Overview of fluoropolymers
Fluoropolymers represent a group of macromolecules with a variety of unique properties, especially good to excellent chemical resistance and stability at high temperatures. As a result, they are increasingly used in applications where most hydrocarbon-based polymers will fail, such as chemical processing, oil wells, motor vehicle engines, nuclear reactors, and space applications. Generally speaking, the unique properties of fluoropolymers are the result of the very strong bonding between carbon and fluorine (Table 1) and the fact that fluorine atoms shield the carbon backbone and that they are fully saturated macromolecules.
| Table 1. Carbon Bond Energies |
|---|
| Bond | Bond energy, kcal/mol | Bond | Bond energy, kcal/mol |
| C-F | 116 | C-Cl | 78 |
| C-H | 99 | C-Br | 66 |
| C-O | 84 | C-I | 57 |
| C-C | 83 | | |
Reference: Iezzi, R. A. in Modern Fluoropolymers (Scheirs, J., Ed.), John Wiley & Sons, 1997.
Fluoropolymers can be divided into two categories: fluoroplastics and fluoroelastomers.
According to whether the polymer is fully fluorinated or contains hydrogen atoms in its structure, the mechanical properties of fluoroplastics can be divided into two categories. Generally speaking, the structure of fluoroplastics containing hydrogen is about 1.5. The strength is twice that of perfluorinated polymers, and the hardness is twice that. On the other hand, perfluorinated polymers show a higher maximum use temperature and greater elongation (Figure 1 and Figure 2).
Fluoroelastomers are mostly based on the combination of vinylidene fluoride (VDF) and other monomers that destroy the typical high crystallinity of PVDF homopolymers. The properties of the resulting elastic material are determined by the short VDF sequence and low or negligible crystallinity. PTFE has a rod-like microstructure, in which the basic fibrils are about 6 nm wide and the molecular chains are fully extended. The elastomers based on VDF and TFE-VDF-HFP are composed of fine particles with a diameter of 16 to 30 nm. Therefore, the properties of VDF/HFP elastomers, such as elasticity and flexibility, may be related to interconnected spherical domains with a diameter of about 25 nm.
Figure 1. Elongation values from commercial fluoropolymers (ASTM D638).
Figure 2. Maximum service temperatures for commercial fluoropolymers (UL-746B).
Reference
- Scheirs, J., Ed., Modern Fluoropolymers, John Wiley & Sons, Ltd., 1997.
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