Field of the Invention
The present invention relates to a process for producing an ester mixture, to the ester mixture produced in this way, to the use of the ester mixture as a plasticizer, and to compositions which comprise this ester mixture.
Discussion of the Background
The use of esters of terephthalic acid, phthalic acid, cyclohexanedicarboxylic acids, adipic acid, succinic acid, citric acid and other organic acids as plasticizers for polymers has been known for some considerable time. These esters are produced primarily by esterification of the acids or derivatives thereof with alcohols.
Depending on the acid used or on the acid derivative used and on the alcohol selected, there are significant variations in the properties of the resultant (unmixed) esters. The resulting esters are indeed suitable as plasticizers for a wide variety of applications but often, in addition to one or two very good properties, also have unfavourable properties, which may then severely restrict the use of the ester in question. For example, dioctyl phthalate, diisodecyl phthalate, diisononyl phthalate, trioctyl trimellitate and triisononyl trimellitate could be used usefully in the various high-temperature applications, since they exhibit high heat stability and low volatility. According to document US 2014/0096703 A1, however, their use in the area of these applications was subject to restrictions, since these esters had low compatibility with the polymers used for these applications. The dibutyl derivatives of these esters exhibited high compatibility with polymers, for example PVC, but were not suitable for high-temperature applications, on account of their low heat stability. In order to be able nevertheless to provide plasticizer systems which exhibit high heat stability in conjunction with high compatibility, document US 2014/0096703 A1 proposes the use of an ester mixture composed of C8 and C10 esters of terephthalic acid comprising not only the unmixed esters but also the mixed ester which has a C8 and a C10-alcohol radical.
Document KR 2013/0035493 A sets out the problem that dibutyl terephthalate, while having a high rate on penetration into the resin and on melting, nevertheless exhibited an undesirably high migration loss, whereas for diethylhexyl terephthalate there was virtually no migration loss, but penetration into the resin, and melting, took an unacceptably long time. For improvement, that document also proposes using ester mixtures comprising the mixed ester containing a C4 and a C8 alcohol radical.
Document WO 2008/140177 A1 proposes the preparation of ester mixtures composed of C8 and C9 esters of terephthalic acid, which again also include the mixed ester, and describes how an improvement was possible in the processing qualities of the plasticizer preparations through the variation of the ratios of the individual esters in the ester mixture.
Also known are ester mixtures comprising mixed esters prepared from cyclohexanedicarboxylic acids with different alcohols (WO 2011/115757 A1).
Also known are mixtures of citric esters including not only the corresponding unmixed esters but also mixed esters with C5 and C9 alcohol radicals (U.S. Pat. No. 8,431,638 B2).
These ester mixtures are provided in the related art—irrespective of their parent structure—by the esterification of the corresponding acids or acid derivatives with an alcohol mixture which provides the alcohol radicals of the unmixed esters and of the mixed ester or mixed esters.
As is, for example, shown in document WO 2008/140177 A1, however, it is not possible to set the distribution of the resultant esters in a targeted way by means of the composition of the alcohol mixture. Accordingly, the ester mixtures produced in the examples of that document do not contain the C8 and C9 alcohol radicals in the proportion in which these radicals are present in the alcohol mixture used and probably also the proportion in which it was intended that they should be introduced into the system. The C8/C8 esters, the C8/C9 esters and the C9/C9 esters as well are not formed in the molar ratios expected on the basis of statistical considerations, but instead in ratios which deviate markedly therefrom. In Example 1 of document WO 2008/140177 A1, with a statistical expectation value of 25:50:25 (assumption: equimolar incorporation of the alcohol radicals), the molar ratio obtained for the esters is 10:54:36, which corresponds to an uncontrollable deviation by (|25−10|+|50−54|+|25−36|=) 30 points (the calculation of the deviation in points will be elucidated later on in the text). While in this example the two alcohols are indeed used in a molar ratio of 1:1, the resulting ester mixture contains the two alcohol radicals in a molar ratio of 37:63. The document ascribes these deviations from the statistical expectation value to the differing reaction rates of the individual esterification reactions.
This problem of the “uncontrollability” of the ester mixture compositions in the case of production by esterification of terephthalic acid is resolved in document KR 2013/0035493 A by separately preparing the esters of the mixture and blending them in the desired ratio. The document, however, includes no disclosure as to how it is possible to produce a mixed ester separately without at the same time forming the two unmixed diesters. Nor does this appear possible at commercially acceptable cost and effort. The provision of the pure mixed ester would have to be accomplished by removal thereof from the unmixed esters during a separation step. If this purified mixed ester is then to be blended with the two unmixed esters, in accordance with the disclosure content of document KR 2013/0035493 A, then the production of an ester mixture requires the performance, for example, of at least three esterification reactions and one subsequent blending—a procedure inconvenient and costly in terms of both apparatus and time.
A different pathway is described by document U.S. Pat. No. 8,431,638 B2 for the production of mixed citric esters. Mixtures comprising these esters can be produced by subjecting citric acid pentyl esters to the transesterification with citric acid nonyl esters. This procedure nevertheless requires three separate process steps: two esterifications and one transesterification.