The conjugation of the double bonds of polyunsaturated fatty acids of the type that are accumulated or present in mixtures of natural fatty acids, for example, is of technological importance. Such mixtures of fatty acids are obtained by the cleaving of the more highly unsaturated fats, for example from soybean oil, sunflower oil, linseed oil, talloil and similar substances. These mixtures of fatty acids contain as compounds that can be conjugated mainly the double unsaturated linoleic acid and the triple unsaturated linolenic acid. The double bonds of these acids are separated from one another respectively by methylene groups.
The conjugation of the double bonds of these polyunsaturated fatty acids converts them into a form capable of chemically stronger reactions. It has been demonstrated that they offer a great advantage in the conjugated form in their main field of application, that is in drying oils. Conjugated double bonds can undergo the Diels-Alder reaction. Consequently, the field of application is expanded for the conjugated fatty acids.
The processes for the conjugation of fatty acids known so far and used in the technology are relatively complex, require expensive catalysts or have a detrimental effect on the environment. Iodine or nickel catalysis are suggested, for example. According to the German Published Application DE-OS No. 22 61 517, complex organic sulfur compounds are used. The U.S. Pat. No. 2,350,583 describes the conjugation with the use of the sodium salts of fatty acids. In this large-scale process the conjugated fatty acid must be released with mineral acid from its sodium salt in a second step, resulting in a full equivalent of an inorganic salt as by-product.
Occasionally the literature also contains references to the effect that gaseous SO.sub.2 can be used for the conjugation. However, SO.sub.2 alone yields demonstrably poor degrees of conjugation beside relatively large amounts of polymers of fatty acids. For example, a mixture of fatty acids of sunflower oil with a content of about 60% linoleic acid is converted into a product with only 10% conjuene fatty acids and 12% polymeric fatty acids by passing SO.sub.2 through for 4 hours at 225.degree. C. A "conjuene" fatty acid is polyunsaturated fatty acid having conjugated double bonds.
U.S. Pat. No. 3,257,377 describes the preparation of emulsifying agents from talloil for the polymerization of unsaturated compounds. For this purpose, talloil is treated with sulfur dioxide first in the temperature range from about 200.degree. C. to 320.degree. C. until the colophonic resin acids are disproportionated and the fatty acids are conjugated, and then it is treated in the presence of an alkaline compound at 250.degree. C. to 330.degree. C. for the further disproportionation and dimerization of the conjugated fatty acids. The alkaline compound also may be added in the first process step of the treatment with sulfur dioxide. The concomitant use of 2 to 5% by weight of the alkaline compound, for example alkali metal hydroxides, carbonates or sulfides is suggested. The SO.sub.2 passed through the talloil that is heated to high temperatures is in the gaseous state. The subsequent processing shows that only small amounts of distillable monomeric conjuene fatty acids are formed by this method, even when the dimerization step is omitted.