Thermal Analysis UserCom 45

The fluid bath DMA 1 option allows the influence of swelling on the dynamic mechanical properties of a sample to be measured in the temperature range 0 to 200 °C. This means that deformation conditions of components that are in direct contact with fluids can be simulated (for example drive or timing belts that permanently run in motor oil).

Introduction

In recent years, the application areas of technical rubber materials have been greatly expanded through the optimization of important properties such as high temperature stability and resistance to media. An excellent example is that of drive belts in motors. A few years ago, drive belts could only be used at temperatures lower than 100 °C and where there was no contact with motor oil. Metal chains were therefore employed for critical parts instead of belts.

Nowadays, the lifetime of belts in permanent contact with hot oil is appreciably longer than that of the motor. In the development of such high-temperature and medium-resistant components, the characterization of the dynamic mechanical properties of materials in direct contact with media such as oils and fuels at high temperatures is of great importance.

This article describes a method that can be used to measure dynamic mechanical data of test samples in contact with a fluid in the temperature range –20 °C to 200 °C.

This includes information about swelling as well as the change in dynamic mechanical properties during swelling.

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Crystalline pharmaceutical substances often decompose immediately before or during melting. To determine the glass transition temperature, the substance must be melted and then cooled as rapidly as possible so that decomposition and crystallization do not occur. In many cases, the heating and cooling rates of conventional DSCs are not high enough for this purpose. The METTLER TOLEDO Flash DSC however offers new possibilities. This is illustrated in this article using prednisolone as an example.

Introduction

Amorphous forms of active pharmaceutical ingredients (APIs) are often preferred in order to obtain the highest level of bioavailablity. Amorphous APIs are only stable in the glassy state below the glass transition temperature (Tg). Above the Tg, they can crystallize. This can have a large effect on bioavailablity.

APIs are often crystallized from solution in order to obtain the purest form. Conversion to the amorphous state can be achieved by grinding the crystals, whereby the temperature of the material being ground must not rise above the Tg during grinding [1, 2]. Knowledge of the glass transition temperature of amorphous APIs is therefore important both from the point of view of storage and from process engineering..

To determine the glass transition temperature of a (crystalline) starting material, the material must be melted and then cooled as rapidly as possible so that no decomposition or crystallization occurs. In many cases, the heating and cooling rates of conventional DSCs are inadequate for this purpose.

The Flash DSC offers new possibilities with heating and cooling rates of up to 40,000 K/s (heating) and 4,000 K/s (cooling). This allows a substance to be heated up to several hundred degrees and then cooled within a few milliseconds [3, 4]. In this short time interval, practically no decomposition can occur. This makes it possible to determine the glass transition temperatures of crystalline substances that would have decomposed in a conventional DSC during the comparatively slow melting process [5]..

In this article, we illustrate the procedure using prednisolone as an example.

Prednisolone is a synthetic corticoid used to treat inflammation. Prednisolone is available as the anhydride in two stable polymorphic forms and as a sesquihydrate [6]. The expected solid-solid transition between the two anhydrides should occur between 120 and 130 °C. This has however previously not been observed [6].

The form stable at room temperature (Form I) melts between 236.5 and 239 °C [6]. The high-temperature form (Form II) melts between 224 and 228 °C [6]. The melt is thermally unstable and decomposes [7]. Form I is however stable at room temperature and is commercially available.

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References

[1] Wildfong, P. L. D. Effects of Pharmaceutical Processing on the Solid Form of Drug and Excipient Materials. In Polymorphism in pharmaceutical solids; Brittain, H. G., Ed.; Informa Healthcare: New York, 2009, pp 510-559.
[2] Descamps, M.; Willart, J. F.; Dudognon, E.; Caron, V. Transformation of pharmaceutical compounds upon milling and comilling: the role of Tg. J. Pharm. Sci. 2007, 96, 1398-1407.
[3] Mathot, V.; Pyda, M.; Pijpers, T.; Vanden Poel, G.; van de Kerkhof, E.; van Herwaarden, S.; van Herwaarden, F.; Leenaers, A. The flash DSC 1, a power compensation twin-type, chip-based fast scanning calorimeter (FSC): First findings on polymers. Thermochim. Acta. 2011, 522, 36-45.
[4] Poel, G.; Istrate, D.; Magon A.; Mathot, V. Performance and calibration of the flash DSC 1, a new, MEMS-based fast scanning calorimeter. J. Therm. Anal. Calorim. 2012, 110, 1533-1546.
[5] Corvis, Y.; Wurm, A.; Schick, C.; M.; Espeau, P. Vitreous state characterization of pharmaceutical compounds degrading upon melting by using fast scanning calorimetry. J. Phys. Chem. B 2015, 119, 6848-6851.
[6] Suitchmezian, V.; Jess, I.; Sehnert, J.; Seyfarth, L.; Senker, J.; Näther, C. Structural, Thermodynamic, and Kinetic Aspects of the Polymorphism and Pseudopolymorphism of Prednisolone (11,17,21-Trihydroxy- 1,4-pregnadien-3,20-dion). Cryst. Growth Des. 2008, 8, 98-107.
[7] Veiga, M. D.; Cadorniga, R. Thermal study of prednisolone polymorphs, Thermochim. Acta. 2005, 96, 111-115.

The interpretation and quantitative evaluation of thermal analysis measurement curves is difficult when several effects take place simultaneously. A number of methods are available that can be used to separate overlapping effects and analyze them individually afterward. Using suitable examples, we discuss strategies for DSC curves. A second article to be published in the next UserCom will cover TGA applications.

Introduction

The interpretation and quantitative evaluation of thermal analysis measurement curves is difficult when several effects occur simultaneously. For DSC measurements, there are four main strategies that can be applied to separate overlapping effects:

a) Variation of the temperature program. This includes the use of different heating and cooling rates as well as heating-cooling-heating cycles.

b) Changing the environmental conditions. This includes using different gases, different crucibles (e.g. highpressure crucibles, platinum crucibles) as well as different methods to seal the crucibles (e.g. hermetically sealed crucibles, lids with 50-μm holes, open crucibles).

c) Modulated techniques. If reversing (e.g. a glass transition) and nonreversing effects (e.g. vaporization, crystallization) overlap, they can be separated using temperature-modulated DSC measurements (IsoStep, ADSC, TOPEM^®).

d) Combined techniques. If the DSC measurement alone is inconclusive, techniques such as DSC-microscopy or DSC-chemiluminescence can be helpful.

In the following sections, we will discuss these strategies and illustrate them with examples.

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The shelf life of a packaged product, for example in the food sector, is often strongly influenced by the properties of the product packaging. An important factor here is the permeability of the product packaging toward water vapor. The ProUmid SPS and Vsorp sorption test systems in combination with special sample holders allow the transmission rate of water vapor through the packaging and the sorption rate of the packaged products to be determined experimentally.

Introduction

Dynamic water vapor absorption using automated sorption test instruments has proven to be the ideal method for investigating the uptake or loss of water of materials, for example in the form of powders, granules, flakes, extrudates or tablets.

Dynamic water vapor sorption analysis is often used to perform stability tests with new products at defined temperatures and relative humidity. Such long-term tests often take several weeks or months and provide valuable information about the influence of temperature and relative humidity on the shelf life of the products. For packaged products, the question also arises as to how long a product can be stored under certain climatic conditions before a critical moisture content is reached that significantly shortens the shelf life of the product or leads to the loss of product-specific properties.

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