Thermogravimetry

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 pointw

eig

ht

chan

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

ΔT

SR

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).