PCTFE Plate, Cut Piece

PCTFE plate in natural and black, thicknesses from 1/8 inch to 1 inch, cut to size and sold by the square foot.

PCTFE - polychlorotrifluoroethylene - is the material the trade still calls Kel-F, after the 3M brand that was discontinued in 1996. It is made today as Neoflon PCTFE and Voltalef.

The reason to specify PCTFE is the bottom of the temperature range. It is serviceable continuously from about -427F, below the boiling point of liquid hydrogen, and its coefficient of expansion falls as it gets colder rather than rising, so a seal set at room temperature stays set at cryogenic temperature. That is why it ends up in LNG, liquid oxygen, liquid nitrogen and liquid hydrogen valve seats.

It is also the best barrier in the fluoropolymer family. Water vapour transmission is about 0.008 g.mm per square metre per day by ASTM F1249, roughly a third of PTFE, and water absorption is nil at 24 hours - among the lowest of any thermoplastic and the lowest of any fluoropolymer. Oxygen permeability runs about 250 times lower than FEP. Quote any barrier figure with its temperature and thickness, because permeability rises about a hundredfold between 77F and 212F.

Against PTFE, which is the usual alternative: PCTFE carries roughly three times the flexural modulus (200,000 to 243,000 psi against 72,000), close to double the hardness (Shore D 75 to 80 against 55), and two to three times the tensile strength. That is the whole point - PTFE cold flows and extrudes out of a gland under load, and PCTFE largely does not, with deformation under load held to 0.2 percent at 24 hours. It gives up some top-end temperature to get there, and its chemical resistance, while very broad, is not quite PTFE's.

On oxygen service, the honest position. PCTFE is the polymer of choice for liquid and gaseous oxygen, is non-flammable with a limiting oxygen index of 95 to 100 percent, and specific grades are listed for oxygen service in NASA KSC-KTI-5210 and specified for aerospace parts under ASTM D7194. This is unbranded stock shape and is not itself certified to either. Oxygen-service parts have to be qualified by the end user and annealed for dimensional stability first - that standard exists because PCTFE valve seats that had cold flowed in service caused a run of oxygen cylinder fires in the 1990s.

Chemically it is close to inert: no measurable attack from 96 percent sulphuric or 70 percent nitric acid at 158F, 50 percent hydrofluoric acid, caustic soda at boiling, alcohols, acetone, aniline or phenol. The exceptions are worth knowing - it swells 5 to 10 percent in carbon tetrachloride, trichloroethylene, chloroform and the aromatics like benzene, xylene and toluene. It also holds up under radiation far better than PTFE does, and outgasses almost nothing in vacuum.

Electrically it is a good insulator but not a high-frequency one. Volume resistivity is about 2 x 10^17 ohm.cm and the dielectric constant 2.6 at 1 kHz, but unlike PTFE the chlorine atom gives the chain a permanent dipole, so the dissipation factor is 0.02 rather than PTFE's near-zero. Good for insulators and connector bodies, wrong for RF dielectrics.

It machines like a hard bronze rather than like PTFE - sharp tooling, positive rake, light finishing passes, and it holds a dimension where PTFE springs back. Stress relieve before finish machining anything critical, and anneal it if the part is going into oxygen service. Annealing is a real choice, not a formality: water-quenched PCTFE comes out around 30 percent crystalline, transparent and tougher, while slow cooled or annealed stock runs near 70 percent, opaque, stronger and more creep resistant. Nothing bonds to it without sodium etching or a fluoropolymer primer, so design for mechanical fastening.