Wool shrinkage control and surface modification by ozone

In treating fine wool top, more ozone is needed to achieve a given level of felting shrinkage control than with coarse wool. Treatment that allows no more than about 20% top shrinkage in three successive washes is usually satisfactory for controlling fabric shrinkage. A detergent rinse after treatment markedly decreases top and fabric shrinkage. Ozonization decreases fiber strength and wet friction against glass but increases fiber-to-fiber adhesion enough to increase yarn strength. Scanning electron microscopy (SEM) of fibers before and after treatment and laundering indicates that ozone makes the fiber surface more plastic (presumably when wet) but more liable to mechanical damage and erosion, which increase with the severity of treatment and with laundering. However, treatments most effective for shrinkage control are not distinguished from less effective treatments by the degree and character of surface changes shown by SEM. Ring dyeing of fiber cross sections shows that effects of ozone are greatest near the surface. X-ray photoelectron spectroscopy (XPS) and amino acid analysis show that cystine is oxidized to cysteic acid and that carbon is also oxidized at the surface. The increased dyeability (rate and uptake) is attributed to increased wettability, changed ionic balance, and crosslink scission, which, with cuticle damage, allow greater swelling and easier penetration. As found earlier with coarse wools, acid milling (leveling) dyestuffs further decrease shrinkage of treated fine wool. The position of wool in the triboelectric series is shifted toward the negative end by ozone treatment, expressing the higher electron work function of the added sulfonic acid groups. This shift is not simply related to shrinkage. Decreased feltability is attributed to softening of the wet cuticle layer and to increased surface charge, but the mechanism needs further study.