Enhancement of flame retardancy and mechanical properties of polyamide 6 by incorporating an aluminum salt of diisobutylphosphinic combined with organoclay

Polymers with highly effective flame retardancy and optimal mechanical properties remain difficult to achieve. In this study, polyamide 6(PA6)/clay nanocomposites with excellent flame retardancy and mechanical properties were prepared through melt compounding of PA6 and a novel salt of phosphinic acid, namely, aluminum diisobutylphosphinate (ABPA), combined with organically modified layered montmorillonite (OMMT). The dispersion of ABPA and OMMT in the flame retardant composites was characterized by X-ray diffraction, transmission electron microscopy, and viscoelastic measurements. The flame retardancy and thermal degradation behavior of PA6/ABPA and PA6/OMMT/ABPA composites were investigated by limiting oxygen index (LOI) assessment, UL94 testing, cone calorimetry, and thermogravimetric analyses. The morphologies and chemical compositions of the char residue were determined by scanning electron microscopy-energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy analyses. Results demonstrated that the ratio of ABPA and OMMT at a constant total flame retardant level of 12 wt% considerably influenced the thermal stability, flame retardancy, and mechanical properties of the resulting PA6 nanocomposites. When ABPA was introduced solely at a loading amount of 12 wt%, the specimens passed the UL-94 V-0 flammability rating, and the LOI reached 34.8%. When OMMT (ABPA: OMMT 5:1, 12 wt% in total) was incorporated into the ABPA/PA6 composites, the samples passed the UL-94 V-0 flammability rating, and the LOI reached 36.0%. The samples successfully passed the UL-94 V-0 flammability rating when incorporated with OMMT as high as 6 wt% (ABPA: OMMT 1:1, 12 wt% in total; considerable decrease in ABPA loading). The cone test results revealed that the introduction of OMMT efficiently reduced the fire behavior parameters such as heat release rate and total heat release but increased the mean effective heat combustion. Although the substitution of ABPA with OMMT reduced the gas activity of the flame retardants, the amount and thermal stability of the char layers were effectively enhanced. The digital photographs, morphological structures, and composition analysis of char residues verified that OMMT promoted the formation of sufficient, compact, and homogeneous char layer on the material surface during burning. This phenomenon led to increased strength of the char layer and improved flame retardancy of PA6 polymeric materials. Moreover, the flame retardant PA6 nanocomposites showed equal or even superior mechanical properties in terms of flexural strength and modulus than pure PA6.