A seminar on

THERMOGRAVIMETRIC ANALYSIS Presented by:

Princy Agarwal Ist SemesterM.Pharma. (QA)

B.N.I.P.S., Udaipur

Guided by:Dr. (Mrs.) Anju Goyal

Professor & HeadDept. of Pharmaceutical Chemistry

CONTENTS :Thermal AnalysisDifferent thermal analytical methodsThermogravimetryTypes of ThermogravimetryPrincipleDescriptionRecording of resultInformation obtained from a TG curveFactors affecting a TG curveInstrumentationApplications

THERMAL ANALYSIS The term “thermal analysis” incorporates those techniques in

which some physical parameters of the system is determined and/or recorded as a function of temperature.

When matter is heated it undergoes certain physical and chemical changes. These changes that take place when an unknown sample is heated provide us with information that enables us to identify the material.

Based on the above definition, the various techniques of thermal analysis are summarised in the table:

S.No NAME OF THE TECHNIQUE

ABBREVIATION OF THE

TECHNIQUE

INSTRUMENT EMPLOYED

PARAMETER MEASURED

GRAPH

1. THERMOGARVIMETRY TG Thermobalance Mass Mass vs. Temperature

or Time

2. DERIVATIVE THERMOGRAVIMETRY

DTG Thermobalance dm/dt dm/dt vs. Temperature

3. DIFFERENTIAL THERMAL ANALYSIS

DTA DTA Apparatus ΔT ΔT vs. Temperature

4. DIFFERENTIAL SCANNING

CALORIMETRY

DSC Calorimeter dH/dt dH/dt vs. Temperature

5. THERMOMETRIC TITRIMETRY

- Calorimeter Temperature Temperaturevs. Titrant Volume

DIFFERENT THERMAL ANALYTICAL TECHNIQUES

S.No NAME OF THE TECHNIQUE

ABBREVIATION OF THE

TECHNIQUE

INSTRUMENT EMPLOYED

PARAMETER MEASURED

GRAPH

6.DYNAMIC

REFLECTANCE SPECTROSCOPY

DRS Spectro-photometer

Reflectance % Reflectancevs. Temperature

7. EVOLVED GA S DETECTION

EGD Thermal conductivity cell

Thermal conductivity

(T.C.)

T.C. vs. Temperature

8. THERMO-MECHANICAL

ANALYSIS (DIALOTOMETRY)

TMA Dilatometer Volume or Length

Volume or Length

vs. Temperature

9. ELECTRICAL CONDUCTIVITY

EC Electrometer or Bridget

Current (I)or Resistance

(R)

I or R vs. Temperature

10. EMANATION THERMAL ANALYSIS

ETA ETA Apparatus Radioactivity (E)

E vs. Temperature

According to Wendlandt,“ Any analytical instrument technique is regarded as a

thermal analysis method if the physical parameter is measured as a function of temperature (or time).

According to this definition, PROTON NUCLEAR MAGNETIC RESONANCE,ELECTRON SPIN RESONANCE,ELECTRON DIFFRACTION,X- RAY DIFFRACTION,MASS SPECTROMETRY,UV, Visible and IR SPECTROPHOTOMETRY

are thermal methods.

THERMOGRAVIMETRYIntroduction: It is a method of thermal analysis in which a physical

property of substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature programmer.

An Internationally accepted definition of thermogravimetry is as follows:

"It is a technique where by weight of substance in an environment heated or cooled at a controlled rate, is recorded as a function of time or temperature.”

TYPES OF THERMOGRAVIMETRY : There are three types of Thermogravimetry: Isothermal / Static Thermogravimetry Quasistatic Thermogravimetry Dynamic Thermogravimetry

1. Isothermal/ Static Thermogravimetry: In this technique the sample weight is recorded as a function of time at constant temperature.

2. Quasistatic Thermogravimetry: In this technique the sample is heated to constant weight at each of the series of increasing temperature.

3. Dynamic Thermogravimetry: In this technique a sample is heated in an environment whose temperature is changing in predetermine manner generally at linear rate.

Most of the studies are generally carried out with dynamic thermogravimetry. Therefore it is generally referred to as thermogravimetry.

PRINCIPLE:- The principle of thermogravimetry is based on the simple fact

that the sample is weighed continuously as it is being heated to elevated temperatures and changes in the mass of a sample are studied.

Changes in temperature affect the sample. Not all thermal changes/events bring a change in mass of sample i.e. melting, crystallization but some thermal events i.e. desorption, absorption, sublimation, vaporization, oxidation, reduction and decomposition bring a drastic change in mass of sample.

It is used in analysis of volatile products, gaseous products lost during the reaction in thermoplastics, thermosets, elastomers, composites, films, fibers, coatings, paints, etc.

 

DESCRIPTION: It is a technique which is studied under thermal analysis and is

employed for detection of such type of materials which undergo mass change (gain or loss) when subjected to thermal events viz. decomposition, oxidation, reduction, etc.

For this reason, it is very significant to optimize those conditions/factors on which the change of mass of sample depend throughout the operation/experiment.

RECORDING OF RESULT: The instrument used for themogravimetry is a programmed precision

balance for rise in temperature known as Thermobalance. Results are displayed by a plot of mass change versus temperature or

time and are known as Thermogravimetric curves or TG curves.TG curves are normally plotted with the mass change (Dm) in

percentage on the y-axis and temperature (T) or time (t) on the x-axis. A typical TG curve has been shown (Figure 1).

Fig.1.Characteristics of a single-stage mass-loss

curve 

There are two temperatures in the reaction, Ti (procedural decomposition temp.) representing the lowest

temperature at which the onset of a mass change is seen Tf (final temp.) representing the lowest temperature at which

the process has been completed respectively.

The reaction temperature and interval (Tf-Ti) depend on the experimental condition; therefore, they do not have any fixed value.

INFORMATION OBTAINED FROM A TG CURVE:

Plateau: A plateau (AB, Fig.2.) is that part of the TG curve where the mass is essentially constant or there is no change in mass. Procedural Decomposition Temperature: The initial temperature, Ti, (B, Fig.2.) is that temperature (in Celsius or Kelvin) at which the cumulative-mass

change reaches a magnitude that the Thermobalance can detect. Final Temperature: The final temperature, Tf, (C, Fig.3.), is that

temperature (in Celsius or Kelvin) at which the cumulative mass change reaches a maximum.

Reaction Interval: The reaction interval is the temperature difference between Tf and Ti.

It can be concluded that Thermogravimetry is concerned with the change in weight of a material as its temperature changes.

First, this determines the temperature at which the material loses weight. This loss indicated decomposition or evaporation of the sample.

Second, the temperature at which no weight loss takes place is revealed, which indicates stability of the material.

These temperature ranges are physical properties of chemical compounds and can be used for their identification.

INSTRUMENTAL FACTORS

SAMPLE CHARACTERISTICS

FACTORS AFFECTING THERMOGRAVIMETRIC CURVE

►HEATING RATE

►EFFECT OF FURNANCE ATMOSPHERE

►SAMPLE HOLDER

► WEIGHT OF THE SAMPLE

►SAMPLE PARTICLE SIZE

►HEAT OF REACTION

►COMPACTNESS OF THE SAMPLE

►PREVIOUS HISTORY OF THE SAMPLE

INSTRUMENTAL FACTORS:EFFECT OF FURNANCE ATMOSPHERE: the test samples are

generally heated in vacuo or in the presence of an inert gas, in order to remove the gases formed during sample heating and also to prevent the occurrence of any undesirable reactions. The common atmospheres involved in thermogravimetry are as follows:

1. Static air: In this type air from atmosphere is allowed to flow through the furnace.

2. Dynamic air: In this type compressed air from a cylinder is allowed to pass through the furnace at a measured flow rate.

3. Inert atmosphere: Nitrogen gas (oxygen free) is used as inert environment.

HEATING RATE: If a substance is being heated at a fast heating rate, the temperature of decomposition will be higher than that obtained at a slower rate of heating. Eg.:- for a 10% decomposition of polystyrene, the temperatures are : 375˚C for a heating rate of 1˚C / min and 394 ˚C for a heating rate of 5˚C / min.

SAMPLE HOLDER: The geometry of the sample holder can change the slope of TG curve. Sample holders range from flat plates to deep crucibles of various capacities. Materials used in their construction may vary from glass, alumina, and ceramic compositions to various metals and metallic alloys.

When the atmosphere is solely the gas, the shape of crucible has no effect on the slope of the curve. Generally shallow dish is preferred as there is a rapid exchange of gases between sample and the surrounding atmosphere.

SAMPLE CHARACTERISTICS:Weight of the sample: If a large sample is used, there occurs a

deviation from linearity as the temperature rises, especially for an exothermic reaction. Eg.: evolution of CO during decomposition of calcium oxalate to CaCO3.

Sample particle size: With the particle size of smaller dimension the decomposition takes place earlier, while with greater particle size the decomposition proceeds only at higher temperatures.

Previous history of the sample: Eg.: TG studies showed that Mg(OH)2 prepared by precipitation method has a different temperature of decomposition from that of the naturally occurring material. This factor shows that one should be sure about the source or method of formation of the sample.

Heat of reaction: This effect was studied by Newkirk. The heat of reaction alters the difference between the sample temperature and the furnace temperature. If the heat effect is exothermic or endothermic, this will cause the sample temperature to lead or lag behind the furnace temperature.

Compactness of the sample: A compressed sample will decompose at higher temperatures than a loose sample.

INSTRUMENTATION: Components of Instrumentation: :  A. Recording balance B. Sample Holder C. Furnace D. Furnace temperature programmer/Controller

E. RecorderTG curves are recorded using a Thermobalance. It consists

of an electronic microbalance, a furnace, a temperature programmer and a recorder (instrument connected to Thermobalance to record the output/curves).

Block diagram of a Thermobalance:

Microbalance: It is the most important component of Thermobalance.

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

a. It should accurately and reproducibly record the change in mass of sample in wide ranges of atmospheric conditions and temperatures.

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

c. The electronic signals should provide rapid response to change in mass. d. It should be stable at high ranges, mechanically and electrically. e. Modern microbalances have the ability to be not affected by vibrations. f. Its operation should be user friendly.

After the sample has been placed on microbalance, it is left for 10-15 min to stabilize. Recorder balances are of two types:

1. Deflection-type instruments and2. Null-type instruments.

Deflection Balances: They are of following types: I. Beam Type  ii. Helical Type iii. Cantilevered Beam iv. Torsion Wire

● Null-Point Balances : It consists of a sensor which detects the deviation from the null point and restores the balance to its null point by means of a restoring force.

Different Types of Deflection Balances

Null Type balance

 Sample Holder or Crucible: The sample to be studied is placed in sample holder or

crucible. It is attached to the weighing arm of microbalance. There are different varieties of crucibles used. Some differ

in shape and size while some differ in materials used.They are made from platinum, aluminum, quartz or

alumina and some other materials like graphite, stainless steel, glass, etc.

Crucibles should have temperature at least 100 K greater than temperature range of experiment and must transfer heat uniformly to sample. Therefore, the shape, thermal conductivity and thermal mass of crucibles are important which depends on the weight and nature of sample and temperature ranges.

There are different types of crucibles. They are:1. Shallow Pans: These are used for such samples in which diffusion is

the rate controlling step. Volatile substances produced during reaction must escape out which is determined as weight loss.

2. Deep Crucibles: These are used in such cases where side reactions are required such as in study of industrial scale calcinations, surface area measurements, etc.

3. Loosely covered Crucibles: These are used in self-generated atmospheric studies. Rate of temperature or weight loss is not important because the studies are done isothermally.

4. Retort Cups: These are used in boiling point studies. It provides single plate of reflux for a boiling point determination.

Different types of crucibles are used for different materials i.e. Flat crucibles with small lip are used for powdered sample whereas walled crucibles are used for liquid samples. Therefore, the form of crucibles used will determine the temperature gradients in sample.

 Furnace (Heater/Boiler/Oven): The furnace should be designed in such a way that it produces a linear

heating range. It should have a hot zone which can hold sample and crucible and its

temperature corresponds to the temperature of furnace. There are different combinations of microbalance and furnace available. The

furnace heating coil should be wound in such a way that there is no magnetic interaction between coil and sample or there can cause apparent mass change.

Coils used are made of different materials with variant temperature changes viz.

Nichrome wire or ribbon for T<1300 K, Platinum for T>1300 K, Platinum-10% rhodium Alloy for T<1800 K.The size of furnace is important. A high mass furnace may have a high range

of temperature and obtain uniform hot zone but requires more time to achieve the desired temperature. Comparatively, a low mass furnace may heat quickly but it’s very difficult to control rise in temperature and maintain hot zone.

Position of furnace with respect to balance

Temperature Measurement:It is done with the help of thermocouple. Different materials are used for measuring different ranges of

temperatures i.e. chromal or alumel (alloys of Platinum) thermocouples are used for T=11000 C, tungsten or rhenium thermocouples are used for higher temperature.

The position of thermocouple is important. It can be adjusted in following ways:

i. Thermocouple is placed near the sample container and has no contact with sample container. This arrangement in not preferred in low-pressures. ii. The sample is kept inside the sample holder but not in contact with it. It responds to small temperature changes

only. iii. Thermocouple is placed either in contact with sample or with sample

container. This method is best and commonly employed.

Position of thermocouple in a Thermobalance

Recorder:The recording systems are mainly of 2 types:1. Time-base potentiometric strip chart recorder.2. X-Y recorder.

In some instruments, light beam galvanometer, photographic paper recorders or one recorder with two or more pens are also used.

In the X-Y recorder, we get curves having plot of weights directly against temperatures.

However, the percentage mass change against temperature or time would be more useful.

Applications:

Thermal StabilityMaterial characterizationCompositional analysis Used to analyze filler content in polymers; carbon black in

oils; ash and carbon in coals.Kinetic StudiesCorrosion studiesAutomatic Thermogravimetric AnalysisEvaluation of gravimetric precipitatesEvaluation of suitable standardsTesting of purity of samplesCurie point determination

REFERENCES:- Skoog, Douglas A, F James Holler and Timothy Nieman, Principles Of Instrumental

Analysis, 5th edition New york 2001. H.H.Willard, L.L Merrit Jr.J.A Dean, F.A Settle Jr.Intrumetal Method Of

Anlysis,Wadsworth Publishing Company,USA 1986 Gurdeep R.Chatwal, Sham K. Anad, Instrumental Method Of Chemical Analysis,5th

edition, Himalaya Publishing house,Page No-2.701. T. Hatakeyama and F.X. Quinn, Jhon Wiley and Sons Publications, 1999. Thermal

Analysis Fundamentals and Applications to Polymer Science: Thermogravimetry, pg. 45-71, 2nd Ed.

Coats, A. W.; Redfern, J. P., 1963. "Thermogravimetric Analysis: A Review", pg. 88: 906–924.

Fleming Polymer Testing And Consultancy, http://www.flemingptc.co.uk/our-services/dsc-tga.

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

Thermal Analysis Dr. S. Anandhan, Asst. Professor, Dept. of Met. And Mat. Engg., NITK

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