38 Chapter 3
Cellulose will reversibly absorb water vapour as the relative humidity (RH) is
increased and decreased. Figure 4 shows a typical absorption/desorption
curve for cellulose. Note that the two curves do not coincide, the desorption
curve lags relative to the absorption curve; this is called hysteresis.
When water is absorbed, the dimensions of the fibres change. As the gen-
eral orientation of the cellulose molecules is along the fibre, most of the
expansion occurs sideways. The increase in dimensions is proportional to the
water content, thus a graph of fibre diameter versus RH has the same shape
as the graph in Figure 4 and hysteresis is again exhibited.
Individual fibres can expand in width up to 20% when going from 0 to
100% RH but lengthen perhaps about one-tenth of that percentage. The paper
does not expand up to 20% because the effect is attenuated by the micro-
structure of the paper. Typical maximum expansion of paper is about 2–3%.
As most fibres are aligned along the MD, the hygroexpansivity of paper (pro-
duced by RH changes) is mostly in the CD. This may be demonstrated on a
piece of paper; by wetting one side of a small square, the paper will form a
U-shape along the MD as the fibres on the wetted side expand sideways while
those on the dry side do not.
Newly-made paper contains dried-in stress introduced during the drying of
the paper. The stress will be removed if the paper goes to a sufficiently high
RH during storage, when the paper may shrink irreversibly. Old papers do not
appear to have dried-in stress, possibly this has been relieved during repeated
small RH fluctuations over many years.
05101520250 10 20 30 40 50 60 70 80 90 100
Relative Humidity %Water Content %Figure 4How cellulose fibres absorb and desorb water with changing relative humidity;
upper curve shows desorption and lower curve shows absorption