Hygrothermal Tensile Testing of Plastics: Methods, Standards, and Applications

Hygrothermal tensile testing is a specialized mechanical evaluation that measures the tensile properties of plastic materials after or during exposure to combined heat and humidity conditions. This test is essential for verifying the long-term reliability and safety of plastic components in automotive, electronic, photovoltaic, and medical applications where temperature and moisture act together to degrade material performance. Accredited third-party laboratories conduct hygrothermal tensile testing in accordance with internationally recognized standards such as ISO 527, ASTM D638, and IEC 60068-2-78 to help manufacturers qualify materials and validate product designs.

Technical Overview

Hygrothermal tensile testing, also known as hot-humid tensile testing or damp heat tensile testing, determines the tensile properties of plastic materials after, or during, controlled exposure to elevated temperature and high relative humidity. Conventional tensile tests described in ISO 527-1 and ASTM D638 are performed on specimens conditioned in a standard laboratory atmosphere, typically 23°C and 50% relative humidity. Such data, however, can significantly overestimate the mechanical performance of plastics in real service environments where heat and moisture act simultaneously.

Moisture penetrating a polymer matrix acts as a plasticizer, lowering the glass transition temperature, softening the material, and reducing stiffness and strength. In polyesters such as PET, PBT, and PLA, as well as in polycarbonate and polyurethanes, absorbed water combined with heat can trigger hydrolysis, cleaving polymer chains and causing permanent embrittlement. In glass-fiber or mineral-filled compounds, moisture attacks the fiber-matrix interface and degrades coupling agents. Heat accelerates each of these mechanisms and adds physical aging and residual stress relaxation effects. Hygrothermal tensile testing quantifies these changes and provides the realistic design data required for safety-critical applications.

Two complementary methodologies are recognized. In the sequential method, specimens are first brought to a defined hygrothermal state in a climate chamber or by immersion and are then tensile tested. In the in-situ method, the tensile test is executed inside an environmental chamber fitted to a universal testing machine, capturing the stress-strain response of a fully saturated specimen at the specified temperature and humidity. Accredited third-party laboratories routinely provide both configurations.

Testing Samples

Specimen geometry is selected according to the material form and the governing product standard. All specimens should be free of flash, notches, voids, and visible defects, and should be molded under documented conditions, since processing history strongly influences moisture sensitivity.

Rigid and Semi-Rigid Thermoplastics

Injection-molded or machined dumbbell specimens conforming to ISO 527-2 Type 1A or 1B, or ASTM D638 Type I, are the most common formats. Typical materials include PA6, PA66, PBT, PET, PC, ABS, POM, PP, PE, PVC, PPS, and PEEK. Because polyamides are strongly hygroscopic, they are among the most frequent subjects of hygrothermal tensile studies.

Fiber-Reinforced and Filled Compounds

Glass-fiber or carbon-fiber reinforced and mineral-filled grades are tested using ISO 527-2 Type 1A or ASTM D638 Type I specimens. Tabbed or reinforced gripping sections may be required for high-strength compounds to prevent premature grip failure. Interface degradation under hot-humid conditions is a key failure mode investigated with these samples.

Plastic Films and Sheets

Thin films used in packaging, lamination, and photovoltaic encapsulation are tested with strip or dumbbell specimens defined in ISO 527-3. Photovoltaic encapsulant films such as EVA and POE, together with fluoropolymer-based backsheets, are commonly evaluated after 1000 hours of damp heat exposure.

Thermosets and Molding Compounds

Epoxy, phenolic, and unsaturated polyester molding compounds, as well as cast sheets, are machined or molded into ISO 527-2 Type 1B or ASTM D638 Type I or Type V specimens. Although thermosets absorb less water than polyamides, epoxy systems used in electronics and composites remain sensitive to hot-wet conditions.

Testing Items

The following tensile parameters are measured before and after hygrothermal conditioning to characterize the material response:

• Tensile strength (stress at maximum force)
• Tensile stress at yield and tensile stress at break
• Tensile strain at yield
• Tensile strain at break (elongation at break)
• Tensile modulus, determined between defined strain limits such as 0.05% and 0.25% per ISO 527-1
• Complete stress-strain curves at each conditioning state
• Moisture uptake of the specimen, determined gravimetrically
• Property retention rate, expressing each parameter as a percentage of the unconditioned reference value

Testing Methods

Initial Conditioning and Reference Testing

Reference specimens are conditioned in a standard atmosphere per ISO 291, ASTM D618, or GB/T 2918 at 23°C and 50% relative humidity, and then tensile tested to establish baseline properties. Depending on the specified protocol, companion specimens are either dried to constant mass or tested in the as-molded state.

Hygrothermal Conditioning

Exposure is performed in climatic chambers following the damp heat steady-state regime of IEC 60068-2-78 or the cyclic regime of IEC 60068-2-30. Widely used conditions include 40°C/93%RH for general electrical applications and 85°C/85%RH, the classic accelerated condition adopted in photovoltaic qualification per IEC 61215. Exposure durations typically range from 24 hours to 1000 hours or longer. For some programs, immersion in temperature-controlled water, based on the principles of ISO 62 and ASTM D570, is used instead of humid air. Specimens are weighed at intervals until the mass change between successive weighings is negligible, confirming moisture equilibrium; accelerated conditioning of polyamides may follow ISO 1110.

Tensile Test Execution

Tensile testing is performed per ISO 527-1 and ISO 527-2, ASTM D638, or the equivalent GB/T 1040 series. A calibrated universal testing machine with an appropriate load cell and grips is used. The modulus is measured at a low crosshead speed, typically 1 mm/min, using a strain gauge extensometer or video extensometer, followed by a higher speed, commonly 5 mm/min or 50 mm/min, to capture yield and break. At least five valid results per condition are required. Testing after humid conditioning is usually carried out at 23°C/50%RH within a short, defined time window after removal from the chamber to prevent moisture loss, and specimens may be sealed in moisture-barrier bags during transfer.

In-Situ Hot-Humid Tensile Testing

Where the actual design point corresponds to a hot and wet environment, the tensile test itself is conducted inside an environmental chamber integrated with the testing machine. The specimen is soaked at the set point, for example 85°C and 85%RH, until thermal and moisture equilibrium are achieved, and the stress-strain curve is recorded under those exact conditions. This method eliminates moisture desorption during transfer and reflects true service behavior.

Data Evaluation

For each parameter, the retention rate is calculated as the ratio of the mean value after hygrothermal exposure to the mean value of the reference condition, expressed as a percentage. Engineering specifications frequently require, for example, a tensile strength or elongation retention of at least 50% after 1000 hours at 85°C/85%RH. Test reports should include individual and mean values, standard deviations, conditioning details, moisture content, specimen geometry, test speeds, and complete stress-strain curves.

Applications

• Automotive: qualification of under-hood connectors, sensors, coolant-contact components, and structural parts made from polyamide and polyester compounds operating in hot, wet environments.
• Electronics and electrical engineering: reliability assessment of housings, insulating parts, cable ties, and connectors subjected to humid heat based on IEC 60068-2 specifications.
• Photovoltaics: evaluation of encapsulant films, backsheets, junction boxes, and cable materials after 1000-hour or 2000-hour damp heat aging at 85°C/85%RH in support of IEC 61215 and IEC 61730 qualification.
• Medical devices: verification of catheters, surgical instrument components, and device housings exposed to humidity and steam-based sterilization environments.
• Packaging: performance confirmation of barrier films and laminates intended for tropical climates with sustained high temperature and humidity.
• Material development: comparison of hydrolysis-resistant grades, anti-hydrolysis additives, coupling agents, and surface treatments under accelerated hot-wet conditions.

FAQ

Q: What is the difference between hygrothermal tensile testing and standard tensile testing?

A: Standard tensile testing per ISO 527 or ASTM D638 evaluates specimens conditioned at 23°C and 50% relative humidity. Hygrothermal tensile testing first exposes specimens to defined hot-humid conditions, such as 85°C and 85%RH, and then measures how much the tensile properties change. It answers how the material will actually perform in a humid thermal environment rather than in an ideal laboratory climate.

Q: What does the 85°C/85%RH condition mean?

A: It defines an exposure environment of 85 degrees Celsius with 85 percent relative humidity maintained continuously in a climatic chamber. This condition, specified in IEC 60068-2-78 and adopted by photovoltaic standards such as IEC 61215, is a widely accepted accelerated stress for evaluating moisture and heat sensitivity.

Q: How long should specimens be conditioned before tensile testing?

A: Conditioning time depends on specimen thickness and material diffusivity. Specimens are weighed periodically until equilibrium moisture content is reached; thick polyamide parts may take several weeks, while films may saturate within hours or days. Qualification programs often prescribe fixed durations such as 24 h, 96 h, 168 h, 500 h, or 1000 h instead of equilibrium saturation.

Q: Why does moisture reduce the tensile strength of plastics?

A: Absorbed water plasticizes the amorphous phase, lowering the glass transition temperature and stiffness, and it can hydrolyze ester or carbonate bonds in susceptible polymers, permanently shortening molecular chains and embrittling the material. In reinforced compounds, water also attacks the fiber-matrix interface. The net effect is usually lower strength and modulus, with either increased ductility when plasticization dominates or reduced elongation when hydrolysis dominates.

Q: Which standards are commonly applied?

A: Tensile testing follows ISO 527-1, ISO 527-2, ISO 527-3, ASTM D638, or GB/T 1040. Conditioning and atmosphere control follow ISO 291, ASTM D618, GB/T 2918, ISO 1110, ISO 62, and ASTM D570. Damp heat exposure follows IEC 60068-2-78 and IEC 60068-2-30, with sector-specific requirements in standards such as IEC 61215 for photovoltaic modules and the UL 746 series for polymeric materials in electrical equipment.

Q: Can finished products be tested instead of molded specimens?

A: Standardized specimens are preferred for comparability. However, strip specimens can often be cut from films, sheets, pipes, or finished parts, and in-situ chamber testing can evaluate complete components when representative coupons are unavailable. Any deviation from the standardized geometry should be clearly documented in the test report.