THE NEXT TIME you use an electronic device, take a moment to contemplate the contributions of flame retardants in it. Embedded in the sleek polymer components, such as the body of the device and the power cable, are chemicals designed to suppress fire. Should an electrical short, overheating, or any other event threaten to ignite the polymers, these additives will counteract combustion.
Flame retardants are not new. The ancient Egyptians used to impregnate wood for construction with a hydrated aluminum salt to slow its rate of burning. Inorganic flame retardants, such as aluminum hydroxide and magnesium hydroxide, are used to this day. More recent inventions are bromine-based and phosphorus-based compounds.
The chemical diversity of flame retardants reflects the fact that there is no one-size-fits-all flame retardant, explains Frederik Wurm, who investigates flame retardants at the Max Planck Institute for Polymer Research. “For each plastic matrix, you need a different flame retardant because you have to adjust the properties of the flame retardant to the polymer matrix you put it in,” he says.
Because of their small size and intricate shapes, connectors, switches, and related structures are some of the most demanding electric components to produce, says Patrick Jacobs, Lanxess polymer additives manager in Antwerp, Belgium. As a result, these components are usually made from polyamide or polyester engineering thermoplastics. The temperatures required to extrude these plastics during manufacturing would quickly decompose a metal hydroxide flame retardant. So polymeric brominated flame retardants, some of which are stable up to almost 400 °C, are typically used for this application, Jacobs explains.
With a number of small-molecule, brominated flame retardants recently removed from the market because of their potential to leach out of the material, various alternatives are gaining in popularity, says David Sikora, head of polymer additives technology at Lanxess in North America. For example, polymeric flame retardants are fixed into the polymer matrix far more securely than small molecules because of their higher molecular weight. There is also growing consumer demand for halogen-free alternatives. “We have a substantial R&D program looking at nonhalogen flame retardants,” Sikora says. The team is working on phosphorus-based compounds that match or even surpass the performance of brominated flame retardants. Wurm and his team also study phosphorus-based flame retardants, with a focus on compounds that will naturally degrade at the end of their useful life.
The other key trend, Sikora says, is in reactive flame retardants. These compounds are added as monomers during the initial polymerization reaction and become chemically incorporated into the backbone of the polymer matrix. Covalently bound to the polymer matrix, the compound cannot escape into the environment. In the area of electronic devices, the reactive flame retardant tetrabromobisphenol A can be incorporated into the epoxy polymer coating used on printed wire circuit boards. “While we are continuing to develop our existing small molecule halogenated products, our strategic focus for innovation is nonhalogen, polymeric, and reactive flame retardants,” Sikora says.
How flame retardants stop fires
Combustion is a gas-phase process. It starts when a heat source, such as an electrical fault, heats up the polymer until it begins to decompose and release volatile molecules. These molecules serve as fuel and, through increasing heat generation, combust in a reaction with oxygen. The reaction involves a rapid free-radical cascade.
Polymers in external casing and internal components of electronic devices are potentially flammable substances because they are based on hydrocarbons. Flame retardants, incorporated into the polymer during manufacturing, significantly slow the burning process should a fire occur and give people time to extinguish the fire or escape to safety.
Flame retardants suppress one or more steps in the combustion process. They are typically metal hydroxides or bromine- or phosphorus-based materials. Metal hydroxides absorb heat energy by endothermic breakdown to release water, which adds a further cooling effect. Bromine is an effective free-radical scavenger. It traps high-energy hydroxyl and hydrogen radicals that are key components of the radical-chain mechanism of combustion. Some phosphorus-based flame retardants also scavenge these radicals. The phosphorus-based flame retardants’ primary effect is to induce cross-linking and carbonization of the polymer, producing a solid carbonaceous char that reduces the release of volatile molecules and slows combustion.