Thermal Analysis

Thermal Analysis DTA & DSC Dr. Prafulla Kumar Sahu M.Pharm., Ph.D. Raghu College of Pharmacy, Visakhapatnam.

 Thermal analysis •

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 Thermal  Thermal analysis includes a group group of techniques techniques in which which specific physical properties of a material are measured as a function of  temperature.  The techniques include the measurement measurement of temperatures temperatures at which which changes may occur, the measurement of the energy absorbed (endothermic transition ) or evolved (exothermic (exothermic transition ) during a phase transition or a chemical reaction, and the assessment of  physical changes resulting from changes in temperature.

Introduction •

Examples of properties:  –

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Heat-related Heat-related phase changes and degradations, crystallizations, heat capacities, heats of reaction, glass transitions, curing rates for adhesives, and weight changes.

 The properties properties are are observed either by monitoring temperature or heat flow in and out of the sample or by monitoring the sample weight during the process.

Classification •

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Differential thermal analysis (DTA) is a technique in which the temperature difference between the sample tested and a reference material is measured while both are subjected to the controlled temperature program. Differential scanning calorimetry (DSC) is a technique in which the heat flow difference between the sample and reference material is monitored while both are subjected to the controlled temperature program. Thermogravimetric analysis (TGA) is a technique in which the weight of a sample is monitored during the controlled temperature program.

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Thermomechanical analysis (TMA)

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Dynamic mechanical analysis

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Enthalpimetric analysis

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Thermometry

 Trends  Trends •

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Various environments (vacuum, inert, or controlled gas composition) and heating rates from 0.1 to 500°C/min are available for temperatures ranging from −190 to 1400°C.  The analysis analysis of gas(es) gas(es) released released by the specimen as a function of temperature is possible when thermal analysis equipment is coupled with Fourier Fourier-transform -transform infrared detection or with a mass spectrometer. spectrometer.

Applications •

Environmental measurements: vapor pressure, thermal stability, flammability, softening temperatures, temperatures, and boiling boili ng points.

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Compositional analysis: phase diagrams, free versus bound water, solvent retention, additive analysis, mineral characterization, and polymer system analysis.

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Important area of product reliability: heat-capacity data, liquid-crystal transitions, solid fat index, purity, polymer cures, polymer quality control, glass transitions, Curie point, and fiber properties.

Applications •

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Information on stability stability:: modulus changes, creep studies, expansion coefficients, and antioxidant evaluation. Dynamic properties of materials: visco-elastic measurements, impact resistance, cure characteristics, elastic modulus, loss modulus, and shear modulus. Chemical reactions: heats of transition, reaction kinetics, catalyst evaluation, metal– gas reactions, and crystallization phenomena.

DTA & DSC

Principles •

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Both of these methods relate to the monitoring of the heat absorbed or evolved during the heating of a sample and a reference reference in equivalent environments. environments. Differential thermal analysis (DTA) monitors temperature difference, difference, while differential scanning calorimetry (DSC) measures the power supplied. supplied.

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If an inert sample, such as alumina alumina,, is heated at a constant rate of 10°C min−1, the temperaturetemperature-against-time against-time curve is practically a straight line. line. A sample that reacts or melts within the temperature range studied will give small changes on its temperature-tim temperature-time e curve. curve. By heating both a reactive sample and an inert reference together at the same rate, these small differences may be detected and amplified as a function of  temperature. Example: If 10 mg of metallic of metallic indium are heated as sample and a similar amount of alumina as reference, reference, both heat at nearly the same rate until around 156°C the indium starts to melt. melt . This absorbs energy and the temperature of the indium rises less fast. This goes on until all the indium has melted when the temperatures of the liquid indium and alumina again rise at the same rate.

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 Two  Two alternative strategies can now be adopted.  –

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Differential thermal analysis or DTA strategy Power-compensated DSC strategy

DTA strategy •

If the temperatures of sample S and reference R are DTA measured and the temperature difference recorded: DTA strategy  ΔT = TS − TR

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A downward peak (i.e. a minimum) is recorded. Under carefully controlled instrumental conditions, this may be related related to the enthalpy change for the thermal event:

Where, A Where, A is the area area of the temperature-time temperature-time peak from from initial (i) to final ( f ) point .  This leads to quantitative quantitative or heat-flux differential scanning calorimetry (heat-flux DSC). The negative sign is required required since the enthalpy change on melting is positive, but ΔT but ΔT for melting melting is negative. negative.

Power-compensated DSC strategy •

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 The second strategy is to contro controll the amount amount of  heat supplied to sample and reference so that their temperatures stay as nearly the same as possible. Using separate heaters for sample and reference allows measurement of the difference in power ΔP power ΔP to be measured. With proper control and calibration, this will give the enthalpy change of the peak directly:

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A reference material that does not melt in the temperature range. Its temperature would match the temperature of  the surroundings (TE) for the entire temperature program.

 ΔT = TS − TR TR) between Consider plotting the difference ( ΔT the temperature of the sample (TS) and the temperature of  the reference material (TR) vs. the temperature of the surroundings. surroundings. Initially, there would be no difference, ΔT = 0, since the sample and surroundings are heated equally. However, when the sample melts, TS lags behind TR temporarily, making ΔT  negative. After melting is complete, the sample catches up such that After the two temperatures are again equal, ΔT = 0.

 ΔT vs. TE then results a negative peak in the DTA A plot of  ΔT vs. curve when the sample melts.

Endothermic curve •

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 The negative peak shown shown is a result result of an endothermic process (a process that absorbs heat) such as melting. Other endothermic processes, processes, other than melting, would also produce a negative peak. Examples: a chemical reaction or a decomposition.  The particular characteristics characteristics of this peak (shape, (shape, width, sharpness, smoothness, etc.) provide clues concerning the sample composition and properties that are the object of a DTA.

Exothermic curve DTA curve of  the exothermic process

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Exothermic Exothermic processes (processes that evolve heat) may also occur during the experiment.  This would produce produce a surge surge in the sample sample temperature, temperature, it would produce a positive peak in the  ΔT vs.  ΔT vs. TE plot. Exothermic Exothermic processes include crystallization as well as some chemical and decomposition reactions.

DTA curve of kaolinite

Schematic diagram of DTA or DSC apparatus

Instrumentation •

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 The temperature, both for the sample and the reference reference and also the furnace is measured by thermocouples, or resistance sensors. Higher sensitivity and greater stability are obtained if multiple sensors of inert material are used.

Factors influencing thermal analysis •

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The sample is generally about 10 mg of powder, fibers or reactants such as monomers for plastic production. These crucible,, which should be unreactive are placed into the crucible and stable over the temperature range used. Platinum, silica, aluminum, or alumina crucibles are commonly used. The sample and reference pans (either with alumina powder or sometimes an empty pan) are placed in their holders within the furnace, generally a wire-wound electrical heater controlled by the computer program. The rate of heating is user-determined, often about 10 K  min−1, but for the best approach to equilibrium, low heating rates are needed, and isothermal experiments may also be carried out. High heating rates save time, and can simulate situations like burning.

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The atmosphere surrounding the samples can be controlled. A slow flow of  nitrogen gas will give an almost inert atmosphere and sweep away harmful products. Oxygen may be used to study the oxidative stability of polymers. Carbon dioxide will react with some oxides to form carbonates. The mass of the sample, together with its volume and packing is important since these determine the heat transfer and the diffusion of gases across the sample.

Applications •

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Unlike other thermal methods, DTA and DSC are not compound-specific, they are still most  important test methods for a wide variety of  disciplines and materials. Inorganic materials, salts and complexes complexes : physical properties, properties, chemical changes and qualitative thermal behavior. Minerals and fuel (coal and oil) New materials (e.g. liquid crystals) are discovered, discovered, DSC is frequently used to test.  The greatest use is in the pharmaceutical pharmaceutical and polymer industries.

Applications •

One special use of DSC for physical changes is the determination of purity.  –

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While a pure substance melts sharply, perhaps over a few tenths of a degree near its true melting point, an impure sample may start to melt several degrees below this temperature, and will give a broad peak.

Computer analysis of the shape of this peak allows an estimation of purity, but does not provide any information on the nature of the impurities.

Applications •

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Many studies of inorganic complexes, complexes, of  polymer degradations and reactions between samples and reactive gases. Oxidation of polyethene is tested by heating samples in oxygen or holding them isothermally at around 200°C and then changing the surrounding surrounding atmosphere to oxygen and noting the time at which oxidative reaction reaction starts. This is a most useful test for blue polyethene water pipes.