Determination of Heat Release from Plastic Combustion: Test Methods, Standards, and Applications

Heat release is widely regarded as the single most informative indicator of the fire hazard posed by polymeric materials. Determination of heat release from plastic combustion quantifies both the total thermal energy a plastic releases when burned and the rate at which that energy is liberated, providing critical data for flame-retardant development, product certification, and fire safety engineering. This article explains the testing principles, applicable international standards, sample requirements, measured parameters, and typical applications of heat release testing for plastics.

Technical Overview

When plastics burn, the chemical energy stored in their polymer chains is converted into thermal energy, which drives flame spread, accelerates the ignition of adjacent materials, and ultimately governs the severity of a fire. The quantity and rate of this energy release are therefore central to evaluating the combustion behavior and fire performance of polymeric materials.

Modern heat release measurement is dominated by oxygen consumption calorimetry, which is based on Thornton's rule. This empirical principle states that, for most organic materials, approximately 13.1 MJ of heat is released per kilogram of oxygen consumed, regardless of the specific polymer involved. By continuously measuring the depletion of oxygen in the exhaust gases of a controlled combustion process, a calorimeter can compute the instantaneous heat release rate with an accuracy generally accepted to be within about ±5%.

Two complementary techniques are commonly used for plastics. The cone calorimeter (ISO 5660-1, ASTM E1354, GB/T 16172) measures the dynamic heat release rate of a specimen exposed to a controlled radiant heat flux, simulating realistic fire scenarios. The oxygen bomb calorimeter (ISO 1716, ASTM D240) measures the gross heat of combustion, that is, the total caloric value released under complete combustion in pure oxygen under pressure.

Testing Samples

Cone Calorimeter Specimens

Standard specimens are square plaques measuring 100 mm × 100 mm, with a thickness representative of the end-use product, typically up to 50 mm. Specimens are conditioned to constant mass at 23 ± 2 °C and 50 ± 5% relative humidity before testing. A minimum of three replicates is required, although six specimens are strongly recommended because fire tests are inherently variable.

Molded and Machined Plastic Parts

Sections cut from injection-molded, extruded, or thermoformed components can be tested directly, provided they can be trimmed to the required plaque geometry without altering their structure or surface condition.

Granules, Powders, and Compounds

For bomb calorimetry, granules or powders are ground and pressed into a stable pellet, or placed in a crucible with a known combustion aid, typically 0.5 g to 1.0 g of material per determination. Loose granules can also be tested in the cone calorimeter using a special holder that maintains a defined exposed surface.

Films, Foams, Coatings, and Composites

Thin films may be stacked to reach a measurable mass or mounted on a substrate that reflects the actual application. Flexible foams require edge-wrapping with aluminum foil to prevent dripping, and thermoset composites are tested at their nominal laminate thickness.

Testing Items

Heat release testing generates a comprehensive set of combustion parameters, including:

• Heat Release Rate (HRR), expressed in kW/m², as a function of time
• Peak Heat Release Rate (pHRR), the maximum instantaneous HRR
• Total Heat Release (THR or THE), expressed in MJ/m²
• Effective Heat of Combustion (EHC), expressed in MJ/kg, derived from the ratio of heat released to mass lost
• Time to Sustained Ignition (TTI), expressed in seconds
• Fire Growth Rate Index (FIGRA), the ratio of pHRR to the time at which it occurs
• Mass Loss Rate (MLR), expressed in g/s·m²
• Smoke Production Rate (SPR) and Total Smoke Production (TSP), together with Specific Extinction Area (SEA)
• Carbon monoxide and carbon dioxide yields, in mg/g or kg/kg
• Gross heat of combustion (higher heating value) and, where required, net calorific value, expressed in MJ/kg

Testing Methods

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

The conditioned specimen is mounted on a load cell within a holder, with the exposed surface facing a cone-shaped radiant heater set to a prescribed irradiance level, commonly 25, 35, 50, or 75 kW/m². An electric spark igniter positioned above the specimen initiates piloted ignition, though non-piloted (auto-ignition) tests are also possible. All combustion gases are drawn into an exhaust duct at a constant flow rate, where instruments continuously measure oxygen, carbon monoxide, carbon dioxide concentrations, smoke optical density, and gas temperature. Using oxygen consumption calorimetry, the system computes the instantaneous heat release rate throughout the test, which typically runs until the specimen is fully consumed or a defined end criterion is reached.

Oxygen Bomb Calorimeter Method (ISO 1716 / ASTM D240)

A weighed quantity of plastic, usually about 1 g, is sealed in a steel pressure vessel (the bomb) charged with oxygen to approximately 3 MPa. The bomb is submerged in a calibrated water bath, and the sample is ignited electrically. The resulting temperature rise of the water jacket, combined with the effective heat capacity of the calorimeter determined by benzoic acid calibration, yields the gross heat of combustion. This method provides the total caloric potential of the material but does not reflect burning rate or fire dynamics.

Supporting and Sector-Specific Methods

Other established techniques include the OSU rate-of-heat-release apparatus (ASTM E906) specified for aircraft cabin materials under FAR 25.853 requirements, the Single Burning Item test (EN 13823) used in European building product classification, and large-scale calorimetry such as EN 50399 for cables. Selection depends on the product sector, regulatory framework, and fire scenario of interest.

Data Evaluation

For each specimen, the full HRR curve is recorded, and averaged values of pHRR, THR, TTI, EHC, and MLR are reported together with their standard deviations. Results are interpreted against the applied irradiance level, since higher fluxes shorten ignition time and typically increase peak heat release. Comparative assessment of flame-retardant formulations is usually based on reductions in pHRR, FIGRA, and THR, and on improvements in ignition delay.

Applications

Heat release data for plastics support a wide range of industrial and regulatory purposes:

• Flame-retardant research and development, comparing polymer formulations and synergist systems through HRR curve analysis
• Building product fire classification under EN 13501-1 (Euroclasses) and GB 8624 frameworks
• Railway and mass transit material qualification under EN 45545-2
• Aircraft interior material certification against FAR 25.853 and equivalent CS-25 requirements
• Wire and cable performance assessment, including large-scale heat release tests on installed cable bundles
• Input data for computational fire modeling and fire dynamics simulation of real-scale scenarios
• Quality control during polymer compounding and incoming material verification in accredited third-party laboratories
• Scientific publication, safety data documentation, and insurance or fire-engineering risk assessments

FAQ

What is the difference between gross heat of combustion and heat release rate?

Gross heat of combustion, measured by bomb calorimetry, represents the total caloric energy released under complete combustion. Heat release rate, measured by cone calorimetry, describes how quickly that energy is released under realistic fire conditions. A material may have a moderate total calorific value yet still present high fire hazard if its peak heat release rate is very sharp and high.

Which heat flux level should be selected for cone calorimeter testing?

The selection depends on the intended fire scenario. Fluxes of 25–35 kW/m² represent slowly developing fires, 50 kW/m² is the most widely used reference condition, and 75 kW/m² simulates developed fires or exposure to nearby burning objects. Many testing programs run multiple flux levels to characterize ignition and burning behavior across a range of scenarios.

How many specimens are required?

ISO 5660-1 requires a minimum of three specimens per condition, but six replicates are recommended to account for the natural variability of polymer combustion. Bomb calorimeter determinations are typically performed in duplicate or triplicate until results agree within specified repeatability limits.

How long does a typical test take?

A single cone calorimeter run usually takes 10 to 30 minutes of burning time depending on material thickness and flux, with additional time for setup, calibration checks, and conditioning. Bomb calorimetry is faster per determination, generally completed within 15 to 20 minutes after bomb preparation.

Can heat release results be used directly for regulatory certification?

Cone calorimeter data serve as primary input for certain classification schemes and are widely accepted in R&D and fire safety engineering, while specific sectors such as aviation, rail, and building products may additionally require designated large-scale or configuration-specific tests. Accredited third-party laboratories can advise on the exact standard combination required for a target market.