Introduction to Fluoropolymers: Materials, Technology, and Applications

By Sina Ebnesajjad

Introduction to Fluoropolymers, Second Edition, provides a comprehensive overview of the history, principles, properties, processing and applications of fluoropolymers, supporting their development and utilization in high-performance applications, components, and products. This second edition has been updated and expanded to include new in-depth chapters on manufacturing and applications of PTFE and melt processible fluoropolymers.

The book begins by demonstrating the role of fluoropolymers in everyday life, before introducing the history and basic principles of fluoropolymers. This is followed by detailed coverage of the main fluoropolymer types. Properties and applications are illustrated by real-world examples as diverse as waterproof clothing, vascular grafts and coatings for aircraft interiors. The different applications of fluoropolymers show the benefits of a group of materials that are highly water-repellant and flame-retardant, with unrivalled lubrication properties and a high level of biocompatibility. Health and safety and environmental aspects are also covered throughout the book, with a final chapter examining safety, disposal, and recycling in detail.

This book is an essential resource for anyone looking to understand or use fluoropolymer materials in their products. This includes engineers, product designers, manufacturers, scientists, researchers, and other professionals, across industries such as automotive, aerospace, medical devices, food and beverages, high performance apparel, oil and gas, renewable energy, solar photovoltaics, electronics and semiconductors, pharmaceuticals, and chemical processing. This is also a valuable introductory guide for academic researchers and advanced students in plastics engineering, polymer science, and materials science.

• Introduces and demystifies fluoropolymers for a wide audience of engineers, designers, professionals, and researchers, across industries and disciplines
• Covers a broad range of materials, including polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), vinylidene fluoride polymers, fluoroelastomers, and more
• Focuses on properties, processing methods and advanced industrial applications of fluoropolymers

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 8, 2020
• ISBN: 9780128192993

Sina Ebnesajjad

Sina Ebnesajjad is the series editor of Plastics Design Library (PDL) published in the William Andrew imprint of Elsevier. This Series is a unique series, comprising technology and applications handbooks, data books and practical guides tailored to the needs of practitioners. Sina was the editor-in-chief of William Andrew Publishing from 2005 to 2007, which was acquired by Elsevier in 2009. He retired as a Senior Technology Associate in 2005 from the DuPont fluoropolymers after nearly 24 years of service. Sina founded of FluoroConsultants Group, LLC in 2006 where he continues to work. Sina earned his Bachelor of Science from the School of Engineering of the University of Tehran in 1976, Master of Science and PhD from the University of Michigan, Ann Arbor, all in Chemical Engineering. He is author, editor and co-author of fifteen technical and data books including five handbooks on fluoropolymers technology and applications. He is author and co-author of three books in surface preparation and adhesion of materials, two of which are in their second editions. Sina has been involved with technical writing and publishing since 1974. His experiences include fluoropolymer technologies (polytetrafluoroethylene and its copolymers) including polymerization, finishing, fabrication, product development, failure analysis, market development and technical service. Sina holds six patents.

Book preview

2020

1

Fluorine, Fluorocarbons, and Fluoropolymers in Human Life—A Day With the Smiths

Sina Ebnesajjad,    FluoroConsultants Group, LLC, United States

Abstract

A review of a day in the life of Mr. and Mrs. Smith who live in Tucson, Arizona, will illustrate the essential role of fluorine, fluorocarbons, and fluoropolymers in society.

Keywords

Fluorine; fluorocarbons; fluoropolymers; Smith; polytetrafluoroethylene; polyvinylidene fluoride

Outline

Outline

References 5

A review of a day in the life of Mr. and Mrs. Smith who live in Tucson, Arizona, will illustrate the essential role of fluorine, fluorocarbons, and fluoropolymers in society.

Mr. Smith begins the day shaving using a fancy razor, which has a strip of polytetrafluoroethylene (PTFE) fluoropolymer as a part of the safe design of this razor. Taking eggs out of a refrigerator kept cool with a fluorocarbon refrigerant hidden inside its compressor. Mr. Smith makes an omelet for breakfast. He uses a nonstick frying pan to cook, which is nonstick because of the fluoropolymer coating on its surface. Mr. Smith only needs to use a few drops of olive oil for cooking an omelet (low fat) in the nonstick pan. The pan is also easy to wash, saving on detergent and water.

After breakfast, everyone in the family flosses using floss made of PTFE fluoropolymer (e.g., Oral-B Glide made by WL Gore and Associates for Proctor & Gamble) that does not scar gums. The toothpaste contains fluoride to protect the teeth. Fluorine compounds are added to the toothpaste, to help prevent tooth decay. Pain, loss of teeth, gum disease, and disfigurement associated with tooth decay have been reduced since the introduction of fluoridation practice almost 60 years ago.

Community water fluoridation and its effect in reducing the burden of tooth decay are considered one of the 10 public health achievements in the 20th century [1]XXX. Numerous studies since 1945 have illustrated the impact of community water fluoridation in the prevention of tooth decay [2]XXX. For example, in 1993, the results of 113 studies in 23 countries were compiled and analyzed [3]XXX (59 out of the 113 studies analyzed were conducted in the United States). This review provided effectiveness data for 66 studies in primary teeth and for 86 studies in permanent teeth. The decay reductions observed were in the range of 40%–60%. Recent evidence continues to indicate that the economic benefit of community water fluoridation exceeds the intervention cost. Further, the benefit–cost ratio increases with the community population size [4]XXX.

Mrs. Smith is dressing the children. Rain is in the forecast, so to stay dry the children dress in water-repellent coats that have an expanded PTFE fluoropolymer fabric inside of them, best known as Gore Tex (by WL Gore and Associates). These coats keep the water out but breathe thus keeping the children cool. A similar material is used in hospitals.

They pick up their cell phones and laptop computers on their way out of the house. Every one of these devices contains coaxial cables that contain insulation made from fluoropolymers and components, which depend on the unique dielectric properties of PTFE. Modern electronics age has depended on fluoropolymers. The silicon chips are made using fluorine-containing gases for etching and chamber cleaning. Fluoropolymers are used as construction materials for equipment used in factories (Fab) that fabricate semicon chips.

Mrs. Smith takes her son’s asthma drug along to school. A fluorinated chemical enables safe delivery of metered doses of the asthma medicine out of the metal can. A thin layer of fluoropolymer coats the inside of the metal can to prevent the drug from sticking to the surface. Metered Dose Inhaler, as it is called, allows the drug to be administered in a targeted and precise dose. This keeps the exposure of the child limited to the required amount of the asthma drug.

Because of its unique chemical properties, fluorine has been instrumental in the development of novel medicines. Approximately, 150 (20%) of all marketed drugs contain fluorine including three of the 10 best-selling drugs (Lipitor, Advair, and Crestor) contain fluorine. Seven out of 35 new drugs approved by FDA in 2011 contained fluorine. Fluorine-enhanced compounds have better pharmaceutical properties. Nearly 70 years ago, Fried replaced 9α-hydrogen in cortisone with fluorine and unexpectedly found the presence of a single fluorine could improve bioactivity by a factor of 11 times [5]XXX. Ever since fluorine has played a multifaceted role in pharmaceuticals.

Mr. Smith is the last person to leave the house. He turns off his fluorine-containing displays on his desktop computer and plasma display television. Before leaving, he checks on the thermostat to ensure reduced air conditioning operation while the house is empty. The central air unit in his house contains a fluorocarbon refrigerant called hydrofluoroolefin (HFO) that has no impact on the ozone layer and slight impact on global warming. This material complies with Montreal Protocol by having unique stability and reduced global warming potential. It also helps the air conditioning work efficiently. It would be impossible to envision today’s standards of living without air conditioning.

Smith’s house is partially powered by photovoltaic cells that the family installed several years ago. It is a good deal because after they paid for the installation of the cells, they have been enjoying free power for some of the electric needs of the family. A special fluoropolymer called polyvinyl fluoride (PVF) plays an important role in photovoltaic units by protecting them from damage and increasing their useful life.

The power plants that supply the rest of the house’s electricity have many fluoropolymer and fluorocarbon-based components, which help reduce carbon dioxide and other emissions. For example, fluoropolymer-coated bag filters remove harmful particles (fly ash) from the smoke discharged by coal burning plants. There would be a huge decrease in particulate emissions if every coal burning plant in the world installed fluoropolymer-coated bag filters.

Mr. Smith looks at the house admiring the landscaping and the crisp looking aluminum siding and steel roof that still looks as bright and clean as it did 10 years ago (Fig. 1.1). What is great is that he has not had to do a thing to keep the exterior looking good! The roof is coated with a paint made with a fluoropolymer called polyvinylidene fluoride (PVDF). This paint endures all the elements of climate, is maintenance free, and is expected to last 30 years.

Figure 1.1 Photograph of a PVDF-painted house roof using Kynar. Courtesy Kynar® is a trademark of Arkema Corp, www.gulfcoastsupply.com, June 20, 2020.

Later, Mr. Smith will travel overseas on a Boeing 787 Dreamliner airplane that is equipped with Halon 1301 fluorocarbon fire extinguishers in its cargo compartment to prevent passive fires started by phantom sparks. There are a few hundred miles of wires and cables in this wide body aircraft that allow the plane fly and function. Boeing 777 and Airbus 380 contain 176 and 525 km of wire and cable, respectively. The wires and cables are insulated with fluoropolymers or composite materials that contain fluoropolymers. They allow safe performance of the electrical and signal systems of the aircraft over its lifetime (Fig. 1.2).

Figure 1.2 Aerospace data cables insulated with PTFE and melt-processible fluoropolymers. Courtesy Harbour Industries, www.harbourind.com.

Mr. Smith may not realize that the interior of the aircraft is surfaced by a composite of fluoropolymer PVF, which is fire safe, stain resistant, durable, and stands up to the harsh cleaning chemicals and disinfectants (Fig. 1.3).

Figure 1.3 Ceiling and stow bins surfaced with Tedlar PVF Declam. Courtesy Heath Techna Corp, www.HeathTechna.com.

Mrs. Smith’s 78-year-old mother is recovering from an operation. A part of one of her arteries was replaced by vascular grafts made of expanded polytetrafluoroethylene (ePTFE) material like the one seen in Fig. 1.4. She suffered no pain during the operation and had a normal anesthetic experience thanks to the use of a fluorocarbon gas. Mrs. Smith’s mother has been given an excellent prognosis for recovering from the surgery and is adapting well to her new ePTFE veins. She will never know that anesthesia from which she recovered was a fluorocarbon gas. She will go on with her active life and enjoy her grandchildren for a long time.

Figure 1.4 Stretch vascular grafts made from Gore-Tex ePTFE. Courtesy Gore Medical, WL Gore and Associates, www.GoreMedical.com.

Dr. Charles Suckling attempts to prepare the ideal inhalation anesthetic gas that lead to the successful development of Halothane in 1951. It had the desired characteristics including effectiveness, nontoxicity, inflammability, and rapid detoxification. Halothane was the first fluorinated inhaled anesthetic that was extremely successful, rapidly displacing all other potent inhaled anesthetics. Efforts to develop other halogenated anesthetics with more of the characteristics of the ideal inhaled anesthetic agent than halothane led to the introduction of isoflurane, desflurane, sevoflurane, enfluorane, isoflurane, desflurane, sevoflurane, and methoxyflurane [6]XXX.

In 2012, over 232 million surgical procedures were performed in the world. The number of surgeries in the United States in 2016 was over 53 million. A significant number of operations required general inhalation anesthetic. Fluorocarbon compounds such as Sevoflurane and Isoflurane have drastically reduced the long-standing anesthesia risks [7]XXX. During the 1990s, decade estimates for the number of deaths attributed to anesthesia have dropped by greater than 25-fold from 1 in 10,000 anesthetics to 1 in 200,000 to 1 in 300,000 in 2013. Today, surgeons save lives thanks to safe fluorocarbon anesthetics pioneered by Charles Suckling.

There are many more similar everyday life examples that save lives and enhance the quality of human life. More exotic examples include parts in airplanes and spacecrafts from the early days of space exploration to today’s International Space Station. Fluorocarbons whether chemicals or plastics are invisible and often go unnoticed. The reason is that they are inside the systems that have enabled today’s societal human standards.

Out of sight leads to out of mind! This metaphor taken to extreme may narrow the focus to the challenges of fluorocarbons without placing due weight on their critical roles in human life. Fluorocarbon and fluoropolymer industries and governments have been working together for decades on reducing the negative impact of these products on the environment, on global warming, and on human health [8]XXX. A good example of success in reduction of the negative impacts of fluorocarbons is the development of ozone safe and low global warming potential (

References

1. Allukian Jr M, Carter-Pokras OD, et al. Science, politics, and communication: the case of community water fluoridation in the US. Ann Epidemiol. 2018;28:401–410.

2. Fluoridation Facts, American Dental Association. ; 2005.

3. Murray JJ. Efficacy of preventive agents for dental caries. Caries Res. 1993;27(Suppl. 1):2–8.

4. Ran T, Chattopadhyay SK. Economic evaluation of community water fluoridation. Am J Prev Med. 2016;50(6):790–796.

5. Fried J, Sabo EF. 9α-Fluoro derivatives of cortisone and hydrocortisone. J Am Chem Soc. 1954;76:1455–1456.

6. Edmond Eger II E. Characteristics of anesthetic agents used for induction and maintenance of general anesthesia. Am J Health Syst Pharm. 2004;61.

7. ; 2018.

8. Ebnesajjad S. Fluoroplastics: non-melt processible fluoropolymers. vol. 1 2nd ed. Oxford: Elsevier; 2015.

2

From Fundamentals to Applications

Sina Ebnesajjad,    FluoroConsultants Group, LLC, United States

Abstract

Fluorine is unique and very different from all other halogens. Halogens have a common characteristic, that is, all possess seven electrons in the outer shell of their atomic structure. Their valence is 1 in their reactions with hydrogen and metals. The reactivity of halogens has a reverse relationship with their atomic number. The lightest halogen is fluorine and the heaviest is astatine, respectively, at the top and bottom of group 17 in the periodic table of elements. Substituting fluorine for another element, such as hydrogen, in the chemical structure of an organic compound has profound impact on its properties. The reason is fluorine has the highest reactivity of all elements [1]. It has an electronegativity of 4—the maximum value on the Pauling scale (0.74, dimensionless). Most chemically resistant elements like platinum are susceptible to fluorine attack. For example, platinum reacts with F2 gas and forms platinum hexafluoride (PtF6).

Keywords

Fluorine; fluoropolymers; polyolefin; polytetrafluoroethylene; linear fluoropolymer

Outline

Outline

2.1 Introduction 7

2.2 What makes fluorine unique? 7

2.3 Fluorine Characteristics 8

2.3.1 Fluorination 9

2.3.2 Reactivity—an extreme element 9

2.3.3 Preparation of Fluorine 10

2.3.4 Organic Chemistry 10

2.3.5 Fluorine and Nature 11

2.4 What are fluoropolymers? 11

2.5 Fundamental properties of fluoropolymers 12

2.6 Developmental history of fluoropolymers 13

2.7 Examples of Uses of Fluoropolymers 13

References 17

2.1 Introduction

Fluorine is unique and very different from all other halogens. Halogens have a common characteristic, that is, all possess seven electrons in the outer shell of their atomic structure. Their valence is –1 in their reactions with hydrogen and metals. The reactivity of halogens has a reverse relationship with their atomic number. The lightest halogen is fluorine and the heaviest is astatine, respectively, at the top and bottom of group 17 in the periodic table of elements. Substituting fluorine for another element, such as hydrogen, in the chemical structure of an organic compound has profound impact on its properties. The reason is fluorine has the highest reactivity of all elements [1]. It has an electronegativity of 4—the maximum value on the Pauling scale (0.7–4, dimensionless). Most chemically resistant elements like platinum are susceptible to fluorine attack. For example, platinum reacts with F2 gas and forms platinum hexafluoride (PtF6) (Fig. 2.1).

Figure 2.1 Periodic table of elements. Reproduced by permission of International Union of Pure Applied Chemistry. IUPAC periodic table of the elements. © 2017 International Union of Pure Applied Chemistry [2]XXX.

2.2 What Makes Fluorine Unique?

Other elements such as carbon form strong bonds with fluorine as a result of its highest electronegativity among all elements. The strength of carbon–fluorine bond (C–F) is the fundamental reason polytetrafluoroethylene (PTFE) is one of the most stable and inert plastics known to man. Yet, its monomer tetrafluoroethylene is flammable and highly explosive thanks to the diversity of fluorine effect. McGraw-Hill Encyclopedia of Chemistry points out about the stability of halogenated compounds: Organic halogen compounds generally show progressively increased stability in the order iodine, bromine, chlorine, and fluorine [3]XXX.

Fluorine is relatively easy to substitute for hydrogen (and other elements) in organic compounds due to its extreme affinity for grabbing electrons. Substituting fluorine for hydrogen in a chemical compound gives rise to a variety of unique and useful effects. Examples include increased polarity, decreased polarity, chemical activity, chemical neutrality, increased biological activity for pharmaceuticals and agro chemicals, greater thermal and oxidative stability, and increased chemical resistance.

An interesting example is fluorination of the surface of a polyolefin (e.g., polypropylene) film [4]XXX. Slight fluorination renders the neutral surface of a polyolefin film polar. Further increases in the fluorine content of the surface result in total neutrality of the film surface. In practice, minor fluorination of polyolefin surfaces is used to make them adherable. Inside surfaces of plastic pesticide and herbicide bottles are routinely fluorinated, extensively, to prevent permeation of the agents through the thickness of the container. The fluorination prevents the loss of material, emissions, and human and other exposures. Sometimes fluorine gas is added to the blow molding gas in order to combine the bottle fabrication and fluorination steps.

A number of partially and fully fluorinated polymers have been developed because of the unique effect of fluorine on their properties. Some of the common polymer chemistries include polyolefins, fluorinated elastomers, polymethyl siloxane, acrylic and methacrylic polymers, and perfluoroether polymers. The impact of increasing the fluorine content of olefinic polymers on their properties is listed in Table 2.1.

Table 2.1

Fluorinated compounds have varied and unusual properties, a number of which are quite useful to the development of commercial materials for a broad range of applications including plastics, electronics, agriculture, pharmaceuticals, and medicine.

2.3 Fluorine Characteristics

Fluorine ranks 13th in abundance among the Earth’s rocks, present at an average concentration of 0.1% by weight [1]. Fluorine abundance is 0.08% compared to 0.05% in the Earth lithosphere [5]XXX. Fluorine is considered the most dominant halogen when the whole Earth is considered.

The most abundant natural sources of fluorine are fluorspar (CaF2) and cryolith [also called cryolite (Na3AlF6)]. Enamel of teeth is very hard, mechanical strong, and has long-term durability, mainly because of fluoroapatite [Ca5 (PO4)3F or 3Ca3(PO4)2⋅CaF2], along with hydroxyapatite being its major components. Fluoride is considered a trace element because only small amounts are present in the body (about 2.6 g in adults) and because the daily requirement for maintaining dental health is only a few milligrams a day. About 95% of the total body fluoride is found in bones and teeth. Fluoride's primary function in the human body is to strengthen the bone and it is known to prevent tooth decay. Experts contend that fluoride strengthens the teeth's enamel by strengthening the mineral composition of the teeth themselves [6XXX,7]XXX.

About 40% of fluorspar [1] is used as metallurgical flux in the steel industry, some of which is recovered as synthetic fluorspar. The highest grade of fluorspar (>97% CaF2) is reacted with sulfuric acid for the production of HF, which is the starting point of organic fluorinated compounds. Some fluorspar is consumed in uranium processing, petroleum alkylation, and stainless steel pickling [8]XXX.

Fluorine is a gas with a green-yellow color, a boiling point of –188.1°C, and a melting point of –219.6°C [9]. Its pungent odor is perceptible at a concentration of 10 parts per million. Fluorine is highly toxic, corrosive, and oxidizes nearly every element, including noble gases xenon and krypton. In contrast to HF, dry fluorine does not etch glassware but reacts with hot platinum and gold. To reduce its reactivity and hazard, fluorine is diluted with nitrogen; a 10% F2 in nitrogen can be stored and transported in passivated steel bottles. Some basic facts about fluorine are presented in Table 2.2.

Table 2.2

2.3.1 Fluorination

Fluorinating agents used to introduce fluorine atoms into target molecules. Selectfluor, manufactured by Air Products in the United States [10]XXX, is perhaps the best-known electrophilic fluorinating agent. Pharmaceutical researchers use this reagent to fluorinate steroids. Diethylaminosulfur trifluoride transforms hydroxyl and carbonyl groups into CF and CF2 moieties, while triethylamine tris-hydrofluoride provides a pH neutral, nonvolatile equivalent of hydrogen fluoride, and is a source of fluoride ions for various nucleophilic reactions. In addition, trifluoromethyltrimethylsilane, CF3SiMe3, is a useful CF3 source that reacts with carbonyl systems to yield trifluoromethylated alcohol derivatives.

An important technique for commercial preparation of fluorocarbons is an electrochemical fluorination method called the Simons Process. Professor Simons discovered the process in the 1930s while working on a research project sponsored by 3M Corporation. The process is based on the electrolysis of a solution of an organic compound, such as a hydrocarbon, in a solution of hydrofluoric acid [11]XXX.

2.3.2 Reactivity—An Extreme Element

As has often been stated, fluorine is truly a material of extremes [12]XXX. Fluorine is the most reactive element known to man. It reacts with nearly everything including glass. Nobel gases such as xenon and krypton and precious metals like gold and platinum are not exceptions; all react with fluorine. Moissan has been credited for the first synthesis of fluorine [13]XXX.

Moissan conducted an experiment to illustrate the extreme reactivity of fluorine. Oil of turpentine, in the solid state, is attacked by liquid fluorine. To perform this experiment, he placed a small amount of oil of turpentine at the bottom of a glass tube surrounded with boiling liquid air. As soon as a small quantity of fluorine was liquefied on the surface of the solid, a combination reaction took place accompanied by an explosive force. After each explosion, a slow current of fluorine gas was kept up leading to the formation of a fresh quantity of liquid fluorine. Successive detonations occurred at intervals of 6–7 minutes. Finally, after a longer interval of about 9 minutes, the amount of liquid fluorine formed reached the sufficient quantity to cause complete destruction of the apparatus. Every time drops of liquid fluorine landed on the floor, accidentally, the wood instantly caught fire.

2.3.3 Preparation of Fluorine

Interest in fluorine is literally centuries old even though its successful preparation is relatively recent. A number of unsuccessful efforts to prepare fluorine were made in the past. In 1529, Georigius Agricola described the use of fluorspar (CaF2) as a flux. In 1670, Schwandhard found that glass was etched when exposed to fluorspar treated with acid. Fluorine is one of the last elements to be isolated because of its extreme reactivity. In 1764, Margraff synthesized HF by reacting sulfuric acid and fluorspar. The severe redox potential of fluorine prevented its synthesis because of the lack of a suitable oxidant [9].

The eminent French chemist Henri Moissan first prepared fluorine on June 26, 1886. He used Fremy’s Salt (after the venerable French chemist Edmond Fremy, 1814–1894) technique to make and isolate pure fluorine by electrolysis of KF in anhydrous hydrofluoric acid. Moissan performed electrolysis of a 1–12 mixture of KF and HF in a U-shaped platinum tube cooled at 23°C in a MeCl bath. Henri Moissan was awarded the Nobel Prize in chemistry in 1906. The principle of Moissan’s electrolytic preparation of fluorine is still in use for the industrial manufacture.

2.3.4 Organic Chemistry

Carbon forms its strongest bond with fluorine and the credit for demonstrating the stability of C–F bond goes to the French chemists Dumas and Peligot who heated dimethyl sulfate with potassium fluoride and obtained methyl fluoride (Eq. 2.1) [9].

(2.1)

The first nucleophilic replacement of another halogen by fluorine has been attributed to an unfathomable genius, the Russian musician and chemist Alexander Borodin [14]XXX. He was probably composing the Polovtsian Dances from the great opera Prince Igor (later finished by Nikolai Rimsky-Korsakov and Aleksandr Glasunov) while synthesizing benzoyl fluoride by replacement of chlorine in benzoyl chloride using Fremy’s Salt (KF+HF) [9]. This reaction, known as Halex (short for halogen exchange), remains the most important commercial route to the synthesis of C–F bonds [15].

Fluoroaromatics were better understood in the latter part of the 19th and early part of the 20th centuries. Aliphatic fluorine chemistry lagged behind until the pioneering work of the Belgian Chemist Frederic Swarts. He carried out halogen exchange on polychlorides and polybromides using antimony trifluoride and bromine (SbF3+Br2). Swarts demonstrated dehalogenation using Zn and dehydrohalogenation using K2CO3 would preferentially eliminate halogens other than fluorine, forming fluorinated olefins. Swarts has been credited for the first synthesis of CCl2F2 by Midgley and Henne of the Frigidaire Co. (part of General Motors), who pioneered the use of fluorinated hydrocarbons in the refrigeration industry [15].

The years before and during World War II brought important advances to organic fluorine chemistry that facilitated the use of fluorocarbons. The discovery of the catalytic activity of HgF2 by Simons and Block [15] on to allow nonexplosive reaction of carbon and fluorine paved the way for the synthesis of saturated perfluorohydrocarbons. These compounds resisted the highly reactive UF6 and were used as buffer fluids in the gas-diffusion process of the Manhattan Project in the early 1940s.

Beginning in the 1950s and 1960s, fluorocarbons were studied and developed for biological activity. For example, a number of fluorocarbons such as Fluroxene (CF3CH2OH=CH2) began to revolutionize the field of inhalation anesthetic because of their extreme nonflammability. By the late 1970s, fluorocarbons began to dominate the area of inhalation anesthesiology. Fluorine-containing compounds such as Sevoflurane, Enflurane, and Isoflurane are among the common present-day anesthetics. Other developments of this era include organic fluorine-containing pharmaceuticals, artificial blood, respiratory fluids, and chemical weapons.

In the 1980s, the semiconductor fabrication industry began to use fluorinated gases for plasma etching processes and fluorinated chemicals as cleaning fluids. Two important etchants are NF3 and BF3.

In the 1980s and 1990s, fluorine-containing compounds have made monumental contributions to the agricultural industry, with bioactive compounds used as herbicide, pesticide, fungicide, and plant growth regulators. Today, some 10% of these compounds contain fluorine in some form. Even though the aromatic fluorine chemicals are more expensive than nonfluorinated analogs, their activity and effectiveness leads to the consumption of smaller quantities of fluorinated compounds.

Fluorinated liquid crystals were incorporated in the design of active matrix liquid crystal displays in the 1990s. Development work on the 157 nm photolithography technology during the 2000s required fluorinated photoresists for the manufacture of integrated electronic circuits.

This section is not a complete history of organofluorine chemistry. Much more can be learned about organic fluorine and its characteristics of fluorine by studying the sources cited in this section elsewhere.

2.3.5 Fluorine and Nature

The epic challenge in taming fluorine is to generate C–F bonds by enzyme catalysis. Nature has hardly prepared a biochemistry of fluorine; the plant toxin fluoroacetate described earlier is the most common naturally occurring fluorine-containing compound. Some exceptionally toxic plants, mostly from Africa, can accumulate fluoroacetate at concentrations up to 8000 ppm (8 mg/g). The bacterium Streptomyces cattleya can mediate the biotransformation of inorganic fluoride to fluoroacetate and also to the amino acid 4-fluorothreonine. Researchers at the University of St Andrews and the Queen's University of Belfast have shown [16]XXX that fluoroacetaldehyde is an intermediate in synthesizing both these metabolites. S. cattleya is apparently capable of generating fluoroacetaldehyde in vivo from metabolic intermediates, presumably by reacting them with inorganic fluoride.

Nature certainly seems to find fluoride difficult to manipulate. The reason may be its poor nucleophilicity in aqueous media. Researchers at the University of British Columbia have reported [17]XXX forming C–F bonds from inorganic fluoride by using mutant bacterial trans-glycosidase enzymes. Scientists removed the nucleophilic carboxylate group from a glutamate residue of the natural β-glycosidases by replacing the latter with glycine, alanine, or serine residues.

The ability of these mutant enzymes to generate fluoroglycosides is explained by the presentation of fluoride to the reactive center on the sugar. In the case of the serine mutant, a hydrogen bond forms between F and the hydroxyl group. These are some of the first examples of organofluorine compounds produced by purified, albeit engineered, proteins [18XXX–22]XXX.

2.4 What Are Fluoropolymers?

Traditionally, a fluoropolymer or fluoroplastic is defined as a polymer consisting of carbon (C) and fluorine (F). Sometimes these are referred to as perfluoropolymers to distinguish them from partially fluorinated polymers, fluoroelastomers, and other polymers that contain fluorine in their chemical structure. For example, fluorosilicone and fluoroacrylate polymers are not referred to as fluoropolymers. An example of a linear fluoropolymer is tetrafluoroethylene polymer (PTFE):

A simplistic analogy would be the chemical composition of polyethylene [(–CH2–CH2–)n] where all the hydrogen atoms have been replaced by fluorine atoms. Of course, in practice PTFE and polyethylene are prepared in totally different ways. There are branched fluoropolymers such as fluorinated ethylene propylene polymer (FEP):

Oxygen (O) and chlorine (Cl) are present in the chemical structure of some commercial fluoropolymers. Examples include perfluoroalkoxy and polychlorotrifluoroethylene:

Rf is usually a perfluorinated group consisting of carbon and fluorine. Introduction of nonlinearity, oxygen and side chains, or chlorine invoke a variety of polymer properties which will be dealt with later in this book.

There is a second class of fluoropolymers called partially fluorinated in contrast to perfluorinated polymers. These molecules include hydrogen (H) in addition to fluorine and carbon.

Examples include polyvinylfluoride (PVF), polyvinylidene fluoride (PVDF), and ethylene tetrafluoroethylene copolymer:

Partially fluorinated fluoropolymers are significantly different from the perfluoropolymers with respect to properties and processing characteristics. For example, perfluoropolymers are more thermally stable but physically less hard than partially fluorinated polymers. The former has much higher hardness than the latter.

2.5 Fundamental Properties of Fluoropolymers

The basic properties of fluoropolymers arise from the atomic structure of fluorine and carbon and their covalent bonding in specific chemical structures. These properties become weaker as the chemical structure becomes less perfluorinated, as in PVDF. Because PTFE has a linear structure, it is a good subject for discussion of extreme properties. The backbone is formed of carbon–carbon bonds and carbon–fluorine bonds. Both are extremely strong bonds (C–C=607 kJ/mol and C–F=552 kJ/mol) [23XXX,24]XXX. The basic properties of PTFE stem from these two very strong chemical bonds. The PTFE molecule resembles a carbon rod completely blanketed with a sheath of fluorine atoms [25,26].

The size of the fluorine atom allows the formation of a uniform and continuous sheath around the carbon–carbon bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. The fluorine sheath is also responsible for the low surface energy (18 dynes/cm) [27]XXX and low coefficient of friction (0.05–0.8, static) of PTFE [25,26]. Another attribute of the uniform fluorine sheath is the electrical inertness (or nonpolarity) of the PTFE molecule. Electrical fields impart only slight polarization in this molecule, so volume and surface resistivity are high. Table 2.3 summarizes the fundamental properties of PTFE.

Table 2.3

The basic properties of PTFE result in beneficial attributes with high commercial value (Table 2.4).

Table 2.4

2.6 Developmental History of Fluoropolymers

The development of fluoropolymers began with the invention of PTFE in 1938 and continued to 1992 when a soluble perfluoropolymer (Teflon AF) was introduced. Table 2.5 summarizes the timeline for the development of fluoropolymers that have brought about major changes in properties and/or fabrication processes.

Table 2.5

PTFE, Polytetrafluoroethylene; TFE, tetrafluoroethylene; PCTFE, polychlorotrifluoroethylene; CTFE, chlorotrifluoroethylene; FEP, fluorinated ethylene propylene; HFP, hexafluoropropylene; PDD, Perfluoro-2,2-dimethyl-1,3,-dioxole; PVP, polyvinylfluoride; PVDF, polyvinylidene fluoride; ECTFE, ethylene chlorotrifluoroethylene; PFA, perfluoroalkoxy; ETFE, ethylene tetrafluoroethylene; VDF, vinylidene fluoride; E, ethylene; PAVE, perfluoroalkylvinylether; VF, vinyl fluoride; PDD, 2,2-bistrifluoromethyl-4,5 difluoro-1,3-dioxole.

aHexafluoropropylene (CF2=CF−CF3).

bVinyl fluoride (CH2=CHF).

cVinylidene fluoride (CH2=CF2).

dEthylene (CH2=CH2).

ePerfluoroalkylvinylether (CF2=CF−O−Rf).

fPerfluoro-2,2-dimethyl-1,3,-dioxole.

The discovery of PTFE was a major leap forward in material science. Yet, the new polymer could not be fabricated by melt processing. The next two forms of PTFE, fine powder and dispersion, were also not melt-processible. The pursuit of a more easily processible polymer led to FEP, which could be melted in an extruder. Compared with PTFE, the major disadvantage of FEP is its reduced mechanical properties at elevated temperatures and thus maximum continuous use temperature (200°C) (Table 2.5). PFA, which was introduced in 1973, offers both melt-processing and the same upper continuous use temperature as PTFE (260°C).

Ethylene tetrafluoroethylene (ETFE) addresses the need for a mechanically stronger polymer, albeit at a loss of fluoropolymer properties because of the presence of hydrogen in its molecule.

Compared to PTFE, ETFE has lower continuous use temperature (150°C), less chemical resistance, and a higher coefficient of friction. Mechanical properties, including tensile strength, elongation at break, and tensile modulus are increased, leading to cut-through resistance in wire insulation.

Teflon AF is an amorphous polymer, which is soluble in certain halogenated solvents. It can be applied as a solution, followed by the removal of the solvent. The remaining coating will be as resistant to almost as many chemicals as PTFE. The thickness of the coating can range upward from less than a micrometer.

There are a number of other polymers in this family including polychlorotrifluoroethylene (PCTFE), PVF, PVDF, ethylene chlorotrifluoroethylene (ECTFE), tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride copolymers, perfluoroacrylates, fluorinated polyurethanes, and chlorotrifluoroethylene/vinylether copolymers. Typical properties of commercial fluoropolymers are listed in Table 2.6.

Table 2.6

Brk, Break; RT, room temperature; PTFE, polytetrafluoroethylene; CTFE, chlorotrifluoroethylene; FEP, fluorinated ethylene propylene; PVF, polyvinylfluoride; ECTF3, ethylene chlorotrifluoroethylene; PFA, perfluoroalkoxy; ETFE, ethylene tetrafluoroethylene.

2.7 Examples of Uses of Fluoropolymers

The consumption of PTFE has increased over the years as technological advancement has required the properties of fluoropolymers. The applications of PTFE and fluoropolymers in general span all facets of human life, from household uses to the aerospace and electronic industries. Basic properties (Table 2.3) of PTFE lead directly to its applications: chemical resistance, thermal stability, cryogenic properties, low coefficient of friction, low surface energy, low dielectric constant, high volume and surface resistivity, and flame resistance. Applications for fluoropolymers always exploit one or more of the properties (Table 2.7) that set them apart from other materials, particularly other plastics.

Table 2.7

PTFE, Polytetrafluoroethylene.

In the chemical process industry, for example, fluoropolymers are selected for their unmatched resistance to chemical attack. They serve as linings for carbon steel vessels, and for piping and other fluid handling components. They provide durable, low maintenance, and economical alternatives to exotic metal alloys. In these applications, fluoropolymers also offer thermal stability for use at high temperatures. And because they do not react with process streams, they help prevent contamination of products.

Fluoropolymers are the materials of choice for many materials integrity management applications within the microelectronics [28]XXX, food, beverage, pharmaceutical, and biopharmaceutical industries.

Electrical properties of fluoropolymers are highly valuable in electronic and electrical applications. In data communications, for example, FEP is used to insulate cables installed in air-handling spaces (plenums) in office buildings. FEP provides the excellent dielectric properties these cables require to perform well at high data transmission rates as well as long-term stability so performance will not change over the life of the cabling system. Most importantly, FEP helps these cables meet strict building code requirements for low flame spread and low smoke generation.

Fluoropolymers are used to insulate wire for critical aerospace and industrial applications where chemical and thermal resistance is essential. They are also materials of construction for connectors for high-frequency cables and for thermocouple wiring that must resist high temperatures.

In the automotive, office equipment, and other industries, mechanical properties of fluoropolymers are beneficial in low-friction bearings and seals that resist attack by hydrocarbons and other fluids. In food processing, the Food and Drug Administration (FDA)-approved fluoropolymer grades are fabrication material for equipment due to their resistance to oil and cleaning materials, and their antistick and low-friction properties.

In houseware, fluoropolymers are applied as nonstick coatings for cookware and appliance surfaces. These applications depend on thermal and chemical resistance as well as antistick performance. PTFE and ETFE are chosen to insulate appliance wiring that must withstand high temperatures.

Medical articles such as surgical patches and cardiovascular grafts rely on the long-term stability of fluoropolymers as well as their low surface energy and chemical resistance.

For airports, stadiums, and other structures, glass fiber fabric coated with PTFE is fabricated into roofing and enclosures. The architectural fabric is supported either by cables or by air pressure, thus forming a range of innovative structures. PTFE provides excellent resistance to weathering, including exposure to ultraviolet rays in sunlight, flame resistance for safety, and low surface energy for soil resistance and easy cleaning.

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