Surface roughening by water: Gloss relaxation processes

A new instrument is used to measure simultaneously the gloss and the moisture content of a sample of paper as it is subjected to changes in relative humidity. Measurements made with five different papers confirm the overall inverse relationship between gloss and moisture content reported in the literature, although there are reversals (gloss increasing with moisture content) which are unexplained. A significant result of this work is that gloss lags behind moisture content after a step change in relative humidity, apparently reflecting the continuing structural rearrangement of the fibres in the bulk of the sheet as moisture diffuses into fibre walls. This explains why, in order to stabilize sheet gloss to moisture effects, it is not enough to stabilize the surface only, as is done in surface treatment processes involving a thermal gradient.