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Sunday, August 4, 2019

COTTON FIBRE LENGTH EVALUATION

COTTON FIBRE LENGTH EVALUATION:


The " Fibre length" is a self-explanatory word.  It clearly tells us how long is the fibre. It is measured in inches or millimetres. It is one of the most important characteristics of cotton fibres. 

Now we come on to the topic "cotton fibre length. A cotton lint consists of many fibres.  These fibres have different- different length individually.  We can say many groups of lengths lie in the cotton lint.  Since the cotton lint has many groups of fibre length, it gets very difficult to find the exact fibre length of any cotton variety.  The fibre length is the most important and critical characteristic of cotton fibre. It plays a decisive role to determine the cost of any cotton variety. The fibre length directly influences many yarn parameters like the tensile strength, elongation,  evenness and hairiness of the yarn to be spun. The spinner chooses the correct fibre length to achieve desired yarn parameters to be spun. These yarn parameters directly affect the fabric to be woven. 

It is very difficult to determine the exact value of the fibre length of cotton by using any apparatus. There are many terms used to express the fibre length of the cotton.
Some of the popular terms expressing the fibre length of cotton are given below:

Mean length.

Upper quartile.

Effective length.

Modal length.

2.5% span length.

50% span length.

Mean length:

The mean length of cotton fibre is expressed in millimetres, centimetres or inches. " The arithmetic mean( average)  of the length of all the fibres present in a representative sample of cotton lint is called the mean length of a cotton fibre”.  This quantity can be an average according to either number or weight.

Upper quartile length:

It is that value of fibre length for which 3/4 of all the observed values in the whole representative sample are lower, and 1/4 of all the observed value in the whole representative sample are higher.

Effective length:

It is very difficult to define effective length technically. “ The upper quartile of a numerical length which is determined by making an arbitrary construction of a representative simple. This upper quartile length is called the effective length of fibres”. In this arbitrary construction, the fibres eliminated are shorter than half of the effective length.

Modal length:

It is the most regularly appearing length of the fibres in the representative sample. It is related to mean and median for curve distributions, as exhibited by fibre length, in the following way.

(Mode-Mean) = 3(Median-Mean)
where,
Median is the particular value of length above and below which exactly 50% of the fibres lie.

COTTON SAMPLE ANALYSIS BY SHIRLEY COMB SORTER:

The complete sample analysis procedure can be divided into the following steps:

Sample preparation:

The sample preparation is a very important task during the evaluation of fibre length. The representative sample is divided into 4 parts.  Sixteen small tufts are taken from each corner at random of 20 mg. in size. Now all the tufts are reduced to half four times. These tufts are discarded,  the fibres are parallelized and straightened alternatively with the help of left and right hand. Thus sixteen tufts from each corner are obtained. Now, these sixteen tufts are combined together. Each set of sixteen tufts is combined separately.  Thus four new tufts are obtained. Each new tuft is divided into four parts again. One tuft from each group is combined together.  Thus sixteen tufts are converted into four new tufts.  A quarter of each new tufts is taken and combined together.  Now we get the final tufts of cotton fibres for testing. 20 mg.  of cotton is taken for testing from final tufts. This method of sample preparation is known as the zoning technique. 

PREPARATION OF FIBRES ARRAY:

The representative sample of cotton is parallelized and the fibres are straightened by right and left hand alternatively. Many groups of fibre length are prepared.  If we sort the fibre length groups manually,  the sorting process gets very lengthy.  A Shirley comb sorter apparatus is used to find the groups of various fibre length. These groups of fibre length are arranged on a velvet pad in descending order of fibre length.  The one end of the fibre length group is arranged in such a manner that this end touches the baseline. All the groups of fibre length are arranged accurately on the baseline. Extra care is taken during arranging the groups having very short fibre length. Finally, we get the required fibres array of a representative sample as illustrated in the below figure:



       NL'  = (1/4) LL'

ANALYSIS OF ARRAY OF FIBRES:

The above fibre's array is analysed as follows:
OA = Maximum fibre length
LL' = effective length of fibre










CONCEPT OF SPAN LENGTH OF COTTON FIBRE:

2.5% Span length:

This parameter is determined with the help of a digital fibrograph.  The fibres of a representative sample are made parallel to each other.  They are straightened up to the optimum level.  The fibres in the specimen are distributed randomly. The initial scanning point is assumed as 100 ℅ fibres. Now the scanning is conducted at a place where only 2.5 ℅ fibres of the whole representative sample are present.  Now the distance between the initial point ( 100 ℅ fibres) and 2.5 ℅ fibres are recorded.  This distance is termed as the 2.5 ℅ span length of the fibres.

50 ℅ span length:

This parameter is determined with the help of a digital fibrograph.  The fibres of a representative sample are made parallel to each other.  They are straightened up to the optimum level.  The fibres in the specimen are distributed randomly. The initial scanning point is assumed as 100 ℅ fibres. Now the scanning is conducted at a place where only 50 ℅ fibres of the whole representative sample are present.  Now the distance between the initial scanning point ( 100 ℅ fibres) and 50 ℅ fibres are recorded.  This distance is termed as 50 ℅ span length of the fibres.
Please see the below graphic to understand the concept of the span length of the fibre.:






The South India Textile Research Association (SITRA) find the following equation based on observation and experience to find the Effective Length and Mean Length from the Span Lengths.
Effective length = (1.013 x 2.5% Span length) + 4.39
Mean length = ( 1.242 x 50% Span length )+ 9.78

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