What are the tensile strengths of PTFE and graphite packing? Key Differences & Selection Guide

PTFE and graphite packing materials exhibit distinct tensile strengths, with PTFE generally demonstrating higher values. PTFE packing shows tensile strengths ranging from 2,000 to 4,970 PSI (or 22 MPa per ASTM D4894), while graphite packing typically measures around 650 PSI. These differences stem from their material compositions—PTFE being a synthetic polymer with excellent chemical resistance and low friction, and graphite being an inorganic carbon-based material prized for thermal conductivity and self-lubrication. The choice between them depends on application requirements beyond just tensile strength, including temperature ranges, chemical exposure, and electrical properties.

Key Points Explained:

1. Tensile Strength Ranges

   • PTFE Packing:
     • Minimum reported: 2,000 PSI
     • Maximum reported: 4,970 PSI (or 22 MPa per ASTM D4894)
     • O-ring variants: 3,625–4,496 PSI
   • Graphite Packing:
     • Consistent reported value: ~650 PSI
   • Discrepancies in PTFE values may reflect testing methods (e.g., ASTM standards) or material formulations.

2. Material Characteristics Influencing Strength

   • PTFE:
     • Synthetic fluoropolymer with high molecular weight
     • Exhibits elongation at break up to 220–400%, enhancing durability under stress
   • Graphite:
     • Naturally occurring carbon layers with weaker van der Waals bonds
     • Prioritizes thermal/electrical conductivity over mechanical strength

3. Performance Trade-offs

   • PTFE Advantages:
     • Superior chemical inertness (resists acids/gases)
     • Low friction (0.04–0.10 coefficient) reduces shaft wear
   • Graphite Advantages:
     • Withstands extreme temperatures (up to 1,000°C in inert atmospheres)
     • Self-lubricating in wet/dry conditions

4. Selection Criteria Beyond Tensile Strength

   • Consider temperature limits:
     • PTFE softens above 260°C
     • Graphite maintains integrity at higher temperatures
   • Evaluate chemical compatibility:
     • PTFE fails with molten alkali metals
     • Graphite reacts with strong oxidizers
   • Assess electrical needs:
     • PTFE is insulating; graphite conducts electricity

5. Industry Standards & Testing

   • ASTM D4894 governs PTFE testing, ensuring reproducibility
   • Graphite packing often follows API 622 for valve stem applications
   • Elongation data (300–400% for PTFE) indicates flexibility during installation

For dynamic sealing applications requiring mechanical resilience, PTFE’s higher tensile strength and elongation make it preferable. In high-temperature or conductive environments, graphite’s lower tensile strength may be offset by its thermal stability. Always cross-reference manufacturer datasheets, as additives (e.g., glass fiber in PTFE) can modify properties significantly.

Summary Table:

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