In this tutorial, you will learn about the principles of DSC and its sensor technology, measurement possibilities, plus DSC industries and applications. These include isothermal curing, temperature modulated DSC, oxidation induction time (OIT), crystallinity, glass transition temperature, oxidation onset temperature (OOT), degree of cure and kinetics, safety studies and polymorphism.
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Differential Scanning Calorimetry
Ladies and Gentlemen
Welcome to this seminar on differential scanning calorimetry – or DSC as it is usually called.
DSC is the most important technique in thermal analysis. It can be used to study material properties which are temperature like melting, crystallization, decomposition, etcetera.
In this seminar, I would like to explain the basic principles of differential scanning calorimetry and at the same time introduce a high-performance DSC instrument. I also want to point out a number of important design features and explain their functionality. Finally, I will present several examples to illustrate the different application possibilities of the DSCtechnique.
Principle of DSC
Differential scanning calorimetry measures the energy flow of a sample that is subjected to a temperature ramp. During the heating or cooling the sample undergoes one of more phase changes which can be quantified with a DSC instrument. In the picture on the left of the slide the icy surface of Lake Sihl in Switzerland is shown. This lake often freezes over in winter and the water is present in the solid phase. In spring when the temperature rises, the water melts and goes from the solid phase into the liquid phase, exchanging energy with the environment whilst it does. In a DSC instrument the melting of water can be observed and this shows up as a peak with a normalized surface area equal to the melting enthalpy of water.
The diagram on the right shows an example of a real DSC measurement; here the sample is polyethylene terephthalate (PET). The curve shows the heating of PET with the consecutive measurement signals of glass transition, cold crystallization, melting and decomposition.
Principle of DSC
During a DSC experiment the heat flow from instrument to sample is measured relative to a reference which has an identical setup apart from the sample to be analyzed. In the upper drawing on the slide a schematic representation of a heat flux DSC is shown. In this setup the sample and reference are heated from below and the flow of the heat is represented as the red arrows in the drawing. The sample is placed in a sample container, also called crucible, which sits on top of the sensor. It is crucial that the contact between the sensor, crucible and sample is as good as possible as this ensures an optimal heat flow and thus sensitivity.
On the lower left corner of the slide a close-up of a Mettler Toledo sensor is shown with its typical star-shaped arrangement of thermocouples. The star shape is created by the numerous thermocouples in the sensor and these guarantee that the heat flow can be accurately measured.
An example of a resulting curve is given in the lower right corner of the slide where a typical melting peak is shown. From the measured curve information can be extracted about for example the melting enthalpy, the melting point, and the specific heat capacity.
On this slide a schematic drawing is shown of the furnace in a DSC 1 instrument. The heart of the furnace is the DSC sensor, which is depicted in red, just below the furnace lid. Inside the silver furnace a temperature sensor regulates the temperature via the heater just below the silver furnace, depicted in dark blue.
For temperatures lower than room temperature, a cooling option is required. For cooling with an intracooler a cooling flange will be placed around the oven for direct and effective cooling. When a cryostat is used, a cooling flange is provided, which holds the cooling finger of the cryostat.
Depending on the temperature range used during the measurement, one or two gas inlets shown should be used. The standard inlet is the ‘purge gas inlet’; if the instrument is used with a cooling option, the ‘dry gas inlet’ is used additionally to avoid condensation inside the instrument.
DSC 1 – Sensors
The sensors in the DSC 1 are the most important part of the instrument as they determine the quality of the measurement. For the DSC 1 there are two different sensors available. The most commonly used is the FRS5, the full range sensor. For samples with low signal intensity the HSS8 sensor is advised; HSS stands for ‘high sensitivity sensor’.
On the slide, examples of tests for sensitivity and resolution of both sensors are shown. With better sensitivity it is possible to detect smaller effects in the sample; alternatively, lesser amounts of sample have to be used.
The measurements where performed according to the so-called TAWN test guidelines, a universally recognized standard to test the performance of DSC instruments. In the lower left corner the sensitivity measurement is shown on 4,4′-azoxyanisole. From the figure it can be seen that both sensors have excellent sensitivity, even though the HSS8 high sensitivity sensor has a slightly better signal-to-noise ratio than the regular FRS5 sensor, as is to be expected. In the lower right corner the TAWN test for resolution is shown for both sensors. Again, it can be seen that the resolution is very good for both sensors, but for this time the FRS5 full range sensor has better performance.
Another crucial property for any DSC experiment is the baseline, which essentially is the measurement of an empty DSC instrument. The measured baseline should be free of artefacts or drift, as any of these would overlay any real measurements performed and so obfuscate the true sample effects. As can be seen from the graph in the middle of the slide, both DSC 1 sensors show very good baseline performance, making sure that the measurements really show sample effects and not artefacts.
DSC 1 – Crucibles
To achieve the best heat flow from sensor to sample, the crucible in which the sample is contained needs to have optimal conducting properties and also the best possible contact with the sensor. Geometry and material of the crucibles are therefore very important irrespective of the sort of sample to be measured. Different crucibles are offered to suit different types of samples though; some of these are shown on this slide.
The standard DSC crucibles are shown in the left row on the top. These crucibles are very light and made from pure aluminium for good thermal conductivity; they also have a very level base for optimal contact with the DSC sensor. Depending on the requirements of sample and measurement, other crucibles may be needed. If the sample easily evaporates, the same crucible can be used, but now with the lid shown on the right, which has a pre-pierced hole in it with a 50 µm diameter. This hole makes sure that a self-generated atmosphere is created in the crucible and evaporation is reduced. For samples that react with aluminium, also gold plated crucibles are available, or when a reaction is required, for example oxidation, crucibles made from copper can be used. Another common application is measuring in a closed atmosphere and at higher pressure than ambient. These measurements can be done with the high pressure crucibles shown.
On the slide only a small selection of the available crucibles is presented, in total more than 25 different crucibles are available to match the requirements set by the sample.
DSC 1 – Options
DSC instruments themselves can be optimized for specific samples too, this slide shows the 4 optional DSC modes that are offered by Mettler-Toledo. These are, from left to right on the slide, DSC-Microscopy, DSC-Photocalorimetry, High-Pressure DSC, and DSC-Chemiluminescence.
DSC-Microscopy can be used if DSC curves exhibit effects that cannot immediately be understood. In this case, it can be very helpful to actually see what is going on in the crucible. The extra visual information can enable us to identify solid-solid transitions, differentiate between overlapping effects such as melting and decomposition, observe the shrinkage of fibres or films, or simply identify the cause of an artefact in the DSC curve.
DSC-Photocalorimetry allows enthalpy changes in a material to be measured during and after exposure to light. In this manner effects of light on the behaviour of light-sensitive materials used in different industries can be investigated. Also light-activated curing processes, the influence of UV stabilizers and the effect of light intensity on polymer stability can be studied.
High-pressure DSC is used to study pressure influences on physical and chemical changes. For material testing, process development or quality control there is often no alternative to high-pressure DSC measurements. Measurements under pressure have several advantages, some of which are:
- Shorter analysis times as higher pressure accelerates reactions
- Measurements under real process conditions are possible
- Separation of overlapping effects by suppressing evaporation
- Measurements under special atmospheres to promote or avoid oxidation or measurements with toxic or combustible gases are possible
DSC-Chemiluminescence is a technique where the light emitted by a chemical reaction is observed. For example, chemiluminescence originating from oxidative degradation processes in polymers can be observed, and thus test effectivity of stabilizers as polymer additives. In the food and pharmaceutical industries, chemiluminescence is mostly used to gain information about the stability of various products such as oils or fats.
DSC measurements can be performed as a function of temperature, isothermally or with modulated temperature programmes. For some applications a specific atmosphere as for example pure oxygen or pure nitrogen need to be used.
Temperature scans are mainly used to investigate temperature dependant processes such as the glass transition, crystallization, melting and curing reactions. Isothermal temperature experiments are essentially used for oxygen induction time determinations. Temperature modulated experiments can separate reversible and non-reversible effects such as glass transition and melting, if these are otherwise overlapping. Special atmospheres often are used to detect or avoid decomposition of samples.
In the following 4 slides an example for every category is given.
Temperature ramps are the most commonly used type of DSC measurements. In these experiments, the temperature is incremented with a certain rate to observe the different phases of the sample at certain temperatures. On the slide a typical temperature increment measurement of polyethylene terephthalate (PET) is shown.
In the red curve, which is the first heating of the sample, the standard effects upon heating can be observed. The first event shows the glass transition, next cold crystallization and finally melting. If the temperature would be increased even further, also the decomposition of the PET sample would be visible. The temperatures at which these effects take place are characteristic for each material, therefore the DSC curves can be used for fingerprinting in quality control. In the first heating curve of the original sample information about the processing history of the sample is obtained.
In general, it is very useful to additionally measure the cooling of the sample and also to measure a second heating run. In these subsequent runs information about the material itself can be gained.
This can be seen from the curve of the second heating run, which is blue. In this curve the glass transition shows no enthalpy relaxation, as could be seen in the first heating run, and that indicates storage of the sample. No crystallization peak can be observed either, which means that the cooling down of the sample in the cooling run was slow enough to allow crystallisation during cooling. This can also be deducted from the black curve that shows the cooling run of the sample, and where a nice crystallization peak can be seen. In contrast, the original sample was almost fully amorphous since the cooling during manufacturing was too fast for crystallization to occur.
This example shows that a great deal of information can be gathered from just one sample by applying a simple heating cooling heating cycle.
During an isothermal experiment changes in the sample are observed over a certain period of time during which the temperature is kept constant. This measurement mode is used if information about elapsed time or duration is required. In oxygen induction time the parameter of interest is the time elapsed until samples start to degrade. In curing reactions, as with the example on the slide, it is of interest to know the duration of the complete curing reaction.
In the example a curing reaction of a powder coating is shown. A powder coating is usually sprayed onto the substrate and then cured either thermally (typically at about 180 °C) or by means of UV light at lower temperatures. Curing with UV light has the advantage that temperature sensitive materials can be coated, and also that almost no volatile organic compounds are released. In practice, the main question is how long the material has to be exposed to UV light in order to achieve an adequate degree of cure or cross-linking. This is illustrated with several experiments that were performed to measure the degree of cure for different exposure times. The necessary exposure time can then be simply determined from the requirements set for the degree of cure.
Temperature modulated DSC experiments can be used to separate effects originating from latent and sensible heat flow phenomena. In the example on the slide a sample can be seen where vitrification stopped the curing reaction prematurely. During vitrification the glass transition of the material shifts to higher temperature, stopping the curing reaction by lack of mobility. A logical consequence is that glass transition and curing reaction occur at the same temperature and overlap. This can be seen in the green curve which is a normal DSC experiment.
In practice, incomplete curing as a result of vitrification is one of the most frequent causes of failure in composite materials. If incompletely cured material is heated in a DSC, a so-called post-curing peak is observed immediately after the glass transition. To be able to see the glass transition and to determine the post-curing enthalpy (and original degree of cure) a temperature modulated DSC experiment can be performed.
In this particular example TOPEM was used and the glass transition is clearly visible at about 210 degrees in the reversing heat flow curve, coloured red in the slide. The small peak in the non-reversing heat flow curve is therefore due to the post-curing reaction. This peak can be evaluated and used for quality control.
Not only temperature has an influence on sample behaviour; atmosphere also influences how the sample behaves. In this respect, especially oxidation is important. By bringing the sample to a certain temperature and switching from an inert nitrogen atmosphere to reactive oxygen atmosphere, the time can be measured until the sample starts to decompose. This time is called the ‘oxygen induction time’ and gives information about the stability of materials.
In the example on the slide, the oxygen induction time (OIT) of three polyethylene samples stabilised to different extents were measured at 210 °C. The differences in stability toward oxidation can be clearly seen.
Measurements can also be used to distinguish thermally, mechanically or chemically stressed materials from fresh material.
Why use DSC?
From the previous introduction it will have become clear that there is a lot of information to be gained from DSC measurements. The main uses are summarized on this slide.
General characterisation of samples by their material specific properties such as melting, crystallisation, decomposition and enthalpy is an important application. Also the glass transition can be used as characterisation feature, but often this transition is an important physical property in its own right. Thermal stability, amongst others defined by oxygen induction time or decomposition temperature, is an important quality control parameter. Other types of chemical reactions and their reaction kinetics are important for example for glues and thermosets. In the pharmaceutical industry it is important to be aware of different active and inactive polymorphs of the active ingredients and also to check active pharmaceutical ingredients for impurities.
Industries and Applications
DSC has numerous potential applications and can be used in practically all industries.
The overview in this slide illustrates that DSC measurements are used for various analysis, many of which determine the glass transition, curing reactions, melting and crystallization. Depending on the particular industry, other applications will be used as well.
I would now like to present several different application examples that demonstrate the analytical power and versatility of the DSC technique.
This slide shows an example of sample fingerprinting and identification. The different plastics shown in the figure can be identified by measuring their glass transition and melting temperatures. The different polymer peaks clearly differ in their size and position on the temperature axis. The example of polypropylene (designated PP) and polyoxymethylene (designated POM) shows that identification depends both on the melting temperature and on the enthalpy of fusion. If the type of polymer is known, the degree of crystallinity can be determined from the melting peak, as is shown for polypropylene.
Thermal stability of materials is a very important quality characteristic as many materials decompose at higher temperatures and become unstable. Often stabilization agents are added to the product to counteract this effect. To measure the thermal stability, with or without stabilizers, several alternatives are used. All of these utilize a purge gas that reacts with the sample, generally oxygen.
On this slide an example of oxidation onset temperature (OOT) in different olive oils is shown. For an OOT experiment the sample is placed in an open crucible with uninhibited contact to oxygen gas. The temperature is then increased until the exothermal decomposition of the sample is found at the oxidation onset temperature.
In the slide it can clearly be seen that different oils have different thermal stabilities.
Another important application of differential scanning calorimetry is to measure the glass transition and the curing reaction in epoxy resin systems. The figure displays the curing curves of samples previously cured to different extents. The results show that as the degree of cure increases the glass transition shifts to higher temperatures and the postcuring reaction enthalpy decreases. In the figure the amount of cure increases from top to bottom, starting with uncured sample in the black curve to fully cured in the green curve. If the reaction enthalpy of the uncured material is known the degree of conversion before analysis can be calculated from the enthalpy of the postcuring reaction, in the figure the amount of cure is shown on the right-hand side.
This slide summarizes the steps involved in a kinetics evaluation based on the so-called model free kinetics (MFK) procedure. The original measurement data is from a DSC temperature ramp analysis, measured with at least 3 different heating rates.
In Step 1, the reaction peaks are defined and baselines are drawn for the integration. In the second step, we calculate the conversion level curves by step-wise integration of the curves recorded at the three different heating rates. The next step, number 3, shown in the lower right corner, shows the conversion-dependent activation energy, which is calculated from the conversion curves using model free kinetics. The diagram shows that the activation energy changes during the course of the reaction. This indicates that more than one mechanism is involved in the reaction. We refer to the activation energy calculated as the “apparent activation energy”. In the fourth and final step we use the previously generated data to make predictions, for example to predict the conversion curve as a function of time at isothermal temperatures of e.g. 170 degrees. We can then for example read off how long it takes to reach a particular conversion value at a certain temperature, for example 90 percent conversion at 170 degrees. According to the curve, the time required is approximately 30 minutes.
To check the calculated conversion, curve an isothermal measurement at 170 degrees was also performed and overlaid over the calculated curve. Clearly, theory and experiment show a good match.
For chemical reactions it is important to know the reactivity, the reaction rate and the energy released. If these parameters are known one can assess the security measures that have to be taken when processing or storing the investigated substances.
Chemical substances that contain one or more nitro group are well known for their explosive potential. Some of these products, for example picric acid (trinitrophenol), are starting materials for chemical syntheses or are used for analytical purposes. Others find use as propellants or explosives, for example nitrocellulose or ammonium nitrate. The latter as fertilizer has already been the cause of several very serious explosions. The slide shows DSC measurements of some of these thermally hazardous substances. A comparison of the curves demonstrates the influence of the heating rate, sample size and atmosphere restrictions on the enthalpy production during the reaction.
Analysing the melting of a substance is an important method for the quality control of pharmaceutical products. From the melting behaviour information about polymorphism and purity can be obtained. On this slide an example is shown in the red curve where the metastable form of the substance first melts at a lower temperature than the stable form. The melt then crystallizes to the stable form that subsequently also melts. Knowledge of the particular crystalline form present is very important for assessing the physical stability and physiological stability of substances.
From the melting curve also the purity can be determined, as is shown in the blue curve. The example shows the melting curve of the stable form of phenobarbital which is evaluated with the algorithm for purity determination. This algorithm utilises the van ‘t Hoff equation to determine the amount of impurity present in a substance by evaluating the lowering of the melting point.
Summary: DSC 1
This slide summarizes the features and benefits of the DSC 1. Differential scanning calorimetry is an excellent technique for characterizing the thermal properties of materials such as thermoplastics, thermosets, elastomers, adhesives, chemicals, paints and lacquers, foodstuffs, pharmaceuticals, fats and oils and ceramics.
The METTLER TOLEDO DSC 1instrumentmeasures heat flow very reliably with optimal resolution and sensitivity, so that you are able to measure even the weakest effects. For high-throughput applications a sample robot allows easy automation of whole sample series, where even different types of crucibles can be measured in one series with the robot. For different kinds of samples, many different crucibles are available.
Because of the modular concept, accessories like sample changers or cooling devices can still be added after the initial instrument acquisition, should the requirement for these only arises after purchase of the instrument itself.
Flexible calibration procedures allow calibration and adjustment over the complete temperature range from -150 to 700 degrees.
For More Information on DSC
Finally, I would like to draw your attention to further information about 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 UserCom, the well-known METTLER TOLEDO biannual technical customer magazine. Back issues can be downloaded as PDF files from www.mt.com/ta-usercoms as shown at the bottom of the slide.