External influences such as UV radiation (light), temperature, atmospheric oxygen, atmospheric loads (e.g. impurities) or chemical/biological media lead to premature aging in organic materials, which might considerably influence their usage properties or might even lead to the failure of parts in which they are used as a component. The most common cause of chemical aging (e.g. chain degradation) is oxidation, which makes oxidation stability an important criterion for applications with oils, fats, lubricants, fuels or plastics. The oxidation stability can be determined via the Oxidation Induction Temperature / Oxidation Induction Time (OIT) by means of differential scanning calorimetry (DSC) in standardized procedures.
In practice, two different methods are used: dynamic and isothermal OIT tests. In the dynamic technique, the sample is heated at a defined constant heating rate under oxidizing conditions until the reaction begins. The Oxidation Induction Temperature OIT (also called Oxidation Onset Temperature OOT) is the same as the extrapolated onset temperature of the exothermal DSC effect which occurs. In isothermal IOT tests, the materials to be investigated are first heated under a protective gas, then held at a constant temperature for several minutes 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 Inductive Time OIT.
The procedure for the preparation, implementation and evaluation of measurements is described in detail in national and international standards such as ASTM D3895 (polyethylene), DIN EN 728 (plastic pipelines) or ISO 11357-6 (plastics). Generally, either open crucibles or crucibles with multiple piercings in the lids are used. For polyolefins like PE or PP, a longer OIT time allows one to conclude that the oxidation stability is better and the lifetime therefore longer.
Oxidation tests on lubricating oils and greases are usually carried out using a high-pressure DSC instrument. 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 film 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 below), 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 below). 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.