As a key component for enhancing the fire safety of polymer materials (especially PVC), flame retardants play an indispensable role in many industries, such as construction, electronics, automotive, and packaging, etc. As consumers’ demands for material safety, environmental friendliness, and performance balance continue to rise, a thorough understanding of flame retardants (from their classification to their mechanism of action) has become crucial for innovative researchers, engineers, and enterprises. In this article, we will delve into the core knowledge of flame retardants used in PVC injection molding, revealing their working principles and application logic, providing practical insights for your material optimization efforts.
Most polymer materials, whether natural or synthetic, will catch fire when exposed to fire. Flame retardants are a type of additive that can prevent the material from being ignited or inhibit the spread of flames. Flame retardants are mainly used for the flame retardancy of synthetic polymer materials or natural polymer materials. Adding flame retardants to polymer materials can reduce the flammability of the materials, making them burn more slowly when exposed to flames and extinguish quickly after removing the heat source. It should be noted that materials containing flame retardants are not truly non-flammable; they can only reduce the risk of fire but not eliminate it. The requirements for flame retardants dispersed in multiple aspects. People hope that flame retardants can have a lasting flame-retardant effect with rather low dosage; that they are non-toxic and won’t produce toxic gases and thick smoke during combustion; that they have high thermal stability and will not decompose or volatilize when exposed to fire; and that the mechanical properties and physical properties of the base resin will not be lost or reduced due to the use of flame retardants. The best effect ratio/cost should be attained between the flame retardancy and other properties of the material, rather than sacrificing the original good properties of the material too much to blindly meet the overly high requirements for flame retardancy. In addition, while improving the flame retardancy of the material, the amount of toxic gases and smoke generated during thermal decomposition or combustion should be reduced as much as possible. In the field of flame retardants, flame retardancy and smoke suppression are complementary to one another.
Flame retardants are mainly inorganic compounds of phosphorus, halogens, boron, antimony, lead, molybdenum and other elements. According to their usage methods, flame retardants are generally divided into two types: additive type and reactive type. Additive flame retardants are simply added and mixed in the plastic during the plastic processing, mainly used for thermoplastic plastics. Reactive flame retardants participate in the reaction during the polymer synthesis process as a component and bond to the polymer’s molecular chain, mostly used for thermosetting resins. Some reactive flame retardants can also be added during the plastic processing.
Based on their chemical structure, flame retardants fall into two types: inorganic & organic flame retardants. Organic flame retardants include halogenated aliphatic hydrocarbons and aromatic hydrocarbons, organic phosphorus compounds, halogenated organic phosphorus compounds, etc.; Inorganic flame retardants include metal oxides such as aluminum, antimony, zinc, and molybdenum, phosphates, borates, sulfates, etc. Flame retardants can be classified according to the main elements that play a flame-retardant role into halogen-based flame retardants, phosphorus-based flame retardants, and metal oxide-based flame retardants such as antimony, boron, and molybdenum; or they can be classified into bromine-based, phosphorus-based, chlorine-based, and aluminum-based, boron-based, and antimony-based flame retardants, etc. Reactive flame retardants react with the resin, that is, there is a chemical bond between the flame retardant and the resin, so reactive flame retardants are relatively stable in the resin and their inhibitory effect on the flame is usually more persistent than that of additive type, having less impact on the material’s properties, but the operation and processing technology are more complex. While additive flame retardants are simply physically mixed with the resin and have no chemical reaction, they are usually used in large dosage and the operation is relatively convenient, thus becoming a widely used type.
2. Polymer Combustion
Flame-Retardant PVC Tape Rolls
2.1 The Combustion Process of Polymers
The combustion of flame retardants is a very complex and rapid oxidation process, involving a series of physical and chemical changes from the heat absorption and decomposition of materials to intense oxidation, luminescence and heat generation. When polymers are exposed to external heat sources, they are first heated, then degraded, generating volatile flammable gases and other thermal decomposition products. As the concentration of flammable gases increases, when it reaches a certain limit, the polymer begins to burn. In this combustion mode, the polymer is first decomposed to produce flammable gases under heat, i.e., the flammable gases diffuse from the solid phase to the gas phase, then they react with oxygen to start burning. The heat generated by the combustion radiates to the polymer surface and is transferred to the interior of the polymer. Due to the heat effect, the polymer continues to decompose, forming a combustion cycle. So it can be said that flame retardancy exists to inhibit this combustion cycle.
The matter generated by the thermal decomposition of different polymers determines the ease or difficulty of their combustion, so different polymers have different combustion properties. The ease or difficulty of combustion of the same polymer also varies with the addition of different additives. When plasticizers are added to PVC, the products tend to be more flammable, while the addition of flame retardants makes the products less flammable. The purpose of flame retardancy is to increase the difficulty of combustion of the products, reduce the possibility of fire and turn the products into non-flammable materials. Although flame retardancy can reduce the risk of fire for polymers, it cannot completely eliminate the fire hazard. Flame-retarded polymers can still burn fiercely in a large fire.
2.2 Main Factors Affecting Polymer Combustion
a. The thermal decomposition characteristics of polymers
The thermal decomposition characteristics of polymers determine the combustion performance of the polymers. After absorbing sufficient energy, the polymers begin to decompose, generating combustible gases with relatively low molecular weight, non-combustible gases, and carbonized residues. Different polymers have different thermal decomposition properties, namely different thermal decomposition temperatures and different decomposition products, due to their composition and chemical structure. A higher thermal decomposition temperature indicates better thermal stability of the polymer, and more heat needs to be supplied to cause its decomposition. The decomposition products of the polymer determine the ease of ignition of the polymer, and the more combustible gases are contained in the decomposition products, the easier they are to burn.
b. Combustion temperature and ignition temperature
The combustion temperature has a significant impact on the combustion process of polymers. The higher the combustion temperature, the faster the polymer burns and the more heat is released. In fact, the combustion speed of polymers is also controlled by the oxygen diffusion rate.
The ignition temperature of polymers plays a crucial role in combustion. The combustion of polymers depends on the combustible gases produced by thermal decomposition. The ignition temperature of combustible gases is restricted by the activation energy of combustion, so there is no correspondence between the ignition temperature of combustible gases and their chemical structure.
c. Combustion heat
The stable combustion of polymers mainly relies on the released heat (combustion heat) to maintain itself. If the heat dissipation of combustion heat to the surroundings is greater than the heat released by combustion, once the heat source is removed, the combustion will be difficult to sustain. Conversely, the combustion will intensify further. If the two reach a balance, it enters a stable combustion state.
d. Oxygen concentration
Polymer combustion requires sufficient oxygen. Otherwise, combustion cannot occur or cannot maintain stable combustion, and in the latter case, a large amount of incompletely burned smoke will be produced. Different molecular structures of materials require different oxygen concentrations during combustion. In practical applications, many materials with an oxygen index greater than 21 cannot extinguish themselves when burning in air. Therefore, the commonly mentioned flame-retardant materials with self-extinguishing properties have an oxygen index of at least 27.
2.3 Smoke Generation during Polymer Combustion
When polymers burn, not only do they release a large amount of heat energy, but they also often produce a large amount of dust and toxic gases. The smoke contains both black smoke and white smoke, and some of it is very irritating. The more carbon particles there are in the smoke, the darker its color becomes; the more components such as HCl, hydrogen, ammonia, etc. it contains, the greater its irritancy.
The main methods for suppressing the smoke produced during polymer combustion are physical methods and chemical methods. The important physical methods include insulation, cooling, and their essence is to inhibit the combustion of polymers. Since inhibiting combustion also inhibits the generation of smoke; the chemical method involves adding “smoke suppressants”, which can be classified into two types based on their action principles: adsorption type and reaction type. The adsorption type prevents the carbonaceous or graphite-like particles generated by decomposition or combustion from diffusing in the space and forming smoke. The reaction type changes the composition of the products during polymer combustion through catalytic decomposition or combustion reaction patterns, thereby achieving the purpose of suppressing smoke. Calcium carbonate, magnesium hydroxide, aluminum hydroxide, etc. all have smoke suppression effects. The smoke suppression effect of calcium carbonate mainly lies in capturing the hydrogen chloride gas in the smoke, converting it into stable calcium chloride and leaving it in the carbonized layer after combustion.
2.4 Toxicity of Polymer Combustion
The combustion of polymers is a very complex process. The combustion products vary depending on the composition of the polymer, the combustion conditions, and the flame retardant system. The combustion process also changes with the variations of external factors. Therefore, the resulting toxicity can be diverse.
3. Mechanisms of Flame Retardants
The mechanism of flame retardants is rather complex. The same flame retardant may have different mechanisms of action in different polymers. The classification and summary of the flame retardation mechanisms include the following patterns: 1. Inhibition effect, capturing the active free radicals generated during the polymer combustion, thereby inhibiting the chain reaction that generates active free radicals, and weakening the combustion; 2. Chain transfer effect, changing the combustion mode of the polymer material, inhibiting the generation of combustible gases; 3. Coverage effect, the inert gases released by the heating of the flame retardant in the gas phase prevent the contact between combustible gases and oxygen, or form a solid carbon layer or a liquid film on the polymer surface to prevent the escape of combustible gases; 4. Dilution effect, the non-flammable gases produced by the thermal decomposition of the flame retardant dilute the concentration of oxygen and combustible gases, making it impossible for them to reach the necessary conditions for continued combustion; 5. Endothermic effect, the thermal decomposition of the flame retardant absorbs a large amount of combustion heat, making it difficult for the polymer material to increase in temperature.
Flame-Retardant Flexible PVC Air Ventilation Duct
3.1 Mechanisms of Halogen-based Flame Retardants
When using halogen-based flame retardants alone, they mainly delay or prevent the combustion of the polymer in the gas phase. When halogen-based flame retardants decompose at high temperatures, they generate halogenated hydrogen, which can act as a free radical terminator to capture the active free radicals ·OH, ·O, ·H in the polymer combustion chain, generating less active halogen free radicals, thereby weakening or terminating the chain reaction in the gas-phase combustion and achieving the purpose of flame retardancy.
HX + ·H → H₂ + ·X
HX + ·O → ·OH + ·X
2·H + ZnMoO₄ → ZnO + MoO₃ + O₂↑ + H₂O↑
Halogenated chlorine can also dilute the oxygen in the air and cover the surface of the material to block the air, thereby reducing the combustion rate of the material.
Halogen-based flame retardants have a significant synergistic effect with antimony oxide. The reaction of Hydrogen halide with antimony oxide generates antimony halide, which is the key factor determining the flame retardant effect. Antimony halide has excellent flame retardant performance as follows: 1. Halogen-antimony oxide undergoes a decomposition process that is endothermic, which can reduce the combustion temperature and decomposition rate of the polymer; 2. The vapor of antimony halide can remain in the gas phase for a long time, effectively diluting the flammable gas. At the same time, it covers the polymer surface, providing insulation and oxygen isolation; 3. The surface effect of liquid and solid antimony halide particles can reduce the flame energy; 4. In the solid or molten state of the polymer during combustion, antimony halide can promote the char formation reaction, relatively slowing down the decomposition rate of the polymer to generate flammable gases, and at the same time, the formed char layer can seal the polymer, preventing the escape and entry of flammable gases into the combustion zone; 5. Antimony trihalide can capture the active free radicals in the gas phase that maintain the combustion chain reaction in the combustion zone, change the combustion reaction mode of the gas phase, and reduce the reaction heat to extinguish the flame. The ratio of pentabromobenzene to antimony oxide is best within the range of (1-3):1.
3.2 Mechanisms of Phosphorus-based Flame Retardants
Phosphorus-based flame retardants can generate more char when the material is burning, reducing the amount of flammable gas produced, significantly reducing the mass loss rate of the flame-retarded material, but generating a large amount of smoke during combustion. Organic phosphorus-based flame retardants can exert flame retardant effects in both the condensed phase and the gas phase, but they are mainly in the condensed phase. The mechanism of phosphorus-based flame retardants varies depending on their structure, polymer type, and combustion conditions. When polymers are heated and ignited, organophosphorus flame retardants first decompose to form phosphoric acid. Phosphoric acid then dehydrates to produce meta-phosphoric acid, which polymerizes to form metaphosphoric acid. These acids have a catalytic effect on the dehydration and carbonization of hydroxyl-containing polymers, accelerating the carbonization process. The result of char formation is a graphite-like carbon layer on the material surface. This carbon layer is non-flammable, heat-insulating, and oxygen-blocking, thereby reducing the heat transferred to the material surface and slowing down the thermal decomposition. Secondly, the dehydration of hydroxyl polymers is an endothermic reaction. The water vapor formed during dehydration can also dilute the oxygen and combustible gases in the atmosphere, helping to interrupt the combustion. The polymetaphosphoric acid generated during combustion can form a liquid film on the material surface, reducing the permeability of the carbon layer and protecting the carbon layer from further oxidation. This also helps to improve the flame retardancy of the material. The mechanism of the condensed-phase flame retardancy of organophosphorus flame retardants is basically based on hydroxyl polymers. Therefore, organophosphorus flame retardants have a greater flame retardant effect in epoxy resins and polyurethanes, while having a smaller effect on polymers without hydroxyl groups. Phosphorus-based flame retardants have a synergistic effect when used with halogen-based flame retardants and is depend on the type of polymer.
3.3 Mechanisms of Intumescent Flame Retardants
Intumescent flame retardants overcome the shortcomings of traditional flame retardant technologies and have characteristics such as high flame retardancy, low smoke, low toxicity, no generation of corrosive gases, and no dripping behavior. Intumescent flame retardants exert flame retardant effects in the condensed phase by forming a porous foam char layer. The formation of the char layer proceeds in the following steps: 1. At a relatively low temperature (around 150°C, depending on the nature of the acid source and other components), the acid source releases substances that can esterify polyols and act as a dehydrating agent; 2. At a slightly higher temperature, inorganic acids react with polyols (carbon sources) in an esterification reaction, while the amine in the system acts as a catalyst for the esterification reaction, accelerating the reaction; 3. The system melts before or during the esterification reaction; 4. During the reaction, water vapor and non-flammable gases produced by the gas source cause the system to expand and foam, while polyols and esters continue to dehydrate and carbonize, forming inorganic substances and char residues, further expanding and foaming the system; 5. When the reaction is nearly complete, the system undergoes gelation and solidification, eventually forming a porous carbon foam layer.
3.4 Mechanisms of Inorganic Flame Retardants
Aluminum hydroxide and magnesium hydroxide decompose at high temperatures and absorb a large amount of heat. The generated water vapor can dilute the oxygen concentration in the air, thereby slowing down the thermal degradation rate of the polymer, reducing or inhibiting the polymer’s combustion, promoting carbonization, and inhibiting the formation of smoke.
2Al(OH)₃ → Al₂O₃ + 3H₂O↑
Mg(OH)₂ → MgO + H₂O↑
Based on this principle, when selecting metal hydroxides, the decomposition temperature and heat absorption are two important indicators. Although calcium carbonate also has a relatively high heat absorption, due to its decomposition temperature being much higher than that of polymers, it cannot be used as a flame retardant. Even if it reacts with the HCl generated during the polymer decomposition, since calcium carbonate is in the solid phase and HCl is in the gaseous phase, the reaction speed and process are restricted, and there is no obvious flame retardant effect. Although aluminum hydroxide and magnesium hydroxide have much higher flame retardant efficiency than calcium carbonate, they still need to be added at 60% to achieve a significant effect. Zinc borate can function in both the condensed phase and the gaseous phase as a flame retardant. In the condensed phase, zinc borate can melt and dehydrate under the action of fire to form a glassy coating, further generating an inorganic carbon layer, and at the same time promoting the polymer to form carbon, thereby slowing down the decomposition of the polymer and the generation of combustible gases, achieving the effect of flame retardancy and smoke suppression; in the gaseous phase, zinc borate absorbs heat due to decomposition and generates water vapor, which traps free HO· and H· in the gas phase to exert the gas-phase flame retardant effect when combined with halogen-based flame retardants or used in halogen-containing resins.
2ZnO·3B₂O₃ + 12HCl → Zn(OH)Cl + ZnCl₂ + 3BCl₃↑ + 3HBO₂ + 4H₂O↑
The solid molten zinc has a smoke suppression effect on PVC. A system containing 3.5% – 4% zinc has excellent smoke suppression performance. This is because the Lewis acid ZnCl₂ generated during the combustion can promote the hydrogen-decomposition reaction of PVC to form trans-olefin structures, which is conducive to the intermolecular cyclization reaction and the generation of carbides.
3.5 Smoke-Suppression Mechanisms
The smoke emission of polymers is caused by incomplete combustion or the generation of graphite-like particles. The better the flame retardant performance, the less complete the combustion of the polymer, and the more smoke is generated. Therefore, flame retardancy and smoke suppression are essentially a pair of contradictions. First, during the combustion of polymers, a large amount of smoke is released; second, when halogen-antimony flame retardants or phosphate flame retardants are added, the smoke emission increases. In the former case, an inhibitor needs to be added to suppress the smoke emission process; in the latter case, it is advisable to avoid using flame retardants that increase the smoke emission. According to the mechanism of polymer combustion and smoke generation, suppressing the smoke essentially means suppressing the diffusion of combustible gases from the polymer to the air, accelerating the process of converting combustible gases in the gas phase into water and CO₂, promoting the carbonization reaction in the liquid phase and adsorbing the carbonized particles on the burning material surface.
PVC Flame Retardant Cable Wires
Conclusion
Flame retardants not only ensure the safety of polymer materials, but also are a key factor in promoting the sustainable development of related industries. By mastering their classification standards and action mechanisms, practitioners can make more accurate choices and applications of flame retardant solutions, thereby achieving a perfect balance between fire safety, product performance, and environmental compliance. As the reputable injection molding manufacturer n China, KingStar is dedicated to providing professional technical support and customized solutions(OEM/ODM) for the formulation optimization and manufacturing of plastic products. Please look forward to more in-depth analyses of industry knowledge, and let us help your innovation journey with our professional knowledge based on scientific evidence, email at sales@kingstarmold.com.
