The Relationship of Moisture Vapor Transmission To The Structure of Textile Fabrics.

Maintenance of thermal comfort at high activity levels under both hot and cold weather conditions depends on diffusion of Water vapor through textile systems. A maximum of approximately one liter per hour of moisture is available to the average man for evaporative cooling, which is equivalent to a maximum heat loss of 333 kcal/m² hr. However, the amount of heat dissipated in this manner may be limited by clothing, which impedes convective evaporation. Water vapor transfer is thus an important property to be evaluated in military clothing. This paper discusses several methods for measuring the moisture vapor transmission of textile fabrics and proposes mathematical models to predict moisture vapor transmission in terms of simple, readily available fabric geometric properties. One model was based upon a commonly accepted perforated plate theory which considers the fabric as a perforated structure in which moisture vapor diffuses partly through the interyarn spaces and partly through the "non-perforated" areas of the fabric. The other model considered the fabric as a relatively uniform mixture of fiber and air, a valid assumption for most functional, tightly woven military fabrics. In this model, resistance to diffusion was considered as the sum of the air space and "fiber space" resistance over the entire fabric area. This latter model, With a slight adjustment of the film transmission factor, predicted the resistance of a number of cotton and nylon fabrics as measured experimentally. Thus moisture vapor transport capability, and therefore comfort of a fabric at a specified ambient temperature and humidity, can be predicted if the fabric areal density and thickness are known. [ABSTRACT FROM AUTHOR]