Process
Hammer milling is a high energy process of reducing particle size (comminution) by impact with rapidly moving hammers. The material is fed into the mill’s chamber through a feed chute, typically by gravity, where it is struck by rapidly rotating hammers that strike particles repeatedly until the particles are reduced to a size that can pass through a nearby screen. Hammer mills can use “hard” or “soft” hammers for reducing particle size. Hard hammers are blunt and impact the material with a flat surface in order to smash the particle, leading to wider particle size distributions. Soft hammers use sharp, knife-like edges to cut particle, but are not as effective at particle size reduction as hard hammers.
Promising candidates for hammer milling are materials that are “friable,” or be able to break into smaller pieces upon the application of force. Materials that are not intrinsically friable can be made so through a number of different methods. Intrinsically friable materials usually have microscopic defects, like fractures or gaps, which act as natural places for the material to break apart. Materials that lack these defects can have defects introduced through methods like solvent crazing, sample irradiation, etc. Making a material glassy (i.e., being below the glass transition temperature, Tg) can turn a flexible material into a material that fractures upon impact. This is typically done with liquid nitrogen as referred to in the “Cryo Milling” section.
Changing the morphology of the feed material can also have an impact on the milling behavior. Polymer pellets may bounce off of the hammers and only become slightly rounded, whereas polymer flakes of the same material may break upon collision with the hammers.
To determine if a material is right for hammer milling, there are three basic tests that can be done. The first is to hit the material with a hammer and observe what happens. If it shatters, this is a good candidate for hammer milling. If it mostly sticks together in a mass, this is a less likely candidate. Better yet is to use a desktop mill for initial evaluation. If any fine particles are produced, there is a good chance the material will mill. However, the best method for determining if hammer milling is the right choice for particle size reduction is to run a small-scale test on production hammer mills. Tests as small as five pounds can give an idea of whether or not the material will mill and a rough estimation of the expected resulting particle size. Using production equipment to test the material uses much more energy than other methods and gives data that is relevant for scale-up calculations.
Hammer milling parameters such as screen size, blade selection and rotor speed can be optimized for each application, depending on the equipment used. The typical particle sizes that can be achieved with hammer mills is a D50 between 150 μm and 600 μm. Hammer mills tend to yield less than 25% of particles below 150 μm. Because the end particle size of a milled material is determined by the screen size used on the mill, it is possible to use very small screens to attempt to get smaller particles. However, smaller screens are weaker and tend to break easily, which adds to equipment costs and ruins batches due to contamination.
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