31
definitions
“…. 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
GAS-TIGHT
ENCLOSURE
SAMPLE
HEATER
TEMPERATURE PROGRAMMER
BALANCE
CONTROLLER
POWER FURNACE TEMP.
SAMPLE TEMP.
WEIGHTGAS IN
balance/furnace configurations
example curve
Mass (%) in green, rate of mass loss (%/°C) in blue.
physical limitations on the heating process
CONVECTION
THROUGH
SURROUNDING
ATMOSPHERE
CONDUCTION
THROUGH SAMPLE
PAN AND
INSTRUMENT
RADIATION FROM
FURNACE WALL
EXCHANGE OF GASES:
REACTING GASES IN,
PRODUCTS OUT
INDICATION OF SAMPLE
TEMPERATURE
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
10 mg samples of PTFE, heated at 2.5, 5, 10 and 20 °C/min in nitrogen
isoconversion kinetic treatment
lifetime prediction
effect of atmosphere
CaC2CO4.H2O in air and nitrogen
sources of error
A) MASS
• Classical buoyancy
• Effect temp. on balance
• convection and/or turbulence
• viscous drag on suspension
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.
These are lumped together as the “buoyancy”
correction, and if significant, can be allowed
for by a blank run
NOISY OR ERRATIC RECORDS
CAN ARISE FROM:
• static
• vibration
• pressure pulses in lab.
• uneven gas flow
calibration
MASS - Use standard weights.
Use standard samples to check operation, but unwise to use them
as weight standards.
TEMPERATURE -
Four approaches:
• Observe deflection on Temperature/time curve
• Curie-point standards
• Drop-weight methods
• In simultaneous-type units, use melting standards
• DO NOT use decomposition events to define temperature.
calibration using sample thermocouple
calibration using drop weight
calibration using curie point
weig
ht
ch
an
ge/
mg
temperature /°C
350 370
0.0
0.2
Nickel metal
3°C/min. in nitrogen
calibration using TG-DTA
10mg Gold (99.999%), 10°C/min, alumina pan, air.
polymer stability studies
a = PVC, b= nylon-6, c = LDPE, d= PTFE
compositional analysis of filled rubber
composition of PVAc
LDPE/nylon film
sample controlled TG
comparison of temperature programmes
modulated temperature thermogravimetry
thermogravimetry-EGA by mass
spectroscopy
simultaneous DSC/DTA-thermogravimetry
TR
Δ
T
S
R
simultaneous DSC-thermogravimetry
TGA-MS of PVC
thermogravimetry-EGA by FTIR
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
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).
