8 - Stock Preparation and Additives for Papermaking

Abstract

This chapter discusses preparation of the stock prior to the headbox of the paper machine and describes numerous additives and fillers used to make paper. Wet end chemistry— the chemistry of the dilute aqueous solution of fibers, fillers, and additives— is also discussed. Knowledge of wet end chemistry helps predict retention of additives, drainage during papermaking, and the level of sizing of the final paper. Stock is the mixture of pulp, fillers, other papermaking materials, and water. The combination of all of the materials used to make paper is called furnish. Broke consists of paper trimmings and waste, paper not up to specification, and other paper that is often reused in the paper mill. A mill producing more than 10-15% broke is not operating efficiently. A typical stock preparation system for paper machines includes stock proportioning and stock cleaning. Additives are materials used to improve the finished paper itself or aid in the process of papermaking. Functional additives such as dyes, internal sizing agents, adhesives to increase wet or dry strength, and fillers are used to improve or impart certain qualities to the paper product and must be retained on the sheet to be effective. Control additives as biocides, drainage aids, retention aids, pitch control agents, and defoamers are added to improve the papermaking process, but do not directly affect the product and are not necessarily retained on the product.