PTFE filled with bronze, glass or carbon: differences and uses
Pure PTFE slides beautifully but deforms under load. Fillers improve its strength and life: here is how performance changes with bronze, glass and carbon.
On our product pages you will often see materials such as "PTFE/bronze", "PTFE/glass" or "PTFE/carbon". These are not minor details: the filler added to PTFE significantly changes how a seal or guide ring behaves. Understanding the differences helps you choose the most suitable component and avoid premature wear.
Why fill PTFE
Pure PTFE has outstanding properties: extremely low friction, high chemical inertness, a wide temperature range and no tendency to stick-slip. It has two weak points, though: it slowly deforms under load (creep) and has limited wear resistance. Adding powder or fibre fillers creates a stiffer, more wear-resistant and more stable compound while keeping most of PTFE's advantages.
The most common fillers
Bronze
Bronze-filled PTFE is probably the most widespread in hydraulic seals. It offers high resistance to wear and creep, good thermal conductivity (the heat generated by friction is dissipated better) and excellent behaviour with hydraulic oils. It is widely used for PTFE piston and rod seals and for guide rings in applications with significant pressures and loads. It is not recommended with water or water-based fluids, which can oxidise the bronze.
Glass
Glass fibre improves wear and deformation resistance while keeping good chemical compatibility. It is a versatile filler, suitable for many hydraulic and pneumatic applications. It can be more abrasive to soft mating surfaces, so rod and bore must have adequate hardness and finish.
Carbon and graphite
Carbon, often combined with graphite, gives a compound with good wear resistance even without lubrication, suitable for poorly lubricating fluids, water and dry or semi-dry applications. Graphite further reduces friction.
Molybdenum disulphide
Often added in small amounts together with other fillers, it improves sliding properties and reduces breakaway friction.
Where filled PTFE seals are used
- Piston and rod seals with an elastomer energiser: low friction, no stick-slip, suitable for servo cylinders, presses, machine tools and precision movements.
- Guide rings and guide strip: for high speeds and when friction must be minimal.
- Back-up rings: they protect O-rings and elastomer seals at high pressure.
- Rotary seals: for shafts and swivel joints, where low heat generation is essential.
How to choose the filler
- Fluid: with hydraulic oils bronze is an excellent choice; with water or poorly lubricating fluids carbon or graphite are preferred.
- Load and pressure: high loads require compounds with high creep resistance, such as PTFE/bronze.
- Mating surface: check the hardness and roughness of rod and bore, especially with more abrasive fillers.
- Speed: at high speeds the ability to dissipate heat matters.
A tip on installation
PTFE seals are less elastic than rubber ones: they often require installation and sizing tools to be fitted into the groove without permanent deformation. Always follow the installation instructions and, if in doubt, ask our technical support.
Frequently asked questions
Do PTFE seals leak more than rubber ones?
An energised PTFE seal may leave a slightly thicker oil film in some conditions, but it offers lower friction and great stability. It is often combined with a secondary seal or a double lip wiper.
Can they be reused after dismantling?
It is not recommended: PTFE deforms during installation and operation, and a dismantled seal rarely seals as well as before.
Does the colour indicate the filler?
Bronze-filled PTFE is often golden brown and carbon-filled PTFE dark, but always refer to the product description to be sure.
Are special surfaces needed?
Yes: rod and bore must have suitable hardness and roughness. A surface that is too rough wears PTFE quickly; one that is too smooth can reduce lubrication.
