What flame retardants are available for polystyrene?

Why do polystyrenes need flame retardants?

Polystyrene (PS), a widely used synthetic polymer, plays an integral role in various sectors such as packaging, construction, insulation, and electronics due to its desirable properties of lightweight, affordability, and thermal insulation. In addition, depending on the manufacturing process, there are two types of polystyrene foam, expanded polystyrene foam (EPS) and extruded polystyrene foam (XPS).

However, like most polymers, PS is ignited once it comes into contact with a flame, and can still burn after removing the fire source, with a large heat release, a large amount of smoke, and release of toxic gases. During combustion, PS also melts, flows and drips, which leads to an increased fuel load, which enhances flame spread. Therefore, when PS is used in the manufacture of products requiring fire safety, flame retardant treatment must be carried out.

Flame Retardants for Polystyrene

Different types of flame retardants are used in polystyrene, each offering unique properties and benefits. Some of the commonly used flame retardants in polystyrene include:

• Brominated Flame Retardants

Brominated flame retardants, such as decabromodiphenyl ether (DecaBDE) and hexabromocyclododecane (HBCD), are commonly used PS flame retardants. They are effective in reducing the flammability of polystyrene, but concerns about their environmental impact and toxicity associated with halogenated flame retardants have led to the development of alternative options.

• Phosphorus Flame Retardants

The phosphorus-based flame retardants used in PS are mainly inorganic phosphorus-based flame retardants such as red phosphorus and phosphate esters. For example, aluminum hypophosphite (AP) can be incorporated into PS composites as a single additive by melt blending, and the resulting composites achieved a V-0 rating in the UL-94 test. The study of the flame retardancy mechanism of PS/AP composites reveals the comprehensive activity of AP in the gas phase and condensed phase during the combustion process.

Flame Retardant Mechanism of PS/AP Composites [1]

• Metal Hydroxides

Aluminum hydroxide (ATH) and magnesium hydroxide (MH) flame retardants are the main metal hydroxide flame retardants. They have the advantages of non-toxicity, non-corrosion, good stability, non-volatility, and no toxic gas generation at high temperatures. They are flame retardants that integrate the three functions of flame retardancy, smoke suppression, and filling. However, ATH and MH usually require a large amount of addition to obtain a suitable flame retardant effect, which will reduce the mechanical properties of the material and deteriorate the processing performance. Therefore, it needs to be properly treated on the surface before use, or used in conjunction with other flame retardants to reduce its dosage.

• Silicon Flame Retardants

Silicone-based flame retardant is a low-toxic, anti-melt droplet, environmentally friendly halogen-free flame retardant, and it is also a char-forming smoke suppressant. Silicone flame retardants used in PS mainly include silicone oil, silicone resin, silicone rubber, siloxane, etc. In addition, inorganic silicon flame retardants (such as layered silicate, montmorillonite) can not only improve the mechanical properties, gas barrier properties and solvent resistance of polymers, but also have potential flame retardancy and combustion self-extinguishing characteristics.

Reference

1. Aloshy Baby, et al. Molecules, 2020, 25(17), 3779.