Magnetic Stirring -Key Points
PTFE is the material of choice for encapsulation because of its almost total chemical resistance and its wide range of working temperature -200°C to +280°C
It is difficult to quantify the most effective shape for a particular stirring application, some shapes are self-evident, for example, and oval or egg shape for round bottom containers or a flat and triangular shape where a scraping action is required, large containers generally require large stirring bars. For very viscous liquids a vaned stirrer at slow speed is required while stirring in shallow dished a long thin stirrer at slow speed is effective. Often, however, the selection is a matter of choice or trial with various shapes
Particle Formation and Abrasion:
PTFE is a relatively soft material and the rubbing action against the surface of the container may generate small particles. In an application where the generation of such particles must be avoided, the stirring system must be evaluated before actual use. Generation of particles is reduced by careful selection of the shape of the stirrer, ensuring the contact surfaces are smooth and even, not using an over powerful stirring bar or use suspended stirring.
De-coupling in the form of spin-out, tumbling or migration is generally due to weakness in the strength of the magnetic circuit, a mismatch in sizes of drive magnet and stirrer bar or a stirring speed too high.
Rare Earth Magnets:
Rare Earth Magnets may behave in an erratic manner due to increased strength and may migrate to one of the poles to the drive magnet or tumble with great ease and must, therefore, be selected with great care. Due to the strength of Rear Earth Magnets, there may be an increase in abrasion between the container and stirrer, which may cause increased particle generation.
PTFE stirring bars can be sterilised by chemical or thermal means, but not by gamma radiation