From chemical point of view halons are organic carbon compounds in which hydrogen atoms were replaced totally or partially by atom halogens. Halons are composed of fluorine called freon.
Halons terminology is compatible with Geneva terms nevertheless it is quite complicated. In order to simplify halons terminology the corresponding labeling system was implemented and examples given below:
Hereby we get halon 1211.
Pure halon in the nominal temperature is chemically neutral that is why it does not show corrosive features or they are minor. Halons activity increases under the humidity influence.
Halons are colourless substances of chloroform’s smell. In normal conditions occur as gases or liquids. In an aeriform halons can be described, as quite heavy, it’s crucial when it comes to fire extinction. In view of its low temperature of coagulation halons can be used in the temperatures below zero. Also does not conduct current and does not lose its features in case of long time storage.
FIRE - EXTINGUISHER FEATURES
Halons fire-extinguishers features depend on its continence and structure in particular halogens assortment. Upon appropriate measurements the following was proved. Each particular halogens atom is connected with carbon atom by relatively weak bindings. At the same time was proved that numerical binding’s value vary and range from 0.16MJ /mol for banging’s between the carbon atom and the iodine atom, to 0.43 MJ/mol for banging’s between the carbon atom and fluorine atoms. That makes the iodine atoms and bromine coming off easier from fluorine and chlorine atoms. Accordingly, the iodine and bromine halons are the most effective in fire extinguishing whereas fluorine and chlorine halons are less effective. Dependence upon the energy bindings and efficiency of fire extinguishing is not unambiguous as halons fire-extinguishing mechanism is composed of many elements:
- halons fumes are heavier then air (implemented on fire isolates the air admission)
- catalytic acts in reference to combustion process
- inhibition acting
HALON & THE COMBUSTION PROCESS
Combustion is a chemical process concerning the reactions occurred in the gas phase with the essential participation of the oxygen coming from the air as an oxidizing factor. Gas and liquid’s steams combustion does not proceed as a result of direct fuel’s molecule and oxidant collision but indirectly. In case of hydrogen’s combustion in the oxygen, the combustion is not a result of a direct hydrogen and oxygen reaction but it is a result of many indirect reactions accompanied by hydrogen’s free radicals, oxygen and hydroxide. Radicals perform special part in the process – energetically unbalanced molecules with high chemical activity. General speed of combustion reaction can be high only when the concentration of free radicals is properly high. There from, the reduction of combustion speed can be achieved by the reduction of the free radicals or oxygen concentration. Although, the fast concentration of oxygen reduction is strongly limited, the free radicals limitation can be achieved by the use of combustion inhibitors. Such properties own i.e. halons. After bringing the halon 1301 (CF3Br) to the flame the fast reaction with free radicals follows as below:
CF3Br + H - CF3Br + HBr H + Br - H2 + Br Br + H + M - HBr + M
As stated above, in the schematic process the Holon’s molecular react on the free radical creating a hydrogen bromide which reacts on the next free radical creating not active molecular and bromine’s atom. Then, the bromine’s atom again reacts on the free radical creating a hydrogen bromide molecular and in this way the reaction chain is initiated and it leads to the no end reformation of radicals. As a result arisen the hydrogen bromide and becomes recombination of free radicals.
Talking about halons influence on people, the real conditions that should be taken into consideration, include:
- natural products of combustion generated during the fire,
- toxicity of not decomposed agent,
- products created as a result of thermal decomposition.
Toxicity of not decomposed (Halons) centre was defined by the Laboratories USA through the approximate danger concentration definition "LCT (O) 15 min." for most frequent applied halons. Based on this, halons were divided into toxicity groups (halons are in groups between 1-6), where the more higher group, the more harmful effect of the given concentration in a certain time is lower. For extinguish purposes the less toxic halons are used, they are in groups between 4-6 (accordingly to the UL USA). For instance, halon 1301 with concentration of 2-% does not cause any effects even after 2 hours (group 6). The most toxic halon is carbon’s tetrachloride which product resulting from the decomposition is compound particularly dangerous for humans COCl2phosgene). Based on experimental research the established safe value of volumetric concentration of halon 1301 is 10% and halon 1211 4 – 5%. (
Technical and usable requirements for halons were specified by adequate regional norms:
- series ISO 7201 (7201-1 and 7201-2) (European Union)
- ASTM D5632-01 (Type I) (1301) (USA)
- ASTM D5632-01 (Type II) (1301) (USA)
- MilSpec-B-38741 (halon 1211) (USA)
Technical requirements specify, in particular the physical and chemical features of each substance (molecular weight, boiling point, freezing point, critical pressure, specific weight, cleanness, toxicity, fire extinguishing concentration etc.). Except that, each norm describes proper way of handling each substance.