Differential Scanning Calorimetry (DSC)

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Differential scanning calorimetry

Fundamentals of the DSC method

Dynamic differential scanning calorimetry (DSC) is a testing method frequently used in plastics analysis to measure the thermal energy of a sample during heating (see also: thermal conductivity), cooling, or isothermal storage [1‒2]. The fundamentals of DSC are summarised in a number of textbooks from both a polymer physics and an application technology perspective [3–10].

Fig. 1:

Structure of a DSC measuring cell (heat-flow principle) with disc measuring system (a) and structure of a TzeroTM sensor (TA Instruments, USA) (b)

Basic measurement principles

DSC devices are built according to two basic measuring principles: the heat-flux principle and the power compensation principle. Two lens-sized crucibles (‘pans’, ‘cups’) containing the sample and inert reference material are heated simultaneously according to a selected linear temperature programme. Air is often used as the reference material. In the heat flow method, the sample and the reference sample are placed in a cylindrical furnace. If the arrangement is thermally symmetrical, there is no temperature difference between the crucibles when the furnace is heated. However, if the specific heat capacity of the sample changes during heating, a temperature difference develops, which, in the ideal case, is proportional to the change in specific heat capacity.

The arrangement (Fig. 1a) can be calibrated and used to measure the specific heat capacity. The introduction of Tzero^TM technology serves to improve the resolution in the heat flow method. Compared to the conventional heat flow method with a disc measuring system (Fig. 1a), in which the temperatures of the sample and reference are measured, this method uses a sensor that includes an additional thermocouple (Fig. 1b). This additional temperature sensor measures the so-called base temperature and enables better correction of thermal asymmetries in the furnace [11].

In power-compensated DSC, the sample and reference are completely separated. The sample and reference crucibles have their own heating element and temperature sensor. With the aid of a control device, the sample and reference substance are heated at the same speed in such a way that no temperature difference arises between the two. If the specific heat capacity of the sample changes, more (in endothermic processes) or less (in exothermic processes) sample heating power is supplied in order to avoid a temperature difference. In DIN EN ISO 11357 [12], both methods are summarised under the term dynamic differential thermal analysis (DSC). The amount of heat supplied can be calculated using

$dQ=m\cdot c_{p}(T)dT$

(1)

und

$Q=m\cdot \Delta H=m\int _{T_{1}}^{T_{2}}c_{p}(T)dT$

(2)

Q	heat quantity
m	sample weighting
T	temperature
cp	specific heat
H	enthalpy

the enthalpy and specific heat as a function of temperature determine.

During phase transitions, the temperature dependencies of specific heat (see also: thermal conductivity) and enthalpy show characteristic changes in the curve, as shown schematically in Fig. 2. In the dependence of specific heat on temperature, a step (Fig. 2 left) can be seen in the glass transition range (see also: glass transition temperature), and a peak (Fig. 2 right) in the melting range. A general overview of the physical and chemical causes of DSC peaks is summarised in Table 1.

Determination of glass transition temperature (T_g determination)

The glass temperature T_g is determined in accordance with ISO 11357 based on the dependence of specific heat on temperature, as shown schematically in Fig. 2 on the left. In contrast to metallic materials, semi-crystalline plastics have a relatively wide melting range. The melting process and thus the course of the melting curve depend heavily on the thermal and mechanical history of the plastic material.

Fig. 2:

glass transition with Tg determination (left) and melt region (right)

Table 1:

Physical and chemical background for the formation of peaks during a DSC measurement [13]

Physical reasons			Chemical reasons
	endotherm	exotherm		endotherm	exotherm
melt	⃝		chemiesorption		⃝
crystallise		⃝	desolvation	⃝
evaporate	⃝		dehydration	⃝
sublimate	⃝		decomposition	⃝	⃝
adsorption		⃝	oxidative degration		⃝
absorption	⃝		redox-reaktion	⃝	⃝
desorption	⃝		solid phase reaction	⃝	⃝
Curie point transitions	⃝		combustion		⃝
liquid crystal transitions	⃝		polymerisation		⃝
glass transition	no peak, only offset		curing, cross-linking		⃝

In plastics, the melting point is defined as the temperature at which most crystallites melt, i.e. the temperature T_m of the endothermic maximum in the dependence c_p = f(T) or (dQ/dt)/m = f(T) (Fig. 2, right).

Areas of application

One of the main areas of application for DSC in quality assurance is the identification of plastics, which is generally carried out via the transition temperatures, i.e. via T_g in amorphous plastics and via T_m in semi-crystalline plastics. This method often allows for reliable identification. Homo- and copolymers (see also: polymer blends) can be clearly differentiated based on their different melting temperatures.

A special method of dynamic differential thermal analysis is temperature-modulated DSC (TMDSC) [3–7, 14]. This method can be used to distinguish reversible effects (glass transition, melting) from irreversible effects (cross-linking, decomposition, evaporation, etc.). This allows overlapping or closely successive processes to be separated and poorly pronounced glass transitions, e.g. in semi-crystalline thermoplastics, to be evaluated significantly. In addition, the specific heat capacity is determined in a single measurement.

Another possible application of DSC is the comparative evaluation of the resistance of plastics to thermo-oxidative degradation by determining the oxidative induction time or temperature (OIT). There are two different methods: dynamic measurement with comparatively low sensitivity, in which the DSC measurement is carried out in an oxygen or air atmosphere and the temperature at which exothermic oxidation begins is determined, and the so-called static method, in which the sample is heated under inert gas conditions to a defined temperature above T_m. This temperature is maintained and, once equilibrium has been reached, the atmosphere is switched to an oxidative one. This method measures the time until the oxidation reaction occurs. The static OIT method is standardised in ASTM D 3895 [15]. Figure 3 shows the influence of oven post-exposure on the oxidative induction time, determined at 190 °C and a pressure of 3.4 MPa on POM homopolymer granulates.

Fig. 3:

Influence of oven exposure time on the oxidative induction time in POM homopolymer granules [16]

The oven exposure for setting defined ageing states was carried out at 140 °C. The use of a pressure DSC cell proved necessary for the investigations on polyoxymethylene (abbreviation: POM) materials in order to initiate oxidation. Under standard conditions, depolymerisation dominates in this material. An increasing exposure time leads to an increasing stabiliser consumption, which results in a decreasing induction time. TGA measurements verified that only stabiliser consumption occurs and no thermal damage to the chains takes place [16]. Although this method can only be used as a comparison method with the same stabilisers and no conclusions can be drawn about long-term behaviour, it has proven itself in practice, particularly in the quality assurance of polyolefin cable sheathing.

See also

• Temperature-modulated differential scanning calorimetry (TMDSC)
• Temperature conductivity
• Thermal conductivity
• Crystallinity
• Glass transition temperature

References

[1]	Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 283–299 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 22)
[2]	Ehrenstein, G. W., Riedel, G., Trawiel, P.: Praxis der thermischen Analyse von Kunststoffen. Carl Hanser, Munich (2003) 2nd Edition (ISBN: 978-3-446-210011; see AMK-Library under C 10-2)
[3]	Höhne, G. W. H., Hemminger, W. F., Flammersheim, H.-J.: Differential Scanning Calorimetry. Springer, Berlin Heidelberg (1996) (ISBN 978-3-5405-9012-5)
[4]	Brown, M. E.: Introduction to Thermal Analysis – Techniques and Applications. Springer, Berlin Heidelberg (2001) (ISBN 978-1-4020-0472-8)
[5]	Wunderlich, B.: Thermal Analysis of Polymeric Materials. Springer, Berlin Heidelberg (2005) (ISBN 978-3-540-23629-0; e-Book ISBN 978-3-540-26360-9)
[6]	Hatakeyama, T., Quinn, F. X.: Thermal Analysis: Fundamentals and Applications to Polymer Science. John Wiley & Sons, Inc., 2nd Edition, Indianapolis (1999) (ISBN 978-0-471-98362-0)
[7]	Sorai, M. (Ed.): Comprehensive Handbook of Calorimetry and Thermal Analysis. John Wiley & Sons, Inc., Indianapolis (2004) (ISBN 978-0-470-85152-4)
[8]	Ehrenstein, G. W.: Thermische Analyse. Brandprüfung, Wärme- und Temperaturleitfähigkeit, DSC, DMA. Erlanger Kunststoffanalyse, Carl Hanser, Munich (2020), (ISBN 978-3-446-46258-8, e-Book ISBN 978-3-446-46424-7; see AMK-Library under C 44)
[9]	Haines, P.: Principles of Thermal Analysis and Calorimetry. Royal Society of Chemistry (2002) (ISBN 978-0-85404-610-2)
[10]	Groenewoud, G. W.: Characterisation of Polymers by Thermal Analysis. Elsevier Science, Amsterdam (2001) (e-Book ISBN 978-0-0805-2893-9)
[11]	Höhne, G. W. H., Kunze, W.: Ein Quantensprung in der DSC. LaborPraxis Dezember (2001) pp. 38‒42
[12]	ISO 11357 Plastics – Differential Scanning Calorimetry (DSC) Teil 1 (2023-02): General Principles Teil 2 (2020-03): Determination of Glass Transition Temperature and Step Height Teil 3 (2025-05): Determination of Temperature and Enthalpy of Melting and Crystallization Teil 4 (2021-02): Determination of Specific Heat Capacity Teil 5 (2025-05): Determination of Characteristic Reaction-curve Temperatures and Times, Enthalpy of Reaction and Degree of Conversion Teil 6 (2025-06): Determination of Oxidation Induction Time (Isothermal OIT) and Oxidation Induction Temperature (Dynamic OIT) Teil 7 (2022-03): Determination of Crystallization Kinetics Teil 8 (2021-02): Determination of Thermal Conductivity
[13]	Vogel, J.: Erfahrungen bei der Nutzung der Thermischen Analyse in der Kunststoffforschung. Tagungsband: Thermische Analyse an polymeren Werkstoffen im Rahmen der Qualitätssicherung. Beiträge zum LabTalk-Seminar von Mettler Toledo (1996) 79‒90
[14]	Androsch, R.: Reversibles Kristallisieren und Schmelzen von Polymeren. Habilitation, Martin-Luther-Universität Halle-Wittenberg (2005)
[15]	ASTM D 3895 (2019) Standard Test Method for Oxidative-Induction Time of Polyolefine by Differential Scanning Calorimetry
[16]	Archodoulaki, V.-M.: Eigenschaftsänderungen von Polyoxymethylenen induziert durch Verarbeitung, Alterung und Recycling. Fortschrittberichte VDI Reihe 5: Grund- und Werkstoffe/Kunststoffe. VDI-Verlag, Düsseldorf (2005)