Method for producing polyacetal

Abstract

The invention relates to a method for producing polyacetal. The present invention addresses the problem of providing a method with which it is possible to produce a polyacetal having good thermal stability even when a boron trifluoride ether complex is diluted and then used, and to produce such a polyacetal with good yield and stability. A method for producing a polyacetal, said method comprising a polymerization step in which (a) at least one cyclic ether and / or cyclic formal containing trioxymethylene and optionally a non-cyclic low-molecular-weight acetal compound are supplied; (b) a polymerization catalyst comprising boron trifluoride or a boron trifluoride complex; and (c) an organic solvent having an ester structure represented by formula (1), the water content of (a) being 0.01-30 ppm by mass, and the water content of (c) being 1-50 ppm by mass. # imgabs0 # (1) (In formula (1), R1 and R2 each independently represent a monovalent hydrocarbon group having 1-5 carbon atoms).

Description

Technical Field

[0001] The present invention relates to a method for producing polyacetal. Background Art

[0002] Polyacetal has an excellent balance of mechanical properties, chemical resistance, sliding properties, etc., and is easy to process, so it is widely used as a representative engineering plastic, mainly in electrical parts, electronic parts, automobile parts, and other various mechanical parts. In recent years, as its scope of use has expanded, it has gradually tended to require high mechanical properties.

[0003] As a method for producing polyacetal, cationic polymerization of cyclic ethers such as 1,3,5-trioxane (hereinafter, sometimes simply referred to as trioxane) is widely used.

[0004] Generally, as a polymerization catalyst for trioxymethylene, an ether complex of boron trifluoride (e.g., boron trifluoride ethyl ether complex, boron trifluoride dibutyl ether complex) is often used, and in this case, the catalyst is often diluted with a solvent before use. For example, the following Patent Document 1 discloses a method for producing polyacetal using a cyclohexane solution of a boron trifluoride ethyl ether complex.

[0005] Patent Document 2 listed below discloses a method for producing polyacetal using a dibutyl ether solution of a boron trifluoride dibutyl ether complex.

[0006] Patent Document 3 below discloses a method for producing polyacetal using a mixed solution of boron trifluoride dimethyl ether complex or boron trifluoride ethyl ether complex with a carboxylic acid ester as a method for producing polyacetal having good thermal stability and good mechanical properties at a high conversion rate without containing an aromatic solvent.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2022 / 085280

[0010] Patent Document 2: International Publication No. 2020 / 137414

[0011] Patent Document 3: International Publication No. 2018 / 134075 Summary of the invention

[0012] Problems to be solved by the invention

[0013] For example, in order to obtain a polymer having a high molecular weight and a controlled molecular weight distribution in a twin-shaft paddle type continuous polymerization machine, it is necessary to disperse the polymerization catalyst in the monomer component (e.g., trioxymethylene) without losing the activity of the polymerization catalyst, and to perform polymerization as uniformly as possible in the polymerization machine.

[0014] For this purpose, it is necessary to design the polymerization catalyst to be diluted with a solvent and then dispersed in the monomer component, and after the polymerization catalyst is dispersed in the monomer, polymerization is initiated by contact between the polymerization catalyst and the monomer.

[0015] However, when the ether complex of boron trifluoride is diluted with an organic solvent before use, for example, the thermal stability of the produced polymer may be reduced. Even if the thermal stability of the obtained polymer is relatively good, the yield is low.

[0016] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for stably producing polyacetal in a high yield, wherein the polyacetal produced has good thermal stability even when the ether complex of boron trifluoride is used after dilution.

[0017] Means used to solve problems

[0018] The present inventors have conducted repeated studies to solve the above-mentioned problems and, as a result, have found that when at least one cyclic ether and / or cyclic formal containing trioxymethylene and optionally an acyclic low molecular weight acetal compound (hereinafter, sometimes referred to as "monomer component"), a polymerization catalyst containing boron trifluoride or a boron trifluoride complex, and an organic solvent are supplied to a polymerization reactor and polymerization is carried out, by using a specific carboxylic acid ester as the above-mentioned organic solvent and adjusting the water content of the carboxylic acid ester and the water content of the monomer component to specific ranges, the above-mentioned problems can be solved, thereby completing the present invention.

[0019] That is, the present invention is as follows.

[0020] [1] A method for producing polyacetal, comprising a polymerization step, wherein in the polymerization step:

[0021] (a) at least one cyclic ether and / or cyclic formal, and optionally a non-cyclic low molecular weight acetal compound comprising trioxymethylene;

[0022] (b) a polymerization catalyst comprising boron trifluoride or a boron trifluoride complex; and

[0023] (c) an organic solvent having an ester structure represented by the following formula (1), and performing a polymerization reaction,

[0024] (1)

[0025] (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms)

[0026] The water content of (a) is 0.01 ppm to 30 ppm by mass,

[0027] The water content of (c) is 1 to 50 ppm by mass.

[0028] [2] The production method according to [1], wherein the content of the carboxylic acid contained in (c) which is the same type as the carboxylic acid generated by hydrolysis of the organic solvent having the ester structure represented by the above formula (1) is 1 to 20 ppm by mass.

[0029] [3] The production method according to [1] or [2], wherein the (c) is an organic solvent in which R1 in the formula (1) is a methyl group.

[0030] [4] The production method according to any one of [1] to [3], wherein the (c) is ethyl acetate.

[0031] Effects of the Invention

[0032] According to the present invention, polyacetal having excellent thermal stability can be produced stably with good yield. DETAILED DESCRIPTION

[0033] Hereinafter, an embodiment for carrying out the method for producing polyacetal of the present invention (hereinafter simply referred to as “this embodiment”) will be described in detail.

[0034] The method for producing polyacetal disclosed in the present embodiment is characterized in that the method includes a polymerization step, and in the polymerization step, supplying:

[0035] (a) at least one cyclic ether and / or cyclic formal, and optionally a non-cyclic low molecular weight acetal compound comprising trioxymethylene;

[0036] (b) a polymerization catalyst comprising boron trifluoride or a boron trifluoride complex; and

[0037] (c) an organic solvent having an ester structure represented by the following formula (1), and performing a polymerization reaction,

[0038] (1)

[0039] (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms)

[0040] The water content of (a) is 0.01 ppm to 30 ppm by mass,

[0041] The water content of (c) is 1 to 50 ppm by mass.

[0042] According to the above production method, polyacetal having excellent thermal stability can be produced stably with good yield.

[0043] Hereinafter, the compounds used in the method for producing polyacetal according to the present embodiment will be described.

[0044] <(a) at least one cyclic ether and / or cyclic formal, and optionally a non-cyclic low molecular weight acetal compound>

[0045] In the method for producing polyacetal of the present embodiment, as monomer components, (a) at least one cyclic ether and / or cyclic formal containing trioxymethylene, and an optional non-cyclic low molecular weight acetal compound are used. Cyclic ether and / or cyclic formal refers to a cyclic compound having at least one carbon-oxygen bond or carbon-oxygen-carbon bond, and further, a polymer (trimer, tetramer, etc.) of formaldehyde. Examples of cyclic ether and / or cyclic formal include trioxymethylene, tetraoxymethylene, ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, styrene oxide, oxetane, 1,3-dioxolane, ethylene glycol formal, propylene glycol formal, diethylene glycol formal, triethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal, 1,6-hexanediol formal, etc. The cyclic ether and / or cyclic formal may be used alone (that is, trioxymethylene alone), or two or more thereof may be used in combination.

[0046] At least one cyclic ether and / or cyclic formal used in the method for producing polyacetal of the present embodiment comprises trioxymethylene. It should be noted that trioxymethylene is a cyclic trimer of formaldehyde, which is usually obtained by reacting a formaldehyde aqueous solution in the presence of an acidic catalyst. The trioxymethylene sometimes contains impurities such as water, methanol, formic acid, and methyl formate that cause chain transfer, so it is preferred to remove these impurities in advance by methods such as distillation and purify.

[0047] When trioxane is purified as described above, the total amount of impurities causing chain transfer is preferably adjusted to 1×10 -3 mol or less, more preferably adjusted to 0.5×10 -3 By reducing the amount of impurities within the above range, the polymerization reaction rate can be sufficiently increased for practical use, and various properties such as thermal stability of the produced polymer can be improved.

[0048] As the cyclic ether and / or cyclic formal other than trioxane, 1,3-dioxolane and 1,4-butanediol formal are particularly preferred.

[0049] In addition, when trioxymethylene and a cyclic ether and / or cyclic formal other than trioxymethylene are used in combination as at least one cyclic ether and / or cyclic formal, the amount of the cyclic ether and / or cyclic formal other than trioxymethylene added is preferably in the range of 1 mol% to 20 mol%, more preferably in the range of 1 mol% to 15 mol%, further preferably in the range of 1 mol% to 10 mol%, and further preferably in the range of 1 mol% to 5 mol%, relative to 1 mol of trioxymethylene.

[0050] (Low molecular weight acetal compound)

[0051] In the method for producing polyacetal of the present embodiment, a non-cyclic low molecular weight acetal compound can be optionally used as a monomer component. The low molecular weight acetal compound functions as a chain transfer agent in the polymerization step described later, and is an acetal compound having a molecular weight of 200 or less, preferably 60 to 170. Preferred examples of non-cyclic low molecular weight acetal compounds include: methylal, methoxymethylal, dimethoxymethylal, and trimethoxymethylal. The low molecular weight acetal compound can be used alone or in combination of two or more. It should be noted that the low molecular weight acetal compound in the present invention is non-cyclic.

[0052] When at least trioxymethylene is used as a monomer component and a low molecular weight acetal compound is used as a monomer component, from the viewpoint of controlling the molecular weight of the polyacetal within an appropriate range, the amount of the low molecular weight acetal compound added is preferably 0.1×10 -4 Mole~0.6×10 -2 within the molar range.

[0053] The water content of these monomer components (including cyclic ether and / or cyclic formal, acyclic low molecular weight acetal compound) is preferably 0.01 mass ppm or more, more preferably 1 mass ppm or more, and on the other hand, is preferably 30 mass ppm or less, more preferably 10 mass ppm or less. When the water content of the monomer components exceeds 30 mass ppm, the thermal stability of the produced polyacetal may decrease.

[0054] The method for adjusting the water content of the monomer component is not particularly limited, and the conditions when the monomer component is purified can be adjusted so that it contains a desired amount of water, for example, the water content of the monomer component used in the polymerization can be measured, and the insufficient water can be added. It should be noted that the water content of the monomer component can be measured using a Karl Fischer titrator.

[0055] <(b) Polymerization Catalyst>

[0056] The (b) polymerization catalyst used in the method for producing polyacetal of the present embodiment comprises boron trifluoride or a boron trifluoride complex. Among the boron trifluoride complexes, a boron trifluoride dialkyl ether complex is preferred, and a boron trifluoride ethyl ether complex or a boron trifluoride di-n-butyl ether complex is more preferred. In addition, the boron trifluoride ethyl ether complex has a large number of moles per unit volume and is therefore particularly preferred.

[0057] The amount of the polymerization catalyst (b) added is preferably 1×10 -9 Mole ~ 1×10 -2 molar range, more preferably 2×10 -9 Mole ~ 1×10 -2 molar range, more preferably 5×10 -9 Mole ~ 1×10 -3 When the amount of the polymerization catalyst added is within the above range, the polymerization reaction can be more stably carried out for a long time.

[0058] <(c) Organic solvents>

[0059] In the method for producing polyacetal of the present embodiment, (c) an organic solvent (hereinafter, sometimes simply referred to as “organic solvent”) having an ester structure represented by the following formula (1) is supplied to carry out a polymerization reaction.

[0060] (1)

[0061] (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms.)

[0062] The organic solvents may be used alone or in combination of two or more.

[0063] In the method for producing polyacetal of the present embodiment, by using a compound having an ester structure represented by the above formula (1) as an organic solvent, the flow rate of the solution (b) containing a polymerization catalyst (catalyst solution) in the polymerization reaction is stabilized, and long-term continuous production can be implemented. Although the mechanism for obtaining such an effect is not clear, it is presumed that the polymerization catalyst forms a unique complex with the organic solvent and is stabilized for a long time, thereby suppressing the generation of tar-like precipitates caused by deterioration of the polymerization catalyst.

[0064] In the above formula (1), R1 preferably has 1 carbon atom, i.e., a methyl group. The organic solvent represented by the above formula (1) in which R1 is a methyl group has a low boiling point and is therefore less likely to remain in the final product and reduce the purity of the product. Specific examples of such an organic solvent include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and isopropyl acetate.

[0065] On the other hand, when the total number of carbon atoms of R1 and R2 is too small, the boiling point of the organic solvent becomes low, and poor mixing may occur before mixing with the higher temperature monomer component. Therefore, as the organic solvent, ethyl acetate, propyl acetate, butyl acetate, and isopropyl acetate are more preferred. As the organic solvent, ethyl acetate is most preferred.

[0066] When the above-mentioned (c) organic solvent is used as a solvent for the polymerization catalyst, hydrolysis occurs due to the reaction between the water in the solvent and the polymerization catalyst to generate carboxylic acid. The carboxylic acid generated in this way not only acts as a chain transfer agent, but also sometimes remains in the polymer, thereby deteriorating the thermal stability of the final product. Since the water contained in the above-mentioned (c) organic solvent causes the generation of carboxylic acid, its content is preferably managed separately from the water content contained in the above-mentioned monomer. From the perspective of not deteriorating the thermal stability, the water content of the organic solvent is preferably less. The water content of the organic solvent is 50 mass ppm or less, preferably 40 mass ppm or less, and more preferably 10 mass ppm or less. On the other hand, the boron trifluoride or boron trifluoride complex used as the (b) polymerization catalyst in the method for producing polyacetal of the present embodiment generates protons by reacting with water in the system. It is generally believed that the protons will initiate the polymerization of monomers such as trioxane, and it is necessary to contain a small amount of water within a range that does not damage the thermal stability of the produced polyacetal. Therefore, the water content of the above-mentioned organic solvent is 1 mass ppm or more, preferably 1.5 mass ppm or more, and more preferably 2 mass ppm or more. By adjusting the water content of the organic solvent to the above range, the generation of tarry precipitates caused by the polymerization catalyst can be reduced, and polyacetal with excellent thermal stability can be continuously produced stably for a long period of time. It should be noted that the water content of the organic solvent can be measured using a Karl Fischer titrator.

[0067] Usually, the organic solvent used in the method for producing the polyacetal of the present embodiment has a polar molecular structure and is therefore prone to contain water. When using an organic solvent with a high water content, it is preferred to dehydrate the organic solvent in advance using an appropriate method. The dehydration method can use a known method, such as a method of contacting with a desiccant such as a molecular sieve, a method of separating water by a distillation operation, etc. When the water content of the organic solvent is small, the water content can be adjusted by adding water to the organic solvent.

[0068] The content of the carboxylic acid in the organic solvent is preferably 1 to 20 ppm by mass. The carboxylic acid referred to herein is a compound represented by the following formula (2).

[0069] R1COOH (2)

[0070] (In the formula, R1 represents the same group as R1 in formula (1).)

[0071] It should be noted that, as described above, the carboxylic acid contained in the organic solvent is a carboxylic acid generated by hydrolysis through the reaction of water in the solvent and the polymerization catalyst. However, the organic solvent may also contain the same type of carboxylic acid as the carboxylic acid generated by hydrolysis through the reaction of water in the solvent and the polymerization catalyst before use. In such a case, it is preferred that the above content is the sum of the two.

[0072] Therefore, when, for example, ethyl acetate is used as the organic solvent having an ester structure, it is preferable to adjust the acetic acid content to 1 to 20 ppm by mass.

[0073] The carboxylic acid contained in the above organic solvent acts as a chain transfer agent in the polymerization reaction to generate a polyacetal having a carboxylic acid residue at the terminal, and the terminal carboxylic acid residue of the polyacetal may decompose to impair the thermal stability of the polyacetal. Therefore, it is preferred to adjust the amount of the carboxylic acid contained in the organic solvent having an ester structure to be within the above range.

[0074] On the other hand, the carboxylic acid contained in the above-mentioned organic solvent sometimes reacts with boron trifluoride or a boron trifluoride complex as a polymerization catalyst like water to generate protons, and the protons can initiate the polymerization of monomers such as trioxymethylene. In order to efficiently initiate the polymerization reaction, it is preferred to contain a small amount of carboxylic acid within a range that does not damage the thermal stability of the manufactured polyacetal. Therefore, from the viewpoint of excellent polymerization yield, the carboxylic acid contained in the above-mentioned organic solvent is preferably 1 mass ppm or more, more preferably 1.5 mass ppm or more, and further preferably 2 mass ppm or more. In addition, from the viewpoint of thermal stability, the carboxylic acid contained in the above-mentioned organic solvent is more preferably 15 mass ppm or less, and further preferably 10 mass ppm or less.

[0075] The amount of carboxylic acid in the organic solvent can be measured by known methods such as gas chromatography. When the amount of carboxylic acid is more than the above range, the amount of carboxylic acid can be adjusted to a desired amount by known methods such as distillation separation. In addition, when the amount of carboxylic acid is less than the above range, the amount of carboxylic acid can be adjusted by appropriately adding carboxylic acid.

[0076] The polymerization catalyst and the organic solvent can be pre-mixed to prepare a catalyst solution for use. Regarding the ratio of the polymerization catalyst to the organic solvent, the concentration of the polymerization catalyst in the catalyst solution comprising the polymerization catalyst and the organic solvent is preferably 0.01% by mass or more, preferably 10.0% by mass or less. If the concentration of the polymerization catalyst in the catalyst solution is 0.01% by mass or more, the polymerization reaction can be more reliably carried out, and if the concentration of the polymerization catalyst in the catalyst solution is 10.0% by mass or less, a violent reaction can be suppressed, and a stable, long-term polymerization reaction can be more effectively implemented.

[0077] In addition, in the method for producing polyacetal of the present embodiment, other comonomer components capable of forming a block, branched, or crosslinked structure may of course be used in combination in addition to the above-mentioned components.

[0078] In addition, in the stabilized polyacetal produced by the production method of the present embodiment, antioxidants, formic acid scavengers, weather (light) stabilizers, mold release (lubricating) agents, reinforcing agents, conductive agents, thermoplastic resins, thermoplastic elastomers, pigments, plasticizers, peroxide decomposers, alkaline auxiliary agents, antistatic agents, flame retardants, dyes, fillers, etc., which are known additives commonly used, can also be mixed as desired. In addition, in the polyacetal of the present embodiment, other polymers can also be mixed within the range that does not damage its physical properties. The mixing ratio of these compounding agents is within an appropriate range.

[0079] <Steps Included in the Method for Producing Polyacetal>

[0080] The steps included in the method for producing polyacetal according to the present embodiment will be described.

[0081] The method for producing polyacetal of the present embodiment includes a polymerization step, in which a polymerization reaction is carried out using (a) at least one cyclic ether and / or cyclic formal containing trioxymethylene, and optionally a non-cyclic low molecular weight acetal compound; (b) a polymerization catalyst containing boron trifluoride or a boron trifluoride complex; and (c) an organic solvent having an ester structure represented by the above formula (1). In addition, a deactivation step for deactivating the polymerization catalyst, a drying step, etc. can be optionally and appropriately included. As described above, in the production method of the present embodiment, the generation of tarry precipitates caused by the polymerization catalyst can be reduced, and polyacetal with good thermal stability can be continuously produced stably for a long time. In addition, in the production method of the present embodiment, a polymerization yield equal to or higher than that of the past can be achieved.

[0082] (Polymerization process)

[0083] As the polymerization method in the polymerization step, any of a bulk method and a melt method can be adopted. In addition, the polymerization step can be carried out in a polymerization reactor, and there is no particular restriction on the shape (structure) of the polymerization reactor used. Among them, as the polymerization reactor, it is preferred to use any of a stirring and mixing polymerization device of a twin-shaft paddle type or a screw type with a jacket through which a heat medium can pass.

[0084] In the polymerization step, it is preferred that (b) the polymerization catalyst is dissolved in (c) the organic solvent (the polymerization catalyst is diluted with the organic solvent) to prepare a catalyst solution, the catalyst solution is appropriately stabilized, and then added to (a) at least one cyclic ether and / or cyclic formal, and optionally a non-cyclic low molecular weight acetal compound, to carry out the polymerization reaction. In this case, the generation of tarry precipitates caused by the polymerization catalyst can be further reduced, and continuous production can be further stabilized.

[0085] The temperature of the polymerization reactor (maintaining temperature of the reaction system) when implementing the polymerization step is preferably in the range of 63°C to 155°C, more preferably in the range of 70°C to 145°C, and further preferably in the range of 70°C to 140°C. In addition, the residence (reaction) time in the polymerization reactor (in the reaction system) is preferably 0.1 minute to 30 minutes, more preferably 0.1 minute to 25 minutes, and further preferably 0.1 minute to 20 minutes. If the temperature and the residence time are respectively within the above ranges, there is a tendency to continue a stable polymerization reaction. It is also preferred to appropriately sample the polymer and set the residence (reaction) time to achieve the desired yield.

[0086] In addition, after the polymerization, the residual monomers can be removed from the obtained polyacetal and the catalyst can be deactivated. The removal and deactivation can be carried out by the methods proposed in the past. For example, the polyacetal discharged from the polymerization reactor can be put into water alone, or into an aqueous solution containing at least one deactivating agent selected from amines such as ammonia, triethylamine, tri-n-butylamine, hydroxides of alkali metals or alkaline earth metals, inorganic salts, organic acid salts, etc., in order to deactivate the catalyst efficiently, and the residual monomers in the polyacetal can be removed and the catalyst can be deactivated while continuously stirring for several minutes to several hours in the range of room temperature to 100°C or less. From the viewpoint of improving the cleaning and removal efficiency of the catalyst, when the polyacetal is in a large block, it is also preferred to crush the polyacetal to make it fine.

[0087] The polyacetal obtained by the above polymerization method often has a hydroxyl terminal which is easily a starting point of thermal decomposition. In such a case, for example, the hydroxyl terminal can be stabilized by reacting an organic acid anhydride with the hydroxyl terminal, or the easily decomposable components or sites can be preliminarily decomposed by heating the polyacetal before using it as a product.

[0088] The terminal groups of the polyacetal obtained by polymerization are often thermally unstable. Therefore, from the perspective of suppressing the decomposition of the polyacetal during melt processing, it is preferred to stabilize the polyacetal by decomposing and removing the unstable terminal groups after the catalyst is deactivated.

[0089] As a method for stabilizing by decomposing and removing the unstable terminal portion, it is preferable to use a decomposition agent to treat the thermally unstable terminal portion [-(OCH2) n -OH group] is implemented. The decomposition treatment agent is not particularly limited, and examples thereof include: aliphatic amine compounds such as ammonia, triethylamine, and tributylamine; hydroxides of alkali metals or alkaline earth metals such as sodium, potassium, magnesium, calcium, or barium; inorganic weak acid salts of alkali metals or alkaline earth metals such as carbonates, phosphates, silicates, and borates; organic acid salts of alkali metals or alkaline earth metals such as formates, acetates, stearates, palmitates, propionates, and oxalates; and other alkaline substances. Among these, aliphatic amine compounds are preferred, and triethylamine is more preferred.

[0090] There are no particular restrictions on the method for decomposing and removing the unstable terminal portion, and examples thereof include the following method: heat treatment is performed in the presence of a decomposition treatment agent such as triethylamine at a temperature above the melting point of the polyacetal (e.g., above 170° C.) and below 260° C. in a molten state of the polyacetal. Examples of the heat treatment method include a method using a single-screw extruder or a twin-screw extruder equipped with a vent pressure reducing device, and a method using a twin-screw extruder is preferred.

[0091] (Drying process)

[0092] In addition, a drying step can be performed to obtain a crude polyacetal after polymerization. Specifically, in the drying step, the crude polyacetal after the polymerization step or the deactivation step can be filtered using a centrifugal separator and dried under nitrogen. Thus, the target polyacetal (polymer) can be obtained.

[0093] The method for drying the crude polyacetal in the drying step is not particularly limited, and for example, the crude polyacetal may be dried by heating it using a hot air dryer, a vacuum dryer, or a dryer having a stirring function through which a heat medium can pass.

[0094] The drying temperature in the drying step may be at least the boiling point of the crude polyacetal monomer and not more than the temperature at which the crude polyacetal does not melt, that is, preferably in the range of 115°C to 165°C, more preferably in the range of 120°C to 155°C.

[0095] The polyacetal manufactured by the manufacturing method of the present embodiment and the composition mixed with the above-mentioned compounding agent according to the situation can be made into molded products and parts through various moldings and used for various purposes. The polyacetal manufactured by the manufacturing method of the present embodiment can be used as the known uses of polyacetal without special restrictions, for example, it can be used for gears (gear), cams, sliders, levers, arms, clutches, pulleys, rollers, drums, key cores, key caps, shafts, bearings, guide rails and other parts as electrical and electronic parts and industrial parts. In addition, it can also be used as automotive parts, and as the automotive parts used, it can be used for: fuel peripheral parts represented by fuel tanks, fuel pump modules, valves, fuel tank flanges, etc.; door peripheral parts represented by door locks, door handles, window regulators, speaker grids, etc.; seat belt peripheral parts represented by seat belt slip rings, buttons, etc.; combination switch parts, switches.

[0096] Although the mode for carrying out the present invention has been described above, the present invention is not limited to the above-described embodiment and various modifications can be made to the present invention without departing from the gist of the present invention.

[0097] Example

[0098] The following examples illustrate the present embodiment but do not limit the present embodiment.

[0099] <Polymerization Yield>

[0100] The polymerization yield was calculated by dividing the discharge amount per unit time of the crude polyacetal discharged from the polymerization reactor by the supply amount per unit time of all monomers.

[0101] In addition, the polymerization yield was calculated after 1 hour from the start of polymerization.

[0102] <Method for Determining the Content of Carboxylic Acid in Organic Solvent>

[0103] The content of carboxylic acid in the organic solvent was measured using gas chromatography under the following conditions and calculated using a calibration curve prepared in advance.

[0104] Gas chromatograph: GC-2014 (manufactured by Shimadzu Corporation)

[0105] Chromatographic column: SH-PolarWax (manufactured by Shimadzu Corporation)

[0106] Inlet temperature: 240°C

[0107] Detector temperature: 200°C

[0108] Detector: Hydrogen flame ionization detector

[0109] Carrier gas: Nitrogen

[0110] Column temperature: maintain at 50°C for 5 minutes, then increase the temperature to 240°C at a rate of 5°C / min

[0111] Determination sample injection volume: 1μL

[0112] Preparation method of the test sample: dissolve about 0.1 g of sample in about 1 g of acetone.

[0113] <Evaluation of Thermal Stability of Polyacetal>

[0114] The thermal stability of the obtained polyacetal was determined by using a differential thermal analyzer (Thermo plus EV02 series, manufactured by Rigaku Corporation) to determine the weight loss rate after heating at 200° C. for 90 minutes in a nitrogen atmosphere. The closer the value is to 0, the smaller the weight loss rate is, indicating better thermal stability.

[0115] <Method for Determining Water Content of Monomer Components and Organic Solvents>

[0116] The monomer components and the water content of the organic solvent were quantified by Karl Fischer method under the following conditions.

[0117] Equipment: CA-200 (Mitsubishi Chemical Analytical Tech Co., Ltd.)

[0118] Anolyte: AQUAMICRON (registered trademark) AKX

[0119] Cathodic liquid: AQUAMICRON (registered trademark) CXU

[0120] Determination method: Weigh about 1g of sample into a syringe and inject it into the device. After the sample is injected, divide the total value of the water content detected until the detection speed reaches 0.02μg / second by the amount of sample injected to calculate the water content (mass ppm).

[0121] <Preparation of organic solvent>

[0122] Taking ethyl acetate as an example, the preparation examples of carboxylic acid content and water content are shown. According to the carboxylic acid content and water content of the obtained organic solvent (ethyl acetate and butyl acetate in this example), the methods of Preparation Examples 1 to Preparation Examples 6 were carried out to obtain organic solvents with water content and carboxylic acid content shown in Table 1.

[0123] [Preparation Example 1]

[0124] Molecular sieve 4A (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was heated at 400°C for 8 hours and then cooled in a dry nitrogen atmosphere. The molecular sieve 4A was added to ethyl acetate in a dry nitrogen atmosphere and immersed for 24 hours. The water content of ethyl acetate was 6 mass ppm and the acetic acid content was 1 mass ppm.

[0125] [Preparation Example 2]

[0126] Ethyl acetate with a water content of about 300 mass ppm was added to a three-necked flask with an internal volume of 1 L, and a Weiss fractionating column and a distillation head were installed to perform distillation operation, and the ethyl acetate as a fraction was recovered. Molecular sieve 4A, which had been heated at 400° C. for 8 hours and cooled in a dry nitrogen atmosphere, was immersed in the ethyl acetate for 24 hours, thereby obtaining ethyl acetate with a water content of 2 mass ppm and an acetic acid content of 28 mass ppm.

[0127] [Preparation Example 3]

[0128] Molecular sieve 4A (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was heated at 400°C for 8 hours and then cooled in a dry nitrogen atmosphere. The molecular sieve 4A was added to ethyl acetate (water content of 774 mass ppm, acetic acid content of 18 mass ppm) under a dry nitrogen atmosphere and immersed for 24 hours. A Weiss fractionation column and a distillation head were installed in a 300 mL glass eggplant flask and distillation was performed to recover ethyl acetate as a fraction. The water content of the obtained ethyl acetate was 6 mass ppm and the acetic acid content was 0.7 mass ppm.

[0129] [Preparation Example 4]

[0130] Ethyl acetate having a water content of about 300 mass ppm was added to a three-necked flask having an internal volume of 1 L, and a Weiss fractionating column and a distillation head were installed to perform distillation operation, and the ethyl acetate as a fraction was recovered. Molecular sieve 4A, which had been heated at 400° C. for 8 hours and cooled in a dry nitrogen atmosphere, was immersed in the ethyl acetate for 24 hours, thereby obtaining ethyl acetate having a water content of 11 mass ppm and an acetic acid content of 4 mass ppm.

[0131] [Preparation Example 5]

[0132] Molecular sieve 4A, which was heated at 400°C for 8 hours and then cooled in a nitrogen atmosphere, was added to ethyl acetate having a water content of about 770 mass ppm and an acetic acid content of 18 ppm, and the mixture was immersed for 30 hours. The water content of the obtained ethyl acetate was 6 mass ppm and the acetic acid content was 15 mass ppm.

[0133] [Preparation Example 6]

[0134] The ethyl acetate obtained in Preparation Example 5 was added to a three-necked flask with an internal volume of 1 L, and a Weiss column and a distillation head were installed to perform distillation operation, and the ethyl acetate was recovered as a fraction. The water content of the obtained ethyl acetate was 6 mass ppm, and the acetic acid content was 8 mass ppm.

[0135] [Example 1]

[0136] A solution obtained by mixing a boron trifluoride ether complex (boron trifluoride complex) as a polymerization catalyst and ethyl acetate as an organic solvent was used as a catalyst solution. At this time, the concentration of the polymerization catalyst in the catalyst solution was adjusted to be 3.0% by mass. The ethyl acetate used here was ethyl acetate obtained by the method of Preparation Example 1, and had a water content of 6 mass ppm and an acetic acid content (acetic acid amount) of 1 mass ppm.

[0137] The temperature of a twin-shaft paddle-type continuous polymerization reactor (manufactured by Kurimoto Iron Works, diameter 2B, L / D=14.8) equipped with a jacket through which a heat medium can pass was adjusted to 80°C as a polymerization reactor.

[0138] The monomer components of trioxane, 1,3-dioxolane (4.2 mol% of 1,3-dioxolane relative to 1 mol of trioxane), and methylal as a low molecular weight acetal compound were continuously mixed in a pipeline to obtain a mixed solution, and the mixed solution and the above-mentioned catalyst solution were continuously supplied to a polymerization reactor by separate methods, and a polymerization reaction was carried out to obtain a crude polyacetal. At this time, the amount of the polymerization catalyst relative to 1 mol of trioxane was 2.0×10 -5 The flow rate was appropriately adjusted in terms of mole. The water content in the monomer component was 6 mass ppm.

[0139] The crude polyacetal discharged from the polymerization reactor was sampled into a triethylamine aqueous solution (0.5 mass %) and stirred at room temperature for 1 hour, then filtered using a centrifugal separator and dried at 120° C. for 3 hours under nitrogen.

[0140] As an accelerated test when using the obtained polyacetal, the polyacetal was immersed in 0.1 mass % ammonia water, heated at 120° C. for 3 hours, filtered using a centrifugal separator, and dried at 120° C. for 10 hours under nitrogen, and the thermal stability of the polyacetal was evaluated. The results are shown in the "Thermal Stability" item in Table 1.

[0141] [Examples 2 and 3]

[0142] The same method as in Preparation Example 1 was performed using butyl acetate instead of ethyl acetate. In Example 2, butyl acetate having a water content of 20 mass ppm and an acetic acid content of 2 mass ppm was used, and in Example 3, butyl acetate having a water content of 44 mass ppm and an acetic acid content of 4 mass ppm was used. Polyacetal was obtained by the same method as in Example 1, and thermal stability was evaluated. The results are shown in Table 1.

[0143] [Example 4]

[0144] The same method as in Example 1 was used except that ethyl acetate having a water content of 2 mass ppm and an acetic acid content of 28 mass ppm obtained in the same method as in Preparation Example 2 and a monomer component having a water content of 8 mass ppm was used. The results are shown in Table 1.

[0145] [Example 5]

[0146] The same method as in Example 1 was used except that ethyl acetate having a water content of 27 mass ppm and an acetic acid content of 6 mass ppm obtained in the same method as in Preparation Example 1 and a monomer component having a water content of 25 mass ppm was used. The results are shown in Table 1.

[0147] [Example 6]

[0148] The same method as in Example 1 was used except that ethyl acetate having a water content of 27 mass ppm and an acetic acid content of 6 mass ppm obtained in the same method as in Preparation Example 1 and a monomer component having a water content of 3 mass ppm was used. The results are shown in Table 1.

[0149] [Example 7]

[0150] The same method as in Example 1 was carried out except that the ethyl acetate obtained in Production Example 3 was used. The results are shown in Table 1.

[0151] [Example 8]

[0152] The results are shown in Table 1.

[0153] [Example 9]

[0154] The results are shown in Table 1.

[0155] [Example 10]

[0156] The results are shown in Table 1.

[0157] [Comparative Examples 1 to 4]

[0158] The same method as in Example 1 was carried out using organic solvents having different water contents and acetic acid amounts and monomer components having different water contents. The results are shown in Table 1.

[0159] [Table 1]

[0160]

[0161] From the results shown in Table 1, it is understood that Examples 1 to 10 in which the water content in the monomer components and the water content in the organic solvent were adjusted within the range of the present invention can obtain polyacetal having excellent thermal stability in good yield.

[0162] On the other hand, it is understood that when the water content in the monomer exceeds 30 mass ppm like in Comparative Example 4, the thermal stability is reduced.

[0163] In addition, it is understood that when the water content in the organic solvent exceeds 50 mass ppm as in Comparative Example 1, the thermal stability is reduced.

[0164] It is found that in Comparative Examples 2 and 3 in which the water content in the organic solvent is small, although the obtained polyacetal has good thermal stability, the polymerization yield is low, and the object of the present invention to produce a polyacetal with good thermal stability in high yield cannot be achieved.

[0165] As can be seen, by adjusting the water content in the monomer components and the water content in the organic solvent within the ranges of the present invention, polyacetal having high thermal stability can be produced in high yield.

Claims

1. A method for producing polyacetal, wherein: The production method comprises a polymerization step, in which: (a) at least one cyclic ether and / or cyclic formal, and optionally a non-cyclic low molecular weight acetal compound comprising trioxymethylene; (b) a polymerization catalyst comprising boron trifluoride or a boron trifluoride complex; as well as (c) an organic solvent having an ester structure represented by the following formula (1), and performing a polymerization reaction, （1） In formula (1), R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms, The water content of (a) is 0.01 ppm to 30 ppm by mass, The water content of (c) is 1 to 50 ppm by mass.

2. The manufacturing method according to claim 1, wherein: The content of the carboxylic acid contained in the above (c) which is the same type as the carboxylic acid generated by hydrolysis of the organic solvent having the ester structure represented by the above formula (1) is 1 to 20 ppm by mass.

3. The manufacturing method according to claim 1, wherein: The (c) is an organic solvent in which R1 in the above formula (1) is a methyl group.

4. The manufacturing method according to claim 1, wherein: The (c) is ethyl acetate.

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