FIBRE TESTING
FIBRE ELONGATION:
There are three types of elongation
- Permanent elongation: the length which extended during loading did not
recover during relaxation
- Elastic elongation:The extensions through which the fibres does return
- Breaking elongation:the maximum extension at which the yarn breaks
i.e.permanent and elastic elongation together Elongation is specified as a percentage of
the starting length. The elastic elongation is of deceisive importance, since textile
products without elasticity would hardly be usable. They must be able to deforme, In order
to withstand high loading, but they must also return to shatpe. The greater resistance to
crease
for wool compared to cotton arises, from the difference in their elongation. For cotton it
is 6 -10% and for wool it is aroun 25 - 45%. For normal textile goods, higher elongation
are neither necessary nor desirable. They make processing in the spinning mill more
difficult, especially in drawing operations.
FIBRE RIGIDITY:
The Torsional rigidity of a fibre may be defined as the torque or
twisting force required to twist 1 cm length of the fibre through 360 degrees and is
proportional to the product of the modulus of rigidity and square of the area of
cross-section, the constant of proportionality being dependent upon the shape of the
cross-section of the fibre. The torsional rigidity of cotton has therefore been found to
be very much dependent upon the gravimetric fineness of the fibres. As the rigidity of
fibres is sensitive to the relative humidity of the surrounding atmosphere, it is
essential that the tests are carried out in a conditional room where the relative
humidity is kept constant.
THE SLENDERNESS RATIO:
Fibre stiffness plays a significant role mainly when rolling,
revolving, twisting movements are involved. A fibre which is too stiff has difficulty
adapting to the movements. It is difficult to get bound into the yarn, which results in
higher hairiness. Fibres which are not stiff enough have too little springiness. They do
not return to shape after deformation. They have no longitudinal resistance. In most cases
this leads to formation of neps. Fibre stiffness is dependent upon fibre substance and
also upon the relationship between fibre length and fibre fineness. Fibres having the same
structure will be stiffer, the shorter they are. The slendernesss ratio can serve as a
measure of stiffness,
slender ratio = fibre length /fibre diameter
Since the fibres must wind as they are bound-in during yarn formation in
the ring spinning machine, the slenderness ratio also determines to some extent where the
fibres will finish up.fine and/or long fibres in the middle coarse and/or short fibres at
the yarn periphery.
TRASH CONTENT:
In additon to usuable fibres, cotton stock contains
foreign matter of various kinds. This foreign material can lead to extreme disturbances
during processing. Trash affects yarn and fabric quality. Cottons with two different trash
contents should not be mixed together, as it will lead to processing difficulties.
Optimising process paramters will be of great difficulty under this situation, therefore
it is a must to know the amount of trash and the type of trash before deciding the mixing.
SHIRLEY TRASH ANLAYSER:
A popular trash measuring device is the Shirley Analyser, which
separates trash and foreign matter from lint by mechanical methods. The result is an
expression of trash as a percentage of the combined weight of trash and lint of a sample.
This instrument is used
- to give the exact value of waste figures and also the proportion of clean
cotton and trash in the material
- to select the proper processing sequence based upon the trash content
- to assess the cleaning efficiency of each machine
- to determine the loss of good fibre in the sequence of opening and
cleaning.
Stricter sliver quality requirements led to the gradual evolution of
opening and cleaning machinery leading to a situation where blow room and carding
machinery were designed to remove exclusively certain specific types of trash particles.
This necessitated the segregation of the trash in the cotton sample to different grades
determined by their size. This was achieved in the instruments like the Trash Separator
and the Micro Dust Trash Analyser which could be considered as modified versions of the
Shirley Analyser.
The high volume instruments introduced the concept of optical methods of
trash measurement which utilised video scanning trash-meters to identify areas darker than
normal on a cotton sample surface. Here, the trash content was expressed as the percentage
area covered by the trash particles. However in such methods, comparability with the
conventional method could not be established in view of the non-uniform distribution of
trash in a given cotton sample and the relatively smaller sample size to determine such a
parameter. Consequently, it is yet to establish any significant name in the industry.
RAW MATERIAL AS A FACTOR AFFECTING SPINNING:
Fineness determines how many fibres are present in the cross-section
of a yarn of particular linear density. 30 to 50 fibres are needed minimum to produce a
yarn fibre fineness influences
- spinning limit
- yarn strength
- yarn evenness
- yarn fullness
- drape of the fabric
- lustre
- handle
- productivity
productivity is influenced by the end breakage rate and twist per inch
required in the yarn
Immature fibres(unripe fibres) have neither adequate strength nor
adequate longitudinal siffness. They therefore lead to the following,
- loss of yarn strength
- neppiness
- high proportion of short fibres
- varying dyeability
- processing difficulties at the card and blowroom
Fibre length is one among the most important characteristics. It
influnces
- spinning limit
- yarn strength
- handle of the product
- lustre of the product
- yarn hairiness
- productivity
It can be assumed that fibres of under 4 - 5 mm will be lost in
processing(as waste and fly). fibres upto about 12 - 15 mm do not contribute to strength
but only to fullness of the yarn. But fibres above these lengths produce the other
positive characteristics in the yarn.
The proportion of short fibres has extremely great influence on the
following parameters
- spinning limit
- yarn strength
- handle of the product
- lustre of the product
- yarn hairiness
- productivity
A large proportion of short fibre leads to strong fly contamination, strain on
personnel, on the machines, on the work room and on the air-conditioning, and also to
extreme drafting difficulties.
A uniform yarn would have the same no of fibres in the cross-section, at
all points along it. If the fibres themeselves have variations within themselves, then the
yarn will be more irregular.
If 2.5% span length of the fibre increases, the yarn strength also
icreases due to the fact that
there is a greater contribution by the fibre strength for the yarn strength in the case of
longer fibres.
Neps are small entanglements or knots of fibres. There are two types of
neps. They are 1.fibre neps and 2.seed-coat neps.In general fibre neps predominate, the
core of the nep consists of unripe and dead fibres. Thus it is clear that there is a
relationship between neppiness and maturity index. Neppiness is also dependent on the
fibre fineness, because fine fibres have less longitudinal stiffness than coarser fibres.
Nature produces countless fibres, most of which are not usable for
textiles because of inadequate strength.
The minimum strength for a textile fibre is approximately 6gms/tex (
about 6 kn breaking length).
Since blending of the fibres into the yarn is achieved mainly by
twisting, and can exploit 30 to 70% of the strength of the material, a lower limit of
about 3 gms/tex is finally obtained for the yarn strength, which varies linearly with the
fibre strength.
Low micronaire value of cotton results in higher yarn tenacity.In
coarser counts the influence of micronaire to increase yarn tenacity is not as significant
as fine count.
Fibre strength is moisture dependent. i.e. It depends strongly upon the
climatic conditions and upon the time of exposure. Strength of cotton,linen etc. increases
with increasing moisture content.
The most important property inflencing yarn elongation is fibre
elongation.Fibre strength ranks seconds in importance as a contributor to yarn elongation.
Fibre fineness influences yarn elongation only after fibre elongation and strength. Other
characters such as span length, uniformity ratio, maturity etc, do not contribute
significantly to the yarn elongation.Yarn elongation increases with increasing twist.
Coarser yarn has higher elongation than finer yarn. Yarn elongation decreases with
increasing spinning tension. Yarn elongation is also influenced
by traveller weight and high variation in twist insertion.
For ring yarns the number of thin places increases, as the trash content
and uniformity ratio increased For rotor yarns 50%span length and bundle strength has an
influence on thin places.
Thick places in ringyarn is mainly affected by 50%span length, trash
content and shor fibre content.
The following expression helps to obtain the yarn CSP achievable at
optimum twist multiplier with the available fibre properties.
Lea CSP for Karded count = 280 x SQRT(FQI) +
700 - 13C
Lea CSP for combed count = (280 x SQRT(FQI) + 700 - 13C)x(1+W)/100
where,
FQI = LSM/F
L = 50% span length(mm)
S = bundle strength (g/tex)
M = Maturity ratio measured by shirly FMT
F = Fibre fineness (micrograms/inch)
C = yarn count
W = comber waste%
Higher FQI values are associated with higher yarn strength in the case
of carded counts but in combed count such a relationship is not noticed due to the effect
of combing
Higher 2.5 % span length, uniformity ratio, maturity ratio and lower
trash content results in lower imperfection. FQI does not show any significant influence
on the imperfection.
The unevenness of carded hosiery yarn does not show any significant
relationships with any of the fibre properties except the micronaire value. As the
micronaire value increases, U% also increases. Increase in FQI however shows a reduction
in U%.
Honey-dew is the best known sticky substance on cotton fibres. This is a
secretion of the cotton louse. There are other types of sticky substances also. They are
given below.
- honey dew - secretions
- fungus and bacteria - decomposition products
- vegetable substances - sugars from plant juices, leaf nectar,
overprodcution of wax,
- fats, oils - seed oil from ginning
- pathogens
- synthetic substances - defoliants, insecticides, fertilizers, oil from
harvesting machines
In the great majority of cases, the substance is one of a group of
sugars of the most variable composition, primarily but not exclusively, fructose, glucose,
saccharose, melezitose, as found, for example on sudan cotton. These saccharides are
mostly, but not always, prodced by insects or the plants themselves, depending upon the
influence on the plants prior to plucking. Whether or not a fibre will stick depends, not
only on the quantity of the sticky coating and it composition, but also on the degree of
saturation as a solution. Sugars are broken down by fermentation and by microorganisms
during storage of the cotton. This occurs more quickly the higher the moisture content.
During spinning of sticky cotton, the R.H.% of the air in the production are should be
held as low as possible.
The following table shows the degree of
correlation between the various cotton fibre quality characteristics and those of the
yarns into which these fibres are spun - RING
SPUN YARNS
|
yarn evenness |
imperfection and classimat faults |
breaking tenacity |
breaking elongation |
hairiness |
| fibre length |
|
|
|
|
|
| micronaire value |
|
|
|
|
|
| nep, trash, leaf, microdust, fibre fragments |
|
|
|
|
|
| 1/8" breaking strength |
|
|
|
|
|
| 1/8" elongation |
|
|
|
|
|
| colot/reflectance |
|
|
|
|
|
significant correlation
good correlation
little or no correlation
The following
table shows the degree of correlation between the various cotton fibre quality
characteristics and those of the yarns into which these fibres are spun - ROTOR SPUUN YARNS.
| |
yarn evenness |
imperfections and classimat faults |
breaking tenacity |
breaking elongation |
hairiness |
| fibre length |
|
|
|
|
|
| micronaire value |
|
|
|
|
|
| nep, leaf, trash,microdust, fibre fragments |
|
|
|
|
|
| 1/8" breaking strength |
|
|
|
|
|
| 1/8" breaking elongation |
|
|
|
|
|
| color/ reflectance |
|
|
|
|
|
significant correlation
good correlation
little or no correlation
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