Stabilisation of red phosphorus to prevent moisture absorption and suppression of phosphine release

Red phosphorus (RP) is an essential ingredient to generate smoke for multi-spectral obscuration and is receiving wide attention throughout the world for military applications. However, oxidation of phosphorus occurs in moist air resulting in the formation of various phosphorus acids and subsequently the evolution of the toxic gas, phosphine (PH₃), even in the sealed systems. Entrapped moisture leads to deterioration of the stores. The commercial-RP at ambient temperature and 95 per cent RH conditions indicates 15 per cent moisture absorption and 13 per cent acidity development with the generation of more than 100 ppm of phosphine gas. Efforts were made to overcome this problem by suppressing acid formation. The technology adopted was: (i) use of stabiliser, and (ii) doping stabiliser-coated particles with polymeric substance. The aim to use metal oxide as stabiliser was to neutralise the phosphorus acids, which are catalysing the phosphine generation. MgO due to its basic nature seems to be faster than amphoteric Al₂O₃, in removing the acid from site of reaction, thus suppressing the phosphine release effectively. Though MgCO₃ as stabiliser suppresses moisture absorption and acidity development, phosphine release is not controlled effectively. Study of RP coated with these materials independently and monitoring them at different RH conditions indicated superiority of MgO in suppressing acidity and phosphine formation.