Page 38 - Basic Principles of Textile Coloration

FABRIC MANUFACTURE 27

treated with an aqueous solution or emulsion of the size, and dried. The amounts
of size applied vary from 10–15% solids for cotton (based on the weight of yarn),
to 3–5% for synthetic fibres. Optimised recipes give minimum yarn rupture during
weaving and are often closely guarded secrets. The size must be removed during
preparation for dyeing (Section 2.4), since it interferes with wetting and
penetration of dyes and chemicals into the yarns [4].

The perpendicular warp and weft yarns characterise a woven fabric. Knitted
materials, however, are constructed of interlocking loops of a single yarn, or a
series of parallel yarns. In simple knitted styles such as jersey, each loop, in a series
of loops from a single yarn, passes through a loop of the preceding row. Each loop
is produced by the action of its own needle carrying the yarn. To minimise friction
with the needles and guides, the yarns used in knitting contain a considerable
amount of lubricating oil that must be removed before dyeing. The fabric is
constructed row after row across the width of the material, or around in a circle, to
form what is called a weft knit. When laid flat, circular knitted fabrics have two
layers. The higher the number of loops (wales) and rows (courses) per unit
distance, the greater the weight and rigidity of the material, the better its recovery
after stretching, and the less it is likely to shrink.

Knitting produces fabric much faster than weaving. Knitted materials are flexible,
elastic and relatively crease resistant so that clothing made from them fits well.
Simple weft-knitted fabrics, however, deform easily in all directions and their
dimensional stability is often poor since the yarn is inserted under considerable
tension. The shrinkage observed after wetting can be significant. They often have a
pronounced tendency to curl at the edges. A yarn breakage can cause the
disappearance of a whole series of loops and the formation of a ‘run’. Their handling
during wet processing therefore requires more care than for a woven fabric.

In warp knitting, a series of parallel yarns is fed into the machine from a beam,
similar to a warp beam in weaving. Each yarn passes through its own needle. Warp
knits have vertical columns of loops, but each yarn loops into columns to the left
and right in a zigzag pattern. This gives greater resistance to deformation than for
a simple knit and a material that is more snag resistant. Interlock knits have a
construction intermediate between weft and warp knits.

Besides simple woven and knitted fabrics, there are a large number of other
types that have a pile of surface loops or cut loops. These are manufactured by
knitting or weaving, or by techniques that combine the features of these two
methods. Other types have complex woven or knitted patterns, or may have more
than one warp or filling.