This webinar explains how these combination techniques work, including DSC-microscopy, and illustrate each of them with a number of exciting applications.
The instrument systems can be quickly configured by attaching suitable optical accessories to the METTLER TOLEDO DSC.
The DSC-microscopy system
DSC-microscopy consists of a DSC with a microscope positioned directly above the sample crucible. A CCD camera mounted on the microscope allows us to capture images of the sample while it is heated or cooled in the DSC. The images help us understand what is happening to the sample and enable us to correctly identify and interpret effects observed on the DSC curve.
The DSC-chemiluminescence system
DSC-Chemiluminescence is the combination of a DSC with an optical accessory for recording chemiluminescence. The highly sensitive CCD camera records the chemiluminescence emitted by a sample as it is measured in the DSC. Chemiluminescence is mainly used to investigate the oxidation behavior of polymers and other materials.
The DSC-photocalorimetry system
DSC-photocalorimetry allows us to expose a sample to light of a particular wavelength range and intensity for a defined time and record the heat flow from the sample. This technique is mainly used to study light-induced curing reactions.
DSC-Microscopy, Chemiluminescence, and Photocalorimetry
Slide 0: DSC-Microscopy, Chemiluminescence, and Photocalorimetry
Ladies and Gentlemen
Welcome to this seminar on DSC-Microscopy, DSC Chemiluminescence, and DSC Photocalorimetry.
In the course of the webinar, I would like to explain how these combination techniques work, and illustrate each of them with a number of exciting new applications.
The instrument systems can be quickly configured by attaching suitable optical accessories to the METTLER TOLEDO DSC.
Slide 1: Overview of DSC Optical Techniques
The slide presents an overview of the three techniques.
The DSC-Microscopy System consists of a DSC with a microscope positioned directly above the sample crucible. A CCD camera mounted on the microscope allows us to capture images of the sample while it is heated or cooled in the DSC. The images help us understand what is happening to the sample and enable us to correctly identify and interpret effects observed on the DSC curve.
The DSC-Chemiluminescence System is the combination of a DSC with an optical accessory for recording chemiluminescence. The highly sensitive CCD camera records the chemiluminescence emitted by a sample as it is measured in the DSC. Chemiluminescence is mainly used to investigate the oxidation behavior of polymers and other materials.
Finally, the METTLER TOLEDO DSC-Photocalorimetry System allows us to expose a sample to light of a particular wavelength range and intensity for a defined time and record the heat flow from the sample. This technique is mainly used to study light-induced curing reactions.
Slide 2: An Example of the Use of DSC-Microscopy
Let me begin with DSC-microscopy.
In practice, DSC curves often exhibit effects that cannot immediately be interpreted. If we want to understand the behavior of the sample, we need additional information. One possibility is to simultaneously observe the sample under a microscope.
The diagram in the slide shows three DSC cooling curves of identical polymer samples that had been cured beforehand at 120 degrees Celsius. The cooling curves recorded from plus 90 to minus 60 degrees show the glass transition at about 52 degrees. Immediately after, many small spikes appear that cannot be readily explained.
But what causes the spikes? Visual inspection of the sample during the experiment shows that several cracks occur below the glass transition temperature. In fact, on cooling, the polymer solidifies and forms a glass. Considerable stress builds up between the glassy sample and the aluminum crucible due to their different expansion coefficients. Stress release leads to the formation of cracks and in turn to movement of the crucible. This is the origin of the artifacts on the DSC curve. Crack formation and the movement of the sample crucible can easily be followed by DSC-microscopy. This simple example illustrates the power of DSC-microscopy: it lets us see what we cannot see on the DSC curve.
By the way, this particular problem can be overcome by using just a small amount of sample, for example 1 mg, or by wrapping the sample in a piece of aluminum foil. In the latter case, surface stresses between the aluminum foil and the sample will still occur, but they are no longer transferred to the crucibles.
Slide 3: Advantages of DSC Microscopy
This next slide summarizes some of the most important advantages of DSC-microscopy.
Besides identifying artifacts as illustrated in the previous slide, we can also see color changes that, for example, might indicate a solid-solid transition or a decomposition reaction. We can easily identify melting and distinguish between overlapping effects such as melting and decomposition. Furthermore, DSC-microscopy allows us to analyze two samples simultaneously under exactly the same conditions by placing two samples in the same crucible. In this case, the DSC measures the sum of the effects in both samples. The visual information, however, immediately shows any differences between the thermal stability of two samples. Sometimes, we can see effects that produce only very weak DSC signals such as outgassing or crystallization from solutions.
Altogether, DSC-microscopy helps us to identify and interpret effects on DSC curves.
Slide 4: The DSC-Microscopy System
What then are the main elements of a DSC-microscopy system?
The slide shows a schematic view of the METTLER TOLEDO DSC-Microscopy system.
The standard furnace lid is replaced by a lid with windows in order to gain optical access to the sample. The microscope with the CCD camera is mounted above the sample side on an adjustable slide fixed to a stand. The sample is measured in reflectance; the light needed to illuminate the sample is guided into the DSC cell by means of a glass-fiber bundle connected to a suitable light source. Images are captured at user-specified temperature or time increments. The recording and analysis of images is performed using a dedicated software control and image processing package.
All our current DSC instruments as well as most of our previous standard or high-pressure DSCs can be equipped with the DSC-microscopy accessory. It can be installed or removed in a few minutes. The camera is connected to the computer via a standard USB connector and requires no special adapters.
Slide 5: Applications of DSC-Microscopy
The following slide presents an overview of possible applications of DSC-microscopy.
We see that DSC-microscopy is a technique that can be used to advantage in almost any industry and for many different kinds of application problems. Experiments can be performed using both isothermal and dynamic temperature programs
I will now illustrate this with three application examples.
Slide 6: Application 1: DSC curves of a powder coating
The first application displays DSC heating curves of an epoxy powder used for making powder coatings.
The red curve shows the glass transition with enthalpy relaxation at about 60 degrees Celsius. The broad exothermic peak at 170 degrees corresponds to the curing reaction. In addition, the curve exhibits a ripple-like effect between about 100 and 110 degrees. If we zoom this region, we see that this is in fact a small endothermic peak. The corresponding blue curve was measured using a second sample specimen. It proves that the effect is repeatable and that it must have to do with the behavior of the sample.
But what is the origin of the effect?
Slide 7: Application 1: Microscope images of a powder coating
In this slide, we see that DSC-microscopy gives us the answer.
After the glass transition, the epoxy powder is in a rubbery elastic state. At 85 degrees Celsius, the individual grains of powder are still clearly visible. From about 95 degrees onward, the grains begin to flow and slowly agglomerate. At about 105 degrees, the viscosity of the epoxy powder is so low that the particle coalesce and “melt”. This is completed at about 115 degrees and the sample behaves like a liquid. During this process, the thermal contact between the sample and the crucible changes and gives rise to the small peak at about 110 degrees.
Slide 8: Application 2 DSC melting curve of a Dynema® fiber
The second application has to do with the melting behavior of fibers. Fibers often consist of highly stretched polymers that exhibit a high degree of crystallinity.
The DSC curve in the slide shows the melting behavior of a DyneemaÒ fiber. This is a very strong polyethylene fiber with a tensile strength of about 3.5 giga-pascals. The fibers are used to make bullet-proof protective clothing. The DSC curve shows a large melting peak at about 144 degrees Celsius and a second peak at about 147 degrees.
But what causes this second peak?
Slide 9: Application 2 Microscope images of Dynema® fibers
The four images captured between 140.2 and 146.2 °C show how DyneemaÒ fibers behave during melting. To ensure good thermal contact between the sample and the bottom of the crucible, several pieces of fiber with a total weight of about 74 micrograms were placed in the crucible and covered with a glass disk. On heating, the fibers start to shrink from about 140 °C onward. Comparison with the DSC curve shows that melting also begins at this temperature. We see that the individual fibers now form a loose bundle. Movement of the fiber bundle causes the glass disk on top of it to shift inside the crucible and gives rise to the second small peak in the DSC curve. The peak therefore has nothing to do with the actual melting process but is an artifact caused by the movement of the melted bundle of fibers and the glass disk.
Slide 10: Application 3 Dehydration of CoCl2 hexahydrate
The third application concerns color changes observed during the dehydration of cobalt chloride hexahydrate. Anhydrous cobalt chloride is added to silica gel as a moisture indicator for the desiccant. In the dry state, it is blue. When it absorbs water, cobalt chloride hexahydrate is formed and its color changes to ruby-red. The color of the silica gel therefore indicates whether it can still be used as a drying agent. Moist silica gel can be regenerated by heating. This eliminates the water of crystallization and the silica gel turns blue again. The process can be investigated by DSC-microscopy.
Here, the main question is to investigate the change in color of the cobalt chloride on heating and to determine whether other hydrates exist.
Slide 11: Application 3 Microscope images of CoCl2 hexahydrate
The slide shows images of a sample of cobalt chloride hexahydrate that was heated from 30 degrees Celsius in an open crucible at 1.5 K/min. The images were captured at different temperatures and illustrate the dehydration process.
The color of the sample was initially ruby-red, which is characteristic for the hexahydrate. The color becomes lighter and lighter until it suddenly changes at about 55 degrees. Further color changes are observed at higher temperatures - for example, from violet to dark blue between 100 and 120 degrees, and to light blue at 160 degrees. These differences can be quantified by calculating an average brightness for all the images in the image range studied. The brightness can then be displayed as a function of temperature.
Slide 12: Application 3 DSC and brightness curves of CoCl2 hexahydrate
In this next slide, the brightness curve is shown in red and the DSC curve in blue. The numbers one to six marked alongside the brightness curve correspond to the six images shown in the previous slide.
Up to about 80 degrees Celsius, the DSC curve shows a broad endothermic peak with a sharp peak superimposed on it at about 55 degrees. Two further endothermic peaks are observed at about 104 and 130 degrees. The brightness of the images increases slowly and then suddenly decreases in intensity at about 55 degrees. With increasing temperature, it increases again in several steps.
The two completely different curves obviously tell the same story. We see that the sample loses water right from the beginning and that the loss rate increases with increasing temperature. At about 55 degrees, the crystal structure of the material changes. This leads to the sudden color change and the sharp endothermic peak on the DSC curve between 53 and about 60 degrees. If we examine the images of the sample around 55 degrees more closely, we see tiny droplets of water on the surface of the crystals. This means that at least part of the remaining water of crystallization was driven out of the crystal lattice to the surface of the crystals during the solid-solid transition. The color of the crystals, and by this their brightness, changes continuously as the crystals gradually dry. This process is completed by about 80 degrees. The two step-like changes in the brightness and the two broad endothermic peaks on the DSC curve are also due to the loss of water of crystallization.
Slide 13: Application 3 TGA curves of CoCl2 hexahydrate
We then used thermogravimetric analysis, TGA; to verify our interpretation of the DSC curve.
The slide shows the weight of a sample of cobalt chloride hexahydrate as a function of temperature. The red curve is the weight loss curve, the blue curve is its first derivative (the so called DTG curve), and the green curve is the DSC signal, which is also measured by the TGA instrument. We see that the sample loses water from room temperature onward. Two further weight loss steps occur that start at about 100, and 120 degrees Celsius. Quantitative evaluation of the steps shows that cobalt chloride hexahydrate loses its six molecules of water in steps of four, one and one.
If we look at the first derivative of the weight loss curve, we see that there is a momentary increase in the rate of loss of weight at the temperature at which the solid-solid transition is observed on the DSC curve. As we proposed in the previous slide, this can be explained by assuming that there is partial elimination of water of crystallization to the surface of the crystals during the solid-solid transition. Water on the surface of the crystals evaporates more quickly than water that first has to reach the surface through diffusion. This produces the short-term increase in the rate of loss of weight.
Slide 14: Summary / DSC-Microscopy
Let’s now briefly consider what conclusions we can draw from this information.
First, the availability of DSC-microscopy allows us to identify, clarify, or verify effects on DSC curves. This applies to both real effects and artifacts. DSC-microscopy is therefore a very powerful tool for interpreting DSC curves and for understanding the thermal behavior of samples.
Another interesting possibility is that it enables us to measure more than one sample in a single experiment. This allows us to compare the thermal behavior of two or more samples under exactly the same conditions.
DSC-microscopy can be used in the temperature range from –90 to 450 degrees Celsius with all the current and most older METTLER TOLEDO DSC instruments. It can be installed and removed by the user within a couple of minutes.
Slide 15: What is Chemiluminescence?
This next slide brings me to the second technique that I would like to present, namely DSC chemiluminescence.
Luminescence is the general name used to describe different phenomena in which molecules become excited and then return to the ground state with the emission of light. In chemiluminescence, excitation is due to a chemical reaction. Chemiluminescence is even observed in nature, for example from fireflies, glowworms or rotting wood.
Usually however, chemiluminescence emission is very weak and can only be detected by sensitive detectors such as CCD cameras or photomultipliers. CCD cameras are advantageous because they yield information about the distribution of the chemiluminescence emission across the sample.
Nowadays, chemiluminescence is mainly used for investigating the oxidative stability of polymers. The advantage of chemiluminescence compared with DSC OIT measurements is its specificity - the chemiluminescence is produced only by the oxidation reaction, so that unlike DSC, it is not influenced by thermal effects such as vaporization or evaporation. Furthermore, oxidative stability can be measured by chemiluminescence at temperatures at which DSC is insensitive.
Slide 16: Chemiluminescence Possible reaction mechanisms
Most polymers decompose under natural conditions as a result of oxidation by oxygen in the air. The slide shows possible mechanisms for the reaction process.
In a first step, unstable alkyl radicals are formed in the polymer under the influence of heat, light or mechanical stress. These alkyl radicals react with oxygen to form peroxy radicals. Peroxy radicals can also be produced during the production of the plastic and be present as an undesired byproduct in the plastic. In the presence of oxygen, the peroxy radicals accelerate the decomposition of polymers in a chain reaction. The reaction step in which the chemiluminescence occurs is not fully understood. A mechanism often described in the scientific literature assumes that chemiluminescence occurs when two peroxy radicals recombine to form oxygen and an excited carbonyl radical (Russell mechanism).
Chemiluminescence measurements can therefore be used to study the oxidation of polymers and hence to investigate the effect of stabilizers in general.
Slide 17: The DSC-Chemiluminescence System
In this slide, we see the basic experimental setup of the METTLER TOLEDO DSC-Chemiluminescence System.
Obviously, to make chemiluminescence measurements, the sample must be in complete darkness. In this and other respects, the high-pressure DSC is ideal. The chemiluminescence accessory is therefore only available for the HP DSC. The standard high-pressure cover of the HP DSC is replaced by a special cover. This includes a window through which the sample is observed using an extremely sensitive CCD camera equipped with powerful optics. The special cover allows the sample to be subjected to a pressure of up to 20 bar. The camera is mounted directly on the high-pressure cover by means of a light-tight flange. The entire system can be assembled or disassembled within a few minutes. A dedicated software program is used to control the camera and evaluate the images.
Slide 18: Applications of Chemiluminescence
This slide presents an overview of possible DSC chemiluminescence applications. As a rule, chemiluminescence experiments are performed under isothermal conditions.
The table shows that the main applications of chemiluminescence have to do with the investigation of the stability of plastics, foodstuffs, paints and pharmaceuticals. If the oxidation 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, we refer to it as the oxidation onset temperature, or OOT.
In the following slides, I want to describe some typical application examples involving chemiluminescence measurements.
Slide 19: Application 1 PP with and without copper
The first application has to do with the oxidative degradation of polypropylene. It is generally well known that copper can initiate and accelerate the oxidative degradation of polymers.
The slide shows chemiluminescence measurements performed with two test specimens of a sample of polypropylene. These were simultaneously exposed to pure oxygen in the same crucible at 140 degrees Celsius. A small triangular piece of copper was placed on one of the two specimens.
The false-color images indicate that the sample material near the piece of copper starts to emit light after about 70 minutes. In contrast, this begins about 20 minutes later in the second specimen. This confirms that oxidation of the sample material is accelerated by the presence of copper. Furthermore, decomposition of the copper-free specimen does not begin everywhere at the same time but preferentially where the two specimens are in contact. It seems as if the intact test specimen is “infected” by the test specimen already undergoing degradation. The images also show that oxidation of the copper-free specimen not only begins where it is in contact the specimen with copper but also at other isolated locations. This could indicate that the distribution of the stabilizer is not homogeneous within the sample or that the sample is contaminated.
Slide 20: Application 1 PP with and without copper
The images captured by the CCD camera shown as examples in the previous slide allow us to calculate the course of the relative chemiluminescence intensity with respect to time.
The slide shows chemiluminescence intensity curves for the test specimens with and without copper at different temperatures. We see that at 120 degrees Celsius, the copper has practically no influence on the oxidative decomposition. However, with increasing temperature, the influence of the copper on the oxidation time becomes more and more apparent. For example, at 150 degrees, the OIT of the test specimen with copper is about 16 minutes, and for the test specimen without copper about 40 minutes.
Slide 21: Application 2 Chemiluminescence of an oil
This slide shows chemiluminescence measurements performed on a synthetic motor oil.
The sample was measured isothermally at 200 degrees Celsius in oxygen at a pressure of 10 bar.
The aim here is to obtain information about the thermal stability of oils and to investigate the influence of different stabilizers.
Here 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. In this case, the interesting point to note is that the chemiluminescence intensity steadily decreases before the oil suddenly oxidizes. This is the result of continuous oxidative degradation of the stabilizers in the oil. This cannot be detected on 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 22: Summary / DSC Chemiluminescence
What conclusions can we draw from all this? The slide summarizes the main features and benefits of DSC chemiluminescence measurements.
- Chemiluminescence is very selective to oxidative decomposition of the sample. Due to the outstanding sensitivity of the CCD camera, we can perform OIT measurements at lower temperatures than with DSC. Such measurements have greater practical relevance.
Furthermore, since the chemiluminescence is recorded with a CCD camera, we have images of the sample. This allows us to study the initiation, growth and propagation of oxidative degradation processes.
- In addition, several samples can be analyzed simultaneously under exactly the same conditions. The combination of chemiluminescence and high-pressure DSC makes it possible to work at pressures up to 20 bar. This allows us to investigate the degradation of oils and lubricants under realistic conditions.
Slide 23: Photocalorimetry
We now come to the third technique, namely DSC photocalorimetry.
Photocalorimetry allows us to measure the influence of visible or UV light on materials.
The applications mainly concern systems that cure under the influence of light such as adhesives, coatings involving paints and lacquers, or dental composite fillings. The technique also allows us to investigate the effect of UV stabilizers in plastics or the influence of light on foodstuffs.
Slide 24: Photocalorimetry Advantages and applications
As I have just mentioned, one of the most important applications of photocalorimetry is the investigation of the curing of paints, lacquers or adhesives using UV or visible light.
The advantages of light curing are
- the high rates at which light-curing reactions proceed,
- the fact that light curing usually takes place at ambient temperatures so the substrate materials are not subjected to thermal stress,
- and not least, the environmental compatibility of light curing – there is no solvent emission and it saves energy because energy-intensive drying processes are not necessary.
Slide 25: Photocalorimetry The photocuring process
What actually happens during light curing? This is explained schematically in this next slide using a coating as an example.
Initially, the product we want to use, for example as a coating, is in the liquid state and typically consists of a mixture of oligomers, a polymeric crosslinking agent, and photoactive additives. The mixture is applied to the substrate and exposed to light of the desired type and intensity. Very often UV light is used. Under these conditions, the photo-initiators react with the oligomers and the crosslinking agent to form a solid coating. This process is referred to as curing, and in this specific case, photocuring, because the reaction occurs under the action of light.
Slide 26: The DSC-Photocalorimetry System
The slide shows a schematic view of the METTLER TOLEDO DSC-Photocalorimeter system.
Both the sample and the reference side are exposed to the same light intensity. This is done using a branched fiber-optic light guide. One end is connected to a suitable light source. The two exits are fixed in a holder and positioned directly above the sample and reference crucibles of the DSC. The holder can be mounted and demounted in about 2 minutes. This means we can use the DSC both for photocalorimetric and for normal DSC measurements. The holder fits all METTLER TOLEDO DSC instruments except the high-pressure DSC.
Slide 27: Applications of DSC Photocalorimetry
The slide summarizes the potential applications of DSC photocalorimetry.
Photocalorimetric experiments are normally performed under isothermal conditions at temperatures close to room temperature. The main applications include the curing of paints, lacquers and adhesives in different fields such as polymers, cosmetics, electronics, and dentistry. Besides this, the photocalorimeter-system can also be used to investigate the light stability of foodstuffs, paints, polymers or cosmetic products. The effect of different UV stabilizers on polymers can also be studied.
I will now illustrate this with two application examples.
Slide 28: Application 1 Curing of a cycloaliphatic epoxy resin
The slide describes the curing of a cycloaliphatic epoxy resin using UV light. Epoxy systems like this are often used for coating metals or as an overprint varnish.
In the upper right diagram, we see the DSC curves for the first and second light exposures. The blue curve in the larger diagram corresponds to the difference between the two curves. It records the actual curing process of the system.
The figure shows that the curing takes only a few seconds. The heat flow during curing is very high and can result in the sample temperature momentarily exceeding the program temperature.
Slide 29: Application 2 Influence of UV light on an olive oil
This application example illustrates the use of photocalorimetry to investigate the stability of materials under the influence of light, in this case a foodstuff.
The diagram shows cooling curves of an olive oil that had previously been exposed to UV light with an intensity of 100 milli-watts per square centimeter for different times at room temperature. The light degrades the olive oil by breaking the chains into shorter segments. As a result of this, the crystallization temperature of the olive oil shifts to lower temperatures with increasing exposure time.
In experiments like this, the photocalorimeter system is used to condition the sample.
Slide 30: Summary / DSC Photocalorimetry
The slide summarizes the advantages of using DSC photocalorimetry.
The two application examples show that photocalorimetry is an extremely versatile technique. It can be used to study the effect of light intensity, time and temperature on reactions and materials. As we have seen, these kind of investigations can be carried out quickly in the laboratory and result in a considerable saving of time and money compared with trial and error experiments in production or long-term measurements in the field.
Slide 31: For More Information
Finally, I would like to draw your attention to information that you can download from METTLER TOLEDO Internet pages. An entire set of specific UserCom articles related to DSC microscopy, chemiluminescence and photocalorimetry is available. We publish articles on thermal analysis and applications from different fields twice a year in our well-known UserCom customer magazine.
Slide 32: Thank you
This concludes my presentation on DSC optical accessories. Thank you very much for your interest and attention.