Determination of Combustion Performance Indicators of Foam Plastics: Standards, Methods and Applicat

Foam plastics such as polyurethane, polystyrene, polyethylene and PVC foams are widely used in thermal insulation, packaging, furniture and transportation, yet their low density and large specific surface area make them highly combustible. Determining the combustion performance indicators of foam plastics, including limiting oxygen index, horizontal and vertical burning behavior, smoke density and heat release, is essential for fire safety compliance, product certification and material development. This article explains the sample types, key testing items, standardized test methods and typical applications involved in foam plastics flammability evaluation.

Technical Overview

Foam plastics, also known as cellular plastics or expanded plastics, are polymeric materials structured around a gas-filled cellular matrix. Their low density, low thermal conductivity, excellent energy absorption and design flexibility make them indispensable in building insulation, upholstered furniture, automotive interiors, packaging, appliances and electronics. However, the same cellular structure that provides these advantages, combined with high specific surface area and often open interconnected cells, renders most foam plastics readily ignitable and capable of rapid flame spread, intense heat release and dense smoke generation.

Because foam plastics are directly implicated in fire risk across buildings, vehicles and consumer products, virtually every major market regulates their flammability. Standards organizations including ISO, IEC, ASTM, UL, EN, GB and JIS have established standardized laboratory methods for quantifying combustion performance. These methods measure ignitability, flame propagation speed, self-extinguishing capability, dripping behavior, heat release rate, smoke production and mass loss, producing reproducible indicators that support material selection, flame-retardant formulation development, quality control and regulatory classification.

Accredited third-party laboratories routinely perform these determinations on foam plastics to demonstrate conformity with classification schemes such as GB 8624 in China, EN 13501-1 in Europe, and UL 94 ratings for components of devices and appliances. A sound understanding of the applicable indicators and methods is therefore essential for manufacturers, specifiers and regulatory professionals working with cellular plastic materials.

Testing Samples

Combustion performance testing applies to the full range of thermoplastic and thermoset cellular plastics, in both flexible and rigid forms. The most commonly tested sample types include the following families.

Polyurethane Foams (PU and PIR)

Rigid polyurethane and polyisocyanurate foams are used for building, appliance and pipe insulation, while flexible polyurethane slabstock and molded foams are found in upholstered furniture, mattresses and vehicle seating. These materials represent one of the largest categories of flammability test submissions due to their extensive use in regulated fire-safety applications.

Polystyrene Foams (EPS and XPS)

Expanded polystyrene bead boards and extruded polystyrene boards are dominant thermal insulation materials for walls, roofs and floors. Their combustible nature places them under strict classification requirements in building codes, making burning performance determination a routine necessity.

Polyolefin Foams (PE, PP and EVA)

Cross-linked and non-cross-linked polyethylene foams are widely used in protective packaging and cushioning, expanded polypropylene serves in automotive energy-absorbing components, and ethylene-vinyl acetate foams appear in footwear midsoles, sports mats and padding. These foams are frequently tested for dripping behavior and burning rate.

Polyvinyl Chloride Foam (PVC)

Rigid PVC foam sheets are used in construction panels, signage and marine applications, while flexible PVC foams serve upholstery and sealing functions. Their inherent chlorine content modifies burning behavior, and testing verifies smoke production as well as flame spread.

Phenolic and Melamine Foams

Phenolic foam is specified where high fire performance insulation is required, and melamine foam is used for acoustic absorption. Even inherently flame-resistant foams require verification of oxygen index, smoke density and classification performance.

Composite and Modified Foam Products

Faced insulation boards, foam cores in sandwich panels, and foams compounded with halogenated, phosphorus-based or intumescent flame retardants are tested to confirm that treatments achieve their intended ratings without unacceptable trade-offs.

Testing Items

Depending on the product, end use and target market, accredited laboratories determine one or more of the following combustion performance indicators for foam plastics.

• Limiting oxygen index (LOI), expressed as the minimum volume percentage of oxygen in an oxygen-nitrogen mixture that just supports candle-like combustion.
• Horizontal burning rate in mm/min and the extent of burned distance between reference marks.
• After-flame time and afterglow time following removal of the ignition source.
• Vertical burning characteristics, including flame height, burning time and mass loss of rigid foams in the vertical position.
• UL 94 and equivalent classifications for cellular materials, including HF-1, HF-2 and HBF ratings, and V-0, V-1 or V-2 ratings where applicable.
• Ignitability under small flame impingement, including time to sustained ignition.
• Dripping behavior and whether flaming droplets or particles ignite a cotton indicator.
• Smoke production, including specific optical density, smoke density rating and smoke growth rate index.
• Heat release characteristics from cone calorimetry, including peak heat release rate, total heat released, effective heat of combustion and mass loss rate.
• Fire growth rate index derived from the single burning item test for building products.
• Burning performance classification under GB 8624 (classes A, B1, B2 and B3) or EN 13501-1 (Euroclasses), together with additional classifications for smoke production and flaming droplets.

Testing Methods

Limiting Oxygen Index (LOI)

The oxygen index method is performed according to ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2. A vertically mounted specimen is ignited at its top edge inside a glass chimney through which a controlled mixture of oxygen and nitrogen flows upward. The concentration of oxygen is progressively adjusted to identify the minimum level at which the specimen sustains burning for a defined period or over a defined distance after removal of the ignition flame. Rigid specimens of typically 80 to 150 mm length with 10 mm square cross-sections are commonly used, with thickness selected to represent the actual cellular product. LOI values above approximately 26 to 28 percent generally indicate materials that are difficult to ignite or self-extinguishing in normal air, which contains about 21 percent oxygen.

Horizontal Burning Test for Cellular Plastics

Small-specimen horizontal burning is determined in accordance with ISO 9772, ASTM D4986 and GB/T 8332. A rectangular foam specimen, typically 150 mm by 50 mm at the product thickness, is supported horizontally and exposed to a small laboratory flame applied to its free end for a specified time. The operator records the time for the flame front to travel between reference marks, any after-flame and afterglow, and whether flaming drips fall. For automotive interior applications, horizontal burning rate is measured on larger 356 mm by 100 mm specimens over a 254 mm gauged length in accordance with GB 8410 and ISO 3795, with results reported in mm/min or as self-extinguishing behavior.

Vertical Burning Test for Rigid Foams

Vertical burning behavior of rigid cellular plastics is evaluated using ASTM D3014 and GB/T 8333. A specimen mounted vertically is exposed to a standard burner flame, and the test captures flame height, after-flame time, afterglow characteristics and percentage loss of weight. These parameters are particularly valuable for rigid insulation foams because vertical orientation reflects many real installation geometries and accentuates upward flame propagation.

UL 94 and IEC Small-Flame Classifications

The UL 94 test program assigns foams the horizontal ratings HBF, HF-2 and HF-1, where HF-1 is the most stringent. HF-1 and HF-2 impose limits on after-flame time, afterglow and burn extent, with HF-1 additionally prohibiting ignition of the cotton indicator by flaming droplets, while HBF tolerates slower burning behavior. Where foam materials serve as parts of electronic devices and appliances, the 50 W vertical and horizontal tests of IEC 60695-11-10 and UL 94 yield V-0, V-1 or V-2 classifications, in which V-0 requires individual after-flame times not exceeding 10 seconds and no flaming drips that ignite cotton. IEC 60695 series glow-wire tests may also apply to components in electrotechnical assemblies.

Smoke Density Testing

Smoke production is a decisive safety indicator because smoke obscures escape routes and carries toxic combustion products. In the smoke chamber methods of ASTM E662 and ISO 5659-2, specimens of approximately 75 mm by 75 mm are exposed in flaming and non-flaming modes while a light beam traverses the accumulating smoke, yielding specific optical density values. The building materials method of GB/T 8627 uses small 25 mm by 25 mm specimens in a similar enclosed chamber and reports a smoke density rating, which is frequently specified alongside burning classification for insulation products.

Cone Calorimeter Testing

Cone calorimetry per ISO 5660-1, ASTM E1354 and GB/T 16172 provides the most comprehensive quantitative fire data. A 100 mm by 100 mm specimen is exposed to a controlled radiant flux, commonly 25, 35 or 50 kW per square meter, simulating developing fire conditions. Instrumented oxygen consumption analysis yields peak heat release rate, total heat released, effective heat of combustion, mass loss rate, ignition time and smoke production rate. These results feed fire hazard engineering assessments and comparative flame-retardant development studies.

Burning Performance Classification for Building Products

Insulation foams destined for construction must be classified under the relevant reaction-to-fire frameworks. In China, GB 8624 assigns classes A, B1, B2 and B3 based on a test battery that includes the single flame source test of GB/T 8626, the single burning item test of GB/T 20284, gross heat of combustion measurement and, where applicable, smoke and dripping supplementary classifications. In Europe, EN 13501-1 assigns Euroclasses using EN ISO 11925-2 single flame ignition and the EN 13823 single burning item test. Exterior wall insulation products in China are typically required to achieve at least class B1, making these determinations a routine part of market access for EPS, XPS, PU and PIR foams.

Specimen Conditioning and Preparation

Prior to testing, specimens are conditioned to standard atmospheres in accordance with ISO 291, typically 23 degrees Celsius and 50 percent relative humidity, for a minimum period agreed with the applicable method. Specimens are cut with clean edges at the representative product thickness, and their orientation relative to foaming direction is recorded because cellular anisotropy can measurably influence burning rate. Most methods require a minimum of five specimens per condition, with additional specimens retained for verification or retest.

Applications

Combustion performance determination of foam plastics supports safety, compliance and product development across many industries.

• Building and construction: verification of GB 8624 or EN 13501-1 classification for EPS, XPS, PU, PIR and phenolic insulation boards used in walls, roofs, floors and façade systems.
• Furniture and bedding: evaluation of flexible polyurethane foams for ignition resistance and burning behavior in upholstered furniture and mattresses under applicable furniture flammability requirements.
• Automotive and transportation: horizontal burning rate testing of seat, headliner, trim and acoustic foams per GB 8410 and ISO 3795, and fire performance assessment of rail vehicle materials under the EN 45545 series.
• Electronics and appliances: UL 94 and IEC 60695 based classification of foam components, gaskets and cushioning installed in devices, enclosures and appliances.
• Packaging and logistics: confirmation that protective polyolefin foams meet customer and regulatory flammability expectations for air freight and sensitive goods transport.
• Material development: comparison of flame-retardant formulations using LOI screening, UL 94 ratings and cone calorimeter heat release data to guide compound optimization.
• Regulatory compliance and market access: generation of test reports by accredited third-party laboratories supporting product certification, tender submissions and import inspection.

FAQ

What does the limiting oxygen index value indicate?

The LOI represents the minimum oxygen concentration required to sustain combustion of the foam. Since ambient air contains about 21 percent oxygen, foams with LOI values below 21 burn readily in air, while values above approximately 26 to 28 percent generally indicate self-extinguishing or flame-retarded behavior. LOI is best used for comparative screening rather than as a standalone pass-fail criterion.

What is the difference between HF-1, HF-2 and HBF ratings?

All three are UL 94 horizontal ratings for cellular materials. HBF is the least demanding and tolerates slower horizontal burning. HF-2 and HF-1 require rapid self-extinction with strict limits on after-flame and burn extent, and HF-1 additionally prohibits flaming driplets or particles that ignite the cotton indicator, making it the most stringent of the three.

Which classification must building insulation foam achieve?

Requirements depend on the jurisdiction, product system and installation location. In China, exterior wall thermal insulation products are generally required to reach at least class B1 under GB 8624, while specific applications may demand class A. In Europe, required Euroclasses under EN 13501-1 vary by building element and usage scenario, commonly B, C or E depending on the application and associated smoke and droplet classifications.

How should foam samples be conditioned before testing?

Specimens are normally conditioned at 23 degrees Celsius and 50 percent relative humidity per ISO 291 for the period specified by the test standard, and they should be tested promptly after removal from the conditioning atmosphere. Correct conditioning is essential because moisture content and residual blowing agents can significantly influence ignition time and burning behavior.

How many specimens are required for a determination?

Most small-flame methods require a minimum of five specimens per orientation or thickness, while oxygen index testing typically consumes a larger series of specimens because the oxygen concentration is bracketed iteratively. Cone calorimeter and smoke chamber tests generally use at least three replicate specimens per exposure condition.

Do flame retardants affect other foam properties?

Yes. Flame retardant additives can influence foam density, cell structure, mechanical strength, thermal aging behavior and smoke composition. A complete evaluation therefore often combines combustion performance testing with mechanical and thermal property measurements to confirm that fire performance gains do not compromise fitness for purpose.