Fig. 3 Changes of LSO viscosity as a function of temperature (25, 40, 60, 80, 100, 120oC) over a time span of 168 h. (25 and 40oC designated as Group A and 60, 80, 100, 120oC designated as Group B).
Using the GC-MS data collected regarding the changes of LSO non-oxidized saturated fatty acids (SAFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) (Table 1) and data for two LSO oxidized products, hydroperoxides and aldehydes (Fig. 2), we made a semi-quantified estimation of the changes of the level of polymers/epoxides during the 168 hours of LSO autoxidation (see details and graphic description for all temperatures in Supplemental material 4). This estimation demonstrates that when LSO samples were heated to 25oC and 40oC together with continuous air supply, no significant polymers/epoxides products could be observed above threshold of 10% all over the time frame of the experiment. In LSO samples heated to 60oC and higher temperatures plus air pumping the level of polymers/epoxides significantly increased above the threshold to 28, 27, 41 and 50%, respectively for 60, 80, 100, 120oC at the end time of 168 hours
NMR self-diffusion (D) experiments of the same LSO samples during thermal oxidation at increasing temperatures (25, 40, 60, 80, 100, 120oC) (Fig. 4). The self –diffusion results suggest two autoxidation reaction rates as a function of temperature. One group consists of LSO autoxidation at low temperatures (25 and 40oC) that may relate to a slow rate of oxidation (Slow Ox). The second group of LSO treated at higher oxidative temperatures (80, 100, 120oC) resulting in rapid oxidation rates Rapid Ox). Furthermore, the self-diffusion analysis may also suggest an intermediate oxidation mechanism at 60oC, which may be the intermediate temperature point in going from slow to rapid autoxidation of LSO.
It should be noted that viscosity values change only several hours in a limited scale of 0.05 to 0.8 Pa.s in between Group A and Group B depending of heating temperature. This data corresponds with the findings described and explained above for the induction period of LSO oxidation.