Abstract
In the present study, thermal degradation kinetics of polyurethane (PU) powder coatings reinforced with organo-modified montmorillonite (OMMT) was investigated. PU nanocomposites were prepared in different concentrations of 1, 3, and 5 wt.% of OMMT via the extrusion method. The microstructure of the nanocomposites was observed by scanning electron microscope (SEM) illustrating uniform dispersion of OMMT nano-clay platelets in the PU matrix except for the sample containing 5 wt.% nano-palates. Thermal degradation kinetics of the PU nanocomposite was investigated using thermogravimetric analysis (TGA) at different heating rates of 5, 10, and 20 °C/min. The results showed that the initial decomposition temperatures were shifted toward higher values (more than 40 °C for T5% and up to 20 °C for T10%) by introducing the nano-clay to the PU matrix. Friedman, Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and modified Coats-Redfern iso-conversional methods were applied to model the decomposition reaction and the activation energy of the nanocomposite powder coatings. Overall, the presence of nano-clay increased the activation energy of the PU degradation up to 45 kJ/mol, when compared to the blank PU, which suggests very high thermal stability of nanocomposites. The Sestak-Berggren approach proposed a good approximation for the reaction model, especially at low temperatures. Thus, PU decomposition was detected as an autocatalytic reaction, which was suppressed by the barrier effect of OMMT nano-palates intercalated with polymer chains.
Highlights
All the polymers exposed to the environmental conditions are highly susceptible to undergo degradation over the long-term
Every factor that affects the cross-linking density of the polymer should be taken into account when it comes to studying the degradation kinetics in order to evaluate the underlying reaction mechanisms
Organo-modified montmorillonite (OMMT) nanocomposites prepared by in-situ polymerization show enhanced decomposition temperature, according to the results presented by Rehab et al [10]
Summary
All the polymers exposed to the environmental conditions are highly susceptible to undergo degradation over the long-term. Their protection has drawn a great interest in order to prolong their lifetime, for insulation and outdoor applications [1,2]. Developing proper methods of protection is pertinent to the comprehensive understanding of reactions taking place when the polymer is subject to destructive conditions. In the case of thermoset polymers under well-defined conditions, there is a direct relationship between the cross-linking density and network degradation. Every factor that affects the cross-linking density of the polymer should be taken into account when it comes to studying the degradation kinetics in order to evaluate the underlying reaction mechanisms
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