Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids
such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, this high-strength
material was first commercially used in the early 1970s as a replacement for steel in
racing tires. Typically it is spun into ropes or fabric sheets that can be used as such
or as an ingredient in composite material components.
Currently, Kevlar has many applications, ranging from bicycle tires and racing sails to body
armor because of its high tensile strength-to-weight ratio; by this measure it is 5 times stronger
than steel on an equal weight basis. It is also used to make modern drumheads that withstand
high impact. When used as a woven material, it is suitable for mooring lines and other
underwater applications. A similar fiber called Twaron with roughly
the same chemical structure was developed by Akzo in the 1970s; commercial production
started in 1986, and Twaron is now manufactured by Teijin.
History
Poly-paraphenylene terephthalamide – branded Kevlar – was invented by Polish-American
chemist Stephanie Kwolek while working for DuPont, in anticipation of a gasoline shortage.
In 1964, her group began searching for a new lightweight strong fiber to use for light
but strong tires. The polymers she had been working with at the time, poly-p-Phenylene-terephthalate
and polybenzamide, formed liquid crystal while in solution, something unique to those polymers
at the time. The solution was "cloudy, opalescent upon
being stirred, and of low viscosity" and usually was thrown away. However, Kwolek persuaded
the technician, Charles Smullen, who ran the "spinneret", to test her solution, and was
amazed to find that the fiber did not break, unlike nylon. Her supervisor and her laboratory
director understood the significance of her accidental discovery and a new field of polymer
chemistry quickly arose. By 1971, modern Kevlar was introduced. However, Kwolek was not very
involved in developing the applications of Kevlar.
Production Kevlar is synthesized in solution from the
monomers 1,4-phenylene-diamine and terephthaloyl chloride in a condensation reaction yielding
hydrochloric acid as a byproduct. The result has liquid-crystalline behavior, and mechanical
drawing orients the polymer chains in the fiber's direction. Hexamethylphosphoramide
was the solvent initially used for the polymerization, but for safety reasons, DuPont replaced it
by a solution of N-methyl-pyrrolidone and calcium chloride. As this process had been
patented by Akzo in the production of Twaron, a patent war ensued.
Kevlar production is expensive because of the difficulties arising from using concentrated
sulfuric acid, needed to keep the water-insoluble polymer in solution during its synthesis and
spinning. Several grades of Kevlar are available:
Kevlar K-29 – in industrial applications, such as cables, asbestos replacement, brake
linings, and body/vehicle armor. Kevlar K49 – high modulus used in cable
and rope products. Kevlar K100 – colored version of Kevlar
Kevlar K119 – higher-elongation, flexible and more fatigue resistant
Kevlar K129 – higher tenacity for ballistic applications
Kevlar AP – 15% higher tensile strength than K-29
Kevlar XP – lighter weight resin and KM2 plus fiber combination
Kevlar KM2 – enhanced ballistic resistance for armor applications
The ultraviolet component of sunlight degrades and decomposes Kevlar, a problem known as
UV degradation, and so it is rarely used outdoors without protection against sunlight.
Structure and properties
When Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa, and
a relative density of 1.44. The polymer owes its high strength to the many inter-chain
bonds. These inter-molecular hydrogen bonds form between the carbonyl groups and NH centers.
Additional strength is derived from aromatic stacking interactions between adjacent strands.
These interactions have a greater influence on Kevlar than the van der Waals interactions
and chain length that typically influence the properties of other synthetic polymers
and fibers such as Dyneema. The presence of salts and certain other impurities, especially
calcium, could interfere with the strand interactions and care is taken to avoid inclusion in its
production. Kevlar's structure consists of relatively rigid molecules which tend to form
mostly planar sheet-like structures rather like silk protein.
Thermal properties Kevlar maintains its strength and resilience
down to cryogenic temperatures; in fact, 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 about
10% reduction in strength occurs after 500 hours. At 260 °C 50% strength reduction occurs
after 70 hours. Applications
Protection Cryogenics
Kevlar is often used in the field of cryogenics for its low thermal conductivity and high
strength relative to other materials for suspension purposes. It is most often used to suspend
a paramagnetic salt enclosure from a superconducting magnet mandrel in order to minimize any heat
leaks to the paramagnetic material. It is also used as a thermal standoff or structural
support where low heat leaks are desired. Armor
Kevlar is a well-known component of personal armor such as combat helmets, ballistic face
masks, and ballistic vests. The PASGT helmet and vest used by United States military forces
since the 1980s both have Kevlar as a key component, as do their replacements. Other
military uses include bulletproof facemasks used by sentries and spall liners used to
protect the crews of armoured fighting vehicles. Even Nimitz-class aircraft carriers include
Kevlar armor around vital spaces. Related civilian applications include Emergency Service's
protection gear if it involves high heat, and Kevlar body armor such as vests for police
officers, security, and SWAT. Personal protection
Kevlar is used to manufacture gloves, sleeves, jackets, chaps and other articles of clothing
designed to protect users from cuts, abrasions and heat. Kevlar based protective gear is
often considerably lighter and thinner than equivalent gear made of more traditional materials.
Sports equipment
It is used as an inner lining for some bicycle tires to prevent punctures. In table tennis,
plies of Kevlar are added to custom ply blades, or paddles, in order to increase bounce and
reduce weight. It is used for motorcycle safety clothing, especially in the areas featuring
padding such as shoulders and elbows. In kyudo or Japanese archery, it may be used
as an alternative to more expensive hemp for bow strings. It is one of the main materials
used for paraglider suspension lines. In fencing it is used in the protective jackets,
breeches, plastrons and the bib of the masks. Tennis racquets are often strung with Kevlar.
It is even used in sails for high performance racing boats.
It is increasingly being used in the "peto", the padded covering which protects the picadors'
horses in the bullring. Shoes
With advancements in technology, Nike used Kevlar in shoes for the first time. It launched
the Elite II Series, with enhancements to its earlier version of basketball shoes by
using Kevlar in the anterior as well as the shoe laces. This was done to decrease the
elasticity of the tip of the shoe in contrast to nylon used conventionally as Kevlar expanded
by about 1% against nylon which expanded by about 30%. Shoes in this range included LeBron,
HyperDunk and Zoom Kobe VII. However these shoes were launched at a price range much
higher than average cost of basketball shoes. It was also used as speed control patches
for certain Soap Shoes models. and the laces for the adidas F50 adiZero Prime football
boot. Music
Audio equipment Kevlar has also been found to have useful
acoustic properties for loudspeaker cones, specifically for bass and midrange drive units.
Additionally, Kevlar has been used as a strength member in fiber optic cables such as the ones
used for audio data transmissions. Bowed string instruments
Kevlar can be used as an acoustic core on bows for string instruments. Kevlar's physical
properties provide strength, flexibility, and stability for the bow's user. To date,
the only manufacturer of this type of bow is CodaBow.
Kevlar is also presently used as a material for tailcords, which connect the tailpiece
to the endpin of bowed string instruments. Drumheads
Kevlar is sometimes used as a material on marching snare drums. It allows for an extremely
high amount of tension, resulting in a cleaner sound. There is usually a resin poured onto
the Kevlar to make the head airtight, and a nylon top layer to provide a flat striking
surface. This is one of the primary types of marching snare drum heads. Remo's "Falam
Slam" Patch is made with Kevlar and is used to reinforce bass drum heads where the beater
strikes. Woodwind reeds
Kevlar is used in the woodwind reeds of Fibracell. The material of these reeds is a composite
of aerospace materials designed to duplicate the way nature constructs cane reed. Very
stiff but sound absorbing Kevlar fibers are suspended in a lightweight resin formulation.
Other uses Fire dancing
Wicks for fire dancing props are made of composite materials with Kevlar in them. Kevlar by itself
does not absorb fuel very well, so it is blended with other materials such as fiberglass or
cotton. Kevlar's high heat resistance allows the wicks to be reused many times.
Frying pans Kevlar is sometimes used as a substitute for
Teflon in some non-stick frying pans. Rope, cable, sheath
The fiber is used in woven rope and in cable, where the fibers are kept parallel within
a polyethylene sleeve. The cables have been used in suspension bridges such as the bridge
at Aberfeldy in Scotland. They have also been used to stabilize cracking concrete cooling
towers by circumferential application followed by tensioning to close the cracks. Kevlar
is widely used as a protective outer sheath for optical fiber cable, as its strength protects
the cable from damage and kinking. When used in this application it is commonly known by
the trademarked name Parafil. Electricity generation
Kevlar was used by scientists at Georgia Institute of Technology as a base textile for an experiment
in electricity-producing clothing. This was done by weaving zinc oxide nanowires into
the fabric. If successful, the new fabric would generate about 80 milliwatts per square
meter. Building construction
A retractable roof of over 60,000 square feet of Kevlar was a key part of the design of
Montreal's Olympic stadium for the 1976 Summer Olympics. It was spectacularly unsuccessful,
as it was completed ten years late and replaced just ten years later in May 1998 after a series
of problems. Brakes
The chopped fiber has been used as a replacement for asbestos in brake pads. Dust produced
from asbestos brakes is toxic, while aramids are a benign substitute.
Expansion joints and hoses Kevlar can be found as a reinforcing layer
in rubber bellows expansion joints and rubber hoses, for use in high temperature applications,
and for its high strength. It is also found as a braid layer used on the outside of hose
assemblies, to add protection against sharp objects.
Particle physics A thin Kevlar window has been used by the
NA48 experiment at CERN to separate a vacuum vessel from a vessel at nearly atmospheric
pressure, both 192 cm in diameter. The window has provided vacuum tightness combined with
reasonably small amount of material. Smartphones
The Motorola RAZR Family and the Motorola Droid Maxx have a kevlar backplate, chosen
over other materials such as carbon fiber due to its resilience and lack of interference
with signal transmission. Marine Current Turbine and Wind turbine
The Kevlar fiber/epoxy matrix composite materials can be used in marine current turbine or wind
turbine due to their high specific strength and light weight compared to other fibers.
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. Typical applications include
monocoque 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. See also
Innegra S Vectran
Spider silk Ultra high molecular weight polyethylene
References
External links Kevlar Home Page
Aramids Kevlar – Design Dictionary. Illustrated
article about Kevlar Matweb material properties of Kevlar
U.S. Patent 5,565,264 Kevlar
Kevlar in body armor Synthesis of Kevlar
Aberfeldy Footbridge over the River Tay Kevlar at Plastics Wiki