Glass fibre is the most widely used filler for PTFE. It is cheap, white or off-white, electrically insulating and effective at the main weakness of the polymer, deformation under load.
Typical content
Fifteen to twenty-five percent by weight is the usual window. Ten percent gives a modest improvement. Above about 30 percent the compound becomes brittle and loses sealing ability.
What it improves
- Wear rate. Often by two orders of magnitude compared with virgin.
- Creep and cold flow. Substantially lower, so rod guides, back-up rings and piston rings hold their size.
- Compression strength and stiffness. Higher modulus allows higher load.
- Dimensional stability. Thermal expansion falls compared with unfilled.
- Electrical insulation. Retained.
What it costs you
- Counterface wear. Glass is abrasive. Soft shafts (aluminium, brass, mild steel) can be scored. Hard-chrome, nitrided or hardened steel at a suitable finish is normal.
- Friction. Slightly higher than virgin PTFE.
- Chemical limits. Hydrofluoric acid, hot concentrated alkalis and some fluorinating agents attack glass. In those fluids use carbon or graphite filler instead.
- Permeation. Marginally higher.
- Lower elongation. It will not stretch to fit as virgin does.
Typical applications
Rod and piston seals in hydraulic and pneumatic equipment, wear rings, back-up rings, gaskets in non-aggressive service, valve seats and insulating parts.
Choosing the surface
Match the counterface hardness to the filler. A common guide is at least 50 HRC and a ground or lapped finish of about Ra 0.2 to 0.4 micrometres for dynamic sealing. Softer or rougher surfaces shorten life.
Specification tips
State the percent glass by weight, fibre length class if relevant, and the standard for filled PTFE moulding. Ask for density: 15 percent glass gives about 2.20 to 2.26 g/cm3, 25 percent about 2.2 to 2.3.
