The three cotton fiber
properties most often considered which are micronaire, length and strength. Naps may also be considered
for applications where visual appearance is important.
Micronaire
Micronaire is an airflow
measurement of fiber fineness. It is performed on a weighed test specimen,
which is compressed to a specific volume in a chamber. Air is forced through
the specimen and the resistance to the airflow is measured. This resistance is
proportional to the linear density of the fibers (expressed in micrograms per
inch), adjusted for the maturity of the fiber (because micronaire and maturity
are highly correlated within each cotton variety). If the exact linear density
of the fibers needs to be determined, the maturity of the fibers must be
determined by another measurement. The micronaire range for upland cotton is
3.0 to 5.5.
Fiber Length
Cotton fiber length varies
genetically and any sample of cotton fiber shows an array, or distribution, of
fiber length. The HVI reports fiber length as the mean length of the longer
half of the fibers in the sample. Figure
shows a typical fiber length array.
Fiber lengths normally are between 1.0 and 1.25 inches for U.S. upland
raw cotton, as long as 1.6 inches for
Pima cotton, and less than 0.5 inches
for linters and comber noils (the portion of shorter fibers removed by the
combing operation).
Fiber Strength
The HVI system measures fiber
strength by clamping a bundle of fibers, with 1/8 inch between the two sets of
jaws, and measuring the force required to break the fibers.
Results are reported as grams
per tex or grams per denier. A “tex” is a unit equal to the weight in grams of
1,000 meters of fiber. Therefore, the strength reported is the force in grams
required to break a bundle of fibers one tex unit in size.
Neps
A “nep” is a small knot of
tangled fibers, often caused biologically or by mechanical processing. Neps can
detract from the visual appearance of fabrics by causing white specks. Neps can
be measured with the Zellweger Uster Advanced Fiber Information System (AFIS)
nep tester and are reported as total neps per gram of cotton and mean nep
diameter in millimeters. Nep formation during processing can be minimized
through the use of appropriate equipment and settings.
The AFIS equipment
can also be used to measure fiber length and trash content. It is very
effective at measuring small amounts of residual trash present in bleached
cotton.