Heat Resistant Silicone Rubber: How to Select Compounds for 200°C to 300°C Service

When elastomers are specified for high-temperature applications, failure can occur rapidly if the material is not properly matched to the service conditions. For instance, an elastomer rated for 150°C may harden, crack, and lose sealing force within weeks when exposed to temperatures of 220°C. This guide addresses the critical challenge of selecting the right heat resistant silicone rubber compounds for continuous service in the 200°C to 300°C range. It is designed for mechanical engineers and procurement professionals sourcing elastomers for applications such as oven and appliance gaskets, turbocharger and charge-air hoses, steam sterilization seals, industrial high-temperature gaskets, and EV battery cooling loops near hot zones. By understanding continuous service temperature ratings and the factors that contribute to material degradation, you can make informed decisions to ensure long-term performance and reliability.

Service Temperature Ratings: Standard VMQ, Phenyl Silicone PVMQ, and Fluorosilicone

When selecting heat resistant silicone rubber compounds, it is crucial to understand the difference between continuous service temperature ratings and short-term peak temperatures. Continuous service ratings, which are based on extended exposure (thousands of hours), provide a more realistic indication of material performance in high-temperature environments. Here are the key silicone rubber grades and their temperature ratings:

These ratings assume air oven aging per ASTM D573. For more detailed information on silicone rubber compounds, visit our heat resistant silicone rubber compounds page.

What Actually Fails at High Temperature: Oxidation, Hardening, and Compression Set

At high temperatures, silicone rubber can degrade through two main paths: thermal oxidation and reversion. Thermal oxidation, which is dominant above approximately 200°C in air, involves the oxidation of methyl side groups. This process crosslinks the polymer network, leading to hardening, embrittlement, and surface cracking. Reversion, on the other hand, is dominant in confined hot air or steam-poor environments and involves chain scission, which results in softening and tackiness. Both degradation paths compromise the sealing force of the material.

To measure the effects of high-temperature exposure, compression set at temperature (ASTM D395 Method B) and heat aging retention (ASTM D573) are critical metrics. For example, a good silicone rubber compound should have a compression set of ≤30-50% after 22 hours at 175°C and should retain at least 70% of its tensile strength and 60% of its elongation after 70 hours at 225°C in an air oven. The table below provides typical values for these properties:

Additionally, the thermal gravimetric analysis (TGA) onset of degradation is approximately 350°C in nitrogen. For more on the properties and test standards for high-temperature seals, visit our how to choose the right silicone rubber material page.

Compounding for Heat Resistance: Stabilizers, Cure Systems, and Processing

To enhance the heat resistance of silicone rubber, several strategies can be employed:

1. Confirm the real continuous service temperature and medium (hot air, steam, oil, or ozone-rich air each require different approaches).
2. Add 1-3 phr red iron oxide (Fe2O3) as the classic heat stabilizer (or cerium hydroxide for platinum-cured compounds) to scavenge radicals and delay hardening.
3. Choose PVMQ when service exceeds 220-250°C or low-temperature flexibility below -60°C is also required.
4. Select the appropriate cure system: peroxide DBPH (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) for thick sections and platinum-cure-compatible stabilizers for addition-cured LSR.
5. Avoid acidic fillers and sulfur contamination that poison platinum cure and accelerate aging.
6. Post-cure for 4 hours at 200°C to drive off volatiles and stabilize crosslink density.
7. Validate each lot by ASTM D573/D865 aging plus compression set at the actual service temperature.

For more on compounding for heat resistance, visit our how to choose the right silicone rubber material page.

Key Properties and Test Standards to Specify for High-Temperature Seals

When specifying high-temperature seals, several key properties and test standards should be considered. These include hardness, tensile strength, elongation at break, tear strength, compression set, heat aging, continuous service temperature, brittleness point, TGA degradation onset, specific gravity, and ozone resistance. The table below provides typical specifications for these properties:

Excellent ozone resistance (no cracking at 50 pphm) is also a key property for high-temperature seals.

Selecting the right heat resistant silicone rubber compound is crucial for ensuring the longevity and reliability of your high-temperature applications. For further assistance, please Contact Our Engineering Team.

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