This test method covers procedures for all models of the Digital Fibrograph, hereafter referred to as Fibrograph.
Fibro graph measurements provide a relatively fast method for determining the length uniformity of the fibers in a sample of cotton in a reproducible manner.
Results of fibro graph length test do not necessarily agree with those obtained by other methods for measuring lengths of cotton fibers because of the effect of fiber crimp and other factors.
Fibro graph tests are more objective than commercial staple length classifications and also provide additional information on fiber length uniformity of cotton fibers. 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.
Fibro graph measurements are based on the assumptions that a fiber is caught on the comb in proportion to its length as compared to total length of all fibers in the sample and that the point of catch for a fiber is at random along its length.
Equipments for fiber testing
FIBRE FINENESS: Fineness Maturity Tester Method ASTM D1447-89(1994)e1Fiber 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 fibers will have a larger number of fibers in its cross-section and hence it will be more even and strong than that spun from the sample with coarser fibers.Fineness denotes the size of the cross-section dimensions of the fiber. AS the cross-sectional features of cotton fibers are irregular, direct determination of the area of crosection is difficult and laborious. The Index of fineness which is more commonly used is the linear density or weight per unit length of the fiber. 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 micrograms per inch and the various air-flow instruments developed for measuring fiber fineness are calibrated in this unit.
Following are some methods of determining fiber fineness.
1- gravimetric or dimensional measurements
2- air-flow method
3- vibrating string method
Some of the above methods are applicable to single fibers while the majority of them deal with a mass of fibers. As there is considerable variation in the linear density from fiber to fiber, even amongst fibers of the same seed, single fiber methods are time-consuming and laborious as a large number of fibers 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 fiber maturity, which is another important characteristic of cotton fibers.
AIR-FLOW METHOD(MICRONAIRE INSTRUMENT): The resistance offered to the flow of air through a plug of fibers is dependent upon the specific surface area of the fibers. Fineness tester has been evolved on this principle for determining fineness of cotton. The specific surface area which determines the flow of air through a cotton plug is dependent not only upon the linear density of the fibers in the sample but also upon their maturity. Hence the micronaire readings have to be treated with caution particularly when testing samples varying widely in maturity.
In the micronaire instrument, a weighed quantity of 3.24 gms of well opened cotton sample is compressed into a cylindrical container of fixed dimensions. Compressed air is forced through the sample, at a definite pressure and the volume-rate of flow of air is measured by a rotometer type flow meter. The sample for Micronaire test should be well opened cleaned and thoroughly mixed( by hand fluffing and opening method). Out of the various air-flow instruments, the Micronaire is robust in construction, easy to operate and presents little difficulty as regards its maintenance.
Fineness, Maturity and Micronaire
The chart shows the relationship between micronaire, fiber fineness, maturity ratio, and theoretical fiber diameter.
Fiber cross-section is assumed to be circular, which makes the fiber diameter lines on the graph approximate. Curves for micronaire level are empirical and therefore contain experimental error.
Fiber fineness (fiber linear density) is expressed in micrograms/in. or in millitex (µg/m). The practical range of fineness for U.S. Upland cotton is about 125-225 millitex.
Micronaire is expressed in dimensionless micronaire units. The practical micronaire range for U.S. Upland cotton is 2.0 – 6.0.
Maturity ratio is a measure of the relative amount of cellulose in the fiber cross-section. Values are dimensionless numbers in the 0.7 – 1.2 range.
FIBRE MATURITY:
Fiber maturity is another important characteristic of cotton and is an index of the extent of development of the fibers. As is the case with other fiber properties, the maturity of cotton fibers varies not only between fibers of different samples but also between fibers of the same seed. The causes for the differences observed in maturity, is due to variations in the degree of the secondary thickening or deposition of cellulose in a fiber.
A cotton fiber consists of a cuticle, a primary layer and secondary layers of cellulose surrounding the lumen or central canal. In the case of mature fibers, the secondary thickening is very high, and in some cases, the lumen is not visible. In the case of immature fibers, due to some physiological causes, the secondary deposition of cellulose has not taken sufficiently and in extreme cases the secondary thickening is practically absent, leaving a wide lumen throughout the fiber. Hence to a cotton breeder, the presence of excessive immaturefibers in a sample would indicate some defect in the plant growth. To a technologist, the presence of excessive percentage of immature fibers in a sample is undesirable as this causes excessive waste losses in processing lowering of the yarn appearance grade due to formation of neaps, uneven dyeing, etc.
An immature fiber will show a lower weight per unit length than a mature fiber of the same cotton, as the former will have less deposition of cellulose inside the fiber. This analogy can be extended in some cases to fibers belonging to different samples of cotton also. Hence it is essential to measure the maturity of a cotton sample in addition to determining its fineness, to check whether the observed fineness is an inherent characteristic or is a result of the maturity.
DIFFERENT METHODS OF TESTING MATURITY:MATURITY RATIO:The fibers after being swollen with 18% caustic soda are examined under the microscope with suitable magnification. The fibers are classified into different maturity groups depending upon the relative dimensions of wall-thickness and lumen. However the procedures followed in different countries for sampling and classification differ in certain respects. The swollen fibers are classed into three groups as follows
1. Normal : rod like fibers with no convolution and no continuous lumen are classed as "normal"
2. Dead : convoluted fibers with wall thickness one-fifth or less of the maximum ribbon width are classed as "Dead"
3. Thin-walled: The intermediate ones are classed as "thin-walled"
A combined index known as maturity ratio is used to express the results.
Maturity ratio = ((Normal - Dead)/200) + 0.70 where, N - %ge of Normal fibersD - %ge of Dead fibers
MATURITY CO-EFFICIENT: Around 100 fibers from Baer sorter combs are spread across the glass slide (maturity slide) and the overlapping fibers are again separated with the help of a teasing needle. The free ends of the fibers are then held in the clamp on the second strip of the maturity slide which is adjustable to keep the fibers stretched to the desired extent. The fibers are then irrigated with 18% caustic soda solution and covered with a suitable slip. The slide is then placed on the microscope and examined. Fibers are classed into the following three categories
1. Mature : (Lumen width "L")/(wall thickness") is less than 1
2. Half mature : (Lumen width "L")/(wall thickness "W") is less than 2 and more than 1
3. Immature : (Lumen width "L")/(wall thickness "W") is more than 2
About four to eight slides are prepared from each sample and examined. The results are presented as percentage of mature, half-mature and immature fibers in a sample. The results are also expressed in terms of "Maturity Coefficient"
Maturity Coefficient = (M + 0.6H + 0.4 I)/100 Where,
M is percentage of Mature fibersH is percentage of Half mature fibersI is percentage of Immature fibers
If maturity coefficient is
less than 0.7, it is called as immature cotton
between 0.7 to 0.9, it is called as medium mature cotton
above 0.9, it is called as mature cotton
AIR FLOW METHOD FOR MEASURING MATURITY:
There are other techniques for measuring maturity using Micronaire instrument. As the fineness value determined by the Micronaire is dependent both on the intrinsic fineness (perimeter of the fiber) and the maturity, it may be assumed that if the intrinsic fineness is constant then the Micronaire value is a measure of the maturity
DYEING METHODS: Mature and immature fibers differ in their behavior towards various dyes. Certain dyes are preferentially taken up by the mature fibers while some dyes are preferentially absorbed by the immature fibers. Based on this observation, a differential dyeing technique was developed in the United States of America for estimating the maturity of cotton. In this technique, the sample is dyed in a bath containing a mixture of two dyes, namely Diphenyl Fast Red 5 BL and Chlorantine Fast Green BLL. The mature fibers take up the red dye preferentially, while the thin walled immature fibers take up the green dye. An estimate of the average of the sample can be visually assessed by the amount of red and green fibers.
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