Dynamic differential calorimetry

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DSC diagram of a PVC / PVDC sample after the 2nd heating
DTA and DSC crucibles for analysis
DSC sample carrier made of aluminum oxide / platinum

The differential scanning calorimetry or differential thermal analysis ( DSC , English differential scanning calorimetry , DSC ) is a method of thermal analysis for the measurement of emitted or a recorded amount of heat a sample in heating, cooling or isothermal process.

Measurement principles

An encapsulated container, called a crucible, with a sample (5–40 mg) and a second identical container without content (reference) are exposed to the same temperature change program in a heat bath . As a result of the heat capacity of the sample and exothermic or endothermic processes or phase changes such as melting or evaporation, there are temperature differences between the sample and reference, since thermal energy flows into or out of the sample in the process being investigated .

In contrast to the older differential thermal analysis (DTA), with the DSC this temperature difference is not used directly as a measurement signal, but inferred from it about the heat flow as a measured variable. There are two methods available for this.

Dynamic heat flow differential calorimetry

With this type, also known as heat flux DSC , the enthalpy changes (heat flow) are calculated by integrating the Δ T - T Ref curve. There are storage areas for sample and reference in the furnace on a disk ( disk type measuring system ), which has good thermal conductivity and under which the temperature sensors are located. If the oven is heated, the heat flows through the sample / reference into the pane and is picked up there by the sensors:

  • if the sample and reference are the same, heat flows of the same size flow through the pane and the difference in heat flow is therefore zero.
  • if a sample changes during the measurement, e.g. B. by conversion, melting or evaporation, there is a difference in the heat flow, which is proportional to the temperature difference:

With

  • Φ FP the heat flow of the sample
  • Φ FR is the heat flow of the reference
  • Δ T is the temperature difference.

Dynamic power difference calorimetry

With this type, also known as power compensating DSC , the sample and reference crucible are placed in thermally insulated ovens and these are regulated so that the temperature is always the same on both sides. The electrical power required for this is recorded as a function of temperature.

Applications

With the DSC u. a. the following determinations are carried out:

Another typical field of application is the determination of the purity of substances based on the change in melting point caused by impurities. A purity test using the change in melting point is only possible if the pure substance forms a eutectic mixture with the impurity .

precision

Round robin test data are available for dynamic heat flow differential calorimetry . The comparative standard deviation s R of the enthalpy of fusion depends on the material and linearly on the value of the measurand . Their relative value is typically between 7 and 13%. For temperature determinations, s R values ​​of 1.1 to 2.1 ° C can be expected. s R is a good estimate of the standard uncertainty.

Simultaneous applications

For special examinations, there are now also possibilities to examine a sample gravimetrically during the DSC measurement. This combination is called DSC-TG (TG = thermogravimetry ) or STA ( simultaneous thermal analysis ). In addition to the DSC signal, the mass loss is also recorded. In addition, the gases released can be analyzed using infrared spectroscopy or mass spectrometry .

literature

  • DIN 53765, DIN 51007, ASTM E 474, ASTM D 3418
  • DIN EN ISO 11357-1: Plastics - Dynamic differential thermal analysis (DSC) Part 1: General principles. (2008)
  • ISO 11357-2: Plastics - Dynamic differential calorimetry (DDK). Part 2: Determination of the glass transition temperature. (1999)
  • ISO / DIS 11357-3: Plastics - Dynamic differential calorimetry (DDK). Part 3: Determination of the melting and crystallization temperature and the melting and crystallization enthalpy. (2009)
  • ISO 11357-4: Plastics - Dynamic differential thermal analysis (DSC). Part 4: Determination of the specific heat capacity. (2005)
  • WF Hemminger, HK Cammenga : Methods of thermal analysis. Springer, ISBN 3-540-15049-8 .
  • G. Höhne, W. Hemminger, H.-J. Flammersheim: Differential Scanning Calorimetry - An introduction for practitioners. Springer, Berlin 1996, ISBN 978-3-540-59012-5 .
  • Vincent BF Mathot (Ed.): Calorimetry and Thermal Analysis of Polymers. Hanser, ISBN 3-446-17511-3 .
  • A. Müller-Blecking: Investigations of phase equilibria of binary systems: theory and practice of dynamic differential calorimetry (DSC). Verlag G. Mainz, ISBN 3-89653-364-9 .
  • Gottfried W. Ehrenstein, Gabriela Riedel, Pia Trawiel: Practice of thermal analysis of plastics. Hanser, 2003, ISBN 3-446-22340-1 .

Individual evidence

  1. ↑ Bruno Wampfler, Samuel Affolter, Axel Ritter, Manfred Schmid: Measurement uncertainty in plastics analysis - determination with round robin test data . Hanser, Munich 2017, ISBN 978-3-446-45286-2 , pp. 49-56 .