Fluoropolymers

They are found in virtually all industries and exhibit a diverse range of uses ranging from refrigerants through high performance greases to a mass of solid state applications.

From our point of view the fluoropolymers of interest consist entirely, or almost entirely, of carbon and fluorine. These polymers are characterized by along chain molecular structure and have very high molecular weights.

Commercially the most important members of this group are PTFE (PolyTetraFluoroEthylene) in its base and modified forms; FEP and PFA.

Basic PTFE is a linear polymer of tetrafluoroethylene CF2=CF2.

Two co-monomers are used to modify PTFE; these are hexafluoropropylene (HFP) and perfluoropropylene vinyl ether (PPVE). Sufficient amounts of the co-monomers are incorporated in the basic PTFE chain to give modified end products which are thermoplastics; these are fluorinated ethylene propylene = FEP; and Per-fluoroalkoxy=PFA.

A version of PTFE using smaller quantities of co-monomer is also commercially important; this material is known as PTFE-TFM™ has properties which may be conveniently regarded as midway between basic PTFE and its melt processable form. PFA has typically 3% – 15% PPVE while TFM™ has less than 0.1% PPVE.

Fluoropolymers have a quite unique range of useful properties including:

• Virtually total chemical resistance
• Total Insolubility
• Extreme thermal durability
• Exceptional electrical properties
• Low coefficients of friction
• This information document will attempt to identify, clarify and explain some of the important traits of these materials.

Chemical Structure related to Chemical Resistance Insolubility

The structure of the long chain fluoropolymer is essentially a central carbon skeleton surrounded by a shell of fluorine atoms. The strength of the C-F bond at 460kJ/mol is one of the strongest bonds in organic chemistry and this coupled with the shielding of the carbon atoms by fluorine atoms accounts for the almost total chemical inertness of these materials, especially polytetrafluoroethylene, and their almost total insolubility.

The chemical reactivity of PTFE is virtually total. Molten or dissolved alkali metals such as sodium in liquid ammonia will abstract fluorine from the molecule while at elevated temperature attack by fluorine, some fluorine containing compounds, alkali earth and alkali metal oxides and carbonates has been noted.

The polyfluorocarbons, especially PTFE, normally are regarded as completely insoluble and as mentioned above for very much the same reason as they are inert. However, dissolution in materials such as cyclic polyfluorocarbon oligomers at 300°C and atmospheric pressure has been recorded and it is also known that other perfluorocarbons, perfluorocarbon ethers, perhalocarbons, sulphur hexafluoride and carbon dioxide will dissolve PTFE under the right conditions of temperature and pressure.

Chemical Structure related Thermal Properties

All the fluoropolymers have exceptional thermal stability; the behavior of FEP and PFA show a behavior somewhat similar to more general melt processable polymers with well defined melting points accompanied by a readily discernible phase change typical of these materials.

This is not the case with PTFE and TFM™, the thermal behavior of these materials is best described as complex. The behavior of PTFE has been widely investigated and the prominent points to be noted in heating from 0°C are:

Phase transitions accompanied by significant volume changes at 19°C and 30°C.
Large and variable coefficients of thermal expansion.
No easily discernible melting point. The notional melting point of PTFE determined, for example, by depolarization of light indicates a melting point of 325°C – 340°C.
The liquid state is characterized by a very high melt viscosity of about 1010 Pa.sec which means that PTFE and TFM™ are rigid and retain their shapes in a gel like condition.
There is a large, and reversible, increase in volume.
The thermal stability of fluoropolymers is generally credited to the strength of the C-F bond; many of the other characteristics mentioned above, especially for PTFE, are also attributable to the molecular structure of the material. The phase changes at 19°C – 30°C are explained by the stretching or straightening of the helical structure required to accommodate the effect of increase in temperature on the steric needs generated by the large fluorine atoms in the chain structure.

Below 19°C the CF2 groups are equally spaced on a helical chain with a chain repeat distance of 16.8A between 19°C and 30°C, the repeat distance increases by a twisting mechanism to 19.5A. Above 30°C increasing disorder is noted with chain rotation and displacement increasing by variable amounts as the temperature increases.

The high melting points of PTFE and TFM™ are due to the rigidity of the fluorocarbon chain and restriction caused by the fluorine atoms. The retention of significant parallel chain order in the melt explains the very high melt viscosity of these materials.