During production, converting and usage, paper and board products may be exposed to environmental conditions of both constant and variable nature. As these paper and board products are ultimately composites of the natural polymers cellulose, hemicelluloses and lignin, they are, due to their hydrophilic nature and also to the existence of thermal transitions, highly influenced by the surrounding environment. The paper properties are accordingly affected by moisture and temperature and it is these changes of a physical nature that are discussed in this paper.
In this paper, an attempt is made to present a philosophy of how the total performance of paper products can be reduced to the question of how moisture and temperature interact with wood polymers on a molecular level. The multicomponent nature of the wood fibre and the consequences of the build up of dried-in stresses are particularly emphasised. Thus the sorptive properties of the wood polymers are described from the standpoint of molecular interaction and a plasticizing effect which reduces the glass transition temperatures of the wood polymers. The consequences that this softening have on mechanical properties and on hygroexpansivity are presented together with descriptions of the effect of drying stresses and the creep behaviour during moisture cycling.
Practical examples are given, discussing high temperature processes such as hot calendering, corrugating and press drying. Effects related to printing and converting operations, such as dimensional stability, surface roughening and linting are also commented on. Finally some ideas regarding future research are presented.