Polymerisation inhibitors

Abstract

The present invention relates to a mixture containing at least one substituted hydroxy-pyrazole and at least one polymerisable compound.

Description

[0001] Polymerization inhibitors

[0002] The present invention relates to a mixture containing at least one substituted 5-hydroxypyrazole and at least one polymerizable compound.

[0003] Chemical compounds that contain one or more ethylenically unsaturated groups have a pronounced tendency toward radical polymerization. Such compounds are therefore also referred to as polymerizable compounds. Due to their tendency toward radical polymerization, these compounds are used as monomers for the production of polymers. However, the pronounced tendency of these compounds toward radical polymerization is disadvantageous in that it can lead to undesirable, spontaneous radical polymerization during storage and transport, as well as during chemical and / or physical processing, such as distillation or rectification, particularly under the influence of energy, such as heat and / or light.Such uncontrolled polymerizations can lead to the gradual formation of polymer deposits, for example, on heated surfaces, necessitating their removal and thus often resulting in a reduction in operating times. Uncontrolled polymerizations can even be explosive.

[0004] It is therefore common practice to add compounds that inhibit or at least slow down unwanted, spontaneous radical polymerization to ethylenically unsaturated compounds that have a tendency toward radical polymerization, or to mixtures containing such compounds, both during storage and transport and during chemical and / or physical processing. Such substances are referred to as polymerization inhibitors.

[0005] Polymerization inhibitors can be used as individual chemical compounds or as mixtures of compounds. Depending on the application, specific requirements apply to the polymerization inhibitor. For a polymerization inhibitor to be suitable as a transport and / or storage stabilizer for ethylenically unsaturated compounds, it is important that the efficiency of the polymerization inhibitor, i.e., the extent of polymerization inhibition, can be regulated.Under the storage and / or transport conditions of the ethylenically unsaturated compounds, the polymerization inhibitor should sufficiently prevent or slow down the undesired, spontaneous radical polymerization, whereas the desired radical polymerization of the ethylenically unsaturated compounds should be possible under appropriate polymerization conditions without the need to first separate the polymerization inhibitor used during storage and / or transport. If the stabilizer used during storage and / or transport is not separated during the desired polymerization, it is important that it does not adversely affect the desired polymerization, for example, by acting unintentionally as a regulator.

[0006] Measured by global production volume, acrylic acid is certainly one of the most important ethylenically unsaturated compounds. Acrylic acid is typically stabilized against unwanted, spontaneous radical polymerization during storage and / or transport with 0.018 to 0.022 wt.% hydroquinone monomethyl ether (MEHQ), based on the amount of acrylic acid. For adequate stabilization of acrylic acid against unwanted, spontaneous radical polymerization using MEHQ, it is necessary that oxygen is dissolved in the acrylic acid in sufficient quantities. Sufficient amounts of oxygen are usually dissolved in the acrylic acid when acrylic acid is stored and / or transported under an atmosphere containing 5 to 21 vol.% oxygen.During the desired polymerization of acrylic acid, the oxygen content dissolved in the acrylic acid is reduced, thereby reducing the efficiency of MEHQ to inhibit polymerization to such an extent that acrylic acid can be polymerized in the presence of MEHQ.

[0007] In addition to its use as a polymerization inhibitor during storage and / or transport of acrylic acid, MEHQ is also used as a polymerization inhibitor during storage and / or transport of methacrylic acid, acrylic acid esters and / or methacrylic acid esters or mixtures containing one or more of the aforementioned compounds.

[0008] Due to its wide use, MEHQ is one of the most important storage and / or transport stabilizers for polymerizable compounds, especially for acrylic acid, methacrylic acid, acrylic acid esters and methacrylic acid esters.

[0009] Polymerizable compounds are stored in suitable, permanently installed containers, such as storage tanks or storage containers (see, for example, Acrylic Acid, A Summary of Safety and Handling, 4th Edition 2013, 7 Bulk Storage Facilities and Accessories). It is preferred that the polymerizable compounds remain in the respective containers for at least one minute during storage, more preferably at least 10 minutes, and most preferably at least 60 minutes. Although the storage period is theoretically unlimited under appropriate conditions, the storage period is generally reduced to a minimum for economic reasons. It is preferred that the polymerizable compounds remain in the respective containers for a maximum of 180 days, more preferably for a maximum of 90 days, and most preferably for a maximum of 30 days.Particularly preferred ranges result from any combination of the previously mentioned lower and upper limits.

[0010] Polymerizable compounds are usually transported in suitable, transportable containers such as tanks or containers by ship, railcar, and / or truck (see, for example, Acrylic Acid, A Summary of Safety and Handling, 4th Edition 2013, 9 Safe Transport of Acrylic Acid). Transportable containers can also be permanently installed on the respective means of transport, such as ship tanks or rail tank cars. It is also possible for the containers to be stored at a specific location for a certain period of time before being transported to another location. Polymerizable compounds can also be transported in pipelines or hoses, for example, after purification of the polymerizable compounds to the storage tank, during loading from the storage tank into a transportable container, and / or during loading from one transportable container to another.It is preferred that the polymerizable compounds remain in the respective containers for at least 10 seconds, preferably at least one minute, more preferably at least 10 minutes, and most preferably at least 60 minutes, during transport. Although the duration is theoretically unlimited under appropriate conditions, the duration is generally reduced to a minimum for economic and safety reasons. It is preferred that the polymerizable compounds remain in the respective containers for a maximum of 180 days, more preferably for a maximum of 90 days, and most preferably for a maximum of 30 days, during transport. Particularly preferred ranges result from any combination of the aforementioned lower and upper limits.

[0011] The object of the present invention was to provide mixtures containing at least one polymerizable compound, in particular acrylic acid, methacrylic acid, acrylic esters, and / or methacrylic esters, and at least one polymerization inhibitor. The polymerization inhibitors are intended to ensure sufficient stabilization of the polymerizable compounds against unwanted radical polymerization and to represent an alternative to MEHQ. However, in the case of desired radical polymerization, there should be no need to separate the polymerization inhibitors from the mixture prior to polymerization. Therefore, the polymerization inhibitors should not act as polymerization regulators and / or polymerization inhibitors in the desired radical polymerization.

[0012] The object is achieved by mixtures containing at least one compound of the general formula (I) where

[0013] R 1 H, methyl or ethyl, preferably methyl or ethyl,

[0014] R 2 H, methyl or ethyl, preferably methyl or ethyl, and

[0015] R 3 H, methyl or ethyl, preferably H, and at least one polymerizable compound.

[0016] Geeignete Verbindungen der allgemeinen Formel (I) sind beispielsweise 5-Hydroxy-pyrazol, 1- Methyl-5-hydroxy-pyrazol, 3-Methyl-5-hydroxy-pyrazol, 4-Methyl-5-hydroxy-pyrazol, 1-Ethyl-5- hydroxy-pyrazol, 3-Ethyl-5-hydroxy-pyrazol, 4-Ethyl-5-hydroxy-pyrazol, 1 ,3-Dimethyl-5-hydroxy- pyrazol, 1 ,4-Dimethyl-5-hydroxy-pyrazol, 3,4-Dimethyl-5-hydroxy-pyrazol, 1 ,3-Diethyl-5-hydroxy- pyrazol, 1,4-Diethyl-5-hydroxy-pyrazol, 3,4-Diethyl-5-hydroxy-pyrazol, 1,3,4-Trimethyl-5-hyd- roxy-pyrazol, 1,3,4-Triethyl-5-hydroxy-pyrazol, 1-Methyl-3-ethyl-5-hydroxy-pyrazol, 1-Methyl-4- ethyl-5-hydroxy-pyrazol, 1-Ethyl-3-methyl-5-hydroxy-pyrazol, 1-Ethyl-4-methyl-5-hydroxy-py- razol, 3-Methyl-4-ethyl-5-hydroxy-pyrazol, 3-Ethyl-4-methyl-5-hydroxy-pyrazol, 1-Methyl-3,4- diethyl-5-hydroxy-pyrazol, 1 ,3-Dimethyl-4-ethyl-5-hydroxy-pyrazol, 1 ,4-Dimethyl-3-ethyl-5-hyd- roxy-pyrazol, 1-Ethyl-3,4-dimethyl-5-hydroxy-pyrazol, 1 ,3-Diethyl-4-methyl-5-hydroxy-pyrazol und 1 ,4-Diethyl-3-methyl-5-hydroxy-pyrazol.

[0017] The compounds of general formula (I) exist in solution in equilibrium with their keto form (tautomerism). For example, 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole. The tautomer equilibrium is generally shown below for compounds of general formula (I). For the purposes of the present invention, whenever one of the tautomers of the compounds of general formula (I) is mentioned, this also includes the other tautomers, in accordance with the equilibrium below. This applies in particular to the details of quantities. In this context, one of the tautomers or any mixture of two or three of the tautomers shown can be used for the mixture according to the invention, whereby the equilibrium is established.

[0018] The compound of general formula (I) is most preferably 1,3-dimethyl-5-hydroxypyrazole or its keto form 1,3-dimethyl-5-pyrazolone.

[0019] The tautomer equilibrium of 1,3-dimethyl-5-hydroxypyrazole is shown below as an example. The compounds are, in the order shown, 1,3-dimethyl-1H-pyrazol-5-ol, 2,5-dimethyl-2,4-dihydro-3H-pyrazol-3-one, and 2,5-dimethyl-1,2-dihydro-3H-pyrazol-3-one (synonymous nomenclature).

[0020] In the mixture, the total amount of the compounds of the general formula (I) is preferably from 0.0001 to 0.1000 wt.%, preferably from 0.0002 to 0.0750 wt.%, particularly preferably from 0.0005 to 0.0500 wt.%, very particularly preferably from 0.0007 to 0.0250 wt.%, especially preferably from 0.0008 to 0.0100 wt.%, very especially preferably from 0.0009 to 0.0050 wt.%, particularly preferably from 0.0010 to 0.0025 wt.%, in each case based on the total amount of polymerizable compounds.

[0021] In the mixture, the total amount of polymerizable compounds is preferably at least 5% by weight, preferably at least 50% by weight, particularly preferably at least 80% by weight, very particularly preferably at least 98% by weight, especially preferably at least 99% by weight, in each case based on the total weight of the mixture.

[0022] Polymerizable compounds are preferably mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acids, mono-, di- or triethylenically unsaturated C3 to Cs aldehydes, mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acid esters having 1 to 20 carbon atoms in the ester groups, mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acid amides, mono-, di- or triethylenically unsaturated C3 to Cs nitriles, mono-, di- or triethylenically unsaturated C3 to Cs carboxylic acid anhydrides, vinyl esters of saturated C2 to C2o carboxylic acids, vinyl ethers of saturated C1 to C8 alcohols, vinyl aromatics, vinyl heteroaromatics, vinyl lactams having 3 to 10 carbon atoms in the ring, open-chain N-vinylamide compounds and N-vinylamine compounds, vinyl halides, aliphatic, optionally halogenated hydrocarbons with 2 to 8 carbon atoms and 1 or 2 ethylenic double bonds,Vinylidenes or any mixtures of two or more of the aforementioned compounds.

[0023] Particularly preferred are mono-, di- or triethylenically unsaturated C3- to C5-carboxylic acids, for example acrylic acid, methacrylic acid, dimethacrylic acid, ethacrylic acid, citraconic acid, methylenemalonic acid, crotonic acid, fumaric acid, mesaconic acid, itaconic acid and maleic acid, mono-, di- or triethylenically unsaturated C3- to C5-carboxylic acid esters having 1 to 20 carbon atoms in the ester groups, for example acrylic acid esters with C1- to C20-alkyl, methacrylic acid esters with C1- to C20-alkyl, dimethacrylic acid esters with C1- to C20-alkyl, ethacrylic acid esters with C1- to C20-alkyl, citraconic acid esters with C1- to C20-alkyl, methylenemalonic acid esters with C1- to C20-alkyl, crotonic acid esters with C1- to C20-alkyl, fumaric acid esters with C1- to C20-alkyl, me- saconic acid esters with Ci- to C2o-alkyl, itaconic acid esters with Ci- to C2o-alkyl and maleic acid esters with Ci- to C2o-alkyl, mono-, di- or triethylenically unsaturated C3- to Cs-carboxylic acid amides, for example acrylamide, methacrylamide,Dimethacrylamide, ethacrylamide, citraconicamide, methylenemalonicamide, crotonicamide, fumaramide, mesaconicamide, itaconicamide and maleamide, mono-, di- or triethylenically unsaturated C3 to Cs nitriles, for example acrylonitrile and methacrylonitrile, and mono-, di- or triethylenically unsaturated C3 to Cs carboxylic anhydrides, for example acrylic anhydride, methacrylic anhydride, itaconic anhydride and maleic anhydride.

[0024] Very particular preference is given to acrylic acid, methacrylic acid, acrylic acid esters with Ci- to Cs-alkyl, such as methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate, and methacrylic acid esters with Ci- to Cs-alkyl, such as methyl methacrylate.

[0025] Further suitable polymerizable compounds are dipropylene glycol diacrylate, tripropylene glycol diacrylate, polyethylene glycol diacrylate, glycerol triacrylate, ethoxylated glycerol triacrylate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, butanediol monoacrylate, dicyclopentadienyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate, and 2-hydroxypropyl acrylate. The present invention further provides for the use of at least one compound of general formula (I), as defined above, for inhibiting the polymerization of at least one polymerizable compound.

[0026] The present invention further provides a process for storing and / or transporting a mixture comprising at least one compound of general formula (I) as defined above and at least one polymerizable compound.

[0027] The mixture is usually stored and / or transported under an oxygen-containing atmosphere.

[0028] The mixture is preferably stored in a container under an oxygen-containing atmosphere with an oxygen content of 5 to 10 vol.%, and the mixture is regularly circulated within the container, for example, by completely pumping the tank contents around at least once a week. The relatively low oxygen content prevents flammable gas mixtures in the container. Circulation replaces consumed dissolved oxygen in the liquid polymerizable compound.

[0029] Examples

[0030] Preparation of 1,3-dimethyl-5-hydroxypyrazole

[0031] 1,3-Dimethyl-5-hydroxy-pyrazole was prepared as described in Fakhraian, H., Nafari, Y; Preparative, mechanistic and tautomeric investigation of 1-phenyl and 1-methyl derivative of 3-methyl-5-pyrazolone; J Chem Sci 133, 40 (2021) (DOI: https: / / doi.org / 10.1007 / s12039-021-01902-9).

[0032] Experiments on polymerization inhibition

[0033] The monomer used was distilled twice to remove the polymerization inhibitor MEHQ. The resulting monomer was then treated with the specified amount of the polymerization inhibitor.

[0034] 0.5 ml of each mixture was filled into a 1.8 ml ampoule and stored at the specified temperature in a convection oven. In each test series, three ampoule(s) of each mixture were filled and tested, with the mean time to complete polymerization being recorded visually. The relative efficacy is calculated from the quotient of the time to polymerization of the sample with the polymerization inhibitor under investigation and the time to polymerization of the sample with MEHQ. For MEHQ itself, the efficacy is thus 100%.

[0035] The results are summarized in Tables 1 and 2:

[0036] Table 1 : Mixtures with acrylic acid

[0037] MEHQ: hydroquinone monomethyl ether DMHP: 1,3-dimethyl-1H-pyrazole-5-ol

[0038] TBHP: 3-(tert-Butyl)-1-methyl-1H-pyrazole-5-ol

[0039] TM HP: 1,3,4-Trimethyl-1H-pyrazole-5-ol

[0040] MHP: 1-Methyl-1H-pyrazole-5-ol

[0041] IM HP: 1 -isopropyl-3-methyl- 1 H-pyrazol-5-ol *) Comparison

[0042] The polymerization inhibitors according to the invention have improved efficacy compared to MEHQ and the derivatives not according to the invention.

[0043] Polymerization experiments

[0044] Under a nitrogen atmosphere, 450 g of water were placed in a reaction vessel. The reaction mixture was heated to 95°C with stirring. Once the temperature reached 95°C, three streams were added while stirring and maintaining the temperature.

[0045] Stream 1 : Dosage over 5 h, 500 g acrylic acid

[0046] Stream 2: Dosage over 4.75 h, 15 g sodium hypophosphite in 35 g deionized water Stream 3: Dosage over 5.25 h, 5 g sodium peroxodisulfate in 66.4 g deionized water

[0047] After addition of the three streams, the reaction mixture was stirred for another hour at 95°C. The reaction mixture was then cooled to room temperature and 80 g of water were added.

[0048] The acrylic acid used in stream 1 was stabilized with 200 ppm by weight of 1,3-dimethyl-5-hydroxypyrazole or 200 ppm by weight of MEHQ.

[0049] The obtained polymers were analyzed by GPC (calibration with Na-PAA standard, eluent 0.01 mol / l phosphate buffer, pH=7.4 in distilled water, with 0.01 M NaN3).

[0050] Table 2: Polymerization of acrylic acid

[0051] MEHQ: hydroquinone monomethyl ether

[0052] DMHP: 1,3-Dimethyl-5-hydroxy-pyrazole

[0053] *) Comparison

[0054] The polymerization inhibitor according to the invention resulted in a polyacrylic acid with comparable molecular weight.

Claims

Patent claims 1. Mixture containing at least one compound of general formula (I) where R 1 H, methyl or ethyl, R 2 H, methyl or ethyl, and R 3 H, methyl or ethyl, and at least one polymerizable compound.

2. Mixture according to claim 1, wherein R 1 is methyl or ethyl, R 2 is methyl or ethyl and R 3 H is.

3. Mixture according to claim 1, wherein the compound of general formula (I) is 1,3-dimethyl-5-hydroxypyrazole.

4. Mixture according to one of claims 1 to 3, wherein the total amount of the compounds of the general formula (I) in the mixture is from 0.0001 to 0.1000 wt.%, based on the total amount of polymerizable compounds.

5. Mixture according to one of claims 1 to 3, wherein the total amount of the compounds of the general formula (I) in the mixture is from 0.0008 to 0.0100 wt.%, based on the total amount of polymerizable compounds.

6. Mixture according to one of claims 1 to 5, wherein the total amount of polymerizable compounds is at least 5 wt.%, based on the total weight of the mixture.

7. A mixture according to any one of claims 1 to 5, wherein the total amount of polymerizable compounds is at least 98% by weight, based on the total weight of the mixture.

8. Mixture according to one of claims 1 to 7, wherein the polymerizable compound is acrylic acid and / or methacrylic acid.

9. Use of at least one compound of general formula (I) as defined in any one of claims 1 to 3 for inhibiting the polymerization of at least one polymerizable compound.

10. A method for storing and / or transporting a mixture according to any one of claims 1 to 8.

11. The method according to claim 10, wherein the mixture is stored and / or transported under an oxygen-containing atmosphere.

12. The method according to claim 11, wherein the mixture is stored in a container under an oxygen-containing atmosphere with an oxygen content of 5 to 10 vol.% and the mixture is regularly circulated in the container.

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