Page 281 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 255

consists of two cylinders, each with alternating rows of hard rubberized bats and stiff
wire brushes. These cylinders rotate to grab cotton stalks. As stalks pass between these
cylinders, cotton bolls are separated from the stalk and harvested. Spindle harvesting,
on the other hand, has a direct impact on the fibers in the cotton bolls. The picker is
equipped with barbed spindles that literally detach bolls from the stalk and grab lint
and remove it from the boll itself. Unfortunately, we are unaware of reliable studies
conducted on the effects of the harvesting techniques on fiber strength and elongation
of seed cotton. The difficulty of performing such a study stems from the need to use
the same cotton field to provide experimental control on the many variables that are
expected to be involved in such study. On the other hand, comparing the methods
of harvesting after ginning would be largely of less value due to the addition of
numerous variability in all fiber attributes on ginning, particularly in the drying and
lint cleaning processes. Because of these limitations, the following fundamental
question remains unanswered: Does the method of harvesting alter the tensile behavior
of cotton fibers through, perhaps, cyclical mechanical conditioning or pulling fibers
beyond their elastic limits?

7.14 The ginning process

The ginning process aims at separating the seed from the fibers in such a way that it
protects the seed from crushing and cause minimum fiber fragmentation. This was
typically the way when ginning was done manually using hand ginning. Mechanical
ginning was considered as one of the main driving forces to the industrial revolution
in the 18th century. There are two common types of mechanical ginning: saw ginning
and roller ginning. The saw gin utilizes a series of circular saws mounted closely and
projected slightly between bars or ribs, which are so, spaced that they prevent the seed
from going forward. The saws pull away the fibers from the seeds at high speed. The
roller gin picks and moves the cotton fibers between a rotating roller and a fixed knife
preventing the seeds from passing through. The free ends of the fibers are gripped, in
the grooves of the rotating roller, and dragged forward till the seeds reach the edge of a
stationary knife. Using a downward motion of the moving knife, the seeds are detached
from the cotton fibers.
Per the above principles of ginning, the only reliable way to test the tensile behavior
of seed cotton fibers as well as the force required to detach the fibers from the seed
would be to design tensile instruments that can simulate the seedefiber separation
action during ginning. This would make a good research project for young researchers
in the field. Traditionally, the pulling force of the fibers from the seed has been
measured using classic tensile instruments (e.g., the Instron tester) in which the seed
is clamped and the fibers are pulled mechanically under a constant rate of extension.
Some studies (Byler, 2006) indicated that at 7% moisture content, the force required to
separate fibers from their seed is about 55% of their tensile strength, or conversely, the
mean fiber tensile strength is about 1.8 times the fibereseed separation force. It is the
opinion of the authors of this chapter that this level of fibereseed separation force is
considerably high and permanent plastic deformation is likely to occur at this level.
Nevertheless, further research should be made in this area with the objective being
to design gentler ginning processes.