Rubber compounds in the raw state do not demonstrate any elastomeric properties. In order to obtain these, the molecular chains should be crosslinked together, therefore the rubber compounds have to undergo a vulcanization process. A rheometer is a laboratory device designed for measuring viscoelastic properties of rubber compounds during the vulcanization process.
There are two types of rheometers in the rubber industry, ODR (oscillating disc rheometer) and MDR (moving die rheometer), but their purpose and role are the same.
We insert a piece of raw unvulcanized rubber into the cavity of the rheometer and close the rheometer. So the rubber compound is exposed to pressure at a certain constant temperature, which depends on the type of compound and the method of crosslinking (vulcanization). The ODR rheometer has a biconical rotor inside the cavity that oscillates left and right, usually by ± 3 °. During the vulcanization process, the sensor records the resistance with which the rubber resists the oscillating rotor. The torque required for oscillation depends on the stiffness or viscoelastic properties of the compound.
During the vulcanization process, which takes place in the cavity, the hardness of the compound increases. At the same time increase also the force and the torque on the rotor. Namely, the rubber has non-Newtonian properties and thus exhibits a viscous and elastic component. The shear forces are directly proportional to the degree of vulcanization. The rheometer thus measures a degree of vulcanization as a function of time at a given temperature. The graph of the torque curve on the rotor as a function of time represents a vulcanization curve or rheometric curve.
The rheometric curve is an imprint of the vulcanizing and processing properties of rubber compounds. Parameters like ML, MH, Ts2 and t’90 tell us much about this.
First, a compound at room temperature is inserted into a preheated cavity of the rheometer. As the compound heats up, its viscosity drops, which is reflected in a decreased torque (resistance) on the rotor. The lowest value of the torque curve, expressed in dN*m, is called ML (Moment Lowest). It is a measure of the toughness of an unvulcanized compound at a given temperature.
As the vulcanization progresses, the torque increases. The time from the start of the test to the moment when the torque value increases for 2 dN * m above the ML value is called Ts2. The Ts2 value is measured in time units and indicates when the compound starts to vulcanize.
The closer we approach the finally crosslinked compound, the higher is the torque. The slope of increase depends on the type of crosslinking used. After a certain time the curve normalizes. The highest value of the torque curve, expressed in dN*m, is called MH (Moment Highest).
In fact, there are three situations possible. With certain types of compounds (NR, CR), the opposite effect and the risk of over-vulcanizing the compound are possible, which is reflected in the insufficient hardness of the final products. However, it may happen that the curve continues to rise, which is typical for EPDM compounds.
The time from the start of the test to the point where 90 % of the MH value is reached is called t’90.