Aminoxyl (nitroxyl or nitroxide) radical formation by the action of ozone on squalene containing secondary aromatic amine antioxidants

Abstract

Squalene, a model compound of polyisoprene or natural rubber, was employed as solvent to study the reaction between ozone and a series of N,N′‐substituted‐p‐phenylenediamines (PPDs) and other secondary aromatic amines. In all cases, it was found that nitroxyl radicals were formed nearly quantitatively by the action of ozone on the squalene solutions containing dissolved the selected additives. The resulting nitroxyl radicals from PPDs or from other secondary aromatic amines were easily detected by electron spin resonance spectroscopy at room temperature and the isotropic and anisotropic hyperfine interaction constants of the said nitroxides were determined. The FT‐IR spectroscopy was employed to show the easy oxidation of unprotected squalene when exposed to an ozone stream, while in presence of PPDs, ozone is quantitatively scavenged by the PPDs forming the resulting nitroxides and protecting completely squalene from ozonolysis reactions. The nitroxyl radicals formed by PPDs or secondary aromatic amine ozonation exert a permanent antioxidant activity entering in a redox cycle already known to ensure a long flex fatigue life or the given rubber compound.

Molecular Insights into Antioxidant Efficiency of Melanin: A Sustainable Antioxidant for Natural Rubber Formulations

“…In addition to the binding energy ( E binding ), the solubility parameter can be used to evaluate the compatibility of the antioxidant and NR, where the latter could not only dictate the permeability rate of O 2 inflowing the NR matrix but also ensure the reactivity among oxygen and the NR chain. − Therefore, suitable dispersibility becomes essential for screening antioxidant molecules. The solubility parameters could be computed using a semiempirical approach as follows. Δδ = | δ antioxidant − δ NR | …”

“…External stimuli, including light, heat, ozone, and oxygen, could significantly deteriorate the physical or chemical properties of NR products. − Thus, enhancing the NR’s oxidative aging resistance is of primary importance for physical or chemical property enhancement and for economic reasons. N -phenyl- N ′-(1,3-dimethyl butyl)- p -phenylenediamine (6-PPD) is a worldwide antioxidant commonly added to delay the thermo-oxidative degradation of tire rubbers. − 6-PPD functions by forming a protective layer on the tire surface and thus preferentially reacts with atmospheric ozone and oxygen, resulting in various transformation products (TPs). Moreover, 6-PPD migrates easily from the NR matrix at higher temperatures due to its low molecular weight , and loses its antiaging efficiency.…”

Conventional and Green Rubber Plasticizers Classified through Nile Red [E(NR)] and Reichardt’s Polarity Scale [ET(30)]

“…The micropolarity of polystyrene and poly(lactic acid) films was also studied, whereas the E(NR) value reported by Dutta et al [43] was adopted for PMMA. Regarding the rubber and other polymers studied, there is a logical trend from apolar to polar compounds starting with the liquid model compounds squalane and squalene (squalane is the liquid analog of the ethylene-propylene copolymer, while squalene can be considered a liquid model compound of natural rubber or cis-1,4-polyisoprene as discussed elsewhere [44]) as the most apolar, followed by the liquid polyisoprene and polybutadiene and then by the thin solid films of polybutadiene (BR) and polyisoprene (IR). The copolymer styrene-butadiene rubber (S-SBR) with 21% styrene content is found to be more polar than either BR or IR as expected.…”

Conventional and Green Rubber Plasticizers Classified through the Nile Red [E(NR)] and Reichardt’s Polarity Scale [E_T(30)]

“…Regarding the rubber and other polymer studied, the trend from apolar to polar compounds is absolutely logic starting with the liquid model compounds squalane and squalene (squalane is the liquid analogous of the ethylene-propylene copolymer while squalene can be considered a liquid model compound of natural rubber or cis-1,4-polyisoprene as discussed elsewhere [42]) as the most apolar, followed by the liquid polyisoprene and polybutadiene and then by the thin solid films of polybutadiene (BR) and polyisoprene (IR). The copolymer styrene-butadiene rubber (S-SBR) with 21% styrene content is found more polar than either BR and IR as expected.…”