Flame Retardants FR and Smoke Suppressants SS

PVC-U formulations have low flammability due to the chlorine content. The addition of plasticiser in PVC-P formulations necessitates the use of flame retardant and smoke suppressant additives. These additives are known as functional fillers and a correct balance is necessary to achieve all the end-use specification requirements. They are predominately used in cable, conveyer belting and roofing membrane formulations to give resistance to fire initiation and propagation. It is also important to reduce dripping in a fire situation and that as little smoke as possible is generated. Antimony trioxide has been used extensively, usually in combination with phosphate ester plasticisers, giving excellent fire performance and mechanical properties. 

The most recent work has involved reducing smoke emissions, a previous weakness of FR PVC formulations. A review of this area covering additives and testing is available (78). 

Aluminium trihydrate (ATH) and magnesium hydroxide (MDH) have been shown to create low smoke levels and much reduced toxic gas emission (245,390). The activity of these materials is a function of the vaporisation of their water of hydration. 

Zinc borate is also an effective SS in combination with phosphate ester plasticiser (130). The combination of ammonium octamolybdate and zinc stannate is also beneficial (435). It has been shown that ATH and MDH functional fillers coated with zinc hydroxystannate give significantly increased combustion resistance and lower levels of smoke evolution (386). Copper (I) complexes have been suggested as smoke suppressants (241). Copper oxides have also been investigated with positive results (60). 

Ethylene copolymers (high molecular weight flexibilisers) have been proposed to replace liquid plasticiser for low smoke FR PVC cable formulations (238). 

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