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【文献】Part I: Discovery of Polytetrafluoroethylene

2019-03-04

Part I: Discovery of Polytetrafluoroethylene  

Many people around the world, regardless of whether or not they have been directly involved in the creation and production of polymer, are familiar with the origins of fluoropolymers. Teflon®, DuPont’s trademark name for polytetrafluoroethylene (PTFE), is world renowned. The classic story of the discovery of fluoropolymers is replete with the magical combination of curiosity, perseverance, and serendipity. It is helpful to note the environment and context in which polymer science began. This section places the invaluable discovery made by Roy Plunkett [1b] inthe context of the times and of the events and personalities that shaped science, industry, and the world in the 1930s and 1940s. Plunkett’s finding is even more impressive when viewed through the prism of this context.

 

1.1 Discovery of Polytetrafluoroethylene

For those who do not already know the story, let us begin with the ending. By 1938, Dr. Roy Plunkett had been working at DuPont for 2 years, developing new fluorinated refrigerants that were safer than old gases because of being nonflammable, nontoxic, colorless, and odorless. He reacted tetrafluoroethylene (TFE) with hydrochloric acid (HCl) for synthesis of a refrigerant, CClF2eCHF2 [2]. As he had done on many other occasions, on the morning of April 6, 1938, Plunkett checked the pressure on a full cylinder of TFE. He was surprised to find no pressure, and yet the weight of the cylinder was the same as it had been the previous day. Plunkett and his technician removed the valve and shook the cylinder upside down. When they cut open the gas
cylinder, they recovered a small amount of a slippery white substance (
Fig. 1.1). They analyzed the waxy powder and named this new substance polytetrafluoroethylene, later trademarked as Teflon® by the DuPont Company. The rest, as they say, is history (Fig. 1.2). 

 

1.2 Roy Plunkett’s Story

Roy was born into a poor farm family in New Carlisle, Ohio. When the Great Depression began he was a student at Manchester College in North Manchester, Indiana, where he shared a room with an older student named Paul Flory. Roy graduated with a bachelor of arts in chemistry in 1932 and followed Paul to graduate school at Ohio State University.
Within 2 years of one another, Roy and Paul both earned Masters and PhD degrees from Ohio State University. In 1936, Roy joined DuPont Central Research, where Paul had been working since 1934. Roy quickly advanced to Kinetic Chemical Co., a joint venture that DuPont and General Motors (GM) had set up to produce safe refrigerants to replace ammonia and sulfur dioxide. Roy was given a laboratory in DuPont’s Jackson Laboratory on the shore of the Delaware River in Deep Water, New Jersey. Roy’s laboratory was across the hall from a laboratory run by a colleague named Charlie, whose research focused on synthesizing new organic compounds. Roy was trying to expand the line of fluorocarbons, known as Freon, to meet the needs brought on by the explosive growth of automobile production at GM. Excitement erupted in Roy’s lab on April 6, 1938, when he found no pressure in the TFE cylinder and discovered the strange new substance inside. What was this slippery white powder? Because he had time, knowledge, and curiosity, he paused to ask
questions.

He was not working under relentless pressure to meet next month’s deadline because companies like DuPont, that funded research like Roy’s, understood that success in research required a
reasonably low-stress work environment. One wonders how history might have been different had Roy had been given an inflexible objective. What would have happened if Roy had, upon determining that the foreign substance had no properties that would further his fluorocarbon research, wiped up the powder and carried on with his daily tasks? But Roy was a well-trained scientist with the freedom and curiosity to investigate this unexpected finding. When Charlie heard the racket across the hall, he walked over to Roy’s lab to investigate. He later said, “I noticed commotion in the laboratory of Roy Plunkett, which was across the hall from my own. I investigated and witnessed the sawing open of a cylinder from which was obtained the first sample of
Teflon
® fluoropolymer.” This is the description of that day at Jackson Laboratory that Charlie Pedersen shared in his 1987 Nobel Lecture.

Pedersen (Fig. 1.3) went on to invent new crown ether compounds, for which he was awarded the Nobel Prize in Chemistry. After logging the results of his discovery that day, Roy Plunkett continued with his research. Several years later, wartime needs rescued his discovery from oblivion. The Manhattan Project was a covert program whose aim was to develop an atom bomb before Nazi Germany. Lieutenant General Leslie RichardGroves, who led the project, made critical decisions to prioritize the various methods of isotope separation; and he acquired the raw materials needed by the scientists and engineers working on the project. In the course of his search for new materials to meet the novel needs of the Manhattan Project, General Groves came across PTFE. After hearing about the properties of PTFE and its resistance to different chemicals, General Grove is purported to have said that the cost, even at $100 a pound, was a bargain! Scientists working on the project badly needed corrosion-resistant materials for the uranium enrichment process. U-235 had to be separated from U-238 using differential diffusion of UF6. UF6 is highly corrosive to most metals, but PTFE stands up to it. Once the scientists involved in the Manhattan Project verified its properties, the US Patent Office placed PTFE under a national “secrecy order” and from then on it was referred to as “K-416.” Only one patent, with minimal content, was issued to DuPont in 1941 to recognize its rights to the invention [4b]. The next time anyone outside of DuPont heard of PTFE was after World War II, in 1946, under the now-famous trademark of Teflon®. DuPont learned a great deal about PTFE during its intense efforts to
produce it for the Manhattan Project. When resources formerly reserved for the war effort became available again to scientists and manufacturers in the US and around the world, it was time to move the production of PTFE from pilot plant to a commercial manufacturing operation
dand DuPont was ready. Dr. Plunkett’s own words describe the impact of his discovery: “The discovery of polytetrafluoroethylene (PTFE) has been variously described as (1) an example of serendipity, (2) a lucky accident and (3) a flash of genius. Perhaps all three were involved.
There is complete agreement, however, on the results of that discovery. It revolutionized the plastics industry and led to vigorous applications not otherwise possible”
[4a]. 

 

1.3 Commercialization of Polytetrafluoroethylene

Efficient monomer synthesis methods, polymerization technologies, and various forms of PTFE had to be developed. The fact that large-scale monomer synthesis and controlled polymerization had not been fully developed was a technical impediment to commercialization of the new polymer. Intensive studies resolved these problems, and small-scale production of Teflon® began in Arlington, New Jersey, in 1947. In 1950, DuPont scaled up the commercial production of Teflon® in the US with the construction of a new plant in Parkersburg, West Virginia. In 1947, Imperial Chemical Industries built the first PTFE plant outside the US, in the United Kingdom. PTFE cannot be dissolved in any solvent, acid, or base, and when melted it forms a stiff clear gel with no flow. Special processing techniques normally used for molding metal powders were modified to fabricate parts from PTFE. Another process, called paste extrusion, was borrowed from ceramic processing. Roy Plunkett’s discovery of PTFE was just the beginning. Throughout this embryonic stage of polymer science there was much excitement and curiosity and debate in scientific circles, and many scientists around the world built upon this discovery. Scientists devoted a great deal of effort, from the 1940s through the 1960s, to developing technologies to fabricate useful objects from the three forms of PTFE: granular, fine powder, and dispersion. Over time, through the 1980s, a variety of TFE copolymers were developed that could be processed by melt extrusion techniques and solution processing [5].