Abstract:
The instant invention involves a process for preparing acrylic acid esters which comprises coreacting an acrylic acid and an aliphatic mono or polyol by maintaining one of the reactants in a reactor at elevated temperatures and adding the other reactant to the reactor over a period of time. By employing this process esterification times are reduced significantly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of copending application Ser. No. 06/891,991 filed on 8/1/86. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates to hemical reaction processes. More particularly, this invention relates to highly efficient processes for preparing acrylic acid esters. 
     2. Prior Art 
     The preparation of carboxylic acid esters has been carried out employing a wide variety of chemical reaction processes. For example, carboxylic acid esters have been prepared by reacting corresponding carboxylic acids, acid anhydrides or acid chlorides with various hydroxyl containing materials. 
     One important category of esterification reactions involves the formation of acrylic or methacrylic acid esters of mono or polyhydric alcohols. U.S. Pat. Nos. 4 053,504 and 4,059,721 are typical of the patents relating to processes for forming acrylate esters. Both of these patents involve processes wherein essentially &#34;cold&#34; reactants are mixed together in a reactor and heated to reaction temperature. As the reaction proceeds, the water of reaction is removed and the temperature continues to rise until the reaction is completed. 
     In general, carboxylic acid-based esterification reactions are relatively slow, often requiring as long as 12 to 24 hours to complete. 
     Accordingly, it is an object of this invention to describe an esterification process which reduces the time necessary to carry esterification reactions to completion. 
     It is another object of this invention to reduce the costs of conducting esterification reactions by reducing the amount of time necessary to carry out the reaction. 
     Another object of this invention is to prepare esters of polyhydroxy materials such as the acrylate or methacrylate esters of pentaerythritol, trimethylolethane, or trimethylolpropane. 
     These and other objectives are obtained by carrying out the process of the instant invention. 
     SUMMARY OF INVENTION 
     Disclosed herein is a process for the expeditious preparation of carboxylic acid esters which involves the co-reaction of a carboxylic acid and an aliphatic mono or polyol by maintaining one of the reactants in the reactor at elevated temperatures and by adding the other reactant to the reactor over a period of time. Using this process it is possible to significantly improve the speed of esterification. The process of the instant invention is particularly applicable in preparing polyacrylate esters of polyols which products are, in turn, useful in forming ultraviolet curable protective coatings. 
     DETAILED DESCRIPTION OF INVENTION 
     The process of the instant invention is generally applicable to the formation of essentially monomeric esters using as starting materials any aliphatic alcohols and acrylic type acids which do not contain groups which otherwise interfere with the esterification reaction. 
     The process of the instant invention may be used to form mono esters by reacting an aliphatic alcohol containing a single hydroxyl group with an acrylic acid. Examples of the hydroxy materials which may be esterified according to the instant invention include C 1  -C 18  aliphatic monohydroxy compounds. Examples of such materials include alcohols such as methanol, ethanol, isopropanol, butanol and the like. Also encompassed within the &#34;aliphatic alcohol&#34; definition as used herein are aliphatic alcohols wherein the hydroxyl group maintains its aliphatic character even though it is part of a molecule which is substituted with an aromatic group such as, for example, 1-hydroxy-2-phenylethane,1-hydroxy2-phenylpropane and the like. 
     Aliphatic polyols may also be employed in the instant invention. In general, the polyols of the instant invention contain from about 2 to about 10 hydroxyl groups per molecule and up to about 36 carbon atoms, preferably about 2 to about 5 hydroxyl groups and up to about 18 carbon atoms. The polyols should be liquid at a temperature below about 300° C. and should contain no other groups which interfere with the esterification reaction. Examples of such polyols include the glycols including ethylene glycol, propylene glycol, hexanediol and the like; the triols such as glycerine, trimethylolpropane, trimethylolethane; and the tetraols, such as pentaerythritol. Useful polyols may also be formed by etherifying the above-mentioned polyols. Examples of these materials include the polyglycols such as diethylene and triethylene glycols and the polypentaerythritols such as di and tri pentaerythritol. 
     As used herein the phrase &#34;acrylic acid&#34; means acids which are based upon acrylic acid or modifications thereof and includes acrylic acid, methacrylic acid and ethacrylic acid. 
     Most preferred among the aliphatic polyols are trimethylolpropane, trimethylolethane and pentaerythritol. Most preferred among the acrylic-based acids are acrylic and methacrylic acids. 
     In carrying out the esterification reaction specified above, an esterification catalyst is employed. The esterification catalyst should be added in the range of about 0.1-10 percent, preferably about 1-5 percent, based on the total weight of the reactants. Suitable esterification catalysts include paratoluene sulfonic acid (PTSA) and methane sulfonic acid. 
     The esterification catalyst may be removed from the reaction medium by means of a cation exchange resin. This resin may be added directly to the reaction mixture and then filtered off, or the finished product may be passed through a cation exchange resin of the tertiary amine type. In the alternative, the insoluble salt of the reaction catalyst is formed such as by adding ammonia to a PTSA catalyst system. The reaction mixture is then filtered to remove the salted catalyst. In some cases, the esterification catalyst can be retained in the product when its presence will not have an adverse impact on the properties of the final product. 
     About 0.01-3.0 percent by weight, based on the total reaction mixture weight of a polymerization inhibitor is added to reduce the formation of polymer during esterification. Examples of such materials include the quinones, such as hydroquinone and its monomethyl ether, the various phenols, p-tert-butylcatechol, p-methoxyphenol, 2,4-dichloro-6-nitrophenol, n-propyl gallate, di-tert-butyl-p-cresol, 2,2&#39;-methylenebis(4-methyl-6-tert-butylphenol), 1-amino-7-naphthol, p-benzoquinone, 2,6-dichloro-p-benzoquinone, 2-amino-1,4-napthoquinone, 3-aminoanthraquinone, diphenylamine, p-nitrosodimethylaniline, α and β-naphthylamine, nitrobenzene, phenothiazine, hexamethylphosphoramide, n-dodecyl mercaptan, benzenethiol, 2,2-diphenyl-1-picrylhydrazyl (phenyl hydrazine), divinylacetylene, and various antimony and copper salts. Most preferred among the inhibitors are paramethoxyphenol, phenothiazine and nitrobenzene. 
     In order to activate the polymerization inhibitor, it is necessary to employ an oxygen containing sparge, such as an air sparge, particularly during the early stages of the esterification reaction. 
     The instant invention achieves significant reductions in esterification processing times for virtually any esterification reaction which is carried out. Reductions can range from as little as 20% in reaction time to as high as 400 to 500% or more. There are two basic alternative means of carrying out the process of the instant invention. One involves the addition of the acrylic acid to the hydroxyl reactant which has previously been charged into the reactor and heated to below its boiling point. The other alternative process involves the addition of the hydroxyl reactant to the preheated acrylic acid in the reactor. 
     In carrying out the process of the instant invention, regardless of whether the acrylic acid or the hydroxyl material is initially charged into the reactor, it is essential that the contents of the reactor be heated to from about 100° C. to about the boiling point of the lowest boiling reactant. If the reaction temperature which is chosen exceeds the boiling point of the acrylic-type acid, the acid will reflux out of the reactor and may polymerize in the reflux condensor. If a temperature of less than about 100° C. is chosen, the reaction speed is substantially reduced. Preferably the temperature is from about 110° C. to about 10° below the boiling point of the lowest boiling reactant. According to the process of this invention the material which is added to the reactor may also be heated up to the boiling point of the lowest boiling reactant although this step is not required. 
     The material which was added to the reactor is added over a period of time which is at least equal to the time necessary to remove, at reaction temperature, at least about 65 percent, preferably about 75 percent of the water of reaction from the reactor. Normally the minimum period of time will range from about 45 minutes to about one hour and 20 minutes. The maximum amount of time over which the material is added to the reactor depends upon the desired speed of the reaction. As the speed at which the material is added to the reactor is slowed, overall processing time will also be slowed. In general, the material should be added over to the reactor the fastest possible time consistent with the above standards. 
     The reaction in the reactor is preferably carried out in the presence of a reflux solvent in order to facilitate the removal of water. The reflux solvent which is chosen should have a boiling point sufficient to form an azeotrope with water at above about 100° C. Examples of such solvents include xylene, toluene and other aromatic and aliphatic hydrocarbons having similar boiling points. 
     The reactor which is utilized to carry out the reaction is a conventional esterification reactor equipped with an addition reservoir to allow addition of the reactant over an extended period of time. The addition reservoir should be provided with a heating means to allow addition of the reactant at the desired temperature. The reactor should also be provided with a mechanical agitator, thermometer, sampling tube and water removal condensor. 
     In one alternative process of the instant invention, after the reactants have been added to the reactor, they are heated under reflux conditions for up to about two hours. By providing for a reflux holding period, it is possible to insure that the condensate from the reactor fully separates into two phases - one a water phase and the second a solvent phase with the solvent phase being returned to the reactor and the water being removed. If the reflux holding period is not employed, in some cases an undesirably large amount of acid may be removed from the reactor. When employing this alternative process, it is necessary for the reactor to be equipped with a means to switch from a reflux condensor in which the refluxing materials are condensed and returned to the reactor to a conventional water take-off condensor wherein water is removed from the reactor during processing. 
     In order to insure that rapid esterification occurs, the reactants are maintained at the highest possible temperature consistent with efficient water removal. The temperature in the reactor is increased as rapidly as possible to allow sufficient removal of the water and, when the esterification is completed as measured the acid value of the reactants being less than about 10, the reaction is deemed complete. 
     The product prepared according to the method of the instant invention may then be processed as any conventional ester by carrying out various purification steps including filtering, washing, straining, distillation and the like. These esters find particular utility as compositions for coatings and plastics, particularly for ultraviolet curable systems. 
     In the following examples all part and percentages are by weight unless otherwise indicated. 
    
    
     EXAMPLES 
     EXAMPLE 1 
     Into a reactor equipped with a mechanical agitator sampling device, thermometer, heating means and water take off condensor were added 1 mole of trimethylolpropane and 3.3 moles of acrylic acid. Also added to the reactor was 3.5 percent by weight of toluene, 0.5 percent by weight of methane sulfonic acid and 0.1 percent by weight of hydroquinone. An air sparge was started into the reactor. The contents of the reactor were heated over a 10 hour period to 135° C. A product resulted having an ester rank of 2.8. 
     EXAMPLE 2 
     Example 1 was repeated except that the trimethylolpropane was preheated to about 135° to 150° C. and added to the reactor over a 110 minute period to the acrylic acid which had been heated to 120° C. During the first 50 minutes of addition no distillate was recovered from the reactor but was returned to the reactor via a reflux condensor. At this point a Dean Stark trap was placed in the system and the water distillate was removed. The entire reaction period was about 5 hours. The resulting product exhibited an acid number of 0.11, a weight percent of free acrylic acid of 0.013 and a viscosity of 135 cps. 
     The FIGURE attached shows a comparison of the distillate collected versus processing time for Example 1 and Example 2. Two conclusions are readily apparent. The first is that the processing time, i.e., the time which is required to proceed to the same point in the reaction sequence is much quicker for Example 2 than it is for Example 1 after an initial induction phase. The second conclusion which can be drawn from the figure is that the reaction kinetics for the esterification in question have been significantly altered as between Example 1 and Example 2. 
     EXAMPLE 3 
     Into a reactor equipped as in Example 2 were added 780.1 parts of methacrylic acid, 35.0 parts of toluene, 5.0 parts of methane sulfonic acid and 1.0 parts of phenothiazine. An air sparge was begun. The contents of the reactor were heated to about 118° C. and the addition of 368.3 parts trimethylolpropane (TMP) maintained at 128° C. was begun. The TMP was added to the reactor over a 100 minute period and the reaction was held at reaction temperature for about 5 hours producing a finished polyacrylate ester. 
     EXAMPLE 4 
     Into a reactor equipped as in Example 2 were added 406.9 parts of trimethylolpropane, 35 parts of toluene, 0.5 parts of hydroquinone and 1.0 part of methane sulfonic acid. The contents of the reactor were heated to 122° C., an air sparge was begun and the addition of 721.5 parts of acrylic mixed with 4.0 parts of methane sulfonic acid and 0.5 parts of hydroquinone was carried out over a 4.5 hour period. The reaction was continued at 122° to 134° C. for two more hours to produce a completed reaction product.