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The TMS rheometer.

The TMS rheometer

Possibly the most serious omission in the rubber testing laboratory is the routine measurement of processability - that almost indefinable quantity upon which the production of a saleable item is dependent. This situation still exists despite the high cost of waste material and the existence of instruments capable of measuring processing characteristics. Coupled with increased quality awareness in the polymer industry, there is a necessity for more efficient production to meet today's demanding specifications. The philosophy has to be "get it right first time." Many instruments have been made available for assessing processability with an ever increasing degree of sophistication over the years. In particular, there has been a reliance on Mooney viscometry as a measure of processability that far outweighs the rather vague rheological measurement obtained. It was against this background that several years ago, three workers at a UK tire company put their thoughts together to develop a variable speed rotor-in-die configuration instrument to assess processability. This has now developed to the point of commercialization. Those three workers were Turner, Moore and Smith - the instrument is the TMS Rheometer.

This instrument is similar to a Mooney viscometer in that both involve the rotary shear deformation of a sample enclosed in a heated cavity. However, the TMS differs in that cavity filling is by injection, the rotor is bi-conical to give uniform shear and the drive is by a stepper motor giving programmable speed control. Because of the uniform shear rate, results can be expressed in fundamental units enabling comparison with other rheometers. The variable rotor speed enables results to be collected over a range of shear rates with the versatility of speed control opening the way to a range of tests. Speed profiles are possible and closed-loop control of rotor torque can be achieved. It is only with microprocessor control of testing and data acquisition that the advantages of the TMS can be fully exploited. While the instrument's full potential has yet to be realized, results follow showing how the TMS can assess viscous flow and viscoelastic characteristics of polymeric materials as well as measuring the wall slip characteristics of practical extrusion compounds.

The TMS Rheometer is a bi-conical rotor instrument designed to give comprhensive information on the processing characteristics of raw elastomers, unvulcanized rubber compounds and other polymers. It represents a distinct departure from single-point testing in that shear stress and shear rate can be varied over a wide and useful range. In conjunction with excellent temperature control, the conditions found in practical processes can be simulated and evaluated.

The rotor is driven from a stepper motor which enables closed-loop control of either shear stress or shear rate to be programmed from the microprocessor controller. Testing sequences and data acquisition can be accomplished directly from the microprocessor, or an additional measure of versatility and data analysis can be achieved by linking a second level computer.

If we look at some of the instruments available in the marketplace (table 1) today, we can see how these can be tabulated according to the shear rate used and whether results are obtained in empirical or fundamental units. Clearly this indicates that the TMS does fill a gap in the existing technology.

Moving on to consider the shear rates found in practical processing operations we can see how (figure 1) the TMS does cover a wide and useful range. Because results are obtained in fundamental units, these can complement results from other test methods. This also illustrates the inadequacy of single point testing.

Four basic testing modes are available on the TMS; three of these are illustrated in figure 2.

Looking at transient flow, the transient high shear stresses are caused by rapid changes of shear rate. In processes such as transfer or injection molding, or the feeding of masterbatch onto a two-roll mill or into an internal mixer, the transient flow behavior can be more important than steady state flow. Transient flow behavior can be characterized by the TMS using test routines that record, at high data acquisition rates, the stress peak following a step increase of shear rate.

Moving on to consider steady state flow, the relationship between shear stress and shear rate under conditions of steady-state viscous flow can be obtained between shear rates of [Mathematical Expression Omitted] at temperatures between 50 [degrees] C and 200 [degrees] C. Test routines have been developed where the required number of shear rate steps are programmed into the instrument together with the run time at each step. The shear stress at each step is then recorded at a specified programmable data acquisition rate.

Finally, stress relaxation characteristics are measured by stopping the rotor after a conditioning run and recording the decay in shear stress with respect to time. The data shown was obtained from a commercially available butyl elastomer tested in a routine utilizing closed-loop shear-rate control. To study stress relaxation in detail the conditioning run is carried out in a routine involving closed-loop shear-stress control. This ensures that a standard programmable initial stress is used.

The fourth testing mode is used to study wall slip. A number of additives in rubber compounds promote wall-slippage (or boundary lubrication) between the compound and metal surfaces during processing, and we are all probably familiar with various process aids designed for this purpose. By comparison of results (figure 3) from a grooved rotor which inhibits slippage and a smooth rotor which promotes slippage, the TMS can measure wall slip velocity. Looking at the diagram we can see that a natural rubber compound considered difficult to extrude exhibits virtually no wall slip, whereas a high degree of slip is shown by an EPDM compound considered to have very good extrusion characteristics. Results of this type illustrate how the TMS can, and indeed is, being used to predict extrusion performance.

The data acquisition rate has been mentioned several times. This can be varied over a wide range with a maximum rate of 100 data points per second; these high rates are needed, of course, to study transient flow and stress relaxation.

The applications of the TMS can be split into four areas:

* raw material testing,

* mixed compound testing,

* materials and compound development,

* mold and die design.

Raw material testing: Variations in molecular weight, molecular weight distribution and chain branching all have an important influence on processability; this influence extends over the full range of flow rates encountered in practical processes. Clearly a single point measure may not identify such variations, whereas measurements over a range of conditions certainly will. With the TMS rheometer it is possible to design compact testing routines which with reference to the process being operated can monitor the critical aspects of raw elastomer behavior.

Mixed compound testing: In conjunction with cure testing, the TMS rheometer provides a means of controlling the consistency of material supplied to downstream processes such as extrusion and injection molding. It will also enable, in conjunction with the expertise of the rubber technologist, corrective action to be taken at the mixing and material supply stage.

Material and compound development: With growing experience of the use of the TMS it will be possible to define rheological properties for optimum processing behavior. Raw elastomer and rubber compound development can then be directed toward these ideal properties.

Mold and die design: There is a substantial body of information available on the quantitative design of extrusion dies and injection molds for high productivity. In order to use the design techniques effectively it is essential to have rheological properties measured over a range of conditions and expressed in fundamental engineering units. The testing modes and routines indicated earlier can provide this data. As stated earlier, the TMS is a bi-conical rotor instrument, with the sample (figure 4) being injected into the test cavity. This method ensures that:

* the sample size is not critical,

* a known and controllable hydrostatic pressure is applied during the test,

* rubber is eliminated from the sealing faces of the die,

* newly formed rubber surfaces are presented to the rotor.

These points are all conducive to good test-to-test repeatability, with the last point in particular being essential for viable wall slip measurements.

The characteristics of the TMS Rheometer offer a machine with the sophistication required by the research technologist coupled with the ease of operation required for a process control system. [Table Omitted] [Figure 1 to 4 Omitted]
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Title Annotation:measure of processability of rubber
Author:Trim, R.S.
Publication:Rubber World
Date:Nov 1, 1989
Previous Article:Crosslinking system effect on processing behavior and performance profile of HNBR.
Next Article:Developments in Rubber Technology, vol. 4.

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