Thermoanalytical techniques

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THERMOANALYTICAL TECHNIQUES

PRESENTED BY: DEEPALI JADHAV, M. PHARMORIENTAL COLLEGE OF PHARMACY,SANPADA,NAVI MUMBAI.GUIDED BY: Mrs. SNEHAL KANE

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IntroductionThermal Analysis is the term applied to a group of methods and techniques in which chemical or physical properties of a substance, a mixture of substances or a reaction mixture are measured as function of temperature or time, while the substances are subjected to a controlled temperature programme.

THERMOANALYTICAL TECHNIQUESDIFFERENTIAL SCANNING CALORIMET

RY(DSC)

Thermogravimetry(TG)

Differential Thermal Analysis

(DTA)

Thermo-Mechanical Analysis(TMA)

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DIFFERENTIAL SCANNING CALORIMETER

Principle : It measures the heat into or out of a sample. The difference in the amount of heat required to

increase the temperature of a sample and reference is measured as a function of temperature.

– The differences in heat flow occur with the occurrence

of two major events:

1) The heat capacity of the sample which increases with

temperature.

2) Transitions occur.

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Instrumentation Basics: 

• Types of DSC Instruments: 

  1. Power compensated DSC

2. Heat flux DSC

3. Modulated DSC  

4. High sensitivity DSC (HS-DSC)

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1.Power compensated DSC:

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• In power compensated DSC, the base of the sample holder unit is in direct contact with a reservoir of coolant.

• Resistance sensor measures the temperature of the base of the holder and a resistance heater.

• Phase changes in sample temperature difference is detected electrical power gets supplied to bring temperature difference below threshold value.

• Temperatures of both are increased or decreased linearly.• The power needed to maintain the sample temperature equal to the

reference temperature is measured.

Advantages over Heat flux DSC:

Its response time is more rapid than heat flux DSC. It is also capable of higher resolution.

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2.Heat flux DSC:

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• In heat flux DSC, the difference in the heat flow into the sample and reference is measured while the sample temperature is changed at a constant rate.

• Both sample and reference are heated by a single heating unit.

• Heat flows into both the sample and reference material via an electrically heated constant thermoelectric disk is given by

where,

f (T, t) is the kinetic response of the sample in J mol ˉˡ

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3.Modulated DSC Instruments (MDSC) : 

• In MDSC, a sinusoidal function is superimposed on the overall temperature program.

• And produces a micro heating and cooling cycle as the overall temperature is increased or decreased.

• The overall signal is mathematically deconvoluted into two parts

1. A reversing heat flow signal- Associated with the heat capacity component of the thermogram .

2. Non- reversing heat flow signal- Associated with the kinetic processes. Step transitions appear only in the reversing heat flow signal. Exothermic and endothermic events may appear in either or in both

signals.

• It can be useful in resolving polymer glass transitions, which are difficult to analyze owing to concomitant solvent evaporation.

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Sample Preparation• If possible, clean and dry your sample prior to DSC analysis.• Wear gloves or use forceps when handling your DSC sample.• DSC samples should be small enough.• Powder samples or flat solid samples (less than 2 mm tall) work best.• Sample weight should be between 0.5 and 100 mg.

DSC sample preparation procedures :• Weigh sample about 0.5 mg with the analytical balance. Record the weight. • Use forceps to place sample. • Use forceps to place the aluminium pan lid on top of your sample. • Use forceps to load the aluminium pan and sample into the sample encapsulating

press .• Align the sample pan in the encapsulating press, and press down on the handle to

seal the aluminium pan. • Use the empty pan as a reference sample. • Need inert gas like nitrogen to avoid oxidation or decomposition.

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Precaution :

Sample Decomposition

The DSC is capable of heating samples to 600°C.

Many materials may decompose during the heating, which can generate hazardous by-products.

WARNING: If you are using samples that may emit harmful gases, vent the gases in an appropriate manner. In general, samples should not be heated above their decomposition temperatures to prevent the release of hazardous materials.

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Instrument Operation

• Powering up the instrument, • Loading your sample, • Setting your testing conditions, • Running a scan and collecting

data • Analyzing your results.

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Applications

Quantitative application includes the determination of heat of fusion and the extent of crystallization for crystalline materials.

Glass transition temperatures and melting points are useful for qualitative classification of materials.

DSC is used to study ‘aging’ and shelf life of pharmaceuticals, as well as other basic research and development.

In the Food Industry, DSC has numerous applications to monitor thermal events discussed earlier such as melting, crystallization, etc, as well as decomposition, denaturation , dehydration, polymorphism, oxidation, etc.

Evaluation of metal catalysts by pressure DSC . Measurement of heat capacity

Measurement of thermal conductivity

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1

T

T

T

T

pp dT

T

CSdTCH

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DIFFERENTIAL THERMAL ANALYSIS(DTA)

• DTA is a technique in which the difference in temperature between a substance and a reference material is measured as a function of temperature while the substance and reference material are subjected to a controlled temperature program.

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Fig: Differential thermogram showing types of changes encountered with polymeric materials.

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PrincipleIt is a technique in which the temperature of the substance under

investigation is compared with the temperature of a thermally inert

material.

Initial increase in ΔT is due to the glass transition.

Tg is the charecteristic temperature at which glassy amorphous

polymers become flexible or rubberlike.

When heated to the glass transition temperature, the polymer changes

from a glass to a rubber and results in no change in enthalpy.

The heat capacity of rubber is different from that of glass , which results

in the lowering of the baseline. No peak appears during this transition

because of the zero enthalpy change.

19Fig.DTA INSTRUMENTATION

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INSTRUMENTATION

Sample holder Sample and reference cells (Al)

SensorsPt or chromel/alumel thermocouples one for the sample and one for the reference joined to differential temperature controller

FurnaceAlumina block containing sample and reference cells

Temperature controllerControls for temperature program and furnace atmosphere

Recording system

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Applications

Qualitative and Quantitative Identification of Minerals: Detection

of any minerals in a sample

Polymeric Materials: DTA useful for the characterization of

polymeric materials in the light of identification of thermo physical ,

thermo chemical, thermo mechanical and thermo elastic changes or

transitions.

Measurement of Crystalline: Measurement of the mass fraction of

crystalline material in semi crystalline polymers.

Analysis of Biological Materials: DTA curves are used to date bone

remains or to study archaeological materials.

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Thermogravimetry (TG) :

• It is the study of weight changes of a specimen as a function of temperature. • This technique is useful for transformations involving the absorption or evolution

of gases from a specimen consisting of a solid phase.

• Reactant(s) Product(s) + Gas (a mass loss)• Gas + Reactant(s) Product(s) (a mass gain)

• A plot of mass versus temperature permits evaluation of thermal stabilities, rate of reaction, reaction processes, and sample composition.

• Measurements of changes in sample mass with temperature are made using thermobalance. The balance should be in a suitably enclosed system so that the atmosphere can be controlled.

  Processes occurring without change in mass (e.g., the melting of a sample)

obviously cannot be studied by TG.

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Perkin Elmer TGA7 Thermogravimetric Analyser

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Instrumentation

1. Recording balance

2. Furnace

3. Furnace programmer or controller

4. Recording device

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1)Microbalance:

It is used to record a change in mass of sample/substance.An ideal microbalance must possess following features:

• It should have accuracy and reproducibility and rapid response.

• It should provide electronic signals to record the change in mass using a recorder.

• It should have the capacity of auto-weight/mass adjustment.

• It should be mechanically and electrically stable and user friendly.

 • The thermobalance has a clamp which is used to hold the microbalance

arm. After the sample has been placed on microbalance, it is left for 10-15 min to stabilize.

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2)Sample holder Available with different size and shape depending upon the weight of sample.Made up of glass, quartz, aluminium, stainless steel etc.

2 TYPES 1)Shallow pan for holding samples which eliminates gas, vapours or volatile matter by diffusion during heating.

2)Deep crucible

3)Furnace

4)Furnace temperature programmero These are the controller which can provide gradual rise of temperature at a fixed rate.o This device has a coarse and fine control knobs through which desired temperature with respect to rate/ time can be obtained.o This controlling is done by increasing voltage through the heated element by motor driven variable transformer or by different thermocouples.

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INTERPRETATION OF TG

• We can consider that the TG curve directly indicates the characteristic of that compound.

• This is due to the sequence of physicochemical events that occur under particular conditions over the temperature range

• Ex:1. calcium carbonate

decomposes –single step(800oC–900o C)

forms calcium oxide (a stable solid) & gas CO2

CaCO3 (s) CaO (s) + CO2 (g)

• Ex: 2 ammoniun nitrate

NH4NO3 (g) N2O (g) + 2H20 (g)

(300 Oc) (two volatile prods)

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• Ex: calcium oxalate monohydrate (CaC2O4.H2O) is heated in air up to 1000 oC.

• We see a well separated three thermal decompositions

Step 1: hydrate water is lost:

CaC2O4. H2O (s) CaC2O4 (s) + H2O

Step 2 : Anhydrous salt decomposes

CaC2O4 (s) CaCO3 (s) + CO (g)

Step 3: carbonate decomposes.

CaCO3 (s) CaO (s) + CO2 (g)

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APPLICATIONS OF TG :

Determining the purity and thermal stability of both primary and secondary standards.

Investing the correct drying temperature and the suitability of various weighing forms for gravimetric analysis.

Determining the composition of alloys and mixtures.

Eg. For calcium and strontium binary mixture. Here the decomposition temperature for CaCO3 is 65J C to 85J C.

Where for strontium carbonate the decomposition temperature is 95JC to 115J C.

Both carbonates decompose to their oxides with the evolution of CO2.

Hence amount of Ca and strontinum present in the mixture may be calculated from weight losses due to the evolution of CO2 at the lower and higher temperature respectively.

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Thermo-Mechanical Analysis(TMA)

It provides measurements of penetration, expansion, contraction and extension of materials as a function of temperature.

Volume expansion characteristics of samples are measured by placing the sample in a quartz cylinder fitted with a flat tipped quartz probe in a cylinder piston arrangement.

Sample volume changes are translated into linear motion of the piston. In this technique, dimensional changes in a sample are the primary

measurement, while the sample is heated, cooled, or studied at a fixed temperature.

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Principle: A dilatometer is used to determine the linear thermal expansion of a

solid as a function of temperature. In this a small load acts on the specimen . The measured expansion of the specimen can be used to determine

the coefficient of linear thermal expansion . The 1st heating phase yields information about the actual state of

the specimen, including its thermal and mechanical history. When thermoplastics soften, especially above the glass transition, orientations and stresses may relax, as a result of which post crystallization and recrystallization processes may occur.

To determine the coefficient of expansion as a material characteristic, the material must undergo reversible changes and so forth during a 2nd heating phase that has followed controlled cooling.

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Instrumentation:

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The sample sits on a support within the furnace. Resting upon it is a probe to sense changes in length, which are measured by a sensitive position transducer, normally a Linear Variable Displacement Transducer (LVDT).

The probe and support are made from a material such as quartz glass (vitreous silica), which has a low, reproducible, and accurately known coefficient of thermal expansion, and also has low thermal conductivity, which helps to isolate the sensitive transducer from the changing temperatures in the furnace.

The quartz probe contains a thermocouple which measures the sample temperature.

Any movement of the sample is translated into a movement of the transformer core and results in an output that is proportional to the displacement of the probe, and whose sign is indicative of the direction of movement.

The temperature range is from liquid nitrogen to 850 JC. In this technique (TMA), dimensional changes in a sample are the primary

measurement, while the sample is heated, cooled, or studied at a fixed temperature.

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Typical probes:

•Where the specimen is large enough, a macroprobe with a contact area of approx. 28 mm2 is used.• For smaller specimens, for example, with an edge length less than 6 mm, a normal probe with a contact area of approx. 5 mm2 is suitable.

•Penetration probes with a small contact area of just about 0.8 mm2 are suitable for determining the glass transition temperature.

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Calibration• The TMA apparatus must be calibrated in terms of length and

temperature. If the loading force is applied electromagnetically, it may be calibrated with the aid of known weights.

• A new calibration must be performed every time a probe is changed. Calibration and actual test measurements must be performed using the same probe geometry, applied load, heating rate, and temperature range (starting temperature). The specimen should be the same size as the calibration specimen.

• The calibration factor, F, is calculated as follows:

F=

Calibrating the Length Calibrating the Temperature

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Samples

• TMA samples should be of 2-6 mm in diameter and 2-10 mm in

height. • Samples for thermal expansion and glass transition measurements

should have parallel and flat top and bottom surfaces. • NOTE: To avoid making a mess with molten thermoplastics, you

should encapsulate thermoplastic polymer samples in aluminum foil.

• To avoid reactions between molten metals and the quartz probe and stage, you should sandwich your metallic sample between two plates of aluminum oxide.

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Instrument Operation

• Powering up the instrument, • Loading your sample, • Setting your testing conditions, • Running a scan and collecting data, and Analyzing your results.

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Precaution:

Sample Decomposition. The TMA is capable of heating samples to 1000°C.

Many materials may decompose during the heating, which can generate hazardous by products.

WARNING: If you are using samples that may emit harmful gases, vent the gases in an appropriate manner. In general, samples should not be heated above their decomposition temperatures to prevent the release of hazardous materials or contamination of the TMA

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APPLICATIONS:

Thermo mechanical measurements are routinely used to investigate mechanical stability and measure thermal expansion coefficients.

Use of the technique on the small scale as a micro-analytical tool to identify the distribution of two materials within a matrix and also on a larger scale to investigate the firing of a ceramic material.

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REFERENCES

1. Mita, T., “Recent Advances in Heat-Resistant Polymers.General Technical Centre

“Publication,No 8,p.117.

2. D. A. Skoog et al., “Principles of instrumental analysis”, fifth edition, Harcourt

Publishers, 2001.

3. Willard.”Instrumental methods of analysis”.seventh edition.pg. 761-776

4. Andrew J.Paasztor.”Handbook of instrumental techniques for analytical

chemistry”.pg.909-927

5. H.Kaur.”Instrumental methods of chemical analysis”.pg.979-991

6. Gurdeep Chatwal.”Thermal methods”.pg.2.701-2.759

7. Blaine,R.L.,Du Pont “Thermal Analysis Technical Report”,No.TA-40

Wilmington,DE,Du pont,1989

8. Sharma B.K. Goel Publishing House “Instrumental Methods of Analysis” pg. 234-237

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