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Material and Function of Soap

Soap’s raw materials are mainly oil, synthetic fatty acid, and alkali. In addition, in order to improve the comprehensive properties of soap and meet various application requirements, some auxiliary materials and filling materials are usually added.


(1) Oil

Oil is the basic raw material for soap production. Its composition and properties play a direct decisive role in soap quality. The grease used in soap making is mainly animal and vegetable oils.


1) Composition of grease. The main components of oil are esters formed by the combination of fatty acids and glycerol.


Glycerin Fatty Acid Oil Water

The properties of oils depend on the properties of fatty acids that make up oils. It can be seen that the properties of soaps are closely related to the types and properties of fatty acids. There are many kinds of fatty acids in natural oils. Saturated fatty acids, unsaturated fatty acids and structurally abnormal fatty acids are commonly used in soap making.


2) The nature of oils and fats. Oils and fats are insoluble in water, with a specific gravity of 0.91-o.976 and no fixed melting point, but they are far below 100 C. Some of them have color and odor, and decolorization and deodorization are necessary for soap making.

Under the action of alkali, oil can be hydrolyzed thoroughly to form the reaction of glycerol and fatty acid salts (soap), which is called saponification reaction.


Oil, caustic soda, soap and glycerin

Under certain conditions, unsaturated oils can be hydrogenated to make their double bonds disappear and reduce the degree of unsaturation. The reaction can be changed from liquid to solid, that is, hydrogenation reaction, also known as oil hardening. After hardening, liquid grease can be used as raw material for soap making with better texture.

After storage for a period of time, oil is easily oxidized and hydrolyzed due to the action of oxygen and microorganisms in the air. The phenomenon of bad odor is called grease rancidity. Therefore, the storage room should be clean, cool and dry.


3) Oils and fats commonly used in soap making and the properties of soap. According to the coagulation point of oil, there are three main types: solid oil, lauric acid oil and soft oil.

  • The solidification point of solid grease is above 40 C, such as cattle and sheep fat, palm oil, paint, tar, wood oil, hardening oil, etc. This kind of grease can ensure enough hardness of soap because it contains more saturated fatty acids .
  • Lauric acid grease. There are mainly coconut oil, palm kernel oil and so on. This kind of oil contains more saturated fatty acids. In addition to increasing its hardness in soap, it mainly increases the whiteness and foam of soap. Coconut oil, for example, is the main ingredient of soap.
  • Soft oils mainly include lard, cottonseed oil, peanut oil, tea oil, rapeseed oil, castor oil and so on. This kind of oil contains more unsaturated fatty acids, which mainly regulate the hardness of soap and increase the plasticity of soap.


(2) Oil substitutes

  1. Synthetic fatty acids. Synthetic fatty acids are made from petroleum by chemical synthesis. At present, they are mainly made from paraffin, a by-product of petrochemical industry. The amount of soap can be up to 60%, which can save a lot of natural oils and fats. With the development of petrochemical industry, synthetic fatty acids will become the main raw materials for soap production.
  2. rosin. Rosin is an unsaturated compound obtained by distillation of turpentine from rosin. Its main components are rosin acid and rosin anhydride. In soap, it can increase foam force and stabilize foam, improve solubility, prevent rancidity, make soap body fine, lighten frost and lubricate skin.


(3) Inorganic raw materials

  1. Alkali. There are mainly sodium hydroxide and potassium hydroxide. Sodium hydroxide is the main raw material for solid soap, which is used to saponify oil and fat to produce soap. Many properties of potassium hydroxide are similar to sodium hydroxide. Saponified oil and fat produce soft soap, which is the raw material for making potassium soap.
  2. Sodium chloride (salt). When making soap, it is used for salting out because soap is insoluble in brine and glycerol and pigments are soluble in brine, which can separate glycerol and pigments from the soap solution and improve the quality of soap.
  3. Sodium carbonate and sodium silicate. Sodium carbonate is the filler of soap. It can not only directly saponify oil or rosin, reduce the dosage of caustic soda, but also increase the reflectivity of soap and prevent rancidity. Sodium silicate, also known as sodium silicate or sparkling alkali, is an important detergent for laundry soap. It can increase the hardness of soap, reduce the shrinkage and deformation of soap, soften hard water and prevent soap from rancidity.
  4. Spices
    Spices are essential ingredients for soap making. Laundry soap can be made without spices or with less low-grade spices. Spices not only have aromatic odor, but also arouse people’s spiritual pleasure and excitement. More importantly, spices have good disinfection and bactericidal effect . Their bactericidal efficacy is much stronger than that of carbolic acid, a commonly used disinfectant.
  5. Other raw materials
    Colors are used to decorate the color of soap and increase its beauty; medicines are used as disinfectants in soap.