Silicone rubber is used for seals, gaskets and extrusions because it remains flexible in both hot and cold conditions. But in high-temperature applications, the critical question is not just can it cope today but how long will it last over prolonged use? Our test data shows that a general-purpose grade can harden by more than 15 Shore A points after just 28 days at 200°C, while a high-temperature grade under the same conditions remains almost unchanged. This process is known as thermal ageing of silicone rubber and it’s the hidden factor that determines service life in high-temperature applications.
For purchasing managers, design engineers and production teams, this is a practical concern. Premature hardening leads to seals that leak, gaskets that crack and unplanned downtime in general. This article explains what thermal ageing is, how it affects silicone rubber and how to specify the right grade for longer performance.
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Understanding Thermal Ageing in Silicone Rubber
Thermal ageing of silicone rubber refers to the gradual changes that occur when the material is exposed to high heat for long periods. Silicone is chosen for its wide temperature range: general-purpose grades operate from about −60°C to +200°C, while high-temperature silicone rubber grades extend up to +300°C.
Even within these limits, long exposure causes the rubber to change. It may stiffen, lose flexibility or shrink. Seals may no longer compress properly, gaskets lose their tight fit and extruded components fail earlier than expected.
Why Thermal Ageing Matters
Understanding how silicone rubber responds to long-term heat exposure is important for two reasons:
- Performance in service – even small changes in hardness or flexibility can affect how well silicone rubber seals, protects or insulates. If this isn’t considered during material selection, components can fail earlier than expected, causing leaks, safety issues or costly downtime.
- Specifying the right material – knowing how different grades behave under heat makes it easier to match the material to the job. For short-term heat spikes up to 200°C, general-purpose grades (GP40–GP80) are often the most cost-effective choice. But where there’s continuous exposure, or in cases where equipment runs above 200°C, high temperature grades (HT40W–HT70W) give longer service life and more consistent performance.
As Villeater Gladman, Technical Director at Advanced Materials, explains:
“Thermal ageing isn’t always visible – at first. Our team understands how different grades behave over time, so we know which materials keep their properties in real-world conditions. That lets us guide customers towards grades that won’t just perform on day one, but will continue to work reliably after months or years in service.”
How Thermal Ageing Changes Physical Properties
Advanced Materials’ testing data shows that prolonged heat exposure changes the way heat resistant silicone rubber behaves in use. Signs of thermal ageing include:
- Hardening – the rubber becomes stiffer and resists compression
- Loss of stretch – elongation at break reduces, increasing the risk of cracks
- Reduced tensile strength – the material can no longer handle the same load
- Shrinkage and weight loss – parts may no longer fit as intended
- Surface cracking – sealing faces lose integrity
When ageing sets in, extruded silicone rubber profiles may harden and stop sealing effectively. Gaskets can shrink or crack, which often results in leaks and costly interruptions.
Chemical Mechanisms Behind Thermal Degradation
The thermal ageing of silicone rubber follows a series of changes when exposed to sustained heat:
- Extra crosslinking begins – at temperatures higher than 180°C, side groups on the silicone backbone break down, creating free radicals. These free radicals form new bonds within the polymer chains (crosslinking), making the material progressively harder and more brittle.
- Volatile compounds are lost – leading to slight weight loss and shrinkage of the silicone rubber part.
- Mechanical properties decline – as crosslinking and shrinkage increase, tensile strength and flexibility fall, reducing overall silicone rubber heat resistance.
Key Factors Influencing Silicone Rubber Thermal Ageing
Several conditions determine how quickly silicone rubber changes under heat:
- Operating temperature – the higher the exposure, the faster ageing occurs. The silicone rubber temperature range for general-purpose (GP) grades is around -60°C to +200°C. For harsher environments, high temperature (HT) silicone rubber extends up to +300°C.
- Grade selection – GP grades may handle short spikes, but HT grades are formulated with stabilisers that slow ageing and maintain performance.
- Initial hardness – softer grades generally last longer. Since thermal ageing of silicone rubber makes the material harder, starting lower helps preserve flexibility.
- Formulation additives – heat stabilisers improve silicone rubber heat resistance, supporting reliable performance in continuous high-temperature service.
| Access our GP and HT Technical data sheets for further information here |
Indicators and Measurement of Ageing
Thermal ageing of silicone rubber is usually monitored using standard test methods, such as Shore A hardness checks, tensile strength tests and elongation at break measurements. These tests are widely used across the industry and give a reliable picture of when material properties are declining.
The key indicators are:
- Hardness – a steady rise in Shore A hardness shows the rubber is stiffening. In seals and extrusions this means less compression and a higher risk of leaks.
- Elongation at break – when the ability to stretch drops, the rubber becomes brittle. Components under movement or vibration are most at risk.
- Tensile strength – falling strength values indicate the material can no longer resist load or stress, shortening part life.
- Visual condition – shrinkage, surface cracking or distortion confirm the loss of flexibility and sealing ability.
Advanced Materials’ ageing test data confirms the difference.
At 250°C, HT40W (40 Shore A) increased gradually in hardness and remained below 80 Shore A even after 6,000 hours, while HT70W (70 Shore A) hardened to nearly 90 Shore A in under 2,000 hours. See Chart A below:
Chart A
At 300°C, the same pattern was seen: HT40W rose steadily from 40 to about 75 Shore A over 600 hours, but HT70W jumped from 70 to almost 90 Shore A in less than 200 hours. This shows that softer high temperature grades maintain usable flexibility for far longer, while harder grades degrade much faster. See Chart B below:
Chart B
GP vs HT Silicone Rubber – Which One Lasts Longer?
General-purpose grades can handle heat up to 200°C, but high temperature silicone rubber is needed when parts are exposed to continuous or extreme conditions. These HT grades are formulated with stabilisers that slow down thermal ageing, keeping material properties stable for longer.
Advanced Materials’ test data at 200°C (a temperature still inside the stated operating range for GP grades) shows the difference clearly. Over a period of around 700 hours, GP60 hardened rapidly from 62 Shore A to nearly 78. In the same test, HT60W remained stable, only moving from 60 to about 63 Shore A. See Chart C below.
Chart C
“This performance gap matters in silicone rubber high temperature application, because a general-purpose profile may appear suitable based on datasheet values, but it will not hold up under prolonged heat. In contrast, a high-temperature silicone rubber extrusion continues to seal, flex and perform over much longer service periods.”
– Villeater Gladman, Technical Director at Advanced Materials
Practical Tips to Maximise Silicone Rubber Service Life
There are a few straightforward ways to reduce failures linked to thermal ageing of silicone rubber:
- Select the right grade – for applications running hotter than 180°C, specify heat resistant silicone rubber, rather than relying on general-purpose materials.
- Start softer – because ageing naturally increases hardness, choosing a lower Shore A at the outset helps maintain flexibility in service.
- Build in tolerances – allow for some shrinkage or stiffening in the design so parts continue to fit and seal effectively.
- Use data-driven selection – suppliers, like Advanced Materials, who provide test evidence on silicone rubber heat resistance can guide you towards materials that will last longer in your specific application.
Advanced Materials’ technicians use detailed ageing data to advise our customers. By interpreting changes in hardness, tensile strength and elongation across different grades, they can identify which materials are most suitable for continuous high-temperature use. This practical knowledge helps customers avoid costly failures and choose silicone rubber grades that will perform reliably over time.
Partnering for Reliability and Longevity
Thermal ageing of silicone rubber is the key factor that determines how long parts will last in use. Even when operating within datasheet limits, general-purpose grades can harden quickly and lose flexibility under continuous heat. By contrast, high temperature silicone rubber is formulated with stabilisers that slow down ageing, giving seals, gaskets and profiles a much longer working life.
For buyers and engineers, the practical takeaway is clear: when specifying a silicone rubber extrusion for applications such as ovens, autoclaves, fryers or any use with sustained heat, grade selection makes the difference between early failure and long-term performance.
At Advanced Materials, our team uses detailed ageing data to guide customers toward the most suitable material. This ensures that parts perform reliably, downtime is reduced and costs are controlled.
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FAQs: Thermal Ageing of Silicone Rubber
What is thermal degradation of silicone rubber?
Thermal degradation, also known as thermal ageing of silicone rubber, is the change that happens when silicone is exposed to high heat for long periods. The rubber gradually hardens, loses flexibility and its mechanical properties decline, reducing service performance.
Does silicone rubber degrade over time?
Yes. All grades change gradually with age, but heat accelerates the process. At room temperature, the effect is slow. At elevated temperatures, general-purpose grades can harden and lose flexibility in weeks, while high temperature silicone rubber maintains its properties for much longer
How does thermal oxidation ageing affect silicone rubber sealing performance?
When silicone rubber is exposed to both heat and oxygen, the material hardens faster. This is called thermal oxidation ageing, and it’s one of the main reasons seals and gaskets lose flexibility in service.
What temperature does silicone degrade?
General-purpose grades are rated up to about 200°C. Advanced Materials’ data shows that at this level, GP grades still age quickly and may fail in continuous service. For applications with repeated exposure to higher levels of heat, high temperature silicone rubber grades, formulated with stabilisers, can operate continuously up to 300°C with a slower rate of property change
Advanced Materials is a leading supplier of silicone rubber extrusions. To find out more about our silicone rubber manufacturing services contact our expert and friendly team on +44 (0)1903 713566 or sales@amsilicones.co.uk