Kinetics of Cross-Linking Reaction of Epoxy Resin with Hydroxyapatite-Functionalized Layered Double Hydroxides

<p dir="ltr">The cure kinetics analysis of thermoset polymer composites gives useful information about their properties. In this work, two types of layered double hydroxide (LDH) consisting of Mg²⁺ and Zn²⁺ as divalent metal ions and CO₃²⁻ as an anion intercalating agent were synthesized and functionalized with hydroxyapatite (HA) to make a potential thermal resistant nanocomposite. The curing potential of the synthesized nanoplatelets in the epoxy resin was then studied, both qualitatively and quantitatively, in terms of the <i>Cure Index</i> as well as using isoconversional methods, working on the basis of nonisothermal differential scanning calorimetry (DSC) data. Fourier transform infrared spectroscopy (FTIR) was used along with X-ray diffraction (XRD) and thermogravimetric analysis (TGA) to characterize the obtained LDH structures. The FTIR band at 3542 cm⁻¹ corresponded to the O–H stretching vibration of the interlayer water molecules, while the weak band observed at 1640 cm⁻¹ was attributed to the bending vibration of the H–O of the interlayer water. The characteristic band of carbonated hydroxyapatite was observed at 1456 cm⁻¹. In the XRD patterns, the well-defined (00l) reflections, i.e., (003), (006), and (110), supported LDH basal reflections. Nanocomposites prepared at 0.1 wt % were examined for curing potential by the <i>Cure Index</i> as a qualitative criterion that elucidated a <i>Poor</i> cure state for epoxy/LDH nanocomposites. Moreover, the curing kinetics parameters including the activation energy (<i>E</i>_α), reaction order, and the frequency factor were computed using the <i>Friedman</i> and Kissinger–Akahira–Sunose (<i>KAS</i>) isoconversional methods. The evolution of <i>E</i>_α confirmed the inhibitory role of the LDH in the crosslinking reactions. The average value of <i>E</i>_α for the neat epoxy was 54.37 kJ/mol based on the <i>KAS</i> method, whereas the average values were 59.94 and 59.05 kJ/mol for the epoxy containing Zn-Al-CO₃-HA and Mg Zn-Al-CO₃-HA, respectively. Overall, it was concluded that the developed LDH structures hindered the epoxy curing reactions.</p><h2>Other Information</h2><p dir="ltr">Published in: Polymers<br>License: <a href="https://creativecommons.org/licenses/by/4.0/" target="_blank">https://creativecommons.org/licenses/by/4.0/</a><br>See article on publisher's website: <a href="https://doi.org/10.3390/polym12051157" target="_blank">https://doi.org/10.3390/polym12051157</a></p><p dir="ltr"><br></p><p dir="ltr">The University of Doha for Science and Technology replaced the now-former College of the North Atlantic-Qatar after an Amiri decision in 2022. UDST has become and first national applied University in Qatar; it is also second national University in the country.</p>