Fluoropolymers are polymeric materials containing fluorine atoms bonded to carbon atoms in their structure. They have high resistance to chemical agents such as organic solvents, acids and inorganic bases. The different starting monomers contribute to the diversification of fluoropolymers and their respective properties.
When all hydrogen atoms are substituted with fluorine we have perfluorinated polymers. Although structurally interesting, industrial applications are scarce. Polyfluoroolefins are very malleable and easily reduced into transparent foils and wires. Those derived from perfluoro-propylene, pentafluoro-propylene and chlorotrifluoroethylene are used mixed with basic fillers and inert material to make rubbers with special properties and usable up to 400 °C. Many other fluorinated aromatic polymers are very stable to heat and oxidation.
The main characteristic of fluorinated polymers lies in the fact that most of the chemical bonds present are C-F (Carbon-Fluorine), one of the highest energy covalent bonds. It follows that the molecules are very stable, able to withstand high levels of thermal stress and chemical aggression, more than other polymers can. On the other hand, their cost ranges widely, from tens of thousands to several millions per kilogram. This explains why the applications of fluoropolymers are still very limited: these materials, in fact, are used when no other polymer is able to meet application requirements of high severity to extreme.
They present exceptional properties of chemical and thermal stability, excellent dielectric characteristics, particularly low refractive index, high resistance to UV, flame and solvents; also the surface properties are of great interest: low friction coefficient, good lubricating characteristics, hydrophobicity.
Fluoropolymers are in fact very high performance (and high cost) “high tech” materials. The main characteristics of fluoropolymers can be summarized as follows:
- Low adhesion: the surface energy is very low, so they offer excellent performance against wetting and adhesion of foreign substances.
- Environmental resistance: they are transparent to UV rays, extremely resistant to oxidation and maintain their properties even at very low temperatures; fluoropolymers are also resistant to attack by microorganisms and absolutely not biodegradable.
- Light transmission: they have high light transmission values and a very low refractive index.
- Absence of contaminants: they are intrinsically pure and, therefore, do not give rise to chemical pollution.
- Corrosion resistance: they resist chemical aggression over a wide temperature range.
- Heat resistance: among fluorinated polymers there are grades that offer a continuous service temperature of 260°C, with higher peaks for short periods.
- Fire resistance: the optical density of the fumes produced in the event of combustion is also low.
- Resistance to wear: they are among the materials with the lowest friction coefficient and this implies a low abrasion.
- Electrical resistance: they have an excellent set of properties, such as low loss factor, high arc resistance, high puncture resistance; these characteristics persist in a wide range of environmental conditions.
- Long service life: they are characterized by excellent resistance to aging, even in the presence of high temperatures and aggressive chemicals; high is also the resistance to dynamic stresses, such as vibration or bending.
The forefather of fluorinated polymers is polytetrafluoroethylene (PTFE), which was discovered on April 6, 1938 by Roy J. Plunkett, an employee of the DuPont laboratory in Jackson (USA). It was a completely accidental discovery: Plunkett was carrying out experiments on gaseous fluorinated refrigerants of the Freon family. One test involved a sample of tetrafluoroethylene (TFE, whose chemical formula is CF2=CF2, i.e. all hydrogen atoms in ethylene are replaced by fluorine atoms) kept under pressure at low temperature. It was realized that the gaseous product had given rise to a spontaneous polymerization, turning into a solid mass, white and waxy appearance.
The first tests on the characteristics of the new polymer showed that it was in the presence of a material with very unique properties: it was not attacked by any chemical reagent, its surface was so slippery that no material was able to adhere to it, it was absolutely hydrophobic. In addition, it did not degrade when exposed to light and had a very high melting point; contrary to known thermoplastic resins, moreover, the polymer did not flow at temperatures above its melting point.
Plunkett and his collaborators realized that the new material could have considerable application possibilities; in a short time it was understood that it could be transformed into the desired form by means of a technology conceptually similar to that of the processing of metallurgical powders: that is, obtaining blocks by sintering that could then be processed with a tool. Thus was born Teflon (registered trademark DuPont), marketed since the 1940s and still one of the most successful polymers in use today.
Teflon was used in World War II as a coating for metals to protect them from corrosion. Only later in the Manhattan Project was it used to contain the fluorine used for the enrichment of uranium 235.
For more than a decade after the war Teflon was not much used. The same DuPont in 1960 was the first to make non-stick cookware. Since then the uses of Teflon in the fields have been the most varied:
- production of computer chips
- insulation for cables in the field of telecommunications
- coating of architectural structures
- space suits.
Sinterable fluoropolymers: PTFE
PTFE can be produced by means of two polymerization processes, in suspension and in emulsion, respectively. The polymerization in aqueous suspension allows to produce polymers with high molecular weights, which through various finishing treatments are transformed into powders. These powders, even when heated to high temperature (e.g. 370 °C) still have a viscosity too high to be processed as plastomers. The transformation takes place mainly through compression molding at room temperature, followed by a sintering treatment at high temperature, thus obtaining finished or semi-finished pieces in the form of plates or cylinders (for prototypes and small series). This is a very slow technology that, in the case of large pieces, may require sintering cycles of several days, since the ramps of temperature rise and fall must be slow in order not to generate stress in the piece, with consequent risk of breakage.
A very important parameter in the transformation of PTFE powders is their morphology; particles with an average size in the order of 25 microns are available, defined “non-free-flowing” as they give problems of transport. By agglomeration, particles of larger size are obtained, reaching 400-500 microns, defined as “free-flowing”. Agglomeration can be used for the production of large articles, while free-flowing powders must be used for the production of small parts, where the feeding of particles into the mould is more difficult. It should be noted that molded parts often have to go through an additional refinement step at the tool in order to be forged into the desired shape.
Emulsion polymerization (with non-ionic emulsifiers), on the other hand, produces polymers with lower molecular weight, which are more suitable to be transformed, after suitable additives with lubricants, into pastes that can be extruded for the coating of electrical cables, thin-walled pipes, profiles, coatings for metal pipes, etc.. Emulsions can also be used (after concentration and additivation) for the impregnation of technical textiles, in particular glass fiber fabrics, or for the coating of metal surfaces.
From both polymerization processes, after grinding, micropowders with an average size of less than 10 microns are obtained, used as additives. They are particularly appreciated in the production of lubricants, oils and greases, or in printing inks and protective coatings, where they impart anti-friction and anti-adhesion properties.
Remaining in the field of high polymers, they are indicated to improve the extrudability of polyolefins, in particular LLDPE films. In this case, the low friction coefficient of PTFE is essentially exploited: the addition of 500-1,000 ppm is sufficient to eliminate the “stick and slip” effect that produces phenomena known as “snake skin” or “shark skin” in films. In addition, the addition of fluoropolymers increases plant productivity. When, on the other hand, it is only necessary to improve the cleanliness of the die, concentrations of 100-150 ppm are sufficient.