Section 2 - Myths
The most commonly asked question regarding fire helmet materials is which makes the 'best' helmet. Unfortunately, there is no simple answer to that question. The high heat resistance of modern thermoplastic materials has effectively eliminated what had previously been the primary advantage of thermoset composites, namely their inability to melt. The temperatures required to melt Ultem and Radel are so high as to cause chemical degradation of most thermoset resins. At the same time, these new materials exhibit the high quality of surface finish and the resilience and impact resistance that has long been associated with thermoplastics.
Composite materials still offer benefits, however: their cross-linked molecular structure, in addition to preventing melting, also gives these materials an exceptional resistance to chemical attack, while their traditional inferiority in impact resistance is being increasingly offset by new developments in high strength resins and reinforcements.
Many commonly held beliefs are the result of experience with products in the field. While certainly valid, these experiences in many cases have occurred decades ago and may not accurately represent the nature of modern materials and products. Below, you will find some of the more common beliefs as well as the "rest of the story".
Composites
True or False: Composite (fiberglass) helmets do not degrade under conditions that affect thermoplastic helmets.
False: In fact, fiberglass will often degrade more rapidly under similar conditions. The thermoset resin, which holds the glass fibers together, degrades when exposed to extreme temperatures. Each exposure to high heat attacks and weakens the molecular structure of the resin. Lower temperatures can also degrade the resin but over a much longer period of time. Ultraviolet (UV) exposure from sunlight has a similar effect on these resins. An example of how this degradation takes place can be seen in corrugated fiberglass paneling used in deck roofs. Over time, the sun will bake the resin out of the fiberglass, leaving glass fibers exposed and degrading both the strength and appearance of the material. High heat exposures are acceleration of this type of degradation.
True or False: Composites remain rigid when exposed to high heat conditions that soften thermoplastic materials.
False: Even though they do not melt, thermoset resins used in composite do soften significantly at high temperatures such as those involved in structural firefighting service. In addition, unlike their plastic cousins, thermoset resins are not well suited to being repeatedly heated to the softening point and re-cooled. Each heat exposure weakens the resin from its original state. Thermoplastic materials, being made into products by a process of melting and re-cooling, are designed to accept this re-heating.
Thermoplastics
True or False: Thermoplastic helmets cannot stand up to the heat of normal firefighting activities.
False: Manufacturers, such as General Electric (GE), have developed specialized high performance plastics that perform at temperatures exceeding 500° F (260° C). These new plastics offer the well-known durability that comes with plastic and endure heat as well as many composite materials.
True or False: Thermoplastic helmets will not withstand chemical exposure.
Partly False: The new breeds of plastics used today are extremely resistant to most chemicals. There are, however, some chemical solvents, which may be more harmful to thermoplastic helmet shells than to composites. The most chemical sensitive part of any fire helmet assembly is the faceshield or goggle lens. If you are experiencing a high rate of non-impact related cracking in faceshields or goggle lenses in service, you may be working in a chemical environment in which a composite shell would provide better service than a thermoplastic.
Actual Firefighting Temperatures
Ask 100 firefighters what temperatures they typically experience during actual firefighting and you will get 100 different answers. Most believe that they spend large periods of time in temperatures in excess of 350ºF. Although there are short periods of time when firefighting exposures exceed these temperatures, the majority of firefighting is done in temperatures less than 300ºF. In fact, a larger portion of time is in temperatures below 250ºF.
Studies in controlled fire training exercises have demonstrated that firefighters, on their knees, will enter rooms that involve pallet fires with temperatures around 250º at approximately 4’ above the floor. Heat stratifies and quickly escalates to temperatures from 500º-1000ºF above the 6’ level. Firefighters are trained to stay low to avoid exposure to these extreme and deadly heat levels. Unlike real fire exposures, firefighters in training exercises will often expose themselves to high levels of heat for long periods of time to experience the different characteristics of these fires. In real fire environments, firefighters are trained to change the environment and lower the heat exposure with use of agents such as water, foam and/or positive pressure ventilation. Introducing these agents into a fire environment has proven to reduce temperatures in seconds by 200º-300ºF.
Many components worn for protection have limited performance life when exposed to temperatures above 500ºF. NFPA Standards have established that these components must endure 500ºF for 5 minutes; however, it is understood that endurance does not mean that these components survive without sustaining damage. Their endurance is to insure that the firefighter is given that window of opportunity to escape. Equipment exposed to this level of heat and time will most often need to be retired from additional service.
Product Life
How long should a helmet last in the field: 3, 4, 5, 10 years? The answer is yes. Depending on the level of activity, exposures, and preventative maintenance, a helmet's life could be as short as one time use or as long as 10+ years. The primary issue is how well the helmet is maintained and what the helmet has been subjected to while in service.
Any fire helmet that was involved in direct flame contact or that experienced an impact from a falling object should be removed from service. This could happen the first time the helmet was put into service. A helmet used by an active fire training officer with several months each year of live fire exposure could well reach its full and useful life in 1-2 years. The helmet would not be the only item that would exceed useful life in that time under those conditions. Coats and trousers would also be subject to retirement. Helmets in a rural volunteer department that did not see live fire but once or twice a year may easily last 10 years.