Diffusion of Water Vapor Through Textiles

One factor influencing the suitabilty of fabrics for clothing is resistance to the passage of water vapor. Measurement of the low resistances of woven fabrics requires a different technique from that for the high resistances of waterproofing and packaging materials, since the resistances are close to that of air itself, which may be used as a convenient comparison standard.

The resistance of a woven fabric depends on the kind of fiber, the thickness, and its tightness of weaving. With glass and vinyon, the resistance increases rapidly with density, showing that the main pathway through these materials is the air spaces. With cotton, viscose rayon, and wool, the tightest weaving produces resistances only four to eight times that of air, whereas those of ordinary fabrics range from two to four times that of air. This indicates that significant amounts of water vapor travel through the substance of the fibers themselves, which is confirmed by ex periments on viscose film (cellophane). Nylon and cellulose acetate are intermediate. The effects of resistant fibers are seen when the fraction of the volume occupied by fiber exceeds 40 percent; below this level all fabrics are closely similar, since the low-resistance pathway through the air spaces is sufficiently available.