Chemical Resistance of Fluoropolymers

A = Excellent chemical resistance B = Limited chemical resistance C = Not chemical resistant 

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The best way for the reader to learn is to study actual examples of parts made by the blow molding of fluoropolymers. A majority of such parts have a multilayer construction to combine the properties of other plastics in a composite structure. The most common technique for producing multilayer sheet/film is coextrusion. The chemical resistance of fluoropolymers makes them attractive materials for inner layers of containers that come in contact with aggressive chemicals that can swell or degrade thermoplastics. 

Table A.34 Chemical resistance of fluoropolymers S = swelling temperature is provided in [32], temperature is not provided in [1039]...

The heat resistance and chemical resistance of the fluoropolymers is mainly dependent on the extent of fluorination and stability of the crosslinks. For example, most fluorocarbons have fluorine contents of 50-70%, more chemically resistant types 65-69%. For comparison, fluorosilicones contain about 36% fluorine. 

Polytetrafluoroethylene (PTFE Teflon) was discovered accidently by PlunkettCZ nd commercialized by DuPont in the 1940 s. This polymer has a solubility parameter of about 6H and a high melting point of 327°C and is not readily moldable. Poly-chlorotrifluoroethylene (CTFE, Kel-F), the copolymer of tetrafluoroethylene and hexafluoropropylene (FEP), polyvinylidene fluoride (PVDF, Kynar), the copolymer of tetrafluoroethylene and ethylene (ETFE), the copolymer of vinylidene fluoride and hexafluoroisobutylene (CM-1), perfluoroalkoxyethylene (PFA) and polyvinyl fluoride (PVF, Tedlar) are all more readily processed than PTFE. However, the lubricity and chemical resistance of these fluoropolymers is less than that of PTFE. 

This chapter has been divided based on the fluorine content of fluoropolymers, that is, perfluorinated and partially fluorinated. In general, resistance of polymers to chemicals of all types increases with an increase in their fluorine content. Therefore, the chemical resistance of ETFE, ECTFE, and PVDF is generally inferior to that of perfluorinated polymers such as PFA and FEP. 

In addition to manufacturer-supplied data, information is provided by a number of independent sources. Appendix V contains comprehensive data showing the chemical resistance of various fluoropolymers to numerous reagents under different conditions. These data have been compiled by the Plastics Design Library (PDL) from manufacturers literature. A rating system used by the PDL is applied to the data for efficiently determining potential compatibility. 

Selecting Fluoropolymers for Corrosion Control Table 4.5. Chemical Resistance of Filled PTFE Compounds ]... 

Chemical Resistance of Halar Fluoropolymers, Supp. Tech. Rep. AHH, Ausimont. 

Chemical Resistance of Halar Fluoropolymer, supplier technical report (AHH), Ausimont. 

The chemical resistance of PEEK is shown in Table 6.3. PEEK exhibits a remarkable chemical resistance, comparative to fluoropolymers. PEEK is approved by the FDA. PEEK undergoes crosslinking by irradiation in vacuum under stress. The tensile properties of PEEK sheets after UV radiation show a tendency to embrittlement. This is caused not only by crosslinking but also by the orientation of molecular chains resulting from the temperature rise of the specimens. Furthermore, the tensile stress applied during exposure accelerates molecular scission and disturbs the crosslinking. ... 

A fundamental property of fluoropolymers is their resistance to organic and inorganic chemicals (Fig. 12.1). Increased content of fluorine enhances the chemical resistance of the polymer. The overwhelming majority of the applications of fluoropolymers take advantage of their inertness to chemicals. Chemical properties of fluoropolymers are not affected by fabrication conditions. Another aspect of the interaction of these plastics with chemicals is permeation. Even though a reagent may not react with a fluoropolymer, it may be able to permeate through the polymer structure. The extent and rate of permeation is dependent upon the structure and properties of the plastic article as well as the type and concentration of permeant. Temperature and pressure usually influence the permeation process. This chapter reviews chemical compatibility of fluoropolymers and their permeation behavior towards different chemicals. 

It is necessary to modify the surface of fluoropolymers to obtain stronger adhesive bonds. Modification or surface treatment alters the structure of the polymer at the surface enabling formation of true adhesive bonds. Mechanical abrasion imparts little improvement and chemical etching is required. Chemical resistance of perhalogenated polymers such as PTFE, PFA, FEP, and PCTFE mandates the use of highly potent agents. 

A 50 50 mol/mol copolymer of hexafluoroisobutylene (CH2 = C(CF3)2) and vinylidene fluoride was made available by Allied Chemical in the mid-1970s as CM-1 Fluoropolymer. The polymer has the same crystalline melting point as PTFE (327°C) but a mueh lower density (1.88g/cm ). It has excellent chemical resistance, electrical insulation properties and non-stiek characteristics and, unlike PTFE, may be injeetion moulded (at 380°C). It is less tough than PTFE. 

Non-Metallic Materials Numerous engineering thermoplastics have been commercialised including materials such as polyetherether ketone (PEEK) and polyether sulphate (PES) with much improved thermal/chemical resistance. The usage of FRP equipment has increased, and fluoropolymer lining technology/applications have come of age. Of particular interest is the development of stoved, fluoropolymer coating systems for process industry equipment. 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

One of the first fictional fluoropolymers was poly-1,2,2-trifluorostyrene. On one hand, it has much better oxidation and chemical resistance in comparison with common hydrocarbon polymers and, on the other hand, a wide range of functional groups can be attached to the aromatic ring. A sulfonated polymer was successfully used as a membrane for fuel cells by General Electric Co.3... 

Nonstickiness, low friction, low wettability, and high thermal- and chemical-resistance are the major properties of PTFE, which was accidentally discovered in the DuPont laboratories in 1938, and these properties are more or less typical of other fluoropolymers that have been developed since. 

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by... 

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