CAN THE YELLOWING OF PAPER BE AVOIDED

In order to be used in high quality printing- and writing papers, mech anical and high-yield pulps must be prevented from yellowing. The choi ce of a good raw material and defibration under suitable conditions, i ncluding chemical treatment, are important, as is the removal of detri mental metal ions. Because lignin is mainly responsible for the yellow ing, stabilization can be achieved by chemical derivatisation, e.g. by alkylation or acylation, though these methods are hardly economical. The use of antioxidants also suffers from this drawback. In contrast, the use of UV screens is a more practical approach. Likewise reagents with unknown modes of action (probably complexing) such as polyethylen e glycols offer good opportunities for stabilizing the lightness and c olour of papers.