Plastic aging hardness change testing measures how the hardness of polymeric materials evolves after exposure to heat, UV radiation, humidity, and other environmental stressors. By comparing hardness values measured before and after accelerated aging, manufacturers can evaluate material durability, screen formulations, and predict long-term service performance. This article explains the test principle, applicable standards, sample requirements, test methods, data evaluation, and typical applications.
Technical Overview
Plastics are inherently susceptible to degradation when exposed to heat, ultraviolet radiation, oxygen, moisture, and mechanical stress over time. These environmental factors trigger molecular-level changes such as chain scission, oxidative crosslinking, post-crystallization, and plasticizer migration or volatilization. Each of these mechanisms directly affects the surface and bulk mechanical response of the material, and hardness is one of the most sensitive and convenient indicators of such changes.
Depending on the dominant degradation mechanism, aging may cause a plastic to harden (for example, through crosslinking or plasticizer loss) or to soften (for example, through chain scission or hydrolytic degradation). Plastic aging hardness change testing therefore quantifies the direction and magnitude of hardness drift after a defined aging exposure. The basic test sequence is: measure the initial hardness of conditioned specimens, subject them to a controlled aging environment, recondition them, measure the final hardness, and calculate the hardness change and retention rate. The procedure is governed by a family of international and national standards covering both the aging exposure and the hardness measurement.
Testing Samples
Thermoplastic Resins and Compounds
Common test materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), ABS, polymethyl methacrylate (PMMA), polycarbonate (PC), polyamide (PA), polyoxymethylene (POM), and polyethylene terephthalate (PET). Specimens are usually prepared as compression-molded or injection-molded plaques with flat, smooth surfaces, free of sink marks, flash, and surface contamination.
Thermosetting Plastics
Phenolic resins (PF), epoxies (EP), unsaturated polyesters (UP), melamine-formaldehyde resins, and rigid polyurethanes are typically supplied as cured molded sheets or laminates. Because thermosets are already crosslinked, aging tends to cause surface oxidation, embrittlement, and hardening rather than softening.
Elastomers and Thermoplastic Elastomers
Soft materials such as thermoplastic polyurethane (TPU), thermoplastic elastomers (TPE), and rubber-plastic blends are evaluated predominantly on the Shore A scale. For these materials, hardness change is often accompanied by loss of elongation and is a key acceptance criterion in quality specifications.
Finished Products and Components
Pipes, profiles, cable insulation and sheaths, gaskets, automotive trim, and equipment housings can be tested directly, provided a sufficiently flat measurement area is available. Components may be tested as whole parts or by cutting plaque specimens from representative locations.
Specimen dimensional requirements follow the hardness method used. For durometer measurements per ISO 868 and ASTM D2240, the test area should be flat and the thickness should typically be at least 6 mm for Shore A and at least 3 mm for Shore D; thinner sheets may be stacked to reach the minimum thickness, provided they do not separate during measurement. Ball indentation hardness per ISO 2039-1 requires a flat surface of at least 20 mm in width with a thickness of at least 4 mm.
Testing Items
The following measurements and derived results are typically reported in a plastic aging hardness change test program:
- Initial hardness of the unaged specimen (Shore A, Shore D, Shore OO, Rockwell, ball indentation, or Barcol as appropriate)
- Hardness after a defined aging exposure (single-point or multi-point time series)
- Absolute hardness change (delta value, in hardness units)
- Relative hardness change percentage
- Hardness retention rate after aging
- Aging environment parameters: exposure temperature, irradiance, humidity, cycle profile, and total exposure duration
- Optional complementary indicators: color change, tensile property retention, mass change, and surface condition (cracking, chalking, blistering)
Testing Methods
Heat Aging in Air (Air Oven) Method
Specimens are exposed to circulating hot air in a forced-ventilation oven at a temperature selected according to the material class and its intended service condition, commonly in the range of 70 °C to 150 °C, with exposure durations from 24 hours up to several thousand hours. Heat aging is performed in accordance with ISO 188, ASTM D573, GB/T 3512, and GB/T 7141. Intermediate hardness measurements at planned intervals allow the construction of a hardness-versus-time curve, which can be used with Arrhenius-type extrapolation to estimate long-term behavior. This approach is consistent with the relative temperature index concept described in UL 746B.
Xenon Arc Weathering Method
Xenon arc exposure using filtered lamps provides the best available laboratory simulation of full-spectrum sunlight. Testing is conducted per ISO 4892-2, ASTM G155, and GB/T 16422.2, with controlled irradiance (commonly 0.35 W/m² at 340 nm or 0.51 W/m² at 420 nm, depending on the filter system), black panel or black standard temperature, relative humidity, and optional water spray cycles. Hardness is measured at cumulative exposure points expressed in total radiant energy or exposure hours.
Fluorescent UV Weathering Method
Fluorescent UV lamps, typically UVA-340 lamps, reproduce the short-wavelength UV portion of sunlight, which is primarily responsible for photodegradation of most plastics. Exposure follows ISO 4892-3, ASTM G154, and GB/T 16422.3, alternating UV irradiation at a controlled temperature with condensation cycles. This method is widely used for coatings, PVC products, and general outdoor durability screening.
Damp Heat Aging Method
For materials used in humid or tropical environments, or for electronics-related polymers, steady-state damp heat exposure such as 85 °C / 85 % RH or 40 °C / 93 % RH is applied per IEC 60068-2-78, sometimes combined with cyclic humidity per IEC 60068-2-30. Moisture can plasticize some polymers (apparent softening) or catalyze hydrolysis in polyesters and polyamides, both of which are reflected in the hardness change results.
Hardness Measurement and Data Evaluation
After aging, specimens are reconditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity per ISO 291 (typically for at least 24 hours) before the final hardness is measured. Hardness is determined using the same method, instrument type, and measurement locations as the initial test. Commonly used measurement standards include:
- Shore durometer hardness: ISO 868, ASTM D2240, GB/T 2411, and JIS K 6253
- Ball indentation hardness: ISO 2039-1 and GB/T 3398.1
- Rockwell hardness: ASTM D785 and ISO 2039-2
- Barcol hardness for reinforced thermosets: ASTM D2583
The results are expressed as the hardness change (delta H = final hardness minus initial hardness), the relative change percentage ((final minus initial) divided by initial, multiplied by 100), or the hardness retention rate (final divided by initial, multiplied by 100). A positive delta indicates aging-induced hardening; a negative delta indicates softening. Acceptance limits are defined by the product specification, purchase agreement, or relevant material standard.
Applications
Plastic aging hardness change testing supports material selection, formulation development, and product qualification across many industries:
- Automotive: interior trim, dashboard skins, seals, exterior trim, and under-hood components exposed to heat and sunlight
- Wire and cable: insulation and sheathing materials requiring retention of mechanical properties after thermal aging
- Construction: PVC pipes, fittings, window and door profiles, roofing membranes, and geomembranes exposed to outdoor weathering
- Electronics and electrical equipment: enclosures, connectors, and insulating parts evaluated after damp heat and thermal aging
- Consumer goods: appliance housings, footwear components, sporting goods, and furniture materials
- Medical devices: polymer components requiring validated performance stability throughout shelf life and service life
- Research and development: stabilizer package screening, recycled-content evaluation, and comparative resin studies
FAQ
Why does plastic hardness change after aging?
Aging drives molecular rearrangement in the polymer. Crosslinking and the loss of plasticizers or volatile additives generally increase hardness, while chain scission, oxidation of the surface layer, hydrolysis, and moisture absorption can decrease hardness. In practice, a combination of mechanisms operates, which is why direct measurement before and after exposure is necessary.
Which hardness scale should be selected?
Soft elastomers and flexible plastics are measured with Shore A (or Shore OO for very soft gels), semi-rigid and rigid plastics with Shore D, and very hard or reinforced plastics with ball indentation (ISO 2039-1), Rockwell (ASTM D785), or Barcol (ASTM D2583). The scale must be chosen so that the measured value falls within the effective range of the instrument.
How are the aging conditions and duration selected?
Conditions are selected to accelerate the actual service environment without introducing degradation mechanisms that do not occur in use. Heat aging temperatures are usually set at least 20 °C to 40 °C above the intended service temperature, while weathering parameters follow ISO 4892 or ASTM G151-series profiles. Multi-point testing with several exposure durations is recommended so that the trend, not a single value, is evaluated.
Can finished products be tested directly?
Yes, provided the part offers a flat, smooth measurement area of sufficient thickness and area. Curved or thin sections may require cutting a specimen from the part or preparing molded plaques from the same compound. The sampling location should be documented because surface orientation and processing history can influence results.
How long does a typical test take?
Short screening exposures of 72 to 168 hours are common for quality control, while qualification programs may require 500, 1000, or more hours of exposure, plus conditioning time before and after aging. Xenon and UV weathering tests are additionally defined by total radiant exposure, so the calendar duration depends on the selected irradiance level.
How should the results be interpreted?
A small hardness change (for example, within a few Shore units) generally indicates good aging resistance, whereas a large change signals significant degradation, even if the absolute hardness remains acceptable. Hardness data are most valuable when evaluated together with tensile retention, elongation at break, and visual surface assessment, and when testing is performed by accredited third-party laboratories following the referenced standards.