1. The combustion and flame retardant combustion of rubber are natural phenomena in the objective world. For example, lightning strikes can cause forest fires. Combustion is also an artificial phenomenon required in human life or production. For combustion to occur and proceed, three conditions are required, none of which are indispensable. Any substance at a certain temperature can ignite only after the ambient temperature reaches the ignition point. The burning point of different characteristics is different, and the difficulty of burning is also different, so there are flame-retardant and flammable points. Oxygen is a combustion accelerant and is an indispensable factor to ensure that the combustion continues. Combustible substances are the bodies that can be burned, generally hydrocarbons, biological materials (such as grass, wood) and synthetic polymer materials (such as rubber, plastics and fibers, etc.).
1.1 The combustion of rubber The combustion of rubber is similar to the combustion of other materials, but it also has special features. As a polymer material, the combustion process of rubber is more complicated, and its combustion temperature is also higher than that of ordinary substances. Even if the fire is ignited, the temperature should reach above 3160. After rubber catches fire, its combustion process can usually be divided into three stages.
(1) After thermal decomposition reaches the burning point (different rubber types have different burning points, such as NR is 6200C ~ 6700C), it first begins to soften and melt, and decompose into low molecular compounds. No open flame is visible at this stage, which can be regarded as a prelude to burning.
(2) The thermal decomposition product of combustion reacts violently with oxygen in the atmosphere to form a flame, which marks the official start of combustion. With the release of light and heat, new low-molecular-weight combustibles (such as CO), non-combustibles (such as CO2) and smoke are generated.
(3) Continue to burn This stage can be continued until all combustibles are burnt out. Most rubber types go through these three stages, but it is possible for halogenated rubbers to go through only the second stage, because the hydrogen halide produced in the combustion acts as an inhibitor.
1.2 The grade of rubber burning is usually distinguished by the difficulty of burning, which can be divided according to the oxygen index (see Table 1). Table 1 Oxygen index flame retardant grade of rubber The oxygen index range of the rubber is not flame retardant <20 Combustible rubber species (such as natural rubber), without flame retardants, generally flame retardant >20<30 If the commonly used rubber types are measured by the oxygen index (OI), the order of combustion (from easy to difficult) is shown in Table 2. Table 2 Oxygen index of commonly used rubber types EPDMBR, INRRSBR, NBRCSM, CHRCR1718～192021～2227～3038～41 Natural rubber SBR- styrene-butadiene rubber NBR- nitrile rubber EPDM- EPDM rubber It can be seen from Table 2 that the oxygen index of halogen-containing rubber is higher than that of halogen-free rubber; the oxygen index of pendant groups in halogen-free rubber is higher than that of non-halogenated rubber Halogenated rubber. The basis of rubber burning is a hydrocarbon-based structure (all kinds of rubber are combustible materials), only the difficulty of burning is different. The only exception to the flame-resistant properties of halogen-free rubber is silicone rubber. Its main chain structure is composed of silicon and oxygen atoms, which has a certain value in flame retardancy, but its physical properties are poor, so its application range is narrow. Its own flame retardancy is not the only way for rubber to obtain flame retardant effect. If flame retardant is added to non-flame retardant rubber, it can also be used for general flame retardant. Of course, adding a flame retardant to the flame retardant rubber can further improve the flame retardant grade, just like the icing on the cake.
1.3 Selection of flame retardant rubber type Halogen-containing rubber is generally the first choice, and its flame retardancy is beyond reproach, but the combustion is the hydrogen halide gas produced, which is corrosive and toxic, and its products are only suitable for applications in open spaces; Places with limited space, such as vehicles or underground facilities, are less safe. The higher the halogen content of the raw rubber, the higher the oxygen index, but the safety increases. Although halogen-free rubber is not flame retardant, it can still meet the flame retardant requirements after adding a certain amount of flame retardant. Some resins may also improve the flame retardancy of rubber when it is blended with rubber due to the halogen in its structure (eg PVC).
1.4 The single and combined use of rubber The single and combined use of the main material in flame retardant rubber is very common.
1.4.1 There are examples of single-use focus halogen rubber, especially CR. Halogen-free rubber is not flame retardant by itself, but it can be compensated by adding flame retardants. In recent years, the use of halogen-containing rubber has been prohibited in some occasions. For example, the United States prohibits the use of halogen-containing rubber as the main material on single-use facilities, aircraft, and ships. The halogenated hydrogen generated in the flame retardant acts as a suppressor and adds a large amount of halogen-free resistors. fuel.
2. Flame retardant Flame retardant is a type of special rubber additives, suitable for all rubber products that require flame retardant. In addition to being flame retardant, some flame retardants also have the functions of plasticization and filling.
2.1 Mechanism of action of flame retardants Flame retardants can play one or more of the following roles. The thermal decomposition and oxidation reactions during cooling and endothermic combustion will lead to a large amount of heat generation, and the heat provides conditions for continued combustion, while the effect of flame retardants is the opposite. For example, some flame retardants generate water when they act, and water will absorb the surrounding heat during the vaporization process after being heated. For example, aluminum hydroxide is a typical representative. Two molecules of aluminum hydroxide can release 3 molecules of water, and its mass is equivalent to 36.4% of aluminum hydroxide. Blocking the oxygen source Some flame retardants will decompose incombustible gases N2, CO2, etc. during combustion. These gases surround the combustion material, block the oxygen source, and inhibit the spread of the fire. Another example is the phosphate ester flame retardant, which generates phosphoric acid or metaphosphoric acid in case of fire, and coats a hard and transparent protective layer on the rubber surface. HCI or HB decomposed by antimony trioxide and halogen-containing flame retardants generate SbCI3 or SbBr3, which is very important and deposited on the rubber surface to form a flame retardant barrier. Inhibiting the flammability of rubber The decomposition products of some flame retardants will make the rubber lose its flammability, such as halogen free radicals released by oxidized paraffin.
Source: Suzhou Dongtuo Chemical Co., Ltd.
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