External influences such as UV radiation (light), temperature, atmospheric oxygen, mechanical load or chemical/ biological media lead to premature aging in materials, resulting in a change in their chemical and physical properties. Proper aging inhibitors (stabilizers) slow down the aging process and prolong the induction period, i.e. the time span preceding the onset of thermo-oxidative decomposition (chain degradation, technical failure). An important indicator of the oxidation stability of oils, fats, lubricants, fuels or plastics is the Oxidation Induction Temperature or Oxidation Induction Time (O.I.T.), which can be determined by means of DSC in standardized procedures.
In practice, two different methods are used: dynamic and isothermal O.I.T. tests. In the dynamic technique, the sample is heated at a defi ned constant heating rate under oxidizing conditions until the reaction begins. The corresponding Oxidation Induction Temperature is the same as the extrapolated onset temperature of the exothermal DSC effect which occurs.
In isothermal O.I.T. tests, the materials to be investigated are first heated under a protective gas, then held at a constant temperature for several minutes unter protective gas to establish equilibrium, and subsequently exposed to an atmosphere of oxygen or air. The time span from the first contact with oxygen until the beginning of oxidation is called the Oxidation Induction Time. This is shown in figure 1.
Many national and international standards – such as ASTM D 3895 (polyethylene), DIN EN 728 (plastic pipelines), ISO 11357-6 (plastics) and ASTM D 525 (aircraft fuel) – give recommendations for sample preparation and proper selection of the measurement conditions.
Oxidation tests on lubricating oils and greases are usually carried out using a high-pressure DSC instrument (see fi gure 2). A back-pressure is generated – generally 35 bar – in an attempt to prevent evaporation of the sample. In oxidation reactions, however, the oxygen not only serves for pressure generation, but also as a reaction partner. For this reason, both the pressure and the gas flow must be regulated with the utmost precision.
Determination of the oxidation stability is “surface-sensitive”. This means that the oil or grease fi lm to be investigated should ideally exhibit a smooth, uniform surface in order to ensure high reproducibility of the test results. Very well suited for such investigations are SFI crucibles (SFI stands for Solid Fat Index; see diagram in figure 3), as recommended in ASTM D 5483 for lubricating greases and ASTM D 6186 for lubricant oils.
In crucibles with a flat bottom, oils and greases often creep to the rim zones at higher temperatures. The effective surface of the sample which can interact with the surrounding atmosphere is thus reduced in size. This affects the O.I.T. result (see figure 5). When the analysis is conducted in an open standard aluminum crucible (blue curve), the O.I.T. time (extrapolated onset) amounts to 64.6 min. In comparison, when analyzed in an SFI crucible (green curve), the O.I.T. is shortened considerably (to 46.4 min) due to the larger effective surface.