Introduction to several common flame retardants for polypropylene (PP) plastics

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This post was last edited by Desert Fish on 2016-12-15 at 16:27. There are a wide variety of flame retardants that can be used for PP polypropylene, and they can be classified into two main categories based on their chemical composition: organic flame retardants and inorganic flame retardants； Based on the method of use, they are further divided into reactive and additive types. Representative flame retardants include bromine-based, phosphorus-nitrogen-based, phosphorus-based agents, as well as aluminum hydroxide and magnesium hydroxide. 1. Bromine-based flame retardants: Bromine-based flame retardants experienced a period of rapid development during the 1970s to mid-1980s. Due to the relatively low bond energy of the C-Br bond, most bromine-based flame retardants decompose at temperatures between 200–300°C, which happens to be the same temperature range at which polypropylene decomposes. Therefore, when polypropylene decomposes under heat, the bromine-based flame retardants also begin to decompose. It undergoes decomposition and is capable of capturing the free radicals generated by its degradation reactions, thereby slowing down or stopping the chain reaction of combustion. The HBr released simultaneously is itself a non-flammable gas; it has a high density and can cover the surface of materials, acting as a barrier to prevent flammable gases from reaching the surface and thus suppressing the combustion of the materials. Such flame retardants can also be used in combination with other compounds (such as antimony trioxide), thereby significantly enhancing the flame-retardant effect through a synergistic effect. Brominated flame retardants play an important role in the flame-retardant treatment of polypropylene. The main products currently in use include decabromodiphenyl ether, tetrabromobisphenol A, tetrabromodipentyl ether, brominated polystyrene, pentabromotoluene, and hexabromocyclododecane. The main drawback of bromine-based flame retardants is that they reduce the UV stability of the material being flame-retarded, and they produce large amounts of smoke, corrosive gases, and toxic gases during combustion, which limits their use to some extent. 2. Phosphorus-nitrogen-based flame retardants: Phosphorus-nitrogen-based flame retardants are also known as intumescent flame retardants. When polymers containing such flame retardants are heated, a uniform layer of carbonous foam forms on their surface, which serves to insulate, prevent the entry of oxygen, suppress smoke generation, and avoid the formation of molten droplets; hence, they exhibit excellent flame-retardant properties. An intumescent flame-retardant system generally consists of three components: an acid source (dehydrating agent), a carbon source (char-forming agent), and a gas source (nitrogen source, foaming agent). Expansive flame retardants primarily exert their flame-retardant effect in the condensed phase by forming a porous foam carbon layer. Phosphorus-nitrogen-based flame retardants have the advantages of being halogen-free, producing low levels of smoke, and having low toxicity. 3. Phosphorus-based flame retardants function as flame retardants by promoting dehydration and carbonization during the initial decomposition of polymers. This dehydration and carbonization step must rely on the oxygen-containing groups present in the polymer itself, for polymers whose structure already contains such oxygen-containing groups. Their flame-retardant effect will be better. For polypropylene, since its molecular structure does not contain oxygen-containing groups, the flame-retardant effect is poor when phosphorus-based flame retardants are used alone. However, by combining them with substances such as (OH)3 and Mg(OH)2, a synergistic effect is achieved, resulting in a good flame-retardant performance. Commonly used organic phosphorus-based flame retardants include triphenyl phosphate, trimethylphenyl phosphate, tris(xylene) phosphate, phenylpropane phosphate esters, and styrenebutadiene phosphate esters. Phosphates are characterized by their dual functions of flame retardancy and plasticization. It enables flame retardants to be halogen-free; its plasticizing effect improves the flowability of plastics during molding, and it helps to suppress the residues left after combustion. Fewer toxic and corrosive gases are produced compared to halogen-based flame retardants. Its main advantage is high efficiency ； Has little effect on light stability or the function of light stabilizers ； Low corrosivity during processing and combustion ； It helps to prevent recurrence ； The mass of flame-retardant materials is minimally or not increased at all. However, most phosphate-based flame retardants also have some drawbacks. Such as poor heat resistance, high volatility, inadequate compatibility, and the formation of droplets during combustion. The main products of phosphorus-containing inorganic flame retardants include red phosphorus flame retardants, ammonium phosphates, and ammonium polyphosphates. As the use of halogen-free flame retardant materials increases, the amount of red phosphorus flame retardants used also increases. Red phosphorus has a better flame-retardant effect than phosphate esters. Phosphorus-containing inorganic flame retardants are widely used due to their advantages such as good thermal stability, non-volatility, no generation of corrosive gases, long-lasting effectiveness, and low toxicity. 4. Aluminum hydroxide, Al(OH)3 (abbreviated as ATH), decomposes at temperatures between 200–300°C; the heat absorbed during this process is 1,967.8 J/g. It is a flame retardant that combines three functions: flame retardancy, smoke suppression, and filling. It has the advantages of being non-toxic, non-corrosive, having good stability, not being volatile, and not producing toxic gases at high temperatures; it is also inexpensive and widely available. However, as a flame retardant, it also has the disadvantages of high filler content, reduced mechanical properties, and poor processability. The temperature at which Al(OH)3 begins to decompose falls within the range during which the polymer transitions from a solid phase to a liquid phase; therefore, it helps to suppress the rise in temperature of the polymer material at early stages. When the mass fraction of added Al(OH)3 is 40%, it can significantly slow down the thermal decomposition rate of the material, providing flame-retardant effects and reducing smoke production. 5. Magnesium hydroxide: Magnesium hydroxide decomposes at temperatures between 340–490°C; the heat absorbed during this process is 782.9 J/g. It possesses good thermal stability, as well as excellent flame-retardant and smoke-suppressing properties, making it particularly suitable for polypropylene materials that are processed at high temperatures. When used in PP (at addition levels greater than 50%), Mg(OH)2 exhibits good flame-retardant effects, outperforming Al(OH)3. At the same filling level, the flame-retardant effects of different ratios of aluminum hydroxide and magnesium hydroxide show little difference; however, using both together yields a better result than using either one alone, as although both involve dehydration reactions, there are differences in their decomposition temperatures and heat absorption amounts. Magnesium hydroxide requires higher temperatures to undergo dehydration, with carbonization occurring simultaneously. Magnesium hydroxide, on the other hand, has a relatively lower heat absorption capacity, as its ability to prevent an increase in the material’s temperature is not as effective as that of aluminum hydroxide. When used together, the two compounds complement each other, resulting in better flame-retardant properties than when used alone. However, Mg(OH)2 also has disadvantages such as poor acid resistance, as well as poor dispersibility and compatibility, necessitating the development of new varieties with better compatibility.