Impact of calcium, zinc or aluminum dimethylphosphate coordination polymers on flame retardancy and rheological properties of poly(butylene succinate)

Abstract

In this paper the impact of three metal diorganophosphates (MtDOPs) coordination polymers: calcium bis(dimethyl phosphate) (CaDMP), zinc bis(dimethyl phosphate) (ZnDMP) and aluminum tris(dimethyl phosphate) (AlDMP) on the flame retardancy and rheological properties of poly(butylene succinate) (PBS) is discussed in detail. The incorporation of 20 wt% of MtDOPs into PBS improved self-extinguishing properties, since all investigated composites achieved a V-2 rating in the UL-94 test. In the limited oxygen index (LOI) test the best effect was demonstrated by CaDMP, which enhanced the LOI value from 23.6 vol% for neat PBS to 27.3 vol% for PBS/20%CaDMP. We also found that AlDMP effectively suppressed melt-dripping in UL-94 and LOI tests due to a strong increase in complex viscosity and the existence of a relative high apparent yield stress for the PBS/20%AlDMP composite, which we observed in the rheological tests. In the mass loss calorimeter (MLC) test, MtDOPs reduced peak heat release rate (pHRR), total heat evolved (THE) and enhanced char yield (CY). The best result is offered by ZnDMP, in which case pHRR and THE decreased by 57 and 24%, respectively. Based on MLC test results, solid residue characterization as well as evolved gas analysis by means of thermogravimetry coupled with FTIR spectroscopy, the possible flame retardancy mechanism of MtDOPs in PBS was proposed. MtDOPs improve the fire response of PBS acting in the gas phase by liberating trimethyl phosphate (TMP) and in the condensed phase by increasing CY. Formation of a foamed and compact char acting as a barrier for heat and mass transport was also observed during combustion of a ZnDMP-containing composite.