Chemical, physical and electrical properties of aged dodecylbenzene 2: thermal ageing of single isomers in air

Abstract

The linear isomer of dodecylbenzene (DDB), 1-phenyldodecane, was aged at temperatures of 105 and 135 o C in air and the resultant products were analyzed using a range of analytical techniques. On ageing, the 1-phenyldodecane darkened, the acid number, dielectric loss and water content increased and significant oxidation peaks were detected in the infrared spectrum. When aged in the presence of copper, a characteristic peak at 680 nm was also detected by UV/visible spectroscopy but, compared with previous studies of a cable-grade DDB, the strength of this peak was much increased and no appreciable precipitate formation occurred. At the same time, very high values of dielectric loss were recorded. On ageing in the absence of copper, an unusually strong infrared carbonyl band was seen, which correlates well with the detection of dodecanophenone by gas chromatography / mass spectrometry and nuclear magnetic resonance spectroscopy. It was therefore concluded that the ageing process proceeds via the initial production of aromatic ketones, which may then be further oxidized to carboxylic acids. In the presence of copper, these oxidation products are present in lower quantities, most of these oxidation products being combined with the copper present in the oil to give copper carboxylates. The behavior is described in terms of a complex autoxidation mechanism, in which copper acts as both an oxidizing and a reducing agent, depending on its oxidation state and, in particular, promotes elimination via the oxidation of intermediate alkyl radical species to carbocations.

Effect of aging on the physical, chemical and dielectric properties of dodecylbenzene

“…Figure 1 contains spectra from homogenized samples aged without copper. The absorption edge shifts to longer wavelengths (arrowed) and the oil becomes visibly yellowed [6,7,18,[20][21][22][23]; the wavelength of 50 % transmission after 336 h aging is 500 nm, which agrees with previous comparable studies [18,20]. In homogenized oils aged with copper (Figure 2a), aging is accelerated and an additional absorbance is present centered at 680 nm after 72 h aging (arrowed).…”

“…In homogenized oils aged with copper (Figure 2a), aging is accelerated and an additional absorbance is present centered at 680 nm after 72 h aging (arrowed). This has previously been associated with the presence of copper carboxylates, an aging product that is unique to DDB [18,20,21]. Further aging then results in absorption over all wavelengths as a precipitate is formed [7,8,18,[20][21][22].…”

“…Separate samples were then removed from the oven after the prescribed aging time had elapsed. The copper concentration, aging temperature and times were chosen to ensure consistency with earlier studies [7,18,[20][21][22][23]. Some oils were tested after centrifuging (4000 RPM, 10 min) to remove the precipitate, whereas others were homogenized to include this material.…”

“…It has excellent biodegradability in the environment [16,17] and is therefore gradually replacing mineral oil in paper/oil cable systems [18] whereas in transformers it is more cost effective to utilize a vegetable oil [3]. The aging behavior of DDB has been previously studied [19][20][21][22] and the aging chemistry has been elucidated in the simpler system of single isomer 1-phenyldodecane [21]. More sustainable replacements for mineral oils in high voltage plant are vegetable oils and silicone oils and the aging behavior of each has also been studied [23,24].…”

“…Whilst still not ideal, this approach allows the effects of reduced oxygen availability (oxygen which might for example, arise due to degradation of cellulose) to be studied whilst maintaining the same aging temperature used in previous laboratory aging studies. DDB, like mineral oil, is also prone to precipitate formation, particularly when aged in the presence of copper [18,[20][21][22] and therefore the electrical properties of the particulate was also studied. UV/Vis spectroscopy was used to determine the extent of aging, infrared spectroscopy (IR) and measurements of water content were used to identify chemical changes whilst dielectric loss measurements were complemented here by electrical breakdown and conductivity measurements.…”