A material spec lands on your desk that reads “LOI ≥ 28, per ASTM D2863.” A supplier sends two flame-retardant masterbatches with a data sheet claiming one hits an oxygen index of 32 and the other 29, and asks which grade you want to qualify. Before you sign off on either number, it pays to know exactly what that single figure measures, what it does not, and how far you can trust it to predict how your part behaves in a real fire. The Limiting Oxygen Index is one of the most quoted flammability numbers in plastics, and one of the most misread.
The short version: The Limiting Oxygen Index (LOI), also called the oxygen index, is the minimum oxygen concentration in volume percent that will keep a material burning once it is lit, measured in a controlled oxygen/nitrogen stream under ASTM D2863 or the technically equivalent ISO 4589-2. Air is about 20.9% oxygen, so a polymer with an LOI below ~21 burns freely in open air, while one above ~21 tends to self-extinguish once the ignition source is removed. Many engineers treat an LOI of roughly 27 to 28 as the working bar for a “good” flame-retardant grade. Untreated commodity plastics sit low (PP and PE around 17 to 18, PS around 18), engineering polymers higher (nylon 24 to 29, polycarbonate 25 to 28), and halogen- or fluorine-rich polymers very high (rigid PVC 45 to 49, PTFE around 95). Phosphate-ester flame retardants raise LOI two ways: by quenching flame radicals in the gas phase and by driving protective char in the condensed phase. Treat LOI as a screening and ranking tool, not a fire-code pass; UL 94 and cone calorimetry measure different things, and none of them is a guarantee of real-fire performance.
What the Limiting Oxygen Index actually measures
LOI is the minimum oxygen concentration, by volume, in a flowing oxygen/nitrogen mixture that will *just* sustain flaming combustion of a specimen. The method was first proposed by Fenimore and Martin in the 1960s and is now standardized as ASTM D2863 in the US and ISO 4589-2 internationally; the two are technically equivalent when the same direct oxygen-measurement setup is used.
The test itself is simple to picture. A bar of the material, typically 80 to 150 mm long, 10 mm wide and around 4 mm thick, is clamped vertically inside a glass chimney while a metered oxygen/nitrogen stream flows up past it. The top edge is ignited like a candle, and the operator steps the oxygen fraction up or down across repeated specimens until they find the lowest concentration at which the bar keeps burning for at least three minutes or consumes more than 50 mm of its length. That oxygen percentage is the LOI. A higher number means it took richer oxygen to keep the flame alive, which is what we mean by harder to burn.
Because the result depends on specimen geometry, conditioning and the exact apparatus, an LOI figure is only meaningful alongside the standard and test conditions it was run under. When you compare two grades, confirm both were measured to the same standard and bar thickness before you trust the gap between them.
Reading the number: the 20.9% air benchmark
The reason LOI is intuitive is that it lives on the same scale as the air you breathe. Atmospheric oxygen is about 20.95%, usually rounded to 21%. That single fact sets the interpretation:
- LOI below ~21: there is more than enough oxygen in ordinary air to keep the material burning. It is readily combustible and will keep burning after the igniter is removed.
- LOI just above 21 (roughly 21 to 27): marginally stable to slow-burning. It needs slightly oxygen-enriched conditions to sustain a flame, so it tends to self-extinguish in still air, but the margin is thin.
- LOI above ~27 to 28: generally treated as self-extinguishing and a genuine flame-retardant grade. High-risk sectors such as aerospace and rail transport often specify an LOI above 28.
Here is the honesty the data sheets usually skip. LOI is a *ranking and screening* number, not a verdict on real-fire safety. The test is downward, candle-like burning of one small bar in a quiet chamber with no external radiant heat, no realistic ventilation, and no account of how the material drips, smokes, or releases heat. Real fires, and the codes written for them, care about those things. UL 94 is a Bunsen-burner orientation-and-dripping test that returns a pass/fail class (such as V-0); the cone calorimeter measures heat-release rate, peak heat release, and total heat released. Published comparisons find only limited, inconsistent correlation between these methods and full-scale fire behavior, so a high LOI does not by itself promise a UL 94 V-0, and a V-0 material is not guaranteed to post a high LOI. Use LOI to rank candidate formulations and bracket flame-retardant loadings, then qualify the final material to the actual code test that governs your application.
LOI values for common polymers
The table below gives approximate ambient-temperature LOI ranges for unfilled base polymers, measured under ASTM D2863 / ISO 4589-2. Treat them as a starting map, not a spec: grade, molecular weight, fillers, glass fiber, plasticizer, and colorants all shift the number.
| Polymer | Typical LOI (vol% O₂) | Behavior in air (20.9% O₂) |
|---|---|---|
| Polypropylene (PP) | 17–18 | Burns readily |
| Polyethylene (PE) | 17–18 | Burns readily |
| Polystyrene (PS) | ~18 | Burns readily |
| ABS | 18–19 | Burns readily |
| Cured epoxy | 19–23 | Flammable to marginal |
| PET (polyester) | 20–21 | Marginal / slow-burning |
| Nylon 6 / 6,6 (PA) | 24–29 | Self-extinguishing tendency |
| Polycarbonate (PC) | 25–28 | Self-extinguishing tendency |
| Rigid (unplasticized) PVC | 45–49 | Self-extinguishing |
| PTFE | ~95 | Essentially non-flammable in air |
Two patterns are worth pulling out. First, aromatic and halogenated backbones burn harder than aliphatic ones: the carbon-rich aromatics (PC, the char-formers) and the chlorine and fluorine carriers (PVC, PTFE) sit far up the scale, while the simple hydrocarbon chains (PP, PE, PS) sit at the bottom. Second, the table is for *rigid* PVC. Add plasticizer and the LOI of flexible PVC falls back toward the low-to-mid 20s, because the plasticizer is itself fuel. That is the recurring trade-off in flame-retardant design: anything you add for flexibility, flow, or cost is usually something else for the fire to consume.
How phosphorus flame retardants raise LOI
Phosphorus-based flame retardants are the main lever for moving a polymer’s LOI up without adding halogen, and they work through two complementary routes.
Gas phase (radical quenching). As the additive decomposes, it releases phosphorus species such as PO•, PO₂•, and HPO• into the flame. These scavenge the reactive H• and OH• radicals that drive the combustion chain reaction, slowing or interrupting flame propagation. Aryl phosphate esters lean toward this mechanism, which is part of why even a few weight-percent of an active phosphorus additive can lift LOI by several points.
Condensed phase (char formation). On heating, the same chemistry breaks down to phosphoric and polyphosphoric acid, which catalyze dehydration and cross-linking of the polymer surface into a stable, carbon-rich char. That char is a physical barrier: it shields the polymer underneath from heat and chokes off the flow of flammable decomposition gases feeding the flame. This is the dominant route for intumescent systems built around ammonium polyphosphate (APP), where APP supplies the acid source that drives charring, usually paired with a carbon source and a blowing agent.
In practice many phosphate esters do both, and the balance shifts with chemistry. Liquid aryl and alkyl-aryl phosphates such as triphenyl phosphate (TPP), cresyl diphenyl phosphate (CDP), 2-ethylhexyl diphenyl phosphate (DPOP), and isopropylated triphenyl phosphate (IPPP) act largely in the gas phase while also plasticizing the matrix. Tricresyl phosphate (TCP) and the low-viscosity triethyl phosphate (TEP) are widely used as phosphate-ester flame retardants and plasticizers, with TEP often chosen as a reactive or processing-friendly additive in low-viscosity systems such as polyurethane foams. The general rule: char-forming polymers reward condensed-phase phosphorus, while non-charring polyolefins usually need an intumescent package or a higher loading to move the needle.
Selecting a flame retardant by polymer and target LOI
Picking an FR is never just about chasing the highest LOI. You match the chemistry to the polymer, the target LOI set by your governing code test, and every other spec the additive touches: glass transition and heat-deflection temperature, volatility and migration, clarity, electrical properties, processing window, and end-use approvals.
A few practical anchors:
- Low-viscosity or foam systems: TEP blends in easily and carries phosphorus efficiently where viscosity matters.
- Rigid and engineering thermoplastics: solid TPP preserves stiffness; liquid CDP, DPOP, and IPPP double as plasticizing FRs in PVC and engineering blends.
- Intumescent coatings and polyolefins: APP-based char systems do the heavy lifting where the base polymer will not char on its own.
The trade-off to weigh out loud: liquid aryl phosphates plasticize, which can lower modulus and heat-deflection temperature and raise migration or volatility over the part’s life, while solid additives like TPP and APP protect stiffness but demand attention to dispersion and processing. There is no free LOI point. Set the target LOI from the code test that actually governs your part, then choose the *lowest* loading of the right chemistry that clears it while holding your other specs, and validate on your own compound rather than on a base-resin literature value.
A note on chlorinated phosphate esters. If your selection includes chlorinated organophosphates such as TCEP, TCPP (tris(1-chloro-2-propyl) phosphate), or TDCPP, factor in their regulatory trajectory. In the US, the EPA has finalized a TSCA risk evaluation for TCEP and has assessed the chlorinated-phosphate-ester cluster. In the EU, TCEP, TCPP, and TDCPP have all drawn substance-of-very-high-concern scrutiny, including a 2018 ECHA screening that identified exposure risks for children from these substances in childcare articles and upholstered furniture; TCEP appears on the ECHA Candidate List. Status and permitted uses differ by jurisdiction and end-use sector and continue to change, so confirm the current position for your jurisdiction and application before you design one of these in.
Buying phosphate-ester flame retardants
RawSource supplies the phosphate-ester flame-retardant and plasticizer range used to raise LOI across plastics, coatings, cable, and foam — TEP, TPP, TCP, CDP, DPOP, IPPP, and APP — for industrial manufacturing and coatings formulators, in drums, IBCs, and bulk with CoA documentation. Tell us your base polymer, your target LOI and the governing code test (UL 94, FAR, EN, or another), and your constraints on plasticization, volatility, and approvals, and request a sample to qualify the LOI lift on your own system. If your flame-retardant work sits in epoxy, our companion guide to amine blush in epoxy covers a related cure-side failure mode worth knowing before you load an additive into the system.
Frequently asked questions
What is a good LOI value?
As a working rule, a Limiting Oxygen Index above roughly 27 to 28 is treated as a solid flame-retardant grade, because the material then needs meaningfully more oxygen than the 20.9% in air to keep burning. Anything below about 21 burns freely in air, and the 21-to-27 band is marginal or slow-burning; aerospace and rail often specify above 28. Always read “good” against the code test that governs your part, not as an absolute.
Is LOI the same as a UL 94 rating?
No. LOI is a quantitative number, the minimum oxygen percentage that sustains downward candle-like burning under ASTM D2863 / ISO 4589-2. UL 94 is a pass/fail class (V-0, V-1, V-2) based on Bunsen-burner behavior including dripping and orientation. They measure different aspects of flammability, correlation between them is limited, and a high LOI does not automatically earn a UL 94 V-0. Most qualifications report both.
What LOI is self-extinguishing?
A material with an LOI above the ~20.9% oxygen content of air will tend to self-extinguish once the ignition source is removed, because ordinary air no longer holds enough oxygen to sustain the flame. The threshold engineers rely on for confident self-extinguishing behavior is higher, around 27 to 28, which builds in a margin above the air benchmark. This is still-air, small-specimen behavior, not a real-fire guarantee.
How do flame retardants raise LOI?
Phosphorus flame retardants raise LOI through two mechanisms. In the gas phase, phosphorus radicals (PO•, PO₂•, HPO•) released as the additive decomposes scavenge the H• and OH• radicals that drive combustion, slowing the flame. In the condensed phase, the additive forms phosphoric and polyphosphoric acid that dehydrate and cross-link the polymer surface into a protective char barrier that blocks heat and flammable gases. Many phosphate esters do both, and a few weight-percent can lift LOI by several points.
Does a higher LOI mean the material is safe in a fire?
No. LOI is a laboratory screening and ranking number measured on a small bar in a controlled oxygen stream; it is not a measure of fire safety, smoke, toxicity, heat release, or how a finished product performs in a real fire. Use it to compare and screen candidate formulations, then qualify the material to the actual fire-test standard and building or product code that applies to your end use. This article makes no safety claim about any material or additive.
Why is the LOI of air about 21%?
Atmospheric air is roughly 20.9% oxygen (often rounded to 21%), with most of the balance nitrogen. Because LOI is reported as the oxygen percentage needed to sustain burning, that 20.9% figure is the natural dividing line: a material whose LOI is below it has enough oxygen in ordinary air to keep burning, while one whose LOI is above it does not, and so tends to stop burning when the ignition source is taken away.
Editorial note. This article is general technical guidance for plastics, coatings, cable, and foam formulation professionals. LOI values depend on the specific grade, fillers, plasticizers, specimen geometry, and test conditions and must be validated on your own system; the Certificate of Analysis governs the grade you buy, and the governing fire-test standard and code govern your end use. Nothing here is a fire-safety, medical, health, or environmental claim, and no flame retardant referenced is characterized as “safe,” “non-toxic,” or “green.” Regulatory status for flame retardants — particularly chlorinated phosphate esters such as TCEP, TCPP, and TDCPP — varies by jurisdiction, end use, and date and is actively changing; statements here reflect publicly available EPA and ECHA information at the time of writing and must be re-confirmed for your jurisdiction and application before use. Products are sold for industrial and professional use only. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
