Chapter 24 The Development and Application of Kevlar
Introduction
As Du Pont claims on its company website, “Kevlar®brand fiber is an innovative technology from Du Pont that combines high strength with light weight to help dramatically improve the performance of a variety of consumer and industrial products. Groundbreaking research by Du Pont scientists in the field of liquid crystalline polymer solutions in 1965 formed the basis for the commercial preparation of the Kevlar31 aramid fiber. Lightweight and flexible, Kevlar®has evolved over four decades of innovation to do everything from helping save thousands of lives around the world to helping make safer homes and vehicles to helping land spacecraft on Mars. If it needs to be light, strong, or safe, it needs to be Kevlar ®. ”
As for the uses and applications which can also be found on Du Pont’s website,“the worldwide recognition Kevlar®brand fiber has earned as a high-performance material for helping to protect human life extends far beyond the body armor used by the military and law enforcement community. A unique combination of properties makes Kevlar®the first choice for an ever-growing number of applications where a reduction of weight, increase in strength and resistance to corrosion produce significant improvements in safety and efficiency. Today, Kevlar®is used in everything from airplane parts to reinforced suspension bridge structures to suspension bridge cables to fiber optic cables, not to mention a variety of consumer goods. Chances are you either use Kevlar or come into contact with it on a regular basis. ”
The development of Kevlar®serves as an excellent example of the innovation process which demonstrates that between discovery and commercialization there are major difficulties and several reality gaps, to overcome which requires a multi-disciplinary approach. In addition, it also shows in the Kevlar®innovation that questioning conventional wisdom and an environment that encourages risk taking are very important in industrial revolution. These were clearly pointed out by David Tanner in his 1995 paper on the development of Kevlar®. Difficulties Involved in Kevlar^ Development and Their Solutions.
With the invention of nylon and subsequent fibers, Du Pont continued to find better fibers with the heat-resistance of asbestos and the stiffness of glass. Kevlar®, one of the modem world’s most readily recognized and widely used materials, was invented by Stephanie Kwolek, a research scientist at Du Pont’s Pioneering Research Laboratory. Kevlar®is an aramid, which is an abbreviation for aromatic polyamide, belonging to the same family of polymers as nylons. For her invention of Kevlar®, Kwolek was inducted into the United States Inventor’s Hall of Fame in July 1995. During the course of developing Kevlar®fibers, Kwolek and her colleagues encountered quite a few difficulties. However, these problems were solved one by one through cooperative, multi-disciplinary efforts.
Kwolek was a polymer chemist who specialized in low-temperature condensation polymerization. In 1965, she successfully prepared the first pure monomers, then polymerized and solubilized Para-aminobenzoic acid, resulting in polybenzamide, which represented an entirely new branch of synthetics, liquid crystalline polymers. To Kwolek’s disappointment at first, the solution of polybenzamide she synthesized appeared opaque and could not be clarified by conventional methods such as heating or filtration. As she has recalled, “when I dissolved the PBA (poly-p-aminobenzamide) polymer at 10% concentration in tetramethylurea with 6. 5% Li Cl, the solution was unusually fluid, turbid, stiropalescent, and butter-milk-like in appearance. ” And it seemed that the polymer solution would not be useful for fiber spinning because of possible clogging problem. However, acting on instinct, Kwolek went against suggestions from experienced fiber technicians and insisted that the solution be extruded anyway. Interestingly, the spinning process went smoothly. The resulting fiber showed astonishing stress-strain profiles, and it had excellent properties that Du Pont was trying hard to look for. Now we all know that the polymer formed a lyotropic liquid crystal in the solution, leading to the opacity. The polyamide begins to form a liquid crystalline phase above a critical concentration (approximately 5%- 10%by weight) depending on polymer molecular weight and the solvent. The formation of the liquid crystalline phase in solution actually facilitated the fiber spinning process due to the decrease in solution viscosity.
The second difficulty was that it was too expensive to use the para-aminobenzoic acid raw material for large-scale production. With the understanding of the physical properties of liquid crystalline solutions, the researchers at Du Pont developed a suitable polymer called PPD-T from less costly starting materials p-phenylene diamine and terephthalic acid. PPD-T also had good properties comparable to those of PBA. Finally, PPD-T became the basis of Kevlar, and the problem of high material costs was resolved.
The third obstacle in the large-scale production of high-performance fibers from PPD-T lied in the spinning solvent used. The researchers had to use 100% sulfuric acid, which resulted in a very viscous solution and it seemed impossible to obtain the high spinning speed required for an economically viable process. In addition, this unconventional spinning solvent was highly corrosive, while the yields and throughput of the process were quite low, and the investment would be very high. Consequently, it would be too risky for the production of PPD-T fibers. Du Pont research scientist Herb Blades increased the concentration from a typical 10% to 20% and tried spinning at elevated temperatures. The ability of spinning at such a high polymer concentration was attributed to the unexpected formation of a crystalline complex by the PPD-T polymer and sulfuric acid under these conditions. Blades’ discovery was a major breakthrough, and such high concentrations of the spinning solution made the spinning process much more economic. In his second important contribution, Spinneret Blades found that an air gap between the spinneret and cold water in the quench bath made a fiber with extraordinary tensile properties and developed the revolutionary air gap spinning process in 1970.
There were also difficult challenges faced by the scientists and engineers of various disciplines that were assembled to help the complex scale-up development. One of the hurdles involved the polymerization solvent, hexamethylphosphoramide (HMPA) because of its animal carcinogenicity. Fortunately, a combination of N-methylpyrrolidone (NMP) and calcium chloride was found to be the best option of the solvent to replace HMPA. This mixture solvent provided identical fiber properties and was compatible with the expensive equipment designed for polymerization with HMPA.
The demonstration of the market potential of Kevlar was an even greater challenge in order to justify a full-scale commercialization plant, which required a huge amount of investment. Fortune magazine regarded Kevlar as “a miracle in search of a market”. As Kevlar®is very different from other fibers, it was proposed to use them to replace more conventional materials. One possible market was in rope applications, especially in replacing steel wire ropes which were used extensively in offshore drilling platforms because the specific strength of Kevlar®is more than 20 times that of steel in sea water. Even in this market development stage, there were still a number of difficulties to overcome. The first problem was that the lifetimes of Kevlar® ropes were only 5%- 10% that of steel ropes. This problem was solved through three main improvements: using three Kevlar® strands of different diameters in a rope, which resulted in a five-fold improvement in lifetime; lubricating the strands which were jacketed with fluorocarbons-impregnated braids, leading to a six-fold improvement; optimizing twisting angles, which decreased radial squeezing forces and brought an additional two-fold improvement. Finally, the Du Pont engineering team developed a rope with more than three times the life of steel in severe laboratory tests and more than five times in real-life experience. Today, a wide range of Kevlar® rope applications have been developed for other areas.
Why Is Kevlar “ So Strong?-Chemical and Physical Aspects of Kevlar®
Then where does the exceptional strength of Kevlar®come from? There are many components contributing to the strength of Kevlar®, including the following five key factors:
1. The aramid polymer for producing Kevlar®is a long-chain molecule, which consists of covalently bonded repeating units. The covalent bond along the polymer chain afforded the polymer with high strength along the chain direction.
2. Again, Kevlar®is an aromatic polyamide which contains both aromatic and amide groups. These molecular groups are often present in other polymers with a high breaking strength. The rigid rod-like nature of the aromatic polyamide provides the fibers with high strength and high modulus.
3. A Kevlar®fiber is an array of long-chain molecules which are oriented parallel to one another similar to a package of uncooked spaghetti. Such an orientation assists the formation of a crystalline arrangement with the polymer chains oriented parallel to the fiber axis when the molten Kevlar®is extruded through the spinneret and spun into fibers. The crystallinity of the Kevlar* polymer strands makes a significant contribution to the unique strength and rigidity of Kevlar®. Heat treatment could increase orientation and crystallization of the polymer chains, thus enhance properties of the resulting fibers.
4. The polar amide groups in PPD-T are able to form hydrogen bonds between the polymer chains so that the individual polymer strands of Kevlar®are held together. 5. The aromatic components of Kevlar®polymers have a radial (spoke-like) orientation, which allows for a high degree of symmetry and regularity in the internal structure of the fibers.