Testing the Tensile Strength Reduction Rate of Plastics After Aging: Standards, Methods, and Applica

The tensile strength reduction rate after aging is a critical durability index that quantifies how much breaking strength a plastic or elastomeric material loses following exposure to heat, UV radiation, ozone, or other degrading environments. This article explains the underlying degradation mechanisms, relevant international standards, specimen preparation, laboratory test procedures, and the calculation of the strength reduction rate, helping manufacturers and engineers select the right evaluation approach through accredited third-party laboratories.

Technical Overview

The tensile strength reduction rate (also referred to as breaking strength change rate) is a quantitative index used to evaluate the aging resistance of polymeric materials. It is defined as the relative decrease in the maximum tensile stress a specimen can sustain before fracture after exposure to a defined aging environment, compared with an unaged control specimen, and it is expressed as a percentage.

During service, plastics and elastomers undergo thermal-oxidative degradation, UV photo-oxidation, hydrolysis, ozone attack, and plasticizer or additive migration. These processes cause macromolecular chain scission, secondary crosslinking, or a combination of both, leading to embrittlement, surface cracking, and a measurable loss of load-bearing capacity. Because tensile strength is highly sensitive to such molecular-level changes, the strength reduction rate is one of the most widely used indicators of long-term durability.

The parameter is typically calculated as: Reduction Rate (%) = (σb0 − σb1) / σb0 × 100, where σb0 is the tensile strength at break of the unaged specimen and σb1 is the tensile strength at break after aging. The corresponding retention rate is σb1 / σb0 × 100. Tensile testing is generally performed in accordance with ISO 527 or ASTM D638 for plastics and ISO 37 or ASTM D412 for rubber-like materials, while the aging preconditioning is conducted according to ISO 188, the ISO 4892 series, ISO 1431, ASTM G154, ASTM G155, and the corresponding GB standards such as GB/T 1040, GB/T 528, GB/T 3512, and GB/T 16422.

Testing Samples

Thermoplastic and Thermosetting Resins

Common test materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), ABS, polyamide (PA), polycarbonate (PC), polyoxymethylene (POM), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), polyurethane (PU), epoxy, and phenolic compounds, including formulations containing stabilizers, fillers, reinforcements, and colorants.

Elastomeric and Rubber-Plastic Materials

Thermoplastic elastomers (TPE/TPV), EPDM, NBR, SBR, chloroprene rubber (CR), silicone rubber, and rubber-plastic blends are frequently evaluated, since these materials are especially sensitive to oxidative and ozone-induced degradation.

Finished and Semi-Finished Products

Insulation and sheathing compounds for wire and cable, hoses, belts, seals and gaskets, films, sheets, geomembranes, pipes and fittings, footwear components, and automotive trim can be assessed either directly or via representative compound plaques.

Specimens are usually dumbbell-shaped, die-cut or machined from compression- or injection-molded sheets, following ISO 527 specimen types 1A/1B, ASTM D638 types I through V, ISO 37 types 1 to 4, or ASTM D412 type C. A sufficient number of specimens, typically at least five per condition, is prepared, and a matched set of unaged control specimens is retained for baseline comparison.

Testing Items

The core deliverable of the evaluation is the tensile strength reduction rate after aging, supported by the following measured parameters:

• Tensile strength at break of unaged control specimens
• Tensile strength at break of aged specimens
• Tensile strength reduction rate or retention rate after aging
• Elongation at break and its percentage change after aging
• Tensile stress at specified elongation (modulus) and its change, particularly for elastomeric materials
• Yield strength and yield strain changes for ductile thermoplastics
• Supplementary observations such as hardness change, mass change, surface cracking, and color difference

Testing Methods

Step 1: Baseline Tensile Testing

Control specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity for at least 16 hours, or longer for hygroscopic materials such as polyamide. A calibrated universal testing machine with an appropriate load cell and extensometer applies a constant crosshead speed specified by the relevant standard, and the maximum force at rupture together with the cross-sectional area is recorded for each specimen.

Step 2: Aging Exposure

The aging environment is selected to simulate the dominant degradation mechanism expected in service:

• Air-oven heat aging per ISO 188, ASTM D573, or GB/T 3512, typically at temperatures from 70 °C to 150 °C for durations ranging from 24 hours to several thousand hours
• Xenon-arc weathering per ISO 4892-2, ASTM G155, or GB/T 16422.2, combining filtered xenon radiation with water spray and humidity cycles
• Fluorescent UV-condensation weathering per ISO 4892-3, ASTM G154, or GB/T 16422.3, reproducing UV photodegradation and condensation moisture effects
• Ozone aging per ISO 1431-1 or ASTM D1149, with specimens held under static elongation in an ozone-containing chamber
• Natural outdoor weathering per ISO 4607, ASTM D1435, or GB/T 3681 for direct environmental correlation
• Chemical or liquid immersion aging per ISO 175, ISO 1817, or ASTM D471 for service in contact with oils, fuels, or aqueous media

Step 3: Post-Aging Conditioning and Retesting

After exposure, specimens are allowed to recover in the standard atmosphere, typically for 16 to 96 hours depending on the material, and their dimensions are re-measured because oxidation and shrinkage may alter the cross-section. The aged specimens are then tested under exactly the same tensile conditions as the controls so that the only variable is the aging history.

Step 4: Calculation and Reporting

The tensile strength of each specimen is calculated as the maximum force divided by the original cross-sectional area, and the reduction rate is determined from the mean or median values as required by the applicable standard. The test report should state the material and formulation, specimen type and preparation method, aging method and parameters, conditioning details, testing speed, individual and statistical results, and any anomalies such as grip failures. Accredited third-party laboratories ensure traceable calibration of the testing machine, oven temperature uniformity, and weatherometer irradiance control, which are essential for reproducible results.

Applications

Measurement of the post-aging tensile strength reduction rate supports material qualification, quality control, and design validation across many industries:

• Wire and cable: verification of insulation and sheathing compounds after thermal and UV aging, as required by most cable product standards
• Automotive: under-hood rubber and plastic components, exterior trim, hoses, belts, and weather seals exposed to heat, ozone, and sunlight
• Construction and infrastructure: geomembranes, roofing membranes, window profiles, pipes, and fittings requiring decades of outdoor service life
• Footwear and consumer goods: outsoles, straps, and soft-touch components evaluated after flexing, heat, and light exposure
• Packaging and agriculture: films assessed for retention of mechanical performance after UV exposure
• Medical devices and rail transit: accelerated aging studies supporting shelf-life claims and long-term reliability documentation

Beyond compliance verification, the reduction rate is a powerful research tool for optimizing antioxidant and light-stabilizer packages, comparing candidate materials, and, when combined with elevated-temperature aging data and Arrhenius analysis, extrapolating expected performance over the product service life.

FAQ

Q: What is the difference between the strength reduction rate and the retention rate? A: The two express the same comparison from opposite directions. The reduction rate describes how much strength was lost, for example 25%, while the retention rate describes how much remains, for example 75%. Product specifications may cite either form, so laboratories typically report both.

Q: What reduction rate is considered acceptable? A: Acceptance limits depend entirely on the product standard and application. General-purpose specifications commonly require a retention of at least 70% to 80% after the specified aging period, while critical applications such as cables, geomembranes, and safety components may demand retention of 85% or higher. The governing product standard should always be consulted.

Q: How long does the aging treatment take? A: Durations vary widely. Routine quality-control aging may last 72 to 168 hours at elevated temperature, whereas qualification programs frequently run 500 to 3000 hours or longer in ovens and weatherometers. Natural weathering exposures typically extend over one to several years.

Q: How should the aging method be selected? A: The method should mirror the dominant degradation driver in service: heat aging for thermal environments, xenon or UV weathering for sunlight, ozone aging for elastomers under static strain near electrical equipment or in outdoor atmospheres, and immersion aging for chemical contact. When in doubt, accredited third-party laboratories can recommend a suitable protocol based on the intended application.

Q: How many specimens are required? A: For plastics, at least five valid specimens per condition are recommended in line with ISO 527 and ASTM D638. For rubbery materials, ISO 37 permits three specimens in certain cases, but five or more improve statistical confidence, and an equal number of unaged controls is required.

Q: Can finished products be tested directly instead of molded plaques? A: Yes, whenever geometry permits dumbbell specimens to be die-cut from the product itself, for example from cable sheathing, sheet, or membrane. Otherwise, test sheets are molded from the same compound under controlled conditions so that the evaluation remains representative.

Q: Can the test results predict actual service life? A: Accelerated aging provides a comparative measure of aging resistance rather than an absolute lifetime. When multi-temperature aging data are analyzed using Arrhenius or time-temperature extrapolation, and the accelerated environment is validated against natural weathering, reasonably confident lifetime estimates can be obtained.