Paper Functional Chemicals : Sizing Advice

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. • Rosin: The average addition rate is 1 % solid calculated on paper, the individual rate depending on the required degree of sizing. With these products this degree can be adjusted very precisely and is fully obtained directly after the paper has been dried. The product is usually added to the stock suspension upstream of the headbox and then fixed chemically, in the simplest case with aluminum sulfate (alum). The retention of rosin size is in the region of 70 % if the usual fixing agents are employed. Rosin sizes are not usually suitable for surface ap¬plications.
. • AKD: As little as 0.005–0.008 % reacted AKD is believed to yield sufficient monomolecular coverage of the paper stock, while dosage rates are considerably higher and typically vary from 0.05 to 0.2 %, calculated as solid AKD wax based on paper stock. The stability to hydrolysis of most commercial products has been improved significantly in recent years. The ketone resulting from the hydrolytic decomposition of AKD is a solid (melting point 80–85 °C) which can be retained in the sheet but does not contribute to sizing. The risk of the formation of deposits is relatively low. For cost efficient sizing, the curing (orientation of AKD particles) must be initiated appropriately because chemical reaction alone is not sufficient. The curing can be improved by drying the paper web at high contact temperatures and to a very low moisture content e. g. before the size press, typically to 1–3 %, especially on fast-running machines. Another way to speed up the curing before converting is to carry out AKD sizing in the presence of a so-called promoter resin, e. g. polyamide epichlorhydrin, modified polyethyleni¬mine, polyvinylamine.
. • ASA: This will not dissolve in water and, prior to application in papermaking, it must be emulsified on site at the paper mill. There, a small amount (3–6 %) of an activator is added, plus cationic starch and a synthetic cationic polymer which serve as stabilizers. Activators are surface-active agents that promote effective emulsification at low mechanical energy. Emulsion stability is best with syn¬thetic polymer, and sizing efficiency is best with cationic starch, improving with the amount of starch used. The ratio of starch to ASA is usually in the range 2:1 to 4:1. Emulsification is primarily carried out in a continuously operated special automated equipment, and can be affected by both low and high shear proce¬dures. Particle sizes in the range 0.5 to 2 mm can be obtained. Emulsification imparts a cationic charge on the oil droplets which helps to increase the stability as well as improving ASA retention on anionic fibers and sizing efficiency.

Linear hydrocarbons are more hydrophobic than branched ones. However, the greater the hydrophobicity of the ASA molecule, the more difficult is the prepa¬ration of a stable quality ASA emulsion. ASA undergoes the normal reactions of acid anhydrides. Of particular interest in conjunction with sizing is the reaction with alcoholic hydroxy groups to yield an ester, and the hydrolysis with water. Both reactions occur in the papermaking system. ASA is highly reactive, and the reactions proceed rapidly and irreversi¬bly. Although this would provide satisfactory development of sizing on a paper machine, the hydrolysis of ASA is undesirable. The hydrolysis accelerates with pH, time, and temperature and leads to deposits and runnability problems on the paper machine. In order to limit the hydrolysis of ASA emulsions, the pH can be lowered immediately after the emulsification by addition of aluminum sulfate. High solids retention is extremely important in ASA sizing. If this is guaranteed and ASA is applied correctly, a relatively small dosage of 0.1–0.2 % based on solid furnish is required to produce the desired degree of sizing. Careful selec¬tion of retention aid is therefore an important part of the ASA sizing process. The great advantage in favor of ASA is the high rate of cure in comparison with AKD size. The reaction with cellulose hydroxy groups takes place rapidly in the dryer section of the paper machine at less than 5 % web humidity. More than 90 % of the attainable sizing potential is, in most cases, achieved before the size press unit. The reaction rate is influenced by the same parameters as the hydrol¬ysis, that is primarily by pH, previous residence time, and stock temperature. The best conditions obviously depend upon the balance between the reaction rates with cellulose hydroxy groups and hydrolysis of ASA, and optimum sizing can be achieved at neutral rather than at higher pH, depending on the full set of system conditions.

• PSA: For internal sizing (wet-end addition) quantities of 0.5 to 1 % (solid product on dry paper stock) are required to obtain a hard sized sheet, depending on the nature of the stock. The product is best metered continuously to the stock sus¬pension before the pressure screens. The polymer must be prediluted with an in-line mixer to a concentration of 1–5 %. A cationic retention aid, e. g. modified polyethylenimine, cationic polyacrylamide or polyvinylamine improves the fixa-tion/retention and therefore gives a better sizing effect.
For surface application, first the compatibility with the other chemicals used in the preparation and also the shear stability have to be checked (e. g. with starch, optical brightener, dyes). It is advisable to meter the undiluted polymer by means of a piston pump to the circuit of the size-press, e. g. before the feed pump. To achieve a good effect, the PSA must penetrate sufficiently into the paper, e. g. an adequate quantity of sizing solution must be taken up. Quantities of 2–4 g l–1 solid polymer in combination with 40–80 g l–1 starch are normally sufficient to achieve a hard sized paper sheet; 1–2 g l–1 solid polymer is suffi¬cient for final sizing of a paper that has been presized internally.

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