Direct evidence of amine-metal reaction in epoxy systems: An in situ calorimetry study of the interphase formation

Abstract

Epoxy resins are ubiquitously encountered in industrial applications as in adhesives and composites. The properties of epoxy-amine networlcs are directly impacted by the presence of metal (bydr-oxidized) surfaces, leading to a modificati on of their glass transition temperature T 8 • We propose here an innovative experimental approach, investigating the interaction of DET A amine and DGEBA epoxy with Al and Cu powder substrates (partially (hydr)oxidized). We explored for the first tiine the formation of the amine-metal interphase by in situ mixing calorimetry to evaluate the energetics of interaction. While DGEBA interacted only sl ightly with Al-based surface, the reaction with OETA was associated with a high exothermic enthalpy of reaction. The enhancing role of surfaoe hydroxylation was also evidenced by comparing boehmited Al to a simply oxidired counterpart. An even larger exothermic effect was measured with copper, which was related to the high chelating power of Cu compared to Al. The possible underlying mechanism of amine-metal interphase formation was discussed with a generalized schematic.

Further insight on amine-metal reaction in epoxy systems

“…The T g endset also globally followed this trend. It may be noted that, on the contrary, previous observation showed that pre-contacting DGEBA with Al did not lead to significant T g modification [24]. This effect is thus related here to the amine-metal interaction.…”

“…It may be noted that this experimental value is lower than ~0.600 that might be considered a priori for such systems by analogy with other epoxy systems [30], although values down to 0.42 have for example been reported for DGEBA-based polyepoxys cured with di-amines [30]. Taking into account the measured values of T g∞ = 131 • C and T g0 = -42 • C (see Fritah et al [24]), application of Eq. (3) with λ = 0.374 then leads to the α = f (T g ) curve plotted on Fig.…”

“…Amine-metal interaction was followed in situ by mixing calorimetry at 40 • C thanks to a Setaram C80 calorimeter equipped with Calvet 3D sensors and using two-compartment Hastelloy® mixing cells, as described elsewhere [24]. Briefly, the heat flow associated with the studied reaction occurring in the sample calorimeter cell was recorded over time taking the second empty calorimeter cell as reference.…”

“…These works pointed to the need to further explore such systems by microscopic simulation approaches, so as to better understand relative molecular interactions. An acid-base mechanism of amine-aluminum interaction was tentatively proposed from experimental data [17][18][19], which was lately extended to a more general case [24], involving the formation of surface complexes and chelate release. But these steps still need to be explored in depth.…”

“…But these steps still need to be explored in depth. Recently, amine/metal (hydr/oxide) interphase formation was examined for the first time by direct observations via in situ mixing calorimetry [24], using DETA as a model polyamine curing agent. Interphase formation was found to be associated with a net enthalpy of interaction clearly exothermic (contrarily to the epoxy/metal (hydr/oxide) system rather unreactive), and the nature of the metal (Al, Cu) and its oxidation state were directly linked to the…”