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 rings 2

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 glass 1 Experiments revealed such a material must contain stiff chain aromatic polyamides – Such material escaped scientists due to extreme insolubility and intractability (knottiness) 1

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• In 1965, research scientist Stephanie Kwolek from DuPont discovered p-aminobenzoic acid could polymerize and solubilize to yield a ridged rod-shaped spinnable polymer 1 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 stiffness 1

Kevlar Chemistry and Development

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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 4 aminoaniline 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 chains 2

• 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 more 1

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 heated 1 Complex melts at 70 °C 1

• 2 nd 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 properties 1 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 properties 2

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, pi stacking, and stiff chain crystallinity why is Kevlar so damage tolerant?

Drona?

Ductile compressive failure mode! Correct!

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• • • 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 Kevlar 1 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 hours 4

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 applications 3

• 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 cut resistant gloves, in life protection uses such as helmets, vehicular armoring, and plates, and as rubber reinforcement in tires and automotive hoses 3

• 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 applications 3 Dupont

• Kevlar® 100 – Producer-colored Kevlar® yarns, used in ropes and cables, tapes and strappings, gloves and other protective apparel, and sporting goods 3

• Kevlar® 119 – Higher-elongation, flexible-fatigue–resistant yarn types found in mechanical rubber goods, such as tires, automotive belts, and hoses 3 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 industry 3

• 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 soldier 3

• 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