Phosphorus intercalation of halloysite nanotubes for enhanced fire properties of polyamide 6

The fire performance of polyamide 6 (PA6) blended with phenyl phosphonic acid functionalized halloysite nanotubes (PPA-HNTs) is evaluated in order to determine if there is any fire performance gains over the unmodified composite. In the continuing search for safe halogen-free fire retardants for polymers, and the desire to more fully capture the advantages of nano-materials, this research seeks to utilize any synergy which might result in improved fire performance of PA6, via the known advantages offered by nano particles and phosphorus-based compounds. HNTs offer the ability for functionalization by exchange at their hydroxyl sites, while PPA can readily attach at these sites because of its acidity. In this context, the nanotubes act as a carrier for the acid within the polyamide polymeric matrix because it is readily intercalated and functionalized with the acid. The intercalated nano particles are added to the polyamide via melt extrusion and then analyzed using thermal analysis and cone calorimetry. The key finding from this work is that functionalization of HNTs with PPA, followed by blending into PA6 results in a significant improvement in fire performance by altering the combustion dynamics of the polymer. This work proposes a new synergistic fire retardant system for polyamides utilising relatively safe and inexpensive components. More broadly, the concept of using a carrier such as HNTs, which is so easily functionalized, can be applied to other thermoplastic materials, and other additives, for improvements in material properties other than fire.