Thermal Analysis UserCom 17

Introduction

This article describes the modification of a TGA instrument to measure dynamic water vapor sorption processes (Dynamic Vapor Sorption, DVS). Instrumentation of this type allows the change in mass of a sample in powder form to be measured as a function of relative humidity and time [1]. This is of particular interest because many water-soluble and powdered foodstuffs are moisture sensitive. If such foodstuffs are stored under conditions where the humidity is too high, they can become lumpy, undergo phase changes and recrystallization, lose aroma or go bad due to the growth of molds.

The DVS method described here is one of many methods that can be used to characterize the behavior of powder systems with regard to humidity [1]. Compared with conventional methods, the DVS technique has the advantage that it is rapid and less labor-intensive [1]. With a DVS instrument, it is possible to measure a complete sorption isotherm quasi fully automatically with a single sample, and at the same time follow the sorption process dynamically, i.e. as a function of time. A commercially available DVS instrument, however, costs several times more than a standard TGA instrument. We therefore decided, together with METTLER TOLEDO, to modify a TGA instrument to a DVS instrument so that it could be used for either DVS or TGA measurements as desired. An additional advantage is that important thermodynamic quantities such as the enthalpy of sorption can be determined from a combined DVS/ TGA measurement. This is not directly possible with conventional DVS instruments.

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Literature

[1] Gál, S., Recent Advances in Techniques for the Determination of Sorption Isotherms, in Water Relations in Food, R.B. Duckworth, ed., Academic Press, London (1975).

Introduction

Kinetic studies have become an important application in thermal analysis. Kinetic data from chemical reactions can be determined and used to make predictions about the reaction behavior of chemical substances outside their practical measurement range. This article describes how the out-life of a carbon-fiber-reinforced prepreg can be determined using the Advanced Model Free Kinetics (advanced MFK) software option. Prepreg is the name given to uncured composite thermosetting materials consisting of fibers embedded in a matrix of uncured resin and prepared in the form of sheets ready for forming. Prepregs are usually stored in a freezer and have a so-called storage life at this temperature (e.g. at -18 °C). The out-life is the accumulated time that the prepreg spends out of the freezer, e.g. at ambient temperature (20 °C) prior to use.

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Introduction

Up until now, bone fractures have been treated by fixing them in position while they mend by encasing them in bandages impregnated with plaster of Paris. Such plaster casts are however relatively heavy and lose their rigidity if they become wet. In recent years, people have been on the lookout for alternatives to plaster of Paris. A very promising material is the copolymer of poly-ε-caprolactone and polytetrahydrofurane (PCL-PTHF), which is synthesized by polycondensation of mixtures of PLC and PTHF diols in appropriate ratios with diisocyanatohexane (HDI). Thermoanalytical methods have supplied valuable information in the development of this material. For example, the influence of the composition of the copolymer on the processing temperature can be investigated by differential scanning calorimetry (DSC) and the mechanical properties of different copolymers can be compared using dynamic mechanical analysis (DMA).

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Introduction

Thermogravimetric experiments provide important quantitative information for the characterization of the thermal behavior of materials.

Simple reactions, such as a dehydration process, can usually be completely accounted for by a TGA measurement. To elucidate more complex reactions, the TGA is often coupled to a mass spectrometer (MS) or an infrared spectrometer (IR) to identify the gaseous products evolved during the decomposition of the sample. When complex materials such as the starting materials for syntheses, polymers or bitumens decompose, mixtures of gases are produced whose individual constituents cannot be identified with absolute certainty by MS or FTIR analysis. In such cases, the online coupling of a TGA with a GC-MS system offers important advantages. Gases simultaneously evolved are first separated by gas chromatography (GC) and then identified by MS.

In this article, we describe the online combination of a TGA/SDTA851e and a Hewlett Packard (HP) GC-MS (HP6890 and HP5973) system.

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Introduction

Thin layers of nickel oxide have been used for some time as optically active elements in electrochromic components [1].

Electrochromic layers change their optical transmission when a low electrical voltage is applied to the layer. Chemically, the oxidation state of the molecules in the layer changes, which in turn changes their interaction with light and hence their transmission behavior. These types of layer are used mainly for electrochromic windows and mirrors whose transmission or reflexion behavior automatically adapts to the actual brightness. To prepare such layers, a thin film of nickel acetate is applied to the substrate in a dipping process. This is then heated and held isothermally for a certain time at a particular temperature (i.e. baked). An electrochromic layer of nickel oxide is formed. The properties of the layer are influenced by the temperature and duration of the baking process: if the temperature is too high or the baking time too long, the electrochromic properties degrade to complete inactivity. If the layer is not properly thermally treated, there is the risk that the active layer separates from the substrate.

Thermogravimetry can be used to investigate the influence of temperature and time on the formation of the electrochromic layer. The difficulty is that the layers formed on the substrate are very thin. This results in extremely small weight changes that are often close to the detection limit of the thermobalance. The powder form of the starting material used for the layers is therefore often investigated. Compared with the actual layers, there are however large differences in the microstructure of the powdered starting material. This of course makes a comparison of the results more difficult.

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Literature

[1] C. G: Granqvist: Handbook of Inorganic Electrochromic Materials, Elsevier Science, Amsterdam, 1995, p. 4, 39.