Novel polymeric acrylate-based flame retardants (FR 1-4) containing two phosphorus groups in different chemical environments were synthesized in three steps and characterized via nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS). Polylactic acid (PLA) formulations with the synthesized compounds were investigated to evaluate the efficiency of these flame retardants and their mode of action by using TGA, UL94, and cone calorimetry. In order to compare the results a flame retardant polyester containing only one phosphorus group (ItaP) was also investigated in PLA regarding its flame inhibiting effect. Since the fire behavior depends not only on the mode of action of the flame retardants but also strongly on physical phenomena like melt dripping, the flame retardants were also incorporated into PLA with higher viscosity. In the UL94 vertical burning test setup, 10% of the novel flame retardants (FR 1-4) is sufficient to reach a V-0 rating in both PLA types, while a loading of 15% of ItaP is not enough to reach the same classification. Despite their different structure, TGA and cone calorimetry results confirmed a gas phase mechanism mainly responsible for the highly efficient flame retardancy for all compounds. Finally, cone calorimetry tests of the flame retardant PLA with two heat fluxes showed different flame inhibiting efficiencies for different fire scenarios.
Keywords: acrylate-based flame retardant; phosphorus-based flame retardant; polylactic acid.