Lube-Tech
As AW/EP additives are typically based on Phosphorus 31
P, the observation of the 31 P isotope with NMR is
a preferable choice. Furthermore, it is possible to track the concentration of different additives such as thiophosphates, phosphates, phosphoric esters and amides as well as zinc-dialkyldithiophosphates due to their different moieties leading to different chemical shifts in the 31
P-NMR.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.147 page 4
engine. Samples of several field test vehicles have been submitted. The evaluation for this report is performed exemplary on the data gained from the specimen of one vehicle. The lubricant used in all vehicles was from the same brand and type, a market available oil in the SAE 5W-30 viscosity class with an ACEA C3 approval as well as holding the approvals of several OEMs at the time of the testing. It is intended for prolonged oil drain intervals (ODI), so called “long life”, according to the relevant OEM specifications. A fresh oil sample is required as reference for the determination of the Zn-dtp degradation.
The used lubricant samples were drawn and subsequently submitted to OELCHECK for analysis.
In addition to the standard analysis programme, 31
Figure 2: 31 P-NMR Spectrum of Common AW/EP Additives.[19]
Results and Discussion For reasons of simplicity, proofing the applicability of the method, the observation has been limited to the Phosphorus contained in Zinc dithiophosphate (Zn-dtp). Of course, Zn-dtp describes not a single compound but a class of different strain, optimised for the intended purpose. [20] It is a well-known AW additive class typically contained in any modern lubricant intended for the use in internal combustion engines. [21] [22] In addition to its proven anti-wear properties, Zn-dtp acts as a secondary anti-oxidant. Thus, it is subjected to depletion by two independent mechanisms of lubricant ageing. [23]
Accordingly, it is not possible to distinguish whether the Zn-dtp structure has been altered by tribological effects or other influences.
Lubricant samples of a light commercial vehicle have been drawn on a regular basis in the course of the field-testing of a newly designed 4-cylinder diesel
P-NMR has been measured to track the behaviour of the Zn-dtp concentration. The measurements were conducted using a 300 MHz Bruker SB Avance Nanobay NMR instrument with a BBFO sample head (5mm). The samples were diluted 1:1 using CDCl3
. Sample tubes have been sealed after dilution.
The initial measurement of the reference oil revealed that Zn-dtp decomposition products were already present in the fresh oil (Figure 3, blue curve, resonances at 80 ppm). The reason for this is not clear; it may result from the production process.
The sample data show, that Zn-dtp is fully consumed during the operating interval. According to the manual, 30,000 km is the intended oil operating time.
The conversion to so called poly phosphates which can be found in the NMR spectrum at around 0 ppm has been achieved. Intermediate or by-products of the conversion have also been detected in the samples submitted for analysis. However, no elucidation of the structure has been made.
The runtime of the lubricant was subsequently prolonged until a total of 70.000 km. During the
LUBE MAGAZINE NO.176 AUGUST 2023 33
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