Magnesium hydroxide derivatives as stabilisers and flame retardants for plasticised poly (vinyl chloride)

Abstract

The potential of magnesium hydroxide, hydromagnesite and layered double hydroxides (LDHs) as heat stabilisers and flame retardants for plasticised poly (vinylchloride) (PVC) was studied. These inorganic hydrated fillers feature flake-shaped particles with a strong tendency to agglomerate. Filler particles must be homogeneously distributed and individually dispersed in the polymer matrix in order to attain the best performance. For this reason the first step in the investigation was to explore the use of a stearic acid coating in order to improve the dispersability of these fillers in liquids. The platelet morphology-type flame retardants were coated with approximately a monolayer of stearic acid using a solvent technique. Compared to the uncoated powders, the BET surface area was lower, the powder packing density was improved, and the thickening effect on white oil was significantly reduced. The latter two observations were rationalised in terms of a reduction in the attractive interactions between the powder particles. The viscosity of white oil slurries containing 25 wt.% solids showed shear-thinning non-Newtonian behaviour. The coated powders showed significantly lower viscosities at low shear rates although the difference diminished at high shear rates. The lower viscosities shown by the coated powders indicate that the surface modification facilitated the break-up of agglomerates and aided the dispersion of individual particles in the fluid. The thermal decomposition of these hydrated fillers is central to their flame retardant action. At elevated temperatures they endothermically release inert gases. The latter dilute the atmosphere surrounding the burning sample while the endothermic decomposition cools the substrate. These two effects are responsible for the flame retardant action of these fillers. The detailed behaviour of the present samples was studied using thermogravimetric analysis and spectroscopic methods. The decomposition mechanisms, proposed in the literature for these flame retardants, were confirmed. This includes the mass loss, enthalpy of decomposition, and the nature of evolved gases for temperatures up to 1 000 °C. The magnesium hydroxide decomposed endothermically at temperatures well above 250 °C releasing only steam. The LDH decomposed between 225 ºC and 450 ºC and the hydromagnesite between about 220 °C and 500 °C. Both initially released water vapour followed by carbon dioxide. Next the utility of the magnesium hydroxide, hydromagnesite and LDH as combination heat stabilisers and flame retardants for plasticised PVC was studied. Emulsion grade PVC was plasticised with 100 parts per hundred parts of resin (phr) diisononyl phthalate (DINP) and filled with 30 parts per hundred parts of resin (phr) filler additive. Thermomat static heat stabilities were determined at 200 °C by following the time dependence of hydrogen chloride evolution. Fire retardancy was studied using a cone calorimeter at a radiant flux of 35 kW m-2. The layered double hydroxide outperformed the other fillers with regard to improving heat stabilisation and also with respect to most fire retardancy indices. Since the layered double hydroxide performed best it was decided to see whether slight composition variations could improve performance. Derivatives of the standard LDH compound ([Mg0.667Al0.333(OH)2](CO3)0.167·0.44H2O) were synthesised using a hydrothermal method. Again, emulsion grade PVC was plasticised with 100 phr diisononyl phthalate and stabilised with 30 phr of the LDH filler additive derivatives. The heat stability and fire resistance of these compounds were studied. Heat stabilities were determined at 200 °C. The dynamic heat stability tests were performed on the plastisols using the torque rheometer method. Static heat stability was evaluated on the fused compounds. It was evaluated from discoloration profiles of strips exposed for various lengths of time to high heat in a Metrastat oven. The time dependence of hydrogen chloride evolution was followed with a Metrohm Thermomat instrument. The conventional LDH provided the best dynamic heat stability. However, partial replacement of the magnesium with copper significantly delayed the release of volatile HCl. If instead the replacement was done using zinc, better colour retention was achieved. The fire performance was determined at a radiant flux of 35 kWm?2 in a cone calorimeter. The conventional magnesium-aluminium LDH lowered the peak heat release rate of the plasticised PVC from 623 ± 8 kW m?2 to 389 ± 9 kW m?2 and reduced the smoke release by 37 %. Partial replacement of the aluminium with iron resulted in a red pigmented additive that was more effective as a flame retardant reducing the peak heat release rate (pHRR) to as little as 253 ± 5 kW m?2. This additive also showed better smoke suppression (reduction of 44 %) but the best smoke suppression was achieved by replacing part of the magnesium with copper reduction by 49 %).