Monika Kannadaguli and Ramaiah Kotra
Nylon was the first truly synthetic fiber to be commercialized (1939).
Nylon was developed in the 1930s by scientists at Du Pont, headed by an
American chemist Wallace Hume Carothers(1896-1937). It is a polyamide fiber,
derived from a diamine and a dicarboxylic acid, .Because a variety of
diamines and dicarboxylic acids can be produced, there are a very large
number of polyamide materials available to produce nylon fibers. The two
most common versions are nylon 66 (polyhexamethylene adiamide) and nylon 6 (Polycaprolactom,
a cyclic nylon intermediate). Raw materials for these are variable and
sources used commercially are benzene (from coke production or oil
refining), furfural (from oat hulls or corn cobs)
or 1,4-butadiene(from oil refining). The
chemical reaction are as following :
Fiber types are produced commercially in various parts of the world.
Nylon 66 has been preferred in North American markets, whereas nylon 6 is
much more popular in Europe and elsewhere. Nylon is produced by meltspinning
and is available in staple, tow, monofilament, and multi-filament form. The
fiber has outstanding durability and excellent physical properties. Nylons
are semi-crystalline polymers. The amide group -(-CO-NH-)- provides hydrogen
bonding between polyamide chains, giving nylon high strength at elevated
temperatures, toughness at low temperatures, combined with its other
properties, such as stiffness, wear and abrasion resistance, low friction
coefficient and good chemical resistance. These properties have made nylons
the strongest of all man-made fibers in common use. Because nylons offer
good mechanical and thermal properties, they are also a very important
engineering thermoplastic. For example, 35% of total nylon produced is used
in the automobile industry . There are several commercial nylon products,
such as nylon 6, 11, 12, 6/6, 6/10, 6/12, and so on. Of these, the most
widely used nylon products in the textile industry are formed of nylon 6 and
nylon 6/6. The others are mainly used in tubing extrusion, injection
molding, and coatings of metal objects .
Nylon's outstanding characteristic in the textile industry is its
versatility. It can be made strong enough to stand up under the punishment
tire cords must endure, fine enough for sheer, high fashion hosiery, and
light enough for parachute cloth and backpacker's tents. Nylon is used both
alone and in blends with other fibers, where its chief contributions are
strength and abrasion resistance. Nylon washes easily, dries quickly, needs
little pressing, and holds its shape well since it neither shrinks nor
One of the most important factors in polymer processing is viscosity,
which is a function of molecular weight. The number-average molecular weight
of polymer suitable for textile fiber production ranges from 14,000 to
20,000. Since polycaprolactam can be regarded at equilibrium as a
polycondensation polymer, the number-average molecular weight alone is
sufficient for its characterization. Two-step melt spinning, comprised of
spinning and drawing, is considered to be the conventional method to
manufacture nylon filaments. After melting, filtering, and deaerating, the
molten polymer is extruded through a spinneret into a chamber where the melt
solidifies into a filament form. At this stage, the filaments have little
molecular orientation, and their slight birefringence is due to shear forces
set up during extrusion. In order to achieve desirable properties through
molecular orientation and crystallinity, the newly formed filaments must be
drawn. Since the Tg of nylon is below room temperature, nylon can be cold
Hot drawing is also frequently used. Nylon filaments are drawn
approximately four times their initial length. The effect of drawing on
birefringence, a measure of molecular anisotropy, can be seen in Table I.
Also, the elastic modulus increases significantly with increasing
orientation as shown in Table I. Other physical properties, such as density
equilibrium, moisture sorption, tenacity and elongation-at-break, are also
affected by drawing.
Rather than two-step spinning (extrusion) and drawing, a one-step,
high-speed spinning process is being used increasingly. In high-speed
spinning, filament windup speed relative to the extrusion speed is very high
and orientation and crystallization occur in elongation flow along the spin
line. When drawing as-spun fibers, the molecules are arranged randomly in
amorphous regions and as folded chains in crystalline region as shown in
In essence, cold drawing stretches chains in amorphous regions, but
molecular folds are restricted and the molecules orient themselves along the
fiber axis direction, resulting in enhanced orientation and high
crystallinity. In the case of nylons, which have sheet-like crystal
structures, drawing may enable the hydrogen-bonded polyamide sheets to slip
past each other and form more oriented structure . Hot drawing is a
procedure using high temperature during drawing and annealing under
restraint after drawing. Exposure to high temperature helps to increase the
draw ratio, and higher moduli and tenacity can be achieved.. Ultradrawing of
solidified crystalline material induces a high degree of chain extension
(Figure 2), which leads to very high tensile strength and modulus. This
results in a so-called high-performance fiber.
A skin-core structure, mostly depending on spinning speed, is generally
formed within melt-spun fibers. At a constant feeding rate, higher spinning
speeds will produce more extended chains in the melt and form a finer
filament. Therefore, the finer fiber usually has higher modulus and
tenacity. Fine filament cannot be drawn as much as a coarse filament,
because partial orientation on the outer parts of the filaments is formed
when the molten fluid is drawn over the sides of the orifice. As a result,
finer filaments have a greater proportion of 'skin' to bulk, i.e., better
orientation has already been formed. Naturally, there is not much space for
an improvement by cold drawing within fine filaments. The filaments become
lustrous and strong.
The melt viscosity of the polymer can be represented as a function of
molecular weight by the relationship [5, 6]:
where is the zero shear viscosity, Mw it the weight average
molecular weight, K and a are constants dependent upon the polymer and
temperature. In the case of nylons, the value of exponent a normally is in
the range of 3.4-3.8.
It has long been known that moisture has a strong effect on the
rheological behavior of nylons. Generally, high moisture levels cause
degradation and foaming, and relatively low levels of moisture act as
plasticizer in nylon 6 during melt processing. All nylons absorb moisture.
the extent of moisture absorption depends on temperature, crystallinity, and
humidity. Therefore, before processing of nylon resins, the polymer pellets
must be dried to moisture levels below 0.2 wt%, in order to avoid bubble
formation and significant polymer degradation during processing. A recent
study  found that zero shear melt viscosity is affected by the drying
temperatures used. The result is shown in Table II.
Non-conventional Spinning Techniques
Alternative to conventional melt spinning, various solution spinning
techniques have been introduced [8,9]. Solution spinning techniques (gel,
wet, dry) enable the spinning of high molecular weight polyamides, leading
to high tenacity filaments (tenacity 100cN/tex).
As an innovation on fiber formation, new technologies producing
microfiber have been developed and reported . Microfibers are produced
primarily by direct spinning and mechanical and solvent splitting.
Electrospinning  represents another approach to fiber spinning, when
electrical forces on polymer melt or solution surface overcome the surface
tension and cause an ejection from an electrically charged jet. The diameter
of the fibers produced by this technique is of the order of nanometers.
Frequently, there are produced fibers that are electrically charged.
Both nylon 6 and nylon 66 are semi-crystalline polymers. These linear
aliphatic polyamides are able to crystallize mostly because of strong
intermolecular hydrogen bonds through the amide groups (Figure. 3), and
because of van der Waals forces between the methylene chains. Since these
unique structural and thermo-mechnical properties of nylons are dominated by
the hydrogen bonds in these polyamides, quantum chemistry can be used to
determine the hydrogen bond potential. The left side of the figure shows
hydrogen-bonding planes, and the right side shows the view down the chain
axis. For the -form of nylon 6, adjacent chains are antiparallel and the
hydrogen bonding is between adjacent chains within the same sheet (bisecting
the CH2 angles). For the -form of nylon 6, the chains are
parallel and the hydrogen bonding is between chains in adjacent sheets. . In
nylon 66, the chains have no directionality. Research results have shown
that the stable crystalline structure is the -form comprised of stacks of
planar sheets of hydrogen-bonded extended chains. It also appears that
Young's modulus of the -form is higher than the -form.
Mechanical, thermal and optical properties of fibers are strongly
affected by orientation and crystallinity. Basically, higher fiber
orientation and crystallinity will produce better properties. Crystallinity
of nylons can be controlled by nucleation, i.e., seeding the molten polymer
to produce uniform sized smaller spherulites. This results in increased
tensile yield strength, flexural modulus, creep resistance, and hardness,
but some loss in elongation and impact resistance. Another important benefit
obtained from nucleation is decrease of setup time during processing .
The dyeability of nylon fibers is enhanced due to the end groups -COOH
and -NH2, which exhibit polar and hydrophilic characteristics. Dye diffusion
into fibers is closely related to the rate of dyeing, level of dyeing
through dye migration, wet fastness properties of dyes, etc. It is generally
believed that dye diffusivity is independent on dye concentration, with some
exception. T. Shibusawa  studied the diffusion of most disperse dyes on
nylon 6 and found that the actual diffusivity on nylon 6 fibers is not
always independent on dye concentration. Kim et al.  have reported that
both dyeing rate and dye saturation of 1,4 -diaminoanthraquinone (1,4-DAA)
were improved considerably in the presence of didodecyldimethlammonium
bromide (DDDMAB). The amount of DDDMAB adsorbed on nylon 6 fiber is roughly
20 times higher than that a conventional dispersing agent. This suggests
that there might be fairly strong interaction between DDDMAB and the fiber
by virtue of electrostatic and hydrophobic interactions.
There have been many attempts to improve nylon's dyeability or at least
to point out the factors and mechanisms acting in nylon dyeing. It has been
shown that acrylonitrile and styrene radiation grafting on the polymer could
improve the dyeability of nylon. Another approach to higher dyeability
of nylon 6 is by copolymerization . In this case, the dyeability can be
improved at the expense of a decrease of specific viscosity and of heat and
Other treatments, such as plasma etching  and superheated steaming
 have proved to decrease nylon dyeability. In the former treatment,
outer structures, not normally susceptible to dyes, are etched away whereas
the crystalline phases inside the fiber are not as much affected.
Superheated steaming of the fibers leads to higher shrinkage and to higher
crystallinity and crystal size, which contribute to decrease dyeability.
The -COOH and -NH2 end-groups in nylons are sensitive to
light, heat, oxygen, acids and alkali. When exposed to elevated
temperatures, unmodified nylons undergo molecular weigh degradation, which
results in loss of mechanical properties. The degradation is highly
time/temperature dependent. By adding heat stabilizer, nylon can be used at
elevated temperature for long-term performance. Exposure to UV light results
in degradation nylon over an extended period of time, it appears that adding
carbon black can reduce the radiation degradation. Nylons are chemical
resistance to hydrocarbons, aromatic and aliphatic solvents, but they are
attacked by strong acids, bases, and phenols. They also are gradually
attacked hydrolytically by hot water. Newly developed sulfonation of nylon 6
fiber  by 2,5 dichlorobenzene sulfonyl chloride (DSBC) has a great
effect on the heat and chemical stability of the fibers. It reported that
the modified fiber is non-melting up to 1000oC, and does not burn
when put it in direct flame (but chars without losing fiber form). It does
not dissolve in formic acid and concentrated mineral acid. Its glass
transition temperature is about 500oC.
Properties of Nylon 66
-Tenacity-elongation at break ranges from 8.8g/d-18% to 4.3 g/d-45%. Its
tensile strength is higher than that of wool, silk , rayon, or cotton.
- 100% elastic under 8% of extension
-Specific gravity of 1.14
-Melting point of 263oC
-Extremely chemically stable
-No mildew or bacterial effects
-4 - 4.5% of moisture regain
-Degraded by light as natural fibers
-Permanent set by heat and steam
-Lustrous- Nylon fibers have the luster of silk
-Easy to wash
-Can be precolored or dyed in wide range of colors, dyes are applied to
the molten mass of nylon or to the yarn or finished fabric.
-Filament yarn provides smooth, soft, long lasting fabrics
-Spun yarn lend fabrics light weight and warmth
Properties of Nylon 6
The main difference between nylon 6 and nylon 6,6 is nylon 6 has a much
lower melting point than nylon 66. This is a serious disadvantage, as
garments made from it must be ironed with considerable care.
The fiber has outstanding durability and excellent physical properties.
Like PET fiber, it has a high melting point, which conveys good high-
temperature performance. The fiber is more water sensitive than PET; despite
this fact, nylon is not considered a comfortable fiber in contact with the
toughness makes it a major fiber of choice in carpets, including
needlepunched floor-covering products.
Because of its relatively high cost, nylon has somewhat limited use in
nonwoven products. It is used as a blending fiber in some cases, because it
conveys excellent tear strength. The resiliency and wrinkle recovery
performance of a nonwoven produced from nylon is not as excellent as that
from PET fiber.
In certain applications, the performance of nylon fiber is hard to beat.
However, because of its higher cost, it is used in specialized applications
where its performance can justify the increased cost. It is used as a
blending fiber in some cases, because it conveys excellent tear strength.
The resiliency and wrinkle recovery performance of a nonwoven produced from
nylon is not as excellent as that from PET fiber. This polymer is used in
moderate quantities, because it is more expensive than polyester,
polypropylene, or rayon. Some particular applications are as follows:
- It can be mostly found in garment interlinings and wipes where it
supplies strength and resilience.
- In Ni/H and Ni/Cd batteries, nylon fibers are used as nonwovens
- Nylon fibers are used for the manufacture of splittable-pie fibers.
These fibers find application in high performance wipes, synthetic suede,
heat insulators, battery separators and speciality papers.
- Nonwovens developed from nylon are found in automotive products,
atheletic wear and conveyor belts. >
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