The solubility of metal soaps of different fatty acids in mineral oil (non-polar liquid) is different, which has a great influence on the difficulty of making lubricating grease and the structural properties of the product. Fatty acid soaps with fewer carbon atoms have lower solubility in mineral oil, and are not easily dispersed in oil to form a stable system. The resulting grease is easy to separate oil and has low consistency. Fatty acid soaps with more carbon atoms have greater solubility in oil and greater dispersion in oil, making it easier to form a more stable dispersion system, resulting in less oil separation and greater consistency. However, if the fatty acid has too many carbon atoms, the solubility of its soap in the oil is too large to be easily crystallized to form a structure, and the resulting grease has a very small consistency, or even a fluid. Therefore, when making grease, it is generally best to use fatty acids with carbon atoms of 12-18.

  Some people have studied the phase transition of different fatty acid sodium soaps and the phase transition of the soap-oil system, and found that the carbon chain length of the fatty acid is related to the phase transition point of the soap and the soap-oil system. The longer the carbon ripple, the lower the low temperature phase transition point of soap and soap-oil system; on the contrary, the higher it is. The phase transition temperature change of fatty acid soaps with less than 12 carbon atoms is quite high enough to reduce the soap-oil solubility so that it cannot be made into grease. On the contrary, if the number of carbon atoms in the fatty acid is too large, the solubility of the soap in the oil is too large, so that it is not easy to crystallize in the oil to form a structure. When the fatty acid above C18 is used as the raw material for saponification, the amount of soap must be increased to make the grease have a larger consistency. Soap-based greases made from fatty acids with fewer carbon atoms have a higher dropping point but are easier to separate oil. On the contrary, the grease made of fatty acid soap with more carbon atoms is softer, but has good colloidal stability and is not easy to separate oil. In addition, naphthenic acid soap is more soluble in mineral oil than fatty acid soap, and the resulting grease is softer, but has good colloidal stability.

  When there are double bonds in the fatty acid carbon chain, the solubility of soap in oil is greater, and it is easier to form grease. However, compared with saturated fatty acid soaps, the resulting grease has a lower dropping point and a smaller consistency, but it has a great ability to expand the oil.

  In the manufacture of grease, animal fat is generally considered to be a good raw material for saponification, because it contains saturated and unsaturated fatty acid glycerides with appropriate carbon atoms, and the solubility of its soap in oil is not too large or too small, and it is easy to form fat. And the oxidation stability is good. Vegetable oils contain more unsaturated components, and are generally easier to form fats, resulting in lower product consistency and poor oxidative stability. Some vegetable oils (such as coconut oil, palm tung oil) contain more lauric acid (C12 fatty acid) glycerides, and their soaps are not easy to obtain a colloidally stable system in mineral oil. In order to improve the degree of saturation of vegetable oils, hydrogenation can be adopted. . Hydrogenated vegetable oils and their hydrolysates (corresponding fatty acids) are good raw materials for soap-based greases.

  When animal and vegetable oils are used as raw materials, soap and glycerin are simultaneously generated during the saponification process. If the grease is directly added to the base oil after saponification, the glycerin remains in the grease. Glycerin can act as a structure improver in calcium-based, sodium-based greases, etc., but the inclusion of glycerin in aluminum-based greases tends to soften the grease. In addition, glycerol itself is easy to be oxidized, especially at high temperature and with catalyst, and the low molecular acid generated by oxidation will corrode metals. Therefore, in order to prepare a grease with better oxidation stability, it is necessary to reduce the glycerol content as much as possible, and it is better to use fatty acid as the raw material