Perfluorinated Chemicals (PFCs): Contaminants of Concern

Chapter 1

What Fluoropolymers Are

1.1 Introduction

Fluorine-based polymers are referred to as fluoropolymers. These are man-made products that impart certain attributes and properties to coatings used in industrial, household, and construction products, as well as in firefighting foam applications. The qualities of fluoropolymer resins and oligomeric additives in coatings make them useful in applications requiring a high resistance to solvents, acids and bases, and most importantly, an ability to greatly reduce friction.

The use of surfactant additives reduces surface energy while increasing chemical, UV, moisture, grease and dirt resistance, and surface lubricity. In addition to more common fluorinated olefin-based polymers, specialty fluoroacrylates, fluorosilicone acrylates, fluorourethanes, and perfluoropolyethers/perfluoropolyoxetanes exhibit properties beneficial to various coatings applications. Coatings containing fluorochemicals find broad applications in electronics such as photomask covers, anti-reflection coatings; in construction as protective coatings for exterior substrates; as cool-roof coatings and optics such as antifouling coatings for eyeglass lenses and liquid crystal displays. Other coatings that often contain fluoropolymers include floor polishes, wood stains, and automotive clear coats, as well as ink jet inks, pigment dispersions, and adhesives.

At the heart of these products is the chemical fluorine. Unique characteristics of the fluorine atom impart certain properties to polymers that contain it. Fluorine is a fairly small atom that has very low polarizability and high electronegativity. Because there is a high degree of overlap between the outer orbitals of fluorine and the corresponding orbitals of second period elements, bonds formed between carbon and fluorine are very strong. The higher bond energy of the C-F bond compared to the C-H bond leads to greater thermal stability.

A perfluorinated chemical (PFC) is an organofluorine compound containing only carbon-fluorine bonds (no C-H bonds) and C-C bonds but also other heteroatoms. PFCs have properties that represent a blend of fluorocarbons (containing only C-F and C-C bonds) and the parent functionalized organic species. For example, perfluorooctanoic acid functions as a carboxylic acid but with strongly altered surfactant and hydrophobic characteristics. Perfluoropolymers, which contain only C-F bonds, have excellent chemical and weather resistance. The small dipole moment of these compounds contributes to their oil and water-repellency, as well as low surface tension, low refractive index, low friction coefficient, and reduced adhesion to surfaces. Even partially fluorinated polymers exhibit a strong electron-attracting ability, resulting in a high dielectric constant and optical activity. In small molecules, this attribute leads to enhanced acidity, lipophilicity, and the ability to block metabolic pathways, making fluorine-substituted compounds well-suited for pharmaceutical applications.

Other characteristics of fluoropolymers, which are determined by the strength of the C-F bond and the low polarizability and high electronegativity of fluorine, include soil resistance, insulating properties, and the ability to act as a gas barrier.

Commercial fluoropolymers are generally classified according to morphology (crystalline, semi-crystalline, and amorphous categories) and perfluorinated and partially fluorinated. See Figure 1.1.

Figure 1.1 The major types of today’s commercial fluoropolymers.

1.2 Evolution of Fluoropolymers and the Markets

The following is a timeline of the evolution of fluoropolymers and the market applications.

Since the 1990s, trademarked ranges of fluoropolymers have expanded in various forms to meet the needs of emerging technologies in construction, electronics, and energy sectors. For the most part, these have consisted of modified formulations or newly processed forms of existing fluoropolymer blends. Effort continues to be invested in developing new types and blends of fluoropolymers, particularly in the energy and electronics sectors. Table 1.1 lists some of the most important commercial fluoropolymers that are part of a worldwide annual market that is upwards of 200,000 tons.

Table 1.1 Important Commercial Fluoropolymers.

PTFE (Polytetrafluoroethylene), historically and through present times, is the most widely produced of all the fluoropolymers with demand steadily increasing. Current manufacturers producing a range of fluoropolymer resins and products include DuPont, Asahi Glass, Solvay Solexis, 3M, Dyneon, Honeywell, and Daikin.

1.3 PFAS Compounds

1.3.1 General Description

Any organic or inorganic substance that contains at least one fluorine atom is referred to as fluorinated substances as a general term. However, their chemical, physical, and biological properties could differ significantly. A subset of fluorinated substances are the highly fluorinated aliphatic substances that contain one or more carbon atoms on which the fluorine atoms have replaced the hydrogen atoms that would normally be found in nonfluorinated substances. These subset substances contain the perfluoroalkyl moiety with the form of CnF2n+1 – and are referred to as perfluoroalkyl or polyfluoroalkyl substances having the acronym PFAS.

PFASs comprise a large group of chemicals that are both chemically and thermally stable and are both lipophobic (have no affinity for oils) and hydrophobic (have no affinity for water), making them very useful in surfactants and as polymers. However, PFASs are composed of two main parts; one that is formed out of a hydrophobic alkyl chain and a hydrophilic (strong affinity to water) functional group. A total of 146 perfluorochemicals and 469 fluorochemicals are potentially able to degrade to PFCAs. The most investigated classes of PFASs are the perfluorocarboxylateacids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs). The most studied PFCA compound is perfluorooctanoic acid (PFOA) and for PFSA it is perfluorooctane sulfonate (PFOS).

Because of the use of PFASs in many industries and the difficulty of natural processes to degrade the compounds, environmental contamination is a global concern. PFASs have been found to be able to bioaccumulate (become concentrated inside the body) and biomagnify (the concentration increases at each trophic level through the food web) in arctic, temperate, and subtropical systems.

Due to the persistence of these chemicals in the environment and multiple studies that showed adverse health effects at very low concentrations in both the environment and humans, many manufacturing companies voluntarily removed products from the market in the early 2000s. The USEPA entered into an agreement with fluorochemical manufacturers to comply with a PFOA/PFOS Stewardship program and cease production of all fluorinated compounds with an eight carbon chain (C8) base before 2015. However, this measure only prevents the problems from spreading but does nothing to address historical or legacy pollution. PFOS has a reported environmental half-life between 4 and 41 years, and hence contaminated groundwater poses a potential risk for some communities. Sites currently contaminated with PFAS will remain contaminated well into the future. Figure 1.2 provides examples of PFAS classes of compounds. Figure 1.3 shows the chemical structures of major PFCs that are discussed in subsequent chapters.

Figure 1.2 Examples of PFAS classes of compounds.

Figure 1.3 Shows chemical structures of major PFCs.

1.3.2 How They Are Made

Electrochemical Fluorination

The technology that was extensively used to create many of these unique man-made compounds is called electrochemical fluorination. Electrochemical fluorination, or ECF, is a technology used for preparing fluoro organic compounds. There are at least three distinct methods of ECF, namely the Simons process, selective electrochemical fluorination, and the Philips process. These processes have evolved over the past six decades. For perfluorinated compounds, the Simons process has historically through present times played a key role in the synthesis of these materials. By this method, a wide range of organic substrates can be perfluorinated in a single step. For nearly 60 years, electrochemical fluorination has been used as the preferred method in the production of perfluorinated compounds bearing different functional groups.

The Simons process for the electrochemical perfluorination (ECPF) of organic compounds in anhydrous hydrofluoric acid was developed during World War II. More precisely, in 1944, the electrochemical fluorination (ECF) process was developed by Simons and coworkers. The 3M Company has used this route of production since 1956 to manufacture perfluoroalkylsulfonates. In the ECF process, the organic compound is dissolved or dispersed in anhydrous hydrogen fluoride. A direct electric current is passed through the hydrogen fluoride, causing all the hydrogen atoms on the organic compound to be replaced by fluorine. The overall reaction is shown below. Perfluoro-1-octane sulfonylfluoride (POSF) is the starting product for the range of products based on perfluorooctylsulfonates (or C8-perfluoroalkylsulfonates). This compound is made to react with methyl or ethylamine, and subsequently with ethylene carbonate to form N -methyl (N -MeFOSE) or N -ethylper- fluorooctanesulfonamidoethanol (N-EtFOSE). These two compounds are the primary building blocks for the perfluorochemistry of 3M.

The ECF process is an impure process, meaning that the reaction leads to several by-products. As noted, in 2000 the 3M Company decided to phase out the perfluorooctyl chemistry. However, for some applications the production of PFAS is being continued, but otherwise it is believed that 3M will or has replaced the perfluorooctyl chemistry with the butyl