The Influence of Syringic Acid and Erucic Acid on the Antioxidant Properties of Natural Rubber: Experimental and Molecular Simulation Investigations

Abstract

In this work, the influence of syringic acid (SA) and erucic acid (EA) on the oxidation resistance of natural rubber (NR) was investigated by combining experimental and computational methods. The antioxidant activities of SA and EA were predicted by calculating the enthalpy of bond dissociation (BDE), the anti-migration ability of antioxidants (AOs) in the rubber matrix by calculating the mean square displacement (MSD), and the effect of antioxidants on oxygen barrier properties of rubber materials by calculating the permeability coefficient (P). The predicted result is that EA has a better comprehensive performance than SA. The DPPH (2,2-diphenyl-1-picrylhydrazyl) test and mechanical properties test demonstrated the results predicted by the simulations. Both SA and EA can protect natural rubber, while EA has a better comprehensive effect.

Keywords: antioxidative activity; molecular dynamics simulation; natural phenolic antioxidants; natural rubber; quantum mechanics simulation.

Figure 1

Thermo-oxidative aging process of RH and the anti-oxidation reaction of AH.

Figure 2

Example of the thermodynamic cycle for SA. (White spheres, red spheres, and gray spheres represent H atoms, O atoms, and C atoms, respectively).

Figure 3

Construction processes of three types of amorphous cells (take syringic acid, for example, and the purple, white, and red spheres represent C atoms, H atoms, and O atoms, respectively).

Figure 4

The bond-breaking positions of hydrogen in (a) syringic acid, (b) erucic acid, and (c) natural rubber repeat unit.

Figure 5

MSD curves of SA and EA in NR matrix.

Scheme 1

Principal mechanism of radical scavenging reaction.

Figure 6

DPPH radical scavenging rates of syringic acid (SA), erucic acid (EA), and BHT.

Figure 7

The mechanical properties of NR and AO/NR systems at different aging times: (a) tensile strength; (b) elongation at break.

Figure 8

(a–c) The FT-IR spectra of NR composite before aging and after aging 1, 3, and 5 days and (d) the carbonyl increase ratio of A_(C=O)/A_(CH2) for NR and AO/NR composites at 100 °C.