Effect of the Curative on the Thermal Decomposition Behavior and Kinetics of Polyester-Based Polyurethanes

Abstract

In this work, three different types of polyester-based polyurethanes (PUs) were synthesized from a hydroxyl-terminated polyester and three different curatives, namely methylenediphenyl diisocyanate (MDI), isophorone diisocyanate (IPDI), and 2,2,4-trimethyl hexamethylene diisocyanate (TMDI) as curing agents, by a polymerization reaction and these PUs are designated by PU/MDI, PU/IPDI, and PU/TMDI, respectively. The formation of urethane linkages in the PUs was confirmed by our Fourier Transform Infrared (FTIR) study. Chemical stability and thermal stability based on gas evolved and thermal decomposition by heat flow were studied by means of a vacuum stability tester (VST) and thermogravimetric analysis (TGA) techniques, respectively. The VST measurements were carried out at 100 °C for 40 h. The results revealed that the volume of gases evolved by these PUs was less than 2.0 mL/g according to Standardization Agreement (STANAG) 4147, indicating that they are chemically stable. The TGA method results indicated that the thermal stability of the polyester-based PU followed the order PU/TMDI > PU/IPDI ≃ PU/MDI. The thermal decomposition kinetics was studied through the TGA method at various heating rates, 2, 5, 10, and 20 °C/min, under nitrogen atmosphere by using the Kissinger–Akahira–Sunose (KAS) and Starink methods. The activation energy values calculated by the isoconversional KAS method for the thermal degradation of PU/MDI, PU/IPDI, and PU/TMDI were 226.5, 206.8, and 183.6 kJ/mol, respectively, with extents of conversion being 0.55–0.95. The activation energies obtained from the KAS method were similar to those obtained from the Starink method. The results indicated that there was significant variation in the thermal stability and activation energy with varying the above-mentioned three curing agents in the polyester-based PUs.