Hollow Bar in PTFE CP25 2000 mm natural

PTFE CP25 is PTFE reinforced with 25 percent carbon fiber. The result is a tube material that takes the same extremely aggressive chemical environments as pure PTFE, but that handles considerably higher surface load, has lower creep and wears several times more slowly in dry running. That is the reason carbon fiber filled PTFE is the first choice when seals and slide rings have to hold up in hydraulics, compressors and pumps where pure PTFE would give up on creep or cold flow.

The density of the material sits around 2.1 grams per cubic centimeter and the color is dark gray to black from the carbon fiber. The service temperature follows the curve of the PTFE base resin and sits at about 260 degrees continuously. Downward the material goes all the way to cryogenic temperatures without turning brittle, which is unusual among engineering plastics and one of the reasons PTFE grades are used in cryogenic applications. The flammability rating is V0, that is, the highest UL class for self extinguishing materials. The friction remains very low, somewhat higher than pure PTFE but in exchange for a dramatically better wear figure. The thermal conductivity is also raised by the carbon fiber, which is an advantage in dynamic seals where friction heat otherwise collects in the contact surface and accelerates the wear.

Chemically PTFE CP25 is in principle impossible to attack. Acids, bases, solvents, oils, ozone, UV; the material stays put. The exceptions are the same as for pure PTFE: molten alkali metals, elemental fluorine under pressure and a few fluorine compounds at high temperature. Water absorption is negligible, which means the dimensions hold even in a damp environment and the material is dimensionally stable over time. The carbon fiber also raises the electrical conductivity compared with pure PTFE, which can be worth knowing in applications where static charge is a problem.

The tubular format is machined into seal rings, piston rings, bearing sleeves, plain bearings and guide bands. Typical uses are in hydraulic cylinders, high pressure compressors, valve stems, dosing pumps and process valves. In dynamic applications with a high PV factor, CP25 is often the only PTFE grade that meets the service life requirement without cold flowing out of the groove. It also handles dry running better than most other plastics, which is valuable at start-up and when the lubrication is interrupted. In rotating seals for pump shafts, CP25 rings are a standard choice when the medium is aggressive and pure PTFE would wear out too fast.

Machining is done on a lathe with sharp, polished tools. Tube in PTFE is press sintered and has a slightly different character than extruded plastic, which means a calm feed and a small parting cut give a fine surface finish. The carbon fiber is not as abrasive as glass fiber but still wears HSS edges harder than pure PTFE, so carbide is preferable for longer production runs. The material is not welded and not bonded conventionally; mechanical fastening or etching is required if the surface needs adhesion. The chips are dry and clean and require no cutting fluid, but cooling with air or water works fine to keep the tube wall cool during heavier cuts.

The thickness of the tube wall governs both pressure rating and machinability. Thicker walls give a higher possible clamping force and more margin for deep grooves, thinner walls more flexibility and lower weight per meter. For a press fit in a metal housing the thermal expansion should be counted in so that the fit holds across the whole working temperature, since PTFE grades shrink and expand more than steel and bronze. For bolted joints with PTFE seals a retightening after the first heating cycle can be wise, since the material settles somewhat under heat and pressure.

In storage, PTFE CP25 is insensitive to UV, ozone and normal temperature swings and requires no special handling beyond supporting the tubes along their whole length so that they do not go oval over time. That matters especially for large outer diameters with thin walls. The material is food approved in its base form, but the carbon fiber means the CP25 grade is usually used in technical processes rather than in food contact; for food applications pure PTFE or a glass filling is more common. The electrical conductivity from the carbon fiber means CP25 can be used in seals in environments with a risk of static charge, such as when pumping flammable liquids.

For the designer, PTFE CP25 is a middle segment in the PTFE family: more expensive than pure PTFE and the GF grades, but cheaper than more specialized fluoropolymers such as PFA and PVDF. It therefore often becomes the choice when pure PTFE is not enough mechanically but you do not need to go up in price to a completely different plastic family.

When choosing between grades: pure PTFE for minimal friction without a mechanical load, PTFE GF for better stiffness in static seals, PTFE CP25 when the component both has to take load and move. If you want to discuss dimension, wall thickness or a specific bushing or sealing task, get in touch.

With 25 percent carbon powder, more than in the sibling bar, the rings take even harder wear, and at 2000 mm in natural you get several out of a single piece.