Effect of copper on the curing and structure of a DICY-containing epoxy composite system

Abstract

ABSTRACT:The curing and structure of an epoxy system containing dicyandiamide (DICY) as hardener were studied as a function of temperature and the presence or absence of copper with the use of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy in photoacoustic mode (FTIR-PAS). Spectroscopic analysis of specimens taken from the DSC helped to clarify the reaction mechanism in terms of the different schemes that have been proposed. The initial stages, corresponding to the first peak in the DSC exotherm, involve the usual epoxide-amine reactions closely followed by a reaction between DICY nitrile groups and hydroxyl groups to form structures containing iminoether and urea groups. These reactions are slightly retarded in the presence of copper. At higher temperatures, corresponding to the second peak in the exotherm, these structures are transformed into others believed to contain urethane ester groups. This reaction, which may be considered to constitute a form of degradation, is significantly accelerated in the presence of copper. The effect is particularly large around 180°C, a temperature commonly used to cure such systems, so the results have important practical implications, for example, in the lamination of circuit boards.

Cure monitoring of epoxy films by heatable in situ FTIR analysis: correlation to composite parts

“…However, a slight shift in the wavenumber with temperature can be observed. Because a significant influence of hydrogen bonds on imine band can be excluded, this indicates the formation of the various intermediate species postulated for the established curing mechanism . A carbonyl signal at 1690 cm −1 is formed during the curing process and vanishes at higher temperatures, and finally, a carbonyl signal at 1762 cm −1 is formed at higher temperatures.…”

“…On the one hand, bands vanished, such as the nitrile signals at 2205 and 2160 cm −1 as well as the oxiran ring signal at 914 cm −1 ; on the other hand, new bands were formed, such as the signals at 1750, 1690, or 1647 cm −1 . The latter is attributed to newly formed imine signals, whereas the signals between 1680 and 1750 cm −1 are correlated with various carbonyl species, such as those present in cyclic or linear urea or urethane species . The characteristic ether signals between 1300 and 1000 cm −1 change in position and intensity, indicating the formation of a newly created network with changed flexibility of various bonds.…”

“…This substitution of nitrogen by oxygen at the central carbon atom in the urea derivative can occur only through trans‐etherfication reaction by a hydroxyl group. In the literature, a hydrolysis reaction is described instead of this trans‐etherfication reaction . However, since no water should be present under the conditions of these measurements, we did not consider this reaction.…”

“…The material we use in this study is an epoxy mixture of a resin based on diglycidyl ether of Bisphenol A (DGEBA) polymer and a hardener based on dicyandiamide (DICY). It is well established that the crosslinking reaction of the oxiran ring of resin with the primary amine group of the hardener occurs first . But different mechanisms come to bear in the further progress of curing.…”

Online near‐infrared measurements of an epoxy cure process compared to mathematical modeling based on differential scanning calorimetry measurements

“…Besides etherification and amine addition reactions, different cyclization reactions can occur, complicating an exact description of the cure reactions. Detailed descriptions of the cure reactionfor this complicated system can be found in refs 2–4, 18, 22,. and23.…”