There are many types of fluoropolymers used in coatings, but they usually consist of several general types, which can be divided into three groups: perfluorinated semi-crystalline polymers, partially fluorinated semi-crystalline polymers, and amorphous fluoropolymers.
Examples of perfluorinated resins are semi-crystalline homopolymers and copolymers of tetrafluoroethylene, and examples of partially fluorinated semi-crystalline resins are polyvinylidene fluoride (PVDF) and its copolymers. Amorphous resins may be fully or partially fluorinated, and may have a glass transition temperature lower or higher than room temperature.
The reasons for incorporating fluoropolymer additives into coatings are similar to the reasons why they are added to bulk plastics:
Perfluorinated resins have excellent friction and mold release properties, and are thermally stable at high temperatures, which makes them very suitable for cooking utensils and wire coatings. In addition, they also have some advantages as additives in non-fluoropolymer coatings.
Both suspension polymerization and dispersion polymerization can obtain high molecular weight polytetrafluoroethylene, and the powder particles produced by the two polymerization processes are very different in shape. The powder separated from the dispersion polymerization is composed of agglomerates of hundreds of microns of spherical primary particles of 200 ~ 270 nm. Suspension polymerized PTFE has a fairly large irregular particle size, directly from polymerization, and is usually "cut" or ground before being sold or used. Compared with the use of suspension polymerized high molecular weight PTFE as a coating component, the use of dispersion polymerized polymers is more common. Although high-molecular-weight PTFE has been replaced by low-molecular-weight PTFE in many coatings, high-molecular-weight resins are still the first choice when coatings with high abrasion resistance are required.
Generally, low-molecular-weight PTFE is used as an additive in many coatings. Because low-molecular-weight PTFE can be ground into small particle sizes that are easier to disperse in coating formulations and provide adequate performance in most cases. The production of low-molecular-weight PTFE is usually accomplished by heat treatment or irradiation of high molecular weight PTFE resin from dispersion or suspension polymerization. Low-molecular-weight PTFE "fine powder" or " fluoroadditives " can be ground to any particle size suitable for a particular coating.
Partially fluorinated fluoropolymers used in coatings are generally based on vinylidene fluoride (CF2=CH2 or VDF). The melting point of homo-polymer PVDF (155-192 °C) is much lower than PTFE (317-337 °C), FEP (260-282 °C) or PFA (302-310 °C), which provides some advantages for the preparation of coatings. The common comonomers of VDF are tetrafluoroethylene, HFP and CTFE. Adding comonomers will reduce crystallinity, increase flexibility and lower the melting point and the end use temperature of the resin. Compared with perfluorinated resins, PVDF resins generally have superior mechanical properties, but have relatively poor friction and mold release properties. The main reasons for using PVDF in coatings are their solubility in certain solvents (especially ketones), good weather resistance and good mold release properties.
Amorphous resin does not have a crystalline melting point, but has a glass transition (Tg) that has a significant impact on its mechanical properties. Amorphous resins are generally harder and stiffer below their Tg. Most amorphous fluoropolymer resins have a Tg lower than room temperature, which are dominated by fluoroelastomer resins.
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