Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-08-24
2002-08-20
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S224000
Reexamination Certificate
active
06437172
ABSTRACT:
The present invention relates to an improved process for the preparation of (meth)acrylates by esterifying a high-boiling compound having one or more alcoholic hydroxyl groups with (meth)acrylic acid in the presence of an acidic esterification catalyst and of an entraining agent with discharge of the water of reaction and temperature-controlled recycling of the entraining agent into the reaction mixture.
The esterification of high-boiling compounds having aliphatic hydroxyl groups with (meth)acrylic acid has become very important industrially owing to the wide use of esterification products for radiation-curable finishes and coatings. For this purpose, hydroxy compound, (meth)acrylic acid, catalyst, stabilizer and entraining agent are initially taken in the batch process. After heating to the boil, the water of reaction formed during the esterification reaction is distilled off as an azeotropic mixture comprising entraining agent and water. The azeotropic mixture distilled off is condensed and is separated into water and entraining agent in a downstream phase separator. The entraining agent separated off is then recycled directly to the reaction vessel to remove further water of reaction. After the end of the esterification reaction and the end of removal of liberated water of reaction, all the entraining agent is removed from the reaction mixture by distilling off (cf. for example DE-A-3316593, DE-A-3704098, DE-A-3836370, EP-A-646567). In the conventional esterification process, the boiling point corresponds to the reaction temperature and, as a result of the method of water removal, the content of entraining agent in the reaction mixture is constant.
It has long been desired to reduce the long duration of the esterification equilibrium reaction. An increase in the reaction rate can be achieved, for example, by increasing the reaction temperature, which however increases the danger of polymerization of the unsaturated monomers during the esterification reaction. It must also be borne in mind that most of the conventional stabilizer systems readily lead to discoloration of the product at high esterification temperatures. This limits the choice of high esterification temperatures because resulting discolorations can be eliminated only by tedious measures such as distillation, washing out, etc. For example, in the case of the otherwise advantageous inhibitor system comprising copper(II) chloride and hydroquinone monomethyl ether, an esterification temperature of about 105° C. should therefore not be exceeded in order to obtain high-quality colorless products.
The temperature of the reaction mixture is a function of the boiling point of the entraining agent used. High-boiling entraining agents, such as toluene, have the advantage that they have a good entraining effect for water and lead to relatively high boiling points of the mixture, with the result that the esterification range can be increased. The disadvantage is that the high-boiling entraining agents are difficult to separate from some end products. In addition, they must be used in high concentrations since otherwise the resulting boiling point of the mixture will be too high and there is a danger of polymerization. Low-boiling entraining agents, e.g. cyclohexane, have the disadvantage of too low an entraining effect and a reduction in the reaction temperature and hence in the reaction rates. However, they have the advantage that they can be used in lower concentrations and can be more easily removed after the end of the reaction.
However, the boiling point of the reaction mixture is also a function of the composition of the mixture. Since the composition of the mixture changes during the reaction as a result of the reaction of hydroxy compound and acid to form the ester, the reaction temperature also increases during the esterification. With a constant amount of an entraining agent it is necessary to carry out the esterification reaction initially at low temperatures and at a low reaction rate in order to avoid reaching, at the end of the reaction, a temperature at which there is a danger of polymerization. Consequently, esterification times which are undesirably long for economic reasons result.
In principle, it is possible to regulate the boiling point by controlling the pressure, but this requires working in special expensive pressure-resistant apparatuses.
It is an object of the present invention to provide a process for the preparation of esters of (meth)acrylic acid with high-boiling alcohol compounds, in which a good constant product quality is achieved in combination with a very uniform optimum reaction temperature. It is a further object of the invention to achieve an increase in the reaction rate and hence a reduction in the reaction time in the esterification reaction.
We have found that this object is achieved by a process for the preparation of (meth)acrylates by esterifying a high-boiling compound having one or more alcoholic hydroxyl groups with (meth)acrylic acid in the presence of an acidic esterification catalyst, of an entraining agent for the water of reaction and of a polymerization inhibitor while heating the reaction mixture to the boil and discharging the water of reaction with the aid of the entraining agent and feeding entraining agent to the reaction mixture during the esterification reaction, wherein the entraining agent is metered batchwise into the reaction mixture under temperature control during the predominant duration of the esterification reaction so that the boiling point of the reaction mixture is kept essentially constant, essentially within a range of ±2° C.
In the novel process, the usual inevitable recycling of entraining agent from the phase separator in the removal procedure is therefore omitted. Instead, the entraining agent is metered into the reaction mixture under temperature control or under control by the respective temperature of the reaction mixture in such a way that, through the boiling point of the reaction mixture, a constant internal temperature of the reaction mixture results during the predominant duration (in particular during more than 70%, preferably more than 80% or 90% of the total duration) and preferably during the essentially total duration of the esterification reaction. Preferably, the variations in the boiling points, apart from initial settling variations, should be no greater than ±2° C. In this way, the procedure can be carried out at a constant optimum reaction temperature during the total esterification reaction, which, as stated, leads to constantly good product quality and to a reduction in the reaction times of the esterification.
Preferably, the boiling point of the reaction mixture, including entraining agent, is from 70 to 110° C., particularly preferably from 75 to 105° C., low-boiling entraining agents being used. The boiling point of the reaction mixture is preferably more than 5° C., in particular more than 10° C., higher than the boiling point of the pure entraining agent (at the same pressure).
A further advantage of the novel process is that the reaction kettle for the esterification reaction can be filled to a higher level with the reaction mixture from the outset, since less volume is required for the entraining agent at the beginning of the esterification reaction. If the volume of the reaction mixture decreases owing to discharged water of reaction, further entraining agent is metered in.
Particularly suitable entraining agents are aliphatic and cycloaliphatic saturated hydrocarbons which are capable of forming an azeotropic mixture with water and have a boiling point of from 40 to 120° C., in particular from 70 to 110° C. Suitable low-boiling entraining agents are, for example, n-hexane, n-heptane, cyclohexane and methylcyclohexane, special gasolines and commercial aliphatic hydrocarbon mixtures having boiling points in the stated temperature ranges. A preferred entraining agent is cyclohexane. In general, the entraining agents are used in an amount of from 1 to 80, preferably from 1 to 35, % by weight, based on th
Beck Erich
Geisendörfer Matthias
Leidinger Kurt
Leube Hartmann F.
BASF - Aktiengesellschaft
Killos Paul J.
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