Because lignin rejects water and resists bonding, it is often undesirable, and its presence in paper is proved to contribute to chemical degradation (Turner 1998, p 21).
Heitner (1993) summarizes the photochemistry of lignin, which is believed to be involved in yellowing. Lignin is branched compound and cab be described as a network of phenyl groups that have oxygen-containing substituents such as "OH and "OCH3 on them, connected by short alkyl or vinyl chains. As lignin absorbs ultraviolet radiation, phenoxy free radicals are formed that are able to abstract a hydrogen atom from other phenyl group. These active groups can cause breaking of bonds and the formation of coloured moieties (Schaeffer 2001, p. 58). So, papers or paper-based collections made from mechanical wood pulp were used for products that were intended to be cheap and short-lived that the long-term durability of it was not a major concern, so it is easy to discover them will become brittle and overall yellowing after a period of time.
Damages Found on Papers or Paper-Based Collections.
Paper or paper-based collections may not show distinct property changes in the initial stages of photochemical degradation. However, it will lose its strength and flexibility, and then the physical integrity of a paper will usually be compromised after a period of time. The appearance of them begins to change, such as discoloration, both overall yellowing or fading. They will gradually alter the uniform and eventually its strength and whiteness will be lost and to tent not to withstand handling and display. In the other words, photochemical damage is less visible, but often more sever and detrimental to collections' structural stability. Damage caused by light especially ultraviolet radiation is cumulative and irreversible and therefore the next chapters will introduce how typical damages will react and influence to paper and paper-based collections.
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