Determination of Heat Release from the Combustion of Cellular Plastics: Principles, Methods, and Sta

Determining the heat released during the combustion of cellular (foam) plastics is one of the most important measurements in fire-safety engineering, because heat release rate directly drives flame spread, flashover, and overall fire hazard. This article explains the oxygen-consumption principle behind modern heat release measurement, the standard test methods such as ISO 5660-1, ASTM E1354, and GB/T 16172, and the sample types, measured parameters, and typical applications involved in foam plastic heat release testing.

Technical Overview

Cellular plastics—including polyurethane, polystyrene, polyolefin, and phenolic foams—are widely used as thermal insulation, cushioning, packaging, and lightweight core materials. Because of their low density, high surface-to-mass ratio, and often high hydrocarbon content, foamed polymers can release heat very rapidly when ignited. The quantification of this heat release is therefore the cornerstone of fire-hazard assessment, material classification, and fire-simulation modeling.

Modern heat release measurement is almost universally based on the oxygen-consumption principle formulated by Huggett: for most organic materials, the heat released per unit mass of oxygen consumed is approximately constant at 13.1 MJ/kg of oxygen, with an uncertainty of about ±5 percent. By measuring the oxygen depletion (and, for improved accuracy, the CO and CO₂ concentrations) in the exhaust duct of a fire test apparatus, the heat release rate can be determined continuously in real time without needing to capture all of the combustion products in a calorimeter vessel.

For foam plastics, the primary laboratory instrument is the cone calorimeter, which exposes a small, horizontally or vertically oriented specimen to a controlled radiant heat flux and records ignition time, heat release rate, total heat release, effective heat of combustion, smoke production, and mass loss. Supplementary techniques—such as bomb calorimetry for gross heat of combustion, OSU calorimetry for aircraft interiors, and intermediate-scale room-corner tests—complete the assessment across a range of scales and regulatory contexts.

Testing Samples

Heat release testing is applicable to virtually all thermoset and thermoplastic cellular plastics, in both neat and flame-retarded grades, as well as foam-containing composites and sandwich panels. Typical sample categories handled by accredited third-party laboratories include the following:

Rigid Polyurethane (PUR) and Polyisocyanurate (PIR) Foams

Rigid PUR and PIR foams used as building insulation boards, spray foams, pipe insulation, and sandwich-panel cores. PIR formulations generally exhibit lower peak heat release rates and higher char formation than standard PUR, and cone calorimeter data are frequently used to compare and optimize these formulations.

Expanded and Extruded Polystyrene Foams (EPS and XPS)

Thermoplastic insulation foams that melt and shrink away from the heat source before igniting. Because of this behavior, specimen preparation (edge framing, grid use, and retainer frames) significantly influences the measured heat release, and standardized mounting procedures must be strictly followed.

Flexible Polyurethane Foams

Flexible slabstock and molded foams used in upholstered furniture, mattresses, and automotive seating. These low-density, highly combustible foams are commonly tested as received or after application of cover materials and flame-retardant treatments, often at multiple irradiance levels.

Polyolefin and Other Thermoplastic Foams

Polyethylene (PE), polypropylene (PP), ethylene-vinyl acetate (EVA), and cross-linked olefin foams used in packaging, sports goods, automotive components, and expansion joints.

Phenolic, PVC, Melamine, and Specialty Foams

High char-forming or inherently flame-resistant foams, such as phenolic insulation foam and melamine acoustic foam, whose low heat release and low smoke contribution must be demonstrated for high-fire-performance applications.

Specimen Preparation and Conditioning

For cone calorimeter testing, specimens are typically cut to 100 mm × 100 mm squares with a representative thickness (commonly up to 50 mm), wrapped on the underside and edges with aluminum foil, and placed in a retainer frame where required. Unless otherwise specified, specimens are conditioned to constant mass at 23 ± 2 °C and 50 ± 5 % relative humidity prior to testing. For materials that intumescence, melt, or shrink, standardized mounting accessories such as grids and edge frames are used to ensure reproducibility.

Testing Items

A heat release test campaign on cellular plastics typically reports the following parameters, each of which characterizes a different aspect of burning behavior:

• Time to sustained flaming ignition (TTI, s) at the selected irradiance level.
• Heat release rate per unit area (HRR, kW/m²), recorded continuously throughout the test.
• Peak heat release rate (pk-HRR, kW/m²), the single most widely quoted fire-hazard indicator for foamed polymers.
• Average heat release rate over defined intervals (e.g., 60 s, 180 s, 300 s after ignition).
• Total heat release (THR, MJ/m²), representing the fuel contribution of the specimen.
• Effective heat of combustion (EHC, MJ/kg), the ratio of heat released to mass lost, reflecting combustion efficiency.
• Mass loss rate (MLR, g/s·m²), obtained from the integrated load cell beneath the specimen.
• Smoke production rate and total smoke release (SPR, TSR), measured optically across the exhaust duct.
• Carbon monoxide and carbon dioxide yields (kg/kg), indicating smoke toxicity potential and incomplete combustion.
• Gross heat of combustion / calorific value (PCS, MJ/kg), determined separately by oxygen bomb calorimetry.

Testing Methods

The choice of method depends on the required scale, the regulatory framework, and whether heat release rate data or total calorific value is needed. The principal standardized methods are described below.

Cone Calorimetry — ISO 5660-1 / ASTM E1354 / GB/T 16172

The cone calorimeter is the internationally recognized bench-scale reference method. A 100 mm × 100 mm specimen is irradiated by a conical heater at a selected flux—commonly 25, 35, 50, 75, or 100 kW/m², chosen to simulate small, developing, or fully developed fire exposure. A spark igniter induces piloted ignition, and all combustion products are drawn into an exhaust duct at a standardized flow rate (typically 24 L/s). Oxygen, CO, and CO₂ concentrations, duct temperature, pressure differential across the measurement orifice, optical smoke density, and specimen mass are recorded continuously, allowing calculation of instantaneous and cumulative heat release via the oxygen-consumption equations. Testing continues until all flaming has ceased and mass loss has stabilized. ISO 5660-2 extends the same apparatus to dynamic smoke production measurement. The corresponding Chinese national standard is GB/T 16172 (Parts 1 and 2), which is technically aligned with ISO 5660.

Oxygen Bomb Calorimetry — ISO 1716 / GB/T 14402 / EN ISO 1716

Where the regulatory question is the total energy content of the material rather than the burning rate—for example, classification of building products as non-combustible or limited-combustible—the gross heat of combustion (PCS) is determined in an isothermal or adiabatic oxygen bomb calorimeter. A small, precisely weighed powdered specimen burns in pure oxygen at elevated pressure, and the temperature rise of the calorimeter yields the calorific value in MJ/kg. For cellular plastics containing flame retardants, gas washing and correction procedures account for acids and other combustion residues.

OSU Calorimetry for Aircraft Interiors — ASTM E906 / 14 CFR Part 25

Aviation regulations administered by civil aviation authorities require interior materials, including certain foams, to pass a rate-of-heat-release test in the Ohio State University (OSU) calorimeter. A 150 mm × 150 mm specimen is exposed to a radiant flux of 35 kW/m², and both the maximum heat release rate achieved during the first five minutes and the total heat released in the first two minutes are compared against regulatory limits. This method differs from cone calorimetry in its heat-exchange-based rather than oxygen-consumption-based measurement principle.

Intermediate- and Full-Scale Heat Release Tests

For building products, EN 13823 (Single Burning Item, SBI) derives the FIGRA fire-growth-rate index from heat release measured on wall and ceiling specimens exposed to a burner, and feeds directly into European reaction-to-fire classification. ISO 9705 and comparable room-corner reference scenarios measure the heat release of complete linings at full scale. For upholstered furniture containing flexible foam, furniture calorimeters per ASTM E1474 or equivalent methods determine the heat release of complete composites of foam, batting, and cover fabric.

Microscale Combustion Calorimetry — ASTM D7309

When only milligram quantities of material are available, or for rapid screening of foam formulations, microscale combustion calorimetry pyrolyzes the specimen in nitrogen and combusts the pyrolysis products in excess oxygen. The method yields specific heat release rate, peak heat release capacity, and total heat of combustion, and correlates well with cone calorimeter peak HRR for many polymers, making it valuable in research and formulation development.

Applications

Heat release data for cellular plastics underpin safety decisions across many industries:

• Building product classification: Demonstrating compliance of insulation foams with reaction-to-fire classes under Euroclass systems, GB 8624, and equivalent national schemes, where cone calorimeter and bomb calorimeter results provide supporting and classification data.
• Transportation: Qualification of seating foams, headliners, and insulation for rail vehicles (e.g., EN 45545-2), road vehicles, ships, and aircraft, where peak and total heat release limits are strictly controlled.
• Furniture and bedding: Assessment of flexible foam composites for open-flame and smoldering performance in support of flammability regulations and voluntary safety programs.
• Fire engineering and modeling: Supplying cone-calorimeter-derived ignition temperatures, HRR curves, effective heat of combustion, and yields as input data for zone models and computational fluid dynamics fire simulations used in performance-based design.
• Research and formulation development: Ranking flame-retardant systems (halogenated, phosphorus-based, nitrogen-based, mineral-filled, and reactive) by their effect on peak HRR, THR, and char yield.
• Quality control and batch conformity: Verifying that production foam batches retain the heat-release characteristics qualified during type approval, particularly for safety-critical applications.

FAQ

Why is heat release rate considered the most important fire property of foam plastics?

Heat release rate governs how much additional fuel and energy a burning material feeds back into the fire, which in turn controls flame spread, the rate of fire growth, and the time available for occupants to escape. Two foams with identical ignition resistance can pose very different hazards if their peak heat release rates differ substantially; hence regulators and fire engineers place HRR at the center of material assessment.

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

Gross heat of combustion, measured by bomb calorimetry, is the total chemical energy per unit mass released under complete combustion in pure oxygen—it is a material property. Heat release rate, measured by cone or OSU calorimetry, describes how quickly that energy is released under realistic, ventilation-limited flaming conditions. Both are needed: the calorific value indicates fuel potential, while HRR indicates fire severity.

Which irradiance level should be selected for cone calorimeter testing of foams?

The level is chosen to represent the scenario of interest: 25 kW/m² approximates exposure from a small ignition source, 35 kW/m² corresponds to many regulatory screening conditions, and 50–75 kW/m² simulate developing or post-flashover environments. For foam plastics, 35 kW/m² and 50 kW/m² are the most common choices, and testing at multiple levels is recommended to characterize the material across the fire growth range.

How do melting and shrinkage of foams such as EPS and XPS affect the results?

Thermoplastic foams may melt, drip, and shrink away from the heater, changing the exposed area and the effective heat flux. Standardized mounting—aluminum foil backing, retainer frames, and specimen grids when specified—keeps the melt in the irradiation zone and improves reproducibility. Test reports should always state the mounting arrangement used.

How long does a typical test take, and how should results be reported?

A single cone calorimeter run typically lasts from 10 to 30 minutes of burning time plus instrument warm-up and calibration; a full test series with replicates and multiple irradiance levels usually requires several days. Reports should identify the standard applied, irradiance level, specimen orientation and thickness, mounting details, ignition time, peak and average HRR, THR, EHC, smoke and gas yields, and the mean and scatter of at least three replicate determinations.

Can heat release results from bench-scale tests predict real fire behavior?

Bench-scale HRR data are the primary input for predictive fire models and correlate well with large-scale outcomes for many geometries, but they are not standalone pass/fail predictors. Regulatory frameworks therefore combine cone or bomb calorimeter results with intermediate-scale tests, such as the Single Burning Item test or room-corner scenarios, to confirm performance at the scale of actual application.