Thermal Analysis UserCom 48

This article describes how the heat of combustion of carbonaceous materials can be determined using simultaneous TGA and DSC. In this method, the substance is heated in air or oxygen at normal pressure to 900 °C and held isothermally until the weight change due to the loss of combustion gases has stopped and the production of heat has finished. The weight loss steps also show whether other processes besides combustion occur, for example vaporization, and whether constituents such as moisture and ash are present in the material. The exothermic DSC measurement of the combustion can thereby be related to specific weight losses.

Introduction

The heat released when a substance burns is known as the heat of combustion. During combustion, oxides are formed that are volatile under normal conditions (e.g. CO₂), that condense and become liquid (e.g. water) or that form solid compounds (e.g. ash, metal oxides). Normally, the heat of combustion is determined using a bomb calorimeter.

The materials are completely oxidized in excess oxygen under pressure at constant volume under adiabatic conditions and then cooled to room temperature. The heat of reaction and possible transformation is determined from the measured temperature increase. In each case, it is important to check whether combustion was complete.

With heating systems, the terms higher and lower calorific or heating values (and other terms) are used, depending on whether combustion occurs with condensation of the combustion products (usually water) or without condensation.

The lower calorific value, LCV (or lower heating value, LHV) is the value when condensation does not occur and only volatile gaseous products are formed.

The higher calorific value, HCV (or higher heating value, HHV) is the value that results when all the products of combustion are cooled to the original pre-combustion temperature of 25 °C and any vapors formed are condensed.

The upper heating value is higher than the lower heating value because when condensation occurs (mostly water) the heat of condensation is released.

The following measurements show how the heat of combustion of small amounts of carbonaceous inorganic materials can be quantitatively determined by TGA/DSC.

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This article presents a new predictive micro-scale solubility method for predicting the formation of amorphous solid dispersions (ASDs) by hot-melt extrusion. It enables the temperature-dependent solubility of active pharmaceutical ingredients (API) in polymer matrices to be characterized using glass transition temperatures (T_g). The application of a complex mathematical model (BCKV equation) to describe the fractiondependent course of the T_g of API/polymer mixtures allows API solubility to be predicted at ambient conditions (25 °C). This is illustrated using ball-milled physical mixtures of indomethacin in poly(vinylpyrrolidone-covinyl acetate) (copovidone).

Introduction

Hot-melt extrusion is widely used for the preparation of amorphous solid dispersions (ASDs) to increase the bioavailability of sparingly soluble active pharmaceutical ingredients (APIs). Differential Scanning Calorimetry (DSC) is often employed as a screening tool to determine the most suitable polymer for forming ASDs.

This article presents a new and rapid method for the determination of temperature- dependent phase diagrams of APIs in polymer matrices using DSC measurements. It involves an indirect measurement of the API solubility using glass transition temperatures (T_g) and the BCKV fit [1]. This allows the dissolved concentration of the API to be estimated at a desired temperature.

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Laminated iron cores in electric motors or transformers are manufactured by stamping out thin sheets of a particular shape from electrical steel sheet precoated with a bonding varnish. A stack of these sheets is then “glued” together by applying mechanical pressure at a suitable temperature. This results in the formation of a solid laminated metal core on cooling. In this article, we show how thermal analysis can contribute to a better understanding of the process.

Introduction

In electric motors and transformers, alternating magnetic fields generate eddy currents which reduce electrical performance. To minimize eddy current losses, the iron cores used are manufactured by bonding stacks of thin insulated sheets together to form a laminate structure.

The individual laminate sheets are stamped out in any desired shape from precoated cold-rolled soft magnetic sheeting made of iron-silicon alloys. The sheets are coated on both sides with a thin coating of a special varnish or resin. Three-dimensional components are made by stacking the individual precoated sheets on top of one another and applying mechanical pressure at a particular temperature.

The varnish between the sheets cures and bonds the sheets together. The result of this process is a solid laminated iron core whose metal layers are insulated from one another by a film of hardened varnish typically about 10-μm thick (Figure 1).

During the bonding process, the varnish layers cure. At the same time, the stack of sheets is subjected to mechanical pressure. This can result in a certain amount of varnish being squeezed out from between the sheets. If this happens, droplets can form on the sides of the component and possibly make mechanical post-processing necessary. Two important questions arise in practice:

1. At what temperature should the curing reaction be performed and how long should it last?
   
2. How can the formation of droplets due to the squeezing out of varnish be avoided?
   

In this article, we show how thermal analysis can address these questions.

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This article describes how the Flash DSC was used to investigate the influence of calcium carbonate on the crystallization of filled polypropylene (PP) in the temperature range 120 °C to 0 °C. At low supercooling (above 80 °C), the presence of calcium carbonate reduces the crystallization rate of the α-phase. Between 45 °C and 80 °C, the crystallization of the α-phase is accelerated. The mesophase formation at low temperatures is not influenced by the filler.

Introduction

The crystallization of polypropylene (PP) under non-isothermal and isothermal conditions has been studied in detail [1].

The introduction of the Flash DSC with its high cooling rates has made measurements possible that provide new information about crystallization processes over a large temperature range and in particular at high cooling rates and high supercooling [2].

It is well-known that in PP two different crystallization mechanisms occur. Above 60 °C, the monoclinic α-phase is formed through heterogeneous nucleation [3].

At low temperatures, the so-called mesophase is formed through largely homogeneous nucleation [3]. This phase is said to be conformationally disordered. It has properties that lie between those of the crystalline and the amorphous states [1]. Fillers can affect the crystallization of polymers and have been classified as active or inactive with regard to their nucleation activity.

The influence of fillers on the crystallization process of PP is often investigated by conventional DSC at low supercooling or low cooling rates. DSC measurements at crystallization temperatures that also occur in polymer processing (e.g. injection molding) can be performed by Flash DSC.

As far as we are aware, we do not know of any investigations dealing with the crystallization behavior of filled PP at supercooling levels relevant to industrial processes. In this article, we investigate the influence of calcium carbonate (CaCO₃), an inactive filler, on the crystallization process of PP using DSC and Flash DSC.

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References

[1] R. Androsch, M.L. Di Lorenzo, C. Schick, B. Wunderlich, Mesophases in polyethylene, polypropylene, and poly (1-butene), Polymer, 51 (2010) 4639–4662.
[2] J.E.K.Schawe, Influence of processing conditions on polymer crystallization measured by fast scanning DSC, J. Thermal. Anal. Calorim. 116 (2014) 1165–1173.
[3] C. Silvestre, S. Cimmino, D. Duraccio, C. Schick, Isothermal crystallization of isotactic poly (propylene) studied by superfast calorimetry, Macromol. Rapid Commun. 28 (2007) 871– 881.
[4] J.E.K. Schawe, P.A. Vermeulen, M. van Drongelen, A new crystallization process in polypropylene highly filled with calcium carbonate, Colloid and Polymer Science, 293 (2015) 1607–1614.

It is nowadays very important to identify and analyze the constituents of unknown products and to quantitatively determine their individual contents [1]. Combined techniques such as TGA-MS (thermogravimetric analysis-mass spectrometry), TGA-FTIR (thermogravimetric analysis-infrared spectroscopy) [2], and TGAGC/ MS (thermogravimetric analysis-gas chromatography-mass spectrometry) [3] are very useful for this type of work.

Introduction

This example shows how easy it is to analyze the contents of an unknown plastic material using a TGA-FTIR. The technique allows both the quantitative composition and the chemical nature of the constituents to be determined. To characterize unknown materials, a useful first step is to measure an ATR infrared spectrum of the material.

This enables qualitative information about the main constituents to be obtained within a few minutes. For detailed analysis, we recommend the use of a TGA coupled to a suitable gas analyzer. In this article, a white molded component of unknown composition was analyzed by ATR spectroscopy, TGA, and TGA-FTIR in order to determine its constituents.

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