Differential scanning calorimetry (DSC) - A192

Differential scanning calorimetry (DSC)
Differential scanning calorimetry, TA Instruments
Document Type	Article
Document Identifier	192
Themes	Quality/inspection management
Relevant Class	Equipment
Tags	Material characterization Test lab
Factory Cells	Test lab
Prerequisites	Glass transition temperature (Tg) Testing

Introduction

A Differential Scanning Calorimeter (DSC) measures the heat flow in and out of a sample relative to a reference during a predefined thermal cycle. This test provides quantitative and qualitative insights into physical and chemical transformations during endothermic and exothermic processes. The samples are placed in a controlled temperature environment alongside the reference pan (see Figure 1). Thermocouples embedded in the test chamber allow temperature to be recorded with time. The temperature of the test chamber is controlled by cooling accessories and heating elements surrounding the chamber. The electrical power consumed by the equipment to maintain the predetermined thermal cycle is recorded and represented to the user as Heat Flow.

Standard aluminum sample pans and lids - TA instrument

In many cases, the linear temperature ramps in dynamic tests are clubbed with sinusoidal heating profiles such that while the sample experiences an average increase in temperature, it undergoes instantaneous heating and cooling. This modulation is sufficient to determine the glass transition temperature.

While a DSC provides detailed reliable data on cure kinetics for thermosetting polymers, it is important to not exceed the temperature beyond the degradation temperature. Hence, oftentimes, an extra sample will be prepared for testing in the Thermogravimetric Analyzer (TGA) before beginning the series of DSC tests.

Features

The DSC environmental chamber is designed to measure properties like glass transition temperature, crystallization, phase changes, product stability, oxidative stability, cure and cure kinetics among others. By comparing against an empty 'reference' pan, the DSC is able to log heat flow into and out of the pan containing the sample with high sensitivity while temperature in the chamber is controlled.

Cooling Accessories

Different cooling accessories exist that allow different temperature ranges to be scanned. For example, Refrigerated Cooling System (RCS) and Discovery Liquid Nitrogen Pump Accessory (LN2P) are provided by TA Instruments, which can achieve temperatures as low as -90 °C and -120 °C, respectively.

Auto-Samplers

Some DSCs are equipped with auto-samplers that allow for tests to be run sequentially. This consists of a built-in XYZ gantry system and gripper to move samples from a tray to the test chamber. Grippers can be sensitive to dimensional changes of the pans so care must be taken to prevent any damage during sample preparation.

Other

Several other accessories are available to allow the collection of more qualitative data. Some examples include a microscope camera that can be used to take pictures of the sample as it experiences thermal events. Other optical can be used to capture the optical characteristics of samples.

Uses and test types

As noted previously, a DSC is used to quantitatively determine the heat energy absorbed/released by the sample, thereby allowing the computation of DOC (non-reversible heat flow) and Tg (reversible heat flow – corresponds to specific heat capacity of the sample). It does this through three types of tests.

Dynamic Tests

Dynamic tests are the default test type used in DSC. The sample undergoes a linear temperature ramp during heating and cooling, producing a heat flow vs temperature graph. Tm, Tc, and Tg are some properties that can be determined from this test.

Isothermal Tests

An isothermal test is simply a temperature hold. Temperature holds are frequently added to dynamic tests to allow the material to reach thermal equilibrium before the next temperature ramp. Isothermal tests can also be valuable for analyzing thermoset polymers (Thermoset polymers). The thermoset sample undergoes the cure reaction (Cure of thermosetting polymers) while exposed to a constant temperature for extended periods of time. The heat released by the cure reaction, reaction rate, and total cure time can be determined from this procedure. These tests more accurately represent the cure cycles used by industry.

Modulated DSC Tests

MDSC tests include a sinusoidal heating profile superimposed on a temperature ramp with constant rate. This allows properties that would normally be masked by other effects (such as Tg) to be more accurately defined. For example, in a regular DSC test, thermal events frequently occur during the same temperature range which can sometimes mask weaker events such as the glass transition. MDSC allows the data to be analyzed separately by separating total measured heat flow into reversing and non-reversing heat flow. Modulation also increases sensitivity, allowing the detection of weaker transition periods. Test parameters should be set to optimize the machine’s sensitivity for the specific thermal events expected of the material. Adjustments can be made based on which events are being studied. Care should be taken to not exceed the degradation temperature of the material.

Sample preparation

The samples are commonly sealed in aluminum pans through a crimping process, to prevent sample and volatile loss. Typical sample masses are between 5-20 mg. Metallic samples are recommended to be around 5 mg while polymeric samples are recommended to be on the higher end around 15 mg. Samples should be kept as flat and thin as possible to maximize the contact surface with the pan, which results to more efficient heat transfer by conduction. A variety of sample pans are available and should be selected depending on the maximum temperature and internal pressure they can withstand. Commonly, hermetic aluminum pans are used. Great care should be taken to prevent any damage to the sample pan as they are very fragile. Damage can affect the quality of heat transfer, leading to inaccurate results.

Analysis of Results

A typical DSC graph consists of normalized heat flow on the y-axis and temperature on the x-axis. As the material experiences exothermic and endothermic reactions, transition regions marked by peaks and slope differences appear and can be analyzed using the provided software to characterize the material.

Sample graphs of Heat Flow (Normalized) vs Temperature

In Figure 2, common thermal events have been marked. Point 1 displays a typical glass transition region without overlapping thermal events. This shows how weak this event can appear. Point 2 displays a typical melting temperature peak. This is marked as a sharp endothermic peak. Point 3 displays a typical crystallization event marked by a sharp exothermic peak. For an example of a cure reaction in a thermoset polymer, please refer to Figure 3 shown below.

An example of a heat flow curve obtained from differential scanning calorimetry (DSC) measurement of a resin sample starting from an unreacted state. The total heat of reaction is the area under the heat flow curve.

For more information, please see page Heat of reaction.

Reaction Enthalpy

The equipment software can be used to integrate over exothermic and endothermic peaks. By selecting the onset points on either end of the peak and choosing to integrate over time, enthalpy can be determined. For thermoset polymers, this feature can be used to determine the enthalpy of cure. For thermoplastic polymers, this can be used to determine the fusion enthalpy.

Glass Transition Temperature (Tg)

Although modulated DSC tests are more common for determining Tg, this property can still be determined through regular DSC testing. It is marked by a step change in heat capacity. A region with two linear trends with differing slopes will be observed (see region 1 of Figure 2). The slope of the line connecting these two linear trends provides Tg. This temperature value represents the range over which the glass transition occurs. Care must be taken when analyzing this property as it is affected by the heating and cooling rates in addition to other physical properties.

Special Case 1: Characterization of thermoset polymers via DSC

Property of Interest: Degree of Cure

DSC provides detailed, reliable data on cure kinetics for thermosetting polymers. Cure kinetics modeling is based on measuring the heat of reaction during cure as a function of various temperatures or heating rates. The exothermic peak is integrated to find the residual heat of reaction. Using the following equation, the DOC can be found.

\( \text{DOC}(t) = 1- \frac{\Delta H(t)}{\Delta H_R} \)

In which ∆H(t) is the measured enthalpy of cure reaction at time t, and ∆H_R is the total enthalpy of cure reaction (fully cured thermoset). This property is useful for quality control purposes and process optimization.

Special case 2: Characterization of thermoplastic polymers via DSC

Property of Interest: Crystallization Temperature (Tc)

Found from the temperature at the exothermic peak of the cooling cycle in a heat flow (normalized) vs temperature graph. With slower temperature ramps, several peaks may be visible in close proximity to one another. These may represent the formation of different crystal structures. It is worth noting that crystallization is a kinetic process and the formations that occur are greatly affected by the test parameters.

Property of Interest: Melting Temperature (Tm)

The sharp endothermic peak on the heating cycle of a normalized heat flow vs temperature graph would be considered the melting temperature of the material. Secondary melting temperatures can be determined using the same method on endothermic peaks occurring on following heat cycles after a cooling cycle.

Additional information on thermal events of polymers can be found here: Thermal phase transitions of polymers.

Combining Equipment

Thermogravimetric Analyzer (TGA) (TGA):

It is crucial to always run a TGA test to determine the temperature at which the material degrades before performing DSC testing. Degradation of the sample during testing can cause corrosion of the pan and interfere with results.

Dynamic Mechanical Analyzer (DMA) (DMA):

To provide a more realistic view of a material’s behavior during manufacturing, dynamic testing can be performed to determine Tg and Tm with a dynamic and static load. This results in a lower melting temperature and is a useful property for understanding the material’s limits and true behavior. Performing DSC testing prior to DMA allows one to more efficiently set temperature targets and test parameters.

References

[1] TA Instruments, “TA Instruments Discovery DSC Brochure,” 2024. https://www.tainstruments.com/pdf/brochure/TA-Instruments-Discovery-DSC-Brochure-EN.pdf (accessed May 27, 2025).

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Differential Scanning Calorimetry (DSC).

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Polymerization of thermoset resins is an exothermic reaction and heat is generated during the curing process. A thermosetting resin has the potential to release a certain amount of energy while curing. This is called the total heat of reaction, H_R, with a unit of J/g (SI units).

The heat of reaction during polymerization is measured using a Differential Scanning Calorimeter (DSC) equipment measuring much energy/heat comes out of the reaction for a small resin sample.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.