Mooney index

Figure 3 shows that the delta Mooney indexes show an abrupt change from negative to positive values in the same region. Thus, there is a clear relationship between the onset of dominant crosslinking and processability. The deterioration in physical characteristics is not coincidental with the formation of gel since, in both of these examples, the gel point occurs at conversions greater than 60%. 

Figure 3. Delta Mooney indexes (no oil) as a function of conversion for series A (a) and series B (O) (3)...

The Mooney index is usually measured under the conditions, i.e., time and temperature, where the material behaviour is rubbery and not flow. Therefore, what is measured is not a viscosity. Yet, it is erroneously called Mooney viscosity. The deformation and fracture may be observable using coloured markers on the rubber specimen. 

In production control, GPC data are sometimes collected routinely along with measurements of the Mooney index [14]. When these two sets of data are compared for hundreds of production lots, in only about 50% of cases may a correlation be found. This is partly because the Mooney index is a very crude measure of rubber property, but the uncertainties described above on GPC data are also responsible. 

The Mooney index is often called Mooney viscosity hut there is no rationale that the index represents viscosity. It is an expression of some aspect of viscoelasticity in that there may be some relationship to the processability. The method uses an instrument with a particular design, which provides a simple operation and a quick result. Compared to the dilute solution method, the sample size required is much larger so that it is more representative of the commercial lot. The Mooney index has been used worldwide as a specification for gum rubbers. 

However, there is no firm theoretical basis for this index. It is only a very crude method of differentiation between samples for the same value of the index, the processability of two gum rubbers may be different. In other cases, with rather wide differences in the value of the Mooney index, two rubbers may process almost equally well. 

There have been attempts to interpret the meaning of the Mooney index. Often it is assumed that the measurement concerns steady state viscosity at a prescribed temperature. 

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. 

An example with an ethylene-propylene rubber of Mooney index 40, is shown in Figures 6.10a and 6.10b. 

On the other hand, an NBR with a Mooney index of 35 and acrylonitrile content of 33% does not obey equation (6.35), see Figure 6.11a, but obeys the non-linear equation (6.37), see Figure 6.11b. 

The samples were two commercial ACMs having a Mooney index of about 50 [14]. The carbon black used for the mill processability was a standard N330 [37] 50 parts per 100 parts of rubber by weight was used. 

Type Trade Mark Code for gum Code for compound Mooney index of gum " % gel of gum Carbon black, N550 (phr)... 

Sample Designation Catalyst Type cis Content Mooney Index of gum Filler Loading (phr)... 

The sample used was Hycar 4051EP (ACM), a product of the BFGoodrich Company. The Hycar homoploymer, has a glass-transition temperature of -14 °C and a Mooney index (ML-(1 + 4) at 100 °C), in the range of 35-50. 

The formulation is given in Table 10.1. The raw elastomer was an NBR copolymer powdered rubber with a Mooney index of 80 (ASTM D1646 [7]). The powder was preblended with a high-intensity Henschel mixer. 

Because the Mooney index [11] of SIR-10 was about 100, it was necessary to masticate the compound for several minutes to bring it down to 70 to 75. This may be called the zeroth stage. 

There are two aspects in the question of uniformity. The first one is a black swan , so-to-speak. If the specification is for the shape of the bird, a black one will not be noticed. This is illustrated in Figure 13.2, by two torque-time curves obtained in the usual Mooney index measurements [2]. These two rubbers give exactly the same torque at 4 minutes of the rotation, e.g., ML (1+4), and yet their behaviours are very different. If the specification is the value of ML (1+4) only, the black swan goes unnoticed. 

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 ... 

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