Why is adipic acid classified as a Class B substance

Why is adipic acid classified as a Class B substance? It is mainly classified as a Class B substance based on its fire hazard classification. According to the "Code for Fire Protection Design of Buildings", Class B substances refer to liquids with a flash point greater than or equal to 28 ℃ but less than 60 ℃, gases with a lower explosive limit greater than or equal to 10%, oxidants that do not belong to Class A, flammable solids that do not belong to Class A, combustion supporting gases, floating dust and fibers that can form explosive mixtures with air. Although adipic acid is a solid at room temperature, it has a certain degree of flammability and may cause dangerous situations such as fires under specific conditions, so it is classified as a Class B substance.
From a chemical perspective, adipic acid has a high flash point, generally around 196 ℃. However, it decomposes under heating and other conditions to produce flammable vapors. When mixed with air, it can form an explosive mixture within a certain concentration range, which meets the partial determination criteria for flammability of Class B substances.
Adipic acid undergoes complex oxidation reactions during combustion, with a combustion heat of approximately 2795.5 kJ/mol. This high combustion heat indicates that it will release a large amount of energy once burned, which may lead to the spread and expansion of fires. Based on this, the fire hazard is classified as Class B. According to relevant experimental data, adipic acid has relatively poor thermal stability in high temperature environments. When the temperature reaches 200 ℃ or above, it gradually undergoes decomposition reactions, producing some flammable small molecule compounds that increase the possibility of fire. Therefore, it is classified as a Class B substance. During the production, storage, and transportation of adipic acid, improper handling such as exposure to open flames, high temperatures, or strong oxidants can easily lead to combustion accidents. According to relevant industry statistics, most of the small fires caused by adipic acid are due to illegal operations on site, which is also one of the actual reasons for its classification as a Class B substance. From the perspective of dust explosion, adipic acid generates dust during processes such as crushing and screening. The lower explosive limit of the dust is about 30g/m ³. When the concentration of adipic acid dust in the air reaches this value, it may explode when exposed to a source of fire, which is consistent with the characteristics of Class B substances regarding floating dust that can form explosive mixtures with air.
The oxidation reactivity of adipic acid also affects its classification to a certain extent. In some specific oxidative environments, it can react with oxidants and the reaction process may release heat, which poses a potential danger and classifies it as a Class B substance. From the perspective of fire engineering, there are specific fire safety requirements for the storage and handling of adipic acid. Because it belongs to Class B substances, the storage site needs to be equipped with corresponding fire-fighting facilities, such as fire extinguishers, fire hydrants, etc., to cope with possible fire accidents. Research has shown that the combustion rate of adipic acid is relatively moderate, with a flame propagation speed of approximately a few centimeters to ten centimeters per second under certain conditions. This combustion characteristic is neither as rapid and intense as that of Class A substances, nor has a certain degree of danger, so it is classified as Class B. Adipic acid increases the risk of combustion when mixed with certain metal powders. For example, when mixed with aluminum powder, a violent chemical reaction may occur under specific conditions, and the hazardous characteristics of this mixture are also a consideration for its classification as a Class B substance.
In industrial production, the production process of adipic acid involves multiple stages, each of which carries a certain risk of fire. For example, in esterification reactions and other processes, improper control of temperature, pressure, etc. may cause the combustion or explosion of adipic acid. Based on the hazard assessment of the entire production process, it is classified as a Class B substance.