#184 from R&D
Innovator Volume 4, Number 11
Dr. Tanner is
president, Tanner & Associates, Inc. and executive director,
Edward de Bono International Creative Forum in Wilmington,
Delaware (phone 302-478-5177).
Previously, he was R&D director of duPont Industrial
Fibers Division and director, duPont Center for Creativity &
Innovation. Dr. Tanner was research director, Pioneering Research
Laboratory during the development phase of Kevlar, and technical
director during Kevlar’s commercialization phase.
Kevlar is an
aramid fiber having about 5 times the strength of steel at equal
weight. Applications include bullet-proof vests, fire-blocking
fabrics, cables, and reinforcement of tires, break linings, and
high-performance composites for aircraft.
How Kevlar was
developed is an excellent example of the innovation process,
showing that the period from discovery to commercialization had
tough hurdles and several reality gaps that had to be overcome.
A multi-disciplinary approach was almost always required to
overcome these obstacles. The
Kevlar innovation also illustrates the value of questioning
conventional wisdom as well as the value of an environment that
encourages risk taking.
The invention of
nylon and subsequent fibers, provided duPont with a direction to
continually find better fibers.
A goal was defined for a fiber with the heat-resistance of
asbestos and the stiffness of glass.
Stephanie Kwolek, a research scientist at duPont’s Pioneering
Research Laboratory, found that para aminobenzoic acid could be
polymerized and solubilized.
But it didn't seem that this polymer solution would be
useful for spinning into fibers because the solution was opaque
and couldn't be clarified either by heating or by filtration.
This implied that there was some inert matter which would
plug the spinneret holes. The
experienced fiber technicians advised against trying to spin it.
against conventional wisdom and insisted on extruding it through
the spinneret anyway. Surprisingly,
it spun well. We now
know that the opacity was due to the formation of polymer liquid
crystals. The stress-strain curve of this fiber was startling!
The physical test lab had to run it several times before
anyone would believe the results.
This fiber was very different, had the desired
extreme stiffness and heat resistance, and breaking strength
many-fold higher than existing fibers.
The first reality
gap was soon encountered. The
para aminobenzoic acid raw material was too costly to justify
scale-up. A major
program was launched to understand the physical chemistry of
liquid crystalline solutions.
With that knowledge, a suitable polymer, called PPD-T, was
developed from lower-cost ingredients.
This polymer eventually became the basis of Kevlar.
Things looked promising as the ingredient’s costs were
reasonable and the product properties were good.
reality gap quickly showed itself.
The spinning solvent had to be 100% sulfuric acid, a very
viscous solution. This
viscosity made it impractical to achieve the spinning speed
necessary for an economic process.
Also, when the researchers described what they had, the
manufacturing and engineering groups felt it was too risky!
The spinning solvent was unconventional and highly
corrosive, process yields and throughput were very low, and the
investment very high. These
are the types of realities that research people don't pay much
attention to when they are at the frontiers of discovery.
“wisdom” was that a concentration of polymer more than 10%
would increase viscosity and make the system less efficient; but
Herb Blades tried twice that amount at elevated temperatures.
This was a major breakthrough since under these conditions,
the PPD-T polymer and sulfuric acid unexpectedly formed a
crystalline complex. This
enabled spinning at much higher polymer concentrations than had
been previously possible. Because of this discovery, the spinning economics now
important contribution was finding that an air gap between the
spinneret and cold water made a fiber with extraordinary tensile
properties. Now PPD-T
was worthy of scale-up. The
product was unique, the process looked scaleable, and the
economics looked satisfactory. Manufacturing and engineering people became part of the team
and accepted the high risk in building and operating a plant based
on 100% sulfuric acid spin solvent.
Translation of a
laboratory discovery to a practical commercial process is one of
the hardest tasks faced by a technology-driven industry. Dozens of scientists and engineers of many disciplines were
assembled. Some moved
from the laboratory in Delaware to the plant site in Virginia.
Transfer of technology by transfer of people was essential
in this complex development.
unexpected hurdles were encountered.
How was the spent sulfuric acid to be disposed of after
spinning? The best
option was converting it into calcium sulfate (gypsum)--useful to
wallboard and cement manufacturers.
involved hexamethylphosphoramide (HMPA), the polymerization
solvent. While HMPA
had been studied for many years and no unusual toxic effects had
been reported, there still had to be a concern about its toxicity.
A major exposure
study with rats was initiated at duPont Haskel laboratories, which
showed some animal carcinogenicity.
Immediate steps were taken in the handling of HMPA to be
certain that there was no hazard to the workers, the community, or
A crash technical
program was initiated to identify an HMPA replacement of low
toxicity that provided identical fiber properties.
It also had to be compatible with the expensive equipment
layout designed for polymerization with HMPA.
The chemistry turned out to be relatively straightforward.
A combination of N-methylpyrrolidone (NMP) and calcium
chloride was the solvent of choice.
change provided new engineering challenges. For instance, there were problems due to the presence of a
low molecular-weight polymer fraction.
A reactor system to eliminate these interfering polymers
required multi-disciplinary chemical and engineering skills.
scale-up phase, the team members were confident and enthusiastic.
These were the underlying success factors.
There was no hurdle that couldn't be jumped--although at
times it took considerable energy, creative thinking, and support
to do so.
Perhaps an even
greater challenge than scale-up was to demonstrate the market
potential of Kevlar. This
was necessary to justify the final step in the innovation
process--a full-scale commercialization plant, requiring a $400
million investment. Therefore,
throughout the development there was intensive parallel effort to
find practical applications for this new fiber.
Kevlar was a
unique fiber that wouldn't automatically fit into existing
magazine referred to Kevlar as “a miracle in search of a
"systems" engineering approach requiring the combined
talents of people in many disciplines was necessary.
It was also essential to enter into early partnerships with
potential customers to determine what was really needed. This part of the program was as scientifically challenging
and exciting as the para aminobenzoic acid discovery itself.
Kevlar is a good
example to illustrate the systems approach. In sea water, the specific strength of Kevlar is more than 20
times that of steel. Steel
wire ropes, cycling over pulleys, were used extensively in
offshore drilling platforms.
Kevlar ropes, therefore should be lighter and more easily
however, Kevlar rope lifetimes were only 5 to 10% that of steel
effort was initiated to improve the rope. One change involved utilizing three different diameter
strands in a rope. This
compacted the structure and spread the lateral loads uniformly
over a greater area, and resulted in a five-fold improvement in
A second change
was to lubricate the strands by jacketing each with a braid
impregnated with fluorocarbons.
This reduced friction, heat buildup, abrasion, and internal
shear stresses. The
result was a six-fold improvement.
approach was to optimize twisting angles to minimize radial
squeezing forces without seriously affecting other rope
properties. This gave an additional two-fold improvement.
The result was a rope having more than three times the life
of steel in severe laboratory tests, and more than five times in
real-life experience with many oil rigs.
Now, a wide variety of Kevlar rope applications have been
developed for other applications.
innovation story exemplifies the kind of hurdles that must be
overcome and the interdisciplinary skills and systems approach
necessary to overcome them in order to bring a laboratory
discovery into commercial reality.
The story is still unfolding.
For her invention of Kevlar, Stephanie Kwolek was inducted,
July 1995, into the United States Inventor’s Hall of Fame.