Flammability of Polymer Materials and UL94 Explained

What is the UL94 flammability test that are referred by number of manufacturers using plastic polymer materials such as moulding packages? Omnexus, the material selection platform for plastic material by SpecialChem provides an overview of the test in details.

The UL 94 flammability test classifies how plastic materials burn after exposure to a specified ignition flame under controlled laboratory conditions. This article explains the horizontal, vertical, thin-film and foam classifications, their limitations, and how design engineers and component purchasers should use them when selecting housings, encapsulants, insulation and PCB materials.

Key Takeaways

• UL 94 is a small-scale material flammability classification, not proof that a component or finished product is fire-safe.
• V-0 permits no more than 10 s of afterflame after either flame application; V-1 and V-2 permit no more than 30 s. Other criteria, including total afterflame time and cotton ignition, also apply.
• V-0 and V-1 do not mean that no material can drip. The classification criteria prohibit ignition of the cotton indicator by flaming particles or drops.
• A classification must be checked against the recognised material grade, colour, thickness and applicable conditions. A rating established at a thicker section cannot simply be transferred to a thinner design.
• VTM ratings for thin flexible materials and HF ratings for foams are separate classification families. VTM-0 is not equivalent to V-0.
• Glow-wire performance, thermal endurance, electrical tracking resistance and environmental compliance require separate checks; none is established by a UL 94 rating alone.

UL 94 Flammability Test: Preliminary Screening of Polymer Materials

UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, provides small-scale methods for comparing the burning behaviour of plastic specimens. Depending on the method, the observations include burning rate, afterflame time, afterglow, flaming particles and burn-through. These results support material preselection for components and equipment; they do not reproduce every fire scenario in a finished assembly. [1]

Afterflame is the period of continued flaming after the ignition source is removed. Afterglow is continued glowing combustion without a flame. These are separate observations, and a material must satisfy all the criteria of the claimed classification rather than only a headline extinguishing time. [1, 2]

The scope is particularly relevant to plastic parts in electronic devices and appliances: connector housings, transformer bobbins, component cases, insulating supports and similar parts. A material classification does not establish the acceptability of the complete component, its installation or the finished equipment. UL recognition remains subject to the relevant conditions of acceptability. [1, 3]

What UL 94 Does Not Establish

Fire behaviour involves more than whether a small specimen stops flaming. Relevant characteristics include the following, but they are not all measured by UL 94. [2, 8]

• Ease of ignition: how readily a material ignites under a particular heat or ignition source.
• Flame spread: how combustion propagates over a surface or through an assembly.
• Barrier performance: whether a construction resists fire penetration.
• Heat release: how much energy combustion releases and how quickly.
• Extinction behaviour: how flaming and glowing combustion cease after removal of an ignition source.
• Smoke production and the toxicity of combustion gases.

These differences explain why research into polymer combustion considers the burning mechanism, methods of reducing flammability and the selection of suitable test methods together. A formulation that improves one fire-performance measure does not necessarily improve every other measure. In particular, UL 94 does not provide a smoke-density or smoke-toxicity classification. [2, 8]

Main Test Types and Flammability Ratings

UL 94 includes separate test families for ordinary plastic specimens, thin flexible materials and low-density foams. The specimen arrangement and ignition exposure differ, so the methods must not be described using one universal geometry, exposure time or conditioning schedule. Table 1 separates the principal families. [1]

Test family	Classifications	Principal application and observations	Ignition exposure overview
Horizontal burning	HB	Horizontal plastic specimen; burning rate or cessation of burning	One application of a 20 mm flame for up to 30 s; removed earlier if the flame front reaches the first reference mark
Vertical burning	V-0, V-1, V-2	Vertical specimen; afterflame, afterglow, cotton ignition and burning to the clamp	20 mm flame; two applications of 10 s
500 W burning	5VA, 5VB	Bar and plaque testing; combustion behaviour and plaque burn-through	125 mm flame; repeated applications under the 5V method
Vertical thin material	VTM-0, VTM-1, VTM-2	Thin material formed into a cylindrical specimen; afterflame, afterglow, cotton ignition and propagation to a reference mark	20 mm flame; two applications of 3 s
Horizontal foamed material	HF-1, HF-2, HBF	Low-density foam; burning behaviour, damaged length and, for HF classifications, cotton ignition	38 mm flame; 60 s application

UL Solutions groups the twelve classifications into six commonly used for enclosures, structural parts and insulators, three for low-density foams and three for thin films. Within the ordinary-plastics group, the sequence 5VA, 5VB, V-0, V-1, V-2 and HB indicates increasingly less demanding classifications, but it is not a universal ranking across foams, films and all possible fire hazards. [1]

Related international methods include IEC 60695-11-10 for 50 W horizontal and vertical flame testing, IEC 60695-11-20 for 500 W flame testing, ISO 9772 for cellular plastics and ISO 9773 for thin flexible specimens. A related method number does not remove the need to check the exact classification and certification requirement specified for the application. [1, 3]

Vertical Burning Tests for Classifying Materials

Vertical tests assess how a specimen behaves after a prescribed flame exposure, including whether combustion continues and whether falling burning material ignites an indicator. The V, VTM and 5V methods use different arrangements and exposures; their similar-looking designations must not be treated as interchangeable. [1]

UL 94 V Ratings: V-0, V-1 and V-2

In the V method, the specimen is held vertically from its upper end and a burner is placed below it. The flame is applied for 10 s, removed while the afterflame period is measured, and applied again for 10 s after the first flaming period ends. The assessment includes individual and total afterflame times, combined afterflame and afterglow after the second application, cotton ignition and propagation to the clamp. [1, 2]

• V-0: each afterflame period is no longer than 10 s, with no cotton ignition by flaming particles or drops, and all other V-0 criteria satisfied.
• V-1: each afterflame period is no longer than 30 s, with no cotton ignition by flaming particles or drops, and all other V-1 criteria satisfied.
• V-2: each afterflame period is no longer than 30 s; cotton ignition by flaming particles or drops is permitted, provided the other V-2 criteria are satisfied.

Criterion	V-0	V-1	V-2
Afterflame time after either application, for each specimen	≤ 10 s	≤ 30 s	≤ 30 s
Total afterflame time for a set of five specimens, comprising ten flame applications	≤ 50 s	≤ 250 s	≤ 250 s
Afterflame plus afterglow after the second application, for each specimen	≤ 30 s	≤ 60 s	≤ 60 s
Cotton ignition by flaming particles or drops	Not permitted	Not permitted	Permitted
Flaming or glowing combustion reaching the holding clamp	Not permitted	Not permitted	Not permitted

The distinction between dripping and cotton ignition matters. A V-0 or V-1 classification does not mean that no molten material can fall from the specimen. Conversely, cotton ignition is allowed for V-2 but is not required: the word permitted describes an acceptance limit, not a behaviour that every V-2 material must exhibit. [1, 2]

Why Thickness, Colour and Grade Matter

A UL 94 classification belongs to a particular material designation and recognition scope, not to a generic polymer name. The UL Yellow Card identifies the supplier and grade and associates flame classifications with minimum thickness and colour coverage. Other properties and explanatory footnotes may also depend on thickness or the recognised formulation. [3]

For illustration, a statement such as V-0 at 1.5 mm does not establish V-0 performance at 0.8 mm. Likewise, V-0 at 0.8 mm is meaningful only when the specified grade, colour and recognition conditions cover the material being purchased. These thickness values illustrate how to read a specification; they are not test results for a particular compound.

Do not interpret minimum thickness as a requirement for an exact numerical match between a drawing and a Yellow Card row. Instead, confirm that the finished part’s minimum thickness, including manufacturing tolerance, lies within the permitted recognition scope. Do not extrapolate to a thinner section or assume that an unlisted formulation, colour or processing variant has the same recognition. [3]

UL 94 VTM Ratings: Thin Flexible Materials

The VTM method addresses thin materials whose behaviour or flexibility prevents assessment using the ordinary V method. The specimen is formed around a mandrel and the mandrel is removed before testing, leaving a cylindrical specimen. The ignition flame is applied twice for 3 s rather than twice for 10 s. [1, 2, 9]

The method assesses propagation to a reference mark on the specimen rather than simply substituting the holding-clamp criterion from the ordinary V test. VTM-0, VTM-1 and VTM-2 therefore need their own specification: UL Solutions explicitly states that they are not equivalent to V-0, V-1 and V-2 respectively. This distinction is important for flexible insulation, films and flexible printed-circuit substrates. [1, 9]

Criterion	VTM-0	VTM-1	VTM-2
Afterflame time after either application, for each specimen	≤ 10 s	≤ 30 s	≤ 30 s
Total afterflame time for five specimens	≤ 50 s	≤ 250 s	≤ 250 s
Afterflame plus afterglow after the second application, for each specimen	≤ 30 s	≤ 60 s	≤ 60 s
Cotton ignition by flaming particles or drops	Not permitted	Not permitted	Permitted
Flaming or glowing combustion reaching the reference mark	Not permitted	Not permitted	Not permitted

UL 94 5V Testing: 5VA and 5VB

The 5V method uses a 500 W, 125 mm flame and evaluates bar and plaque specimens. It applies a more severe flame exposure than the ordinary V method and distinguishes whether a hole forms in the plaque. The resulting classifications are 5VA and 5VB; 5V describes the test family rather than an additional classification to specify alongside them. [1]

Criterion	5VA	5VB
Afterflame plus afterglow after the fifth flame application	≤ 60 s	≤ 60 s
Flaming particles or drops	Not permitted	Not permitted
Burn-through hole in the plaque	Not permitted	Permitted

A 5VA classification does not make a material non-combustible, establish a fire-resistance duration for an enclosure or replace the evaluation of openings, joints and installed components. Its value is the defined performance demonstrated by the recognised material in that particular test. [1, 3]

Horizontal Burning Tests for Classifying Materials

Horizontal testing includes the HB method for ordinary plastic specimens and a separate method for low-density foams. HB is principally a burning-rate classification, whereas the foam classifications also distinguish short afterflame behaviour and cotton ignition. [1, 2]

UL 94 HB Rating

For HB testing, the horizontal specimen is exposed to the specified flame for up to 30 s. The flame is removed earlier if the flame front reaches the first reference mark. Acceptance depends on the burning rate for the relevant thickness, or on combustion stopping before the 100 mm reference mark. [1, 2]

Acceptance route	Requirement	Classification
Specimen thickness from 3 mm to 13 mm	Burning rate ≤ 40 mm/min	HB
Specimen thickness below 3 mm	Burning rate ≤ 75 mm/min	HB
Combustion stops before the 100 mm reference mark	Self-extinguishing acceptance route, independent of the rate limits above	HB

An HB classification must not be described as equivalent to V-0 or as proof of self-extinguishing behaviour in every orientation. The horizontal result answers a different, specifically defined screening question. [1]

UL 94 HBF, HF-1 and HF-2: Foamed Materials

The foam method applies to low-density foamed materials, including materials used for acoustic functions. Foam density is part of the test context and must not be omitted when comparing grades. HBF can be obtained when the burning rate does not exceed 40 mm/min over a 100 mm span, or when flaming or glowing combustion stops before the 125 mm reference mark. [1, 2]

Criterion	HF-1	HF-2
Afterflame time after removal of the ignition source	At least four of five specimens: ≤ 2 s; remaining specimen: ≤ 10 s	At least four of five specimens: ≤ 2 s; remaining specimen: ≤ 10 s
Afterglow time for each specimen	≤ 30 s	≤ 30 s
Cotton ignition by flaming particles or drops	Not permitted	Permitted
Damaged length of each specimen	< 60 mm	< 60 mm

How Plastics Achieve Flame-Retardant Performance

A compound’s burning behaviour depends on the polymer itself and the complete formulation, including flame retardants, fillers and other additives. Some engineering polymers have inherent flame-retardant behaviour, while others need an additive or reactive flame-retardant system. Classification must therefore be checked for the finished compound rather than inferred from the polymer family. [2, 8]

Halogenated Flame-Retardant Systems

Brominated and chlorinated flame-retardant systems can suppress combustion through gas-phase mechanisms. Their performance and environmental status depend on the specific chemistry, polymer matrix and formulation; halogenated materials should not be treated as one uniform regulatory category. [8]

For EU electrical and electronic equipment, RoHS restricts specified substances, including polybrominated biphenyls and polybrominated diphenyl ethers. This is not a general prohibition of every brominated or chlorinated flame retardant, and a RoHS-compliant material is not necessarily halogen-free. [7]

Halogen-Free Flame-Retardant Systems

Halogen-free systems include phosphorus-based, nitrogen-based and mineral approaches, such as aluminium trihydroxide and magnesium hydroxide. Depending on the chemistry, they may promote protective char, modify decomposition or absorb heat. Loading and property trade-offs depend on the system; not every halogen-free solution requires a higher loading than every halogenated alternative. [8]

Halogen-free does not, by itself, certify low smoke density or low combustion-gas toxicity. Comparative research shows that the outcome depends on the polymer and flame-retardant combination. Where smoke or toxicity limits matter, specify the relevant test results separately instead of inferring them from the flame-retardant label or UL 94 classification. [8]

For component housings, bobbins, connector bodies and PCB materials, selection should balance the required flammability classification with electrical insulation properties, thermal endurance, mechanical behaviour and processing compatibility. Environmental declarations and customer restrictions are an additional selection requirement, not a substitute for those engineering checks. [3, 4, 7, 8]

UL 94 in Context: Related Tests and Design Application

UL 94 addresses only part of material and product qualification. Table 7 separates several parameters often listed alongside a flame classification so that thermal, electrical and ignition properties are not mistaken for one another. [1, 3–6]

Test or property	Standard or method	What it establishes	Design use and limitation
UL 94 classification	UL 94	Burning behaviour under a specified small-scale flame exposure	Material screening; does not certify the complete product
Glow-wire end-product test, GWEPT	IEC 60695-2-11	End-product behaviour under a defined electrically heated ignition source	Applied when required by the relevant product evaluation
Glow-wire flammability index, GWFI	IEC 60695-2-12	Material behaviour under the defined glow-wire flammability procedure	Material preselection; not the same test as GWEPT
Glow-wire ignition temperature, GWIT	IEC 60695-2-13	Material ignition behaviour under the defined glow-wire ignition procedure	Material preselection alongside GWFI where applicable
Limiting oxygen index, LOI	ISO 4589 series	Oxygen concentration needed to sustain specified combustion	Material comparison and formulation work; not interchangeable with UL 94
Relative thermal index, RTI	UL 746B	Long-term retention of specified electrical or mechanical properties under thermal ageing	Thermal endurance selection; separate from flame classification and short-term processing temperature
Polymeric material use in electrical equipment	UL 746C	Evaluation requirements associated with material use and relevant exposures	Application suitability; not the standard assigning RTI
Comparative tracking index, CTI	IEC 60112	Susceptibility to electrical tracking under the specified test	Input to insulation and creepage assessment; not dielectric strength or a complete spacing rule

Glow-Wire Material Data Versus End-Product Testing

IEC 60695-2-11 is an end-product test. IEC 60695-2-12 and IEC 60695-2-13 provide material data for GWFI and GWIT respectively. Material preselection can support an end-product assessment where the applicable rules permit it, but the three results must not be reported as though they were the same test. [5, 6]

This distinction matters for parts exposed to potential electrical heating faults, including insulating structures near current-carrying connections. Whether glow-wire testing is required, and whether material data can be used in place of a particular product test, depends on the applicable product requirements rather than the UL 94 designation. [5, 6]

Thermal Endurance Is Not Flame Resistance

RTI concerns retention of specified properties during long-term thermal ageing. Yellow Cards can list different electrical, mechanical-impact and mechanical-strength RTI values. These values are not interchangeable with glass-transition temperature, T_g, or with a short-duration soldering or processing limit. A material must satisfy the relevant thermal and flammability requirements independently. [3, 4]

Design and Selection Checklist

Selection begins with the part’s function and the applicable product requirements, not with choosing the highest available UL 94 designation. For power and SMPS assemblies, give particular attention to insulating structures near heat-producing parts and electrical connections; for signal and data assemblies, do not assume that low signal power removes all product-level fire-safety requirements. The component’s role and installation determine which evidence is needed. [1, 3, 5]

1. Identify the equipment category, target market and applicable product evaluation. Establish the required classification or permitted alternative for the particular part rather than assuming one universal V-0 requirement.
2. Specify the minimum finished wall or insulation thickness, including tolerances. For foams, also confirm the density covered by the reported classification.
3. Record the manufacturer, exact material grade, colour designation, flame classification, relevant thickness and UL recognition file or equivalent accepted evidence.
4. Compare the supplier datasheet with the recognised material record and its footnotes. Resolve discrepancies before releasing the material or component.
5. Check electrical RTI and the relevant mechanical RTI separately, together with CTI, dielectric properties, moisture behaviour and application-specific mechanical requirements.
6. Determine whether glow-wire end-product testing or material preselection data are required. Keep GWEPT, GWFI and GWIT distinct in the specification.
7. For catalog components, obtain the component’s approval information and conditions of use as well as the material designation where needed. Do not replace a component-level assessment with a generic resin datasheet.
8. For custom housings, bobbins and encapsulated assemblies, agree the compound, colour, processing controls, minimum section and qualification evidence with the supplier before tooling or production release.
9. Request current chemical-compliance declarations and any separately specified halogen-free declaration. Do not use RoHS compliance as a synonym for halogen-free.
10. Control substitutions and engineering changes. Review changes to grade, colour, formulation, thickness and relevant processing against the recognition scope and product qualification requirements.

This checklist is an engineering release workflow based on the separation between material properties, recognition conditions and product evaluation. It does not create an additional certification requirement. A normal change of production lot does not, by itself, prove that recognition has been lost; the important questions are material identity, traceability and continued conformity to the approved specification. [3]

Conclusion

UL 94 provides a useful common language for the burning behaviour of plastic materials under specified laboratory conditions. Its classifications become meaningful in design only when the correct test family, material grade, colour and thickness are identified and the full acceptance criteria are understood.

For electronic components and assemblies, select the classification required for the part’s actual function and installation, then verify thermal endurance, tracking resistance, glow-wire performance and chemical compliance separately. A recognised flame-retardant compound supports qualification; it does not replace component approval or end-product fire-safety assessment.

FAQ

How to Select a UL 94-Rated Material for an Electronic Component

Further Reading

The following articles provide related background on polymer selection, PCB construction and component-level fire-safety considerations.

• Polymer Materials and Processing — background on connector housing polymers, reinforcement and moulding processes.
• Basic PCB Technology Overview — explains how laminates, prepregs and copper form a PCB construction.
• PCB Manufacturing, Test Methods, Quality and Reliability — connects material selection with manufacturing control and qualification.
• Schurter: Remember the New Fuseholder Standard — a historical example of why component installation and glow-wire requirements must be considered separately from resin flame ratings.

References

The following sources support the classification summaries and selection guidance. The applicable standard and recognition conditions govern qualification; the overview tables are not substitutes for laboratory procedures.

1. UL Solutions, Combustion (Fire) Tests for Plastics, UL Solutions, undated online technical guidance.
2. SpecialChem, Flammability UL94, SpecialChem, updated 2025.
3. UL, Yellow Card for Plastics: Add More Properties, Win New Customers, UL Performance Materials, 2016.
4. UL, Faster Time to Market with UL’s Specialized Long Term Thermal Aging Polymer Variation Test Method, UL, 2013.
5. International Electrotechnical Commission, IEC 60695-2-11:2021: Glow-Wire Flammability Test Method for End Products, IEC, 2021.
6. International Electrotechnical Commission, IEC 60695-2-12:2021: Glow-Wire Flammability Index Test Method for Materials, IEC, 2021.
7. European Commission, Restriction of Hazardous Substances in Electrical and Electronic Equipment, Directorate-General for Environment, undated online guidance.
8. Feuchter and co-authors, The Impact of Halogen Free Phosphorus, Inorganic and Nitrogen Flame Retardants on the Toxicity and Density of Smoke from 10 Common Polymers, Fire and Materials, Wiley, 2023.
9. Avery Dennison, Flammability in Automotive Adhesives, Avery Dennison, undated online technical article.