Cotton fibre click here
Physical properties of cotton fibre click here
Chemical composition of cotton fibre click here
Cotton fibre and its chemical structure click here
Consumers love cotton click here
Cellulose chemistry click here
Cottons unique fibre morphology click here
Cotton
Raw cotton contains cotton
fiber
along with small plant parts and field trash that are not removed by the
ginning process. At this stage, the cotton fiber has a coating of oils and
waxes that make it hydrophobic. Raw fiber is suitable for making nonwovens to
be used in industrial products in which absorbency and aesthetics are not
important. In some cases, nonwoven fabrics made with raw fiber can be wet
processed in the same manner as woven and knitted fabrics.
Cotton is the most important apparel fiber. It is
important because of its many favorable properties. One of these properties is
the care of cotton. When we use talk about care we are
referring to the measures required to keep the cotton's or any other textile's
original appearance. One reason for cotton's popularity is its easy care properties. Cotton is
a hydrophilic fiber is very absorbent. Cotton has a poor resiliency which means
that it wrinkles very easily.
Physical Properties of Cotton
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.
Chemical Composition
of Cotton Fiber
The chemical
composition of cotton fiber consists of ninety-five percent cellulose, one
point three percent protein, one point two percent ash, point six percent wax,
point three percent sugar, and 0.8
percent organic acids, and other chemical compounds that make up three point
one percent. The non-cellulose chemicals of cotton are usually located in the
cuticle of the fiber.
Ø The non-cellulose chemicals of cotton consist of protein,
ash, wax, sugar and organic acids. Cotton wax is found on the outer surface of
the fiber. The more wax found on cotton the greater the surface area of cotton
there is; finer cotton generally has more cotton wax . Cotton wax is primarily
long chains of fatty acids and alcohols. The cotton wax serves as a protective barrier for the cotton fiber. Sugar makes up point three percent of
the cotton fiber, the sugar comes from two sources plant sugar and sugar from
insects. The plant sugars occur from the growth process of the cotton plant. The plant sugars consist
of monosaccharide, glucose and fructose. The insect sugars are mainly
for whiteflies, the insect sugars can cause stickiness, which can lead to problems in
the textile mills. Organic acids are
found in the cotton fiber as metabolic residues. They are made up of malic acid and
citric acid.
Ø The non-cellulose chemicals of cotton are removed by using
selective solvents. Some of these solvents include: hexane, chloroform, sodium hydroxide
solutions, non-polar solvents, hot ethanol, and plain water.
Ø After removing all the non
cellulose chemicals,
the cotton fiber is approximately ninety-nine percent cellulose.
Cellulose
|
95%
|
Protein
|
1.3%
|
Ash
|
1.2%
|
Wax
|
0.6%
|
Sugar
|
0.3%
|
Organic acid
|
0.8%
|
Other chemical
compound
|
3.1%
|
Cotton Fiber and its
Chemical Structure
The chemical
composition of cotton, when picked, is about 94 percent cellulose; in finished fabrics is it 99 percent cellulose. Cotton contains carbon, hydrogen, and oxygen with
reactive hydroxyl groups. Glucose is the basic unit of the cellulose molecule. Cotton may have as many as 10,000 glucose monomers
per molecule. The molecular chains
are arranged in long spiral linear chains within the fiber. The strength of a
fiber is directly related to chain length.
Hydrogen bonding occurs between cellulose chains in
a cotton fiber. There are three
hydroxyl groups that protrude from the ring formed by one oxygen and five
carbon atoms. These groups are polar meaning the electrons surrounding the
atoms are not evenly distributed. The hydrogen atoms of the hydroxyl group are
attracted to many of the oxygen atoms of the cellulose. This attraction is
called hydrogen bonding. The bonding of hydrogen's within the ordered regions
of the fibrils causes the molecules to draw closer to each other which
increases the strength of the fiber. Hydrogen bonding also aids in moisture
absorption. Cotton ranks among the most absorbent fibers because of Hydrogen bonding
which contributes to cotton's comfort.
The chemical
reactivity of cellulose is related to the hydroxyl groups of the glucose unit.
Moisture, dyes, and many finishes cause these groups to readily react. Chemicals
like chlorine bleaches attack the oxygen atom between or within the two ring
units breaking the molecular chain of the cellulose.
Consumers Love Cotton
Consumers
perceive cotton as soft, natural and comfortable. The preferred
fiber for clothing and home textiles, cotton is also preferred by consumers for wipes, feminine
hygiene, and adult incontinence products. Cotton is naturally
hypoallergenic, lending an additional comfort level to consumers with sensitive
skin.
Cotton is an environmentally
friendly fiber
Cotton has been around for thousands of years making it one of the world’s
oldest fibers. It is annually renewable, biodegradable, and regulated by the
USDA as a food crop. Modern farming practices and agri-science advancements
continue to optimize production so that market demand can be met without
sacrificing the future integrity of our natural resources.
Cotton is a performance fiber by
its very nature
Its structure allows for superior absorption and release capabilities,
making it a viable delivery system for cleansers, moisturizers, or efficacious
formulation found in baby, personal care, and hygiene products. Another unique
attribute of cotton is its superior wet-strength. Cotton is naturally
stronger when wet than dry lending enhanced tear resistance to
all wet wipes even those used for tough household cleaning jobs. This trait
makes cotton an ideal component for any nonwoven product that requires exposure
to water or other liquids.
Cellulose Chemistry
Ø
Cellulose
is a macromolecule –– a polymer made up of a long chain of glucose molecules
linked by C-1 to C-4 oxygen bridges with elimination of water.
Ø
After
scouring and bleaching, cotton is 99% cellulose.
Ø
The anhydroglucose units are linked together
as beta-cellobiose; therefore, anhydro-beta-cellobiose is the repeating unit of
the polymer chain.
Ø
The number of repeat units linked together to
form the cellulose polymer is referred to as the “degree of polymerization.”
Wood pulp, rayon and
cellophane (all three derived from wood cellulose) are also constructed of
cellulose polymers. Cotton cellulose differs from wood cellulose primarily by having a higher
degree of polymerization and crystallinity. Crystallinity indicates that
the fiber molecules are closely packed and parallel to one another). Higher degree of
polymerization and crystallinity are associated with higher fiber strengths.
The cellulose chains
within cotton fibers tend to be held in place by hydrogen bonding. These hydrogen bonds occur
between the hydroxyl groups of adjacent molecules and are most prevalent
between the parallel, closely packed molecules in the crystalline areas of the
fiber.
The three hydroxyl groups,
one primary and two secondary, in each repeating cellobiose unit of cellulose
are chemically reactive. These groups can undergo substitution reactions in
procedures designed to modify the cellulose fibers or in the application of
dyes and finishes for cross-linking. The hydroxyl groups also serve as
principal sorption sites for water molecules. Directly sorbed water is firmly chemisorbed on the
cellulosic hydroxyl groups by hydrogen bonding.
In the case of regenerated
and derivative cellulose fibers, strength generally decreases with increasing
moisture content.
In contrast, the strength of cotton generally increases with increased moisture. This difference among fibers
in their response to moisture is explained in terms of intermolecular hydrogen
bonding between cellulose chains and their degree of crystallinity .
Cotton's Unique Fiber Morphology
o A mature cotton fiber has
the following six parts.
o
The “cuticle” is the outer waxy layer, which
contains pectins and proteinaceous materials. It serves as a smooth,
water-resistant coating, which protects the fiber. This layer is removed from
the fiber by scouring.
o
The “primary wall” is the original thin cell wall.
Mainly cellulose, it is made up of a network of fine fibrils (small strands of
cellulose).
This makes for a well-organized system of continuous, very fine capillaries. It
is well known that fine capillaries rob liquids from coarse capillaries. The
fine surface capillaries of each cotton fiber contribute greatly to cotton’s
wipe-dry performance.
o
The “winding layer” (also called the S1 layer) is
the first layer of secondary thickening. It differs in structure from both the primary wall and the
remainder of the secondary wall. It consists of fibrils aligned at 40 to
70-degree angles to the fiber axis in an open netting type of pattern.
o
The “secondary wall” (also called the S2 layer)
consists of concentric layers of cellulose, which constitute the main portion
of the cotton fiber.
After the fiber has attained its maximum diameter, new layers of cellulose are
added to form the secondary wall. The fibrils are deposited at 70 to 80-degree
angles to the
fiber
axis, reversing angle at points along the length of the fiber. The fibrils are
packed close together, again, forming small capillaries.
o
The “lumen wall” (also called the S3 layer)
separates the secondary wall from the lumen and appears to be more
resistant to certain reagents than the secondary wall layers.
o
The “lumen” is the hollow canal that runs the length
of the fiber.
It is filled with living protoplast during the growth period. After the fiber
matures and the boll opens, the protoplast dries up, and the lumen naturally
collapses, leaving a central void, or pore space, in each fiber.
o
Figure shows a schematic structure of a mature
cotton fiber, identifying its six parts.
o Throughout the fiber structure,
there are variously sized pores or capillary spaces between the variously sized
fibrils in each of the six fiber parts. Thus, the cotton fiber can be viewed as
a microscopic physical sponge with a complex porous structure. This internal
structure makes cotton fibers accessible to liquids and vapors. The capillary
action of the fibrils pulls liquid in, where it is held in pores between the
fibrils. This structure accounts for cotton’s wickability and unique absorbing
capacity.
o
The
cotton fiber, when observed in its entirety, is a flat, twisted ribbon, with 50
to 100 convolutions per inch. The fiber is tapered on one end and fibrillated
on the other, where it was joined to the cottonseed. This provides the fiber
with a soft touch or feel, because it has no sharply cut ends, as do synthetic
staple fibers.