INPLANE DIFFUSIVITY OF MOISTURE IN PAPER

Any nonuniformity in local moisture content of paper which develops du ring drying because of locally nonuniform drying rates provides a driv ing force for in-plane diffusion of moisture, which in turn acts to re duce this moisture nonuniformity. As no data have appeared for the in- plane diffusivity of moisture during desorption from paper over the ra nge of conditions existing during papermachine drying, an investigatio n was undertaken to obtain this information. Moisture diffusivity was determined to be a very strong function of the extent and state of wat er in the sheet, increasing exponentially with paper moisture content. The presence or absence of liquid water at the sheet boundary would e ffect moisture diffusivity when there is water in the pores but the di rection of moisture transport in paper was found to be of overriding i mportance. In-plane moisture diffusivity is very much greater than tha t in the thickness direction, indicating that the non-isotropic nature of paper structure is a key factor. A microscale view of the mechanis m of moisture transport in the thickness and inplane directions was de veloped, consistent with the enormous difference in effect of moisture content on diffusivity in the two directions.