The Characteristics of and Applications for Fluoropolymer Coatings
The desirable features of fluoropolymer coatings are mainly due to the atomic structures of carbon and fluorine as well as the covalent bonding in their unique chemical structures.
Fluoropolymer coatings possess a useful combination of desirable properties, including weatherability and corrosion barrier protection, chemical inertness, flame retardancy, non-stick, low friction, and a high dielectric constant. As a result, these coatings have found applications in numerous industries.
What is a Fluoropolymer Coating?
In order to understand fluoropolymer coatings, we must first become conversant with the terms polymer and fluoropolymer.
A polymer is a molecular structure made up of a large number of similar chemical compounds called monomers or units, which are strongly bonded or linked together. Rubber is an example of a hydrocarbon polymer.
Fluoropolymers are a class of polymers that have a carbon-carbon backbone with fluorine attached by strong fluoride-carbon bonds. Its extraordinary advantage lies in its ability to resist high and low temperatures, severe corrosive environments and most chemicals. These are polymers where fluorine atoms have completely or partly replaced the hydrogen atoms of hydrocarbon polymers.
The fluoropolymers (consisting of fluorine and carbon) are also known as perfluoropolymers to distinguish them from fluoroelastomers and partially fluorinated polymers. Different fluorocarbon products may have different molecular weights that also influence their melting points.
Characteristics of Fluoropolymers
Characteristics of common fluoropolymers include:
- Nonreactivity (inertness) to most alkalis, acids and other chemicals
- Nonstick (non-adhesiveness) or quick release property
- Low coefficient of friction
- Resistance to corrosion and abrasive wear
- Resistance to extreme weather
- Dielectric (electrical insulation) characteristics largely independent of power frequency and temperature
- Cryogenic (very low temperature) capability
- High temperature stability
- Moisture resistance (nonwetting)
The Chemistry of Fluoropolymers
The characteristic features of the fluoropolymers are mainly due to the atomic structures of carbon and fluorine as well as the covalent bonding in their unique chemical structures. The atomic size of fluorine helps it to form its unique covering around the bonds between carbon atoms, which protect the backbone of the carbon-carbon bonds from corrosive chemical attack and provides stable chemical resistance. (However, polymeric materials are not corrosion-proof. Read The Corrosion of Polymeric Materials for more information.)
Most applications of fluoropolymers require the ability to withstand specific operating environments, for which the manufacturers have developed custom-designed products as well as innovative modifications of pure fluoropolymers.
Some of the popular fluoropolymers include:
|fluorinated ethylene propylene||FEP|
Producing Fluoropolymers through Polymerization
The term fluoropolymer represents a family of different polymers, custom-produced by manufacturers.
Fluoropolymers are produced by the process of polymerization. For example, PTFE is produced by the polymerization of TFE (tetrafluoro ethylene), creating the bonded fluorine-carbon molecules that repeat several thousand times in the chemical structure. Generally PTFE is stable up to 260°C (500°F).
However, a special category of PTFE polymers with a high molecular weight (due to high crystallinity) can have a higher melting point up to 342°C (648°F). Also, some manufacturers have developed a modified PTFE by adding other monomers below 1% by weight of PTFE. This has been done to meet certain specific operational requirements.
Applications for Fluoropolymers
General applications of this family include:
- PTFE coating for cookware
- PFA coating for wires and semiconductor parts
- FEP coating for wires and cables
- ETFE roofing material
- EFEP fuel tubes
- PCTFE waterproof films
Some industry-specific uses are described in the following sections.
Building and Architectural Applications
In the architectural field, the fluoropolymer coating systems have facilitated innovative aesthetic designs, while at the same time lowering maintenance costs and cooling energy costs.
ETFE is used as a component of structures such as domes and stadia. Bridges use PTFE bearing pads due to their inherent low friction and low maintenance. Unique examples of fluoropolymer applications include the Sony Centre in Berlin and the Wimbledon Centre Court’s retractable roof.
Power Utility and Chemical Manufacturing
In the power and chemical production sectors, fluoropolymers are used to prevent emissions and leaks, as well as to minimize corrosion.
Typical applications include:
- Piping and tubing
- Vessels, filters and pumps
- Tank and component linings,
- Data and electric power cables
- Nuclear fluid handling and filtration systems
In the renewable energy sector, fluoropolymer coatings provide high and low temperature resistance, corrosion and chemical resistance, and abrasive wear resistance to various parts.
Important applications include:
- Nonstick film for composite turbine blades
- Protective coatings on the front and back sheets of photovoltaic cells
In medical field, fluoropolymers help extend the lifespan of medical equipment and implantable devices as well as contribute to minimizing medical complications.
Notable applications for these polymers include:
- Membranes for venting and filtering
- Filters for sterile containers and diaphragm pumps
- Systems for needle retrieval
- Catheter guides (wires and tubing)
Aerospace and Automotive
In the aircraft industry, fluoropolymers are used for electric wire insulation because they minimize fire hazards. They are also used on fuel lines and hoses, cylinder head gaskets, hydraulic systems, and electronic systems. (Learn more about preventing corrosion in hydraulic systems in the article Understanding the Prevention of Corrosion in Hydraulic Systems.)
In the automotive sector, fluoropolymers are helping to achieve improved engine performance and fuel efficiency, reduce component weight, and minimize leaks. These coatings are applied on auto parts to protect them from high friction, wear and tear, and corrosive deterioration.
Certain reinforced fluoropolymers with ceramic fillers eliminate wear and increase the component’s lifespan, often up to 500 percent. Fluoropolymer coatings thus help extend the useful life and increase the performance of automotive parts. On bearings, gears and other moving parts, the coatings reduce friction, save energy, and minimize wear and heat generation. In some cases fluoropolymer coatings act as a solid dry film lubricant. (You can learn more about these lubricants in An Introduction to Corrosion Inhibiting Dry Film Lubricants.)
Application Methods for Fluoropolymer Coatings
Fluoropolymer coatings can be applied on clean surfaces free from oils and other contaminants. Careful surface preparation is required to ensure proper adhesion. A critical factor is the relatively high curing temperature that these coatings require, which is between 80°C to 420°C (176°F to 788°F), based on the characteristics of the coating chosen. The surface being coated must be able to withstand this curing temperature for the required curing duration without deterioration.
While most metals can be coated with fluoropolymers, copper can often oxidize at these high curing temperatures, thus forming an oxide layer below the coating film. Brasses and bronzes have similar issues related to high temperature curing, while the high nickel-copper alloys can face adhesion problems.
Compressed air sprays are commonly used to apply fluoropolymer coatings. These coatings can be manufactured and transported as liquids and then poured by gravity into spray system containers.