There are many flame retardants for plastic pellets that are available these days. An old example “ which I knew when I was student” was antimony trioxide with brominated or chlorinated compounds. The presence of halogen atoms is what effects its retardation action. In the literature & websites , you will find several organic halogen compounds suggested & were used for this purpose.

For reasons that cannot be discussed here, there has been a trend to switch into inorganic flame retardants . Those which showed good performance are the hydrated minerals. The most common inorganic flame retardants are the hydroxides of aluminum & magnesium. Al(OH)3 –made from alumina – is the most widely used flame retardant. Mg(OH)2 is quite good for plastics which require high processing temperatures.

I am adding this MSc thesis about flame retardants in the file below.