Flame Retardant Modification of Plastics

June 13, 2018

 

 1. Grade of flame-retardant

 

Flame retardancy is a process that prevents or suppresses the rate of physical / chemical change during combustion of polymer materials. The flame-retardant grade of plastics was gradually increased from HBV-2V-1 to V-0, and the flame-retardant grade higher than V-0 was 5V / 5VBV / 5VA.

 

The higher the flame retardant grade, the better?

 

In general, people think so, and hope that the flame retardant level reaches the highest, but the actual situation is not the case. First, the existing flame retardant testing methods have limitations, and the conclusion is relative. Furthermore, in some application conditions, the flame retardant grade of low grade plastics is better. For example, in some electrical products, the material is required. It is resistant to electric arc igniting, when the use of V-2 grade plastics will be better than the V-0 grade plastic. This is because the V-2 grade plastic does not form electrical conductivity under electric action and coking, thus reducing the possibility of ignition, and V-0 grade plastics may have ignition possibility.

 

2. Classification of flame retardants

 

Flame retardant can be divided into two categories: additive type and reactive type. The former is only physically dispersed in the substrate and is mostly applied to thermoplastic polymers, while the latter is mainly used as monomer or auxiliary reagent to participate in the synthesis of polymers, and finally becomes the structural unit of polymers, and is mostly applied to thermosetting polymers.

 

The commonly used flame retardants are halogen, phosphorus, nitrogen and halogen-phosphorus, phosphorus-nitrogen, antimony, magnesia-aluminum, boron, silicon, molybdenum and so on, as well as intumescent flame retardants. They are mostly phosphorus-nitrogen compounds; and a nano-inorganic compound, mainly layered silicate.

 

3. Status of development of flame retardants

 

Organic halogen flame retardant

 

At present, organic halogen flame retardants are still one of the most important flame retardants. There are nearly 100 kinds of flame retardants, mainly bromine flame retardants and chlorine series flame retardants, among which there are more than 70 kinds of bromine flame retardants. Because bromine flame retardants have the advantages of high flame-retardant efficiency, low dosage, little influence on material properties, wide application fields and moderate price, three mostly used organic flame retardants, decabromodiphenyl ethane, etrabromobisphenol A and hexabromocyclodiane are rominated  flame retardants.

 

However, the environmental and safety problems of halogen flame retardants are becoming increasingly apparent. Tests carried out by Swiss and German scientists have shown that PBDPOs and their flame-retardant materials produce a toxic carcinogen, polybrominated dibenzo-dioxins (PBDDDDs) and polybrominated dibenzofurans (PBDFs) during pyrolysis and combustion, and, Flame retardant polymers with bromine flame retardants produce a large amount of soot and corrosive gases during hot cracking and combustion. At present, Apple, Samsung, Hewlett-Packard, Dell and other companies have promised to phase out brominated flame retardants, requiring strict or even strict regulation of the 10 Br < 900ppm. BrCl < 900ppmBrCl < 1500 ppm.

 

Halogen-free environmental flame retardant

 

At present, non-halogen environmental flame retardants have been widely studied and used, such as inorganic flame retardants, phosphorus flame retardants, nitrogen flame retardants, silicon flame retardants, intumescent flame retardants, nano-flame retardants and synergistic flame retardants.

 

At present, the industry is focusing on the development of new low-toxic, low-smoke, and non-polluting flame retardants and flame retardant materials. Although inorganic flame retardants have no environmental protection problems, because of their low flame-retardant efficiency and large amount of addition, they are easy to reduce the relationship between the mechanical properties of the materials themselves, and are often used in materials which do not require high mechanical strength properties. It is often necessary to add bromine flame retardants to improve the synergistic flame retardation efficiency, and its application field is limited. Organophosphorus flame retardants have the advantage of environmental protection and are one of the priority alternatives to organic halogen flame retardants. The synergistic effect of new halogen-free flame retardants and different flame retardants was studied, and a new type of watch was developed. Surface modifier and new surface modification technology make the flame retardant and polymer matrix have a suitable compatibility. Building a moderately flexible, strong binding interface structure, is the current development direction of flame retardants.

 

4. Flame-retardant principle

 

1) Endothermic action

 

The amount of heat emitted by any combustion in a shorter time is limited. If a portion of the heat released from the fire source can be absorbed in a shorter time, the flame temperature will decrease. Radiation to the combustion surface and the amount of heat acting on the pyrolysis of the gasified combustible molecules into free radicals will decrease and the combustion reaction will be inhibited to a certain extent.

 

At high temperature, the flame retardant has a strong endothermic reaction, which absorbs some of the heat released by combustion, reduces the surface temperature of the combustible material, and effectively inhibits the formation of combustible gas. The flame retardant mechanism of the flame retardant, Alo OH3, is to increase the heat capacity of the polymer, so that it can absorb more heat before reaching the thermal decomposition temperature, so as to improve the flame retardancy of the polymer. This kind of flame retardant gives full play to its heat absorption property when it binds to water vapor, and improves its own flame retardant ability.

 

2) Coverage

 

When the flame retardant is added to the combustible material, the flame retardant can form a glass-like or stable foam coating at high temperature, insulate the oxygen, insulate the oxygen and prevent the combustible gas from escaping out, thus achieving the purpose of flame retardation.

 

For example, organic phosphorous flame retardants can produce more stable cross-linked solid matter or carbonized layer when heated.

 

On the one hand, the formation of carbonation layer can prevent the further pyrolysis of polymers, on the other hand, it can prevent the thermal decomposition products from entering the gas phase to participate in the combustion process.

 

  1. Inhibition of chain reaction

 

According to the chain reaction theory of combustion, free radicals are needed to maintain combustion. The flame retardant can act on the gaseous combustion zone, capture the free radicals in the combustion reaction, so as to prevent the flame from propagating, decrease the flame density in the combustion zone, and finally make the combustion reaction speed decrease until the end of the combustion reaction.

 

For example, the evaporation temperature of halogen-containing flame retardant is the same or close to that of polymer decomposition. When the polymer is decomposed by heat, the flame retardant also volatilizes. When halogenated flame retardants and pyrolysis products are in the gaseous combustion zone at the same time, halogen can capture the free radicals in the combustion reaction, thus preventing the flame from spreading and decreasing the flame density in the combustion zone. Finally, the combustion reaction rate decreases until the end.

 

4) Asphyxiation of gas body

 

When the flame retardant is heated, it decomposes the non-gas body and dilute the concentration of the combustible gas to the lower limit of combustion. At the same time, it can dilute the oxygen concentration in the combustion zone, prevent the combustion from going on, and achieve the function of flame retardation.

 

PVC plastics are non-flammable plastics, but they are not nonflammable. When the fire is severe, PVC will burn and cut out toxic gases. Therefore, the environment with special requirements requires the use of flame retardant PVC formula system products.

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