Typical heat exposures due to firefighting fall well within the range of performance for composite and thermoplastic helmets. Concerns should be raised when helmets are exposed to high heat such as flashover chambers, training fires, and direct flame impingement.

Resilience

Repeated heat exposure under the 250°F range will have limited effect on helmet shells. The difference begins to take place above this range. Thermoplastics can deal with repeated exposures well above 400°F with little damage, whereas repeated exposures will cause composites to break down over time.



Degradation

Heat exposures as they have been described will have less of an impact on thermoplastic helmets. Both composite and thermoplastic helmets will perform well when new. The degradation is a time/heat combination that has greater impact on composites.



Blistering/Bubbling

Thermoplastic helmets can exhibit blisters or bubbling due to heat exposures. The types of bubbles that you might see on these helmets can range from small ballpoint pen size blisters to large thumbnail size blisters depending on the type of heat as well as the length of time of the exposure. Thermoplastic is much like a sponge with the ability to absorb moisture via humidity. The blistering or bubbling in thermoplastic helmets is a reaction due to moisture trapped in the material that vaporizes and expands. The softened plastic stretches to accommodate this expansion of gas causing the blister/bubble to form.



Radiant Heat

Radiant heat is the line-of-sight heat found in high heat exposures such as fuel fires. This heat can develop in structural fires due to the synthetic compositions of the material found in structures today. This type of heat exposure is like a laser beam. Reports have shown where 2 firefighters have been within inches of each other and one would receive damaging heat exposure to some portion of the ensemble while the other firefighter did not experience any serious heat. Any blistering or bubbling of a thermoplastic helmet when exposed to radiant heat usually results in small pen size blisters in a very localized area of the helmet shell. This reaction to radiant heat can result after only seconds of exposure.



Convective Heat

Convective heat is the type of heat that envelops the room or rooms involved with a fire. This is much like walking into an oven. Heat is everywhere. Since this heat stratifies, the time it takes to boil moisture inside a thermoplastic helmet shell becomes shorter the greater the distance from the floor. Unlike radiant heat, convective heat that causes blistering or bubbling often requires several minutes of exposure.