PTFE Grades: Virgin, Glass, Carbon & Bronze Filled

PTFE is available in six principal grades — virgin unfilled plus five filler systems — each targeting a different mechanical weakness of the base resin while preserving PTFE's core advantages of low friction, high-temperature stability, and broad chemical resistance. Selecting the wrong grade for a seal or bearing application leads to premature dimensional change (cold flow), wear failure, or unexpected chemical attack. This page identifies each grade's property profile and the applications it best serves.

At a Glance

Virgin PTFE: highest purity, FDA-compliant, best chemical resistance, lowest mechanical strength
Glass-filled (15% / 25%): reduced creep, improved stiffness — standard for gasketing and thrust bearings
Carbon-filled (15%): best wear life in dynamic seals, electrically conductive
Bronze-filled (60%): highest compressive strength and thermal conductivity, for high-load bearings
MoS₂-filled (2–5%): marginal friction reduction over virgin, used in dry sliding
No single grade is universally optimal — match filler to load, chemical environment, and FDA requirements

Grade-by-Grade Breakdown

Virgin PTFE

Virgin PTFE contains no filler or processing aid beyond the PTFE resin itself. It is the reference-grade material: lowest coefficient of friction (0.05–0.10), maximum chemical resistance, and FDA 21 CFR 177.1550 compliance for food contact. Electrical resistivity exceeds 10¹⁸ Ω·cm, making it the correct choice for insulation applications.

Limitations: poor creep resistance under sustained compressive load (cold flow), low tensile strength (2,500–4,500 psi), and limited PV (pressure × velocity) in bearing applications. For a static gasket with periodic bolt re-torquing, virgin PTFE is adequate. For a thrust bearing under continuous load, it is not.

Best for: chemical process gaskets, electrical insulation, lab ware, FDA-regulated food and pharmaceutical contact, valve stem packing in corrosive service, and non-stick surfaces.

Glass-Filled PTFE — 15% and 25%

Glass fiber reinforcement in PTFE significantly improves compressive modulus and creep resistance without dramatically increasing friction. A 25% glass-filled compound reduces deformation at a given compressive stress by approximately 50–60% compared to virgin. The fiber also increases wear resistance in sliding applications.

Chemical resistance note: glass fibers are attacked by hydrofluoric acid and strong caustic (>30% NaOH at elevated temperature). Specify virgin PTFE for those service conditions.

15% glass: a balanced grade — moderate improvement in creep with a smaller friction penalty than 25% glass. Common for pump rings, piston guides, and bearing pads.

25% glass: the heavier reinforcement for applications with sustained compressive loads, such as bridge bearing plates, flange gaskets in high-bolt-load joints, and structural slide plates.

Best for: flange gaskets, guide rings, slide plates, bearing pads, pump valve seats.

Carbon-Filled PTFE — 15%

Carbon (graphite or carbon fiber, depending on formulation) improves PTFE's tribological performance beyond any other common filler. Wear life in dynamic seals increases by 10–100× compared to virgin material in the same conditions. Carbon also improves thermal conductivity slightly, helping to dissipate frictional heat in high-speed bearing applications.

Critical limitation: carbon-filled PTFE is electrically conductive. Volume resistivity drops to the 10²–10⁴ Ω·cm range — suitable for static-discharge applications but completely disqualifying it from electrical insulation service.

FDA status: FDA does not list carbon-filled PTFE under 21 CFR 177.1550. Not for direct food contact without specific regulatory review.

Best for: compressor piston rings, rotary lip seals, hydraulic piston seals, pump bushings, dry-running bearing applications.

Bronze-Filled PTFE — 60%

Bronze-filled grades (typically 40–60% by weight) provide the highest compressive strength and best dimensional stability of the standard PTFE compounds. Thermal conductivity is 5–8× that of virgin PTFE, which dissipates heat efficiently in heavy bearing applications and reduces the risk of thermal runaway. The filled composite machines well and holds tighter tolerances than softer grades.

Limitations: bronze corrodes in oxidizing acids (nitric, sulfuric above ~30%), chlorinated solvents, and certain halogen environments. Not suitable where the process fluid contacts the bearing and is oxidizing. Bronze-filled PTFE is also significantly denser (up to ~3.15 g/cc vs 2.15 g/cc for virgin), which matters in weight-sensitive applications.

FDA status: bronze-filled PTFE is not FDA-compliant for food contact.

Best for: hydraulic rod seals, heavy-duty thrust washers, ball valve seats with high-cycle counts, gear guides, structural bearing plates.

MoS₂-Filled PTFE — 2–5%

Molybdenum disulfide (MoS₂) is a solid lubricant that marginally reduces the coefficient of friction below virgin PTFE levels and improves wear resistance in slow, dry sliding contact. The effect is less dramatic than carbon or glass fills, and MoS₂-filled PTFE is a specialty grade rather than a mainstream stock material.

Best for: precision linear slide bearings, slow-speed pivot bushings, and dry-running applications where the lowest possible friction is required. Often seen in semiconductor and clean-room equipment alongside virgin PTFE.

Grade Comparison Table

Grade Selection Decision Logic

Is it a food or pharmaceutical contact application? → Virgin PTFE only (or confirm FDA status of filler)
Is it an electrical insulator? → Virgin PTFE or glass-filled only (not carbon or bronze)
Will it contact hydrofluoric acid or strong caustic? → Virgin or carbon-filled only (not glass or bronze)
Is sustained compressive load or creep a concern? → Glass-filled (25%) or bronze-filled
Is it a dynamic seal or high-wear bearing? → Carbon-filled for best wear life; bronze-filled for high compressive load

For specifications and size availability by grade, see the PTFE specifications page. For machining considerations by grade, see the PTFE machining guide.

Request pricing on virgin or filled PTFE stock shapes

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