Thermogravimetry
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Thermogravimetry. “…. 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)

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Thermogravimetry
Thermogravimetry

“…. 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.


Instrumentation

BALANCE

CONTROLLER

GAS IN

WEIGHT

SAMPLE TEMP.

FURNACE TEMP.

POWER

TEMPERATURE PROGRAMMER

instrumentation

GAS-TIGHT

ENCLOSURE

SAMPLE

HEATER



Example curve
example curve

Mass (%) in green, rate of mass loss (%/°C) in blue.


Physical limitations on the heating process
physical limitations on the heating process

EXCHANGE OF GASES: REACTING GASES IN, PRODUCTS OUT

CONVECTION THROUGH SURROUNDING ATMOSPHERE

RADIATION FROM FURNACE WALL

CONDUCTION THROUGH SAMPLE PAN AND INSTRUMENT

INDICATION OF SAMPLE TEMPERATURE


Factors that affect the results
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

For a given instrument, careful standardisation of experimental procedures leads to highly reproducible results.


Effect of heating rate
effect of heating rate

10 mg samples of PTFE, heated at 2.5, 5, 10 and 20 °C/min in nitrogen




Effect of atmosphere
effect of atmosphere

CaC2CO4.H2O in air and nitrogen


Sources of error
sources of error

  • A) MASS

  • Classical buoyancy

  • Effect temp. on balance

  • convection and/or turbulence

  • viscous drag on suspension

  • NOISY OR ERRATIC RECORDS

  • CAN ARISE FROM:

  • static

  • vibration

  • pressure pulses in lab.

  • uneven gas flow

These are lumped together as the “buoyancy” correction, and if significant, can be allowed for by a blank run

B) TEMPERATURE

Temperature calibration difficult to carry out accurately.

Many methods exist, but none totally satisfactory.

Best accuracy from simultaneous TG-DTA or TG-DSC instrument.


Polymer stability studies
polymer stability studies

a = PVC, b= nylon-6, c = LDPE, d= PTFE








Summary

PROCESS WEIGHT GAIN WEIGHT LOSS

Ad- or absorption 

Desorption, drying 

Dehydration, desolvation 

Sublimation 

Vaporisation 

Decomposition 

Solid-solid reactions (some) 

Solid-gas reactions 

Magnetic transitions 

summary


Recommended reading
recommended reading

D. M. Price, D. J. Hourston & F. Dumont, “Thermogravimetry of Polymers”, R. A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, John Wiley & Sons Ltd., Chichester (2000) pp. 8094-8105.

G. R. Heal, “Thermogravimetry& Derivative Thermogravimetry”, in P.J. Haines (ed.) Principles of Thermal Analysis & Calorimetry, ch. 4, Royal Society of Chemistry, Cambridge (2002) pp. 10-54.

C. M. Earnest (Ed.), Compostional Analysis by Thermogravimetry, ASTM STP 97, American Society for Testing and Materials (1988).