Page 37 - Basic Principles of Textile Coloration

26 FIBRES AND TEXTILES: PROPERTIES AND PROCESSING

The characteristics of a woven fabric depend on the type of fibres present, the
fineness and twist of the yarns, the number of yarns per centimetre, and the
pattern or weave of the interlaced yarns. The warp yarns, wound side by side on a
beam, are threaded through the loom under tension. They run along the length of
the fabric. During weaving, raising some warp yarns and lowering others creates a
gap for inserting the weft or filling yarn from the side. The positions of the warp
yarns then change ready for the next insertion of filling yarn. The two types of
yarn are thus woven according to a specific pattern. Winding the warp yarns onto
the beam and weft insertion, in the correct colour sequences, is vital when
weaving coloured patterns with dyed yarns.

The newer weaving technologies use small projectiles, rapiers, or water or air
jets to insert the filling between the warp yarns rather than the older, classical
shuttles. They generate much less noise and vibration and increase production
speeds.

A woven fabric exhibits maximum resistance to extension in the warp and weft
directions. The selvages, running along the edges of the fabric, usually have a
more robust and compact woven structure. They stabilise the shape, prevent
unravelling and form an area where the fabric can be gripped during
manufacturing operations.

During weaving, there is considerable abrasion of the warp yarns from their
guided movement through the loom, and from the repeated rapid lifting and
lowering required to separate them for insertion of the filling. To avoid excessive
abrasion and yarn breakage, the warp yarns usually have a greater twist and a
smooth film of size coating their entire length to reinforce them. Sizing chemicals
of various types are used depending upon the kinds of fibres present. Good size
adhesion on cotton, which has a polar hydrophilic surface, requires use of a polar
hydrophilic polymer such as starch. For the hydrophobic surfaces of synthetic
fibres, less polar synthetic polymer sizes are preferred. Sizing mixtures include one
or more film-forming polymers such as starch, carboxymethyl cellulose (cellulose
with some of its hydroxyl groups converted into Cell–O–CH2–CO2Na) or
polyvinyl alcohol (1, in Figure 2.3), as well as wax lubricants, anti-static agents,
preservatives, emulsifying agents, and anti-foaming agents. The yarns are usually

CH2 CH CH2 CH CH2 CH

n

OH OH OH

1

Figure 2.3 Polyvinyl alcohol, used as a film-forming agent in size mixtures