JP7722835B2 - Polyoxymethylene Composition - Google Patents

Description

The present invention relates to a polyoxymethylene composition.

Traditionally, polyoxymethylene has been widely used as a material for many molded products, including mechanical parts for automobiles, electrical appliances, and machinery, due to its high mechanical strength, rigidity, creep resistance, solvent resistance, and sliding properties. However, as the range of uses for polyoxymethylene has expanded in recent years, as exemplified by its use as a replacement for metal materials, there has been a trend toward increasingly more advanced properties. There is also a demand for resistance to use in harsh conditions such as high temperatures and acidic conditions.

For example, resin materials for power window gears, which are mechanical components in automobiles, are becoming increasingly smaller, and while they are required to have higher torque and creep resistance, there is also a demand for materials with high impact resistance to ensure good operational durability even when a starting torque greater than the rated torque is momentarily applied during operation. Furthermore, there is also a demand for resistance to thermal degradation caused by the acidic components contained in the grease and frictional heat during gear operation.

To date, the use of boron nitride as a nucleating agent has been investigated as a method for improving the creep resistance of polyoxymethylene. For example, Patent Document 1 states that adding boron nitride at a specific concentration improves high-temperature, low-stress creep properties and suppresses variation in Charpy impact strength.

JP 2019-183080 A

However, while the technology disclosed in Patent Document 1 improves creep properties under low stress, there is still room for improvement in creep properties under high stress. Furthermore, while the variation in Charpy impact strength itself is suppressed, further improvement is desired in terms of ensuring sufficient impact strength. Generally, polyoxymethylene to which a nucleating agent has been added tends to have lower impact strength due to increased rigidity caused by the nucleating agent, but also increased brittleness as the nucleating agent increases the crystallinity of the polyoxymethylene. Therefore, there is a trade-off between creep resistance and impact resistance in systems to which a nucleating agent has been added. Furthermore, the technology disclosed in Patent Document 1 is not expected to contribute significantly to improving the thermal stability or acid resistance of polyoxymethylene compositions.

The present invention therefore aims to provide a polyoxymethylene composition that has high high-stress creep resistance and Charpy impact strength, as well as excellent acid resistance and thermal stability.

As a result of extensive research into the above-mentioned problems, the inventors discovered that the above-mentioned problems could be solved by creating a polyoxymethylene composition containing a specific amount of boron nitride with an average particle size within a specific range, leading to the completion of the present invention.

In other words, the present invention is as follows:

[1]
(A) 100 parts by mass of polyoxymethylene; and (B) 0.02 to 0.5 parts by mass of boron nitride,
The polyoxymethylene composition is characterized in that the boron nitride (B) has an average particle size of 10 to 800 nm for at least 100 particles in a field image of 3.0 × 3.0 μm obtained by a scanning electron microscope at a magnification of 50,000 times, as determined using image processing and analysis software ("A-zo-kun" manufactured by Asahi Kasei Engineering Co., Ltd.).
[2]
The polyoxymethylene composition according to [1], wherein the average particle size of the (B) boron nitride is 80 to 500 nm.
[3 ]
The polyoxymethylene composition according to [1] or [2] , further comprising: (C) 0.01 to 3.0 parts by mass of a poly-β-alanine polymer containing 30 to 70 mol % of primary amide groups.
[ 4 ]
The polyoxymethylene composition according to any one of [1] to [ 3 ], further comprising 0.001 to 0.1 parts by mass of (D) a polyamide polymer having a melting point of 140 to 230°C.
[ 5 ]
The polyoxymethylene composition according to [ 4 ], wherein the mass ratio (D)/(B) of the polyamide polymer (D) to the boron nitride (B) is 0.04 to 1.0.

The present invention provides a polyoxymethylene composition that has excellent high-stress creep resistance, impact resistance, acid resistance, and thermal stability.

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

[Polyoxymethylene composition]
The polyoxymethylene composition of this embodiment contains (A) polyoxymethylene and (B) boron nitride. The polyoxymethylene composition of this embodiment may further contain (C) a poly-β-alanine polymer containing 30 to 70 mol% of primary amide groups (hereinafter simply referred to as "(C) poly-β-alanine polymer"), (D) a polyamide polymer having a melting point of 140 to 230°C (hereinafter simply referred to as "(D) polyamide polymer"), and various other additives.

<(A) Polyoxymethylene>

The polyoxymethylene (A) contained in the polyoxymethylene composition of the present embodiment is not particularly limited and may be any of a polyoxymethylene homopolymer, a polyoxymethylene copolymer, and a mixture thereof. Among these, a polyoxymethylene homopolymer is preferred.
The polyoxymethylene (A) may be used alone or in combination of two or more.

-Polyoxymethylene homopolymer-
The polyoxymethylene homopolymer is a polymer whose main chain is composed substantially of only oxymethylene units, and preferably 99.8 mol % or more of the main chain excluding both terminals is composed of oxymethylene units, more preferably the main chain excluding both terminals is composed of only oxymethylene units. In particular, a polyoxymethylene homopolymer in which both terminals are blocked by esterification is preferred.

Polyoxymethylene homopolymers can be obtained by homopolymerizing formaldehyde monomer or cyclic oligomers of formaldehyde such as its trimer (trioxane) or tetramer (tetraoxane). For example, they can be produced by feeding formaldehyde as a monomer, a chain transfer agent (molecular weight regulator), and a polymerization catalyst into a polymerization reactor containing a hydrocarbon polymerization solvent, and polymerizing them by a slurry polymerization method.
In this case, the raw material monomers, the chain transfer agent, and the polymerization catalyst may contain components capable of chain transfer (components that generate unstable terminal groups), such as water, methanol, and formic acid, and therefore it is preferable to first adjust the contents of these components capable of chain transfer.
The content of these chain-transferable components is preferably in the range of 1 to 1000 ppm by mass, more preferably 1 to 500 ppm by mass, and even more preferably 1 to 300 ppm by mass, based on the total mass of formaldehyde.
By adjusting the content of the chain-transferable component within the above range, a polyoxymethylene homopolymer having excellent thermal stability can be obtained.

The molecular weight of the polyoxymethylene homopolymer can be adjusted by chain transfer using a molecular weight regulator such as a carboxylic acid anhydride or a carboxylic acid, etc. As the molecular weight regulator, propionic anhydride and acetic anhydride are particularly preferred, and acetic anhydride is more preferred.
The molecular weight modifier may be used alone or in combination of two or more.

The amount of molecular weight modifier incorporated is adjusted and determined depending on the desired properties (particularly the melt flow rate) of the polyoxymethylene homopolymer. For example, the polyoxymethylene homopolymer is preferably adjusted so that its melt flow rate (MFR value (compliant with ISO 1133, 190°C, 2.16 kg load)) is in the range of 0.1 to 100 g/10 min, and more preferably in the range of 1.0 g/10 min to 70 g/10 min. By adjusting the MFR value of the polyoxymethylene homopolymer to fall within the above range, it is possible to obtain a polyoxymethylene homopolymer with excellent mechanical strength.

The polymerization catalyst is preferably an anionic polymerization catalyst, and more preferably an onium salt polymerization catalyst represented by the following general formula (1).
[R ₁ R ₂ R ₃ R ₄ M] ⁺ X ^- (1)
(In formula (1), R ₁ , R ₂ , R ₃ and R ₄ each independently represent an alkyl group, M represents an element having a lone electron pair, and X represents a nucleophilic group. R ₁ , R ₂ , R ₃ and R ₄ may be the same as or different from one another.)
The polymerization catalyst may be used alone or in combination of two or more.

Among the 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 the onium salt polymerization catalyst such as a quaternary phosphonium salt compound or a quaternary ammonium salt compound added is preferably 0.0003 to 0.01 mol, more preferably 0.0008 to 0.005 mol, and even more preferably 0.001 to 0.003 mol, per 1 mol of formaldehyde.

The hydrocarbon polymerization solvent is not particularly limited as long as it does not react with formaldehyde. Examples of the hydrocarbon polymerization solvent include pentane, isopentane, hexane, cyclohexane, heptane, octane, nonane, decane, and benzene, with hexane being particularly preferred.
These hydrocarbon polymerization solvents may be used alone or in combination of two or more.

In producing polyoxymethylene homopolymer, it is preferable to first obtain crude polyoxymethylene homopolymer by polymerization, and then, as described below, to perform a stabilization treatment to remove unstable terminal groups.

The polymerization reactor for producing crude polyoxymethylene homopolymer is not particularly limited as long as it can simultaneously supply the monomer formaldehyde, chain transfer agent (molecular weight regulator), polymerization catalyst, and hydrocarbon polymerization solvent, but from the standpoint of productivity, a continuous polymerization reactor is preferred.

The terminal groups of the crude polyoxymethylene homopolymer obtained by polymerization are thermally unstable. Therefore, it is preferable to cap these unstable terminal groups with an esterifying agent or etherifying agent, etc., to perform a stabilization treatment.

The stabilization treatment of the terminal groups of the crude polyoxymethylene homopolymer by esterification can be carried out, for example, by charging the crude polyoxymethylene homopolymer and an esterifying agent and/or an esterification catalyst into an end-group stabilization reactor containing a hydrocarbon polymerization solvent, and allowing them to react.
The reaction temperature and reaction time at this time are preferably 130 to 155°C and 1 to 100 minutes, more preferably 135 to 155°C and 5 to 100 minutes, and even more preferably 140 to 155°C and 10 to 100 minutes.

As the esterifying agent for blocking and stabilizing the terminal groups of the crude polyoxymethylene homopolymer, an acid anhydride represented by the following general formula (2) can be used.
R ₅ COOCOR ₆ ...(2)
(In formula (2), _R5 and _R6 each independently represent an alkyl group. _R5 and _R6 may be the same or different.)

Examples of the esterifying agent 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 alone or in combination of two or more.

The esterification catalyst is preferably an alkali metal salt of a carboxylic acid having 1 to 18 carbon atoms, and the amount added can be appropriately selected within the range of 1 to 1000 ppm by mass relative to the mass of the polyoxymethylene homopolymer.
Examples of alkali metal salts of carboxylic acids having 1 to 18 carbon atoms include alkali metal salts of carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caprylic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, and stearic acid, and examples of the alkali metal salts include lithium, sodium, potassium, rubidium, and cesium. Among these alkali metal salts of carboxylic acids, alkali metal salts of lithium acetate, sodium acetate, and potassium acetate are preferred.

The end groups of the crude polyoxymethylene homopolymer may be capped and stabilized by etherification.
The etherifying agent may 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, and may be stabilized with a Lewis acid type catalyst such as a medium strength organic acid such as p-toluenesulfonic acid, acetic acid and oxalic acid, or a medium strength mineral acid such as dimethyl sulfate and diethyl sulfate.

When the end groups of crude polyoxymethylene homopolymer are blocked and stabilized by etherification, examples of solvents used in the etherification reaction include low-boiling aliphatic organic solvents such as pentane, hexane, cyclohexane, and benzene; alicyclic and aromatic hydrocarbon organic solvents; and halogenated lower aliphatic solvents such as methylene chloride, chloroform, and carbon tetrachloride.

The polyoxymethylene homopolymer whose end groups have been stabilized by the above method can be dried using a dryer such as a hot air dryer or vacuum dryer, with air or nitrogen gas adjusted to 100-150°C enclosed to remove moisture, thereby obtaining the desired polyoxymethylene homopolymer.

-Polyoxymethylene copolymer-
The polyoxymethylene copolymer can be produced, for example, by using, as a comonomer, a glycol such as 1,3-dioxolane or 1,4-butanediol formal, a cyclic ether such as a cyclic formal of diglycol, or a cyclic formal, and copolymerizing these with a monomer such as trioxane.

The ratio of the comonomer to be copolymerized is preferably 0.03 to 20 mol%, more preferably 0.03 to 15 mol%, and even more preferably 0.04 to 5 mol%, per 1 mol of the trioxane. If the comonomer ratio is within the above range, polyoxymethylene pellets with superior mechanical strength can be obtained.

Examples of the polymerization catalyst for the polymerization of the polyoxymethylene copolymer include cationic active catalysts such as Lewis acids, protonic acids, and esters or anhydrides thereof.
Examples of Lewis acids include halides of boric acid, tin, titanium, phosphorus, arsenic, and antimony, and specific examples thereof include boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentafluoride, phosphorus pentachloride, antimony pentafluoride, and complex compounds or salts thereof.
Examples of protonic acids and their esters or anhydrides include perchloric acid, trifluoromethanesulfonic acid, perchloric acid-tert-butyl ester, acetyl perchlorate, and trimethyloxonium hexafluorophosphate.
Among these, boron trifluoride; boron trifluoride hydrate; and coordination complex compounds of boron trifluoride and an organic compound containing an oxygen atom or a sulfur atom are preferred, and examples of preferred compounds include boron trifluoride diethyl ether and boron trifluoride di-n-butyl ether.

The amount of boron trifluoride added is preferably 0.10× ¹⁰ mol or less, more preferably 0.07× ¹⁰ mol or less, and even more preferably 0.03 to 0.05× ¹⁰ mol, per 1 mol of trioxane. When the amount of boron trifluoride added is within the above range, a polyoxymethylene composition with excellent thermal stability can be provided.

As the polymerization method for the polyoxymethylene copolymer exemplified above, in addition to the above-mentioned slurry polymerization method, for example, bulk polymerization method may be used, and either a batch method or a continuous method may be applied.
Examples of the polymerization apparatus include a co-kneader, a twin-screw continuous extrusion kneader, a self-cleaning extrusion kneader such as a twin-screw paddle type continuous mixer, and the like.
Molten monomers are fed into the polymerization apparatus, and as the polymerization proceeds, a solid block of polyoxymethylene copolymer is obtained.

The polyoxymethylene copolymer obtained by the above polymerization may contain thermally unstable terminal groups [-( _OCH2 ) _n -OH groups], so it is preferable to carry out a treatment to decompose and remove these unstable terminal groups. The decomposition and removal method for the unstable terminal groups can be a known method.

<(B) Boron Nitride>
The polyoxymethylene composition of the present embodiment contains (B) boron nitride as a crystal nucleating agent.
From the viewpoint of the size of the spherulites of the polyoxymethylene (A) formed by the boron nitride (B) functioning as a crystal nucleating agent, the average particle size is 10 to 800 nm, preferably 80 to 500 nm, and more preferably 100 to 500 nm.
The average particle size of (B) boron nitride is determined by extracting particles of (B) boron nitride from the polyoxymethylene composition, and then using image processing and analysis software ("Asahi Kasei Engineering Co., Ltd."'s "A-zo-kun") to obtain a field image of a 3.0 × 3.0 μm area obtained at a magnification of 50,000 times using a scanning electron microscope. The average particle size of (B) boron nitride is determined as the average value of the longest particle sizes of at least 100 particles in the field image, specifically, by the method described in the Examples below.
Methods for extracting the boron nitride particles (B) from the polyoxymethylene composition include methods in which organic components in the polyoxymethylene composition are decomposed and removed by incineration or heat treatment under a nitrogen gas atmosphere.

As for (B) boron nitride, (B) boron nitride having an average particle size within the above range may be used as is, or boron nitride having a larger particle size may be subjected to a pulverization treatment and processed to have an average particle size within the above range before use. In other words, the particle size of (B) boron nitride can be controlled by the pulverization treatment.
Examples of machines for pulverization include dry jet mills, wet jet mills, hammer mills, vibration mills, roller mills, tumbling mills, pin disc mills, bead mills, impact shear mills, high-pressure fluid impingement mills, dry ball mills, and wet ball mills.
The pulverization method may be a dry method alone, a combination of a wet method and a dry method, or a wet method alone.

(B) Boron nitride may be in any shape, such as a scale shape, a flat shape, a disk shape, or a sphere shape.
The crystal structure of (B) boron nitride includes a hexagonal graphite structure, a cubic zinc blende structure, and a hexagonal wurtzite structure. (B) boron nitride is preferably hexagonal boron nitride with a hexagonal graphite structure as a crystal nucleating agent for polyoxymethylene.
(B) Boron nitride not only functions as a crystal nucleating agent, but also contributes to improving the thermal conductivity, heat resistance, corrosion resistance, electrical insulation, lubricity, and mold releasability of the polyoxymethylene composition.

(B) The type of boron nitride is not particularly limited, but examples thereof include XGP (manufactured by Denka Co., Ltd.), SGP (manufactured by Denka Co., Ltd.), MGP (manufactured by Denka Co., Ltd.), GP (manufactured by Denka Co., Ltd.), HGP (manufactured by Denka Co., Ltd.), SP-2 (manufactured by Denka Co., Ltd.), SP-3 (manufactured by Denka Co., Ltd.), SGPS (manufactured by Denka Co., Ltd.), AP-10S (manufactured by MARUKA Co., Ltd.), AP-20S (manufactured by MARUKA Co., Ltd.), AP-100S (manufactured by MARUKA Co., Ltd.), SL-170-20OG (manufactured by MARUKA Co., Ltd.), SL-170-20-WA (manufactured by MARUKA Co., Ltd.), UHP-S2 (manufactured by Showa Denko K.K.), UHP-1K (manufactured by Showa Denko K.K.), UHP-2 (manufactured by Showa Denko K.K.), UHP-G1H (manufactured by Showa Denko K.K.), and the like. Wadenko Co., Ltd.), PCT-UFB (Saint-Gobain), PCTP2 (Saint-Gobain), PCTF5 (Saint-Gobain), PCTP8 (Saint-Gobain), PCTL30 (Saint-Gobain), PCTP12 (Saint-Gobain), PCTP16 (Saint-Gobain), PCTP30 (Saint-Gobain), PCTP30D (Saint-Gobain), PCTL5MHF (Saint-Gobain), PCTL7MHF (Saint-Gobain), PCTL20MHF (Saint-Gobain), PCTH7MHF (Saint-Gobain), PCTH10MHF (Saint-Gobain), CTS7M (Saint-Gobain), CTS25M (Saint-Gobain) ), IDL100 (manufactured by Saint-Gobain), IDL200 (manufactured by Saint-Gobain), IDL300 (manufactured by Saint-Gobain), IDL400 (manufactured by Saint-Gobain), IDL500 (manufactured by Saint-Gobain), IDL600 (manufactured by Saint-Gobain), IDL700 (manufactured by Saint-Gobain), IDL800 (manufactured by Saint-Gobain), IDL900 (manufactured by Saint-Gobain), PHPP325 (manufactured by Saint-Gobain), PHPP325B (manufactured by Saint-Gobain), MCFP (manufactured by Saint-Gobain), PSHP325 (manufactured by Saint-Gobain), PSHP605 (manufactured by Saint-Gobain), PCPS3005 (manufactured by Saint-Gobain), PCPS302 (manufactured by Saint-Gobain), PCPS308 (manufactured by Saint-Gobain), PCPS3012 (manufactured by Saint-Gobain), PCPS3016 (manufactured by Saint-Gobain), PCPS330 (manufactured by Saint-Gobain), PUHP3008J (manufactured by Saint-Gobain), PUHP1109J (manufactured by Saint-Gobain), PT110 (manufactured by MOMENTIVE), PT120 (manufactured by MOMENTIVE), PT350 (manufactured by MOMENTIVE), PT670 (manufactured by MOMENTIVE), PTX25 (manufactured by MOMENTIVE), PTX60 (manufactured by MOMENTIVE), ZSA-200 (manufactured by JIX Industrial Co., Ltd.), ZSA-20 (manufactured by JIX Industrial Co., Ltd.), ZSA-5 (manufactured by JIX Industrial Co., Ltd.), SCP1 (ESK CERAMICS), SX (ESK CERAMICS), S1 (ESK CERAMICS), S3 (ESK CERAMICS), SRF (ESK CERAMICS), SHP-2 (Mizushima Ferro Alloy Co., Ltd.), SHP-3 (Mizushima Ferro Alloy Co., Ltd.), SHP-4 (Mizushima Ferro Alloy Co., Ltd.), SHP-5 (Mizushima Ferro Alloy Co., Ltd.), SHP-5L (Mizushima Ferro Alloy Co., Ltd.), SHP-6 (Mizushima Ferro Alloy Co., Ltd.), SHP-7 (Mizushima Ferro Alloy Co., Ltd.), SHP-9 (Mizushima Ferro Alloy Co., Ltd.), HP-1 (Mizushima Ferro Alloy Co., Ltd.), HP-P1 (Mizushima Ferro Alloy Co., Ltd.), HP-2 (Mizushima Ferro Alloy Co., Ltd.), HP-4W (Mizushima Ferro Alloy Co., Ltd.), HP-6 (Mizushima Ferro Alloy Co., Ltd.), HP40J (Mizushima Ferro Alloy Co., Ltd.), HP40MF (Mizushima Ferro Alloy Co., Ltd.), FS-1 (Mizushima Ferro Alloy Co., Ltd.), FS-3 (Mizushima Ferro Alloy Co., Ltd.), Platelets 001 (manufactured by 3M), Platelets 003E (manufactured by 3M), Platelets 003SF (manufactured by 3M), Platelets 003 (manufactured by 3M), Platelets 006 (manufactured by 3M), Platelets 007HS (manufactured by 3M), Platelets 0075 (manufactured by 3M), Platelets 009 (manufactured by 3M), Platelets 012 (manufactured by 3M), Platelets 012P (manufactured by 3M), Flakes 500-3 (manufactured by 3M), L551 (manufactured by HONGWUNEWMA TERIAL), L553 (manufactured by HONGWUNEWMA TERIAL), L556 (manufactured by HONGWUNEWMA (manufactured by TERIAL), Hex-Boron Nitride Powder (hBN, 99% Pure, APS: 70nm) (manufactured by LOWER FRICTION), Hex-Boron Nitride Powder (hBN, 98% Pure, APS: 0.5 micron) (manufactured by LOWER FRICTION), Hex-Boron Nitride Powder (hBN, 98% Pure, APS: 1.5 micron) (LOWER FRICTION), Hex-Boron Nitride Powder (hBN, 98% Pure, APS: 5micron) (LOWER Hex-Boron Nitride Powder (hBN, 98% Pure, APS: 30 microns) (manufactured by LOWER FRICTION Co., Ltd.), and the like.
(B) The boron nitride may be used alone or in combination of two or more kinds.

(B) A typical method for producing boron nitride is to react molten boric anhydride (B ₂ O ₃ ) with nitrogen or ammonia in the presence of calcium phosphate (CaPO ₄ ) as a catalyst. Other methods include reacting boric acid or an alkali borate with an organic nitrogen compound such as urea, guanidine, or melamine in a high-temperature nitrogen-ammonia atmosphere, reacting molten sodium borate (Na ₃ BO ₃ ) with ammonium chloride in an ammonia atmosphere, reacting boron trichloride (BCl ₃ ) with ammonia at high temperatures, CVD (chemical vapor deposition), thermal CVD, and PVD (physical vapor deposition), among others.

The boron nitride (B) used in this embodiment may contain impurities such _{as B2O3} , boric acid, and carbon components, but the purity of the boron nitride (B) is preferably 60% or more, more preferably 80% or more, and most preferably 90% or more.

In the production of the polyoxymethylene composition of this embodiment, boron nitride (B) may be added as a solid or dispersed in a dispersion medium. Pretreatments such as surface oxidation, surface modification, calcination, dopant intercalation, and cleavage may also be performed as appropriate.

The content of (B) boron nitride in the polyoxymethylene composition of the present embodiment can be measured by quantitative analysis of boron element by inductively coupled plasma mass spectrometry (ICP-MS), and specifically, can be measured by the method described in the examples below.
As a pretreatment for ICP-MS measurement, pellets, powder, or molding cuttings of the polyoxymethylene composition and a mixed acid of hydrofluoric acid and nitric acid are placed in a pressurizable container, and the mixture is subjected to pressurized acid decomposition in a closed system using microwave irradiation. This decomposes all of the boron nitride (B) in the polyoxymethylene composition, allowing quantitative analysis of the boron element by ICP-MS.
The content of (B) boron nitride in the polyoxymethylene composition of this embodiment is, from the viewpoint of the effective amount of crystal nucleating agent, 0.001 parts by mass or more, preferably 0.002 parts by mass or more, more preferably 0.005 parts by mass or more, even more preferably 0.007 parts by mass or more, even more preferably 0.01 parts by mass or more, and most preferably 0.02 parts by mass or more, per 100 parts by mass of (A) polyoxymethylene. On the other hand, from the viewpoint of thermal stability and the influence of (B) boron nitride as a foreign matter serving as a fracture starting point, the content of (B) boron nitride is 0.5 parts by mass or less, preferably 0.3 parts by mass or less, more preferably 0.2 parts by mass or less.

By using (B) boron nitride within the above-mentioned specific particle size and content range, it is possible to minimize the spherulite size of (A) polyoxymethylene, improving creep resistance under high stress, and, more surprisingly, providing a polyoxymethylene composition with excellent impact resistance and acid resistance. While the exact mechanism is unknown, it is thought that impact resistance is achieved by eliminating the anisotropy of the crystalline structure of the surface layer of the molded product due to an increased crystallization rate, while acid resistance is achieved by the nitrogen atoms in (B) boron nitride acting as a base.

<(C) Poly-β-alanine polymer>
The polyoxymethylene composition of the present embodiment may contain (C) a poly-β-alanine polymer containing 30 to 70 mol % of primary amide groups.
The poly-β-alanine polymer (C) is mainly composed of a structural unit (3) represented by the following formula (3) and a structural unit (4) represented by the following formula (4).
[-CH ₂ -CH ₂ -CONH-] ...(3)
[-CH ₂ -CH-] ...(4)
｜
CONH ₂
The phrase "mainly consisting of the structural unit (3) and the structural unit (4)" means that the total content of the structural unit (3) and the structural unit (4) in the poly-β-alanine polymer (C) is 95 mol % or more.
The poly-β-alanine polymer (C) may be used alone or in combination of two or more.

As described above, the structural unit (4) contains a primary amide group, and the content of the primary amide group in the poly-β-alanine polymer (C) is 30 to 70 mol %, more preferably 35 to 65 mol %, and even more preferably 40 to 60 mol %. When the content of the primary amide group is within this range, excellent productivity is achieved.
The primary amide group content is measured by first placing a sample polymer and a 40% by mass aqueous potassium hydroxide solution in a flask equipped with a stirrer, and heating the mixture at 105-110°C for 20 minutes while stirring to hydrolyze the primary amide groups to ammonia. The contents of the flask are then cooled to below 50°C, and methanol is added to extract the ammonia together with the methanol. The extract is absorbed in a 0.1 N aqueous sulfuric acid solution, and neutralization titration is performed with a 0.1 N aqueous sodium hydroxide solution using methyl red as an indicator to determine the primary amide group content.

The poly-β-alanine polymer (C) may be any of a low molecular weight polymer soluble in formic acid and a high molecular weight polymer insoluble in formic acid. Preferably, the poly-β-alanine polymer has a molecular weight corresponding to a reduced viscosity (ηsp/c) of 0.5 to 15 dL/g, more preferably 1 to 10 dL/g, and even more preferably 2 to 5 dL/g.
The reduced viscosity (ηsp/c) was measured by first adding 5 g of polymer to 100 mL of formic acid and stirring at room temperature for 2 hours to dissolve the polymer. The solution was then filtered under reduced pressure to obtain a formic acid solution. 500 mL of methanol was added to the formic acid solution to precipitate a formic acid solution. The precipitate was then filtered and dried under reduced pressure in a vacuum dryer at 80°C for 10 hours. The resulting sample was dissolved in formic acid with a purity of 99% or higher and passed through a 200-mesh filter to prepare a sample solution with a sample concentration of 1 g/dL. The drop time of the sample solution at 35°C was measured using an Ostwald viscometer, and the reduced viscosity ηsp/c (dL/g) was calculated using the following formula:
ηsp/c=(t/t ₀ -1)/c
(In the formula, t is the drop time of the sample solution (seconds), _t is the drop time of the formic acid solution (seconds), and c is the concentration of the sample solution (g/dL).)

(C) Poly-β-alanine polymer can be produced, for example, by polymerizing acrylamide using an alcoholate of an alkaline earth metal as a catalyst.
The polymerization reaction of (C) poly-β-alanine polymer may be carried out in the presence or absence of a solvent, such as aromatic hydrocarbons such as benzene, toluene, and xylene, and halogenated aromatic hydrocarbons such as chlorobenzene and o-dichlorobenzene.
The polymerization is carried out, for example, by adding a predetermined amount of catalyst to dehydrated and purified acrylamide and heating the mixture in an inert gas atmosphere. Polymerization methods that can be used include batch solution polymerization, batch bulk polymerization, continuous solution polymerization, and continuous bulk polymerization.
The reaction temperature for this polymerization reaction is usually preferably in the range of 70 to 150°C, more preferably 80 to 130°C.

Furthermore, as the (C) poly-β-alanine polymer, those with a crosslinked structure can also be used. Examples of those with a crosslinked structure include those crosslinked with methylenebisacrylamide. The crosslinked structure improves cohesion and further improves continuous productivity.

From the viewpoint of thermal stability, the content of (C) poly-β-alanine polymer is preferably 0.01 parts by mass or more, and more preferably 0.05 parts by mass or more, per 100 parts by mass of (A) polyoxymethylene. Furthermore, from the viewpoint of the impact on mechanical properties as a foreign matter, the content is preferably 3.0 parts by mass or less, more preferably 1.0 part by mass or less, and even more preferably 0.5 part by mass or less.

<(D) Polyamide Polymer>
The polyoxymethylene composition of the present embodiment may contain (D) a polyamide polymer having a melting point of 140 to 230°C.
From the viewpoint of productivity, the polyamide polymer (D) has a melting point of 140 to 230°C, preferably 150 to 200°C, and more preferably 150 to 180°C.
It is believed that the melting point being in the vicinity of the processing temperature range of (A) polyoxymethylene reduces the resin pressure in the extruder, improving productivity.
The melting point can be measured by differential scanning calorimetry (DSC) in accordance with JIS K7121, specifically by the method described in the examples below.

(D) As the polyamide polymer, any copolymerized polyamide obtained by polycondensation of lactams, diamines, dicarboxylic acids, and ω-aminocarboxylic acids, either alone or in combination, can be used, and it is preferable to use at least one of these.

(D) Polyamide polymers can be obtained by, but are not limited to, ring-opening polymerization of lactams, polycondensation of diamines and dicarboxylic acids, or self-condensation of aminocarboxylic acids.

Specific examples of lactams include ε-caprolactam, enantholactam, and ω-laurolactam.

Diamines can be broadly classified into aliphatic, alicyclic, and aromatic diamines, and specific examples include tetramethylenediamine, hexamethylenediamine, undecamethylenediamine, dodecamethylenediamine, tridecamethylenediamine, 2,2,4-trimethylhexamethylenediamine, 2,4,4-trimethylhexamethylenediamine, 5-methylnanomethylenediamine, 1,3-bisaminomethylcyclohexane, 1,4-bisaminomethylcyclohexane, m-phenylenediamine, p-phenylenediamine, m-xylylenediamine, and p-xylylenediamine.

Dicarboxylic acids can be broadly classified into aliphatic, alicyclic, and aromatic dicarboxylic acids, and specific examples include adipic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, 1,1,3-tridecanedioic acid, 1,3-cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, and dimer acid.

Specific examples of aminocarboxylic acids include ε-aminocaproic acid, 7-aminoheptanoic acid, 8-aminooctanoic acid, 9-aminononanoic acid, 11-aminoundecanoic acid, 12-aminododecanoic acid, and 13-aminotridecanoic acid.

Examples of the polyamide polymer (D) include polyamide 6, polyamide 11, polyamide 12, polyamide 610, polyamide 612, polyamide 6/612, polyamide 6/66/610, etc. Among these, polyamide 6, polyamide 12, polyamide 610, and polyamide 6/66/610 are preferred, and polyamide 12 and polyamide 6/66/610 are more preferred from the viewpoint of productivity.
These polyamide polymers may be used singly or in combination of two or more.

The polyamide polymer (D) preferably has a relative viscosity of 3.0 or less, more preferably 2.5 or less. When the relative viscosity is 3.0 or less, productivity is excellent.
The relative viscosity is measured in accordance with JIS K6920-1: 2000. Specifically, the relative viscosity is a value expressed by "ηr = t 1 /t 0 ", where t ₁ is the flow time measured at 25°C by dissolving a polyamide polymer in 98% concentrated sulfuric acid at a concentration of ₁ g/100 _cm ³ using an Ostwald viscometer and t ₀ is the flow time of 98% concentrated sulfuric acid alone at 25°C.

The content of (D) polyamide polymer is preferably 0.001 to 0.1 parts by mass, more preferably 0.005 to 0.08 parts by mass, and even more preferably 0.005 to 0.05 parts by mass, per 100 parts by mass of (A) polyoxymethylene. Addition of 0.001 parts by mass or more improves productivity, while addition of 0.1 parts by mass or less maintains a low level of retained coloration.

The mass ratio (D)/(B) of (D) polyamide polymer to (B) boron nitride is preferably in the range of 0.04 to 1.0, more preferably 0.07 to 0.7, and even more preferably 0.1 to 0.5. Having the mass ratio (D)/(B) in the above range tends to provide a better balance between strength and acid resistance. This is hypothetical, but it is thought that this is because (B) boron nitride and (D) polyamide polymer both contain nitrogen atoms and contribute to acid resistance as bases. The interaction between two different types of bases—(B) boron nitride present as a solid base in (A) polyoxymethylene and (D) polyamide polymer compatible with (A) polyoxymethylene—effectively neutralizes the acid.

When the contents of (A) polyoxymethylene, (B) boron nitride, (C) poly-β-alanine polymer, and (D) polyamide polymer in the polyoxymethylene composition of this embodiment are each within the specific ranges described above, it is possible to provide a polyoxymethylene composition that has significantly superior creep resistance, impact resistance, acid resistance, and thermal stability, although the detailed mechanism is unknown.

<Other additives>
The polyoxymethylene composition of the present embodiment may further contain various additives, such as pigments, dyes, various reinforcing materials, antioxidants, heat stabilizers, stabilizers such as formaldehyde and/or formic acid scavengers, weathering stabilizers, release agents, lubricants, conductive agents, thermoplastic resins, thermoplastic elastomers, inorganic fillers, or organic fillers, within the range that does not impair the properties of the polyoxymethylene (A).
These additives may be used alone or in combination of two or more.
The content of the additive in the polyoxymethylene composition is preferably 10 parts by mass or less per 100 parts by mass of the polyoxymethylene (A).

<Method of producing polyoxymethylene composition>
The polyoxymethylene composition of this embodiment can be produced by mixing the above-mentioned essential components (A) and (B), optional components (C) and (D), and other additive components using, for example, a Henschel mixer, a tumbler, a V-shaped blender, or the like, and then melt-kneading the mixture using a kneader such as a single-screw extruder, a twin-screw extruder, a heating roll, a kneader, or a Banbury mixer. This produces a product in various forms, such as strands or pellets. Alternatively, each component can be continuously fed into the extruder individually or in batches using a metering feeder or the like, without premixing. Alternatively, a high-concentration masterbatch composed of each component can be prepared in advance and then diluted with polyoxymethylene (A) during extrusion melt-kneading.
The kneading temperature may be determined in accordance with the preferred processing temperature of the polyoxymethylene (A) used, and is generally in the range of 180°C to 240°C, preferably 190°C to 230°C.

The polyoxymethylene composition pellets obtained as described above may be dried before use in the production of a molded article. The drying method is not particularly limited, but examples thereof include drying methods using a box dryer (normal pressure, vacuum), a rotary and ventilated rotary dryer, a grooved agitator dryer, a fluidized bed dryer, etc.
The drying temperature, as the temperature of the heat medium, is preferably 80° C. or higher, and more preferably 100° C. or higher. The drying time, when the drying start time is defined as the time when the product temperature of the polyoxymethylene resin composition pellets reaches 100° C., is preferably 0 to 10 hours, more preferably 0 to 6 hours, and even more preferably 1 to 6 hours.

<Molded Article of Polyoxymethylene Composition>
The molded article of the present embodiment contains the polyoxymethylene composition of the present embodiment described above. The desired molded article can be molded using the polyoxymethylene composition product, such as pellets, obtained as described above.
The method for producing the molded body is not particularly limited, and any of the commonly used known molding methods can be applied, such as injection molding, extrusion molding, vacuum molding, blow molding, injection compression molding, decorative molding, other material molding, gas-assisted injection molding, foam injection molding, low-pressure molding, ultra-thin-wall injection molding (ultra-high-speed injection molding), in-mold composite molding (insert molding, outsert molding), etc.

The present invention will be described in more detail below with reference to Examples , Reference Examples and Comparative Examples, but the present invention is not limited to these Examples in any way.

[Various evaluation methods]
The details of the various evaluation methods used in the Examples , Reference Examples and Comparative Examples are described below.

(1) Average particle size of boron nitride The average particle size of the boron nitride contained in the polyoxymethylene compositions of the Examples , Reference Examples , and Comparative Examples was determined by taking images of the boron nitride observed with a scanning electron microscope and analyzing the images. The measuring device, conditions, and method were as follows:
Apparatus: Scanning electron microscope (SEM) SU8220 manufactured by Hitachi High-Technologies Corporation
Acceleration voltage: 1.0 kV
Detector position: Top Detection target: Secondary electron image Image processing and analysis software: A-zo-kun manufactured by Asahi Kasei Engineering Co., Ltd. Analysis application: Particle analysis Analysis parameters: (Particle brightness) Bright, (Binarization method) Manual, (Area specification) None, (Outer edge correction) Four sides, (Hole filling) Yes, (Area of small shapes removed) 10 pixels, (Correction method) Manual, (Noise removal filter) Yes, (Shading) Yes, (Shading size) 180, (Measurement item selection) Area Boron nitride was sprinkled on carbon tape attached to the SEM sample stage and fixed, followed by osmium plasma coating, and SEM observation was performed. An image of a field of view of 3.0 μm × 3.0 μm obtained by magnifying 50,000 times was edited into a binarized image using the image processing and analysis software (the threshold was manually adjusted so that the dark black areas represented particles and the light black areas represented grain boundaries), and the longest particle diameter of at least 100 boron nitride particles in the SEM image separated into particles and grain boundary areas was measured, and the average value thereof was taken as the average particle diameter (nm).
When measuring the average particle size of boron nitride contained in a polyoxymethylene composition, the organic components in the polyoxymethylene composition can be decomposed and removed by means of incineration, heat treatment under a nitrogen gas atmosphere, or the like, to extract boron nitride, which can then be subjected to the above-mentioned analysis.

(2) Boron Nitride Content The boron nitride content in the polyoxymethylene compositions of the Examples , Reference Examples , and Comparative Examples was quantitatively analyzed for boron element using an inductively coupled plasma mass spectrometer (ICP-MS) according to the following method, and the boron nitride content (parts by mass) per 100 parts by mass of polyoxymethylene was calculated from the amount of boron. The measuring device and method were as follows:
Apparatus: Inductively coupled plasma mass spectrometer (ICP-MS) manufactured by Thermo Fisher Scientific, Model X Series 2
First, as a pretreatment, 0.2 g of polyoxymethylene composition pellets and 6 mL of a mixed acidic solution of nitric acid and hydrofluoric acid were placed in a Teflon (registered trademark) pressure-resistant sealed container, and a heated, pressurized decomposition treatment was performed by microwave irradiation using a microwave sample decomposition apparatus (Milestone, model: ETHOS UP). The decomposed solution was adjusted to a constant volume with ultrapure water to prepare a sample solution, and mass analysis of boron element was performed by ICP-MS measurement. The boron nitride content was calculated from the obtained boron mass. A calibration curve for boron element was created by diluting a boron ICP-MS standard solution (100 μg/mL in water tr. NH ₄ OH) manufactured by AccuStandard with ultrapure water, and then adding an acidic solution of the same concentration as the sample solution to prepare a standard solution, and then performing ICP-MS measurement.

(3) High-temperature, high-stress creep properties (time to failure)
Using a Toshiba IS (100GN) injection molding machine, strip-shaped test specimens measuring 110 mm x 6.5 mm x 3 mm were molded from pellets of the polyoxymethylene compositions of the Examples , Reference Examples, and Comparative Examples at a cylinder temperature of 200°C, injection pressure of 50 MPa, injection speed of 30%, injection time of 15 seconds, cooling time of 25 seconds, and mold temperature of 70°C. These specimens were then left at an ambient temperature of 23±2°C and humidity of 50±10% for at least 24 hours. After leaving the specimens, they were subjected to a creep test using a creep tester (Toyo Seiki Co., Ltd., C200-6) at a set temperature of 80±2°C and a tensile stress of 22 MPa. The time until the specimens broke (time to break (hr)) was measured. Three test specimens were prepared for each polyoxymethylene composition. The break times in Tables 1 and 2 represent the arithmetic mean time for n = 3.
The longer the time until failure, the better the creep resistance against high stress loads.

(4) Charpy Impact Strength ISO dumbbell test specimens (notched) for Charpy impact testing were molded from pellets of the polyoxymethylene compositions of the Examples, Reference Examples , and Comparative Examples using a 5-ounce molding machine (manufactured by Toshiba Machine Co., Ltd., product name "IS-100GN") set at a cylinder temperature of 215°C under conditions of a mold temperature of 90°C, an injection time of 35 seconds, and a cooling time of 15 seconds. Six test specimens were prepared for each polyoxymethylene composition. Charpy impact tests were performed on these six test specimens in accordance with ISO 179/1eA to measure the Charpy impact strength (kJ/m ² ). The values shown in Tables 1 and 2 are arithmetic averages for n=6.
The higher the Charpy impact strength value, the better the impact resistance.

(5) Thermal Stability The thermal stability of the polyoxymethylene composition pellets of the Examples , Reference Examples , and Comparative Examples was evaluated. Using a differential thermobalance TG-DTA (Rigaku's "Thermo plus EVO2"), the pellets were heated from 40°C to 225°C at a heating rate of 30°C/min under a nitrogen gas flow rate of 500 mL/min, and then held at 225°C for 60 minutes. The mass loss rate (percentage) relative to the charged amount of the polyoxymethylene composition was determined and used as an index of thermal stability.

(6) Acid Resistance ISO dumbbell test specimens for Charpy impact testing were molded from pellets of the polyoxymethylene compositions of the Examples, Reference Examples , and Comparative Examples using a 5-ounce molding machine (manufactured by Toshiba Machine Co., Ltd., product name "IS-100GN") set at a cylinder temperature of 215°C under conditions of a mold temperature of 90°C, an injection time of 35 seconds, and a cooling time of 15 seconds. Four test specimens were prepared for each polyoxymethylene composition. 10 mL of sulfuric acid was added to a cylindrical glass container with a radius of 15 mm and a length of 200 mm, and the test specimens were immersed in the container. After storing the specimens in a thermostatic chamber at 23°C for 1 hour, the specimens were rinsed with water and their dimensions were measured. A dimensional change of 5% or less before and after sulfuric acid immersion was evaluated as "Excellent," a change of more than 5% but not more than 8% was evaluated as "Good," a change of more than 8% but not more than 10% was evaluated as "Fair," and a change of more than 10% was evaluated as "Poor." The dimensional change rate before and after immersion in sulfuric acid is an arithmetic average value of n=4.

[Components used in Examples , Reference Examples , and Comparative Examples]
(A) Polyoxymethylene A formaldehyde homopolymer stabilized by terminal acetylation (acetic acid esterification) and having an MFR value (in accordance with ISO 1133, 190° C., 2.16 kg load) of 2 g/10 min was used.

(B) Boron nitride (B-1)
Sold by EM Japan, product name: Boron nitride nanoparticles NP-BN-2
Average particle size: 80 nm.
(B-2)
Sold by ALLIANCE Biosystems, product name: ultrafine boron nitride powder BN-NANO5
Average particle size: 70 nm.
(B-3)
Manufactured by Mizushima Ferroalloy Co., Ltd., product name: BN Boron nitride FS-1 for special industrial applications
Average particle size: 250 nm.
(B-4)
Manufactured by Denka Co., Ltd., product name: Denka Boron Nitride SP-2
Average particle size: 500 nm.
(B-5)
Showa Denko K.K., product name: Boron nitride powder UHP-S2
Average particle size 700nm.
(B-6)
Manufactured by Mizushima Ferroalloy Co., Ltd., product name: BN boron nitride HP-P1 for industrial special applications
Average particle diameter 1000 nm.

(C) Poly-β-alanine polymer (C-1)
2,400 g of acrylamide and 1.08 g of calcium n-propylate (1/5,000 mol relative to acrylamide) as a catalyst were added to a 5-L batch reactor equipped with a stirrer, and the mixture was reacted at 125°C for 4 hours with stirring in a _N2 stream. After completion of the reaction, the solid matter was pulverized, washed with acetone, and dried. The primary amide group content and reduced viscosity of the resulting polymer were as follows:
The content of primary amide groups is 45.0 mol%
Reduced viscosity is 2.3 dL/g
(C-2)
2,400 g of acrylamide and 0.81 g of calcium n-propylate (1/7,500 mol relative to acrylamide) as a catalyst were added to a 5 L batch reactor equipped with a stirrer, and the mixture was reacted at 125°C for 4 hours with stirring in a _N2 stream. After completion of the reaction, the solid matter was pulverized, washed with acetone, and dried. The primary amide group content and reduced viscosity of the resulting polymer were as follows:
The content of primary amide groups is 63.0 mol%
Reduced viscosity is 2.31 dL/g
(C-3)
2,400 g of acrylamide and 0.54 g of calcium n-propylate (1/10,000 mol relative to acrylamide) as a catalyst were added to a 5 L batch reactor equipped with a stirrer, and the mixture was reacted at 125°C for 4 hours with stirring in a _N2 stream. After completion of the reaction, the solid matter was pulverized, washed with acetone, and dried. The primary amide group content and reduced viscosity of the resulting polymer were as follows:
The content of primary amide groups is 75.0 mol%
Reduced viscosity is 2.4 dL/g
(Measurement of Primary Amide Group Content)
The content of primary amide groups in the poly-β-alanine polymer was determined as follows. First, the poly-β-alanine polymer and a 40% by mass aqueous solution of potassium hydroxide were placed in a flask equipped with a stirrer, and the mixture was heated at 105 to 110°C for 20 minutes while stirring, to hydrolyze the primary amide groups to ammonia. Next, the contents of the flask were cooled to below 50°C, and methanol was added to extract the ammonia together with the methanol. The extract was absorbed in a 0.1 N aqueous sulfuric acid solution, and neutralization titration was performed with a 0.1 N aqueous sodium hydroxide solution using methyl red as an indicator to determine the content of primary amide groups.
(Measurement of reduced viscosity)
The reduced viscosity of the poly-β-alanine polymer was determined as follows. First, 5 g of poly-β-alanine polymer was added to 100 mL of formic acid and dissolved by stirring at room temperature for 2 hours. The solution was then filtered under reduced pressure to obtain a formic acid solution. 500 mL of methanol was added to the formic acid solution to precipitate a formic acid solution. The precipitate was filtered and then dried under reduced pressure at 80°C for 10 hours in a vacuum dryer. The obtained sample was dissolved in formic acid with a purity of 99% or higher and passed through a 200-mesh filter to prepare a sample solution with a sample concentration of 1 g/dL. The falling time of the sample solution at 35°C was measured using an Ostwald viscometer, and the reduced viscosity ηsp/c (dL/g) was calculated using the following formula:
ηsp/c=(t/t ₀ -1)/c
(In the formula, t is the drop time of the sample solution (seconds), _t is the drop time of the formic acid solution (seconds), and c is the concentration of the sample solution (g/dL).)

(D) Polyamide Polymer (D-1) Polyamide 6/66/610 Copolymer 0.45 kg of an equimolar salt of adipic acid and hexamethylenediamine, 0.32 kg of an equimolar salt of sebacic acid and hexamethylenediamine, 1.67 kg of ε-caprolactam, and 2.5 kg of pure water were charged into a 5 L autoclave and stirred thoroughly. After thorough _N2 replacement, the temperature was raised from room temperature to 220°C over approximately 1 hour while stirring. During this process, the internal pressure of the autoclave reached 18 kg/cm2-G due to natural pressure caused by water vapor within the autoclave. Heating was continued while removing water from the reaction system to prevent the pressure from exceeding 18 kg/ ^{cm2 -G} . When the internal temperature reached 230°C, overheating was stopped, the autoclave's exhaust valve was closed, and the mixture was allowed to cool to room temperature over approximately 8 hours. After cooling, the autoclave was opened, and approximately 2 kg of polymer was removed and pulverized to obtain a powder. The melting point was 150°C and the relative viscosity was 2.0.
(D-2) Polyamide 12
Daicel-Degussa Co., Ltd., trade name: DIAMID L1700
Since the pellet form had a problem with dispersibility, it was pulverized into powder form. The melting point was 178°C and the relative viscosity was 1.7.
(D-3) Polyamide 66
2.5 kg of an equimolar salt of adipic acid and hexamethylenediamine and 2.5 kg of pure water were charged into a 5 L autoclave and stirred thoroughly. After thorough _nitrogen replacement, the temperature was raised from room temperature to 220°C over approximately 1 hour while stirring. During this process, the internal pressure of the autoclave reached 18 kg/ ^cm2 -G due to natural pressure caused by water vapor. Heating was continued while removing water from the reaction system so that the pressure did not exceed 18 kg/ ^cm2 -G. When the internal temperature reached 260°C, heating was stopped, the autoclave's exhaust valve was closed, and the autoclave was cooled to room temperature over approximately 8 hours. After cooling, the autoclave was opened, and approximately 2 kg of polymer was removed and pulverized to obtain a powder. The melting point was 260°C and the relative viscosity was 2.0.
(Melt point measurement)
The melting point of the polyamide polymer was measured using a differential scanning calorimeter (PerkinElmer, "DSC 8000") in accordance with JIS K7121. 8 mg of polyamide polymer was precisely weighed and used for the measurement. The measurement conditions were as follows: under a nitrogen atmosphere, the temperature was increased from 50°C to 300°C at a heating rate of 20°C/min, held at 300°C for 5 minutes, then decreased to 50°C at a heating rate of 20°C/min, and then increased from 50°C to 300°C at a heating rate of 20°C/min. The endothermic peak that appeared was taken as the peak indicating melting of the polyamide polymer, and the temperature (°C) at the highest endothermic peak was taken as the melting point of the polyamide polymer.
(Measurement of relative viscosity)
The relative viscosity of the polyamide polymer was determined in accordance with JIS K6920-1:2000 by dissolving the polyamide polymer in 98% concentrated sulfuric acid at a concentration of ¹ g/100 cm3, measuring the flow time at 25°C using an Ostwald viscometer as _t1 , and using the formula "relative viscosity ηr= _t1 / _t0 " where _t0 is the flow time of 98% concentrated sulfuric acid alone at 25°C.

(E) Hindered phenol-based antioxidant: Irganox (registered trademark) 245, manufactured by BASF Corporation

[Example 1]
100 parts by mass of (A) polyoxymethylene, 0.02 parts by mass of (B-1) boron nitride, 0.1 parts by mass of (C-1) poly-β-alanine polymer, 0.02 parts by mass of (D-1) polyamide polymer, and 0.2 parts by mass of (E) antioxidant ((D)/(B) mass ratio = 1) were mixed uniformly using a Henschel mixer to obtain a mixture. The mixture was melt-kneaded in a 30Φ single-screw extruder, extruded into a strand shape, cooled, and pelletized to obtain pellets of a polyoxymethylene composition. The extrusion conditions were a cylinder set temperature of 200°C, a discharge rate of 5 kg/hr, a screw rotation speed of 50 rpm, and a vent vacuum of -720 mmHg. The obtained pellets were dried at 80°C for 4 hours. The dried pellets were used to evaluate (2) the boron nitride content, (3) high-temperature, high-stress creep properties (time to failure), (4) Charpy impact strength, (5) thermal stability, and (6) acid resistance using the various evaluation methods described above. The evaluation results are shown in Table 1.

[Examples 2 to 14, 17 to 22 , Reference Examples 15 and 16 ]
The component compositions of the polyoxymethylene compositions were changed as shown in Tables 1 and 2. Other than that, the same procedures as in Example 1 were carried out to obtain dried polyoxymethylene composition pellets, which were then evaluated in the same manner. The evaluation results are shown in Tables 1 and 2.

[Comparative Examples 1 to 5]
The component composition of the polyoxymethylene composition was changed as shown in Table 2. Otherwise, the same procedure as in Example 1 was carried out to obtain dried polyoxymethylene composition pellets, which were then evaluated in the same manner. The evaluation results are shown in Table 2.

As is clear from the results in Tables 1 and 2, by incorporating (A) polyoxymethylene and (B) boron nitride having a specific particle size in a specified content, a polyoxymethylene composition was obtained that combines excellent high-stress creep resistance, impact resistance, acid resistance, and thermal stability.

As described above, the polyoxymethylene composition of the present invention eliminates the trade-off between creep resistance and impact strength caused by the addition of a crystal nucleating agent, and possesses high creep resistance and high impact strength under high stress. It also has excellent acid resistance and thermal stability, making it suitable for use in mechanical parts for automobiles and other applications where reliability is required, as well as in mechanical parts for equipment used under harsh conditions.

Claims (5)

(A) 100 parts by mass of polyoxymethylene; and (B) 0.02 to 0.5 parts by mass of boron nitride,
The polyoxymethylene composition is characterized in that the boron nitride (B) has an average particle size of 10 to 800 nm for at least 100 particles in a field image of 3.0 × 3.0 μm obtained by a scanning electron microscope at a magnification of 50,000 times, as determined using image processing and analysis software ("A-zo-kun" manufactured by Asahi Kasei Engineering Co., Ltd.). The polyoxymethylene composition according to claim 1, wherein the average particle size of the boron nitride (B) is 80 to 500 nm. 3. The polyoxymethylene composition according to claim 1, further comprising: (C) 0.01 to 3.0 parts by mass of a poly-β-alanine polymer containing 30 to 70 mol % of primary amide groups. The polyoxymethylene composition according to any one of claims 1 to 3 , further comprising: (D) 0.001 to 0.1 parts by mass of a polyamide polymer having a melting point of 140 to 230°C. 5. The polyoxymethylene composition according to claim 4 , wherein the mass ratio (D)/(B) of the polyamide polymer (D) to the boron nitride (B) is 0.04 to 1.0.