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We demonstrate how terahertz time-domain spectroscopy (THz-TDS) can be used to quantify water absorption in hygrothermally aged simple and commercial epoxy systems supported by conventional gravimetric analysis.

In this study, terahertz time domain spectroscopy (THz-TDS) is used to analyze how the geometry of a compact affects its pore structure (pore shape and orientation). By using flat-faced and biconvex compacts, it was evident from our analysis that pores tend to assume specific shapes and orientations based on the compact's geometry and this was found to significantly affect the extracted optical properties of samples prepared by mixing a material with polyethylene (PE) as diluent and subsequent compaction. However, such sensitivity to the pore properties opens a number of industrial applications such as for quality testing of pharmaceutical tablets. A comparison made between the PE based compacts and a set of pharmaceutical tablets prepared from the same formulation has revealed that flatfaced and biconvex compacts possess different pore geometries and hence different optical properties.

Terahertz pulsed imaging (TPI) coupled with a customised flow cell has previously been used successfully to study liquid transport in porous media, such as powder compacts. In some samples, the tracking of the liquid front remains challenging when the refractive indices of liquid and sample are similar and hence little contrast is observed. In this work, we found that even when the relative difference between the refractive indices of liquid and sample is high, it can still be challenging to track the reflection peak of water in some samples while it works fine in others. It is hypothesised that this is due to the formation of a water gradient in these samples due to their specific microstructure and we explore methods to overcome this limitation.

This study demonstrates the analysis of anisotropic pore structures of highly porous pharmaceutical powder compacts by combining terahertz time-domain spectroscopy and in-situ measurements of the liquid penetration using terahertz pulsed imaging.