Antioxidants 31
Owing to the limitation of laboratory setup, a single bench test cannot address all oxidation
aspects of a real world scenario. The large variation in test conditions, particularly test tempera-
ture, use of catalyst, performance parameter, oxidation mechanism, and targeted oxidation stage,
etc., makes it rather diffi cult or even impractical to correlate one test with another. It is therefore
a common practice to run multiple tests at a time when characterizing a lubricant formulation
and its additives. This section selectively reviews oxidation bench tests more closely related to the
characterization of antioxidants. These tests have been standardized by some of the international
standardization organizations such as ASTM and the Co-ordinating European Council (CEC),
etc., and are more widely used in the industry. It is important to note that there are a number of
custom- tailored test methods designed for specifi c needs that have been proven to be advantageous
in certain circumstances. The value of these tests should not be underestimated.
1.11.1 THIN-FILM OXIDATION TEST
1.11.1.1 Pressurized Differential Scanning Calorimetry
Differential scanning calorimetry (DSC), including PDSC, is an emerging thermal technique
for rapid and accurate determination of thermal-oxidative stability of base oils and performance
of antioxidants. PDSC has been a more sought after technique for two main reasons. First,
high pressure elevates boiling points, thus effectively reducing experimental errors caused by
volatilization losses of additives and light fractions of base oil; second, it increases the saturation
of the reacting gases in sample, allowing the use of lower test temperature or shorter test time at
the same temperature [207].
PDSC experiments can be run in an isothermal mode to measure oxidation induction time
(OIT) corresponding to the onset of oil oxidation or in a programmed temperature mode to measure
the onset temperature of oxidation. The temperature technique has been utilized to study deposit-
forming tendency of fi ve engine oils, and the results obtained were consistent with their engine test
ranking [208]. The OIT technique, however, is more commonly used for its simplicity and speed.
Its early use can be traced back to the 1980s when Hsu et al. [209] tested a number of engine oils
and found the induction periods of the samples to be indicative of the sequence IIID viscosity break
points. Soluble metals consisting of lead, iron, copper, manganese, and tin together with a synthetic
oxidized fuel were included as catalysts to promote oil oxidation.
The CEC L-85 and the ASTM D 6186 [210,211] are two standard methods that are based on OIT
technique. Key test conditions of the methods are listed in Table 1.5. The CEC L-85 test method
was originally developed for European Association des Constructeures Europeens de l’Automobile
(ACEA) E5 specifi cation for heavy-duty diesel oils and has been incorporated in the current E7
specifi cation. The test is capable of differentiating between different quality base oils, additives,
indicating antioxidant synergies and correlating with some bulk oil oxidation tests [212]. With
appropriate modifi cations to the standard methods, PDSC has been successfully utilized in the
characterization of various lubricants in addition to automotive engine oils. These include, but not
limited to, base oils [213,214], greases [215], turbine oils [214], gear oils [216], synthetic ester lubri-
cants [217], and biodegradable oils [218,219]. Using PDSC to study the kinetics of base oil oxidation
[220] and antioxidant structure–performance relationship [221] has also been reported.
1.11.1.2 Thermal-Oxidation Engine Oil Simulation Test (ASTM D 6335; D 7097)
The TEOST was originally developed to assess the high-temperature deposit-forming characteris-
tic of API SF quality engine oils under turbocharger operating conditions [222]. The original test
conditions were specifi ed as the 33C protocol and subsequently standardized in the ASTM D 6335
method [223]. In this test, oil containing ferric naphthenate is in contact with nitrous oxide and
moist air and is cyclically pumped to fl ow past a tared depositor rod. The rod is resistively heated
through 12 temperature cycles, each going from 200 to 480°C for 9.5 min. After the heating cycle