Equipment Description:Cationic Starch
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Cationic starch can be added to paper making stock to improve the dry strength of the finished sheet, to improve the retention of fines and fillers, or for a combination of both of these. Cationic starch is preferred because the positive charge that has been introduced onto the starch molecule chain tends to form an electrostatic bond with negative charge sites on the fibres and fines. This in turn results in better retention of the starch in the paper web. |
Cationic starch is supplied as a free flowing white powder. Cationic starch for the wet end is typically either potato or maize based. The starch manufacturer extracts the native starch from the plant source, then carries out a further modification step to introduce cationic charges onto the starch molecule. The number of cationic groups typically range from 2 per 100 starch monomers up to 10. The number of cationic groups per hundred starch monomers is called the degree of substitution (ds), expressed as a decimal fraction. Typically, most cationic wet end starches have ds between 0.03 and 0.06 (meaning between 3 and 6 cationic groups per 100 starch monomer units). |
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Cationic starch has to be dissolved in water and cooked prior to use. If strength improvements are the key requirement, the cooked starch is added as far back in the system as possible, typically into the thick stock. If retention improvements are desired, the cooked starch is added into the thin stock loop, close to the flow box. Cationic starch induced flocs are very sensitive to hydraulic shear forces, the starch is added as late in the system as possible. Some mills use a split addition in an attempt to maximise both strength improvement and retention. Cationic starch is added in doses from 0.5 up to 1.5% dry starch on dry fibre. Excess cationic starch can result in the overall charge of the wet end system crossing over from anionic to cationic. If this happens, drainage and formation of stock on the wire is severely disrupted. Cationic starch is supplied in 25 kg bags, in 1 tonne big bags, or in bulk form into a storage silo. Starch should be stored in cool, dry environment. Constant mixing is required for starch slurries, otherwise the granules will settle out. Starch dust carries an explosion risk |
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