The oxygen index (Limiting Oxygen Index, LOI) test is the primary method for quantifying the flame-retardant performance of resin fiberglass boards such as FR-4, G10, G11 and epoxy glass cloth laminates used in electrical insulation and structural applications. This article explains the testing principle, applicable standards (ISO 4589-2, ASTM D2863, GB/T 2406.2), specimen preparation, step-by-step procedures and result interpretation for resin fiberglass board oxygen index determination.
Technical Overview
Resin fiberglass boards are thermoset composite laminates in which woven glass fiber cloth or glass mat is impregnated with a cured resin system, typically epoxy, phenolic or unsaturated polyester. They are widely used as electrical insulation materials, printed circuit board substrates and structural components because of their high dielectric strength, mechanical rigidity, heat resistance and dimensional stability.
The oxygen index, also known as the Limiting Oxygen Index (LOI), is defined as the minimum concentration of oxygen, expressed as a volume percentage in a flowing mixture of oxygen and nitrogen, that will just support candle-like combustion of a vertically mounted specimen after ignition at room temperature. Because normal air contains approximately 20.9% oxygen, a material with an LOI above 21% will not continue to burn under ambient conditions once the ignition source is removed. Higher oxygen index values therefore indicate better flame-retardant performance, which is a critical acceptance parameter for resin fiberglass boards used in electrical equipment, rail transit, aerospace and marine environments.
Oxygen index determination provides a quantitative, reproducible and sensitive measure of burning behavior, making it suitable for material comparison, quality control, batch conformity assessment and compliance demonstration for flame-retardant grades such as FR-4 laminates.
Testing Samples
Epoxy Glass Cloth Laminate Boards
This category includes FR-4, G10, G11 and 3240-type epoxy (or epoxy-phenolic) glass cloth laminated sheets. These are the most commonly submitted materials for oxygen index testing, since flame-retardant epoxy grades are required to demonstrate a high LOI value, typically at or above 28%, to support flame-retardant declarations.
Phenolic Resin Fiberglass Boards
Phenolic-bonded glass fabric laminates, such as 3025/3240-type boards in regional specifications, offer good heat resistance and char-forming behavior. Their oxygen index is evaluated using the same procedure to verify consistent flame performance between production batches.
Polyester and Other Thermoset Glass Fiber Laminates
Unsaturated polyester glass mat sheets, modified epoxy systems and specialty high-temperature laminates can also be characterized. Materials with decorative or coated surfaces should be tested in their final delivered condition unless the specification requires otherwise.
Specimen Requirements and Conditioning
Standard specimens are rectangular strips cut from the board, typically 80 mm to 150 mm in length and 10 mm in width, with the preferred thickness of 4 mm; other thicknesses may be tested provided the actual thickness is reported, as results at different thicknesses are not directly comparable. Edges must be smooth and free of cracks, and surfaces must be clean. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or GB/T 2918 prior to testing.
Testing Items
A typical oxygen index determination program for resin fiberglass boards includes the following items:
- Limiting Oxygen Index (LOI) at the specified specimen thickness, expressed as volume percentage of oxygen.
- Burning behavior verification, including afterflame time and whether the specimen reaches the 50 mm reference mark under defined oxygen concentrations.
- Batch-to-batch conformity checking against internal or contractual LOI limits.
- Comparative evaluation of flame-retardant and non-flame-retardant grades or alternative resin systems.
- Optional elevated-temperature oxygen index determination in accordance with ISO 4589-3 for high-temperature applications.
Testing Methods
Applicable Standards
Oxygen index testing of resin fiberglass boards is performed in accordance with the following internationally and nationally recognized standards, which are technically aligned in principle:
- ISO 4589-2: Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test.
- EN ISO 4589-2: European adoption of the ISO method.
- ASTM D2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index).
- GB/T 2406.2: Chinese national standard, technically equivalent to ISO 4589-2, commonly specified for domestic qualification of insulation laminates.
- JIS K 7201-2: Japanese industrial standard equivalent for the ambient-temperature oxygen index test.
- ISO 4589-3: Oxygen index test at elevated temperatures, where required for specific applications.
Test Apparatus
The test is conducted on an oxygen index apparatus comprising a heat-resistant glass combustion chimney, a vertical specimen holder, a precision oxygen and nitrogen gas supply with calibrated flow control and mixing, and a small propane or butane flame igniter. The total volumetric flow of the gas mixture is maintained constant, typically in the region of 10 to 15 liters per minute, while the oxygen/nitrogen ratio is adjusted to set the desired oxygen concentration. Oxygen analyzers or calibrated flow meters are used to ensure concentration accuracy within the tolerance required by the standard.
Test Procedure
The determination follows a bracketing procedure at ambient temperature:
- Condition the specimens and verify their dimensions; apply reference marks 50 mm apart, measured downward from the top edge.
- Mount the specimen vertically in the holder with the top of the specimen exposed below the chimney outlet.
- Purge the chimney with the gas mixture at the initial oxygen concentration for a sufficient stabilization period.
- Apply the igniter flame to the top surface of the specimen until it is well ignited, then remove the igniter and start timing.
- Record whether the specimen "responds", defined as burning for longer than 180 seconds or flames reaching the 50 mm reference mark, whichever occurs first.
- Adjust the oxygen concentration upward if the specimen responds, or downward if it does not, using progressively smaller increments down to approximately 0.2% v/v in the final stage, repeating with fresh specimens until the critical concentration is bracketed.
- Verify the bracketed concentration with additional specimens as required by the standard's calculation method, and report the LOI as the volume percentage of oxygen.
Expression of Results
The final result is reported as the LOI value in volume percent, for example "LOI = 32%". The test report should state the applicable standard, specimen dimensions and thickness, conditioning regime, ambient laboratory conditions, gas flow parameters, individual burning responses and the calculated oxygen index. Testing is typically performed on a series of at least 10 to 15 specimens per condition to satisfy the statistical requirements of the bracketing method.
Factors Influencing Test Results
Several factors must be controlled to obtain valid and comparable data:
- Specimen thickness: LOI values vary with thickness, so comparison is only valid at the same thickness.
- Resin content and curing degree: incompletely cured resin or resin-rich layers can reduce the measured oxygen index.
- Conditioning state: moisture content affects ignition behavior, particularly for hygroscopic resin systems.
- Gas purity and flow calibration: incorrect oxygen concentration calibration is a common source of systematic error.
- Ignition technique: over- or under-application of the igniter flame influences the response judgment and must follow the standard.
Applications
Oxygen index data for resin fiberglass boards directly supports material selection, supplier qualification and regulatory compliance across multiple industries:
- Electrical and electronics: flame retardancy verification for FR-4 printed circuit board substrates, insulation barriers, busbar supports and switchgear components.
- Power equipment: insulation spacers, arc barriers, transformer components and generator slot wedges where self-extinguishing behavior is specified.
- Rail transit and aerospace: qualification of interior and structural laminate components against flame performance requirements.
- Marine and construction: assessment of fiberglass laminate panels where reduced fire propagation is required.
- Quality control: routine batch release testing by manufacturers and incoming material verification by OEMs, frequently performed by accredited third-party laboratories.
FAQ
What oxygen index value indicates a flame-retardant resin fiberglass board?
Any material with an LOI above 21% will not sustain combustion in normal air. In practice, flame-retardant grades such as FR-4 are generally expected to achieve LOI values of at least 28%, with well-formulated brominated or phosphorus-modified systems often exceeding 30%. Contract specifications should define the exact acceptance limit.
How does the oxygen index relate to UL 94 ratings?
The oxygen index and UL 94 are different tests: LOI is a quantitative measure of the minimum oxygen concentration to sustain burning, while UL 94 (for example the V-0 vertical burn test) grades time to self-extinguish and flaming drips under a defined burner flame. A high LOI generally correlates with good UL 94 performance, but there is no direct conversion between the two, and both may need to be reported separately.
Does specimen thickness affect the oxygen index result?
Yes. LOI values depend on specimen geometry, particularly thickness, because heat transfer and combustion behavior change with cross-section. Results are only comparable at the same thickness, and the board should ideally be tested at its actual application thickness or at the standard 4 mm thickness as defined in ISO 4589-2 and GB/T 2406.2.
How many specimens are needed and what is a typical turnaround?
A complete determination normally consumes 10 to 15 specimens, including preliminary screening, bracketing and verification burns. After conditioning, routine testing at accredited third-party laboratories is usually completed within a few working days, depending on the number of thicknesses and conditions specified.
Can oxygen index testing be performed at elevated temperatures?
Yes. ISO 4589-3 extends the method to elevated temperatures, which is relevant for resin fiberglass boards serving near heat sources, such as motor and transformer insulation, where flammability performance at operating temperature must be demonstrated.