Formation Mechanism of a Heterogeneous Network in Epoxy Resins

Abstract

In general, the network structure formed in epoxy resins is heterogeneous, and its extent is dependent on the curing temperature. Based on Fourier-transform infrared spectroscopy in conjunction with coarse-grained molecular dynamics simulations, we herein discuss the origin of mesoscopic heterogeneity in an epoxy resin composed of a typical epoxy base and diamine hardener, which react with each other in two steps. The rate balance of the first and second step reactions, which led to chain extension and branching, respectively, depended on the curing temperature; at a higher temperature, the second-step reaction became faster. Since the reaction proceeded more slowly at lower temperatures, a homogeneous domain was formed as unreacted substances were incorporated into the network. At higher temperatures, on the other hand, the reaction proceeded rapidly before unreacted substances were incorporated into the network, leaving isolated small fragments and nanoscale voids or free spaces in the domain. We also found that an actively reacting domain with a length scale of several tens of nanometers was formed even at a conversion lower than the gel point, regardless of the curing temperature. The connection of domains was also key to better understanding the temperature dependence of the hierarchical heterogeneity. The final network was found to be denser and more homogeneous at a lower temperature, while heterogeneous with many free spaces at a higher temperature. Our molecular picture of the network formation drawn by combining experimental and simulation techniques advances our current understanding of the heterogeneity formed in epoxy resins.

Effect of Condensed Water at an Alumina/Epoxy Resin Interface on Curing Reaction

“…The E a value of the first-step reaction was 60.4 kJ mol –1 , which is higher than that of the second-step reaction by 11.7 kJ mol –1 . This is common for other epoxy and amine systems . The Δ H value was 7.2 kJ mol –1 higher for the first-step reaction.…”

Effect of a Heterogeneous Network on the Fracture Behavior of Epoxy Resins

“…Nevertheless, the T g value was lower in the order of ER353, ER323, and ER296. The network formation during the pre-curing process was dependent on temperature, as suggested by FT-IR spectroscopy, which detected the difference in the generation of the secondary and tertiary amines, in conjunction with coarse-grained molecular dynamics simulations. , At lower temperatures, homogeneous domains were formed, unreacted substances were incorporated into them, and the network grew. At higher temperatures, the reaction proceeded rapidly before unreacted substances were incorporated into domains, leaving isolated small fragments and nanoscale voids, or free spaces, in the network.…”

“…At higher temperatures, the reaction proceeded rapidly before unreacted substances were incorporated into domains, leaving isolated small fragments and nanoscale voids, or free spaces, in the network. The resultant network became more heterogeneous as the pre-curing temperature increased. , That is, there were denser and less dense regions in the network structure. Given that the number density of dangling chains, in which one end attaches to the network and the other is free, increases with increasing network heterogeneity, a decrease in T g with increasing pre-curing temperature can be understood. , …”

Optimizing Epoxy Molding Compound Processing: A Multi-Sensor Approach to Enhance Material Characterization and Process Reliability

“…The dimensions and concentration of the polymer coils increase until a cross-linked structure is formed. This penetrates the system and leads to macrogelation [ 48 , 49 , 50 , 51 ]. Controlling the temperature through the process parameters accelerates the reaction and leads to increased microgel formation due to the reduced gel time.…”