Chapter 1: Industrial Aspects of Fluorinated Oligomers and Polymers Free

In the early 1930s, researchers of IG-Farbenindustrie in Frankfurt (Germany) studied systematically the first polymerizations of fluoroethenes; the Hoechst researchers had already prepared polychlorotrifluoroethylene (PCTFE) and polytetrafluoroethylene (PTFE), including copolymers, recognizing the outstanding properties of these polymers.¹  The first patent application for a fluoropolymer was filed in October 1934 by Schloffer and Scherer.²  PTFE was also discovered in 1938 in the USA by Plunkett of E. I. DuPont de Nemours while investigating fluorinated refrigerants. The unique properties of PTFE were recognized during the Manhattan Project, where there was an urgent need for a material that would withstand the highly corrosive environment during the process of separating the isotopes of UF₆ for the first atomic bomb. PTFE apparently fulfilled all the needs, spurring the development of processing and production methods for this unique polymer. In 1946, PTFE was commercialized by E. I. DuPont de Nemours under the trade name Teflon.³ 

The unique properties of fluoropolymers are due to the fact that the polymer backbone is formed by strong carbon–carbon bonds (C–C ∼340 kJ mol⁻¹) and extremely stable carbon–fluorine bonds (C–F ∼490 kJ mol⁻¹; for comparison, C–H ∼420 kJ mol⁻¹). Substitution of fluorine for hydrogen in a material improves three key physical properties:

• increased service temperatures and reduced flammability;

• low surface energy, providing non-stick properties/anti-adhesiveness, low coefficient of friction, self-lubricating effects and lower solubility in hydrocarbons;

• excellent electrical and optical properties resulting in low high-frequency-loss rates and low refractive indices.

PTFE, PCTFE and all other fluoropolymers (see Table 1.2) gained immediate acceptance during commercialization in the various markets.

During the following decades, many fluoropolymers, including fluorothermoplastics and fluoroelastomers, were developed. The worldwide annual sales volume of fluoropolymers is today more than 230 000 tonnes (world consumption of fluoroplastics in 2012 was ∼216 000 tonnes;⁴  world consumption of fluoroelastomers in 2009 was ∼20 000 tonnes⁵ ). The total market value is more than US$6 billion.

In contrast to the higher molecular weight polymers, oligomers are characterized by a low number of repeating units, usually less than 50, and a low molecular weight, often not higher than 20 000 Da (as measured by gel permeation chromatography).

Many synthetic routes to oligomers have been described, including radical oligomerization, oligocondensation, ionic oligomerization and ring-opening reactions.⁶  Telomerization is an oligomerization by a chain-transfer reaction, carried out in the presence of a large amount of chain-transfer agent, so that end-groups are essentially fragments of the chain-transfer agent.⁷ 

In Sections 1.2 and 1.3, some of the results of the research and development work carried out at 3M using functionalized fluorinated oligomers are discussed.

All industrial routes for the synthesis of the five major C₂/C₃ fluoromonomers are based on chlorination/fluorination of C₁/C₂ hydrocarbons, mostly including a de(hydro)chlorination step at high temperature⁸  (Scheme 1.1).

Some of these manufacturing processes are fairly energy consuming (e.g. the preparation of 1 ton of TFE requires >10 000 kWh). Also, special care has to be taken in producing and handling TFE owing to its tendency to self-decompose into carbon and tetrafluoromethane. In Table 1.1 an overview of monomers to produce fluoropolymers is given.

A key-intermediate in the preparation of vinyl ethers and their oligomers is hexafluoropropylene oxide (HFPO). HFPO is prepared from HFP via direct oxidation with oxygen, by electrochemical oxidation or by reaction with hypochlorides or hydrogen peroxide:⁸ 

HFPO reacts readily with nucleophiles; for example, in the presence of fluoride salts (e.g. NaF, KF, CsF) it forms the intermediate perfluoropropyl oxide salt, which reacts with the next HFPO to form an acid fluoride after elimination of a fluoride ion. This compound is the precursor for perfluoro(propyl vinyl ether) (PPVE), which is obtained by reaction with alkali/alkaline earth metal carbonates and subsequent pyrolysis (Scheme 1.2).⁹ 

HFPO can also be oligomerized to produce higher molecular weight (up to 15 000 Da) perfluorinated polyethers having following general structure:

Other important perfluorinated vinyl ethers are synthesized by reaction of fluorinated alkoxides with HFPO followed by pyrolysis; the fluorinated alkoxides are usually prepared in situ from the corresponding acid fluorides (Scheme 1.3).⁸ 

An attractive, alternative route to perfluoro(methyl vinyl ether) (PMVE) is based on the reaction of perfluoromethyl hypofluoride and dichlorodifluoroethene followed by dehalogenation (Scheme 1.4).⁹  Solvay Specialty Polymers has mastered this synthesis and transferred it into large scale production.

Starting materials with functional groups can be prepared by direct or electrochemical fluorination (ECF) or by standard synthesis¹⁰  (Scheme 1.5).

The synthesis of comonomers for the preparation of perfluorinated amorphous polymers with high glass transition temperatures (T_g) involves multiple steps and has recently been described in detail¹¹  (Figure 1.1).

Fluorochemical building blocks containing a perfluorinated chain can be made on an industrial scale by methods including TFE/C₂F₅I telomerization, ECF and direct fluorination.¹⁷ 

Telomerization allows the synthesis of oligomers of the type C_nF_2n+1–CH₂CH₂I, whereas ECF produces perfluoroalkyl carbonyl fluorides, R_fCOF, or sulfonyl fluorides, R_fSO₂F. The perfluorinated chain may contain from one up to 16 carbon atoms. Telomer iodides and the carbonyl and sulfonyl fluorides can be converted on an industrial scale into alcohols and (meth)acrylate monomers.^12–17  The acrylic monomers have the general structures shown in Scheme 1.6.

The first surfactants and textile treatments, containing perfluoroalkyl chains, were commercialized by 3M in the 1960s.³⁴  Since then, major applications that were developed included fire-fighting agents, emulsifiers for fluoropolymers, oil and water repellents and paint and coating additives. Most of the fluorochemicals contained C₆F₁₃, C₇F₁₅ or C₈F₁₇ perfluoroalkyl groups. In the late 1990s, PFOS and related compounds were identified at parts per billion levels in the sera of the general population. In May 2000, 3M announced the manufacturing phase-out of “C₈” chemistry. During the following years, new materials based on C₄F₉ technology that addressed the bioaccumulation and toxicity concerns associated with longer chain functional perfluoroalkyls, were developed and commercialized in selected markets.⁵⁶  Environmental groups and governmental agencies are monitoring the use of higher perfluoroalkyl homologs and low molecular weight fluorochemicals very closely.

Many of the functionalized fluorinated oligomers used on an industrial scale are made by ring-opening reactions of HFPO, photo-oxidation of fluoroolefins and telomerization of fluorinated (meth)acrylates with functional mercaptans.

Perfluoropolyethers (PFPEs) are a class of low molecular weight polymers (500–15 000 Da) that were originally developed in the mid-1960s.¹⁸  Functionalized PFPEs are commercially available, for example under the trade names Krytox (E. I. DuPont de Nemours), Demnum (Daikin Industries) and Fomblin (Solvay Specialty Polymers). Their synthesis is summarized in Scheme 1.7, where X represents a COF group.^16,18 

These PFPE carbonyl fluorides can be further converted into other functional groups, such as hydroxy, (meth)acrylate, nitrile and trialkoxyalkylsilane.⁴⁵  A few examples are discussed below.

The synthesis of functionalized fluorinated oligomers can be carried out by radical oligomerization of acrylic monomers in the presence of a functional mercaptan, such as 2-mercaptoethanol.^19–23  A similar strategy was used to prepare oligomers with molecular weights ranging between 1500 and 10 000 Da and one or more functional groups, such as hydroxy, carboxy, amine or trimethoxysilane.^24–29  The oligomers have an aliphatic backbone with a plurality (usually between 4 and 16) of pendant fluoroaliphatic groups and the majority of them are endcapped with the functional group(s). The oligomers contain a mixture of compounds with a range of repeating units; for example, if a monomer to mercaptan molar ratio of 4 : 1 is used, a mixture of compounds with molar ratio ranging from 1 : 1 to about 8 : 1 is obtained after oligomerization. The synthesis is summarized in Scheme 1.8.

Using the same synthetic procedure, mixed co-oligomers can also be prepared using fluorine-free hydrophilic and hydrophobic comonomers.²⁴  In the case of hydrophilic monomers containing alkylene oxide segments or an ionic group, such as salts of an acid (as present in, for example, sodium acrylate) or a quaternary ammonium group (as present in, for example, diethylaminoethyl methacrylate hydrochloride salt), surfactants can be prepared having an anionic, cationic, amphoteric or non-ionic character.^30,31  Such surfactants provide efficient and effective lowering of the static and dynamic surface tension of liquids and increase the wetting of a coating on a substrate surface.⁵⁶ 

Functional oligomers can be reacted and blended with many different compounds through their functional group(s). For example, fluorinated oligomeric alcohols can be used in combination with isocyanates to make (poly)urethanes^24,26  or in combination with carboxylic acids to make (poly)esters.³²  Oligomeric acids can be condensed to (poly)amides.³³  These condensates can be used as surface modifiers for commodity polymers such as poly(methyl methacrylate) (PMMA) or polyamide 6 (PA 6).²⁸  In the following, examples and applications are discussed.

Fluorochemical oil and water repellents for textile fabrics were discovered in the 1950s by researchers at 3M³⁴  and, since then, many commercial products have been developed for a wide variety of surfaces by different companies.^35–37  The repellent properties are the result of the low surface energy, typically between about 12 and 15 mN m⁻¹, of a textile fabric treated with a fluorochemical repellent material. Water and oily substances will not be able to wet and spread on such a treated surface, resulting in water and oil repellency of the treated fabric³⁸  (Figure 1.2).

Many fluorine-containing repellents are based on poly(meth)acrylates. These acrylic polymers can be visualized as consisting of pendant perfluoroalkyl groups (R_f) and hydrocarbon groups (R_h), an acrylic polymer backbone and non-fluorinated linkages between the two. The composition and ratio of the comonomers in such polymers affect the repellent properties. Comonomers with a crosslinking function, such as 2-hydroxyethyl acrylate, glycidyl methacrylate or N-methylolacrylamide, are used to increase the durability of the repellent treatment.^35–37  Such copolymers can be represented as shown in Figure 1.3.

It was observed that organizing the fluorinated groups, R_f, into small domains improves their efficiency and effectiveness as oil and water repellents.²⁴  This organization into fluorochemical domains can be achieved by using fluorochemical-functionalized oligomers and chemical methods to connect the oligomers to a backbone (Figure 1.4).

Much of the research and development work at 3M involved the combination of hydroxy-functionalized oligomers with isocyanates and fluorine-free mono-, di- or polymeric alcohols, amines, thiols and other isocyanate-reactive materials to form urethanes, ureas, thioureas or their polymeric analogs such as polyurethanes.^24–27,39  These reactions are carried out in organic solvents, such as ethyl acetate, usually in the presence of catalysts such as certain Sn-compounds (e.g. dibutyltin dilaurate). Crosslinking of such urethane derivatives can be achieved by incorporating specific blocking agents such as oximes or imidazoles, thus forming thermolabile urethane groups. These thermolabile groups decompose at the curing temperature of the fabric treatment, typically 150–170 °C, generating in situ isocyanate functionalities that will react with any hydroxy, amino or carboxy group present on the fiber surface, creating a chemical bond between the textile fabric and the fluorochemical agent, resulting in improved laundering and dry-cleaning resistance.³⁷ 

An interesting class of isocyanate-derived repellents are polycarbodiimides,^40,41  since the carbodiimide group, –NCN–, itself can react with functional groups present at the surface of the textile fabric.⁴²  Polycarbodiimides containing fluorinated oligomer segments were prepared and found to have very good durable repellent properties without the use of an isocyanate blocking group.²⁴ 

Another important finding was the discovery of functionalized fluorinated spacer oligomers, prepared by radical oligomerization of spacer monomers with functional mercaptans.⁴³  The fluorinated spacer monomers can be prepared by combining a fluorinated alcohol, e.g. C₄F₉SO₂N(CH₃)CH₂CH₂OH, with a diisocyanate, e.g. MDI (4,4′-diphenylmethane diisocyanate) and a hydroxy-terminated alkyl(meth)acrylate, such as 2-hydroxyethyl methacrylate.

Fluorochemical textile treatments provide excellent oil and water repellency and stain-repellent finishes, but for the release of soil and stains, water needs to displace the contaminants and the laundering detergent must be able to wet the fabric. Treatments with both hydrophilic and hydrophobic/oleophobic segments were developed, which in dry conditions provide repellent properties (due to the fluorochemical tails), but in water the structure “inverts” and exposes the hydrophilic parts, resulting in good soil-release (“flip-flop” mechanism).^27,34,57 

By applying fluorinated compounds, especially fluorinated trialkoxysilanes, to siliceous surfaces, such as glass or ceramics, such substrates can be given a low surface energy, typically around 10–15 mN m⁻¹, even when solutions with very low concentrations of 0.01–0.2% by weight are used.⁴⁴  Very high thermal and oxidative stabilities were also observed. Fluorinated silanes have the ability to form chemical bonds with the hydroxy groups present on the glass through the formation of Si–O–Si bonds.¹⁵ 

Perfluoro(polyether silanes) can be easily prepared by reaction of the corresponding esters with aminopropyltrialkoxysilanes⁴⁵  or the corresponding alcohols with isocyanatopropyltrialkoxysilanes.⁴⁶  Some structures are shown in Figures 1.5 and 1.6.

These silanes can be easily applied to siliceous surfaces, such as shower panels or bathroom ceramics, by applying dilutions in alcohols, such as ethanol or 2-propanol, in combination with catalytic amounts of acid. In a first step, the trialkoxysilanes are hydrolyzed into silanols, which then undergo condensation reactions (silanols reacting with themselves) and crosslinking, where the fluorochemical is chemically bonded to the hydroxy groups of the siliceous surface. The PFPE layer is very thin (about 20–100 nm) and provides excellent repellent and easy-to-clean properties and very good durability against aggressive chemicals, such as acids or bases, and against mechanical abrasion.⁴⁶ 

Methods for aqueous delivery of PFPE-silanes were also developed. One method consists of making a non-aqueous concentrate containing the PFPE-silane and a fluorosurfactant, diluting the concentrate in water and applying the aqueous formulation to the siliceous surface.⁴⁷  Another way is to prepare cationic perfluoro(polyether silanes), which are readily soluble or dispersible in water, and applying them to the siliceous surface followed by room temperature drying and curing.⁴⁸ 

A wide variety of fluoropolymers have been developed and produced on an industrial scale for a broad range of applications.^49,50  However, except for fluorosilicones,⁵¹  fluoropolymers with a low T_g are not widely available.

Recently, chemistries and methods were developed to prepare fluoroelastomers with perfluoropolyether segments and having a T_g of less than −40 °C. Starting materials include low molecular weight perfluoro(polyether dinitriles), such as shown in Figure 1.7, and fluorinated amidines, such as H₂NC(NH)(CF₂)₈C(NH)NH₂, or other reagents that allow the formation of triazine groups.^58–60 

Fluoroelastomers containing perfluoropolyether segments and triazine groups were obtained after curing; their T_g was about −112 °C.⁵²  Fluoroelastomers containing perfluoropolyether segments and low T_g were also prepared starting from PFPE-diiodides, such as ICF₂O(CF₂O)_n(CF₂CF₂O)_mCF₂I, according to a radical curing mechanism,^53,54  and PFPE-dinitriles using click chemistry, involving azides or alkynes.⁵⁵  The fluoropolyether elastomers obtained have unique attributes as far as T_g and other physical properties are concerned (Table 1.2).

The principal method for synthesizing fluoropolymers is free-radical polymerization, as other typical methods, e.g. cationic polymerization, are ineffective owing to the electrophilic nature of fluoroolefins. Fluoroolefins can be polymerized using anionic catalysts, but termination by fluoride ion elimination prevents the formation of high molecular weight polymers. Coordination catalysts do not lead to polymerization of fluoroolefins.

The free-radical polymerizations are mostly water based, either as aqueous suspension polymerization (mostly applied for PTFE polymers) or as aqueous emulsion polymerization in the presence of emulsifiers, most preferably in the presence of fluorinated emulsifiers.

In the past, most commonly the ammonium salt of perfluorooctanoic acid (PFOA) (C₇F₁₅COO⁻NH₄⁺) was used as an emulsifier. However, owing to environmental concerns, the US Environmental Protection Agency (EPA) initiated a program to reduce the emissions of PFOA and to work towards the elimination of PFOA by 2015.^61–63  Therefore, fluoropolymer producers have implemented PFOA replacements (see Chapter 13) and have also developed technologies using hydrocarbon emulsifier^64–72  or even emulsifier-free technologies.^73–76 

In early times, radical copolymerization of fluorinated olefins in chlorinated fluorocarbon solvents (e.g. R113, CF₂Cl–CFCl₂) was widespread; also, many work-up processes (e.g. agglomeration steps) used chlorofluorocarbons. Owing to the high emissions of these ozone-depleting solvents and to the Montreal Protocol, these processes had to be changed so as to use either environmentally friendly solvents [e.g. CF₃(CF₂)₄CF₂H] for ETFE polymerizations⁷⁷  or water-based systems.

Polymerization in supercritical (sc) media (e.g. in scCO₂) – originally introduced as an alternative “green” polymerization technology – did not find broad applicability.^3,78  Other free-radical polymerization processes, e.g. in the gas phase or in ionic liquids,⁷⁹  are currently not widely adopted in industrial manufacturing processes.

In 2012, world consumption of PTFE and fluorothermoplasts reached 216 000 tonnes plus more than 20 000 tonnes of elastomers.^4,5  The world production capacity was about 305 000 tonnes, due to the addition of large manufacturing capacities in China.

Table 1.3 gives an overview of the most important commercially available fluoropolymers. Details of polymerization, processing and product properties are provided in some excellent review articles.^3,80–83 

Alternating copolymers of HFIB (hexafluoroisobutylene) and VDF have been prepared, offering outstanding creep resistance and excellent mechanical and chemical properties; however, these copolymers have not attracted much commercial interest.⁴⁹  Owing to the larger scale availability of R1234yf (CF₃–CFCH₂), a new class of fluoropolymers might be developed.^87–89 

Table 1.4 provides an overview of fluoropolymers with functional end-groups, often present in side chains. The most important fluoropolymer classes in this category are as follows:

• (Per)fluoro ionomers, bearing SO₃⁻ and/or COO⁻ groups; these polymers were originally developed as membranes for applications in the NaCl/HCl electrolysis. Nowadays, such polymers have attracted much interest in applications for energy conversion/storage, e.g. in fuel cells, water electrolyzers and redox flow cells. Important ionomers are shown in Figure 1.8.^84,85 

• TFE/CTFE-vinyl ether copolymers (FEVE); these copolymers contain hydrocarbon vinyl ether units, and some of them have OH and/or COOH groups. From CTFE, Asahi Glass has pioneered this class of fluoropolymers under the Lumiflon brand name.⁸⁶  Those based on TFE are marketed by Daikin Industries under the trade name Zeffle. The main application is in coatings. In Asia, there is a capacity of more than 5000 tonnes available.

  FEVE copolymers were the first solvent-soluble fluoropolymers for weather-resistant coating applications that can be cured at room temperature. These polymers are completely amorphous and combine various characteristics, such as solubility in organic solvents, film-forming ability and transparency of the resulting films. FEVE-polymers can also be applied as aqueous dispersions or powders.

  The molecular design of FEVE polymers offers the possibility of tailor-made resins for various coating applications. Compared with TFE, CTFE improves the solubility of the polymer. The fluorinated olefins and the hydrocarbon vinyl ethers are highly alternating, thus protecting the rather unstable vinyl ethers present in the backbone. Alkyl-, cycloalkyl- or alkylene-substituted vinyl ethers provide the required solubility and glass transition temperature. Hydroxy-containing vinyl ethers permit curing of the polymer with isocyanates. Compatibility with pigments can be achieved by partially converting the hydroxy groups into carboxylates by using acid anhydrides. Copolymers with a high COOH content are soluble in aqueous media after neutralization with organic amines.

Owing to the outstanding, unique product properties, fluoropolymers are indispensable materials and consequently their socioeconomic value is extremely high. Fluoropolymers are now serving highly demanding applications in a diverse range of industries, which no other class of polymers can achieve (Table 1.5).

For decades, the ammonium salts of perfluorooctanoic acid (PFOA) and perfluorooctylsulfonic acid (PFOS) have been used in aqueous emulsion polymerization to produce fluoropolymers (cf. Section 1.4.1). With the recognition of the environmental and health concerns associated with long-chain functional perfluoroalkyls, fluoropolymer manufacturers began the development of alternative emulsifiers and different polymerization techniques using less or no fluorinated emulsifier. The challenge was to insure that fluoropolymers could still be safely manufactured while minimizing emulsifier emissions and use. Ultimately, the goal became the replacement of PFOA and related materials with emulsifiers that had an improved hazard profile and still met polymerization process needs. These objectives became part of an EPA-initiated PFOA Stewardship Program that called for zero emissions during production and zero product content of PFOA and related substances by 2015. The initiative also suggested that manufacturers replace these substances in their manufacturing processes with compounds having an improved toxicity and ecotoxicity profile or, even better, develop polymerization techniques that require less or no fluorinated emulsifier.^61–63 

Meanwhile, most fluoropolymer manufacturers have been pursuing PFOA replacements. According to the literature, different PFOA replacements have been reported by the companies mentioned in Figure 1.9.⁹⁰ 

Since the beginning of the 1990s, Dyneon (previously owned by Hoechst and now part of 3M) was already pioneering potential options to recover/remove and recycle fluorinated emulsifiers, including PFOA, from off-gases, waste water streams, aqueous dispersions and products. By the mid-1990s, a large-scale facility to recover PFOA from off-gas streams was implemented by using scrubbing systems. During the following years, large-scale units to recover PFOA from aqueous waste water streams and aqueous fluoropolymer dispersions, using anion-exchange methods, were implemented; a recycling facility to reuse the recovered PFOA was installed in parallel.

With the so-called “containment strategy” (Figure 1.10), new state-of-the-art technology, with a proven record of robustness and large-scale applicability, was established. These recovery/recycling techniques were used for PFOA and today are used for a variety of replacement fluoroemulsifiers and are installed in a number of fluoropolymer manufacturing facilities around the world. The overall recapture rate for fluoroemulsifiers is approximately 98%.

Increased environmental awareness requires the complete life cycle of products to be considered, and consequently recycling of polymers comes into the focus. There are numerous different sources and different types of fluoropolymers for recycling purposes. Consequently, one has to consider these issues from various angles.

The amounts of scrap, wet waste materials and off-specification materials of unfilled PTFE from manufacturers are usually in the lower percentage range. In contrast, the amounts of waste resins from processors and compounders in making semi-finished parts or end-use articles are usually in the range of 10–30%,⁹¹  and in some areas even above 50%. The large amounts of waste are due to the very specific processing technologies for PTFE (molding, sintering, machining and cutting). For unfilled PTFE resin, three established recycling paths exist today:

1. Sintered, unfilled PTFE resin is cleaned from all contaminants and milled into certain particle size classes, which can be reused e.g. in ram extrusion applications. This so-called repro-PTFE can also be mixed with virginal PTFE to a certain content; such repro-PTFE materials have specific designations and are typically used for less demanding end-use applications.

2. Clean, unfilled PTFE can be thermally degraded into low molecular weight PTFE. Such processes are used on a commercial scale. The thermal degradation of high molecular weight waste PTFE occurs at about 500 °C in ovens, kneaders or preferably extruders.^92,93  The low molecular weight PTFE materials obtained are further milled into very small particle sizes of a few microns; such materials are often called PTFE micropowders.

3. Alternatively, clean and unfilled PTFE can be degraded by high-energy radiation such as with X-ray, gamma-ray or electron beam techniques.^3,94  The degradation of high molecular weight PTFE by electron beam irradiation is commercially widespread, and in practice continuous processes are used to improve the economics. After irradiation, the material is milled to the desired particles size.

The thermal/radiation degraded PTFE micropowders are mostly used as additives in plastics, inks, oils, lubricants and coatings to introduce fluoropolymer-like properties such as reduced wear rates and friction.

In contrast to clean, unfilled PTFE, where some recycling opportunities are well established and the recycling rates reach a significant level, there are no large-scale recycling technologies for PTFE compounds. This is primarily due to the presence of a large variety of different fillers (e.g. glass fiber, graphite, carbon, metal compounds, ceramics, organic fillers, pigments) and to the variable amounts of fillers in the PTFE compounds. For a significant proportion of waste streams (at least 10–30%), landfilling is not a sustainable option, especially since it is becoming increasingly regulated in Europe. Therefore, PTFE compound waste tends to be recycled into TFE/HFP monomers (see below).

Clean, unfilled, unpigmented and uncured waste from the manufacturing and processing of perfluorinated/partially fluorinated thermoplastics or elastomers is generated in the low percentage range. Nearly all of these materials are recycled back into the corresponding processes; the end-use properties are almost unaffected.

In some cases, used perfluorinated fluoropolymers (e.g. PTFE, PFA) are recycled by special cleaning processes and end up in the repro-PTFE or PTFE micropowder market. Perfluorinated thermoplastics (e.g. PFA) are reused in applications where the quality requirements (e.g. lot traceability) are much lower. Overall, the major share of used fluoropolymers ends up in landfills, in incineration plants or in blast furnaces. Communal waste incinerators can tolerate only very limited amounts of fluoropolymers owing to the high corrosiveness of the hydrofluoric acid formed during the process.

The manufacture of TFE/HFP monomers consumes large amounts of energy (>10 000 kWh per tonne of TFE). In the past, many attempts were made to recover perfluorinated monomers from waste materials. All of the approaches were based on the finding that perfluorinated polymers can be pyrolyzed under high temperature conditions into TFE/HFP^95–101  (Scheme 1.9).

The pyrolysis of polymers into monomers is seldom practiced in industry; there are only a few other polymer classes [e.g. polystyrene, poly(methyl methacrylate)] that can be converted back into monomers by heating.

This process is a convenient route to prepare small quantities of TFE on a laboratory scale, but to our knowledge, no industrial facility has been installed. However, this may soon change, as new energy-efficient and robust processes for the high-temperature conversion (HTC) of perfluoropolymers have been developed, which are also suitable for large-scale HTC units.^99,100,102  The new HTC process designs are capable of pyrolyzing all kinds of perfluoropolymers back into TFE/HFP with yields of more than 80%.¹⁰⁰  The process is particularly well suited to convert filled PTFE compounds into monomers. So far, no viable technology for recycling PTFE compounds exists. The HTC pyrolysis can recycle all kinds of filled fluoropolymer compounds (e.g. containing carbon, graphite, glass fiber and metal compounds such as sulfides and oxides) and unfilled materials from manufacturing, processing and end-of-life materials.

The HTC process is complementary to existing recycling methods for unfilled PTFE (e.g. repro-PTFE, micropowder). These processes may finally close the loop for perfluorinated polymers by avoiding landfilling and combustion (Figure 1.11). In 2015, 3M/Dyneon implemented an industrial-scale HTC unit in Germany to demonstrate its feasibility and robustness. The environmental benefit in pursuing this recycling path is showing significant reductions in raw material usage, waste streams and the overall CO₂ balance (Figure 1.12).

Partially fluorinated polymers (e.g. THV, ETFE, PVDF) and also perfluorinated polymers in combination with larger amounts of hydrocarbon-containing polymers (e.g. blends, laminates) can be recycled by HTC processes into monomers in only low/moderate yields;¹⁰³  usually, these EOL polymers are landfilled or incinerated in small portions owing to the corrosive nature of HF.

In Europe, and particularly in Germany, CaF₂ is considered as one of the most important of 14 strategic raw materials.¹⁰⁴  This classification has spurred investigations to optimize the incineration/combustion of fluoropolymer-containing materials to recover HF and/or CaF₂ from the off-gases. Such processes have been discussed in the literature;¹⁰⁵  the challenge is to establish robust and energy-efficient processes and to apply the know-how from thermal oxidizers of fluorocarbon gases in order to recover HF and/or CaF₂ in an efficient manner for reuse. The implementation of such processes would finally close the remaining loop (Figure 1.13).

The fluorochemical and fluoropolymer industries have already mastered successfully many of the challenges relating to environmental issues (e.g. banning of chlorofluorocarbons, PFOA/PFOS phase-out). Life cycle assessments, including eco-balances (e.g. according to ISO 14040/14025), have also been initiated to demonstrate the value of specific fluorochemicals/fluoropolymers. Although a comprehensive overview of the impact of fluorochemical/polymer manufacturing can be difficult to assemble, as the worldwide existing data from the various databases (e.g. ProBas,¹⁰⁶  EcoInvent¹⁰⁷ ) are not always consistent, these data will definitely be consolidated, as the understanding of the environmental impact of fluoropolymer manufacture remains a priority for national and international stakeholders.

Based on all these achievements, it is only a matter of time before the industry will establish closed manufacturing loops throughout the whole value chain (with close to zero emissions). A closed fluorine cycle (Figure 1.13) will be one such target, with optimized raw material streams and energy balances and reduced environmental burdens.

The realization of this vision will also ensure further growth opportunities and will stimulate the development of new materials with unique properties.

ADONA

Ammonium 4,8-dioxa-3H-perfluorononanoate

APFDO

Ammonium perfluoro-3,6-dioxaoctanoate

APFO

Ammonium perfluorooctanoate

CTFE

Chlorotrifluoroethene

FEVE

Fluorinated ethylene vinyl ether

HFP

Hexafluoropropene

HFPO

Hexafluoropropylene oxide

MV4S

1,1,2,2,3,3,4,4-Octafluoro-4-[(trifluoroethenyl)oxy]butane-1-sulfonyl fluoride

PBVE

Perfluoro(4-vinyloxy-1-butene)

PFOA

Perfluorooctanoic acid

PFOS

Perfluorooctylsulfonic acid

PFPE

Perfluoropolyether

PFSF, PSEPVE

Perfluoro(4-methyl-3,6-dioxooct-7-ene)sulfonyl fluoride

PMVE

Perfluoromethyl vinyl ether

PPD

2,2-Bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole

PPVE

Perfluoropropyl vinyl ether

R22

Chlorodifluoromethane

R113

1,1,2-Trichlorotrifluoroethane

R142b

1-Chloro-1,1-difluoroethane

R152b

1,1-Difluoroethane

TFE

Tetrafluoroethene

TTD

2,2-Bis(trifluoromethyl)-4-fluoro-5-trifluoromethoxy-1,3-dioxole

VDF, VF2

Vinylidene fluoride

VF

Vinyl fluoride

ECTFE

Ethene–chlorotrifluoroethene copolymer

ETFE/ET

Ethene–tetrafluoroethene copolymer

FEP

Fluorinated ethene–propene copolymer

FKM, FFKM

Fluoroelastomers, perfluoroelastomers

MFA

Methylfluoroalkoxy copolymer

PCTFE

Polychlorotrifluoroethylene

PFA

Perfluoroalkoxy/propylfluoroalkoxy copolymer

PTFE

Polytetrafluoroethylene

PVDF

Poly(vinylidene fluoride)

PVF

Poly(vinyl fluoride)

TFEP

Tetrafluoroethene–propene copolymer

THV

Tetrafluoroethene–hexafluoropropene–vinylidene fluoride terpolymer