Quantitative Analysis of Epoxy Resin Using NIR Spectroscopy
Epoxy resins have become so ubiquitous in modern society that the word epoxy has evolved into a generic term to describe industrial-strength adhesives. Unfortunately, this often leads to confusion since many industrial-strength adhesives are not epoxies, and not all epoxies are adhesives. Epoxy resins are widely used in various manufacturing applications since, when cured, they result in high-performance thermosets, highly crosslinked polymers. In addition, these resins are used as a background matrix in a wide range of composite materials and are finding increasing applicability in dual-cure additive manufacturing. For example, some of the newer high-performance 3D printer resins rely on interpenetrating polymer networks (IPNs) which are created by mixing photocurable (acrylate) resins with epoxy resins. Therefore, parts can be first “green printed” using vat polymerization or direct ink write methods and then thermally post-cured to kick off the epoxy polymerization process. While this incurs an extra processing step, IPN resins produce final parts with far superior thermal and mechanical properties when compared to traditional photo-cured resin systems.
Strictly speaking, epoxies are a broad class of molecules containing a triangular ring with one oxygen atom covalently bound to two carbon atoms. When combined with a molecule containing reactive functional groups, typically amines, it causes the ring to open and initiate the polymerization process. A prototypical two-step epoxy/amine reaction is shown in figure 1, whereby an epoxy monomer first reacts with a primary amine opening the ring and forming a secondary amine. Then the remaining secondary amine reacts with a second epoxy monomer creating a tertiary amine linking the two epoxy monomers together into a dimer. It should be noted that while this is somewhat of a simplification of the epoxy/amine polymerization process, ignoring effects such as etherification, it is more than sufficient or gaining an intuitive understanding of the process.