A Primer Non-Halogen Fire Retardant Resins

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:

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 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.

For processes such as RTM and LRTM, ATH at levels up to 40% by weight are successfully used to meet many FR standards. To put this in perspective, an open mold FR resin can be used with ATH at levels greater than 60% by weight. However, ATH cannot be used in the infusion process because the material’s high viscosity. The infusion process is under vacuum, and a filled resin cannot flow through the reinforcement as it is filtered out. Therefore, the one of the biggest challenges for resin suppliers is the development of non-halogen FR resins that don’t require ATH.

During fabrication, FRP panels are typically encapsulated in plastic film to keep the process clean while producing flat, smooth, embossed or corrugated shapes and surfaces. The highest volume throughput process is the high-temperature continuous process. The stationary room temperature, hand lay-up batch process is more suitable for better aesthetics. Resins currently being used successfully are based on ATH-filled systems, but the panel fabrication process faces many of the same challenges as open molding.

In addition, numerous other fabrication methods, including pultrusion and sheet molding compound (SMC), are utilized when manufacturing end-use products with fire retardancy requirements. No matter what fabrication process is used, some of the common challenges with processing FR resins include:

• Meeting the required FR standards
• Handling higher processing viscosities
• The inability to use filled systems
• Increased weight
• Translucency
• Achieving the required mechanical properties

One notable drawback to unfilled FR resins is the increased cost. However, unfilled FR resins can be cost effective overall when you consider advantages such as higher glass reinforcement content, improved mechanical performance, reduced thickness and decreased weight. In addition, unfilled resins can be processed easier, which leads to an increased production rate.

During the past decade, research and development has come a long way on environmentally friendly, non-halogenated alternatives. By working closely with fabricators, resin suppliers have solved many fabrication challenges. Given the exceptional properties of some emerging FR resins, there is a bright future for the next generation of fire-retardant composites produced using fabrication processes that are not suitable or ideal at this time with existing FR technologies.

Michael Siegel is product leader for corrosion and fire-retardant resins at AOC. Email comments to mike.siegel@aocresins.com.

Disclaimer: Opinions, statements and technical information within the Tech Talk column are that of the authors. ACMA makes no warranty of any kinds, expressed or implied, with respect to information in the column, including fitness for a particular purpose. Persons using the information within the column assume all risk and liability for any losses, damages, claims or expenses resulting from such use.