Aramid fiber, or known by many as Kevlar (DuPont's brand name,) belongs in a family of synthetic products characterized by strength (some five times stronger than steel on an equal weight basis) and heat-resistance (some more than 500 degrees Celcius). It is appropriate for various applications such as composites, ballistics, aerospace, automotive, protective clothing against heat/radiation/chemicals, asbestos substitute, telecommunications (optical fiber cables) and many other.
The word aramid comes from a blend of the words 'aromatic' and 'polyamide' and is a general term for a manufactured fiber in which the fiber forming substance is a long chain synthetic polyamide, in which at least 85% is of amide linkages (-CO-NH-) attached directly to two aromatic rings, (as defined by the U.S Federal Trade Commission.)
Three aramid fiber manufacturers share most of the aramid market worldwide: DuPont in US makes aramid under the brand name “Kevlar” and “Nomex”. Teijin in Japan makes aramid by the brand name “Twaron” and “Technora”. Last, Kolon Industries in South Korea makes aramid by the brand name “Heracron”. Fibermax is a reliable Twaron and Kevlar aramid fiber processor and offers readymade fabrics, for composites and ballistic protection, with worldwide shipping capability.
The development of aromatic polyamides changed dramatically in good direction with the discovery of lyotropic liquid crystalline aramid. Aramid fiber first commercial applications appear in the early 1960s. Stephanie Kwolek did great work while working at DuPont in 1961. It took a long time to figure out how to make anything useful out of aramid (because it wouldn’t dissolve in anything,) but finally, the company introduced the para-aramid fiber with the brand name “Kevlar”. For her great invention of Kevlar, Stephanie Kwolek was inducted on July 1995 into the United States Inventor’s Hall of Fame.
- Fiber structure: A series of synthetic polymers in which repeating units containing large phenyl rings are linked together by amide groups. Amide groups (CO-NH) form strong bonds that are resistant to solvents and heat. Phenyl rings (or aromatic rings) are bulky six-sided groups of carbon and hydrogen atoms that prevent polymer chains from rotating and twisting around their chemical bonds.
- Fiber properties: They are characterized by medium to ultra-high strength, medium to low elongation and moderately high to ultra-high modulus with the densities ranging from 1.38g/cm3 to 1.47g/cm3. Heat-resistant and flame-resistant aramid fibers contain high proportion or meta-oriented phenylene rings, whereas ultra-high strength high-modulus fibers contain mainly para-oriented phenylene rings.
- Chemical properties: All aramids contain amide links that are hydrophilic. However, not all aramid products absorb moisture the same. The PPD-T (poly-phenylene terephthalamide) fiber has very good resistance to many organic solvents and salt, but strong acids can cause substantial loss of strength. Aramid fibers are difficult to dye due to their high Tg. Also, the aromatic nature of para-aramid is responsible for oxidative reactions when exposed to UV light, that leads to a change in color and loss of some strength.
- Thermal properties: Aramid fibers do not melt in the conventional sense but decompose simultaneously. They burn only with difficulty because of Limited Oxygen Index (LOI) values. It should be mentioned that at 300oC some aramid types can still retain about 50% of their strength. Aramids show high crystallinity which results in negligible shrinkage at high temperature.
- Mechanical properties: Aramid yarn has a breaking tenacity of 3045 MPa, in other words more than 5 times than this of steel (under water, aramid is 4 times stronger) and twice than this of glass fiber or nylon. High strength is a result of its aromatic and amide group and high crystallinity. Aramid retains strength and modulus at temperatures as high as 300oC. It behaves elastically under tension. When it comes to severe bending, it shows non-linear plastic deformation. With tension fatigue, no failure is observed even at impressively high loads and cycle times. Creep strain for aramid is only 0.3%.
To sum up, aramid general characteristics are:
Resistance to absorption
Resistance to organic solvent, good chemical resistance
No melting point
Excellent heat, and cut resistance
Sensitive to acids and ultraviolet radiation
Aramid fiber applications are divided into two categories: A) Reinforcement in composites like sport goods, aircraft, military vehicles and many other. B) Fabrics in clothing such as fire protection clothes or bullet proof vests. More elaborative uses of aramid are:
Various forms of composite materials
Protective gloves, helmets, body armor
Filament wound pressure vessels
Flame and cut resistant clothing
Ropes and cables
Optical fiber cable systems
Jet engine enclosures
Tennis strings and hokey sticks
Wind instrument reeds
Reinforced tyres and rubber goods
Circuit board reinforcement
Although every application meets its own requirements, almost all of them share aramid's major characteristics: high strength, high modulus, high toughness, thermal dimensionality stability, low creep and light weight.
HYBRID ARAMID APPLICATIONS
Aramid/Kevlar fibers can be used as reinforcement alone, but also combined with other fibers. This way the properties of the composing fibers are enhanced by synergy. Synergy is the interaction of multiple elements in a system to produce an effect different from or greater than the sum of their individual effects [definition from wikipedia]. In other words, when two or more fibers are combined, the resulting material has a tendency to keep the «good» properties and drop the «bad» ones. For this reason, hybrid fabrics woven in many styles are usually a favorite choice of many composites manufacturers and are used extensively. Aramid fibers can be woven to hybrid fabrics with:
Carbon fiber examples
Vectran fiber (V200T2)
Polyester-Diolen fiber (KP170P)
Other more complex combinations (CKPG251P)
ADVANTAGES / DISADVANTAGES
Aramid main advantages are high strength and low weight. Like graphite, it has a slightly negative axial coefficient of thermal expansion, which means aramid laminates can be made thermally stable in dimensions. Unlike graphite, it is very resistant to impact and abrasion damage. It can be made waterproof when combined with other materials like epoxy. It can be used as a composite with rubber retaining its flexibility. High tensile modulus and low breakage elongation combined with very good resistance to chemicals make it the right choice for different composite structural parts in various applications.
On the other side, aramid has a few disadvantages. The fibers absorb moisture, so aramid composites are more sensitive to the environment than glass or graphite composites. For this reason, it must be combined with moisture resistant materials like epoxy systems. Compressive properties are relatively poor too. Consequently, aramid is not used in bridge building or wherever this kind of resistance is needed. Also, aramid fibers are difficult to cut and to grind without special equipment (e.g. special scissors for cutting, special drill bits). Finally, they suffer some corrosion and are degradated by UV light. For this reason they must be properly coat.
ARAMID IN COMPOSITES
In a world where lightweight and durable composites are increasingly replacing conventional materials, aramid and para-aramid fibers play an important role. They are essential for reinforcing composites where weight reduction and excellent damage tolerance are required. Many different kinds of composite goods are reinforced with aramid because of the strength, stiffness and dimensional stability of laminates that contain it. Aramid and Kevlar fibers are compatible or can be used with many types of resin systems. The best choice of resin system is epoxy as it adheres best to the fiber surface. Vinyl ester, and isophthalic polyester may also be used. Orthopthalic polyester should be avoided as it does not provide sufficient adhesion to the fiber.