Kevlar
Derek Leas
What is Kevlar???
Wikipedia
• Kevlar is an aromatic polyamide.
• In 1974 the United States Federal Trade
Commission adopted the generic term
“aramid”
– A manufactured fiber in which the fiber-forming
substance is a long chain synthetic polyamide in
which at least 85% of the amide (—CO—NH—)
linkages are attached directly to two aromatic
rings2
Wikipedia
History
• In the early 1960s, DuPont was interested in
developing “super fibers,” due to the prior
valuable invention of nylon
– A fiber with the heat-resistance of asbestos and
the stiffness of glass1
• Experiments revealed such a material must contain
stiff chain aromatic polyamides
–
Such material escaped scientists due to extreme insolubility
and intractability (knottiness)1
1
• In 1965, research scientist Stephanie Kwolek
from DuPont discovered p-aminobenzoic acid
could polymerize and solubilize to yield a
ridged rod-shaped spinnable polymer1
Wikipedia
• p-aminobenzoic acid proved to be too costly
thus Kwolek developed a similar polymer
using more cost-efficient ingredients
– Used p-pheylenediamine and terephtalic acid
(PPTA) thus developing Kevlar
– Aside from lower cost ingredients it also had
highest symmetry, and theoretically the highest
stiffness1
Kevlar Chemistry and Development
2
What kind of polymerization process is
this???
Marshall?
Correct! Condensation Polymerization!
• You make a solution of the terephthaloyl chloride
in an organic solvent, and a solution of 4aminoaniline in water. You carefully float one
solution on top of the other in a small beaker,
taking care to get as little mixing as possible.
• Kevlar forms at the boundary between the two
solutions. If you pick up the boundary layer with
a pair of tweezers, you can pull out an amazingly
long tube of Kevlar from the beaker
• Kevlar forms liquid crystals and can thus be
spun very well.
• Its ridged-rod properties in solution make it
behave differently than flexible polymers.
• Flexible polymers have high level of
entanglement at high concentrations, when
spun they develop only partial extended
chains2
• Ridged polymers like p-aramids don’t form
very high levels of entanglement at high
concentration, at a certain concentration they
can no longer populate a polymer solution
randomly
– In order to pack more molecules into solvent they
must align themselves parallel forming liquid
crystalline domains.
– When spun this creates fully extended chains and
increases crystallinity even more1
1
• Initially highly concentrated (more crystalline Kevlar
product) polymer solutions couldn’t be created
• Breakthrough occurred when the polymer solution
containing the 100% sulfuric acid spinning solvent was
found to form a crystalline complex at polymer
concentration of 20% when heated1
• Complex melts at 70 °C1
• 2nd breakthrough occurred in spinning
process. An air gap between the spinneret
face and the quench bath resulted in greater
orientation in the polymer and greater tensile
properties1
1
• Once in air gap elongation occurs
due to making the velocity of the
fiber as it leaves the quenching
bath higher than the velocity of
the polymer emerging from the
spinneret hole → draw ratio- can
be fine tuned to get higher
tenacities and moduli
• The stretch in the air gap perfects
the alignment in the liquid crystal
domains; fiber orientation is
attained via the cold water bath
• The higher the polymer
orientation in the coagulation
medium the greater the
mechanical properties2
My own thoughts...
Adjusting draw ratio orientates the polymer
to either the more stable antiparallel
configuration or the less stable parallel
configuration
1
Initial/Finished Appearance
Aside from H-bonding, pistacking, and stiff chain
crystallinity why is Kevlar
so damage tolerant?
Drona?
Ductile compressive failure mode!
Correct!
3
• This occurs by compressive buckling of p-aramid chains ~0.5%
compressive buckling!
• This causes molecular rotation of the amide C-N bonds or a
shift from trans to cis
• Results in a yielding to the imposed stress without actual
cleavage thus better explaining the compressive properties of
Kevlar1
1
• Kevlar fiber has a tensile strength of about 3,620
MPa
• Kevlar maintains its strength and resilience down
to cryogenic temperatures (−196 °C); it is slightly
stronger at low temperatures.
• At higher temperatures the tensile strength is
immediately reduced by about 10–20%, and after
some hours the strength progressively reduces
further.
– For example at 160 °C (320 °F) about 10% reduction in
strength occurs after 500 hours. At 260 °C (500 °F)
50% strength reduction occurs after 70 hours4
Tensile strength (TS)
• The maximum stress that a material can
withstand while being stretched or pulled
before failing or breaking
Kevlar Applications
• Kevlar® AP
– A next-generation fiber that offers advanced
performance, value, and increased design
flexibility in many applications3
• Kevlar 29
– The original family of product types of Kevlar®, having similar tensile
properties with many deniers and finishes. These yarns are used in
ballistic applications, ropes and cables, protective apparel such as cutresistant gloves, in life protection uses such as helmets, vehicular
armoring, and plates, and as rubber reinforcement in tires and
automotive hoses3
• Kevlar® 49 (K49)
– High-modulus type used primarily in fiber optic
cable, textile processing, plastic reinforcement,
ropes, cables, and composites for marine sporting
goods and aerospace applications3
Dupont
• Kevlar® 100
– Producer-colored Kevlar® yarns, used in ropes and
cables, tapes and strappings, gloves and other
protective apparel, and sporting goods3
• Kevlar® 119
– Higher-elongation, flexible-fatigue–resistant yarn
types found in mechanical rubber goods, such as
tires, automotive belts, and hoses3
Sears.com
• Kevlar® 129
– Lightweight, high-performance, and high-tenacity
type of yarns used in motorcycle racing gear, life
protection accessories, ropes and cables, and
high-pressure hoses used in the oil and gas
industry3
• Kevlar® KM2
– Woven into fabric meeting performance
requirements for helmets and vests for military
and high-performing UDs for spall liners. “Kevlar
KM2 is much softer and flexible than current
Kevlar 129 vests” said a soldier3
• Kevlar® KM2 Plus
– High tenacity, high toughness, and finer denier
fiber used in vests and helmet for both military
and law enforcement officers.
Composite Materials
•
•
Aramid fibers are widely used for reinforcing composite materials, often in
combination with carbon fiber and glass fiber.
The matrix for high performance composites is usually epoxy resin.
– bodies for F1 racing cars, helicopter rotor blades, tennis, table tennis,
badminton and squash rackets, kayaks, cricket bats, and field hockey, ice
hockey and lacrosse sticks
Bibliography