How does it work?
There are two types of rheometers used in rubber industry, an ODR (oscillating disc rheometer) and an MDR (moving die rheometer), their main purpose and role, however, are the same.
A test piece of raw rubber compound is placed into the cavity of the rheometer, the rheometer is closed which puts the rubber compound under positive pressure at a specific constant temperature, which depends on the type of compound and the cross-linking system used. In case of ODR rheometer, a biconical rotor inside the cavity oscillates back and forth by a rotary motion, usually by +/-3˘. This action exerts a shear strain on the test piece. A torque required to oscillate the disc depends on the stiffness (i.e. shear modulus) of the rubber compound.
As the cure rate of the compound in the cavity progresses, the stiffness of the specimen and therefore also the torque on the rotor increases. Since rubber is a non-Newtonian material, it exhibits a viscous and elastic behaviour. The shear modulus is directly proportional to the cross-linking density. Rheometer thus measures a degree of cure, over time, at a given temperature. A plot of measured torque value, over time, produces a typical curing curve.
What does the rheometric curve tell me? What are ML, MH, Ts2 and t’90?
The obtained rheometric curve is a fingerprint of compound's curing and processing behaviour. Parameters like ML, MH, Ts2 and t’90 can tell much about it.
First, a compound at a room temperature is inserted into the cavity of the rheometer. As the compound gets heated under pressure, the viscosity drops and the torque exerted on the rotor decreases. The lowest torque value recorded on the graph, measured in dN*m, is called ML (Moment Lowest). It is a measure of stiffness of uncured rubber compound at a given temperature.
As the curing starts, the torque rises. When the torque increases 2 dN*m units above ML value, the time is recorded. Ts2 is the time from the beginning of the test to the time the torque has increased 2 units above ML value. It is measured in time units and provides an information about scorch time or at which point the curing actually starts.
As the curing progresses, the torque increases further. The slope depends on the compound and curing system used. After some time the torque typically attains maximum value and it plateaus out. The highest torque recorded on the graph, measured in dN*m, is called MH (Moment Highest).
In fact, as the torque starts to flatten out, there are 3 scenarios possible. With certain types of compounds (NR, CR), a danger to over-cure the compound is possible. This is known as a reversion in cure, typically flowed by drop in hardness and other physical properties. It can also happen that the torque continues to rise. Such phenomenon is called marching and is typical for EPDM based compounds.
Time from the start of the test to the point where 90% of the MH value is reached is called t’90. It is measured in time units.