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

Zohre Karami,Maryam Zarghami Dehaghani,Mohammad Reza Saeb,Józef T Haponiuk,Inamuddin Inamuddin,Ahmad Mohaddespour,Mohammad Reza Ganjali,Sajjad Habibzadeh,Mustafa Aghazadeh,Maryam Jouyandeh,Amin Esmaeili,Krzysztof Formela

doi:10.3390/polym12051157

Abstract

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 Cure Index 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 Cure Index as a qualitative criterion that elucidated a Poor cure state for epoxy/LDH nanocomposites. Moreover, the curing kinetics parameters including the activation energy (Eα), reaction order, and the frequency factor were computed using the Friedman and Kissinger–Akahira–Sunose (KAS) isoconversional methods. The evolution of Eα confirmed the inhibitory role of the LDH in the crosslinking reactions. The average value of Eα for the neat epoxy was 54.37 kJ/mol based on the KAS 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.

Highlights

Epoxy resins, well-known thermosetting polymers, have been extensively used as versatile coatings and adhesives due to their promising features such as corrosion resistance, appropriate mechanical strength, chemical stability, and desirable adhesion to different surfaces/substrates [1,2,3].achieving high mechanical properties and the thermal stability of thermosetting epoxy for high performance coatings has been a matter of controversy [4,5]
A diglycidyl ether of bisphenol A epoxy resin was used as the main matrix of the nanocomposite and cured with triethylenetetramine (TETA) hardener that was acquired from Sigma Aldrich Co., Milan, Italy
The crystalline structure of the synthesized layered double hydroxide (LDH) was examined by X-ray diffraction (XRD) analysis and the calculated basal spacing and intermetallic distance of the synthesized structures

Summary

Introduction

Achieving high mechanical properties and the thermal stability of thermosetting epoxy for high performance coatings has been a matter of controversy [4,5]. Layered double hydroxide (LDH) falls into the category of lamellar solid materials composed of stacking brucite-like layers of metal hydroxide possessing positive charges [7,8]. These positivelycharged sheets are exfoliated and neutralized by anions located between the layers as well as water molecules [9]. The introduction of LDH into the epoxy matrix as reinforcement has attained considerable attention due to the nontoxicity, layered structure, and high content of water [12]

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Conclusion