Neutral-alkaline papermaking

In a previous publication in this series (8), Hagemeyer set alkaline papermaking in the context of future demand for paper, and dealt briefly with some of the technical consequences. Since then, more mills have converted to the new method, and the aim of this chapter is to inform the reader in some detail about the reasons for changing to neutral/alkaline papermaking, some of the consequences for the production and properties of paper, and how the change impinges on... 

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. 

M. Strutz, C. Sweeney, Natural Ground Calcium Carbonate, Neutral/Alkaline Papermaking Short Course October 1990, Tappi Reprint... 

In 1945, cationic urea resins were introduced and quickly supplanted the anionic resins, since they could be used with any type of pulp (62). Although they have now become commodities, their use in the industry has been steadily declining as the shift towards neutral and alkaline papermaking continues. They are commonly made by the reaction of urea and formaldehyde with one or more polyethylene—polyamines. The stmcture of these resins is very compHcated and has not been deterrnined. Ammonia is evolved during the reaction, probably according to the following ... 

However, the general trend is towards neutral and alkaline papermaking, so currently the predominance is with cationic rosin sizes for neutral and ASA/AKD sizes for alkaline. The difference in the three types can be described by ... 

Today, there is a wide variety of fillers suited for acid, neutral, and alkaline papermaking conditions. The amoimt of pigment added to paper normally varies... 

CaCOs in the form of PCC, as true for GCC, is naturally soluble under add conditions and therefore requires a near neutral to slightly alkaline pH environment Residual calcium hydroxide Ca(OH)2 in the PCC can require extra measures for pH control of the final product and the paper mill wet end system. Despite reported developments in providing so-called acid tolerant PCC, intended to remain stable against dissolution under acid (< pH 7) conditions, papermakers, it seems, continue to make the conversion to slightly alkaline or alkaline papermaking when considering the use of CaCOs. 

The main classes of chemicals and their consumption as a proportion of the total worldwide usage of 350000 tons dry p. a. is shown in Fig. 3.10. Because of the trend to neutral or slight alkaline papermaking and the increasing use of calcium carbonate as a filler and coating pigment, the proportion and also the absolute volume of rosin will decline further and the synthetic products will steadily grow. 

Uses Emulsifier for NovaSize ASA for optimal sizing performance in neutral and alkaline papermaking systems including bleached board cylinder board unbleached virgin Kraft linerboard Properties Opaque liq. vise. 600 cps pH 3.0 15.5% NV Novawet DOSS [DyStar]... 

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. 

The change to alkaline—neutral papermaking is laudable and will assist in the preservation of books published in the future. (The production of alkaline—neutral paper is surveyed in Chapter 1 of this volume.) However, the difficulty with books printed since 1850 remains. Methods of deatiditying paper are critically evaluated in Chapter 2, and the potential of graft copolymerization as a means of strengthening paper is described in Chapter 3. 

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. 

---