Mooney Cure Time

Mooney Cure Time is defined as the total running time to reach a viscosity of 35 imits above the minimum (tss). It is used to estimate curing time at the test temperature. 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

A similar sol-gel process method, via NR rubber solution, was also conducted to study the effect of in situ silica content, which was varied from 15 to 65 phr, on the cure characteristics and mechanical, dynamic mechanical and thermal properties of the silica-NR nanocomposite.Both the Mooney viscosity and cure time of the sol-gel silica-NR compound increased with increasing silica contents and were lower than those of the commercial silica-filled NR compound at the same amount of silica. This is attributed to the fewer amounts of silanol groups in the sol-gel silica as compared to the commercial silica. Better reinforcement of the in situ silica, compared to the normal silica, was confirmed when higher moduli and improved compression set were observed for the sol-gel silica NR vulcanizate. This observation is consistent with the Guth and Gold equation as well as the TEM micrographs results. The sol-gel silica vulcanizate has lower storage modulus but better thermal stability then the commercial silica vulcanizate. 

Figure 9.1 Effect of blend ratio on Mooney viscosity (a) and scorch time (is2) along with optimum cure time (tc90) (b) of the compounds.

The Mooney scorch times of ENR mixes can be increased by the use of commercial prevulcanization inhibitors (PVI) once the base has been added. The response of these materials depends on the cure system employed and parallels that observed with NR and other unsaturated polymers. 

Mooney viscosities were determined with the large rotor after a one minute warm-up, followed by four minutes at 212°F. Mooney scorch tests were made with the small rotor at 250°F. Time to scorch was three units above the minimum and cure time was taken at eighteen units above the minimum. 

When added to a conventional accelerator/sulphur compound of NR containing 60 pphr precipitated amorphous white silica, the bifunctional organosilane bis(3-triethoxysilylpropyl)tetrasulphane (TESPT) had a profound effect on the viscosity and cure properties. The Mooney viscosity decreased when 7 pphr silane/60 pphr silica was mixed with the compound. The scorch time shortened when 3 pphr silane/60 pphr silica was added to the mix and the optimum cure time was also reduced when 7 pphr silane/60 pphr silica was introduced into the rabber. The rate of cure of the mix accelerated with 7 pphr silane/60 pphr silica. 11 refs. 

The potential use of palm oil fatty acid additive (POFA) as a new rubber ingredient in carbon black-filled NR compounds is examined. It is found that the cure time (t90) and scorch time decrease with increasing POFA concentration. The Mooney viscosity of the rubber compounds also decreases with increasing POFA concentration. The mechanical properties of the carbon black-filled NR compounds are enhanced by the addition of POFA, especially at a concentration of 2 phr. Results of the swelling measurement and scanning electron microscopy indicate that POFA has some effect on crosslink density and also improved filler dispersion. 9 refs. 

This article describes the performance of a new curative for polychloroprene, a derivative of 2,5-dimercapto-1,3,4-thiadiazole (DMTD). This curative provides long Mooney scorch times, rapid cure rate resulting in short cure times and an exceptionally flat cure plateau. Other performance characteristics are presented, including the effect of other additives in a typical polychloroprene formulation. 5 refs. 

Mooney viscometer is also used to measure the time it takes, from initial exposure of the compound to a particular temperature, to the time of onset of cure at that temperature [2]. This is known as the scorch time. Scorch time is an important parameter to the rubber processor, as a short time may lead to problems of premature vulcanization. As the test is taken past the onset of cure, the rotor tears the cured rubber, and therefore this device cannot be used to investigate rheological properties after the scorch time. 

A development of plasticity determination by means of the Mooney viscometer. The same instrument is used but the temperature of operation is higher and the test is continued until the sample vulcanises. A curve of Mooney reading against time is drawn, from which the scorch and cure characteristics are estimated (BS 903-A58). 

An early cure instrument which combined in one operation the functions of a Mooney viscometer and of a curometer or vulcameter, i.e., it measures in one quick test the plasticity (viscosity) of the (uncured) mix, its scorch time and cure rate. Now superseded by instruments such as the moving die rheometer. 

The Mooney viscometer offers a more convenient way of measuring scorch and even rate of cure, and a standard method for scorch is given in ISO 289-238 and the BS and ASTM equivalents39 40, as mentioned in Section 3.3 of this chapter. This is essentially the method of ISO 289-1 continued until the viscosity reaches a specified number of Mooney units above its minimum value, either 5 or 3 units depending on whether the large or small rotor is used. The minimum viscosity and the time to reach 5 or 3 units above minimum viscosity are reported. In earlier versions of this test the difference between times to 5 units and 35 units above minimum viscosity was taken as an indication of the rate of cure, but this measure of cure rate is now been superceded. 

Mooney viscosity—measure of the resistance of raw or unvulcanized rubber to deformation, as measured in a Mooney viscometer. A steel disc is embedded in a heated rubber specimen and slowly rotated. The resistance to the shearing action of the disc is measured and expressed as a Mooney viscosity value. Viscosity increases with continued rotation, and the time required to produce a specified rise in Mooney viscosity is known as the Mooney scorch value, which is an indication of the tendency of a rubber mixture to cure, or vulcanize, prematurely during processing. 

Whatever the reason, it will be assumed that the cure reaction obeys a first-order reaction, as said by various authors. And, thus, a reference temperature with reference time will not be necessary or even useful. However, it is useful to know the time at which the cure reaction starts for a rubber sheet. The Mooney viscometer is pretty well described in Reference [3]. 

As noted [15], when a compounded rubber is subjected to a plasticity measurement at a high enough tanperature and for a long time, it will cure, and consequently a plasticity test can be used as a test for scorch or rate of cure. Thus, the Mooney viscometer is used to measure scorch (i.e., the onset of vulcanization), and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure, as weU as the iuCTease in stiffness as curing takes place. Moreover, tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanized material. 

Parallel plate compression plastimeters have been used for measuring the rate of cure at various times and temperatures. The test pieces are heated for various times and then tested in the plastimeter. The change in plasticity is then plotted against time of heating to give a scorch curve. The Mooney viscometer offers a more convenient way of measuring scorch, and a standard method is given in ISO 667 [7]. 

As discussed earlier, the Mooney scorch test can measure time to scorch and provide some information regarding the rate of cure. However, because the Mooney rotor rotates continuously. slipping and tearing of the specimen occur well before the ultimate state of cure is reached. It is not possible to observe the entire cure curve with the Mooney viscometer. 

Mooney scorch is the time to incipient cure a rubber compound in a test in the Mooney shearing disk viscometer. 

The use of the amine hexadecylamine (HDA) to devulcanise sulfur-cured EPDM rubbers has been described by Dijkuis and co-workers [50]. Conventionally and efficiently vnlcanised (EV) EPDM rubbers were devulcanised in a Brabender Plasticorder with HDA under various conditions. The effects of temperature, time, shear and amonnt of reclaiming agent on devnlcanisation were examined nsing a design of experiments approach, and the devulcanisates were characterised by Mooney viscosity measnrements, solvent extraction, crosslink density determinations and light microscopy. 

T5 The scorch time at lower temperature is of importance too. This can be obtained by using a Mooney Viscometer at lower temperature. A Mooney Viscometer is also used to measure the viscosity of the compounds (important for dictating injection-moulding behaviour). The viscometer is also used to assess the tendency to scorch, and sometimes the rate of cure of a compound. A useful estimate of scorch behaviour is represented by T5, the time taken from the beginning of the warm-up period to that at which the Mooney value rises five units above the minimum value. 

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