Improvement of the flame retardancy of polyamide 6 by the incorporation of UiO-66 and UiO-66/melamine

• UiO-66 is synthesized assisted by the addition of a template following a solvent-free method. • Using melamine as template promotes the formation of crystalline phase of hcp UiO-66. • The flame retardancy of PA6 is improved obtaining a V-2 classification with the addition of only 4 wt% of flame retardant. • Melamine/UiO-66 composite achieves very low extinguishing times in UL-94 test. The flame retardancy of polyamide 6 (PA6) has been investigated through the incorporation of the metal–organic framework (MOF) UiO-66 and a composite of UiO-66/melamine during the extrusion process. The prepared materials have been characterised by various techniques: XRD, XPS, FTIR, TGA, DSC, SEM, EDX and mechanical properties. Moreover, to examine the efficiency of the flame retardants, the composites have been tested using the UL-94 vertical and, horizontal burning test, as well as the limiting oxygen index (LOI). The UL-94 V-2 standard has been achieved with only a 4 wt% load of either UiO-66 or UiO-66/melamine, compared to pure polyamide, which is not classified. The best results have been achieved with the UiO-66/melamine sample at 7 wt%, which prevents the agglomeration of UiO-66 alone. The highest LOI value was also obtained with this sample, achieving a value of 35.5 %, compared to pure PA6 and with UiO-66, which reached values of 22.4 % and 23.2 %, respectively. A flame suppression mechanism of the UiO-66/melamine composite is proposed, based on the formation of zirconium oxide, gas adsorption by the MOF and release of NH 3 .