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

12.5
Processing wool and wool blends on the
cotton spinning system
There are a number of reasons why short staple (cotton) spinning system is
preferred for processing wool blends. Cotton has a much larger share of the
total fibre market than wool. This means that cotton processing machines are
lower in cost, more widely available and more automated. Major cotton
processing machines, including carding and spinning machines, operate at
higher speed than their worsted and woollen counterparts. In addition, the
carded cotton spinning route involves fewer processing steps than the worsted
spinning route. When short wools are spun on the cotton system, the mean
number of fibres in the yarn cross-section can be set to about 60–80 fibres
(Harry and Robinson, 1977a), while 100 to 120 fibres are typically required
for the woollen spinning system.
The wide use of the cotton system for processing wool and wool blends
has been mainly limited by the quantity and price of short wool available.
Short-shorn wools and combing noils are much cheaper than short wool
converted from wool tops. There have been large-scale installations of cotton
processing machines for processing wool blends in China in recent years.
This has raised the demand for short wools. A sector of the Chinese cashmere
dehairing industry started to separate the fine fibre components (called
‘wooldown’) from wools grown on some Chinese traditional breed sheep.
These traditional wools have very broad fibre diameter distributions and
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contain a significant quantity of fine fibres for commercial separation using
low-cost dehairing machinery modified from cotton cards. Wooldown now
makes up a considerable proportion of the short wool supplies in China.
12.5.1 Cleaning and carding of short shorn wool
on cotton card
Owing to the high bulk of wool fibres, it has been found difficult to form all-
wool laps on some old cotton blowroom machinery and to unroll the laps
when being fed to the card. Addition of cotton or synthetic fibres could
improve the situation. In modern cotton spinning mills, the cotton/wool
blend is fed to the carding machine by a direct chute feeding system and the
problems associated with forming and unrolling the lap are avoided. Hopper
feeding to the cotton card, in a similar fashion as in woollen carding, has
been an economical option for many of the short staple facilities in
China.
The blending of scoured wool in the cotton blowroom can be problematic
owing to inadequate cleaning, high fibre breakage and other processing
difficulties. Woollen and worsted cards are installed with sophisticated cleaning
systems to deal with vegetable matter (VM) in scoured wool. The cotton
card, on the other hand, is not designed to deal with the vegetable matter in
wool. Wools with a high VM content produce high waste, which increases
with the wool content in the cotton/wool blend (Spencer and Taylor, 1979).
On the other hand, blends of cotton and low VM wool (such as wool converted
from tops) produced less waste in comparison with processing cotton
alone.
Wool fibre breakage by the cotton card is difficult to accurately quantify
when blends are processed. In one experiment on pure wool top fed to a
cotton card, a mean fibre length reduction of 25% (from 40.1 to 30 mm) was
observed when ignoring the effect of carding waste (Aldrich, 1975). Major
fibre breakage took place at the feed roller/licker-in region, while the main
carding elements (flats and cylinder) caused little fibre breakage. Fibre breakage
was reduced by reducing the speeds and opening settings in the feed roller
and licker-in region. Owing to the lower fibre-to-fibre cohesion, all-wool
webs processed on cotton card were found to sag prior to its entry into the
trumpet to form a sliver. This problem was resolved by installing a web
supporting pan (Louis and Pardo, 1980). The processing performance of
wool and wool blends was improved by some other minor changes on the
cotton card, including a 5–6% reduction in tension draft between the doffer
and the coiler (Harry and Robinson, 1977a), replacing the licker-in wires
with wires that were normally used for processing synthetic fibres, increasing
the flat-cylinder setting and by running the card at a lower production
rate.
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Advances in wool technology
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12.5.2 Square-cut and stretch-broken wool tops
Wool tops are converted into short wools to process on the cotton system for
different reasons, including processing efficiency and product quality.
Converted short wool also provides the option of sliver blending on the
cotton drawframe, avoiding the difficulties of processing blends in blowroom
and carding processes mentioned above.
Cutting of wool tops and card slivers can be carried out using a variety of
highly productive guillotine and rotary cutters. Stretch breaking, on the other
hand, is usually carried out on machines consisting of essentially a roller
drafting system (Eley et al., 1980). Stretch-breaking acts only on fibres
longer than the effective roller ratch while cutting applies to all fibres.
Consequently, the coefficient of variation of fibre length of cut wool is
higher than that of stretch-broken wool.
 
The required level of roller loading
in stretch breaking largely depended on the size of the slivers fed to the
stretch breaker. A minimum draft of 1.3 is required to break wool fibres, but
breaking efficiency increases with increasing draft up to an optimum value
between 2.7 and 3. The resultant mean fibre length (MFL) was approximately
proportional to the setting of the last ratch (MFL = 0.55 
× final ratch).
Lupton (1980) compared 60/40 wool/polyester blends composed of cut-
top and 6-month shorn wools. The mean fibre length of the shorn short wool
(30.2 mm) was appreciably longer than the cut-top (25.9 mm). It was concluded
that fabrics containing cut-top wool in general exhibited higher strength, but
lower resistance to flex abrasion, than their short-shorn wool counterparts.
Other fabric properties such as tear strength and laundering shrinkage were
similar. Short wools produced by stretch-breaking (51 mm setting) and square
cutting (38 mm setting) were compared by Louis and Pardo. (1980). The
stretch-broken top had greater mean fibre length and smaller amount of short
fibres than the cut top. However, after blending with 40% or more cotton, the
difference in wool fibre length did not transfer into any significant differences
in fibre bundle strength and yarn strength.
Stretch-breaking of wool tops has also been carried out in steps using a
combination of modified worsted gilling machines and modified cotton
drawframes (Kim, 1997). The drawn sliver is then ready for roving and
spinning. When woollen-type fibres (such as lambswool and cashmere) are
used, the fibres are carded on a woollen card into a sliver, which is then fed
to a series of modified cotton drawing frames to stretch-break the longer
fibres (Kim, 2002). Three to four passages of drawing were recommended.
The system, known commercially as AFAY (All Fibres for All Yarns), has
been used to produce blends of wool and short staple fibres commercially.
(Anon, 2003).
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12.5.3 Drawing, roving and ring spinning
Slivers of short length wool and wool blend can be processed on cotton
drawframes, speedframes and ringframes without major changes. Wools with
a staple length of 40–45 mm were processed in this way without major
alteration. Roller ratch in the drafting systems can usually be adjusted to
accommodate the relatively long fibres from the wool component. In case
such adjustment is not sufficient, a recess may be cut in the back top roller
of the front drafting zone to allow for slip draft of the longer fibres (Harry
and Robinson, 1977a). Slip drafting was used to process wools with a mean
fibre length up to 53 mm. Wool slivers with a mean fibre length of 65 mm
were processed by removing the top middle roller (Ellis and Robinson,
1979). A thicker apron spacer was used to accommodate the high bulk of
wool at the speedframe and ringframe. A lower twist in roving (twist factor
= 950 – 1100 T/m Tex ) was required for a wool/cotton blend than for pure
cotton (Harry and Robinson, 1977b). Yarn twist levels used on wool/cotton
blends were set according to pure cotton yarns (Harry and Robinson, 1977b).
12.5.4 Open-end rotor spinning
The open-end rotor spinning system offers an even shorter processing route
with higher levels of automation and productivity than the conventional
cotton ring spinning system. The twist insertion rate of rotor spinning is five
to ten times higher than that of the ring frame. Rotor spinning is especially
suitable for producing medium to coarse count carded cotton yarns. The best
known application of rotor spun cotton yarns is found in denim fabrics. The
productivity advantage of rotor spinning reduces when longer fibres are
processed because correspondingly large rotors are required, which limits
the rotor speed. The presence of long fibres can also cause excessive spinning
breaks due to the interference of these fibres with the yarn tail in the rotor
groove. Special long staple rotor spinning machines fitted with large rotors
were designed in the 1970s and used commercially for spinning wool
(Landwehrkap, 1979). Spinning of short wools (lambswool, combing noils,
etc.) using relatively small diameter rotors has been investigated in recent
years. A higher twist factor is required to achieve optimum yarn strength in
rotor spun yarns than in ring spun yarns, and even then, yarn and fabric
strengths, abrasion performance and yarn evenness will be reduced (Louis
and Pardo, 1980; Lehmann and Philippen, 1998; Schmidt, 2002).