Abstract:
In a very thick application such as tire, not only the design of the vulcanisation system but also
the technique used to measure the level of vulcanisation is important. A traditional method employed to
inspect the curing efficiency is a direct temperature measurement by thermocouple. The disadvantage of
this measurement is that the tire must be destroyed. Therefore, an innovative technique to determine the
vulcanising level using ultrasonic waves was proposed to solve the problem. The aim of this study was
to follow the crosslinking determination of rubber vulcanisates by using ultrasonic testing which is an
application of nondestructive technique.
The first section of this study focused on the capability of ultrasonic measurement to determine
crosslink density of rubber. It was apparent that ultrasonic velocity was not sensitive to the difference in
crosslinking in this range. The change in attenuation of ultrasonic signal was only observed. It indicated
that better propagation of ultrasound wave occurred when the chemical network was more densely
produced. Good accuracy and precision of the ultrasonic measurement could be obtained by controlling
the applied force against the transducer to rubber sample. The highest sensitivity to the variation in
crosslinking was achieved by using 5 MHz frequency of transducer and the deviation was reduced when
the measurement was performed at fifty times. For the effect of thickness, a decrease in attenuation
coefficient with increasing specimen thickness might be due to the poor distribution of crosslinks in
rubber. However, the 9 mm thick rubber sample was the most suitable for ultrasonic testing in this
study. Furthermore, the small difference in crosslinking would be clearly determined by ultrasonic
measurement when the elastomer was completely vulcanised.
The second part of this study focused on monitoring the change in crosslinking of vulcanisates
with the rubber additives by ultrasonic measurement. The results revealed that the ultrasonic technique
could not be employed to investigate the variation in crosslinking in filled natural rubber compounds
with an increase in surface area of filler and carbon black loading because the obtained attenuation
results were interfered by the effect of filler-elastomer and filler-filler interactions inside rubber
molecule. Furthermore, the crosslinks in oil filled natural rubber compounds could be followed by
ultrasonic measurement when oil loading was varied. However, types of oil did not affect the change in
the loss of ultrasound intensity