Flame-retardant PP Materials in Industry Working Principle

Polypropylene (PP) itself is a highly flammable hydrocarbon with a limiting oxygen index (LOI) of only 17.8%. It will continue to burn even after being removed from the fire source. The core principle of flame-retardant PP is to interrupt or delay its combustion cycle through physical and chemical means. Combustion requires the simultaneous existence of three elements: combustible material, heat and oxygen. The function of flame retardants is to destroy this "burning triangle".

In industry, flame retardancy is mainly achieved by adding flame retardants to PP. Different types of flame retardants function through the following mechanisms:

1. Gas-phase flame retardant mechanism

This is one of the most common mechanisms, especially applicable to traditional halogen-based flame retardants. When flame retardants are heated and decomposed, they can capture the free radicals (such as H· and HO·) that maintain the combustion chain reaction in the combustion reaction zone (flame), causing their concentrations to drop sharply and thus interrupting the combustion.

2. Condensed phase flame retardant mechanism

This is the most mainstream mechanism of halogen-free flame-retardant PP. Flame retardants promote the formation of a uniform and dense carbon layer on the surface of polymers. This layer of carbon has three major functions. The first step is to prevent external heat from entering the interior of the polymer. Secondly, it prevents the escape of flammable gases inside and the entry of external oxygen. Finally, it inhibits the further pyrolysis of the polymer and the generation of smoke.

When a fire occurs, the acid source promotes the dehydration, cross-linking and carbonization of the carbon source. Meanwhile, the large amount of gas produced by the decomposition of the gas source causes the softened carbon layer to expand, eventually forming a porous, dense and strong foam carbon layer, which protects the underlying PP like "armor".

3. Cooling/heat absorption mechanism

Flame retardants absorb a large amount of heat during the decomposition process, reducing the surface temperature of polymers and making it difficult for them to continuously pyrolyze and produce flammable gases. Typical representatives include aluminium hydroxide (ATH) and magnesium hydroxide (MH). When they decompose, they absorb a large amount of heat (endothermic reaction) and release water vapor. The water vapor can not only dilute flammable gases but also play a cooling role.

4. Dilution mechanism

Flame retardants decompose to produce a large amount of non-flammable gases (such as water vapor and CO₂, etc.), which can dilute the concentration of flammable gases and oxygen near the polymer surface, making combustion unsustainable. Both the gas sources of metal hydroxides and intumescent flame retardants have this function.

In conclusion, the working principle of flame-retardant PP in industry is a complex process involving the synergy of multiple mechanisms. Modern flame-retardant PP technology is developing towards halogen-free, low smoke, low toxicity and high efficiency. Among them, the condensed phase flame-retardant mechanism represented by intumescent flame retardants (IFR) is the core of current research and application. By carefully designing flame-retardant formulas, the best balance can be achieved among flame-retardant efficiency, material mechanical properties, processing performance and cost.