Halloysite nanotubes loaded with liquid organophosphate for enhanced flame retardancy and mechanical properties of polyamide 6

Aiming at achieving polyamide 6 with enhanced flame retardancy and maintained mechanical properties, halloysite nanotubes (HNT) and bisphenol-A bis(diphenyl phosphate) (BDP) flame retardant were introduced to prepare PA6/HNT/BDP flame retardant nanocomposites. Efficient loading of BDP inside the HNT lumen was demonstrated. Qualitative and quantitative characterization of BDP-loaded HNT revealed that BDP was mainly encapsulated inside the central lumen of halloysite and also located in the small pores on the outer surface of HNT. The use of only BDP (up to 4 wt%) or HNT (up to 10 wt%) cannot improve the UL-94 of the PA6 to reach V-0 classification. By combining use of HNT and BDP, the nanocomposites achieved V-0 rating with significant suppression in peak heat release rate and total heat release from cone calorimeter. The negative effect of BDP on strength and rigidity of PA6 composites was obviously minimized by encapsulating some portions of BDP in the nanotube. As a result, the tensile strength and thermal stability of the PA6 retained without loss of stiffness. During burning process, the BDP in the bulk polymer performed its function in the early period of the process, whereas the BDP inside the nanotube possessed a prolonged release for flame retardation at the later stage of combustion.