Tailor-Made Fats for Industry

R. O. Feuge.

In the manufacture of erasers, an elastic, rubberlike material with just the right degree of crumbliness is needed. Technicians have found that the light-colored plastic formed when castor or rapeseed oil is treated with sulfur compounds meets the requirements perfectly.

Makers of high-grade linoleum also need a plastic ingredient, but their exacting requirements can be met only by the solid obtained when linseed oil or another suitable drying oil is allowed to react slowly with air while warm.

The textile industry uses a number of specially prepared oils. One such oil, used in finishing fabrics, must possess the usual properties of an oil, so that it makes the treated fabric attractive and soft but at the same time it must be soluble in water, so that it can be applied, as well as removed, with water.

Thus, many fatty materials intended for industry are modified to give them special properties or the characteristics that better adapt them to the process or final product.

The new characteristics given them by chemists may be entirely different from their original, natural characteristics. Sometimes they are changed to resemble another natural product for example, cottonseed oil is made into various plastic products, which could pass for tallow or other hard fats. Many special fats are prepared for industrial uses uses so diversified and numerous that nobody has been able to list them all. Often the fats lose their identity during processing and conversion to finished products. In oil-modified plastics or finishes, for example, the fat disappears as such and is unrecognizable in the final product.

Let us consider here the types of tailor-made fats and oils that are manufactured in quantity not necessarily just the compounds chemically identical with some of the constituents of a natural fat, but those that are derived from natural fats or oils and fall within the broad definition of an oil. We include fatty acids, fatty alcohols, and their monoesters, but not alkali and heavy-metal soaps.

The special fats most important from the standpoint of volume are the ones produced by catalytic hydrogenation of the carbon-to-carbon double bonds in the molecules of the fatty acid portion of the oil. This consists simply of the addition of hydrogen at the double bonds, or, as the chemist says, at the points of unsaturation. The reaction is called catalytic because it is enormously accelerated in the presence of certain metallic surfaces (usually nickel), which remain unchanged at the end of the reaction.

Here we have illustrated the hydrogenation reaction of a very simple molecule, that of ethylene, a well-known hydrocarbon. The ethylene molecule consists of two carbon atoms joined by a double bond and otherwise surrounded by hydrogen atoms, each of which is connected to a carbon atom by a single bond. The double bond enables the molecule to take up two more hydrogen atoms, so that the two carbon atoms will also be joined only by a single bond.

A natural fat molecule is made up of three long hydrocarbon chains attached to a molecule of glycerol to form a triglyceride. Any one or all of the long chains may contain one or more carbon-to-carbon double bonds, or so-called ethylenic linkages.

When one or more of these carbon-to-carbon double bonds is present in each of the three or even in two of the three long chains in the fat molecule, the product will be a liquid at room temperature; that is, it will have a low melting point. As the number of the carbon-to-carbon double bonds is reduced by the addition of hydrogen, the melting point will increase, and eventually, when enough of the double bonds disappear, the liquid oil will become a solid fat at room temperature.

The formula for a fat molecule--triglyceride is shown in the accompanying diagram.

This particular fat molecule contains three different fatty acid residues, each with a different number of carbon-to-carbon double bonds. The formula shows how the various atoms are combined to form a molecule of a simple fat, or triglyceride, which in this case is called oleo-linoleo-linolenin, because it contains one residue each of oleic, linoleic, and linolenic acid. It is one of many triglycerides found in natural fats or oils, which are merely mixtures of such glycerides whose molecules contain different numbers and kinds of fatty acid residues. The C, H, and O represent carbon, hydrogen, and oxygen atoms. Arrows that point to the carbon-to-carbon double bonds have been added to facilitate subsequent discussion.

With the diagram before us, we can discuss the changes which a fat undergoes during hydrogenation. This triglyceride contains six carbon-to-carbon double bonds, any one or all of which may add hydrogen, with a resulting decrease in the number of such bonds and a consequent change in properties. The change in properties is affected not only by the number of double bonds that are saturated with hydrogen, but also by the particular carbon-to-carbon bonds at which the addition occurs.

For example, if hydrogen is added at the carbon-to-carbon double bonds labeled 5 and 6, this portion of the molecule will look exactly like the oleic acid residue (at the top of the formula). If it is added at the position labeled 3, then the middle portion of the molecule will look like the oleic acid residue.

Complete hydrogenation of all the double bonds would convert all of the fatty acid residues to stearic acid residues and the resulting fat would be solid at room temperature. If the hydrogen is added at the double bond labeled 2, or only at the bond labeled 5, the resulting product would differ chemically and physically from any natural fat.

During hydrogenation, the hydrogen tends to add to the fat in a more or less specific manner. The carbon-to-carbon double bond farthest from the glycerin residue in the more reactive and is hydrogenated first. Also linolenic acid is much more reactive than linoleic acid, which in turn is more reactive than oleic acid. The relative reactivities, however, do not strictly control the actual addition of hydrogen. Some completely hydrogenated acid residues are formed simultaneously with the conversion of linolenic and linoleic acid residues to less highly unsaturated forms.

Investigations by A. E. Bailey and his coworkers in the Department of Agriculture showed that under selective conditions (conditions that favor the conversion of highly unsaturated acids to oleic acids but not oleic acids to stearic acid) the relative reactivities of the unsaturated acids toward hydrogen may be approximately represented by the following whole numbers : Oleic acid, 1; iso-oleic acid, 1; isolinoleic acid, 3; linoleic acid, 20; linolenic acid, 40. In nonselective hydrogenation, the ratio of reactivity of linoleic acid to that of oleic acid may be as low as 5, as compared with 20 or above under selective conditions.

The investigators also observed that, under commercially used conditions, the formation of completely saturated stearic acid is repressed, and the formation of iso-oleic acid is simultaneously favored, by increasing the temperature, increasing the concentration of the catalyst, decreasing the pressure of hydrogen, and decreasing the agitation. The nature of the nickel catalyst, as influenced by its method of preparation, was also found to have a marked effect on the composition of the hydrogenated product.

Margarine oil or fat produced from liquid vegetable oils is one illustration of an industrially important oil whose special characteristics are the result of carefully controlled hydrogenation.