Why do old books turn yellow ?

 

Paper is made from pulp that contains cellulose. Another organic substance that binds the cells, fibers and blood vessels that make up wood is lignin. Apparently, it is the most abundant renewable carbon source after cellulose. Cellulose, a colorless substance, reflects light so well that it is perceived as white. To produce fine white paper (bond paper), the pulp undergoes a chemical process that separates and removes the lignin. That's why the paper is usually white. However, when paper is made, it also contains some lignin. Paper used as newsprint must be economical, so it contains more lignin than other papers.  The brown paper and cardboard used for packaging and paper bags contain a lot of lignin, so they are hard and strong. paper is made from these ingredients, but it does yellow over time, at least when exposed to oxygen. However, when lignin is exposed to light and ambient air, its molecular structure changes. Lignin is a polymer. H. It consists of batches of identical molecular entities bound together. In the case of lignin, these repeating units are alcohols, composed of oxygen and hydrogen, with some intercalated carbon atoms. , these molecules change the structure of the polymer. The added oxygen molecules break the bonds that hold these alcohol subunits together, creating molecular regions called chromophores. Chromophores reflect specific wavelengths of light that our eyes perceive as colors.