CN115894829A - Method for producing polyacetal resin - Google Patents
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- CN115894829A CN115894829A CN202211162407.5A CN202211162407A CN115894829A CN 115894829 A CN115894829 A CN 115894829A CN 202211162407 A CN202211162407 A CN 202211162407A CN 115894829 A CN115894829 A CN 115894829A
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Abstract
The present invention relates to a method for producing a polyacetal resin. The present invention provides a method for producing a polyacetal resin, which can suppress the generation of tarry precipitates caused by a polymerization catalyst and can continuously produce a polyacetal resin stably for a long period of time. A method for producing a polyacetal resin, comprising the following polymerization steps: a polymerization reaction is carried out using (a) a cyclic ether and/or a cyclic formal, (b) boron trifluoride or a boron trifluoride complex, and (c) an organic solvent, wherein the cyclic ether and/or the cyclic formal (a) contains at least trioxane, and the organic solvent (c) is a compound represented by the specified formula (1).
Description
Technical Field
The present invention relates to a method for producing a polyacetal resin.
Background
Polyacetal resins are resins excellent in rigidity, mechanical strength such as toughness, slidability, creep properties, and the like, and are widely used in a range centered on automobile parts, electric/electronic devices, and various mechanical parts.
Automobile parts, electric/electronic devices, and various mechanical parts using the polyacetal resin are important parts in many cases, and it is important to stabilize the quality, that is, to reduce the variation in the quality of parts obtained by molding. In order to achieve this quality stabilization, it is important to perform stable operation for a long period of time in the production of the polyacetal resin.
Conventionally, in the polymerization of a polyacetal resin, as a cause of hindering the long-term stable operation, there is a case where the polymerization operation becomes unstable due to the generation of tar-like precipitates caused by a polymerization catalyst.
As a prior art for suppressing the generation of tar-like precipitates caused by a polymerization catalyst, the following techniques are known: in a solution containing a complex as a polymerization catalyst, the amount of the catalyst relative to the amount of an organic compound that does not form a complex is specified (for example, see patent document 1).
As another method, for example, patent document 2 discloses the following technique: a premixing step of premixing a part of the raw material and the polymerization catalyst is provided, and the raw material and the polymerization catalyst are supplied to the polymerization reactor while being maintained in a specific temperature range.
The above techniques are all techniques capable of realizing a long-term polymerization stable operation in the production of a polyacetal resin.
Documents of the prior art
Patent document
Patent document 1: japanese laid-open patent publication No. 10-81722
Patent document 2: japanese patent No. 6034572
Disclosure of Invention
Problems to be solved by the invention
However, the techniques disclosed in patent documents 1 and 2 cannot sufficiently suppress the occurrence of tarry precipitates caused by the polymerization catalyst, and there is room for improvement from the viewpoint of realizing more stable continuous production of the polyacetal resin for a long period of time.
Accordingly, an object of the present invention is to provide a method for producing a polyacetal resin, which can suppress the occurrence of tarry precipitates caused by a polymerization catalyst and can continuously produce a polyacetal resin stably for a long period of time.
Means for solving the problems
As a result of intensive studies to solve the above problems, the present inventors have found that when a monomer component of a polyacetal resin, a polymerization catalyst and an organic solvent are supplied to a polymerization reactor and polymerized, the use of an organic compound having a specific chemical structure as the organic solvent can reduce the generation of tarry precipitates by the polymerization catalyst and can continuously produce a high-purity polyacetal resin stably for a long period of time, thereby completing the present invention.
Namely, the present invention is as follows.
[1]
A method for producing a polyacetal resin, characterized in that,
the method for producing a polyacetal resin comprises the following polymerization steps: feeding (a) a cyclic ether and/or a cyclic formal, (b) boron trifluoride or a boron trifluoride complex and (c) an organic solvent, and carrying out a polymerization reaction,
the (a) cyclic ether and/or cyclic formal contains at least trioxymethylene, and
the (c) organic solvent is a compound represented by the following formula (1),
(in the formula (1), R 1 And R 2 Each independently a hydrocarbon group having 1 or 2 carbon atoms, R 3 A hydrocarbon group having 2 to 5 carbon atoms, and n is 1).
[2]
Such as [1]]The method for producing a polyacetal resin, wherein R in the formula (1) 1 And R 2 The number of carbon atoms of (2) is 1.
[3]
The process for producing a polyacetal resin according to [1] or [2], wherein the organic solvent (c) is 1, 2-dimethoxyethane.
Effects of the invention
In the present invention, when the polymerization is carried out using the monomer component of the polyacetal resin, the polymerization catalyst and the organic solvent, by using the compound having the specific chemical structure as the organic solvent, the occurrence of tarry precipitates caused by the polymerization catalyst can be reduced, and the high-purity polyacetal resin can be continuously produced stably for a long period of time.
Detailed Description
Hereinafter, a mode for carrying out the present invention (hereinafter, referred to as "the present embodiment") will be described in detail. The present invention is not limited to the following description, and can be implemented by being variously modified within the scope of the gist thereof.
[ method for producing polyacetal resin ]
The method for producing a polyacetal resin according to the present embodiment includes the steps of: a polymerization reaction is carried out using (a) a cyclic ether and/or a cyclic formal, (b) boron trifluoride or a boron trifluoride complex and (c) an organic solvent, wherein the cyclic ether and/or the cyclic formal (a) contains at least trioxane and the organic solvent (c) is a predetermined compound. In the method for producing a polyacetal resin according to the present embodiment, other components such as a low-molecular-weight acetal compound may be further used in addition to the above components.
(Material)
Materials used in the method for producing a polyacetal resin according to the present embodiment will be described.
< A) Cyclic Ether and/or Cyclic Formaldehyde >
In the present embodiment, (a) a cyclic ether and/or a cyclic formal is used as a monomer component. Cyclic ether and/or cyclic formal refers to a cyclic compound having at least one carbon-oxygen bond or carbon-oxygen-carbon bond, and in addition, cyclic ether and/or cyclic formal comprise multimers (trimers, tetramers, etc.) of formaldehyde. Examples of the cyclic ether and/or cyclic formal include: trioxymethylene, tetrapolyoxymethylene, 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 and the like. (a) The cyclic ether and/or cyclic formal may be used alone only one kind (i.e., trioxane alone), or two or more kinds may be used in combination.
As described above, the cyclic ether and/or cyclic formal (a) used in the present embodiment contains at least trioxane. Trioxymethylene is a cyclic trimer of formaldehyde, and is generally obtained by reacting an aqueous formaldehyde solution in the presence of an acidic catalyst. The trioxymethylene may contain water, methanol, formic acid, methyl formate and other impurities which cause chain transfer, and therefore it is preferable to purify the trioxymethylene by removing these impurities in advance by a method such as distillation.
In the case of purifying trioxymethylene as described above, it is preferable to adjust the total amount of impurities to be subjected to chain transfer to 1X 10 relative to 1 mole of trioxymethylene -3 The molar ratio is preferably adjusted to 0.5X 10 or less -3 The mole is less. By reducing the amount of impurities to the above value, the polymerization reaction rate can be sufficiently increased in practical use, and various characteristics such as thermal stability of the polymer to be produced can be improved.
As the cyclic ether and/or cyclic formal other than trioxane, 1, 3-dioxolane and 1, 4-butanediol formal are particularly preferable.
In the case where trioxymethylene and a cyclic ether and/or a cyclic formal other than trioxymethylene are used in combination as (a) the cyclic ether and/or the cyclic formal, the amount of the cyclic ether and/or the cyclic formal other than trioxymethylene to be 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% with respect to 1 mol of trioxymethylene.
Boron trifluoride or boron trifluoride complex
In the present embodiment, (b) boron trifluoride or a boron trifluoride complex (hereinafter, sometimes referred to as (b) a polymerization catalyst) is used. The polyacetal resin can be obtained by a cationic polymerization method, and examples of the cationic polymerization catalyst include a lewis acid, a protonic acid, and an ester or an anhydride thereof, but in the present embodiment, boron trifluoride or a boron trifluoride complex (b) is used as the polymerization catalyst. (b) The polymerization catalyst may be used alone or in combination of two or more. The boron trifluoride includes boron trifluoride hydrate. The complex is a coordination complex of an organic compound containing an oxygen atom or a sulfur atom and boron trifluoride.
For example, when trioxymethylene is used as the component (a), the amount of the polymerization catalyst (b) to be added is preferably 1X 10 relative to 1 mole of the trioxymethylene -9 mole-1X 10 -2 In the molar range, it is more preferably 2X 10 -9 mol-1X 10 -2 In the molar range, the molar ratio is more preferably 5X 10 -9 mole-1X 10 -3 In the molar range. When the amount of the polymerization catalyst (b) added is within the above range, the polymerization reaction can be carried out more stably for a long period of time.
< Low molecular weight acetal Compound >
In the present embodiment, a low molecular weight acetal compound may be further used. The low molecular weight acetal compound functions as a chain transfer agent in a 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 low-molecular-weight acetal compounds include: methylal, methoxy methylal, dimethoxy methylal and trimethoxy methylal. The low-molecular-weight acetal compound may be used alone or in combination of two or more.
When at least trioxymethylene is used as a monomer component and a low-molecular acetal compound is used, the amount of the low-molecular acetal compound added is preferably 0.1 × 10 relative to 1 mole of trioxymethylene from the viewpoint of controlling the molecular weight of the polyacetal resin to an appropriate range -4 molar-0.6X 10 -2 In the molar range.
Organic solvent (c)
In the present embodiment, (c) an organic solvent is used. The organic solvent (c) is an ether compound having no hydroxyl group and not participating in the polymerization reaction or exerting an adverse effect, and specifically, a compound represented by the following formula (1) is used,
(in the formula (1), R 1 And R 2 Each independently a hydrocarbon group having 1 or 2 carbon atoms, R 3 A hydrocarbon group having 2 to 5 carbon atoms, and n is 1). (c) The organic solvent may be used alone or in combination of two or more.
In the present embodiment, the compound having the above chemical structure is used as the (c) organic solvent, and the (b) polymerization catalyst (boron trifluoride or a boron trifluoride complex) forms a specific complex with the (c) organic solvent, thereby stabilizing the compound for a long period of time and suppressing the generation of tarry precipitates due to the deterioration of the polymerization catalyst. As a result, the flow rate of the solution containing the polymerization catalyst (catalyst solution) in the polymerization reaction is stable, and continuous production can be performed for a long period of time.
In addition, a part of the organic solvent may be mixed into the polymer and remain. When the organic solvent remains in the polymer product, the organic solvent also remains in the final product, which causes odor emission from the product. In this regard, in the present embodiment, by using the above-mentioned compound as the (c) organic solvent, the amount of the organic solvent remaining in the polymerization reaction product is small, and a high-purity polymer (polyacetal resin) can be obtained.
N in the formula (1) is 1. Specific examples of the above-mentioned compounds include: 1, 2-dimethoxyethane, propylene glycol dimethyl ether, butanediol dimethyl ether, pentanediol dimethyl ether, ethylene glycol ethyl ether methyl ether, ethylene glycol diethyl ether.
R in the formula (1) 1 And R 2 Saturated hydrocarbon groups are preferable, and in the present embodiment, methyl groups and ethyl groups are exemplified. In addition, R in the formula (1) 1 And R 2 More preferably, the number of carbon atoms is 1. Namely, R in the formula (1) 1 And R 2 Preferably both are methyl.
R in the formula (1) 3 Preferably saturated hydrocarbon groups, e.g.Such as the following: ethylene, 1, 2-propylene, 1, 3-propylene, various linear or branched butylene, various linear or branched pentylene, and the like. In addition, R in the formula (1) 3 The number of carbon atoms is preferably 2 to 3.
The organic solvent (c) is more preferably 1, 2-dimethoxyethane or propylene glycol dimethyl ether. As the (c) organic solvent, 1, 2-dimethoxyethane is most preferable.
For example, when trioxymethylene is used as the component (a), the amount of the organic solvent (c) to be added is preferably 0.1X 10 relative to 1 mole of trioxymethylene -3 The molar ratio is in the range of 0.2 mol to 0.2 mol, more preferably 0.2X 10 mol -3 The molar amount is in the range of 0.1 mol. When the amount of the organic solvent (c) added is within the above range, the amount of scale formation in the supply part of the polymerization reactor can be more effectively reduced, and the polyacetal resin can be obtained in high yield. When the amount of the organic solvent added is too large, the monomer concentration as a whole is relatively lowered, and the reaction may be slowed. In addition, when the amount of the organic solvent added is too small, the polymerization reaction may rapidly proceed, resulting in the generation of scales.
The ratio of the (b) polymerization catalyst to the (c) organic solvent is preferably 0.01 to 10.0 mass% of the concentration of the (b) polymerization catalyst in the catalyst solution containing the (b) polymerization catalyst and the (c) organic solvent. If the concentration of the polymerization catalyst (b) 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 (b) in the catalyst solution is 10.0% by mass or less, the rapid reaction can be suppressed, and the stable polymerization reaction can be more effectively carried out for a long time.
(step (1))
The respective steps included in the method for producing a polyacetal resin according to the present embodiment will be described.
The method for producing a polyacetal resin according to the present embodiment includes a polymerization step of carrying out a polymerization reaction using (a) a cyclic ether and/or a cyclic formal (containing at least trioxymethylene), (b) a polymerization catalyst (boron trifluoride or a boron trifluoride complex) and (c) an organic solvent (a compound represented by formula (1)), and may optionally include a deactivation step of deactivating the polymerization catalyst, a drying step, and the like as appropriate. As described above, in the production method of the present embodiment, the occurrence of tarry precipitates due to the polymerization catalyst can be reduced, and the polyacetal resin can be continuously produced stably 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.
< polymerization step >
As the polymerization method in the polymerization step, either a bulk method or a melt method can be used. The polymerization step may be carried out in a polymerization reactor, and the shape (structure) of the polymerization reactor to be used is not particularly limited. However, any of a biaxial paddle type or screw type stirring and mixing type polymerization apparatus with a jacket through which a heat medium can pass is suitably used as the polymerization reactor.
In the polymerization step, it is preferable to prepare a catalyst solution by dissolving the polymerization catalyst (b) in the organic solvent (c) (diluting the polymerization catalyst (b) with the organic solvent (c)), and after stabilizing the catalyst solution as appropriate, add the cyclic ether and/or cyclic formal (a) and perform polymerization reaction. In this case, the generation of tar-like precipitates caused by the polymerization catalyst can be further reduced, and further stabilization of the continuous production can be achieved.
The temperature of the polymerization reactor (holding temperature of the reaction system) when the polymerization step is carried out is preferably in the range of 63 to 155 ℃, more preferably 70 to 145 ℃, and still more preferably 70 to 140 ℃. The residence (reaction) time in the polymerization reactor (in the reaction system) is preferably 0.1 to 30 minutes, more preferably 0.1 to 25 minutes, and still more preferably 0.1 to 20 minutes. When the temperature and the residence time are within the above ranges, stable polymerization reaction tends to continue.
< inactivation step >
The crude polyacetal resin is obtained through the polymerization step, and then the deactivation step may be carried out. Examples of the method for deactivating the polymerization catalyst include the following methods: the crude polyacetal resin discharged from the polymerization reactor is put into an aqueous solution or an organic solution containing at least one neutralizing/deactivating agent such as ammonia, amines such as triethylamine and tri-n-butylamine, hydroxides of alkali metals or alkaline earth metals, inorganic salts, and organic acid salts, and is continuously stirred in a slurry state at room temperature to 100 ℃ or lower for several minutes to several hours. In this case, when the crude polyacetal resin is in the form of a large lump, it is preferable to carry out the treatment by crushing after the polymerization step.
< drying step >
Further, a drying step of drying the obtained crude polyacetal resin may be carried out. Specifically, the crude polyacetal resin after the polymerization step or the deactivation step may be filtered by a centrifugal separator and dried under nitrogen. Thus, the objective polyacetal resin (polymer) was obtained.
The method for bringing the crude polyacetal resin into a dry state in the drying step is not particularly limited. For example, the crude polyacetal resin can be heated to a dry state by using a hot air dryer, a vacuum dryer, a dryer having a stirring function capable of passing a heat medium, or the like.
The drying temperature may be a temperature which is not lower than the boiling point of the monomer of the crude polyacetal resin and not higher than the temperature at which the crude polyacetal resin does not melt. That is, it is preferably in the range of 115 ℃ to 165 ℃. More preferably, it is in the range of 120 to 155 ℃.
In the method for producing a polyacetal resin according to the present embodiment, it is needless to say that other comonomer components capable of forming a block structure, a branched structure or a crosslinked structure may be used in combination with the above components.
Examples
The present invention will be described in detail below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples.
The evaluation methods in examples and comparative examples are as follows.
< polymerization yield >
The polymerization yield was calculated by dividing the discharged amount per unit time of the crude polyacetal resin discharged from the polymerization reactor by the fed amount per unit time of the whole monomers.
The polymerization yield was calculated 1 hour after the start of the polymerization.
< Condition of precipitate in catalyst solution >
After the continuous operation was completed, the state of precipitates in the catalyst solution containing the polymerization catalyst and the organic solvent was visually confirmed. Further, the following evaluations were performed: if no two-layer separation and precipitation of the solution were observed by visual observation, the operation was stable.
< State of deposit on wall surface of catalyst solution supply line >
After the completion of the continuous operation, the polymerization reactor was opened, and the presence or absence of the deposit on the wall surface of the catalyst solution supply line was visually checked. Further, the following evaluations were performed: the less the amount of the adhering matter such as viscous tarry matter is visually recognized, the more stable the operation is.
< amount of residual solvent in Polymer >
A dried polymer (polyacetal resin) obtained by polymerization was charged into a glass-made closed vessel, and distilled water (manufactured by Fuji film and photo-fabrication) was added in an amount of 1.5ml per 1g of the polymer, and the vessel was closed. The closed vessel containing the polymer and distilled water was heated at 90 ℃ for 12 hours, whereby the component of (c) organic solvent remaining in the polymer was extracted. The extract was measured by gas chromatography, and the amount of the residual solvent in the polymer was quantified. The measurement conditions of the gas chromatography are as follows.
The device comprises the following steps: GC-2014 manufactured by Shimadzu
Column: TC-1 (inner diameter 0.25 mm. Times. Film thickness 0.25 μm. Times. Length 30 m) manufactured by GL science
Carrier gas: nitrogen gas
Sample introduction amount: 1 μ l
Gas flow rate: 1.38 ml/min
Inlet temperature: 250 deg.C
Column temperature: heating from 50 deg.C to 300 deg.C at 5 deg.C/min
A detector: FID
[ example 1]
A biaxial paddle type continuous polymerization reactor (manufactured by chestnut iron, diameter 2B, L/D = 14.8) with a jacket capable of passing a heat medium was adjusted to 80 ℃.
A solution obtained by mixing boron trifluoride-diethyl ether complex (boron trifluoride complex) as the polymerization catalyst (b) and 1, 2-dimethoxyethane as the organic solvent (c) was used as a catalyst solution, and the concentration of the polymerization catalyst in the catalyst solution was adjusted to 3.0 mass%.
A mixed solution obtained by continuously mixing trioxymethylene as the component (a), 1, 3-dioxolane (4.2 mol% of 1, 3-dioxolane based on 1 mol of trioxymethylene) as the component (a) as well as methylal as a low-molecular-weight acetal compound in a pipe, and the catalyst solution were continuously supplied to a polymerization reactor through the respective pipes and subjected to polymerization reaction, whereby a crude polyacetal resin was obtained. At this time, the flow rate was adjusted appropriately so that the amount of the polymerization catalyst was 2.0X 10 relative to 1 mole of trioxymethylene -5 And (3) mol.
The crude polyacetal resin discharged from the polymerization reactor was sampled into an aqueous triethylamine solution (0.5 mass%), stirred at room temperature for 1 hour, filtered by a centrifugal separator, and dried at 140 ℃ for 3 hours under nitrogen, to obtain a polyacetal resin.
The polymerization yield and the amount of the residual solvent in the polymer were calculated from the polyacetal resin obtained by the above-mentioned method. After 240 hours from the start of the polymerization, the state of precipitates in the catalyst solution and the state of deposits on the wall surface of the catalyst solution supply line were evaluated. The evaluation results are shown in table 1.
[ example 2]
The polymerization catalyst of example 1 (b) was changed to boron trifluoride-di-n-butyl ether complex (boron trifluoride complex). Polyacetal resin was obtained under the same conditions as in example 1, and the same evaluation was made. The evaluation results are shown in table 1.
[ examples 3 and 5]
The organic solvent (c) of example 1 was changed to the organic solvent (c) shown in table 1. Polyacetal resin was obtained under the same conditions as in example 1, and the same evaluation was made. The evaluation results are shown in table 1.
[ examples 4 and 6]
The organic solvent (c) of example 2 was changed to the organic solvent (c) shown in table 1. Polyacetal resin was obtained under the same conditions as in example 2, and the same evaluation was made. The evaluation results are shown in table 1.
Comparative example 1
The organic solvent of example 1 was changed to benzene. Polyacetal resin was obtained under the same conditions as in example 1, and the same evaluation was made. The evaluation results are shown in table 1.
Comparative example 2
The organic solvent of example 2 was changed to cyclohexane. Other conditions were the same as in example 2, and a polyacetal resin was obtained and evaluated in the same manner. The evaluation results are shown in table 1.
Comparative example 3
As shown in table 1, the organic solvent (c) in example 1 was changed to diethylene glycol dimethyl ether (corresponding to the compound of formula (1) in which n is 2). Polyacetal resin was obtained under the same conditions as in example 1, and the same evaluation was made. The evaluation results are shown in table 1.
Comparative example 4
As shown in table 1, the organic solvent (c) of example 2 was changed to diethylene glycol dimethyl ether (corresponding to the compound of formula (1) in which n is 2). Polyacetal resin was obtained under the same conditions as in example 2, and the same evaluation was made. The evaluation results are shown in table 1.
As shown in table 1, in examples 1 to 6, no precipitate was observed in the catalyst solution even after the operation for a long period of time, and no deposit was present on the wall surface of the supply line, and the polyacetal resin could be stably produced for a long period of time. In addition, the amount of the residual solvent in the polymer is sufficiently small, and high purity of the polyacetal resin can be achieved.
On the other hand, in comparative examples 1 and 2, the catalyst solution at the stage immediately after mixing the polymerization catalyst and the organic solvent was uniform and transparent, but the catalyst solution after completion of the operation had brown precipitates (tar-like substances) or transparent precipitates at the bottom. In addition, the whole wall surface of the catalyst solution supply line after the completion of the operation was adhered with brown and viscous tar-like substances. Therefore, under the conditions of these comparative examples, precipitates were formed after 240 hours of operation, and in a longer industrial-scale operation, the amount of precipitates formed also increased, and the possibility of operation stoppage due to sudden clogging of the piping became high.
In comparative examples 3 and 4, although brown and viscous tar-like substances were not precipitated, the amount of residual solvent in the polymer was increased as compared with examples 1 to 6, and as a result, the purity was lowered.
Industrial applicability
The present invention has industrial applicability as a method for producing a polyacetal resin, which enables a polyacetal resin to be produced stably for a long period of time.
Claims (3)
1. A process for producing a polyacetal resin, which comprises,
the method for producing a polyacetal resin comprises the following polymerization steps: feeding (a) a cyclic ether and/or a cyclic formal, (b) boron trifluoride or a boron trifluoride complex and (c) an organic solvent, and carrying out a polymerization reaction,
the (a) cyclic ether and/or cyclic formal contains at least trioxymethylene, and
the (c) organic solvent is a compound represented by the following formula (1),
in the formula (1), R 1 And R 2 Each independently a hydrocarbon group having 1 or 2 carbon atoms, R 3 Is a hydrocarbon group having 2 to 5 carbon atoms, and n is 1.
2. The method for producing a polyacetal resin according to claim 1, wherein R in the formula (1) 1 And R 2 The number of carbon atoms of (2) is 1.
3. The method for producing a polyacetal resin according to claim 1 or 2, wherein the organic solvent (c) is 1, 2-dimethoxyethane.
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