Meeting Component Demand for Aggressive Environments with Filled Polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) exhibits excellent low-friction, hydrophobic, corrosion-resistant properties, which make it a highly desirable engineered polymer in the fabrication of component parts. The performance capabilities of this polymer are further enhanced to meet the demands of more aggressive environments by filling other compounds within the PTFE matrix.

This article discusses the major types of filled PTFE compounds, especially the enhanced properties imparted by some key material combinations in detail.

PTFE

Since its accidental discovery in 1938, PTFE has been successfully used in a variety of applications, ranging from cookware and industrial applications to electronics, healthcare, and aerospace applications. This thermoplastic polymer is created by the free- radical polymerization of tetrafluoroethylene.

The material is white in color at room temperature and broadly considered as possessing one of the lowest co-efficiencies of friction ever measured in a solid material, holding the third rank with a co-efficiency rating of 0.05 to 0.10.

PTFE consists wholly of high-bonded carbon and fluorine with a high molecular weight. This material is almost completely non-reactive as well as completely hydrophobic.

These properties allow it to be used as a tribological material to lower energy consumption in friction-intensive machinery, as well as in corrosive and reactive applications.

The performance capabilities of PTFE are much better than rival materials, including acetal, nylon, and other engineering plastics, and are comparable to ultra-high-molecular-weight-polyethylene (UWHMPE) in the fabrication of application-specific component parts.

However, PTFE has low strength and modulus, poor wear properties, tendency to creep, and inferior thermal expansion and conductivity ratings, making it less appealing especially for use in extreme conditions.

Filled PTFE

Leading engineered polymer manufacturers are involved in the production of filled PTFE in order to fulfill an increasing demand for component parts that have all the key advantages of PTFE, but are free from the associated drawbacks.

The performance limitations of PTFE can be reduced by integrating a carefully balanced mixture of alternative compounds and embedding them within the PTFE matrix, creating a polymer capable of performing well in highly aggressive and very specific environments.

Graphite, carbon, zinc oxide, molybdenum disulfide, copper, bronze, and glass fiber are some of the widely used compound additives in the production of filled PTFE.

Filled PTFE grade materials are often characterized by complex formulations designed for very specific applications. This makes it difficult for specifiers to identify the appropriate compound mixture suitable for their project.

There is no substitute for a deep-rooted relationship with a reputable manufacturer where the exchange of materials information can be done on a project-by-project basis. However, having a general working knowledge of the commonly used compound additives in the production of filled PTFE and their associated performance benefits can aid the specification process and often the specifier or client relationship too.

Key Filler Compounds

The following are some of the key  filler compounds that are currently available that meet the requirements for a variety of specialist, often highly aggressive, applications.

Carbon and Graphite Filled PTFE

The thermal expansion and wear resistance properties of PTFE can be improved when graphite and carbon are added. The resulting material exhibits between two and eight times more efficient thermal expansion properties and up to 1,000 times more wear resistance in applications such as air compressors with a discharge pressure of up to 20 bar.

It should be noted that the quality of a specific additive can influence the performance of the material and its suitability for certain applications. As a result the specification is even more challenging because of the need to consider multiple variables when selecting an appropriate product.

The use of graphite and carbon is an example of this. A high carbon and graphite filled PTFE exhibits a low coefficient of thermal expansion and can be used in the fabrication of water turbine bearings and labyrinth seals (Figure 1). The same thermal expansion properties cannot be expected when using slightly less carbon graphite (a medium to high filler).

The resulting material offers optimum wear rates, making it ideal for use in air compressor applications. A premium medium to high carbon and graphite filled PTFE that takes advantage of lower porosity is suitable for light gases in lubricated high pressure applications up to 100 bar. Standard quality medium graphite filled PTFE is an ideal material for any application where flexibility is essential.

A high carbon and graphite filled PTFE offers low coefficient of thermal expansion, making it ideal for the manufacture of water turbine bearings and labyrinth seals.

Figure 1. A high carbon and graphite filled PTFE offers low coefficient of thermal expansion, making it ideal for the manufacture of water turbine bearings and labyrinth seals.

Bronze

The wear resistance of a bronzed filled PTFE is not comparable with that of the carbon-graphite counterpart, but a bronzed filled PTFE still offers good wear resistance and is ideal for air compressors where the gas pressure is above 20 bar, especially for air compressors with piston temperatures, due to the preferential thermal conductivity of bronze as a compound (Figure 2).

A bronze filled counterpart can deliver up to ten times greater thermal conductivity ratings than traditional PTFE. The wear resistance of a medium bronze filled PTFE can be further improved when a special filler is added, and the resulting material can deliver effective performance in applications involving high air temperature and pressure.

A bronzed filled PTFE is more suitable for air compressors where gas exceeds 20 bar

Figure 2. A bronzed filled PTFE is more suitable for air compressors where gas exceeds 20 bar

Glass Fiber

Glass fiber is used in combination with various other compounds to manufacture filled PTFE grades ideal for chemically aggressive environments and for applications where a low co-efficiency of thermal expansion is required.

For example, a medium glass fiber and copper filled PTFE offers low thermal expansion, while the embedding of glass fiber on its own is capable of creating an almost chemically inert PTFE material suitable for oxygen-focused applications (Figure 3).

Addition of glass fiber on its own can create a PTFE material which is almost chemically inert and suitable for oxygen-focused applications.

Figure 3. Addition of glass fiber on its own can create a PTFE material which is almost chemically inert and suitable for oxygen-focused applications.

Conclusion

Knowing the variables at play during the development of filled PTFE materials will help any specifier seeking to identify or implement the design of component parts for extreme conditions. It also helps to optimize relationships with producers, who spend time on understanding each of the projects of their customers to ensure that components specified fulfill the criteria of a complex project brief.

This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.

For more information on this source please visit Morgan Advanced Materials.

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