O o d h e a d p u b L i s h I n g L i m I t e d

12.6
Wool–filament composite yarns
Continuous filaments or a pre-spun yarn can be spun together with wool or
wool blend to produce a composite yarn. With the availability of increasing
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number of high-performance synthetic filaments, composite yarn spinning
has become a useful vehicle for developing wool blend products that are
both highly functional and comfortable to wear. Composite yarns may fall
into several types of structures, for example wrap yarn, bicomponent yarn,
core yarn, and intermingled composite yarn.
Wrap yarns can be produced on a hollow spindle wrap spinning machine
or by twisting a pre-spun singles yarn and a filament yarn in the opposite
direction. Hollow spindle wrap spinning machines can offer cost advantages
over ring spinning in some coarse count yarns. Owing to the parallel lying of
the wool fibres, wrap yarns generally offer higher bulk and softer handle
than conventional ring spun yarns.
There are several methods to produce wool/filament bicomponent yarns.
The most commonly used method is the SiroFil method, which is a variation
of the SiroSpun yarn system, by replacing one of the two rovings with a
filament yarn. The filament appears as a helix (parallel to the staple fibres)
on the surface of the final yarn. SiroFil provides the opportunity to spin a
fine count, low hairiness and strong yarn for use in lightweight fabrics.
However, the appearance of filament on the yarn surface can be a distraction
for some products. A further development in this direction is the Treotek or
LincSpun yarn produced from two fine filament yarns and a wool roving.
The two filament yarns are fed to the sides of the wool strand at the
front roller nip. Treotek yarns are used in high abrasion resistance and
lightweight upholstery fabrics, especially for passenger airlines. Wool/filament
bicomponent yarns can also be produced by twisting a wool yarn with a
filament yarn.
12.6.1 Core yarns
Core yarns have a layered core-and-sheath structure. The core component
fed to the yarn formation zone can be a monofilament, a multifilament yarn,
a pre-spun staple fibre yarn, or another strand of staple fibres. The sheath or
cover material is usually staple fibres, but it can also be in other formats,
such as filaments and prespun yarns. Core yarns can be produced on a
spinning machine or a twisting machine. Besides the ring spinning system,
friction spinning, rotor spinning and air-jet spinning systems have been used
to produce core yarns, but ring spinning dominates. Core yarns can be made
on most ring spinning machines with relatively minor changes. Two types of
roving feeding can be adopted, the conventional single roving feeding and
the SiroSpun type of double-roving feeding. Besides the ratio of surface
fibres used, the correct placement of the core filament relative to the rovings
at the front roller nip, the tension applied to the core filament, and the yarn
tension in the spinning zone are important conditions that influence how
well the core is covered by the staple fibres in the resultant core yarn.
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Although core yarns can be produced relatively easily, perfect coverage
of the core component by the surface fibres is not always achieved, especially
when a relatively heavy core component is used. In a poorly covered core
yarn, the bare core is exposed on the yarn surface, leading to an undesirable
barber’s pole effect and poorer product performance. The level of yarn coverage
can be assessed subjectively or objectively by using appropriate image analysis
techniques. Maximum visibility of core exposure is obtained when the core
and surface fibres are in highly contrast colours. Commercial flatbed scanners
and image analysis techniques have been used to quantify the degree of core
yarn coverage (Sawhney et al., 1992; Miao and Barnes, 2006).
Even when complete yarn coverage is achieved, the cohesion between the
covering fibres and the core material may still be low, and the surface fibres
can slip along the core when the yarn is subjected to rubbing. This problem
is caused by the lack of penetration between the surface fibre layer and the
filament core. Different types of yarn abrasion tests could be adapted to test
core yarn resistance to surface fibre stripping (Miao et al., 1996). The surface
fibre stripping resistance was found to improve with the level of twist inserted
(Jou et al., 1996) and the level of pre-tension applied to the core filament
(Miao et al., 1996). The use of a filament yarn pre-twisted in the opposite
direction also brought improvement to core yarn strip-back resistance in
friction spun core yarns (Miao et al., 1996).
12.6.2 Elastane core yarns
Stretch fabrics can be made from either two separate yarns (an inelastic yarn
and an elastic yarn), or from a composite yarn consisting of an elastic component
and an inelastic component. The composite yarn route is suitable for both
knitting and weaving, and it is the more convenient route for fabric
manufacturers. Several types of wool/elastane composite yarns can be made,
including twisted yarns produced from an elastane
 
yarn and a wool spun
yarn, wrap yarns produced using a hollow spindle wrap spinner, and core
spun yarns produced from wool rovings and an elastane yarn on a worsted
spinning frame. The core yarn type is preferred for its soft, smooth fabric
handle and uniform fabric appearance that it produces.
In the spinning of elastic yarns, a positively driven creel is commonly
used to maintain a constant feeding rate of the elastane core. The draw ratio
between the positive feed roller and the front roller is normally between two
and five and is decided according to the fabric stretch required. A roller
with a V-shaped groove above the front top roller is commonly used as a
guide to the elastane core. Su et al. (2004) found that increasing the
wrap angle of the elastic core on the top front roller could improve yarn
coverage.
Special attention is required in the winding and splicing of elastane
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core yarns. In the process of preparing yarn ends for splicing, the elastic
core tends to retract from the cut ends. Longer joint length and higher splicing
air pressure could be used to achieve a satisfactory result (Anon, 2005b).
Winding of an elastane core yarn on a standard winding machine can cause
the inner layers on the package flanks to bulge out due to pressure created
by the yarn tension in the outer layers. Special winding and splicing
machines that avoid these problems are now available (Schlafhorst & Co.,
2002a,b).
12.6.3 Intermingling filaments with staple fibres
Core yarns are often found to possess low stripping resistance because of the
lack of penetration between the filaments and the surface staple fibres. One
solution to this problem is to intermingle the filaments and the staple fibres
before twisting them together to form a composite yarn. Such intermingling
can be achieved by spreading the filaments across the width of the staple
fibre strand at the front roller nip of a ring spinning machine. Filament yarns
can be spread mechanically (Hicks, 1958), electrostatically (Uchiyama
et al., 1972; Jou et al., 1996) and aerodynamically (Arnold et al., 2004). Up
to 12 mm filament spread width could be achieved using an electrostatic
spreader (Wu and Lee, 1995), but 6 mm was optimum, giving yarns with
higher resistance to stripping back and lower surface hairiness than regular
core yarns, but similar tensile properties.