Effect of Hysteresis on Paper

what is hysteresis?

Hysteresis represent the history dependence of physical systems. If you push on something, it will yield: when you release, does it spring back completely? If it doesn't, it is exhibiting hysteresis, in some broad sense.

same like paper, when the moisture equilibrium achieve, by increasing/decreasing the Rh (Relative Humidity) will follow the different trend.
refer to the graph shown below;
it means the rate of swelling from dry to wet, and wet to dry is different
this is cause by the effect of hysteresis.

for wet to dry, it will follow the upper line swelling rate,
and for dry to wet, lower line is used.

iwk205 - Chemical Additives and Paper Properties
lecturer: Dr Leh