If the length of a fiber is higher than that of another fiber, it is apparent that the longer fiber will have greater surface area than the shorter fiber (Fig. 7.4). This implies that the longer fiber can trap more dead air on its surface (boundary air layer) than the shorter fiber. Hence, a fabric with long fibers will always possess greater thermal insulation than a fabric with short fibers [509,515-517].
Fig. 7.4 Longer and shorter fibers in a fiber mixture.
Crimp is defined as the natural wave formation in a fiber. The crimp is another important property that affects thermal insulation [518,519]. It has been found that a fiber with a highly crimped structure is composed of several loops on its surface, and these loops can trap dead air on the fiber’s surface (Fig. 7.5) . On the other hand, anon/ semi-crimped fiber has no/few loops in its structure. Therefore, a fabric with non/ semi-crimped fibers traps less dead air than a fabric with highly crimped fibers. In turn, a crimped fiber-based fabric possesses better thermal insulation characteristics than a non/semi-crimped fiber-based fabric.
Fig. 7.5 Crimped and non/semi-crimped fibers.