Determination of Mass Loss Rate in Burning Plastics: Principles, Test Methods, and Applications

Determination of the mass loss rate (MLR) of burning plastics provides a quantitative measure of how quickly a polymeric material decomposes into combustible fuel when exposed to a controlled radiant heat source. As a core reaction-to-fire parameter obtained with the cone calorimeter in accordance with ISO 5660-1, ASTM E1354, and GB/T 16172, mass loss rate data underpin flame-retardant development, fire hazard modeling, and material qualification across the building, transportation, electrical, and electronics industries. This article explains the measurement principle, specimen requirements, test procedures, data outputs, and practical applications of plastic burning mass loss rate determination.

Technical Overview

The mass loss rate (MLR) of a burning plastic is the negative time derivative of the specimen mass during exposure to a defined radiant heat flux, typically expressed in grams per second (g/s) or normalized to the exposed surface area as grams per square meter per second (g/(m²·s)). For a standard 100 mm × 100 mm cone calorimeter specimen, the exposed area is 0.01 m². Because virtually all combustible volatiles released by a decomposing polymer leave the specimen as gaseous products, the MLR is a direct and continuous indicator of the rate at which the material generates fuel for the flame.

Physically, the measurement captures the complete thermal decomposition sequence of the polymer: radiant heating, softening or melting, pyrolytic chain scission, evolution of combustible volatiles, flaming gas-phase combustion, and finally char formation or burnout. Charring materials exhibit a progressively decaying MLR curve as a protective residue builds up, while non-charring thermoplastics often show a sharp peak shortly after full flame spread across the surface.

The primary instrument for this determination is the cone calorimeter. The specimen rests on a precision load cell (typically with 0.01 g resolution and accuracy better than ±0.1 g) located beneath a conical radiant electric heater, with an electric spark igniter positioned above the exposed face. Mass is recorded continuously, generally at intervals of one second or finer, and the MLR is computed by numerically differentiating the smoothed mass-versus-time curve.

The mass loss rate is scientifically significant because it links directly to fire intensity: the heat release rate equals the mass loss rate multiplied by the effective heat of combustion of the volatiles. A reduced peak MLR therefore indicates reduced heat release, slower fire growth, and lower flashover propensity. The governing methodology is defined in ISO 5660-1, with equivalent or closely related procedures in ASTM E1354 and the Chinese national standard GB/T 16172. Complementary techniques include the Fire Propagation Apparatus method of ASTM E2058 and milligram-scale thermogravimetric analysis per ISO 11358 and ASTM E1131.

Testing Samples

Thermoplastics

Commodity and engineering thermoplastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), high-impact polystyrene (HIPS), ABS, rigid and flexible PVC, PMMA, polycarbonate (PC), polyamide 6 and 66, PET, and POM are routinely tested. Specimens are prepared as flat 100 mm × 100 mm plaques, injection-molded or cut from sheet, with thickness reported to the nearest millimeter (typically up to 50 mm). For materials that melt and drip, the standard permits the use of a retaining wire grid in horizontal orientation to keep molten polymer on the load cell platform, and this must be documented in the report.

Thermosets, Elastomers, and Foams

Cross-linked polymers such as epoxy resins (including glass- and carbon-fiber laminates), phenolics, unsaturated polyesters, silicone elastomers, and rigid or flexible polyurethane foams are tested in the same geometry. For foams, thickness and bulk density must be recorded because they strongly influence heating rate and MLR behavior. Low-density and highly intumescent materials may require additional edge retention and additional replicate specimens to obtain representative results.

Composites, Coatings, and Multilayer Assemblies

Fiber-reinforced laminates, intumescent coating systems applied to their actual substrate, and multilayer structures with barrier films or adhesive layers can be evaluated as complete assemblies. Because heat and mass transfer through the stack are thickness- and substrate-dependent, results apply only to the tested construction and cannot be simply generalized to other build-ups.

Specimens Cut from Finished Products

Flat sections cut from electrical and electronic equipment enclosures, cable management components, automotive interior trim, railway and aircraft interior panels, and building products may be tested, provided a representative 100 mm × 100 mm surface can be obtained. The orientation relative to any anisotropic structure, the exposed face, and the presence of skins, textures, or printed layers must be recorded, as they measurably affect time to ignition and peak MLR.

Unless otherwise specified by the product standard, all specimens are conditioned to equilibrium at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours, wrapped on the bottom and edges with aluminum foil, and mounted in the specimen holder, with or without the retainer frame as required by the standard and the material behavior.

Testing Items

A mass loss rate determination according to ISO 5660-1, ASTM E1354, or GB/T 16172 delivers the following quantitative outputs:

• Mass loss rate as a function of time (full MLR curve)
• Peak mass loss rate (peak MLR), in g/s or g/(m²·s)
• Average mass loss rate over defined periods, commonly 60 s, 180 s, and 300 s after ignition
• Specific mass loss rate normalized to the exposed surface area
• Total mass loss in grams and as a percentage of initial specimen mass
• Residual mass fraction (char or ash yield)
• Time to sustained ignition, time to peak MLR, and time to flame extinction
• Maximum average MLR indices used for material ranking
• Companion data from the same test run, when instrumented: heat release rate, effective heat of combustion, smoke production rate, and CO/CO₂ yields

Testing Methods

Cone Calorimeter Method (ISO 5660-1 / ASTM E1354 / GB/T 16172)

The conditioned specimen is placed on the load cell assembly and exposed to a uniform radiant flux from the conical heater, most commonly at 25, 35, 50, or 75 kW/m², selected to represent the intended fire scenario. Horizontal orientation is standard for MLR-focused work, while vertical orientation is used for specific applications such as wall linings. An electric spark igniter pilots the evolved volatiles, and the exhaust system operates at the standardized flow of approximately 24 L/s. The load cell records specimen mass continuously throughout the test, which continues until all flaming has ceased and the mass has stabilized. The MLR curve is obtained by differentiating the mass–time record after applying a centered moving-average or polynomial smoothing routine, and all derived values are calculated exactly as prescribed by the standard.

Fire Propagation Apparatus Method (ASTM E2058)

The Fire Propagation Apparatus exposes smaller specimens to controlled radiant fluxes in a co-flowing oxidizer stream while a load cell tracks mass. In addition to MLR curves, this method supports derivation of fundamental flammability parameters such as critical mass flux at ignition and heat of gasification, making it valuable for research and fire model input.

Thermogravimetric Analysis (ISO 11358 / ASTM E1131)

Thermogravimetry measures mass loss of milligram-scale samples at programmed heating rates under nitrogen or air. It is an efficient screening and kinetics tool for comparing degradation onset and residue between formulations, but it is not a reaction-to-fire test and cannot substitute for cone calorimeter MLR data in regulatory contexts.

Quality Assurance and Data Integrity

Reliable results depend on rigorous verification: calibration of the load cell with certified reference masses, mapping of heater flux uniformity with a calibrated heat flux meter, confirmation of exhaust flow, and documented igniter function. Accredited third-party laboratories typically test a minimum of three replicate specimens per condition and report mean values with scatter, since relative variability of 10% to 20% is normal for peak MLR even under well-controlled conditions. Known bias sources, including moisture uptake, soot deposition on the holder, edge burning, and dripping losses, are controlled or explicitly noted in the test report.

Applications

Mass loss rate data from plastics are used across the full product lifecycle, from research and development to regulatory conformity:

• Flame-retardant formulation development, comparing peak and average MLR of halogen-free, intumescent, and mineral-filled compounds against baselines
• Verification of charring efficiency through residual mass fraction and MLR curve shape
• Input data for pyrolysis models and computational fluid dynamics fire simulations used in performance-based design
• Technical support for reaction-to-fire evaluation of building products, railway and aircraft interior materials, and marine outfitting components
• Material preselection and qualification support for electrical and electronic equipment enclosures under frameworks aligned with IEC 60695 fire hazard testing guidance and UL 94-style small-flame classifications
• Incoming material quality control, supplier verification, and batch-to-batch consistency monitoring
• Academic and industrial research on polymer degradation kinetics and structure–flammability relationships

FAQ

What is the mass loss rate of a burning plastic?

It is the rate at which a plastic specimen loses mass during controlled exposure to radiant heat under flaming combustion, expressed in g/s or g/(m²·s). Because the mass loss corresponds to the generation of combustible volatiles, the MLR is a direct measure of fuel supply to the flame and a primary indicator of burning intensity.

Which standards govern this determination?

The principal method is the cone calorimeter procedure of ISO 5660-1, which is technically equivalent to ASTM E1354 and aligned with the Chinese national standard GB/T 16172. ASTM E2058 offers an alternative apparatus-based approach, while ISO 11358 and ASTM E1131 cover thermogravimetric screening at the milligram scale.

What specimen size and conditioning are required?

The standard specimen is a flat square of 100 mm × 100 mm with thickness, typically up to 50 mm, measured and reported. Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours unless a product standard specifies otherwise, and are foil-wrapped and mounted in the standard holder before testing.

How is the irradiance level selected?

The heat flux is chosen to represent the fire scenario of interest: 25 kW/m² represents a mild early-fire exposure, 35 kW/m² a general reference condition, 50 kW m² a developing-room-fire condition, and 75 kW m² a fully developed post-flashover exposure. Comparing MLR curves at several fluxes reveals how material performance degrades with escalating fire severity.

How does mass loss rate relate to heat release rate?

The instantaneous heat release rate is approximately the product of the mass loss rate and the effective heat of combustion of the released volatiles. A lower peak MLR therefore generally corresponds to a lower peak heat release rate, slower fire growth, and reduced hazard, provided the combustion efficiency of the volatiles is comparable.

Can melting, dripping, or intumescent plastics be tested accurately?

Yes, with precautions. A retaining grid may be used for materials that flow or drip so that molten polymer remains on the weighing platform, and intumescent materials may be tested with an open frame or reduced restraint as the standard allows. Such deviations must be clearly identified in the report because they influence the measured MLR.

How many specimens are needed and what repeatability can be expected?

At least three replicate specimens per test condition are recommended. Even in experienced accredited third-party laboratories, relative scatter of roughly 10% to 20% on peak MLR is normal, so statistically meaningful comparisons between formulations require replicate testing and, ideally, identical specimen preparation and thickness.

How long does a typical test take?

A single determination, including instrument checks, specimen mounting, exposure, and cool-down, typically requires 20 to 40 minutes, with longer runs for thick, charring, or slowly burning materials. Added to this are conditioning time and calibration intervals performed as part of routine quality assurance.