Oxidation Induction Time

Antioxidants to prevent Polymer Oxidation

OIT- Assessment of oxidative stability of polymeric material

Oxidation Induction Time (OIT) – A method for life-time prediction of polymer materials or a quick analytical tool for assessing thermal stability and material consistency?

Oxidative Induction Time (OIT) measured by Differential Scanning Calorimetry (DSC) on molten polymer samples is widely used in the plastics industry to assess the thermal stability of polyolefins. It is simple, it requires small samples and it is relatively fast.

OIT is defined as a method for the determination or assessment of the oxidative stability of a (polymeric) material at elevated temperatures under controlled, highly automated and precise conditions.

In OIT analysis (e.g. ASTM D3895 – 19 Standard Test Method for Oxidative-Induction Time of Polyolefins by Differential Scanning Calorimetry) the analysis temperature is set arbitrarily at 200°C, unless otherwise specified. Alternative temperatures from 180-220°C may also be used for samples with varying thermal stability to obtain thermal curves which can be analyzed and easily interpreted.

Samples are typically obtained from 10 mil compression molded sheet. Sample “disks” are cut from the sheet with a bore-hole cutter to obtain samples of uniform size/weight/thickness. A variant of the method is the High Pressure OIT- (HP-OIT / ASTM D5885 / D5885M-17), the Standard Test Method for Oxidative Induction Time of Polyolefin Geosynthetics by High-Pressure Differential Scanning Calorimetry.

The temperature used in this high-pressure test method is 150°C while the chamber pressure is maintained at 3.4 MPa (500 psi) using a constant volume test condition.

It is especially important to mention that certain antioxidants may provide poor or weak improvements in antioxidant activity based upon interpretation of OIT experimental results even though they may provide adequate or exceptional performance at their intended use temperatures, and vice versa.

Basics of OIT – Principle of Measurement

Samples are initially heated at a constant rate in an inert gas environment to the prescribed temperature.
When the specified temperature is reached, the environment is changed to oxygen at the same flow rate.
The material is then held at constant temperature until the oxidation reaction is manifested on the thermogram.

Although the method is very simple, the results are depending on many variables.

Remark: In the alternative dynamic method, the sample is heated at constant rate under oxygen. The Oxidation Induction Temperature is determined by the onset of the exothermic peak.

Variables Affecting the OIT Method and Results
Measurement parameters including….

Example of Influence of OIT Environment: Air vs. Oxygen

PP containing 0.10% Irganox B 225 (Phenol/Phosphite 1/1 ratio)

OIT – Utility of the Method

Quick method for the determination of stabilizer efficiency with DSC equipment at the conditions of the OIT measurement
OIT provides the ability to measure, and therefore can differentiate, material oxidative stability at the conditions of the OIT test.
With properly established controls and procedures, OIT can be used as an analytical tool for the direct quantification of additive concentration.
Similar to many other analytical methods, interferences from other additives can affect the accuracy of the measurement.

OIT as Supplemental Analytical Tool

OIT can be used in certain instances as a means of characterizing stabilizer consumption and/or activity as a supplement to other evaluations. i.e. it can detect residual AO activity/oxidative stability in samples which show little or no AO via direct chemical analysis.
OIT can “detect” additives via oxidative stability measurements relative to some reference material or control. i.e. extracted and isolated polymer as reference.

Determination of Stabilizer Content with OIT - Phenol Calibration is System Dependent

The OIT value is often a linear function of the antioxidant concentration, at least in the concentration range of practical relevance.

Different polymer manufacturing technologies, catalysts, processing histories, and other additives can affect OIT values.

Cautionary Statements Regarding OIT

OIT - Does not usually allow a direct prediction of the long-term effect of the stabilizers under end-use conditions.
Life-time prediction is not generally possible based upon experience and correlation with other assessment methods.

Why are correlations often difficult to establish? There are often issues related to time and temperature exposure conditions that are significantly different between highly accelerated high temperature/short time duration tests and actual field or use conditions. Extrapolation or establishing other mathematical functions to correlate these datasets can often be difficult to establish.

Stabilizer Temperature Range vs. OIT Temperature Range

Thiosynergists do not generally show any influence on the effect of phenols in the OIT test. However, this combination of antioxidants is very efficient at low test temperatures and in real-world applications.
Light Stabilizers (HALS) do not generally act as radical scavengers at 200°C even though they show a good contribution to long-term thermal stability in oven aging at 100°C (see figure below). They do not contribute to processing stability of polymer melts….
Co-Stabilizers, such as anti-acids, can influence the OIT without typically contributing to long-term thermal stability at more moderate temperatures.
Colorants have been shown to affect thermal stability and OIT of polymers.

Comments on OIT versus LTTS Data

The correlation between OIT and LTTS can be relatively poor which should limit its utility as a predictive tool for predicting product lifetimes and durability.
- Specific, detailed correlations may be attainable with development efforts/experimental validation
- Generic correlations are suspect
High temperature testing conditions tends to exclude or limit emerging new technologies from R&D as well as traditional products, including Hindered Amines, which are shown to provide benefits as thermal stabilizers and in retaining physical properties in durable applications at lower temperatures which are more representative of end-use conditions.
Material Factors Sample (Polymer) Morphology which Influence Results
- OIT Measurements are done on the substrate in a liquid state whereas the final application is in the solid state.
- In partially crystalline thermoplastics, the stabilizers are dissolved in the amorphous regions.
- In the melt, the stabilizers are evenly distributed throughout the material so that a “dilution” occurs.
- In the liquid state and higher temperature, the mobility and the solubility of the stabilizers increases.
- Oxidative reactions in a solid phase or in a highly viscous liquid depend of the diffusion of the Gas. (Partial Pressure of the Gas).

Antioxidants to prevent polymer oxidation