General principles

Terminology/General principles

Bound moisture: Bound moisture in a solid is that liquid which is retained in a manner that exerts a vapor pressure less than that of the free liquid at the given temperature.

Liquid may become bound to the solid—

-         by retention in small capillaries

-         by solution in cells or fiber walls

-         by homogeneous solution throughout the solid

-         by chemical or physical adsorption on solid surfaces.

Unbound moisture: Unbound moisture exerts a vapor pressure equivalent to that of the free liquid. Thus in a non-hygroscopic material, all the liquid is unbound and in a hygroscopic material, the unbound moisture is the liquid in excess of the equilibrium moisture content (EMC), corresponding to saturation humidity.

Free moisture: Free moisture is the liquid in excess of the equilibrium moisture content at a given temperature and humidity. Thus under condition of saturation humidity, the free moisture is the same as the unbound moisture, but in certain circumstances the free moisture may consist of both unbound and bound moisture.