Identifying the Influences on Network Formation in Structural Isomers of Multifunctional Epoxies Using Near-Infrared Spectroscopy

The network formation of four epoxy-rich formulations of the structural isomers of triglycidyl aminophenol and diaminodiphenyl sulfone has been monitored by using two complementary techniques, near-infrared spectroscopy and resin temperature monitoring. The differences between these networks have been described using the concentration of epoxide, primary amine, secondary amine, and tertiary amine functional groups and the actual temperature of the resin compared to the oven temperature during the cure schedule. It was found that initially, the 3,3′-diaminodiphenyl sulfone (33′DDS) formulations were more reactive and primary amines were completely consumed before the 4,4′-diaminodiphenyl sulfone (44′DDS) formulations. Secondary amines were formed more quickly in 33′DDS formulations compared to 44′DDS. The triglycidyl-meta-aminophenol (TGmAP) formulations consumed secondary amines and produced tertiary amines faster than the triglycidyl-para-aminophenol (TGpAP) formulations, indicating higher levels of cross-linking occurred earlier in the curing reaction. Etherification occurred much earlier in the TGpAP formulations than in the TGmAP formulations. Results suggest that internal cyclization occurs in the three meta isomer-containing formulations, and a corresponding lack of this effect in TGpAP/44′DDS results in a more homogeneous cross-linked network.