There are many kinds of defoamers with different characteristics. From ordinary hydrocarbon oil, polysiloxane oil, fatty acid and its salt, to polyoxyethylene series, etc., defoamers are often divided into several categories, the common ones are classified according to the chemical composition of defoamers and the classification according to the paper production process. The classification of these two methods is introduced below.            ① According to the chemical composition of defoamer, it can be divided into: higher alcohols, fatty acids and their salts, phosphates, hydrocarbon oils, polyethers, organosilicon polymers, amides, etc.            ② According to the classification of papermaking process, defoamers can be divided into three categories: pulp defoamer, paper defoamer and coating defoamer.            a. Defoamer for pulping: this kind of defoamer refers to the chemicals used to eliminate the foam in the pulping process. For example, in the unbleached alkaline and sulfate pulping, there are a lot of soluble organic matters; the waste pulp also contains saponification after deinking. In the process of washing, screening and bleaching, a large amount of foam is often accumulated. When the black liquor is evaporated and concentrated, it will produce serious foam, which will affect the washing and concentration effect. The chemicals composed of hydrocarbon oil solvents and lipophilic surfactants are pulp defoamers, which have the characteristics of alkali resistance and high temperature resistance. When added into pulp or black liquor, they can reduce or eliminate foam, improve the washing, screening and concentration effect, save water, and improve the dryness of pulp after dehydration.            b. Paper defoamer: refers to the chemical used to eliminate the foam on the paper machine. In the process of papermaking, due to unreasonable sizing and washing of bad pulp, the use of alkaline fillers in the acid system, and excessive or other improper additives in the pulp, foam is accumulated in the wet end systems of the paper machine, such as the grit chamber, the high-level box, the mesh front box, and the white puddle. The paper material containing foam, when making paper, will form cavities on the paper surface, etc., so defoamer needs to be added. The defoamer used in papermaking machine should not form oil point and affect water resistance. It is usually an emulsion defoamer composed of silicone oil, fatty amide calcium (sodium) soap or propylene oxide derivative, and also a degasifier composed of non-ionic surfactant. These defoamers can reduce the pinholes, improve the smoothness, enhance the water filtration and improve the wet strength of the paper.            c. Coating defoamer: refers to the defoaming chemicals used in the coating process. There are two kinds of antifoam and defoamer, which are used to prevent air absorption in the coating process and produce foam. The antifoam is used to eliminate the foam generated in the coating system. These defoamers are required to dry the coated paper without "fish eye" and will not affect the brightness and printing performance of the coating. The common ones are: organic phosphate, such as tributyl phosphate, tripropyl phosphate, etc.; there are defoamers composed of fatty amide, polyoxyethylene ester, alcohol, ether and other surfactants.