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“Innovations in Cure Meter and Mooney Viscometer Technology” : Page 5
New Curemeter Innovations
Although the improvements in curemeter design described above solved many problems, we continued to be concerned about several problems and limitations that remain unresolved. One of these problems involves the selection of strain for rheometer testing.
The “dirty die/rotor” effect places severe restrictions on the selection of the optimum strain (arc) for a particular specimen. The strain must be low for stiff stocks (± 0.5°) but for soft stocks the strain can be much higher (±3.0°) without causing the slippage which occurs when there is excessive interfacial strain between the die / rotor and test specimen.
Thus, the operator is faced with a largely subjective decision on selecting strain (arc) for a particular rubber compound. A complicating factor in making this selection is the desire to also obtain useful viscosity data during the pre—cure period. At low strain (arc) this data is of limited value.
Auto-Strain Adjustment:
The Wise Spring
Therefore, it is desirable to have the test instrument adjust the imposed strain to reflect the stiffness of the specimen throughout the overall process. This can be easily accomplished, with features incorporated in the XDR®, by subjecting the test specimen to high strains during the pre—cure period and subsequently reducing the strain as cure proceeds and the stiffness (torque) increases, i.e., reduce the strain in proportion to the stress.
The Wallace—Shawbury curemeter [4,5] did exactly that. However, there was no provision for extracting the stress / strain values. It yielded a desirable expanded curve prior to the onset of cure but an insensitive curve afterward.
This auto—strain feature can be effected by:
(1) introducing a deformable member (spring) between the rotor and the drive system;
(2) measuring the strain at the rotor shaft, the XDR® has a strain transducer for this purpose;
(3) providing two continuous recordings of torque versus time throughout the test:
  (a) torque vs. time and
  (b) torque / strain vs. time (virtual modulus).
This is accomplished with the XDR® as shown in Figure 9.
In this rheograph, the initial arc under no load conditions was ± 2.1°. The deformable member had a spring constant that will reduce the arc to ± 0.5° at 40 in—lb torque. Note that the sensitivity in the viscous region and better defined scorch point could be further enhanced by using a higher initial arc (± 6.0°) and a deformable member that will reduce the arc at a higher torque (± 0.5°) in the 50 in—lb range.
Also note how the strain is reduced in proportion to the stress, eliminating the need to preselect the arc, one instrument setting could be used for all samples.
There is a caution, however, that should be exercised when operating at very high strains. The temperature of the test specimen can increase above the temperature set—point due to hysteresis, particularly prior to crosslinking and / or in very hysteretic compounds, for example with butyl rubber.
As a historical note in this regard, a curemeter design based on this concept (temperature probe) was patented in 1971 [17]. In view of this situation, use discretion when operating at high strains (arc) particularly with hysteretic materials.
Determination of Crosslink Type:
The Coran Button
A novel method was devised by Coran [20] to determine the relative amount of exchangeable (polysulfidic) crosslinks and nonexchangeable (monosulfidic and carbon—carbon) crosslinks using a curemeter.
This simple extension of conventional cure testing is easily accomplished by stopping the test at the peak of a strain cycle and measuring the logarithmic decay of the stress signal. Figure 10 shows a cure curve on the NR formulation, defined in Table I, with the decay curve superimposed.
This largely overlooked technique allows curemeter testing to be extended beyond the determination of physical properties and cure characteristics, making vulcanizate crosslink structure information also available.
Clearly, the compounder, who is charged with altering the compound composition to meet curing requirements, is equally concerned with the effect these changes have on the nature of the crosslink structure in the resulting vulcanizate.
The XDR® software incorporates a selectable “Coran Button” that provides this analysis at the end of a cure test if desired.
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Copyright © 2006 CCSi, Inc. • All Rights Reserved • Published February, 2006
Corporate Consulting, Service & Instruments, Incorporated
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