HP DSC – Material Characterization at Elevated Pressures
On Demand Webinar

HP DSC – Material Characterization at Elevated Pressures

On Demand Webinar

A HP DSC system is ideal for studying the influence of gas pressure and atmosphere on a sample

HP DSC
HP DSC

Increased pressure influences all physical changes and chemical reactions in which a change in volume occurs. For material testing, process development, and quality control there is often no alternative to DSC measurements under pressure.
High-pressure DSC allows you to measure samples under defined atmospheres at up to 10 MPa as a function of temperature or time. Higher temperature and pressure accelerates reactions and shortens the analysis time.

In this Webinar, we will discuss the basic principles of high-pressure DSC and present some interesting applications.

25:45 min
English

The Webinar covers the following topics:

  • What is high-pressure DSC?
  • Basic principles and examples
  • The HP DSC 1 and its options
  • Why use High-pressure DSC?
  • Industries and applications
  • Practical applications
  • Summary

HP DSC is widely used in many different industries such as the chemical, pharmaceutical, petrochemical, plastics, paints and adhesives, electronics, and food industries and in academia.

 

Why use HP DSC?

Increased pressure influences all physical changes and chemical reactions in which a change in volume occurs. This means that increased pressure has a direct effect on many processes and reactions.

 

The most important reasons for using HPDSC are:

  • Higher pressures and temperatures accelerate the rate of chemical reactions and result in shorter analysis times.
  • HPDSC allows practical reaction environments to be simulated so that measurements can be performed under real process conditions.
  • Finally, higher pressure suppresses vaporization. Overlapping effects in DSC curves can be better separated because the effect due to vaporization is shifted to higher temperature. This simplifies DSC curves and makes them easier to interpret.

 

HP DSC Options

 

A METTLER TOLEDO HP DSC can be fitted with two important optional accessories for specific applications:

 

The HP DSC 1 Microscopy system allows you to observe a sample visually while it is heated or cooled in the DSC. Valuable information can be obtained about changes due to relaxation, melting, solid-solid transitions, the shrinkage of fibers and films, or reaction processes.

 

The HP DSC 1 Chemiluminescence system simultaneously detects the light emission and the heat flow from a sample subjected to a precisely controlled gas pressure at a particular temperature. Chemiluminescence measurements of materials provide information about local oxidation rates and the influence of stabilizers in different materials.

High-pressure differential scanning calorimetry

Slide 0: Introduction

Ladies and Gentlemen

Welcome to the METTLER TOLEDO webinar on High-Pressure Differential Scanning Calorimetry, or HPDSC as it is called for short.

DSC measurements performed at higher gas pressures expand the possibilities of thermal analysis. For material testing, process development, and quality control there is often no alternative to DSC measurements under increased gas pressure.

 

Slide 1: Contents

In the course of this seminar,

I would like to explain what we mean by high-pressure DSC and discuss the basic principles involved.

I then want to summarize why we use HPDSC, present some practical examples, and introduce the METTLER TOLEDO HP DSC 1.

I also want to briefly describe two important HPDSC options, namely the HP DSC-1-Microscopy System, and HP DSC1-Chemiluminescence System.

Finally, I will present examples that illustrate some of the different application possibilities.

 

Slide 2: What is HPDSC?

High-pressure DSC is an excellent technique for studying the influence of gas pressure and atmosphere on materials as a function of temperature.

A good example is the measurement of the Oxidation Induction Time, or OIT. The OIT is a relative measure of the oxidative stability of a material at the isothermal temperature of the test. The test is important in the polymer and petrochemical industry and is usually performed according to a standard test method, for example ASTM D6186.

The diagram on the right displays the OIT curves of two different types of engine oil. The samples were held at 180 degrees Celsius under a constant oxygen pressure of 3.5 mega-pascals (or 35 bar) until exothermic oxidation began. The Oxidation Induction Time is evaluated as the intercept of the horizontal baseline with the inflectional tangent, the so-called Onset.

The red curve in the diagram was obtained from a synthetic oil and the black curve from a mineral oil. The results show that 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. HPDSC is 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, this also suppresses vaporization of the oil.

If a conventional DSC instrument were used to measure the same sample at normal pressure, the isothermal temperature would have to be higher. The oil sample would then very likely vaporize before it could oxidize.

 

Slide 3: What is HPDSC?      Conventional DSC versus HPDSC

Let’s have a look at another example that clearly illustrates the use of high-pressure DSC.
The slide compares the measurement curves obtained from the curing reaction of a phenolic resin using conventional DSC, and high-pressure DSC.

The dashed black curve at the top of the diagram shows the conventional DSC curve of a sample measured in a 40-microliter crucible with a pierced lid. The curve exhibits a glass transition with enthalpy relaxation followed by melting processes and a polycondensation reaction. At ambient pressure, the effects due to the exothermic condensation reaction and the endothermic evaporation of water released during the reaction overlap. This makes it difficult to interpret this part of the curve.

The solid black curve is the curve obtained when the experiment was repeated using high-pressure gold-plated 40-microliter crucibles. The curve shows the curing reaction more clearly, but the effects are broader and poorly separated due to the large mass of the crucible.

The HP DSC 1 curve is shown in red at the bottom of the diagram. The sample was measured in a standard 40-microliter aluminum crucible using the same temperature program from 30 to 200 degrees Celsius at a heating rate of 10 degrees per minute, but at a nitrogen pressure of 3 mega-pascals. The peaks in the curve are sharper and the different effects are better separated. This is important when studying the kinetics of curing reactions in which water is released.

 

Slide 4: The HP DSC 1 System

The measurements described in the previous slides were performed using a METTLER TOLEDO high-pressure DSC.

The picture on the left shows the latest version, the HP DSC 1. The module comprises a stainless-steel pressure cylinder containing the furnace, the DSC measuring sensor, and fittings for the gas inlet and outlet. A pressure gauge displays the pressure, and a built-in safety rupture-disc protects the cell against excessive pressure.

The diagram on the right shows a cross section of the cell.

 

Slide 5: HP DSC 1 Options

This slide shows the HP DSC 1 equipped with an important optional accessory, the PC 10 Pressure Controller, or PC 10 for short.

The PC 10 option ensures that the pressure remains precise and constant throughout the analysis during heating and cooling segments. It also controls the auxiliary gas flow.

 

Slide 6: HP DSC 1 Options

The HP DSC 1 can be fitted with two important optional accessories for specific applications.
The slide shows the high-pressure DSC-Microscopy System on the left and the high-pressure DSC-Chemiluminescence System on the right.

The HP DSC 1 Microscopy System allows you to observe a sample visually while it is heated or cooled in the DSC. Valuable information can be obtained about changes due to relaxation, melting, solid-solid transitions, the shrinkage of fibers and films, or reaction processes. The visual data is recorded and makes it easier to interpret the DSC curves.

The HP DSC 1 Chemiluminescence System simultaneously detects the light emission and the heat flow from a sample subjected to a precisely controlled gas pressure at a particular temperature. Chemiluminescence measurements of materials provide information about local oxidation rates and the influence of stabilizers in different materials.

 

Slide 7: Why Do We Use HPDSC?

Before we move on to the applications, let’s briefly summarize why we use HPDSC:

Basically, it has to do with the fact that increased pressure influences all physical changes and chemical reactions in which a change in volume occurs. This means that increased pressure has a direct effect on many processes and reactions.

The most important reasons for using HPDSC are:

  • Higher pressures and temperatures accelerate the rate of chemical reactions and result in shorter analysis times.
  • HPDSC allows practical reaction environments to be simulated so that measurements can be performed under real process conditions.
  • Finally, higher pressure suppresses vaporization. Overlapping effects in DSC curves can be better separated because the effect due to vaporization is shifted to higher temperature. This simplifies DSC curves and makes them easier to interpret.

 

Slide 8: Industries and Applications

The table summarizes the main applications and the industries in which high-pressure DSC is used.
The applications in the chemical and pharmaceutical industries and in academia include the study of physical-chemical processes such as catalytic reactions, heterogeneous reactions, adsorption and desorption, and reactions with reactive gases.

HPDSC is also an important method for studying oxidation stability and reactions with reactive gases in the petrochemical, plastics and food industries.

 

Slide 9: Application 1            Vapor pressure diagram of water

In the following slides, I want to present different application examples that demonstrate the analytical potential of high-pressure DSC.

Let me begin with the vapor pressure diagram of water. The transition from the liquid to the vapor state is strongly pressure-dependent. A liquid boils when its vapor pressure equals that of its surroundings. To illustrate this, samples of 2 to 20 milligrams of water were measured at different pressures in 40-microliter aluminum crucibles using lids pierced with a 50-micron hole. The DSC curves in this diagram show the boiling of water as a sharp endothermic peak at a temperature that depends on the total pressure.
By the way, the small exothermic effect observed immediately after the vaporization peak in the curves measured at higher pressures is not an artifact. It is due to the reaction between the water vapor and the aluminum crucible. A similar measurement in a gold crucible, shown in red at the bottom right of the diagram, displays only the effects due to vaporization.

The diagram in the top left corner is the vapor pressure diagram plotted from the peak temperatures at different pressures according to the modified Clausius-Clapeyron equation. The enthalpy of vaporization is calculated from the slope of measured vapor pressure curve and agrees well with the literature value.

 

Slide 10: Application 2                  Oxidative stability of PE

Most polymers tend to degrade slowly due to chemical attack by atmospheric oxygen. A wide variety of compounds has therefore been developed to stabilize polymers.
In practice, different standard test methods are employed to check the stability of polymers toward oxygen. For example, the ASTM E1858 standard is used to measure the oxidation induction time and is recommended for testing polyolefines and their stabilizers.
In this particular example, samples of stabilized and non-stabilized polyethylene and polypropylene were measured in oxygen at a pressure of 3.5 mega-pascals under isothermal conditions at temperatures of 175 and 165 degrees Celsius respectively. The time from the start of the measurement to the onset of oxidation is the so called oxidation induction time, or OIT, which we discussed earlier.
The OIT values shown in the slide allow us to compare the oxidative stability of the materials and to assess how effective different stabilizers are. We see that the OIT values of the stabilized samples are much higher. A longer OIT indicates better stability.

 

Slide 11: Application 3          Influence of CO2 pressure on Tg

I would now like to discuss the influence of carbon dioxide pressure on polyvinyl pyrrolidone, or PVP for short.
The temperature range in which the glass transition occurs is usually very important for the processing of polymers and their application in finished products. Plasticizers are added to polymers to lower their glass transition temperature. Conventional plasticizers cannot always be used, for example if they give rise to adverse biological effects.
Carbon dioxide can act as a plasticizer in some materials, for example in PVP. The DSC curves on the left and the plot on the right show that increased carbon dioxide pressure causes the glass transition of PVP to shift to lower temperatures. The material is then softer and can be more easily molded.

 

Slide 12: Application 4          Stability of fish oils by HPDSC

High-pressure DSC is also 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 pure oils are sensitive to oxidation and rapidly become rancid. Rancid oils have an unpleasant taste and smell and are unsuitable for further use. This problem can be overcome by adding antioxidants as stabilizers and using microencapsulation techniques. This slows down the oxidation process and improves the stability of the oils.

The diagram in the slide displays typical OOT and OIT curves of a DHA50-SA fish oil sample measured in oxygen at a pressure of 3.5 mega-pascals. This particular fish oil is enriched with 50% docosahexaenoic acid, which is also an omega-3-fatty acid. The oxidation measurements were performed using 5 milligrams of fish oil in a 40-microliter aluminum crucible without a lid. The blue OOT curve shown in the upper part of the diagram was measured at a heating rate of 10 degrees per minute. The red OIT curve was measured isothermally at 90 degrees.
The results obtained by HPDSC are comparable to values determined by other techniques used to classify the fish oils. The main purpose of using HPDSC is to speed-up quality control and simplify sample preparation.

 

Slide 13: Application 5  Curing kinetics of a urea formaldehyde resin

This application concerns the measurement of reaction kinetics by HPDSC at constant pressure using the curing reaction of a urea formaldehyde resin as an example.
The crucible containing the sample was sealed with a lid with a 50-micron hole. This creates a self-generated atmosphere inside the crucible and restricts the evaporation of water and formaldehyde.
Approximately 14 milligrams of sample was heated to 200 degrees Celsius at various heating rates in a nitrogen atmosphere at a pressure of 6 mega-pascals. The kinetic evaluation was performed using Model Free Kinetics, called MFK for short, and by Advanced Model Free Kinetics, AMFK. Both methods deliver similar results, as shown in the slide.
The curves in the upper left diagram are the dynamic heating runs. MFK or AMFK analysis requires at least three measurements to be performed at different heating rates.
Conversion curves were then evaluated from the three heating curves and are displayed in the lower left diagram. The conversion curves are used to calculate the activation energy as a function of conversion, as shown in the upper right diagram. This allows predictions to be made about the curing behavior. For example, in the lower right diagram, we see that it takes approximately 10 minutes to achieve a conversion of 90% at an isothermal curing temperature of 120 degrees.
The advantage of using HPDSC for this particular type of sample is that the evaporation of water is suppressed - the exothermic peak arising from the curing reaction is not masked by the endothermic peak due to the evaporation of water.

 

Slide 14: Applications of HPDSC Microscopy

Let’s now turn our attention to the combination of HPDSC with microscopy. As I explained earlier, the high-pressure DSC 1 Microscopy System allows us to observe the sample visually while it is heated or cooled in the DSC.

The overview shows that HPDSC microscopy can be used to study a wide range of topics and problems that are likely to occur in almost any industry. Experiments can be performed using both isothermal and dynamic temperature programs.

I will now describe a typical application that illustrates the use of this technique.

 

Slide 15: Application 6 HPDSC microscopy analysis of a Dyneema® fiber

The application concerns the melting behavior of a DyneemaÒ fiber, a very strong polyethylene fiber with a tensile strength of about 3.5 giga-pascals. The fiber is for example used to make bullet-proof protective clothing. In general, fibers often consist of highly stretched polymers that exhibit a high degree of crystallinity.

Several lengths of fiber with a total weight of approximately 74 micrograms were placed in the crucible, covered with a quartz disk and pressed down to ensure good thermal contact between the sample and the bottom of the crucible.

The HPDSC curve in the slide records the melting behavior of the DyneemaÒ fibers at a heating rate of 4 Kelvin per minute. The curve exhibits a large peak due to melting at about 144 degrees Celsius followed by a second, smaller peak at about 147 degrees.

But what is the cause of the small peak?

This was investigated by recording microscope images across the melting region. The three images captured at 140.2, 143.4, and 146.2 degrees illustrate the behavior of the DyneemaÒ fibers during melting.

The images show that the fibers start to shrink from about 140 degrees onward to form a loose bundle. Comparison with the DSC curve shows that melting also begins at this temperature. Movement of the fiber bundle caused the quartz disk on top of it to move within the crucible and resulted in the small peak in the DSC curve.
This peak is therefore not directly related to the actual melting process, but is an artifact caused by movement of the molten fiber bundle and the quartz disk.

 

Slide 16: Applications of HPDSC Chemiluminescence

The other important option I mentioned earlier on was the combination of HPDSC with chemiluminescence. The high-pressure DSC 1-Chemiluminescence System allows you to simultaneously measure light emission and heat flow from a sample at a precisely controlled gas pressure. Chemiluminescence experiments are usually performed at constant temperature.

The slide presents an overview of applications of this technique.
The main applications of chemiluminescence are concerned with the stability of plastics, foodstuffs, paints and pharmaceuticals. When the oxidative stability is determined isothermally, it is characterized by a parameter called the Oxidation Induction Time, or OIT. Experimentally, this is the time when the chemiluminescence signal deviates significantly from the baseline. Under dynamic conditions, ( a programmed temperature ramp), this is referred to as the Oxidation Onset Temperature, or OOT.

In the next slide, I want to describe a typical application of chemiluminescence.

 

Slide 17: Application 7          HPDSC chemiluminescence of an oil

The slide shows the results of DSC-chemiluminescence measurements performed on a synthetic motor oil. The sample was measured isothermally at 200 degrees Celsius in an oxygen atmosphere at a pressure of 1.0 mega-pascal.
The purpose of the experiment was to obtain information about the thermal stability of oils 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 indicating that the oil begins to oxidize.

The onset of oxidation can also be determined from the chemiluminescence measurement curve. In this case, it is interesting to note that the chemiluminescence intensity initially gradually decreases before it suddenly increases when the oil oxidizes. The gradual decrease is the result of continuous oxidative degradation of the stabilizers in the oil. The effect cannot be detected in the DSC curve. We also notice from the false-color images that the oil is concentrated around the edge of the crucible due to surface tension.

 

Slide 18: Summary

This next slide summarizes the features and benefits of high-pressure DSC.

The technique is ideal for studying the influence of gas atmospheres and pressure on a sample or for separating an effect that is overlapped by vaporization.

HPDSC is widely used in many different industries such as the chemical, pharmaceutical, petrochemical, plastics, paints and adhesives, electronics, and food industries and in academia.

 

Slide 19: For More Information on HPDSC

Finally, I would like to draw your attention to information about high-pressure differential scanning calorimetry that you can download from the Internet.

METTLER TOLEDO publishes articles on thermal analysis and applications from different fields twice a year in Thermal Analysis UserCom, the well-known METTLER TOLEDO technical customer magazine.

The UserCom articles given on the slide are specifically related to HPDSC, HPDSC microscopy, and HPDSC chemiluminescence.

Back issues of UserCom can be downloaded as PDFs from the Internet address at the bottom of the slide.

 

Slide 20: For More Information

 

You can also download information about webinars, application handbooks or topics of a more general nature from the list of Internet addresses given on this slide.

 

Slide 22: Thank You

This concludes my presentation on high-pressure differential scanning calorimetry.

Thank you for your interest and attention.

 

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