FIBRE TESTING

Yarn Fiber Testers

IMPORTANCE OF RAW MATERIAL IN YARN MANUFACTURING:
Raw material represents about 50 to 70% of the production cost of a short-staple yarn. This fact is sufficient to indicate the significance of the raw material for the yarn producer. It is not possible to use a problem-free raw material always, because cotton is a natural fiber and there are many properties which will affect the performance. If all the properties have to be good for the cotton, the raw material would be too expensive. To produce a good yarn with these difficulties, an intimate knowledge of the raw material and its behavior in processing is a must.
Fiber characteristics must be classified according to a certain sequence of importance with respect to the end product and the spinning process. Moreover, such quantified characteristics must also be assessed with reference to the following

1. what is the ideal value?
2. what amount of variation is acceptable in the bale material?
3. what amount of variation is acceptable in the final blend

Such valuable experience, which allows one to determine the most suitable use for the raw material, can only be obtained by means of a long, intensified and direct association with the raw material, the spinning process and the end product.
Low cost yarn manufacture, fulfilling of all quality requirements and a controlled fiber feed with known fiber properties are necessary in order to compete on the world's textile markets. Yarn production begins with the raw material in bales, whereby success or failure is determined by the fiber quality, its price and availability. Successful yarn producers optimize profits by a process oriented selection and mixing of the raw material, followed by optimization of the machine settings, production rates, operating elements, etc. Simultaneously, quality is ensuredby means of a closed loop control system, which requires the application of supervisory system at spinning and spinning preparation, as well as a means of selecting the most suitable bale mix.

BASIC FIBRE CHARACTERISTICS: A textile fiber is a peculiar object. It has not truly fixed length, width, thickness, shape and cross-section. Growth of natural fibers or production factors of manmade fibers are responsible for this situation. An individual fiber, if examined carefully, will be seen to vary in cross-sectional area along it length. This may be the result of variations in growth rate, caused by dietary, metabolic, nutrient-supply, seasonal, weather, or other factors influencing the rate of cell development in natural fibers. Surface characteristics also play some part in increasing the variability of fiber shape. The scales of wool, the twisted arrangement of cotton, the nodes appearing at intervals along the cellulose natural fibers.

Following are the basic characteristics of cotton fiber

1. fiber length
2. fineness
3. strength
4. maturity
5. Rigidity
6. fiber friction
7. structural features

STANDARD ATMOSPHERE FOR TESTING: The atmosphere in which physical tests on textile materials are performed. It has a relative humidity of 65 + 2 per cent and a temperature of 20 + 2° C. In tropical and sub-tropical countries, an alternative standard atmosphere for testing with a relative humidity of 65 + 2 per cent and a temperature of 27 + 2° C may be used.

FIBRE LENGTH: The "length" of cotton fibers is a property of commercial value as the price is generally based on this character. To some extent it is true, as other factors being equal, longer cottons give better spinning performance than shorter ones. But the length of cotton is an indefinite quantity, as the fibers, even in a small random bunch of cotton, vary enormously in length. Following are the various measures of length in use in different countries
mean length
upper quartile
effective length
Modal length
2.5% span length
50% span length
Mean length: It is the estimated quantity which theoretically signifies the arithmetic mean of the length of all the fibers present in a small but representative sample of the cotton. This quantity can be an average according to either number or weight.
Upper quartile length: It is that value of length for which 75% of all the observed values are lower, and 25% higher.
Effective length: It is difficult to give a clear scientific definition. It may be defined as the upper quartile of a numerical length distribution eliminated by an arbitrary construction. The fibers eliminated are shorter than half the effective length.
Modal length: It is the most frequently occurring length of the fibers in the sample and it is related to mean and median for skew distributions, as exhibited by fiber 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 fibers lie.
2.5% Span length: It is defined as the distance spanned by 2.5% of fibers in the specimen being tested when the fibers are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH"
50% Span length: It is defined as the distance spanned by 50% of fibers in the specimen being tested when the fibers are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".
The South India Textile Research Association (SITRA) gives the following empirical relationships to estimate 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

FIBRE LENGTH VARIATION: Even though, the long and short fibers both contribute towards the length irregularity of cotton, the short fibers are particularly responsible for increasing the waste losses, and cause unevenness and reduction in strength in the yarn spun. The relative proportions of short fibers are usually different in cottons having different mean lengths; they may even differ in two cottons having nearly the same mean fiber length, rendering cotton more irregular than the other. It is therefore important that in addition to the fiber length of cotton, the degree of irregularity of its length should also be known. Variability is denoted by any one of the following attributes
1. Co-efficient of variation of length (by weight or number)
2. irregularity percentage
3. Dispersion percentage and percentage of short fibers
4. Uniformity ratio
Uniformity ratio is defined as the ratio of 50% span length to 2.5% span length expressed as a percentage. Several instruments and methods are available for determination of length. Following are some
· Shirley comb sorter
· Baer sorter
· A.N. Stapling apparatus
· Fibro graph
uniformity ration = (50% span length / 2.5% span length) x 100uniformity index = (mean length / upper half mean length) x 100

SHORT FIBRES: The negative effect of the presence of a high proportion of short fibres is well known. A high percentage of short fibers are usually associated with, - Increased yarn irregularity and ends drown which reduce quality and increase processing costs - Increased number of naps and slubs witch is detrimental to the yarn appearance - Higher fly liberation and machine contamination in spinning, weaving and knitting operations. - Higher wastage in combing and other operations. While the detrimental effects of short fibers have been well established, there is still considerable debate on what constitutes a 'short fiber'. In the simplest way, short fibers are defined as those fibers which are less than 12 mm long. Initially, an estimate of the short fibers was made from the staple diagram obtained in the Baer Sorter method
Short fiber content = (UB/OB) x 100
While such a simple definition of short fibers is perhaps adequate for characterizing 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 fibers or its equivalent which does not take into account the effect of short fibers. In this regard, the concept of 'Floating Fiber Index' defined by Hertel (1962) can be considered to be a better parameter to consider the effect of short fibers on spinning performance. Floating fibers are defined as those fibers which are not clamped by either pair of rollers in a drafting zone.
Floating Fiber Index (FFI) was defined as
FFI = ((2.5% span length/mean length)-1)x(100)
The proportion of short fibers has an extremely great impact on yarn quality and production. The proportion of short fibers has increased substantially in recent years due to mechanical picking and hard ginning. In most of the cases the absolute short fiber proportion is specified today as the percentage of fibers shorter than 12mm. Fibro graph is the most widely used instrument in the textile industry

More on: FIBROGRAPH MEASUREMENT: ASTM D1447-89(1994)e1 Standard Test Method