Determination of mass loss of plastics upon thermal aging is a gravimetric test method used to evaluate the thermal stability and formulation quality of polymer materials by measuring the change in specimen mass after exposure to a controlled elevated temperature in a forced-air oven. This test quantifies the loss of plasticizers, volatiles, and other low-molecular-weight components that directly affects long-term performance. This article explains the technical principles, applicable standards, sample requirements, test procedures, calculation methods, and typical industrial applications of thermal aging mass loss testing for plastics.
Technical Overview
Thermal aging mass loss testing is a gravimetric evaluation method in which conditioned plastic specimens are exposed to a specified elevated temperature in an air-circulating oven for a defined period, and the resulting change in mass is measured and expressed as a percentage of the initial mass. The mass lost during heat aging primarily consists of plasticizers, lubricants, stabilizers, residual monomers, solvents, moisture, and volatile thermal degradation products that migrate to the surface and evaporate from the polymer matrix.
This test is one of the most important quality indicators for plasticized thermoplastics, particularly flexible polyvinyl chloride (PVC) compounds. Excessive mass loss during thermal aging signals poor formulation stability and predicts premature embrittlement, shrinkage, hardening, loss of elongation, and degradation of electrical insulation performance during service. For this reason, maximum allowable mass loss values are written directly into numerous product and material specifications.
The method is governed by several international and national standards, and testing is routinely performed by accredited third-party laboratories, manufacturer quality control departments, and research institutions. The most commonly referenced frameworks include:
- GB/T 7141, which specifies general procedures for heat aging tests of plastics
- IEC 60811-401, covering thermal ageing in an air oven for insulating and sheathing materials
- ASTM D3045, the standard practice for heat aging of plastics without load
- ISO 188, addressing accelerated ageing and heat resistance of rubber and thermoplastic elastomers
- ISO 2578 and the IEC 60216 series, which use heat exposure data to establish time-temperature limits and thermal endurance indices
- UL 746B, which defines long-term thermal aging programs for polymeric materials, including relative temperature index (RTI) evaluation
Testing Samples
Molded Specimens and Sheets
Compression-molded or injection-molded plaques and sheets are the preferred specimen form for material qualification. A nominal thickness of approximately 1 mm is frequently specified so that the surface-area-to-mass ratio remains consistent between batches, since this ratio directly governs the rate of volatile loss.
Cable Insulation and Sheathing Compounds
Flexible PVC compounds used for wire and cable insulation and sheathing represent the largest application area for this test. Specimens may be prepared as molded sheets or taken directly from insulated conductors and finished cables, then aged under conditions tied to the temperature class of the compound, such as 168 hours at 80 °C or 100 °C.
Raw Materials and Pellets
For incoming quality control of resins and compounds, pellets are melt-pressed into films or small plaques before testing. This allows buyers to verify plasticizer retention and volatile content before materials enter production.
Finished and Semi-Finished Products
Representative specimens can be cut from pipes, profiles, films, sheets, gaskets, and molded components to assess the actual thermal stability of the delivered product rather than the base compound alone.
Sample Preparation and Conditioning
Before testing, specimens are conditioned in a standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 hours, or dry-conditioned in a desiccator where the governing standard requires it. A minimum of three identical specimens is normally tested, and each specimen is marked, dimensioned, and weighed on an analytical balance with a readability of 0.1 mg.
Testing Items
Depending on the specification being applied, a thermal aging mass loss determination program typically includes the following test items and reported parameters:
- Mass loss (or mass change) after thermal aging, expressed as a percentage of the initial mass at specified temperature and duration
- Retention of tensile strength and elongation at break after the same aging exposure, which is usually required together with mass loss in cable material standards
- Visual assessment of aged specimens for cracking, discoloration, blistering, warping, or surface deposits
- Optional supplementary properties such as hardness change, dimensional change, and thermal stability time
- Complete reporting of initial mass, final mass, calculated mass loss, aging temperature, aging duration, and oven operating conditions
Testing Methods
Apparatus and Equipment
The core equipment set comprises a forced-convection air oven with demonstrated temperature uniformity, typically within ±2 °C of the set point, and a controlled air-exchange rate, commonly in the range of 8 to 20 air changes per hour for cable material testing. Additional equipment includes an analytical balance with 0.1 mg readability, a desiccator charged with fresh desiccant, inert specimen holders or racks that permit free air circulation on all sides, and calibrated temperature loggers or reference thermometers for verifying oven performance.
Test Procedure
- Step 1: Condition the specimens in the specified standard atmosphere and verify their dimensions and freedom from surface contamination.
- Step 2: Weigh each conditioned specimen on the analytical balance and record the initial mass (m0).
- Step 3: Preheat the oven to the specified aging temperature and confirm stable, uniform conditions before loading.
- Step 4: Mount the specimens on racks with adequate spacing, commonly at least 10 mm apart and never in contact with oven walls or with one another, and place them in the oven within its qualified working zone.
- Step 5: Maintain the specimens at the specified temperature for the specified duration, with typical industrial conditions being 168 hours at 80 °C, 100 °C, 105 °C, 121 °C, or 135 °C depending on the temperature rating of the material.
- Step 6: Remove the specimens at the end of the exposure period, transfer them to a desiccator, and allow them to cool to room temperature without absorbing ambient moisture.
- Step 7: Reweigh each specimen and record the final mass (m1).
- Step 8: Inspect the aged specimens visually and proceed to any paired mechanical testing while the specimens remain in the conditioned state.
Calculation and Expression of Results
Mass loss is calculated as: Mass loss (%) = (m0 − m1) / m0 × 100, where m0 is the initial conditioned mass and m1 is the mass after aging and desiccator cooling. A positive value indicates mass loss from volatilization, while a small negative value may indicate oxidation-driven mass gain. Results are reported as the median or mean of the specimen set as defined by the applicable standard, together with the complete aging conditions.
Acceptance Criteria and Typical Limits
Acceptance limits are set by the governing product or material standard and are tied to the temperature class of the compound. For general-purpose PVC insulation materials tested in accordance with IEC-based cable standards, a maximum mass loss of 2.0 % after 168 hours at 80 °C is a widely applied limit, with corresponding sheathing compounds often limited to 1.5 % under identical conditions. National specifications for cable-grade PVC compounds define analogous limits that become more stringent for higher temperature classes, and the aging temperature itself is increased for heat-resistant grades.
Quality Control and Influencing Factors
Key factors that must be controlled for valid results include oven temperature accuracy and uniformity, air-exchange rate, total oven loading, specimen spacing, specimen surface-area-to-mass ratio, and cooling regime after aging. Specimens of different formulations should not be aged simultaneously in the same oven chamber because volatile plasticizers from one material can migrate into another and distort the results. Regular oven calibration, temperature mapping, and the use of reference specimens are essential elements of a compliant quality system, and accredited third-party laboratories follow these controls when issuing test reports for certification purposes.
Applications
Thermal aging mass loss determination is applied across the full plastics value chain, from compound development to finished product certification:
- Wire and cable manufacturing: qualification and routine release testing of PVC insulation and sheathing compounds against IEC, EN, GB, and UL-based material requirements
- Automotive engineering: evaluation of wire harness insulation, interior trim compounds, and under-hood polymer components subjected to sustained elevated temperatures
- Building products: thermal stability assessment of pipes, profiles, window gaskets, flooring, and roofing membranes
- Research and development: comparison of plasticizer systems, stabilizer packages, and polymer blends during formulation optimization
- Incoming material inspection: verification of resin and compound quality at goods receipt before production release
- Compliance and certification: generation of test data for type approval, factory acceptance, and long-term thermal endurance programs supporting temperature index and RTI ratings
FAQ
What causes mass loss during thermal aging of plastics?
Mass loss is caused by the evaporation and migration of plasticizers, lubricants, stabilizers, residual monomers, solvents, absorbed moisture, and low-molecular-weight degradation products from the heated polymer matrix. The rate of loss accelerates with increasing temperature and larger surface-area-to-volume ratios.
What are the most common test conditions?
The most widely used industrial condition is 168 hours of exposure in an air oven, with the temperature selected according to the material class, typically 80 °C for general-purpose PVC insulation compounds and 100 °C to 135 °C for heat-resistant grades. Product standards specify the exact temperature-duration combination that applies.
How is the mass loss result calculated and reported?
The result is the difference between the initial conditioned mass and the final mass after aging, divided by the initial mass and multiplied by 100 to express a percentage. The test report must state the aging temperature, duration, number of specimens, individual and statistical results, and the standard applied.
How is this test different from thermogravimetric analysis?
Oven-based mass loss testing ages larger specimens under realistic service-simulating conditions defined by product standards, whereas thermogravimetric analysis heats milligram-scale samples under precisely controlled ramp rates to characterize decomposition onset and constituent mass fractions. The two methods are complementary and serve different specification purposes.
What does a high mass loss value indicate?
A high value indicates excessive volatile content or poor plasticizer retention, which predicts hardening, embrittlement, shrinkage, cracking, and reduced flexibility or insulation life in service. It is a direct signal of formulation deficiency or processing contamination.
How long does a typical test take?
For the standard 168-hour exposure, the complete workflow, including conditioning, weighing, aging, cooling, and reweighing, typically requires about eight to ten working days. Extended programs for thermal endurance evaluation may involve multiple temperature points and exposure periods lasting several weeks or months.
In summary, the determination of mass loss of plastics upon thermal aging is a straightforward yet powerful gravimetric method that links laboratory heat exposure to real-world durability. When performed with calibrated ovens, controlled specimen geometry, and strict adherence to the governing GB, IEC, ISO, ASTM, or UL-based procedures, it delivers reliable data for material qualification, certification, and long-term performance prediction.