Page 143 - Handbook of Properties of Textile and Technical Fibres

Physical, chemical, and tensile properties of cashmere, mohair, alpaca 121

It is obvious from the above reports that considerable variation exists in the surface
morphology of rare animal fibers. However, there is a general, if qualitative, accep-
tance that these fibers have flatter cuticle cell profiles, resulting in a smoother fiber sur-
face. These differences in physical structure of the surface of rare animal fibers,
compared with wool, will be at least partially responsible for the greater difficulty
in processing mohair and cashmere, and will also contribute to desirable aesthetic ad-
vantages such as softness and lustre and different friction properties.
As nutrition affects cashmere cuticle properties, it is not surprising that the fiber
friction index (Onions, 1962) indicated differences in friction between cashmere
from different nutrition treatments (McGregor and Liu, 2017). The lowest friction in-
dex of 13.0 was obtained with the most productive nutrition treatment, compared with
the least productive nutrition treatments, which had the highest friction index of 21.0
and 23.0.

4.2.2.6 Lustre

Fibers with high lustre reflect a high proportion of light that is exposed to the fiber sur-
face. Lustre has been studied particularly for mohair and mink. The main explanation
given for the high lustre of mohair is the relatively large surface cuticle scales and the
low-scale edge height relative to other animal fibers, which results in a lustre peak
reflection (Hunter, 1993). However, in wool Orwin and Woods (1983) determined
that increased within-fiber diameter variability and increased ellipticity of fibers
reduced lustre with lesser reductions in lustre related to increased crimp frequency, fi-
ber alignment, and cuticle scale edge frequency.
Lustre can be measured objectively by visible light reflectance at a range of obser-
vation angles although methods differ (Hunter, 1993). Vassis et al. (2003) evaluated
two methods of measuring reflection from wool, mohair, and cashmere. They found
the reflections from mohair and cashmere to be complex. Lustre of mink, based on
pelts, has been examined in detail (Rasmussen, 2001), and there may be methods
developed for mink pelts that have application with rare animal fibers. New methods
of measuring lustre in Suri alpaca have been reported (Lupton and McColl, 2011).

4.2.2.7 Regain
The surface of animal fibers repels water owing to the surface fatty layer on the cuticle
cells. However, the proteins in keratin fibers attract water. The regain of the fiber is the
mass of water absorbed/mass of dry fiber. For animal fibers, the regain is generally

15%e18% under standard laboratory conditions (20 C and 65% relative humidity).
Mohair takes up to 32% by mass of water in going from completely dry to completely
wet (Fig. 4.4, lower curve). The regain of mohair and other fibers follow a curvilinear
response to relative humidity, and there is hysteresis, whereby the moisture content is
higher at a given relative humidity when the fiber is drying out (desorption, upper
curve Fig. 4.4) compared with when the fiber is becoming wetter (adsorption).
Regain affects many fiber attributes including measurement of fiber diameter.
When wool absorbs water it swells and the swelling is entirely radial with little in-
crease in length (Edmunds, 1992). The cross-sectional area of wool fibers increases
about 30% as relative humidity increases from 0% to 100%. Increasing water content