Screening natural antioxidants for polypropylene through molecular simulation and elucidation of their antioxidant mechanisms

Polypropylene (PP) is prone to oxidative degradation during practical applications, thereby significantly limiting its service life. The present study integrates multiscale simulation techniques with experimental validation to systematically screen and identify highly effective antioxidants from 45 natural phenolic compounds, while elucidating their underlying protective mechanisms in mitigating the thermo-oxidative degradation of PP. The O-H bond dissociation energy (BDE), molecular mobility, and oxygen barrier properties were comprehensively evaluated through a combination of quantum mechanics (QM), molecular dynamics (MD) and Monte Carlo (MC) simulations. Solubility parameter (δ) analysis showed that thymol, eugenol, and carvacrol exhibited the highest compatibility with PP, with thymol showing the smallest δ difference (1.366 (J·cm^-³) ¹^/²). Simulations demonstrated that thymol possesses outstanding free radical trapping ability, as evidenced by its relatively high bond dissociation energy (BDE: 163.728 kJ/mol), along with a low diffusion coefficient (D) (0.0112 × 10^-⁶ cm²/s), low permeability coefficient (P) (1.493 × 10^-⁹ cm²·s ^-¹·kPa^-¹), and a reduced reaction energy barrier, indicating long-term stability and highly efficient. The optimal antioxidant efficacy of thymol was systematically validated through thermo-oxidative aging experiments, employing advanced analytical techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), melt flow rate (MFR) measurement, mechanical property characterization, and oxidation induction time (OIT) determination. DSC results showed a 2.9 °C increase in PP melting temperature, supported by XRD data indicating enhanced crystallinity. XPS measurements indicated that thymol exhibited the minimal alteration (47.13 %) in the oxygen-to-carbon (n(O)/n(C)) ratio following aging. Mechanical testing showed that thymol/PP retained 62.73 % elongation at break after 5 days of aging, significantly outperforming pure PP (failure after 3 days) and other antioxidant systems. OIT tests revealed a 173.86 % increase in PP's OIT value with thymol. This study provides a robust theoretical foundation and practical guidance for developing eco-friendly antioxidants for PP.