The way to achieve flame retardant

Gas phase flame retardant mechanism: that is, to inhibit the free nature of the role of chain growth in the combustion reaction, and the flame retardant effect.

Condensed phase flame retardant mechanism: it is in the solid phase to prevent the thermal decomposition of the polymer and prevent the polymer from releasing the role of flammable gas.

Interruption of heat exchange mechanism: the heat generated by the polymer is taken away and not fed back to the polymer, so that the polymer will not continue to decompose.

Classification of flame retardants

According to the element type, it is divided into halogen, organophosphorus and halogen-phosphorus, nitrogen, silicon, aluminum-magnesium, molybdenum, etc. According to the flame retardant effect, there are expansion type flame retardants, carbon flame retardants, etc. According to the chemical structure, there are inorganic flame retardants, organic flame retardants, polymer flame retardants, etc. According to the relationship between flame retardants and flame retardant materials can be divided into additive flame retardants and reactive flame retardants, reactive flame retardants involved in the chemical reaction of polymers.

1、Halogen flame retardants

Halogen flame retardants are one of the largest production of organic flame retardants in the world. Halogen flame retardants are mainly used in the electronics and construction industries, and about 50 to 100 kinds of halogen-containing flame retardants cover most of the market demand.

The main reason why halogen flame retardants are valued by people is that halogen flame retardants have high flame retardant efficiency and moderate price, and their performance-to-price ratio is difficult to compare with other flame retardants, plus the variety of halogen flame retardants and a wide range of applications, so they are favored by people. However, halogen flame retardants in the thermal cracking or combustion to generate more smoke and corrosive gases and to be plagued by the problem of dioxins.

More than 80% of halogenated flame retardants are used in electronic/office equipment and construction industry, and the main application varieties are styrenics and their copolymers, thermoplastic engineering plastics and epoxy resins.

2. Phosphorus-containing flame retardants

Phosphorus-containing compounds can be used as flame retardants for thermoplastics, thermosets, fabrics, paper, coatings and adhesives. Such flame retardants include red phosphorus, water-soluble inorganic phosphates, insoluble ammonium polyphosphate, organic phosphate esters and phosphonates, phosphine oxide, hydrogen bomb hydrocarbon-based phosphate esters and brominated aromatic phosphate esters.

Although halogen flame retardants have high flame retardancy, but there are problems such as environmental pollution and toxicity, while phosphorus-based flame retardants are very effective flame retardants in addition to several types of polymers such as styrene and polyolefins, and the secondary pollution is small, so the flame retardants have received attention.

Ammonium phosphate class

Monoammonium phosphate (MAP) and diammonium phosphate (DAP) are still effective flame retardants for fibers and fabrics, nonwovens, paper, wood and many other cellulose. They can form phosphoric acid, which makes cellulose hydroxyl esterification, and the resulting cellulose decomposes into carbon, changing the course of thermal degradation, thus achieving the purpose of flame retardant. They are easily soluble in water, so their flame retardancy can not last. It is currently one of the main components of intumescent flame retardants.

Red phosphorus

Red phosphorus is an extremely effective flame retardant for oxygenated polymers such as PC, PET, PBT, PPE, and is used more often in Europe as a flame retardant for nylon parts. Because red phosphorus reacts with water in the atmosphere to form toxic phosphine, industrial products need to be stabilized and coated.

Trihydroxyphosphine (phosphine) esters

Trihydroxyphosphine (phosphine) ester is used as a diluent when unsaturated polyesters are highly filled (e.g. aluminum hydride, calcium carbonate). It is also used as a co-effector in halogenated polyesters and is less effective than antimony oxide, but has good processability. The less volatile trihydroxy phosphate esters are tributyl phosphate, trioctyl phosphate and phosphate ester of tributyl oxyethyl. Methyl dimethyl phosphate contains up to 25% phosphorus, is an extremely effective flame retardant. Its high volatility limits the application. Suitable for polyurethane rigid foam, highly filled thermosetting resins. Also used as a viscosity diluent in highly filled thermoset resins. Ethyl diethyl phosphate is more stable under the condition that urethane has blowing agent and amine catalyst.

Aryl phosphate esters

Aryl phosphate esters are mainly used in industry as non-combustible plasticizers for PVC and cellulose acetate and as non-halogen flame retardants for engineering plastics such as PPE, PC/ABS alloys.

Alkylated phosphate triphenyl esters

Aryl phosphate esters are liquid, the main alkylation products are made from synthetic isopropyl propanol or isobutylphenol. The method of preparation is to make phenol alkylation, and then react with phosphoryl chloride. Therefore, alkylated triaryl phosphate esters are a mixture. Among them, tributyl phenyl phosphate has better antioxidant properties, but less plasticity. Methyltriphenyl phosphate is the most effective flame retardant for PVC. However, it is also the most volatile and is used in Europe. Alkyl diaryl phosphonate has better plasticity and is used in PVC with better low temperature, it also has less smoke generation than triaryl phosphonate.

2-Ethylhexyl diphenyl phosphate

2-Ethylhexyl diphenyl phosphate has been approved by the U.S. Food and Drug Administration for use in food packaging. Isodecyl diphenyl phosphate is used in PVC cables, especially high-voltage cables, because it has a long hydrocarbon group that is not volatile.

Dimer aryl phosphates are less volatile and have linking groups of resorcinol, hydroquinone or bisphenol. They are used in thermoplastics (e.g. PPE, PC/ABS).

Modified polyphenylene ethers (PPE)

Modified polyphenylene ether (PPE) is a blend of PPE and high-impact polystyrene (PS-HI). Depending on the grade, it generally contains 55% to 65% PS-HI. Although phosphate ester does not enable PS-HI to achieve UL 94 flame-retardant rating, it is used in industry to flame retard PPE resin and achieve UL 95 V-0 rating. The phosphoric acid generated by the phosphate ester makes PPE carbonized for flame retardant purposes. The flame retardants used in industry are liquid alkylated triaryl phosphonates or dienes.

Triaryl phosphonates

Triaryl phosphonates volatilize when processed, causing stress fractures. The use of less volatile diphosphonates reduces stress fractures. Bisphenol A can be a linking group for diphosphonoquinone.

Halogen-containing phosphate esters

Halogenated alkyl phosphate esters are mainly used in polyurethane harder foam products. However, the thermal stability is poor. But because of the halogen-containing phosphate ester due to the toxicity of the base, has been banned in most countries such as the EU, China is also not recommended.

Phosphorus oxides

Phosphorus oxides with hydrolysis stable P-C bond, they contain more P than aryl phosphate esters, and are therefore more effective flame retardants. Their diols and triols are active flame retardants for polyurethane and epoxy resins.

Phosphorus compounds as flame retardants can inhibit free radical reactions in the condensed phase, and they themselves can also generate glassy substances until the barrier effect of heat and mass transfer. Organophosphorus flame retardants are considered to be one of the most promising species to replace halogenated flame retardants.

3、Inorganic hydroxide

Inorganic hydroxide is a very important flame retardant. Inorganic hydroxides are easy to handle, relatively non-toxic, do not produce toxic, corrosive gases, and smoke suppression, and more importantly, cheaper than halogen and phosphorus flame retardant systems. This type of flame retardant using the appropriate formulation can make the material meet a variety of test requirements. Inorganic aluminum hydroxide is the most sold flame retardant of inorganic hydroxide and is mainly used for synthetic rubber, thermosetting resins and thermoplastics with processing temperature below 200℃. The application of inorganic hydroxide flame retardants has continued to grow considering the environmental concerns of halogen-containing and phosphorus-containing flame retardants.

Aluminum hydroxide (ATH)

Aluminum hydroxide is one of the most widely used flame retardants. It is a non-toxic, white to light white powder with a relative density, Mohs hardness of. When the temperature is heated above 320°C, aluminum hydroxide loses its weight due to water loss.

For polymers processed at temperatures lower than the decomposition temperature of aluminum hydroxide (190~230℃), aluminum hydroxide is an excellent flame retardant material, and it is worth noting that the size of particle diameter has an effect on thermal stability.

Aluminum hydroxide is used as a flame retardant material in elastomers, thermosetting resins and thermoplastics, etc. It is also used in large quantities in the production of styrene-butadiene latex for flame retardant carpets, in the production of flame retardant insulated rubber cables, insulation foam, conveyor belts and hoses, etc. It can be used in all unsaturated resins, such as laminated roofs and walls, lavatory appliances, decorative wall panels, various covers, automobile shields, truck parts, etc., and electronic components including insulators and circuit boards. It also includes building construction tools, etc.

Aluminum hydroxide is used in epoxy and phenolic resins, including adhesives, laminated parts, circuit boards, imitation marble and ceramic appliances, etc.

Aluminum hydroxide is used in cross-linked acrylic resins for flame retardant and decorative purposes, such as roofs, sinks, lavatory panels, decorative materials and wall panels.

In thermoplastics, aluminum hydroxide is receiving increasing attention due to the environmental impact of halides, especially in Europe. The scope of its application is also increasing. In soft and hard PVC, ethylene propylene rubber, EPDM, EVAC, ethylene – ethyl acrylate copolymer, PE-LD, PE-HD, blends of PE and PP, plastics made using metallocene catalyst technology, etc. In addition, it is commonly used in wires and cables, conduits, pipes, adhesives, laminates for construction, and insulation foam.

Aluminum hydroxide can provide formulations with low raw smoke. This formulation is less toxic and less corrosive in terms of smoke generation than formulations containing a mixture of halides and antimony oxide. Depending on the polymer and the requirements, the mass fraction of aluminum hydroxide ranges from 5% to 75%, and in non-halogenated systems, it is generally 35% to 65%. In this application range sometimes increases the viscosity of the mixture and has a negative impact on the physical properties of the resin. The use of suitable additives and reasonable mixing techniques can disperse the aluminum hydroxide sufficiently and greatly reduce the above effects. Aluminum hydroxide with proper surface treatment can also reduce the effects due to high filling rate, which may increase the cost of the material.

Magnesium hydroxide

Magnesium hydroxide is the second most sold inorganic hydroxide flame retardant. It is a white to light white crystalline powder with relative density, Mohs hardness. Weight loss (mass fraction) due to loss of water when heated to above 450°C.

When magnesium hydroxide as a flame retardant, its purity requirements are quite high, at least the magnesium hydroxide contained, many grades higher than. Most flame retardant grades are white powders with particle diameters ranging from μm. The surface area is 7~15 sqm/g. Most of the magnesium hydroxide used as flame retardant is surface treated to improve its dispersion and distribution in the polymer. Magnesium hydroxide is used at the same high addition level as aluminum hydroxide, typically 50% to 70%. The high purity requirements and surface treatment requirements make magnesium hydroxide more expensive than precipitated grade aluminum hydroxide.

Magnesium hydroxide is used for processing thermoplastics and thermosetting resins with temperature from 200~225℃. It is mainly used for EVAC, PP and blends, ABS and its blends, fluoropolymers, PPE and blends, polyimide, etc. Can not be used for thermoplastic polyester. Magnesium hydroxide is used together with aluminum hydroxide to meet different usage requirements. Wires and cables, racks, construction of multi-layered panels, pipes, electrical appliances and other parts will also use magnesium hydroxide.

4、Silicon compound

Silicon compound is a new flame retardant. It can be completely independent of halogen and phosphorus compounds and play a flame retardant role. Recent articles and patents on silicon flame retardants have become the new hot spot. All kinds of composition of silicon is used as a flame retardant research. Silicon-containing compounds, whether as additives to polymers or in blends with polymers, have significant flame retardant properties.

The practical silicon containing compounds flame retardant techniques are:

1) The introduction of silicon atoms or silicon groups in polymers through grafting reactions;
2) Addition of silicone resin powder;
3) Addition of high molecular weight silicone oil with organometallic compounds and silica;
4) silicone rubber with metal compounds;
5) polymer/clay nanocomposites;
6) Addition of primary silicate;
7) silica gel in combination with potassium carbonate;
8) low melting point glass containing silicon.

Silicon containing flame retardant and its flame retardant technology is now widely studied, the polymer containing silicon flame retardant is large how much smoke non-toxic, low calorific value of combustion, slow flame propagation, and thus is valued, its development potential and application prospects are very huge and broad.