The invention: A synthetic fiber made from polyacrylonitrile that has become widely used in textiles and in the preparation of high-strength carbon fibers.
The people behind the invention:
Herbert Rein (1899-1955), a German chemist Ray C. Houtz (1907- ), an American chemist
A Difficult Plastic
“Polymers” are large molecules that are made up of chains of many smaller molecules, called “monomers.” Materials that are made of polymers are also called polymers, and some polymers, such as proteins, cellulose, and starch, occur in nature. Most polymers, however, are synthetic materials, which means that they were created by scientists.
The twenty-year period beginning in 1930 was the age of great discoveries in polymers by both chemists and engineers. During this time, many of the synthetic polymers, which are also known as plastics, were first made and their uses found. Among these polymers were nylon, polyester, and polyacrylonitrile. The last of these materials, polyacrylonitrile (PAN), was first synthesized by German chemists in the late 1920′s. They linked more than one thousand of the small, organic molecules of acrylonitrile to make a polymer. The polymer chains of this material had the properties that were needed to form strong fibers, but there was one problem. Instead of melting when heated to a high temperature, PAN simply decomposed. This made it impossible, with the technology that existed then, to make fibers.
The best method available to industry at that time was the process of melt spinning, in which fibers were made by forcing molten polymer through small holes and allowing it to cool. Researchers realized that, if PAN could be put into a solution, the same apparatus could be used to spin PAN fibers. Scientists in Germany and the United States tried to find a solvent or liquid that would dissolve PAN, but they were unsuccessful until World War II began.
Fibers for War
In 1938, the German chemist Walter Reppe developed a new class of organic solvents called “amides.” These new liquids were able to dissolve many materials, including some of the recently discovered polymers. When World War II began in 1940, both the Germans and the Allies needed to develop new materials for the war effort. Materials such as rubber and fibers were in short supply. Thus, there was increased governmental support for chemical and industrial research on both sides of the war. This support was to result in two independent solutions to the PAN problem.
In 1942, Herbert Rein, while working for I. G. Farben in Germany, discovered that PAN fibers could be produced from a solution of polyacrylonitrile dissolved in the newly synthesized solvent dimethyl-formamide. At the same time Ray C. Houtz, who was working for E. I. Du Pont de Nemours in Wilmington, Delaware, found that the related solvent dimethylacetamide would also form excellent PAN fibers. His work was patented, and some fibers were produced for use by the military during the war. In 1950, Du Pont began commercial production of a form of polyacrylonitrile fibers called Orlon. The Monsanto Company followed with a fiber called Acrilon in 1952, and other companies began to make similar products in 1958.
There are two ways to produce PAN fibers. In both methods, polyacrylonitrile is first dissolved in a suitable solvent. The solution is next forced through small holes in a device called a “spinneret.” The solution emerges from the spinneret as thin streams of a thick, gooey liquid. In the “wet spinning method,” the streams then enter another liquid (usually water or alcohol), which extracts the solvent from the solution, leaving behind the pure PAN fiber. After air drying, the fiber can be treated like any other fiber. The “dry spinning method” uses no liquid. Instead, the solvent is evaporated from the emerging streams by means of hot air, and again the PAN fiber is left behind.
In 1944, another discovery was made that is an important part of the polyacrylonitrile fiber story. W. P. Coxe of Du Pont and L. L. Winter at Union Carbide Corporation found that, when PAN fibers are heated under certain conditions, the polymer decomposes and changes into graphite (one of the elemental forms of carbon) but still
keeps its fiber form. In contrast to most forms of graphite, these fibers were exceptionally strong. These were the first carbon fibers ever made. Originally known as “black Orlon,” they were first produced commercially by the Japanese in 1964, but they were too weak to find many uses. After new methods of graphitization were developed jointly by labs in Japan, Great Britain, and the United States, the strength of the carbon fibers was increased, and the fibers began to be used in many fields.
As had been predicted earlier, PAN fibers were found to have some very useful properties. Their discovery and commercialization helped pave the way for the acceptance and wide use of polymers. The fibers derive their properties from the stiff, rodlike structure of polyacrylonitrile. Known as acrylics, these fibers are more durable than cotton, and they are the best alternative to wool for sweaters. Acrylics are resistant to heat and chemicals, can be dyed easily, resist fading or wrinkling, and are mildew-resistant. Thus, after their introduction, PAN fibers were very quickly made into yarns, blankets, draperies, carpets, rugs, sportswear, and various items of clothing. Often, the fibers contain small amounts of other polymers that give them additional useful properties.
A significant amount of PAN fiber is used in making carbon fibers. These lightweight fibers are stronger for their weight than any known material, and they are used to make high-strength composites for applications in aerospace, the military, and sports. A “fiber composite” is a material made from two parts: a fiber, such as carbon or glass, and something to hold the fibers together, which is usually a plastic called an “epoxy.” Fiber composites are used in products that require great strength and light weight. Their applications can be as ordinary as a tennis racket or fishing pole or as exotic as an airplane tail or the body of a spacecraft.
See also Buna rubber; Neoprene; Nylon; Plastic; Polyester; Polyethylene; Polystyrene.