Lignin
Also: lignin
The natural polymer that bonds wood cells together. Removed in chemical pulping, retained in mechanical pulping.
Lignin is the second most abundant wood polymer after cellulose, accounting for 15 to 35% of wood mass. Chemically, it is a cross-linked aromatic polymer that acts as the glue between cellulose fibers in wood. To make paper, lignin either has to be dissolved (chemical pulping) or softened and broken mechanically (mechanical pulping).
Paper made from pulp that retains lignin (newsprint, directory paper) yellows with age because lignin oxidizes under UV light. Paper made from lignin-free pulp (woodfree, bleached kraft) can stay white for a century.
Related
- Kraft pulp. Chemical pulp produced by cooking wood chips with sodium hydroxide and sodium sulfide. The dominant pulp process globally.
- Mechanical pulp. High-yield pulp made by mechanically separating wood fibers, retaining lignin. Used in newsprint and magazine papers.
- Bleaching. Chemical removal of residual lignin from chemical pulp to produce white pulp. ECF (chlorine dioxide) and TCF (oxygen, ozone, peroxide) are the two major systems.