Why do polyolefins need flame retardants?
Polyolefins, including polyethylene (PE) and polypropylene (PP), are versatile plastic materials widely used in various industries due to their excellent properties such as low cost, light weight, and chemical resistance. However, one of their major limitations is their high flammability, which makes them prone to ignition and rapid flame spread. To address this safety concern, flame retardants are extensively applied to polyolefins, enhancing their resistance to fire and reducing their contribution to potential fire hazards.
Flame retardants play a pivotal role in improving the fire resistance characteristics of polyolefins, making them suitable for numerous critical applications, including construction, automotive, electrical, and consumer goods. These additives help mitigate the risks associated with flammable polymers by delaying ignition, reducing flammability, and controlling the spread of fire. By introducing flame retardants, the thermal stability and fire properties of the polyolefin are significantly enhanced, allowing them to meet stringent fire safety regulations and standards.
Modes of Flame Retardant Action
Flame retardants exert their fire-suppressing effects through various mechanisms, depending on the chemical compositions of the additives. These mechanisms can be broadly categorized into gas phase flame inhibition, solid-phase carbonization, and heat sink effect.
- Gas phase flame inhibitors interfere with the combustion process by producing non-flammable gases, interrupting the chain reaction, and diluting the concentration of flammable gases.
- Solid-phase carbonization occurs when the flame retardant forms a protective char layer on the surface of the polyolefin. This char layer acts as a barrier, preventing the propagation of the flame and reducing the release of combustible gases.
- The heat sink effect refers to the ability of certain flame retardants to absorb and dissipate heat energy, thereby lowering the temperature of the material and reducing its flammability.
Mode of action for combustion [1]
Flame Retardants for Polyolefins
There are several flame retardants available for polyolefins. It is important to note that the choice of flame retardant depends on various factors including application requirements, regulatory compliance, and environmental considerations.
Brominated flame retardants such as decabromodiphenyl ether (DBDPE) and tetrabromobisphenol A (TBBPA) are widely used in polyolefins to provide flame retardant properties.
Phosphorus-based flame retardants like red phosphorus, ammonium polyphosphate (APP), and phosphinate are commonly used in polyolefins. They work by releasing phosphoric acid during combustion, which has a charring effect and helps in reducing the flammability.
- Halogen-free flame retardants
Various halogen-free flame retardants, such as aluminum diethyl phosphinate (AlPi), melamine polyphosphate (MPP), and Zinc borate, are also used in polyolefins. These flame retardants provide good fire resistance without the use of halogenated compounds.
Metal hydroxides, such as aluminum hydroxide (ATH) and magnesium hydroxide (MH), are also commonly used flame retardants for polyolefins. However, MH or ATH usually needs to be added in a large amount to obtain the desired flame retardancy, which may affect the mechanical properties of the material.
Reference
- Li Y, et al. Polymers, 2022, 14(14): 2876.
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