ISO 188 Accelerated ageing heat resistance test on rubbers and elastomers

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ISO 188 Rubber, vulcanized or thermoplastic. Accelerated ageing and heat resistance tests

ISO 188 defines air‑oven (hot‑air) ageing procedures for vulcanized rubber and thermoplastic elastomers (TPEs). It specifies for conducting accelerated ageing tests and heat resistance tests on vulcanized rubber and thermoplastic elastomers, serving as a fundamental testing specification for evaluating rubber’s thermal stability and long-term service performance.

Test Principle

The core principle is to expose rubber specimens to controlled elevated temperatures, atmospheric pressure and regulated air circulation inside a hot-air oven. Physical properties of specimens are measured before and after thermal exposure, and property variations are analyzed to assess thermal degradation.

Two core test types are defined:

Accelerated Ageing Test: Specimens are exposed to temperatures higher than their actual service temperature. This accelerates oxidative degradation to simulate long-term natural ageing within a short testing period. The acceleration effect varies by rubber type and tested properties.

Heat Resistance Test: Specimens are kept at the typical operating temperature of the final product for long-duration exposure, to evaluate performance stability under real working thermal conditions.

ISO 188 Accelerated ageing heat resistance test on rubbers and elastomers

Specific test methods defined

Method	Oven type	Airflow character	Air exchange (ventilation)	Typical airflow speed	Key notes
Method A	Cell oven or cabinet oven without fan (low air speed)	Essentially laminar, driven by natural convection / slow flow	3–10 changes/hour	Not fan‑forced; speed “only depends on air exchange”	Most conservative; slower temperature uniformity; risk of local O₂ depletion / volatile buildup if poorly managed
Method B	Cabinet oven with forced air circulation	Laminar flow past test pieces (minimum face to flow)	3–10 changes/hour	0.5–1.5 m/s	Workhorse for reproducible lab ageing
Method C	Cabinet oven with forced air circulation	Turbulent flow + rotating test‑piece carrier (≈5–10 rpm)	3–10 changes/hour	Avg ≈ 0.5 m/s ± 0.25 m/s	Improves exposure uniformity around pieces
Method D (added emphasis in modern practice)	Cabinet oven, turbulent flow, stationary pieces	Turbulent	>30 changes/hour	0.25–3.0 m/s	Intended mainly for short‑term exposures (≤ 168 h); wider tolerance but less suited for very long runs

ISO 188 test specimen:

Specimens are prepared and conditioned per ISO 23529.

Only specimens with the same dimensions and exposed area should be compared.

Identify pieces with a method that survives exposure and does not contaminate airflow (e.g. heat‑resistant tags / thread; some inks bleed or affect ageing).

ISO 188 Rubber Accelerated ageing heat resistance test equipment and device:

General Ageing heating Oven Requirements

Volume rule: The total volume of test specimens shall not exceed 10% of the oven’s free internal space.

Specimen spacing: Specimens are suspended with at least 10 mm gap between each other, and a minimum 50 mm distance from oven walls (for cabinet and forced-air ovens).

Temperature control: A calibrated temperature sensor is placed near specimens to monitor actual ageing temperature. Heating chambers must not contain copper or copper alloys.

Pre-heating: Inlet air shall be heated to the target temperature with a tolerance of ±1 °C before contacting specimens.

Core Test Parameters

Temperature

Temperature tolerance: ±1 °C for temperatures ≤ 100 °C; ±2 °C for 125 °C to 300 °C (per ISO 23529).

Resolution: 0.1 °C for Method A/B/C; 1 °C is acceptable for Method D.

Rule: Excessively high ageing temperatures may alter rubber degradation mechanisms and invalidate test results.

Calibrated sensors near specimens are used to set oven temperatures.

Air Circulation Parameters

Air exchange rate: 3–10 changes/hour (Method A/B/C); >30 changes/hour (Method D).

Air velocity: 0.5–1.5 m/s (Method B); 0.5 m/s ± 0.25 m/s (Method C); 0.25–3.0 m/s (Method D).

Ageing Duration

Determined by rubber type, test purpose and product specifications.

The duration shall not cause irreversible damage to specimens to ensure post-ageing property measurement is feasible.

Method D is limited to a maximum exposure of 168 hours.

Key Test Stipulations

Atmosphere: All tests are conducted under standard atmospheric pressure; no light exposure during ageing.

Equipment Calibration: All ovens, thermometers and timers must be calibrated regularly in accordance with Annex D (Normative Calibration Plan) and ISO 18899.

Post-Ageing Conditioning: After ageing, specimens are cooled and conditioned in a strain-free state for 16 hours to 6 days in ambient air before property testing.

Property Selection: If no specific properties are required, the standard mandates testing four core indicators: tensile strength, stress at intermediate elongation, elongation at break (per ISO 37) and hardness (per ISO 48-2).

ISO 188 Rubber Heat Ageing Test Procedure:

1, Oven Preheating: Heat the selected oven to the target ageing temperature and stabilize it.

2, Specimen Placement: Suspend specimens inside the oven following spacing rules; ensure specimens are strain-free and fully exposed to hot air. For multi-cell ovens (Method A), place only one rubber compound per cell.

3, Ageing Exposure: Close the oven and start timing for the specified duration under set temperature and airflow conditions.

4, Specimen Removal & Conditioning: After ageing, take out specimens and condition them in ambient air for 16 hours to 6 days without applied strain.

5, Property Testing: Measure physical properties of both unaged and aged specimens per corresponding ISO standards.

6, Data Calculation & Recording: Compute property changes; compile all data into a formal test report.

ISO 188 Test Application (Industry Fields):

Because it’s the canonical hot‑air thermal durability screen, you’ll see it everywhere rubber/TPEs must survive heat:

Automotive: engine‑bay seals/gaskets, hoses, mounts, damping bushes, CVJ boots, wire‑harness insulation (validates material capability at 70–150°C+).

Aerospace / defense: seals & elastomeric components exposed to high skin temps or hot compartments.

Industrial: conveyor belts, rolls, industrial hose, pump/valve seals, oilfield packers.

Electrical / cable: insulation/sheath ageing qualification.

Appliance & consumer: oven‑door seals, dishwasher/water‑heater seals, kettle/gasket parts.

R&D + QC: compound development (antioxidant/plasticizer/cure system optimization) and batch release.

Related test standard:

ISO 37	Rubber, vulcanized or thermoplastic – Determination of tensile stress-strain properties (for tensile strength and elongation tests).
ISO 48-2	Rubber – Determination of hardness (Part 2: 10 IRHD to 100 IRHD) (for rubber hardness testing).
GB/T 3512	Rubber, vulcanized or thermoplastic―Accelerated ageing and heat resistance tests

Electricity hot blast drying oven is mainly used to measure the dimension change of plastic material after heating, suitable for plastic pipe, plastic sheet etc.

Q1. What is ISO 188 and why is it so widely used? What does ISO 188 actually do, and why do most rubber specifications reference it?

A: ISO 188 is the international standard for hot‑air oven ageing of rubber and thermoplastic elastomers. It exposes test pieces to elevated temperatures under controlled airflow, then measures how key properties (hardness, tensile strength, elongation, modulus) change.

It is important because:

Rubber often fails due to thermal oxidation long before mechanical wear.

It provides a repeatable, comparative screen for material selection, formulation tweaks, and incoming QC.

Without it, you cannot objectively compare different compounds or suppliers.

Q2. Is ISO 188 an “accelerated life test” that predicts real service life? Can I use ISO 188 results to guarantee a product will last 10 years?

A: Not directly. ISO 188 is a simulation, not a perfect predictor. The standard itself warns:

Ageing in an oven ≠ ageing in a real application (which may include stress, ozone, UV, fluids, or dynamic movement).

Different polymers respond differently to temperature.

To estimate service life or maximum use temperature, you must test at multiple temperatures and apply Arrhenius analysis per ISO 11346. Even then, it remains a qualified estimate, not a warranty.

Q3. What’s the difference between “accelerated ageing” and “heat resistance” in ISO 188? Are these two different tests?

A: Same procedure, different temperature intent:

Accelerated ageing: Use a temperature higher than the expected service temperature to speed up degradation (e.g., 125°C for a part running at 90°C).

Heat resistance test: Use the actual service temperature to confirm the material survives its real operating condition.

Both follow the same ISO 188 steps—only the chosen temperature differs.

Q4. Do I really have to worry about oven copper parts? The standard says “no copper or copper alloys in the heated chamber.” Is that serious?

A: Yes. Copper acts as a pro‑oxidant catalyst for many rubbers and can drastically accelerate degradation, giving misleading failure times. Even small copper fixtures or racks can ruin test validity.

Q5. Can I put different rubber types in the same oven at the same time? To save time, can I age EPDM, NBR, and Silicone together?

A: Strongly discouraged. The standard warns against mixing compounds because:

Plasticizers, antioxidants, sulfur, peroxides, etc., can migrate through the air and cross‑contaminate other samples.

If unavoidable, only mix materials with the same polymer family, cure system, antioxidant type, and plasticizer type.

Best practice: use separate ovens or dedicated cells.

Q6. Why does the standard care so much about air speed? Isn’t temperature enough?

A: No. Air speed affects:

Oxygen supply (too slow → oxygen depletion, slower ageing).

Volatile removal (too slow → degradation products accumulate).

Oxidation rate (too fast → faster ageing, plasticizer loss).

That’s why ISO 188 defines strict airflow ranges and requires air‑speed measurement (Appendix A) for Methods B, C, and D.

Q7. Why is ISO 188 important for my industry? I make automotive seals—why should I care?

A: Because under‑hood temperatures keep rising, and heat‑ageing failures cause leaks, noise, and safety issues. ISO 188 helps you:

Select the right compound for 125°C+ environments.

Set realistic specification limits (e.g., “≤30% tensile loss after 1000 h at 125°C”).

Avoid premature field failures and warranty claims.

It is equally critical in aerospace, industrial machinery, cables, appliances, and any product where rubber sees heat.

Q8: How to measure air velocity inside forced-air circulation ovens?

A: Fix a transparent plastic plate on the oven door, insert an anemometer through reserved holes, and measure wind speed at 9 specified positions at the height of specimen centers. Calculate the average value as the final wind speed.

Q9: How to determine ageing temperature and duration?

A:For heat resistance tests: The temperature is set to the actual long-term operating temperature of the product.

For accelerated ageing tests: Select temperatures higher than service temperature (multiple temperatures are recommended for material comparison).

Ageing duration is determined by product specifications or agreements between parties. The duration shall not cause irreversible damage to specimens to ensure post-test property measurement.

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