Transcript Thermogravimetry

Thermogravimetry
Duncan Price
IPTME, Loughborough University
© Copyright: [email protected] (2006)
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 IN
WEIGHT
BALANCE
CONTROLLER
GAS-TIGHT
ENCLOSURE
SAMPLE
HEATER
SAMPLE TEMP.
POWER
FURNACE TEMP.
TEMPERATURE PROGRAMMER
balance/furnace configurations
example curve
Mass (%) in green, rate of mass loss (%/°C) in blue.
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
A) INSTRUMENTAL
B) SAMPLE-RELATED
• heating rate
• mass
• furnace atmosphere and flow-rate
• particle size
• geometry of pan and furnace
• sample history/pre-treatment
• material of pan
• 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
NOISY OR ERRATIC RECORDS
• Classical buoyancy
CAN ARISE FROM:
• Effect temp. on balance
• static
• convection and/or turbulence
• vibration
• viscous drag on suspension
• pressure pulses in lab.
These are lumped together as the “buoyancy”
• uneven gas flow
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.
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
weight change/ mg
calibration using curie point
0.2
Nickel metal
0.0
3°C/min. in nitrogen
350
370
temperature /°C
calibration using TG-DTA
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
Δ
T
TR
S
R
simultaneous DSC-thermogravimetry
TGA-MS of PVC
thermogravimetry-EGA by FTIR
summary
PROCESS
Ad- or absorption
WEIGHT GAIN
WEIGHT LOSS

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).