#184 from R&D Innovator Volume 4, Number 11          November 1995

The Kevlar® Innovation
by David Tanner, Ph.D.

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. 

Research

In 1965, 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.

Stephanie went 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.

The second 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.

The lab “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 dramatically improved.

Blades' second 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.

Scale-up

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.

Several 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.

Another problem 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 the customers.

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.

However, that 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.

Throughout this 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.

Market Development

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 applications.  Fortune magazine referred to Kevlar as “a miracle in search of a market.”  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 handled.  Surprisingly, however, Kevlar rope lifetimes were only 5 to 10% that of steel rope. 

An intensive 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 lifetime.

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.

The third 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.

The Kevlar 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.