Increasing the thermal stability of a vinyl aromatic...

Chemistry of hydrocarbon compounds – Product blend – e.g. – composition – etc. – or blending process... – With nonhydrocarbon additive

Reexamination Certificate

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C585S832000, C585S004000, C208S002000, C208S0480AA, C208S348000, C208S349000

Reexamination Certificate

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06395942

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for increasing the thermal stability of a vinyl aromatic polymerization inhibitor by adding an inhibitor such as N,N′-di-2-butyl-N,N′-dinitroso-1,4-diaminobenzene to a stabilizer compound such as N,N′-di-2-butyl-1,4-diaminobenzene. The mixture of inhibitor and stabilizer can be used in a process for inhibiting the polymerization of vinyl aromatic compounds such as styrene during its distillation.
BACKGROUND OF THE INVENTION
Styrene is one of several vinyl aromatic compounds which has considerable commercial utility. Styrene is polymerized into polystyrene, which is a clear, readily colored and easily fabricated plastic with many uses. The efficiency of the polymerization process is dependent on the purity of the monomer starting material. Since the processes for producing styrene, and other vinyl aromatic compounds, will contain various reaction products including benzene, toluene, etc., the mixture is distilled to separate these undesirable contaminants. Unfortunately, the temperatures required for distillation, typically 90° C. to about 150° C., leads to the polymerization of the vinyl aromatic compounds. In order to minimize or prevent polymerization of the vinyl aromatic monomer, it is common to add a polymerization inhibitor to the distillation mixture.
The art discloses a variety of compounds, which are claimed to inhibit polymerization. These include U.S. Pat. No. 4,050,993, which discloses the use of N,N-nitroso-methylaniline as a polymerization inhibitor. U.S. Pat. No. 3,988,212 which disclose the use of N-nitrosodiphenyl amine in combination with dinitro-o-cresol. U.S. Pat. No. 4,013,580, which discloses the use of N-nitroso aniline derivatives. U.S. Pat. No. 4,341,600, which discloses the use of a mixture of dinitro-p-cresol and N-nitroso-diphenyl amine. U.S. Pat. No. 4,654,451, which discloses mixtures of alkyl substituted p-nitroso phenols and p-nitroso phenol. U.S. Pat. No. 5,034,156, which discloses N-nitrosophenyl-hydroxylamine plus hydroquinone monomethyl ether. U.S. Pat. No. 5,396,004, which discloses a phenylenediamine compound plus a hydroxyalkylhydroxyl-amine compound. U.S. Pat. No. 5,254,760, which discloses 1-oxyl-2,2,6,6-tetramethylpiperidine plus an aromatic nitro compound. U.S. Pat. No. 4,929,778, which discloses a phenylenediamine compound plus a hindered phenol compound. U.S. Pat. No. 5,312,952, which discloses the use of the reaction product of a C
9
-C
20
alkyl phenol with sulfuric and nitric acid and optionally an aryl or alkyl-substituted phenylenediamine. WO9503263, which discloses 3,5-di-tert-butyl-4-hydroxy-N,N-dimethyl benzyl amine. EP-697386-A2, which discloses 4-acetylamino-2,2,6,6-tetramethyl piperidine N-oxyl in combination with 4-nitroso phenol. JP0701910-A which discloses phosphite compounds, nitrosoamine compounds or phenol compounds. JP05310815-A, which discloses the ammonium salt of N-nitrosophenyl hydroxylamine. JP03149205-A, which discloses nitrosophenols plus dicyclohexylammonium nitrate. J01226858-A, which discloses the use of substituted nitrosobenzene. U.S. Pat. No. 4,967,027, which discloses p-nitroso phenol plus p-t-butyl catechol. J59029624-A, which discloses the use of N-nitroso compound, e.g., N-nitroso-diphenylamine and a catechol, e.g., p-t-butylcatechol. Finally, U.S. Pat. No. 5,869,717, discloses the use of N-nitroso derivates of unsubstituted or dialkyl substituted phenylenediamine.
Applicants have now found that the thermal stability of the mono or di-nitroso compounds described in U.S. Pat. No. 5,869,717 can be increased by adding to the nitroso containing compound a stabilizer such as an unsubstituted or substituted phenylenediamine (diaminobenzene) or a substituted phenolic compound. The importance of this discovery is that it extends the temperature range over which the inhibitor can be used. This in turn means that the combination of inhibitor and stabilizer is able to withstand higher temperature disruptions or swings.
SUMMARY OF THE INVENTION
As stated, this invention relates to a process for increasing the thermal stability of various polymerization inhibitors and to a process for inhibiting the polymerization of a vinyl aromatic compound during its distillation. One embodiment of the invention is a process for increasing the thermal stability of a polymerization inhibitor selected from the group consisting of:
and mixtures thereof, where R and R
1
are each independently hydrogen, cyclohexyl, phenyl and an alkyl group having from 1 to 18 carbon atoms, the process comprising adding to the organic compound an effective amount of a stabilizer selected from the group consisting of:
and mixtures thereof, where R
2
is selected from the group consisting of hydrogen, phenyl, cyclohexyl and an alkyl group having from 1 to 18 carbon atoms, and R
3
is selected from the group consisting of hydrogen, an alkyl group having from 1 to 18 carbon atoms, cyclohexyl, and an aromatic group having from 6 to 10 carbon atoms.
Another embodiment of the invention is a process for inhibiting the polymerization of a vinyl aromatic compound during the distillation of the vinyl aromatic compound comprising adding to the compound an effective amount of a mixture of at least one polymerization inhibitor and at least one stabilizer, where the inhibitor is selected from the group consisting of:
and mixtures thereof, where R and R
1
are each independently hydrogen, cyclohexyl, phenyl, or an alkyl group having from 1 to 18 carbon atoms, and the stabilizer is selected from the group consisting of:
and mixtures thereof, where R
2
is selected from the group consisting of hydrogen, cyclohexyl, phenyl, and an alkyl group having from 1 to 18 carbon atoms, and R
3
is selected from the group consisting of hydrogen, an alkyl group having from 1 to 18 carbon atoms, cyclohexyl, and an aromatic group having by from 6 to 10 carbon atoms.
These and other objects and embodiments will become more evident after a detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for increasing the thermal stability of a polymerization inhibitor and to a process for inhibiting the polymerization of a vinyl aromatic compound during its distillation. The polymerization inhibitors are selected from the group consisting of:
The R and R
1
groups can each independently be hydrogen or an alkyl group having from 1 to 18 carbon atoms. Illustrative of the alkyl groups which can be used are methyl, ethyl, propyl, butyl, pentyl, octyl, decyl, sec-butyl, isopropyl and isopentyl. Included in structures I and II above are the o, m and p isomers. Preferred compounds encompassed within structures I and II are the following compounds having structure IA and IIA.
These nitroso compounds are prepared by reacting nitrous acid with an amine according to the procedure set forth in U.S. Pat. No. 5,869,717 which is incorporated by reference. The reaction of nitrous acid with an amine is well known in the art and is included here for completeness. Generally, the amine is reacted with nitrous acid at a temperature of about −10° C. to about 120° C. The nitrous acid can be added or it can be generated in situ by adding sodium nitrite and hydrochloric acid. Further, in the case of a diamine, the amount of nitrous acid added will determine whether the mono or dinitroso compounds are obtained. However, even if a 1:1 stoichiometric amount of nitrous acid:amine is added, a mixture of the mono and dinitroso compounds may still be obtained.
It has been found that the thermal stability of these polymerization inhibitors can be increased by adding to the inhibitor an effective amount of a stabilizer selected from the group consisting of:
and mixtures thereof and R
2
is selected from the group consisting of hydrogen, cyclohexyl, phenyl, and an alkyl group having from 1 to 18 carbon atoms, and R
3
is selected from the group consisting of hydrogen, an alkyl group having from 1 to 18 carbon atoms, cyclohexyl and an aromatic group having from 6 to 10 c

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