The importance of fibre fineness to spinning and yarn quality

w45…SPINNING AND YARN QUALITY
as shown schematically in Figure 2(c). It is
now clear that the natural variation in the
number of fibres in the yarn cross-section
is now more exaggerated with the ‘thin’
place identified with the vertical line in Figure
2(c) being much more exaggerated. )
Figure 4(a) illustrates the effect of making
the same yarn from cottons with different
fibre linear density/fineness values.
As the cotton becomes coarser, its linear
density/fineness increases, the average
number of fibres in the yarn cross-section
reduces (the peak of the curve in Figure 4
moves to the left). It is important that the
yarn is strong enough at all points along
its length so that it does not break during
spinning or fabric manufacture.
Figure 4(b) plots the relative number
of particularly thin places along the yarn
(each with less than half the average
number of fibres in the yarn cross-section.
It shows that increasing the linear density
(fineness) by a relatively small amount can
have a relatively big effect on the ability
to spin the yarn. For example, increasing
the fibre linear density by only 10 per cent
from 160mtex to 180mtex increases the
number of critical thin spots by a factor of
300 per cent.
The 160 or 180mtex fibre might easily
spin and produce a commercially acceptable
yarn. But the spinner might not be able
to spin the same yarn from the 200mtex
fibre due to the very large number of breakages
and yarn unevenness.
These effects are well known to the
spinner and so he chooses the input fibre
quality with some care. As noted above,
the linear density of synthetic fibres is routinely
available and fibre diameter (micron)
is used by the wool industry.
Spinners carefully use this data to
choose appropriate raw materials for spinning
either synthetic or wool yarns. In the
case of cotton, unfortunately, fibre linear
density or fineness is not available to the
trade, which instead relies on the micronaire
value as a proxy for fibre linear density
(fineness).
The industry discount for high micronaire
is indeed a direct manifestation of
the problems highlighted in the example
Figure 4: Illustration of the effect of fibre linear density
(fineness) on (a) the number of fibres in the yarn cross-section
and (b) the number of thin places in the yarn
Figure 5: The relationship between fibre fineness (linear
density) and micronaire
above of trying to spin a yarn if the fibre
is too coarse.
The limitations of micronaire
As noted in Part 1 of this series (van der
Sluijs et al, 2008) micronaire is a mixture
of both fibre linear density (fineness) and
also fibre maturity. Figure 5 shows the relationship
between these three parameters.
Note that it is not possible to accurately
estimate fibre linear density (fineness) from
the micronaire value alone.
For example as shown in the Figure, a
micronaire value of 4.1 can be obtained
from cottons with fibre linear density values
over the range 160 to 200 mtex for
fibre maturity values in the range 0.95
down to 0.75. While a thorough study of
range of fibre maturity for Australian cotton
has not been undertaken, this is likely
to be a representative range.
Note that the range of fibre fineness values
(ie 160 to 200 mtex) in this example
can give rise to quite different behaviour
in spinning as illustrated in Figure 4. It is
thus not surprising those spinners of fine
yarns who are largely the customers for
Australian cotton adopt a risk management
strategy of discounting high micronaire
cotton.
It would therefore be useful to have a
technique to routinely measure cotton fibre
linear density or fineness directly as part of
the determination of the quality and value
of a cotton. The Cottonscan instrument under
development at CSIRO and supported
financially by the Australian cotton industry
through research funding from the CRDC
is designed to undertake this task.
Conclusion
Fibre linear density (sometimes called fibre
fineness) is a key parameter that can
significantly impact on the spinner’s ability
to manufacture fine yarns suitable for high
value-added fabrics, the desired market for
Australian cotton.
The ambiguity in interpreting the fibre
linear density or fineness of a sample of
cotton from the micronaire value leads to
potential major impacts on spinning efficiency
and yarn quality. So it would be
useful if the industry were able to directly
measure fibre linear density or fineness of
cotton samples.
Each coloured line represents a different fibre maturity ratio (mat) value covering the common commercial
range. The vertical line highlights that a particular micronaire value (in this case 4.1) can represent cottons
with a range of different fibre linear density (fineness) values depending of the fibre maturity
46 — THE AUSTRALIAN COTTONGROWER APRIL–MAY 2008