An approach for predicting the true density of powders based on in-die compression data

Helium pycnometry, a commonly used technique for measuring the true density of powders, is sensitive to the release of volatiles during measurement. This can lead to over-estimated true density, and as such, an accurate method for determining the true density of powders containing volatile components is needed. Here, a method based on in-die compression data obtained with a compaction simulator was assessed. Specifically, the stress transmission coefficient (STC), measured using an instrumented die, was used to predict the in-die Heckel mean yield pressure (P_y). A true density was derived by repeatedly performing a Heckel analysis using iteratively estimated true density values until the predicted P_y value from the measured STC value is obtained from in-die density - pressure data. This novel method was validated using a set of water-free powders. Using crystalline hydrates, we further showed that the calculated true densities were closer to values calculated from crystal structure than those from helium pycnometry. Hence, this method may be used for determining the true density of powders from their STC values.