Chemical Resistance - Fluoropolymers
By chemical resistance we mean that PTFE/PFA fluorocarbon resins can be in continuous contact with another substance with no detectable chemical reaction taking place.
In general, PTFE/PFA fluorocarbon resins are chemically inert. Nevertheless, this statement, like all generalisations, must be qualified if it is to be perfectly accurate. The qualification will not lead to confusion, however, if one keeps in mind the basic facts about the behaviour of PTFA/PFA resins.
The usual descriptive summary of various test data can be misleading, for it may lump together fundamentally different types of “chemical” behaviour. If the description is to be clear, it must distinguish between strictly chemical reactions and physical actions such as absorption. The description must enable the user to take into account the inter-relationships of physical and chemical properties which may affect a particular application.
For example, PTFE/PFA resins will be unaffected by immersion in aqua regia. Yet if the temperature and resultant pressure of this reagent become high, absorption of the components of the reagent into the resin will also increase. Subsequent fluctuations, such as sudden pressure loss, can then be physically damaging due to expansion of the vapours absorbed in the resin.
Within normal use temperatures, PTFE/PFA resins are attacked by so few chemicals that it is more practical to describe the exceptions rather than tabulate the chemicals with which they are compatible. These reactants are among the most violent oxidiser and reducing agents known. Elemental sodium in intimate contact with fluorocarbons removes fluorine from the polymer molecule. This reaction is widely used in anhydrous solutions to etch the surfaces of PTFE/PFA so that the resins can be adhesive bonded. The other alkali metals (potassium, lithium, etc.) react similarly.
Intimate blends of finely divided metal powders (e.g., aluminium or magnesium) with powdered fluorocarbon resins can react violently when ignited, but the ignition temperatures are far above the published recommended maximum service temperature for PTFE/PFA resins.
The extremely potent oxidisers, fluorine (F2) and related compounds (e.g., chlorine trifluoride, ClF3 ), can be handled by PTFE/PFA only with great care and recognition of potential hazards. Fluorine is absorbed into the resins, and with such intimate contact the mixture becomes sensitive to a source of ignition such as impact.
In some instances at or near the maximum service temperature of 260°C for Teflon PTFE and PFA, and 200°C for Teflon FEP, a few chemicals at high concentrations have been reported reactive. Attack similar to the sodium etch has been produced at such high temperatures by 80% NaOH or KOH, metal hydrides such as boranes (e.g.,B2H6), aluminium chloride, ammonia (NH3), and certain amines (R-NH2) and imines (R=NH). Also, slow oxidative attack has been observed by 70% nitric acid under pressure at 250°C. Special testing is required when such extremes of reducing or oxidising conditions are approached.
Hence, with exceptions as noted, PTFE/PFA resins exhibit a very broad range of chemical and thermal serviceability. But the purchaser or specifier of components of PTFE/PFA also needs to know and understand its limitations in regard to the more usual chemical environments. Unlike the limitations of metals, these normally are not chemical but physical in nature. The effects of temperature, pressure, and absorptivity of the chemicals in PTFE/PFA, and their interaction, are what in time usually limits the conditions under which PTFE/PFA will perform satisfactorily. Since this is different from almost any other material of construction, it requires careful consideration.
[We wish to acknowledge the help of DuPont in compiling the above information.]