Reaction pathways for surface activated rubber particles

Abstract

This paper studies molecular level interaction mechanisms involved in surface activation of rubber particles when exposed to a newly developed bio-modifier and microwave irradiation. It further examines how surface activation of rubber promotes its interaction with asphalt molecules to reduce phase separation in rubberized asphalt-binder. Using quantum-based calculations, chain reactions between an amide-rich bio-modifier and irradiated molecules of rubber is examined. The density functional theory (DFT)-based molecular modeling was performed on amide-type molecules as representatives for the bio-modifier. Comparing interactions of amidyl radical (RCO-N•H) of bio-modifier with sulfur-centered and carbon-centered radicals of rubber showed that amidyl radicals have higher affinity to bind to carbon-centered radicals than to sulfur-centered radicals. While the spine triplet state for ‒N…S‒ reaction coordinates encounters the obstacle of spin block and activation barrier, no spin block is observed on the ‒N…C‒ reaction pathway. The latter reaction was further observed via FTIR spectra where a new peak appeared at 1040 cm−1 indicating formation of CN bonds. Aforementioned reactions led to grafting highly polar molecules of bio-modifier onto the surface of rubber increasing rubber’s surface polarity. This was evidenced by the increase in acid-base component of rubber’s surface energy from 2.9 mJ/m2 in control rubber to 14.6 mJ/m2 in treated rubber. The increased polarity increased the interactions of rubber with asphalt molecules reducing the phase separation commonly known as segregation in rubberized asphalt. The latter was reflected in reduction of differences in complex modulus (G*) between top and bottom parts of rubberized asphalt specimens from 53.69% to 0.05%.