The rheometric characteristics of NR mixes containing two
different adhesion promoters are given inTable 2. Manobond-16,
a very common grade of cobalt boron acylate, provides satisfac-tory adhesion between NR and brass-coated steel cord. It was
chosen as a base line to evaluate the performance of the kaolin
modified resin (KMR), as an adhesion promoter. Scorch time (tS2)
is a measure of the time at which vulcanization of the rubber mix
begins. It is clear formTable 2that the addition of adhesion
promoters reflects better scorch time safety up to 1.5 phr of the
promoter. The further increase in the promoter concentration
results in a decrease in scorch safety. The optimum cure time
(tC90) is the vulcanization time required to obtain optimum
physical properties. For mixes containing manobond promoter,
it can be seen that the optimum cure time (tC90) decreases
significantly with increasing the manobond concentration in the
rubber mix. On the other hand, for rubber mixes containing KMR
promoter, the cure time does not show a considerable response
with the variation of the KMR concentration.
The cure rate index (CRI) is the measure of the vulcanization
rate based on the difference between optimum vulcanization time
and scorch time. It can be calculated from the relation:
CRI¼100=ðtC90tS2