Determination of Hardness Change in Plastics After Aging: Methods, Standards, and Applications

Determining the hardness change of plastics after aging is a fundamental method for evaluating the long-term durability and performance stability of polymeric materials exposed to heat, light, and humidity. This article provides a comprehensive technical overview of hardness change determination in aged plastics, covering sample preparation, aging exposure techniques, hardness measurement standards such as ISO 868, ASTM D2240, and ISO 2039-1, calculation of hardness variation, and typical industrial applications.

Technical Overview

Plastic materials undergo progressive physical and chemical changes when exposed to thermal, photo-oxidative, and humid environmental conditions over time. These aging processes—including post-crosslinking, chain scission, oxidation, plasticizer migration and loss, changes in crystallinity, and moisture absorption—directly alter the surface and bulk mechanical properties of the material. Hardness, being one of the most convenient and reproducible indicators of such changes, is widely used to quantify the degree of material degradation after aging.

Determination of hardness change in plastics after aging involves exposing test specimens to controlled accelerated aging conditions, measuring their hardness before and after exposure, and calculating the absolute or relative change. The aging exposure is typically performed according to standards such as ISO 188, ASTM D573, ISO 4892-2, ISO 4892-3, ASTM G154, ASTM G155, GB/T 3512, and GB/T 16422 series, while hardness measurement is conducted in accordance with ISO 868, ASTM D2240, ISO 2039-1, ISO 2039-2, ASTM D785, and ASTM D2583. A systematic increase in hardness after thermal aging often indicates post-curing, oxidation-induced embrittlement, or plasticizer loss, whereas a decrease may reflect hydrolysis, excessive chain scission, or plasticization by absorbed moisture.

Testing Samples

Molded Sheets and Plates

Compression-molded or injection-molded flat sheets are the preferred specimen form for hardness change determination. For Shore hardness testing in accordance with ISO 868 and ASTM D2240, the recommended minimum thickness is 4 mm; thinner specimens may be stacked, but not more than three layers, with each layer having a smooth and uniform surface. For ball indentation hardness per ISO 2039-1, a minimum thickness of 4 mm is also generally specified so that the indentation is not influenced by the supporting surface.

Pipe and Profile Sections

For pipes, cables, and extruded profiles, hardness is measured directly on the curved or flattened outer surface, provided the radius of curvature is sufficiently large to support the pressure foot of the durometer. When the curvature is excessive, ring segments or longitudinally cut strips may be used, and this deviation shall be documented in the test report.

Film and Thin Sheets

Flexible films and thin sheets are typically tested by stacking multiple layers to reach the required total thickness. Care must be taken to exclude trapped air between layers, as entrapped air will produce artificially low hardness readings. This specimen type is common for flexible PVC, polyolefin films, and coating evaluations.

Finished Products and Components

Actual manufactured components—such as gaskets, seals, footwear soles, dashboard skins, and cable jackets—may be tested directly on representative flat areas. Testing finished parts provides the most realistic assessment of in-service aging behavior, although the measurement location must be clearly defined to ensure repeatability between the initial and post-aging readings.

All specimens shall be conditioned prior to initial hardness measurement in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 40 hours. A sufficient number of specimens shall be prepared so that destructive or non-destructive measurements can be made at each aging interval, with a minimum of three specimens per interval and at least five measurement points per specimen.

Testing Items

• Initial hardness of the unaged specimen (Shore A, Shore D, ball indentation, Rockwell, or Barcol hardness)
• Hardness after a specified aging period or after a series of incremental exposure intervals
• Absolute hardness change (ΔH) expressed in hardness units
• Relative hardness change or hardness retention rate expressed as a percentage of the initial value
• Time required to reach a defined hardness change threshold, used as a failure criterion in durability studies
• Supplementary observations, including surface cracking, discoloration, chalking, and blooming, recorded together with hardness data to support degradation diagnosis

Testing Methods

Aging Exposure Methods

Thermal aging is performed in forced-ventilation air ovens in accordance with ISO 188, ASTM D573, ASTM D3045, or GB/T 3512, at temperatures selected according to the intended service environment and the material's softening behavior. Artificial weathering is conducted using xenon-arc lamps in accordance with ISO 4892-2, ASTM G155, or GB/T 16422.2, or fluorescent UV lamps in accordance with ISO 4892-3, ASTM G154, or GB/T 16422.3, with controlled irradiance, temperature, humidity, and water spray cycles. Damp-heat aging may be carried out in constant temperature and humidity cabinets to evaluate hydrolytic effects, particularly on polyester- and polyurethane-based materials. Specimens are withdrawn at predefined intervals—for example, 24 h, 72 h, 168 h, 500 h, and 1000 h—depending on the test program.

Hardness Measurement Methods

Shore hardness, measured with a durometer per ISO 868 or ASTM D2240, is the most widely applied method: Shore A is used for soft and elastomeric plastics, while Shore D is used for harder plastics. Ball indentation hardness, determined in accordance with ISO 2039-1, applies a 5 mm diameter steel ball under a fixed load for 30 seconds and is suited to relatively hard, rigid moldings. Rockwell hardness per ISO 2039-2 or ASTM D785 is applied to rigid thermoplastics and thermosets. Barcol hardness per ASTM D2583 is commonly used for reinforced thermosetting materials and hard coatings. After aging, specimens shall be reconditioned in the standard laboratory atmosphere, typically for at least 3 hours but preferably longer for hygroscopic materials, before the post-aging hardness is measured at the same locations as the initial readings.

Calculation of Hardness Change

The absolute hardness change is calculated as the difference between the post-aging hardness and the initial hardness, and the relative change is calculated as this difference divided by the initial hardness and expressed as a percentage. Hardness retention is the ratio of post-aging hardness to initial hardness expressed as a percentage. For each aging interval, the median of the measurement points is reported, together with the range, the aging method and conditions, the hardness scale used, the specimen thickness, and any visual changes observed. A result is generally considered significant when the change exceeds the inherent measurement repeatability of the selected hardness method, and acceptance criteria—such as a hardness change of no more than ±10 % or ±5 Shore units—are defined by product specifications or contractual agreements.

Quality Control Measures

To ensure valid comparisons, the same durometer type, indenter geometry, and applied force shall be used before and after aging. The instrument shall be verified against calibrated reference blocks, and measurement points shall be spaced sufficiently apart—at least 6 mm from each other and at least 9 to 12 mm from the specimen edge, depending on the standard—so that indentations do not interact. Testing performed by accredited third-party laboratories provides traceable calibration, validated environmental chambers, and standardized reporting suitable for certification and regulatory submission.

Applications

Hardness change determination after aging is applied extensively across industries where the long-term functional performance of polymeric materials is critical:

• Automotive interior and exterior components, including instrument panel skins, trims, seals, and tires, where heat and light exposure alter surface hardness and tactile properties
• Wire and cable insulation and sheathing materials, for which thermal aging resistance is a core qualification requirement in product standards
• Building materials such as PVC window profiles, door and window gaskets, roofing membranes, and waterproofing sheets exposed to outdoor weathering
• Footwear components, hoses, seals, gaskets, and O-rings in industrial equipment, where compression performance depends on stable hardness
• Packaging films and medical device components requiring validation of shelf-life performance under accelerated storage conditions
• Material research and development, comparative formulation studies, stabilizer evaluation, and incoming material qualification in quality control laboratories

FAQ

Why does plastic hardness increase or decrease after aging?

An increase in hardness after aging is typically caused by post-crosslinking or post-curing, loss of plasticizers or volatile additives, oxidation-induced embrittlement, and increased crystallinity. A decrease in hardness generally results from hydrolytic degradation, extensive chain scission, or absorption of water acting as a plasticizer. The direction and magnitude of the change help identify the dominant degradation mechanism.

How is hardness change calculated and reported?

Hardness change is reported as the difference between the median post-aging hardness and the median initial hardness, in the units of the selected hardness scale, and optionally as a percentage change or retention relative to the initial value. The test report shall state the aging method, exposure conditions and duration, hardness standard and scale used, specimen details, and conditioning procedures.

Which hardness method should be selected for aged plastics?

The method is selected according to material stiffness: Shore A for soft, flexible, and elastomeric plastics; Shore D for hard plastics; ball indentation hardness for rigid moldings requiring improved repeatability; Rockwell hardness for very rigid engineering plastics; and Barcol hardness for reinforced thermosets and coatings. The same method must be used for both initial and post-aging measurements.

How long should the aging exposure be?

The exposure duration depends on the test objective and the material class. Routine quality control programs commonly use intervals of 72 h to 1000 h of thermal or weathering exposure, while durability studies may extend to several thousand hours with periodic sampling until a defined failure criterion, such as a specified hardness change, is reached.

Can hardness change results be correlated with other aging properties?

Yes. Hardness change is frequently evaluated together with tensile strength and elongation retention, color change, mass loss, and visual assessment of cracking. Correlating hardness data with these complementary properties provides a more complete picture of the aging mechanism and supports reliable lifetime prediction of the material.