Plastic char length determination testing measures the extent of carbonized damage that develops on a plastic specimen after exposure to a standardized ignition source, providing a quantitative indicator of flame retardancy. The test supports compliance verification against flammability standards such as UL 94, IEC 60695-11-10, GB/T 2408, and ISO 3795 for applications spanning electronics, automotive interiors, and building products. Accredited third-party laboratories conduct these determinations to help manufacturers classify materials, validate flame-retardant formulations, and demonstrate regulatory conformity.
Technical Overview
Char length, also referred to as damaged length or carbonized length, is the linear distance measured on a plastic test specimen from the edge exposed to the ignition source to the farthest point of visible thermal damage, including charred, embrittled, melted, or consumed material. It is one of the most intuitive quantitative indicators of how effectively a polymer resists flame propagation under controlled laboratory conditions.
In practice, char length is not evaluated in isolation. Accredited third-party laboratories record it alongside afterflame time, afterglow time, flaming drips, and ignition of a cotton indicator, because these parameters together determine the flammability classification of a material under schemes such as UL 94 (V-0, V-1, V-2, 5VA, 5VB), IEC 60695-11-10, GB/T 5169.16, and GB/T 2408. A shorter char length generally indicates that the material forms a protective carbonaceous layer, self-extinguishes quickly, and limits the spread of combustion.
- Char length and damaged length provide direct evidence of the combustion zone extent on the specimen.
- The measurement complements timed parameters such as afterflame and afterglow duration.
- Results feed directly into material classification, quality control, and regulatory compliance documentation.
- Determinations are performed under standardized burner, airflow, and conditioning conditions to ensure reproducibility.
Testing Samples
Molded Test Bars and Plaques
For vertical burning tests under UL 94 and IEC 60695-11-10, standard bar specimens measure 125 mm in length and 13.0 mm in width, with a preferred thickness of 3.0 mm. Materials intended for commercial use should be tested at both the minimum and maximum thicknesses declared by the manufacturer, since thin sections typically burn more readily than thick sections. Two sets of five specimens are generally required to establish a classification.
Thin Films and Flexible Sheets
Plastic films below approximately 1 mm in thickness may buckle or distort in the standard vertical test. These materials are evaluated using the thin-material vertical burning method (VTM-0, VTM-1, VTM-2 per UL 94, or ISO 9773), in which the specimen is wrapped around a mandrel to maintain geometry during flame exposure. Char length and damaged length are then measured after the specimen is unwrapped.
Foamed Plastic Materials
Automotive Interior Specimens
For vehicles, interior trim materials are tested as rectangular specimens of 356 mm by 100 mm, positioned horizontally in a combustion chamber per GB 8410, ISO 3795, and the equivalent North American regulation. The burned length along the specimen is the key measurement used to calculate the horizontal burning rate in millimeters per minute.
Finished Components and Parts
Where cutting standard bars from a finished product is not feasible, laboratories may test complete parts, sections of parts, or specially prepared plaques molded from the same compound under agreed processing conditions. The sampling plan should be documented because processing history, orientation, and thickness can measurably influence char formation.
Testing Items
A typical char length determination report includes the following measured and derived parameters:
- Char length (damaged length), reported in millimeters for each specimen and as the maximum value.
- Afterflame time t1 after the first flame application and t2 after the second application.
- Total afterflame time for the set of specimens.
- Afterglow time following extinction of visible flame.
- Observation of flaming drips and whether they ignite the cotton indicator placed below the specimen.
- Horizontal burning rate in mm/min, calculated from burn length and time.
- Whether afterflame or glowing combustion reaches the clamped upper edge of the specimen.
- Burn-through or hole formation for 5VA and 5VB plaque evaluations.
- The resulting flammability classification (for example V-0, V-1, V-2, HB) under the applicable standard.
Testing Methods
Specimen Conditioning and Apparatus
Specimens are conditioned at 23 ± 2 °C and 50 ± 10 % relative humidity for at least 48 hours before testing. The apparatus consists of a laboratory fume hood or enclosure with controlled airflow, a specified burner producing a 20 mm blue flame, a specimen clamp, timing devices, and a cotton indicator pad. Gas flow and flame height are verified before each series to keep the ignition source consistent across laboratories.
Vertical Burning Method (50 W)
Following UL 94, IEC 60695-11-10, and GB/T 5169.16, the bar specimen is clamped vertically with its long axis downward. The burner flame is applied to the lower edge for 10 ± 0.5 seconds, removed, and reapplied for a second 10-second interval after combustion ceases. Afterflame time, afterglow time, dripping behavior, and the extent of carbonized damage are recorded. Class V-0, for example, requires short afterflame times, no cotton ignition from flaming drips, and no combustion reaching the clamp.
Horizontal Burning Method
Under ASTM D635, ISO 1210, and the horizontal method of GB/T 2408, the specimen is mounted horizontally and ignited at one marked end. The time for the flame front to travel between reference marks, or the self-extinguishment of the flame before the first mark, is used to calculate the burning rate. The damaged length between the marks is measured and reported, supporting HB classifications for enclosure-grade materials.
Thin-Material and Foamed-Material Methods
Thin films wrapped on mandrels and foams supported on wire grids are exposed to the same 20 mm flame geometry adapted to their configuration. Because these materials can shrink, curl, or melt away from the flame, char length is measured after carefully restoring the specimen geometry, and melt-through or separation events are documented separately.
Automotive Interior Horizontal Test
Per GB 8410 and ISO 3795, the specimen is mounted horizontally with the exposed face facing downward, and the burner flame is applied to the free end for a defined period. If burning does not cover the entire specimen, the burned length is measured and divided by the elapsed time to obtain the burning rate. Most automotive specifications require the rate not to exceed 100 mm/min, or the material must self-extinguish within the timing window.
Char Length Measurement Procedure
After the flame exposure sequence, the specimen is allowed to cool, and loose soot and non-adherent char are gently removed without disturbing the damaged zone. For flexible materials, a controlled tearing technique adapted from ASTM D6413 may be applied so that the boundary between damaged and undamaged material is clearly revealed. The distance from the original exposed edge to the furthest point of damage is measured with a calibrated ruler to the nearest 1 mm, and the maximum value among all specimens is reported together with photographs and specimen-level data.
Applications
Char length determination supports material qualification and regulatory submission across many industries:
- Electronics and electrical appliances, where enclosure plastics must meet UL 94 or IEC 60695-11-10 classifications for product safety certification.
- Automotive engineering, where interior trim, headliners, and seat components are verified against GB 8410, ISO 3795, and FMVSS 302 burning rate limits.
- Wire and cable manufacturing, where insulation and sheathing compounds are screened for flame spread and char formation.
- Construction and building products, where flame-retardant panels, pipes, and films require documented damaged-length evidence.
- Rail transit and public transport, where strict fire performance regimes demand short char lengths and low smoke hazard.
- Research and development, where char length trends guide the optimization of halogen-free and intumescent flame-retardant formulations.
- Incoming quality control, where batch-to-batch consistency of flame-retardant masterbatches is monitored.
FAQ
What exactly is char length in plastic flammability testing?
Char length is the measured distance from the specimen edge exposed to the burner flame to the farthest point of carbonized or otherwise damaged material after the test. It quantifies how far the combustion and pyrolysis zone advanced along the specimen.
How is char length different from afterflame time?
Afterflame time measures how long the specimen continues to burn after the ignition source is removed, while char length measures the spatial extent of damage. A material can have short afterflame times yet still show a long damaged zone, so both parameters are reported together for a complete assessment.
Which standards govern char length determination for plastics?
The principal references include UL 94, IEC 60695-11-10 and IEC 60695-11-5, GB/T 5169.16, GB/T 2408, ISO 9772, ISO 9773, ISO 1210, ASTM D635, ASTM D3801, ASTM D6413 for tearing-based char measurement, and GB 8410 together with ISO 3795 for automotive interiors.
How many specimens are required for a valid test?
Vertical classification tests typically require two sets of five bar specimens, one set in the as-received condition and, where relevant, one set after conditioning or at an alternative thickness. Automotive horizontal tests usually require multiple specimens cut from both machine and cross directions of the material.
Can finished products be tested instead of molded bars?
Yes. When standard bars cannot be cut from a part, accredited third-party laboratories can test the finished component or plaques molded from the identical compound, provided the thickness, colorant, and processing condition are documented, since all of these factors influence flammability performance.
What do typical acceptance criteria look like?
For a V-0 classification, individual afterflame times must not exceed 10 seconds, the total for a set must not exceed 50 seconds, flaming drips must not ignite cotton, and combustion must not reach the clamp. For automotive interiors, the horizontal burning rate must not exceed 100 mm/min, or the specimen must stop burning before the burn front reaches the reference mark.
How should samples be prepared and submitted for testing?
Submit flat, warp-free specimens at the declared production thickness with sufficient quantity for both sets and any retests, clearly identified by grade, color, and lot number. Package samples to prevent contamination, bending, or surface damage, and provide the intended standard and classification target so the laboratory can configure the correct burner geometry and conditioning regime.