What is Cryogenic Milling?
Cryogenic milling, or cryo-milling, is the process of using liquid nitrogen to lower the temperate of the material and/or the milling process. AVEKA‘s cryogenic hammer mills can typically reduce the particle size from small pellets down to the 100s of microns. Cryo-milling can be used for:
- Reducing the material temperature below its glass transition temperature (Tg), which is the point at which a polymer goes from a brittle, glassy state (low temperature) to an elastic, rubbery state (higher temperature). Materials below their Tg are generally friable to some degree.
- Keeping the mill and material from overheating. Hard to mill materials generate a lot of heat during milling, and materials must stay below their melt temps or softening points to be friable.
- Temperature control to prevent hazardous decomposition in the high energy environment of the mill
- Oxygen (or other gases) exclusion to prevent degradation of the milled material when new surfaces are formed
The milling of polymers is usually difficult without the use of some liquid nitrogen since many polymers melt in the high energy environment of the mill, especially after a mill has warmed up to its running temperature. The biggest reason, however, is for keeping polymers below their Tg. Many times, being below the Tg is sufficient to render polymers friable. In some cases, the polymers may need further pre-processing in the form of irradiation or solvent crazing.
Water-sensitive materials can be cryo-milled because when liquid nitrogen expands from its liquid state to its gaseous state, its volume increases 694 times. This is enough to quickly exclude any other gas, including water vapor, from being in the atmosphere of the mill, as long as the liquid nitrogen is still flowing. Special care must be taken with any milled material that may come out cold enough to condense water on the surface; sealing up the milled material immediately after completion of milling is usually sufficient.
