Two in One: Recycled Cobalt Aluminate as a Pigment and Synergistic Flame-Retardant Agent for Polylactide

Cobalt aluminate (CoAl₂O₄) pigment, synthesized from recycled aluminum obtained from can seals and transformed into the boehmite phase, was combined with ammonium polyphosphate (APP422) to produce an efficient flame-retardant material for polylactide (PLA) while simultaneously imparting coloration to the polymer matrix. The chemical structure of the pigment was investigated using X-ray photoelectron spectroscopy and X-ray diffraction prior to its integration into PLA in combination with ammonium polyphosphate (APP422). Thermal gravimetric analysis highlights the superior effect of the APP422/CoAl₂O₄ combination that enables obtaining a greater amount of char, presenting improved thermal stability and an enhanced protective effect, as clearly evidenced by Mass Loss Cone test results. A reduction of 70% in peak heat release was observed when APP422 was combined with CoAl₂O₄, in contrast to a reduction of 31% when only APP422 was used at a similar incorporation level. The enhanced flame-retardant properties of the combined APP422 and CoAl₂O₄ additives can be attributed to a rapid formation of a homogeneous char layer at the surface of the burning material when both additives are used together. This results from the interaction between Co³⁺ and APP422, which leads to the formation of the thermally stable Co₃(PO₄)₂ phase.