What are the Different Types of Composite Flame Retardants?

Composite flame retardants are an indispensable part of modern material science. They combine two or more different types of flame-retardant components in a specific way to create a synergistic effect, achieving a level of flame retardancy that a single agent cannot. This synergistic action not only boosts flame-retardant efficiency but also reduces the amount of additive needed, minimizing negative impacts on the material's physical properties, such as mechanical strength and processability.

1. Classification by Flame-Retardant Mechanism

The core advantage of composite flame retardants lies in the synergy of their multiple flame-retardant mechanisms. Based on their primary mode of action, they can be categorized as follows:

1. Halogen-Inorganic Composite Flame Retardants

   • Core Components: Primarily consist of halogenated flame retardants (like decabromodiphenyl ethane, brominated epoxy resins, etc.) and inorganic flame retardants (such as antimony trioxide, magnesium hydroxide, aluminum hydroxide, etc.).

   • Mechanism: Halogenated flame retardants release halogen radicals during combustion, which capture the radicals produced by the polymer's thermal decomposition, interrupting the combustion chain reaction. Inorganic compounds like antimony trioxide ($S{b}_2{O}_3$) act as a synergist here. It reacts with the halogenated flame retardant to form more efficient antimony halides (like $SbC{l}_3$ or $SbB{r}_3$), further enhancing the gas-phase flame-retardant effect. Furthermore, inorganic hydroxides like magnesium and aluminum hydroxide absorb heat as they decompose and release water vapor to dilute flammable gases, forming a physical barrier that provides solid-phase flame retardancy.

   • Applications: Mainly used in thermoplastics like polystyrene and polypropylene, as well as in cable insulation and other insulating materials.

2. Phosphorus-Nitrogen Composite Flame Retardants

   • Core Components: Primarily composed of phosphorus-containing compounds (like red phosphorus, phosphate esters, polyammonium phosphate—PAP, etc.) and nitrogen-containing compounds (such as melamine, melamine cyanurate—MCA, guanidine, etc.).

   • Mechanism: The synergistic effect of this type of flame retardant is highly significant. Phosphorus-containing compounds dehydrate when heated to form a char layer, which creates a dense barrier on the material's surface. This barrier isolates the material from heat, oxygen, and flammable gases, serving as a solid-phase flame retardancy mechanism. At the same time, nitrogen-containing compounds decompose at high temperatures to produce non-combustible gases (like $N_2$ and $N{H}_3$). These gases effectively dilute the concentration of flammable gases, achieving a gas-phase flame-retardant effect. The nitrogen-containing compounds also promote the formation of the char layer, further boosting the flame-retardant performance.

   • Applications: Widely used in polyurethanes, epoxy resins, polyolefins, and other fields, especially where environmental protection is a key consideration, such as in electronics, building materials, and transportation.

3. Intumescent Composite Flame Retardants (IFR)

   • Core Components: IFRs are inherently a composite system, usually containing three key components:

     • Acid Source: Dehydrates the carbon source for char formation, such as polyammonium phosphate (APP), boric acid, or phosphoric acid.

     • Carbon Source: A substance that can be catalyzed by the acid source to form a char layer at high temperatures, like pentaerythritol, starch, or sorbitol.

     • Gas Source: Decomposes at high temperatures to produce non-combustible gases, causing the char layer to swell and foam, such as melamine or guanidine.

   • Mechanism: The mechanism of IFRs is a classic example of solid-phase flame retardancy. When heated, the acid source produces acid, which causes the carbon source to dehydrate and form char. Simultaneously, the gas source decomposes and produces gases that cause the forming char layer to foam and expand. This results in a thick, non-combustible, porous foam layer on the material's surface. This foam layer not only insulates the material from oxygen and heat but also prevents the release of flammable gases, achieving a highly effective flame-retardant result.

   • Applications: Widely used in engineering plastics, textiles, coatings, and adhesives. They are highly favored for their halogen-free and eco-friendly properties.

2. Classification by Flame Retardant Form and Compatibility

In addition to their mechanism, composite flame retardants can also be categorized by their physical form and compatibility with the base material:

1. Powder Composite Flame Retardants

   • Characteristics: Two or more flame retardants are simply blended together as micron- or nano-sized powders, typically a mixture of inorganic and organic flame retardants.

   • Advantages: Simple production process and relatively low cost.

   • Disadvantages: Can suffer from uneven powder dispersion, which affects the stability of the flame-retardant effect.

   • Examples: A mixture of antimony trioxide and decabromodiphenyl ethane.

2. Masterbatch Composite Flame Retardants

   • Characteristics: Multiple flame retardants are pre-dispersed into a polymer carrier to create high-concentration pellets (masterbatches).

   • Advantages: The flame retardants are uniformly dispersed within the base material, improving the stability and consistency of the flame-retardant effect. The masterbatch form also makes handling and processing easier and reduces dust pollution.

   • Disadvantages: Relatively high production cost, requiring careful selection of an appropriate carrier resin.

   • Examples: A flame-retardant masterbatch made by mixing phosphorus-nitrogen flame retardants with a polypropylene carrier.

3. Microencapsulated Composite Flame Retardants

   • Characteristics: Flame retardants are encapsulated within a polymer or other microcapsule wall material, forming a core-shell structure at the micron level.

   • Advantages: Solves the problem of poor compatibility between flame retardants and the polymer matrix, reducing migration and bleeding of the additives. It also protects the flame retardant from heat and moisture, improving its thermal stability.

   • Disadvantages: The preparation process is complex and costly.

   • Examples: Microencapsulated red phosphorus, where the outer shell effectively prevents the oxidation and hydrolysis of the red phosphorus, solving safety issues during its use.

Conclusion

Composite flame retardants (synergistic flame retardant systems) have become a crucial direction in the development of flame retardant technology due to their unique synergistic effects. They improve the flame-retardant performance of materials while considering environmental friendliness and processability. As the demand for eco-friendly and high-performance materials continues to grow, future research will focus on developing new, efficient, halogen-free, low-smoke, and low-toxicity composite systems. These systems will incorporate advanced technologies like nanotechnology and microencapsulation to achieve breakthroughs in more high-value-added applications.