Flame retarded polymer systems based on the sol-gel approach: recent advances and future perspectives

The ease of flammability of polymers represents a key issue that limits their applications in different advanced sectors. In this context, a reliable and effective solution regards the use of flame retardants, i.e., additives that are able to slow down (or even stop) the flame propagation and to enhance the resistance to an irradiative heat flux. Among the different flame retardants designed, synthesized, and applied up-to-now, the utilization of inorganic particles, inorganic and hybrid organic-inorganic coatings has gathered a great interest from either the academic and industrial community, as these structures can provide remarkable flame retardant features to different polymer systems, in which they are embedded or applied onto. In particular, the in situ generation (through sol-gel processes, i.e. hydrolysis and condensation reactions from tailored alkoxide precursors) of ceramic phases, either in the form of particles or as surface coatings, has clearly demonstrated its effectiveness in creating a physical barrier that limits the degradation of the polymer when subjected to the application of a flame or an irradiative heat flux. It also lowers the heat and mass transfer from the degrading polymer to the surroundings and vice versa, hence providing an overall enhancement of heat and fire resistance. This review work seeks to provide an up-to-date overview of the most recent advances in the use of sol-gel methods for conferring flame retardant features to bulk polymers, cellulosic textiles (cotton), and polymer foams. In addition, the current limitations and the potential progresses of these approaches are discussed.