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.
Key Takeaways
- UL 94 is a widely used flammability test for plastic materials, assessing their ability to extinguish flames and dripping behavior.
- The standard evaluates material flammability but does not predict performance in actual fire scenarios, requiring context from relevant product standards.
- UL 94 classifies plastics with ratings based on tests such as vertical and horizontal burning tests, indicating their flammability performance.
- Understanding UL of plastic materials helps design engineers select appropriate flame-retardant additives and ensure compliance with safety standards.
- High UL 94 ratings depend on the material’s thickness and must be verified against the UL Yellow Card for accuracy.
Preliminary Screening of Plastic Polymer Materials for Fire Performance
UL 94 (Underwriters Laboratories test standard UL 94) is one of the most widely used flammability tests for determining relative flammability for plastic materials. It measures the ability of plastic part to extinguish the flame after ignition and its dripping behavior in response to a small open flame or radiant heat source under controlled laboratory conditions.
The tests results are intended to serve as a preliminary indication of the materialโs suitability with respect to flammability for a particular application, such as plastic materials used for parts in electronic devices and appliances.
UL 94 is published by Underwriters Laboratories (UL), an independent safety certification organization. The standard does not evaluate a material’s performance in an actual end-product fire scenario โ it is solely a material-level screening test. For system-level fire safety, UL 94 ratings must be read in context of the applicable end-product standard (e.g. IEC 62368-1 for audio/video and IT equipment, IEC 60601-1 for medical devices, or IEC 60335-1 for household appliances). Passing a high UL 94 rating does not automatically mean the end product meets those standards, and vice versa.
The flammability is influenced by several factors such as:
- Ease of ignition โ how rapidly a material ignites
- Flame spread โ how rapidly fire spreads across a polymer surface
- Fire endurance โ how rapidly fire penetrates a wall or barrier
- Rate of heat release โ how much heat is released and how quickly
- Ease of extinction โ how rapidly the flame chemistry leads to extinction
- Smoke evolution
- Toxic gas generation
Hence, the study of how plastics burn has been, and continues to be, a major area of research to examine:
- The nature of the burning phenomenon in plastics
- Various methods to reduce plastics flammability, and
- Methods to test flammability
Main Types of Testing & Flammability Ratings to Classify Plastics
The UL standard classifies plastics according to the minimum thickness at which it stops burning when tested in a horizontal or vertical orientation.
Types of testing/flammability ratings covered in the UL 94 standard for flammability of plastic materials include:
- Vertical burning tests (V-0, V-1, V-2, 5V, 5VA, 5VB, VTM-0, VTM-1, VTM-2)
- Horizontal burning tests (HB, HBF, HF-1, HF-2)
| Parameter | Vertical (V / VTM / 5V) | Horizontal (HB / HBF / HF) |
|---|---|---|
| Specimen dimensions | 125 ร 13 mm bar | 125 ร 13 mm bar |
| Number of specimens per rating | 5 (minimum) | 5 (minimum) |
| Flame source | 20 mm (Bunsen) or 125 mm (5V) | 20 mm Bunsen |
| Flame application time | 10 s ร 2 applications | Until flame front reaches 25 mm mark |
| Pre-conditioning | 48 h at 23 ยฐC / 50% RH + 168 h at 70 ยฐC | Same |
| Cotton indicator pad | Yes (below specimen) | No |
Note: VTM specimens are wrapped around a mandrel (12.7 mm diameter) instead of being tested as a flat bar.
(Source: W. L. Gore & Associates, Inc.)
The UL test methods are aligned with IEC 60707, IEC 60695-11-10, IEC 60695-11-20, ISO 9772 and ISO 9773 standards used to determine fire properties.
Vertical Burning Test for Classifying Materials
UL 94 V Rating: V-0, V-1, V-2
This test measures the self-extinguishing time of the vertically oriented polymer specimen. The top of the test specimen is clamped to a stand and the burner is placed directly below the specimen. The test evaluates both the burning and afterglow times and dripping of the burning test specimen.
The material will be rated:
- V-0: if flame extinguishes within 10 seconds with no dripping
- V1: if flame extinguishes within 30 seconds with no dripping
- V2: if flame extinguishes within 10 seconds with dripping
| Flammability rating UL 94 V | |||
| Test Criteria | V-0 | V-1 | V-2 |
| Burning time of each individual test specimen (s) (after first and second flame applications) | โค10 | โค30 | โค30 |
| Total burning time (s) (10 flame applications) | โค50 | โค250 | โค250 |
| Burning and afterglow times after second flame application (s) | โค30 | โค60 | โค60 |
| Dripping of burning specimens (ignition of cotton batting) | no | no | yes |
| Combustion up to holding clamp (specimens completely burned) | no | no | no |
(Source : UL LLC)
Important: UL 94 Ratings Are Thickness-Dependent
A material’s UL 94 classification is always tied to the minimum tested thickness. The same polymer compound may achieve V-0 at 1.5 mm wall thickness but only V-1 or V-2 at 0.8 mm. When reading component or material datasheets, always check the thickness value alongside the rating โ for example “V-0 @ 0.8 mm”. Ratings published on UL’s Yellow Card (the official UL certification database at iq.ul.com) list all tested thicknesses and are the authoritative reference for compliance verification. A rating printed on a product datasheet is only valid if it matches the thickness used in your actual design.
UL 94 VTM Rating: VTM-0, VTM-1 and VTM-2 – Thin Material Vertical Burning Testing
This test method is used for thin specimen or materials that are too flexible or may distort, shrink or flex during the traditional vertical testing (failure to pass the UL 94 V test is a precondition of all VTM ratings). This test is similar to the vertical burning test except that the material is wrapped around a mandrel before clamping to the stand and the flame application is reduced. In this test, the flame is applied only for 3 seconds.
| Flammability rating UL 94 VTM | |||
| Test Criteria | VTM-0 | VTM-1 | VTM-2 |
| Burning time of each individual test specimen (s) (after first and second flame applications) | โค10 | โค30 | โค30 |
| Total burning time (s) (10 flame applications) | โค50 | โค250 | โค250 |
| Burning and afterglow times after second flame application (s) | โค30 | โค60 | โค60 |
| Dripping of burning specimens (ignition of cotton batting) | no | no | yes |
| Combustion up to holding clamp (specimens completely burned) | no | no | no |
(Source : UL LLC)
UL 94 5V Rating: 5V, 5VA, 5VB – Vertical Testing is done on both bar and plaque specimens
For any material to achieve 5V classification, the test specimens must not burn with flaming and/or glowing combustion for more than 60 sec after the fifth flame. Also, the test specimens must not drip.
| Flammability rating UL 94 | ||
| Test Criteria | 5VA | 5VB |
| Burning and afterglow times of specimens after fifth flame application (s) | โค60 | โค60 |
| Dripping of burning specimens | no | no |
| Hole formation | no | yes |
(Source : UL LLC)
Horizontal Burning Test for Classifying Material
Specimens molded from the plastic material are oriented in either a horizontal depending on the specifications of the relevant test method, and they are subjected to a defined flame ignition source for a specified period of time.
UL 94 HB Rating
A material having a thickness between 3 and 13mm will be classified as a HB material if it does not have a burning rate exceeding 40mm per minute. For material with thickness less than 3mm the burning rate should not exceed 75mm per minute. The material will also be rated as HB if it ceases to burn before the 100mm mark independent of thickness.
| Test criteria | Burning rate in V | Flammability rating |
| Test specimen thickness 3-13mm | โค40 mm/min | HB |
| Test specimen thickness < 3 | โค75 mm/min | HB |
| Flame is extinguished before first mark | โก 0 mm/min | HB |
(Source: UL LLC)
UL 94 HBF: HBF, HF-1 and HF-2 – Horizontal Burning Foamed Material
This test grants three different types of ratings depending on burning rates and or after flame times. A material will be classified as a HBF material if it does not have a burning rate exceeding 40mm per minute over a 100 mm span or if ceases to burn before flaming or glowing reach the 125mm mark.
| Flammability rating UL 94 HBF | ||
| Test criteria (test specimens) | HF-1 | HF-2 |
| Burning time (s) | 4/5 โค 2 1/5 โค 10 | 4/5 โค 2 1/5 โค 10 |
| Burning and afterglow time of each individual specimen (s) | โค 30 | โค 30 |
| Dripping of burning specimen (ignition of cotton batting) | no | yes |
| Damaged length of individual specimen (mm) | < 60 | < 60 |
(Source: UL LLC)
How Do Plastics Achieve High UL 94 Ratings? Flame Retardant Chemistry
The UL 94 rating of a polymer is primarily determined by the choice and concentration of flame retardant (FR) additives incorporated by the material compounder. Two main families are used in electronics-grade polymers:
Halogenated flame retardants (HFR): Bromine- or chlorine-based additives that interrupt the combustion chain reaction in the gas phase. They are highly effective at achieving V-0 even at thin sections, but raise environmental concerns. Brominated FR materials may be subject to restrictions under RoHS, REACH SVHC, and certain industry green-procurement policies.
Halogen-free flame retardants (HFFR): Phosphorus-, nitrogen-, or mineral-based (e.g. aluminium trihydroxide, magnesium hydroxide) systems. These are preferred for applications requiring low smoke/toxicity and in products targeting eco-design regulations. They typically require higher additive loadings, which can affect mechanical, electrical (e.g. CTI), and thermal properties.
For passive component housings, connector bodies, and PCB substrates, the choice between HFR and HFFR is often driven by a combination of UL 94 rating requirement, operating temperature (Tg/RTI), CTI class, and compliance with customer environmental specifications.
UL 94 in Context: Related Tests and Design Application
UL 94 is one of several flammability-related tests relevant to electronic component and assembly design. The table below shows how it compares with other commonly referenced methods:
| Test | Standard | What it measures | Typical use |
|---|---|---|---|
| UL 94 | UL 94 | Small-flame ignitability & self-extinguishing of material | Material screening for parts in devices |
| Glow-wire | IEC 60695-2-11/12/13 | Resistance to ignition from a hot glowing element | End-product testing for unattended appliances |
| Oxygen Index (LOI) | ISO 4589 | Minimum Oโ concentration sustaining combustion | Material R&D and comparative ranking |
| UL 746C / RTI | UL 746C | Long-term thermal endurance of plastics | Relative temperature index for material selection |
| CTI | IEC 60112 | Resistance to tracking/creepage โ indirectly affected by FR additives | Insulation and creepage design |
For design engineers selecting housing materials or PCB laminates, the practical hierarchy is:
- Identify the UL 94 rating required by the end-product standard (e.g. V-0 for most IT/telecom PCB substrates per IEC 62368-1).
- Match the required rating at the actual minimum wall thickness of your part.
- Verify the rating on the UL Yellow Card (not just the manufacturer datasheet).
- Check whether the application additionally requires glow-wire performance (common for household appliances and industrial equipment).
- Confirm material environmental compliance (RoHS/REACH) if halogenated FR systems are involved.
Conclusion
UL 94 remains the cornerstone flammability screening standard for plastic materials used in electronic components, devices, and appliances. Understanding its rating hierarchy โ from the most stringent 5VA down to the basic HB โ allows design engineers and material selectors to make informed decisions early in the product development cycle. The key takeaways are that ratings are always thickness-dependent, must be verified against the UL Yellow Card rather than relying solely on manufacturer datasheets, and represent only one layer of a broader fire-safety compliance picture that typically also includes end-product standards, glow-wire testing, and environmental regulations such as RoHS and REACH. As the electronics industry continues to push for thinner walls, higher operating temperatures, and halogen-free material systems, selecting the right UL 94-rated polymer with an appropriate flame retardant chemistry becomes an increasingly critical engineering decision โ one that directly affects product reliability, regulatory approval, and time to market.
FAQ
V-0 is the most stringent vertical burn rating. It means the tested material self-extinguishes within 10 seconds after each flame application, produces no dripping particles, and does not burn up to the specimen clamp. It is the rating most commonly required for housings and insulators in IT, telecom, and industrial electronics.
Not necessarily for every application. 5VA is the highest classification, but V-0 is sufficient for most electronic component housings. Selecting an unnecessarily high rating can increase material cost and reduce other properties such as impact strength or CTI. Always match the rating to the end-product standard requirement.
No. UL 94 is solely a small-flame ignitability and self-extinguishing test on material specimens under controlled lab conditions. It does not evaluate smoke density, toxicity of combustion gases, heat release rate, or fire spread in an actual product assembly. Additional tests such as glow-wire (IEC 60695-2) or end-product fire standards are needed for full fire safety compliance.
UL 94 ratings are thickness-dependent. The same polymer compound can achieve V-0 at 1.5 mm but only V-2 at 0.8 mm. The rating is only valid at the minimum tested thickness, so designers must confirm the datasheet rating matches the actual wall thickness in their design.
Halogenated flame retardants (bromine/chlorine-based) are highly effective at achieving V-0 at thin sections but raise environmental concerns under RoHS and REACH. Halogen-free systems (phosphorus, nitrogen, mineral-based) are preferred for eco-design compliance but typically require higher additive loadings, which can affect dielectric properties, CTI, and mechanical strength.
How to Select the Right UL 94 Flammability Rating for Your Electronic Component Design
- Step 1: Identify the applicable end-product standard
Determine which system-level fire safety standard governs your end product โ e.g. IEC 62368-1 for IT/AV equipment, IEC 60601-1 for medical devices, or IEC 60335-1 for household appliances. This standard will specify the minimum UL 94 rating required for each part in the assembly.
- Step 2: Determine the minimum wall thickness of the plastic part
Measure or specify the minimum wall thickness of the housing, connector body, or insulator in your design. Remember that a UL 94 rating is only valid at the tested thickness โ a material rated V-0 at 1.5 mm may not hold that rating at 0.8 mm.
- Step 3: Verify the rating on the UL Yellow Card
Do not rely solely on the manufacturer’s datasheet. Go to the official UL Product iQยฎ database (iq.ulprospector.com), find the material’s Yellow Card, and confirm the rating at the specific thickness used in your design.
- Step 4: Check whether glow-wire compliance is also required
For unattended appliances and industrial equipment, end-product standards often additionally require glow-wire ignition temperature (GWIT) or glow-wire flammability index (GWFI) performance per IEC 60695-2-11/12/13. UL 94 alone is not sufficient in those cases.
- Step 5: Confirm environmental compliance of the flame retardant system
If your customer or market requires halogen-free materials (e.g. for RoHS, REACH SVHC, or green-procurement specifications), verify whether the UL 94-rated compound uses a halogen-free flame retardant system. Check the material’s safety data sheet (SDS) and supplier declaration of conformity.
- Step 6: Re-validate if design or material changes
If the wall thickness, material formulation, or processing conditions change during development, re-verify the UL 94 rating. A change in thickness or resin lot can invalidate a previously confirmed classification.
