Section 3 - Heat Exposure
Composite and Thermoplastic Helmets
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 250ºF 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 400ºF with little damage. Composites however, will begin to breakdown 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 a 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 boils and in turn gases. When the water becomes a gas it 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 in seconds of the 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 distance from the floor. Unlike radiant heat, convective heat that causes blistering or bubbling most often requires several minutes of exposure.
Technical Bulletin - Heat Rise Test
Urethane Foam Impact Liners Provide Radiant Heat Protection
Does a fire helmet manufactured with a urethane foam impact inner liner provide better protection than a fire helmet without an inner liner? Of course, in addition to impact protection, the inner liner plays a vital role in protecting the head from heat.
To prove this point, we subjected a Bullard helmet with a urethane inner liner and a competitor’s helmet, which has no inner liner to the NFPA 500°F oven test.* A non-conductive head-form with a thermalcouple attached was used to measure the heat build up inside the helmets. The results of the test are shown in the graph below. Can you afford not to wear a helmet without a urethane foam inner liner?
*The oven test shall be a horizontal flow-circulating oven with minimum interior dimensions so that the specimens can be suspended and are a least 5.08 cm (2 in.) from any interior oven surface or other test specimens. The test oven shall have an airflow rate of 38 m/min to 76 m/min (125 ft/min to 250 ft/min) at the standard temperature and pressure of 21°C (70°F) at 1 atmosphere, measured at the center point of the oven. A test thermocouple shall be positioned so that it is level with the horizontal centerline of a mounted sample specimen. The thermocouple shall be equidistant between the vertical centerline of a mounted specimen placed in the middle of the oven and the oven wall where the airflow enters the test chamber. The thermocouple shall be an exposed bead, Type J or Type K, No. 30 AWG thermocouple. The test shall be heated and the test thermocouple stabilized at 260°C, +6° / -0°C (500°F, +10°/-0°F) for a period of not less than 30 minutes.