Plastic Melt Flow Index (MFI) Aging Test: Principles, Standards, Methods, and Applications

A plastic melt flow index (MFI) aging test measures the melt flow rate (MFR/MVR) of a thermoplastic before and after controlled thermal aging, quantifying chain scission, oxidative degradation, or crosslinking over time. Performed in accordance with ISO 1133, ASTM D1238, and GB/T 3682, this test is essential for evaluating long-term thermal stability, stabilizer package performance, and Relative Temperature Index (RTI) qualification of polymer materials. This article explains the technical principles, sample requirements, test methods, and typical applications of melt index aging evaluation for plastics.

Technical Overview

The melt flow index (MFI), also expressed as melt mass-flow rate (MFR, in g/10 min) or melt volume-flow rate (MVR, in cm³/10 min), describes the flowability of a molten thermoplastic extruded through a standard die under a specified temperature and dead-weight load. Because melt viscosity is directly related to average molecular weight, the melt index is one of the most sensitive and widely used indicators of polymer degradation.

A melt index aging test combines two procedures: (1) determination of the baseline melt index of the material in its original state, and (2) re-determination of the melt index after the material has been exposed to a controlled aging environment, typically forced-air oven aging at an elevated temperature for a defined period. The percentage change in melt index before and after aging reflects the degree of molecular-level deterioration caused by thermal-oxidative exposure.

An increase in melt index after aging indicates chain scission and a reduction in average molecular weight, which is the dominant degradation mechanism for polyolefins such as PP and PE. A decrease in melt index suggests crosslinking, long-chain branching, or oxidation-induced structuring of the polymer. Both trends reduce mechanical performance, so product specifications commonly limit the permissible melt index change after a defined heat treatment.

• ISO 1133-1 and ISO 1133-2: determination of melt mass-flow rate and melt volume-flow rate of thermoplastics
• ASTM D1238: melt flow rates of thermoplastics by extrusion plastometer
• GB/T 3682.1 and GB/T 3682.2: Chinese national standards for melt flow rate determination
• ASTM D3045 and ISO 2578: heat aging procedures and determination of time-temperature limits
• UL 746B: long-term property evaluation of polymeric materials, including Relative Temperature Index (RTI) programs
• IEC 60216 series: thermal endurance evaluation of electrical insulating materials

Testing Samples

Suitable test samples include virgin pellets, granules, powders, regrind, compression-molded plaques, or small pieces cut from finished parts. Approximately 5 to 10 g of material is required for each melt index determination, and sufficient material must be reserved for each aging interval and for replicate measurements. Hygroscopic materials must be dried before testing to prevent moisture-induced degradation in the barrel.

Polyolefins (PE, PP)

Polyethylene (LDPE, LLDPE, HDPE) and polypropylene are the most frequently tested materials, since thermal-oxidative chain scission proceeds readily at moderate temperatures. Pipe-grade and cable-insulation PE specifications typically require that the melt index change after a defined heating cycle remain within a narrow percentage band.

Styrenic Resins (PS, ABS, SAN)

Polystyrene, ABS, and SAN grades are evaluated to verify antioxidant effectiveness and to support RTI assignments for appliance housings, electrical enclosures, and automotive interior components.

Engineering Thermoplastics (PC, PA, PBT, PET, POM)

Polycarbonate, polyamides, polyesters, and acetal resins are tested after mandatory drying, because hydrolysis combined with oxidation can sharply alter melt viscosity during both aging and measurement.

Recycled and Compound Materials

Post-consumer recyclate, regrind, and filled or reinforced compounds are tested to quantify processing history and re-stabilization quality, as repeated processing cycles typically shift the melt index measurably.

Testing Items

• Baseline melt mass-flow rate (MFR) of the unaged material, expressed in g/10 min
• Melt volume-flow rate (MVR) of the unaged material, expressed in cm³/10 min, with calculated melt density
• Melt index after aging at each specified temperature and exposure duration (for example, 168 h, 500 h, or 1000 h)
• Percentage change in melt index: ΔMFR (%) = (MFR aged − MFR initial) / MFR initial × 100
• Melt index retention rate for qualification against product specifications
• Multi-temperature aging data sets for Arrhenius time-temperature extrapolation and thermal endurance assessment
• Complementary oxidation induction time (OIT) testing per ISO 11357-6 for stabilizer system evaluation, where required

Testing Methods

Baseline Melt Index Measurement

The specimen is charged into the preheated barrel of a calibrated extrusion plastometer equipped with a standard 2.095 mm die and piston. After the specified preheat period, the selected dead-weight load is applied and molten extrudate is collected. In Method A (ISO 1133-1 / ASTM D1238 Procedure A), timed extrudate cuttings are weighed to obtain MFR directly. In Method B (ISO 1133-2 / ASTM D1238 Procedure B), piston travel is measured automatically to obtain MVR, and MFR is derived using melt density. Commonly specified condition pairs include PE at 190 °C / 2.16 kg (or 21.6 kg for high-load melt index), PP at 230 °C / 2.16 kg, PS at 200 °C / 5.00 kg, ABS at 220 °C / 10.0 kg, and PC at 300 °C / 1.20 kg.

Thermal Aging Exposure

Representative specimens are exposed in a forced-ventilation air oven at one or more aging temperatures selected below relevant transition points and typically spaced in 10 °C increments, for example 80–120 °C for PE, 100–150 °C for PP, and 120–140 °C for PC. Exposure durations are chosen to generate a degradation curve, and aging is performed in accordance with ASTM D3045, ISO 2578, or UL 746B long-term aging protocols. Specimen geometry is controlled because oxidation is diffusion-limited and thin sections degrade more uniformly.

Post-Aging Measurement and Data Evaluation

Aged specimens are conditioned in a standard atmosphere (for example 23 °C / 50 % relative humidity per ISO 291) before melt index re-measurement under conditions identical to the baseline test. The change in melt index is calculated, and where multi-temperature data are available, the logarithm of aging time is plotted against reciprocal temperature to derive a time-temperature limit or to support RTI conclusions. A melt index increase beyond the specified limit is reported as evidence of unacceptable thermal degradation.

Quality Control and Precision

Instrument temperature, load, and die geometry are verified with certified reference materials, and replicate determinations (typically three per condition) are averaged. Accredited third-party laboratories report mean values, percentage changes, and the applicable test conditions, enabling direct comparison against datasheet values and purchase specifications.

Applications

Melt index aging data underpin material qualification, formulation development, and lifetime prediction across the plastics industry. Typical use cases include:

• Long-term thermal endurance studies and Relative Temperature Index (RTI) support under UL 746B
• Verification of antioxidant and light stabilizer packages in polyolefins for pipes, geomembranes, and cable insulation
• Quality control of recycled resins and assessment of re-stabilization after multiple processing cycles
• Automotive under-hood and interior component validation, where sustained elevated temperatures reduce ductility
• Wire and cable jacketing and insulation lifetime assessment in conjunction with IEC standards
• Lot acceptance testing comparing incoming resin against datasheet melt index and degradation limits
• Research into hydrolytic and thermal-oxidative degradation mechanisms of engineering plastics

FAQ

What is a plastic melt flow index aging test?

It is a comparative test in which the melt flow rate of a thermoplastic is measured before and after controlled thermal aging. The percentage change in melt index quantifies the degree of molecular degradation caused by the aging exposure.

Why does the melt index increase after aging?

Thermal-oxidative chain scission breaks polymer chains, lowering average molecular weight and melt viscosity, which allows more material to flow through the die in the same time and therefore raises the melt index.

Can the melt index decrease after aging?

Yes. Crosslinking, oxidative long-chain branching, or structuring reactions increase molecular size and melt viscosity, reducing the melt index. This behavior is observed in some PE grades, peroxide-modified resins, and highly filled compounds.

What sample quantity and form are required?

Approximately 5 to 10 g per determination is typical, supplied as pellets, powder, or small pieces cut from plaques or finished parts. Enough material should be provided for the baseline test plus every aging interval, with replicate measurements recommended.

What level of melt index change is acceptable?

Acceptance limits are set by the relevant product specification. Pipe-grade and cable-grade polyolefin specifications commonly restrict melt index change after a defined heat treatment to roughly 20–30 %, while RTI programs evaluate retention against end-use performance requirements.

How long does the test take?

A single melt index determination requires less than one hour, but the aging program dominates the timeline: exposure may range from several days at elevated temperatures to thousands of hours for thermal endurance and RTI studies.

What is the difference between MFR and MVR?

MFR (g/10 min) is the mass extruded in ten minutes measured by weighing cuttings, while MVR (cm³/10 min) is the extruded volume determined from piston displacement. The two are linked through the melt density at the test temperature.