Investigations on the Dynamics and Mechanics of Milling Processes for

Mechanochemistry

Ciara S. Griffin1; Patrick Cronin1; Alan Ryan2; Ahmad B. Albadarin3 and Gavin M. Walker1, 3

1Synthesis and Solid State Pharmaceutical Centre (SSPC), Department of Chemical Sciences, Bernal

Institute, University of Limerick, Ireland.

2 School of Engineering, University of Limerick, Ireland.

3 Department of Chemical Sciences, Bernal Institute, University of Limerick, Ireland.

Ciara.Griffin@ul.ie

Introduction

Mechanochemistry has proven to be successful tool for many fields and applications, such as organic and inorganic synthesis, synthesis of metal organic frameworks (MOFs), mechanical alloying and pharmaceutical materials. However, development of the field is hindered by a lack of understanding of the mechanisms in which drive a mechanochemical reaction (Fischer and Rademann, 2016).
Primarily, a proof of concept approach was undertaken by our group to demonstrate the feasibility of the process for chemical transformation induced by mechanical forces in the absence of solvents, which was followed by optimisation of the milling process, manipulating milling parameters such as type of mill, milling frequency, reaction time, material of construction of mill ball and grinding jars, ball-to- powder weight ratio and free volume of the grinding jar.
The investigation focuses on the influence of the mill ball on properties of impact collisions and aims to track the energy distribution and dissipation throughout the mill for a given milling sequence. The research presented is specific to a Retsch Mixer mill MM 400, with 25mL grinding jar and a single mill ball, 15mm in diameter.
A transparent milling assembly was designed to visually observe the mill process (Figure 1), in particular, the trajectory of the mill ball and interaction of the mill ball and wall of the grinding jar throughout the process. A 25 mL Perspex grinding jar and 3D printed polylactic acid mill ball were used in the study.

Figure 1

Transparent milling assembly designed to visually observe milling process

A study of the kinematics and mechanics of the milling process was performed with the use of an Olympus iSpeed high speed camera (Figure 2). The position of the mill ball in both the xy- and xz- planes were obtained for a three dimensional representation of the milling process. Velocity-time profiles, and hence kinetic energy of the ball, were determined.

Figure 2

High Speed Imaging of Mill Process in xy-plane

The impact energy at various milling frequencies was obtained for mechanochemical reactions performed in a Retsch Mixer Mill MM 400. The resulting impaction energy values indicate the energy required for mechanochemical reactions, and may assist with the successful scale up for industrial applications.

References

Fischer, F. and Rademann, K. (2016) 'Quantitative determination of activation energ1es m mechanochemical reactions t', pp. 23320-23325. doi: 10.1039/c6cp04280e.