Beyond ZDDP

Abstract

Editorial Beyond ZDDPThe lubricant additive zinc dialkyl dithiophosphate (ZDDP) has been used as an anti-wear additive in almost all engine oils for over 60 years. It has thus outlived all of the lubricant chemists and engineers who were active when it was born. It is an extraordinary additive, cheap to produce, very effective and multifunctional -especially in its combination of anti-wear and extreme pressure properties. ZDDP has stimulated an enormous amount of research, and yet many of the details of how it forms a boundary lubricating fi lm remain obscure.During the last 15 years, the future of ZDDP has become very uncertain. Engine exhaust aftertreatment devices are harmed by the presence of sulphur and phosphorus oxides and metal salts in engine exhausts, and ZDDP contains all three of these noxious species. There is thus a strong interest in fi nding additive replacements for ZDDP in its multifarious roles. In terms of ZDDP's anti-wear behaviour, this has proved remarkably diffi cult, although now, some real progress is being made. It now appears that it will be possible, if required, to develop high-performance engine oils without any ZDDP content. It is far less certain, however, whether ZDDP replacements will be wholly free of sulphur, phosphorus and metals.One thing that the possible imminent demise of ZDDP has certainly achieved is a great deal of research, into ZDDP itself, its possible replacements and how to measure the effectiveness of novel low-or zero-ZDDP formulations.This special issue contains fi ve papers that span the gamut of current research on ZDDP replacement. Three originate from primarily academia, while two are from industry. The fi rst paper is an experimental study of fi lm formation by two sulphur-and phosphorus-containing, but metal-free, additives that are suggested as possible replacements for ZDDP.1 The second explains how ZDDP adversely affects after-treatment devices, and then reviews all of the various anti-wear chemistries that might be considered as possible replacements for ZDDP. 2 An alternative approach to replacing ZDDP by other anti-wear additives is to remove the need for an anti-wear additive in the fi rst place, and one possible way to do this is to coat the surfaces of rubbing engine components with highly wear-resistant materials. One very promising material is diamond-like carbon (DLC), and the third paper describes work on a particular type of DLC coating and its very effective interaction with the unusual friction-reducing agent, glycerol. The last two papers address the practical development of ZDDP-free engine oils. One, from a lubricant additive company, describes the impact of partial or full replacement of ZDDP by a metalfree phosphorus-containing additive on friction and wear in bench tests and also on fuel economy and catalyst performance in engine tests. 4 Of particular interest is the fi nding that a non-ZDDP anti-wear additive is less harmful to exhaust catalysts than ZDDP despite it containing as much or even more LUBRICATION SCIENCE

Activation Volume in Shear-Driven Chemical Reactions

“…[12] Since then, many studies have tried to replace ZDDP, but the progress has been very slow. [10,13] This is partially because development of alternatives largely relies on empirical knowledge, rather than a priori first principles. Fundamental knowledge of tribochemical mechanisms could significantly accelerate development of lubricant additives.…”

A Short Review on Polymeric Biomaterials as Additives for Lubricants

“…They are considered to be toxic to aquatic wildlife with long-lasting effects. ZDDP contains sulfur, phosphorus, and zinc, which produce harmful emissions from engines during the tribo-chemical process [8]. An additive like ZDDP does not work well under extreme conditions of pressure and temperature.…”

Substituent Effect on the Reactivity of Alkylated Triphenyl Phosphorothionates in Oil Solution in the Presence of Iron Particles

“…Although the concentration of ZnDTPs in lubricant formulation has already been reduced [11], the development of new additives with lower-or zero-SAPS levels is becoming a necessity [12,13].…”