Starch

Starch is added at the wet end and at the size press. The wet end starch is cationic, has a positive charge, and bonds to the anionic pulp fibers. The wet end starch strengthens the web formed by the fibers. Wet end starches generally have solids of about 6%.

Starch is used at the size press to further strengthen the sheet. It is also used as a substrate for other chemicals. There are some important aspects of starch used at the size press that may be different from that of the wet end. The cooking of the size press starch is dependent on the product that is being cooked. The starch can be made from potatoes, corn, or tapioca. Solids and pH are other variables that also must be controlled.

In my experience wet end starch was fairly issue free. I don't recall ever having a major issue with the wet end starch. There were times when supply was an issue due to the boom in corn use for ethanol. At the size press there were problems from time to time. If not cooked properly the starch can be burned and have a brown appearance. This can affect the shade of the sheet. Increasing solids was also an issue due to the increase in viscosity as solids increased. Other issues that occurred from time to time was foaming in the starch run tanks. Defoamer was added in small amounts to help control this. Too much defoamer would hurt the sizing, reduce Cobb and HST test. Addition of fan nozzle showers in the run tanks also helped. The problems occurred during runs of special grades, such as grease resistent paper.