Flammability UL94

1. What is UL 94?
2. Which materials show flame retardance?
3. What factors influence flammability?
4. Why it is important to examine how plastic burn?
5. What are the main types of flame ratings to classify plastics?
6. What is vertical burning test for classifying materials?
7. What is horizontal burning test for classifying materials?
8. What standards align with UL flame ratings?
9. What are the other tests to determine flammability?

What is UL 94?

UL 94 is a safety standard to determine the flammability of plastic materials. This test is released by Underwriters Laboratories of the United States. It measures:

• The ability of the plastic part to extinguish or spread the flame after ignition.
• Its dripping behavior in response to a small open flame or radiant heat source under controlled laboratory conditions.

This test is a preliminary sign to check the flammability of plastics used in any application. For example, plastic parts used in electronic devices and appliances.

Which materials show flame retardance?

1. Flame Retarded Thermoplastics – View Product List
2. Flame Retarded Thermoplastic Elastomers – View Product List
3. Flame Retarded Thermosets – View Product List
4. Flame Retarded Rubbers – View Product List

What factors influence flammability?

The factors that influence flammability are:

• Ease of ignition – how rapidly a material ignites
• Flame spread – how rapidly fire spreads across a polymer surface
• Fire endurance – how rapidly fire penetrates a wall or barrier
• Rate of heat release – how much heat is released and how quickly
• Ease of extinction – how rapidly the flame chemistry leads to extinction
• Smoke evolution
• Toxic gas generation

Why is it important to examine how plastics burn?

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.

What are the main types of flame 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. The flame ratings covered under these orientations 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)

Class	Orientation of Test Sample	Definition	Time of Burn Allowed	Particle Drop Allowed		Plaque Holes
				Flaming	Non-Flaming
UL 94 HB	Horizontal	Slow Burning	Burning rate of less than 76 mm/min for a specimen less than 3 mm thick and burning stops before 100 mm
UL 94 V-2	Vertical	Burning Stops	30 seconds	Yes	Yes
UL 94 V-1	Vertical	Burning Stops	30 seconds	No	Yes
UL 94 V-0	Vertical	Burning Stops	10 seconds	No	Yes
UL 94 5VB	Vertical	Burning Stops	60 seconds	No	No	Yes
UL 94 5VA	Vertical	Burning Stops	60 seconds	No	No	No

UL Classifications Majorly Used for Plastics
(Source: W. L. Gore & Associates, Inc.)

What standards align with UL flame ratings?

The UL flame ratings are aligned with the following standards to determine fire properties:

• IEC 60707
• IEC 60695-11- 10
• IEC 60695-11-20
• ISO 9772, and
• ISO 9773

What is 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 the flame extinguishes within 10 seconds with no dripping
• V1: if the flame extinguishes within 30 seconds with no dripping
• V2: if the 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)	≤10	≤30	≤30
Total burning time (s) (10 flame applications)	≤50	≤250	≤250
Burning and afterglow times after second flame application (s)	≤30	≤60	≤60
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 VTM Rating: VTM-0, VTM-1 and VTM-2 ‐ Thin Material Vertical Burning Testing

This test method is used for thin specimens or materials that:

• are too flexible, and
• may distort, shrink, or flex.

This occurs 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. The only exception is that the material wraps around a mandrel before clamping to the stand and reduces the flame application. 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)	≤10	≤30	≤30
Total burning time (s) (10 flame applications)	≤50	≤250	≤250
Burning and afterglow times after second flame application (s)	≤30	≤60	≤60
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 seconds 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)	≤60	≤60
Dripping of burning specimens	no	no
Hole formation	no	yes

(Source : UL LLC)

What is horizontal burning test for classifying materials?

Specimens molded from plastic material orient either horizontally or vertically. This orientation depends on the specifications of the relevant test method. They are subjected to a defined flame ignition source for a specified period of time.


Horizontal Burning Test (Source: ScienceDirect)

UL 94 HB Rating

A material having a thickness between 3 and 13 mm will be classified as an HB material if it does not have a burning rate exceeding 40 mm per minute. For material with a thickness of less than 3 mm, the burning rate should not exceed 75 mm per minute. The material will also be rated as HB if it ceases to burn before the 100 mm mark independent of thickness.

Test criteria	Burning rate in V	Flammability rating
Test specimen thickness 3-13mm	≤40 mm/min	HB
Test specimen thickness < 3	≤75 mm/min	HB
Flame is extinguished before first mark	≡ 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 an HBF material:

• if it does not have a burning rate exceeding 40 mm per minute over a 100 mm span or
• if it ceases to burn before flaming or glowing reaches the 125 mm mark.

Flammability rating UL 94 HBF
Test criteria (test specimens)	HF-1	HF-2
Burning time (s)	4/5 ≤ 2 1/5 ≤ 10	4/5 ≤ 2 1/5 ≤ 10
Burning and afterglow time of each individual specimen (s)	≤ 30	≤ 30
Dripping of burning specimen (ignition of cotton batting)	no	yes
Damaged length of individual specimen (mm)	< 60	< 60

(Source: UL LLC)


Check out an interesting video about UL 94 Horizontal & Vertical Flammability Tester, Flammability Test Chamber
(Source: TESTEX Testing Equipment)

What are the other tests to determine flammability?

The limiting oxygen index is another test to measure the relative flammability of materials such as:

• plastics and
• composite materials.

This is done by burning them in a controlled atmosphere consisting of a mixture of oxygen and nitrogen. Know more about Limiting Oxygen Index (LOI) here »