Scorch Time Measure

When the componnded rubber is heated at a high enough temperature and for long enough time, it will cure. Hence, there is not a clear distinction between a plasticity 

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]. 

Other laboratory instruments able to measure the curing characteristics, the so-called curemeters, are widely used for the routine control of fully compounded rubbers. 

Two types of curemeters are in common use the reciprocating paddle type, such as the Wallace-Shawbury curometer, and the osaUating disc type, such as the Monsanto rheometer. 

In the reciprocating paddle, a small paddle anbedded in the rubber, which is itself enclosed in a heated cavity, is reciprocated. Either the change in amplitude of oscillation at constant force or the change in force to prodnce constant amplitude is monitored as a measure of change in stiffness. 

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Another study by Sui et al. shows the vulcanization curves of the neat NR and NR composites. In Figure 21.11 it can be seen that the scorch time (measurement of premature vulcanization) decreased after adding CNTs or carbon black into the NR. This may be because when CNTs are added, they would absorb the basic accelerator species and delay the onset of vulcanisation. The reason for this is that the addition of the CNTs could improve thermal... 

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. 

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 cure characteristics for the compounds were measured according to the procedure described in Sect. 4.1.2. From the curemeter data shown in Table 13 it becomes clear that the SBR compounds containing coated sulfur show longer scorch times (ts2) and optimal curing times (t90) than the compound with uncoated sulfur. The only exception is for sample PPASg-4, which gives a similar optimal curing time to uncoated sulfur (Sg). 

The model described in Eq.(l) not only can predict the cure behavior measured by standard curometers, but also can explain filler effects on the cure reaction. The model enables one to predict scorch time and cure time of elastomers at various filler loadings and cure temperatures. In the following discussion, this kinetic model of cure will be extended to explain and predict the modulus or viscosity of elastomers/thermosets during... 

Measurements were taken on two natural rubber compounds rubber A with a very short scorch time, and rubber B with a very long one. For both rubbers, experiments were conducted at 120 C, 140°C, 160°C, 180°C, and 200 C. A Monsanto rheometer and a Wallace-Shawbury curometer were used, as well as isothermal apparatuses (see Table 1.1). 

The scorch time, which is the measure of premature vulcanization, and the optimum cure of vulcanizates increased. ... 

Cure characteristics, which include scorch time, cure rate and torque values, were measured over a 20 minute period at 160 °C using a Monsanto oscillating-disk rheometer according to ASTM method D1084 [22],... 

Mooney Scorch Time is the total running time to reach a viscosity of five or ten units above the minimum (ts or tio). It is used as a rough measure of the processable life of a compound under normal processing conditions -the greater the scorch time, the longer the processable life. A common test temperature for Mooney Scorch measurements is 121 C (250°F), but some processors may use different temperatures because of particular factory experience. 

The ODR measures the force (torque) required to oscillate a biconical disk (rotor) back and forth within a shallow cavity filled with rubber compound. As the compound cures, its viscosity increases and more torque is needed to move the rotor. The instrument plots a continuous curve of torque versus time. This curve is often called an ODR trace. From an ODR trace, the rubber compounder can estimate stock viscosity, scorch time, cure rate and cure state. Important test variables are the arc through which the disc oscillates, the speed of oscillation, and the test temperature. The usual arc is 1° (actually, this means 1 on either side of the starting point, or a total swing of 2°) some laboratories use 3 or 5 . Normal oscillation rate is 100 cycles per minute. Test temperature should be close to the expected cure temperature. 

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. 

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. 

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. 

The measurement of pre-vulcanisation properties is given in ISO 289-238. This is essentially a matter of running a test on a fully compounded material (including curatives) as in ISO 289-1 until the viscosity reaches a specified number of units above the minimum (usually 5 units). The time to this point is designated as the scorch or pre-vulcanization time. The temperature of test is chosen to suit the process in question. 

Tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanised material. The latter type of test estimates degree of cure by measuring the physical properties of test pieces vulcanised for various times, tensile properties, swelling and set measurements being the parameters most commonly used. 

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. 

The unique character of Neoprene vulcanisation is that rubber accelerators as a class are not effective. As a matter of fact several have measurable retarding effects and it has to be understood that mercaptobenzothiazole is an extremely potent retarder in Neoprene vulcanisation. Salicylic acid is a fast accelerator leading many a times to scorching or premature vulcanisation at higher dose levels [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. 

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. 

Scorch resistance is usually measured by the time required, at a given temperature, for the onset of crosslink formation, as indicated by an abrupt increase in viscosity. The Mooney viscometer has been used for this... 

Scorch resistance is usually measured by the time at a given temperature required for the onset of crosslink formation as indicated by an abrupt increase in viscosity. The Mooney viscometer is usually used [4]. During this test, fully mixed but unvulcanized rubber is contained in a heated cavity. Imbedded in the rubber is a rotating disc. Viscosity is continuously measured (by the torque required to keep the rotor rotating at a constant rate) as a function of time. The temperature is selected to be characteristic of rather severe processing (extrusion, calendering, etc.). 

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