“…The initiation step can involve alcoholysis of the anhydride; alcoholysis is a nucleophilic reaction and, hence, the presence of tertiary amines accelerates the reaction due to the positive partial charge on the carbon adjacent to the nitrogen atom [29]. The initiation step can also involve the reaction of tertiary amines with epoxy groups, giving rise to a zwitterion, as proposed by Matejka et al [30] and Fedtke and Domaratius [31]. The resultant salt will then act as the initiator for the curing process.…”
Epoxy-based systems are used widely as dielectrics in electrical applications and, in such systems, the chosen stoichiometry is important in determining the nature of the molecular network that forms and, hence, the physical properties of the final system. However, the inclusion of nanoparticles with large interfacial areas into epoxy systems may introduce additional chemical reactions between moieties on the nanoparticle surfaces and the reactants, or may alter the rate and sequence of the various chemical pathways that occur during curing, with the consequence that the cross-linked network that forms within the matrix polymer will be altered. This study set out to investigate the effects of resin stoichiometry on material properties, including the glass transition temperature and electrical breakdown strength of an epoxy resin and its nanocomposites filled with silica particles, which were introduced using a Nanopox masterbatch system. The results indicate that the nanosilica affects network formation in complex ways and that the concept of an optimum stoichiometry is highly dependent upon the property of interest. From the perspective of breakdown strength, a stoichiometric excess of anhydride hardener is detrimental; the introduction of nanoparticles has a comparable effect to an excess of hardener.
“…The initiation step can involve alcoholysis of the anhydride; alcoholysis is a nucleophilic reaction and, hence, the presence of tertiary amines accelerates the reaction due to the positive partial charge on the carbon adjacent to the nitrogen atom [29]. The initiation step can also involve the reaction of tertiary amines with epoxy groups, giving rise to a zwitterion, as proposed by Matejka et al [30] and Fedtke and Domaratius [31]. The resultant salt will then act as the initiator for the curing process.…”
Epoxy-based systems are used widely as dielectrics in electrical applications and, in such systems, the chosen stoichiometry is important in determining the nature of the molecular network that forms and, hence, the physical properties of the final system. However, the inclusion of nanoparticles with large interfacial areas into epoxy systems may introduce additional chemical reactions between moieties on the nanoparticle surfaces and the reactants, or may alter the rate and sequence of the various chemical pathways that occur during curing, with the consequence that the cross-linked network that forms within the matrix polymer will be altered. This study set out to investigate the effects of resin stoichiometry on material properties, including the glass transition temperature and electrical breakdown strength of an epoxy resin and its nanocomposites filled with silica particles, which were introduced using a Nanopox masterbatch system. The results indicate that the nanosilica affects network formation in complex ways and that the concept of an optimum stoichiometry is highly dependent upon the property of interest. From the perspective of breakdown strength, a stoichiometric excess of anhydride hardener is detrimental; the introduction of nanoparticles has a comparable effect to an excess of hardener.
“…The reaction proceeds through a chainwise polymerization comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Figure . â 1 (a) Initiation and (b) propagation and chain termination steps in the anhydrideâepoxide copolymerization initiated by tertiary amines.…”
The best models available in the literature to analyze gelation in the diepoxide-cyclic anhydride copolymerization are based on the mean-field approach. A kinetic model was developed to account for the nonrandom steps of the copolymerization. Two kinetic schemes were employed with one including chain transfer steps. Predicted gel-point conversions as a function of the initiator concentration were compared with experimental results obtained for a system based on diglycidyl ether of bisphenol A, methyltetrahydrophthalic anhydride, and benzyldimethylamine, as initiator. Both kinetic schemes explained the observed first-order kinetics for monomer consumption, after an induction period. However, experimental gel-point conversions could only be fitted by assuming the presence of a chain transfer step regenerating an active species.
“…Unlike other epoxy systems, i.e. anhydridecured epoxy, where the curing process can proceed via different competitive chemical routes [30][31][32], a previous study [4] showed that the crosslinking process in this amine-based epoxy system is mainly due to the reaction between the epoxy and amine groups. Therefore, this system was chosen for this investigation as its networking mechanism is simple and any potential variations in the crosslinking density can be directly related to the resin/hardener stoichiometry.…”
The addition of nanofillers can have a significant influence on the resin stoichiometry of thermosetting polymer systems. Based on differential scanning calorimetry (DSC) results, it is estimated that the inclusion of 2 and 5 wt% of silicon nitride nanofiller displaces the resin/hardener stoichiometry of an epoxy/amine network by 6.5 and 18%, respectively. Dielectric spectroscopy results confirm the above findings, in that the spectra of the nanocomposite samples were found to be equivalent to the spectra of unfilled samples when the above stoichiometric effect was taken into account. Therefore, this study provides clear evidence that the presence of a nanofiller can directly and significantly affect the curing process of an epoxy network. Consequently, this should always be considered when introducing nanofillers into thermosetting matrices. These results indicate the presence of covalent bonding between the nanoparticles and the surrounding polymer and, therefore, provide an opportunity to explore the influence of this bonding on the molecular dynamics of the polymer layer around the particles. However, the obtained DSC and dielectric spectroscopy results suggest that, in the system considered here, either the covalent bonding does not have an appreciable influence on the segmental dynamics of the polymer, as revealed by these techniques, or that the thickness of the affected layer is less than 1 nm and therefore too small to be distinguished from experimental uncertainties.
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