There are two main types of rheometers currently in use: the ODR and the MDR. The older of these, the Oscillating Disk Rheometer (ODR), builds on the Mooney Viscometer’s rotor-based design. An ODR gauges the amount of torque (twisting force in pounds per inch, lb/in, or deciNewtons per meter, dN/m) needed to oscillate a rotor within the rubber sample. Whereas a viscometer rotor relies on full rotation, the ODR rotor only moves back and forth across a small arc. This oscillation is less degrading to the material than in the viscometer, where destruction of the sample is typical.

ODR test results are also more reflective of actual cure conditions because constant high pressure and the desired vulcanization temperature are maintained on the sample. As testing progresses, the sample begins to behave in predictable ways. Viscosity briefly drops as the sample first heats up, but the chemical reaction soon starts. The rubber becomes more viscous due to crosslinking of the macromolecular chains. As a result, the amount of torque that is required to internally shear (deform) the sample increases. Using this increasing torque as a gauge, the ODR plots a cure curve (see Table 17) illustrating the state of cure for a given time and temperature.

Though the Monsanto ODR was for many years the most-used rheometer, a more recent development is the Moving Die Rheometer (MDR, see Figure 45). Whereas the ODR uses an embedded rotor to torque the rubber sample, an MDR holds the sample between a pair of heated dies (metal plates forming a cavity). As one of the dies moves across a small arc, the other die gauges the reaction torque generated in the sample. This again results in a cure curve that can show the optimum cure time for the desired blend of properties. Since the MDR does not insert a rotor into the sample, many molders feel the MDR is less intrusive to the curing process and thus more objective and accurate than the ODR.

BATCH TESTING MAIN PAGE