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Dry strength additives increase sheet strength by increasing the strength and/or the number of fiber-to-fiber bonds in the sheet. Dry strength additives are natural or synthetic polymers that are added when refining cannot achieve the required dry strength levels. They also function as retention and drainage aids due to their bonding abilities with fibers and fiber fines.
The most widely used dry strength additives are based on starch, mostly corn starch in North America and Western Europe. Starch is a carbohydrate, a polymer of glucose that is produced and used by all plants to store energy. The starch is often modified by heat or chemicals before it is delivered to the mill as a dry powder, granules, or flakes. At the mill, the dry starch is mixed with water and cooked with steam before it is diluted and added to the stock. The type of starch used and the addition point must take into account the other furnished components, including other additives.
Synthetic polymers can also be used as dry strength additives. Anionic and cationic polyacrylamides are the most common, and they are sometimes used in combination with starch. Guar gum, carboxymethyl cellulose, and methyl cellulose can also be used, but they tend to be more expensive.