Milling Options to Break Down Agglomerates
Ball milling is a grinding technique that uses media to effectively break down pigment agglomerates and aggregates to their primary particles. Using a rotor or disc impeller to create collisions of the grinding media, the impact and force created by the bead mills collisions break down the pigment agglomerates. The media can consist of either stainless steel, glass, or ceramic materials. The higher the bead hardness or density, the greater the collision force. The ball-milling process uses a higher concentration of grinding media to mill base in which the chambers are designed to maximize the energy transfer.
When a particle size has to be reduced below 10 microns, bead milling is the technique to use. However, if the material has a very low viscosity, ball milling is a better dispersing process than using a high shear mixing (vertical) system.
Since organic pigments are more difficult to break down to their primary particles compared to inorganic pigments, ball milling is the recommended choice.
Currently, the VMA-Getzmann company offers 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. Depending upon the model, sample quantities can be as low as 20 ml or up to 20,000 ml.
Our 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 selected to run as a single pass or as a recirculation configuration.
One of the unique features 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 the operator can control the volume, through 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.
To learn more about SL Bead mill models with “C” technology, visit our Laboratory Disperser page.
Basket bead milling is a relatively new design for ball milling applications. The grinding media is contained in a cylinder (basket), and 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 standard diameter size of the grinding media is 1.0 mm. however, it can be ordered to use 0.3 mm bead size.
Since the Getzmann basket mill is attached to a High-Speed Disperser model, those with an adapter allow the user to switch between the basket mill system and a motor shaft for high-shear dispersing easily.
The basket mill itself is immersed in the mill base. The sample first enters the basket mill from the top, then is pumped into the milling chamber, and finally exits out from the bottom.
Attached to the bottom of the basket is a cowles blade that rotates at high speed. The purpose of the cowles blade is to circulate the mill base to ensure all materials enter the basket mill. When you have created the desired particle size, the basket mill is then raised out of the sample container, while the grinding media stays in the basket.
One of the primary advantages of both the basket and grinding media is that they can easily be cleaned and require 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 third system for ball milling applications is the APS (air pressure system). The APS is attached to a high shear disperser. It consists of a sample container with a sieve filter at the bottom, a stand to elevate the sample container, along with a sealing system around the motor shaft, and a 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 depending on the desired particle size. 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; the air is applied to force the sample through the sieve filter separating the mill base from the grinding media. Aside from the ability to produce small quantities of less than 25 milliliters, another advantage of the APS system is their ease of cleaning.
The BYK-Gardner Dispersion Team is ready to answer any questions you may have about our products. To speak with a dispersion specialist contact us at (800) 343-7721 or request a catalog.