Why are some papers darker than others? What prolongs the useful life of the material most used in schools, offices and student assignments?
The main component of wood, besides cellulose, is a macromolecule called lignin. This substance is present, in large quantities, in some types of paper such as brown paper (used in bread bags) and cardboard.
These papers have characteristics in common, among them, the dark coloration. The presence of a large amount of lignin in the composition confers this property. However, lignin makes the paper look aged over time, that is, it accelerates its degradation. Lignin molecules undergo oxidation when exposed to oxygen in the air, undergo changes and become less stable.
To make white colored paper, the lignin must be almost completely extracted. This is possible through bleaches (whitening chemicals), see Paper manufacturing process. During the process, these chemicals are added to the cellulose pulp which separates and discards the lignin. The result is a white paper with a longer shelf life.
By Líria Alves
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/lignina-na-composicao-papel.htm