Char length determination quantifies the extent of carbonized thermal damage along a plastic specimen after controlled flame exposure and is a key parameter in flammability evaluation and classification. This technical article explains the measurement principle, sample requirements, test procedures, and reporting practices for char length testing under standards such as UL 94, IEC 60695-11, ISO 1210, ASTM D635, and GB/T 2408. It also summarizes typical applications in electronics, automotive, construction, and flame-retardant material development.
Technical Overview
Char length is defined as the linear distance between the edge of a plastic specimen exposed to the ignition source and the furthest point of charred, burned, or consumed material, measured after the specimen has cooled and all combustion, including afterglow, has ceased. Depending on the governing standard, the measurement may be described as char length, damaged length, or extent of burning, but the measurement principle is the same: it records how far the flame front and associated thermal degradation progressed along the specimen. Soot deposits, smoke staining, and glossy melted regions that never ignited are generally excluded from the measured length unless the applicable standard states otherwise.
Char length is a direct quantitative indicator of flame propagation tendency and self-extinguishing behavior. A short, well-defined char region demonstrates that the material resisted fire spread, while a long char length or complete burn-off indicates poor flame retardancy. Charring itself is also an important condensed-phase flame-retardant mechanism: intumescent, phosphorus-nitrogen, and mineral-filled systems promote the formation of a protective char layer that insulates the underlying polymer from heat and oxygen. Char length data therefore complement afterflame time, afterglow time, and flaming drip behavior when classifying a material.
Char length determination is performed within the framework of internationally recognized flammability standards, including UL 94, the IEC 60695-11 series, ISO 1210, ISO 9772, ISO 9773, ISO 10351, ASTM D635, ASTM D3801, ASTM D4804, ASTM D4986, ASTM D5048, GB/T 2408, GB/T 5169.16, and the corresponding JIS adoptions of the IEC 60695-11 series. Accredited third-party laboratories conduct these tests for product certification, regulatory compliance, and research and development purposes.
Testing Samples
Rigid Thermoplastics and Thermosets
Standard bar specimens are cut or injection moulded from materials such as ABS, PS, PP, PE, PA, PC, PBT, PVC, modified PPO, epoxy, phenolic, and unsaturated polyester compounds. Specimens are typically 125 mm in length and 13 mm in width, tested at the end-use wall thickness and, where required by the classification scheme, at additional thicknesses such as 3.0 mm. Because pigments, fillers, and processing conditions can influence results, specimens should represent the final commercial formulation and colour.
Foamed Plastics
Flexible and rigid cellular materials, including polyurethane and polystyrene foams, are tested under horizontal burning methods such as ISO 9772, ASTM D4986, and the foam classifications of UL 94 (HBF, HF-1, HF-2). For these materials, damaged length limits, for example a maximum damaged length of 60 mm for HF-1 and HF-2 classifications, are central acceptance criteria.
Films and Thin Flexible Materials
Plastic films and thin flexible sheeting below the thickness range of rigid bar tests are evaluated with dedicated methods such as ISO 9773 and ASTM D4804, which use special specimen holders to maintain reproducible flame contact. The damaged length is again the primary measure of fire performance.
Specimen Preparation and Conditioning
Edges should be smooth and free of burrs, flash, and cracks that could influence flame behaviour. Unless otherwise specified, specimens are conditioned before testing at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 48 hours in accordance with ISO 291 or the equivalent conditioning practice referenced by the test method. The number of specimens is defined by each standard, typically five bars per thickness for horizontal tests and five or ten bars per thickness for vertical tests, plus additional sets if retest provisions apply.
Testing Items
Char length determination is normally reported together with the full set of flammability parameters required by the selected standard, including:
- Afterflame time after the first and second flame applications (t1 and t2) and total afterflame time
- Afterglow time after flame extinction
- Char length, damaged length, or extent of burning, reported in millimetres for each specimen and as an average
- Horizontal burning rate in mm/min measured between reference marks
- Whether the flame front reached the upper clamp or the end reference mark
- Ignition of the surgical cotton indicator by flaming drops or particles
- Flammability classification result, such as V-0, V-1, V-2, HB, HB40, HB75, 5VA, 5VB, HBF, HF-1, or HF-2
- Visual observations such as melting, dripping, warping, and intumescence, supported by photographs of the charred specimens
Testing Methods
Horizontal Burning Method
In the horizontal test of ASTM D635, ISO 1210, IEC 60695-11-10 Method A, and GB/T 2408, a bar specimen is clamped horizontally and a small premixed blue flame, typically 20 to 25 mm in height, is applied at 45 degrees to one end for 30 seconds or until the flame reaches the 25 mm reference mark. If burning passes the 25 mm mark, the burning rate between the 25 mm and 100 mm marks is calculated; the damaged length from the exposed edge to the furthest point of char is recorded in all cases. This method yields HB, HB40, or HB75 classifications, with thick slow-burning materials accepted if they self-extinguish before the 100 mm mark.
Vertical Burning Method (50 W Flame)
In the vertical test of UL 94 V, IEC 60695-11-10 Method B, ASTM D3801, and GB/T 2408, the specimen is clamped vertically with dry absorbent cotton placed beneath it. A 20 mm blue flame from a 50 W premixed burner is applied to the lower edge for 10 seconds, removed, and then reapplied for a second 10-second interval after flaming stops. Afterflame times, afterglow, and cotton ignition are recorded, and after the specimen has cooled, the char length is measured from the original lower edge to the furthest damaged point, frequently using a graduated metal template laid over the specimen. For a V-0 or V-1 classification the specimen must not burn with flaming or glowing combustion up to the clamp, which is verified by the measured damage length.
500 W Flame Method (5VA and 5VB)
The 5V procedures of UL 94, IEC 60695-11-20, and ASTM D5048 apply a 500 W flame repeatedly, five applications of five seconds each, to both bar specimens and horizontal plaques. The charred area and burn-through behaviour of the plaque are assessed, with 5VA requiring no burn-through of the plaque and 5VB permitting burn-through only if all other criteria are met. These classifications represent the most stringent requirements in the series and are widely specified for enclosures of electrical equipment.
Char Length Measurement Procedure
After the burning test is complete and the specimen has cooled, any residual afterglow is extinguished and loose carbonaceous debris and molten beads are gently removed without disturbing the damage boundary. The damaged zone is identified as the region in which the material is charred, burned, or consumed, excluding soot and smoke stains and excluding melted material that never supported combustion. A calibrated template or ruler with 1 mm resolution is used to measure from the original flame-exposure edge to the furthest point of damage, and each specimen is documented photographically. The individual values, average, and maximum char length are reported together with the classification result, specimen thickness, and conditioning history.
Applications
Char length and the associated flammability classifications are used across many industries in which plastics are exposed to potential ignition sources:
- Electrical and electronic equipment: enclosures, switches, relays, connectors, and printed wiring board laminates requiring V-0 or 5VA ratings
- Wire and cable: insulation and sheathing compounds evaluated for flame propagation resistance
- Home appliances and lighting: housings, diffusers, and internal components near live parts
- Automotive engineering: interior trim, wiring ducts, connectors, and electric vehicle battery housing and module materials
- Construction products: wall panels, foam insulation, pipes, and fittings, often alongside reaction-to-fire methods such as EN 13501-1 and ISO 11925-2
- Rail and aircraft interior materials, where strict limits on char length and burn length are commonly specified by transport-sector specifications
- Flame-retardant research and quality control: development of halogen-free, intumescent, and mineral-filled formulations and incoming material verification
Accredited third-party laboratories provide these determinations for regulatory submissions, product safety certifications, supplier qualification, and comparative material screening during research and development.
FAQ
What is the difference between char length and damaged length?
The two terms describe the same measurement concept: the distance from the flame-exposed edge to the furthest point of combustion damage. UL 94 and ASTM methods commonly use the term damaged length or extent of burning, while char length is the preferred term in vertical burn-type specifications used for transport interior materials. All of them are expressed in millimetres.
Do melted or warped areas count toward the char length?
In general, only material that actually charred, burned, or was consumed is measured. Soot staining, smoke marks, and glossy melted material that never ignited are excluded, although the applicable standard always governs the exact definition. Any borderline judgement should be documented with photographs in the test report.
How many specimens are required for a valid result?
Most horizontal and vertical methods require five bar specimens per thickness as a baseline, with additional sets of five used when a retest is triggered by borderline results. Classification standards also require testing at multiple thicknesses when the material is supplied in a range of wall thicknesses.
Why is char length important for flame-retardant development?
A short, coherent, and intumescent char indicates effective condensed-phase protection of the polymer, while long char lengths reveal insufficient barrier formation. Char length data allow formulators to compare loadings of phosphorus, nitrogen, and mineral flame retardants quickly and objectively.
How long does char length testing take?
After specimen conditioning of at least 48 hours, the burning sequence and char measurement for a standard set of specimens are usually completed within one working day, with the full report issued shortly afterwards, subject to laboratory scheduling and the number of thicknesses and colours tested.