## 1 Introduction  
As a consequence of the trend towards machinery with an increasing performance, while scaling down in size, the appropriate selection of the lubricant quality, composition, and, hence, its performance profile, to fulfil the requirements of lubrication is becoming a more critical issue in the major fields of application. The base oil composition with regard to the application plays a crucial role in these considerations. Accordingly, group I base oils--among the five groups of base oils as defined by the American Petroleum Institute (API)--are in decline. In turn, mineral oil based group II and III base oils, which are more intensively refined petroleum crude oils, as well as fully synthetic polyalphaolefines (PAO) representing group IV, can be found in an increasing number of commercial lubricant formulations. Group V summarizes all of the other types of base oils such as silicones, phosphate esters, polyalkylene glycol (PAG) ethers, biolubes, and various synthetic esters [1,2]. With a global market size of USD 3.46 billion and a production amount of 652.2 thousand tons in 2015, the synthetic lubricants market represents a considerable economic factor [3]. In our paper, the focus is placed on ester-based oils according to their technical-economical potential and impact. Besides the growing market for environmentally friendly products, e.g., for hydraulic oils, an increasing demand from the heavy machinery industry is noticeable. Amongst the most important application fields, compressor oils have to be mentioned with a market share of 40.7% of 2013's global synthetic ester-based oil volume, followed by hydraulic fluids, engine oils, metalworking fluids, and other application fields including high temperature chain, aviation, and food lubricants. The highly diverse fields of application of ester-based lubricants can be explained by their unique set of properties: they offer advantageous properties for high temperature and high stability applications, as well as a high flash point, low volatility, and good thermo-oxidative stability. Besides, they display an advantageous solubility for additives and an adequate inherent lubricity. Their low toxicity and good biodegradability are additional advantages [2; 4]. Ester-based base oils can be distinguished in three main groups: (1) natural esters, including vegetable crops and animal fats; (2) synthetic oleochemical esters made by the reaction of alcohols and natural fatty acids; and (3) petrochemical esters derived from synthetic alcohols and acids [4]. Besides the above-mentioned general benefits of esters compared to high-quality hydrocarbons, they can exhibit a high thermal decomposition temperature of 275 \({}^{\circ}\)C according to ASTM D2879, a superior trade-off between a viscosity index (VI) of 124 and pour point of \(-68\)\({}^{\circ}\)C, as exemplarily given for the bis(2-ethylhexyl) adipate ester [2]. Despite the widespread applications of esters at high temperatures and under harsh conditions, the knowledge about their thermo-oxidative stability behaviour still remains a critical issue. Nowadays, an increasingly important application field where the thermo-oxidative stability of esters is in focus is that of renewable biolubricants being employed as esters based on synthetic and vegetable oils, opening a wide field for eco-friendly and biodegradable lubricants [5]. Thereby, a knowledge of structure-stability relationships lays the foundations for the adjustment of thermo-oxidative stability based on the application-related modification of molecule geometry and targeted synthesis [6; 7; 8; 9]. The thermo-oxidative stability of esters is highly linked to the hydrogen configuration of the \(\beta\)-carbon, especially of the alcohol part, the second carbon atom next to the functional group of the C-O bond in the alcohol (see Figure 1). Hydrogen atoms which bond to these \(\beta\)-carbons are prone to thermal decomposition, as a six-membered cyclic intermediate can be built and decomposes to acid and alkene.