Differential Scanning Calorimetry

DSC (Differential Scanning Calorimeters) and DTA (Differential Thermal Analyzer) quantitatively determine conversion temperatures and enthalpies for solids and liquids by measuring the heat flows to both the sample and to a reference as a function of temperature and time.

Due to its versatility and the high significance of its analytical output, differential scanning calorimetry (DSC) is the most often employed method for thermal analysis.

It can be used to investigate a great variety of materials:

Compact solids (granulates, components, molds, etc.) such as plastics, rubbers, resins or other organic materials, ceramics, glass, composites, metals and building materials
Powders such as pharmaceuticals or minerals
Fibers, textiles
Viscous samples such as pastes, creams or gels
Liquids

Typical Information That Can Be Derived from DSC Measurements:

Characteristic temperatures (melting, crystallization, polymorphous transitions, reactions, glass transition)
Melting, crystallization, transformation and reaction heats (enthalpies)
Crystallinity of semi-crystalline substances
Decomposition, thermal stability
Oxidative stability (OIT, OOT – oxidative-induction time and oxidation onset temperature, respectively)
Degree of curing in resins, adhesives, etc.
Eutectic purity
Specific heat (cp)
Compatibility between components
Influence of aging
Distribution of the molecular weight (peak form for polymers)
Impact of additives, softeners or admixtures of re-granulates (for polymer materials)

Funcional Principle of a Heat-Flux DSC

A DSC measuring cell consists of a furnace and an integrated sensor with designated positions for the sample and reference pans. The sensor areas are connected to thermocouples or may even be part of the thermocouple. This allows for recording both the temperature difference between the sample and reference side (DSC signal) and the absolute temperature of the sample or reference side.

Due to the heat capacity (cp) of the sample, the reference side (usually an empty pan) generally heats faster than the sample side during heating of the DSC measuring cell; i.e., the reference temperature (TR, green) increases a bit faster than the sample temperature (TP, red). The two curves exhibit parallel behavior during heating at a constant heating rate – until a sample reaction occurs. In the case shown here, the sample starts to melt at t1. The temperature of the sample does not change during melting; the temperature of the reference side, however, remains unaffected and continues exhibiting a linear increase. When melting is completed, the sample temperature also begins to increase again and, beginning with the point in time t2, again exhibits a linear increase.

The differential signal (ΔT) of the two temperature curves is presented in the lower part of the image. In the middle section of the curve, calculation of the differences generates a peak (blue) representing the endothermic melting process. Depending on whether the reference temperature was subtracted from the sample temperature or vice versa during this calculation, the generated peak may point upward or downward in the graphs. The peak area is correlated with the heat content of the transition (enthalpy in J/g). DIN 51007 and ISO 11357-1 recommend the portrayal of endothermic processes with upward ordinate amplitude. In, for example, ASTM E793 and E794, downward application of the endothermic direction is suggested. This is why the NETZSCH Proteus^® software allows for the direction of application for endothermic and exothermic processes to be selected.

What Is the Difference Between DSC and DTA?

According to DIN 51 007, differential thermal analysis (DTA) is suited for the determination of characteristic temperatures, while differential scanning calorimetry (DSC) additionally allows for the determination of caloric values such as the heat of fusion or heat of crystallization. This can be done with two different measuring techniques: heat-flux differential scanning calorimetry or power-compensated differential scanning calorimetry. Since all NETZSCH DSC instruments are based on the heat-flux principle, only this method will be discussed in more detail in the following sections.

For both DTA and heat-flux DSC, the primary measuring signal during a measurement is the temperature difference between a sample and reference in µV (thermal voltage). For DSC, this temperature difference can be converted into a heat-flux difference in mW by means of an appropriate calibration. This possibility does not exist for a purely DTA instrument.

Definition

Temperature Modulated Differential Scanning Calorimetry (TM-DSC) is a DSC technique whereby a sample is subjected to a superposition of a linear and a periodic temperature program. This means: a constant heating rate is overlapped by a further modulated temperature control.

The sinusoidal temperature perturbation leads to a resulting sinusoidal heat flow curve (response).

In consequence of the perturbation (modulated heating rate), the sample temperature is also being oscillating in a sinusoidal manner, resulting in a fluctuating heat flow signal.

By evaluating the average value as well as the amplitude and phase shift, the measuring signal is separated into an alternating (reversing) and a non-alternating (non-reversing) part. The raw data is then subjected to a Fourier analysis.

Usually, there is a phase shift (delay) between the perturbation (modulated heating rate) and the response (DSC signal). TM-DSC mathematically deconvolutes this response by means of the Fourier analysis into two types of signals, a reversing and a non-reversing one.

Reversing - Non-Reversing

Reversing heat flow:
The reversing part is dependent on the heat capacity (thermodynamic component) and heating rate. Crystallization processes and most of chemical reactions are showing practically no temperature dependence and therefore no contribution to the reversing signal.

„reversing“ ≠ „reversible“, Melting is reversible from the physical meaning, but non-reversing, because not the entire melting enthalpy is in the reversing DSC.

Non-reversing heat flow:
The non-reversing heat flow is the kinetic component and the part of the heat flow which does not react on the heating rate modulation.

Non-reversing Φ= total Φ – reversing heat flow

⇒ It is possible to separate overlapping effects if one is reversing and the other is non-reversing!

TM-DSC Result

Which Effects Can Be Separated with TM-DSC?

Effects which are not frequency-dependent:
- Always visible in the reversing (alternating) DSC curve:
  - c_p change during a glass transition
  - c_p change during a structural change
  - c_p change during a chemical reaction
- Time-dependent processes are always visible in the non-reversible (non-alternating) DSC curve:
  - Relaxation
  - Re-crystallization
  - Curing
  - Slow chemical reactions
  - Decomposition
  - Evaporation

Which Effects Can Not Be Separated?

Solid-solid transitions are often too slow to be alternating (reversing) referring to the time scale of the measurement.

Effects which are frequency-dependent:
- Partly within the reversing (alternating) DSC curve and within the non-reversing (non-alternating) DSC curve are visible:
  - Fast chemical reactions
  - Fast melting processes (metals)
  - Melting of polymorphic forms
  - Melting of polymers (heat only mode)
  - Melting and crystallization of polymers (heat-cooling mode)
  - etc.

In these cases, the choice of the modulation parameters must be taken into consideration. They can have a decisive influence on the measurement result.

O.I.T. for Cable Insulations, Tubes and More

DSC allows the determination of the Oxidative Induction Time (O.I.T.) according to well-established standards (ASTM D 38 95, ASTM D 6186, EN 728 und ISO 11357-3).

O.I.T. - Temperature Program, Isothermal and Dynamic O.I.T.

O.I.T. - Temperature Program

Isothermal O.I.T.

Dynamic O.I.T.

O.I.T. - Temperature Program
Isothermal O.I.T.:
OIT measurement according to DIN EN 728, ISO/TR 10837, ASTM D3895 (normally with open Al or Cu pans; Cu pans for cables). The isothermal temperature depends on the standard.
Dynamic O.I.T.

Usually, the beverage company purchases bottles and locking caps from different suppliers.

If quality problems occur, the bottle and locking cap producers have to prove that the used materials or manufacturing processes are not responsible for it.

The results of the O.I.T. tests indicate that the thermal damage of the locking caps occurred after the manufacturing process was finished.

The O.I.T. method is a very quick and cheap procedure for failure control analysis and incoming goods inspection. The method is standardized for polyolefins (isothermal), but can also be used for some other polymers under conditions optimized for these materials and their application. Long-term predictions (for several years) based on O.I.T. measurements should be considered critical.

Detailed Insight Into the World of Thermal Analysis

All NETZSCH DSC instruments work in accordance with the heat-flux principle and feature high detection sensitivity and long service lives – ideal conditions for successful application in research and academia, material development and quality control.

We offer various DSC models, covering a broad temperature range from -180°C to 1750°C. The DSC 404 F1 and F3 Pegasus^® are two versions for the precise determination of specific heat and caloric effects, particularly in the high-temperature range. The DSC 204 F1 Phoenix^® is our premium device for the temperature range from -180°C to 700°C. It unites excellent performance and highest flexibility. Coupled to a UV add-on (Photo-DSC 204 F1 Phoenix^®), it allows for monitoring the light-induced curing of, for example, paints, adhesives and resins. With the DSC 204 HP Phoenix^®, measurements under increased pressure can be carried out (up to max. 150 bar). The DSC 214 Polyma is a completely new concept. Designed especially for the characterization of polymer materials, its integral approach consists not only of the DSC instrument alone, but also incorporates the entire analytical process chain from sample preparation to evaluation. All differential scanning calorimeters discussed here operate based on the respective instrument standards as well as application or material testing specifications, including ISO 113587, ASTM E968, ASTM E793, ASTM D3895, ASTM D3417, ASTM D3418, DIN 51004, DIN 51007 and DIN 53765.

Knowledge Compact – The Handbook DSC on Polymers – Essential for the Analysis of Plastics, Rubber or Resins

The handbook DSC on Polymers provides you quickly and competently with helpful tips to enable you to carry out measurements on thermoplastics, elastomers, thermoplastic elastomers or thermosets and interpret the results. For 64 polymers, we have compiled measuring plots, measurement parameters, interpretation of the results and general material properties. Also chemical structures (if available), processing possibilities and application ranges, all of which are presented in a clearly structured and practice-oriented overview.

The introductory chapters offer an easy-to-understand introduction to DSC (including special measuring techniques such as temperature-modulated DSC, OIT, specific heat), lists national and international standards important for DSC measurements and describe their recommendations for the evaluation of polymer measurements. The DSC handbook is available in German and English. Reserve your personal copy today.