Rubber Combustion Vertical and Horizontal Flammability Testing: Standards, Methods, and Classificati

Rubber combustion vertical and horizontal testing evaluates the flammability, afterflame behavior, dripping characteristics, and burning rate of rubber materials under strictly controlled laboratory conditions. Based on internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 3795, and GB/T 10707, these tests are essential for compliance in the automotive, wire and cable, electronics, and industrial rubber sectors. This article explains the test principles, specimen requirements, classification criteria, and typical applications of vertical and horizontal burning tests for rubber materials.

Technical Overview

Rubber combustion vertical and horizontal testing is a fundamental fire-safety evaluation used to determine how rubber materials ignite, sustain combustion, and self-extinguish when exposed to a small open flame. Because rubber compounds are organic polymer materials, many of them will burn when a sustained ignition source is applied, and regulatory frameworks across the automotive, electrical, and electronics industries therefore require objective flammability data before products can be placed on the market.

In a vertical burning test, the specimen is clamped so that its long axis is vertical and a standard laboratory flame is applied to the lower edge. This is the more severe orientation because hot combustion gases rise along the specimen surface and preheat the material ahead of the flame front. In a horizontal burning test, the specimen lies horizontally and the flame is applied to one end, allowing the measurement of a linear burning rate as the flame propagates from the ignited end toward the far end.

The most widely applied frameworks include the UL 94 small-flame test method, the equivalent IEC 60695-11-10 procedure, the ISO 3795 and FMVSS 302 horizontal burn tests for road vehicle interior materials, and GB/T 10707, which specifically addresses the flammability of vulcanized or thermoplastic rubber. Testing is typically performed in an enclosed combustion chamber using a calibrated 50 W premixed burner flame, with surgical cotton placed beneath the specimen to detect flaming drips or particles.

Testing Samples

Standard Sheet and Plate Specimens

For UL 94 and IEC 60695-11-10 testing, specimens are cut from molded or extruded rubber sheets to dimensions of 125 mm in length by 13 mm in width, with the thickness being the minimum intended wall thickness of the end product, commonly 1.5 mm to 3.0 mm or greater. A minimum of six specimens per thickness is generally required so that two full sets of three specimens can be evaluated, including any necessary retest series.

Automotive Interior Components

For ISO 3795, FMVSS 302, and GB 8410 horizontal burn testing, specimens are cut from finished components such as door seals, floor mats, grommets, hose covers, and interior trim to dimensions of 356 mm in length by 100 mm in width, tested at the actual product thickness up to the maximum defined by the standard.

Seals, Gaskets, and Molded Parts

O-rings, gaskets, vibration mounts, diaphragms, and other molded rubber articles are usually evaluated by preparing flat plaques of the same compound and wall thickness, since the standard specimen geometry must be preserved for a valid classification. Where a part cannot be flattened, testing is performed on representative samples of identical material and thickness.

Wire and Cable Insulation and Sheath Compounds

Rubber insulation and sheathing materials for cables are commonly screened with vertical and horizontal small-flame tests prior to full-scale cable flame tests, using molded strips of the insulation or jacket compound at the specified production thickness.

Specimen Conditioning

Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours, and each specimen is tested within a short, defined period after removal from the conditioning atmosphere to ensure reproducible moisture content.

Testing Items

The vertical and horizontal combustion test program measures and reports the following key parameters:

• Individual afterflame time after the first and second flame applications (t1 and t2 in vertical testing)
• Total afterflame and afterglow time for the complete specimen set
• Afterglow duration after extinction of visible flaming
• Whether flaming drips or particles are released and whether they ignite the indicator cotton placed below the specimen
• Linear burning rate in millimeters per minute in horizontal testing
• Distance burned between the 25 mm and 100 mm reference marks in horizontal testing
• Whether the specimen burns completely to the clamping device or extinguishes before the upper clamp
• Final flammability classification, for example V-0, V-1, V-2, or HB for UL 94 procedures

Testing Methods

Vertical Burning Test (UL 94 V-0/V-1/V-2 and IEC 60695-11-10)

The specimen is clamped vertically with its lower edge approximately 10 mm above the burner tube. A 20 mm high blue flame from a calibrated burner is applied to the center of the bottom edge for 10 seconds and then withdrawn. The afterflame time t1 is recorded. Once flaming stops, the flame is applied a second time for 10 seconds, and the afterflame time t2 and afterglow time are recorded. A layer of dry surgical cotton is positioned 300 mm below the specimen to capture flaming drips. The classification depends on the afterflame times, the total afterflame time across five specimens, cotton ignition, and whether the specimen burns to the clamp.

• V-0: each afterflame time ≤ 10 s, total afterflame ≤ 50 s for the set, no flaming drips igniting the cotton, and no afterglow igniting the specimen
• V-1: each afterflame time ≤ 30 s, total afterflame ≤ 250 s, no flaming drips igniting the cotton
• V-2: each afterflame time ≤ 30 s, total afterflame ≤ 250 s, brief flaming drips permitted only if they do not ignite the cotton

Horizontal Burning Test (UL 94 HB and ASTM D635)

The specimen is clamped horizontally with reference marks at 25 mm and 100 mm from the ignited end. A burner flame is applied at a defined angle to the free end for 30 seconds or until the flame front reaches the 25 mm mark, whichever occurs first. Timing begins when the 25 mm mark is reached, and the burn rate between the 25 mm and 100 mm marks is calculated. Materials are classified HB if the burning rate does not exceed 75 mm/min for thicknesses of 3 mm to 13 mm, does not exceed 40 mm/min for thicknesses below 3 mm, or the specimen extinguishes before the 100 mm mark.

Automotive Horizontal Burn Test (ISO 3795 and FMVSS 302)

The specimen is mounted horizontally in a U-shaped frame inside the chamber with the exposed face facing downward. A 38 mm high flame is applied to the free end for 15 seconds. If burning stops before the 60 mm mark, a pass is recorded with no burn-rate value. If the flame front passes the 60 mm mark, the burning rate between the 60 mm and 274 mm marks is measured. Passenger vehicle interior materials are generally required to exhibit a burning rate no greater than 102 mm/min, or to self-extinguish within the defined distance.

Vulcanized Rubber Vertical Test (GB/T 10707)

This method is specifically designed for rubber and applies a standard flame to vertically oriented vulcanized rubber specimens to determine afterflame and afterglow behavior, providing classification results consistent with the small-flame vertical philosophy used in international practice. It is frequently specified in Chinese national and industry requirements for rubber components.

Quality Assurance of Results

Accredited third-party laboratories perform these tests in ventilated enclosures with calibrated gas flow, verified flame height and temperature, calibrated timing instrumentation, and documented control of ambient conditions. Full test reports include specimen geometry and thickness, conditioning history, burner parameters, all recorded times, dripping observations, and the resulting classification.

Applications

Vertical and horizontal combustion test data support design, compliance, and quality decisions across many industries, including:

• Automotive engineering: door and window seals, floor mats, hose covers, grommets, boots, and interior trim evaluated against ISO 3795, FMVSS 302, and GB 8410 requirements
• Wire and cable manufacturing: insulation, bedding, and sheath compounds screened with UL 94 and IEC 60695-11-10 before larger-scale cable flame propagation tests
• Electronics and electrical equipment: rubber feet, keypads, seals, connector gaskets, and insulating parts requiring UL 94 V-0 or equivalent classifications
• Railway and aerospace interiors: seals and elastomeric components subject to stringent fire-safety specifications
• Industrial products: conveyor belts, hydraulic hose covers, vibration isolators, and protective rubber sheeting
• Consumer goods and medical devices: appliance gaskets, wearable components, and enclosures where reduced flammability supports product safety claims
• Research and development: benchmarking flame-retardant rubber formulations, comparing halogen-free compounds, and validating compound changes before production release

FAQ

What is the difference between vertical and horizontal combustion testing?

Vertical testing exposes the specimen to flame at its lower edge, so rising hot gases preheat the material above the ignition point and combustion is more severe; the result is an afterflame-based classification such as V-0, V-1, or V-2. Horizontal testing ignites one end of a horizontally mounted specimen and measures how fast the flame travels along it, producing a burning rate in millimeters per minute or an HB classification. A material that passes a vertical test will almost always satisfy horizontal requirements, while the reverse is not true.

What do the classifications V-0, V-1, V-2, and HB mean?

V-0 is the most demanding small-flame vertical rating, meaning the specimen self-extinguishes within 10 seconds after each flame application and produces no igniting drips. V-1 allows up to 30 seconds of afterflame with no igniting drips, while V-2 also allows up to 30 seconds but tolerates brief flaming drips that do not ignite the cotton indicator. HB denotes a horizontal burning rating in which the burning rate stays within defined limits or the specimen self-extinguishes before the 100 mm mark.

Can rubber materials achieve a V-0 rating?

Yes. Natural and many standard synthetic rubber compounds burn readily, but flame-retardant formulations based on chloroprene, chlorosulfonated polyethylene, silicone, EPDM with mineral fillers, or compounds containing halogenated and phosphorus-based flame retardants, often combined with aluminum hydroxide or magnesium hydroxide, can achieve V-0, particularly at wall thicknesses of 3 mm and above.

Does specimen thickness affect the result?

Thickness has a strong influence. Thin rubber sections ignite and burn faster than thick sections, so classifications are only valid at the thickness actually tested. UL 94 and IEC 60695-11-10 classifications are reported together with the tested thickness, and thinner sections of the same compound often require retesting to confirm the rating.

Which standard should I choose for automotive rubber parts?

Interior rubber components for road vehicles are typically evaluated with ISO 3795, FMVSS 302, or the equivalent GB 8410 horizontal burn procedure, which measures burn rate along a 356 mm by 100 mm specimen. For electrical and electronic rubber parts inside the vehicle, UL 94 or IEC 60695-11-10 vertical ratings are commonly requested in addition.

Why is cotton placed under the specimen during vertical testing?

The cotton indicator detects flaming drips or burning particles that fall from the specimen. Dripping flaming debris can ignite adjacent materials in real installations, so standards distinguish between grades that do not drip (V-0 and V-1) and those that produce brief igniting or non-igniting drips (V-2), which is a critical safety distinction for enclosures and wiring environments.

How should samples be submitted for testing?

Submit flat specimens of the production compound at the intended end-use thickness, generally at least six pieces of 125 mm by 13 mm for UL 94 or IEC 60695-11-10 testing, or 356 mm by 100 mm pieces for automotive horizontal testing, along with material identification, color, and molding information. Accredited third-party laboratories handle conditioning, instrument calibration, and reporting in accordance with the selected standard.

How long does a typical test program take?

The combustion testing itself is completed quickly, with each vertical specimen exposed for only two 10-second flame applications, but the full program includes 48 hours of conditioning, specimen preparation, and possible retest series. A complete report from an accredited laboratory is commonly issued within one to two weeks of sample receipt, depending on scope and queue conditions.