Mooney relaxation curve

The so-called Mooney relaxation curve is another test, which has been the subject of discussion recently. The meaning of this relaxation curve is rather complex and will be discussed in section 13.4. 

A proposal to use the Mooney relaxation curve, arises from a realisation that the standard Mooney index alone is not adequate to specify a gmn rubber. However, the proposal is based on a convenience rather than a scientific rationale. Since the standard Mooney measurement is made routinely for a given rubber, by stopping the rotation at some time at or after 4 minutes and by holding the rotor at that position, it is hoped that the relaxation curve yields additional information on the material. But what information is gained from the relaxation curve  

As explained earlier the peak of the curve is a failure point [9]. The mode of failure appears like a slippage at the interface between the rotor of the rheometer and the rubber specimen, when it is marked with a magic marker . Because the surfaces of cavity and rotor are serrated, some fracture must have occurred even though the overall behaviour appears to be a slippage. Once the failure occurs, the remaining strain is an unknown quantity. Therefore, the scientific meaning of the relaxation curve becomes obscure. 

When the rubber is in the melt state, the stress-over-shoot (see section 4.3) occurs and it is not a failure point. A rubber s behaviour may be in the melt state if it is a low MW rubber, at a high temperature or at a low deformation rate. 

What the relaxation curve means if the relaxation curve is obtained by stopping the deformation before, at and after the peak of overshoot, had been described in Chapter 6 [10, 11]. The sample was EPR and temperature was 100 C. The relaxation curve was 

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Ag (or Aq) = 0 represents the contribution of the glassy-relaxation process to the relaxation modulus Gp t). For samples A and B, the curves calculated without both the glassy relaxation and Rouse-Mooney normal modes are also shown the area between a thus calculated curve and that with Ag = 0 represents the contribution of the Rouse-Mooney normal modes to Gp t). Also indicated in the figure are the positions of the relaxation times of the Rouse normal modes in sample C and of the Rouse-Mooney normal modes in samples A and B. 

Fig. 14,17 Comparison of the Gp(t) curves of samples A (middle figure), B (bottom one) and C (top one) at individual glass transition points or AT = 0. In each figure, the relaxation times of the Rouse-Mooney normal modes (for samples A and B) or the Rouse normal modes (for sample C) are indicated (+) the short-dash line is calculated with Aq or Aa = 0 and the long-dash line is calculated with Aq = 0 and the contribution of the Rouse-Mooney normal modes set to zero. The vertical dotted line represents the structural-relaxation time ts = 1,000 sec. The individual points in time at which the three Gp t) curves cross the horizontal dotted lines at 10 dyn/cm represent the structural- (or a-) relaxation times as usually defined, which are all very close to ts = 1,000 sec.

The majority of published work on extrusion behaviour deals with compounded stock. Those papers reporting work on raw rubbers have usually been on the use of capillary rheometers to determine extrusion properties at higher shear rates than are possible with Mooney viscometers. Capillary rheometers are, in principle, quite simple to use, and the application of electronic, minicomputer and laser technology has reduced the operation and data analysis to a routine task. There are no standard ASTM or other test procedures, but under a specific set of conditions, once a material is characterized, the data can be used as standard for comparison of all subsequent batches. It is readily possible to characterize a raw rubber by an extrusion experiment to determine the viscosity/shear rate curve, extrudate swell, and stress relaxation.Both Sezna and Karg have shown how the Monsanto Processability Tester (MPT), a modified, computerized extrusion rheometer, can be used in predicting mixing behaviour. The MPT (shown schematically in Fig. 7) is a most versatile instrument. It has a larger than conventional barrel for minimal pressure drop in the barrel, a pressure transducer at the entrance to the orifice, a microprocessor system, and a laser device for... 

The standard procedure [14] requires 1 minute of preheating before the rotation is started. Because the temperature is set at for example, 100 °C, the sample is not even in temperature equilibrium. The procedure is obviously a compromise for obtaining the data quickly. The rotational speed being 2 rpm, most of the rubber at 100 °C shows stress-over-shoot (see section 4.3) [6]. The torque value is recorded after 4 minutes of rotation and expressed as the Mooney unit. At 4 minutes, the torque is usually past the peak and in the relaxation part of the curve but not in the steady state. This is shown in Figure 6.1, for somewhat exaggerated examples, which gave the same Mooney index but vary in viscoelastic behaviour. 

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