Abstract:
Vinyl aromatic compounds are stabilized against undesired polymerization by adding to the vinyl aromatic compounds small amounts of at least one N,N-diarylhydroxylamine and at least one N,N&#39; dimer of phenothiazine or substituted phenothiazine.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the stabilization of ethylenically unsaturated compounds and more particularly to the inhibition of undesired polymerization of vinyl aromatic compounds during storage, shipping or processing. 
     BACKGROUND 
     Vinyl aromatic compounds such as styrene undergo undesired spontaneous polymerization (i.e. polymerization of monomers due to heat or the random generation of free radicals in the monomers) during storage, shipping or processing. The problem is particularly acute during purification operations carried out at elevated temperatures such as distillation. Spontaneous polymerization is disadvantageous not only because it causes fouling of distillation column reboilers and other equipment used for processing the vinyl aromatic monomer but also because it usually renders the monomer unfit for use without further treatment. Accordingly, it is desirable and often necessary to inhibit the spontaneous polymerization of vinyl aromatic monomers. 
     PRIOR ART 
     To prevent spontaneous polymerization of vinyl aromatic monomers it is common practice to add to the monomers compounds which have polymerization inhibiting activity. A wide variety of such compounds, known as polymerization inhibitors, have been used for this purpose. Sulfur has been widely used in the past to inhibit polymerization of vinyl aromatic compounds; however, sulfur usage is undesirable because large quantities of sulfur are required for effective polymerization inhibition. This presents a waste removal problem when the monomer is separated from the sulfur-monomer mixture, which is accomplished by distillation. The distillation bottoms product, which contains higher molecular weight hydrocarbons, polymer and sulfur, cannot be burned due to the air pollution hazard caused by sulfur oxides. Thus, this material must be disposed of by burial in a waste dump. 
     In recent times, many chemical compounds have been developed as substitutes for sulfur in polymerization inhibiting applications. These compounds have been used as polymerization inhibitors for vinyl aromatic monomers with varying degrees of success. U.S. Pat. No. 3,148,225, issued to Albert, employs dialkyhydroxylamines for inhibiting popcorn polymers formation in styrene-butadiene rubbers. The dialkylhydroxylamine compounds appear to react with and terminate free radicals which cause undesired formation of polymers. U.S. Pat. No. 2,965,685, issued to Campbell, discloses inhibiting polymerization by adding about 5 ppm to 5 percent dialkylhydroxylamine to styrene monomer. Sato et al, in U.S. Pat. No. 3,849,498, teach the use of diethylhydroxylamine as a polymerization inhibitor for an alcoholic solution of unsaturated aldehydes. Mayer-Mader et al, U.S. Pat. No. 3,878,181, employ diethylhydroxylamine either alone or in combination with a water soluble amine such as triethanolamine to terminate the aqueous emulsion polymerization of chloroprene. U.S. Pat. No. 3,426,063 discloses the use of N-nitrosoaralkyldroxylamines to inhibit thermal polymerization of ethylenically unsaturated hydrocarbons. U.S. Pat. Nos. 4,061,545 and 4,177,110 issued to Watson, disclose the use of a combination of tertiary-butylcatechol and phenothiazine as a polymerization inhibitor system for vinyl aromatic compounds. U.S. Pat. No. 3,539,515, issued to McCabe, discloses the use of phenothiazine dehydrocondensates as antioxidants for lubricating oils. The phenothiazine dehydrocondensates are prepared by reacting phenothiazine or a substituted phenothiazine with an organic peroxide. 
     It has now been discovered that mixtures of N,N&#39; dimers of phenothiazine or substituted phenothiazine and N,N-diaromatic-substituted hydroxylamine provide outstanding polymerization inhibiting activity for vinyl aromatic monomers. Thus, because of the synergistic effect of these mixtures it is now possible to provide unexpectedly superior polymerization inhibiting protection with the same total equivalent weight of mixtures of the N,N&#39; dimers of phenothiazine or substituted phenothiazines and N,N&#39;-diaromatic-substituted hydroxylamines than is obtainable by the use of members of either of these groups of compounds by themselves. 
     Accordingly, it is object of the invention to present stable compositions of vinyl aromatic monomers. It is another object of the invention to present a method of effectively and economically inhibiting spontaneous polymerization of styrene and other vinyl aromatic monomers. These and other objects of the invention are set forth in the following description and examples of the invention. 
     SUMMARY OF THE INVENTION 
     According to the invention the protection of vinyl aromatic monomers against spontaneous polymerization is accomplished by incorporating into the monomers a polymerization inhibiting system comprised of at least one N,N&#39; dimer of phenothiazine or a substituted phenothiazine and at least one N,N-diaromatic-substituted hydroxylamine. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The term vinyl aromatic monomer as used in this description includes any of the readily polymerizable vinyl aromatic compounds, e.g. styrene, alpha alkyl styrene, such as alpha methyl styrene, ring alkyl substituted styrene such as p-methyl styrene, diethylenically substituted benzene compounds, such as divinylbenzene, etc. and mixtures thereof. 
     The N,N&#39; dimers of phenothiazine or the substituted phenothiazines most useful in the invention have the structural formula ##STR1## wherein m is an integer of 0 to 4, i.e. there may be from 0 to 16 R substituents on each molecule of the dimer, and some or all of the R&#39;s may be identical or all of the R&#39;s may be different. In the preferred embodiment the m&#39;s are integers having values of 0, 1 or 2. Each R may be a halogen atom or an unsubstituted or a halogen-substituted alkyl group having 1 to 20 and preferably 1 to 6 alkyl carbon atoms. 
     When all of the m&#39;s in the above structural formula are 0, the compound is the N,N&#39; dimer of phenothiazine. This is the preferred dimer for use in the invention since unsubstituted phenothiazine is less expensive as a starting material than the substituted phenothiazines. In an alternate embodiment the m&#39;s may be integers of 1 to 4, in which case the compound is a dimer of an alkyl-substituted or haloalkyl-substituted derivative of phenothiazine. Typical hydrocarbon alkyl substituents include methyl, ethyl, isopropyl, butyl, hexyl decyl, hexadecyl, etc. groups. There may be up to 4 such substituents on each benzene ring portion of the dimer. Since the dimer contains 4 benzene nuclei there may be up to 16 identical or different substituents. If all of the phenothiazine component starting material is phenothiazine or a single derivative of phenothiazine, the dimer is composed of two identical moieities. However, if the phenothiazine component starting material is composed of two or more different phenothiazine derivatives a mixture of products would result some of which could have 16 different substituents if the monomeric starting materials had all dissimilar substituents. 
     Typical phenothiazine dimers are 10,10&#39;diphenothiazine; 1,1&#39;-dimethyl-10,10&#39;diphenothiazine; 2,6,2&#39;6&#39;tetramethyl-10,10&#39;-diphenothiazine; 2,2&#39;-dimethyl-8,8&#39;dipropyl-10,10&#39;diphenothiazine; 3,4&#39;-dimethyl-6,7&#39;dihexyl-10,10&#39;-diphenothiazine; 2,2&#39;-dichloro-10,10&#39;-diphenothiazine; 3,3&#39;,7,7&#39;-tetrabromo-10,10&#39;diphenothiazine; 4,4&#39;-bis(2-chloroethyl)-10,10&#39;-diphenothiazine; 3,3&#39;,6,6&#39;-tetrakis(4-fluorobutyl)-10,10&#39;-diphenothiazine; 1,1&#39;,2,2&#39;,3,3&#39;,4,4&#39;,6,6&#39;,7,7&#39;,8,8&#39;,9,9&#39;hexadecylmethyl-10,10&#39;-diphenothiazine; etc. From the standpoint of preparation, cost and utility, the preferred phenothiazine dimers are phenothiazine dimer and the alkylsubstituted phenothiazine dimers having up to two substituents on each benzene nucleus, each substituent having 1 to 4 carbon atoms in each alkyl group. Examples of preferred substituted phenothiazine dimers are 1,1&#39;-dimethyl-10,10&#39;-diphenothiazine; 2,2&#39;-dimethyl-4,4&#39;-diethyl-10,10&#39;-diphenothiazine; 2,2&#39;,6,6&#39;tetramethyl-3,3&#39;-diethyl-10,10&#39;-diphenothiazine, etc. 
     The term &#34;phenothiazine component&#34; as used herein represents phenothiazine or any of the substituted phenothiazines included in the above definition. 
     Phenothiazine and some hydrocarbon-substituted phenothiazines are available commercially. Others may be prepared by well-known techniques, such as alkylation. The preparation of the phenothiazine component forms no part of the invention. 
     The phenothiazine dimers used in the invention are prepared by heating the phenothiazine component in the presence of an organic peroxide. The optimum reaction temperature employed will vary depending upon the particular phenothiazine compound used as the starting material and the particular organic peroxide used. In general, temperatures in the range of about 25° to 300° C. are effective to produce the desired result. 
     Any of the common organic peroxides can be used to effect the dimerization. The peroxide chosen will depend upon the desired reaction temperature. Typical organic peroxides include benzoyl peroxide, lauroyl peroxide, ditertiary-butyl peroxide, tertiary-butyl hydroperoxide, tertiary-butyl peroctoate, acetyl peroxide, etc. 
     The amount of peroxide present in the reactor relative to the amount of phenothiazine component in the reactor will determine the rate of reaction. Usually it is preferred to add the peroxide to the reactor containing the charge of phenothiazine component at a controlled rate to maintain the reaction speed at the desired rate. The amount of peroxide in the reactor is usually maintained in the range of about 1 to 50 mole percent and preferably in the range of about 5 to 25 mole percent, based on the total number of moles of phenothiazine component present in the reactor. 
     The dimers can be prepared by heating the phenothiazine component and organic peroxide directly, but since the phenothiazine component and many organic peroxides are solid, it is usually preferable to carry out the reaction in the presence of a solvent or diluent. Typical diluents include the lower alkanes; petroleum distillate; kerosene; such as methyl ethyl ketone; aldehydes, such as benzaldehyde, etc. Ideally the solvent or diluent is a substance which will not interfere with the intended end use of the product so that there will be no need to recover the dimer from the solvent or diluent prior to the end use. When the reaction is carried out in the presence of a solvent or a diluent, the solvent or diluent is generally present in amounts of about 70-97%, based on the total weight of components in the reaction mixture. 
     In a typical procedure for preparing the dimers used in the invention the phenothiazine component and solvent or diluent are charged to a suitable reactor. The desired amount of organic peroxde is then charged to the reactor and the reactor contents are heated to the reaction temperature. If desired, the reaction may be carried out under a nitrogen blanket. As the peroxide is consumed additional peroxide is added to the reactor, either continuously or incrementally, at a rate to control the progress of the reaction. Since the reaction is exothermic it may be necessary to cool the reactor during the course of the reaction. It is usually complete in about 2 to 24 hours, depending, of course, on the reaction conditions. Excess peroxide may be added to the reactor to ensure that all of the phenothiazine component is reacted. Upon completion of the reaction, the product may be recovered from the solvent or used as is. 
     Phenothiazine and some hydrocarbon-substituted phenothiazines are available commercially. Others may be prepared by well-known techniques, such as alkylation. The preparation of the phenothiazine component forms no part of the present invention. 
     The N,N-diaromatic-substituted hydroxylamine compounds used in the invention have the structural formula 
     
         R&#39;R&#34;NOH 
    
     wherein R&#39; and R&#34; are the same or different aromatic hydrocarbon radicals having the structural formula ##STR2## wherein R&#34;&#39; is a straight- or branched-chain alkylene radical having 1 to 10 carbon atoms, R&#34;&#34; is hydrogen or a straight- or branched-chain alkyl group having 1 to 6 carbon atoms and n is 0 or 1. In preferred embodiments R&#39; and R&#34; are phenyl or aromatic radicals as described above wherein R&#34; has 1 to 6 carbon atoms and R&#34;&#34; is hydrogen. When n is 0, R&#34;&#39; does not exist and the phenylene group is attached to the nitrogen atom. Although N,N-diaromatic-substituted hydroxylamines having more than the above specified number of alkyl or alkylene carbon atoms in each aromatic substituent group may be useful in the invention it is preferred that compounds containing the above-stated upper limits be used in the invention because the latter compounds provide completely satisfactory results and they are easier to prepare. Mixtures of two or more of such diaromatic substituted hydroxylamines can also be advantageously used in the compositions of the invention. 
     Suitable diaromatic substituted hydroxylamines include N,N-diphenylhydroxylamine; N,N-dibenzylhydroxylamine; N-phenyl-N-(3-phenylpropyl)hydroxylamine; N,N-bis(2-methyl-8-phenyloctyl)hydroxylamine; N,N-bis[4-(1,1-dimethylethyl)phenylhydroxylamine; N,N-bis(3-hexylphenyl-4-butyl)-hydroxylamine; N-phenyl-N-(4-ethylphenyl-2-ethyl)hydroxylamine; etc. Prepared diaromatic-substituted hydroxylamines include N,N-bis(3-phenylpropyl)hydroxylamine, etc. As noted above, two or more of these compounds may be used in combination, if desired. 
     Some N,N-diaromatic-substituted hydroxylamines are available commercially. Those N,N-diaromatic hydroxylamines which are not commercially available may be prepared by any of the well known techniques. The preparation of these compounds forms no part of the present invention. 
     The relative concentrations of the N,N&#39; dimer of the phenothiazine component and N,N-diaromatic-substituted hydroxylamine used in the invention are generally in the range of about 10 to 90 weight percent of the N,N&#39; dimer of the phenothiazine component and about 90 to 10 weight percent N,N-diaromatic-substituted hydroxylamine, based on the total combined weight of these components. In preferred embodiments the concentrations generally fall in the range of about 25 to 75 weight percent of the N,N&#39; dimer of the phenothiazine component and 75 to 25 weight percent N,N-diaromatic-substituted hydroxylamine, based on the total combined weight of these components. 
     The polymerization inhibiting system of the invention is particularly well suited for protecting the reboiler sections of a distillation column during distillation of vinyl aromatic monomers because of the high boiling point of the inhibitor compounds in the system. The inhibitor system may be used at temperatures up to about 400° C. or higher at atmospheric pressure. Since the boiling point of various members of each of the two classes of compounds, i.e. the N,N&#39; dimers of the phenothiazine component and the N,N-diaromatic-substituted hydroxylamines are different, compounds which have the desired boiling point can be easily selected from each class. To make up for the inhibitor which is left behind during distillation, additional inhibitor can be added to the vinyl aromatic monomer after it is distilled from heavier hydrocarbons. In some cases it may be desirable to use lower boiling polymerization inhibitors in combination with the inhibitor system of the invention. For example, when distilling a vinyl aromatic monomer from higher boiling hydrocarbons it may be advantageous to add a polymerization inhibitor which has a boiling point near or lower than the boiling point of the vinyl aromatic compound. This will provide protection to the overhead portion of the column. It may also be desirable to add with the polymerization inhibiting system of the invention other agents, such as corrosion inhibitors, to provide additional protection to process equipment. 
     The inhibitor system of the invention can be introduced into the monomer to be protected by any conventional method. It is generally introduced just upstream of the point of desired application by any suitable means, such as by the use of a proportionating pump. It can be added to the monomer as a single composition containing all of the desired inhibitor compounds, or the individual components can be added separately or in any other desired combination. The composition may be added as a concentrate, if desired, but it is preferable to add it as a solution which is compatible with the monomer being treated. Suitable solvents include kerosene, naphtha, the lower alkanes such as hexane, aromatic solvents, such as toluene, alcohols, polyols or ketone, etc. It is often preferred to dissolve the inhibitors of the invention in the monomer to which the inhibitor is being added to avoid introducing additional impurities to the monomer. The concentration of inhibitor system in the solvent is desirably in the range of about 1 to 30 weight percent and preferably about 5 to 20 weight percent based on the total weight of inhibitor and solvent. 
     The polymerization inhibitor system is used at a concentration which is effective to provide the desired protection against spontaneous polymerization. It has been determined that amounts of inhibitor in the range of about 0.5 to 1000 ppm based on the weight of the monomer being treated affords ample protection against undesired polymerization. For most applications the inhibitor system is used in amounts in the range of about 5 to 500 ppm. 
     The components of the polymerization inhibiting system can be easily removed from the vinyl aromatic monomer prior to polymerization by caustic washing. Such procedures are well known and commonly practiced to separate phenolic type inhibitors, such as tertiary butylcatechol, from monomers. 
     The following examples will serve to further illustrate the invention. Unless otherwise stated, parts and percentages are on a weight basis. In the examples styrene, which is representative of vinyl aromatic monomers, was used as the test monomer. In the tests sodium ion, in the form of sodium hydroxide, and benzoyl peroxide were added to the test samples to provide a more intensive test of the ability of the inhibitor compositions of the invention to inhibit spontaneous polymerization. Sodium ions and benzoyl peroxide are both known addition polymerization catalysts for vinyl aromatic monomers. 
     EXAMPLE I (Control) 
     To distilled styrene was added sufficient sodium hydroxide (as a 50% aqueous solution) to produce a mixture containing 17 mg of sodium hydroxide per each 1000 grams of styrene monomer. This concentration of sodium hydroxide in the monomer is equivalent to a sodium ion concentration of 10 ppm. One hundred grams of the styrene monomer mixture was introduced into a 250 ml Erlenmeyer flask fitted with a ground glass stopper. Two hundred ppm, based on the weight of styrene, of benzoyl peroxide was added to the flask and the flask was then purged of air by bubbling nitrogen gas through the monomer. After the nitrogen purge the ground glass stopper was inserted into the flask and the flask was placed in an oven. The temperature of the oven was raised to and maintained at a temperature of 90°±2° C. for two hours. At the end of the two hour period a sample was drawn from the flask and tested to determine the amount of styrene polymer formed by the following procedure: a 10 ml sample of styrene monomer was introduced into 100 ml of cold methanol to quench the polymerization reaction; the methanol-monomer mixture was heated sufficiently to coagulate the polymer formed; and the polymer was recovered from the methanol by filtration, dried overnight at a temperature of 100° F. and weighed. The percentage of polymer formed was determined and reported in the Table in the Run 1 row. 
     EXAMPLE II (Comparative) 
     The procedure and test of Example I were repeated except that 500 ppm of dibenzylhydroxylamine was added to the Erlenmeyer flask just prior to the initial nitrogen purge. The styrene monomer was tested for polymer formation as indicated in Example I. The results are tabulated in the Table in the Run 2 row. 
     EXAMPLE III 
     To a 500 ml three neck flask equipped with a stirrer, a condenser and a thermometer was charged 200  grams of a 50:50 weight mixture of acetophenone and benzaldehyde and 199.28 grams of phenothiazine. The reaction mixture was stirred at 25° C. for 30 minutes and to this was added 199.3 grams of lauroyl peroxide at a rate of addition sufficiently slow to maintain the resultant exotherm below 35° C. The reaction was considered to be complete after two hours with the color of the initial reaction solution changing from a yellowish-green to a brilliant red. The reaction solvent was then removed from the product by evaporation. Mass spectral analysis showed the product to contain substantial amounts of 10,10&#39;-diphenothiazine. 
     EXAMPLE IV (Comparative) 
     The procedure and test of Example II were repeated except that 500 ppm of phenothiazine dimer prepared in Example III was substituted for the dibenzylhydroxylamine. The results are tabulated in the Table in the Run 3 row. 
     EXAMPLE V 
     The procedure and test of Example II were repeated except that 500 ppm of a mixture consisting of 20 weight percent of dibenzylhydroxylamine and 80 weight percent of the 10,10&#39;-diphenothiazine prepared in Example III was substituted for the dibenzylhydroxylamine. The results are tabulated in the Table in the Run 4 row. 
     
                       TABLE______________________________________                              Weight %                 Inhibitor Concen-                              PolymerRun   Inhibitor       tration, ppm Formed______________________________________1     none            --           *2     dibenzylhydroxylamine                 500          8.33     10,10&#39;-diphenothiazine                 500          0.284     dibenzylhydroxylamine/                 500           0.036 10,10&#39;-diphenothiazine______________________________________ *Too viscous to pipette from test vessel. 
    
     The benefit of the use of the polymerization inhibitor compositions of the invention are shown in the Table. In the Table the uninhibited monomer contained so much polymer after two hours that the sample could not be pipettetted from the reaction vessel; the Run 3 monomer sample, which was inhibited by 500 ppm of 10,10&#39;-diphenothiazine, the most effective comparative inhibitor system listed, contained 0.28 percent polymer at the end of the two hour period and the two hour analysis of the Run 4 sample, which contained one of the inhibitors of the invention, a 20:80 weight percent mixture of dibenzylhydroxylamine and 10,10&#39;-diphenothiazine, showed a polymer concentration of only 0.036 percent at the end of the two hour test period. 
     Although the invention is described with particular reference to specific examples, it is understood that the invention includes obvious variants. For example, the inhibitor system can be formulated to contain more than one dimeric derivative of phenothiazine. The scope of the invention is limited only by the breadth of the appended claims.