Method of producing polyacetal

Abstract

To provide a method of producing a polyacetal stably at high yield, the polyacetal having good thermal stability even when a boron trifluoride ether complex is used in a diluted form.SOLUTION: A method of producing a polyacetal comprises a polymerization step of performing a polymerization reaction by supplying: (a) at least one cyclic ether and / or cyclic formal, including trioxane, and optionally an acyclic acetal compound having a low molecular weight; (b) a polymerization catalyst including boron trifluoride or a boron trifluoride complex compound; and (c) an organic solvent having an ester structure represented by the following formula (1), wherein the water content of the above (a) is 0.01 to 30 ppm by mass and the water content of the above (c) is 1 to 50 ppm by mass. (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms.)SELECTED DRAWING: None

Description

[Technical field]

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

[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 for electrical parts, electronic parts, automotive parts, and various other machine parts. As the range of its use expands in recent years, there is a tendency for increasingly high mechanical properties to be required.

[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 trioxane, an ether complex of boron trifluoride (e.g., boron trifluoride diethyl etherate, boron trifluoride dibutyl etherate) is often used, and in such a case, the catalyst is often used after being diluted with a solvent. For example, the following Patent Document 1 discloses a method for producing polyacetal using a cyclohexane solution of boron trifluoride diethyl etherate.

[0005] The following Patent Document 2 discloses a method for producing polyacetal using a dibutyl ether solution of boron trifluoride dibutyl etherate.

[0006] Furthermore, the following Patent Document 3 discloses a method for producing a polyacetal that does not contain an aromatic solvent and has a high conversion rate and good thermal stability and good mechanical properties, in which boron trifluoride dimethyl etherate or boron trifluoride diethyl etherate is used as a mixed liquid with a carboxylate ester. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2022 / 085280 [Patent Document 2] International Publication No. 2020 / 137414 [Patent Document 3] International Publication No. 2018 / 134075 Summary of the Invention [Problem to be solved by the invention]

[0008] For example, in order to obtain a polymer with a high molecular weight and a controlled molecular weight distribution in a twin-shaft paddle type continuous polymerization reactor, it is necessary to disperse the polymerization catalyst in the monomer component (e.g., trioxane) without deactivating the catalyst, and to carry out polymerization as uniformly as possible in the polymerization reactor.

[0009] For this purpose, it is necessary to design the polymerization catalyst so that it is diluted with a solvent and dispersed in the monomer component, and polymerization is initiated by contact between the polymerization catalyst and the monomer after the polymerization catalyst is dispersed in the monomer.

[0010] However, when the boron trifluoride ether complex is used after diluting with, for example, an organic solvent, the thermal stability of the produced polymer may decrease, or the yield may be low even if the obtained polymer has relatively good thermal stability.

[0011] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for stably producing a polyacetal with good yield, which has good thermal stability even when a boron trifluoride ether complex is used in a diluted form. [Means for solving the problem]

[0012] The present inventors have conducted extensive investigations to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by supplying at least one kind of cyclic ether and / or cyclic formal containing trioxane, 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 compound, and an organic solvent to a polymerization reactor to carry out polymerization, using a specific carboxylic acid ester as the organic solvent, and setting the water content of the carboxylic acid ester and the water content of the monomer component within specific ranges, thereby completing the present invention.

[0013] That is, the present invention is as follows.

[0014] [1] (a) at least one cyclic ether and / or cyclic formal, and optionally acyclic low molecular weight acetal compounds, including trioxane; (b) a polymerization catalyst containing boron trifluoride or a complex compound of boron trifluoride; (c) an organic solvent having an ester structure represented by the following formula (1); [ka] (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms.) A polymerization step of supplying The water content of the (a) is 0.01 to 30 ppm by mass, A method for producing polyacetal, wherein the water content of (c) is 1 to 50 ppm by mass.

[0015] [2] The method according to [1], wherein the content of a carboxylic acid contained in (c) is 1 to 20 ppm by mass, the carboxylic acid being the same as the carboxylic acid generated by hydrolysis of the organic solvent having an ester structure represented by formula (1).

[0016] [3] The method according to [1] or [2], wherein (c) is an organic solvent in which R1 in the formula (1) is a methyl group.

[0017] [4] The method according to any one of [1] to [3], wherein (c) is ethyl acetate. Effect of the Invention

[0018] According to the present invention, polyacetal having excellent thermal stability can be produced stably with good yield. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment for carrying out the method for producing polyacetal of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail.

[0020] The method for producing polyacetal disclosed in this embodiment includes the steps of: (a) at least one cyclic ether and / or cyclic formal, and optionally acyclic low molecular weight acetal compound, including trioxane; (b) a polymerization catalyst containing boron trifluoride or a complex compound of boron trifluoride; (c) an organic solvent having an ester structure represented by the following formula (1); [ka] (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms.) A polymerization step of supplying The water content of the (a) is 0.01 to 30 ppm by mass, The water content of the (c) is 1 to 50 ppm by mass. According to the above-mentioned production method, polyacetal having good thermal stability can be produced stably with good yield.

[0021] The compounds used in the method for producing polyacetal according to the present embodiment will be described below.

[0022] <(a) At least one cyclic ether and / or cyclic formal, and optionally acyclic low molecular weight acetal compound> In the method for producing polyacetal of the present embodiment, (a) at least one cyclic ether and / or cyclic formal containing trioxane, and optionally a non-cyclic low molecular weight acetal compound are used as monomer components. The cyclic ether and / or cyclic formal refers to a cyclic compound having at least one carbon-oxygen bond or carbon-oxygen-carbon bond, and also includes a formaldehyde polymer (trimer, tetramer, etc.). Examples of the cyclic ether and / or cyclic formal include trioxane, tetraoxane, ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, styrene oxide, oxatane, 1,3-dioxolane, ethylene glycol formal, propylene glycol formal, diethylene glycol formal, triethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal, and 1,6-hexanediol formal. The cyclic ether and / or cyclic formal may be used alone (that is, only trioxane) or in combination of two or more kinds.

[0023] At least one kind of cyclic ether and / or cyclic formal used in the method for producing polyacetal of this embodiment includes trioxane. Trioxane is a cyclic trimer of formaldehyde, and is generally obtained by reacting an aqueous solution of formalin in the presence of an acid catalyst. This trioxane may contain impurities such as water, methanol, formic acid, and methyl formate that cause chain transfer, so it is preferable to remove these impurities in advance by a method such as distillation to purify the trioxane.

[0024] When purifying trioxane as described above, the total amount of impurities to be chain-transferred is 1×10 -3 mol or less, and more preferably 0.5×10 -3 By reducing the amount of impurities to within the above range, the polymerization reaction rate can be increased sufficiently for practical use, and various properties such as the thermal stability of the produced polymer can be improved.

[0025] As the cyclic ether and / or cyclic formal other than trioxane, 1,3-dioxolane and 1,4-butanediol formal are particularly preferred.

[0026] When trioxane and a cyclic ether and / or a cyclic formal other than trioxane are used in combination as the at least one cyclic ether and / or cyclic formal, the amount of the cyclic ether and / or the cyclic formal other than trioxane added is preferably in the range of 1 to 20 mol%, more preferably in the range of 1 to 15 mol%, even more preferably in the range of 1 to 10 mol%, and even more preferably in the range of 1 to 5 mol%, relative to 1 mol of trioxane.

[0027] (Low molecular weight acetal compounds) In the method for producing polyacetal of this embodiment, a non-cyclic low molecular weight acetal compound may 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. Specific examples of suitable non-cyclic low molecular weight acetal compounds include methylal, methoxymethylal, dimethoxymethylal, and trimethoxymethylal. The low molecular weight acetal compound may be used alone or in combination of two or more kinds. The low molecular weight acetal compound in the present invention is non-cyclic.

[0028] When at least trioxane is used as a monomer component and a low molecular weight acetal compound is used as a monomer component, the amount of the low molecular weight acetal compound added is 0.1×10 to 1 mol of trioxane from the viewpoint of controlling the molecular weight of the polyacetal within a suitable range. -4 ~0.6×10 -2 The molar range is preferred.

[0029] These monomer components (including cyclic ethers and / or cyclic formals, and non-cyclic low molecular weight acetal compounds) preferably have a water content of 0.01 ppm by mass or more, more preferably 1 ppm by mass or more, and preferably 30 ppm by mass or less, more preferably 10 ppm by mass or less. If the water content of the monomer components exceeds 30 ppm by mass, the thermal stability of the produced polyacetal may decrease.

[0030] The method for adjusting the water content of the monomer component is not particularly limited, and the monomer component may be adjusted so that a desired amount of water is contained depending on the conditions for purifying the monomer component. For example, the water content of the monomer component used in polymerization may be measured and the shortage of water may be added. The water content of the monomer component may be measured with a Karl Fischer moisture meter.

[0031] <(b) Polymerization catalyst> The polymerization catalyst (b) used in the method for producing polyacetal of the present embodiment includes boron trifluoride or a complex compound of boron trifluoride. Among the complex compounds of boron trifluoride, boron trifluoride dialkyl ether complex is preferred, and boron trifluoride diethyl ether complex or boron trifluoride di-n-butyl ether complex is more preferred. In addition, boron trifluoride diethyl ether complex is particularly preferred because of its large mole number per unit volume.

[0032] (b) The amount of the polymerization catalyst added is 1×10 -9 ~1×10 -2 mol range is preferred, more preferably 2×10 -9 ~1×10 -2 mol, more preferably in the range of 5×10 -9 ~1×10 -3 When the amount of the polymerization catalyst added is within the above range, the polymerization reaction can be carried out more stably for a long period of time.

[0033] <(c) Organic Solvent> In the method for producing polyacetal according to the present embodiment, a polymerization reaction is carried out by supplying (c) an organic solvent (hereinafter sometimes simply referred to as an "organic solvent") having an ester structure represented by the following formula (1). [ka] (In the formula, R1 and R2 each independently represent a monovalent hydrocarbon group having 1 to 5 carbon atoms.) The organic solvent may be used alone or in combination of two or more kinds.

[0034] In the method for producing polyacetal according to the present embodiment, the use of a compound having an ester structure represented by the above formula (1) as an organic solvent stabilizes the flow rate of the solution (b) containing the polymerization catalyst (catalyst solution) during the polymerization reaction, enabling long-term continuous production. Although the mechanism by which such an effect is achieved is unclear, it is presumed that the polymerization catalyst and the organic solvent form a specific complex, resulting in long-term stabilization, and thus suppressing the generation of tar-like deposits resulting from the deterioration of the polymerization catalyst.

[0035] In the above formula (1), R1 preferably has one carbon atom, i.e., is a methyl group. The organic solvent represented by the above formula (1) in which R1 is a methyl group has a relatively low boiling point, and is therefore unlikely to remain in the final product and cause a decrease in the purity of the product. Specific examples of such organic solvents include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and isopropyl acetate.

[0036] On the other hand, if the total number of carbon atoms of R1 and R2 is too small, the boiling point of the organic solvent is low, and there is a possibility that poor mixing occurs before mixing with the higher temperature monomer component. Therefore, more preferred organic solvents are ethyl acetate, propyl acetate, butyl acetate, and isopropyl acetate. The most preferred organic solvent is ethyl acetate.

[0037] When the (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, and a carboxylic acid is generated. The carboxylic acid thus generated not only serves as a chain transfer agent, but may also remain in the polymer and deteriorate the thermal stability of the final product. Since the water contained in the (c) organic solvent generates a carboxylic acid, its content is preferably managed separately from the water content contained in the monomer, and from the viewpoint of not deteriorating the thermal stability, it is preferable that the water content of the organic solvent is small. 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, it is generally said that the (b) polymerization catalyst, boron trifluoride or a complex compound of boron trifluoride, used in the method for producing polyacetal of this embodiment generates protons by reacting with water in the system, and these protons initiate the polymerization of monomers such as trioxane, and therefore it is necessary to contain a small amount of water within a range that does not impair the thermal stability of the produced polyacetal. Therefore, the water content of the organic solvent is 1 ppm by mass or more, preferably 1.5 ppm by mass or more, and more preferably 2 ppm by mass or more. By setting the water content of the organic solvent within the above range, the generation of tar-like precipitates caused by the polymerization catalyst can be reduced, and polyacetal with excellent thermal stability can be stably and continuously produced for a long period of time. The water content of the organic solvent can be measured with a Karl Fischer moisture meter.

[0038] In general, the organic solvent used in the method for producing polyacetal according to the present embodiment is likely to contain water due to its polar molecular structure. When using an organic solvent with a high water content, it is preferable to dehydrate the organic solvent in advance by a suitable method. The dehydration method can be a known method, such as contacting the organic solvent with a desiccant such as a molecular sieve or separating water by distillation. When the organic solvent has a low water content, the water content can be adjusted by adding water to the organic solvent.

[0039] The content of the carboxylic acid in the organic solvent is preferably 1 to 20 ppm by mass. The carboxylic acid here is a compound represented by the following formula (2). R1COOH (2) (In the formula, R1 represents the same group as R1 in formula (1).) As described above, the carboxylic acid contained in the organic solvent is a carboxylic acid produced by hydrolysis caused by a reaction between the moisture in the solvent and the polymerization catalyst. However, there are also cases where the organic solvent contains the same type of carboxylic acid as the carboxylic acid produced by hydrolysis caused by a reaction between the moisture in the solvent and the polymerization catalyst before use. In such cases, it is preferable that the combined content of both is set to the above-mentioned amount.

[0040] Therefore, when, for example, ethyl acetate is used as the organic solvent having an ester structure, it is preferable that the amount of acetic acid contained is 1 to 20 ppm by mass.

[0041] The carboxylic acid contained in the organic solvent acts as a chain transfer agent during the polymerization reaction, producing a polyacetal having a carboxylic acid residue at its terminal, and the terminal carboxylic acid residue of the polyacetal may decompose, impairing the thermal stability of the polyacetal. Therefore, the amount of the carboxylic acid contained in the organic solvent having an ester structure is preferably within the above range. On the other hand, the carboxylic acid contained in the organic solvent, like water, generates protons by reacting with boron trifluoride or a complex compound of boron trifluoride, which is a polymerization catalyst, and these protons may initiate polymerization of monomers such as trioxane. In order to efficiently initiate the polymerization reaction, it is preferable to contain a small amount of carboxylic acid within a range that does not impair the thermal stability of the polyacetal produced. Therefore, from the viewpoint of excellent polymerization yield, the carboxylic acid contained in the organic solvent is preferably 1 mass ppm or more, more preferably 1.5 mass ppm or more, and even more preferably 2 mass ppm or more. In addition, from the viewpoint of thermal stability, the carboxylic acid contained in the organic solvent is more preferably 15 mass ppm or less, and even more preferably 10 mass ppm or less. The amount of carboxylic acid in the organic solvent can be measured by a known method 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 a known method such as distillation separation. When the amount of carboxylic acid is less than the above range, the amount of carboxylic acid can be adjusted by adding a suitable amount of carboxylic acid.

[0042] The polymerization catalyst and the organic solvent can be mixed in advance and used as a catalyst solution. Regarding the ratio of the polymerization catalyst to the organic solvent, the concentration of the polymerization catalyst in the catalyst solution containing the polymerization catalyst and the organic solvent is preferably 0.01% by mass or more and 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 promoted, and if the concentration is 10.0% by mass or less, a rapid reaction can be suppressed and a stable long-term polymerization reaction can be more effectively carried out.

[0043] In the method for producing the polyacetal of this embodiment, it is of course possible to use, in addition to the above components, other comonomer components capable of forming a block, branched or crosslinked structure.

[0044] The stabilized polyacetal produced by the production method of this embodiment can also be blended with commonly used known additives such as antioxidants, formic acid scavengers, weather (light) stabilizers, release (lubricants), reinforcing agents, conductive agents, thermoplastic resins, thermoplastic elastomers, pigments, plasticizers, peroxide decomposers, basic assistants, antistatic agents, flame retardants, dyes, and fillers, as desired. The polyacetal of this embodiment can also be blended with other polymers to the extent that the physical properties of the polyacetal are not impaired. The blending ratio of these blending agents is within an appropriate range.

[0045] <Steps included in the manufacturing method of polyacetal> The steps included in the method for producing polyacetal according to this embodiment will be described. The method for producing polyacetal of the present embodiment includes a polymerization step of carrying out a polymerization reaction using (a) at least one kind of cyclic ether and / or cyclic formal containing trioxane, and optionally acyclic low molecular weight acetal compound, (b) a polymerization catalyst containing boron trifluoride or a complex compound of boron trifluoride, and (c) an organic solvent having an ester structure represented by the above formula (1). Optionally, the method may further include a deactivation step for deactivating the polymerization catalyst, a drying step, etc. As described above, the production method of the present embodiment can reduce the generation of tar-like precipitates caused by the polymerization catalyst, and can stably and continuously produce polyacetal with good thermal stability for a long period of time. In addition, the production method of the present embodiment can also achieve a polymerization yield equal to or higher than that of the conventional method.

[0046] (Polymerization process) As the polymerization method in the polymerization step, either the bulk method or the melt method can be adopted. The polymerization step can be carried out in a polymerization reactor, and the shape (structure) of the polymerization reactor used is not particularly limited. However, as the polymerization reactor, any of the two-shaft paddle type or screw type stirring and mixing type polymerization apparatuses equipped with a jacket through which a heat medium can pass is preferably used.

[0047] In the polymerization step, it is preferable to dissolve (b) the polymerization catalyst in (c) an organic solvent (by diluting the polymerization catalyst with an organic solvent) to prepare a catalyst solution, appropriately stabilize the catalyst solution, and then add (a) at least one kind of cyclic ether and / or cyclic formal, and optionally an acyclic low molecular weight acetal compound to carry out the polymerization reaction. In this case, the generation of tar-like deposits caused by the polymerization catalyst can be further reduced, and continuous production can be further stabilized.

[0048] The temperature of the polymerization reactor (reaction system holding temperature) during the polymerization step is preferably in the range of 63 to 155°C, more preferably in the range of 70 to 145°C, and even more preferably in the range of 70 to 140°C. The residence (reaction) time in the polymerization reactor (reaction system) is preferably 0.1 to 30 minutes, more preferably 0.1 to 25 minutes, and even more preferably 0.1 to 20 minutes. If the temperature and residence time are within the above ranges, the polymerization reaction tends to continue stably. It is also preferable to appropriately sample the polymer and set the residence (reaction) time that results in the desired yield.

[0049] After polymerization, residual monomers can be removed from the polyacetal obtained and the catalyst can be deactivated. For the removal and deactivation, methods proposed in the past can be used. For example, the polyacetal discharged from the polymerization reactor can be added to an aqueous solution containing only water or at least one deactivator such as amines such as ammonia, triethylamine, and tri-n-butylamine, hydroxides of alkali metals or alkaline earth metals, inorganic salts, and organic acid salts for the purpose of efficiently deactivating the catalyst, and the monomers and catalyst remaining in the polyacetal can be removed and deactivated while continuously stirring for several minutes to several hours at room temperature to 100°C or lower. From the viewpoint of increasing the efficiency of washing and removing the catalyst, when the polyacetal is in the form of a large block, it is also preferable to pulverize the polyacetal to make it fine.

[0050] Polyacetals obtained by the polymerization methods described above often have terminal hydroxyl groups that are likely to become the starting points for thermal decomposition. In such cases, the polyacetal can be stabilized, for example, by reacting the terminal hydroxyl groups with an organic acid anhydride, or the polyacetal can be heated before use as a product to decompose the easily decomposed components and sites in advance.

[0051] The polyacetal obtained by polymerization often has thermally unstable terminal groups, and therefore, from the viewpoint of preventing the decomposition of the polyacetal during melt processing, it is preferable to stabilize the polyacetal by decomposing and removing the unstable terminal groups after the catalyst is deactivated.

[0052] As a method for stabilizing the polyacetal by decomposition and removal of unstable terminals, the thermally unstable terminals [-(OCH2) n It is preferable to carry out the decomposition of the --OH group by using a decomposition treatment agent. The decomposition treatment agent is not particularly limited, and examples thereof include aliphatic amine compounds such as ammonia, triethylamine, and tributylamine, inorganic weak acid salts of alkali metals or alkaline earth metals such as hydroxides, carbonates, phosphates, silicates, and borates of alkali metals or alkaline earth metals such as sodium, potassium, magnesium, calcium, and barium, and organic acid salts of alkali metals or alkaline earth metals such as formates, acetates, stearates, palmitates, propionates, and oxalates. Among these, aliphatic amine compounds are preferable, and triethylamine is more preferable.

[0053] The method for decomposing and removing the unstable terminals is not particularly limited, and examples thereof include a method of heat treating the polyacetal in a molten state in the presence of a decomposition agent such as triethylamine at a temperature equal to or higher than the melting point of the polyacetal (e.g., 170° C. or higher) and equal to or lower than 260° C. 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.

[0054] (drying process) After the polymerization, a crude polyacetal is obtained, and a drying step can be carried out to dry the obtained crude polyacetal. Specifically, in the drying step, the crude polyacetal after the polymerization step or the deactivation step can be filtered with a centrifuge and dried under nitrogen. This allows the desired polyacetal (polymer) to be obtained.

[0055] The method for drying the crude polyacetal in the drying step is not particularly limited. For example, the crude polyacetal can be dried by heating it using a hot air dryer, a vacuum dryer, or a dryer having a stirring function and capable of passing a heat medium.

[0056] The drying temperature in the drying step may be any temperature that is equal to or higher than the boiling point of the monomer of the crude polyacetal and equal to or lower than the temperature at which the crude polyacetal does not dissolve. That is, the drying temperature is preferably in the range of 115°C to 165°C, and more preferably in the range of 120°C to 155°C.

[0057] The polyacetal produced by the manufacturing method of this embodiment, and the composition containing the above-mentioned compounding agents, etc., can be used for various purposes as molded products or parts through various moldings. Although not particularly limited, the polyacetal produced by the manufacturing method of this embodiment can be used for known polyacetal applications, such as gears, cams, sliders, levers, arms, clutches, pulleys, rollers, rollers, key stems, key tops, shafts, bearings, guides, etc., as electric and electronic parts and industrial parts. It can also be used as automobile parts, and examples of the automobile parts used include fuel-related parts such as gasoline tanks, fuel pump modules, valves, and gasoline tank flanges, door-related parts such as door locks, door handles, window regulators, and speaker grills, seat belt slip rings, seat belt peripheral parts such as press buttons, combination switch parts, and switches.

[0058] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The present invention can be modified in various ways without departing from the gist of the present invention. EXAMPLES

[0059] The following examples are intended to illustrate the present embodiments without, however, limiting them.

[0060] <Polymerization yield> The amount of crude polyacetal discharged from the polymerization reactor per unit time was divided by the amount of all monomers fed per unit time to calculate the polymerization yield. The polymerization yield was calculated one hour after the start of polymerization.

[0061] <Method for measuring the content of carboxylic acid in organic solvent> The content of the carboxylic acid in the organic solvent was measured by gas chromatography under the following conditions, and calculated using a calibration curve prepared in advance. Gas chromatography: GC-2014 (Shimadzu Corporation) Column: SH-PolarWax (Shimadzu Corporation) Inlet temperature: 240℃ Detector temperature: 200℃ Detector: Flame ionization detector Carrier gas: Nitrogen Column temperature: After holding at 50°C for 5 minutes, increase the temperature to 240°C at a rate of 5°C / min. Measurement sample injection volume: 1μL Method for preparing the measurement sample: Approximately 0.1 g of the sample was dissolved in approximately 1 g of acetone.

[0062] <Evaluation of thermal stability of polyacetal> The thermal stability of the obtained polyacetal was evaluated by measuring the weight loss rate after heating at 200°C for 90 minutes in a nitrogen environment using a differential thermal analyzer (Thermo plusEV02 Series, manufactured by Rigaku Corporation). The closer the value is to 0, the smaller the weight loss rate is, and the better the thermal stability is.

[0063] <Method for measuring the water content of monomer components and organic solvents> The water content of the monomer components and the organic solvent was quantified by the Karl Fischer method under the following conditions. Equipment: CA-200 (Mitsubishi Chemical Analytical Co., Ltd.) Anolyte: Aquamicron® AKX Catholyte: Aquamicron® CXU Measurement method: Approximately 1 g of sample was weighed into a syringe and injected into the device. After the sample was injected, the total amount of moisture detected until the detection speed reached 0.02 μg / sec was divided by the amount of sample injected to calculate the moisture content (ppm by mass).

[0064] <Preparation of organic solvent> Preparation examples of the carboxylic acid content and water content are shown below using ethyl acetate as an example. Depending on 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 6 were carried out to obtain organic solvents with the water content and carboxylic acid content shown in Table 1.

[0065] [Preparation Example 1] Molecular sieve 4A (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was heated at 400°C for 8 hours and then cooled under a dry nitrogen atmosphere. The molecular sieve 4A was added to ethyl acetate under a dry nitrogen atmosphere and immersed for 24 hours. The water content of the ethyl acetate was 6 ppm by mass, and the acetic acid content was 1 ppm by mass.

[0066] [Preparation Example 2] Ethyl acetate with a water content of about 300 ppm by mass was placed in a 3-neck flask with an internal volume of 1 L, and a Vigreux tube and a T-shaped tube were attached to perform a distillation operation to recover ethyl acetate as a fraction. Molecular sieve 4A that had been heated at 400°C for 8 hours and cooled under a dry nitrogen atmosphere was immersed in the ethyl acetate for 24 hours to obtain ethyl acetate with a water content of 2 ppm by mass and an acetic acid content of 28 ppm by mass.

[0067] [Preparation Example 3] Molecular sieve 4A (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was heated at 400°C for 8 hours and then cooled under a dry nitrogen atmosphere. The above-mentioned molecular sieve 4A was added to ethyl acetate (water content 774 mass ppm, acetic acid content 18 mass ppm) under a dry nitrogen atmosphere and immersed for 24 hours. A Vigreux tube and a T-shaped tube were attached to a 300 mL glass eggplant-shaped flask to perform a distillation operation, and ethyl acetate was collected 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.

[0068] [Preparation Example 4] Ethyl acetate with a water content of about 300 ppm by mass was placed in a 1 L three-neck flask, and a Vigreux tube and a T-shaped tube were attached to perform a distillation operation to recover ethyl acetate as a fraction. Molecular sieve 4A that had been heated at 400°C for 8 hours and cooled under a dry nitrogen atmosphere was immersed in the ethyl acetate for 24 hours to obtain ethyl acetate with a water content of 11 ppm by mass and an acetic acid content of 4 ppm by mass.

[0069] [Preparation Example 5] Molecular sieve 4A that had been heated at 400°C for 8 hours and then cooled under a nitrogen atmosphere was added to ethyl acetate having a water content of about 770 ppm by mass and an acetic acid content of 18 ppm by mass, and the mixture was immersed for 30 hours. The water content of the resulting ethyl acetate was 6 ppm by mass, and the acetic acid content was 15 ppm by mass.

[0070] [Preparation Example 6] The ethyl acetate obtained in Preparation Example 5 was placed in a 3-neck flask with an internal volume of 1 L, and a Vigreux tube and a T-shaped tube were attached to perform a distillation operation to recover ethyl acetate as a fraction. The water content of the obtained ethyl acetate was 6 ppm by mass, and the acetic acid content was 8 ppm by mass.

[0071] [Example 1] A catalyst solution was prepared by mixing boron trifluoride-diethyl etherate (a complex compound of boron trifluoride) as a polymerization catalyst and ethyl acetate as an organic solvent. The concentration of the polymerization catalyst in the catalyst solution was adjusted to 3.0% by mass. The ethyl acetate used here was the ethyl acetate obtained by carrying out the method of Preparation Example 1, and had a water content of 6 ppm by mass and an acetic acid content (acetic acid amount) of 1 ppm by mass. As a polymerization reactor, a jacketed two-shaft paddle type continuous polymerization reactor (manufactured by Kurimoto, Ltd., diameter 2B, L / D=14.8) through which a heat transfer medium can pass was adjusted to 80°C. A mixture of trioxane, 1,3-dioxolane (4.2 mol% relative to 1 mol of trioxane) as monomer components, and methylal as a low molecular weight acetal compound was continuously mixed in a pipe, and the catalyst solution was continuously supplied to a polymerization reactor through separate pipes to carry out a polymerization reaction, thereby obtaining a crude polyacetal. At this time, the amount of polymerization catalyst relative to 1 mol of trioxane was 2.0×10 -5 The flow rate was appropriately adjusted so that the water content in the monomer component was 6 ppm by mass. The crude polyacetal discharged from the polymerization reactor was sampled in an aqueous triethylamine solution (0.5% by mass), and then the sample was stirred at room temperature for 1 hour, filtered using a centrifuge, and dried under nitrogen at 120° C. for 3 hours.

[0072] As an accelerated test for using the obtained polyacetal, the polyacetal was immersed in 0.1% by mass ammonia water, heated at 120°C for 3 hours, filtered with a centrifuge, and dried under nitrogen at 120°C for 10 hours to evaluate the thermal stability of the polyacetal. The results are shown in the "Thermal Stability" section of Table 1.

[0073] [Examples 2 and 3] Butyl acetate was used instead of ethyl acetate, and the same method as in Preparation Example 1 was carried out. Polyacetal was obtained in the same manner as in Example 1, except that butyl acetate with a water content of 20 ppm by mass and an acetic acid content of 2 ppm by mass was used in Example 2, and butyl acetate with a water content of 44 ppm by mass and an acetic acid content of 4 ppm by mass was used in Example 3, and thermal stability was evaluated. The results are shown in Table 1.

[0074] [Example 4] The same procedure as in Example 1 was carried out, except that ethyl acetate having a water content of 2 ppm by mass and an acetic acid content of 28 ppm by mass, obtained by the same procedure as in Preparation Example 2, and a monomer component having a water content of 8 ppm by mass were used. The results are shown in Table 1.

[0075] [Example 5] The same procedure as in Example 1 was carried out, except that ethyl acetate having a water content of 27 ppm by mass and an acetic acid content of 6 ppm by mass, obtained by the same procedure as in Preparation Example 1, and a monomer component having a water content of 25 ppm by mass were used. The results are shown in Table 1.

[0076] [Example 6] The same procedure as in Example 1 was carried out, except that ethyl acetate having a water content of 27 ppm by mass and an acetic acid content of 6 ppm by mass, obtained by the same procedure as in Preparation Example 1, and a monomer component having a water content of 3 ppm by mass were used. The results are shown in Table 1.

[0077] [Example 7] The same procedure as in Example 1 was carried out, except that the ethyl acetate obtained in Preparation Example 3 was used. The results are shown in Table 1.

[0078] [Example 8] The procedure was the same as in Example 1, except that the ethyl acetate obtained in Preparation Example 4 was used. The results are shown in Table 1.

[0079] [Example 9] The procedure was the same as in Example 1, except that ethyl acetate obtained in Preparation Example 5 was used. The results are shown in Table 1.

[0080] [Example 10] The procedure was the same as in Example 1, except that ethyl acetate obtained in Preparation Example 6 was used. The results are shown in Table 1.

[0081] [Comparative Examples 1 to 4] Organic solvents with different water contents and acetic acid contents, and monomer components with different water contents were used, and the same method as in Example 1 was carried out. The results are shown in Table 1.

[0082] [Table 1]

[0083] The results shown in Table 1 show that in Examples 1 to 10, in which the water content in the monomer components and the water content in the organic solvent were within the ranges of the present invention, polyacetal having good thermal stability could be obtained in good yield.

[0084] On the other hand, it is understood that when the water content in the monomer exceeds 30 ppm by mass as in Comparative Example 4, the thermal stability decreases.

[0085] It is also clear that when the water content in the organic solvent exceeds 50 ppm by mass, as in Comparative Example 1, the thermal stability decreases.

[0086] In Comparative Examples 2 and 3, in which the water content in the organic solvent was low, the thermal stability of the obtained polyacetal was good in some cases, but the polymerization yield was low, and it is clear that the object of the present invention, which is to produce polyacetal having good thermal stability with good yield, was not achieved.

[0087] Thus, it is evident that by setting 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 good yield.

Claims

1. (a) at least one cyclic ether and / or cyclic formal, and optionally acyclic low molecular weight acetal compound, comprising trioxane; (b) a polymerization catalyst containing boron trifluoride or a complex compound of boron trifluoride; (c) an organic solvent having an ester structure represented by the following formula (1); 【Chemistry 1】 (In the formula, R １ , R ２ each independently represents a monovalent hydrocarbon group having 1 to 5 carbon atoms. A polymerization step of supplying The water content of the (a) is 0.01 to 30 ppm by mass, The method for producing polyacetal, wherein the water content of (c) is 1 to 50 ppm by mass.

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

3. The (c) is R in the formula (1). １ The method according to claim 1 , wherein the organic solvent is a methyl group.

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

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