Ball milling is using grinding media to break down the pigment agglomerates and aggregates to their primary particles. The bead mills use a rotor or disc impeller to create collisions of the grinding media. The impact and shear force created by the collisions break down the pigment agglomerates. The media can consist of stainless steel, glass, and ceramic materials. The higher the bead hardness or density creates a greater collision force. The ball milling process uses a higher concentration of grinding media to mill base. The milling chambers are designed to maximize the energy transfer.
Bead milling is used when the particle size has to be reduced below 10 microns. If the material has a very low viscosity, ball milling is a better dispersing process than using a high shear mixing (vertical) system. Organic pigments are more difficult to break down to their primary particles compared to inorganic pigments. For the best results for organic pigment dispersing, ball milling is the recommended choice.
The VMA-Getzmann company has three product lines for bead milling. They can be dedicated stand-alone systems or accessories that can be added to the high-speed vertical disperser models. Sample quantities can be as low as 20 ml up to 20,000 ml depending upon the model.
Horizontal Bead Mills
The Dispermat SL model line is the current horizontal bead mill system. Milling chamber sizes can start at 50 ml to save on raw material costs. The beads are separated from the mill base by a dynamic gap system. The standard gap uses 1.0 mm diameter grinding media; an optional gap is available to use beads down to 0.3 mm diameter. The Dispermat SL can be user selectable to run as a single pass or as a recirculation configuration. A unique feature is an independent pumping system to feed the mill base into the milling chamber. Instead of the speed of the milling rotor controlling the sample volume through the milling chamber, the operator can control the volume by the mixing system pump that fits on top of the milling chamber. Separating the rotor speed from the sample feed system provides more control over the milling process. There are SL Bead mill models with the “C” technology described on Laboratory Disperser page. The milling chamber and rotor can be ordered with a ceramic lining for corrosive samples.
Basket bead milling is a relatively new design for ball milling applications. The grind media is contained in a cylinder (basket). The mill base is circulated through the basket. The VMA-Getzmann basket mill consists of a stainless steel cylinder with an opening at the top and a sieve filter on the bottom. The basket mill is immersed in the mill base. The sample enters the basket mill from the top, it is pumped into the milling chamber, and exits out from the bottom. Attached to the bottom of the basket is a cowles blade that rotates at a high speed. The cowles blade circulates the mill base to make sure all of the material enters the basket mill. When the particle size has been achieved the basket mill is raised out of the sample container. The grinding media stays in the basket. The basket and grinding media can be easily cleaned by immersing the basket mill in a cleaning solution. The standard diameter size of the grinding media is 1.0 mm. A Getzmann basket mill can be ordered to use 0.3 mm bead size. The dispersion results from the basket mill are equivalent to a horizontal bead mill. The primary advantages of a basket mill are quicker clean up and lower maintenance. The Getzmann basket mill is attached to a High Speed Disperser model. The Disperser with an adapter will allow for switching between the basket mill system and a motor shaft for high shear dispersing.
The APS System
A third system for ball milling applications is the APS (air pressure system). The APS is attached to a high shear disperser. It consists of sample containerwith a sieve filter at the bottom, a stand to elevate the sample container, a sealing system around the motor shaft, and container lid. The mill base and grinding media is mixed 50/50 in the container. A disk impeller or pearl mill impeller is immersed into the mixture and rotated from 500 to 5000 rpms. After the dispersion is completed the stop cock that covers the sieve filter is removed, the lid is clamped tight over the vessel. The lid has an air connection, air is applied to force the sample through the sieve filter separating the mill base from the grinding media.
The main advantages of the APS system are:
- Easy to clean
- Can ball mill very small quantities – less than 25 milliliters