Tips & Tricks

Determination of the Oxidation Induction Time or Temperature: OIT and OOT

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 OxidationOxidation can describe different processes in the context of thermal analysis.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 OxidationOxidation can describe different processes in the context of thermal analysis.Oxidation Induction Temperature / OxidationOxidation can describe different processes in the context of thermal analysis.Oxidation Induction Time (Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.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 Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.OOT) is the same as the extrapolated onset temperature of the ExothermiqueA sample transition or a reaction is exothermic if heat is generated.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 to conclude that the oxidation stability is better and the lifetime therefore longer.

Determination of the Oxidation Stability of Fats and Oils

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 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.

Diagram of an SFI crucible with sample (green)

An example of these would be panshaped aluminum crucibles with an outer diameter of 6.7 mm and a volume of 85 μl which can be shaped with a sealing tool (built into a standard crucible press).

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). 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.