Physics/Chemical Engineering/Math Kevlar - Ashley Macner Physics/Chemical Engineering/Math Clarkson University

History DuPont’s brand name for a light and strong aramid fibre. Created in 1965 in a DuPont lab by Stephanie Kwolek and Herbert Blades First used commercially in early 1970’s -Aramid fibre is a fire resistant and strong synthetic fiber. Used as an asbestos substitute

Result: Polymer exhibits liquid-crystalline behaviour Overall Reaction 1,4-phenyl-diamine terephthaloyl chloride poly-paraphenylene terephthalamide -Heteropolymer composed of the two monomers 1,4-phenyl-diamine and terephthaloyl chlorides -This reaction is considered to be a condensation polymerization reaction because there is a loss of a small molecule of HCl, which is common for these type of reactions. -An amide solvent is one that is typically used for the process. -Lower temperatures are used in order to induce the polycondensation reaction. After the mixture gels, low temperatures are no longer necessary. Condensation Polymerization Result: Polymer exhibits liquid-crystalline behaviour

Polymer contains aromatics and amides Kevlar fibers are oriented parallel to each other (Orderly and untangled, as in a crystalline structure) Polymer contains aromatics and amides -The crystallinity is obtained through a process termed wet-spinning, which will be discussed in a bit. -It has been known that polymers which contain aromatics and amides have a high breaking strength.

Individual polymer fibers are held together by hydrogen bonds -The hydrogen bonds form in between the carbonyl groups and the protons of neighboring chains. The hydrogen bonds act like glue. -There is also partial pi-stacking of the benzenoid aromatic stacking interactions between stacked strands. In other words, due to the strong van der Waals forces between the flat aromatic rings, then tend to stack themselves up like coins. Pi-stacking is a noncovalent bonding situation in which the p-orbitals of adjacent aromatic compounds (conjugated pi-bonds) overlap. This intermolecular interaction is obviously stronger, the more pi-bonds there are.

Aromatic components have a radial orientation -The radial orientation gives a high degree of symmetry and regularity to the internal structure of the fibers. This crystalline regularity is the largest contributor to the strength of Kevlar. -These photos were taken using bright synchrotron radiation. Basically, synchrotron radiation is Electromagnetic radiation that is generated by ultrarelativistic electrons moving through magnetic fields.

Stereochemistry trans cis -In the trans amide, the hydrocarbon groups are on the opposite sides of the amide bond -In the cis amide, both of the hydrocarbon groups are on the same side of the amide bond. trans cis

cis conformation trans conformation -The cis conformation will obviously not form due to steric hindrance inbetween the aromatic groups. They want to be as far apart as possible so that they don’t get in the way of each other. Hence, Kevlar mainly exists in the trans conformation.

Wet-spun from a polymer solution in concentrated sulfuric acid Processing Wet-spun from a polymer solution in concentrated sulfuric acid -Wet-spinning involves pumping the polymer solution into a spinneret. A spinneret is the element through which the polymer is extruded and usually has several holes. The polymer melt is put into filament form by extrusion through a die. The viscous filament upon exiting, precipitates from solution, solidifies and aligns itself because of the viscous flow. -Page 525 gives a nice schematic of the wet-spinning process.

Failure Modes - Alkaline Conditions Degradation - UV Light - Chlorine Sensitive to the environment Buckles under compression Kevlar is hard to cut -An alkaline is basically a basic, ionic salt of an alkali metal or an alkali earth metal element (Groups 1 and 2 of the periodic table). -Kevlar is sensitive to the environment in that it absorbs moisture. It’s ability to stop projectiles and bullets degrades when it is wet. It has to be specifically waterproofed in order to prevent this. -The cutting of Kevlar is obviously not a failure mode but cutting up cured laminates without fraying is very difficult.

Useful Characteristics Low Elongation to Break High Modulus Excellent Dimensional Stability High Tensile Strength at Low Weight High Cut Resistance Low Electrical Conductivity Useful Characteristics -Orange(Structural Rigidity) -Also good on impact…which is why its used Low Thermal Shrinkage High Toughness (Work-To-Break) High Chemical Resistance, Flame Resistant, Self-Extinguishing

Uses of Kevlar -Good for canoes due to its resistance to puncture -Good for sails because it will not tear or puncture -Puncture proof tires. Used to make them out of solid rubber to achieve this process which is obviously extremely heavy.

Tensile Strength [GPa] Odds-N-Ends Grade Density [g/cm3] Tensile Modulus [GPa] Tensile Strength [GPa] Tensile Elongation % 29 1.44 83 3.6 4.0 49 131 3.6-4.1 2.8 149 1.47 186 3.4 2.0