The joint research group led by Taiki Hoshino, a full-time researcher of the synchrotron radiation imaging application system development team at the RIKEN SPring-8 Center, succeeded in elucidating the mechanism for formation of a highly heat-resistant epoxy resin, a type of thermosetting resin, by using analytical methods such as X-ray photon correlation spectroscopy (XPCS).
Epoxy resin is employed in various situations such as in adhesives, semiconductor wiring protectants, and automobile / aircraft parts, and it is an indispensable material in modern industries. Thermosetting resin is cured by mixing the resin and curing agent and heating them. The heat resistance (hardness) differs depending on the heating conditions, but the mechanism for this was unknown.
According to Hoshino, "Denso (a major manufacturer of automobile parts) has been conducting analyses focusing on the heat resistance and network structure of epoxy resin. Since we focused on molecular motility, this led to our joint research. At the beginning of the research, we could not obtain a unified interpretation as we did not know the time scale to be considered owing to the molecular motility changing significantly during the curing process." The research group acquired as many X-ray scattering images as possible via XPCS measurement during the curing process, utilizing the large-scale radiation facility SPring-8 (1.8 million images at intervals of 4 milliseconds) and analyzed them in detail. From the analysis, it was found that when the epoxy resin was cured at a low temperature (100° C), the curing process was divided into three stages.
Specifically, it was shown that the reaction of the resin progresses sequentially in three stages, appearing as a clear difference in motion: (1) Oligomerization via bonding of monomers, (2) gelation via cross-linking reactions between oligomers, and (3) densification of the cross-linked structure, In contrast, when the resin was cured at a high temperature (150° C), no boundary was observed in the change in motion, and it was observed to gradually slow down. When the chemical reactions in the curing process were examined via infrared spectroscopy, about 80% of the reactive groups were found to react in the low-temperature curing process, whereas only about 40% of them reacted in the high-temperature curing process. Furthermore, when the density of the resin network structure after curing was examined via pulse nuclear magnetic resonance, the number of densely crosslinked components in the case of low-temperature curing was found to be three times greater than that in the case of high-temperature curing.
Hoshino said, "In this study, we were able to show that the resin with a dense network-like structure suppresses dynamic fluctuations, which is deeply related to the performance of the material. This is an important phenomenon occurring not only in resins but also in a wide range of substances such as glass, and we would like to further clarify its relationship with physical properties."
This article has been translated by JST with permission from The Science News Ltd.(https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.