Halogen mainly refers to the elements of main group VIIA in the periodic table, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). As astatine (At) is a synthetic radioactive element, it almost does not exist in nature, so the halogen usually refers to the first four items. The nature of halogen
Halogen is a lively non-metallic element. As shown in the figure below, the outermost layer of halogen has 7 electrons. The similarity of structure determines the similarity of chemical properties, resulting in carbon halide bond (CX) becoming a polar covalent bond. Polarization is induced by the electric field of the polar reagent, causing it to easily split and cause various chemical reactions.
These four elements generally exist in the form of organic compounds in polymer materials.
Organic halides have excellent flame retardancy, solubility and reactivity, and are widely used in many fields.
The principle of halogen flame-retardant is that when a halide burns, it will release halogenated gas on the surface to isolate the oxygen, thereby extinguishing the fire. Halogen flame retardants can be mainly divided into brominated flame retardants (BFR) and chlorine flame retardants (CFR).
Common brominated flame retardants such as tetrabromobisphenol A (TBBPA), polybrominated biphenyls (PBBs), polybrominated diphenyl ethers (PBDEs), etc. The main advantages are:
(1) The flame retardant efficiency is high, the addition amount is small, and the processing performance and physical and chemical properties of the material are small;
(2) Good dispersibility and good compatibility with materials;
(3) Excellent thermal stability and water insolubility.
Common chlorine-based flame retardants such as chlorinated paraffins and chlorinated alicyclic hydrocarbons are still widely used flame retardants due to their low prices.
PVC (Polyvinyl Chloride)
PVC is a synthetic plastic made from polyvinyl chloride resin and adding other additives. Unlike other plastic products with hydrocarbons as the main element, chlorine accounts for about 56.8% of PVC. The presence of chlorine makes PVC compatible with many other substances and helps delay the burning of plastics.
Harm of halogen
(1) Halogenated gases (fluorine, chlorine, bromine, and iodine) released during the burning of halides have a strong toxicity that affects the human respiratory system, and the resulting reduced visibility makes it impossible for personnel to identify the escape route;
(2) When this gas encounters water vapor, it will generate hydrogen halide, causing corrosion to some equipment and buildings;
(3) Dioxins will be produced when chlorinated plastics are incompletely burned;
(4) According to relevant studies, the destruction of the ozone layer and the appearance of ozone holes are mainly caused by people emitting large amounts of waste gas into the air. Such as CFCs (chlorofluorocarbons), Halon (perbromofluorocarbons), CCl4, methyl chloroform, bromopropane and so on. Once these substances enter the atmosphere, they are difficult to remove, causing long-term damage.
In recent years, international green environmental protection directives led by the European Union and numerous regulations and conventions have restricted the production and use of halides. At present, the industry's control requirements for halogens only apply to chlorine and its compounds, and bromine and its compounds, except for special customer requirements. For many halogens from different sources, most organizations adopt the IEC 61249-2-21:2003 standard developed by the International Electrotechnical Commission (IEC).
Halogen detection method
After a certain quality of sample is aerobic and airtightly burned in an oxygen bomb, the halogenated compounds in it can be effectively converted into fluoride, chloride, bromide, and iodide that can be absorbed or dissolved in the aqueous solution, and then quantitatively measured by ion chromatography. analysis.