The Chemistry of Ageing Processes of Paper and Cellulose

Throughout the ages, paper has always been a privileged vehicle for artistic expression, for the acquisition and dissemination of knowledge and culture; it is the material support for a significant part of humanity's memory. It is also an extremely durable material, since many works on paper have survived for centuries, often reaching us in good condition.

The chemical and physical properties of paper vary according to the raw materials and production methods used in different historical periods. Papermaking technology, which first appeared in China, probably in the 2nd century B.C., evolved as it gradually spread, first to the Middle East from the 8th century onwards, then to the West. However, it was Western papermaking innovations in the 19th century that most radically altered the material's composition and manufacturing methods. The transformation from an artisanal production method to a modern industrial paper mill had a considerable socio-economic and cultural impact, but also had adverse consequences for the material's stability and durability. The two main causes were the replacement by wood of the sources formerly used as raw materials (oriental and middle-eastern annual plants, and later western linen, hemp and cotton rags), and the massive use of rosin-alum sizing replacing traditional sizing, most often based on gelatin or starch. The first industrially-manufactured papers used for printing, from the mid-19th to the mid-20th century, often posed serious conservation problems, unlike contemporary pulps which, depending on their use, can be very durable, the factors influencing the material's lifespan now being better known.

Like all organic materials, paper is subject to aging. Aging is defined as a phenomenon of slow, irreversible change in the structure and/or composition of a material due to its own instability, interaction with the environment (UV irradiation, ionizing radiation, pollution, etc.) or mechanical stress [AM 3150] . Chemical ageing, which will be developed in particular in this article, concerns any ageing mode involving modifications to the chemical structure of cellulose macromolecules.

Thus, the plant source of cellulose and the paper manufacturing process play the main roles in the durability and ageing of the finished product. On the other hand, extrinsic factors such as ambient storage conditions (thermohygrometry, light, air quality), atmospheric pollution or biological contamination and handling also influence...