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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)

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Thermogravimetry

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  1. 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.

  2. BALANCE CONTROLLER GAS IN WEIGHT SAMPLE TEMP. FURNACE TEMP. POWER TEMPERATURE PROGRAMMER instrumentation GAS-TIGHT ENCLOSURE SAMPLE HEATER

  3. balance/furnace configurations

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

  5. 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

  6. 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.

  7. effect of heating rate 10 mg samples of PTFE, heated at 2.5, 5, 10 and 20 °C/min in nitrogen

  8. isoconversion kinetic treatment

  9. lifetime prediction

  10. effect of atmosphere CaC2CO4.H2O in air and nitrogen

  11. 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.

  12. polymer stability studies a = PVC, b= nylon-6, c = LDPE, d= PTFE

  13. compositional analysis of filled rubber

  14. composition of PVAc

  15. LDPE/nylon film

  16. sample controlled TG

  17. comparison of temperature programmes

  18. modulated temperature thermogravimetry

  19. 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

  20. 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).

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