Overview of Fluoropolymers
Fluoropolymers are unique materials with exceptional properties that arise from the nature of the carbon-fluorine bond. The extreme difference in electronegativity between carbon and fluorine results in the formation of a strong chemical bond that is extremely difficult to thermally or chemically break. The electron withdrawal towards the fluorine atom in the C-F bond has the added benefit of strengthening the carbon-carbon bonds that make up the polymer backbone by pulling the electron density away from the C-C bonds. Additionally, the fluorine electronegativity forces the atoms to repel one another which in turn forces the polymer configuration into a rod-like helical structure. Because the fluorine atoms are smaller than carbon atoms, this results in a fluorine sheath uniformly surrounding the C-C backbone. The shielding of the polymer backbone by the fluorine atoms directly contributes to the extremely low coefficient of friction (COF) and surface energy properties of fluoropolymers.
The chemistry of commercial fluoropolymers ranges from fully fluorinated or “perfluoro” to partially fluorinated varieties. Partially fluorinated polymers are produced by incorporating hydrogen- or chlorine-containing monomers into the polymer chain. Replacing fluorine with hydrogen or chlorine results in some improvement in properties relative to perfluoropolymers including stiffness, tensile strength and barrier performance but at the expense of other properties such as chemical resistance, electrical properties, COF and surface energy.
Advantages of Fluoropolymer Tubing
There are three main advantages of using fluoropolymer tubing over other plastic tubes – chemical resistance, thermal stability and permeability. Perhaps most importantly for many applications, fully fluorinated fluoropolymers are resistant to all common solvents and most chemical species with the exception of extremely aggressive agents such as molten alkali metals, gaseous fluorine and certain halogenated compounds. Because fluoropolymers are so chemically resistant, they maintain physical properties even after exposure to harsh chemicals and are less susceptible to chemical stress cracking, even at elevated temperatures. Additionally, perfluoropolymers have excellent thermal stability due higher melt and degradation temperatures compared to other thermoplastics and rubbers. The high degree of crystallinity combined with the comparably high molecular weight results in higher melt temperatures and thus higher continuous service temperatures. FEP has a maximum recommended operating temperature of 200°C while PTFE can be used at higher temperatures up to 260°C.
Thermal stability also translates to low temperature performance, including cryopreservation applications. The majority of thermoplastic tubing becomes brittle and cracks between 0°C and -100°C, but fully fluorinated fluoropolymers maintain flexibility and impact resistance at liquid nitrogen storage temperatures of -196°C.
Lastly, fluoropolymers have very low water vapor transition rates and are less permeable to other gases such as CO2 and oxygen when compared to hydrocarbon thermoplastics on a thickness for thickness basis. Because the vast majority of fluoropolymers are semi-crystalline, the individual polymer chains tend to be more organized and more tightly packed in the crystalline regions. The organization of the denser crystalline regions prevents even small molecules such as helium, water or carbon dioxide from passing through.
Perfluoropolymers tend to have limited mechanical properties because the relatively nonpolar nature limits the attraction between individual polymer chains. Incorporation of some hydrogen-containing monomers into the polymer chain improves mechanical properties. For this reason, the partially fluorinated polymers have better mechanical properties. The addition of the hydrogen functionality increases polarity because hydrogen is less electronegative than fluorine, so individual chains are more attracted to each other. This serves to increase stiffness and tensile properties and often even translates to better barrier properties. It is vitally important to understand the trade-off of improved mechanical properties at the cost of chemical resistance, flammability and surface energy in order to select the best material for the application.