Ryan Sloan
Real World Chemistry
Ann Glenn
What is Kevlar?
poly(p-phenyleneterephtalamide)
Kevlar is a man-made organic fiber with many unique properties such as high tensile strength. Kevlar is employed in making bulletproof vests, belts for radial tires, cables, reinforced composites for aircraft panels and boat hulls, flame-resistant garments (in blends with another similar fiber Nomex), sports equipment such as golf club shafts and lightweight bicycles, and in asbestos replacements in clutches and brakes (3).
Kevlar is a carbon based aramid, (a term invented as an abbreviation for aromatic polyamide, or aromatic nylon fiber). An aramid is a manufactured fiber related in chemical composition to the nylon family, yet its properties vary greatly from nylon. Aramids are prepared by condensation of a diamine and terephthalic acid, a carboxylic acid that contains a hexagonal benzene ring in its molecules. "The close packing of the aromatic polymer chains produced a strong, tough, stiff, high-melting fiber, good for radial tires, heat- or flame-resistant fabrics, bulletproof clothing, and fiber-reinforced composite materials" (4). Kevlar is only the trademarked name of poly-para-phenylene terephthalamide. It is distinguished by the structure of it’s polymer chains, which contains para-oriented phenyl rings. "Common nylons, such as nylon 6,6, do not have very good structural properties, so the para-aramid distinction is important. The aramid ring gives Kevlar thermal stability, while the para structure gives it high strength and modulus" (1).
The polymer used for Kevlar and related compounds, is wet-spun from a hot, high-solids solution of concentrated sulfuric acid. Nylon intended for ultrahigh-strength end use requires hot drawing. Fibers can be drawn either as an integral part of the spinning operation or in a separate step.
Fibers such as nylon and polypropylene can be drawn without applying external heat (or at a temperature no greater than about 70º C [160º F])--a process referred to as cold drawing. Other fibers, such as polyester, that are spun at extremely high rates yield what is known as partially oriented yarns (POY)--i.e., filaments that are partially drawn and partially crystallized and that can be drawn at a later time during textile operations (2).
Aramids can be hot-drawn to give a tensile strength nearly double that of the as-spun product. Kevlar 29, is drawn at a temperature over 400º C (750º F) to produce Kevlar 49, a fiber with nearly double the stiffness of the undrawn product (2). There are three grades of Kevlar available: Kevlar 29, Kevlar 49, and Kevlar 149. An example of Kevlar 49 is cloth. The table below shows the differences in material properties among the different grades.
|
Grade |
Density |
Tensile |
Tensile |
Tensile |
|
29 |
1.44 |
83 |
3.6 |
4.0 |
|
49 |
1.44 |
131 |
3.6--4.1 |
2.8 |
|
149 |
1.47 |
186 |
3.4 |
2.0 |
Like nylons, Kevlar filaments are made by extruding the material’s precursor through a spinneret. "The rod form of the para-aramid molecules and the extrusion process make Kevlar fibers anisotropic--they are stronger and stiffer in the axial direction than in the transverse direction" (1). Because of the rod-like structure of the para-oriented aramids, a "liquid-crystalline" solution is obtained that pre-orients the molecules even before they are spun. This is what leads to as-spun fibers of ultrahigh strength and ultrahigh stiffness. Kevlar’s high tensile strength makes it five times stronger per weight than steel.
Kevlar was developed at DuPont by Stephanie Kwolek, Herbert Blades, and Paul W. Morgan in 1965. DuPont began production in 1971. "Originally developed as a replacement for steel in radial tires, Kevlar is now used in a wide range of applications" (1). Kevlar is not produced in as high a volume as nylon and polyester, but due to its high unit price it represents a large business. "End uses for Kevlar in the home are few, but industrial uses are increasing as designers of products learn how to exploit the properties offered by this unusual material" (2). The table below gives a listing of the properties of Kevlar.
|
General Features : Kevlar |
||
|
High Tensile Strength at Low Weight |
Low Elongation to Break |
|
|
High Modulus (Structural Rigidity) |
Low Electrical Conductivity |
|
|
High Chemical Resistance |
Low Thermal Shrinkage |
|
|
High Toughness (Work-To-Break) |
Excellent Dimensional Stability |
|
|
High Cut Resistance |
Flame Resistant, Self-Extinguishing |
|
Kevlar does also have a few disadvantages. The fibers themselves absorb moisture, making Kevlar composites more sensitive to the environment. Although tensile strength and modulus are high, compressive properties are relatively poor. Kevlar is also very difficult to cut. Special scissors are needed for cutting dry fabric and special drill bits for drilling cured laminates (1). But despite its disadvantages, Kevlar’s unique properties has helped it to become one of the most important man-made organic fibers ever developed.
Bibliography
1.http://composite.about.com/industry/composite/library/weekly/aa050597.htm?iam=mt&terms=%2Bkevlar
2.http://search.britannica.com/bcom/search/results/?p_query0=Kevlar
3.www.dupont.com/afs/kfeatures.htm
4.http://www.umr.edu/~wlf/Synthesis/kevlar.html