Rotation plastimeters

Figure 6-4. Rotation plastimeter geometry, (a), (b) and (c) Coaxial cylinder types (d) and (e) concentric disc types ((d) is the Mooney geometry). A is usually the stator and B the rotor, C is the rotating shaft and r is the cylinder radius (much larger than the clearance between A and B). xy indicates the mid-plane along which the chamber can be opened for filling.

The main advantage of rotation plastimeters over compression instruments is that shearing at constant rate can be continued for as long as required so that thixotropic or structure effects can be studied. Rather higher shear rates are possible, although the Mooney operates at only about 1 sec 1 (the shear rate varies across the diameter of the rotor). A practical difficulty is to avoid slippage of the rubber over the metal parts and this is why the Mooney operates with a positive hydrostatic pressure and has grooves cut in the metal surfaces. 

A number of plastimeters of this type have been used for rubbers, often for research purposes, but one instrument, the Mooney viscometer, gained virtually universal acceptance and has been extensively used for routine quality control purposes for several decades. The principle of the Mooney is shown in Figure 6.4 together with several other possible geometries for a rotational instrument. The rotor turns at a constant rate inside a closed cavity containing the test piece so that a shearing action takes place between the flat surfaces of the rotor and the walls of the chamber. The torque required to rotate the rotor is monitored by a suitable transducer. 

The Mooney plastimeter is a rotational shearing-disc viscometer (Figure 9.2). The temperature of the machine cavity dies can be preset to typical... 

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