The Potential of Vegetable Oils for Lubricants - Green Materials from Plant Oils - page 284

Table 11.3 TGA Noack volatilities, PDSC data (at 10 1Cmin 1 in air under 200 psi
pressure), and PP values of dihydroxy and diester products, and
vegetable oils. T onset and SM values are averages of three experiments
with a standard error of 1 1C. (Reproduced from ref. 47 with kind
permission from Springer Science and Business Media from Springer
and the AOCS Press.)

Onset
temperature
(T onset )/1C

Signal maximum
temperature
(SM)/1C

TGA Noack
volatility/%

Pour point
(PP)/1C Test fluid

SBO 2.6 174 186 9

DiOH-SBO 3.6 163 213 —

Ace-SBO 2.7 174 213 3

But-SBO 2.5 169 208 15

Isobut-SBO 3 157 241 —

Hex-SBO 2.4 161 234 21

ESO — 188 215 0

HOSBO — 186 201 21

MO — 191 217 4

considered reliable for base oils as well as finished lubricants. 25 The PDSC
results, the onset temperature (T onset ) and the signal maximum temperature
(SM) for the diester samples are shown in Table 11.3. The diester samples
showed higher oxidation stability than the dihydroxy derivatives and also
SBO. This shows that oxidation stability is considerably improved by the
removal of unsaturation. The T onset decreases with the chain length of the
branching group. Ace-SBO had the highest value (174 1C) of T onset , while Hex-
SBO had the lowest (1611C). Longer side-chains are more prone to oxidative
cleavage than small compact ones. This observation is supported by another
study on synthetic esters, 26 where they mentioned that short-chain acids are
more stable than long-chain acids.
Table 11.3 shows the PDSC T onset data which is higher for moringa oil (MO)
than for the soybean oils. Similar results were obtained for the oxidative
stability index (OSI) measured using the Rancimat method EN 14112. The
PDSC technique uses a microgram amount of the sample, while Rancimat
uses 3 g of the sample. PDSC experiments were run in triplicate and the values
shown are an average of three measurements. Moringa oil showed a higher
coecient of variation (8.4%) than the other oils (0.3-2.3%). This shows that
oxidative stability data generated using PDSC is more repeatable than OSI. It
has been shown in an earlier study that higher stability oils like moringa oil
show poor repeatability in Rancimat tests, 27 while the PDSC method can be
used for low, as well as very high oxidative stability oils. The PDSC T onset shows
good correlation with the Rancimat OSI values (R 2 value of 0.91). The higher
oxidative stability shown by moringa oil is ascribed to its low unsaturation
number (78) compared to other oils and the presence of naturally occurring
anti-oxidants. 19 This low unsaturation number is a result of the higher
amounts of monounsaturated fatty acids (MUFAs) and saturated fatty acids
(SFAs) present in it. The unsaturation number correlates very well with the