PTFE Fillers & Addititives - Overview

PTFE Fillers & Addititives - Overview

PTFE fillers don't act like elastomer fillers, which become chemically bonded to the elastomer. With polytetrafluoroethylene, the high shear modulus fillers are encapsulated and bound by the low shear modulus PTFE.

Here's an overview of PTFE fillers:

Glass is the most common filler for PTFE. Widely used in hydraulic piston rings, glass gives good wear resistance, low creep, and good compressive strength. Glass also has excellent chemical compatibility. The major disadvantage is that glass-filled PTFE compounds are abrasive to mating surfaces, especially in rotary applications.

Molybdenum disulfide (MoS2) improves wear resistance and further lowers the coefficient of friction. ôMoly" is typically combined with other fillers (such as glass and bronze).

Carbon (powder or fiber) imparts excellent compression (low deformation under load) and wear resistance, good thermal conductivity (heat dissipation), and low permeability. Carbon-filled PTFE compounds are not as abrasive as glass-filled compounds, but they are still more abrasive than polymer-filled compounds. Carbon-filled compounds have excellent wear and friction properties when combined with graphite. Carbon fiber lends better creep resistance than carbon powder, but fiber is more expensive.

Graphite is a crystal modification of high purity carbon. Its flaky structure imparts excellent lubricity and decreased wear. Graphite is often combined with other fillers (especially carbon and glass).

Bronze (a copper-tin alloy) lends excellent wear resistance and thermal conductivity. Bronze-filled materials have higher friction than other filled PTFE compounds, but that can be improved by adding moly or graphite. Bearing and piston ring applications often use compounds containing 55% bronze – 5% moly. Bronze-filled compounds have poorer chemical resistance than other PTFE compounds.

Stainless steel supplies high wear resistance and load bearing capability, along with better chemical resistance than bronze-filled PTFE. Stainless steel is especially good in steam service.

Wollastonite (calcium silicate) is a mineral filler giving properties similar to glass (minus the abrasiveness). The FDA has approved it for food service.

PPS (polyphenylene sulfide, trade name Ryton®) was the first polymeric material used to improve PTFE's wear and abrasion properties. PPS-filled compounds also exhibit excellent deformation and extrusion resistance, making them good for use in back-up rings.

Ekonol® is a thermally stable aromatic polyester. When blended with PTFE, it produces a composite material with excellent high temperature and wear resistance. Ekonol® will not wear mating metal surfaces, making it good for rotary applications. Ekonol®-filled materials are also good for food service.

Polyimide is another polymeric filler offering superior wear and abrasion resistance. Polyimide-filled PTFE compounds have about the lowest friction properties of all filled PTFE materials, so they provide great performance in non-lubricated (dry) applications. They will not abrade mating surfaces (even soft materials such as brass, stainless steel, aluminum, and plastic). Polyimide is one of the most expensive PTFE fillers, however.

Other fillers include calcium fluoride (CaF2), which is specifically used in hydrofluoric acid (HF) service, and alumina (Al2O3), which can improve the mechanical properties of compounds destined for high voltage applications. Alumina-filled compounds are very abrasive.