Why do book pages turn yellow?

Book pages turn yellow over time due to a combination of factors related to the materials used in paper production and environmental conditions. The main reasons for the yellowing of paper are:

1. Lignin: Paper made from wood pulp contains lignin, a complex organic polymer that acts as a natural binder for the cellulose fibers. When exposed to light, air, and moisture, lignin breaks down and oxidizes, causing the paper to turn yellow or brown.
2. Acid hydrolysis: As paper ages, the cellulose fibers break down due to a process called acid hydrolysis. This process releases acids that further catalyze the yellowing of the paper.
3. Sizing: Traditional paper sizing agents, such as animal glues or starch, can also contribute to yellowing as they degrade over time.
4. Environmental factors:
   • Light exposure: Ultraviolet (UV) radiation from sunlight and artificial lighting can cause chemical reactions that lead to yellowing and embrittlement of paper.
   • Heat and humidity: High temperatures and humidity levels can accelerate the chemical processes that cause yellowing and accelerate the breakdown of paper fibers.
   • Air pollution: Pollutants like ozone, nitrogen oxides, and sulfur compounds can react with paper components, leading to discoloration and deterioration.
5. Impurities: Traces of impurities in the paper, such as residual lignin, metal particles, or chemical contaminants, can also contribute to the yellowing process through oxidation reactions.

To minimize yellowing, modern book papers are often made from more chemically stable materials, such as cotton or linen fibers, and treated with alkaline buffers to neutralize acids. Additionally, proper storage conditions with controlled temperature, humidity, and light exposure can help slow down the yellowing process in books.