Mechanical Properties Testing of Plastics After Thermal Aging

Thermal aging is one of the most decisive factors affecting the long-term service life of plastic materials, driving oxidation, chain scission, and progressive embrittlement that degrade mechanical performance over time. Mechanical properties testing after thermal aging quantifies the retention of tensile, flexural, and impact performance, delivering essential data for material selection, quality control, and durability prediction. Accredited third-party laboratories perform these evaluations in accordance with ISO, ASTM, IEC, and GB standards to support manufacturers in the automotive, electrical, electronics, and consumer goods sectors.

Technical Overview

Plastics are polymeric materials whose mechanical properties inevitably change during long-term exposure to elevated temperatures. Heat accelerates physical processes such as post-crystallization, relaxation of processing stresses, and plasticizer loss, as well as chemical processes including thermo-oxidative degradation, chain scission, and crosslinking. The net effect is typically a reduction in ductility, a shift in impact behavior from tough to brittle, and changes in strength and modulus. Mechanical properties testing after thermal aging is therefore a core tool for assessing the thermal endurance of plastics and validating their suitability for extended service at elevated temperatures.

The test sequence consists of two stages. First, specimens are exposed in forced-ventilation air ovens at selected temperatures and durations in accordance with standards such as ISO 188, ASTM D3045, and GB/T 7141. Second, aged and unaged reference specimens are tested mechanically under identical conditions using standardized methods such as ISO 527 or ASTM D638 for tension, ISO 178 or ASTM D790 for flexure, and ISO 179, ISO 180, or ASTM D256 for impact. The results are expressed as property retention values, and multi-temperature data can be extrapolated using Arrhenius analysis to determine time-temperature limits in line with ISO 2578, IEC 60216, or the relative temperature index (RTI) approach of UL 746B.

Testing Samples

Multipurpose Injection-Molded Specimens

Molded bars conforming to ISO 3167 (Type A multipurpose specimens) or ASTM D638 Type I dumbbells are the preferred sample format. These specimens permit direct machining of tensile, flexural, and impact test pieces from a single geometry, ensuring consistent material history across all aged and unaged comparison groups.

Compression-Molded or Extruded Plaques

For materials supplied as sheets or plaques, test pieces are machined or die-cut from the same lot. The grain or flow direction of the material is recorded because anisotropy can significantly influence the measured retention of mechanical properties after aging.

Finished Products and Components

Where the objective is to qualify an actual part, small standardized coupons or mini-specimens are cut from the component in defined locations and orientations. Testing real products captures the influence of wall thickness, processing-induced stresses, and local composition on aging behavior.

Testing Items

• Tensile strength, tensile stress at yield, and tensile modulus (ISO 527-1/-2, ASTM D638)
• Elongation at break and elongation at yield (ISO 527-1/-2, ASTM D638)
• Flexural strength and flexural modulus (ISO 178, ASTM D790)
• Charpy impact strength, notched and unnotched (ISO 179-1, GB/T 1043)
• Izod impact strength, notched and unnotched (ISO 180, ASTM D256, GB/T 1843)
• Compressive strength and compressive modulus (ISO 604, ASTM D695)
• Shore and Ball hardness (ISO 868, ISO 2039, ASTM D2240)
• Property retention rate after aging, expressed as a percentage of the unaged value

Testing Methods

Thermal Aging Exposure

Specimens are placed in forced-ventilation ovens with controlled air exchange, meeting the requirements of ISO 188 and ASTM D3045 for temperature uniformity and air velocity. Exposure temperatures are selected above the intended service temperature but below the point where the material undergoes non-representative transitions such as melting. For thermal endurance characterization, at least three or four temperatures separated by approximately 10 °C to 20 °C are used, with sampling intervals arranged to capture the degradation curve. Specimens are removed at prescribed times, cooled to room temperature, and conditioned at 23 °C and 50 % relative humidity in line with ISO 291 or ASTM D618 before mechanical testing.

Tensile Testing

Tensile tests are performed on aged and unaged specimens using universal testing machines at standardized crosshead speeds, typically 1 mm/min for modulus determination and 50 mm/min for strength and elongation. Ductile-brittle transition is often most clearly reflected in elongation at break, which is why this parameter is frequently adopted as the critical end-of-life criterion, commonly at 50 % retention.

Flexural Testing

Three-point bending tests are conducted with defined support spans and loading rates according to ISO 178 or ASTM D790. Flexural modulus and strength provide sensitive indicators of post-crystallization and oxidative stiffening in semi-crystalline plastics such as polypropylene and polyamide.

Impact Testing

Notched and unnotched impact tests are executed on pendulum impact testers using Charpy (ISO 179-1) or Izod (ISO 180, ASTM D256) configurations. Impact strength is one of the earliest properties to decline during thermal aging because embrittlement initiates at stress-concentrating surface oxidation layers, making it a reliable early-warning indicator.

Data Evaluation and Thermal Endurance Analysis

For each exposure condition, retention values are calculated as the aged property divided by the unaged property, multiplied by 100. When aging is conducted at multiple temperatures, the time to reach the failure criterion is plotted against reciprocal absolute temperature in an Arrhenius diagram. Linear extrapolation yields the time-temperature limit (TI) per ISO 2578 or IEC 60216, or supports RTI assignments under UL 746B for long-term temperature ratings of polymeric materials.

Applications

• Automotive: durability validation of under-hood plastics, interior trim, connectors, and lighting housings exposed to sustained engine-bay heat
• Electrical and electronics: thermal endurance qualification of enclosures, switchgear components, coil formers, and circuit-breaker parts in line with IEC 60216 and UL 746 requirements
• Wire and cable: aging assessment of insulation and sheathing compounds before mechanical verification
• Household appliances: confirmation that plastic components in heaters, ovens, and kettles retain adequate strength and toughness after prolonged operation
• Construction and piping: long-term performance verification of pipe fittings, valve bodies, and profiles intended for hot-water or solar exposure
• Material development: comparison of stabilization packages, flame retardants, and base polymers during formulation optimization
• Quality control and incoming inspection: verification of lot-to-lot consistency against specified retention requirements

FAQ

How are aging temperature and duration selected?

The exposure temperature should be high enough to induce measurable degradation within a practical timeframe yet low enough to preserve the same degradation mechanism as real service. A common practice is to test at three to four temperatures above the intended service temperature, bracketing expected lifetimes from tens of hours to several thousand hours, and to extrapolate the results using Arrhenius analysis.

What is the property retention rate?

The retention rate is the ratio of the mean property value measured on aged specimens to the mean value measured on unaged control specimens, expressed as a percentage. Specifications frequently require retention of at least 50 % for ductility-related properties such as elongation at break and notched impact strength, which degrade faster than strength or modulus.

Why is elongation at break often used as the failure criterion?

Elongation at break responds very sensitively to oxidative embrittlement and molecular weight loss, often falling by more than half while tensile strength remains almost unchanged. It therefore serves as a conservative and repeatable end-point for defining thermal endurance.

How many specimens are required?

For each aging condition, a minimum of five valid specimens per mechanical test is generally required, together with an equivalent unaged control group. Because multiple exposure times and temperatures are usually involved, laboratories typically prepare sufficient specimens at the outset to cover every withdrawal point without interrupting the aging program.

Can finished products be evaluated instead of molded test bars?

Yes. Coupons can be machined from actual components, or the aging program can be followed by functional testing of the complete part. Testing real products accounts for processing history, wall thickness, and geometry effects, although comparison against standardized retention criteria is most rigorous when molded reference specimens of the same compound are tested in parallel.