1,2,3-Triazole mediated, non-halogenated phosphorus containing protective coatings from castor oil: Flame retardant and anti-corrosion applications

Abstract

Halogenated flame retardants are under regulatory scrutiny despite their outstanding flame retardancy. The present study focuses on the development of non-halogenated flame retardant and corrosion-resistant polyurethane coatings from castor oil. Towards this attempt, a non-halogenated, hydroxy functionalized, triazole and phosphorus-containing reactive flame retardant (HTP-FR) with anti-corrosive properties was designed and synthesized by azide-alkyne cycloaddition of 2-azidoethanol, and Diphenyl prop-2-yn-1-yl phosphate. By adding HTP-FR to castor oil in 50 and 100 weight percentages, CO-HTP-PUs (CO-HTP-50, CO-HTP-100, where HTP-FR was loaded as 50 and 100 wt%, respectively) were made and their flame-retardant and corrosion resistance properties were tested. The parent castor oil and MDI-based polyurethane (CO-PU) was concurrently synthesized and investigated as control for comparative appraisal. The FR behaviour of the PUs was examined using bomb-calorimetric testing and lab-scale flame tests. From the lab-scale flame test, it was found that the intumescent char length of the CO-HTP-100 after burning is longer than that of the CO-HTP-50 due to its high HTP-FR content. A substantial decrease in gross heat of 8.23, and 13.17 % was observed for the CO-HTP-50, and CO-HTP-100, respectively compared to the control CO-PU. The intumescent FR mechanism of CO-HTP-PUs was studied using Thermogravimetric Analysis coupled with Mass Spectroscopy (TGA-MS) analysis and Energy-dispersive X-ray spectroscopy (EDX) elemental analysis with mapping. The increase in P content of the post-burn sample in comparison to their unburn counterparts, and the expulsion of nitrogen gas from the triazole moiety from CO-HTP-PUs corroborated the intumescent FR mechanism. High char yields at 700 °C of CO-HTP-100 (20.3 wt%) and CO-HTP-50 (15.5 wt%) when compared to control CO-PU (3.6 wt%) also substantiated the FR behaviour of CO-HTP-PUs. The performance of these protective coatings against corrosion was investigated by the electrochemical methods. The significantly low corrosion current (Icorr) values (2.60 × 10−9 A/cm2), high polarization resistance (42.7 × 104 Ω cm2), bulk resistance values (10.4 × 106 Ω cm2) and low corrosion rate (3.0 × 10−5 mm/year) of CO-HTP-100 demonstrate the excellent corrosion resistance property of HTP-FR.