Page 275 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 249

made at standard ambient conditions. However, the preconditioning of the cotton fiber
may still yield different moisture contents of fibers per the phenomenon of “regaine
time hysteresis” discussed above. In a study by Byler et al. (2002), an attempt was
made to correct HVI cotton fiber strength by adjusting HVI values with measured
values of moisture content using empirical equations. Many classing offices in the
United States utilize these equations to provide realistic values of fiber strength.
Other studies were concerned about the length of the preconditioning time. In
practice, a period of 24e72 h preconditioning time of cotton fibers prior to testing
can be too long. In many situations, production pressure and demand for quick
information force violation of these standard periods. Some find it ironic that with
instrumentation systems that can provide data within seconds, one must wait many
hours to get the data. As a result, more rapid preconditioning techniques were devel-
oped. The first rapid conditioning concept was introduced into USDA cotton
classification facilities in 1993 as an improved means of conditioning cotton samples
(Shofner et al., 1997; Alldredge and Knowlton, 1995; Earnest et al., 1997). The under-
lying concept of rapid conditioning is to draw conditioned air down through cotton
samples to enable them to reach the proper moisture content level for HVI testing.
A typical rapid conditioning unit is composed of a wire mesh conveyor equipped
with a sheet metal plenum that is connected to the facility’s mechanical system. While
plastic trays filled with cotton samples move along the mesh conveyor for approxi-
mately 10 min, conditioned air from the surrounding room is drawn down through
the samples and into the plenum. The air is then returned through the mechanical
system where it is reconditioned before being delivered back into the room as
conditioned air. Moisture readings are taken regularly during shift operation to verify
that cotton samples leaving the rapid conditioning unit are within the allowable
moisture range. Samples not achieving this moisture level are returned back to the
loading end of the unit to be processed again.

7.12.1 Dynamic testing of the tensile behavior of cotton fibers
The dynamic tensile behavior of cotton fiber implies the response of the cotton fiber to
repeated loading and unloading using forces that are far below the breaking force of the
fiber and examining the extent of recovery of the fiber or the permeant (plastic
deformation) that may result from these applied forces. As indicated earlier, the
crystallinity of cotton fiber may range from 60% to 80% (by X-ray diffraction analysis).
The crystalline regions in the fiber provide a great deal of elasticity under low external
loading; but cotton crystallinity is not perfect, and the fiber may suffer permeant or
plastic deformation under low levels of loading. As indicated earlier, a slight stress
applied on a cotton fiber can result in a significant plastic deformation. This deformation
can be attributed to a permeant displacement in the interatomic structure of the fiber.
This phenomenon is commonly called “slip” to indicate a slip occurring when planes
of densely packed atoms slide over one another. The broken individual bonds are
reformed with new atoms in a stepwise fashion until the permanent deformation occurs.
The importance of the dynamic tensile behavior of cotton stems from the fact that
during processing, cotton fibers are subjected to perhaps millions of stretch-release