​How To Select The Optimal Flame Retardant Solution for Polystyrene (PS)? An In-Depth Look at Four Major Flame Retardants

How to Select the Optimal Flame Retardant Solution for Polystyrene (PS)? An In-Depth Look at Four Major Flame Retardants

Polystyrene (PS) is widely used in packaging, home appliances, automobiles, and daily necessities due to its excellent transparency, dimensional stability, electrical insulation, and ease of processing. However, PS and its related polymers, such as high-impact polystyrene (PS-HI) and acrylonitrile-butadiene-styrene (ABS), are highly flammable. During combustion, these materials exhibit severe dripping and emit large amounts of black smoke. Therefore, improving their flame retardancy and smoke suppression capabilities is critical for enhancing their safety in applications.

Currently, two primary methods dominate flame retardancy research for PS: incorporating various flame retardants into the PS matrix and chemically modifying the PS polymer chain to inherently improve its flame retardant properties. A key trend in this field is the avoidance of halogen-containing materials or additives due to the toxic and corrosive gases released by halogenated flame retardants during combustion, which pose significant risks to human health and the environment. As a result, developing halogen-free, efficient, low-smoke, and low-toxicity flame retardant products has become a major focus.

I. Flame Retardancy via Additives

Adding flame retardants offers flexibility in adjusting the flame retardancy level of materials, with simple processing techniques and relatively low costs. Common halogen-free flame retardant systems for PS include phosphorus-based, nitrogen-based, silicone-based, and metal hydroxide-based agents.

1. Phosphorus-Based Flame Retardant Systems

Phosphorus-based flame retardants are categorized into organic and inorganic types, with inorganic phosphorus compounds such as red phosphorus and phosphate esters being predominant in PS applications. Red phosphorus, a pure flame retardant element, delivers excellent performance. When red phosphorus-containing resins combust, they generate phosphorus oxides that promote resin dehydration and carbonization, reducing flammable pyrolysis products. Additionally, phosphoric acid, hypophosphoric acid, and polyphosphoric acid form a glassy melt layer on the resin surface, effectively isolating oxygen and inhibiting flame spread.

However, red phosphorus has limitations due to its vivid color, hygroscopic nature, and poor adhesion to PS resins. Microencapsulated red phosphorus (MRP), coated with one or more protective layers, overcomes these drawbacks by enhancing weather resistance, thermal stability, and interfacial adhesion with polymer matrices, making it more widely applicable. Studies indicate that combining red phosphorus with phenolic compounds improves flame retardancy. For instance, using MRP alongside phenolic epoxy resin (NR) in PS-HI achieves a synergistic effect, elevating the oxygen index to 28.8% and attaining a V-0 rating in UL94 vertical burning tests.

Phosphate esters serve dual roles as flame retardants and plasticizers. During combustion, they form phosphoric and pyrophosphoric acids that provide insulation in the condensed phase. The volatilization of phosphate esters and their pyrolysis products also contributes to flame retardancy in the gas phase. For example, combining phosphate ester P30 oil with flame retardant, char-forming polyphenylene oxide (PPO) in PS-HI leverages synergistic effects to match the performance of brominated flame retardants while enhancing the mechanical properties of the composite material.

Similarly, pairing melamine resin (Novolac) with triphenyl phosphate (TPP) exhibits significant synergistic effects in flame retarding and smoke suppressing PS-HI.

2. Nitrogen-Based Flame Retardant Systems

Nitrogen-based flame retardants, though developed later than others, offer advantages such as being halogen-free, low-toxicity, non-corrosive, effective, and affordable. Organic nitrogen-based compounds are commonly used in PS, often in combination with phosphorus.

Phosphorus-nitrogen flame retardants, also known as intumescent flame retardants, typically consist of an acid source (e.g., ammonium polyphosphate, APP), a carbon source (e.g., pentaerythritol, PER), and a gas source (e.g., melamine, MEL). When heated, these components form a uniform foamed carbon layer on the resin surface, which insulates, oxygenates, suppresses smoke, and prevents dripping, thereby demonstrating excellent flame retardancy.

Upon heating or combustion, APP generates polyphosphoric acid, a strong dehydrating agent that carbonizes PER. The viscous carbonized material expands under the influence of gases like NH3 and H2O released by MEL, forming a porous barrier layer. This layer prevents heat transfer and the diffusion of flammable volatile products and oxygen, achieving flame retardancy. Currently, phosphorus-nitrogen synergistic flame retardants, such as phosphotriazine, polyphosphoramidate, phosphoric urea, and phosphoric guanidine, have garnered significant attention, with phosphotriazine compounds being frequently applied in PS flame retardancy.

Recent studies both domestically and internationally on phosphotriazine polymers highlight their phosphorus-nitrogen-phosphorus coordinated structure, which delivers remarkable flame retardant performance. For example, blending low molecular weight phosphotriazine compounds like bis(phenoxyl)phosphoryl tris(phenoxyl)phosphotriazine (NDTPh) with PS improves its flame retardant properties. Unlike TPP, NDTPh does not phase separate from PS and has minimal impact on mechanical properties within a certain concentration range, making it a novel halogen-free flame retardant.

The phosphorus-nitrogen-phosphorus coordinated structure in NDTPh enhances flame retardancy in two ways: phosphoric esters in the condensed phase promote carbon film formation, preventing further combustion, while nitrogen acts as an expanding flame retardant, generating a porous carbon layer that inhibits polymer thermal degradation and the release of volatile flammable components.

When combined with other flame retardants such as metal oxides, metal hydroxides, inorganic fillers, and epoxy resins, these flame retardant systems exhibit synergistic effects, significantly improving flame retardant performance compared to single-component systems.

3. Metal Hydroxide Flame Retardant Systems

Aluminum hydroxide (ATH) and magnesium hydroxide (MH) are primary metal hydroxide flame retardants. They are non-toxic, non-corrosive, stable, non-volatile, and do not release toxic gases at high temperatures. These flame retardants integrate three functions: flame retardancy, smoke suppression, and filling. Their flame retardant mechanism involves endothermic dehydration at high temperatures, which removes heat generated during combustion. The water vapor produced also dilutes oxygen. The metal oxides formed from dehydration catalyze carbonization and generate highly active metal oxide layers with large surface areas, capable of absorbing smoke particles, combustible particles, and even free radicals. This endows composite materials with excellent smoke suppression properties.

Although ATH and MH undergo dehydration reactions, differences in decomposition temperatures and heat absorption capacities exist. Using them in combination allows complementary benefits, resulting in better flame retardant performance than when used alone. However, achieving adequate flame retardancy typically requires large quantities of ATH and MH, which can degrade the mechanical and processing properties of materials. Surface modification prior to use and combination with other flame retardants can mitigate these issues and reduce filler content.

For instance, modified ATH combined with red phosphorus (RP) and modified polyphenylene oxide (MPPO) in PS-HI demonstrates excellent synergistic flame retardant effects. The resulting composite material achieves a vertical burning rating of FV-0 and an oxygen index of 27.5%.

4. Silicone-Based Flame Retardant Systems

Silicone-based flame retardants are categorized into inorganic and organic types. They are environmentally friendly and enhance not only the flame retardancy of base materials but also improve other properties, contributing to their rapid development in recent years.

Inorganic silicon compounds are abundant and easily sourced. Polymers incorporating these flame retardants are mostly non-toxic, low-smoke, low-flame, and slow in flame spread. Research on inorganic silicone-based flame retardants has focused on silica, glass fibers, porous glass, silicone gel/potassium carbonate, and polymer-layered silicate (PLS) nanocomposites.

Introducing layered silicates such as organoclay (OMMT) into polymer matrices improves mechanical properties, gas barrier performance, and solvent resistance. These materials also exhibit potential flame retardancy and self-extinguishing characteristics, making them environmentally friendly flame retardants.

Studies on PS/OMMT composites prepared via in-situ polymerization reveal reduced heat release rates, smoke release rates, and mass loss rates, indicating enhanced flame retardancy and smoke suppression.

Pure PS samples leave virtually no carbon residue after combustion, while PS/OMMT composites maintain their original shape, forming thick carbon layers. Research suggests that the carbon layers formed during the thermal decomposition of PS/OMMT composites act as barriers and insulators, contributing to flame retardancy. The protective role of these carbon layers may also explain the smoke suppression mechanism.

The flame retardant effect of PLS nanocomposites primarily stems from the physical barrier effect of MMT inorganic layers, which has limitations in improving flame retardant performance. To meet practical application requirements, these materials are often combined with other flame retardants. Studies indicate that while the flame retardant performance of PS-HI/OMMT nanocomposites is somewhat improved, the enhancement is limited. However, when OMMT, RP, and phenolic resin (PFR) are combined into a composite flame retardant system and introduced into PS-HI matrices, the resulting PS-HI/OMMT/RP/PFR composites exhibit further reductions in heat release rates, peak values, mass loss rates, and smoke release rates. The fire performance index is significantly improved, demonstrating low-smoke and high-efficiency flame retardant characteristics.

Organic silicone-based flame retardants are low-toxicity, drip-resistant, and environmentally friendly halogen-free flame retardants, as well as charring smoke suppressants. Organic silicone flame retardants used in PS include silicone oils, silicone resins, silicone rubbers, and siloxanes. During combustion, these flame retardants melt and migrate to the surface of the base material through gaps, forming a dense, stable siliceous char layer. This layer enhances heat and oxygen insulation, prevents melt dripping, and achieves flame retardancy. However, the flame retardant efficiency of organic silicone flame retardants is generally low when used alone and often requires combination with other flame retardants such as ATH and MH to achieve ideal results.

II. Chemical Modification for PS Flame Retardancy

PS tends to burn easily and emit large amounts of black smoke due to its decomposition and release of styrene monomers during combustion. As temperatures rise, C-C bonds in PS break first, forming free radicals that further decompose.

Chemically modifying PS through methods such as grafting, crosslinking, and copolymerization, by introducing flame retardant or smoke suppression functional groups or side chains into the polymer backbone or side chains, can effectively prevent decomposition, achieving flame retardancy and smoke suppression. This method is a highly effective alternative to adding flame retardants. It not only enhances flame retardancy and smoke suppression but also avoids the adverse effects of excessive flame retardant additives on the physical and mechanical properties and processability of polymer materials.

Typically, adding divinylbenzene and trivinylbenzene to PS for crosslinking modification reduces volatility and promotes charring, thereby achieving flame retardancy. Grafting copolymers onto PS backbones also significantly improves thermal stability. Additionally, grafting monomers containing other flame retardant elements or functional groups can greatly enhance flame retardant performance.

Furthermore, copolymerizing styrene monomers with phosphorus-containing olefin monomers to alter the chemical structure of the PS polymer backbone can significantly improve PS flame retardancy. Studies show that introducing phosphorus into the PS molecular structure enhances charring during combustion, modifies the material's condensed phase structure, and significantly improves flame retardant performance. The chemical environment of phosphorus also significantly impacts PS flame retardancy, with organic phosphates outperforming inorganic phosphates.

The demand for PS and its related polymers continues to grow due to their extensive applications. Research into their flame retardancy and smoke suppression is becoming increasingly urgent. Although halogenated flame retardants and materials are highly efficient and widely used, they release large amounts of smoke and corrosive gases during combustion, severely polluting the environment.

Driven by environmental protection and sustainable development, halogen-free flame retardant systems hold immense potential. The research and development of halogen-free, efficient, low-smoke, and low-toxicity novel PS flame retardant materials represent a critical direction. In this field, Yinsu Fire Retardant Company stands out with its advanced technology and rigorous innovation. It has developed several high-performance flame retardants suitable for PS. Among them, the Yinsu red phosphorus flame retardants FRP-950X and FRP-750 demonstrate exceptional flame retardant performance and excellent processability, providing robust fire protection for PS materials. Additionally, the antimony trioxide replacement T3 serves as an environmentally friendly flame retardant synergist. When combined with various flame retardants, it enhances the efficiency and environmental performance of flame retardant systems. These products not only align with the trends of being halogen-free, efficient, low-smoke, and low-toxicity but also exhibit superior comprehensive performance in practical applications, making them a significant driving force in the advancement of PS flame retardant technology.