Flame retardancy in isotactic polypropylene modified with crystalline coordination polymers based on calcium, zinc or aluminum dimethyl phosphates

A novel variety of metal organophosphates (MOPs) coordination polymers: calcium bis(dimethyl phosphate) (CaDMP), zinc bis(dimethyl phosphate) (ZnDMP) and aluminum tris(dimethyl phosphate) was examined for the first time to act as potential flame retardants in isotactic polypropylene (iPP) materials and their flame-retarding activity compared with widely used commercial flame retarder ammonium polyphosphate (APP). All iPP composites containing 20 wt.% of MOPs as with APP achieve V-2 in the UL-94 test and LOI values even higher (25.5 vol.% for iPP/20%CaDMP) than determined for APP-containing material accompanied by maintaining or slightly degrading mechanical properties. Also in mass loss calorimeter test (MLC) the MOPs demonstrated a positive effect on the fire response of the obtained composites leading to a reduced, in the case of ZnDMP, the peak of heat release rate (pHRR), total heat evolved (THE), average effective heat of combustion (av-EHC), fire growth rate (FIGRA) by 57%, 35%, 29%, 63% respectively when compared with pure iPP. Moreover the detailed analysis of the solid and liquid pyrolysis products of MOPs was done via FTIR. The FTIR spectra and PRXD studies of the char residues after combustion indicated the formation of a solid, inorganic condensed phosphates and release of trimethyl phosphate (TMP) into the gas phase. Finally, the flame retardancy mechanism of MOPs was proposed based on results of MLC test and of the pyrolysis experiment. The gas phase activity of all three MOPs was confirmed by the decrease of av-EHC most probably due to release of trimethyl phosphate (TMP) into flame, whereas, in particular ZnDMP acted strongly in the condensed phase by forming a intumescent, protective layer.