Increase in Stiffness during Cure Isothermal MDR

2 INCREASE IN STIFFNESS DURING CURE ISOTHERMAL MDR 3.2.1 Presentation oe the Technique 

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. 

The most obvious changes in a rubber mix when vulcanization sets in are an iuCTease in stiffness and an increase in the elastic component of the visco-elastic deformation. The Mooney viscometer offered a convenient way of measuring scorch and even rate of cure according the standard method given in ISO 667. 

Laboratory measur ent of curing characteristics have been somewhat revolutionized by the introduction of the cure meters. There were two types of cure meter the reciprocating paddle type as in the Wallace-Shawbury curometer, and the oscillating disc type such as the Monsanto rheometer. In the oscillating disc type (ODR) a biconical disc is embedded in the rubber in a closed cavity. The disc is oscillating through constant angular displacement, and the torque required monitored. 

The oscillating die rheometer (ODR) and the moving die rheometer (MDR) have been developed and maiketed by Monsanto, the MDR being introduced in 1985 [16]. In the MDR, a thin sheet of rubber, around 2 mm thick, is placed between the two dies kept at the desired temperature the lower disc oscillates and a reaction torque/ pressure transducer is positioned above the upper disc. It has been found that the MDR gives shorter times of cure than the ODR because of better heat transfer and higher torque values, owing to the die design. 

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