Designed boron chemistry for tribological systems
Boron compounds can be environmentally friendly alternatives for commercially available lubricant additives. Some boron compounds are already used as friction modifiers, corrosion inhibitors, antioxidants and antiwear additives in lubricants. The focus of the present work is on the development and tribological investigations of new boron compounds for these applications. In this work, boron compounds based on dithiophosphates (DTPs), dithiocarbamates (DTCs) and ionic liquids (ILs) were designed, synthesized and tribologically optimized. The work was carried out in the following steps: (1) Synthesis of novel boron based compounds and ionic liquids; (2) Characterization of the compounds with FTIR, liquid-state (1H, 13C, 31P, 11B) and solid-state magic-angle-spinning (MAS) NMR spectroscopy, electrospray ionization mass spectrometery (ESI-MS), elemental analysis, thermal analyses (TG/DTG, DTA, DSC and QMS), viscosity, density and conductivity; (3) Tribological evaluation using four ball and pin-on-disc tribometers; (4) Surface analysis using an optical profiler and Scanning Electron Microscopy coupled with X-ray Energy Dispersive Spectroscopy (SEM/EDS).
(i) Antiwear performance of S-di-n-octoxyboron-O,O’-di-n-octyldithiophosphate (DOB-DTP) was found considerably better in comparison with O,O’-di-n-butyl-dithiophosphato-zinc(II) (ZnDTP). However, since DOB-DTP contains B─S bond, which is sensitive to moisture and may be hydrolyzed by water, a novel class of alkylborate-ethyl-dithiophosphates was synthesized, in which boron is covalently bonded to the dialkyldithiophosphate group through the ethyl linker in order to avoid hydrolysis. (ii) Antiwear performance of an alkylborate-ethyl-dithiophosphate with decyl substitutes at both DTP and the borate groups of the molecule (DDB-EDTP) was superior compared with that of the octyl (DOB-EDTP) and the pentyl (DDB-EDTP) analogues. Residues of one representative compound from this class, DPB-EDTP, after thermal analyses were characterized by multinuclear 13C, 31P and 11B MAS and 31P CP/MAS NMR spectroscopy. Solid-state NMR data suggest that a dominant part of the solid residue of DPB-EDTP consists of borophosphates. (iii) It was found that a novel class of compounds containing alkylborate and dithiocarbamate groups with alkyl or methylbenzyl- substitutes in one molecule provide better antiwear performance compared with commercially available packages of ZnDTPs. (iv) Eight compounds belonging to a novel class of halogen-free chelated orthoborate ionic liquids (hf-BILs) with different phosphonium cations were synthesized. Important physical properties of the ILs including glass transition temperatures, density, viscosity and ionic conductivity were measured. As lubricants, these hf-BILs exhibit considerably better antiwear and friction reducing properties under boundary lubrication conditions for steel-aluminum contacts as compared with fully formulated (15W-50 grade) engine oil. (v) Five compounds belonging to another class of hf-BILs comprised of two chelated orthoborate anions, bis(mandelato)borate and bis(salicylato)borate, and three different nitrile cations (cholinium, pyrrolidinium and imidazolium) were prepared. Solid-state NMR spectroscopy was employed to probe interactions between cations and anions in the salts, which are solid at room temperature. Solid-state NMR data suggested a strong interaction between BScB anions and cholinium cations [Chol]+. This interaction was further confirmed by the X-ray diffraction single crystal structure of this salt as hydrogen bonding between OH of [Chol]+ and the carbonyl group of [BScB]-. The [BScB]- exhibited a strong interaction with imidazolium cation [EMIm]+, whereas a weaker interaction was suggested between pyrrolidinium cation [EMPy]+ and [BScB]-.
It was found that the novel boron based DTPs and DTCs have considerably better antiwear performance and higher stability of the coefficient of friction with time as compared with ZnDTP. They also have a high thermal stability, good miscibility with oils and positive environmental issues all that make them an attractive alternative to ZnDTPs and other commercial additives to lubricants. It was found that novel halogen-free boron based ionic liquids may be also potentially used as alternative lubricants for ferrous and non-ferrous contact.