Neutral—alkaline papermaking process

The repercussions of running neutral/alkaline on the total process are widespread and merit a separate article. Whilst in the present context it is not necessary to deal with the topic in detail, to understand the relationship of permanent paper to neutral/alkaline papermaking as a whole, the reader needs to have some appreciation of what it means to change to the new process. In particular, this understanding is very useful for effective communication between manufacturer and user. 

The most commonly used sizing agents today are rosins, commonly used in acidic papermaking systems, and synthetic sizing agents such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), as shown in Figure 7.18. The latter two components are suitable for papermaking processes under neutral and alkaline conditions. 

Biocides are added to the wet end process to prevent slime formation. Introduction of neutral or alkaline sizing instead of acidic papermaking, the closed water circuits, and the increasing proportions of recycled paper have required changes in biocide types in order to control different microbial populations. 

Acidity. For many years the pH of the paper has been considered an important characteristic affecting peramnence. Permanency specifications exist which call for paper with a pH of 6.5 or higher. Other specifications say that the pH should not be lower than 5.5. There is no disagreement, however, about the poor permanency of paper when it is under pH 5.5 (4). In this context, paper pH is determined by the extraction procedure already described. The acidity that is present in the paper will promote acid hydrolysis of the glucosidic bonds of cellulose. To prevent this type of degradation and to produce paper with a high extract pH, the paper machine process water system must be controlled at a neutral to alkaline pH with a minimum of total acidity, preferably with an excess of alkalinity. The term total acidity is used to describe the concentration of all dissolved ions and particles in the process water that contribute to acidity in the system and a low extract pH in the paper. For example, papermaker s alum dissociates to alumi-... 

The neutral or alkaline pH of the paper machine system necessary for a high extract pH in the paper is not compatible with the rosin size-alum sizing system. To set rosin size properly on fiber, alum is required. Since total acidity is an approximate measure of the amount of alum in solution, it is often used as a control in connection with rosin sizing. The most effective size-alum precipitate forms at a papermaking pH between 4.2 and 4.8 and a total acidity between 90 and 150 ppm. The low pH of the process water, high total acidity, excess alum, and the size-alum precipitate all place a severe limit on the permanency properties of paper. 

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