Dimensional Stability of Woven Wool Fabrics: Hygral Expansion

Abstract

In worsted fabrics, one factor contributing to lack of dimensional stability is the reversible change of dimensions that takes place with regain changes, here termed hygral expansion. The hygral expansion of ten worsted fabrics is studied, and it is shown that the yarn and fabric behavior can be explained in terms of the bending and straightening of single fiber arcs. The observed change of arc radius from dry to wet is about 50%, and when this figure is applied to the geometry of the weave, quantitative predictions of the hygral expansion of the yarn can be satisfactorily made. Such a large change of curvature in single fibers cannot arise from radial swelling alone, and it is suggested that with regain changes, the fibers behave in a way that is analogous to the behavior of bimetal strips with temperature changes. Such a mechanism is possible because of the bilateral structure of the fibers, with different longitudinal swelling of the two segments. Since the fibers are preferentially oriented in the fabric, this mechanism can play a dominant role in yarn and fabric behavior. The hygral expansion increases with increased setting, probably because the yarns no longer tend to revert to their straight form at high regain. Correspondingly, there is a reduction in the reaction forces between yarns at the points of intersection; the higher these reaction forces are, the greater the restriction that is put upon hygral expansion. Some applications of hygral expansion—to measurements of relaxation shrinkage and assessment and effectiveness of mill machines—are discussed.