Rheometer

Elastic seals are manufactured by cross-linking rubber mixtures, which consist of raw rubber and other mixture components. The molecular chains are cross-linked with one another under the influence of temperature, pressure and time, giving the elastomer material its final, elastic properties.

What is a rheometer and what is it used for?

A rheometer is a laboratory device for measuring the viscoelastic properties of non-Newtonian materials (rubbers) during the cross-linking process. It is used for incoming goods inspection and release testing of rubber compounds before further processing and provides information about the flow and vulcanization properties of the compound . The rheometer is used to determine the degree of cross-linking at a certain temperature and time period.

In a rheometer, a part of a two-part metal holder, into which the round-shaped test specimen of a raw rubber mixture is placed, oscillates back and forth in a rotary motion, usually by +/-3°. The test specimen holder is located in a closed cell, which is heated during the oscillating motion.

A torque curve is measured and graphically displayed across the two axes of temperature and time.

What does the rheometer curve say?

As soon as the rubber in the rheometer is subjected to the oscillating gravity movement under pressure and temperature, the initial viscosity of the raw rubber mixture and thus the torque exerted on the rotor decrease slightly, and the uncrosslinked rubber initially becomes softer. The lowest point of the recorded curve is called ML (Moment Lowest). It is a measure of the stiffness of the uncrosslinked rubber.

As time progresses and cross-linking begins, the torque increases sharply. The slope of the curve depends on the type of rubber and the cross-linking system used .

After some time, the vulcameter curve reaches its highest value and then remains relatively constant. The highest point of the recorded torque curve is called MH (Moment Highest).

The difference between the highest and lowest torque can be used to determine so-called percentage crosslinking rates. The associated time is specified. The time at which, for example, 10% of the torque difference is reached is referred to as tc10. The time available for the problem-free filling of an injection mold can be read at point tc2 and refers to the time before the rubber mixture begins to vulcanize. At tc90, almost complete crosslinking is achieved, which can now be completed by tempering.

If the sharp increase in torque levels off again when complete cross-linking is achieved, two further curves are conceivable. With rubbers ( NR , CR ) a so-called reversion (also called over-cross-linking) is possible. This represents a reversal of the cross-linking and is accompanied by a decrease in hardness and other physical properties. With EPDM, however, the torque can increase even further.

Each rubber compound has its typical values for MH, ML, T2, T10 and T90. The vulcanisation curve therefore represents a kind of fingerprint of the cross-linking and processing behaviour of the rubber.

Synonym: vulcameter curve, rheometer curve