Aramid Fibers
Introduction
In the research labs at E. I. Du Pont de Nemours & Company, Inc., in 1965 two research scientists, Stephanie Kwolek and Herbert Blades, were working in a corporate lab to create a new fiber. The technology they developed had enhanced strength, was lightweight and very flexible. The new fiber, called Kevlar, could be offered in many different forms. One of the most popular uses of Kevlar came in the form of bullet-resistant vests that police officers have relied on for over 25 years. The greatest attribute of the fiber was strength it provided in a very lightweight form, that was both comfortable and gave a wide range of movement to the officer. This discovery came from a very chemically similar compound called Nomex. The creation of this fiber gave birth to thermal technology, which combined heat and flame resistant properties along with advanced textile characteristics.
The production of aramid fibers known under their trademark names Kevlar® and Nomex.® have unique and beneficial properties. These two aramids are similar in basic structure and are sometimes produced in the same production plants. The difference is in their structure, Kevlar® is a para-aramid while Nomex® is a meta-aramid. An aramid is a polyamide where at least 85% of the amide bonds are attached to aromatic rings. The first aramid produced was called Nomex® introduced by Du Pont in 1961. For this report we will dissect each fiber separately.
Kevlar®
History
Kevlar® was originally developed in the 1960’s with the chemical name of poly-paraphenylene terephthalamide; but chemists to this day still do not understand why the fiber is so strong. First introduced commercially by Du Pont in 1972, the fiber has similar competitors in Twaron and Technora. Kevlar was originally developed as tire chord material for belts and carcasses in radial tires. The common uses for Kevlar® today include: adhesives and sealants, ballistics and defense, belts and hoses, composites, fiber optic and Electro-mechanical cables, friction products and gaskets, protective apparel, tires, and ropes and cables. These include items such as trampolines and tennis rackets.
Characteristics
The resounding characteristic of Kevlar is its remarkable strength. This very strong fiber has made its biggest impact in the ballistics defense where it’s used in bulletproof vests. It is stronger than fiberglass and five times stronger than steel on a pound-for-pound comparison. The high tensile strength and modulus are characteristics of all the Kevlar fibers, with Kevlar 49 and Kevlar 149 showing an even higher modulus. Kevlar’s chains are ordered in long parallel chains, and the key structural feat is the benzene aromatic ring that has a radial orientation that gives the molecule a symmetric and highly ordered structure that forms rod-like structures with a simple repeating backbone. This creates an extremely strong structure that has few weak points and flaws. The table provided below shows the various characteristics of Kevlar fibers and where compiled from both the Chemical Economics Handbook and Encyclopedia of Chemical Technology, Vol. 19.
Properties of Commercial Aramid Fibers
| ||||
Fiber type
|
Density, (g/cm3)
|
%Elongation
|
Modulus, Gpa
|
Tenacity
|
Kevlar 29
|
1.43
|
3.6
|
70
|
20-23
|
Kevlar 49
|
1.45
|
2.8
|
135
|
20-26
|
Kevlar 119
|
1.44
|
4.4
|
55
|
N/a
|
Kevlar 129
|
1.45
|
3.3
|
99
|
N/a
|
Kevlar 149
|
1.47
|
1.5
|
143
|
18
|
Nomex
|
1.38
|
22
|
17
|
5.8
|
Notice the much higher modulus and lower % elongation from Kevlar 49 and 149.
All of the general features of Kevlar listed here are taken from Du Pont’s web homepage:
· · 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
These features give a good picture on why Kevlar is a popular choice for all protection and casing purposes; low conductivity and self-extinguishing, flame resisting characteristics have made it a component for wire casing and fire fighting protection. The interesting thing is that it has a high elongation at break at around 4%, however it is commonly used in fiber that includes Lycra spandex.
Chemistry/Manufacture
KEVLAR® is a crystalline molecule that consists of long molecular chains that are highly oriented and show strong intermolecular chain bonding in the para position. It is made from the reaction of para-phenylenediamine (PPD) and molten terephthaloyl chloride. The production of p-phenylenediamine is difficult because of the diazotization and coupling of aniline. The reaction compounds involving the production Kevlar using p-phenylenediamine and terephthaloyl chloride is shown below.
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The PPD and the terephthaloyl chloride are reacted by using N-methylpyrrolidone as a reaction solvent. The structure for poly-paraphenylene terephthalamide is shown below.
The resulting polymer is filtered, washed and dissolved in concentrated sulfuric acid and is extruded through spinnerets. It then passes through a narrow duct and goes through the wet spin process where it is coagulated in sulfuric acid. The filament can take two different paths at this point. It can be formed into a yarn, washed and dried which is wound into spools that produces a modulus of 400-500 g/denier. Conversely, the filament can go under further heat treatment with tension and produce a fiber with a modulus of 900-1000 g/denier. The end product can take several forms. It can form filament yarns, pulp, or spun-laced sheets and papers.
Economic Impact
The production of fibers like Kevlar is really an oligopoly. Du Pont, being the producer of Kevlar is the largest producer para-aramids in the world. Du Pont currently produces in three countries: the United States, Northern Ireland, and Japan. These three sites have a production capacity of 65.9 million pounds of the 94.7 million pounds of total aramid fibers capacity. The other producers are Aramid Products in the Netherlands, which makes Twaron and Teijin Ltd of Japan, who makes Technora. Russia also produces a very low percentage of para-aramids called Fenylene.
Below is a production table for all para fibers in the last two decades. As of 1998, Kevlar accounted for 85% of the global market of para-aramid fibers. Production in Western Europe and Japan has jumped up greatly in the last ten years. All of the production in the United States is done by Du Pont to produce Kevlar. Also Du Pont accounts for about one-third of the total production in Europe and about one-half of the production in Japan.
World Production of Para Fibers (millions of pounds)
| |||||
United States
|
Western Europe
|
Japan
|
Russia
|
Total
| |
1979
|
13
|
0
|
0
|
<1 o:p="">
| |
13
1986
29
<1 o:p="">
0
2
31
1988
29
6
<1 o:p="">
2
37
1990
29
10
1
3
43
1991
26
10
4
2
42
1992
23
11
7
2
43
1993
23
12
7
2
44
1998
31
16
8
3
58
***Figure from this table taken from the Chemical Economics Handbook
Consumption of para-aramids in the three major regions: United States, Western Europe, and Japan hit 39 million pounds in 1993 and increased to 47 million pounds in 1998.
The growth of Kevlar has not yet met it’s full potential. The rapidly growing uses for Kevlar include ballistic protection in Western Europe, truck and bike tires, and with it’s lightweight dielectric properties, tension reinforcement for fiber optic above ground cables and protective coverings for underground and underwater fiber optic cable. Of all the Kevlar imported; 50% is used for tire manufacture, while the rest is used for fiber optics, brake materials, and for industrial fabrics. Dunlop Tire Corp. has begun to make a tire that is 30% lighter than traditional tires and that eliminates the steel belt and bead wire. The only catch that’s holding back a full scale use of Kevlar is its price; 1,500 denier is commonly used for tire cord, hoses and belts costs $12.00 per pound, while the other common grades of Kevlar range in the $13.00 to $15.00 range. Outside of the U.S., the same 1,500 denier fiber costs $23.00-27.00 per pound. Even with the expanding market as it currently is, widespread growth will not be realized until the costs of production falls.
Nomex®
History
NOMEX® was developed by DuPont for in 1961 for products that needed dimensional stability and good heat resistance. Nomex® products are used in protective apparel, hot gas filtration, and automotive hoses, electrical insulation, aircraft parts, and sporting goods.
Characteristics
The properties of Nomex include great electrical insulation properties at high temperatures. Nomex does not flow or melt upon heating and doesn’t degrade or char at temperatures until well over 370 degrees Celsius. The compound that is usually found in fire-fighters coats and airline seat covers is Nomex III, which is a composite of 95% Nomex and 5% Kevlar. The Kevlar adds stability and tear resistance to the material. The general properties of Nomex are listed below.
· · Heat and Flame Resistant
· · High Ultraviolet Resistance
· · High Chemical Resistance
· · Low Thermal Shrinkage
· · Formable for Molded Parts
· · Low Elongation to Break
· · Low Electrical Conductivity
This properties cause paper made by Nomex to be stronger and tougher than regular cellulosic papers. Overall, Nomex® is both thermally and chemically very stable. The difference between Kevlar and Nomex is the location of the amide linkages on the aromatic ring. Those differences cause Nomex to a lower modulus and tensile strength and a higher elongation and solubility in organic solvents.
Chemistry/Manufacture
Nomex®, is a meta-aramid fiber created by Du Pont in 1961. The chemical name of Nomex is poly (m-phenylene isophthalamide), which is produced from the reaction of m-phenylenediamine and isophthaloyl chloride whose structures are shown below.
The solution is dry spun through spinnerets. The remaining solvent is evaporated, the filament is washed and wound into tow, heated, and finally stretching into rolls at a temperature of 150 degree’s Celsius. Nomex can be produced as a continuous filament yarn, staple, spun yarn, floc, pressboard, paper, needle felt, or as a fabric. Next we will take a look at the economics of producing Nomex.
Economic Impact
The growth of meta-aramid fibers has grown steadily over the last 10 years. At the same time the U.S. share of production has fallen 19% from 1990 to 1998 from 81% to 62%. This drop is largely due to the growth of production in Western Europe, from no production in 1990 to 21% of the market share in 1998. The table below shows production patterns of meta-aramids since 1979.
World production of Meta-Aramid fibers (millions ofpounds)
| ||||||
United States
|
Western Europe
|
Japan
|
Russia
|
Total
| ||
1979
|
12
|
0
|
<1 o:p="">
| |||
<1 o:p="">
12
1986
18
0
2
1
21
1988
20
0
2
2
24
1990
21
0
4
2
26
1991
23
0
4
1
28
1992
24
0
4
neg
28
1993
26
2
4
neg
32
1998
26
9
5
2
42
***Figures taken from the Chemical Economics Handbook
The world production has more than tripled in the last three decades while consumption in the U.S. only grew 60%. This is due the great increase of consumption in Western Europe and growth in Japan. The uses of this consumption is largely for the production of paper electrical uses, as insulators in dry transformers, motors, and transformers which account for 49% of all U.S. consumption. In the textile industry, fire resistant fabric accounts for 19% and filtration 17% of all U.S. consumption. Overall, the expected annual growth rate for meta-aramids is suppose to average 3% a year until 2003. The textile industry is responsible for the production of fire-resistant clothing and seat covering in airline seats. It also has established a market in asbestos replacement, thermal insulation and as a fiber that prevent static electricity buildup. The prices for meta-aramid fibers range greatly. The staple 1.5-denier fiber cost $11.50 per pound while continuous filament yarn of 200 denier cost $25.00 per pound. Even more, 1,200 denier filament yarn costs 39.00 per pound!
Summary
In this paper, I have dissected the chemistry and the growing markets of the specialty fibers Kevlar and Nomex. Each of these fibers has shown extensive growth over the last few decades with growth expected to continue over the next several years. This poses the question on whether we should expand into these markets and capitalize on this growth or sit by the wayside. In my opinion, the outlook for polyamids such as Kevlar and Nomex aramids is very good. Du Pont, an established company whose products are well known and trusted, dominates the production of these fibers. Over the next several years, Du Pont is going to profit from the production of these fibers. With the established name brand and quality that Du Pont already holds, the barriers to enter the market are too great for any company to start up and take their strangle hold over the aramid market. The invention of these fibers grew from the research from making very basic items into one of the most structurally sound products made today.
Bibliography
“Aramids.” About.com. 1996. <http://composite.about.com/industry/composite/gi/dynamic/offsite.htm?site=http%3A%2F%2Fwww.psrc.usm.edu%2Fmacrog%2Faramid.htm> (15 Nov. 2000)
Chang, Alen: Hung, Richard; Lew, Katherine, Function and Performance of Kevlar, pdf file, http://www.mse.berkeley.edu/classes/matsci102/Kevlar.pdf
Du Pont Website: “Kevlar” <http://www.dupont.com/afs/kfeatures.htm> (15 Nov. 2000).
Du Pont Website: “Nomex” < http://www.dupont.com/nomex/> (15 Nov. 2000).
“Flame Retardants.” Ullmann’s Encyclopedia of Industrial Chemistry. 1988. Vol 11.
GE website: http://www.complas.com/kevlar.asp
Groce, Donald F. “Cotton, Nylon, Lycra Spandex and Allergies.” Latex Allergy News. Sept. 1996. <http://latexallergylinks.tripod.com/lycra.html> (15 Nov. 2000)
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Reisch, Marc. “What’s that Stuff?” Chemical and Engineering News. 15 Feb 1999 <http://pubs.acs.org/cen/whatstuff/stuff/7707scitek4.html> (15 Nov. 2000)
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University of Missouri-Rolla website: http://www.umr.edu/~wlf/Synthesis/kevlar.html
Last Updated: 30 April 2001
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