Polymerization inhibitor

Details Polymerization inhibitor Polymerization inhibitor

2001-04-13

2002-06-25

Griffin, Walter D. (Department: 1764)

Distillation: processes, separatory

Addition of material to distilland to inhibit or prevent...

For inhibiting or preventing a polymerization reaction

C585S002000, C585S003000, C585S004000, C585S005000, C585S950000, C208S0480AA, C203S008000

Reexamination Certificate

active

06409887

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to free radical scavengers for use as or in connection with inhibitors of olefinic polymerisation, particularly but not exclusively for inhibition of polymerisation of vinyl aromatic compounds.
BACKGROUND OF THE INVENTION
Olefinic compounds such as butadiene, acrylic monomers, styrene and other vinyl aromatic compounds have a strong tendency to polymerise on storage or heating. High temperature techniques such as distillation are commonly used during separation and purification processes and the industrial production of these olefinic compounds.
Although free radical scavengers have been used to prevent olefinic polymerisation, there is a requirement for higher efficiency scavengers which may be used at lower concentrations or for scavengers which are more efficient at the same concentrations as are currently employed. Lower concentrations are economic, less toxic and are environmentally beneficial. More efficient compositions are economic, giving reduced wastage and/or prolongation of the active life of the olefin.
Various compounds and compositions have been used as free radical scavengers to prevent or reduce undesired polymerisation of olefinic compounds during high temperature processes. These inhibitors have given varying degrees of success. In a typical process an olefinic compound may be contacted with the inhibitor before distillation or other processing. However the amount of polymer formed during such processing may be substantially higher than desired, leading to economic loss. In other cases it may be possible to achieve economically desirable low levels of polymerisation but only by use of economically unacceptable quantities of the inhibitor. Sulphur has been widely employed as a polymerisation inhibitor but difficulties in handling and disposal have lead to replacement by non-sulphur inhibitors, referred to as NSIs. SU-A-819078 discloses use of 3,5-ditertiary butyl-4-oxy-N,N-dimethylbenzylamine in combination with a C
10
-C
20
aliphatic carboxylic acid. However the concentrations of this particular NSI are sufficiently large (500 to 2200 ppm) that much of the NSI is not consumed in a typical process. This results in uneconomic quantities of residual NSI being present in the effluent or waste from the process. EP-A-550754 discloses a polymerisation inhibitor for vinyl compounds combining a reaction product of 4-(dimethylamino)methyl-2,6-bis(tert butyl)phenol with an inorganic or carboxylic acid in the weight ratio 1 to 15:3. WO95/03263 discloses reduction in the quantity of this NSI by use in combination with air. However it is undesirable to introduce air into many high temperature processes involving olefinic compounds. Firstly there is an economic cost associated with ensuring a supply of the required quantity of air. Secondly it is often a safety hazard to introduce air into the stream of a hot olefinic compound due to the risk of oxidation, fire or an explosion.
Use of stable free radicals for inhibiting polymerisation by scavenging other free radicals has been disclosed. G M Burnett, Mechanism of Polymer Reactions, Interscience, New York 1954, 76, stated that a sterically hindered nitroxyl radical, 2,2,6,6-tetramethyl piperidinyl-1-oxyl (referred to as TEMPO) is effective against carbon centred radicals. Use of stable free radicals as scavengers was reviewed by E G Rozantsev, Free Nitroxyl Radicals, Plenan Press, 1970, 105. The practical utility of nitroxyl radicals as scavengers to prevent polymerisation was disclosed in GB 1127127 and GB 1218456. The high cost of stable free radicals has restricted their commercial use and there remains a requirement for an economic means of preventing or controlling polymerisation. U.S. Pat. No. 4,670,131 disclosed that certain stable free radicals may be used as polymerisation inhibitors under certain conditions at levels less than 700 parts per billion. However such low usage rates are not of general applicability.
SUMMARY OF THE INVENTION
We have now found that a free radical scavenger composition including an aromatic amine in combination with an organic acid and a stable free radical compound is effective without the economic disadvantages of the previously disclosed compositions.
According to a first aspect of the present invention a monomer composition, stabilised against premature polymerisation, comprises:
a) an ethylenically unsaturated monomer or mixture of monomers polymerisable by free radical initiation, and
b) an effective amount, sufficient to inhibit premature polymerisation of component (a) of a mixture of;
i) 1 to 99% by weight, based on the total weight of components (i) and (ii) of a mixture of at least one aromatic amine and at least one organic acid in a molar ratio of 10:1 to 1:10, and
ii) 99 to 1% by weight based on the total weight of components (i) and (ii) of at least one stable radical compound.
Compositions in accordance with this invention exhibit a synergistic effect in that the inhibition efficiency is greater than that of the two components. Preferred monomers are vinyl aromatic compounds for example styrene, methyl styrene, vinyl toluene or divinyl benzene or vinyl aliphatic compounds including acrylonitrile, acrylic acid, methacrylic acid, acrylate esters, methacrylate esters, butadiene and butenes. The invention is particularly applicable in inhibition of polymerisation of styrene during distillation, purification or storage.
DETAILED DESCRIPTION OF THE INVENTION
In preferred compositions the weight of component i) is 50 to 99% and the weight of component ii) is 50 to 1%. In particularly preferred embodiments of the invention the weight of component i) is 67 to 99% and the weight of component ii) is 33 to 1%.
Percentages and other amounts in this specification are by weight unless indicated otherwise. Percentages and other proportions are selected to total 100%.
The molar ratio of said at least one aromatic amine to said at least one organic acid is 3:1 to 1:3.
In preferred embodiments of the invention the aromatic amine has the formula (1)
wherein Q is O or S or N-Z
and wherein at least one R is an alkyl amine of the structure (2)
in which Y, Z are the same or different and comprise C
1
to C
4
branched or straight chain alkyl or hydrogen and in which X is either a covalent bond or C
1
to C
4
alkylene, and wherein each remaining R is independently benzyl, C
1
to C
4
branched or straight chain alkyl or hydrogen with the provision that two or more R may be connected to form one or more rings.
Q is preferably oxygen.
X is preferably methylene.
Y and Z are preferably methyl.
R
1
and R
5
are preferably tertiary butyl.
A preferred amine has Y and Z as methyl and R
2
and R
4
as hydrogen.
Preferred amines include benzylamine and benzylamine derivatives, for example 3,5-tertiarybutyl-4-hydroxybenzylamine, anilines and phenylene diamines, for example
N,N-dimethyl-1,4-phenylenediamine
3,5-ditertiarybutyl-4-oxy, N,N-dimethylbenzylamine
3,5-ditertiarybutyl-4-oxy-N,N-diethylbenzylamine
N,N-dimethyl-2,6-ditertiarybutyl-4-amino phenol
1,4-phenylenediamine
N,N-dimethyl-4-aminothiophenol
N,N′-bis-(1,4 dimethylpentyl)-1,4-phenylenediamine
In preferred embodiments of the invention the organic acid is a carboxylic acid. C
6
to C
22,
preferably C
6
to C
18
saturated or unsaturated carboxylic acids are particularly efficacious. These may be substituted with groups inert to attack by the nitroxyl radical. Linear or branched chain acids may be employed. Branch chain acids which are liquid at ambient temperatures are convenient to manipulate. Unsubstituted saturated acids are especially preferred.
The stable radical compound is preferably a sterically hindered nitroxyl of the formulae (3)
wherein R is hydrogen, alkyl or aryl and T is a group required to form a ring and wherein two or more T groups may be linked by a linking group E.
Preferred nitroxyl compounds may be selected from:
1-oxyl-2,2,6,6-tetramethylpiperidin,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-one,
1-oxyl-2,2,6,6-tetramethylpiperidin-4

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