Why do newspapers turn yellow over time? (2024)

Paper is made from wood, which is made up mainly of white cellulose. Wood also has a lot of a dark substance in it called lignin, which ends up in the paper, too, along with the cellulose. The exposure of lignin to air and sunlight is what turns paper yellow.

­ Lignin makes wood stiff and trees stand upright. You could say it acts as a glue to bind the cellulose fibers together. It is a polymer, a substance that is formed by the joining of simpler molecules into giant molecules that act differently than the smaller molecules did. Dr. Hou-Min Chang, a professor of wood and paper science at N.C. State University in Raleigh, N.C., compares lignin to the concrete used in buildings, with cellulose as the steel frame. Without lignin, Chang says, a tree could only grow to be about 6 feet tall. Lignin also helps protect the wood from pests and other damage.

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Newsprint, which must be produced as economically as possible, has more lignin in it than finer papers. At the mill, the wood that will be turned into newsprint is ground up, lignin and all.

Paper manufacturers utilize the benefits of lignin in some types of paper, though. Brown kraft paper, the dark brown paper used in grocery store bags, and cardboard are stiff and sturdy because they have more lignin in them, and because those kinds of paper aren't treated with bleaching chemicals. It doesn't matter how dark they are because the printing on them is limited.

To make a fine white paper, the mill puts the wood through a chemical solvent process, which separates and discards the lignin. Pure cellulose is white, and the paper made from it will be white and will resist yellowing.

Lignin eventually turns paper yellow because of oxidation. That is, the lignin molecules, when exposed to oxygen in the air, begin to change and become less stable. The lignin will absorb more light, giving off a darker color. Chang says that if newsprint were kept completely out of sunlight and air, it would remain white. After only a few hours of sunlight and oxygen, however, it will start to change color.

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I'm well-versed in the realm of paper production, particularly the intricate composition and properties of wood-derived materials like cellulose and lignin. The expertise in this field stems from an understanding of the chemical and physical processes involved in papermaking, including the impact of lignin on paper quality, color, and durability.

Cellulose, constituting the majority of wood, forms the structural basis of paper. Its white nature contributes to the initial color of paper. However, lignin, a darker substance found in wood, also becomes part of the paper during the pulping process. Lignin, akin to a natural adhesive, binds cellulose fibers together, imparting stiffness to wood and paper.

This lignin, when exposed to air and sunlight, undergoes oxidation, leading to the yellowing of paper over time. It acts as a protective shield for wood against pests and damage, a bit like the 'concrete' that supports a building (an analogy used by Dr. Hou-Min Chang). Moreover, the amount of lignin in paper varies; newsprint, produced economically, contains more lignin, making it less resistant to yellowing compared to finer papers.

The distinction between papers like brown kraft paper (used in grocery bags) and fine white paper lies in their lignin content. Brown kraft paper and cardboard possess more lignin, hence their sturdiness, while fine white paper undergoes a chemical process that separates and eliminates lignin, ensuring its whiteness and resistance to yellowing.

Oxidation causes lignin molecules to become less stable, absorbing more light and resulting in the yellowing of paper. Dr. Chang highlights that by limiting exposure to sunlight and air, newsprint could remain white, but even a few hours of such exposure kickstarts the discoloration process.

Regarding related concepts in the article, there are several intriguing points to expand upon:

1. Cellulose: The primary component of wood and the basis for paper production due to its fibrous structure and abundance in plants.
2. Lignin: A complex organic polymer found in wood that contributes to paper's color, stiffness, and resistance to pests.
3. Pulping Process: The method of separating fibers from wood to create paper pulp, involving the inclusion of lignin in the resulting paper.
4. Newsprint vs. Fine Papers: Different paper types based on lignin content, impacting their durability and susceptibility to yellowing.
5. Chemical Solvent Process: A method used to produce fine white paper by eliminating lignin from wood fibers.
6. Oxidation and Discoloration: The process by which lignin molecules change when exposed to oxygen, leading to paper yellowing.

Moreover, the references to institutions like N.C. State University's Department of Wood and Paper Science and additional resources like the Encyclopaedia Britannica on lignin and Greenpeace's involvement in environmental aspects of paper production are valuable sources to delve deeper into these topics.