yellowing

Yellowing is the gradual development of a yellow or amber tint in materials, often as a sign of aging or degradation. It can affect textiles, paper, plastics, wood, coatings, and other manufactured or natural materials. The term is descriptive rather than a single chemical process, because multiple pathways can produce similar coloration.

Most yellowing results from chemical changes that alter a material's chromophores or introduce new ones. Photooxidation from exposure to light, especially ultraviolet radiation, is a common driver. Oxidation, heat, humidity, and exposure to pollutants can accelerate reactions in polymers, lignin in wood and paper, and surface coatings. In paper, lignin breaks down to chromophores that absorb blue light, creating a yellow appearance; in plastics, UV-initiated degradation and migration of additives can cause yellow tinting.

Examples by material: paper often yellows with age and acid; textiles can yellow from sweat, body oils, or sun exposure; plastics may yellow due to UV exposure; wood and finishes can amber with age. Yellowing can be localized or pervasive and may be reversible for surface films but often involves irreversible changes in the material's bulk.

Prevention and mitigation include reducing light exposure (protective coatings, UV-filtering, storage in dark, cool conditions), using stabilizers and antioxidants, and careful material choice and handling. For existing yellowing, some surface whitening or bleaching methods may reduce appearance but may not reverse underlying changes; archival storage emphasizes acid-free packaging and controlled environments. Color measurements with spectrophotometry or standardized yellowness indices can quantify the degree of yellowing.