The mechanical properties of composites, such as weight, stiffness and strength, are normally the primary considerations in the design process. However, in applications where the risk of fire is a factor, such as in transportation, building and construction, this becomes the primary consideration when specifying composite solutions.
With the use of composites in demanding applications increasing, improving knowledge of their fire performance is becoming a safety-critical issue, particularly for building and transportation applications.
Generally, there are three main fire performance considerations for manufacturers. These are typically referred to as FST—fire, smoke and toxicity properties. That is, how difficult it is for the material to ignite, how fast the fire spreads and how much heat it generates; next, how much smoke will develop and how quickly; and finally, how toxic is the smoke and how quickly can it cause harm?
With these considerations in mind, it is common for composites manufacturers to incorporate flame retardant additives into the materials to reduce or delay the combustion process. The nature of these additives varies depending on the application in question and relevant regulatory requirements.
Halogenated compounds are a broad category of flame retardant additives that were traditionally seen as the first choice for composites manufacturers because of their high efficiency and relatively low cost. Halogenated additives work by eliminating the highly reactive free radicals generated in the thermal decomposition of polymers during combustion. By hampering oxidation reactions in the combustion process, they can effectively kill the flames.
Brominated and chlorinated additives, occasionally helped with synergists such as antimonytrioxide, are still used in heavily corrosive environments. However, they are increasingly being banned for use in many common applications because of their adverse side effects. Several of the halogenated substances present health hazards, the smokes produced are toxic, and waste is classified as hazardous, making it costly to dispose of.
New trends in adding flame retardant properties to composites are based on the use of non-halogenated substances such as aluminum trihydrate (ATH) and metal hydroxides, including magnesium and phosphorus-based compounds.
ATH additives are now the most widely used non-halogenated flame retardants because, despite being less effective than halogens, they are far less toxic when they burn. They operate by releasing water molecules in an endothermic reaction during the combustion process. These water molecules quench the surface of the surrounding materials, providing flame retardancy and smoke suppression. Furthermore, unlike halogenated products, ATH additives do not give off toxic smoke when burned.
The downside of ATH, being much less effective in hampering the fire, is that higher quantities of it are needed to meet the increasingly stringent fire requirements in many applications. Very high filler loadings negatively affect mechanical performance because there is less room for reinforcing fibers, and also influence long-term behavior in difficult environments. Very high resin viscosities, changed reactivities, and important pultrusion pulling forces require narrow process windows and tight process controls. Special consideration needs to be given to geometrical product details, and the highest level of tooling quality is required to successfully produce such parts.
Ultimately, when specifying composites for applications where fire must be considered, a balance must be struck between the desired mechanical properties and meeting the relevant fire safety tests. While this might initially seem like a daunting prospect, this challenge can be overcome by working with a knowledgeable partner who understands all these considerations and can propose the most suitable solution available.
This is particularly crucial in applications like train transportation, where fire requirements vary depending on whether the train will travel aboveground or underground and whether the composites will be used internally or externally.
As FST demands will affect most other properties and have considerable effect on processing, they should be discussed in very early stages of each new inquiry.
To learn more about fire considerations when specifying composites, visit Exel Composites’ website.
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