Market demand for flame-retardant (FR) products is on the rise, with an expected compound annual growth rate of 3.6% by 2027, according to market research firm Grand View Research. Industries driving the growth of FR applications include electrical and electronics, transportation, and construction and building.

Flame retardants span an expansive range of chemicals to help prevent the spread of fire. There are two main classes of fire-retardant resins: halogenated and non-halogenated. Bromine is the most common halogen used in fire-retardant thermosetting resins because of its ability to control flame spread. However, bromine produces toxic dark smoke that makes its use unacceptable for some end-use applications. Another frequently used halogen, chlorine, produces lighter smoke compared to bromine. However, it’s less effective at controlling flame spread and is still toxic in nature.

Due to environmental concerns with halogen compounds, laws banning or limiting their use in certain end-use applications have been implemented. Therefore, the development of flame retardant resins has focused on environmentally friendly, non-halogenated alternatives.

Common non-halogenated fire retardant (NHFR) resins used in the composites industry are often based on alumina trihydrate (ATH) or intumescent technology. During a fire, ATH releases water. Intumescent systems, which are based on phosphorous and nitrogen compounds, typically rely on fillers, which limits their use in some fabrication processes. They also have a high affinity for water absorption, and therefore should be restricted to interior applications.

Other fillers that can impart some flame retardancy are calcium sulfate and calcium carbonate. All fillers have limited use in some fabrication processes because of the material’s high viscosity, preventing the filler from distributing evenly because it can get filtered out.

As resin suppliers continue to develop new non-halogenated fire-retardant products, they are faced with a dual challenge of adhering to stringent FR requirements, while simultaneously addressing processing issues to make these resins more user-friendly. The latter is critical to manufacturing facilities. Here is a list of common fabrication methods and potential issues with FR resins:

Open Mold Fabrication

The most widely-used process to produce FRP parts, open mold processes include spray-up chopping and hand lay-up. Most FR resins, including highly filled systems, can be used effectively in open mold fabrication processes. However, there are potential processing issues when using a highly filled resin, including poor fiber reinforcement wet-out or low glass content impacting mechanical properties.

Closed Molding Fabrication

Closed molding processes include resin transfer molding (RTM), light resin transfer molding (LRTM) and vacuum infusion. With the closed mold process, using an ATH-filled FR resin is always a process challenge. Because of the filled system’s high viscosity and complex fiber reinforcement configuration, depending on the closed molding process being used the ATH can be filtered out or not flow properly and uniformly.