Paper surface sizing/coating

Figure 16 ToF-SIMS images of total ion current, AKD (alkyl ketene dimer) size, optical brightener and hexosan distribution on multipurpose paper surface after coating with Au-Pd. Reproduced from Fardim and Holmbom [70], with permission from Elsevier. 2005.

Surface Sizing. Surface sizing is generally used for modification of other properties of paper or paperboard such as printabiHty, smoothness, porosity, coefficient of friction, opacity, surface strength, anti-linting or coating holdout. Anionic starch is perhaps the most common additive or co-additive used for surface sizing. 

Hydroxyethylstarch is widely used with synthetic latexes in the surface sizing of paper and as a coating binder. For these uses, the hydroxyethylstarch is acid-thinned, oxidized, or dextrinized. Hydroxypropylstarch is used in foods to provide viscosity stabiHty and to ensure water-holding during low temperature storage. 

Figure 8.1 Scanning electron photomicrograph of the surfaces of (a) surface sized, and (b) coated paper. Scale bar = 50 jum.

Other water-soluble polymers which do not tend to suffer from problems of retrogradation are sometimes used when different properties are required. Soluble cellulose derivatives, particularly car-boxymethyl cellulose, which is prepared by reaction of high purity cellulose with chloroacetic acid in the presence of alkali (equation 8.1), is popular for surface sizing base papers which are subsequently to be coated, because it assists in water removal when the coating mix is applied. 

The main nutrients which are present in a paper mill effluent are carbohydrates from wood pulp or from waste paper, and also products which arise from their degradation. Compositions may be very variable and depend to a large extent upon the type and amount of waste paper, if any, which is being used. Recycled coated and surface-sized papers will introduce significant amounts of soluble polymers, in particular starch, which were used in the original surface treatment. 

Multivalent ions in starch dispersions, particularly those of aluminum, calcium, sulfate and oxalate, will induce retrogradation due to complexation or competition for water of hydration. The ions can be introduced by hard process water or accumulate by leaching from paper during surface sizing or coating. The destabilizing effect of ions follows the Schulze-Hardy rule. 

Application of surface size to paper carries with it the transfer of a substantial quantity of water. As an example, surface sizing of a 75g/m2 (501b/3300fit2) sheet (with 1% residual water content) by a 5% starch solution for a coat weight of 1.5 g/m2 (1 lb/3300ft2/side) will raise the water content of the sheet to 43%. This large quantity of water will weaken the paper. Web breaks at the size press can occur, particularly when the sheet is also weakened as a result of edge cracks or holes. 

Maurer HW, ed. Starch and Starch Products in Surface Sizing and Paper Coating, Atlanta, GA TAPPI PRESS 2001. 

The objectives in the final stage of the total papermaking system are to improve the paper surface, to reduce rolls and sheets in size, to modify paper for special properties, such as coat or emboss, to convert to finished products, such as bags and corrugated boxes, and to package for shipping. 

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