Volume 1, Number 1
Jun 1994

Various Causes for the Darkening of Paper>

1. Reprinted with permission from Progress in Paper Recycling, February 1, 1993, p 88.

Some essential elements must be present to establish whether or not paper will darken. These elements are:

• Light
• Light-absorbing material in paper (chemical component)
• Suitable environment (oxygen, humidity, temperature, etc.)

In paper, the light-absorbing materials are: lignin or lignin products, carbohydrates, extractives, and complex-forming metal ions.

Darkening of paper has been explained by different mechanisms such as acidolysis, alkaline hydrolysis, autohydrolysis, oxidation, and pyrolytic scission of cellulose molecules. It is a very complex phenomenon, and no single pathway is likely to provide all the answers. But some very interesting and logical explanations have been proposed in the literature:

• Wave length of light plays an important role. It is observed that, at less than 340 nm, photoyellowing occurs, and, between 340 to 400 nm, photobleaching occurs.
• Oxidation and other reactions of paper components generate chromophores such as carboxylic groups, ketones, and aldehydes in a conjugated system.
• Rag paper darkens less than chemical wood pulp which, in turn, darkens less than groundwood pulp. Others fall in between.
• Extractives are chromophoric and may have variable effects on paper, depending upon the pulping/bleaching processes used and the amount of extractives left in the pulp.
• Lignin and lignin products are photosensitive and have been reported to contribute most to darkening. Two of the contributing lignin products mentioned are quinones and ferulic acid.
• Hemicelluloses and cellulose in the native form do not significantly absorb light. But they do so when they are oxidized or modified, especially under alkaline conditions.
• Papers containing fines, transition metal ions, sizing agents, coatings, and optical brighteners tend to darken more under favorable environmental conditions, e.g., heat, light, humidity, time, etc.

The darkening of recycled paper during a repulping operation at higher pH can lead to the darkening of the pulp, especially if it contains higher-yield pulps.

As mentioned above, mechanical pulps darken when treated with sodium hydroxide. Thus, pulping of recycled papers containing mechanical pulps under alkaline conditions should be done in the presence of hydrogen peroxide, followed by treatment with a reducing agent. The objective of such a sequence is to maintain the high pulp yield. This form of treatment is often referred to as "lignin-preserving bleaching."

2. H.S. (Doug) Dugal is President and Salman Aziz is Vice-President, Technology, Integrated Paper Services Inc., P.O. Box 446, Appleton, WI 54912-0446, USA.