Physical aging is a general process appearing in all amorphous or partially amorphous materials below the glass transition and is a fundamental characteristic of the glassy state. It is shown here that paper is no exception and the same behaviour is observable as in other materials. The aging process represents the slow movement of a glassy molecular network towards equilibrium and displays the ‘universal’ characteristics of strong age-dependence of some properties, scaling behaviour, reversibility, and relaxations which follow a stretched exponential or Kohlrausch law.
Physical aging in paper is readily reversed by moisture sorption, which lends the aging effect particular practical importance. Small-strain mechanical viscoelastic properties such as creep and stress relaxation rates are strongly affected by age; this has very significant implications for the performance of paper webs or structures subject to short or long term endurance loading. A measurable but less important effect is also demonstrated for a very large-strain property, CD Ring Crush. The rate at which moisture is sorbed is strongly age dependent. Attempts to measure the glass-transition temperature in paper, and its dependence on moisture content, by dynamic mechanical thermal analysis are described; the results were inconclusive.