Theory & Method
A Mooney test loads two discs of rubber around a knurled steel rotor, one above and one below. Pneumatic pressure then closes a heated die cavity around the assembly. The sample preheats for a set interval, typically one minute, until it reaches die temperature. The rotor turns at a fixed 2 rpm against the resisting rubber. Torque on the rotor shaft is sensed and scaled into Mooney units.
Viscosity is always quoted with its full test conditions, as in ML 1+4 at 100 °C. The letters name the test and the rotor size, with L for the large rotor and S for the small one. The first digit gives preheat time in minutes and the second gives running time. Die temperature closes the notation.
Three test types run on the same hardware. A viscosity test reports the Mooney value at a specified running time along with the initial reading. A scorch test holds the compound at elevated temperature and watches for a torque minimum followed by a rise. Reported points are t5 and t35 for the large rotor, or t3 and t18 for the small rotor. Those points mark the onset and progression of premature vulcanization.
Stress relaxation testing stops the rotor at the end of the running period and records torque decay against elapsed time. The procedure follows ASTM D1646 Part B. Decay rate indicates elastic recovery and molecular structure, which a single viscosity number does not capture. Results help separate compounds that share a Mooney value but behave differently during extrusion.
