Introduction to thermogravimetry and applications

1. Thermogravimetric Analysis
Guided By
Prof. (Mr.) Mukesh T. Mohite Sir
Submitted By
Tejas Chandrakant Jagtap
M. Pharm Pharmaceutics
(1st Sem)
Dr. D. Y. Patil College Of Pharmacy,
Akurdi, Pune - 411033

2. Contents -
• Definition
• Principle
• Instrumentation
• Example Curve
• Applications
• Limitations
• Factors Affecting Results
• References

3. Definition
“It is a technique in which the mass of a substance is
measured as a function of temperature, while the
substance is subjected to a controlled temperature
programme.”
“Controlled Temperature Programme” can mean :-
• Heating and/or cooling at a linear rate (by far commonest)
• Isothermal measurements
• Combinations of heating, cooling and isothermal stages
• Other, more modern approaches, in which the temperature
profile is modified according to the behaviour of the sample.
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4. Principle
• Changes in the mass of a sample are studied while the sample
is subjected to a program.
• Changes in temperature affect the sample. All Thermal
changes/events not 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.
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5. Instrumentation
 Sample holder (metallic/ceramic pans)
 Microbalance
 Programmable heater (furnace)
 Temperature control (thermostat)
 Temperature sensor (thermocouple)
 Read-out
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6. Sample Holder
• Geometry , size and material of sample holder or crucible
have marked influence on the shape of a TG curve
• Nature, weight, and maximum temp range to be studied
depend upon size and shape of sample holder
• Materials used for sample holder are: Glass, Quartz, Alumina,
Stainless steel, Platinum, Graphite, Metals and Various Alloys
• Crucibles should have temperature at least 100 K greater
than temperature range of experiment and must transfer
heat uniformly to sample.
• In Practice, Four main types of sample holders have been
used in TGA
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7. A) Shallow Pans B) Deep Crucible
C) Loosely covered crucibles
D)Retort Cups
Fig. Different Types of Sample Holders 5

8. Microbalance
Working
• After a mass change in sample,
• Step 1. balance beam is deflected.
• Step 2. Disturbance of the balance beam will change the shutter
position to develop a current in photodiode as a result of the
upcoming light from the lamp.
• Step 3. So, the imbalance induces a current in the magnetic foil.
• Step 4. The current in the magnetic foil generates a magnetic field
that forces the balance beam to its original position.
• Step 5. Photodiode current is amplified.
• Step 6. The amplified current which is translated to give mass-loss
information.
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9. Furnace
Temperature Range at which furnace is to be operated is up to
2000°C in some cases it may be up to 2400°C.
Construction of furnace is such that, it allow easy access to the
sample holder and also to cool down rapidly after the
completion of run.
Design of the furnace should be able to provide suitable smooth
input so that it can maintain either a linear heating programme
or fixed temp.
For heating or maintaining temp inside furnace Nichrome and
Pt-Rh windings are to be used (1000°c & 1450°C resp).
Also Graphite tube furnace is used for obtaining higher temp,
but difficulty arises in the control and measurement of the
temp.
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10. Balance/furnace Configurations
(a)
Vertically Parallel
(c)
Horizontally
perpendicular
(b)
Vertically
Opposite
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11. Temperature Control
(Thermostat)
A thermostat is a component which senses the temperature of
a system so that the system's temperature is maintained near a
desired set point. The thermostat does this by switching heating
or cooling devices on or off, or regulating the flow of a heat
transfer fluid as needed, to maintain the correct temperature.
Common sensor technologies in use today include:
a) Bimetallic mechanical or electrical sensors.
b) Expanding Wax Pellets
c) Electronic thermistor and semiconductor devices
d) Electrical thermocouples
These may then control the heating or cooling apparatus using:
a) Direct mechanical control
b) Electrical signals
c) Pneumatic signals
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12. Temperature Sensor (Thermocouple)
A thermocouple is an electrical device consisting of two
different conductors forming electrical junctions at
differing temperatures. A thermocouple produces a
temperature-dependent voltage as a result of the thermoelectric
effect, and this voltage can be interpreted to measure
temperature. Thermocouples are a widely used type
of temperature sensor.
Commercial thermocouples are inexpensive, interchangeable,
are supplied with standard connectors, and can measure a wide
range of temperatures. In contrast to most other methods of
temperature measurement, thermocouples are self powered and
require no external form of excitation. The main limitation
with thermocouples is accuracy; system errors of less than one
degree Celsius (°C) can be difficult to achieve.
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13. Read Out
The output from the microbalance and furnace are recorded
using either chart recorder or a microcomputer (work station).
The advantage of microcomputer over chart recorder is that the
microcomputer comes with such software which allows data to
be saved and plotted while performing mathematical problems.
Multiple TG curves can be plotted simultaneously. In such
microcomputers, we get plot of weight change vs temperature
or time.
Some other recoding systems are also available viz. X-Y
recorders and Time-Base Potentiometric Strip Chart Recorder.
In X-Y Recorder, plot of weight directly against temperature is
presented.
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14. Instrumentation
TEMPERATURE PROGRAMMER
BALANCE
CONTROLLER
POWER FURNACE TEMP.
SAMPLE TEMP.
WEIGHTGAS IN
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15. Example Curve
CaC2O4.H2O → CaC2O4 + H2O T ~ 100 °C
CaC2O4 → CaCO3 + CO T ~ 500 °C
CaCO3 → CaO + CO2 T ~ 800 °C
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16. Applications
I. Thermal Stability: related materials can be compared at
elevated temperatures under the required atmosphere. The TG
curve can help to elucidate decomposition mechanisms.
II. Material characterization: TG curves can be used to
"fingerprint" materials for identification or quality control.
III.Compositional analysis: by careful choice of temperature
programming and gaseous environment, many complex
materials or mixtures may be analyzed by decomposing or
removing their components. It is used to analyze e.g. filler
content in polymers; carbon black in oils; ash and carbon in
coals, and the moisture content of many substances.
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17. Applications
IV. Simulation of industrial processes: the thermobalance
furnace is thought as mini-reactor and has ability to perform
operations like some types of industrial reactors.
V. Kinetic Studies: by understanding the controlling chemistry
or predictive studies, a variety of methods can be used to
analyze the kinetic features of weight loss or gain.
VI. Corrosion studies: TG provides a means of studying
oxidation or some reactions with other reactive gases or
vapours.
VII.Polymer Identification and separation: TG used for
determination of various polymers as well as identification
and thermo sensitivity of polymers as well as stability of
polymer
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18. Factors That Affect The Results
A) Instrumental
• Heating rate
• Furnace atmosphere
and flow-rate
• Geometry of pan and
furnace
• Material of pan
B) Sample-Related
• mass
• particle size
• sample history/pre-treatment
• packing
• thermal conductivity
• heat of reaction
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19. Physical Limitations On The Heating
Process
RADIATION FROM
FURNACE WALL
INDICATION OF SAMPLE
TEMPERATURE
CONDUCTION THROUGH
SAMPLE PAN AND
INSTRUMENT
CONVECTION THROUGH
SURROUNDING
ATMOSPHERE
EXCHANGE OF GASES:
REACTING GASES IN,
PRODUCTS OUT
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20. References
1. Gurdeep R. Chatwal, Sham K. Anand, Himalaya Publishing
House, 2002. Instrumental Methods Of Chemical Analysis:
Thermal Methods, pg. 2.701, 5th Ed.
2. 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.
3. Coats, A. W.; Redfern, J. P., 1963. "Thermogravimetric
Analysis: A Review", pg. 88: 906–924.
4. Wikipedia: The Free Encyclopaedia
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21. References
5. B. K. Sharma, GOEL Publishing House, 2013. Instrumental
Methods of Chemical Analysis, pg. M-309.
6. Thermal Analysis Dr. S. Anandhan, Asst. Professor, Dept. of
Met. And Mat. Engg., NITK
7. D. A. Skoog et al., Principles of instrumental analysis, fifth
edition, Harcourt Publishers, 2001.
8. Analytical Compendium, Chapter 5, Section 2,
http://old.iupac.org/publications/analytical_compendium/Ch
a05sec2.pdf
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