BACKGROUND OF THE INVENTION

This invention relates to the stabilization of the esters of unsaturated carboxylic acids with blends of particular N,N'-substituted paraphenylenediamines (PPDA) . More particularly it concerns a method for stabilizing methyl methacrylate or the lower alkyl esters of acrylic acid during the process of manufacturing such esters through the introduction of a specific blend of N,N'-substituted paraphenylenediamines characterized by solubility in the aqueous phase of the process containing water and the ester.

The alkyl esters of the methacrylic and acrylic acid have wide application as raw materials for fibers and plastics. Since these compounds have reactive unsaturated bonds in their molecules they're prone to polymerize under the correct conditions during manufacturing or storage. This polymerization may occur as a result of the presence of heat, light, oxygen and other conditions. As a result it is most desirable to minimize or eliminate the tendency of methacrylic acid or their related esters, such as ethylmethacrylate to polymerize during manufacturing. The elimination of the

tendency to polymerize assures that the vessels and pipes used to transport the material during production remain clean and the reactors free of build up of high viscosity, high molecular weight, polymerized material. It has become customary during the manufacturing of methylmethacrylate(MMA) and other esters of the unsaturated carboxylic acids to add polymerization inhibitors such as hydroquinone and certain derivatives of paraphenylenediamine or phenothiazine. It is known to use certain N,N' -substituted paraphenylenediamines including the particular materials disclosed in Japanese Publication 49-43920 in which N-secondary hexyl N' henyl-p-phenylenediamine and N, ' -di(secondary heptyl)-p-phenylenediamine is used to stabilize unsaturated carboxylic acids or their esters, including ethyl acrylate, 2-ethylhexyl acrylate, methyl acrylate, acrylic acid, methacrylic acid in the organic phase. Methylmethacrylate was not mentioned or tested and there is no appreciation of the significance of stabilizing the aqueous phase. This publication teaches only that certain paraphenylenediamine materials show good activity as polymerization inhibitors in pure liquid (organic) phase of the acid or its ester. In spite of such efficacy demonstrated by laboratory scale testing of the inhibition characteristics in pure organic phase monomer, the large world-scale commercial production units still were plagued with the troublesome formation of polymerized

ester or acid in the various vessels, decanters, distillation columns, pipes, heat exchangers and reboilers of the process. This indicated that the polymerization inhibition was still inadequate. This invention was made when it was realized that the esters(and their precursors) must be protected in both the organic and the aqueous phases. This recognition of the elusive key element of the problem ushered in a diligent search for an inhibitor system -jhich could protect both phases. The second problem recognition was that the aqueous phase protection by various paraphenylenediamine compounds is very dependent upon pH of the aqueous phase. Neither phenomenon had not been previously recognized or addressed. An object of this invention is to provide a stabilization package which is effective in both organic and aqueous phases. A further object is to offer protection even at low pH levels, characteristic of production upset conditions when various inorganic and organic acids lower the acidity of the aqueous phase to pH of 4, 3 or even 2. This invention provides protection to both phases thereby minimizing polymer buildup in the production equipment, thus eliminating many heretofore routine shutdowns of various units in the process for cleanout of the polymerized residue and improving plant productivity. BRIEF DESCRIPTION OF THE INVENTION

One aspect of this invention relates to a method of

-4- inhibiting polymerization of an ester of an unsaturated carboxylic acid in a process stream comprising water and said ester and having an aqueous phase with a pH range from about 1 to about 5 and containing a minority amount of said ester and an organic phase composed of a majority of said ester comprising: adding to said process stream, a stabilizer blend comprising (a) an N,N alkyl- or phenyl-substituted paraphenylenediamine having a solubility greater than 50% in said aqueous phase at a pH of 2; and (b) a second l ^~ , l~ alkyl- or phenyl-substituted paraphenylenediamine having a solubility less than 20% in said aqueous phase at a pH of 2.

Another aspect of the invention relates to a process of manufacture of an ester of an unsaturated carboxylic acid having a plurality of process streams comprising water and said ester, at least one of said streams having an aqueous phase containing water and said ester and at least one stream having said ester in an organic phase essentially free of water, an improved method of inhibiting polymerization of said ester comprising: adding to said plurality of process streams, a stabilizer blend comprising (a) a first N,N substituted paraphenylenediamine of structure (I); and (b) a second N,N substituted paraphenylenediamine of structure (II) .

R- and R ₂ are independently selected from phenyl or C ₅ -Cg alkyl and R ₃ is C^-C^ alkyl, ₄ is phenyl or C _j _-Cq alkyl.

Referring now to structures I and II, it is preferred that R- _j _ is phenyl and R ₂ is C ₅ to C _g alkyl ₃ is C-^ to C ₄ alkyl and R4 is phenyl. In a more preferred combination, R3 and R ₄ are C3 to C ₄ alkyl and R _j is phenyl and R ₂ is Cg to C ₇ alkyl. Throughout this specification the term "alkyl" encompasses linear, branched and cyclic alkyls.

Yet another aspect relates to a stabilizer blend consisting essentially of(a) a first N,N substituted paraphenylenediamine of structure (I); (b) a second N,N substituted paraphenylenediamine of structure (II) and; (c) an alcohol soluble in (a) and (b) . DETAILED DESCRIPTION OF THE INVENTION

In the following discussion, MMA is singled out only for exemplary purpose and it is to be understood that any of the many esters of the unsaturated dicarboxylic acids may be substituted for the MMA, with appropriate modification by the skilled artisan in this

field. The esters of acrylic acid and methacrylic acid are the preferred materials to be stabilized by this method. The alkyl esters, exemplified by methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl are the preferred ones. Most preferred are the methyl, ethyl and propyl, lower alkyl esters of acrylic and methacrylic acid. The most preferred ester, methylmethacrylate (hereinafter sometimes abbreviated as MMA) , is discussed below or the purpose of example only and should not be considered to be limitative of the scope of the claimed invention.

A commercial process for the manufacture of methyl methacrylate is based on basic raw materials acetone, hydrogen cyanide, methanol, and sulfuric acid. Acetone is reacted with hydrogen cyanide to form acetone cyanohydrin.

The chemistry of the process involves the reaction of acetone cyanohydrin with excess concentrated sulfuric acid to form methacrylamide sulfate. The methacrylamide sulfate stream reacts with aqueous methanol to form methyl methacrylate by a combination hydrolysis- esterification reaction.

Modifications of the esterification step are variations in the procedures for the recovery of the crude ester and for the separation of methanol and methacrylic acid for recycling. In addition, process conditions vary in terms of the feed ratios of methanol

and water to methacrylamide and the reactor temperatures and residence times.

In one version, the methacrylamide sulfate stream, excess aqueous methanol, and recycled streams react continuously in a series of steam-jacketed esterification reactors at 80-100°C with a 2- to 4-h residence time (61). The reactor effluent is distilled in an acid stripping column to give crude methyl methacrylate, methanol, and water. The water-washed crude ester is purified in a multicolumn distillation system, and the aqueous methanol is distilled to recover methanol for recycling. Based on ACN, the yield of methyl methacrylate is ca 90% and, based on methanol and depending on the process scheme and reaction conditions, it is 80-90%.

In another version of the esterification step, the reaction is performed under pressures of up to 790 kPA(100 psig) at 100-150°C with residence times of <.lhour depending on the reaction temperature. The product may be recovered as described above or the reactor effluent may be separated into organic and waste-acid phases.

Light ends (low-boiling fractions) are removed from the organic layer in a flash column. The crude ester then is washed with water or with aqueous ammonia to remove methanol and some methacrylic acid; the aqueous raffinate is recycled to the esterification reactor. The washed crude ester is purified in as three-column

distillation system. In the first-stage column, water and methanol are taken overhead and are recycled to the esterification reactor. The finished product is taken overhead in the product column. The bottoms from the product column are stripped to recover compounds for recycling and the residue is incinerated.

TABLE 1 WITH METHYL METACRYLATE

TABLE 2 RECIPROCAL SOLUBILITIES OF METHYL METHACRYLATE AND WATER

This invention resulted from the recognition that the MMA(or any other ester) must be protected in the aqueous phase, due to the high solubility of MMA in water and the tendency of MMA to form azeotropic mixtures with water. Tables 1 and 2 detail these solubilities. The next required step in carrying out this invention is the necessity of evaluating whether a

given stabilizer/inhibitor candidate is soluble in the aqueous phase at the required pH levels. The following testing regime is used to make that determination. AQUEOUS SOLUBILITY TEST METHOD EXTRACTION TESTING

General Description

PURPOSE: To determine the partitioning of polymerization inhibitors between MMA and an aqueous compositions having particular pH

METHOD:

1. Standard samples of pure MMA containing approximately 1000 ppm of each inhibitor were prepared.

2. Aqueous solutions were prepared according to following recipes: water 96.5% methanol 2.0%

MMA 1.5%

Aqueous solutions aliquots were adjusted to desired pH level sulfuric acid.

4. 100 grams of each MMA solution was extracted with 100 grams of an aqueous solution of the desired pH. a) The solutions were placed in 16 oz. jars on a mechanical shaker. The solutions were shaken for 16 hours at slow speed. b) The layers were allowed to separate. When emulsions occurred, they were broken by careful heating with a heat gun.

-10- 5. Each layer was submitted to a chemical characterization laboratory for analysis to determine the level of inhibitor in each phase according to the ANALYTICAL TEST FOR PPDA LEVEL. ANALYTICAL TEST FOR PPDA LEVEL

The method used is based on the oxidation/coupling of quinone with the paraphenylenediamine to produce highly colored solutions. Levels are determined colorimetrically using a visible range spectrophotometer equipped with either a 1 cm or 0.5 cm cell. Standard solutions of paraphenylenediamine and p-benzoquinone are prepared by accurately weighing about 0.1 of each reagent in 100 ml of acetic acid.

Calibration is accomplished by: (1) preparing a reagent blank by pipetting 10ml of MMA into a 50 ml volumetric flask, adding 25 ml of acetic acid followed by 1 ml of the p-benzoquinone solution, then distilled water is added to fill to 50 ml flask; (2) 1,2 and 3 is of the appropriate paraphenylenediamine standard solution is added to marked 50 ml flasks, followed by 10 ml of MMA(as used in the reagent blank) and sufficient acetic acid to make 25 ml total acetic acid, including the pipetted volume, then rapidly add distilled water to 50 ml total volume, mix and obtain the spectrum of the solutions from 5 to 15 minutes after addition of the water.

Analysis of test samples is accomplished by following the calibration directions, except for the

substitution of 10 ml of the MMA solution to be analyzed. A calibration curve is used to obtain micrograms of paraphenylenediamine of each sample. Conversion to parts per million of paraphenylenediamine is done by dividing the microgram amount just determined by weight of MMA per 10 ml (9.34g/10 ml).

Methylmethacrylate is described above as representative example only, other esters within the invention may be evaluated in a fully analagous way. When an ester other than MMA is being protected, the appropriate changes in the foregoing method should be made to account for the differing weight.

TABLE OF INHIBITORS Compound Composition 1 Blend of N-(l,4 dimethylpentyl)- '-phenyl paraphenylenediamine and N-(l-methylρentyl)-N'-phenyl paraphenylenediamine

2 N-(l,4 dimethyl pentyl)-N'-phenyl paraphenylenediamine

3 N,N'-di(isopropyl) paraphenylenediamine

4 N-(isopropyl)-N*-phenyl paraphenylenediamine

5 N,N'di(l,4 dimethylpentyl) paraphenylenediamine Compounds shown in Table 3 are of the general structure R-NH-(C ₆ H ₄ )-NH-R' .

TABLE 3

AQUEOUS EXTRACTION DATA pH=2 CMPD _R R' % Aqueous MMA 1 phenyl ^ ^■ β^- 13 -' 2.3 97.7 C ₇ H ₁₄

phenyl C 7-TH ^π 15 2.3 97.7

C ₃ H ₇ C ₃ H ₇ 97.2

phenyl C ₃ H ₇ *90 10.0

*Aqueous phase contained no methanol.

It can be easily seen from the data in Table 3 that certain of the paraphenylenediamine compounds partition readily into the aqueous phase at low pH levels. For the purposes of this invention, in order to be considered an aqueous phase inhibitor candidate, the compound must partition at least 10%, preferably 30% and most preferably 50% of the inhibitor into aqueous phase by the test method at a pH of 2. It can be seen that the compounds 3 and 4 are such materials. N,N' substituted paraphenylenediamine having at least one lower alkyl (C*-_-C ₄ ) substituent are compounds which are preferred materials as the aqueous phase component of the inhibitor system of this invention. The isopropyl substituted paraphenylenediamine compounds 3

and 4 offer exceptional enhanced protection to the MMA monomer present in the aqueous phase, particularly under very acidic conditions. Such conditions may be normal process conditions or they may be transient conditions during production upset periods. Without the enhanced protection offered by this invention, substantial unwanted polymerization is likely to occur in highly acidic process conditions causing fouling of equipment, and reduced heat exchange efficiency as well as down-time for cleanout.

Compounds of Structure (I) do not meet the aqueous solubility test, while the Compounds of Structure (II) do meet this test and are considered soluble in the aqueous phase. If a candidate material meets the aqueous solubility criteria, it must also demonstrate suitable polymerization inhibition properties. It should be recognized that these materials need not demonstrate the best or optimal activity since the aqueous phase inhibitor is accompanied by an organic phase inhibitor, which will be a very active inhibitor. Also it must be borne in mind that there is relatively little ester actually in the aqueous phase compared to the organic phase in a typical production process stream, so the magnitude of protection required is smaller than in the organic phase (essentially all MMA or other ester) . The onset of polymerization test procedure below is

used to evaluate candidate materials that have passed the aqueous solubility test method evaluation. ONSET OF POLYMERIZATION TEST PROCEDURE

1. Any shelf inhibitor is removed from MMA by distillation.

2. 0.1% stock solutions of test inhibitor in MMA are prepared.

3. Solutions containing 10 ppm test inhibitor are prepared from the stock solutions and put in test tubes.

4. The test tubes, equipped with internal oil tube and thermocouple, are placed in a constant temperature oil bath at 80°C.

5. The first sign of an exotherm, read from a chart recorder, is the "Onset of

Polymerization" expressed in hours.

TABLE 4

ONSET OF POLYMERIZATION DATA

Methyl Methacrylate

Conditions: 80°

The data in Table 4 shows the excellent long-term polymerization inhibitory effect of the paraphenylenediamine compounds of Structure I and II. The Controls A,B,and C provide no more than 20 hours of protection, while the alkyl substituted paraphenylenediamines showed from 60 to 140 hours of protection.

This invention utilizes a stabilizer blend of (a) an aqueous phase inhibitor which at the present time may be represented non-exhaustively by the compounds of Structure I above and (b) an organic phase inhibitor which a* the present time may be non-exhaustively represented by the compounds of Structure II. The stabilizer blend may be present at any polymerization

inhibitory effective level, preferably 50 to 10,000 parts per million(pρm) based on the weight of the ester being stabilized. More preferably 100 to 2000 ppm and most preferably 200 to 1000 ppm may be used for in-process stabilization of the ester and its precursors during manufacture of the ester. The ratio of the components (a) and (b) may broadly range from 5/95 to 95/5, more preferably 30/70 to 70/30 and most preferably 40/60 to 60/40. Referring now to structures I and II, it is preferred that R-^ is phenyl and R ₂ is C ₅ to Cg alkyl R ₃ is C* _j _ to C ₄ alkyl and R is phenyl. In a more preferred combination, R ₃ and R ₄ are C ₃ to C ₄ alkyl and R*-_ is phenyl and R ₂ is C - to C ₇ alkyl. Throughout this specification and claims the term "alkyl" encompasses linear, branched and cyclic alkyls.

The invention is believed to be effective using material outside of the structural limits of (I) and (II) so long as one of the paraphenylenediamines is soluble in the aqueous phase. Since an exhaustive evaluation of this whole class of compounds has not been undertaken, the following alternative formulation of the invention utilizes the Solubility Test Method set forth above to screen compounds for utility in this invention. The stabilizer blend may be (a) an N,N alkyl- or phenyl-substituted paraphenylenediamine having a solubility greater than 50% in said aqueous phase at a pH of 2; and (b) a second N,N alkyl- or phenyl-

substituted paraphenylenediamine having a solubility less than 20% in said aqueous phase at a pH of 2.

INDUSTRIAL APPLICABILITY Small scale laboratory tests do not adequately duplicate the variability of conditions in various vessels and streams of a world scale chemical plant making esters of this invention, most notably methylmethacrylate. Therefore it is most indicative of truly improved performance to run an experimental inhibitor package in a very long duration trial. During such trials it can be determined whether the inhibitor can , in fact, eliminate or at least minimize the buildup of polymerized MMA in various production vessels, piping,heat exchangers, etc. Such a trial was run using a 50/50 blend of compounds of structure I and structure II. In a production column which normally required a major shutdown and cleanout every 2-3 months, it ran without need for cleanout for 9 months-demonstrating dramatically improved polymerization inhibition! Other sections showed markedly improved intervals between cleanouts. Where both aqueous and organic liquid phases are present, this invention showed unexpectedly improved results compared to a single N,N-substituted paraphenylenediamine. Another aspect of this invention is a new and useful form of stabilizer package which was heretofore unknown. It utilizes an alcohol compatible with the

ester manufacturing process with the inhibitor compounds to yield a fully soluble, liquid form even if some of the compounds of Structure I are solid materials.

Specifically there is disclosed: a stabilizer blend consisting essentially of:

(a) a first N,N substituted paraphenylenediamine of structure (I) ;

(b) a second N,N substituted paraphenylenediamine of structure (II) and;

(c) an alcohol soluble in (a) and (b) .

The alcohol may be present in any proportion up to 75% by weight of (a) plus (b) . For methylmethacrylate use, methanol is preferred. The ratio of (a) to (b) is within the ranges previously set forth for the stabilizer blend(ie 5/95 to 95/5).

In view of the many changes and modifications that may be made without departing from principles underlying the invention, reference should be made to the appended claims for an understanding of the scope of the protection afforded the invention.