PTFE has the lowest coefficient of friction of any polymer known to mankind. However, once weve added the fillers and additives needed to improve its mechanical properties, it may not have such low friction.
Consider this: virgin PTFE has a coefficient of dynamic friction on the order of 0.04 to 0.06, but adding carbon fiber raises it up to 0.18 and glass can raise it up to 0.5 (yes, about 10x more friction). Fortunately, there are fillers and additives that can improve the coefficient of friction, and we’re going to take a look at some of the more common ones.
One additive that works wonders for bring the coefficient of friction down is Molybdenum disulfide, sometimes referred to in the industry as Moly. When added to PTFE with 15% glass fiber, the coefficient of friction goes from 0.5 to 0.15, which is much closer to virgin PTFE. When added to PTFE, Moly is typically combined with bronze or glass to reduce friction and improve wear characteristics.
PPS (polyphenylene sulfide) is a polymer that is probably better known by its trade name Ryton®. PPS is another filler that is used to improve the frictional properties of PTFE. In not only reduces the coefficient of friction but also improves wear, deformation, and extrusion resistance.
Polyimide-filled PTFE has an incredibly low coefficient of friction and work extremely well for dry running applications where lubrication is just not feasible. Polyimide also offers improved wear and abrasion resistance. The major drawback of using this poyler as a PTFE filler is its price.
In a previous blog post we talked about graphites ability to improve the mechanical properties of PTFE, but we didnt talk about how it also acts as a dry lubricant. Adding graphite to PTFE will decrease its coefficient of friction and improve its overall wear properties. Its not unusual to see graphite added with glass or carbon.
Filled PTFE is an excellent option for many seal and bearing applications, but sometimes those fillers and additives can adversely affect PTFEs coefficient of friction. However, additional additives such as MoS 2, Ryton, Polyimide, and graphite can counteract the negative impact on PTFEs tribological properties and improve its wear resistance at the same time.