Benefits of radical generators in flame retarded polymers: Review and new developments
Radical generators as synergists in flame retarded polymers have been used in combination with brominated flame retardants for decades. However, due to the low thermal stability of radical generators under the usual polymer processing conditions the use was limited to selected applications e.g. in polystyrene foams. The need to find efficient halogen free flame retardants resulted inter alia in the discovery and commercialization of hindered amine light stabilizers based on alkoxyamines (NOR-HALS). NOR-HALS provide flame retardancy of polypropylene and polyolefin fibers, non-wovens and films. The performance of the NOR molecules depends on their structure i.e. the capability to degrade into nitroxyl plus alkyl or aminyl plus alkoxy radicals. Through formation of radicals a fast degradation of the polymer chain is induced and flame retardancy is achieved by removing the substrate from the flame. On the other hand, the formed radicals are involved in the free radical chemical reactions during the combustion process. Furthermore, alkoxy amines can interact with brominated flame retardants and facilitate the release of bromine, consequently increasing the overall FR performance. Therefore, it is possible with NORs to design flame retardant polyolefin molding compositions with lower levels of halogenated flame retardants and, in addition, to eliminate antimony trioxide. Moreover the hindered amine (HALS) structure provides light and long-term thermal stability of flame retarded formulations. Following the success of NOR based flame retardants a number of alternative radical generator molecules have been identified namely azo compounds, triazenes, hydrazones and azines. Especially combinations of Azo and NOR structures in one molecule show increased performance at loadings as low as 0.5 % in polypropylene films, including light and long-term thermal stability, and act synergistically with halogen, phosphorus and inorganic flame retardants. Although the performance of alkoxyamines in thin sections such as films is well documented and the UL 94 V-2 classification is often obtained in moldings, formulations usually fail in achieving the UL 94 V-0 classification as burning drips cannot be avoided despite short burning times. For the first time a new class of nitrogen based radical generators can be presented whereas in combination with selected phosphorus derivatives flame retarded polypropylene with UL 94 V-0 classification is accessible. Thermal stability and degradation of the new class into radicals is correlated to the molecular structure and can be adjusted in line with the degradation of the polymer and the synergist. Moreover the necessary loadings to achieve UL 94 V-0 are below 10 %.