Reduction of aspect ratio of fluoromica using high-energy milling

Abstract

In recent decades layered natural and synthetic clay minerals have been used in a multitude of applications including rheology modifiers, functional additives for polymers, coatings, cosmetics and even medical device components. To aid in optimisation of performance and study of biological impacts, it would be most advantageous to have a series of well-defined nano-sized clay particles of the same chemistry with controlled aspect ratio. To date previous studies into milled materials have limited analysis by advanced microscopic techniques, and have also not correlated the impact of processing the materials at laboratory versus pilot/commercial scales. In this study semi-synthetic Na+ fluoromica was exposed to high-energy milling using lab scale and commercial scale milling systems, using several configurations and separated into size fractions using centrifugation. These fractions were assessed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR) which were employed to characterise the differences in morphology, particle size and structural changes between the unmilled and milled samples to fully assess the impacts of scale and energy on the process. The effect of a small lab-scale batch mill and a larger semi-commercial scale mill connected to a continuous flow reservoir was compared. The microscopy results showed that the particle size was reduced significantly after high-energy milling using both batch and continuous mill configurations on the lab scale mill, and in addition highly reproducible results were obtained using the larger mill. FTIR spectra revealed that no significant fluoromica crystalline structural changes were detected. Finally, DLS, SEM and TEM techniques were compared in order to identify the most reliable method for determining particle size distribution, and we propose that SEM has more advantages when it comes to characterising clay platelets. Furthermore, according to the SEM results, the larger semi-commercial scale mill resulted in a greater size reduction, which is consistent with existing concepts and theory of horizontal bead mill scale up.