 × 10⁻¹⁶ per unit of the [-OCH2-], which is the main chain repeating unit of the polyoxymethylene polymer. -4 Preferably, it should be 5.0 × 10 units or more. -4 It is even preferable that it be greater than or equal to one unit. While an upper limit is not necessary, in practice, 20.0 × 10 -4 (Units) less than or equal to 10.0 × 10 -4 It is preferable that the value is less than or equal to (unit). Good gas permeability resistance can be obtained by staying within the above range.

[0024] <Anionic polymerization of polyoxymethylene homopolymers> An example of a method for obtaining polyoxymethylene homopolymers by anionic polymerization is described below.

[0025] For example, polymerization can be carried out by a slurry method using purified formaldehyde gas. It can be produced by feeding formaldehyde (a monomer), a chain transfer agent (molecular weight modifier), and a polymerization catalyst into a polymerization reactor containing a hydrocarbon polymerization solvent, and polymerizing by slurry polymerization. Furthermore, it is preferable to use formaldehyde gas that contains as few impurities as possible, such as water, methanol, and formic acid, which can cause polymerization termination and chain transfer during the polymerization reaction. If these impurities are present in excess, unexpected chain transfer reactions may 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.

[0026] The molecular weight of polyoxymethylene homopolymers can be adjusted by chain transfer using molecular weight modifiers such as carboxylic acid anhydride or carboxylic acid. Propionic anhydride and acetic anhydride are particularly preferred as molecular weight modifiers, with acetic anhydride being more preferred.

[0027] The amount of molecular weight modifier introduced may be adjusted and determined according to the properties (especially the melt flow rate) of the target polyoxymethylene homopolymer. 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.

[0028] As the polymerization catalyst, anionic polymerization catalysts are preferred, and onium salt polymerization catalysts represented by the following general formula (I) are more preferred. [R1R2R3R4M] + 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.

[0029] 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.

[0030] The amount of onium salt polymerization catalysts such as these 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.

[0031] Any hydrocarbon polymerization solvent that does not react with formaldehyde is acceptable and is not particularly limited, but 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 combination of two or more.

[0032] 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.

[0033] <Cational polymerization of polyoxymethylene homopolymers> An example of a method for obtaining polyoxymethylene homopolymers by cationic polymerization is described below.

[0034] For example, polymerization can be carried out using a bulk method with trioxane or cyclic tetramers, which are cyclic trimers of formaldehyde. There are no particular limitations on the shape (structure) of the polymerization reactor used, but generally, a twin-screw paddle type or screw type agitated mixing polymerization reactor that allows a heat transfer medium to pass through the jacket can be suitably used.

[0035] As for polymerization methods, for example, a method is used 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 the residence time in the polymerization reactor to the above ranges, the thermal decomposition of the polyoxymethylene polymer can be suppressed more effectively, and there is a tendency to be able to produce a more thermally stable polyoxymethylene polymer.

[0036] 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 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 this 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.

[0037] 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.

[0038] 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-(CH2-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. 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, from the viewpoint of controlling the molecular weight of the target polyoxymethylene polymer to a suitable range, per mol of formaldehyde (for example, the cyclic trimer or tetramer of formaldehyde mentioned above).

[0039] <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 copolymerization components (comonomers).

[0040] The comonomer is a component copolymerizable with the above 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, 1,6-hexanediol formal, etc. Among these, 1,3-dioxolane and 1,4-butanediol formal are preferred as cyclic ethers and / or cyclic formals. These may be used individually or in combination of two or more.

[0041] 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.

[0042] 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.

[0043] <Terminal acetylation> The crude polyoxymethylene homopolymers and copolymers obtained by polymerization have thermally unstable end groups. Therefore, after the catalyst is deactivated, these unstable end groups are blocked and stabilized by reacting them with an esterifying agent or an etherifying agent in a liquid or gas phase, or in the case of a crude polyoxymethylene copolymer, by decomposing and removing the unstable end parts described later, which is preferable in terms of suppressing the decomposition of polyoxymethylene during melt processing.

[0044] Terminal stabilization may be achieved by only one method or by combining two or more methods. Among them, stabilization by esterification that can introduce terminal acetyl groups simultaneously with stabilization is preferable, and when combining methods, it is preferable to select stabilization by esterification for at least one of them.

[0045] The stabilization treatment of the end groups of the crude polyoxymethylene homopolymer by esterification can be carried out, for example, by charging the crude polyoxymethylene homopolymer, an esterifying agent, and an esterification catalyst into a terminal stabilization reactor optionally introduced with a hydrocarbon solvent and reacting them. The reaction temperature and reaction time at this time are preferably, for example, a reaction temperature of 130 to 165 °C and a reaction time of 1 to 100 minutes, more preferably a reaction temperature of 135 to 160 °C and a reaction time of 5 to 100 minutes, and even more preferably a reaction temperature of 140 to 160 °C and a reaction time of 10 to 100 minutes.

[0046] As the esterifying agent for blocking and stabilizing the end groups of the above crude polyoxymethylene homopolymer, an acid anhydride represented by the following general formula (II) can be used. R5COOCOR6···(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. R 5 and R 6 and may be linked to each other to form a cyclic structure.)

[0047] 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.

[0048] Examples of the esterification catalyst include tungsten compounds consisting of 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.

[0049] Examples of alkali metal salts of carboxylic acids having 1 to 18 carbon atoms include, but are not limited to, 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.

[0050] Specific examples of tungsten compounds include phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silitungstic acid, silitmolybdotungstovanadic acid, and their acidic salts. Among these, phosphotungstic acid and silitungstic acid are preferred.

[0051] As an etherifying agent to sequester and stabilize the terminal groups of the crude polyoxymethylene homopolymer described above, one can be selected from orthoesters of aliphatic or aromatic acids and aliphatic, alicyclic, or aromatic alcohols, such as methyl orthoformate or ethyl orthoformate, methyl orthoacetate or ethyl orthoacetate, methyl orthobenzoate or ethyl orthobenzoate, and orthocarbonates, 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.

[0052] When stabilizing the terminal groups of a crude polyoxymethylene homopolymer 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.

[0053] <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.

[0054] There are no particular limitations on the method for decomposing and removing the unstable end portions. For example, one method is to heat-treat the polyoxymethylene copolymer in a molten state at a temperature above the melting point of the polyoxymethylene copolymer and below 260°C in the presence of an end stabilizer such as triethylamine. Examples of heat-treating methods include a single-screw or twin-screw extruder equipped with a vent vacuum device, with a twin-screw extruder being preferred.

[0055] <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. [Examples]

[0056] 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.

[0057] The measurement methods used in the examples and comparative examples are shown below. <Tungsten elemental content evaluation> 0.1 g of polyoxymethylene resin composition was accurately weighed into a fluororesin decomposition vessel, and sulfuric acid (Kanto Chemical Co., Ltd., Ultrapur ultra-high purity sulfuric acid) and nitric acid (Kanto Chemical Co., Ltd., EL grade ultra-high purity nitric acid) 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.

[0058] <Evaluation of acetyl group amount> 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 two hours. The solution was filtered using a disposable filter (0.45 μm) and then subjected to 1H-NMR measurement (JEOL ECZ500, 512 scans). The peak at 4.9–5.25 ppm was identified as the peak derived from the polyoxymethylene main chain -CH2O-, and the peak at 2.15 ppm was identified as the peak 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.

[0059] <Gas Permeability Resistance Evaluation> Polyoxymethylene resin composition pellets were injected using an injection molding machine (Toshiba IS-80A) at a cylinder temperature of 200°C and an injection pressure of 60 kgf / cm². 2 A rectangular molded body measuring 130mm x 13mm x 2mm was formed using an injection time of 15 seconds, a cooling time of 25 seconds, and a mold temperature of 70°C. This molded body was cut into a disc shape with a diameter of 38mm and fitted snugly into the opening of a stainless steel cylindrical container with an inner diameter of 38mm containing 50mL of organic solvent (gasoline, 15 vol% methanol gasoline, or methanol), and 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 over an area of ​​1m². 2 This was converted to the amount of gas that permeates a 1mm thick molded body per day (g / [mm·day·m 2 ]). The smaller the value, the better the gas permeability resistance.

[0060] [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 amounts of polymerization catalyst added are shown in Table 1. The amount of acetic anhydride added was 0.5 × 10⁻⁶ per 1 mol of monomer (formaldehyde in this example). -3 The quantity is in moles. The granular polyoxymethylene slurry of the polymer was filtered through a centering device with a filter cloth, and dried at 60°C for 10 hours under a nitrogen atmosphere to obtain the 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 stabilized at the terminals by stirring at 160°C for 1 hour under a nitrogen atmosphere. The obtained polyoxymethylene polymer was then mixed with Irganox 245 in the amounts shown in Table 1 and fed into 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.) at a rate of 3 kg / hr for pelletization. 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.

[0061] [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.

[0062] [Example 8] Pellets were obtained in the same manner as in Examples 1-6, 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.

[0063] [Comparative Example 1] Pellets were obtained in the same manner as in Examples 1-6, 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, and gas permeability resistance described above. The evaluation results are shown in Table 1.

[0064] [Comparative Example 2] Pellets were obtained in the same manner as in Examples 1-6, 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.

[0065] [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 fed into 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.

[0066] [Comparative Example 4] Crude polyoxymethylene polymers were obtained using the same method 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.

[0067] [Comparative Example 5] In the method described in Example 7, crude polyoxymethylene polymers were obtained in the same manner as described above, except that 1,3-dioxolane was fed in the amount shown in Table 1 during polymerization, and the polymerization catalyst was changed to a solution of boron trifluoride dibutyl ether pre-dissolved in cyclohexane. These polymers were then pelletized. The resulting 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.

[0068] [Table 1]

[0069] The results from Examples 1-8 and Comparative Examples 1-5 demonstrate that polyoxymethylene resin compositions in which the concentrations of polyoxymethylene polymers with acetylated polymer ends and tungsten compounds are within a specific range are polyoxymethylene resin compositions with excellent gas permeability resistance. [Industrial applicability]

[0070] The polyoxymethylene resin composition of the present invention is industrially useful as a polyoxymethylene resin composition with excellent gas permeability resistance.

Claims

1. (a) Polyoxymethylene polymer and (b) comprising a tungsten compound, (a) At least a portion of the polymer ends of the polyoxymethylene polymer are acetylated, The (b) tungsten compound contains 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 claim 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 acid, silicic molybdotungstovanadic acid, and acidic salts thereof.

3. The polyoxymethylene resin composition according to claim 1 or 2, wherein the (a) polyoxymethylene polymer contains 50% by mass or more of a polyoxymethylene homopolymer.

4. The content of acetylated polymer ends is such that the main chain repeating unit of the polyoxymethylene polymer is oxymethylene (-O-CH ２ -) 1.0 × 10 per unit －４ A polyoxymethylene resin composition according to any one of claims 1 to 3, wherein the amount is (in units) or more.

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