Analysis of the Ozone Oxidation Reaction Mechanism of Rubber Antidegradants

Theme Overview

Figure 2 (a) natural rubber and (b) the antidegradant 6PPD.

Rubber products such as those made from natural rubber suffer from ozone-induced degradation, which causes deterioration of physical properties and the formation of cracks (Fig. 1, Fig. 2(a)). [1] To address this, ozone-scavenging antidegradants — additives that capture and consume ozone by reacting with it preferentially — are widely used. A representative example is 6PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine)(Fig. 2(b)) ^[2][3]

Ozone-scavenging antidegradants react with ozone before the rubber does, being converted into a variety of oxidation products and thereby suppressing ozone-induced degradation of the rubber.

Although a wide range of oxidation products has been identified experimentally for 6PPD (Fig. 3)^[4], the mechanism of the ozone oxidation reaction of antidegradants remains insufficiently understood. Elucidating this mechanism is expected to contribute to the development of high-performance, long-life rubber materials.

In this case study, GRRM20 with Matlantis^[5] was used to search for reaction pathways between an ozone-scavenging antidegradant and ozone, in an attempt to elucidate the underlying reaction mechanism.

Figure 3: Oxidation products tentatively identified in the ozonation of 6PPD (adapted from Ref. [4], Fig. 1). The labels A–F denote the pathway classification of Ref. [4]: A, quinone; B, nitroso aryl; C, HDPA; D, aryl hydrolysis; E, formaldehyde addition; F, side chain oxidation.

Calculation Models and Methods

Calculation Models and MethodsReaction pathway search: SC-AFIR^[6-8]/PFP v7.0.0, γ=150

Initial structure: MePPD (N-methyl-N′-phenyl-p-phenylenediamine; Fig.4), a model compound of 6PPD, and one ozone molecule

Figure 4: Structure of MePPD, a model compound of 6PPD.

Results and Future Work

Figure 5: Reaction pathway network obtained by the SC-AFIR calculation. Nodes are colored by energy relative to the most stable structure.

Table 1: Numbers of structures discovered in the SC-AFIR calculation

Items	Number
Equilibrium structures (EQs)	836
Path tops (PTs)	2106

The reaction pathway search revealed a wide variety of reaction pathways that can arise when the antidegradant scavenges ozone (Fig. 5, Table 1). The main pathways among the wide variety that can occur when the antidegradant scavenges ozone are summarized in Table 2.

Among these, the most favorable pathway was found to be cleavage of the central benzene ring of the antidegradant by ozone, which has the lowest activation energy (40.0 kJ/mol) and is the most exothermic (−508.8 kJ/mol) (Fig. 6). This finding is consistent with the experimental results reported in a previous study^[4]. In that study, most of the pathways remained tentative; the present calculations provide activation energies and reaction enthalpies for the main pathways, making it possible to compare how readily they proceed.

Based on the insights obtained here, we plan to proceed with the materials design of higher-performance antidegradants.

Figure 6: Energy profile of the most favorable reaction pathway for the ozone oxidation of MePPD

Products of the pathway leading to quinoids — Table 2: Main reaction pathways identified for the reaction of MePPD with ozone. Path A–D and F correspond to the pathway classification in Fig. 3.

Products that formed a five-membered ring — Table 2: Main reaction pathways identified for the reaction of MePPD with ozone. Path A–D and F correspond to the pathway classification in Fig. 3.

Path	Reaction	Product	Reaction enthalpy [kJ/mol]	Activation Energy [kJ/mol]
Path A	Quinoid formation		-232.7	43.7
Five-membered ring formation		-360.6	68.7
Central benzene ring cleavage		-508.8	40.0
Path B	Ozone scavenging at the amino group		-64.6	69.7
Path F	Ozone scavenging at the alkyl group		-213.4	72.4
Path C	Alkyl group elimination		-23.7	86.3
Path D	Dissociation into two anilines		-44.1	89.5

Computational Details

Items	Details
PFP	v7.0.0
Number of atoms	32
Constituent elements	H,C,O,N
Principal GRRM Options	Gamma＝150.0、NRUN = 200 Note: Specified under the SC-AFIR options

Specified under the SC-AFIR options (details).

Options
SaveDataInDirectory
NRUN=200
DownDC=14
RandDC=14
Add Interaction
Gamma=150.0
END
AddUniversalForce=100.0
ReadBareEnergy
Opt = Redundant
TrafficVolumeCheck
TimeScale=0.00100
Siml_temperature=250.0 300.0 350.0
rTemperature=10000.0
Clustering=BondConnectivity
MinFreqValue=50.0
MatchDecScale=3.0 16.0
EQOnly
KeepLUPPath
sublink = ./kickpfp.py

References

[1] Ando, S., Fukamachi, S., Antioxidants, Nippon Gomu Kyokaishi, 82, 45–49 (2009) (in Japanese).

[2] Yamaki, D., Amine Antioxidants, Nippon Gomu Kyokaishi, 91, 442–446 (2018) (in Japanese).

[3] Otomo, S., Improvement of the Ozone Resistance of Rubber Vulcanizate Using Antidegradant, Nippon Gomu Kyokaishi, 87, 284–291 (2014) (in Japanese)..

[4] Seiwert, B.; Nihemaiti, M.; Troussier, M.; Weyrauch, S.; Reemtsma, T., “Abiotic Oxidative Transformation of 6-PPD and 6-PPD Quinone from Tires and Occurrence of Their Products in Snow from Urban Roads and in Municipal Wastewater”, Water Research, 212, 118122 (2022).

[5] Maeda, S.; Ohno, K.; Morokuma, K., “Systematic Exploration of the Mechanism of Chemical Reactions: The Global Reaction Route Mapping (GRRM) Strategy using the ADDF and AFIR Methods”, Phys. Chem. Chem. Phys., 15, 3683–3701 (2013).

[6] Maeda, S.; Harabuchi, Y., “Exploring Paths of Chemical Transformations in Molecular and Periodic Systems: An Approach Utilizing Force”, WIREs Comput. Mol. Sci., 11, e1538 (2021).

[7] Maeda, S.; Taketsugu, T.; Morokuma, K., “Exploring Transition State Structures for Intramolecular Pathways by the Artificial Force Induced Reaction Method”, J. Comput. Chem., 35, 166–173 (2014).

[8] Maeda, S.; Harabuchi, Y.; Takagi, M.; Taketsugu, T.; Morokuma, K., “Artificial Force Induced Reaction (AFIR) Method for Exploring Quantum Chemical Potential Energy Surfaces”, Chem. Rec., 16, 2232–2248 (2016).