Effect of calcium carbonate filler on the fire and smoke properties of moulded polypropylene

Abstract

Abstract‘Oxygen Index’, ‘Rate of Burning and Extent of Burning of Self‐supporting Plastics in a Horizontal Position’ and ‘Vertical Flammability’ tests were used for examination of polypropylene copolymer HW 607M and the same copolymer filled with 40% w/w calcium carbonate. Room‐scale studies were also carried out using a range of standard ignition sources. In the latter tests, measurements of the levels of smoke generated in the test chamber were monitored continuously, and intermittent measurements were made of the carbon monoxide concentrations. Small‐scale testing showed that addition of the filler to the polypropylene raised the limiting oxygen index of the polymer from 17.8 to 20.3, which is only marginally lower than the oxygen concentration in ambient air (20.9); this introduces the possibility of a filled polypropylene which is not ignitable in air. However, the results of the UL94 and ASTM D635 tests did not show significant differences in flammability and rates of vertical and horizontal flame spread as between the two materials, through the unfilled polymer produced extensive burning droplets early in both tests whilst the filled polymer did not. Such droplets can spread fires to floor coverings, so that the filled plastic might be preferable in application. In our laboratory tests, the filled samples gave considerably lower smoke generation than the unfilled ones. At high heat fluxes they generated considerably less smoke than (smaller) unfilled samples containing the same mass of polymer. This suggests that the calcium carbonate has specific smoke‐suppressant activity and is not merely acting as a polymer diluent. In larger‐scale tests, unfilled samples were readily ignited by a domestic match, whilst the filled samples required an ignition source of 43 kJ (some six times greater than the match source) in order to light them and sustain ignition. The filled polymer did not melt on ignition and there was no lateral spread of flame from the sample. Smoke generation was considerably lower, the average optical density at maximum obscuration being 0.27, compared with 0.84 for the unfilled control. In particular, smoke suppression achieved by use of calcium carbonate at higher heat‐flux levels is considerably greater than that expected for an inert diluent.