Tensile strength testing of rubber after aging is a critical quality control procedure that evaluates how thermal, oxidative, ozone, and environmental exposure affect the breaking strength and elongation properties of vulcanized rubber materials. This article provides a comprehensive technical overview of the aging test methods, applicable standards such as ISO 188, ISO 37, ASTM D412, ASTM D573, and GB/T 3512, sample preparation requirements, calculation of tensile strength change rate, and typical industry applications. Whether you are a rubber product manufacturer, quality engineer, or procurement specialist, this guide will help you understand how accredited third-party laboratories verify the long-term durability of rubber compounds and finished products.
Technical Overview
Rubber is a polymeric elastomer whose macromolecular chains are susceptible to degradation when exposed to heat, oxygen, ozone, ultraviolet radiation, moisture, and chemical media over time. This degradation process, commonly referred to as rubber aging, leads to chain scission, crosslink density changes, surface cracking, hardening or softening, and ultimately a measurable decline in mechanical performance. Among all mechanical properties, tensile strength (breaking strength) and elongation at break are the most widely used indicators for quantifying the degree of aging.
Tensile strength testing after aging involves subjecting rubber test pieces to accelerated aging conditions in laboratory equipment, followed by tensile testing to determine the retained breaking strength and the percentage change compared with unaged properties. The test result is typically expressed as the tensile strength change rate or retention rate, which serves as a key acceptance criterion in material specifications, product standards, and supplier qualification programs.
The most authoritative test methods are published by international and national standards organizations, including ISO 188 and ISO 37 (International Organization for Standardization), ASTM D412 and ASTM D573 (ASTM International), and the Chinese national standards GB/T 3512 and GB/T 528. Accredited third-party laboratories routinely perform these tests for material certification, product compliance, and research and development purposes.
Testing Samples
Standard Dumbbell Test Pieces
Dumbbell-shaped specimens are the most common sample form for tensile strength evaluation. They are die-cut from flat vulcanized rubber sheets with a thickness of 2.0 mm ± 0.2 mm. The standard specifies several specimen types with different narrow-section widths and gauge lengths, and the same specimen type must be used for both the unaged control group and the aged group to ensure comparability of results.
Ring Test Pieces
Small ring specimens, typically with an internal diameter of 44.6 mm and a cross-sectional area of approximately 4 mm by 6 mm, can be used as an alternative to dumbbells. Rings are stretched over pulleys on the tensile machine, and their results are not directly interchangeable with dumbbell results, so a note distinguishing the specimen type must accompany the report.
Cut Specimens from Finished Products
When evaluating actual products such as seals, hoses, vibration isolators, or tire components, specimens may be cut directly from the finished article provided that a uniform test section can be obtained. The thickness of product-derived specimens is measured at three points within the gauge length, and the median value is used for cross-sectional area calculation.
Sample Preparation Requirements
- Testing should not begin earlier than 16 hours after vulcanization, with a preferred interval of 16 to 96 hours between vulcanization and testing.
- Test pieces must be conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for at least 3 hours prior to tensile testing after aging.
- Specimens must be free of surface defects, air bubbles, mechanical damage, and non-uniform thickness that could act as stress concentrators.
- A minimum of three specimens is required for each condition, and five specimens are recommended to improve statistical reliability.
Testing Items
The core testing program for aged rubber tensile evaluation includes the following measurable parameters:
- Tensile strength before aging (breaking strength of the unaged control group), expressed in megapascals (MPa).
- Tensile strength after aging, measured on specimens exposed to the specified aging condition.
- Elongation at break before and after aging, expressed as a percentage.
- Tensile stress at a given elongation (for example, 100% and 300% modulus) before and after aging.
- Tensile strength change rate, calculated as the percentage difference between aged and unaged values.
- Tensile strength retention rate, expressed as the ratio of aged strength to unaged strength.
- Elongation change rate and hardness change after aging, when required by the product specification.
The tensile strength change rate is calculated as follows: the difference between the median aged tensile strength and the median unaged tensile strength, divided by the median unaged tensile strength, multiplied by 100%. A negative value indicates degradation, and a positive value indicates post-aging crosslinking or hardening, which may also be undesirable depending on the application.
Testing Methods
Hot Air Accelerated Aging
Hot air oven aging is the most widely applied preconditioning method and is performed in accordance with ISO 188, ASTM D573, and GB/T 3512. Specimens are suspended in a forced-ventilation aging oven at elevated temperature for a defined period. Commonly selected conditions include 70 °C, 100 °C, 125 °C, or 150 °C for durations of 24, 48, 72, 96, or 168 hours, depending on the service temperature class of the compound. After aging, specimens are cooled to room temperature and tensile tested within 16 to 96 hours, in strict accordance with ISO 37, ASTM D412, or GB/T 528.
Ozone Aging Followed by Tensile Testing
Ozone resistance testing is conducted per ISO 1431, ASTM D1149, and GB/T 7762. Specimens are elongated to a specified strain, commonly 20%, and exposed to an ozone atmosphere at a concentration of 50 pphm at 40 °C under either static or dynamic conditions. The test evaluates surface crack formation, and tensile testing after exposure quantifies the reduction in breaking strength caused by ozone-induced crack growth.
Artificial Weathering and UV Aging
For products intended for outdoor service, specimens are exposed to xenon-arc lamps or fluorescent UV lamps with condensation cycles according to ISO 4892, ASTM G154, or GB/T 16422. After the specified exposure energy or cycle duration, tensile strength retention is determined to assess photodegradation resistance.
Liquid Immersion Aging
When rubber contacts oils, fuels, coolants, or chemicals, immersion aging is performed according to ISO 1817, ASTM D471, or GB/T 1690. Specimens are immersed in the reference fluid at a specified temperature and duration, wiped dry, and then tested for tensile strength and elongation change to quantify chemical degradation and swelling effects.
Tensile Testing Procedure
The tensile test itself is performed on a calibrated universal testing machine equipped with pneumatic or mechanical grips and an extensometer. Dumbbell specimens are pulled at a crosshead speed of 500 mm/min as specified by ISO 37 and GB/T 528. The maximum force at break is recorded, and tensile strength is calculated by dividing this force by the original cross-sectional area of the specimen. Median values from replicate specimens are used for reporting, and the change rate is computed against the unaged control group tested under identical conditions.
Applications
Tensile strength evaluation after aging is essential across numerous industries where rubber components must retain mechanical integrity throughout their service life:
- Automotive industry: engine mounts, seals, gaskets, coolant hoses, brake components, and tire compounds requiring validated heat and fluid aging resistance.
- Aerospace engineering: seals, grommets, and vibration isolators qualified under stringent thermal aging and ozone exposure requirements.
- Construction sector: bridge bearings, expansion joints, waterproofing membranes, and window gaskets exposed to outdoor weathering.
- Electrical and cable industry: insulation and sheathing compounds for wires and cables requiring long-term thermal endurance verification.
- Pipeline and chemical processing: linings, diaphragms, and hose materials in contact with aggressive media.
- Medical and consumer products: latex and elastomeric components that must maintain performance after sterilization or shelf-life aging.
- Material research and development: formulation comparison, antioxidant efficiency screening, and shelf-life prediction using time-temperature extrapolation.
FAQ
What is an acceptable tensile strength change rate after aging?
Acceptance criteria depend on the product standard and application. Typical specifications require the tensile strength change rate to remain within minus 10% to minus 25%, or the retention rate to be no lower than 75% to 90% after the specified aging condition. Always verify the requirement against the applicable product standard or customer specification.
Which aging temperature and duration should be selected?
The aging condition should reflect the intended service environment. For general-purpose rubber, 70 °C for 72 hours or 100 °C for 24 hours is common, while heat-resistant compounds may require 125 °C or 150 °C exposures. Elevated-temperature accelerated aging is often used with the Arrhenius approach to estimate service life at lower operating temperatures.
Can dumbbell and ring specimen results be compared directly?
No. Ring and dumbbell specimens produce systematically different tensile strength values due to different stress distributions. Results from different specimen types must not be compared directly, and all comparative testing, aged versus unaged, must use the same specimen geometry.
How long after aging removal should tensile testing be performed?
According to ISO 188 and GB/T 3512, tensile testing should be carried out no sooner than 16 hours and no later than 96 hours after removal from the aging oven, with specimens conditioned at standard laboratory temperature beforehand.
Why does tensile strength sometimes increase after aging?
For certain sulfur-cured compounds, moderate thermal aging can increase crosslink density, producing an initial rise in tensile strength and hardness accompanied by reduced elongation at break. This apparent strengthening usually precedes further degradation, which is why elongation change and modulus change should be evaluated together with tensile strength for a complete aging assessment.
Which standards govern the complete test sequence?
The complete sequence is governed by paired standards: ISO 188, ASTM D573, or GB/T 3512 for hot air aging; ISO 37, ASTM D412, or GB/T 528 for tensile testing; and ISO 1431, ASTM D1149, or GB/T 7762 for ozone aging. Accredited third-party laboratories can perform the full test program and issue test reports recognized for product certification and regulatory compliance.