Flurochemicals
History
While mineral fluorides were known as early as the 16 th century for etching glass, fluorochemicals were not properly developed until the early-to-mid twentieth century. Henri Moisson’s isolation of elemental fluorinein 1886, for which he received the Nobel Prize in Chemistry, and Fre´de´rec Swarts’ use of SbF3 in a Cl/F exchange reaction a few years later to prepare fluorinated aromatics and the first chlorofluorocarbon gas (CF2Cl2), had shown the interesting properties of fluorine and fluorinated compounds. However, hazards associated with the use of reactive and corrosive reagents hampered the development of fluorine chemistry until the 1920s. In 1928, General Motors Corporation in the US set about finding a replacement for their current stock of dangerous and toxic refrigerants (ammonia and sulphur dioxide). A team led by Thomas Midgley Jr supposed that the desired characteristics could be met using low molecular weight fluoroaliphatic compounds. CF2CCl2 was selected as a first candidate and a sample prepared from CCl4 and SbF3 using Swarts’s original methodology.
Fluorocarbons
The gaseous fluorocarbon proved an ideal refrigerant and in 1931 General Motors, in partnership with E. I du Pont de Nemours & Co., formed a new corporation, Kinetic chemicals Inc., to produce commercial quantities of the newly trademarked product Freon-12.
Throughout the 1930s several other Freons were developed including Freon-114 (CClF2CClF2) a precursor of tetrafluoroethylene (TFE) (Table 1) a compound that was soon to have major significance. During this period researchers from IG-Farbenindustrie in Hoechst/Frankfurt, in Germany, studying polymerisation of fluoroethylenes discovered that TFE could be polymerised to form polytetrafluorethylene (PTFE), an inert, chemical and heat resistant polymer plastic. Realising the important properties of this new material, the first patent for a fluoropolymer was filed in 1934 by Schloffer and Scherer, and granted in 1937.
PTFE Discovery
In 1938, Roy Plunkett, a DuPont chemist working on new types of Freons, independently discovered PTFE while attempting to chlorinate gaseous TFE. He too observed the white waxy polymeric substance to be chemically inert, extremely heat resistant, non-adherent, and virtually insoluble in any solvent. At the time however, these properties, together with its excessive cost, meant that PTFE struggled to find a market in the US. The impending World War would soon provide an impetus for its development. During World War II, the Manhattan Project focussed on the development of the first Atomic bomb. A key component of the process was the use of UF6 to separate U235 fromU238. The problem was thatUF6, prepared from uranium oxide (UO2), HF, and fluorine gas, was difficult to handle and purify and extremely corrosive to metal. As such new corrosion resistant materials were needed if the ‘Project’ was to succeed. PTFE proved the solution and its unique chemical resistance properties under extreme conditions led to a government contract being issued for scale-up production. Following the war, DuPont made PTFE available commercially under the tradename Teflon.
Fluorinated Products
The Manhattan project also produced a variety of other fluorinated products for valves, resistant tubing, and lubricants which were later commercialised and so began the fluoropolymer industry. In 1953 the Kellogg Co. introduced polychlorotrifluoroethylene (PCTFE) under the trade name Kel-F 81. PCTFE, a homopolymer of chlorotrifluoroethylene (CTFE), contained chlorine in the fluoropolymer backbone making it a more processable alternative to PTFE. PCTFE was harder and less permeable than PTFE, possessed outstanding moisture barrier properties, and could be produced in bulk solution and suspension forms. Throughout the 1950s researchers looked at fluoropolymeric blends to explore new fluoroplastic materials. This led in 1960 to the introduction of FEP (fluorinated ethylene propylene) the first copolymer of TFE. FEP contains, 5% hexafluoropropylene to introduce a trifluoromethyl group along the polymer chain. Significantly, FEP was able to be processed by conventional polymer techniques such as moulding and casting and was found to be particularly useful as long lasting and fire resistant wire and cable insulation. The following year Dupont released polyvinylfluoride (PVF) which contained only one fluorine in the ethylene monomer unit, and polyvinyldifluoride (PVDF) which contained two. PVF and PVDF have lower amounts of fluorine compared with other fluoropolymers but maintain many of the chemical and thermal resistance properties of PTFE. They also possess enhanced mechanical strength more akin to hydrocarbon-based polymers. PVF was particularly amenable to lamination processes and the resulting films were extremely tough and resistant to water and sunlight. PVDF turned out to be excellent for spray-on coatings, with the first commercial grade PVDF, Kynar500, introduced in 1965 by Pennsalt Co. PVDF was also found to have interesting electronic properties finding use in the growing electronics industry. In the late 1960s Walter Grot at DuPont discovered Nafion, a copolymer of TFE containing sulfonate groups. Nafion was the first synthetic ionic polymer (ionomer) and was found to be highly conductive to cations. This made it suitable for membrane applications and it soon found use in industrial electrolysis and fuel cells. Modified fluoroionomers such as Flemion and Aquivion were later developed to overcome some of the solvent and operating temperature limitations of Nafion. The 1970s saw the introduction of a perfluoroalkoxy (PFA) copolymer by Dupont. PFA, a mix of TFE and perfluoropropylvinylether (PPVE) was transparent in thin sections and possessed a broad range of properties encompassing both FEP and PTFE. PFA soon found applications in the chemical and semiconductor industries as pipes, fittings, linings, and as specialised films. Around the same time Dupont also introduced ECTFE and ETFE, ethylene (E) copolymers of CTFE and TFE respectively.
Combined Polymers
These polymers were the first fluoropolymers to contain non-fluorinated subunits, and possessed a mix of hydrocarbon and fluorocarbon polymer properties. In addition to improved mechanical properties, ECTFE and ETFE were more flexible and could be crosslinked using high energy radiation. The reduced production cost of these polymers made them attractive materials for high strength tubing, films, and fire-resistant cable insulation. In 1976 Robert Gore patented a process whereby PTFE could be heat stretched to give expanded polytetrafluoroethylene (ePTFE). This process stretched PTFE by up to 800% forming a microporous structure that was, 70% air. The pores could be engineered such that air could pass through but water could not. This new ePTFE material trade-named Gore-Tex would soon have an impact on the outdoor apparel, medical, and music industries. In the early 1980s, Asahi Glass developed fluoroethylenevinylether (FEVE) resins under the Lumiflon trademark, for coating plastics, architecture, and other materials. FEVE resins are composed of fluorinated ethylene (TFE or CTFE) and amixofvinylethers that can be varied depending on application. FEVE resins were the first fluoropolymers to be soluble in organic solvents and can be cured at room temperature. Similar FEVE resins were later introduced by Daikin under the tradename Zeffle. During this period new fluoropolymers were being developed to meet the technology demands of growing industries. Up until this time fluoropolymer plastics were semi crystalline materials with poor solubility or low optical transparency. In the mid-1980s DuPont developed Teflon-AF, a copolymer of TFE and perfluoro-2, 2-dimethyl-1, 3-dioxole (PDD), and Asahi Glass introduced Cytop, a homopolymer of perfluoro-3-butenyl-vinyl ether.
Teflon AF
Both Teflon-AF and Cytop are amorphous high molecular weight perfluoropolymers that in addition to having excellent thermal, chemical, and electrical properties, also possess outstanding optical clarity and the lowest refractive index of all known organic materials. This made them ideal for optical lenses, fibreoptic applications, and high quality transparent coatings. Solvay Solexis later introduced Hyflon A Das a more solution processible alternative to Teflon-AF. At present the consumption of amorphous fluoropolymers is still very small. In 1993, Hoechst partnered with 3M to release THV, a semi crystalline three component terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene luoride. THV is highly flexible, soluble in polar organic solvents, and has excellent adhesive properties making it very useful for thin film coatings and multilayer constructions. Since the 1990s, trademarked ranges of fluoropolymers have been expanded in various forms to meet the needs of emerging technologies in construction, electronics, and energy sectors.
Fluoropolymer Blends
For the most part these have consisted of modified formulations or newly processed forms of existing fluoropolymer blends. Much effort continues to be invested in developing new types and blends of fluoropolymers to meet the needs of advancing research and technology, particularly in the energy and electronics sectors. Current commercial fluoropolymers can be divided into per fluorinated, partially fluorinated, crystalline, semi-crystalline, and amorphous categories. Worldwide annual demand for fluoropolymers is upwards of 200000 tons with fluoroplastics making up most of the market. PTFE is still the most widely produced of all the fluoropolymers with demand steadily increasing. Current anufacturers producing a range of fluoropolymer resins and products include DuPont, Asahi Glass, Solvay Solexis, 3M, Dyneon, Honeywell, and Daikin. The number of companies specialising in the production of one or two fluoropolymers, particularly PTFE, are increasing.