Virgin PTFE has good properties, but they may not be good enough for demanding applications involving low creep, high compressive strength, and excellent wear properties. There are a variety of fillers that can be added to PTFE to solve those problems, however.
Here’s a list of common PTFE fillers:
- Glass
- Carbon
- Graphite
- Molybdenum Disulfide, MoS 2
- Bronze
- Polyimide
In this article, we’ll take a look at each of these PTFE fillers and what they can provide to help you
select the right filler for your application.
Glass
When added to PTFE, glass is used to improve wear resistance, increase compressive strength/reduce deformation under a compressive load, and decrease creep. Its major drawback is its abrasive tendencies, making it a less popular choice for rotary applications where it could abrade the mating surface. While glass is fairly chemically resistant, it shouldn’t be used in alkali environments.
The percent of glass added is usually between 5% and 40%, and E-glass is the typical glass fiber type used.
Carbon
Carbon is added to PTFE to increase compressive strength/reduce deformation under a compressive
load, increase wear resistance, reduce permeability, increase hardness, and increase thermal
conductivity to improve heat dissipation. Note that carbon is naturally very inert, and does not
significantly impact the chemical resistance of PTFE. Carbon tends to be abrasive, but not as abrasive as glass. You’ll often see carbon combined with graphite to further improve wear properties and reduce friction.
Carbon fillers come in two forms:
- powder; and
- fiber
Fiber provides better creep resistance than powder, but powder is much cheaper. The percent of carbon added to PTFE is usually 5% and 15%.
Graphite
Graphite is crystal modified high purity carbon, and it provides the same benefits as carbon. In addition, it greatly decreases friction, acts as a dry lubricant, increases load carrying capabilities, and further decreases wear. You will often see graphite combined with carbon and glass.
The typical percentage of graphite added to PTFE is between 5% and 15%. Like carbon, it is chemically inert.
Molybdenum Disulfide
When added to PTFE, MoS 2 reduces friction, reduces stick-slip behavior, increases hardness, improves wear resistance, increases stiffness, and exhibits good resistance to deformation. You’ll typically see it combined with glass or bronze. It is fairly chemically inert, and only dissolves in strongly oxidizing acids.
Bronze
Bronze is an alloy of copper and tin that is added to PTFE to increase thermal conductivity (and heat
dissipation) and improve its wear resistance. The drawback is that it increases the coefficient of friction; however, the addition of molybdenum disulfide or graphite can help reduce the friction. Another major issue with bronze as a filler is that it reduces the natural chemical resistance of PTFE. It also tends to oxidize.
Polyimide
Adding polyimide to PTFE results in the lowest friction filled PTFE compound. It provides excellent wear and abrasion resistance, and will not harm the mating surface. It works very well in dry running
applications. It is, however, the most expensive of the PTFE fillers.
Conclusion
There are a variety of fillers available to improve characteristics like the strength, thermal conductivity,
hardness, wear resistance, and creep of PTFE. The main issue to watch out for is the chemical
compatibility of the filler with the media it will be in contact with. Otherwise, by combining additives
with PTFE it is possible to get all the positive of features of PTFE and reduce many of its negative
features, like excessive creep and high wear rate.