What is CTE ?
Coefficient of thermal expansion (CTE) measures how a material’s size changes with temperature, typically denoted by the symbol alpha (α). It’s defined as the fractional change in size per degree change in temperature at constant pressure, with units of 10^-6/K or 10^-6/°C. CTE varies significantly between materials, with metals having higher values, ceramics lower, and polymers often the highest. It also varies with temperature, typically increasing with rising temperature. Understanding CTE is crucial in engineering and design applications, including construction, precision instruments, electronic components, and aerospace materials, and can be measured using techniques like dilatometry and interferometry. The coefficient of thermal expansion (CTE) significantly impacts the performance and effectiveness of Teflon encapsulated O-rings in several ways.
Impact on Dimensional Stability
Teflon encapsulated O-rings consist of a Teflon (PTFE) outer layer encapsulating an elastomer core, typically silicone or FKM (Viton). The coefficient of thermal expansion affects how these materials expand or contract with temperature changes
- Teflon (PTFE) CTE: Approximately 70 × 10^-6 °C^-1
- Silicone CTE: Approximately 310 × 10^-6 °C^-1
- FKM (Viton) CTE: Approximately 160 × 10^-6 °C^-1
These differing CTEs can lead to dimensional changes in the encapsulated O-ring as temperatures fluctuate, potentially affecting sealing performance.
Sealing Properties
The coefficient of thermal expansion directly influences the sealing properties of Teflon encapsulated O-rings
- As temperature changes, the O-ring may expand or contract differently than the surrounding materials, potentially causing gaps or excessive compression.
- Improper CTE matching could lead to seal failure, resulting in leaks and system malfunctions.
Temperature Range Considerations
Teflon encapsulated O-rings are often used in applications requiring chemical resistance and high-temperature tolerance. CTE becomes particularly important when dealing with extreme temperatures:
- These O-rings can typically withstand temperatures from -76°F to +400°F (-60°C to +204°C) for FEP encapsulation, and up to +500°F (+260°C) for PFA encapsulation
- The different thermal expansion rates of the Teflon shell and elastomer core must be considered across this wide temperature range to maintain sealing effectiveness.
Design Implications
Engineers must account for coefficient of thermal expansion when designing systems using Teflon encapsulated O-rings
- Special groove design considerations may be necessary to accommodate the O-ring’s thermal expansion and contraction.
- The stiffness of Teflon compared to solid rubber O-rings requires additional design attention.
Material Selection
Understanding CTE helps in selecting the appropriate core material for specific applications
- Silicone cores offer better low-temperature flexibility.
- FKM (Viton) cores provide increased chemical and compression set resistance.
Furthermore, the effect of CTE on O-rings should be considered not only for their dimensional stability but also for their sealing properties. In other words, a seal with an inappropriate CTE could fail, causing leaks and system malfunctions. Therefore, comprehensive research into the CTE of Teflon® encapsulated O-rings is indispensable for engineers and designers working with these components.
M-Cor Inc.’s engineering department has conducted substantial and proprietary research. This research provides valuable insights into the CTE of Encap-O-Seal™ Teflon® encapsulated O-rings. It is not only unique but also applicable, offering a broad-spectrum discussion of the findings and equations to calculate the CTE of various encapsulated O-rings. This knowledge of O-ring behavior under thermal stress is vital for anyone involved in designing and maintaining high-performance sealing systems.
By considering the CTE of Teflon encapsulated O-rings, engineers can ensure optimal performance and reliability in sealing applications across various temperature ranges and environmental conditions.