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After 5 minutes of dry grinding, particles up to 2mm length can be found. By further grinding, we found a maximum particle length of 400µm after 10 minutes. After totally 15 minutes of grinding, sample started sticking strongly to bowl and balls.
Usually sample starts sticking to bowl and balls, when majority of particles reached a fineness of < 20-30µm. Interacting forces between fine ground particles will become bigger as their own g-force. Therefore, particles will stick to each other and just become compressed by the used grinding balls. These clusters of particles also contain bigger particles which will not be ground any further too.
For this, a longer dry grinding time as 10 minutes is not recommended. Only by grinding in suspension (e.g. in alcohol), a better endfineness can be achieved.
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