Process for preparing ester plasticizers

Organic compounds -- part of the class 532-570 series – Organic compounds – Fatty compounds having an acid moiety which contains the...

Reexamination Certificate

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C560S129000, C560S263000

Reexamination Certificate

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06423856

ABSTRACT:

The invention relates to a process for preparing ester plasticizers from ethylene glycol or the dimers, trimers or tetramers of this compound and linear or branched aliphatic monocarboxylic acids having from 3 to 20 carbon atoms in the presence of organic substances which have a boiling point of <112° C. and form azeotropes with water (hereinafter also referred to as azeotrope formers or entrainers) for removing the water of reaction.
Plasticizers are widely employed in many ways in plastics, coatings, sealants and rubber articles. They enter into a physical interaction with thermoplastic high polymers without reacting chemically, preferably by way of their solvent and swelling capabilities. This forms a homogeneous system whose thermoplastic range is shifted to lower temperatures compared to the original polymers, resulting in, inter alia, optimization of its mechanical properties, e.g. moldability, elasticity and strength are increased and the hardness is reduced.
For plasticizers to be suitable for a very wide range of applications, they have to meet a series of criteria. In the ideal case, they should be odorless, colorless and stable to light, cold and heat. In addition, it is expected that they should be insensitive to water, not be readily combustible and have a low volatility and not damage health. Furthermore, the preparation of the plasticizers should be simple and, in order to meet ecological requirements, should avoid the formation of waste materials such as by-products which cannot be utilized further and pollutant-containing wastewater.
Among the most important plasticizers are the esters of dicarboxylic and polycarboxylic acids with plasticizer alcohols, i.e. unbranched or branched primary alcohols having from about 6 to 20 carbon atoms, which are used as individual compounds or as mixtures. The esters are prepared, according to the classic method, by reacting the acids or acid anhydrides with an alcohol or a mixture of different alcohols in the presence of an acid, preferably sulfuric acid, as catalyst.
A specific class of ester plasticizers (referred to by the abbreviation G esters) comprises diols or ether diols, namely ethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol, as alcohol component. They can be prepared in various ways. In addition to the reaction of alcohol and acid in the presence or absence of acid catalysts, further processes are employed in practice for obtaining G esters, for example the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, with the esterification of diol and acid being most preferred. This is because this process can be carried out readily in conventional chemical apparatuses and gives chemically uniform products. On the other hand, ethoxylation requires comprehensive and costly technical measures. Ethylene oxide is a very aggressive chemical substance. It can polymerize in an explosive manner and forms explosive mixtures with air in a very wide range of mixing ratios. Ethylene oxide irritates the eyes and bronchial passages, leads to burns, to liver and kidney damage and is carcinogenic. Its handling therefore requires comprehensive safety measures. In addition, it is necessary for storage facilities and reaction apparatuses to be meticulously clean in order to avoid the formation of undesirable impurities due to secondary reactions of ethylene oxide with extraneous substances. Finally, the reaction with ethylene oxide is not very selective since it leads to mixtures of compounds having different chain lengths.
The direct esterification of alcohols with carboxylic acids is among the basic operations of organic chemistry. To increase the reaction rate, the reaction is usually carried out in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed during the reaction ensures that the equilibrium corresponding to the Law of Mass Action is shifted to the side of the reaction product, i.e. the ester, so that high yields are achieved.
Owing to the quality criteria for ester plasticizers described at the outset, the choice of catalyst and the procedure for removing the water of reaction are very important process features. This is because both these aspects of the process have a considerable influence on organoleptic and optical properties of the final products. The structure of the starting materials, viz. alcohol and acid, in turn has a critical effect on the mechanical and thermal properties of the plasticizers.
Although odor and color of the plasticizers can be matched to the desired requirements by addition of additives, the use of auxiliaries is to be avoided because they can impair other properties of the plasticizers and/or can limit their possible uses, e.g. because of incompatibility with the substrate.
Various methods of removing the water from the reaction formed from ethylene glycol (and its oligomers) and carboxylic acids in the esterification are known. Preference is given to azeotropic distillation in the presence of a water-immiscible solvent, heating the reaction mixture while passing an inert gas through it, and reacting the starting materials alcohol and carboxylic acid under reduced pressure or in the presence of a desiccant.
The removal of water by azeotropic distillation has proven to be particularly useful for adjusting the equilibrium in the preparation of ester plasticizers. However, the known methods and the entrainers used hitherto do not ensure that the high quality standards required for plasticizers are achieved.
It is therefore an object of the present invention to provide a process which makes it possible to prepare plasticizer esters based on ethylene glycol and oligomeric ethylene glycols in high purity and in high yields. In this context, it is particularly important that the process can be implemented using simple engineering means, that it ensures long operating times and that it gives consistently high-quality products over the entire operating time.
The invention provides a process for preparing ester plasticizers by reacting monoethylene, diethylene, triethylene or tetraethylene glycols with linear or branched aliphatic monocarboxylic acids having from 3 to 20 carbon atoms in the presence of an entrainer for removal of the water formed during the reaction as an azeotropic mixture. In the process of the present invention, organic substances having a boiling point of <112° C. are used as entrainers.
The new method is very reliable not only in laboratory and experimental operation, but also especially in industrial plants. It can be carried out easily, both batchwise and continuously, and gives plasticizers of high purity. A particularly notable aspect is the trouble-free and complete removal of the water of reaction and also of the entrainer used for removing the water. The complete removal of the reaction by-product and of the auxiliary results in the excellent color properties and the notable color stability of the ester plasticizers.
A critical feature of the process of the invention is the removal of the water of reaction from the reaction mixture and thus a displacement of the equilibrium in favor of the ester by means of organic substances whose boiling point is <112° C. The azeotrope formers are usually organic solvents which are available at a low price on an industrial scale. However, all other organic substances which have an appropriate boiling point and form azeotropes with water are also suitable. Examples of entrainers used according to the invention are hexane, 1-hexene, cyclohexane and toluene. Cyclohexane has been found to be a particularly advantageous azeotrope former. With water, cyclohexane forms a low-boiling binary system which can easily be distilled off from the mixture of reactants and product. The occu

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