Title 34. THERMAL ANALYSIS.pdf ✅

PHARMD GURU Page 1 INTRODUCTION:  Thermal analysis is a branch of physico-chemical science where the properties of materials are studied and quantified as they change with respect to the change in temperature and/or heat.  The thermal analysis is defined as “a group of techniques in which a physico- chemical property of the material is measured as the function of temperature, while the sample is subjected to a controlled temperature programs like cooling, heating, and/or isothermal process”. They include: 1) Thermo Gravimetric Analysis (TGA), 2) Differential Thermal Analysis (DTA), 3) Differential Scanning Calorimetry (DSC), 4) Thermometric Titration (TT), 5) Direct Injection Enthalpimetry, 6) Dynamic Mechanical Analysis (DMA), and 7) Thermo Mechanical Analysis (AMA).  Thermal analysis is basically used to study the physical properties of the material such as mass, enthalpy, dimension, dynamic characteristics, freezing temperature, boiling temperature, melting temperature, curing rates for adhesives, heat of fusion, heat of vaporization, etc.  Thermal analysis techniques are used in the characterization of inorganic and organic compounds, polymers, cosmetics, pharmaceuticals, metals, alloys, geological samples, ceramics, glasses, and many manufactured products. DIFFERENTIAL SCANNING CALORIMETRY (DSC):  Differential Scanning Calorimetry (DSC) is a thermo-analytical technique which involves the measurement of the difference in the amount of heat required to increase the temperature of the sample and reference as a function of temperature.  Sample and reference both are maintained at almost the same temperature throughout the experiment. THERMAL ANALYSIS
PHARMD GURU Page 2  Calorimetry is the process of measuring the amount of heat released or absorbed during a chemical reaction and the device is known as calorimeter.  Watson and O'Neill developed this thermal analysis in 1960 and then introduced it in 1963 commercially at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. PRINCIPLE: The basic principle involved in DSC is that this technique is used to study what is the effect of heating on the polymers/samples. It examines the thermal transitions of polymer or sample when heated. For example, this technique can be employed to study the effect of heating on a crystalline polymer, glass transitions, and crystallization. The sample and reference materials are heated by separate heaters at the same temperature throughout the experiment. The energy which is required to obtain zero temperature difference between sample and reference is measured. INSTRUMENTATION: FIG: SCHEMATIC REPRESENTATION OF DSC APPARATUS  The DSC measurement requires a sample and a reference.  Modern DSC sample and reference pans are small and usually made of aluminum. They may or may not have lids.  Sample size is generally 1-10 mg and the reference pan is left empty, but an inert reference material may be used.
PHARMD GURU Page 3  Commercial DSC equipment can operate at temperatures from -180°C to 700°C, with specialized instruments capable of maximum temperatures of 1600°C.  The DSC must be able to be heated and cooled in a controlled manner.  To achieve the very low end of the temperature range, a special liquid nitrogen cooling accessory is needed. TYPES OF DSC INSTRUMENTATION: There are two main types of DSC instrumentation: 1. Heat flux DSC 2. Power compensated DSC 1) HEAT FLUX DSC: FIG: HEAT FLUX DSC  In a heat flux instrument the same furnace heats both the sample and the reference.  Time temperature is changed in a linear manner while the differential heat flow into the sample and reference is measured.  The sample and reference pans sit on the heated thermoelectric disk, made of a Cu/Ni alloy (constantan).  The differential heat flow to the sample and reference is monitored by area thermocouples attached to the bottom of the sample and reference positions on the thermoelectric disk.
PHARMD GURU Page 4  The differential heat flow into the pans is directly proportional to the difference in the thermocouple signals.  The sample temperature is measured by the alumel/chromel thermocouple under the sample position.  This temperature is an estimated sample temperature because the thermocouple is not inserted into the sample itself.  The accuracy of this temperature will depend on the thermal conductivity of the sample and its container, the heating rate, and other factors.  The sample and reference pans both have lids and the reference pan is an empty pan. 2) POWER COMPENSATED DSC: FIG: POWER COMPENSATED DSC  The major difference in power compensated DSC instruments is that, two separate heating elements are used for the sample and the reference.  A change in temperature between the sample and the reference serves as the signal to “turn on” one of the heaters, so that the sample and the reference stay at the same temperature.  When a phase change, reaction, glass transition, or similar event occurs in the sample, the sample and reference temperatures become different.  This causes extra power to be directed to the cell at the lower temperature in order to heat it.