UL 94 Flammability Ratings: 12 Classes, Test Criteria and Additive Routes to V-0

UL 94 sorts a plastic compound into 1 of 12 flammability ratings, from HB at the bottom to 5VA at the top, and every rating is valid only for the thickness and colour that were actually tested. The 12 classes come from 4 different test methods with 2 different flame sizes, so which method applies to a given part depends on whether it is a rigid moulding, a cellular foam or a flexible film.

The 4 test methods run in a fixed order of severity: the horizontal burning test produces the single HB class for rigid parts tested lying flat; the 50 W vertical burning test produces V-2, V-1 and V-0 for rigid parts tested standing upright; the 500 W vertical flame test produces 5VB and 5VA for parts resisting a larger ignition source; and the thin-film and foam tests, run under their own ISO standards, produce VTM-2, VTM-1 and VTM-0 for films and HBF, HF-2 and HF-1 for foams.

This page works through all 12 classes in that order, the exact pass criteria for each, how the vertical test is run, and the flame retardant loading a formulator needs to reach V-0 in a given polymer. Fire performance is one property inside the wider set of plastic additives, and the plastic additives category page indexes every other function, from plasticizers to impact modifiers, before this page turns to selection, application requirements and what a rating does not tell a design engineer.

Table 1. The 12 UL 94 ratings at a glance

HBF, HF-1 and HF-2 apply to cellular materials; VTM-0 to VTM-2 apply to flexible thin materials that distort in the standard vertical test.

What Is the UL 94 Flammability Test?#

UL 94 is the Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, and it sorts a compound into a flammability class by measuring how long the material keeps burning after a calibrated flame is taken away. UL Standards & Engagement publishes and maintains UL 94, and a result is a classification, not a pass or fail grade: the test places a material into one of the 12 ratings on this page, and the end-product standard governing the finished device decides which rating that part actually needs.

Manufacturers use UL 94 to compare the small-flame behaviour of candidate materials before a part is designed, and laboratories record the same 12 classes on a datasheet or a UL Yellow Card entry. The test evaluates small laboratory specimens of one thickness under a controlled ignition source, not a finished enclosure in a real fire, which is why thickness and colour matter as much as the material itself.

Which standards are equivalent to UL 94?#

UL 94 is harmonised as 4 international standards: IEC 60695-11-10 for the 50 W horizontal and vertical tests, IEC 60695-11-20 for the 500 W test, ISO 9772 for cellular materials and ISO 9773 for flexible thin materials.

• IEC 60695-11-10, which mirrors the horizontal burning test and the 50 W vertical burning test and produces the HB, V-2, V-1 and V-0 classes
• IEC 60695-11-20, which mirrors the 500 W flame test and produces the 5VB and 5VA classes
• ISO 9772, which covers cellular materials and produces the HBF, HF-2 and HF-1 classes
• ISO 9773, which covers flexible thin materials and produces the VTM-2, VTM-1 and VTM-0 classes

These are harmonised, equivalent methods, not identical certificates: a result generated under IEC 60695-11-10 follows the same procedure as UL 94 but does not automatically transfer as a UL recognition or Yellow Card entry.

The 12 UL 94 Flammability Ratings in 4 Test Methods#

The 12 UL 94 ratings run from HB, the lowest, through the foam classes HBF, HF-2 and HF-1, the thin-film classes VTM-2, VTM-1 and VTM-0, the vertical classes V-2, V-1 and V-0, to 5VB and 5VA at the top. Each class is written with a hyphen throughout this page; the vertical burning section below also covers the unhyphenated form some datasheets use. Table 2 lists the exact pass criteria for every class in this fixed order.

Table 2. Full UL 94 criteria by class

Blank cells mean the criterion is not part of that classification. The foam and thin-film time limits are set in ISO 9772 and ISO 9773 and are not reproduced here.

UL 94 HB: the horizontal burning test#

UL 94 HB is the lowest flammability class: a horizontal specimen under 3 mm thick may burn no faster than 76 mm per minute, or the flame must stop before it reaches the 100 mm mark. The HB test lies the specimen flat with one end lit by a small flame, and burning is timed as the flame front moves along the bar.

HB is the lowest UL 94 class, and many unmodified commodity plastics, including unfilled polypropylene and polyethylene, reach it with no flame retardant at all. Passing HB confirms only that a material does not burn quickly while lying flat; it says nothing about how the same compound behaves standing upright in the 50 W vertical test, where the V classes are measured, or under the more severe 500 W flame that produces the 5V classes.

UL 94 V-0, V-1 and V-2: the 50 W vertical burning test#

The 50 W vertical burning test produces 3 classes: V-0 requires each afterflame to stop within 10 seconds, V-1 allows 30 seconds, and V-2 allows the same 30 seconds but also permits flaming drips that ignite the cotton indicator below the specimen.

All 3 classes share the same test setup: 5 specimens, 2 flame applications each, so the "total" criterion in Table 2 covers 10 flame applications across the full specimen set. The cotton indicator sitting beneath the clamped specimen is what the drip criterion is judged on, and it is the single condition that separates V-2 from V-0 and V-1. Which of the 5 criteria fails most compounds during development? In practice the drip criterion is the hardest one to satisfy, because a resin can meet every afterflame time and still fail the moment a flaming particle falls onto the cotton, which is why anti-drip additives exist as a distinct additive class alongside flame retardants.

What does UL 94 V-0 mean?#

UL 94 V-0 means that each of 5 vertical specimens stopped burning within 10 seconds of each flame application, that the 5 specimens together burned for no more than 50 seconds in total, that afterflame plus afterglow after the second application stayed under 30 seconds, that nothing burned up to the holding clamp, and that no flaming drip ignited the cotton below. A V-0 result is always quoted with a thickness, because the same compound can be V-0 at 3.2 mm and V-2 at 0.8 mm; some datasheets still write the rating as 94V-0.

What is the difference between a V-2 and a V-0 rating?#

A V-2 compound may burn for up to 30 seconds after each flame application and may drop flaming particles that ignite the cotton indicator, while a V-0 compound must stop within 10 seconds and must not drip flame at all. V-1 sits between them on time, allowing the same 30 seconds as V-2, but it forbids the drips that V-2 permits. HB, for comparison, is measured with the specimen lying down; the V classes are all measured standing up.

UL 94 5VA and 5VB: the 500 W flame test#

The 500 W test applies a 125 mm flame 5 times for 5 seconds each and produces 2 classes: 5VB, where burning stops within 60 seconds with no drips, and 5VA, which adds the condition that the plaque must not burn through.

The 500 W flame is far more severe than the 50 W flame used for the V classes, and 5V testing is usually run on a flat plaque rather than a bar, because the burn-through criterion needs a surface to penetrate. 5VA is the most demanding UL 94 class, required where a part has to survive a larger ignition source than the vertical test represents, such as a large enclosure panel.

VTM-0, VTM-1, VTM-2, HBF, HF-1 and HF-2: thin films and foams#

Films and foams have their own UL 94 classes: VTM-0, VTM-1 and VTM-2 for flexible thin materials, and HBF, HF-1 and HF-2 for cellular materials such as foams.

ISO 9773 covers the VTM classes because a thin, flexible film distorts away from a flame in the standard vertical test, so it is wrapped around a mandrel and tested in that configuration instead. VTM-0 is the strictest of the 3 film classes, followed by VTM-1 and then VTM-2.

ISO 9772 covers the HBF, HF-2 and HF-1 classes for cellular materials, including rigid and flexible foams, with HF-1 the strictest and HBF the least strict. The time limits for the film and foam classes follow the same structure as the V classes and are set out in ISO 9773 and ISO 9772 rather than restated here.

How the UL 94 Vertical Burning Test Is Performed#

In the vertical test a 125 by 13 mm bar hangs from a clamp, a 20 mm flame is held under it for 10 seconds and removed, the afterflame time t1 is recorded, the flame is applied a second time for 10 seconds, and t2 and the afterglow time t3 are recorded. This sequence repeats across all 5 specimens in the set, and the totals across the set are what Table 2 reports as the total afterflame criterion.

Which specimen details decide whether the result is repeatable between laboratories? The specimen dimensions, the conditioning time before testing and the exact flame height are the 3 variables that most affect repeatability, which is why the standard fixes each of them rather than leaving them to the tester's judgement.

Specimen size, conditioning and flame application#

A UL 94 vertical test uses 5 bars of 125 by 13 mm at a stated thickness, conditioned for 48 hours at 23°C and 50% relative humidity, with a second set aged 7 days at 70°C.

• Specimen: 5 bars measuring 125 by 13 mm, moulded or cut at the thickness the rating will cover
• Conditioning: 48 hours at 23°C and 50% relative humidity before the first test run
• Heat ageing: a second set of 5 bars conditioned for 7 days (168 hours) at 70°C, which checks that the rating survives heat ageing of the specimen, a condition where volatile or blooming flame retardants tend to fail
• Flame application: a 20 mm (50 W) flame applied twice for 10 seconds each, with the afterflame and afterglow times recorded after each application

Why a UL 94 rating is valid only at one thickness and colour#

A UL 94 rating belongs to a specific thickness, so a compound listed as V-0 at 1.6 mm has said nothing about its behaviour in a 0.8 mm wall.

The Exolit OP 1312 phosphinate blend, for example, gives V-0 across a stated range of 0.4 to 3.2 mm in glass-filled PA6 and PA66, while the Exolit OP 1230 system gives V-0 at exactly 1.6 mm and 0.8 mm in PA 6T/66; neither result can be assumed to extend beyond the thickness range that was actually tested.

UL records every rating against the thickness and the colour of the specimen tested, because colourants and reprocessing can shift a flame retardant's performance enough to change the class. UL recognition and Yellow Cards for plastic compounds explains how a compound is entered onto that listing, and a part that changes thickness, colour or resin lot needs its own confirmed rating rather than one inherited from a similar formulation.

Which Additives Change a UL 94 Rating?#

Three additive groups decide a UL 94 rating: flame retardants, which cut the afterflame time, anti-drip additives such as PTFE, which remove the flaming drips, and reinforcing fibres and some fillers, which make both worse. Each group acts on a different one of the 5 pass criteria in Table 2: flame retardants shorten t1, t2 and the total afterflame time; anti-drip agents stop the flaming-drip criterion from failing; and fibres or fillers can push a formulation the wrong way by wicking molten polymer toward the flame front. A formulation usually needs more than one of these groups working together to move a compound from HB up to V-0.

Flame retardant loadings that reach V-0, by polymer#

Flame retardant loadings for V-0 run from about 15 wt% of the compound for an aluminium diethylphosphinate blend in glass-filled polyamide to 22-30 wt% of the compound for an intumescent ammonium polyphosphate system in polypropylene, and up to 100 phr of resin for aluminium trihydroxide in a flexible PVC cable compound. These loadings vary by polymer chemistry, processing temperature and whether the compound is glass filled, and the classes and mechanisms behind each chemistry are compared in full on flame retardants for plastics, which indexes phosphorus, nitrogen, halogenated and mineral systems by mechanism rather than by polymer.

Table 3. Flame retardant loading for UL 94 V-0, by polymer

Loadings are wt% of the finished compound, except the PVC cable row, which is phr of resin. Every row is a published formulation, not a specification, and the rating has to be confirmed on the actual part thickness.

The phosphinate values come from the Clariant Exolit thermoplastics brochure, the PA6 synergist data from a 2026 study in Polymers (PMC12899044), and the HIPS values from the Albemarle Saytex 8010 datasheet. Formulators who need a ready-rated starting point rather than a loading table can compare flame-retardant plastics and their UL 94 ratings by polymer, which lists compounds already validated at a stated thickness.

Anti-drip additives: the step from V-2 to V-0#

Anti-drip additives are the shortest route from V-2 to V-0, because the two classes differ mainly in whether flaming drips ignite the cotton, and 0.1 to 0.5 wt% of PTFE builds a fibrillar network in the melt that holds the burning material together.

PTFE for this purpose is usually supplied SAN-encapsulated, a form commonly called TSAN, because encapsulation avoids fibrous agglomerates and improves dispersion and surface finish compared with raw PTFE powder. The full set of anti-drip PTFE grades and encapsulation forms is described separately, and the worked PC/ABS case in Table 3 uses 0.4 wt% PTFE alongside 20 wt% aryl bisphosphate to reach V-0.

The patent literature for encapsulated PTFE anti-drip systems covers a range of 0.05 to 10 phr of active PTFE, with 0.1 phr common in the worked examples, and the same mechanism applies across PC, PC/ABS, PBT and PA. Flame retardant synergists and anti-drip agents together cover the full set of additives that push a compound the last step to V-0 without raising the flame retardant loading itself.

Additives, fillers and reinforcing fibres that make a UL 94 rating worse#

Reinforcing glass fibre usually makes a UL 94 rating harder to reach, because the fibres wick molten polymer towards the flame front, so a glass-filled grade needs a higher flame retardant loading than the same unfilled polymer, as the glass-filled rows of Table 3 show against their unfilled counterparts.

• Reinforcing glass fibre, which wicks molten polymer toward the flame front and typically pushes the required flame retardant loading higher than in the unfilled resin; reinforcing fibers for plastics covers the loadings and fibre types in full
• Plasticizers, whose type and level change the fire behaviour of flexible compounds such as PVC, so a cable formulation cannot change its plasticizer without re-checking the rating
• The processing window itself, because engineering polymers running at 240-320°C exclude aluminium trihydroxide, which releases water from about 200°C, and most ammonium polyphosphate systems, whose second decomposition phase starts from about 240°C

How to Select Additives for UL 94 V-0 in 6 Steps#

Select a flame retardant system for UL 94 V-0 in 6 steps: fix the rating and thickness, identify the polymer and its glass content, check the processing temperature, choose halogenated or halogen free, add an anti-drip additive if dripping is the failure mode, then set the loading and confirm it by test.

1. Fix the rating and the thickness the part actually needs, since a V-0 result at 3.2 mm does not confirm performance at 0.8 mm.
2. Identify the polymer and whether it is glass filled, because glass-filled grades usually need a higher loading than the unfilled resin.
3. Check the processing temperature against the flame retardant's decomposition temperature; aluminium trihydroxide releases water from about 200°C and ammonium polyphosphate systems begin their second decomposition phase from about 240°C, which rules both out of most engineering polymers processed at 240-320°C.
4. Choose a halogenated system paired with an antimony trioxide synergist, or a halogen-free phosphorus or nitrogen system, and screen the regulatory status of each candidate before committing to it.
5. Add an anti-drip additive, typically 0.1-0.5 wt% PTFE, if the failure mode is dripping rather than afterflame time.
6. Set the loading, check what it costs in mechanical and electrical properties such as HDT and CTI, and confirm the rating by test at the real wall thickness.

This 6-step sequence follows the same logic as the general framework on how to select plastic additives, applied here specifically to the flame retardant and anti-drip decision.

Formulators working through several candidate systems at once can filter candidates with the flame retardant selector by polymer and UL 94 rating rather than repeating this sequence by hand for every resin grade.

Which UL 94 Rating Does an Application Need? Appliances, Wire and Cable, Automotive and Electronics#

A UL 94 rating is required by the end-product standard, not by UL 94 itself: IEC 60335-1 sets glow-wire values for unattended household appliances, electrical and electronic enclosures are typically specified at V-0 from 0.4 to 1.6 mm, and automotive interiors are governed by FMVSS 302 rather than by UL 94 at all. Table 4 lists the typical requirement for 5 applications, the other fire test that applies, and where each application's additive package is described in full.

Table 4. UL 94 requirements by application

What a UL 94 Rating Does Not Tell You#

A UL 94 rating does not mean a plastic is fireproof: it records how 5 small specimens of one thickness behaved under a laboratory flame, and it says nothing about heat release, smoke, toxic gas or the behaviour of the finished part in a real fire.

• Fire hazard in a full-scale fire, since UL 94 uses small laboratory specimens under a calibrated ignition source rather than a real compartment fire
• Behaviour at other thicknesses or colours, because the rating belongs only to the specimen that was actually tested
• Heat release rate, which the cone calorimeter measures separately; Bernhard Schartel of BAM in Berlin and T. Richard Hull of the University of Central Lancashire showed in a 2007 study in Fire and Materials that peak heat release rate from the cone calorimeter correlates poorly with UL 94 results for intumescent flame retardant systems
• Smoke density and toxic gas evolution, which are measured by separate tests such as ISO 5659-2, ASTM E662 and, for cables, IEC 61034

The limiting oxygen index can also rank the same set of materials differently from UL 94, because V-2 tolerates flaming drips that an oxygen-based test does not penalise at all.

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How Does UL 94 Fit With the Other Fire Tests and Fire Rules?#

UL 94 is one test in a set: an electrical connector also has to pass a glow-wire test and a comparative tracking index test, a cable has to pass the EN 50399 cable fire test, and a car interior is judged by FMVSS 302. Every method that plastics compounds have to pass sits alongside UL 94 in a wider set of fire, mechanical, thermal and optical tests, and testing plastic additives indexes all of them in one place, from flammability through to weathering and impact.

Glow wire, CTI and what IEC 60335-1 requires#

A UL 94 rating alone does not qualify a part for an unattended household appliance: IEC 60335-1 clause 30.2.3 also requires a GWFI of at least 850°C and a GWIT of at least 775°C for parts carrying connections above 0.2 A. The glow wire itself is applied for 30 seconds at a temperature between 550°C and 960°C depending on the test, and GWFI and GWIT are reported as two separate values rather than one combined number. Glow wire test (GWFI, GWIT) explains how the two values are measured and why they diverge.

The same phosphinate blend that reaches V-0 at 0.4-3.2 mm in glass-filled polyamide (Table 3) also reaches GWFI 960°C, GWIT 775°C and CTI up to 600 V, which is why the three numbers appear together on many electrical and electronic datasheets. The trade-off against flame retardant loading is described on comparative tracking index (CTI), which covers how tracking resistance interacts with a phosphorus-based flame retardant package.

LOI, cone calorimeter, smoke density and the cable and automotive fire tests#

Four other fire tests decide whether a UL 94 rating is enough: the limiting oxygen index ranks how much oxygen a material needs to keep burning, the cone calorimeter measures heat release, smoke density measures optical obscuration, and the cable and automotive tests apply their own pass criteria.

Table 5. Sibling fire tests to UL 94

Where to get the UL 94 standard and UL 94 testing#

The UL 94 standard is a copyrighted document published by UL Standards & Engagement and bought from the UL standards store, so the free PDF copies that circulate online are not authorised versions. Testing against UL 94 is carried out by UL Solutions and by independent third-party fire-testing laboratories holding the relevant accreditation, rather than by reading the standard alone. No public source distributes an authorised free copy of the current text, so a formulator confirming a claimed rating needs either a UL Yellow Card record or a certificate of test from an accredited laboratory.

Are the flame retardants and the anti-drip PTFE that deliver V-0 restricted?#

Several of the flame retardants that deliver V-0 are restricted or on the candidate list: decaBDE is a Stockholm Convention Annex A pollutant, HBCD passed its REACH authorisation sunset on 21 August 2015, and TBBPA has been an SVHC since 17 January 2023. Every instrument behind these statuses is dated in full on flame retardant regulations. Not every flame retardant in Table 3 is affected: antimony trioxide, the HIPS synergist, carries a harmonised Carc. 2 H351 classification and an EU 10/2011 migration limit of 0.04 mg/kg as antimony, but it is not an SVHC; the evidence behind these classifications is summarised on flame retardants and human health.

• decaBDE, Stockholm Annex A since COP-8 (2017); EU trace limit for tetra- to decaBDE 10 mg/kg under Delegated Regulation (EU) 2025/1482
• HBCD, past its REACH Annex XIV sunset since 21 August 2015, replaced by polymeric brominated FRs in EPS and XPS
• TBBPA, SVHC since 17 January 2023, harmonised Carc. 1B from 1 September 2025
• DBDPE, added to the SVHC Candidate List on 5 November 2025 as vPvB, not otherwise restricted

Regulation (EU) 2019/2021 has banned halogenated flame retardants in electronic-display enclosures and stands since 1 March 2021. Anti-drip PTFE and PFAS in plastics tracks the pending EU universal PFAS restriction, written here as pending because PTFE falls inside its proposed scope but the restriction is not yet adopted.

Is there a UL 94 V-3 rating?#

No UL 94 V-3 rating exists: the 50 W vertical test produces only V-0, V-1 and V-2, and the classes above V-0 come from the 500 W test and are called 5VB and 5VA. A search for "V0 V1 V2 V3" usually reflects this misconception rather than an actual class.

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