Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
This is characterised by acyl decomposition and a tetrahedral intermediate state. Two molecules of water are required in the intermediate step, one molecule that acts as a proton donator and one as a nucleophile.
Test methods Figure 6. Acid ester decomposition.
The net reaction is shown in Figure 6. The reaction rate depends on the concentration of the reactants, ester and water, as well as on the catalytic converter. The temperature relationship follows the Arrhenius equation. Water is only soluble in esters to a limited extent, which restricts the ester decomposition. However, the acid formed works autocatalytically.
In practice, the hydrolysis of the ester is less problematic than the theoretical considerations suggest. By means of appropriate measures, the hydrolytic stability can be further improved.
Figure 7. The reaction process of the acid ester decomposition.
Thus, a low acid number as well as the freedom of catalyst residues from the esterification and the hydrolysis work in opposite directions.
A conjugated -electron system has a positive effect on the hydrolysis stability as it does in aromatic esters, for example, benzoic acid. Through this the C=O bond resonance is stabilised and reacts at a much slower pace for the tetrahedral transition state (Euranto, 1969). Steric effects also lead to a significant stabilisation of the ester bond, since the bond angles of the esters can be compressed from 120° to 109° in the tetrahedral
RPVOT (ASTM D-2272) The RPVOT (Rotating Pressure Vessel Oxidation Test) was designed for the fast determination of the resistance to oxidation and ageing of lubricants. About 50 g of test substance and 5 g of distilled water are weighed in a reaction vessel. A previously polished copper or iron coil in the oil-water mixture is used as a catalyst. The reaction vessel is screwed tightly into a pressure-tight stainless steel container and connected to a pressure gauge. It is now filled with pure oxygen up to a pressure of 620 kPa. The apparatus is then rotated (60 to 100 rpm) and heated to 150°C in an oil bath. There is initially a pressure rise due to the extension of the media. Over a given time the oxygen reacts with the test medium and the result is a pressure drop proportional to the amount of oxygen consumed. The time it takes the pressure to fall by 175 kPa under the maximum is measured. Oxidation-resistant lubricants, therefore, exhibit a longer time interval. The reproducibility is around 22%.
Beverage Bottle Test (ASTM D-2619 /DIN 51348) The hydrolytic stability of lubricants can be measured with the beverage bottle test. For this purpose, 75ml of the ester oil and 25ml of distilled water are put into a pressure-resistant bottle together with a copper strip as a catalyst. The bottles were clamped in a holder, placed in an oven at 93°C and rotated at 5 rpm around their longitudinal axis. After 48 hours, the increase in the acid number of the organic phase, the acid number of the aqueous phase, as well as the weight and the condition of the copper plate (as specified in ASTM D130) is evaluated.
LUBE MAGAZINE NO.150 APRIL 2019 31
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transition state. If there are large-scale structures in the vicinity of the ester group, the formation of this transition state becomes more difficult. This is the case, for example, with iso acids. This measure is all the more effective, the closer the branching is to the ester bond. However, a branched alcohol is therefore less effective (Leslie R. Rudnick, 1999).
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