Pulp & Paper

When paper is "sized" during the manufacturing process, a glutenous material is used to fill the pores in the surface of the paper. Sizing has traditionally been done using an acid medium such as aluminum sulfate and rosin. However alkaline and neutral sizing are rapidly gaining favor throughout the paper industry. The major alkaline sizing agent is alkylketenedimer (AKD).

Paper in which alkaline sizing has been used is generally stronger, brighter and more resistant to aging. The alkaline sizing process also allows for more filler to be used in the manufacture of the paper, and improves the operating efficiency of the paper-making machine.

Sodium Bicarbonate and the Alkaline Sizing Process
The key to sizing success is to ensure that the cellulose fibers of the paper are as reactive to the sizing as possible. With AKD sizing, reactivity is improved by increasing the final drying temperature and raising and stabilizing the pH of the sizing suspension. An optimum level of alkalinity in the suspension helps stabilize pH.

Sodium bicarbonate is typically the chemical used to raise alkalinity, when required, in the sizing process. The AKD process works best at 150 ppm of bicarbonate alkalinity. Precipitated CaCO3 contributes about 60 ppm of bicarbonate alkalinity. Therefore, supplemental sodium bicarbonate is required in plants with process water of 90 ppm alkalintiy or less

Sodium bicarbonate is also used in neutral starch sizing by adjusting the pH to 7-8 at the final starch sizing press.