Fluoropolymers for Demanding Applications

Fluoropolymers are a family of plastic materials that contain fluorine in their molecular structures.  Fluoropolymers can be divided into two groups: perfluorinated fluoropolymers and partially fluorinated fluoropolymers. The perfluorinated fluoropolymers are homopolymers and copolymers of TFE including PTFE, PFA, and FEP. Partially fluorinated fluoropolymers, including PCTFE, ETFE, ECTFE, and PVDF, have hydrogen, chlorine, or other atoms in their molecular structures in addition to fluorine and carbon.

The mechanical properties and continuous service temperatures for a number of fluoropolymers are shown in the table below. The bottom row of the table shows the relative cost of each material compared with the least expensive fluoropolymer, PTFE.

FLUOROPOLYMERS MECHANICAL PROPERTIES COMPARISON TABLE

Note: The mechanical properties of fluoropolymer materials vary depending on the grade and how the resin is processed.

Fluoropolymers have a number of unique characteristics that make them extremely useful in a wide range of industries including aerospace, spacecraft, semiconductor, scientific instrumentation, pharmaceutical processing equipment, food processing equipment, and electrical devices. Applications include tubing, valve components, seals, pump components, and chemical tanks.

Key characteristics of fluoropolymers include:

• Outstanding chemical resistance
• Toughness and impact resistance
• Resistance to certain types of radiation including resistance to UV light
• Excellent flammability characteristics including high LOI (limiting oxygen index)
• Stain resistance
• Resistance to erosion from atomic oxygen in low earth orbit
• High purity (low leeching, low extractables, low outgassing)
• High continuous service temperatures
• Resistance to bacteria and fungus buildup

When considering various fluoropolymers for an application, it is important to review how each material is unique. Fluoropolymers vary in a number of characteristics as follows.

Mechanical Properties of Fluoropolymers

The graphs below show the tensile strength and flexural modulus for a number of perfluorinated and partially fluorinated fluoropolymers. The perfluorinated materials, PFA, FEP, and PTFE, have relatively low strength and low modulus, which make them good choices for flexible tubing. The low modulus of these polymers also allows them to conform to mating metal surfaces, which makes them outstanding materials for use in sealing applications.

The partially fluorinated fluoropolymers, ETFE, PVDF, PCTFE, and ECTFE, are stronger and stiffer than PFA, FEP, and PTFE. This allows them to be used in applications such as chemical tanks and rigid piping, where load-bearing characteristics are important.

Electrical Properties of Fluoropolymers

Most fluoropolymers have low dielectric constants, which allow them to be used in applications such as radomes, where it is important that the material not attenuate RF signals. One notable exception is PVDF, which has a relatively high dielectric constant compared with other fluoropolymers. This limits its use in certain telecommunications applications.

Processability of Fluoropolymers

With the exception of PTFE, all of the fluoropolymers discussed in this article are weldable via thermoplastic welding and melt-processable using standard thermoplastic extrusion and injection molding equipment with plasticating screws. PTFE can’t be welded into tanks using standard thermoplastic welding equipment and it can’t be processed using standard injection molding and extrusion machinery. Instead, PTFE is processed via ram extrusion or compression molding.

Permeability of Fluoropolymers

The barrier properties of fluoropolymers vary among materials. As shown in the chart below, PCTFE and PVDF have outstanding permeation resistance to a number of gasses. This makes them good choices for high purity fluid handling applications.

GAS PERMEABILITY OF FLUOROPOLYMERS TABLE

Source: Excerpted from Fitz, H., Fluorocarbon Films - Present Situation and Future Outlook, Kunststoffe with German Plastics, vol 70(1) English translation, 1980, pp. 11-16.

Release Characteristics of Fluoropolymers

Most people are familiar with the release characteristics of fluoropolymer-coated cookware. The coating prevents eggs and other food from sticking to pots and pans. The graph below shows the dispersive works of adhesion (a measure of the attraction between surfaces) for a number of liquids on various fluoropolymers. As shown on the graph, the perfluorinated fluoropolymers, PTFE, FEP, and PFA, all have outstanding release characteristics (low work of adhesion values). This is why they are often specified for applications such as composite processing films where easy release from part surfaces is essential.

Friction and Wear Characteristics of Fluoropolymers

Most fluoropolymer materials have moderate coefficients of friction against metal counterfaces. PTFE is an exception, with the lowest coefficient of friction of any polymer when sliding against metal. Filled grades of PTFE are excellent friction and wear materials, especially against soft metals such as stainless steel, where other plastics may perform poorly. PTFE is often added to other polymers such as acetal and PEEK and even to metal plating formulations to reduce the coefficient of friction and improve the wear performance of the base material.

Creep and Stress Relaxation of Fluoropolymers

Creep strain is the deformation of a plastic part over long periods of time. One common example is the bending of a heavily loaded plastic shelf over a period of days or weeks. Stress relaxation refers to the decay of apparent stress when a plastic material is placed under a mechanical stress with a fixed strain.

An example of this is a polymer seal in a plumbing faucet that slowly relaxes, resulting in leak paths.

Creep and stress relaxation resistance are important properties when selecting plastic materials for use in seals or load bearing applications. PTFE has relatively poor stress relaxation characteristics, which is why PTFE is often formulated with glass, carbon, or other additives to improve its stress relaxation behavior when it is used in load bearing applications or high pressure seals.

Differentiating Fluoropolymers

Of course, it is important to consider many different material properties to differentiate individual fluoropolymers. However, the following list of bullet points will help to highlight some of the outstanding features of each material.

PTFE

• Lowest coefficient of friction
• Lowest cost of the fluoropolymers
• Not melt-processable using standard extrusion and injection molding equipment with plasticating screws
• Poor creep and stress relaxation characteristics
• Low strength and stiffness
• High coefficient of thermal expansion
• Mechanical (reprocessed) grades available. Note: Mechanical grades may not have the same level of purity as virgin PTFE. They are not FDA compliant, and the properties are less consistent than those of virgin PTFE.
• A wide range of filled grades are available including glass-filled PTFE and proprietary formulations such as Mitsubishi’s Fluorosint®. These materials are frequently specified for friction and wear applications.
• Excellent release characteristics
• Relatively high permeability compared with other fluoropolymers. “Modified” grades with improved permeation resistance are available.

PFA

• Low strength and stiffness make it an ideal choice for flexible tubing in high purity fluid handling applications.
• Higher service temperature and higher cost than FEP
• Excellent release characteristics

FEP

• Low strength and stiffness make it an ideal choice for flexible tubing in high purity fluid handling applications.
• Lower service temperature and lower cost than PFA
• Excellent release characteristics

ETFE (Tefzel®)

• Highest room temperature strength and stiffness of the fluoropolymers make it an excellent choice for high purity chemical tank and rigid piping applications.
• Good release characteristics compared with other partially fluorinated fluoropolymers

PVDF (Kynar®)

• High strength and stiffness at relatively low cost make it an excellent choice for many high purity chemical tank and rigid piping applications.
• Copolymer grades with lower strength, lower modulus, and higher impact strength available.
• Excellent barrier properties
• Relatively high dielectric constant limits its use in certain radome applications
• Relatively low continuous service temperature compared with other fluoropolymers
• Relatively poor release characteristics compared with other fluoropolymers

PCTFE

• Widely used by NASA and other spacecraft equipment manufacturers for valve components. This is because of its excellent flammability properties and also many years of proven success in space flight applications.
• Most expensive of the fluoropolymer materials
• Excellent barrier properties
• Good low temperature toughness

ECTFE (Halar®)

• Relatively high strength and stiffness make it an excellent choice for high purity chemical tank and rigid piping applications.
• Relatively low continuous service temperature compared with other fluoropolymers
• Relatively poor release characteristics compared with other fluoropolymers
• Exceptional surface smoothness

This article provides general guidelines and is intended for informational purposes only. Because every situation is unique, many factors must be considered when selecting a material. It is the reader’s responsibility to conduct his or her own research and make his or her own determination regarding the suitability of specific products for any given application.