PTFE Plate, 25% Carbon Filled

25% carbon-filled PTFE plate in black, cut to size and sold by the square foot.

This is 25 percent carbon-filled PTFE, and it is the grade to reach for when the problem is load rather than chemistry. Of the mainstream fillers, carbon does the most for PTFE's worst property: deformation under load drops to 5 to 7 percent against virgin's 10 to 13 (ASTM D621). Carbon beats glass on cold flow, and both resin producers and distributors say so in those terms.

It also brings the best thermal conductivity of the common grades - about 4.5 against virgin's 1.70 BTU-in/ft2-hr-F, roughly two and a half times - which matters for a dry-running bearing that has to get rid of frictional heat. Flexural modulus rises from 72,000 to about 160,000 psi, hardness to around 62 Shore D, and thermal expansion drops about 20 percent. It is less abrasive to mating surfaces than glass-filled.

The one thing you must not do with it: carbon-filled PTFE is NOT an electrical insulator. Volume resistivity measures about 10^4 ohm-cm against over 10^18 for unfilled PTFE - fourteen decades down, confirmed by four independent sources. Substituting carbon-filled for virgin PTFE in an insulation application is the most dangerous mistake available in this material family. We are quoting the measured resistivity rather than calling it antistatic or conductive, because which side of that line it falls on depends on the threshold your specification uses.

What it costs you: elongation at break collapses from 200-300 percent to 75-90 percent, so it is appreciably more brittle than any other PTFE here and is the wrong choice for a part that has to stretch or snap into place. Tensile strength is around 1,900 psi. It is black only. Chemically it holds up well, including in hydrochloric acid, but it is not resistant to strong oxidizers, bleaches or halogens - which is a real narrowing of PTFE's usual universal resistance.

No food-contact claim is made for this grade. No mill declaration for carbon-filled PTFE under 21 CFR 177.1550 turned up in our research, and food-contact status does not pass down from unfilled PTFE to a filled compound - the filler has to qualify on its own. If you need food contact with a filled grade, ask us about the wollastonite and mineral-filled options instead.

It goes into bearings and bushings, dry-running and load-bearing wear parts, piston rings specified to reduce cylinder-wall wear, parts that need to shed frictional heat, and static-dissipative or conductive duty where the resistivity figure above meets the spec.

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.