Flame Retardants

Flame retardants prevent or reduce the flammability and combustibility of plastics by intervention in the combustion mechanism, either physically by cooling as well as diluting or chemically by reaction in the gaseous phase (radical interceptor) or in the solid phase (formation of a protecting carbon layer). Flame retardants usually act according to several of these principles. They cannot prevent the burning of plastics, but they reduce the flammability or cause self-extinguishing. A unit of measurement for the flammability is the oxygen index (LOI – limited oxygen index – boundary oxygen concentration). Polyolefins such as polyethylene or polypropylene maintain a once started combustion automatically, whereas polycarbonate and especially polymers with a higher halogen content like PVC, PTFE etc. are self-extinguishing. In order to achieve a flame resistant behaviour polymers are equipped with additives which have to be compatible with the other properties of the plastic material. Because of the partially high additive dosages of 3 – 60 % the mechanical properties of the plastics are completely changed. The diverse range of flame retardants used in plastics are classified in groups: 1.) Aluminium- and magnesium oxidhydrates split-off water in the heat and cool the surface below the ignition temperature. 2.) Special anorganic solids such as zinc borate, ammonium phosphate yield a stop function opposite the flame. In the heat ammonium polyphosphate becomes meta phosphoric acid with charring effect. On the surface a flame retarding thin, thermally insulating barrier film is built-up which denies atmospheric oxygen the entry to the subsoil. Similar properties have mixtures of urea, dicyandiamide, melamine and organic phosphates. The substances which promote the charring process and form an insulating layer, so-called intumescent substances which expand in a foamy way char at 250 – 300 °C, solidify then and form a fine-pored, well insulated melt cushion. Mixtures of ammonium polyphosphates, melamine and dipentaerythrit are suitable for this. 3.) The most important flame retardants are organic chlorine- and bromine compounds whereby the latters are more effective. Chlorinated paraffin, hexa bromine benzole, brominated diphenyl ethane, tetra bromine bisphenol A, tetrabromophthalic anhydride a.o. compounds act as radical interceptors. Together with synergists such as antimony trioxide halogen atoms are released at higher temperatures, which stop the combustion of the maintaining radicals of the chain reactions and retard the combustion process in the gaseous phase. 4.) Halogenated organic phosphorus compounds such as Tri (2,3–dibrompropyl) – phosphate or Tris – (2-brom-4-methylphenyl)-phosphate. In view of the increasing use of plastics the fire resistance of plastic products is getting more and more important. The fire behaviour is classified by the German DIN Norm 4102/ B 1 or B 2, the American UL 94 list V0 or V 2, the French M-classes or according to special tests such as glowing wire test at different temperatures. At present the European Commission is trying to grade the international norms by means of a matrix. The use of flame retardants is not always harmless; in case of fire the reduction of a fire risk faces the possibility of environmental damages. Toxic phosphates might arise. Halogenated flame retardants can charge the atmosphere during combustion and form persistent residues like PCB or chlorine biphenyl a.o. During combustion processes in presence of chlorine compounds the formation of highly toxic dioxines cannot be excluded. Therefore the development of flame retardants which are less dangerous to the environment is pushed forward intensively. Nevertheless the protection of people, especially the gain in time for fleeing in order to save life has to be placed above any discussions about environmental problems. The composition of our masterbatches depends on the special requirements of the end products. The selection of the active ingredients depends on the polymeric system, the effectiveness regarding the burning norms and on the acceptable changes in properties of the plastics.

Halogen-free flame retardants

In order to fulfil burning norms some standard plastics like polyamides and polyester only need small quantities of halogen-free flame retardants so that the polymeric properties are maintained to a great extent. In order to fulfil burning norms with polyolefins with halogen-free flame retardants, there normally have to be added large quantities of active substances which have a very negative influence on the mechanical properties. Only for thin polyolefin final applications there are halogen-free systems which maintain the properties of the plastics. In the meantime there is also an option to use low-halogenated additives for thick-walled polyolefin articles.