Nylon Fibers
Updated: April, 2004 -
Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath
(Monika
Kannadaguli and Ramaiah Kotra)
1. INTRODUCTION
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 Caruthers (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 (Polycaprolactam, 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
reactions are as follows [1].
2. Fiber TYPES
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
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 stretches.
3. Fiber Formation
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 drawn.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.
Table 1: Effect of Drawing on
Birefringence and Elastic modulus
|
Draw Ratio |
Birefringence |
Elastic Modulus
(GPa) |
|
1 |
0.00832 |
1.97 |
|
2 |
0.03297 |
2.74 |
|
3 |
0.05523 |
3.70 |
|
4 |
0.05904 |
4.59 |
|
5 |
0.06381 |
5.77 |
|
60 |
0.06901 |
6.74 |
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 Figure 1.
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 [4]. 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.. Ultra drawing 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.
4. Rheological Behavior
The
melt viscosity of the polymer can be represented as a function of molecular
weight by the relationship [5, 6]:
η=K (Mw) a
Where
is the zero shear viscosity, Mw it the weight average molecular
weight, K and an 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 [7] found that the drying
temperatures used affect zero shear melt viscosity. The result is shown in
Table 2. Effect of Resin Drying Temperature on Zero Melt
Viscosity
|
Drying Temperature (°C)* |
Molecular Weight Exponent |
|
50 |
3.8 |
|
110 |
4.6-5.4 |
5. 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)[8]. As an innovation on fiber formation, new
technologies producing micro fibers have been developed and reported [10].
Primarily direct spinning and mechanical and solvent splitting produce micro
fibers. Electro spinning [11] 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.
6. Crystalline structure
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)[3], and because of Vander
walls forces between the methylene chains. Since these unique structural and
thermo-mechanical properties of nylons are dominated by the hydrogen bonds in
these polyamides, quantum chemistry can be used to determine the hydrogen bond
potential [3]. 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 ant parallel 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 γ-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 [1].
7. Dyeability
The
dyeing efficiency 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 [12] 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. [13] 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 [15]. Another approach to higher dyeability
of nylon 6 is by copolymerization [16]. In this case, the dyeability can be
improved at the expense of a decrease of specific viscosity and of heat and
hydrolysis resistance. Other treatments, such as plasma etching [17] and
superheated steaming [18] 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.
8. Degradation
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
strong acids, bases, and phenols attack aliphatic solvents, but them. They also
are gradually attacked hydrolytically by hot water. Newly developed sulfonation
of nylon 6 fiber [19] 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.
9. 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
-Abrasion
resistant
-Lustrous- Nylon fibers have the luster of silk
-Easy to wash
-Can be pre colored or dyed in wide range of colors; dyes are applied to the molten mass of nylon or to the yarn or finished fabric.
-Resilient
-Filament yarn provides smooth, soft, long lasting fabrics
-Spun yarn lend fabrics light weight and warmth
10. 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.
11. Nonwovens Usage
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 skin. Its
toughness makes it a major fiber of choice in carpets, including needle punched
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.
12. World Consumption of Nylon Fiber
in Nonwovens 1998- 2007
It
is forecasted that global consumption of nonwoven may reach 3.7 million tons by
2005 and 4 million tones by 2007.Consumption of manmade fiber was about 8.1% of
all Textile fibers in 1998.In 2005 it is expected to reach 10% and 10.4 % by
2007. Table 3 shows the forecasted consumption of nylon fiber in nonwoven
Industry till 2007. [14]
Table 3
|
Year |
Consumption in
thousand tons |
|
1998 |
36 |
|
1999 |
39 |
|
2000 |
49 |
|
2001 |
50 |
|
2005 |
56 |
|
2007 |
60 |
Fig. 4
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 separators.
· Nylon fibers are used for the manufacture of split table-pie fibers. These fibers find application in high performance wipes, synthetic suede, heat insulators, battery separators and specialty papers.
·
Nonwovens developed from nylon are found in
automotive products, athletic wear and conveyor belts.
References
[1].
P. G. Galanty, and G. A. Bujtas, “Modern Plastics Encyclopedia” '92, pp
23-30 McGraw Hill 1992
[2].
P. Meplestor, “Modern Plastics”, 74, Jan., 66 (1997)
[3].
S. Dasgupta, W.B.hammond, and W.A. Goddard III, J. Am. Chem. Soc. 118,12291-12301,
(1996)
[4]
A. B. Thompson Fiber structure Edited by J. W. Hearle and R. H. Peters,
Butterworth & Co. Ltd. and the Textile Institute pp 499 (1963)
[5].
E.Mukouyama and A. Takegawa, Kobunshi Kagaku, 13,323 (1956)
[6].
G. Pezzin and G. B. Gechele, J. Appl. Polym. Sci., 8, 2159 (1964)
[7]
Y. P. Khanna, P. K. Han, and E. D. Day, Polym.
[8].
J. Smook, G. J. H. Vos, and H. L. Doppert, J. Appl. Polym. Sci., 41,105-116
1990)
[9].
J. W. Cho, G. W. Lee, and B. C. Chun, J. Appl, Polym. Sci., 62(5),771-8
(1996)
[10].
D. Gintis, “Chemical Engineering World”, 32(3),43-50 (1997)
[11].
D H. Reneker, and
[12].
T. Shibusawa Textile Research Journal, 66,421-428 (1996)
[13].
[14] www.ifj.com
[15].
K. El Salmawi, M. B. El Hosamy, A. M. El Naggar, and A. M. El Gendy, American
Dyestrff Reporter, 82 (5),47-59 (1993)
[16]. M. Nagata and T. Kiyotsukuri, European Polymer Journal, 28 (9),1069-72 (1992)
[17].
T. Okuno, T. Yasuda, and H. Yasuda, Textile Research Journal, 62(8),474-80
(1992)
[18].
L. Han, T. Wakida, and T. Takagishi, Textile Research Journal, 57(9),519-12
(1987)
[19].
S. A. El Garf and S. M. El kemry, Textile Research Journal, 67, 13-17
(1997)