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.