Fluoropolymers
There are wide varieties and ranges of fluorocarbon/fluoropolymers that are commercially important. 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.
PointPTFE C2 F2 Excellent ? 330°C Rigid
TFM as PTFE+0.1% PPVE Excellent ? 330°C Rigid
PFA as PTFE+10.30% PAV Excellent 280°C Melt Processable
FEP as PTFE+HFP Excellent 230°C Melt Processable
Purity
All fluoropolymers can be considered to have very high intrinsic purity.
This factor, coupled with other properties already mentioned has made these materials quite essential for use in certain industries, particularly in the semi-conductor industries and to a lesser extent combinatorial chemistry.
Consideration of the purity issue raises a number of complex questions; however the basic needs for a high purity material are readily met by the fluoropolymers in that free radical initiation is used in the polymerization process and no additives in the form of plasticisers, extenders, stabilizers or the like are required in the commercial material. The raw polymers of PTFE, TFM, PFA and FEP are, therefore, likely to have pretty much the same purity levels in
The end of the line commercial product, however, will have purity/contamination levels very much dependant on the processing method and wide variations are not only possible but will inevitable be encountered. At this point we should identify two types of contaminant; that due to the presence of more or less trace amounts of metals and that due to the presence of organic material (here we are not considering reworked material or non-premium grades of materials where other contaminants may well be present). Where organic matter is present in the raw polymer the processing temperatures for these materials is so high, especially for PTFE and TFM, that organic material is generally reduced to carbon; this may appear as a black mark but it is not usually of significance in purity issues.
We are therefore mainly concerned here with metal contamination either as pure metal or as a metal containing compound and it is now of interest to examine how contamination can occur, how it can be prevented, and how it can be reduced. PTFE and TFM are processed by “cold” molding or extrusion. In the cold process the molding is formed by application of pressure to the material in simple mold. This is followed by sintering at 380?C and if necessary machining to the final form.
Assuming that the raw polymer is essentially free from contamination then pick-up will clearly occur during the molding stage or final stages or both.
Contamination during the molding stage will arise mainly from the press tools, which will usually be mild steel, stainless steel, chrome plated steel, brass or aluminum. The process is carried out in the cold and since the flow properties of PTFE and TFM molding powder is poor there is minimal movement of the powder in the mold. Contamination is therefore likely to be limited to the surface of the molding. This can easily be reduced or eliminated by using press tools lined with PTFE or by removing the outer surface of the part by machining.
To minimize contamination during machining diamond or carbide tools may be used without the use of cutting or cooling fluids.
The processing of PTFE and TFM using “cold” molding lends itself readily to the production of components with very low levels of contamination.
PTFE materials are also processed using extrusion to manufacture tubing and rod. Here contamination is likely to be considerably higher than the cold molding procedures just described since the material is forced under pressure through heated dies and transfer of material to the polymer is inevitable.
FEP and PFA are melt processable and are injection molded, transfer molded and extruded. These processes are carried out at elevated temperatures and all will result in the transfer of metal form the process equipment and molding tool to the polymer. In order to minimize contamination of the polymer (and prevent corrosion of the processing equipment) processing equipment for these materials requires very corrosion resistant metals of construction such as Hastalloy for the extrusion barrels and molds.
It is now clear that material with the least contamination will be “cold” molded PTFE and TFM while other methods of processing and materials are likely to introduce some contamination into the molded material. In literature, claims are made that PFA, especially in its blow-molded form, is the purest material available – this appears to be in contradiction to our comments above.
Literature invariably refers to purify in terms of “EXTRACTABLES” not inherent purity since it is clear that materials processed at high shear and high temperature will inevitably be contaminated.
In assessing purity the criteria used is in terms of “extractability” of metals, the fluoropolymer is immersed in an extraction medium, typically dilute nitric acid, for a given period and the extractables determined by the chemical analysis. On this basis PFA frequently appears as the purest material – the explanation for this however, resides in the physical structure of the materials rather than their inherent purity.
PFA is melt processable to give a homogenous solid with a very smooth surface, penetration into the substructure by outside solvents and reagents is therefore difficult and material can be extracted only from the surface.
PTFE is formed by the compression of the small particles followed by sintering; microscopic voids may be present in the material which means some penetration into the substructure may be possible and material may be extracted from the body of the material as well as the surface – this inevitable leads to a higher level of extractables and an APPARENT lower purity.
TFM has dramatically improved coalescence behavior during sintering leading to a significant reduction in void content. This coupled with high purity cold molding leads to a material which under optimum processing conditions is likely to have the highest purity rating and indeed for certain premium grades this has been clearly demonstrated.
