Overview of Fluoropolymers
The chemical structure of fluoropolymers (also called fluoroplastics) primarily consists of carbon and fluorine. The particular combination of these two chemical elements arranged along the molecular chain imparts a unique set of properties to these types of carbon - fluorine based polymers. The commercially available fluoropolymers are as follows:
1. PTFE (Poly tetra-fluoro ethylene) – is a fully fluorinated polymer available in various unmodified and modified grades. PTFE tubing is processed by compacting the powder under pressure at ambient or slightly higher than ambient temperatures. Methods of forming products include paste extrusion, ram extrusion, molding and calendaring between rolls. Many formed PTFE products are consolidated by sintering in an oven or used in unsintered form ( eg: thread sealant tape). Paste extrusion and calendaring methods are used with fine powder PTFE resins while granular PTFE resins are processed by ram extrusion and molding.
Unlike PTFE, all of the following polymers are melt processable.
2. FEP (Fluorinated
ethylene propylene) – is a fully fluorinated
3. PFA, MFA – Perfluoroalkoxy, fully fluorinated copolymers
4. ETFE – Ethylene tetra-fluoro ethylene, partially fluorinated polymer containing hydrogen. ETFE has the ability to withstand exposure to high temperature withour rapid embrittlement.
5. ECTFE – Ethylene chloro tri-fluoro ethylene, a copolymer of ethylene and chlorotrifluoroethylene
6. PCTFE – Poly chloro tri-fluoro ethylene, copolymer containing chlorine
7. PVDF – Poly vinyledene fluoride, partially fluorinated polymer containing carbon to carbon double bond (which is weaker than single bonds of fully fluorinated polymers).
are also known by their trade names as follows:
PTFE, FEP, PFA - Teflon,® Neoflon®, Hyflon®
MFA - Hyflon®
ETFE - Tefzel®, Neoflon®
ECTFE - Halar®
PCTFE (or CTFE) - Neoflon® (originally Kel-F®)
PVDF - Solef®, Hylar®, Kynar®
Tefzel® are the trademarks of E.I. DuPont
de Nemours Company
Neoflon®, Polyfon® are the trademarks of Daikin America Inc.
Hyflon®, Halar®, Hylar® are the trademarks of Solvay Solexis, Inc.
Kynar® is the trademark of Elf Atochem North America, Inc.
Kel-F® was the trademark of 3M Company ( this trade name is now discontinued)
Material Properties of Fluoropolymers – A Comparison
In general, the chemical resistance of these materials is superior to most other families of plastics. This “chemically inert” characteristic is closely allied to their superior performance in ultra-pure environments. The chemical inertness varies between the fluoropolymers. The fully fluorinated resins such as PTFE, FEP, PFA and MFA exhibit chemical inertness to a wider range of chemicals than do the partially fluorinated polymers such as CTFE (or PCTFE) and ECTFE. A better property in one or two areas is accompanied by a diminished property in others (for example PTFE is better than PVDF in chemical resistance but it has lower mechanical properties at normal ambient temperatures. Fully fluorinated polymers (Perfluoropolymers) such as PTFE, FEP and PFA offer better thermal (higher use temperature) and chemical resistance properties than their partially fluorinated counterparts like ECTFE or PCTFE. However, partially fluorinated resins possess better mechanical properties, such as tensile strength, toughness, abrasion and cut-through resistance at ambient temperatures.
The flex modulus of PVDF tubing is considerably higher than PTFE (relatively most flexible), FEP, PFA or MFA. This makes PVDF tubing considerably more rigid than the other materials; however it has higher tensile strength at ambient temperatures.
The selection of a resin for a specific use is based on criteria for that application; for example permeability at the use temperature may be a critical requirement and may override other features such as chemical resistance and tensile strength. In each case the choice of material is made by comparing the key property requirements and, of course, cost (see Property chart).
The chemical resistance properties of PTFE (also PFA, MFA and FEP) is so broad that its use is not recommended for only a limited list of chemicals. (see Chemical Resistance). All of these fluoropolymers are generally acceptable for a wide variety of industrial and commercial applications. Due to lack of additives and extreme chemical inertness these materials also qualify for ultrapure applications, such as using deionized (DI) water in the semiconductor, biological and pharmaceutical industries.
of Fabrication on PTFE, FEP and PFA tubing
In general, it is safer to assume that fabrication procedures affect some properties of PTFE, FEP and PFA products. Certain physical properties such as tensile strength, permeability and dielectric strength vary with fabrication conditions. Examples of causes of these may be macroscopic flaws, microporosity (for PTFE) and crystallinity. The extent of the variation depends upon the specific conditions of fabrication. Properties of PTFE, FEP and PFA that are relatively unaffected are as follows:
- Chemical resistance
- Long-term weathering
- Low dielectric constant and low dissipation factor
- High arc resistance, surface and volume resistivities
- Flexibility at low temperatures and thermal stability at high temperatures
- Low coefficient of friction