Application of Yellowing Inhibitors on Mechanical Paper in a Size Press

Several yellowing inhibitors (UV absorbers) were applied to paper or board in a laboratory size press and their efficiency in slowing down paper yellowing was measured. The inhibitors were applied in aqueous starch solutions and the concentration and type of starch was varied. The starch had no effect on the rate of yellowing of paper, but it did affect the yellowing of board. The difference is ascribed to the complete penetration of the starch-yellowing inhibitor solutions into paper, but partial penetration into board, keeping more of the inhibitors near the paper surface. For board, the color reversion depends on the viscosity of the solution, which determines the penetration depth into the board. The yellowing inhibition is proportional to inhibitor dosage.