Editor’s Note: This is the first article in a 3-part series on How to Relate Test Results of a Torque Rheometer to Problems in Elastomer Processing
Elastomeric (rubbery) material can be manufactured into a variety of everyday products, including tires, tubes, gloves, adhesives, tapes, carpets, etc. The way the material is processed can affect the outcome of the finished product, however. In addition, the activities surrounding compounding (design, mixing and extrusion) are very costly and time consuming. As a result, more and more manufacturers have started to bring compound-development and processing closer together and are actively looking for methods of linking laboratory-scaled test results with production experience.
To meet these requirements, test methods and development tools have to be meaningful and process-related. The documentation of test results and a comparison to accepted standards and tolerance levels is a must in order to meet the requirements stipulated by different quality standards like e.g. SPC ISO 9000. Often a torque rheometer is used to test materials and help meet quality control demands.
Torque rheometers have been in use in the rubber industry for many decades with great success. They treat small rubber samples in a way similar to the mixing conditions encountered in internal production mixers. They allow the grading of polymers with respect to their behaviour during mastication and show how compound ingredients change viscosity.
Process simulation
Mastication, the process by which the viscosity of natural rubber is reduced to a suitable level, is an important step in the mixing process. Rubber polymers such as Natural Rubber (NR) require mastication to reduce their viscosity/elasticity and to even out variations between different polymer lots. This process can be simulated by a torque rheometer.
Compounds with curing agents are tested for minimum viscosity, the onset of scorching and for the rate of cure.
These tests allow the assessment of the initial flow properties of the sample: the torque peak caused by the high resistance of polymer being sheared while still cold. Then one can evaluate the process of mastication.
The figure below combines the two mastication test runs for an easy comparison:
- Mastication of a natural rubber (“NR“) just under the influence of shear and temperature;
- Mastication as above but with the additional influence of a chemical additive called “Renacit 7“ added to the polymer at a level of 0.5 %. This accelerates the breakdown of the molecular structure and greatly reduces the mixing energy.
The advantage of this test procedure can be easily seen. The compound designer can determine the influence of different additives on the compound by using a small sample volume; the process engineer can optimize the mastication process with respect to the mixing energy and parameters. The biggest advantage attainable as a result of the above test procedure is extensive cost savings by reducing tests on production scaled machines.
Batch differentiation
Rubber polymers are distinctly “non-Newtonian liquids“ (i.e. liquids whose viscosity is strongly dependent on the shear rate). If polymers only differ e.g. by their molecular weight, they can still have equal viscosity levels at high shear. Differences between these polymers are shown more clearly at lower shear rates or rotor speeds.
The below chart illustrates the mastication of three NR samples which originated from three different countries. We used the same torque rheometer setup to test the materials.
The first test segment (0-7 min/70rpm) serves for the mastication of the samples. The high shearing forces are breaking the polymer chains and tear them apart, in order to obtain a lower viscosity. In the diagram this can be identified by the reduction of the torque to a plateau. But at 70 rpm the torque curves of the three samples are too close to each other, thus preventing a differentiation. The torque rheometer can be programmed to reduce the rotor speed automatically after a defined time, in this test down to 5 rpm. At this lower shear rate, the three samples can now be easily differentiated.
For more details, read How to Relate Test Results of a Torque Rheometer to Problems in Elastomer Processing.
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