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Polycarbonate (PC), as one of the five major engineering plastics, is widely used in fields such as electronics, lighting, building materials, automotive parts, food packaging, and medical devices due to its advantages of high strength, high transparency, high impact resistance, and heat resistance. It is expected that the demand for PC in China will reach approximately 2.4 million tons in 2022.
The glass transition temperature of PC is 140-150°C, and the heat distortion temperature is 135°C, with a usage range from -60°C to 120°C, exhibiting good thermal stability and dimensional stability. Although the flame retardancy of PC is better than that of PE and PP, with a Limiting Oxygen Index (LOI) of 21%-24% and a V-2 level flame retardancy, it still cannot meet the higher flame retardancy requirements in special fields, necessitating improvement through the addition of flame retardants. The main types include halogenated flame retardants, silicon-based flame retardants, boron-based flame retardants, sulfonate flame retardants, phosphorus-based flame retardants, and others such as magnesium hydroxide, aluminum hydroxide, and carbon nanotubes.
Halogenated Flame Retardants
Halogenated flame retardants mainly refer to flame retardants containing Br or Cl elements, with brominated flame retardants being the first to be industrially produced. Adding halogenated flame retardants to PC enhances the plasticizing effect of the material, improving its flow properties, and during combustion and melting, some heat can be carried away; at the same time, PC produces a small amount of hydrogen halide gas during combustion, which shields the entry of oxygen; when PC undergoes pyrolysis, it generates H· and HO·, which combine with hydrogen halide gas, blocking further reactions of free radicals with oxygen.
While halogenated flame retardants have good flame retardancy, they produce a large amount of smoke that is toxic, and their usage has been decreasing year by year due to increasing environmental protection requirements.
Silicon-based Flame Retardants
Silicon-based flame retardants can be divided into organic silicon and inorganic silicon based on structural differences. They are environmentally friendly and can effectively provide flame retardancy and smoke suppression with minimal impact on mechanical properties. The flame retardant mechanism is as follows:
(1) At high temperatures, siloxanes migrate to the surface of PC and accumulate, effectively protecting the substrate while preventing combustible gas and oxygen from contacting, thus inhibiting the spread of combustion.
(2) Siloxanes can accelerate the char formation process of PC, and the branched structure of siloxanes helps prevent zipper-like depolymerization, making them applicable in PC products designed to prevent dripping.
Silicon-based flame retardants must be compounded to achieve synergistic flame retardancy, and after combustion, they migrate to form a protective layer, suitable for products designed to prevent dripping.
Boron-based Flame Retardants
Boron-based flame retardants are characterized by good heat resistance, low toxicity, and smoke suppression. Common inorganic boron flame retardants include borax, boric acid, zinc borate, calcium borate, ammonium pentaborate, ammonium borate, sodium borate, ammonium fluoroborate, and zinc fluoroborate; organic boron flame retardants mainly include tri(2,3-dibromopropyl) phosphate and polysiloxane borate.
Boron-based flame retardants achieve flame retardancy through the following four mechanisms:
(1) The flame retardant melts during combustion, absorbing some heat;
(2) The flame retardant covers the polymer surface, isolating combustibles from oxygen;
(3) The flame retardant releases bound water at high temperatures, cooling the material as the moisture evaporates;
(4) The thermal decomposition pathway of combustibles changes, reducing the generation of flammable gases. When used in conjunction with nitrogen or halogen-containing flame retardants, the resulting non-flammable gases NH3 and HX can dilute the concentration of oxygen.
Boron-based flame retardants are typically used in combination with other flame retardants to enhance their cooling and oxygen-isolating effects.
Sulfonate Flame Retardants
Sulfonate flame retardants decompose under combustion conditions to produce a small amount of alkyl salts, promoting rearrangement isomerization reactions in PC, generating a large amount of CO2 and H2O, which are non-combustible, thus reducing the concentration of flammable gases; they can also act as catalysts for the ester exchange reaction of PC, leading to cross-linking reactions that enhance flame retardancy. Due to their low addition levels, sulfonate flame retardants can significantly improve flame retardancy while maintaining the mechanical strength and other properties of the PC substrate.
Sulfonate flame retardants exhibit good self-extinguishing properties and have minimal impact on transparency, but they can cause dripping ignition, necessitating the use of other flame retardants that prevent dripping or have good char-forming properties.
Phosphorus-based Flame Retardants
Phosphorus-based flame retardants are halogen-free, environmentally friendly flame retardants that use phosphorus as the flame-retardant element. They can be classified into inorganic phosphorus flame retardants and organic phosphorus flame retardants. Organic phosphorus flame retardants mainly include phosphonitrile, aryl phosphates, phosphoric esters, phosphonates, organic phosphonium salts, and phosphorus-nitrogen compounds, while inorganic phosphorus flame retardants mainly include red phosphorus, ammonium polyphosphate, and hypophosphites. Most phosphorus-based flame retardants exert their flame-retardant effects primarily in the condensed phase, although some can also act in both the gas and solid phases.
Phosphorus-based flame retardants are environmentally friendly, exhibit good char formation and self-extinguishing properties, but they can lower the glass transition temperature of PC by 20-40°C, are not resistant to hydrolysis, and have poor compatibility.
Source: Internet
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