OIT Measurements by Means of Thermal Analysis
On Demand Webinar

OIT Measurements by Thermal Analysis

On Demand Webinar

OIT measurements are important to optimize the properties and performance of many products

OIT measurements
OIT measurements

OIT measurements determine the oxidative stability of oils, polymers and foods. OIT techniques are thus frequently employed for process development and quality control.

How oxidation affects materials
Oxidation is the reaction between a substance or material and oxygen. The oxidation of materials can lead to failure in their particular use or application.
One of the best known is the slow oxidation of iron to form iron oxide, a brittle brown powdery substance known as rust. During this process, the iron loses its mechanical strength.

The oxidation of polymers causes them to become brittle. Edible oils become rancid, which gives them an unpleasant smell and taste. Mineral oils undergo polymerization and the viscosity of petrochemical products changes.

OIT measurements
The OIT test is a standardized isothermal measurement performed with a Differential Scanning Calorimeter (DSC). 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.

OOT and OIT are accelerated test methods. They allow you to compare and assess the oxidative stability of materials and the performance of stabilizers much more quickly than by traditional methods.

This webinar also describes a number of applications to illustrate the possibilities that the OIT technique offers in various fields.

English

Oxidation Induction Time

Slide 0: Oxidation Induction Time

Ladies and Gentlemen

Welcome to this seminar on the Oxidation Induction-Time or OIT as it is usually called for short.

In the scientific literature, the expression Oxidative-Induction Time is also often used. Both expressions mean the same.

 

The OIT test is a standardized isothermal measurement performed with a Differential Scanning Calorimeter. 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 determination is therefore an accelerated thermal aging test. It is frequently used for studying the oxidative stability of oils, polymers and foods and in process development and quality control.

 

Slide 1: Contents

During this seminar, I will explain why we perform OIT measurements. A related test method called the Oxidation Onset Temperature or OOT will also be introduced.

The basic principles of OIT and OOT will then be discussed and compared.

After this, I will present the METTLER TOLEDO instruments used to perform these measurements.

Finally I want to describe a number of applications to illustrate the possibilities that the OIT technique offers in various fields.

 

Slide 2: Why Oxidation Induction Studies?

Ideally, we expect materials and products to be stable in air for a long time. For example, the polyethylene pipelines shown in the pictures.

Pipelines like this are nowadays widely used to transport water or gas. The pipeline industry expects the lifetime of polyethylene pipes to be at least 10 years above ground and 30 years when installed below ground.

Oxidation studies are important to optimize the properties and performance of such products.

 

Slide 3:Introduction to Oxidation Studies

The slide shows the effects of oxidation on the behavior of materials.

Oxidation is the reaction between a substance or material and oxygen. The oxidation of materials can lead to failure in their particular use or application.

The table shows some examples of the effects of oxidation. One of the best known is the slow oxidation of iron to form iron oxide, a brittle brown powdery substance known as rust. During this process, the iron loses its mechanical strength.

The oxidation of polymers causes them to become brittle. Edible oils become rancid, which gives them an unpleasant smell and taste. Mineral oils undergo polymerization and the viscosity of petrochemical products changes.

 

Slide 4:Introduction to Oxidation Studies

Let’s consider polyethylene pipes again and their manufacture.

Accelerated testing under oxidative conditions is an important method used to analyze materials within an acceptable time. To do this, measurements are performed at elevated temperatures and higher oxygen concentrations.

Oxidation studies play a very important role in both quality control and in research and development in this industry segment. To maintain quality, each lot of polyethylene pellets has to be checked before the pellets are used for production. For R&D studies it is important to compare the effect of different inhibitors and stabilizers on the product in order to select additives that are both suitable and compatible.

 

Slide 5: Introduction to Thermal Analysis

The ICTAC definition of thermal analysis is:

“A group of techniques in which a physical property of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature program”.

The lower half of the slide illustrates typical effects that occur when a sample is heated. For example, initial melting, in which the sample changes from the solid to the liquid state. If the sample is exposed to air or oxygen, it will start to oxidize and finally decompose.

The study of oxidation behavior is an important application in thermal analysis. Preliminary measurements are often performed using a simple linear temperature program as shown in the diagram on the right.

 

Slide 6: Principles of OIT

This slide illustrates the principles involved in the determination of the Oxidation Induction Time of a material, in particular of a polymer.

OIT measurements are commonly performed isothermally by differential scanning calorimetry in an oxidative atmosphere. The DSC measures the heat produced in the oxidation reaction and can hence detect the onset of oxidation.

The most usual way is to insert the sample in an open crucible at room temperature into the DSC instrument. The sample is then rapidly heated to the isothermal test temperature using nitrogen purge gas as shown in the diagram on the left. Once the sample temperature has stabilized, the purge gas is switched to oxygen.

The diagram on the right shows the resulting heat flow as a function of time.

The OIT is the time from the initial exposure to oxygen (t0) to the onset of oxidation (tonset). The onset is evaluated as the point-of-intersection-of-the-tangent- to-the-exothermic-reaction-curve with the extrapolated DSC baseline.

 

Slide 7: Principles of OIT

This slide illustrates the determination of OIT using high-density polyethylene as an example.

Sixteen point six milligrams (16.6 mg) of PE-HD was weighed into a standard forty-microliter (40 μl) aluminum crucible without a lid. The sample was inserted into the DSC and held isothermally at thirty degrees Celsius (30 °C) for 3 minutes under a nitrogen purge gas flow of fifty milliliters per minute (50 mL/min). It was then heated to two hundred degrees (200 °C)at twenty degrees per minute (20 K/min) during which time the polyethylene melted. After 2 minutes, the furnace atmosphere was automatically switched to oxygen at fifty milliliters per minute (50 mL/min) using the gas controller. The temperature was held constant at two hundred degrees (200 °C) until the end of the measurement. The OIT is the time from switching to oxygen to the onset of exothermic oxidation, in this example 15.9 minutes.

 

Slide 8: Principles of OIT: Influencing Factors

To obtain meaningful OIT values, the measurement conditions have to be accurately and reproducibly set, especially if we want to compare similar samples. It is therefore very important to be aware of the factors that influence OIT values and have them under control.

These factors will be discussed in more detail in the following slides.

Let’s begin with the most obvious, namely the influence of the isothermal temperature on the OIT.

This was studied by measuring the OIT of high-density polyethylene at five different isothermal temperatures ranging from two hundred and forty degrees Celsius (240 °C) to two hundred and ten degrees (210 °C). All other experimental parameters such as sample mass, purge gas flow-rate, pressure, and so on, were kept constant.

The values obtained for the OIT at different temperatures increased dramatically, from 3 to 56 minutes. This shows that accurate calibration of the isothermal temperature is essential, especially if the results are to be compared with those from other laboratories.

 

Slide 9: Principles of OIT: Influencing Factors

As we know, reactions proceed more rapidly at higher temperatures and at higher gas pressures. The oxidation reaction is no exception.

Higher pressures help to reduce analysis times and suppress the vaporization of volatile constituents, for example in oils. The slide illustrates the influence of oxygen gas pressure on the OIT of a mineral oil. The diagram shows five OIT curves measured isothermally at one hundred and ninety-five degrees Celsius (195 °C) by high-pressure DSC at different oxygen pressures.

The gas flow was maintained at forty milliliters per minute (40 mL/min) and the oxygen pressure set to different values ranging from zero point five (0.5 MPa) to ten megapascals (10 MPa) by means of a pressure and flow controller.

At an oxygen pressure of zero point five megapascals (0.5 MPa), the OIT is more than 120 minutes. In contrast, at six point eight megapascals (6.8 MPa) the OIT is less than 40 minutes. This shows that the OIT decreases with increasing pressure.

Higher pressure also permits lower test temperatures to be used. Lower temperatures are closer to the actual end-use conditions. Almost all high-pressure OIT measurements are therefore performed at pressures of three point five megapascals (3.5 MPa) or higher.

 

Slide 10: Principles of OIT: Influencing Factors - Copper

The oxidation reaction is also very sensitive to catalytic effects. The polymer used to insulate copper wires in electrical cables may undergo accelerated oxidation.

Measurements are therefore often performed in crucibles made of copper instead of aluminum in order to assess the influence of copper. The OIT measured in a copper crucible is about half that of the OIT measured in an aluminum crucible.

We can see this in the slide: the OIT of a sample of high-density polyethylene measured in an aluminum crucible was 15.9 minutes, whereas in a copper crucible it was just 9.5 minutes.

Copper crucibles can also be used to check the influence of specific stabilizers. In sample preparation, the catalytic effect of copper should be kept in mind when cutting samples with knives made of copper alloys.

 

Slide 11: Principles of OIT: Standard Methods

Several important Standards Organizations describe standard test methods for the determination of OIT at ambient pressure by DSC.

The standards are listed below and mostly concern polyolefins and hydrocarbons. These methods have been developed by specialist committees to provide reliable test methods for comparative testing. They also simplify the procedures used in older manual testing methods.

 

Slide 12: Principles of OIT: Standard Methods

This slide lists a number of standard test methods for the determination of OIT by high-pressure DSC.

These methods are mainly used in the oil, petrochemical, and lubricant industries.

The test methods typically use pure oxygen at pressures of three point five megapascals (3.5 MPa), or 35 bar.

 

Slide 13: Principles of OIT: Repeatability

OIT measurements are generally precise and reproducible.

The diagram shows the OIT curves obtained from the measurement of three, approximately eight-milligram (8-mg) test specimens taken from the same sample of polyethylene at two hundred and twenty degrees Celsius (220°°C). Under the conditions used, the mean OIT was 27.5 minutes and the repeatability standard deviation 0.76 minutes.

The latter value is well within the repeatability standard deviation of 1.3 minutes and reproducibility standard deviation of 4.5 minutes of an interlaboratory test.

 

Slide 14: Principles of OOT

This slide illustrates the principles involved in the determination of the Oxidation Onset Temperature or OOT of a material, in particular of a polymer.

Like OIT, the Oxidation Onset Temperature is also a qualitative measure of the level of stabilization a material toward oxidation. In contrast to OIT, it is measured at a specified heating rate in an oxidative environment of air or pure oxygen.

Typically, the test specimen is inserted into the DSC at room temperature in an open crucible and then heated at ten degrees per minute (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.

The OOT is the time to the onset of oxidation.

 

Slide 15: Principles of OOT

The diagram shows the measurement of the Oxidation Onset Temperature of a sample of olive oil.

The experiment was performed by heating the sample from thirty to three hundred degrees Celsius (30 to 300 °C) at five degrees per minute (5 K/min) using oxygen as purge gas. Evaluation of the curve yielded an OOT temperature of one hundred and seventy-one degrees (171 °C).

This indicates that the olive oil will rapidly oxidize during the frying process, giving it an unpleasant smell and taste. The result means that the olive oil should not be heated above one hundred and fifty degrees (150 °C) and should not be used for frying.

 

Slide 16: Principles of OOT: Standard Method

ASTM E2009 is a standard test method for determining the OOT of hydrocarbons at ambient pressure or at three point five megapascals (3.5 MPa) in air or pure oxygen.

The Oxidation Onset Temperature is also a quick way to determine the optimum temperature for an OIT measurement.

 

Slide 17: Comparison of OIT and OOT

Now that we are familiar with the two techniques, we can compare their relative merits.

Both the OIT and OOT tests are universally accepted methods of accelerated testing that have been approved by standards organizations.

The OOT test is dynamic measurement technique and provides results in a shorter analysis time. It is faster than OIT but cannot differentiate the degree of stability of materials so well. OOT can be used to determine the optimum isothermal temperature for OIT measurements. To date, only one standard is available for OOT.

The OIT is an isothermal measurement technique and takes more time, typically 1 to 2 hours. The temperature often has to be optimized in order to obtain OIT values that are not too short or not too long. However, the longer measurement time allows better differentiation of different degrees of stability. The OIT test method is therefore usually preferred and recommended.

 

Slide 18: Techniques: DSC

Let’s begin with the DSC technique.

DSC is a calorimetric technique that measures the energy absorbed or released by a sample as it is heated or cooled or held at constant temperature.

The standard METTLER TOLEDO DSC 1 instrument with gas controller is ideal for performing OIT and OOT studies of polymers at ambient pressure.

The GC100 Gas Controller allows us to set and automatically switch gas flows, for example from nitrogen to oxygen at controlled flow rates.

 

Slide 19: Techniques: HPDSC

Another useful DSC technique is high-pressure DSC, or HPDSC for short. The METTLER TOLEDO HP DSC 1 instrument can analyze samples under inert or reactive gases at pressures of up to ten megapascals (10 MPa).

The PC10 pressure controller allows you to work at a constant elevated pressure during a DSC experiment at a controlled gas flow-rate. High pressure helps to reduce the undesired vaporization of stabilizers or other additives from oil samples.

The HP DSC 1 with a pressure controller is an excellent instrument for OIT and OOT studies of oils and petrochemical samples at elevated pressure and high oxygen concentrations.

 

Slide 20: Techniques: HPDSC-Chemiluminescence

The METTLER TOLEDO HP DSC 1 can easily be expanded to a high pressure DSC-Chemiluminescence System.

This instrument simultaneously detects the light emission and the heat flow from a sample subjected to a precisely controlled gas pressure at a particular temperature.

Chemiluminescence is the radiation of light that is generated when some materials oxidize. Chemiluminescence measurements of materials provide information about local oxidation rates and the influence of stabilizers as well as additional visual information about the oxidation of the test material.

The pictures show the experimental set up. The standard high-pressure cover of the HP DSC1 is replaced by a special cover. The cover includes a window through which the sample is observed using an extremely sensitive CCD camera equipped with powerful optics and control software. The special cover allows the sample to be subjected to pressures of up to two megapascals (2 MPa) or 20 bar. The system can be assembled or disassembled again for normal DSC measurements within a few minutes.

 

Slide 21: Industries and Application

Oxidation induction studies have many potential applications and can be used for stability testing in practically all industries.

In the automotive industry, oxidation studies are used to determine the temperature stability of lubricants and engine oils.

In the piping industry, OIT and OOT measurements are used to maintain and improve the quality of polymers used for the manufacture of polyethylene pipes.

In the electronics industry, OIT measurement is the best method for checking the stability of polymers used for the insulation of wires and cables as well as for studying the efficiency of inhibitors. The polymers investigated are mainly polyolefins such as polyethylene or polypropylene.

 

Slide 22: Application 1                                       Oxidative stability of PP

I will now discuss several applications that illustrate oxidation testing.

The first application shows the measurement of the OIT of three samples of polypropylene according to ASTM E1858. The OIT curves were measured isothermally at one hundred and ninety five degrees Celsius (195 °C)using samples of about three milligrams (3 mg) and an oxygen flow rate of fifty milliliters per minute (50 mL/min).

Unstabilized polypropylene oxidizes much more quickly than stabilized polypropylene namely in 28 minutes whereas stabilized polypropylene remains stable for more than 120 minutes. The stabilized, but aged PP starts to oxidize after about 67 minutes.

 

Slide 23: Application 2                           OIT of engine oils by HPDSC

The determination of the OIT of oils is an important test in the petrochemical industry. The test is usually performed according to a standard, for example ASTM D6186.

The diagram displays the OIT curves of two different types of engine oil. The samples were held at one hundred and eighty degrees Celsius (180 °C) under a constant oxygen pressure of three point five megapascals (3.5 MPa) until exothermic oxidation began.

The black curve was obtained from a mineral oil and the red curve from a synthetic oil. The mineral oil oxidizes after about 35 minutes and the synthetic oil after about 236 minutes. This indicates that the synthetic oil is much more stable than the mineral oil.

In general, synthetic oils are very stable and take a long time to oxidize. High-pressure DSC is therefore ideal for measuring the OIT of engine oils because the measurement time at a given isothermal temperature can be shortened by increasing the oxygen pressure. At the same time, higher pressure also suppresses vaporization of the oil.

 

Slide 24: Application 3               Stability of a fish oil by HPDSC

High-pressure DSC can also be used to study the stability of fish oils and fish oil products.

Fish oils are widely used as dietary supplements in foodstuffs because of their high content of essential omega-3 fatty acids. Unfortunately, the oils are very sensitive to oxidation and easily become rancid, making them unsuitable for further use. This problem can be overcome by adding antioxidants as stabilizers. This slows down the oxidation process and improves the stability of the oils.

The diagram displays typical OOT and OIT curves of a fish oil sample measured in oxygen at a pressure of three point five megapascals (3.5 MPa) or 35 bar. This particular fish oil was enriched with 50% docosahexaenoic acid without additional antioxidant. The oxidation measurements were performed using about 5 milligrams of fish oil in a 40-microliter aluminum crucible without a lid.

The upper, OOT curve was measured at a heating rate of 10 degrees per minute and the lower, OIT curve isothermally at 90 °C.

The results obtained by HPDSC are comparable to values determined by other techniques used to classify fish oils. The main aim of using HPDSC is to speed-up quality control and simplify sample analysis.

 

Slide 25: Application 4                           Oxidation of vegetable oils

Oxidation causes rancidity in edible oils and fats, giving them an unpleasant odor and taste and making them unsuitable for cooking. Similarly, long-term oxidation may occur during storage and processing.

The oxidative stability of fats or oils depend on their precise nature. Furthermore, used oils can be distinguished from fresh oils.

The diagram shows the OOT curves of soybean oil and palm fat. In each case, about two milligrams (2 mg) of the oil or fat was weighed into forty-microliter (40-uL) standard aluminum crucibles and measured from fifty (50 °C) to two hundred and eighty degrees Celsius (280 °C) at a heating rate of fifteen degrees per minute (15 K/min).

The soybean oil shows that oxidation begins at about one hundred and eighty-eight degrees (188 °C) under oxygen but shows no visible signs of reaction under nitrogen. Similarly, palm fat begins to oxidize at about two hundred and thirteen degrees (213 °C).

Clearly, OOT determination is a useful method for comparing and characterizing edible fats and oils.

 

Slide 26: Application 5               HPDSC-Chemiluminescence of an oil

This slide shows chemiluminescence measurements performed on a synthetic motor oil.

The sample was measured isothermally at two hundred degrees Celsius (200 °C) in an oxygen atmosphere at a pressure of one megapascal (1.0 MPa) or 10 bar.

The aim here was to obtain information about the thermal stability of the oil and to investigate the influence of different stabilizers.

In the diagram, we see that the DSC measurement curve exhibits a strong exothermic effect after about 66 minutes. This indicates that the oil begins to oxidize.

We can of course also determine the onset of oxidation from the chemiluminescence measurement curve. It is interesting to note that the chemiluminescence intensity first gradually decreases and then suddenly increases when the oil suddenly oxidizes. The gradual decrease is the result of continuous oxidative degradation of the stabilizers in the oil. This effect cannot be detected on the DSC curve. We also notice from the color-enhanced images that the oil is concentrated around the edge of the crucible due to surface tension.

 

Slide 27: Summary

This slide summarizes the features and benefits of OIT and OOT measurements.

OOT and OIT are accelerated test methods. They allow you to compare and assess the oxidative stability of materials and the performance of stabilizers much more quickly than by traditional methods.

The operating procedures are relatively simple, reliable and cost-effective. The results are reproducible if the conditions are properly set. The OIT and OOT techniques are universally accepted standard test methods.

The tests help laboratories to ensure constant product quality and are used to compare the effectiveness of stabilizers.

 

Slide 28: For More Information on OIT

Finally, I would like to draw your attention to information that you can download from METTLER TOLEDO Internet pages.

Articles on thermal analysis and applications from different fields are published twice a year in our well-known, METTLER TOLEDO, thermal analysis, UserCom customer magazine. The slide lists different UserCom articles that specifically relate to OIT studies.

 

Slide 29: For More Information on Thermal Analysis

In addition, you can download information about webinars, application handbooks or information of a more general nature from the Internet addresses given on this slide.

 

Slide 30: Thank you

This concludes my presentation on OIT. Thank you very much for your interest and attention.

Many organic compounds are attacked by oxygen and undergo oxidation reactions even at low temperatures.
Materials and substances such as plastics, greases, oils and fats are easily investigated by thermal analysis techniques.
Oxidation stability can be characterized by OIT or OOT. The OIT of a material is measured in an oxygen atmosphere at a particular isothermal temperature whereas the OOT is determined in a dynamic measurement in oxygen. OOT experiments are performed more rapidly.
Thermal analysis is an excellent method for investigating oxidation stability. Only a few milligrams of sample are required and the measurement is quickly performed. It can even be automated.

The Webinar covers the following topics:

  • Introduction
  • OIT and OOT methods
  • Why use TA
  • Industries and applications
  • Different instruments for the measurement of OIT and OOT
    – DSC
    – HP DSC
    – TGA
    – TMA
  • Summary


 

 
 
 
 
 
 
 
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