High-temperature resistance testing of sealants evaluates whether silicone, polyurethane, polysulfide, hybrid, and epoxy sealing compounds can retain adhesion, elasticity, and mechanical strength after sustained thermal exposure. Guided by ISO, ASTM, EN, GB, UL, IEC, and JIS standards, accredited third-party laboratories use oven aging, thermal cycling, and instrumental thermal analysis to qualify sealants for construction, automotive, electronics, and industrial applications. This article summarizes the sample types, test items, testing methods, and acceptance criteria involved in a professional high-temperature performance evaluation.
Technical Overview
Sealants are expected to maintain weathertight, airtight, or structurally bonded joints for service lives of 20 years or more, and elevated temperature is one of the most aggressive drivers of long-term degradation. Sustained heat accelerates oxidation, post-cure reactions, chain scission, secondary crosslinking, and the loss of plasticizers and volatile components. The visible consequences include hardening, shrinkage, chalking, surface cracking, blistering, discoloration, and ultimately cohesive or adhesive failure at the joint interface.
High-temperature resistance testing is an accelerated laboratory simulation of these mechanisms. Fully cured specimens are exposed to controlled elevated temperatures in ventilated air ovens for defined durations, and their mechanical, adhesive, and physical properties are compared before and after exposure. The resulting retention values allow manufacturers, engineers, and certification bodies to classify sealants, verify formulation changes, and confirm suitability for specific service temperature ranges.
The most commonly referenced standards frameworks include:
- ASTM C920 and ASTM C1184 — specifications for elastomeric joint sealants and structural silicone sealants, including thermal exposure requirements
- ISO 11600, ISO 8339, and ISO 8340 — classification and tensile/adhesion test methods for building sealants
- GB/T 13477 and GB 16776 — Chinese standard test methods for building sealants and the specification for structural silicone sealants
- ASTM D573 and ISO 188 — accelerated aging of rubber-like materials in air ovens
- ASTM C719 and ASTM C794 — adhesion and cohesion evaluation of sealants after movement and thermal conditioning
- IEC 60068-2-2 — dry heat testing for electronic components and assemblies
- UL 746B — long-term thermal endurance (relative temperature index) evaluation of polymeric materials
- JIS A 5758 — Japanese performance standard for building sealants
Testing Samples
Test specimens are typically prepared from freshly manufactured product, cured under standard laboratory conditions (usually 23 ± 2 °C and 50 ± 5% relative humidity) for the full cure period specified by the manufacturer. Common specimen configurations include free cured films, die-cut dumbbells, H-type tensile specimens, cured plaques for hardness testing, and sealant beads applied to representative substrates such as glass, anodized aluminum, steel, concrete, or printed circuit board laminates.
Silicone Sealants
One-part (RTV-1) and two-part (RTV-2) silicone sealants, in both neutral and acetoxy cure systems, offer the highest heat capability among common sealant families, with continuous service ratings typically up to 150 °C and specialty high-temperature grades rated to 260 °C or beyond. Heat aging tests focus on post-cure stability, depolymerization resistance, and retention of elastic recovery.
Polyurethane Sealants
Polyurethane and modified polyurethane sealants generally serve in the -40 °C to +90 °C range. Thermal aging tests reveal oxidation of the polyol backbone, secondary isocyanurate reactions, and hardening that reduces movement capability in construction and vehicle body joints.
MS Polymer and Hybrid Sealants
Silyl-modified polyether and polyurethane hybrid sealants typically carry continuous service limits near 100 °C. Heat aging evaluation verifies the stability of the siloxane crosslink network and detects plasticizer migration or loss.
Polysulfide Sealants
Polysulfide sealants are widely used in fuel tank and glazing applications, with conventional grades rated near 90–120 °C and aerospace grades higher. Testing emphasizes heat-aged adhesion, fuel resistance after thermal exposure, and compression set.
Epoxy and Specialty Heat-Resistant Sealants
Epoxy, silicone-epoxy, and other engineered high-temperature sealants used in industrial equipment and power electronics are evaluated for glass transition shift, decomposition onset, and retention of bond strength after aging at 150–250 °C.
Butyl and Hot-Melt Sealants
Butyl and hot-melt sealants have lower thermal limits, so testing concentrates on softening, cold flow, and mass loss at moderate elevated temperatures relevant to insulated glazing and HVAC service.
Testing Items
A typical high-temperature resistance evaluation covers the following test items, measured before and after defined thermal exposures:
- Mass loss (weight loss) after oven heat aging, indicating plasticizer and volatile loss
- Shore A hardness change per ASTM D2240, ISO 7619-1, or ASTM C661
- Tensile strength retention
- Elongation at break retention
- Modulus change (for example, 100% and 200% modulus)
- Adhesion-in-peel strength after thermal exposure with quantified adhesive failure area
- Adhesion and cohesion performance after cyclic movement combined with temperature conditioning
- Surface condition assessment: cracking, crazing, chalking, blistering, and discoloration
- Dimensional change and shrinkage after heating
- Compression set at elevated temperature for gasket and flange sealing applications
- Thermal decomposition onset temperature by thermogravimetric analysis (TGA)
- Glass transition temperature and residual cure behavior by differential scanning calorimetry (DSC)
Testing Methods
Oven Heat Aging
Fully cured specimens are placed in forced-ventilation air ovens in accordance with ASTM D573 or ISO 188. The aging temperature is selected according to the sealant class and declared service temperature, with common set points of 70 °C, 80 °C, 90 °C, 100 °C, 125 °C, 150 °C, 200 °C, and 250 °C, and exposure durations ranging from 72 hours to 1000 hours or longer. After aging, specimens are conditioned back to 23 °C and 50% relative humidity before mechanical testing, and results are expressed as percentage retention relative to unaged controls.
Tensile Property Testing Before and After Aging
H-type tensile specimens are tested in accordance with ISO 8339 or ASTM C1135, while die-cut dumbbells may be evaluated per ASTM D412 or ISO 37. Testing is performed at 23 ± 2 °C at the specified crosshead speed, and tensile strength, elongation at break, and modulus are recorded. Governing specifications define minimum retention limits, and a substantial loss of elongation is typically the earliest and most sensitive indicator of thermal degradation.
Hardness and Mass Loss Determination
Hardness is measured on aged and unaged cured plaques using a Shore A durometer per ASTM D2240, ISO 7619-1, or ASTM C661. Excessive hardening indicates over-crosslinking or oxidation, while softening can signal plasticizer migration or chain scission. Mass loss is determined by weighing specimens to 1 mg precision before and after aging, with typical acceptance limits in the range of a few percent depending on the product standard.
Adhesion-in-Peel and Cohesion After Movement
Adhesion-in-peel testing per ASTM C794 evaluates bond strength to representative substrates after heat exposure and quantifies the percentage of adhesive failure at the interface. Cyclic movement testing per ASTM C719 subjects sealed joints to repeated extension and compression combined with water immersion and temperature extremes, verifying that heat-aged sealants can still accommodate joint movement without cracking or debonding.
Thermal Cycling and Dry Heat Testing
For assembled components and electronics applications, dry heat testing per IEC 60068-2-2 (Test Bb) exposes sealed assemblies to elevated constant temperature, followed by functional and leak inspection. Thermal cycling between high temperature and low temperature (for example, between 125 °C or 150 °C and -40 °C) imposes differential expansion stresses between the sealant and substrate, revealing fatigue cracks and interfacial delamination that constant heat alone may not produce.
Instrumental Thermal Analysis
Thermogravimetric analysis per ISO 11358 or ASTM E1131 determines the decomposition onset temperature and mass-loss steps, establishing the fundamental thermal ceiling of the formulation. DSC per ISO 11357 measures glass transition temperature and residual cure exotherms, and thermomechanical analysis can characterize expansion behavior. These data support long-term temperature ratings, including inputs to relative temperature index programs under UL 746B.
Applications
High-temperature resistance testing supports qualification and quality control across a broad range of industries:
- Structural glazing and curtain walls — verifying heat-aged tensile strength and adhesion of structural silicone sealants used in facade assemblies
- Automotive engineering — engine compartment sealing, headlamp housings, underhood electronics, and transmission components exposed to 125–180 °C service temperatures
- Electronics and power modules — potting compounds, PCB gaskets, LED drivers, and thermal interface sealing validated by dry heat endurance testing
- Aerospace — fuel tank sealants, access panel sealing, and firewall barrier sealants after thermal aging
- Industrial equipment — ovens, furnaces, flue gas ducts, boilers, and solar thermal installations
- Petrochemical and pipeline systems — flange and joint sealants exposed to sustained process heat
FAQ
What aging temperature should be selected for testing?
The aging temperature is chosen based on the declared continuous service temperature with an appropriate safety margin, and it may be fixed by the governing product standard. Building sealants are commonly evaluated at 70–100 °C, automotive grades at 125–180 °C, and specialty silicones at 200–300 °C, with exposure times adjusted to accelerate degradation realistically without introducing non-representative failure modes.
How are pass and fail criteria defined?
Acceptance criteria combine minimum retention of tensile strength and elongation, limits on hardness change and mass loss, a maximum permitted adhesive failure area in peel testing, and the absence of cracking or crazing after aging. The specific limits depend on the applicable specification, such as ASTM C920, ASTM C1184, ISO 11600, GB 16776, or JIS A 5758.
How long does a high-temperature resistance test take?
A typical program includes 1 to 4 weeks of specimen curing, 3 days to 6 weeks of oven aging, and several days of mechanical testing, so the total turnaround is usually 2 to 8 weeks. Long-term thermal endurance programs, such as relative temperature index studies under UL 746B, can require thousands of hours of exposure.
Is constant heat aging better than thermal cycling?
The two methods address different degradation mechanisms. Constant heat accelerates chemical aging such as oxidation, post-cure, and volatile loss, while thermal cycling superimposes mechanical stress from differential thermal expansion. Many specifications require both to demonstrate chemical stability and movement capability under realistic service conditions.
Can testing be performed on finished products or assemblies?
Yes. Sealant beads on actual components, cut sections from cured joints, and complete sealed assemblies can all be evaluated, particularly for dry heat and thermal cycling endurance. Standard laboratory specimens, however, provide better reproducibility and are preferred for quantitative classification and certification purposes.
What are the most common high-temperature failure mechanisms in sealants?
Typical mechanisms include plasticizer and volatile loss, oxidative crosslinking, chain scission, depolymerization in some silicone systems, hydrolysis, and oxidation of the substrate interface. These mechanisms manifest as hardening, shrinkage, loss of elongation, surface cracking, and progressive adhesion failure at the bond line.