The family of halogen-free flame retardants includes inorganic flame retardants, halogen-free expansion flame retardants, organic phosphorus flame retardants, organic silicon flame retardants, and nitrogen flame retardants.
1, Inorganic flame retardant
Inorganic flame retardants have the advantages of good stability, low toxicity or non toxicity, non-volatile and not easily precipitated from plastics during storage, abundant raw material sources, low price, and dual functions of flame retardancy and filling; And environmentally friendly, it is a very promising flame retardant. Inorganic flame retardants mainly include Sb2O3, Al (OH) 3, Mg (OH) 2, inorganic phosphorus series, etc.
1. Antimony trioxide
Antimony trioxide (Sb2O3) is the most widely used type of inorganic flame retardant. Due to its poor effectiveness when used alone, it is often used in combination with organic halides to achieve synergistic effects, known as synergists. It has excellent flame retardant effect and can be widely used in plastics such as PVC, polyolefin, and polyester. But it has a stimulating effect on the nose, eyes, and throat. Inhalation into the body can irritate the respiratory organs, and contact with the skin can cause dermatitis. Caution should be taken when using it.
2. Aluminum hydroxide
Aluminum hydroxide Al (OH) 3 is commonly referred to as hydrated alumina. It is a white fine crystalline powder containing 34.4% crystal water, dehydrated at temperatures above 200 ℃, and can absorb a large amount of heat. In addition, when aluminum hydroxide is added to plastics, the white smoke released during combustion dilutes the black smoke produced by polymer combustion, providing a masking effect and reducing smoke and toxic gases.
3. Magnesium hydroxide
Magnesium hydroxide Mg (OH) 2 has slightly inferior flame retardant properties compared to aluminum hydroxide, and its addition in plastics in large quantities can affect the mechanical strength. After surface treatment with coupling agents, its adhesion with resin can be improved, making it have both flame retardant and filling functions. Commonly used in EP, PF, UP, ABS, PVC, PE, etc.
4. Inorganic phosphorus flame retardants
Inorganic phosphorus flame retardants mainly refer to red phosphorus. It is a high-performance flame retardant with high efficiency, smoke suppression, low toxicity and other flame retardant effects. It has been used as a flame retardant for more than 20 years and is highly valued. It can be widely used in polymer materials such as PA, PP, PE, PET, ABS, rubber, low smoke halogen-free cable materials, etc.
5. Other inorganic flame retardants
Other inorganic flame retardants include flame retardant enhancers, flame retardant smoke suppressants, and some less commonly used flame retardants, mainly including molybdenum compounds, borates, layered silicates, tin compounds (zinc stannate and zinc hydroxystannate), etc. Molybdenum compounds are the best smoke suppressants discovered so far. Usually molybdenum trioxide and ammonium molybdate are used. The United States has developed a series of ammonium free molybdate smoke suppressants that can withstand processing temperatures above 200 ℃.
2, Halogen-free expanding flame retardant
Zero halogen flame retardant (IFR) is a halogen-free flame retardant mainly composed of phosphorus and nitrogen. It has the advantages of high flame retardancy, no melting droplets, excellent resistance to prolonged or repeated exposure to flames, halogen-free, antimony oxide free, low smoke, non-toxic, and non corrosive gas generation.
IFR mainly consists of three parts: the first part is the acid source, also known as a dehydrating agent or carbonization promoter. Usually inorganic acids or inorganic acid compounds, such as phosphoric acid, sulfuric acid, boric acid, ammonium phosphate salts, phosphate esters, and ammonium polyphosphate (APP), can react with resins to promote the formation of carbides; The second part is charcoal source, also known as charcoal agent, mainly composed of some polyhydroxy compounds or carbohydrates with high carbon content, such as starch, pentaerythritol (PER) and its dimers and trimers, etc; The third part is the gas source, also known as the foaming source, which can release inert gases such as nitrogen-containing compounds such as urea, melamine (MEL), melamine, APP, etc.
Due to the hygroscopicity, high dosage, and poor compatibility with plastics of ordinary flame retardants, it is necessary to modify them to improve their compatibility with plastics or to share them with substances with synergistic effects to enhance flame retardancy.
3, Organophosphorus flame retardants
Among various flame retardants, phosphorus based flame retardants, especially organic phosphorus based flame retardants, occupy an important position. The research on organic phosphorus flame retardants began at the beginning of this century. Most organic phosphorus flame retardants have the advantages of low smoke, non-toxic, low halogen, and halogen-free, which are in line with the development direction of flame retardants and have good development prospects. Organic phosphorus flame retardants include phosphate esters, hypophosphite esters, phosphonates, organic phosphorus salts, and oxidized phosphorus, as well as phosphorus heterocyclic compounds and polymer phosphate (phosphonate) esters, but the most widely used are phosphate esters and phosphonates.
Due to the problems of high volatility, poor thermal stability, and toxicity of phosphorus halogen flame retardants in previously industrialized organic phosphorus flame retardants, a large number of new organic phosphorus flame retardants have been developed domestically and internationally in recent years, and some have already been industrialized. In addition, flame retardants using nitrogen and silicon as flame retardant elements have also received widespread attention due to their high efficiency, low smoke, low toxicity, and green environmental protection advantages. Therefore, in the molecular design of organic phosphorus flame retardants, phosphorus nitrogen flame retardants or phosphorus silicon flame retardants obtained by introducing nitrogen or silicon elements can often exert a synergistic flame retardant effect of both.
BDP and RDP are new halogen-free and environmentally friendly organic phosphorus flame retardants developed in recent years, and their structural formulas are shown in the above figure. Compared with traditional organic phosphorus flame retardants, they have the characteristics of high molecular weight, high thermal stability, low volatility, and high flame retardant efficiency. BDP is slightly superior to RDP in thermal stability and hydrolytic stability. As an additive flame retardant, BDP is mainly used in thermoplastic engineering plastics such as PC/ABS blends, polyethylene and foam polyurethane, showing excellent flame retardancy.
4, Organic silicon flame retardant
Organic silicon flame retardant is a new type of high-efficiency, low toxicity, anti melting and anti dripping, environmentally friendly halogen-free flame retardant, and also a char type smoke suppressant. Organic silicon flame retardants not only endow the substrate with excellent flame retardant properties, but also improve the processing performance, heat resistance, and other properties of the substrate. At present, the main types of organic silicon flame retardants are silicone resin flame retardants and polydimethylsiloxane flame retardants. The method of flame retardancy can be to directly add organic silicon flame retardants to plastics, or to embed some functional groups (such as hydroxyl, amino, or epoxy groups) of polysiloxane segments into some polymers.
5, Nitrogen based flame retardants
The research on nitrogen based flame retardants started relatively late, and currently there are not many varieties used, mainly additive type. Commonly used nitrogen based flame retardants include melamine, melamine cyanuric acid (MCA), etc. Nitrogen based flame retardants, as a new and efficient flame retardant, have been widely studied and valued both domestically and internationally in recent years. According to current literature reports, the use of nitrogen based flame retardants alone has poor flame retardant effects, but the flame retardant system formed by combining with phosphorus containing flame retardants has good flame retardant effects.
