Design Properties for Engineers: Fire Behavior (UL 94) and Limiting Oxygen Index (LOI) of High Performance Polymers

 In this post, we discuss the fire behavior and flammability of high performance polymers. 

Generally, fire behavior is measured according to the Underwriter Laboratories (UL) standard ISO 9772 and 9773. Furthermore, there is the limiting oxygen index (LOI) which is estimated according to ISO 4589. Let us have a look at what each of them mean. 

UL 94 flammability test

UL 94 flammability standard consists of two different tests: the horizontal and vertical burning test. In the first, a specimen is burned horizontally. The speed of burning as a function of material thickness is estimated. The classification “HB” means that those materials are easily burning in a horizontal way. Those materials will not be further tested. Materials which did not show burning during the first test will be tested in a vertical way. In this second test, the specimen is placed vertically and burned from the lower end. V-0 rating means that the material passes the strictest test criteria. 

Limiting Oxygen Test (LOI)

The LOI represents the amount of oxygen (in %) needed to keep a tested material burning. The higher the LOI value is the higher is the probability that the material extinguishes once the surrounding atmosphere is lacking oxygen. 

How do high performance polymers behave in terms of flammability rating and LOI?

In general, high performance polymers reach inherently a V-0 flammability rating (graph below). Exceptions are PSU and semi-aromatic Nylons such as PPA and PARA which need additional flame retardants to reach V-0. 

PTFE has the highest LOI which means that an oxygen concentration below 95% will extinguish the flame. Among the other high performance polymers, polyimides (PAI, PEI, PI, PBI) have a slightly better LOI. 

Fire Behavior (UL 94) and Limiting Oxygen Index (LOI) of High Performance Polymers

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Greetings, 

Herwig Juster

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Literature: 

[1] Saechtling Kunststoff Taschenbuch, Hanser