Hygroexpansivity of anisotropic sheets

Dimensional instability, more particularly its component hygroexpansivity, may cause problems in process or end-use situations in which paper or board is in contact with water or subject to changes in ambient relative humidity. Misregistration in printing, curl during copying and calender wrinkles are examples of such defects. In this paper, the in-plane hygroexpansivity of oriented laboratory sheets with different pulps and dried both freely and under restraint is studied. A linear relationship between the drying shrinkage and hygroexpansion coefficient of freely dried laboratory sheets having different fiber orientation anisotropies, was observed. Regardless of both the measurement direction (MD or CD) and the drying options (freely or restraint) all hygroexpansion coefficient values of each pulp type fell quite well on one single power curve as a function of the elastic modulus. Fiber orientation is considered via two different approaches: using fiber orientation anisotropy and using directional variable named as anisotropy index. When the anisotropy index is used, the MD and CD hygroexpansivity or the MD and CD drying shrinkage can be fit on a single curve, while the freely dried and restraint-dried sheets evidently need two different fitting curves. Between the hygroexpansion coefficient and the anisotropy index, a simple power law relationship, with two fitting parameters depending on pulp and drying restraints, is introduced.