Under the microscopic observation, the length of the wool fiber shows a scale structure. The size of the scale varies from very small to comparatively broad and large. As many as 700 scales are found in 1 cm of fine wool, whereas coarse wool may have as few as 275 per cm. Fine wool does not have as clear and distinct scales as coarse wool, but they can be identified under high magnification.
A cross section of wool shows three distinct parts to the fiber. The outer layer, called cuticle, is composed of the scales. These scales are somewhat horny and irregular in shape, and they overlap, with the top pointing towards the tip of the fiber; they are similar to fish scales. The major portion of the fiber is the cortex (composed of cortical cells ); this extends toward the center from the cuticle layer. Cortical cells are long and spindle-shaped and provide fiber strength and elasticity. The cortex accounts for approximately 90 percent of the fiber mass. In the center of the fiber is the medulla. The size of the medulla varies and in fine fibers may be invisible. This is the area through which food reached the fiber during growth, and it contains pigment that gives color to fibers.
Wool fibers vary in length from 3.8 to about 38 cm. Most authorities have determined that fine cools are usually from 3.8 to 12.7 cm; medium wool from 6.4 to 15.2 cm; and long (coarse) cools from 12.7 to 38 cm.
The width of wool also varies considerably. Fine fibers such as Merino have an average width of about 15 to 17 microns; whereas medium wool averages 24 to 34 microns and coarse wool about 40 microns. Some wool fibers are exceptionally stiff and coarse; these are called Kemp and average about 70 microns in diameter.
The wool fiber cross section may be nearly circular, but most wool fibers tend to be slightly elliptical or oval in shape. Wool fibers have a natural crimp, a built-in waviness. The crimp increases the elasticity and elongation properties of the fiber and also aids in yarn manufacturing. It is three-dimensional in character; in other words, it not only moves above and below a central axis but also moves to the right and left of the axis.
There is some luster to wool fibers. Fine and medium wool tends to have more luster than very coarse fibers. Fibers with a high degree of luster are silky in appearance.
The color of the natural wool fiber depends on the breed of sheep. Most wool, after scouring, is a yellowish-white or ivory color. Some fibers may be gray, black, tan or brown.
The tenacity of wool is 1.0 to 1.7 grams per denier when dry; when wet, it drops to 0.7 to 1.5 g/d. Compared with many other fibers, wool is weak, and this weakness restricts the kinds of yarns and fabric constructions that can be used satisfactorily. However, if yarns and fabrics of optimum weight and type are produced, the end-use product will give commendable wear and retain shape and appearance. Fiber properties such as resiliency, elongation, and elastic recovery compensate for the low strength.
Wool has excellent elasticity and extensibility. At standard conditions the fiber will extend between 20 and 40 percent. It may extend more than 70 percent when wet. Recovery is superior. After a 2 percent elongation the fiber has an immediate regain or recovery of 99 percent. Even at 10 percent extension, it has a recovery of well over 50 percent, which is higher than for any other fiber except nylon.
The resiliency of wool is exceptionally good. It will readily spring back into shape after crushing or creasing. However, through the application of heat, moisture and pressure, durable creases or pleats can be put into wool fabrics. This crease or press retention is the result of molecular adjustment and the formation of new cross-linkages in the polymer. Besides resistance to crushing and wrinkling, the excellent resilience of wool fiber gives the fabric its loft, which produces open, porous fabrics with good covering power, or thick, warm fabrics that are light in weight. Wool is very flexible and pliable, so it combines ease of handling and comfort with good shape retention.
The standard moisture regain of wool is 13.6 to 16.0 percent. Under saturation conditions, wool will absorb more than 29 percent of its weight in moisture. This ability to absorb is responsible for the comfort of wool in humid, cold atmospheres. As part of the moisture absorption function, wool produces or liberates heat. However, as wet wool begins to dry, the evaporation causes heat to be absorbed by the fiber, and “chilling” may be experienced, though the chilling factor is slowed down as the evaporation rate is reduced. The property of moisture absorption and desorption peculiar to wool and similar hair fibers is called hygroscopic behavior. Wool accepts colors and finishes easily because of its moisture absorption properties.
Despite the absorption properties of wool, it has an unusual property of exhibiting hydrophobic characteristics. That is, it tends to shed liquid easily and appears not to absorb moisture. The cause is a combination of factors: interfacial surface tension, uniform distribution of pores, and low bulk density. These moisture properties help make wool very desirable for use in a variety of situations where moisture can be a problem to comfort.
Wool fibers are not dimensionally stable. The structure of the fiber contributes to a shrinking and felting reaction during processing, use and care. This is due, in part, to the scale structure of the fiber. When subjected to heat, moisture, and agitation, the scales tend to pull together and move toward the fiber tip. This property is noticeable in yarns and fabrics and is responsible for both felting and relaxation shrinkage.