Horizontal Burning Test of Plastics: Standards, Methods, and Classification

The horizontal burning test of plastics is a widely used fire-safety evaluation method that measures the ignition characteristics, flame spread, burning rate, and afterflame behavior of plastic materials in a horizontal orientation. Governed by standards such as GB/T 2408, IEC 60695-11-10, UL 94, and ASTM D635, this test provides essential flammability classifications for materials used in electrical, electronic, automotive, and consumer products. This article explains the specimen requirements, test procedure, classification criteria, and typical applications of horizontal burning determination.

Technical Overview

The horizontal burning test is a small-scale laboratory flammability test designed to assess the relative burning behavior of plastic specimens when oriented horizontally and exposed to a low-energy (50 W) premixed gas flame. During the test, a Bunsen-type burner applies a small blue flame to the free end of the specimen for a defined period, after which the operator records whether the material ignites, how far the flame front propagates, how long afterflame persists, and whether flaming drips are produced.

The principal purpose of the test is to determine a material's burning rate between reference marks placed 25 mm and 100 mm from the ignited end, and to classify the material accordingly. Depending on the standard applied, results are expressed as an HB rating (UL 94), HB40 or HB75 classification (IEC 60695-11-10), FH classification levels (GB/T 2408), or reported extent and rate of burning values (ASTM D635). Because it is relatively fast, repeatable, and inexpensive, the horizontal burning test is one of the most frequently performed flammability screening methods in plastics development, quality control, and compliance verification at accredited third-party laboratories.

Testing Samples

Standard Bar Specimens

The most common specimen geometry is a rectangular bar measuring 125 mm in length and 13.0 mm in width. The preferred thickness is 3.0 mm; however, the actual end-use thickness may be tested when representative of the final product, subject to standard limits. Typically, a minimum of three specimens is required for each thickness and condition. Specimens must be free from burrs, flash, and visible defects, and reference marks are applied at 25 mm and 100 mm from the end that will be exposed to the flame.

Thin Films and Sheets

Flexible films and thin sheets below 3 mm in thickness can also be evaluated, often using a supporting frame or mandrel to maintain the specified horizontal orientation and 45-degree inclination during testing. Thin materials frequently exhibit faster linear burning rates, which is reflected in the differentiated classification thresholds for specimens below 3 mm.

Rigid Cellular Plastics

Rigid foamed plastics are generally evaluated under a dedicated protocol (for example ISO 9772 or the corresponding GB/T 8332 method), which uses longer specimens and a modified specimen support to accommodate the geometry and burning behavior of cellular materials.

Conditioning Requirements

Prior to testing, specimens are conditioned in a controlled atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity, for a minimum of 48 hours unless otherwise specified by the applicable standard or by agreement between the parties.

Testing Items

The horizontal burning determination typically includes the following measured and observed parameters:

• Ignition behavior, including whether the specimen ignites under the applied pilot flame.
• Afterflame time, defined as the duration of continued flaming after the ignition source is removed.
• Extent of burning, recorded by whether the flame front passes the 25 mm and 100 mm reference marks.
• Linear burning rate (mm/min), calculated from the time required for the flame front to travel between the 25 mm and 100 mm marks.
• Presence of flaming drips or flaming particles that could ignite adjacent materials such as surgical cotton.
• Final classification, such as HB40, HB75, or failure to classify under the chosen standard.

Testing Methods

The general test procedure, harmonized across GB/T 2408, IEC 60695-11-10, and UL 94 HB requirements, proceeds as follows:

• The conditioned specimen is clamped at one end so that its longitudinal axis is horizontal and its transverse axis is inclined at 45 degrees to the horizontal, with the free end exposed.
• A Bunsen or Tirrill-type burner is adjusted to produce a stable premixed blue flame of approximately 20 mm to 25 mm in height, depending on the standard, using methane or natural gas of specified calorific value.
• The flame is applied to the free end of the specimen at a 45-degree angle for 30 seconds, or until the flame front reaches the 25 mm mark, whichever occurs first under the governing standard.
• The burner is withdrawn, and the operator records the afterflame time, whether the flame front crosses the 25 mm and 100 mm marks, and the transit time between the two marks.
• The linear burning rate is calculated by dividing the 75 mm distance between the reference marks by the measured transit time and expressed in mm/min.
• Flaming drips, afterglow, and distortion of the specimen are noted, and the result is compared with the classification criteria.
• The test is repeated for all specimens, and the material is assigned the classification supported by the complete set of results.

Under IEC 60695-11-10, the HB40 classification applies when the burning rate does not exceed 40 mm/min between the 25 mm and 100 mm marks, or when flaming ceases before the 100 mm mark is reached. The HB75 classification applies when the burning rate does not exceed 75 mm/min, which is relevant for thinner specimens. Materials that burn faster than the applicable limit, or that cannot be tested meaningfully due to severe melting or shrinkage away from the flame, are reported as not classified by this method. GB/T 2408 expresses comparable results through its FH classification levels, while ASTM D635 reports the average extent of burning, average time of burning, and average rate of burning without assigning pass/fail categories.

Applications

Horizontal burning determination supports material selection, product safety compliance, and quality control across many industries:

• Electrical and electronic products, where enclosure materials, connectors, and internal structural parts must demonstrate an acceptable flammability rating for compliance with equipment safety standards.
• Household appliances, power tools, and lighting equipment that incorporate polymeric components near live electrical parts.
• Automotive interior and cabin components, complementing vehicle-specific horizontal burn requirements for trim materials.
• Consumer goods, furniture, toys, and packaging, where flame retardancy of plastics is a regulatory or contractual requirement.
• Flame-retardant material development, used to compare resin formulations, filler loadings, and synergist packages during research and development.
• Incoming material inspection and supplier qualification programs in manufacturing supply chains.

FAQ

What is the difference between the horizontal and vertical burning tests?

The horizontal test exposes a specimen oriented horizontally to a small flame and primarily measures burning rate and extent of spread, making it a moderate-severity screening method. The vertical test, performed under standards such as GB/T 2408, IEC 60695-11-10, and UL 94, applies the flame to the lower end of a vertically mounted specimen and evaluates afterflame time, afterglow, and flaming drips to assign more demanding classifications such as V-0, V-1, and V-2. A material passing the vertical test is generally considered to exhibit better flame resistance than one achieving only an HB rating.

What does an HB rating mean?

An HB (Horizontal Burn) rating indicates that the material either extinguished before the flame front reached the 100 mm mark or burned slowly within the rate limit permitted by the standard. It represents the baseline flammability level for plastics in schemes such as UL 94 and is suitable for many less critical applications, but it is not equivalent to the higher vertical ratings required for certain electrical equipment.

How many specimens are required?

Most standards require a minimum of three specimens per thickness and per condition. Additional specimens may be tested when results are inconsistent, when multiple thicknesses must be evaluated, or when both standard and artificially aged material conditions are specified.

Can all plastics be tested by the horizontal burning method?

No. Materials that melt and flow severely, shrink away from the ignition flame, or are too thin to support stable geometry may not produce valid results. In such cases, alternative methods, specialized fixtures, or different flammability standards should be selected, and the limitations should be clearly stated in the test report.

How is the burning rate calculated?

The linear burning rate is the 75 mm distance between the 25 mm and 100 mm reference marks divided by the time the flame front takes to travel that distance. For example, if the transit time is 150 seconds, the burning rate equals 75 mm divided by 2.5 minutes, or 30 mm/min.