Method for producing polyoxymethylene resin composition, and polyoxymethylene resin composition

Abstract

To provide a method for producing a polyoxymethylene resin composition capable of reducing the generation of formaldehyde, and to provide a polyoxymethylene homopolymer resin composition.SOLUTION: A method for producing a polyoxymethylene resin composition comprises an esterification step employing at least an esterifier, an esterification catalyst, and a hydrocarbon solvent, wherein an amount of addition of an alkali metal salt of a carboxylic acid as the esterification catalyst in the esterification step is 250 to 500 ppm by mass relative to a total mass of the esterifier and the hydrocarbon solvent.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing a polyoxymethylene resin composition and a polyoxymethylene resin composition.

Background Art

[0002] Polyoxymethylene is an engineering plastic used in a wide range of fields such as the electrical and electronic material field, the automotive field, and various industrial material fields because it has excellent mechanical properties, moldability, and slidability. Polyoxymethylene generally decomposes by the action of heat, light, oxygen, acids, alkalis, etc. to generate formaldehyde, and there is concern about deteriorating the indoor environment. Among polyoxymethylenes, polyoxymethylene homopolymers are excellent in mechanical properties compared to polyoxymethylene copolymers and are used for gear parts and the like, but the drawback is that they are easily decomposed, and a technique for suppressing the decomposition of polyoxymethylene homopolymers is required. The crude polyoxymethylene homopolymer obtained by polymerization has thermally unstable end groups and easily depolymerizes to generate formaldehyde. Therefore, a method is known in which these unstable end groups are blocked by reacting with an esterifying agent and stabilized (see, for example, Patent Document 1 and Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the content disclosed in Patent Documents 1 and 2, the fact is that it is not possible to fully cope with the strengthening of environmental regulations required for current automotive parts and the like, and there is a need for a method for producing a polyoxymethylene resin composition that further suppresses the generation of formaldehyde compared to the prior art.

[0005] Therefore, an object of the present invention is to provide a method for producing a polyoxymethylene resin composition capable of suppressing the generation of formaldehyde, and a polyoxymethylene homopolymer resin composition.

Means for Solving the Problems

[0006] The stabilization treatment of the terminal groups of the crude polyoxymethylene homopolymer by esterification is carried out by respectively charging the crude polyoxymethylene homopolymer, an esterifying agent, an esterification catalyst, and a hydrocarbon solvent which is an inert solvent into a terminal stabilization reactor and reacting them. Usually, an alkali metal salt of a carboxylic acid is used as the esterification catalyst. However, if the catalyst remains in the polyoxymethylene resin composition, it has an adverse effect on the thermal stability. Therefore, it has been considered that a small amount of these additives is better. As a result of intensive studies by the present inventors on the above problems, by adding a larger amount of the terminal stabilization catalyst than in the prior art, the stabilization due to the progress of the terminal stabilization reaction outweighs the adverse effect of the catalyst residue, and it has been found that the generation of formaldehyde can be suppressed by improving the thermal stability of the polyoxymethylene resin composition as a whole.

[0007] That is, the present invention is as follows.

[0008] [1] Including an esterification step of a polyoxymethylene homopolymer using at least an esterifying agent, an esterification catalyst, and a hydrocarbon solvent, A method for producing a polyoxymethylene resin composition, wherein the addition amount of the alkali metal salt of the carboxylic acid, which is the esterification catalyst in the esterification step, is 250 to 500 mass ppm with respect to the total mass of the esterifying agent and the hydrocarbon solvent.

[0009] [2] The method for producing a polyoxymethylene resin composition according to [1], wherein the alkali metal salt of the carboxylic acid as the esterification catalyst is potassium acetate.

[0010] [3] The method for producing a polyoxymethylene resin composition according to [1] or [2], wherein the mass ratio of the hydrocarbon solvent to the esterifying agent is 1.1 to 2.0.

[0011] [4] The method for producing a polyoxymethylene resin composition according to any one of [1] to [3], wherein in the esterification step, the esterification catalyst is added in an aqueous solution state, and the concentration of the esterification catalyst in the aqueous solution is 30 to 70% by mass.

[0012] [5] A polyoxymethylene resin composition, comprising: (a) a polyoxymethylene homopolymer having acetoxy terminals; and (b) potassium, A polyoxymethylene resin composition containing 0.1 to 2.3 mass ppm of (b) potassium based on the mass of the entire polyoxymethylene resin composition. [Advantages of the Invention]

[0013] The method for producing a polyoxymethylene resin composition of the present invention can suppress the generation of formaldehyde. Further, the polyoxymethylene resin composition of the present invention can suppress the generation of formaldehyde. [Embodiments for Carrying Out the Invention]

[0014] Embodiments for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail below. The present invention is not limited to the following present embodiment, and can be variously modified and implemented within the scope of the gist thereof.

[0015] [Method for Producing Polyoxymethylene Resin Composition] The manufacturing method of the polyoxymethylene resin composition of the present embodiment includes an esterification step using at least an esterifying agent, an esterification catalyst, and a hydrocarbon solvent, and is characterized in that the addition amount of the alkali metal salt of carboxylic acid, which is the esterification catalyst in the esterification step, is 250 to 500 mass ppm with respect to the total mass of the esterifying agent and the hydrocarbon solvent. According to the manufacturing method of the polyoxymethylene resin composition of the present embodiment, the generation of formaldehyde can be suppressed. Further, according to such a method, the yield during the terminal stabilization reaction of the polyoxymethylene homopolymer can also be improved.

[0016] The manufacturing method of the polyoxymethylene resin composition of the present embodiment includes an esterification step of a crude polyoxymethylene homopolymer, and may further include other steps. In the above esterification step, an esterifying agent, an esterification catalyst, a hydrocarbon solvent (inert solvent), etc. are used.

[0017] In the above esterification step, an alkali metal salt of carboxylic acid is used as the esterification catalyst. The alkali metal salt of carboxylic acid is not particularly limited, and examples thereof 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, stearic acid. Further, examples of the alkali metal include lithium, sodium, potassium, rubidium, and cesium. As the alkali metal salt of carboxylic acid, potassium acetate is preferable. The addition amount of the alkali metal salt of carboxylic acid is 250 to 500 mass ppm with respect to the total mass of the esterifying agent and the hydrocarbon solvent. The addition amount of the alkali metal salt of carboxylic acid is preferably 250 to 400 mass ppm, more preferably 300 to 400 mass ppm with respect to the total mass of the esterifying agent and the hydrocarbon solvent. When the addition amount of the alkali metal salt of carboxylic acid is within the above range, the esterification reaction can proceed sufficiently, the generation of formaldehyde can be suppressed, and the yield during the terminal stabilization reaction of the polyoxymethylene homopolymer can be improved.

[0018] The above hydrocarbon solvent may be any solvent that does not react with formaldehyde and is not particularly limited. Examples of the hydrocarbon solvent include solvents such as pentane, isopentane, normal hexane, cyclohexane, heptane, octane, nonane, decane, benzene, etc., and normal hexane is particularly preferred. These hydrocarbon solvents may be used alone or in combination of two or more.

[0019] As the above esterifying agent, it is preferable to use acetic anhydride.

[0020] The mass ratio of the hydrocarbon solvent to the esterifying agent is preferably 1.1 to 2.0, more preferably 1.2 to 1.8, and even more preferably 1.3 to 1.7. When the mass ratio of the hydrocarbon solvent to the esterifying agent is within the above range, the esterification reaction can proceed sufficiently, the generation of formaldehyde can be suppressed, and the yield during the terminal stabilization reaction of the polyoxymethylene homopolymer can be improved.

[0021] In the method for producing polyoxymethylene of the present embodiment, the alkali metal salt of the carboxylic acid as the esterification catalyst is added in an aqueous solution state, and the concentration of the esterification catalyst in the aqueous solution is preferably 30 to 70% by mass. The concentration of the esterification catalyst in the aqueous solution is more preferably 40 to 65% by mass, and even more preferably 50 to 65% by mass. When the concentration of the esterification catalyst in the aqueous solution is within the above range, the esterification reaction can proceed sufficiently, the generation of formaldehyde can be suppressed, and the yield during the terminal stabilization reaction of the polyoxymethylene homopolymer can be improved.

[0022] As the reaction temperature in the esterification step, it is preferably 130 to 165°C, more preferably 135 to 160°C, and even more preferably 140 to 160°C. Also, as the reaction time in the esterification step, it is preferably 1 to 100 minutes, more preferably 5 to 100 minutes, and even more preferably 10 to 100 minutes. Regarding the reaction temperature and reaction time in the esterification step, for example, it is preferable that the reaction temperature is 130 to 165°C and the reaction time is 1 to 100 minutes, more preferably that the reaction temperature is 135 to 160°C and the reaction time is 5 to 100 minutes, and even more preferably that the reaction temperature is 140 to 160°C and the reaction time is 10 to 100 minutes.

[0023] After the esterification of the polyoxymethylene homopolymer, it is preferable to wash and remove the esterification catalyst with an organic solvent. The organic solvent to be used is not particularly limited, and examples thereof include solvents such as acetone, isopropyl alcohol, methanol, normal hexane, and cyclohexane, and normal hexane is particularly preferable.

[0024] From the perspective of productivity, the method for producing the polyoxymethylene resin composition of the present embodiment is preferably a production method using an extruder. The production method using an extruder is not particularly limited and may include melt-kneading using a single-screw or multi-screw extruder. Among them, kneading by an extruder equipped with a vent decompression device is preferable from the perspective of productivity. Also, for stably producing a large amount of the polyoxymethylene resin composition, a single-screw or twin-screw extruder is preferably used. The kneading temperature may follow the preferred processing temperature of the polyoxymethylene resin composition to be used. Generally, it is in the range of 140 to 260 °C, preferably in the range of 180 to 230 °C.

[0025] [Polyoxymethylene resin composition] The polyoxymethylene resin composition of the present embodiment (a) a polyoxymethylene homopolymer having acetoxy terminals, and (b) potassium, and contains With respect to the total mass of the polyoxymethylene resin composition, (b) potassium is contained at 0.1 to 2.3 mass ppm.

[0026] The polyoxymethylene resin composition of the present embodiment contains potassium. This potassium may be derived from the potassium salt of a carboxylic acid which is an esterification catalyst used when polymerizing the polyoxymethylene homopolymer. In that case, by adjusting the amount of the esterification catalyst containing potassium, the potassium content in the polyoxymethylene resin composition can be controlled.

[0027] In the polyoxymethylene resin composition of the present embodiment, with respect to the total mass of the polyoxymethylene resin composition, (b) potassium is contained at 0.1 to 2.3 mass ppm, preferably at 0.5 to 2.0 mass ppm, more preferably at 0.5 to 1.5 mass ppm, and even more preferably at 0.6 to 1.3 mass ppm. When the content of (b) potassium is within the above range with respect to the total mass of the polyoxymethylene resin composition, the generation of formaldehyde can be suppressed.

[0028] For the purpose of improving the physical properties, various known additives can be used in the polyoxymethylene resin composition of this embodiment as needed. Examples of the additives include antioxidants, weathering agents, mold release agents, conductive agents, plasticizers, nucleating agents, pigments, dyes, thermoplastic elastomers, other resins, inorganic fillers, organic fillers, and the like.

[0029] Since the polyoxymethylene resin composition of this embodiment is a homopolymer, its molecular weight distribution is unimodal. The molecular weight distribution of the polyoxymethylene resin composition is obtained by GPC measurement. Specifically, a calibration curve method using HFIP (hexafluoroisopropanol) as the eluent and polymethyl methacrylate as the standard substance is used.

[0030] The polyoxymethylene homopolymer contained in the polyoxymethylene resin composition of this embodiment has acetoxy terminals. The acetoxy terminals can be introduced by reacting with an esterifying agent in the esterification of the polyoxymethylene homopolymer. The presence of acetoxy terminals in the polyoxymethylene homopolymer stabilizes it and suppresses the generation of formaldehyde.

[0031] The mass ratio of the polyoxymethylene homopolymer to 100% by mass of the polyoxymethylene resin composition of this embodiment is preferably 50% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, still more preferably 90% by mass or more, and particularly preferably 100% by mass. When the above other components are included, the above mass ratio of the polyoxymethylene homopolymer may be less than 100% by mass.

[0032] (Method for producing polyoxymethylene homopolymer) An example of the method for producing the polyoxymethylene homopolymer will be described below. A polyoxymethylene homopolymer can be produced, for example, by feeding a monomer formaldehyde, a chain transfer agent (molecular weight regulator), and a polymerization catalyst into a polymerization reactor into which a hydrocarbon-based polymerization solvent has been introduced, and polymerizing by a slurry polymerization method. It is preferable to use formaldehyde gas that contains as little as possible impurities having a polymerization termination and chain transfer action during the polymerization reaction, such as water, methanol, and formic acid. If these impurities are present in excess, the target molecular weight product cannot be obtained due to an unexpected chain transfer reaction. Among them, especially for water, it is preferably 100 mass ppm or less, more preferably 50 mass ppm or less, based on 100% by mass of formaldehyde gas. The polymerization method is not limited to the above, and polymerization can also be carried out by a known method.

[0033] The molecular weight of the polyoxymethylene homopolymer can be adjusted by causing chain transfer using a molecular weight regulator such as an anhydrous carboxylic acid or a carboxylic acid. As the molecular weight regulator, propionic anhydride and acetic anhydride are particularly preferable, and acetic anhydride is more preferable.

[0034] The introduction amount of the molecular weight regulator may be adjusted and determined according to the characteristics of the target polyoxymethylene homopolymer (especially the melt flow rate). For example, the polyoxymethylene homopolymer preferably has a melt flow rate (MFR value (conforming to ISO 1133)) in the range of 0.1 to 100 g / 10 minutes, more preferably in the range of 1.0 to 70 g / 10 minutes. By setting the MFR value of the polyoxymethylene homopolymer within the above range, a polyoxymethylene homopolymer having excellent mechanical strength can be obtained.

[0035] As the polymerization catalyst, an anionic polymerization catalyst is preferable, and an onium salt-based polymerization catalyst represented by the following general formula (I) is more preferable. [R1R2R3R4M] + X - ··· (I) (In general formula (I), R1, R2, R3, and R4 each independently represent an alkyl group, M represents an element having a lone pair of electrons, and X represents a nucleophilic group.) Only one kind of polymerization catalyst may be used alone, or two or more kinds may be used in combination. Among the onium salt-based polymerization catalysts, quaternary ammonium salt-based compounds such as tetramethylammonium bromide and dimethyldistearylammonium acetate are preferred. The addition amount of the above polymerization catalyst (for example, the onium salt-based polymerization catalyst of the above quaternary ammonium salt-based compound) is preferably 0.00001 to 0.01 mol, more preferably 0.00003 to 0.005 mol, and still more preferably 0.00005 to 0.003 mol per 1 mol of formaldehyde.

[0036] The hydrocarbon-based polymerization solvent may be any solvent that does not react with formaldehyde and is not particularly limited. For example, solvents such as pentane, isopentane, normal hexane, cyclohexane, heptane, octane, nonane, decane, and benzene can be mentioned, and normal hexane is particularly preferred. These hydrocarbon-based solvents may be used alone or in combination of two or more.

[0037] By subjecting the crude polyoxymethylene homopolymer to esterification in the above-described esterification step, a polyoxymethylene homopolymer with stabilized end groups can be obtained. Thereafter, by using a dryer such as a hot air dryer or a vacuum dryer, adjusting the temperature to 100 to 150°C, removing moisture, and drying, the target polyoxymethylene homopolymer can be obtained.

Examples

[0038] Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. The present invention is not limited by these Examples.

[0039] The measurement methods for Examples and Comparative Examples are shown below. <Formaldehyde generation amount> 0.5 g of a polyoxymethylene homopolymer was heated and melted at 234 °C under a nitrogen atmosphere, and the formaldehyde gas generated during a residence time of 200 minutes was absorbed into a 1 mol / L aqueous sodium sulfite solution. The sodium hydroxide produced was titrated with 1 / 10 N sulfuric acid to determine the amount of formaldehyde generated.

[0040] <Yield of terminal stabilization reaction> The yield of the terminal stabilization reaction (the reaction yield during the terminal stabilization reaction of the polyoxymethylene homopolymer) was determined from the mass of the polyoxymethylene homopolymer before and after the terminal stabilization reaction.

[0041] <Potassium content> 3 g of a polyoxymethylene homopolymer and 20 g of a 1 mol / L aqueous hydrochloric acid solution were placed in a Teflon crucible, heated at 120 °C for 3 hours to decompose the polyoxymethylene homopolymer, and then the decomposed solution was analyzed with an atomic absorption spectrophotometer (AA-7000, manufactured by Shimadzu Corporation) to determine the amount of potassium in the polyoxymethylene resin composition.

[0042] [Examples 1 to 6] Purified formaldehyde, dimethyldistearylammonium acetate as a polymerization catalyst, and acetic anhydride as a chain transfer agent were added to a normal hexane solution at 60 °C and polymerized. The addition amount of the polymerization catalyst was 5.0×10 -5 mol per 1 mol of the monomer (formaldehyde in this example). The addition amount of acetic anhydride was 0.5×10 -3 mol per 1 mol of the monomer. The granular polyoxymethylene slurry of the polymer was filtered with a centrifuge equipped with a filter cloth and dried at 60 °C for 10 hours under a nitrogen atmosphere to obtain a crude polyoxymethylene homopolymer. To 3.8 g of the crude polyoxymethylene homopolymer obtained as described above, 4.8 g of acetic anhydride as an esterifying agent and normal hexane as a hydrocarbon solvent were added such that the mass ratio of the hydrocarbon solvent to the esterifying agent was the value shown in Table 1. Thereafter, potassium acetate was used as an aqueous solution of 60% by mass as an alkali metal salt of a carboxylic acid which is an esterification catalyst, and was added in the amount shown in Table 1 as the concentration of potassium acetate with respect to the mixed solution of acetic anhydride and normal hexane. Thereafter, esterification was carried out by stirring at 160 ° C. for 1 hour under a nitrogen atmosphere. After filtering the polyoxymethylene homopolymer slurry after esterification and washing three times with normal hexane, it was dried under reduced pressure at 100 ° C. for 2 hours to obtain a polyoxymethylene homopolymer. Using the obtained polyoxymethylene homopolymer, evaluation of the potassium content, the amount of formaldehyde generated, and the terminal stabilization reaction yield was carried out. The evaluation results are shown in Table 1.

[0043] [Comparative Example 1] A polyoxymethylene homopolymer was obtained in the same manner as in Examples 1 to 6, except that no esterification catalyst was added and the mass ratio of the hydrocarbon solvent to the esterifying agent was the condition described in Table 2. Using the obtained polyoxymethylene homopolymer, evaluation of the amount of formaldehyde generated and the terminal stabilization reaction yield was carried out. The evaluation results are shown in Table 2.

[0044] [Comparative Examples 2 to 3] A polyoxymethylene homopolymer was obtained in the same manner as in Examples 1 to 6, except that the addition amount of the esterification catalyst and the mass ratio of the hydrocarbon solvent to the esterifying agent were the conditions described in Table 2. Using the obtained polyoxymethylene homopolymer, evaluation of the potassium content, the amount of formaldehyde generated, and the terminal stabilization reaction yield was carried out. The evaluation results are shown in Table 2.

[0045]

Table 1

[0046]

Table 2

[0047] From the results of Examples 1 to 6 and Comparative Examples 1 to 3, it was shown that by setting the addition amount of the esterification catalyst or the potassium amount in the polyoxymethylene homopolymer within a predetermined range, formaldehyde generation can be suppressed, and the reaction yield during the terminal stabilization reaction of the polyoxymethylene homopolymer can be improved.

Industrial Applicability

[0048] The method for producing the polyoxymethylene resin composition of the present invention can provide a polyoxymethylene resin composition with suppressed formaldehyde generation, and the polyoxymethylene resin composition of the present invention is industrially useful as a polyoxymethylene resin composition with suppressed formaldehyde generation.

Claims

1. including an esterification step of a polyoxymethylene homopolymer using at least an esterifying agent, an esterification catalyst, and a hydrocarbon solvent, wherein the addition amount of the alkali metal salt of carboxylic acid, which is the esterification catalyst in the esterification step, is 250 to 500 ppm by mass based on the total mass of the esterifying agent and the hydrocarbon solvent, a method for producing a polyoxymethylene resin composition.

2. The method for producing a polyoxymethylene resin composition according to claim 1, wherein the alkali metal salt of carboxylic acid, which is the esterification catalyst, is potassium acetate.

3. The method for producing a polyoxymethylene resin composition according to claim 1, wherein the mass ratio of the hydrocarbon solvent to the esterifying agent is 1.1 to 2.

0.

4. In the esterification step, the esterification catalyst is added in an aqueous solution state, and the concentration of the esterification catalyst in the aqueous solution is 30 to 70% by mass. The method for producing a polyoxymethylene resin composition according to claim 1.

5. A polyoxymethylene resin composition, (a) a polyoxymethylene homopolymer having acetoxy terminals, and (b) potassium, and containing 0.1 to 2.3 ppm by mass of (b) potassium based on the total mass of the polyoxymethylene resin composition, a polyoxymethylene resin composition.

Images