Fluoropolymer coatings (also paints or finishes) have also developed based on their stable dispersions obtained by emulsion polymerization. The two coatings with the largest market share are based on polytetrafluoroethylene and polyvinylidene fluoride. One well-known application of these coatings is in non-stick cookware and bake ware. Fluorinated coatings are extensively applied to surfaces in industrial environments to protect them from a variety of corrosive agents [13].
Coatings have the advantage of ease of propagation based on few fluoropolymers by manipulating the formulation. Fluoropolymers can be converted into coatings in two separate ways, as liquids or solids. PTFE must be applied as a liquid coating in contrast to the melt processible fluoropolymers that can also be powder-coated. Fluoropolymer paints are formulated as water-based (aqueous) or solvent-based products. The latter is limited to cases in which aqueous dispersions would not work.
Fluorinated coatings, similar to other paints, are mixtures of a variety of components including liquid carriers, pigments and specific additives in addition to fluoropolymers and additional polymeric binders. Pigments affect the color and appearance of the coatings as well as its functions such as abrasion resistance, thermal conductivity, the extent of porosity and electrical resistance/conductivity. Specific additives are sometimes incorporated to modify the rheology of the paint, wettability of the final coating and facilitate formation of the film at the time of baking. Other polymers are included in the paint to enhance the adhesion of the paint to the substrate or increase its hardness thus overcoming shortcomings of fluoropolymers.
Like other paints those made with fluoropolymers can be applied as one or more “coats” or layers. Flexibility of fluoropolymer content and availability of thickness ranges (2.5-2,000 µm) are two important features of those paints. The base polymers polyvinylidene fluoride and polytetrafluoroethylene provide a broad range of bake temperature ranging from 175 to 440°C. The combination of material properties, coating thickness and process variables determine the performance parameters of the final paint.
Applications of fluorinated paints include chemical process equipment liners, insulating coatings for electronics, non-stick coatings for cookware, surgical patches and glass fiber fabric coatings used for roofing of large structures flue gas heat exchangers, interior of exhaust ductworks and others.
2.6 Summary
HFOs Hydrofluoroolefins (HFOs) are the fourth generation of fluorine-based gases. HFC refrigerants are composed of hydrogen, fluorine and carbon atoms connected by single bonds between the atoms. HFO refrigerants are composed of hydrogen, fluorine and carbon atoms but contain at least one double bond between the carbon atoms. HFO refrigerants have zero ODP and low GWP thus offer a more environmentally friendly alternative to CFCs, HCFCs and HFCs.
DuPont (Chemours) and Honeywell jointly developed HFO 1234yf which is sold under the brand names OpteonTM yf and Soltice® yf. This low GWP fluorocarbon is also a replacement for R134a for use in mobile air conditioning (MAC) systems in the automotive sector.
References
1. Tavener, S.J. and Clark, J.H., Chapter 5: Fluorine: Friend or Foe? A Green Chemist’s Perspective, in Fluorine and The Environment, Vol. 2, A. Tressaud (ed.), Elsevier, 2006.
2. Sato, K., Naturally Occurring Organic Fluorine Compounds, Tokyo Chemical Industry, April 2016, www.tcichemicals.com.
3. US Patent 1,978,840, A. L. Henne, assigned to General Motors Corp, Oct. 30, 1934.
4. US Patent 2,192,143, T. Midgley, Jr., A. L. Henne, assigned to Kinetic Chemicals Co., Feb. 27, 1940.
5. US Patent 2,062,743, H. W. Daudt, M. A. Youker, assigned to Kinetic Chemicals Co., Dec. 1, 1936.
6. Blasing, T.J. and Jones, S., Environmental Sciences Division, Oak Ridge National Laboratory, doi: 10.3334/CDIAC/atg.033, February 2012.
7. Daikin Industries, Fluorocarbon, www.daikin.com/chm/products/fluorocarbon, May 2016.
8. Refrigerants Environmental Data, The Linde Group, www.linde-gas.com, April 2020.
9. Ashford, P. et al., Chapter 7, Emissions of Fluorinated Substitutes for Ozone Depleting Substance, in: Processes and Product Use, vol. 3, IPCC Guidelines for National Greenhouse Gas Inventories, www.ipcc-nggip.iges.or.jp, 2006.
10. ASHRAE Standard, ANSI/ASHRAE Addenda z, ah, ai, and aj to ANSI/ASHRAE Standard 34-2007, www.ashrae.org, Jan 27, 2010.
11. Smith, M.B. and March, J., March’s Advanced Organic Chemistry - reactions, mechanism and Structure, John Wiley & Sons, 2007.
12. Ebnesajjad, S., Introduction to fluoropolymers: materials, technology, and applications, 2nd ed, Elsevier, New York, 2020.
13. Ebnesajjad, S., Fluoroplastics Vol 1 – Non-melt Processible Fluoropolymers, 2nd ed, Elsevier, 2015.
14. Ebnesajjad, S., Fluoroplastics Vol 2 – Melt Processible Fluoropolymers, 2nd ed, Elsevier, 2015.
15. Kynar® PVDF Fluoropolymer Family, Arkema High Performance Polymers, http://americas.kynar.com, 2020.
16. U.S. Patent 2,810,702, M.F. Bechteld and M. I. Bro, assigned to Du Pont Co, October 22, 1957.
17. Fluoropolymers Market - Global Industry Trends & Forecasts to 2019 www.marketsandmarkets.com, May 2016.
3
Fluorine Sources and Basic Fluorocarbon Reactions
Fluorine represents the most extreme of all elements [1]. It is the most reactive element known to man. It reacts with glass and nearly everything else. Even noble gases such as xenon, krypton and gold are not safe because every one of them reacts with fluorine. Fluorine is quite unique among all other elements because of its properties. It carries its uniqueness into organic substances when fluorine substitutes hydrogen and other elements in their molecules. The first synthesis of fluorine has been attributed to Moissan who exhibited the reactivity of fluorine [2].
The impact of fluorine on other materials, nicknamed superhalogen, is more severe than that of other halogens including chlorine. This chapter describes fluorine ores and the basic chemistry of reactions to produce organic fluorine compounds. Narrow aspects of fluorine chemistry related to the preparation of commercial fluorinated alkanes are discussed. An overview of the polymerization of fluorinated oleffinic monomers is also reviewed. The readers should consult the books and articles cited in this chapter for a broader understanding of fluorine chemistry.
3.1 Role of Fluorine in Fluorocarbons
Fluorine is the most electronegative of all elements at electronegativity of 4 in Pauling units. Electronegativity of other elements are 3.4 for oxygen, 3.2 for chlorine, 2.6 for carbon and 2.2 for hydrogen (Table 3.1) [3]. Extreme electronegativity of fluorine renders its covalent bonds highly polarized such as in C-F. Consequently, fluorine gas attacks nearly every substance and chemical because of very high reactivity. It even attacks noble gases like xenon producing XeFx. It is easy to fluorinate hydrocarbons by fluorine gas, but the intensity of this reaction is too severe to control and causes broad decomposition.
The shortest bond is formed between C and H (0.11 nm) followed by C-F at 0.14 nm. Van der Waals radius (rw)