The In-Plane and Out-of-Plane Hygroexpansional Properties of Paper

Abstract

This paper describes some studies of the effects of drying restraints and sheet density on the in-plane and out-of-plane hygroexpansivity of paper. It is shown that drying restraints in the RTE range, i.e. between 20% and 0% moisture content, greatly influence hygroexpansivity, with in-plane hygroexpansivity being lower the lower the RH to which the paper has been dried under restraint. For sheets of high density, the volume expansivity is not of affected by the drying restraint, and the reduction in in-plane hygroexpansivity is compensated by an increase in out-of-plane hygroexpansivity.

For freely dried sheets both the in-plane and the out-of-plane hygroexpansivity increase with increasing density. The volume hygroexpansivity at various densities is similar to that for wood of the same density. For sheets dried under restraint the density has only a slight influence on the in-plane hygroexpansivity. The out-of-plane hygroexpansivity is higher than for freely dried sheets but includes changes which are probably irreversible, particularly at low densities.