Defoamers for pulping and papermaking
In the pulping process, since the pulping raw materials contain natural substances such as fatty acids and resin acids, they react with the added chemicals such as caustic soda and alkali sulfide to generate foaming substances such as fatty acid soaps and resin acid soaps. In addition, the foam stabilizing effect of polymer substances such as cellulose causes a large amount of air to be brought into the pulp system during the flow and mixing of the pulp, forming a large amount of stable foam, which brings great difficulties to subsequent processes such as washing, transportation, and bleaching, causing floating pulp and reducing the quality of pulp. Some factories use “clean water foaming”, which has a certain effect on removing foam, but the defoaming effect is not ideal. The black liquor washed from the pulping process also contains high concentrations of foaming and stabilizing substances such as fatty acid soaps and resin acid soaps. In order to facilitate operation and improve the alkali recovery rate of pulp, the black liquor must also be defoamed.
The slurry in the papermaking process also contains foaming and stabilizing substances similar to those in the pulping process. Air will also be mixed in during operation to produce foam. The presence of foam has a great impact on paper, resulting in poor sizing, poor cross-section quantitative uniformity, spots, light-transmitting points and small holes on the paper surface, and even paper breaks. In addition, if a large amount of foam is generated in the papermaking white water, a defoamer should also be added to remove it to increase the recovery rate of the pulp.
At present, the active ingredients of the defoamers used in the pulping process and papermaking process are mainly fatty acids and their derivatives, polyethers, silicones, etc. Defoamers are generally not a single active ingredient, but a combination of multiple active ingredients to complement each other and obtain good effects. The formula listed in Table 2 is a pulping defoamer with fatty acid derivatives and other substances as active ingredients.
