It might be difficult for you to conjure up an image when I mention the words, aramid fibers, but if I told you that they are the raw materials used within body armor, bullet-proof vest, firefighters uniforms, etc, then you start to get the picture.
In fact, aramid fibers are the superstar family in fiber world. In their most basic of forms, they appear as bright golden yellow filaments (so far more colors are available). The name comes from a combination of two words, an "aromatic polyamide". Due to their outstanding strength-to-weight ratio and heat-resistant properties, aramid fibers can be applied to above mentioned protective-wear applications.
As well as their superb heat-resistance and high strength-to-weight ratio characteristics, they have many more impressive properties (more on those later on). This means that aramid fibers play an essential role in applications such as composites, the automotive industry, military applications and many similar fields.
1. History of Aramid Fibers
It took quite a while to work out how to utilize aramid fibers, mainly because it just can’t be dissolved in anything. This makes the process of working with aramid fibers rather difficult. The dramatic development of aromatic polyamides is mainly down to the discovery of lyotropic liquid crystalline aramid, which in solid form is commercially known as Kevlar®(DuPont’s brand).
In the early 1970s, a Polish-American chemist by the name of Stephanie Kwolek, a DuPont Research Scientist, invented the para-aramid, branded as Kevlar®. In 1964, Kwolek’s team started looking into new lightweight, strong fiber for use in tires. Thanks to the discovery of lyotropic liquid crystaline aramids, it enabled her to develop a novel spinning process for the anisotropic solution, this subsequently leads to the commercialization of Kevlar.
Structure of Kevlar, a para-aramid
Kwolek' s invention of Kevlar was fundamentally groundbreaking. In July 1995, she became the fourth woman to be added to the National Inventors Hall of Fame. In 1995 DuPont also awarded her the Lavoisier Medal for outstanding technical achievement, which even today is remarkable as she is the only female employee to have received the honor.
A Firefighter in Toronto, Canada wears a Nomex hood (2007)
However; aramid fibers themselves became commercialized in the early 1960s, before the invention of Kevlar. The trade name was Nomex®, a meta-aramid fiber produced by DuPont. The credit for this great invention goes to Dr Wilfred Sweeny, a Scottish-born scientist who also worked for DuPont.
This particular fiber features great thermal resistance, which means that it does not melt or catch fire at a common level of oxygen. This material was very quickly used in the manufacturing of protective clothing, air filtration units as well as becoming a substitute for asbestos. Nomex® was marketed in 1967 and has gone on to save millions of lives including firefighters, aircraft pilots, racing car drivers, to name but a few!
Meta-aramids are produced in many other countries including the Netherlands, Japan by Teijin under the trade name Conex, in Korea by Toray under the trade name Arawin, in China by Yantai Tayho under the trade name New Star, by SRO Group (China) under the trade name X-Fiper, a variant of meta-aramid in France by Kermel under the trade name Kermel as well.
There are even more twists and turns in the development of aramid fibers which you can see in the table below, the entire history of aramid fibers.
History of Development of Aramid Fibers
2. About the Aramid Fiber Category
In addition, both Nomex (a meta-aramid) and Kevlar (a para-aramid) have a number of variants each with specific properties.
For Kevlar, other para-aramid yarns can act as the replacement. And they usually come at a lower cost, with Twaron and Technora by Teijin, Heracron by Kolon, as well as Alkex by Hyosung included, each of which can exhibit similar results to Kevlar.
Kevlar® and Para-aramid Filament Yarn Alternatives Compared
|Density||(g/cm3)||1.44||1.44||1.44||1.44 - 1.45||1.39|
|Tenacity||(g/den)||23||23||23.0-24.0||18.7 - 28.3||28.3|
|Modulus||(Gpa)||70.33||70 - 102||8. - 109||60 - 120||74|
|Elongation @ Break||(%)||3.6||2.8 - 4.2||2.8 - 3.6||2.2 - 4.4||4.5|
|Moisture Regain||(%)||7||4.5||not avail||3.2 - 5.0||1.9|
|Decomposition||(°C)||427 - 482||500||not avail||500||500|
|(°F)||800 - 900||932||not avail||932||932|
3. The Molecular Structure of Aramid Fibers
Aramid consists of relatively rigid polymer chains with linked benzene rigs and amide bonds. The structure endows aramid fibers with high tenacity, high modulus and great toughness.
The molecular structure of aramids can be shown as below:
Kevlar is a kind of polyamide. Its amide groups are separated by para-phenylene groups. That is, the amide groups attach to the phenyl rings opposite to each other, at carbons 1 and 4.
3D Model of Kevlar Aramid. Click Here to see.
While Nomex is a polyamide. It has meta-phenylene groups, that is, the amide groups are attached to the phenyl ring at carbons 1 and 3.
Aramid fibers are created with a range of impressive properties. But due to the differences between para-aramid and meta-aramid, here I’d list the two separately.
#1. Para-aramid (typical example: Kevlar)
√ High Strength-to-weight ratio: Para-aramid fibers, like Kevlar and Twaron, are slightly different from the others. The two have outstanding strength-to-weight properties. Plus, they have great tenacity, making it abrasion-resistant.
Ultimate Tensile Strength
|E Glass Fiber||1307||3450||2.57|
|E Glass Laminate||775||1500||1.97|
√ High Young’s Modulus (structural rigidity): 130-179 GPa. While carbon fiber is 300 Gpa and glass 81 GPa.
(Young’s Modulus: Also known as elastic modulus. It defines the relationship between stress and strain in a material)
|Aramid(such as Kevlar and Twaron)||70.5-112.4|
√ Low elongation at break point, meaning that it stretches a little.
√ Para-aramid are usually Nonconductive under normal conditions.
√ Good Resistance to abrasion and cutting.
√ Good Resistance to organic solvents
√ Retain Low flammability, are resistant to thermal degradation and self-extinguishing.
√ Keep Good fabric integrity at elevated temperatures
√ Excellent Dimensional Stability.
#2. Meta-aramid (typical example: Nomex)
√ Heat Resistance: Meta-aramid has long-lasting thermal stability. It can operate for long time at a temperature of 204°C and it maintains excellent dimensional stability. It doesn’t go brittle, soften or melt even if it is briefly exposed to temperatures up to 300°C .
√ Chemical Stability: Meta-aramid has a very stable chemical structure. It's resistant to organic solvents.
√ Radiation Resistance: Good resistance to Ultraviolet, α and β.
√ Mechanical Properties: Meta-aramid is formable for moldable parts.
√ Low elongation at break point as well as para-aramid, meaning that it stretches a little.
The key properties of para-aramid and meta-aramid have been listed above. While the properties among para-aramid variations and meta-aramid variations differ, too.
The table below shows the various characteristics of aramid fibers and compiled from the Chemical Economics Handbook and Encyclopedia of Chemical Technology, Vol.19 and Indian Journal of Fiber and Textile Research.
Properties of Commercial Aramid Fibers
|Extension to Break
5. Major Application of Aramid Fibers
Thanks to the outstanding properties of aramid fibers, they can be used in a wide variety of industries.
√ Flame-resistant clothing: For example, military MIL-G-181188B suits. This includes Heat-protective clothing and helmets.
√ Substitute for asbestos (e.g. brake linings), whose fibers will give rise to pulmonary diseases after being inhaled into the lungs.
√ Hot air filtration fabrics
√ Reinforced thermoplastic pipes
√ Bullet-proof wear: Body armor, competing with PE-based fiber products such as Dyneema and Spectra.
√ Composite materials and they are often combined with carbon fiber.
√ Tires, most recently as Sulfron (sulfur-modified Twaron)
√ Mechanical rubber reinforcement
√ Ropes and cables, although severe weakening under impact limits its use on boats and climbing. Aramid cables more applicable to static load situations, for example, the cables used as guy-wires for hydro tower erection for Hydro Quebec.
√ Wicks for fire dancing
√ Optical fiber cable systems
√ Sailcloth (not necessarily racing boat sails)
√ Sporting equipment
√ Wind instrument reeds, such as the Fibracell brand
√ Loudspeaker diaphragms
√ Boat hull material
√ Fiber-reinforced concrete
√ Tennis strings (e.g. by Ashaway and Prince tennis companies)
√ Hockey sticks (normally in composition with such materials as wood and carbon)
√ Jet engine enclosures
The Bottom Line
Are you now suitably impressed at the versatility of the amazing fiber family?
I bet you are!
Next week I am going to talk more about aramid fibers so keep subscribed or you may lose the chance to pick up a little more materials science than the average guy in the street.