Process for changing the water content of water-absorbing or water-releasing material

摘要

The water content of a material, which tends to either absorb water or to release water, is changed with the aid of a film, which at least partially separates the material from the adjacent space and is impermeable to liquid water but permeable to gaseous water. Polyvinyl alcohol with a particular degree of polymerization and degree of hydrolysis is used as the film material. The film can be mechanically or chemically after-treated. The moistening or de-moistening of the material, which is solid, liquid or gaseous, is effected, utilizing a water vapor pressure gradient, under very mild conditions.