Thermal analysis

Thermal analysis is the application of a precision controlled temperature program that allows quantification of a change in a substance’s properties with change in temperature. Measurements are usually made with increasing temperature, but isothermal measurements or measurements made with decreasing temperatures are also possible. Thermal analysis mainly studies physical changes (crystal transformation, melting, sublimation, adsorption, etc.) and chemical changes (dehydration, decomposition, oxidation, reduction, etc.) during the heating or cooling process, which not only provide thermodynamic parameters of materials such as phase transition temperature, heat capacity, and enthalpy, but also can give a certain reference value of the kinetic data. There are various types of thermal analysis methods, including differential thermal analysis (DTA), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

Differential Scanning Calorimetry (DSC)

In a DSC experiment the difference in energy input to a sample and a reference material is measured while the sample and reference are subjected to a controlled temperature program. DSC requires two cells equipped with thermocouples in addition to a programmable furnace, recorder, and gas controller. In DSC the measured energy differential corresponds to the heat content (enthalpy) or the specific heat of the sample. The thermal effects that occur can be roughly divided into three types:

Endothermic reaction: such as crystallization, evaporation, sublimation, chemical adsorption, decrystallization water, secondary phase change, gaseous reduction.

Exothermic reaction: Such as gas adsorption, oxidative degradation, gaseous oxidation, explosion, recrystallization.

Exothermic or endothermic reaction that may occur: Transformation of crystalline form, chemical decomposition, redox reaction, solid state reaction.

DSC is widely used in materials science research. The melting point of a substance can be determined and is one of the most commonly measured physical properties of DSC. Other measurable property is the glass transition temperature of the material, which is an important characteristic parameter of the material, and many properties of the material change sharply around the glass transition temperature. The crystallinity of the material is determined by DSC. Crystallinity is an important parameter for characterizing the morphological structure and physical properties of crystalline polymers. The thermoelectric and mechanical properties of polymers are closely related to the degree of crystallinity. Generally, this test method can achieve fast measurement speed, accurate quantitative results and good repeatability. Recent years thermal analysis and calorimetry have many applications within the Pharmaceutical disciplines.

Differential Thermal Analysis (DTA)

DTA is a thermoanalytic technique that is similar to DSC. The differential thermal analyzer uses a differential thermocouple to measure the temperature difference between the inert reference and the sample under test. Insert the two hot ends of the differential thermocouple into the reference and the sample. During programmed heating and cooling cycles, if the sample undergoes physicochemical changes, a thermal effect occurs. The DTA curve is a curve of temperature difference between the sample material and the reference material versus temperature or time. Applications of DTA include:

DTA is widely used in the pharmaceutical and food industries

DTA may be used in cement chemistry, mineralogical research, and environmental studies

DTA curves may be used to date bone remains or to study archaeological materials

Generally, the materials can be qualitatively and quantitatively analyzed through DTA. Various physical properties such as recrystallization characteristics, crystal transformation, melting point, crystallinity, glass transition temperature, and heat of fusion can be measured.

Thermogravimetric analysis (TGA)

Thermogravimetry is a technique for measuring the relationship between mass and temperature under different thermal conditions. In TGA the mass loss versus increasing temperature of the sample is recorded. As long as the mass changes when the material is heated, it can be studied by thermogravimetry, such as dehydration, moisture absorption, decomposition, compounding, adsorption and desorption, sublimation and so on. The instrument is easy to operate, sensitive, fast, requires less sample, and has a wide range of scientific information. The basic instrumental requirements are simple: a precision balance, a programmable furnace, and a recorder (Figure 1). Modern instruments, however, tend to be automated and include software for data reduction. In addition, provisions are made for surrounding the sample with an air, nitrogen, or an oxygen atmosphere.

Figure 1. Typical components of a TGA instrument.

TGA can effectively evaluate the thermal stability of the material. By detecting the thermal stability, the composition of the material can be analyzed, and the degradation kinetics of the polymer can also be studied. Although TGA can quickly and effectively study the mass change of materials due to heat, it is impossible to know the mechanism of decomposition because it cannot simultaneously characterize the state of the sample and escape gas components. By using the combination of thermogravimetry and spectroscopy, the gas overflowing during the thermal decomposition process can be detected and analyzed, and the mechanism and process of the test can be inferred. DSC is often used in conjunction with TA to determine if a reaction is endothermic, such as melting, vaporization and sublimation, or exothermic, such as oxidative degradation.

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