In its virgin form, PTFE offers some excellent properties such as extremely low friction and outstanding chemical resistivity. However, it doesnt always have the strength and stiffness needed for applications such as seals or bearings.
Thats where fillers and additives come in: they can be added to virgin PTFE to enhance its mechanical properties, including strength and stiffness. In this post, we will talk about some of the most commonly used PTFE fillers. (Note that anything could theoretically be used as a filler as long as it can handle the sintering temperatures involved between 710°F and 730°F.)
The most common filler used with PTFE is glass. Using glass as an additive will decrease the tensile strength and flexural strength while increasing the compressive strength. It significantly increases the flexural modulus and overall stiffness of PTFE. Glass fibers have no significant effect on impact strength of PTFE, but will increase its hardness. Glass fibers also cause a slight increase in thermal conductivity and a major reduction in the coefficient of linear expansion, resulting in a more dimensionally stable part. Glass has a very positive effect on wear, and has little to no effect on the chemical compatibility of PTFE.
The major drawback of glass is its abrasive nature; it can cause wear on mating surfaces. However, there is an alternative filler that provides many of the same benefits without the abrasiveness: Wollastonite (calcium silicate). This mineral is a valid alternative to glass and has been FDA approved.
Using carbon (either in powder or fiber form) as a PTFE additive will result in significant increases in compressive strength and stiffness, with a slight increase in flexural strength and flexural modulus. It also results in increased hardness and a major increase in thermal conductivity. Carbon has no significant effect on the impact strength of PTFE, and only a slight reduction in wear at 5% carbon and a more significant effect at 15% carbon. Carbon fiber provides much improved creep resistance compared to carbon powder, both resulting in a more dimensionally stable part than can be achieved with virgin PTFE.
Carbon fiber provides much improved creep resistance compared to carbon powder.
Another benefit of carbon is its chemical neutrality; thus it does not compromise the chemical compatibility of PTFE. Note that carbon is not as abrasive as glass, and makes for an excellent alternative to it when the abrasiveness of glass becomes a major issue. You will often see carbon combined with graphite, which results in even better wear and friction properties.
Graphite, is also a common additive for PTFE – usually at about 15%. Many people think graphite and carbon are the same thing, but graphite is actually a crystal modification of very high purity carbon. Graphite reduces the tensile strength and % elongation at failure while significantly increasing the flexural modulus. It slightly increases the hardness of PTFE. In addition, adding graphite to virgin PTFE will decrease the coefficient of linear expansion and significantly increase the compressive strength. It also makes the PTFE much more dimensionally stable and has a very positive effect on wear. Graphite is often combined with carbon or glass.
Many people think graphite and carbon are the same thing, but graphite is actually a crystal modification of very high purity carbon.
Bronze is an alloy of copper and tin, and it is another potential additive for PTFE. It will increase the weight of the final component, and results in a significant decrease in tensile strength. At the same time, it increases the overall stiffness and compressive strength. Adding bronze also significantly increases the hardness. A major reduction in the coefficient of linear expansion results in a very dimensionally stable part. The drawbacks of bronze are higher friction, increased weight, and more susceptibility to chemicals.
While virgin PTFE has outstanding properties in so many areas, it can cause problems with it comes to stiffness, strength, and dimensional stability. The use of fillers such as glass, carbon, graphite, and bronze can have a powerfully positive impact on its mechanical properties.
Interested in more information about PTFE fillers? You may enjoy these additional pieces of content:
- Selecting the Right PTFE Filler for Your Application
- PTFE Seal Factors