Fibre Maturity
Fiber
maturity:
Fiber maturity (of cotton) is a fiber
characteristic which expresses the relative degree of thickening of the fiber
wall. In other words, it is the measure
of primary and secondary wall thickness. It is usually estimated by several
indirect tests which are often used to find out the proportion of fibers
containing a maturity greater than some selected level. A fiber will be mature
if a high degree of wall thickening took place during cotton growth.
Fiber maturity depends on -
§ Weather.
§ Types of soil.
§ Plant diseases.
§ Pests.
§ Dead fibers etc.
Importance of maturity:
1. Nep formation:
The immature thin walled
fibers are more flexible than thick walled fiber. So they blend and tangle more
easily forming neps. If these neps appear in the dyed cloth they show up as
spechs of lighter shade.
One of the main
troubles caused by the presence of these thin walled immature fibres is
nepping. It is created during processing, starting at the gin. It also occurs
for some natural causes like fragments of seed pods which attached to fiber. Where
rubbing between surfaces occurs e.g. during carding, minute knots of tangled fiber
are caused.
2. Dyeing faults:
Immature fibers cannot
be dyed evenly. If in a fabric there is
yarn of immature fibers, shade variation will occur. The thinner the secondary
cell wall, the lighter the shade will be.
3. Fineness:
The immature fibers
cannot produce fine fabric and yarn. Immaturity decreases the weight of unit
length of fibre and thus reduces fineness.
As a result the accuracy of the test is badly affected.
4. Less yarn strength:
Due to immaturity, yarn strength reduces and
for that breakage of yarn occurs during spinning.
5. Less production:
Ends down are increased due to immaturity. As
a result production is less.
6. Fabric quality:
Immature fibers are less absorbent and less
absorbent and have uneven surface. So they
are uncomfortable in handle weaving.
7. Yarn
hairiness:
Immature fiber leads to yarn hairiness.
8.
Problem in spinning:
The immature fiber,
the fragments of seed pod attached to a fiber, which creates great problem
during spinning.
Immature fibre causes the problems:
1.
Nep
formation.
2.
Dyeing
faults.
3.
Less
fineness.
4.
Less
yarn strength.
5.
Less
production.
6.
Less
quality fabric.
7.
Yarn
hairiness.
8.
Problem
in spinning.
Types of fibres:
1. Normal fibre.
2. Thin
walled fibres
3. Dead
fibres.
1.
Normal fibre:
Normal
fibers with a well-developed cell wall and pronounced convolutions in the raw
state and become rod-like after swelling. It is denoted by N. In normal fibers
no empty spaces are seen longitudinal section.
2. Thin walled fibers:
Thin
walled fibers having the structure and characteristics lying between normal and
dead fibers. It is denoted by ‘T’.
3. Dead fibers:
Dead
fibers appear ribbon like even after swelling. They are extremely immature
fiber. If the cell wall is less than one-fifth of the total width of the fiber
is termed as dead fiber.
Maturity ratio:
Maturity
ratio of a method of numerically expressing the maturity of a sample of cotton
fibre. It is the ratio of actual degree
of wall thickening to a standard degree
of wall thickening.
So, Maturity ratio =
In
other words, MaturitY ratio is the ratio
which expresses the actual fiber weight per centimeter H, in
relation to a standard fibre wt. per centirnetel H'.
So, Maturity ratio =
=
Measurement of the fiber
maturity:
One of
the troubles caused by immature fibres was faulty dyeing. This difference
between the dyeing properties of mature and immature fibres is employed in the
Goldthwaite test to give a visual indication of the maturity of a sample of
cotton. Two dyes are used on the same bath, a red and a green dye, mature
fibres are stained red and immaturity fibres green, the red colour being developed
in the cellulose of the secondary wall. Hence little or no secondary wall thickening
- no red.
Light-scattering methods
The
fibre diameter analyser (FDA) [10,11] system is a non-microscopical method of
measuring fibre diameter which operates by light scattering. In the instrument
the fibres are caused to intersect a circular beam of light in a plane at right
angles to the direction of the beam. As the fibre passes through the beam the
intensity of the scattered light reaches a maximum which is closely
proportional to the projected area of the fibre. Only fibres that completely
cross the beam are recorded so that the scattered light pulse is then
proportional to the fibre diameter. The beam diameter is no greater than 200 ^m
in order to reduce the effect of curvature of the fibres due to crimp. In order
to present the fibres to the beam in the correct manner they are cut into short
snippets 1.8mm long and suspended in isopropanol to give a slurry. This is
circulated through a square section channel 2mm deep, as shown in Fig. 3.8, at
a suitable flow rate and concentration so that they intersect the beam one at a
time. A proportion of the snippets do not fully intersect the beam, and these
are rejected by using a detector which is split into two halves, each operating
on one half of the beam. If a fibre does not fully extend across the beam the
signals from the two detectors are unequal and so the result can be rejected.
The system is capable of measuring 50 fibres per second and can produce a mean
fibre diameter and a diameter distribution.
Fibre fineness:
The
fibre fineness is expressed in wt. per unit length or length
per weight. According to "Textile Institute", the fineness of cotton,
silk and manufactured fibres is usually expressed in terms of average linear density.
A
single fibre has variable cross-section along its length and varies in cross-section shape from fibre to fibre.
To overcome their effect in calculating fineness, sarne index of fineness is derived'.
Mass =Volume x Density
= Cross-section area x length x
density'
For a known length or unit length, Mass a
Cross-sectional area.
For
this suitable fineness Index is taken by measuring the weight of a known length
of fibre is called linear density and this is expressed in terms of weight per unit length. Hence,
fineness can be calculated.
Importance of fineness:
1. Lower irregularity of yarn:
With a greater number of fibres in the cross-section the
basic irregularity is reduced, The finer the fibre the higher the number and
the lower the irregularity. Fine fibre gives
more regular yarn than the coarse fibre.
2. Uniformity of count:
A fine fibre can be spun to
finer than coarse fibre measurement of fineness. In other words the finer the
fibre, the higher the yarn count will be.
3.
Uniformity of strength:
In a given
cross-sectional a reat if a given count
is spun a fine and coarse fibre,
a more and a stronger yarn will result from the fine fibre because of being large no of finer fibre.
4. Uniform of fabric characteristics:
As fine fibre gives rnore
uniform yarn, so it gives good quality fabric with uniform property.
5. Less twist:
The finer the fiber the greater the
total surfaces area available for inter fibre contact and consequently, less
twist is needed to provide the necessary cohesion.
6. Spinning performance:
The
fineness of fibre affects several mechanical properties and therefore
influences the behavior of the fiber during processing.
7.Good appearance:
The finer yarn produced
by finer fiber and appearance becomes good.
8. Less neps:
In finer yarn less neps are
present.
Principal of fiber fineness
measurement:
·
Gravimetric Method
·
Optical method
·
Air flow method
·
Vibroscope method
1. Gravimetric method:
For a given fibre (that
is of a fixed density) its mass is proportional to its cross-sectional area:
Mass of a fibre =
cross-sectional area X length X density
Therefore for a known
length of fibre its mass will be directly related to itscross-sectional area.
This relationship is made use of in the gravimetric definition of fibre
fineness in which the mass of a given length of fibre is used as a measure of
its fineness. This is similar to the system of measuring yarn linear density.
The primary unit is tex (g/lOOOm), but it is also common to use:
Decitex = mass in grams
of 10,000 metres of fibre
Millitex = mass in
milligrams of 1000 metres of fibre
Denier = mass in grams
of 9000 metres of fibre
For
fibres with a circular cross-section such as wool the mass per length can be
converted into an equivalent fibre diameter sometimes known as d(grav.) using
the following equation:
Decitex
= 106= p X A
where
A = cross-sectional area in cm2, p = density in g/cm3
See
Table 3.1 for a list of common fibre densities.
For
a circular fibre then:
Decitex
- 10~2 Xp
where
d = diameter of fibre in micrometres
This
reduces to:
Decitex
= 7.85 X 10~3
XpXd 2
Similarly
for denier:
Denier
= 7.07 X 10~3XpXd 2
2. Fibre fineness by
the airflow method
This is an indirect
method of measuring fibre fineness which is based on the fact that the airflow
at a given pressure difference through a uniformly distributed mass of fibres
is determined by the total surface area of the fibres [4]. The surface area of
a fibre (length X circumference) is pro- portional to its diameter but for a
given weight of sample the number of fibres increases with the fibre fineness
so that the specific surface area (area per unit weight) is inversely
proportional to fibre diameter; Fig. 3.5 shows this diagrammatically. Because
the airflow varies with pressure difference it is the ratio of airflow to
differential pressure that is determined by the fibre diameter. Therefore the
method can be used to measure either the airflow at constant pressure or the
pressure drop at constant airflow. The measurement of airflow at constant pressure
is the more usual form of apparatus with wool.For fibres of approximately
circular cross-section and constant overall density such as unmedullated wool,
the estimate of fineness corresponds to the average fibre diameter as
determined by the projection microscope with a good degree of accuracy.
Fiber Strength:
Stronger fiber gives stronger
yarn. Higher productivity can be achived with less end breakage due to stronger
fiber. It is expressed as gram/tex(GPT).
Strength (GPT)
Rating
Below21 Very
Weak
22-24 Weak
25-27 Medium
28-30 Strong
31 & above Very Strong
Breaking strength;
tensile strength
This is the maximum
tensile force recorded in extending a test piece to breaking point. It is the
figure that is generally referred to as strength. The force at which a specimen
breaks is directly proportional to its cross- sectional area, therefore when
comparing the strengths of different fibres, yarns and fabrics allowances have
to be made for this. The tensile force recorded at the moment of rupture is
sometimes referred to as the tensile strength at break [I]. This figure may be
different from the tensile strength defined above as the elongation of the
specimen may continue after the maximum tensile force has been developed as
shown in Fig. 5.1 so that the tensile strength at break is lower than the
tensile strength.
Stress
Stress is a way of
expressing the force on a material in a way that allows for the effect of the cross-sectional
area of the specimen on the force needed to break it:
Stress
=
In the case of textile
materials the cross-sectional area can only be easily measured in the case of
fibres with circular cross-sections. The cross-sections of yarns and fabrics
contain an unknown amount of space as well as fibres so that in these cases the
cross-sectional area is not clearly defined. Therefore stress is only used in a
limited number of applications involving fibres.
Tenacity:
Tenacity is defined as
the specific stress corresponding with the maximum orce on a force/extension
curve. The nominal denier or tex of the yarnor fibre is the figure used in the
calculation; no allowance is made for any thinning of the specimen as it
elongates.
Stress-strain curve:
The stress-strain curves
derived from the load- elongation curves. The general shape of the curves
remains the same but their relative positions have changed. The superior
strength of the nylon is more clearly seen and the compression between the two-types
of fiber made easier.
Fibre strength
Carrying out strength
tests on fibres is difficult and time consuming. This is because, particularly
with natural fibres, the individual strengths of the fibres vary a great deal
and therefore a large number have to be measured to give statistical
reliability to the result. Furthermore individual fibres are difficult to
handle and grip in the clamps of a strength testing machine, a problem that
increases as the fibres become finer. For these reasons single fibre strength
tests are more often carried out for research purposes and not as routine
industrial quality control tasks. Tests on fibre bundles, which overcome the
problems of fibre handling and number of tests needed for accuracy, are carried
out as part of the normal range of tests on cotton fibres.
Single fibre
strength
Tests on single fibres
can be carried out on a universal tensile tester if a suitably sensitive load
cell is available. Also required are lightweight clamps that are delicate
enough to hold fibres whose diameters may be as low as 10-20 ^m. A problem
encountered when testing high-strength fibres is that of gripping the fibres
tightly enough so that they do not slip without causing jaw breaks due to fibre
damage. If the fibres cannot be gripped directly in the testing machine jaws
they are often cemented into individual cardboard frames which are themselves
then gripped by the jaws. The cardboard frames, shown in Fig. 5.20, have an
opening the size of the gauge length required. When they are loaded into the
tensile tester the sides of the frame are cut away leaving the fibre between
the jaws. The cement used is respon- sible for gripping the fibres, therefore
the samples have to be left for a sufficient time in the frames for the cement
to set.
Yarn strength: skein method:
In this method a long
length of yarn is wound into a hank or skein using a wrap reel as would be used
for linear density measurement, the two loose ends being tied together. The
whole hank is then mounted in a strength testing machine between two smooth
capstans, which may be free to rotate. The hank is subjected to increasing
extension while the force is monitored. When one part of the yarn breaks, the
hank begins to unravel. If the yarn was looped over frictionless pulleys, once
one end broke the yarn would then unwrap completely and the strength per strand
that was measured would be that of the weakest spot. Because of the friction
present in the system the force continues to increase until sufficient strands
have broken for the hank to unravel, the force passing through a maximum value
at some point. This maximum force is known as the hank strength. Because the
fric- tion of the yarn against the pulleys plays a large part in the result, the
mea- sured hank strength can vary according to yarn friction and the particular
machine that it is measured on. Measuring the strength of a hank or skein of
yarn is a method that was used in the early days of textile testing but that is
now being replaced by the single strand method, especially since the
development of automatic strength testing machines. The main advantage of the
hank method is that it tests a long length of yarn in one test. The yarn is
expected to break at the weak spots so giving a more realistic strength value
and also the same hank can be used for measuring the yarn count. The
disadvantage of the test is that it is dependent on the friction between the
yarn and the cap- stans which determines how well the load is spread between
the multiple strands of the hank. This means that the results are specific to a
particular machine and yarn combination. The test is considered satisfactory
for acceptance testing of commercial shipments but not for measurements which
have to be reproducible between laboratories. There is a correlation between
the tenacity of yarn measured by the skein method and that mea- sured by the
single strand method. The value for the skein is always lower than that for the
single strand [1O]. Other drawbacks to the method are that there is no measure
of strength variability and no measure of yarn exten-
sion as the distance
moved by the capstans is determined by yarn extension and hank unravelling. The
British Standard [16] specifies a hank of 100 wraps of Im diameter. This is tested
at such a speed that it breaks within 20 ± 3 s, or alternatively a constant
speed of 300mm/min is allowed. If the yarn is spun on the cotton or worsted
systems 10 skeins should be tested and 20 skeins if the yarn is spun on the
woollen system. The method is not used for continuous filament yarns.
The US standard [17]
has three options for hank size:
1 Eighty, 40 or 20
turns on a 1.5m (1.5yd) reel tested at a speed of 300 mm/min. Twenty or 40
turns are to be used when the machine capacity is not great enough to break a
hank of 80 turns.
2 Fifty turns on a 1.0m
(lyd) reel broken at a speed of 300 mm/min.
3 Fifty turns on a 1.0m
(lyd) reel broken in a time of 20s.
The number of samples
tested is the same as that for the British Standard.
The breaking force per
strand increases slightly as the perimeter of the skein is reduced as would be
expected from a change in gauge length. The breaking strength of a lyd skein is
5% higher than that for a 1.5yd skein.
Factors
affecting yarn strength:
1. Staple length:
Longer staple cotton gives higher strength with synthetics
where much longer staple lengths than
cotton are available, the increase levels off after the optimum length.
2. Fiber Fineness:
Fine fiber gives greater yarn
strength than coarse fibers when spun
into a given size.
3. Fiber strength:
Logically, a strong fiber produces
a stronger yarn than a weak fiber.
4. Twist:
For any single spun yarn there is always a twist that gives maximum
strength. A twist less than or greater than this optimum amount results in a
yarn of lower strength.
5. Evenness:
The greater the uniformity of a
spun yarn, the higher is its strength and the more uneven a yarn, the lower is
its strength.
6. Fiber length
distribution:
Variations in the distribution
of fiber lengths will cause a variation in yarn strength. The greater
percentage of short fibers, the lower the strength of the yarn.
7. Fiber finish:
The type and amount of
chemical finish applied to fibers, particularly the manmade fibers, has a very
definite effect on the strength of the yarn as well as on the processing
characteristics of the staple.
8. Maturity:
If maturity of fiber increases
yarn strength also increases.
Neps:
Entanglements of several fibers are called
neps. If neps are more in the fiber, yarn neps will be more. It is expressed as
neps/gram.
Neps/gram Rating
Below 100 Very low
101-200 Low
201-300 Medium
301-450 High
451 & above Very High
Reference:
Net & Class Lecture
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