Determination of the Combustion Heat Value (Gross Calorific Value) of Foam Plastics: Standards, Meth

The combustion heat value of foam plastics, more precisely the gross heat of combustion determined by oxygen bomb calorimetry, is a decisive parameter in the fire classification of thermal insulation and other polymeric construction materials. Accredited third-party laboratories perform this test in accordance with EN ISO 1716, GB/T 14402 and related standards to quantify the total energy a material can release in a fully developed fire. This article explains the test principle, applicable standards, sample preparation, calculation method and typical applications for EPS, XPS, PUR/PIR, phenolic and polyolefin foams.

Technical Overview

The combustion heat value, also known as the calorific value or heat of combustion, expresses the total energy released when a material is completely burned under standardized oxygen-rich conditions. Two quantities are conventionally reported: the gross heat of combustion (PCS), in which the water formed during combustion is condensed and its latent heat is included, and the net heat of combustion (PCI), in which the water remains as vapor. For foam plastics, the gross value is the figure cited in most fire classification schemes and is expressed in megajoules per kilogram (MJ/kg).

Because cellular plastics are organic polymers with very low density and high fuel content, they typically exhibit large calorific values, often comparable to those of wood or even liquid fuels. Measuring this value allows regulators, designers and manufacturers to evaluate fire load, verify compliance with combustibility class limits, and compare the fire performance of competing insulation products. Typical ranges are as follows:

• Expanded polystyrene (EPS): approximately 38 to 42 MJ/kg
• Extruded polystyrene (XPS): approximately 35 to 39 MJ/kg
• Polyurethane (PUR) foam: approximately 26 to 30 MJ/kg
• Polyisocyanurate (PIR) foam: approximately 20 to 26 MJ/kg
• Phenolic foam: approximately 12 to 22 MJ/kg
• Polyethylene foam: approximately 42 to 46 MJ/kg
• PVC foam: approximately 18 to 22 MJ/kg

Flame retardants, inorganic fillers and graphite additives reduce the effective heat of combustion, which is why the test must be performed on the actual commercial formulation rather than the base polymer.

Testing Samples

Testing is performed on representative specimens taken from finished foam products, including boards, blocks, sandwich panel cores, sprayed foams and molded parts. The material is generally reduced to small particles or a powdered, pelletized form so that combustion is complete and reproducible.

Expanded Polystyrene (EPS)

Bead-molded EPS boards and composite products such as graphite EPS. The brittle cellular structure is easily crushed, and static charging during grinding must be controlled.

Extruded Polystyrene (XPS)

Dense, closed-cell boards used for perimeter and roof insulation. Specimens are cut across the board thickness to capture any skin-to-core variation.

Polyurethane (PUR) and Polyisocyanurate (PIR) Foams

Rigid boardstock, laminate cores and sprayed foams. These materials char and intumesce during burning, so careful comminution and, where necessary, the use of a combustion aid are important.

Phenolic Foam

Highly cross-linked, low-calorific insulation foam whose combustion residue content is significant; the test quantifies its substantially lower fuel contribution.

Polyethylene, PVC and Other Cellular Plastics

Flexible and semi-rigid foams used in packaging, cushioning, flotation and sealing, as well as specialty materials such as melamine and silicone foams.

Prior to testing, specimens are conditioned, typically at 23 ± 2 °C and 50 ± 5 % relative humidity, until constant mass is reached, so that the reported value reflects the product as delivered.

Testing Items

accredited laboratories typically report the following quantities for foam plastic samples:

• Gross heat of combustion (PCS) per unit mass, MJ/kg
• Net heat of combustion (PCI) per unit mass, MJ/kg, where required
• Gross heat of combustion per unit area (MJ/m²) of an individual substantially component, used in the classification of multi-component or sandwich products
• Mean value and scatter of at least three valid determinations

The result serves directly as an input to fire classification under EN 13501-1 (Euroclasses) and GB 8624, where defined calorific value ceilings must not be exceeded for a given class.

Testing Methods

Principle of Oxygen Bomb Calorimetry

A weighed specimen of approximately 0.5 g is placed in a sealed stainless-steel bomb charged with pure oxygen at about 3.0 MPa. The specimen is ignited by an electrically heated fuse arrangement, and the heat released by its complete combustion is absorbed by the calibrated calorimeter system, whose temperature rise is measured with a high-resolution sensor. From the temperature rise and the energy equivalent of the apparatus, the heat released per unit mass is calculated.

Applicable Standards

• EN ISO 1716, Reaction to fire tests for products — Determination of the gross heat of combustion (bomb calorimetric method), the reference method for construction products in Europe
• GB/T 14402, the Chinese national standard for the determination of the combustion heat value of building materials, technically aligned with EN ISO 1716
• ISO 1928 and DIN 51900, general bomb calorimetric methods widely used for solid fuels and engineering materials
• ASTM D240 and related ASTM practices, referenced for calorimeter operation and calibration conventions

Sample Preparation

Foam specimens are cut, ground or shredded to small particles and, where appropriate, pressed into a coherent tablet to prevent loss of fragments. Products that melt, shrink away from the igniter, or otherwise burn incompletely are tested together with a weighed quantity of a certified combustion aid such as benzoic acid, cellulose or paraffin of known calorific value; the energy contributed by the aid, established in separate runs, is subtracted from the total.

Calibration and Test Procedure

The energy equivalent of the calorimeter is determined by burning certified reference benzoic acid with a stated heat of combustion of approximately 26.4 MJ/kg, ensuring metrological traceability. In each determination, the prepared specimen is placed in the crucible inside the bomb, the bomb is charged with oxygen, and the assembly is lowered into the calorimeter water jacket. After equilibration, ignition is triggered, the temperature-time curve is recorded through its exponential decay phase, and the corrected temperature rise is computed. Most protocols require three valid determinations, and the mean gross heat of combustion is reported.

Calculation and Corrections

The gross heat of combustion is calculated as the calibrated heat capacity of the system multiplied by the corrected temperature rise, minus the energy contributions of the ignition wire, cotton thread, fuse and any combustion aid, divided by the specimen mass. Where the net heat of combustion is required, the latent heat of vaporization of the water formed from the hydrogen in the polymer, together with any moisture present, is deducted, and corrections for nitric acid formation and sulfur oxidation are applied in accordance with the governing standard. For homogeneous, well-prepared foam samples, repeatability is typically within about one to two percent.

Applications

• Fire classification of building products under EN 13501-1 and GB 8624, where Euroclass A1 and A2 limit the gross heat of combustion of homogeneous products to 2.0 MJ/kg and 3.0 MJ/kg respectively
• Demonstration of compliance with combustibility requirements in national building codes for external wall insulation, sandwich panels and roofing systems
• Input data for fire safety engineering, including fire load calculations and design fire scenarios for performance-based building design
• Research, development and quality control of flame-retarded foam formulations, verifying the calorific reduction achieved by additives and inorganic fillers
• Energy content declarations for end-of-life foam in waste-to-energy recovery and thermal disposal studies
• Comparative material selection support for architects, facade engineers and insulation system manufacturers

FAQ

What is the difference between gross and net calorific value?

The gross value includes the latent heat recovered when the water formed by combustion condenses, while the net value assumes that water leaves the system as vapor. Fire classification thresholds refer almost exclusively to the gross heat of combustion.

How much sample is required for the test?

Each determination consumes roughly 0.5 g of prepared material, but several grams should be submitted to allow conditioning, repeat determinations and, where needed, runs with a combustion aid.

Why do some foam samples need a combustion aid?

Many foams melt, drip, shrink or intumesce before they ignite, preventing complete combustion in the crucible. A weighed, certified aid with a known calorific value ensures full burnout, and its contribution is arithmetically removed from the result.

Can a foam plastic ever achieve a very low calorific value?

Conventional polymeric foams cannot approach the 2 to 3 MJ/kg thresholds for A1 or A2 classification; only heavily filled or mineral-based cellular products can. PIR and phenolic foams achieve moderate reductions, while halogenated or mineral-filled formulations can lower the value further.

How long does the determination take?

A single bomb run requires roughly 20 to 30 minutes of instrument time, but a full report including conditioning, calibration checks and three valid determinations is typically completed within a few working days by an accredited laboratory.