Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Nevertheless, at temperatures above 300°C a pyrolysis of the ester takes place, through which decomposition is triggered by a decline in an olefin and a carboxylic acid. Ester compounds with a ß-hydrogen atom in the polyol are significantly more vulnerable and respond as shown in the equation in Figure 1.
Figure 2. Thermal ester decomposition of free radicals.
Oxidation stability The properties of the hydrocarbon chain play a decisive role when considering the oxidation stability of esters. This applies to the arrangement of the various CH-groups, but also the number and distribution of unsaturated bonds.
Figure 1. The thermal decomposition reaction process.
The reaction proceeds over a six-element cyclic transition state. The reaction starts with the decomposition of the ß-hydrogen atom by the carbonyl oxygen atom. This breaks the oxygen- carbon bond of the alkoxy group. In this conversion, three pairs of electrons are displaced, similar to the Diels-Alder reaction. According to Bredt’s rule there are no double bonds formed to bridge head atoms. If the formation of a conjugated system is possible by the elimination then this is preferable. Otherwise, the Hofmann rule is respected (elimination in the direction of the least substituted carbon atom). This reaction occurs, for example, in the case of natural trigylcerides, such as sunflower oil.
If the ß-hydrogen atom is replaced by alkyl groups, as is the case in the polyol esters, the pyrolysis occurs due to free radicals. A corresponding representation is shown in Figure 2. This type of reaction requires a higher energy supply and only takes places at higher temperatures. Esters based on pentaerythritol have the highest stability, followed by trimethylolpropane and neopentane glycol. Through the formation of free radicals this reaction can be easily prevented with antioxidants.
The influence of unsaturated fatty acids was examined in detail in the autoxidation of lipids in the food sector. The mechanisms shown in Figure 3 were postulated for the progress of this reaction. The alkyl radicals necessary to start the chain can be produced by temperature effects, shear forces, UV-radiation and other influences. The radical intermediate stages formed, especially the peroxy- radical (ROO.), have good stability and can only abstract particularly activated H-atoms of the ally groups (Belitz, et al, 2013).
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Table 1. Oxidation of fatty acids depending on the iodine value (Frankel, 1998).
This step is slow and is, therefore, the speed limiting step in the chain reaction. Thus, the reaction rate increases with the number of double bonds in a fatty acid molecule at an exponential rate (see Table 1). A special position, however, is occupied by linolenic acid, since here the particularly reactive 1.4 Penta-Diensystem is transferred to a stable conjugated DienSystem.
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