Sol-gel derived architectures for enhancing cotton flame retardancy: effect of pure and phosphorus-doped silica phases

Pure and phosphorus-doped silica phases derived from sol–gel processes have been investigated, aiming to assess any enhancement of the thermal and fire stability of cotton fabrics. Indeed, it has already been shown that the combustion behaviour of cellulosic substrates can be strongly affected by the presence of a protective phosphorus-rich silica coating. Thus, in the present work, the performances of coatings consisting of pure silica and phosphorus-doped silica have been investigated and compared by using thermogravimetry and combustion tests (more specifically, resistance to an irradiative heat flux and to a flame application). The results show that the thermal and thermo-oxidative stability of cotton, as well its resistance to an irradiative heat flux of 35 kW/m2, have been enhanced by the deposited coating, irrespective of the presence of phosphorus. However, the best results have been achieved exploiting the joint effect of thermal shielding (exerted by the silica phase) and char-forming (as a consequence of the phosphoric acid source present in the alkoxysilane precursor). On the other hand, only pure silica coatings, despite their very low add-on, have proven to protect cotton from the application of a methane flame for 5 s, favouring the formation of a thermally stable residue.