Sealant tensile strength testing is a critical quality control and conformity assessment procedure that measures the maximum tensile stress a cured sealant can sustain before failure. Performed by accredited third-party laboratories in accordance with recognized standards such as ISO 8339, ASTM C1135, and ISO 37, this testing verifies joint integrity, adhesion performance, and long-term durability for construction, curtain wall, insulating glass, automotive, and industrial sealing applications.
Technical Overview
Sealant tensile strength refers to the maximum tensile stress, expressed in megapascals (MPa), that a cured sealant can withstand before rupture when stretched in a controlled laboratory test. It is one of the most fundamental mechanical performance indicators for elastomeric jointing materials, alongside elongation at break, modulus, and adhesion characteristics. Tensile strength directly reflects the crosslink density, formulation quality, and cure completeness of the sealant, and it governs whether a sealed joint can safely absorb thermal movement, wind loading, substrate displacement, and long-term environmental stress.
In practice, sealant tensile strength testing falls into two complementary families of methods. The first evaluates the bulk (homogeneous) properties of the cured sealant using die-cut dumbbell specimens, typically in accordance with ISO 37 or ASTM D412. The second evaluates tensile behavior in a joint configuration, where the sealant is bonded between two substrate plates and pulled until failure, following standards such as ISO 8339, ISO 8340, and ASTM C1135 for structural sealants. National and regional standards, including GB/T 13477 and GB 16776 in China and EN standards in Europe, align closely with these ISO references and add product-specific requirements.
Test results are used for product classification under systems such as ISO 11600, ASTM C920, and GB/T 14683, for factory production control, for type testing and certification, and for acceptance of project-specific applications. Independent verification by accredited third-party laboratories is essential whenever results support structural design, regulatory compliance, or contractual acceptance, because specimen preparation and failure-mode interpretation strongly influence reported values.
Testing Samples
Bulk Sealant Dumbbell Specimens
For evaluating the intrinsic strength of the cured material, the sealant is dispensed into a flat mold to produce a cured sheet, typically at least 2 mm thick. Dumbbell-shaped specimens are then die-cut or knife-cut from the sheet in accordance with ISO 37 (Type 1, 2, or 3 dumbbells) or ASTM D412 (Die A through F, with Die C being widely used). This specimen geometry concentrates deformation in a defined gauge section so that failure occurs away from the grips, allowing accurate determination of tensile strength and elongation at break of the polymer itself.
Adhesion-in-Joint (H-Type) Specimens
For joint performance, sealant is extruded between two parallel substrate blocks to form a bead of defined cross-section, commonly 12 mm by 12 mm with an approximately 50 mm free length as specified in ISO 8339. Substrates are selected to represent the actual project materials, such as float glass, anodized or coated aluminum, stainless steel, or concrete. ASTM C1135 uses a similar H-shaped configuration for structural silicone sealants. These specimens simultaneously test the cohesive strength of the sealant body and the adhesive bond to the substrate, and the recorded failure mode is as informative as the numerical strength value.
Project-Specific and Production Samples
For curtain wall projects and insulating glass production, samples are often prepared directly from site-mixed or machine-dispensed material to verify mixing homogeneity, ratio accuracy, and cure behavior under real installation conditions. Retained batches, field-cured coupons, and witness panels are tested alongside standard specimens when dispute resolution or retrospective quality assessment is required.
Specimen Preparation and Curing
- Two-part sealants must be mixed at the manufacturer's specified ratio using calibrated equipment, with air entrapment avoided during dispensing.
- One-part sealants are typically conditioned and cured for 21 to 28 days at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291.
- Two-part systems may be tested after a shorter conditioning period, such as 14 days, followed by post-cure verification when specified.
- A sufficient number of specimens, commonly three to five per condition, is prepared to allow statistical evaluation as required by the governing standard.
Testing Items
Depending on the applicable standard and the purpose of the evaluation, a sealant tensile strength test program may include the following measured and derived parameters:
- Tensile strength: the maximum stress sustained up to rupture, calculated as the peak force divided by the original cross-sectional area of the specimen.
- Maximum tensile force: the peak load in newtons (N), often reported for H-type joint specimens.
- Elongation at break: the extension at rupture expressed as a percentage of the original gauge length or joint free length.
- Tensile stress at specified elongation: the modulus value at reference elongations such as 25 %, 60 %, or 100 %, used for movement-capability classification.
- Failure mode assessment: the percentage of cohesive failure within the sealant body versus adhesive failure at the substrate interface.
- Retained tensile strength after conditioning: strength values measured after water immersion, heat aging, or artificial weathering, expressed as a percentage of the initial value.
- Bond performance to specific project substrates, including glass, anodized aluminum, coated metals, and concrete.
Testing Methods
Apparatus and Test Conditions
Testing is performed on a calibrated universal testing machine fitted with appropriate grips or fixtures capable of holding the substrate blocks of joint specimens without slippage or stress concentration. Load measurement accuracy is typically required to be within ±1 % of the indicated value, and an extensometer or crosshead displacement measurement is used to record strain. All specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity before testing, and tests are conducted in the same environment unless otherwise specified.
Dumbbell Test Procedure
Gauge-length marks are applied to the narrow section of each dumbbell, and the specimen thickness and width are measured at several points to determine the minimum cross-sectional area. The specimen is clamped in the grips, aligned axially to avoid bending, and extended at the high constant rate specified by ISO 37 or ASTM D412, commonly 500 mm/min for elastomeric materials. Force and extension are recorded continuously until rupture, and tensile strength, elongation at break, and stress at specified strain are calculated.
H-Type Joint Tensile Test Procedure
Joint specimens are mounted so that the sealant bead is loaded in pure tension, perpendicular to the bond surfaces. ISO 8339 specifies a comparatively slow separation rate of 5 to 6 mm/min to capture quasi-static joint behavior, while ASTM C1135 for structural sealants applies a constant rate of approximately 12.7 mm/min. After rupture, the force-extension curve provides the maximum tensile force, and the fracture surfaces are examined visually to quantify cohesive versus adhesive failure.
Calculation and Evaluation
- Tensile strength is calculated as the maximum force divided by the original minimum cross-sectional area and reported in MPa, or as kPa where joint configurations demand it.
- Elongation at break is calculated as the increase in length at rupture divided by the original length, multiplied by 100.
- Statistical treatment follows the standard, typically reporting the median or mean of valid specimens, with retesting required when premature grip failure or specimen flaws occur.
- Acceptance criteria commonly require that reported strength meets or exceeds the declared product value and that failure is predominantly cohesive, with adhesive failure beyond a small tolerated fraction triggering investigation of surface preparation, primer use, or compatibility.
Post-Aging and Durability Verification
To simulate service exposure, tensile testing is frequently repeated on specimens after immersion in water, exposure to elevated temperature, or artificial weathering with UV radiation using equipment and procedures consistent with ASTM C1442 and ISO 4892. Comparing the retained tensile strength and failure mode against initial values allows laboratories to assess degradation resistance, which is particularly important for structural silicone sealants governed by ASTM C1184 and GB 16776.
Applications
Sealant tensile strength testing supports quality assurance and design verification across a wide range of industries and end uses:
- Structural glazing and unitized curtain walls, where structural silicone sealants transfer wind and dead loads and must demonstrate high tensile bond strength with cohesive failure.
- Insulating glass units, where the secondary edge seal carries mechanical loads and moisture-barrier functions, and tensile verification confirms mixing and cure quality.
- Weatherproofing joints in facades, precast concrete panels, and curtain wall perimeters, supporting movement-class designations under ISO 11600 and ASTM C920.
- Automotive, marine, electronics, and aerospace sealing, where elastomeric adhesives and sealants must maintain strength under vibration, thermal cycling, and fluid exposure.
- Factory production control and incoming material inspection, where routine tensile checks detect batch-to-batch deviation, incorrect mixing ratios, and shelf-life issues.
- Research and formulation development, providing comparative data for polymer selection, filler loading, crosslinker levels, and primer systems.
- Regulatory compliance and certification submissions, including type testing reports issued by accredited third-party laboratories for market access and tender qualification.
FAQ
What is sealant tensile strength and how is it expressed?
Sealant tensile strength is the maximum tensile stress the cured material can sustain before breaking, calculated by dividing the peak force by the original cross-sectional area of the specimen and expressed in MPa or kPa. It indicates the load-carrying capacity of both the sealant body and, in joint tests, its bond to the substrate.
Which standard applies to my sealant product?
Bulk elastomeric properties are typically evaluated with ISO 37 or ASTM D412 dumbbell specimens, while joint performance is evaluated with ISO 8339 for building sealants, ASTM C1135 for structural sealants, and the corresponding national equivalents such as GB/T 13477. Product-specific specifications, including ISO 11600, ASTM C920, ASTM C1184, and GB 16776, define the required values and conditioning sequences.
What is the difference between cohesive and adhesive failure?
Cohesive failure occurs within the body of the sealant, leaving sealant residue on both substrate surfaces, and indicates that the adhesive bond is stronger than the sealant itself. Adhesive failure occurs at the interface, with clean separation from the substrate, and suggests inadequate surface preparation, contamination, incompatibility, or absence of a required primer. Most specifications require predominantly cohesive failure.
How long must specimens cure before testing?
One-part sealants are generally cured for 21 to 28 days at 23 ± 2 °C and 50 ± 5 % relative humidity, while two-part systems may be tested after approximately 14 days unless the product standard specifies otherwise. Insufficient cure is one of the most common causes of understated tensile results.
Can sealant applied on a project site be tested?
Yes. H-type joint specimens can be prepared on site using the actual substrates, primers, cleaning procedures, and dispensing equipment, then delivered to an accredited third-party laboratory for testing. This verifies that installation conditions, mixing quality, and environmental exposure during application meet the design assumptions.
What are typical causes of low tensile strength results?
Common causes include off-ratio or poorly mixed two-part material, expired or improperly stored product, incomplete cure, voids or trapped air in the specimen, contaminated or unprimed substrates, and testing at non-standard temperature or humidity. A structured review of specimen preparation records and failure surfaces usually identifies the root cause.