Temperature Aging and Heat Cycling: How Silicone Parts Change Over Time

Silicone rubber is prized for its wide temperature range and long-term stability, but over time even silicone parts gradually age and change. Under sustained heat, silicone can undergo chemical and physical transformations: it typically hardens, loses elasticity, and shrinks slightly. For example, prolonged exposure causes extra crosslinking (making the material stiffer), volatile loss (leading to weight loss and shrinkage), and eventual drops in tensile strength and elongation. In practice, signs of silicone heat aging include:

Hardening: Shore A hardness increases, so seals become stiffer.
Loss of elasticity: Elongation-at-break declines, making rubber more brittle.
Tensile strength drop: The maximum load the material can bear falls.
Shrinkage: Parts may shrink or lose a bit of mass as low-weight components outgas.
Surface cracking: Micro-cracks or crazing can appear, especially in sealing faces.

These changes, though sometimes subtle at first, ultimately reduce sealing and insulating performance. For example, gaskets that harden and shrink can leak, and components that lose flexibility may fail under vibration.

The image below shows silicone O-rings undergoing an oven-aging test. In such accelerated tests (e.g. ASTM D573 or ISO 188) samples are held at high temperature for hours or days, and changes in hardness, tensile strength, and elongation are measured.

Silicone rubber samples in an oven at ~220 °C for accelerated aging. Standard tests heat specimens for set durations and then measure changes in tensile strength, elongation, and hardness to evaluate temperature aging of silicone.

Temperature Aging (Static Exposure)

Temperature aging refers to chemical and physical changes that occur when silicone is held at an elevated temperature for extended periods. Under static high heat (e.g. oven aging), silicone undergoes oxidative and thermal degradation: free radicals form, extra crosslinks build up, and some polymers break down. This process is often slow but accelerates at higher temperatures. In practice:

Extra crosslinking: At temperatures above ~180–200 °C, silicone side groups break down and create new bonds within the polymer. This makes the rubber progressively harder and more brittle.
Weight loss/volatilization: Heat can drive off plasticizers or small molecules, causing minor weight loss and shrinkage of the part.
Decline in strength: As crosslink density rises and structure changes, tensile strength and flexibility fall.

These effects mean that continuous use temperature must be chosen carefully. Most general-purpose silicones have a continuous-rating around 200–230 °C. Special high-temperature grades extend that up to ~250–300 °C, but even those will slowly age under extreme heat. As a rule, higher temperature means faster aging – “rubber aging is like cooking: the higher the heat, the faster it loses elasticity”. For example, at 180–200 °C silicone parts maintain flexibility for a long time, but above ~220 °C they must be treated as consumable unless high-temp formulations are used.

Typical conditions: Accelerated aging tests (ASTM D573 / ISO 188) often use ovens at 100–200 °C for tens to thousands of hours. Real-world conditions vary: an engine exhaust gasket might see 150 °C continuously, while a silicone oven door seal might see 200 °C intermittently. Even short spikes matter: one expert notes a gasket at 250 °C for 10 minutes may survive, but 250 °C for 8 hours daily causes rapid hardening and cracking.

Standard tests: Test methods like ASTM D573 (heat aging of rubber) and ISO 188 specify how to oven-age samples and measure property changes. In these tests, specimens are exposed at a given temperature for a set time, then evaluated for changes (tensile strength, elongation, hardness, etc.). For example, ASTM D573 might age samples for 168 hours at 150 °C, then compare their elongation and hardness to unaged specimens. Tensile testing is typically done per ASTM D412 (or ISO 37) to quantify strength and elongation. Compression set is measured by ASTM D395 (or ISO 815), which compresses a sample at elevated temperature for hours and sees how much permanent set remains.

Heat Cycling (Thermal Cycling)

While static aging involves steady heat, many applications subject silicone to temperature cycles (repeated heating and cooling). Thermal cycling introduces additional effects due to expansion and contraction, and can accelerate certain failures. Key points about heat cycling:

Thermal stress: Each cycle causes the silicone to expand when hot and contract when cooled. Over many cycles this can fatigue the material and its bond to surrounding parts. Repeated expansion/contraction can eventually cause cracks or weaken interfaces.
Stress relaxation: At high temperature, silicone exhibits viscoelastic stress relaxation (it loses some internal stress when held under constant strain). In cycling tests, researchers found compressive stress in LSR samples jumped during heating and relaxed at 120 °C, then nearly returned on cooling. After multiple cycles, stress relaxation was significant at high T but negligible at low T. In other words, the material softens under heat but largely recovers when cooled.
Accelerated aging: In practice, thermal cycling can accelerate overall aging. Alternating cycles mean silicone spends time at both hot and cold extremes. This causes combined fatigue and chemical effects. One review notes: “Over time, thermal cycling can accelerate the aging of elastomers, degrading their elasticity, flexibility, and resilience.”. Cyclic stresses can cause micro-cracking and increase compression set over many cycles.

Static vs. Dynamic: Because of these differences, static and dynamic aging are treated separately. Static (steady) aging is dominated by chemical changes (crosslinking, oxidation). Dynamic (cycling) aging also involves mechanical fatigue. For example, seals in an engine that repeatedly heat up and cool down will not last as long as identical seals kept at a steady intermediate temperature. In fact, dynamic seals “age faster than static” due to the cycling stresses and higher effective temperature ranges.

Effects on Material Properties

Heat exposure changes silicone’s mechanical properties significantly. Compared to unaged material, aged silicone typically shows:

Hardness: Increases over time. Heated silicones stiffen (Shore A rises), indicating extra crosslinking. For instance, a 40A grade might creep up to ~80A after many hours at high temperature, whereas a 70A grade could reach ~90A much faster. Softer grades usually harden more slowly.
Tensile Strength: Decreases. The maximum stress the rubber can bear typically falls with heat aging. Initially this drop may be gradual, but at higher temperatures (e.g. ≥200 °C) the tensile strength can decline rapidly after extended exposure.
Elongation-at-Break: Decreases. Heated silicone loses stretchability. Under aging, elongation at break typically drops steadily as the material becomes brittle. For example, studies show that phenyl silicone’s elongation gradually fell at 180 °C and plummeted at 210 °C during aging tests.
Compression Set: Increases. Silicone parts tend to take more permanent set after compression if they’ve been heat-aged. (Compression set is measured by ASTM D395/ISO 815 under constant deflection at elevated T.) Though silicone generally resists set better than many rubbers, higher temperature and time still worsen its set. For instance, a rubber sample tested for 70 h at 100 °C showed a 40% compression set (nearly half its original thickness lost). In service, higher compression set means a gasket can’t spring back fully, raising leak risk.
Flexibility/Resilience: Decreases. Overall, aged silicone is less flexible. Reduced elongation and higher hardness mean the material is tougher but less elastic. This reduces its ability to damp vibrations and maintain seals under movement.

Quantitatively, changes depend on formulation and conditions. In a benchmark test at 200 °C, a 62A silicone hardens to ~78A in ~700 h, whereas a specially stabilized silicone at 60A hardly changes (60A→63A) in the same time. The key takeaway is that even if initial data looks similar, continuous high heat can degrade properties significantly over months or years.

Test Methods and Standards

Heat-aging of silicone is assessed by standardized tests. Important standards include:

ASTM D573 / ISO 188 – Oven (static) heat aging: Samples are held at a constant high temperature (with controlled air circulation) for a set time. After aging, tensile strength, elongation, hardness, etc. are measured and compared to unaged values. These tests define material ratings for heat resistance and help predict lifetime.
ASTM D412 / ISO 37 – Tensile test: Used to measure tensile strength and elongation at break of elastomers. A dumbbell-shaped silicone specimen is pulled until it breaks. Testing before and after aging quantifies how much strength and stretch are lost.
ASTM D395 / ISO 815 – Compression set: A rubber sample is compressed (e.g. to 75% thickness) at elevated T for hours, then released. The percentage it fails to recover is the compression set. Lower % means better recovery. Typical tests use 22–70 h at ~70–100 °C (or higher) depending on intended service.
ASTM D2000 – Material specification: Identifies silicone compounds with letters (e.g. Class A or JY for silicone). Includes suffixes for heat aging performance (e.g. “CG” categories in SAE J200).
Others: ASTM D865 (heat aging in oil) or UL 94 (flammability) may be relevant for specific cases.

Using these tests, engineers can rate silicone grades for long-term heat stability. For instance, materials suppliers often provide aging curves or tabulated property retention for 70h/175 °C or 168h/200 °C exposures. These allow sourcing teams to compare “heat aging of silicone rubber” across formulations.

Applications and Examples

In real-world use, silicone parts see a variety of heat-aging scenarios. Some examples:

Automotive (Engine Bay): Under-hood gaskets, seals and hoses may see 120–150 °C continuously, with spikes up to ~200 °C. Temperature cycling (engine on/off) adds thermal stress. Over years, crankcase or thermostat seals can harden, lose compression, and begin leaking. Designers often choose high-temp silicone grades here and include >20 °C safety margins.
Medical Devices (Sterilization): Silicone tubing and gaskets are often autoclaved at 121–134 °C for minutes at a time (steam or dry heat). Repeated autoclave cycles (hundreds or thousands of cycles) gradually age the silicone: some hardening and loss of elasticity may appear after months of frequent sterilization. Although 120 °C is moderate, the cumulative hours can matter; so medical silicones are formulated for good aging stability.
Appliances (Ovens, Dishwashers): Oven door seals, cookware gaskets and heating pad interfaces see direct dry heat up to 200–260 °C intermittently (when pre-heated). Similarly, dishwashers expose seals to hot water (70–80 °C) and steam cycles. Silicone must resist both peak heat and steam aging. Over time, oven seals will harden and may eventually crack if used at the limit daily. Designers may accept that seals above ~220 °C are “consumable” and specify replacement intervals.
Electronics / Aerospace: Silicone potting compounds or encapsulants may hold up well to 150 °C continuous but still slowly degrade. Aerospace wiring uses silicone insulation that must survive cycles from -60 °C to +200 °C; here, silicone’s broad range is ideal, though one monitors elongation loss over mission life.

These cases illustrate that use context matters. For instance, a seal in a steam autoclave (short spike of 121 °C) behaves differently from a seal in a brake system continuously at 150 °C.

Material and Design Tips for Heat Resistance

To maximize silicone longevity under heat:

Choose the Right Grade: Use a high-temperature silicone if continuous use exceeds ~200 °C. Standard grades are best below ~200 °C; above that, look for specialty formulations (silicone or fluorosilicone) with stabilizers.
Durometer (Hardness): Softer silicones generally age slower under heat. A 40A silicone will take longer to harden than a 70A counterpart at the same temperature. So if high-heat flexibility is critical, consider a lower durometer grade.
Design for Less Compression: Lower compression stresses can reduce compression set. For static seals, avoid extreme squeeze and allow some expansion space. Remember silicone will relax some stress over time, so ensure enough initial compression.
Allow Temperature Margin: Do not operate at the absolute limit. Keep working T at least 10–20 °C below the material’s rated maximum to account for hot spots and aging over time.
Test in Advance: If possible, request aging test data at your target temperature. Ask suppliers for ASTM D573 or ISO 188 aging curves. This gives confidence in life expectancy for your specific conditions.
Monitor Compression Set: For dynamic seals (o-rings, rotating shafts), compression set is a key indicator. A silicone that maintains low set (well under 30%) at your service temperature is preferred. High-set materials will fail to seal over time.
Use Fillers and Additives: Many high-temp silicones contain reinforcing fillers (like silica) or antioxidants to improve thermal stability. Ensure additives don’t limit compatibility (e.g. food-safe, if needed).
Accommodate Dimensional Changes: Account for thermal expansion. In design, include appropriate clearances or use adaptive features (spring-loaded seals, expansion joints).
Regular Inspection/Replacement: In critical applications, plan maintenance. For example, if an engine gasket sees 200 °C daily, schedule inspection after a set number of service hours, since even silicone will eventually harden or crack.

Silicone’s thermal stability is much higher than most organic elastomers, but it is not infinite. By understanding temperature aging behavior and designing within limits, engineers ensure silicone parts seal and flex reliably over their intended service life.

References: Industry testing standards (ASTM D573, D412, D395; ISO 188, ISO 815) define how to measure these effects. For deeper details on heat aging and silicone performance, see sources such as Advanced Materials’ guide on thermal ageing and materials guides on silicone rubber properties.