Determination of mass loss in plastics during aging is a fundamental materials testing technique used to evaluate the thermal endurance, weathering resistance, and long-term stability of polymeric materials. By precisely measuring the weight change of plastic specimens before and after exposure to elevated temperatures, UV radiation, and humidity, engineers can assess additive volatilization, oxidative degradation, and expected service life in accordance with international standards such as IEC 60216, ISO 2578, ASTM D3045, and UL 746B. This article provides a comprehensive technical overview of the testing principles, sample requirements, analytical methods, and industrial applications of plastic aging mass loss determination.
Technical Overview
Mass loss determination is one of the most direct and reproducible methods for quantifying the degradation of plastic materials during aging processes. When plastics are exposed to elevated temperatures, ultraviolet radiation, oxygen, or humidity over extended periods, several physicochemical processes occur that alter the total mass of the material. These include the volatilization and migration of low-molecular-weight additives such as plasticizers, stabilizers, lubricants, and antioxidants; the evaporation of residual monomers, solvents, and absorbed moisture; and oxidative chain scission reactions that generate and release volatile decomposition products such as hydrogen chloride from degrading PVC, water, carbon dioxide, and short-chain hydrocarbon fragments.
The measured mass loss serves as a valuable diagnostic indicator because it correlates strongly with the retention of mechanical strength, elongation at break, dielectric strength, and other critical performance properties. In thermal endurance evaluation, mass loss is frequently adopted as the endpoint criterion for Arrhenius-based lifetime extrapolation. Relevant international standards include the IEC 60216 series and its Chinese counterpart GB/T 11026 for the determination of thermal endurance properties of insulating materials, ISO 2578 and ASTM D3045 for heat aging of plastics, the ISO 4892 series and ASTM G154/G155 for artificial weathering exposure, ISO 11358 and ASTM E1131 for thermogravimetric analysis, and UL 746B for the long-term property evaluation of polymeric materials.
Testing Samples
Molded Plaques and Sheet Specimens
Compression-molded or injection-molded plaques with a typical thickness of 1.0 mm to 3.0 mm are the most common sample form. Standardized geometry ensures comparable surface-to-volume ratios between test lots. Plaques can be cut into square pieces, typically 50 mm × 50 mm or smaller, depending on the balance capacity and the required number of replicate specimens.
Film and Membrane Samples
Thin films below 250 μm thickness, such as packaging films, agricultural films, and geomembranes, are well suited to mass loss studies because degradation and additive loss occur relatively rapidly at high surface-to-volume ratios. Film specimens are usually cut into defined rectangles and weighed in stacks to improve measurement precision.
Cut Specimens from Finished Products
Samples may be taken directly from manufactured goods such as cable insulation, connector housings, gaskets, pipes, and automotive trim components. When testing finished products, the specimen should be documented with its original wall thickness, cure or molding history, and any surface layers that may influence volatile loss.
Dumbbell and Bar Test Pieces
When mass loss is evaluated alongside mechanical property retention, standard dumbbell tensile bars (for example, ISO 527-2 type 1A or ASTM D638 Type I specimens) are weighed before and after aging. This combined approach allows direct correlation between mass change and mechanical degradation on identical specimens.
Testing Items
- Mass loss percentage of plastics after forced-convection thermal oven aging at one or more exposure temperatures
- Mass change after artificial weathering using xenon arc light or fluorescent UV lamps combined with water spray or condensation cycles
- Mass loss kinetics, including mass loss rate, time-to-endpoint behavior, and apparent activation energy derived from multi-temperature aging programs
- Thermal endurance characterization based on a defined mass loss endpoint, including temperature index (TI), relative temperature index (RTI), and halving interval determination
- Volatile matter content and thermal degradation parameters measured by thermogravimetric analysis, including onset temperature of decomposition, maximum mass loss rate temperature, and residue yield
- Plasticizer retention and additive volatility studies for flexible PVC and other plasticized compounds
- Moisture uptake or desorption behavior evaluated as mass change under controlled humidity conditioning
Testing Methods
Specimen Conditioning and Initial Weighing
Specimens are first conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618 until mass equilibrium is reached. Hygroscopic materials such as polyamides may require desiccator conditioning at 23 °C over a desiccant to separate moisture effects from true degradation mass loss. The initial mass m₀ of each specimen is measured using an analytical balance with a resolution of 0.1 mg for small specimens or 1 mg for larger pieces. A minimum of three to five replicate specimens per exposure condition is recommended to ensure statistical reliability, and each specimen is individually identified and tracked throughout the program.
Thermal Aging in Forced-Ventilation Ovens
Thermal aging is performed in forced-convection ovens complying with the requirements of IEC 60216-2 or ASTM D3045, with temperature uniformity typically within ±2 °C and a controlled rate of air exchange. Exposure temperatures are selected at intervals of approximately 10 °C to 20 °C above and below the anticipated service temperature, generally spanning a range that produces endpoint failure between a few hundred and several tens of thousands of hours. Common test temperatures for engineering thermoplastics range from 100 °C to 200 °C, while higher-temperature materials such as fluoropolymers may be evaluated up to 300 °C or above. Specimens are positioned to allow free air circulation, avoiding contact between different material types to prevent cross-contamination by volatile additives.
Artificial Weathering Followed by Mass Determination
For photo-oxidative aging studies, specimens are exposed in laboratory light sources in accordance with ISO 4892-2 (xenon arc), ISO 4892-3 (fluorescent UV lamps), ISO 11507, ASTM G154, or ASTM G155. Standard cycles combine irradiance levels of 0.35 W/m² to 0.55 W/m² at 340 nm with alternating light, dark, water spray, and condensation phases. Total exposure is expressed in radiant energy (kJ/m² at 340 nm) or machine operating hours. After defined exposure intervals, specimens are removed, visually inspected, and reconditioned before reweighing to determine mass change attributable to photodegradation and additive loss.
Thermogravimetric Analysis (TGA)
Thermogravimetric analysis in accordance with ISO 11358 or ASTM E1131 provides complementary information on the intrinsic thermal stability of the polymer. Small samples of 5 mg to 10 mg are heated at a controlled rate, typically 10 °C/min, from ambient temperature to 600 °C or higher, under a nitrogen or air purge. The resulting thermogram yields the onset temperature of decomposition, the temperature of maximum mass loss rate, characteristic mass loss steps, and the residual char yield, enabling comparison of stabilizer efficiency and base polymer stability.
Calculation and Data Evaluation
After each aging interval, specimens are reconditioned to equilibrium in the standard atmosphere and weighed to obtain the final mass m₁. The mass loss percentage is calculated as (m₀ − m₁) / m₀ × 100, where m₀ is the conditioned initial mass and m₁ is the conditioned mass after aging. For thermal endurance evaluation, the exposure time required to reach a specified mass loss endpoint, for example 5 % or another criterion agreed for the material, is determined at each aging temperature. An Arrhenius plot of the logarithm of time to endpoint against reciprocal absolute temperature (1/T) is constructed, and the data are extrapolated to estimate the temperature index at 20,000 hours or the halving interval, following the statistical procedures of IEC 60216-1 and IEC 60216-3.
Applications
Mass loss determination during aging is widely applied across the polymer industry for material qualification, formulation development, and lifetime assessment:
- Wire and cable insulation and jacket compounds, where long-term thermal performance is qualified under UL 746B relative temperature index programs and IEC 60216 thermal endurance evaluations
- Electrical insulation systems in motors, transformers, and switchgear, where mass loss serves as a diagnostic property for verifying thermal class and insulation system endurance
- Flexible PVC products, including cables, flooring, hoses, and medical devices, where plasticizer volatility and retention directly determine flexibility retention and compliance requirements
- Automotive interior and under-hood components, where heat aging mass loss relates to fogging, odor, embrittlement, and long-term dimensional stability
- Packaging films, agricultural films, and geosynthetic membranes, where mass loss after accelerated weathering indicates antioxidant and UV stabilizer depletion
- Research and development of stabilization packages, comparing antioxidant systems, HALS, and UV absorber formulations by their ability to suppress mass loss under accelerated aging
- Quality control and incoming material verification performed by accredited third-party laboratories and in-house testing facilities for regulatory submissions and customer specifications
FAQ
What level of mass loss indicates that a material has failed?
There is no universal failure threshold because acceptance criteria depend on the material, application, and governing specification. In thermal endurance studies, mass loss endpoints of 5 % or values agreed between parties are commonly used, while some electrical insulation specifications limit mass loss to 10 % or require that mechanical property retention be evaluated in parallel. The endpoint must always be defined relative to the relevant product standard or customer requirement.
How does oven aging mass loss differ from thermogravimetric analysis?
Oven aging evaluates the long-term behavior of full-size specimens at moderate temperatures over hours to thousands of hours, capturing slow additive migration and oxidative processes relevant to service conditions. TGA heats milligram-scale samples rapidly to decomposition temperatures and measures intrinsic thermal stability and composition steps. The two techniques are complementary: TGA characterizes the material itself, while oven mass loss characterizes durability under realistic aging conditions.
Can absorbed moisture distort the mass loss results?
Yes. Hygroscopic plastics such as polyamide and polyester can gain or lose several percent of their mass through moisture exchange, masking true degradation mass loss. To eliminate this interference, specimens must be conditioned to moisture equilibrium in a defined atmosphere both before the initial weighing and after each aging interval, following ISO 291 or ASTM D618 procedures.
What balance accuracy and specimen size are recommended?
An analytical balance with 0.1 mg readability is recommended for specimens below approximately 50 g, which covers most standard plaques and film stacks. Larger molded parts may be weighed on a balance with 1 mg readability provided the expected mass loss exceeds the measurement uncertainty by a factor of at least ten. Replicate weighing and consistent specimen geometry further improve precision.
Why does specimen thickness affect the measured mass loss rate?
Volatilization of additives and oxidation products is largely diffusion-controlled. Thin specimens with high surface-to-volume ratios lose volatiles and degrade faster than thick specimens of the same compound. For comparability, all specimens within a test program should have identical geometry, and reported results should always state the specimen thickness.