Oxidation Induction Time

A Guide to the Successful Determination of OIT

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Many organic compounds tend to undergo oxidation reaction in the presence of oxygen, even at low temperatures. However some classes of substance, when subjected to isothermal conditions, exhibit a period of induction during which no reaction with oxygen occurs. This time period is called the Oxidation Induction Time or Oxidative Induction Time (OIT). After this period, a reaction with oxygen occurs at an increasing rate.

The classes of substance which show this property are:

Polyolefins (mainly Polyethylene and Polypropylene)
Lubricating greases and oils
Edible oils and fats

OIT is frequently used to study the stability of the above materials in process development and quality control. The test is used to assess the level of stabilization of a material by determining the time to the onset of oxidative decomposition. The measurement is carried out in an oxygen atmosphere at a temperature that is sufficiently high to ensure that decomposition begins within a reasonable time. OIT is performed under accelerated conditions to shorten the measurement duration for practical feasibility.

Oxidation Onset Temperature

Oxidation onset temperature or OOT is the temperature at which onset of the oxidation of a sample is observed. Similar to OIT, OOT is also a relative measure of the stability of the test material toward oxidation. The higher the OOT, the more stable the material.

How to Measure Oxidation Induction Time (OIT) And Oxidation Onset Temperature (OOT)?

OIT is the time from the initial exposure to oxygen to the onset of exothermic decomposition at the isothermal temperature of the test. It gives a relative measure of the stability of the test material toward oxidation. The longer the OIT, the more stable the material. As shown in the diagram, the OIT is the time from the initial exposure to oxygen (t₀) to the onset of oxidation (t_onset).

OOT or Oxidation Onset Temperature is the temperature at the onset of exothermic decomposition measured at a given heating rate in an oxidative environment. The OOT test is a dynamic test and is measured at a specified heating rate in the presence of air or pure oxygen. The sample is heated in a DSC module in an open crucible with a preferable heating rate of 10 K/min until the exothermic oxidation reaction begins. This is shown by the sudden increase in heat flow in the diagram on the right. Evaluation of the onset in the curve gives the value of OOT.

Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) is a versatile measurement technique used in many analytical, QC and R&D laboratories. It is used to measure the heat flow produced in a sample when it is either heated, cooled, or held isothermally at a constant temperature.

OIT measurements are commonly performed isothermally by DSC in an oxidative atmosphere. The instrument measures the heat produced in the oxidation reaction and detects the onset of oxidation accordingly.

The sample is inserted in an open crucible into the instrument and then rapidly heated to the isothermal test temperature, with nitrogen as a purge gas. Once the sample temperature is stabilized, the purge gas is switched to oxygen. The onset is evaluated as the point of intersection of the tangent to the exothermic reaction curve with the extrapolated DSC baseline, as shown in the diagram. This onset is nothing but the OIT value, which is the time between the initial exposure to oxygen and the onset of the oxidation reaction.

High Pressure Differential Scanning Calorimetry (HP DSC) and High Pressure Differential Scanning Calorimetry Chemiluminescence

The HP DSC 2+ with a pressure controller is beneficial for OIT and OOT studies of oils and petrochemical samples at elevated pressure and high oxygen concentrations.

The HP DSC 2+ instrument can analyze samples under inert or reactive gases at pressures of up to 10 MPa. The PC10 pressure controller is used to maintain a constant elevated pressure during the experiment at a controlled gas flow-rate. High pressure helps to reduce the undesired vaporization of stabilizers or other additives from oil samples.

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The HP DSC can easily be expanded to a HP DSC Chemiluminescence System.

Chemiluminescence is the radiation of light that is generated when some materials oxidize. The instrument simultaneously detects the light emission and the heat flow from a sample subjected to a precisely controlled gas pressure at a particular temperature.

In this example of OIT determination of Motor oil, the DSC measurement curve exhibits a strong exothermic effect after about 66 minutes corresponding with the start of oil oxidation.

The onset of oxidation is also evident from the chemiluminescence measurement curve. The intensity of the chemiluminescence first gradually decreases due to oxidative degradation of stabilizers and then suddenly increases when the oil shows oxidation.

Thermo Gravimetric Analysis (TGA) and Thermo Gravimetric Analysis/Differential Scanning Calorimetry (TGA/DSC)

DSC is certainly the most extensively used technique for OOT and OIT determination. However, in some cases, TGA and TGA/DSC instruments can also be considered for OOT studies. In TGA, oxidation of the sample is indicated by mass gain in the TGA curve. Step height and onset temperature can be evaluated at this point and this onset corresponds to the OOT of the sample. For instance, OOT determination of olive oil performed with TGA/DSC under different crucible conditions gives comparable OOT values to those observed in the DSC signal.

Influencing Factors

1. Temperature

The isothermal temperature used in OIT experiments, influences the OIT results. Higher temperature accelerates the oxidation, hence OIT is decreased with increasing temperature when other parameters such as sample mass, purge gas flow-rate and pressure are constant. The values obtained for the OIT of PE-HD decreased dramatically, from 55.44 to 3.03 minutes, as the temperature increased from 210 °C to 240 °C. This shows that accurate calibration of the isothermal temperature is essential for OIT experiments, especially if inter-laboratory results are to be compared.

Webinar – OIT Measurements by Thermal Analysis

2. Pressure

The oxygen pressure applied in the reaction influences rate of oxidation reaction. Higher pressure helps to reduce analysis time and suppresses the vaporization of volatile constituents. It also allows the use of lower test temperatures, which are closer to the actual end-use conditions. OIT of mineral oil is determined isothermally at 195 °C with varying oxygen pressure. The OIT of the oil decreases drastically as the pressure is varied from 0.5 mPa to 6.8 mPa.

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3. Copper

Oxidation is often influenced by the presence of a catalyst, which accelerates the reaction rate. This effect is seen with copper. An OIT experiment performed with PE-HD in a copper crucible gives a lower OIT compared to the same experiment performed with aluminum crucibles, as copper increases the rate of the oxidation reaction.

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Oxidation Induction Time Determination: Standard Test Methods

A standard or norm provides a scientifically designed test method that is globally accepted. Use of these methods ensures uniformity in test procedures as well as a base for comparison of inter-laboratory results. Adopting the standard method minimizes the work of method validation, which is a prerequisite for quality assurance approval. OIT analysis in ambient conditions and in high pressure conditions have dedicated International Standard methods such as ASTM, EN, ISO and CEC. However, for OOT, a single standard method from ASTM is available until date. A list of standard norms associated with Thermal Analysis is given in the link below.

Standardization in Thermal Analysis

OIT Determination
ASTM	ASTM D3895	OIT of polyolefins by DSC
	ASTM E1858	OIT of hydrocarbons by DSC

EN	EN 728	OIT of polyolefin pipes and fittings

ISO	ISO 11357	OIT of plastics by DSC

OIT Determination at Higher Pressure
ASTM	ASTM D5483	OIT of lubricating greases by pressure DSC
	ASTM D5885	OIT of polyolefin geosynthetics by HPDSC
	ASTM D6186	OIT of lubricating oils by pressure DSC

CEC	CEC L-85-T-99	Hot surface oxidation of lubricating oils by pressure DSC

OOT Determination at Higher Pressure
ASTM	ASTM E2009	Oxidation Onset Temperature (OOT) of hydrocarbons by DSC

Adjustment and Calibration

OIT results are influenced by the calibration factors of the instrument. Hence it is important that the instrument is properly calibrated, otherwise a temperature adjustment is needed. Certified reference substances available from LGC and NIST are used for routine calibration and adjustment procedures.

OIT analysis is performed in isothermal conditions at a selected temperature. Hence temperature adjustment for such isothermal conditions is important. For this, a reference substance is used for one specific temperature instead of a temperature range.

The STAR^e software comes with ready-to-use methods for quick checking of the instrument status and dedicated calibration and adjustment methods for a single or combination of different reference substances. The choice of reference will depend on the working temperature range of the user.

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Webinar - Calibration and Adjustment in Thermal Analysis

Thermal Analysis in Practice Handbook

Tips and Hints

Allow the instrument to stabilize before starting the experiment.
For reporting, do not consider the first measurement after switching on the instrument.
Check the instrument calibration before performing experiments. For OIT experiments proper temperature adjustment is important.
Maintain a uniform sample quantity in multiple measurements.
Spread the sample evenly over the bottom of the crucible. For liquid samples, ensure that it covers the bottom rim of the crucible.
Sample should be measured in an open pan or partially closed with a lid pierced with five 1-mm holes, to prevent oxidation products from creeping out.
Considering practical conditions, change the measurement temperature to achieve OIT results within limits.
If a sample or its oxidation products spill onto the sensor during measurements, perform a burnout run for cleaning prior to proceeding with the next sample.

Webinar - Calibration and Adjustment in Thermal Analysis

Thermal Analysis Tips and Hints Guide

FAQ

What is the difference between OIT and OOT?

OIT (Oxidation Induction Time) is the time from the initial exposure to oxygen to the onset of exothermic decomposition at the isothermal temperature of the test. OOT (Oxidation Onset Temperature) is the temperature value at the onset of exothermic decomposition, measured at a given heating rate in an oxidative environment.

What is the importance of the OIT value?

OIT indicates stability of the sample towards oxidative conditions. The longer the OIT, the more stable the sample. OIT also decides shelf life of the material.

How to select temperature for OIT measurement?

Generally, it is advisable to perform an OOT measurement first. Once the onset temperature of the oxidation reaction is obtained, then the isothermal temperature for OIT analysis is selected as OOT – 30 °C. If standard test methods are used, these methods mention the test temperature to use.
In order to obtain reproducible results, OIT should be a minimum of 5 mins. If it is less, the temperature should be lowered by 10 K. If OIT is more than 1 hour, the measurement temperature should be increased by 10 K.

Which crucibles are suitable for OIT experiments?

Aluminum or copper? Both aluminum and copper crucibles can be used for OIT experiments. However, copper acts as a catalyst for oxidation reactions and gives lower OIT values. Hence, copper crucibles are used when the catalytic action is studied.

How to achieve reproducibility during OIT measurements?

For reproducible results, sample mass must be comparable when multiple measurements are performed. OIT results are temperature dependent, hence precise temperature calibration is important.

When to use High-Pressure Differential Scanning Calorimetry for OIT measurements?

If the OIT values are too high and have practical limitations, the use of higher oxygen pressure can increase the rate of reaction. Otherwise, if the use of high pressure conditions are mentioned in the method as per international standards, HP DSC can be used.