Paper is made from pulp or cellulose pulp, a material made up of an aqueous suspension of cellulose fibers held together by hydrogen bonds. Cellulose pulp is obtained mainly from woodbut cotton, linen or hemp fibers are also widely used.
The paper is obtained by drying and hardening the cellulose pulp and in the process other substances are usually added, such as propylene and polyethylene, to achieve some specific characteristics in the paper.
Pure cellulose is White color; We have proof of this in cotton, made up of 90% cellulose fibers. But the most widely used cellulose pulp also contains colored substances, mainly lignina complex organic polymer that is very important in the formation of rigid structures in vascular plants, such as wood and bark, but is also found in some algae.
In general, the higher the lignin content, the darker the paper. A paper with a high lignin content is usually considered to be of poorer quality and to whiten it, it is often subjected to bleaching with chlorine and derived substances. Sometimes a pulp that is high in lignin is used and is not intentionally bleached.
Oxidation and ultraviolet radiation
Whether due to whitening, low lignin content, or both, new paper is often whiter than old paper. As time goes by, the air oxygen and solar radiation, especially ultraviolet radiationthey generate various chromophores, especially aldehyde chromophoresresponsible for the paper acquiring yellowish tones and, in advanced stages, brown tones.
The effects of oxygen and UV radiation are greater on lignin than on cellulose, hence papers with lower lignin content, considered to be of higher quality, yellow more slowly. However, the cellulose also ends up being affected and becoming darker. To prevent this effect, protecting the paper from air and sunlight, together with humidity control, are usually sufficient measures.
On the other hand, the substances used in the paper bleaching process are oxidizing substances that tend to make the cellulose and lignin more susceptible to further degradation, thus they can accelerate the darkening of the paper in the long term.
Acidic compounds used in some stages of papermaking, such as sulfurous acids and aluminum. The most durable papers, both in color and structure, are called acid free paperseither acid-freewhich are manufactured with special precautions to reduce the acidity in the final paper.