Halogen-free flame retardants are widely used and of great significance in polyvinyl chloride (PVC). The following is an analysis from several aspects including application background, common types, mechanism of action, and application effects:
Application background
PVC material has a wide range of applications and features excellent processing performance, low manufacturing cost, corrosion resistance, and insulation. It is mainly used in the production of architectural profiles, cable materials, plates, films, leather, sealing strips, etc. However, PVC material is flammable and releases a large amount of smoke and toxic gases during combustion, posing safety hazards. With the improvement of environmental protection requirements and people’s concern for safety, the application of halogen-free flame retardants in PVC has received increasing attention.
Common types
Phosphorus-based flame retardants
Inorganic phosphorus-based flame retardants: such as red phosphorus, ammonium polyphosphate (APP), etc. Red phosphorus is an excellent flame retardant with high efficiency, smoke suppression, low toxicity and other flame retardant effects. However, it has poor stability when used, is flammable and explosive, easily oxidized into acid, and is red in color, which can easily color products. It is also prone to moisture absorption and has poor compatibility with polymers. Generally, it needs to be used in combination with other additives.
Organophosphorus-based flame retardants: such as phosphate esters, phosphorus compounds, etc. Phosphate esters have both flame-retardant and plasticizing functions and are often used in soft PVC. For instance, triaryl phosphate esters have better flame retardancy than trialkyl phosphate esters, but they have poor compatibility with PVC and are prone to crystallization and precipitation. Alkyl diaryl phosphate esters have better overall performance and also have better safety performance. For example, diphenyl isooctyl phosphate has been approved by the US FDA for use in food flexible packaging materials.
Nitrogen-based flame retardants, such as melamine and melamine-cyanurate (MCA), are often used in combination with phosphorus-based flame retardants.
Inorganic flame retardant
Hydroxides: such as aluminium hydroxide, magnesium hydroxide, etc. Aluminum hydroxide is a white fine crystalline powder, containing 34.4% of crystalline water. It dehydrates above 200℃ and can absorb a large amount of heat. In addition, when ATH is added to plastics, the water vapor and white smoke it releases during combustion dilute the black smoke produced by the burning of polymers, playing a masking role. Therefore, it also has the effect of reducing smoke and toxic gases. Magnesium hydroxide has slightly inferior flame retardant performance than aluminium hydroxide. When added in large quantities to plastics, it will affect the mechanical strength. After surface treatment with a coupling agent, its bonding force with the resin can be improved, making it have the dual functions of flame retardancy and filling.
Other inorganic compounds, such as magnesium oxide, are regarded as ideal candidates for halogen-free flame retardants due to their non-toxicity, harmlessness, low cost and good thermal stability. Its main mechanism of action is that when it is heated and decomposed, it absorbs a large amount of heat, reducing the surface temperature of the material. At the same time, the non-flammable gases such as water vapor and carbon dioxide released can dilute the concentration of flammable gases, thereby suppressing the spread of flames. In addition, magnesium oxide can also promote the formation of a carbon layer. This dense carbonized layer can effectively isolate the contact between oxygen and combustible substances, further enhancing the flame retardant effect.
Intumescent flame retardants (IFR) : Flame retardants that use phosphorus and nitrogen as flame-retardant elements and generally do not contain halogens, thus there is no need to use antimony synergists. When polymers containing this type of flame retardant are subjected to strong heat or combustion, a uniform porous carbonaceous foam layer can be formed on the surface. This foam layer has the functions of heat insulation, oxygen isolation, smoke suppression and preventing the formation of molten droplets, thus having excellent flame retardant and smoke suppression functions. Intumescent flame retardants are composed of three parts: acid source (dehydrating agent), carbon source (carbon-forming agent) and gas source (foaming agent).
Mechanism of action
The mechanism of the isolation protective film: At high temperatures, some flame retardants can form an isolation film on the surface of PVC, isolating air, thereby preventing heat transfer, reducing the release of flammable gases and isolating oxygen, thus achieving the purpose of flame retardancy. For instance, boric acid and hydrated borates decompose into non-volatile and non-oxidizing glassy films at combustion temperatures, which cover the surface of materials. These films can isolate air (or oxygen), reflect heat away or reduce thermal conductivity, thereby achieving a flame-retardant effect.
The mechanism of non-flammable gases: Flame retardants can immediately decompose into non-flammable gases at moderate temperatures, dilute flammable gases, and prevent combustion from occurring.
Cooling mechanism: Flame retardants decompose vigorously at high temperatures, absorbing a large amount of heat energy, lowering the ambient temperature and thereby preventing the combustion from continuing. Such as aluminium hydroxide and magnesium hydroxide, when they are heated and decomposed, they release crystalline water, absorbing a large amount of heat. The water vapor in them reduces the concentration of flammable gases and isolates the material from the air. The heat-resistant metal oxides Al₂O₃ and MgO generated simultaneously will also catalyze the thermal oxygen cross-linking reaction of the polymer, forming a carbonized film on the surface of the polymer, weakening the heat and mass transfer effects during the material’s combustion. Thus, it not only plays a role in preventing combustion but also serves to eliminate smoke.
The mechanism of terminating the chain reaction: The decomposition products of flame retardants are prone to react with active free radicals, reducing the concentration of certain free radicals and preventing the chain reaction that plays a key role in combustion from proceeding smoothly.
Application effect
Improving flame retardant performance: The addition of halogen-free flame retardants can significantly enhance the flame retardant performance of PVC materials and reduce the risk of fire. For example, when trimer O, O-2-hydroxypropyl phosphonitrile is used for flame retardancy of polyvinyl chloride, when the addition amount is 4.88%, the limiting oxygen index reaches 28%, and it has a good flame retardant effect.
Environmental protection and safety: Halogen-free flame retardants do not contain halogen elements. When burning, they do not produce toxic and harmful hydrogen halides and dioxins, etc., making them safer for the environment and human health.
Reducing smoke and toxic gases: Some halogen-free flame retardants can reduce the generation of smoke and toxic gases during combustion, improve visibility in case of fire, and facilitate personnel evacuation and rescue operations. For instance, when aluminium hydroxide is added to plastics, the white smoke of water vapor released during combustion dilutes the black smoke produced by the combustion of polymers, playing a masking role.
