Most plastic materials are made from the same raw ingredient that powers the majority of our vehicles — petroleum. It is not surprising that most plastics will be extremely flammable if given the chance. In essence, the plastics that surround us can be viewed as a solid fuel. To reduce the risk of a plastic part from catching fire and spreading a flame, additives are commonly blended into the plastic. These additives are referred to as flame retardants. Some plastics are inherently more flame resistant than others, such as poly(vinyl chloride) (PVC), and require little-to-no additional flame retardants. To provide the engineer or designer with some knowledge of the plastic’s propensity to ignite and spread a flame, it is commonly given a flammability rating. What does (or does not) this rating actually mean? This article will attempt to provide some insight to this commonly misunderstood rating.
Misconception of the UL 94 Rating
Have you heard or read the phrase “The plastic is UL 94 rated” and understood this to mean that the plastic will not burn? You are not alone. This is a common misunderstanding by many engineers that specify plastics for the parts they are molding or using in assemblies. The UL 94 test standard does not provide definitive guidance on whether the material will burn or not. What the standard does provide is some insight into the ignition characteristic of the plastic material. The UL 94 tests are performed under controlled laboratory conditions on bar or plaque shaped specimens. The conditions rarely emulate actual field conditions. Further, since the geometry of the actual part can play a critical role on whether a part will ignite and/or spread a flame, the test geometry can overstate, or more likely, understate the resistance to igniting and spreading a flame.
The UL 94 Test
The UL 94 test involves the lighting of specimens with a controlled flame under controlled conditions. Following the horizontal burn (HB) test, a 5 in. x 1/2 in. x 0.118 in. specimen is exposed to a flame for 30 seconds while in a horizontal configuration. The plastic material will receive an HB rating if the flame self-extinguishes before or has a burn rate that is less than 3” per minute. The HB rating is considered the easiest UL 94 test to pass. Note, the plastic can get an HB rating if it ignites and spreads a flame. A more difficult rating to achieve, places the sample in a vertical (V) orientation during the testing procedure. The 5 in. x 0.5 in. specimen, with a thickness that is typically 1/32 in., 1/16 in. or 1/8 in., is exposed to a flame for 10 seconds. If the sample does not ignite or the flame self-extinguishes, the sample is exposed to the flame for another 10 seconds. Igniting the vertical sample at its bottom, allows the flame to burn the plastic material above it and feed the flame. This makes it a more aggressive test than the horizontal burn test. A cotton ball is placed below the vertical specimen, which will burn if it comes in contact with any flaming drips of plastic.
The plastic gets a rating if the following results are met:
- V-0: If the sample self-extinguishes within 10 seconds and has no flaming drips.
- V-1: If the sample self-extinguishes within 30 seconds and has no flaming drips.
- V-2: If the sample self-extinguishes within 30 seconds, but flaming drips that light the cotton, are allowed.
It is important to note that a plastic material can get a “V” UL rating, even if does not self-extinguish immediately or has flaming drips that can start another material on fire.
Why UL 94 is an Important Test/Rating
Though the UL 94 test does not indicate if the plastic material will burn or not, it does provide a relative measure on how flammable it can be. It also provides insight to how quickly a flame will spread. In many instances, the goal is not to necessarily stop the flame, but to reduce how quickly it will advance. This allows time for people to exit a structure where a fire may have started. Further, many flame events in electrical appliances are brief, lasting less than several seconds. A material with a UL 94 rating may never ignite under these conditions, whereas, a material that is not rated may ignite and spread the flame quickly and aggressively.
In summary, plastic materials are hydrocarbons that are commonly filled with additives to improve flammability resistance. Perhaps it is most important to understand that a plastic material that has an UL 94 rating can ignite, spread a flame, and contribute to a fire event. It is wrong to assume that if the plastic materials are UL 94 rated, there are no combustible materials in the assembly. This is not to say that the UL 94 rating is not important. On the contrary. The UL 94 rating provides an important metric for the engineer or designer on the plastic material’s propensity to ignite and spread a flame. It is also important to know that there are other test methods (e.g. ASTM, IEC, FMVSS, FAR) used to measure or rate how a plastic material will behave during a fire event.