Table 2 shows the tc90 cure times and torque differences for NR compounds filled with various weight fractions of precipitated silica (PSi) in a hybrid filler of fly ash silica (FASi) and precipitated silica. It was found that the PSi fraction had no effect on the cure time of the silica hybrid-filled NR compounds at low total silica loading (10 phr). The cure time ranged from 5:03 to 5:24 min. For high total silica loading (40 phr), the cure time of NR compounds appeared to increase with the PSi fraction. This can be explained by the acidity of precipitated silica on the filler surface and by interaction with basic accelerators, which caused a delay in the vulcanization reaction [16–17]. As the PSi filler ratio increased, the acidity of the composites increased for a fixed content of polyethylene glycol (PEG). The cure time did not appear to change with different FASi particle sizes in FASi/PSi-filled NR compounds. Delta torque is the difference between the maximum and mini- mum torques obtained from an ODR rheograph, which indicated the rigidity of overall composites, as a result of the filler content, crosslink density, and rubber-filler interactions in the vulcani- zates [17–20]. Therefore, the delta torque values of NR com- pounds filled with high total silica content were higher than those with low total silica content. The incorporation of PSi evidently had a significant effect on delta torque values, which increased with PSi content; this is because PSi particles are much smaller in size than FASi particles, resulting in a greater overall surface area for the filler to interact with the rubber molecules [21,22]. It was also observed that an increase in delta torque indicated an improvement the rubber-filler interaction in rubber compounds filled with small-sized FASi particles; this occurred in the case of a
0% PSi fraction in FASi/PSi-filled NR composites. For silica hybrid- filled rubber vulcanizates with additional PSi, FASi particle sizes had no effect on delta torque.