PTFE Rod, 15% Glass Filled

15% glass-filled PTFE rod in natural, cut to length and sold by the foot.

This is 15 percent glass-filled PTFE - PTFE with chopped glass fiber compounded into it to carry load that unfilled PTFE cannot. It is the most common filled PTFE and the usual choice for dynamic seals, piston rings and wear duty.

What the glass buys: hardness up from around 54 to 60-64 Shore D, flexural modulus roughly tripled from 72,000 to about 190,000 psi, better wear life, thermal conductivity up about 80 percent, and better recovery after a load is removed - permanent deformation drops to 5.0-6.5 percent against virgin's 6-7.5.

What it costs you, and this is under-stated almost everywhere: friction roughly doubles. Static coefficient goes from 0.08-0.10 to 0.17-0.20. Tensile strength drops about 35 percent. And be honest about creep - at room temperature glass barely helps. 25 percent glass measures 9-11 percent deformation under load against virgin's 10-13, and 15 percent glass measures 11-13, which is no improvement at all. The real creep benefit of filling shows up at elevated temperature and long duration, not in a 24-hour room-temperature test. If cold flow is your problem, carbon-filled is the better answer.

Two limits that will cost you a part if you miss them. Glass-filled PTFE is abrasive - the resin producer's own guidance is that it is not suitable for soft metal counterfaces, so keep it off aluminum, brass, bronze, unhardened steel and plated surfaces and run it against hardened steel or ceramic. Where the counterface is soft, specify carbon-filled or a polymer-filled grade and ask us. And glass-filled PTFE is attacked by hydrofluoric acid and does poorly in strong alkali - the glass is the weak link there, not the PTFE. For HF service the published answer is a calcium-fluoride-filled grade.

Electrically it is still an insulator at about 10^15 ohm-cm, three decades down from virgin, but the dissipation factor rises from under 0.0002 to about 0.05 at 1 MHz, which rules it out of RF and microwave work even though it insulates perfectly well at line frequency.

Food contact: glass-filled PTFE is declared food-contact compliant by mills under 21 CFR 177.1550 paragraphs (a)(1) and (b) - the glass filler qualifies under the optional-components paragraph. As with any PTFE, that declaration belongs to the mill and names a specific product; ask us and we will supply it for the material we would ship.

It goes into dynamic seals with rotating or reciprocating movement, hydraulic and pneumatic piston rings, bearings at low PV, wear strips and slide plates against hard counterfaces, and valve seats.

One quirk that causes more tolerance arguments than anything else in this material: PTFE passes through a transition near room temperature that changes its volume by 1 to 1.5 percent - roughly 0.3 to 0.5 percent on a linear dimension. A part measured in a 65F inspection room is genuinely a different size in a 75F process. For critical dimensions, measure at service temperature or compensate for it.

Temperature: 500F continuous is unanimous across every source, and PTFE stays useful down to -328F (-200C), with some sources citing -400F. Heat deflection is 250F at 66 psi but only 132F at 264 psi - read the load with the number, because PTFE softens early under load. It is UL 94 V-0.

It machines easily with sharp tools and good chip clearance, but it is soft and it moves, so it will not hold a tolerance the way acetal does. Heat concentrated at the cut makes it swell locally, which is how you get overcuts, undercuts and tapered holes. For close work, rough it out, leave 0.015 to 0.030 inch, stress relieve, then finish cut.

Thermal decomposition - worth reading even if you have worked with PTFE for years. Continuous service is 500F; above roughly 625F PTFE may give off hazardous fumes, and at 840F and above it can form toxic fumes including hydrogen fluoride and carbonyl fluoride, with perfluoroisobutylene above 890F. That leaves only about 125F between the top of the service range and the start of fume evolution. Inhaling decomposition products causes polymer fume fever, a temporary flu-like illness with fever, chills and cough that normally clears in 24 to 48 hours. Do not overheat it, use local exhaust when machining or grinding, and use coolant. And the one everybody leaves out: do not smoke or use tobacco where PTFE is machined or handled - PTFE residue transferred to tobacco and then burned is a documented cause of polymer fume fever. Exposure limits for the particulate are an OSHA PEL of 15 mg/m3 total and 5 respirable; ACGIH TLV-TWA 10 and 3. Ask us for the Safety Data Sheet.