Effect of Surface Treatment of Halloysite Nanotubes (HNTs) on the Kinetics of Epoxy Resin Cure with Amines

Abstract

The epoxy/clay nanocomposites have been extensively considered over years because of their low cost and excellent performance. Halloysite nanotubes (HNTs) are unique 1D natural nanofillers with a hollow tubular shape and high aspect ratio. To tackle poor dispersion of the pristine halloysite (P-HNT) in the epoxy matrix, alkali surface-treated HNT (A-HNT) and epoxy silane functionalized HNT (F-HNT) were developed and cured with epoxy resin. Nonisothermal differential scanning calorimetry (DSC) analyses were performed on epoxy nanocomposites containing 0.1 wt.% of P-HNT, A-HNT, and F-HNT. Quantitative analysis of the cure kinetics of epoxy/amine system made by isoconversional Kissinger–Akahira–Sunose (KAS) and Friedman methods made possible calculation of the activation energy (Eα) as a function of conversion (α). The activation energy gradually increased by increasing α due to the diffusion-control mechanism. However, the average value of Eα for nanocomposites was lower comparably, suggesting autocatalytic curing mechanism. Detailed assessment revealed that autocatalytic reaction degree, m increased at low heating rate from 0.107 for neat epoxy/amine system to 0.908 and 0.24 for epoxy/P-HNT and epoxy/A-HNT nanocomposites, respectively, whereas epoxy/F-HNT system had m value of 0.072 as a signature of dominance of non-catalytic reactions. At high heating rates, a similar behavior but not that significant was observed due to the accelerated gelation in the system. In fact, by the introduction of nanotubes the mobility of curing moieties decreased resulting in some deviation of experimental cure rate values from the predicted values obtained using KAS and Friedman methods.

Bulk-Surface Modification of Nanoparticles for Developing Highly-Crosslinked Polymer Nanocomposites

“…The outcomes of the two methods are expectedly similar, where the E α plot follows an ascending trend for possible autocatalytic reactions taking place at later stages of cure. Moreover, a shift to higher E α is obvious for systems containing MNPs, particularly for the bulk surface-functionalized MNPs, due to the intensified epoxy ring opening [ 54 ]. The curing reaction of epoxy system is progressed through chemically crosslinking reaction, but, after the occurrence of vitrification, it went through the diffusion-controlled mechanism [ 43 ].…”

Thermal Analysis of Crosslinking Reactions in Epoxy Nanocomposites Containing Polyvinyl Chloride (PVC)-Functionalized Nickel-Doped Nano-Fe3O4

“…The decrease of Eα value at the late stage of curing reaction when the degree of conversion is high would be because of the autocatalytic nature of the reaction of epoxy with hardener. The decreasing of activation energy is due to the facilitation of autocatalytic ring opening fueled by the hydroxyl functional groups generated during the reaction [55]. In addition, by progress in curing reaction and formation of a partially cross-linked network, both the molecular weight and the viscosity of the system increase due to the restricted molecular mobility.…”

Design of halloysite modification for improvement of mechanical properties of the epoxy based nanocomposites

“…Covalent functionalization provides a wide range of possibilities and can be obtained by surface-initiated polymerization or by direct addition of polymers to the nanotube surface [41] . Surface modification methods of HNT include silanization using silanes as modifiers [42,43] , the addition of (Iso)Cyanate esters or isocyanates [44] , esterification using phosphoric or lactic acid [45] , hydrogen bonding [46] , charge transferring [47] , electrostatic modification with surfactants or polyelectrolytes [48] , etching using alkali or acids [43,49] and bidentate coordination using catechol as a modifier [40] . One common method of HNT functionalization is silane coupling (Table 1.…”