Stephanie Kwolek: Inventor of Kevlar

Stephanie KwolekOriginally published in the ebook A Passion for Science: Stories of Discovery and Invention.

by Suze Kundu

In 2001, a police lieutenant, David Spicer, was recovering in hospital after being shot in the chest and arms at point blank range. Spicer was alive to tell the tale, thanks to the Kevlar body armour that he was wearing at the time.

Kevlar thread is strong because it is made of plastic fibres in which matchstick-like molecules line up and stick to one another, giving it a specific tensile strength of over eight times that of steel wire. Kevlar fabric is even stronger because these fibres are then woven tightly together and are very difficult to prise apart. This is why Kevlar is used for body armour: the amount of energy required to break apart multiple layers of Kevlar fabric is greater compared to the energy that a bullet or a knife can impart. The bullet, knife or other weapon is slowed down and deformed by each layer until it is stopped in its tracks within the body armour, or is moving slowly enough to cause much less damage to the victim if it does break through the body armour.

Spicer had the opportunity to thank Stephanie Kwolek, the inventor of Kevlar, for her practical contributions to materials science that had saved his life. Summing up Kwolek perfectly after their conversation, Spicer simply said, “She was a tremendous woman.”

Kwolek’s career as a research scientist spanned four decades, and the impact of her contributions to science, as well as in encouraging and inspiring more women to take up science, continue to be felt to this day.

Of science and fashion

Stephanie Kwolek was born on 31 July 1923 in New Kensington, Pennsylvania, a suburb of Pittsburgh. Her father, John Kwolek – Jan Chwałek, in his native Polish – was a naturalist, which sparked her interest in science from an early age. Together they would explore the woods and fields near their home, creating scrapbooks of their findings. Sadly, he died when she was only 10 years old.

Kwolek’s mother, Zajdel, known as Nellie, worked in fashion, which exposed her daughter to elements of design and sewing, and allowed her to work with a range of fabrics; her interest in materials would extend beyond textiles as her career developed. Kwolek showed an aptitude for fashion design, but Nellie knew that her daughter was a perfectionist and apparently said that if Stephanie were to pursue this as a career she would starve, as she would never be happy enough with the pieces that she created.

Kwolek initially wanted to study medicine, but the high cost of medical school meant that she needed a plan: she decided that she would study chemistry and work in industry until she had enough money to pursue her dream of becoming a medic. She graduated from Margaret Morrison Carnegie College, part of the Carnegie Mellon University, with a degree in chemistry in 1946.

One of the several jobs she applied for was that of chemist at DuPont, where she was interviewed by research director W Hale Charch, who had discovered how to make cellophane waterproof. When he mentioned that they would make a decision within two weeks, she pressed him for a faster decision because another company wanted her answer on their job offer. Faced with the prospect of losing Kwolek, Charch asked his assistant to dictate an offer letter there and then, offering Kwolek the position, which she decided to accept.

Although she initially only saw the job as a means to provide her with enough money to pursue her medical education, Kwolek’s skills as a chemist and her love of teaching the subject led her to decide to stick with chemistry.

Low temperature polymers

Kwolek’s early research focused on finding a new process to create polymers at low temperatures. At that time, the process used for creating polymers through condensation, polycondensation, only worked at high temperatures. Nylon 66, which was first developed also at DuPont in 1935 by Wallace Carothers, formed at 285C, which made it a very energy-intensive and expensive process. Ideally, the industry wanted the polycondensation process to occur between 0C and 40C.

Working on a new process, Kwolek found that nylon can form at the interface of two layers of different precursor chemicals at room temperature. If this layer is pinched with tweezers and pulled away, more nylon forms at the fresh interface, and if it is attached to a device that can rotate, or is able to continually be pulled away from the interface layer, a long nylon thread can be drawn. Kwolek called this the Nylon Rope Trick and won her first award, the American Chemical Society’s prestigious Publication Award, for the paper she published documenting this discovery.

The discovery also gave rise to a whole new area of science – low temperature polycondensation reactions to produce polymers. Her once novel process has become a common fixture in school chemistry labs, and here in the Department of Materials at Imperial College London, undergraduate students extend the ‘trick’ to an investigation as part of their materials science and engineering lab course.

A life-saving discovery

When Kwolek was in her 40s, in 1964, the country faced the prospect of a petrol shortage, which would minimise the volume of crude materials available for manufacturing rubber. Her new research aim was to find a strong but lightweight fibre that could be incorporated into tyres to reinforce them and improve fuel efficiency. She discovered that under certain experimental conditions, the polyamide solutions that she was working with became very runny and behaved strangely. Whilst runny solutions did not normally lead to the production of strong polymer fibres, Kwolek was curious to find out what properties these polymers would have, if indeed fibres could be spun from something so runny. In a speech in 1993, she explained:

“The solution was unusually (low viscosity), turbid, stir-opalescent and buttermilk in appearance. Conventional polymer solutions are usually clear or translucent and have the viscosity of molasses, more or less. The solution that I prepared looked like a dispersion but was totally filterable through a fine pore filter.”

The technician in charge of the fibre spinnerette, Charles Smullen, was initially reluctant to try to spin this polymer, as not only was it runny and difficult to handle, it was also cloudy, which he thought meant that there were tiny particles suspended in the liquid. If that were the case, such particles could become trapped in the fine holes of the spinnerette that the polymer solution was squeezed through during the wet spinning process, the potential aftermath of which he was reluctant to deal with.

Kwolek filtered the solution to test whether it was contaminated – it wasn’t – and, after much persuasion, Smullen agreed to spin some fibres. The results were ground-breaking. The fibre produced from this runny, messy polymer solution was not only lightweight, but also incredibly strong. In an interview, Kwolek said, “I knew that I had made a discovery”. She went on to add, “I didn’t shout ‘Eureka,’ but I was very excited, as was the whole laboratory excited, and management was excited because we were looking for something new, something different, and this was it.”

Unbeknownst to Kwolek, the polymer solution of poly-para-phenylene terephthalamide was displaying properties we now know to be that of a liquid crystal. When the same solution had been synthesised at high temperature, the resultant spun polymer fibres were weak and stiff, but at these much lower temperatures they were five times stronger than steel of the same weight.

This surprised both Kwolek and her colleagues. “Lest a mistake had been made, I did not report these unexpected results until I had the fibers retested several times,” she later said.

Kwolek also found that heat-treating these new fibers increased their strength; the rod-shaped molecules aligned and hydrogen bonds formed between them. This discovery led to a new area of research science on liquid crystal synthetic aromatic polyamide, or aramid, fibres. Thus, Kevlar was born.

Kevlar was launched onto the market in 1975 as a material that was resistant to wear, corrosion, flames and extreme temperatures, both high and low. There are currently more than 200 applications for Kevlar, in sports equipment (archery bows, tennis racquets and skis), transport (boats, cars and aeroplanes) and reinforcement (cables, ropes, tyres and car brakes). Kevlar has recently been incorporated into the Motorola Droid RAZR phone, which boasts a unibody of Kevlar to help protect it from the impact of drops and shocks.

It is also used in pipes to give them strength with flexibility, as my second year undergraduate students finally discovered when presented with a reverse engineering task as part of their lab course. The pipe consists mostly of white plastic, but close examination reveals a layer of a tell-tale yellow fibre. Considering how resistant the material is to most forms of treatment, the students found it very difficult to characterise, but eventually realised what they were dealing with.

Kwolek signed the patent for Kevlar over to DuPont so made no profit from it, and had no influence on its applications either. She was, however, the co-recipient of 17 American patents in total. Although she officially retired in 1986, Kwolek continued to work in consultancy and got involved in schools outreach to encourage children to take up science.

A lasting legacy

In 1994, Kwolek was inducted into the National Inventors Hall of Fame, one of only four women out of 113 members. In 1995, DuPont awarded her with the Lavoisier Medal, awarded for outstanding technical achievement and to this day, she is the only female employee ever to be honoured in such a way. In 1996, she was awarded the National Medal of Technology, acknowledging her general contributions to the area of polymer science, and the practical applications of her research. Again, this was an award rarely given to women, much like the Perkin Medal she received in 1997. Sir William Henry Perkin, for whom the medal is named, was a chemist in the Royal College of Chemistry, one of the constituent colleges of Imperial College. The medal has been awarded to the best American scientists for an “innovation in applied chemistry resulting in outstanding commercial development” since 1908, and is regarded as the highest honour that can be awarded in the US industrial chemical industry. Also in 1997, she was inducted into the Plastics Hall of Fame at the University of Massachusetts, remaining the only woman to be thus honoured until Dr Maureen Steinwall was inducted in 2015.

Kwolek appreciated the under-representation and lack of acknowledgement of women in science, and turned her hand to working with female scientists as a mentor, offering advice and guidance as their careers developed. When Kwolek died, DuPont’s chief executive Ellen Kullman described her as “a creative and determined chemist and a true pioneer for women in science”.

While carrying out my research on Kwolek, I was tempted to contact her, to let her know that I had chosen to write about her, and to perhaps let her know that not only are her contributions to science still valued and highly relevant in the context of modern life, but that her legacy as an early cheerleader for women in science continues to be felt. While awards and accolades are one form of appreciation and validation, during my research, I got the impression that she cared more about the real life impact of her work as a chemist, and as a supporter and mentor for women in science.

I regretfully missed my opportunity to contact Kwolek as she passed away on 18 June 2014 in Wilmington, Delaware at the age of 90. During the week of Kwolek’s death, the one millionth bullet-resistant vest was sold. The vests are sold as part of the standard uniform for many soldiers and law enforcement officers. In a 2007 interview with the Wilmington News Journal, Kwolek said, “At least I hope I am saving lives. There are very few people in their careers that have the opportunity to do something to benefit mankind”.

Further reading

Bregar, B. (2014), “Obituary: Kevlar Inventor Stephanie Kwolek”,

Chemical Heritage Foundation (2014c), “Stephanie L. Kwolek”,

About the author

Dr Sujata “Suze” Kundu is a Teaching Fellow in the Department of Materials at Imperial College London. A nanochemist both literally and professionally, Suze’s research focuses on materials that can capture solar energy. She gives regular public lectures, is a presenter on the Discovery Channel, and is a contributor for Forbes Science and Standard Issue Magazine.

Twitter: @FunSizeSuze

Posted in STEM Stories.