Thermal Analysis UserCom 46

DSC measurements can be performed up to about 700 °C using conventional DSC instruments. If higher temperatures are required, DSC curves can be measured up to 1600 °C using the TGA/DSC. This article compares DSC and TGA/DSC measurements and discusses how quantitative calorimetric measurements are possible in the high temperature region.

Introduction

Thermal analysis methods have been used for the analysis of metals since the end of the 19th century. The first measurements with controlled heating and cooling rates were carried out by differential thermal analysis (DTA). This method allows transition temperatures to be determined. However, the determination of quantitative thermodynamic quantities such as transition enthalpies or specific heat capacities requires a calorimeter.

In the low and medium temperature range, good accuracy can be obtained using conventional DSC. DSC measurements above 700 °C can be performed using simultaneous thermal analysis (TGA/DSC) and the appropriate sensor. The technique allows measurements to be performed from room temperature to 1600 °C.

In this article, we compare DSC and DSC/ TGA and show the possibilities of DSC measurement at high temperatures using metals and metal alloys as examples.

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The first measurements of the thermal conductivity of powders [1] showed that powders can be an interesting alternative to vacuum systems for achieving good thermal insulation. Currently powders of different materials (ceramics or polymers) are used in packaging or for building insulation. On the other hand, the low thermal conductivity of powders entails serious risks in the production and manipulation of energetic powders intended for pyrotechnics or explosives. Knowledge of the thermal conductivity of powders is therefore crucial to avoid spontaneous ignition.

Introduction

Many industrial activities (ceramics, powder metallurgy, food, etc.) involve the heating or cooling of powders.

We have developed a new method for measuring the thermal conductivity, κ, of powders using heat flow DSC. The measurement of κ is made at discrete temperatures that correspond to the melting points of selected pure metal references. Our results indicate that κ can be determined within an error bar of ±10%.

Several methods are currently available for measuring the thermal conductivity, κ, of thin solid slabs by DSC [2, 3, 4]. Camirand’s method [3] involves measuring the melting peak of a reference metal placed on top of a slab of the sample to be analyzed. The thermal conductivity, κ, is obtained from the slope of the lowtemperature side of the peak.

The difficulty with powders is that they have to be packed inside a pan. This causes a dramatic change in the geometry of the path of thermal conduction. We have successfully adapted Camirand’s method to these particular samples.

A spherical reference metal bead is placed on top of the powder filling the pan, as shown in Figure 1. When the pan is heated at a constant rate, the slope of the low-temperature side of the melting peak will depend on the resistance of the powder, R, as given by eq 1:

where R_DSC is the thermal resistance of the DSC sensor measured without the powder. It was shown that after an initial transient period the constant slope given by eq 1 is valid for any pan geometry [5].

For simple geometries such as those of commercially available cylindrical pans [5] or home-made hemispherical pans, κ can be easily determined from R [6] using eq 2:

where D_m is the diameter of the reference metal bead and D_p is the diameter of the pan. The value of the constant, ε, for a hemispherical pan is 1.

The value of ε for cylindrical pans was obtained by solving the heat transport equation in the steady state for a range of different pans with different height-to-diameter ratios (H/Dp) and different bead sizes that cover all practical situations. Some particular values for the correction factor, ε, are given in Table 1.

Finally, we would like to point out that the slope given by eq 1 is only reached after a transient period whose duration depends on several experimental conditions [5]. Figure 2 shows that the melting time for a large metal bead (D_m = 2.26 mm in Figure 2) is longer than the transient period and a constant slope is obtained. Consequently, in this case, R can be correctly measured.

On the other hand, a smaller bead (D_m = 1.29 mm in Figure 2) melts too quickly. In general, it is best to use a low heating rate in order to achieve a constant slope according to eq 1.

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References

[1] Z. Klemensiewicz, Thermal Conductivity of Powders, Nature, 164 (1949) 589-589.
[2] J.H. Flynn and D.M. Levin, A method for the determination of thermal conductivity of sheet materials by differential scanning calorimetry, Thermochim. Acta, 126 (1988) 93–100.
[3] C.P. Camirand, Measurement of thermal conductivity by differential scanning calorimetry, Thermochim. Acta, 417 (2004) 1–4.
[4] R. Riesen, Simple determination of the thermal conductivity of polymers by DSC, UserCom 22, 19–23.
[5] M. Pujula, D.Sánchez-Rodríguez, J.P. López-Olmedo, J.Farjas and P. Roura, Measuring thermal conductivity of powders with differential scanning calorimetry: a simplified method, J. Therm. Anal. Calorim. DOI: 10.1007/s10973-016-5274-4
[6] D.Sánchez-Rodríguez, J.P.López- Olmedo, J.Farjas and P.Roura, Determination of thermal conductivity of powders in different atmospheres by differential scanning calorimetry. J. Therm. Anal. Calorim. 121 (2015) 469–473.

High demands are nowadays put on packaging materials. For example, depending on the application field, the materials must provide optimum barrier properties toward water vapor, oxygen or odorants. In addition, there are requirements regarding tear resistance, transparency and compatibility with the contents of the packaging. In this article, we show how the water vapor transmission rate of materials can be determined using a sorption test system.

Introduction

The water vapor transmission rate of films can be determined gravimetrically with great accuracy. To do this, sample dishes are filled with saturated salt solutions, water or a desiccant and covered with the film under investigation.

The contents of the dish define a certain vapor pressure within the dish. The difference between the partial pressures inside and outside the dish results in water molecules diffusing through the film and being absorbed by the desiccant. This leads to a change in mass of the dish¹.

The mass changes can be determined by periodically weighing the dish. This allows the water vapor transmission rate of the film under investigation to be determined. Measurements like this can be carried out using a ProUmid GmbH sorption test system [1].

Figure 1 shows the preparation of a dish for an experiment. In this example, molecular sieves (about 2 g) are spread over the bottom of the dish. The material whose water vapor transmission rate is to be investigated is secured over the molecular sieves by a clamping ring. The excess film is then removed using a knife (scalpel).

1 By “mass of the dish” we mean the mass of the empty dish plus the mass of the film and that of the molecular sieves

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References

[1] R. Kirsch, Water vapor sorption of product packaging using the ProUmid sorption test systems, UserCom 45, 24–25. http://www.proumid.com or http:// www.mt.com/us/en/home/supportive_ content/specific_overviews/ DynamicVaporSorption.html#ptabs_ tab_custom2_li

The interpretation and 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. In this article, we discuss strategies for TGA curves using suitable examples.

Introduction

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

a) Variation of the temperature program. This includes using different heating rates, performing heatingcooling- heating experiments, and using "MaxRes" (a method in which the heating rate is automatically changed).

b) Changing the environment directly around the sample. This includes using different gases, different gas pressures, and different crucibles.

c) The use of TGA-EGA. If different decomposition products are formed when mass losses overlap, the substances involved can be identified and quantified with the aid of evolved gas analysis techniques such as TGA-MS, TGA-FTIR, TGA-GC/MS, TGA-Micro GC/MS.

We will discuss these strategies in the following sections and illustrate them with suitable examples.

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Knowledge of the polymorphic forms of an active substance is very important, especially in the pharmaceutical industry. In this article, we show how previously unknown polymorphs of menthol can be identified and characterized by Flash DSC.

Introduction

Both the stability of a chemical compound and its solubility in a medium depend on the structure of the compound. For example, in the development of a pharmaceutical substance it is important to identify different polymorphs and assess their stability. DSC is often used for the rapid detection of polymorphs.

Here the choice of the heating and cooling rate has a large influence on whether polymorphs are found and if so which ones. The formation of a structure in a liquid is strongly influenced by the cooling conditions. Depending on the cooling rate, different polymorphs or mixtures of different polymorphs are formed. During heating, reorganization processes occur which indicate the presence of other polymorphs.

Conventional DSC instruments provide heating and cooling rates of up to a maximum of about 300 K/min. In this article, we show how the high heating and cooling rates possible in the Flash DSC allow polymorphs to be identified that cannot be detected by conventional DSC. The substance chosen for this work was menthol. Previously unknown polymorphs were found both in levorotatory levomenthol as well as in the racemate (a 1:1 mixture of levorotatory and dextrorotatory enantiomers).

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The composition of unknown polymer samples can be quickly characterized by means of combined TGA/ DSC-FTIR measurements. This article describes a typical example.

Introduction

A TGA/DSC directly coupled to an FTIR spectrometer can be used to investigate the decomposition products of a material while it is heated [1]. This provides information about the composition of the original material. Further information can be obtained from the DSC curve measured simultaneously during the TGA experiment. In this article, we show how the composition of an unknown black polymer sample can be determined from measurements using a TGA/DSC-FTIR instrument combination.

Experimental details

The experiments were performed using a TGA/DSC 1 equipped with a DTA sensor. The TGA/DSC 1 was coupled to a Thermo Nicolet iS™50 FTIR spectrometer. Both the transfer line and the gas cell in the spectrometer were maintained at 200 °C to prevent condensation of the decomposition products. Our instrument was also equipped with an ATR accessory (attenuated total reflectance), which can be used to measure the infrared spectra of opaque solids.

The measurements were carried out from room temperature at a heating rate of 10 K/min. A purge gas rate of 50 mL/min nitrogen was used up to 700 °C and from then on 50 mL/min air. The IR spectra were recorded at a resolution of 4 cm^-1. Eight spectra were averaged to improve the signal-noise ratio. With these settings, a spectrum was obtained every 2 K at the heating rate used. The sample mass was 6 to 9 mg.

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References

[1] C. Darribère, Evolved Gas Analysis, METTLER TOLEDO Collected Applications Handbook.