Page 35 - Basic Principles of Textile Coloration

24 FIBRES AND TEXTILES: PROPERTIES AND PROCESSING

drawn even more and twisted. This produces quite fine yarns. The open-end and
friction spinning techniques give much faster rates of production since the yarn is
produced directly from card sliver without intermediate drawing. The bobbins of
yarn can also be much larger since the twist is not inserted by rotation of the take-
up bobbin, as in ring spinning. Open-end and friction spun yarns are courser and
cannot be mixed with ring-spun yarns because of their different structures and
twist characteristics.

The final step in yarn production is winding. The yarn is wound into hanks, or
bobbins of various types, whose size depends on its subsequent use. Winding also
allows an opportunity to detect overly thick or thin sections of the yarn and to
eliminate them, and ensures that all the yarn on the bobbin has the same tension.
Ply yarns are produced at this stage by twisting two or more yarns together, in the
opposite sense to their own twist.

During dyeing, it is imperative that all the yarn in hanks or bobbins has equal
access to the circulating dye liquor. Yarn uniformly wound onto perforated
supports gives packages with either parallel sides (cheeses) or slanting sides
(cones). Their permeability must be uniform throughout. Permeability depends on
the type and twist of the yarn, the type and density of winding, and the degree of
swelling that occurs when the yarns are wet. If packages are too dense, the
pressure required to force dye liquor through them is excessive. Obviously, the
package must not deform during dyeing, and the yarn must be easy to unwind.
Low density or poorly wound packages may become unstable during liquor
circulation, or when the direction of circulation changes, and yarn becomes
detached from the body. For these reasons, the preparation of yarn hanks and
bobbins for dyeing merits particular attention.

The two major characteristics of a yarn are its degree of twist and its thickness
or count. The thickness of a yarn, or of continuous filaments, is expressed as the
length of a given weight of yarn, or vice versa. For example, the denier of a
continuous filament is the weight in grams of 9000 m. A considerable number of
older measures gave the yarn count as the number of hanks, containing a defined
length of yarn, obtained from a given weight of fibre. Different standard lengths
were used for different fibres. For example, a cotton count of 40 corresponds to 40
hanks, each containing 840 yd of yarn produced from 1 lb of cotton fibre. The
standard lengths for wool vary from 100 to 560 yd hank–1 depending on the
region and the spinning system used. This type of count increases as the yarn
becomes finer. Since 1960, the tex system has become increasingly popular. In this,
the count of a yarn or filament is the weight in grams of 1 km of yarn. The tex