Heat release rate (HRR) is widely recognized as the single most important quantitative variable in characterizing the fire hazard of burning materials. Determination of the heat release rate of plastics, typically performed using a cone calorimeter in accordance with ISO 5660-1, ASTM E1354, and GB/T 16172, provides essential data on ignitability, fire growth, smoke production, and combustion efficiency. This article presents a comprehensive technical overview of heat release rate testing of plastics, covering sample requirements, measured parameters, test methods, applications, and frequently asked questions.
Technical Overview
Heat release rate (HRR) is defined as the rate at which thermal energy is liberated from a burning material per unit area (kW/m²) and is considered the most informative single measurement in fire science. Because heat released by a burning plastic feeds back into the material and drives further pyrolysis and flame spread, HRR governs fire growth intensity, the time to flashover in a compartment, and the overall hazard posed by a product in a real fire scenario.
Modern heat release rate measurement is based on the oxygen consumption principle. Extensive research has demonstrated that for most organic materials, including virtually all plastics, approximately 13.1 MJ of heat is released per kilogram of oxygen consumed, with a measurement uncertainty of only about 5 percent. By continuously analyzing the oxygen concentration in the exhaust duct of a controlled combustion apparatus, laboratories can calculate heat release rate with high accuracy and reproducibility without the need for direct calorimetric measurement of all heat losses.
The cone calorimeter is the most widely used bench-scale instrument for this purpose. A truncated cone-shaped heater exposes a horizontally or vertically oriented specimen to a controlled radiant flux, typically between 10 and 100 kW/m², simulating a range of fire exposure severities. A spark igniter initiates piloted ignition, and the exhaust system collects all combustion products for gas analysis. Principal reference standards include ISO 5660-1, ASTM E1354, and the Chinese national standard GB/T 16172 for reaction-to-fire testing of building materials and products. Complementary methods include ASTM E906 (OSU calorimeter) and ASTM D7309 (microscale combustion calorimetry).
Testing Samples
Thermoplastics
Common thermoplastic materials submitted for heat release rate testing include polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylonitrile butadiene styrene (ABS), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), polycarbonate (PC), and polyamide (PA). These materials typically exhibit melting and dripping behavior during testing, which may require the use of retainer frames and mesh screens to maintain a stable specimen geometry.
Thermosetting Plastics
Thermosetting materials such as epoxy resins, phenolic resins, unsaturated polyesters, and rigid polyurethanes char rather than melt during combustion. Their heat release characteristics are strongly influenced by char formation, which acts as a thermal barrier and often results in lower peak heat release rates.
Composite, Filled, and Flame-Retarded Materials
Glass-fiber or carbon-fiber reinforced plastics, mineral-filled compounds, and formulations containing halogenated, phosphorus-based, or intumescent flame retardants are frequently evaluated to quantify the effectiveness of flame retardant strategies. Heat release rate curves provide direct evidence of reduced peak values and delayed ignition achieved through additives.
Foams, Films, Sheets, and Coatings
Cellular plastics such as flexible and rigid polyurethane foam, expanded polystyrene (EPS), and extruded polystyrene (XPS) are tested due to their high fire load. Thin films, sheets, and coatings may require stacking to the minimum specimen mass or mounting on a substrate to obtain meaningful results.
Specimen Preparation Requirements
- Standard specimen dimensions of 100 mm × 100 mm, with thickness not exceeding 50 mm for the cone calorimeter
- Conditioning at 23 ± 2 °C and 50 ± 5% relative humidity to constant mass prior to testing
- Aluminum foil wrapping of specimen edges and bottom, with retainer frames used when necessary
- Minimum of three replicate specimens, with at least six recommended when statistical reliability is required
Testing Items
Heat release rate testing generates a comprehensive set of quantifiable fire performance parameters, including:
Peak Heat Release Rate (peak HRR): the maximum instantaneous heat release rate, indicating the greatest thermal threat during combustion
Average Heat Release Rate: mean values calculated over defined periods, such as 180 s and 300 s after ignition
Total Heat Release (THR): the cumulative energy released, representing the total fire load contributed by the specimen
Time to Ignition (TTI): the period of radiant exposure required before sustained flaming occurs
Effective Heat of Combustion (EHC): the ratio of heat release rate to mass loss rate, indicating combustion efficiency
Mass Loss Rate (MLR): the rate of fuel gasification measured continuously by a load cell
Smoke Production Rate (SPR) and Total Smoke Production (TSP): quantified by laser photometry in the exhaust duct
Fire Performance Index (FPI) and Fire Growth Index (FGI): derived ratios used for material comparison and ranking
Yields of carbon monoxide and carbon dioxide, indicating combustion completeness and toxic hazard potential
Residual mass fraction and char yield after test completion
Testing Methods
Cone Calorimeter Method (ISO 5660-1 / ASTM E1354 / GB/T 16172)
The cone calorimeter is the internationally recognized benchmark method. A specimen is placed on a load cell beneath a conical radiant heater and exposed to a set irradiance level, most commonly 25, 35, 50, or 75 kW/m², corresponding to small developing fires through post-flashover conditions. An electric spark igniter initiates piloted ignition, and combustion gases are drawn through an exhaust duct at approximately 24 L/s, where oxygen, carbon monoxide, and carbon dioxide concentrations plus optical smoke density are continuously measured. Heat release rate is calculated from the oxygen depletion using the oxygen consumption principle.
OSU Calorimeter Method (ASTM E906)
The Ohio State University rate-of-heat-release apparatus measures heat release directly through the temperature rise of the exhaust airflow. It remains significant in aviation applications, where regulatory requirements for aircraft interior materials specify maximum heat release rates measured with this instrument.
Microscale Combustion Calorimetry (ASTM D7309)
The microscale combustion calorimeter (MCC) requires only milligram-scale specimens. The plastic is pyrolyzed in a controlled nitrogen atmosphere, and the evolved gases are combusted in a high-temperature furnace. This method rapidly determines specific heat release capacity, peak heat release rate per unit mass, and total heat of combustion, making it valuable for screening, quality control, and research.
Typical Test Procedure
- Calibration of the cone heater irradiance using a heat flux meter, gas analyzers, and the load cell
- Measurement and conditioning of the specimen, followed by wrapping and framing as required by the standard
- Placement of the specimen on the holder with appropriate insulation backing
- Application of the selected radiant flux with simultaneous start of data acquisition and spark ignition
- Continuous recording of heat release rate, mass loss, smoke, and gas concentrations until flaming ceases
- Post-test calculation of derived parameters and preparation of the test report with full HRR curves
Reporting and Data Interpretation
A complete test report includes specimen identification, thickness and mass, test orientation, irradiance level, ignition time, complete heat release rate and smoke production curves, peak and average HRR values, total heat release, effective heat of combustion, and mass loss data. Interpretation typically focuses on peak HRR for flashover propensity, THR for fire load contribution, and TTI for escape time implications.
Applications
Heat release rate data for plastics support a broad range of engineering, regulatory, and research objectives:
Building and construction: reaction-to-fire performance assessment of insulation boards, wall linings, flooring, and piping under GB/T 16172 and related classification frameworks, with data supporting compliance strategies for building codes
Railway transportation: EN 45545-2 fire safety requirements for railway vehicles rely on heat release data obtained with the cone calorimeter for interior materials and components
Aviation: qualification of aircraft cabin interior plastics against heat release limits specified by airworthiness regulations
Wire and cable: evaluation of insulation and jacketing compounds, often combined with smoke and toxicity assessment
Electronics and appliances: fire risk assessment of housings, connectors, and circuit board laminates under IEC fire hazard testing frameworks
Flame retardant development: comparative evaluation of additive packages, polymer modifications, and nanocomposite formulations
Fire modeling and safety engineering: input parameters for zone models and computational fluid dynamics simulations used to predict fire development in real-scale scenarios
Product certification and quality control: periodic verification of material fire performance in accredited third-party laboratories
FAQ
What is heat release rate and why is it important for plastics?
Heat release rate quantifies how quickly a burning plastic liberates thermal energy. Because it controls flame spread, fire growth, and the potential for flashover, it is regarded by fire protection engineers as the most meaningful indicator of fire hazard among all reaction-to-fire properties.
Which standards govern heat release rate testing of plastics?
The principal standards are ISO 5660-1, ASTM E1354, and GB/T 16172 for the cone calorimeter, ASTM E906 for the OSU calorimeter, and ASTM D7309 for microscale combustion calorimetry. Selection depends on specimen availability, regulatory context, and the intended application of the data.
What are the standard specimen dimensions and test conditions?
Cone calorimeter specimens are typically 100 mm × 100 mm with a thickness up to 50 mm. Common exposure irradiances are 25, 35, 50, and 75 kW/m², with 50 kW/m² frequently selected as representative of a well-developed fire.
How long does a typical test take?
Individual specimen runs generally last 10 to 30 minutes depending on material thickness and burning behavior. Including setup, calibration, and replicate testing, a complete evaluation of one material typically requires several hours in the laboratory.
What is considered a good peak heat release rate value?
Values are application dependent. As a general guideline, materials with peak HRR below 200 kW/m² are considered low hazard, values between 200 and 500 kW/m² moderate, and values exceeding 500 kW/m² high hazard, though classification limits specified by product regulations always take precedence.
Can heat release rate results predict real-scale fire behavior?
Bench-scale HRR data correlate well with large-scale fire tests and serve as essential inputs for fire modeling. However, real fire performance also depends on geometry, installation conditions, and interactions with other materials, so full-scale verification is recommended for final safety assessments.