The use of a waste material as flame retardant for polypropylene (PP), i.e. flue gas desulfurization (FGD) gypsum, has been studied. FGD gypsum has been used alone or in combination with melamine cyanurate (MC) and a phosphate ester (SolDP), which acts also as polymer plasticizer. Several formulations have been investigated in order to obtain UL94-V2 rated PP at 3.2 mm thickness.The results have demonstrated that 20 wt% of FGD is the optimal amount to be used together with 15 wt% MC and 3 wt% SolDP to get UL94-V2 PP material. The flame retardancy action of MC and SolDP can be, indeed, further strengthen by the presence of FGD filler, which is essential to guarantee the good fire reaction obtained. The results have also shown that the control of the dripping phenomena is essential and therefore a suitable plasticizer, acting also as flame retardant, shall be used in this system. The content of the plasticizer shall be optimized since, from one side, it favours dripping but, on the other side, it tends to decrease the heat stability of the polymeric matrix when used at high content (>5%). Finally, a synergy between this phosphate ester plasticizer and MC has also been revealed.
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