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Purpose This paper aims to study the thermal oxidation performance of antioxidant additives in ester base oils deeply. Design/methodology/approach ReaxFF molecular dynamics was used to simulate the thermal oxidation process of butyl octyl diphenylamine and octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate as two antioxidant additives act on the Trimethylolpropane trioleate (TMPTO) base oil. Meanwhile, combining with the infrared spectroscopy characterization results of the thermal oxidation test, this paper provides theoretical support for the development of high-performance synthetic lubricants and their antioxidant additives. Findings The results show that butyl octyldiphenylamine easily removes the hydrogen atom on the secondary amine, which promotes the formation of more long carbon chain diene radicals or polyene hydroperoxides from TMPTO. Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate could easily decompose into octadecyl hydroperoxide and 2,6-di-tert-butyl 4-propionylphenol, which could convert into 2-tert-butyl-4-peroxyethyl-6-hydroperoxy-tert-butylphenol in the middle of the thermal oxidation reaction, prompting TMPTO to form more short-chain alkenyl and olefin hydroperoxide or other oxide. Originality/value The main change characteristics of base oil molecules are the first thermal decomposition to form oleic acid groups and ethane cyclopropane methyl oleate. Under the action of butyl octyldiphenylamine and octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate, the deep oxidation and decomposition reaction are slowed down. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2020-0037/

Purpose This paper aims to investigate the thermal oxidation behavior of trimethylolpropane trioleate (TMPTO) base oil when exposed to Fe surfaces. Design/methodology/approach Samples of TMPTO bulk oil were placed in Fe vessels and heated in an oven to accelerate the oxidation at different time intervals, while others were placed in glass vessels and used as experimental controls. Subsequently, the physicochemical properties of the oxidized TMPTOs, including the kinematic viscosity and acid value, were measured and a structural analysis was conducted using the Raman and Fourier transform infrared (FTIR) techniques. Findings The results demonstrate that the TMPTO bulk oil exhibited an exponential increase in the kinematic viscosity along with the increasing acid value over the oxidation time. The Fe surface significantly increased the kinematic viscosity of TMPTO, while only mildly impacting its acid value compared with the experimental controls. The structural analysis results of the TMPTO suggest that the C = C and = C-H bonds were the vulnerable sites. Furthermore, the results suggest that the Fe surface evidently accelerates the chemical reactions of the C = C and the = C-H bonds, and less alcohols and more carbonyl products were identified in the oil samples that were heated in the Fe vessels. Originality/value The results demonstrate that the Fe surfaces affected the oxidation behavior of the TMPTO base oil, and an interaction mechanism between the Fe and the TMPTO is developed.

To prepare a high-performance anti-friction and wear-resistant composite material for friction sub-components in marine equipment, a modification was made by adding different amounts sodium polyacrylate (PAANa) to ultra-high molecular weight polyethylene (UHMWPE). PAANa/UHMWPE-blended powders were prepared at individual weight ratios of 0/100, 3/97, 5/95, 8/92, 13/87, and 18/82 with hot-press molding. In seawater, experiments of PAANa/UHMWPE composites sliding against GCr15 have been conducted with a ball-on-disk configuration in this study. The results show the surface hardness of composites was almost the same with the increase in PAANa proportion, however, the friction coefficient and wear scars of PAANa/UHMWPE composites/GCr15 sliding pairs firstly decrease and then increase. Scanning electron microscopy (SEM) results show that wear mechanism of the composites is mainly plastic deformation and spalling. The composites with PAANa content of 3% and 5% achieves better tribological properties than the pure UHMWPE material.

Determining the nature of the microscopic mechanism of tribological properties by experimental methods for a polymer material surface/interface in the sliding friction process is a challenge. Molecular dynamics simulations were conducted by sliding a rigid indenter over the amorphous polyethylene. The results show that the friction is mainly composed of plough force and adhesion force. The average friction of adhesive contact is greater than that of frictionless contact because of the adhesion effect. The difference of average friction between adhesive contact and frictionless contact increases with increasing indentation depth because of the plough force effect. The elastic deformation of amorphous polyethylene in the cohesive zone is related to van der Waals interaction energy, whereas the plastic deformation was mainly dominated by bond angle energy and dihedral energy of the molecular chain for amorphous polyethylene. Molecular chains of amorphous polyethylene extend along the sliding direction and agglomerate along the indentation direction. The flexibility of the molecular chains increases with the increase of temperature and facilitates the molecular chains returning more easily to their original state.

Available model of 3D-quantitative structure-activity relationship (3D-QSAR, the method of TopomerCoMFA) has been used to establish relationship between antioxidant performances of lubricating oil (trimethylolpropane trioleate, TMPTO) with 15 kinds of amine additives and its molecular structure. Correlation coefficient of the cross-validation (include internal q 2 and external q pred 2 ) and conventional correlation coefficient showed the model to be both stable and good predictive. The electron-donating group and larger steric effects group are adopted at the para-position or meta-position of amido on the benzene ring, and suitably decreasing the branch chain near the para-position, these indicate that the antioxidant performance of amine additives can be enhanced. The results not only can help us understanding the rule of antioxidant performance, but also providing guidance(s) to predict antioxidant performance of existing amine additives and to design or modify new amine additives in lubricating oil.

Some constructions of the molecular model for reactive molecular dynamics (RMD) simulations have neglected the effect of molecular weight on the RMD results. A computational methodology for generating structural models of amorphous ultrahigh molecular weight polyethylene (UHMWPE) with different degrees of polymerization was thus proposed. The methodology was then used in RMD simulations of three types of molecular models for UHMWPE with 20, 40, and 80 vinyl monomers. The accuracy of the model has been discussed by comparing the RMD results with experimental data previously available in the literature. Results have confirmed that the atomic model with larger molecular weight allows (i) exact calculations of the diffusion coefficient and (ii) accurate prediction of the cross-linked reaction. The RMD simulations provide the basic data for understanding and further studying the chemical reaction of other interesting macromolecules.

The research of nanoscale contact problem has important significance for the micro-/nano-machinery, especially for micro-/nano-contact problem of material defects, from which deformation and failure mechanism of materials could be revealed. Molecular dynamics method is a valid approach that describes microscopic phenomenon. Taking the rigid hemispherical surface in contact and indentation process with the perfect and defective monocrystalline copper as the research object, the molecular dynamics model of nanoscale contact was established, after solving and simulating analysis. Results showed that the substrates above the square void collapse at contact depths of 0.20 and 0.37 nm when the depths of the square void ( d) are 0.8 and 1.6 nm, respectively. The dislocations and glide band increase as the d increases; at the same time, a larger depth of square void results in a bigger contact force between the hemispherical surface and the substrate surface. In the process of disengagement, there is a sudden drop in the contact force, and unrecoverable plastic deformation of the copper material occurs. Moreover, with the increase in void depth, the number of high-stress atoms increases between the hemisphere and the surface of substrate. The stress is mainly distributed in the diagonal of the square void, and the contact area generates stress concentration.

The ester base oil of dioctyl adipate (DOA) was oxidized in an oven at 200 °C for 30 h, and variations in the physicochemical and tribological properties were studied. To investigate the thermal-oxidation mechanism, the thermal-oxidation products were analyzed by gas chromatography–mass spectrometry (GC−MS), and the thermal-oxidation process was simulated using visual reactive force field molecular dynamics (ReaxFF MD). The results indicated that the total acid number (TAN) increased significantly because of the presence of 14% carboxylic acids and low molecular weight monoesters. The tribological properties were improved by the formation of the strongly polar carboxylic acids. Additionally, the increase in kinematic viscosity was limited due to the formation of high molecular weight polymerization products and low molecular weight degradation products. Thermal-oxidative degradation and polymerization mechanisms were proposed by combining ReaxFF MD simulations and GC−MS results.