JP2006070196A - Polyacetal resin composition - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyacetal resin material having high heat stability especially strongly required in recent years, capable of being more decreased in an extremely minor component (for example, formaldehyde) emitted from a resin and a molded product than ever, and having mechanical characteristics at the same time. <P>SOLUTION: A polyacetal resin composition is obtained by adding (B) a glass-based filling material in an amount of 3-200 pts.wt., (C) a boric acid compound in an amount of 0.001-3.0 pts.wt., and (D) a triazine derivative having a nitrogen-containing functional group in an amount of two to ten times the amount of the component (C) to (A) a polyacetal resin in an amount of 100 pts.wt., wherein the polyacetal resin (A) has -OH end groups in an amount of ≤5 mmol/kg, the glass-based filling material (B) has a fibrous shape and is subjected to surface treatment with an aminoalkoxysilane, etc., the boric acid compound (C) comprises at least one kind selected from orthoboric acid, metaboric acid, tetraboric acid, and diboron trioxide, and the triazine derivative (D) comprises a melamine compound. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polyacetal resin composition having excellent mechanical properties.

  In order to improve the mechanical strength of polyacetal resin, it is conventionally known to add a glass-based filler. However, since polyacetal resin is poor in chemical activity, a sufficient reinforcing effect does not appear even if glass-based fillers such as glass beads are simply blended into polyacetal resin and melt-kneaded. Rather, these fillers are not blended. The mechanical strength may be lower than that of reinforced polyacetal resin.

  In order to improve this point, use a glass filler whose surface is treated with an epoxy compound, a silane compound, a titanate compound, or the like, or use these compounds in combination with a glass filler. Has been proposed (Patent Documents 1 and 2).

Moreover, the method of improving mechanical strength is proposed by adding a glass-type filler and a boric acid compound to a polyacetal resin (patent documents 3 and 4).
JP-A-62-91551 JP-A-61-236851 Japanese Patent Laid-Open No. 9-151298 JP-A-9-176443

  However, in a method of blending a glass-based filler surface-treated with an epoxy compound, a silane compound, a titanate compound, or the like into a polyacetal resin, or a method of blending these compounds with a glass-based filler in a polyacetal resin However, the mechanical strength of the polyacetal resin could not be sufficiently improved, and it was not yet satisfactory.

  On the other hand, according to the method of adding a glass-based filler and a boric acid compound to a polyacetal resin as described in Patent Documents 3 and 4, a considerable improvement in mechanical strength was possible. As the quality of the polyacetal resin itself is improved, there has been a situation in which sufficient mechanical strength cannot be improved with this composition. The cause is not necessarily clear, but is generally estimated as follows. That is, in recent years, there has been a strong demand for high thermal stability with respect to polyacetal resins and further reduction of extremely small amounts of components (for example, formaldehyde and the like) released from the resins and molded articles. These characteristics are closely related to the types of terminal groups and the amount of unstable terminal parts of the polyacetal resin, and the improvement is to reduce the hydroxyl groups, which are the active terminal groups slightly possessed by the polyacetal resin. Therefore, it is presumed that sufficient adhesion between the glass-based inorganic filler and the polyacetal resin cannot be obtained only by adding boric acid, and as a result, improvement in mechanical strength cannot be achieved.

  The present invention solves the problems of the prior art, provides a polyacetal resin material having further excellent mechanical properties, and also provides a polyacetal resin material having various stability and mechanical properties that have been particularly demanded in recent years. The purpose is to provide.

  As a result of intensive studies to solve such problems and to obtain a reinforced polyacetal resin composition having excellent mechanical properties, the present inventors have found that a small amount of boric acid compounds and amino groups are added to the polyacetal resin together with a glass-based filler. It has been found that such problems can be remarkably improved by blending a plurality of organic compounds (triazine derivatives having nitrogen-containing functional groups), and the present invention has been completed.

That is, the present invention
(A) For 100 parts by weight of polyacetal resin
(B) 3 to 200 parts by weight of glass filler
(C) Boric acid compound 0.001 to 3.0 parts by weight
(D) It relates to a polyacetal resin composition comprising a triazine derivative having a nitrogen-containing functional group added in an amount 2 to 10 times the amount of component (C).

The configuration of the present invention will be described below. The polyacetal resin (A) used in the present invention is a polymer compound having an oxymethylene group (—CH ₂ O—) as a main structural unit, and is basically a polyoxymethylene homopolymer consisting of only a repeating unit of an oxymethylene group, Copolymers (including block copolymers) and terpolymers containing a small amount of other structural units in addition to the oxymethylene group may be used, and the molecule may have a branched or crosslinked structure as well as a linear shape. .

  In general, homopolymers are produced by polymerization of anhydrous formaldehyde or polymerization of trioxane, a cyclic trimer of formaldehyde. Usually stabilized against thermal degradation by end caps.

Copolymers of about 85 to 99.9 mole% -CH ₂ O-repeating group, the general formula:

Wherein R ₁ and R ₂ are each selected from the group consisting of hydrogen, lower alkyl and halogen substituted lower alkyl groups, each R ₃ is methylene, oxymethylene, lower alkyl and haloalkyl substituted methylene, and lower alkyl and haloalkyl substituted Selected from the group consisting of oxymethylene groups, m is an integer from 0 to 3, and each lower alkyl group is one having from 1 to 2 carbon atoms). A high molecular compound having an average molecular weight of 5000 or more, and generally a formaldehyde or a cyclic oligomer of formaldehyde represented by the general formula (CH ₂ O) _n (where n is an integer of 3 or more), for example, trioxane, Manufactured by copolymerizing with cyclic ether and / or cyclic formal, usually removing terminal unstable parts by hydrolysis It is stabilized against thermal decomposition Te. Examples of cyclic ether or cyclic formal for copolymerization include ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, and the like.

  In addition to the above components, a component for adjusting the molecular weight can be used in combination. As a component for adjusting the molecular weight, a chain transfer agent that does not form an unstable terminal, that is, an alkoxy group such as methylal, methoxymethylal, dimethoxymethylal, trimethoxymethylal, and oxymethylene di-n-butyl ether. 1 type or 2 types or more of the compound which has is illustrated. Further, the terpolymer is produced by adding a monofunctional compound capable of forming a branched chain such as a monoglycidyl ether compound or a polyfunctional compound such as a diglycidyl ether compound in the copolymerization, and then polymerizing the terpolymer.

  Although there is no restriction | limiting in particular in the polyacetal resin used for this invention, The thing with few -OH terminal groups is preferable and it is especially preferable that it is 5 mmol / kg or less. When polyacetal resin with a large number of --OH end groups is used, there is an advantage that high mechanical properties can be obtained, but due to instability of end groups, many mold deposits are generated during molding. Also, the emission of formaldehyde from the molded article will increase. In contrast, the use of a polyacetal resin having a small number of —OH end groups is a factor that is disadvantageous from the viewpoint of mechanical properties. However, in the composition of the present invention, the mechanical combination of the compounding components is not limited. Since the physical properties can be maintained at a high level and the number of —OH terminal groups is small, the amount of formaldehyde generated and released from the polyacetal resin and its molded product is greatly reduced, and both characteristics are balanced. Will be.

  The number of —OH terminal groups of such a polyacetal resin can be adjusted as follows.

  The amount of water and the amount of catalyst during the polymerization greatly contribute to the number of —OH terminal groups of the polyacetal resin obtained by polymerization, and the proportion of the amount of catalyst contributed as the amount of water during the polymerization increases It is known that the amount of water during polymerization acts dominantly. In particular, in the production of the copolymer, the amount of water contained in the main monomer such as trioxane is greatly affected. That is, a polyacetal resin having a large number of —OH terminal groups can be obtained by using a raw material monomer having a high water content. On the other hand, as the amount of water contained in the monomer decreases, the proportion of the catalyst amount that contributes to the formation of —OH end groups gradually increases. By reducing the amount of the catalyst, a polyacetal resin having a smaller number of —OH end groups can be obtained.

  Next, as the glass-based filler (B) used in the present invention, there are no particular restrictions on the shape, such as fiber shape, granular shape, powder shape, plate shape, hollow shape, and representative examples thereof are glass fiber, Glass beads, milled glass fibers, glass flakes, and glass balloons are exemplified, and glass fibers are particularly preferable. In the present invention, these glass fillers can be used alone or in admixture of two or more depending on the purpose. In the present invention, the amount of the glass-based filler (B) is 3 to 200 parts by weight, preferably 5 to 150 parts by weight, particularly preferably 10 to 100 parts by weight, based on 100 parts by weight of the polyacetal resin (A). Part. If the amount is less than 3 parts by weight, the mechanical properties are not sufficiently improved. If the amount exceeds 200 parts by weight, the molding process becomes difficult.

  In the present invention, these glass-based inorganic fillers may be untreated, but it is preferable to use those which have been surface-treated with a silane-based or titanate-based coupling agent or the like.

  Examples of the silane coupling agent include vinyl alkoxy silane, epoxy alkoxy silane, amino alkoxy silane, mercapto alkoxy silane, allyl alkoxy silane, and the like, and amino alkoxy silane is particularly preferable.

  Examples of the vinylalkoxysilane include vinyltriethoxysilane, vinyltrimethoxysilane, vinyltris (β-methoxyethoxy) silane, and the like.

  Examples of the epoxyalkoxysilane include γ-glycidoxypropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, and the like.

  Examples of the aminoalkoxysilane include γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-aminopropylmethyldimethoxysilane, γ-aminopropyltrimethoxysilane, and N-phenyl-γ-aminopropyltrimethoxy. Silane etc. are mentioned.

  Examples of mercaptoalkoxysilanes include γ-mercaptopropyltrimethoxysilane and γ-mercaptopropyltriethoxysilane.

  Examples of allylalkoxysilane include γ-diallylaminopropyltrimethoxysilane, γ-allylaminopropyltrimethoxysilane, γ-allylthiopropyltrimethoxysilane, and the like.

  Examples of titanate-based surface treatment agents include titanium-i-propoxyoctylene glycolate, tetra-n-butoxy titanium, tetrakis (2-ethylhexoxy) titanium, and the like. Although any desired surface treatment agent can be used, the desired effect of the present invention can be obtained. For the purpose of the present invention, aminoalkoxysilane is a particularly preferred surface treatment agent.

  The amount of the surface treatment agent used is 0.01 to 20 parts by weight, preferably 0.05 to 10 parts by weight, particularly preferably 0.05 to 5 parts by weight, based on 100 parts by weight of the glass filler.

  Moreover, in glass fiber, what uses a polymer binder, an adhesion promoter, another adjuvant, etc. as a sizing agent is used suitably. As the polymer binder, generally known materials such as organic materials such as water-dispersible / water-soluble polyvinyl acetate, polyester, epoxide, polyurethane, polyacrylate or polyolefin resin, and mixtures thereof are preferably used.

  Next, examples of the boric acid compound (C) used in the present invention include orthoboric acid, metaboric acid, tetraboric acid, and diboron trioxide, and commercially available products can be used. The compounding amount of the boric acid compound (C) in the present invention is 0.001 to 3 parts by weight, preferably 0.005 to 1 part by weight, particularly preferably 0.01 to 0.5 part by weight, based on 100 parts by weight of the polyacetal resin (A). is there. If it is less than 0.001 part by weight, the desired effect cannot be obtained, and if it exceeds 3 parts by weight, thermal stability becomes a problem.

  Examples of the triazine derivative having a nitrogen-containing functional group (D) used in the present invention include guanamine, melamine, N-butylmelamine, N-phenylmelamine, N, N-diphenylmelamine, N, N-diallylmelamine. N, N ′, N-triphenylmelamine, benzoguanamine, acetoguanamine, 2,4-diamino-6-butyl-sym-triazine, ameline, 2,4-diamino-6-benzyloxy-sym-triazine, 2,4- Diamino 6-butoxy-sym-triazine, 2,4-diamino 6-cyclohexyl-sym-triazine, 2,4-diamino 6-chloro-sym-triazine, 2,4-diamino 6-mercapto-sym-triazine, 2, 4-dioxy-6-amino sym-triazine, 2-oxy-4,6-diamino s ym-triazine, 1,1-bis- (3,5-diamino2,4,6-triazinyl) methane, 1,2-bis- (3,5-diamino-2,4,6-triazinyl) ethane [other names ( Succinoguanamine)], 1,3-bis- (3,5) -diamino 2,4,6-triazinyl) propane, 1,4-bis- (3,5-diamino 2,4,6-triazinyl) butane Methyleneated melamine, ethylene dimelamine, triguanamine, melamine cyanurate, ethylene dimelamine cyanurate, triguanamine cyanurate and the like. These triazine derivatives may be used alone or in combination of two or more. Preferred are guanamine and melamine, with melamine being particularly preferred.

  The blending amount of the triazine derivative having a nitrogen-containing functional group as the component (D) is 2 to 10 times (weight ratio), preferably 3 times the amount of the boric acid compound added to the component (C). -8 times, particularly preferably 4 times to 6 times. If it is less than 2 times, it is difficult to obtain the desired effect, and if it is more than 10 times, the additive may ooze out and the physical properties may be lowered.

  The polyacetal resin composition of the present invention may further contain various known stabilizers / additives. Examples of the stabilizer include any one or more of hindered phenol compounds, alkali or alkaline earth metal hydroxides, inorganic salts, carboxylates, and the like. The additive used in the present invention is a general additive for thermoplastic resins, for example, any of coloring agents such as dyes and pigments, lubricants, nucleating agents, release agents, antistatic agents, and surfactants. 1 type or 2 or more types can be mentioned.

  In addition, if it is in a range that does not significantly reduce the performance of the molded product that is the object of the present invention, known inorganic, organic, and metallic fibers other than glass-based fillers, plates, powders, etc. It is also possible to add one or two or more of these fillers in combination. Examples of such fillers include talc, mica, wollastonite, carbon fiber and the like, but are not limited thereto.

  The method for preparing the composition of the present invention is not particularly limited, and it can be easily prepared by a known facility and method generally used as a conventional resin composition preparation method. For example, i) a method in which each component is mixed and then kneaded and extruded by an extruder to prepare pellets, and thereafter molded, ii) pellets having different compositions are once prepared, and a predetermined amount of the pellets are mixed to form. Any method can be used, such as a method of obtaining a molded product having a desired composition after molding, or a method of directly charging one or more of each component into a molding machine. Further, mixing a part of the resin component as a fine powder with other components and adding it is a preferable method for uniformly blending these components.

  The resin composition according to the present invention can be molded by any of extrusion molding, injection molding, compression molding, vacuum molding, blow molding, and foam molding.

The following examples further illustrate the present invention, but the present invention is not limited thereto. The evaluation was performed by the following method.
<Tensile strength and elongation>
A tensile test piece according to ISO 3167 was left for 48 hours under conditions of a temperature of 23 ° C. and a humidity of 50%, and the measurement was performed according to ISO 527.
<Terminal-OH group number of polyacetal resin>
A polyacetal resin dissolved in HFIP (hexafluoroisopropanol) and silylated was subjected to NMR to measure the number of terminal —OH groups.
<Amount of mold deposit>
The sample polyoxymethylene composition was continuously molded for 24 hours under the following conditions using an injection molding machine, and then the amount of the deposit on the mold was visually observed, and A (very small) -B-C-D-E (many Evaluation was made in five stages in the order of (there were deposits on the entire surface).
(Mold)
Disk-shaped molded product with a diameter of 20 mm and a thickness of 2 mm (one-point gate)
(Molding condition)
Injection molding machine; Toshiba IS30EPN (manufactured by Toshiba Machine Co., Ltd.)
Cylinder temperature: 210 ℃
Injection pressure: 750kg / cm ²
Injection time: 4 sec
Cooling time: 3 sec
Mold temperature: 30 ℃
(Evaluation method for mold deposits)
Since it is extremely difficult to quantitatively measure the amount of mold deposits, the range of deposits attached to the mold cavity and the state of the deposits (rainbow color when the deposits form a thin layer) And a white color along with the deposits of deposits) were comprehensively evaluated. That is, the adhering substance range was used as a temporary reference, and the final evaluation was made by adjusting the adhering substance state in consideration of the adhering condition.
A: The range of deposits is less than about 10% in the mold cavity. B: The range of deposits is about 10 to 30% in the mold cavity. C: The range of deposits is about half of the mold cavity ( 30-60%)
D: The range of the deposit is about 60 to 80% in the mold cavity E: The range of the deposit is almost the entire surface in the mold cavity (about 80 to 100%)
[Polyacetal resin used]
-Preparation of polyacetal resin (a-1) Using a biaxial polymerization machine having a jacket for a heat (cold) medium on the outside, adjusting the jacket temperature to 80 ° C and rotating the rotating shafts in different directions at 50 rpm, Polymerization was carried out by continuously supplying trioxane and 1,3-dioxolane, continuously adding boron trifluoride as a catalyst, and further adding methylal as a molecular weight regulator. The feed ratio of trioxane and 1,3-dioxolane was adjusted to 97: 3 by weight, the amount of catalyst added was adjusted to 30 ppm with respect to the total monomers, and the amount of methylal added was adjusted to 1000 ppm. The average residence time in the polymerization reactor was 8 minutes. Further, the water content of trioxane used here was 10 ppm, and the water content of 1,3-dioxolane was 20 ppm.

  The reaction product discharged from the polymerization machine is quickly passed through a crusher to deactivate the catalyst in addition to an aqueous solution at 60 ° C. containing 0.05% by weight of triethylamine, and after separation, washing and drying, crude polyacetal A resin was obtained.

The obtained polymer was dissolved in hexafluoroisopropanol, and the —OH end group of the polymer was silylated and subjected to NMR measurement to confirm that the number of —OH end groups was 4..8 mmol / kg. The melt index (measured at 190 ° C. and a load of 2160 g) was 9.2 g / min.
-Preparation of polyacetal resin (a-2) Polyacetal resin (a-2) was prepared in the same manner as the preparation of polyacetal resin (a-1) except that the amount of catalyst added was reduced to 15 ppm.

The obtained polymer had a —OH terminal group number of 3.1 mmol / kg and a melt index of 9.9 g / min.
-Preparation of polyacetal resin (a-3)-(a-5) Using trioxane which changed water content by adding water to trioxane used for preparation of polyacetal resin (a-1), and of trioxane The polyacetal resin (a-1) except that the amount of the molecular weight modifier added is changed in order to adjust the molecular weight (melt index as a substitute property) of the polymer to be almost the same even when the water content is changed. The polyacetal resins (a-3) to (a-5) were prepared in the same manner as in the above.

  Table 1 shows the water content of trioxane used for the preparation of the polyacetal resins (a-3) to (a-5), the addition amount of the molecular weight regulator (methylal), the number of —OH terminal groups of the obtained polymer, and the melt index. Show.

Examples 1-13, Comparative Examples 1-12
The ratio shown in Table 2 for various glass fibers (B1 to B3), boric acid compounds (C1 to C3) and triazine derivatives (D1 to D3) having a nitrogen-containing functional group shown below in the polyacetal resin (a-1) The mixture was melted and kneaded with an extruder having a cylinder temperature of 200 ° C. to prepare a pellet-shaped composition. Next, a test piece was molded from the pellet-shaped composition using an injection molding machine, and physical properties were evaluated. The results are shown in Table 2.

On the other hand, for comparison, (C) when no boric acid compound is added, (D) when a triazine derivative having a nitrogen-containing functional group is not added, (D) when the amount of component is outside the scope of the present invention, and ( In the case where neither C) nor (D) was added, a pellet-like composition was prepared in the same manner, and physical properties were evaluated. The results are also shown in Table 2.
<Used glass filler>
B1: Glass fiber surface-treated with γ-aminopropyltriethoxysilane
B2: Glass fiber surface-treated with titanium-i-propoxyoctylene glycolate
B3: Glass fiber obtained by further treating epoxide with B1 as polymer binder <Boric acid compound used>
C1: Orthoboric acid
C2: Metaboric acid
C3: Tetraboric acid <Triazine derivative having nitrogen-containing functional group used>
D1: Melamine
D2: Guanamin
D3: Triguanamine

Examples 14-23
Triazine derivatives (D1 to D3) having glass fibers (B1 to B3), boric acid compounds (C1 to C3) and nitrogen-containing functional groups on polyacetal resins (a-1) to (a-5) having different numbers of terminal-OH groups ) Were blended in the proportions shown in Table 3, and melt-kneaded with an extruder having a cylinder temperature of 200 ° C. to prepare a pellet-shaped composition. Next, a test piece was molded from the pellet-shaped composition using an injection molding machine, and physical properties were evaluated. The results are shown in Table 3.

Examples 24-31, Comparative Examples 13-25
The ratio shown in Table 4 for various glass beads (B4 to B7), boric acid compounds (C1 to C3) and triazine derivatives (D1 to D3) having nitrogen-containing functional groups shown below in the polyacetal resin (a-1) The mixture was melted and kneaded with an extruder having a cylinder temperature of 200 ° C. to prepare a pellet-shaped composition. Next, a test piece was molded from the pellet-shaped composition using an injection molding machine, and physical properties were evaluated. The results are shown in Table 4.

On the other hand, for comparison, the same applies when (C) a boric acid compound is not added, (D) a triazine derivative having a nitrogen-containing functional group is not added, and both (C) and (D) are not added. A pellet-shaped composition was prepared, and physical properties were evaluated. The results are also shown in Table 4.
<Used glass filler>
B4: Glass beads without surface treatment agent
B5: Glass beads surface-treated with γ-aminopropyltriethoxysilane
B6: Glass beads surface-treated with vinyltriethoxysilane
B7: Glass beads surface-treated with γ-glycidoxypropyltriethoxysilane

Examples 32-41, Comparative Examples 26-37
Table 5 shows the various milled glass fibers (B8 to B9), boric acid compounds (C1 to C3), and triazine derivatives (D1 to D3) having nitrogen-containing functional groups shown below in the polyacetal resin (a-1). The mixture was blended at a ratio and melt-kneaded with an extruder having a cylinder temperature of 200 ° C. to prepare a pellet-shaped composition. Next, a test piece was molded from the pellet-shaped composition using an injection molding machine, and physical properties were evaluated. The results are shown in Table 5.

On the other hand, for comparison, a pellet-like composition was prepared in the same manner and evaluated for physical properties when (C) a boric acid compound was not added and (D) a triazine derivative having a nitrogen-containing functional group was not added. It was. The results are also shown in Table 5.
<Used glass-based inorganic filler>
B8: Milled glass fiber without surface treatment agent
B9: Milled glass fiber surface-treated with γ-aminopropyltriethoxysilane

Examples 42 to 46, Comparative Examples 38 to 43
The glass flakes (B10), boric acid compounds (C1 to C3) and triazine derivatives having nitrogen-containing functional groups (D1 to D3) shown below are blended in the ratio shown in Table 6 to the polyacetal resin (a-1). A pellet-shaped composition was prepared by melt-kneading with an extruder having a cylinder temperature of 200 ° C. Next, a test piece was molded from the pellet-like composition using an injection molding machine, and the physical properties shown below were evaluated. The results are shown in Table 6.

On the other hand, for comparison, a pellet-like composition was prepared in the same manner and evaluated for physical properties when (C) a boric acid compound was not added and (D) a triazine derivative having a nitrogen-containing functional group was not added. It was. The results are also shown in Table 6.
<Used glass filler>
B10: Glass flakes surface-treated with γ-aminopropyltriethoxysilane

Claims (7)

(A) For 100 parts by weight of polyacetal resin
(B) 3 to 200 parts by weight of glass filler
(C) Boric acid compound 0.001 to 3.0 parts by weight
(D) A polyacetal resin composition comprising a triazine derivative having a nitrogen-containing functional group added in an amount 2 to 10 times the amount of component (C).   (B) The polyacetal resin composition according to claim 1, wherein the glass-based filler is glass fiber.   (B) The polyacetal resin composition according to claim 1, wherein the glass-based filler is selected from glass beads, milled fibers, and glass flakes.   (B) The polyacetal resin composition according to any one of claims 1 to 3, wherein the glass-based filler is surface-treated with aminoalkoxysilane.   (C) The polyacetal resin composition according to any one of claims 1 to 4, wherein the boric acid compound is at least one selected from orthoboric acid, metaboric acid, tetraboric acid and diboron trioxide.   (D) The polyacetal resin composition according to any one of claims 1 to 5, wherein the triazine derivative having a nitrogen-containing functional group is a melamine compound.   The polyacetal resin composition according to any one of claims 1 to 6, wherein (A) the number of -OH terminal groups of the polyacetal resin is 5 mmol / kg or less.