THE CHARACTERIZATION OF HYGROEXPANSIVITY OF PAPER

The hygroexpansion of paper was determined under controlled cyclic humidity conditions using a newly developed hygroexpansimeter. For fine paper samples taken in the machine direction and for handsheets dried under restraint, the relation between hygroexpansion and moisture content was approximately linear and reversible in the lower moisture range. In the higher moisture range, however, the relation was non-linear and showed an irreversible shrinkage after exposure to a relative humidity greater than 65%. The fine paper samples taken in the cross-machine direction and the freely dried handsheets showed an almost linear and reversible response in the whole moisture range tested. An explicit relationship was obtained between hygroexpansion of paper under varying moisture conditions and drying stress on the basis of a viscoelastic constitutive model. The physical significance of the gradient of the curve between hygroexpansion and moisture content is discussed on the basis of the viscoelastic analysis.