High performance fibres-2

High performance fibres-2
High performance fibres, edited by J
W S Hearle, Woodhead Publishing
Limited, 2001

• Most meta-oriented aramid fibers are heat-resistant; they
are regarded as the first generation of high performance
fibers. Para-oriented fibers are considered the second
generation of high-performance fibers; they are composed
mainly of para-substituted residues, instead of the meta-
substituted residues of the first generation.
• Du Pont initiated the second generation with Kevlar, a
successor to the first-generation Nomex. Compared to
meta-oriented fibers, highly sophisticated polymerization
and production techniques are needed for the para-
oriented type to overcome difficulties caused by their even
more rigid molecular structure.
• Para-aramids, such as Kevlar, belong to the family of liquid-
crystalline polymers (LCP).
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Liquid crystalline polymers (LCP)
• Modern fibres based on liquid crystalline
polymers manifest outstanding tensile
mechanical properties. They can reach a
tensile modulus of up to 300 GPa and tensile
strength of up to about 6 GPa.
• Remember that m-aramid have tensile
modulus <18 Gpa and tensile strength around
0.6 GPa.

• A common situation in polymer solutions is that
of randomly coiled polymer chains.
• However, if the chains are relatively stiff and are
linked to extend the chain in one direction, then
they are ideally described in terms of a random
distribution of rods.
• Crystalline solids are ordered in three
dimensions, while liquids are entirely disordered:
liquid crystals lies between these two extreme
cases, i.e. they exhibit long-range order in one or
two dimensions, but not in all three.
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• The molecular asymmetry is the most important requirement
for a macromolecule in order to originate the various possible
LC phases (called mesophases).
• These asymmetry can be manifested either as rods of axial
ratio grater than about 3, or thin platelets of biaxial order.
• Another fundamental requirement is a sufficiently high chain
stiffness.
• Liquid crystals can be divided into thermotropic and lyotropic.
In fact, the liquid crystalline behaviour may occur either in the
diluted state (lyotropic liquid crystal) in a critical
concentration range, or in the molten state (thermotropic
liquid crystals) in the proper temperature range.
• Lyotropic and thermotropic LCPs are probably the ideal
precursors for preparing fibres. In the diluted or molten
states the degree of uniaxial orientation is typically very high
and the extensional flow that is associated with the extrusion
process orients the mesophases in the flow direction.

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• As the concentration of rod-like macromolecules is increased
and the saturation level for a random array of rods is attained,
the system will simply become a saturated solution with
excess non-solved polymer;
• or more interestingly, if the solvent/polymer relationships are
right, additional polymer may be dissolved by forming regions
in which the solvated polymer chains approach a parallel
arrangement. These ordered regions define a mesomorphic or
liquid crystalline state.
• Continued addition and dissolution of polymer forces more
polymer into the ordered state. If the rod-like chains are
arranged in an approximately parallel array but are not
otherwise organised, then the ordered phase is termed
‘nematic’.
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Lyotropic Liquid Crystal

• To retain a minimal volume, and indeed a minimal free
energy, above a certain critical concentration, orientational
order of the rods appears.
• In this case the solution becomes anisotropic. The degree of
this anisotropy will be less than the strict three-dimensional
ordering typical of a crystalline system, but at the same time it
will differ significantly from an isotropic state characteristic of
amorphous systems.
• The concentration threshold defining the transition to the
liquid crystalline state will depend on the degree of shape
asymmetry of the macromolecules, which will be determined
as the ratio of their equilibrium length to their diameter,
termed the ‘axial ratio’.
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• If concentrations above a critical limit are used, spinnability is
affected due to undissolved material; therefore the resulting fibre
has inferior mechanical properties.
• Because these rod-like polymers are rigid, they orientate
themselves with respect to each other, forming a nematic phase
as illustrated in figure, which shows the orientation angle b with
respect to the director n. This phase is dominated by liquid
crystalline domains that contain aligned polymer chains. The
degree of orientation of these polymer chains depends on
solution temperature and polymer concentration.
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Dry-jet wet spinning
• Polymer spinning solutions are extruded through
spinning holes and are subjected to elongational
stretch across a small air gap (of 10-15 mm).
• Under shear, the crystal domains become
elongated and orientated in the direction of the
deformation. Once in the air gap, elongational
stretching takes place. This is effected by making
the velocity of the fibre as it leaves the
coagulating bath higher than the velocity of the
polymer as it emerges from the spinning holes.
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• The precipitation in the quench water bath
freezes the structure in a highly oriented state.
• In comparison to the wet spinning process
used for conventional organic fibres, where
the spinning nozzle is immersed in the
coagulation liquid, the air gap of the dry-wet
spinning process induces a higher degree of
molecular orientation and hence an
improvement of the mechanical properties.

• The resulting stretch in the air gap further
perfects the respective alignment of the liquid
crystal domains. Overall, a higher polymer
orientation in the coagulation medium
corresponds to higher mechanical properties of
the fibre.
• Because of the slow relaxation time of these
liquid crystal systems, the high as-spun fibre
orientation can be attained and retained via
coagulation with cold water.
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• The importance of the orientation induced
during the spinning process clearly emerges
when the effects of the solution concentration
on the mechanical properties are considered.
• In fact, the mechanical properties are
significantly improved only if the solution
concentration is significantly higher than the
critical concentration so that a LC is obtained.

• Fibres prepared by a dry-jet wet-spun process have a
noteworthy response to very brief heat treatment
(seconds between 150-550°C) under tension. These
fibres will not undergo drawing in the conventional
sense, showing an extension of less than 5% even at
temperatures above 500°C, but the crystalline
orientation and fibre modulus is increased by this
short-term heating under tension.
• X-ray diffraction analysis shows that the heat
treatment induces an increase of the apparent
crystallite size and a reduction of the axial crystal
orientation angle from about 15-20° to 10° or less.
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• Fibres can exhibit three possible lateral or
transverse crystalline arrangements and these
are illustrated in the Figure.
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• Poly(p-phenylene-terephthalamide (PPTA), a
typical para-oriented aramid, is formed by the
reaction:
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Para-oriented aramid fiber

• PPTA forms anisotropic solution in strong acids such
as sulphuric acid, chloro- and fluorosulphuric acids,
and hydrogen fluoride.
• At low concentration (below 8%), rod-like PPTA
molecules are randomly oriented in an isotropic
dilute solution.
• When the concentration approaches a critical value
(around 12%), the molecules pack close together and
rearrange is small domains, which remain randomly
oriented.
• When the solution is under flow, shear and
elongational stresses induce an orientation of the LC
domains in the flow direction.

• PPTA provides an example of a polymer that has very limited
solubility in suitable solvents. In the laboratory, para-
phenylene diamide –PPD- is dissolved in an amide solvent in
concentrations up to 0.5 mol/L. The solution is cooled to near
0◦C, stirring vigorously.
• Solid terephthaloyl chloride-TCL- in an equal stoichiometric
quantity is added and within a few minutes the solution
becomes opalescent. Vigorous stirring is continued as the
polymerizing mixture solidifies and then breaks into particles
with the consistency of wet sawdust. This crumb can then be
neutralized with dilute caustic, washed with water, and dried.
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Laboratory synthesis

• Solvents using mixtures of hexamethylphosphoramide -HMPA
- and Methylpyrrolidone –NMP- or HMPA and DMA produce
polymers with higher molecular weight than any of the three
solvents alone.
• Similarly, salts can be added to amide solvents to increase the
solubility of PPTA and thereby increase the molecular weight.
The combination of NMP and CaCl2 is especially useful for
providing high molecular weight PPTA.
• Another approach for increasing the level of molecular weight
that can be attained is the use of an acid acceptor, such as
tertiary amines and tributyl amine.
HMPA
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DMA
NMP

Commercial polymerization process
U.S. Pat. 3850888 (Nov. 26, 1974), J. A.
Fitzgerald and K. K. Likhyani (to E. I. du Pont
de Nemours & Co., Inc.)
TCL teraphthaloyl chloride
ICL isophthaloyl chloride
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Spinning of PPTA
• Unlike MPDI, high molecular weight PPTA is not soluble in
amide solvents, with or without the addition of inorganic
salts. Formation of fibers from PPTA became possible when it
was discovered that concentrated solutions of the polymer in
100% sulfuric acid had relatively low viscosity, could be spun
at moderate temperatures.
• The patent literature U.S. Pat. 3767756 (Oct. 23, 1973), H.
Blades (to E. I. du Pont de Nemours & Co.,Inc.) describes a
spinning process in which PPTA is dissolved in 98–100%
sulfuric acid at a concentration of greater than 18%. The
solution is pumped through a spinneret into an aqueous
coagulating/quenching bath, with an air gap separating the
spinnerets from the bath.
• The fiber is then washed thoroughly with water and dried.
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• Fibers formed by this spinning process are highly
oriented even without the type of high
temperature drawing common for other
polyamide fibers, and have high stiffness (tensile
moduli of 50–75 GPa).
• Even higher moduli (Kevlar 149 has a modulus of
180 GPa) can be obtained by subjecting the fibers
to a stretching process at high temperature.
• This would appear to be the basis for the high
modulus versions of Kevlar and Twaron.
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• Conventional synthetic fibers cannot attain sufficient strength
without drawing, and would not find practical use. However,
when a para-aramid is spun from this concentrated sulphuric
acid solution, the fiber shows high tenacity-high Young’s
modulus without the need for drawing and additionally
exhibits high heat resistance.
• The amorphous phase is virtually absent and a very small
fraction (few per cent) of unoriented crystalline component is
present.
• The crystalline structure of aramid fibres is arranged to form
ordered lamellae, stacks of platelets with approximately 3 nm
spacing perpendicular to the fibre axis.
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The lamellae are loosely connected as microfibrils (about 600 nm wide) with random tie
points between fibrils. The fibrillar structure is superimposed on the crystalline structure.

Skin-core structure
• Aramid fibres also present a skin-core
structure. The surface fibrils are uniformily
oriented in the axial direction, while the fibrils
in the inner core are imperfectly packed.
• Skin and core regions
differ also in terms
of density and void content.

Properties
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Effect of temperature on modulus

Applications of PPDA-1
• Protective Clothing. PPTA fibers have become the standard
from which body armor for the protection of police and
military personnel is made. These protective fabrics must be
thicker than their fire protection cousins, but they still must
be designed to be comfortable.
• PPTA is also used to make cut resistant fabrics for use in
gloves and chain saw chaps.
• Composites. High strength and low weight provides the basis
for the use of PPTA reinforced composites for the strength
members in aircraft, boats, the transportation industry, and
sports equipment. Related applications would include
providing the protective shielding in lightweight helmets for
the military and as rigid armor in military vehicles, police cars,
helicopters, and banks.
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Health and safety
• The safety requirements in the production of aramid fibers are substantial. The
monomers used in these processes are highly toxic and are sometimes handled at
high temperatures. They require protective clothing and great care to avoid any
contact.
• Sulfuric acid is used in the fiber formation of PPTA, and protective clothing is a
requirement for all personnel who enter the spinning area. The high strength of
PPTA fibers makes handling the yarn at high speed a hazardous enterprise, and
special training of production workers is especially important.
• PPTA and MPDI are relatively safe products that present minimal risk to human
health and the environment. MPDI fabrics have been worn for over 30 years
without significant effect on the skin, and PPTA products have a similar history.
• The Food and Drug Administration now provides that many forms of PPTA fiber
may be safely used as components of articles that come in repeated contact with
food.
• During the processing of these fibers some respirable fibrous particles are always
produced, and inhalation of these particles should be minimized. Adherence to
good industrial hygiene practices for ventilation and cleanup will protect against
significant exposure.
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• PPTA is also used in pulp form in a variety of automotive and
industrial applications such as brake and clutch linings,
gaskets, and nonwoven felts, where it has replaced asbestos.
• Ropes and Cables. Here, again, high specific strength and
stiffness are important. Cables based on PPTA fibers anchor oil
rigs and provide ship to shore mooring lines. PPTA fibers also
provide tension reinforcement for fiber optic cables, where
high stiffness and dielectric properties are key advantages.
• PPTA fibers were developed originally to replace polyester and
steel as the reinforcing fiber in tires. That market continues to
be important today, although it never reached the level
envisioned in the 1960s. Key market segments today include
high performance automobile tires, heavy-duty machinery
and aircraft tires and, on the other end of the spectrum,
puncture resistant, high performance bicycle tires.

Tensile properties of high-performance
fibers
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High performance fibres-2

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