FIBRE TESTING - 2
The negative effects of the presence of a high proportion of short fibres is well known. A high percentage of short fibres is usually associated with,
- Increased yarn irregularity and ends dddown which reduce quality and increase processing costs
- Increased number of neps and slubs whiiich is detrimental to the yarn appearance
- Higher fly liberation and machine contttamination in spinning, weaving and knitting operations.
- Higher wastage in combing and other oppperations.
While the detrimental effects of short fibres have been well established, there is still considerable debate on what constitutes a 'short fibre'. In the simplest way, short fibres are defined as those fibres which are less than 12 mm long. Initially, an estimate of the short fibres was made from the staple diagram obtained in the Baer Sorter method
Short fibre content = (UB/OB) x 100
While such a simple definition of short fibres is perhaps
adequate for characterising raw cotton samples, it is too simple
a definition to use with regard to the spinning process. The
setting of all spinning machines is based on either the staple
length of fibres or its equivalent which does not take into
account the effect of short fibres. In this regard, the concept
of 'Floating Fibre Index' defined by Hertel (1962) can be
considered to be a better parameter to consider the effect of
short fibres on spinning performance. Floating fibres are
defined as those fibres which are not clamped by either pair of
rollers in a drafting zone.
Floating Fibre Index (FFI) was defined as
FFI = ((2.5% span length/mean length)-1)x(100)
The proportion of short fibres has an extremely great impact on yarn quality and production. The proportion of short fibres has increased substantially in recent years due to mechanical picking and hard ginning. In most of the cases the absolute short fibre proportion is specified today as the percentage of fibres shorter than 12mm. Fibrograph is the most widely used instrument in the textile industry , some information regarding fibrograph is given below.
Fibrograph measurements provide a relatively fast method for determining the length uniformity of the fibres in a sample of cotton in a reproducible manner.
Results of fibrograph length test do not necessarily agree with those obtained by other methods for measuring lengths of cotton fibres because of the effect of fibre crimp and other factors.
Fibrograph tests are more objective than commercial staple length classifications and also provide additional information on fibre length uniformity of cotoon fibres. The cotton quality information provided by these results is used in research studies and quality surveys, in checking commercial staple length classifications, in assembling bales of cotton into uniform lots, and for other purposes.
Fibrograph measurements are based on the assumptions that a
fibre is caught on the comb in proportion to its length as
compared to toal length of all fibres in the sample and that the
point of catch for a fibre is at random along its length.
Fibre fineness is another important quality characteristic which plays a prominent part in determining the spinning value of cottons. If the same count of yarn is spun from two varieties of cotton, the yarn spun from the variety having finer fibres will have a larger number of fibres in its cross-section and hence it will be more even and strong than that spun from the sample with coarser fibres.
Fineness denotes the size of the cross-section dimensions of the fibre. AS the cross-sectional features of cotton fibres are irregular, direct determination of the area of croo-section is difficult and laborious. The Index of fineness which is more commonly used is the linear density or weight per unit length of the fibre. The unit in which this quantity is expressed varies in different parts of the world. The common unit used by many countries for cotton is microgrammes per inch and the various air-flow instruments developed for measuring fibre fineness are calibrated in this unit.
Following are some methods of determining fibre fineness.
- gravimetric or dimensional measurements
- air-flow method
- vibrating string method
Some of the above methods are applicable to single fibres while the majority of them deal with a mass of fibres. As there is considerable variation in the linear density from fibre to fibre, even amongst fibres of the same seed, single fibre methods are time-consuming and laborious as a large number of fibres have to be tested to get a fairly reliable average value.
It should be pointed out here that most of the fineness determinations are likely to be affected by fibre maturity, which is an another important characteristic of cotton fibres.
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