What is Micronaire Value of Cotton | Relation between Micronaire, Fineness, and Maturity

Last Updated on 20/08/2022

What is Micronaire Value of Cotton?
Micronaire (MIC) is a combination of fibre linear density and fibre maturity. Micronaire is the widely used method to estimate fibre fineness and maturity. The miconaire of a cotton is determined by measuring the resistance to airflow of a sample of fibres of a specified mass compressed to a fixed volume. Low micronaire value indicate fine and/or immature fibres; high values indicate coarse and/or mature fibres. The fineness factor in micronaire is considered more important in spinning, and fibre maturity is thought to have more effect on dye-uptake success. Micronaire value between 3.8 and 4.5 are desirable.

Micronaire is one of the two most important fibre properties for international cotton classers and spinners. Micronaire is an indicator of air permeability. It is regarded as an indication of both fineness (linear density) and maturity (degree of cell-wall development). For a given type of cotton, a relatively low micronaire has been used as a predictor of problems in processing, but a low micronaire may also indicate fine fibres with adequate maturity. Similarly, growers may be discounted for high micronaire when, in fact, the fibers have adequate fineness and good maturity, because high micronaire fibers are normally coarse, which is undesirable from the point of view of spinning and yarn evenness.

Fineness is generally expressed as gravimetric fineness or linear density (wall area times a constant), and maturity is generally expressed as maturity ratio (wall area divided by perimeter squared). One of the first practical tools to measure fineness and maturity was the determination of linear density and maturity ratio on the Shirley Developments Limited Fineness and Maturity Tester (FMT).

Fig: Relation between Micronaire, Fineness, and Maturity

Although linear density, maturity ratio, and micronaire are useful to spinners, all three properties can be viewed for any given cotton in terms of wall thickness and perimeter. Wall area is a function of wall thickness and perimeter. Wall thickness and perimeter are fundamental cross-sectional characteristics of the fibre with respect to wall area, because the function cannot be decomposed further into other geometric measures. If one examines the fibre cross section, the wall thickness is not constant but varies around the fibre, so that points must be sampled to get an averaged value. As a consequence, an averaged wall thickness and perimeter are fundamental with respect to an averaged wall area. Exploring the relationships on a fundamental level can be beneficial by demonstrating how a unique wall thickness and perimeter value together give an equivalent micronaire-fineness-maturity combination.

Cottons with a much greater genetic diversity are being developed, and a greater range of both fibre perimeter and wall thickness, and their combinations, is probable. Consequently, the relationships between micronaire, fineness, and maturity are being modified. This is because the original set of U.S. cottons that were used to calibrate the micronaire instrument had perimeters with a smaller range compared to current cultivars. The original relationships apply best to those cottons having perimeters similar to the calibration samples. For other cottons, these relationships do not apply as well, which results in modified expressions.

Even though micronaire is of great practical value for trade and industry, a literature review indicated no theoretical or experimental studies have been reported that model the three fibre characteristics in terms of the fundamental measures of thickness and perimeter. The specific objectives of this research were to use fineness and maturity components – wall thickness and perimeter to develop models for fineness, maturity, and micronaire; to simulate the interaction of fineness and maturity and the resultant micronaire; to quantify the relative sensitivity of the models to changes in thickness and perimeter; and to demonstrate variability in the coefficients of determination between micron aire and the other variables.

The micronaire module of HVI tester uses the airflow method to estimate the fineness value of cotton. Airflow instruments are widely used for the estimation of fibre fineness. These instruments are based on the principle that, for equal weights of fibre samples, the rate of airflow across the sample would be less for finer fibres than the coarser fibres due to the relatively more surface area in the case of finer fibres that offer a drag on the flow of air. High-volume instruments measure fineness and maturity of cotton. The AFIS-A2 Fineness and Maturity (F&M) module uses scattered light to measure single-fibre cross-sectional areas. The maturity ratio values of fibres ranging from very mature to immature evaluated by HVI are given in below Table.

HVI Maturity Ratio of Cotton:

  • Above 1.0 →Very mature
  • 0.8–1.0 → Mature
  • 0.7–0.8 → Immature
  • Below 0.7 → Uncommon

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