Thermogravimetric Analysis Theory, Operation, Calibration and Data Interpretation

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Thermogravimetric AnalysisTheory, Operation,
Calibration and Data Interpretation

• Prepared by Kadine Mohomed, Ph.D
• Thermal Applications Chemist
• TA Instruments

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Agenda TGA Theory, Operation and Calibration

• Definitions and review of instrument
• Balance, furnace and heat exchanger review
• Mass and temperature calibration
• Purge gas considerations
• Baseline considerations
• Sample preparation and pan selection
• Method development

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TGA The Technique

• Thermogravimetric Analysis (TGA) measures the
  amount and rate of change in the weight of a
  material as a function of temperature or time in
  a controlled atmosphere.
• Measurements are used primarily to determine the
  composition of materials and to predict their
  thermal stability at temperatures up to 1000C.
• The technique can characterize materials that
  exhibit weight loss or gain due to decomposition,
  oxidation, or dehydration.

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What TGA Can Tell You

• Thermal Stability of Materials
• Oxidative Stability of Materials
• Composition of Multi-component Systems
• Estimated Lifetime of a Product
• Decomposition Kinetics of Materials
• The Effect of Reactive or Corrosive Atmospheres
  on Materials
• Moisture and Volatiles Content of Materials

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Calcium Oxalate Example

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Mechanisms of Weight Change in TGA

• Weight Loss
• Decomposition The breaking apart of chemical
  bonds.
• Evaporation The loss of volatiles with elevated
  temperature.
• Reduction Interaction of sample to a reducing
  atmosphere (hydrogen, ammonia, etc).
• Desorption.
• Weight Gain
• Oxidation Interaction of the sample with an
  oxidizing atmosphere.
• Absorption.
• All of these are kinetic processes (i.e. there is
  a rate at which they occur).

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Features of the Q500/ Q50 TGA

• The Q500 is a research grade thermogravimetric
  analyzer, whose leading performance arises from a
  responsive low-mass furnace sensitive
  thermobalance, and efficient horizontal purge gas
  system (with mass flow control). Its convenience,
  expandability and powerful, results-oriented
  software make the Q500 perfect for the multi-user
  laboratory where a wide variety of TGA
  applications are conducted and where future
  expansion of analytical work is anticipated.

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Features of the Q500 TGA

• 1. Q Series Two Point Mass Adjustment
• 200mg range
• 1000mg. range
• No need to do a mass recalibration
• when switching from regular Pt pans to
• Pt pans with Al hermetic pans.
• Mass Loss Reference Materials
• Materials with nominal 2, 50 and 98
• mass loss are available for verification of TGA
• weight calibration.
• 2. Curie Point Transition Temperature
• Calibration
• ASTM 1582
• Curie Temperature Reference Materials
• TA Instruments is the exclusive worldwide
• distributor for a set of six certified and
  traceable
• Curie temperature materials developed by ICTAC

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Q50/Q500 Features and Options
Feature Q500 Q50 Furnace low
mass Standard Standard Furnace EGA
Option Option Temperature Range RT-1000C
RT-1000C MFC / GSA Standard
Option Autosampler Option NA Hi-Res
TGA Option NA Modulated TGA Option
NA Touch-screen display Standard NA TGA
/ MS operation Option Option TGA / FTIR
operation 3rd Party 3rd Party
NA Not Available
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TGA Furnaces

• Standard Furnace
• Low mass
• Used for Hi-Res Runs
• Cools down in lt20min

• EGA Furnace
• Higher Mass
• Used for EGA runs due to quartz liner
• Cools down in 40min

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TGA Purge Gas Flow
10ml/min
40ml/min
90ml/min
60ml/min
EGA Furnace
Standard Furnace
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Standard Furnace
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EGA Furnace Schematic

Low internal Volume 15ml
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TGA How the balance works

• The balance operates on a null-balance principle.
  At the zero, or null position equal amounts of
  light shine on the 2 photodiodes.
• If the balance moves out of the null position an
  unequal amount of light shines on the 2
  photodiodes. Current is then applied to the
  meter movement to return the balance to the null
  position.
• The amount of current applied is proportional to
  the weight loss or gain.

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TGA Q Series MFC and GSA
MFC and GSA standard on Q500 and optional on Q50
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TGA Q-Series Purge Gas Plumbing

• Instruments w/o MFC
• The gas 1 port purges the sample area only.
• The gas 2 port purges the balance area only.
• Instruments w/ MFC
• The gas 1 port purges both sample and balance
  areas.
• The gas 2 port is used when a different purge gas
  is required or gas switching is used.
• Selection of gas on NOTES page is critical for
  proper use of MFC calibration tables.

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Heat Exchanger Cleaning

• Check cleanliness (no algae growth) once every
  3-6 months.
• To clean dump old water, fill with new and add
  conditioner (algae growth suppressor) if
  available.
• For Q series, after filling, in software choose
  Control \ Prime Exchanger.
• For 2xxx, after filling, continue starting a
  dummy run until error 119 (heat exchanger no
  flow) goes away.

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TGA Performance Criteria

• Baseline
• Drift
• Affected by TGA construction, balance quality,
  and buoyancy effect (minimized through proper
  construction techniques and purge gas control)
• Sensitivity
• Affected by TGA balance quality
• Reproducibility
• Affected by balance quality, temperature control,
  and construction quality
• Temperature Accuracy
• Affected by thermocouple placement, calibration
  stability, purge gas interaction

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TGA Performance

• TGA Performance is primarily a function of
  balance sensitivity and baseline stability
• Balance sensitivity is optimized through design
  and construction techniques
• Baseline stability is a function of instrument
  design, as well as purge gas control
• TGA resolution is primarily a function of heating
  rate, but can be optimized using Hi-Res TGA

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Quantifying TGA Baseline Performance
Drift
Unnormailzed Sample Mass
Temperature or Time
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Measuring Q500 TGA Baseline Performance
Drift 19 mg
Q500, 20C/min Ramp
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TGA Calibrations

• Mass (Verify monthly)
• Temperature (Verify monthly)
• Platform (Perform if there is a problem picking
  up pans.)
• Q series instruments w/ MFC will also have
  options to calibrate the sample and balance
  MFCs. These have been calibrated by TA
  Instruments and should not require further
  calibration. Contact TAI if a problem arises.

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TGA Mass Calibration

• Two point mass adjustment 2050, 2950, Q50, Q500
• 100mg. (2XXX modules) or 200mg (Q series) range
  (use 100mg. weight)
• 1000mg. range (use 1000mg. weight)
• Q5000IR 100mg
• Run TGA weight calibration routine
• Follow screen instructions to tare and
  masscalibrate using two calibration weights (if
  known,enter exact mass of calibration weights)

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Mass Loss And Residue Validation
P/N 952540.901 TGA / SDT Mass Loss Reference
Materials Kit 1,760
Mass Loss Reference Materials Materials with
nominal 2, 50 and 98 mass loss are available
for verification of TGA weight calibration.
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Temperature Calibration Curie Point Transition

• Paramagnetic - a material that is susceptible to
  attraction by a magnet
• Curie Point Temperature - that temperature where
  the material loses its magnetic susceptibility
  (defined as offset point)
• Requires a magnet and well characterized
  transition materials
• ASTM 1582 - Standard Practice for Calibration of
  Temperature Scale for Thermogravimetry

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TGA Temperature Calibration
Vertical Balance Configuration - TGA
2050/2950/Q50/Q500
Tare

Sample

Offset
Furnace
temp
Attraction of Sample to Magnet Results in Initial
Weight Gain
Magnet
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TGA Temperature Calibration Important Points

• Clear the Temperature Table before performing
  the calibration runs (TGA only).
• Choose method end condition of Furnace Closed.
  This prevents the potential of the furnace
  opening onto the magnet at the end of the run and
  damaging the TGA.
• Start run and then put magnet under furnace.
  This allows capture of the weight increase
  (decrease) at the beginning.
• Use of a small labjack is recommended for holding
  the magnet in place under the furnace.

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Standards Can Be Run Simultaneously
Alumel 157.00?C
Nickel 368.80?C
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Calcium Oxalate Standard Analysis

• Although Calcium Oxalate is not generally
  accepted as a Standard Material, it does have
  practical utility for INTRA-laboratory use
• Carefully control the experimental conditions
  i.e. pan type, purge gases/flowrates, heating
  rate
• Particularly control the amount (5mg) and the
  particle size of the sample and how you position
  it in the pan
• Perform multiple runs, enough to do a statistical
  analysis
• Analyze the weight changes and peak temperatures
  and establish the performance of YOU and YOUR
  instrument
• When performance issues come up, repeat the
  Calcium Oxalate analysis

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Calcium Oxalate Decomposition

• 1st Step CaC2O4H2O (s) CaC2O4 (s) H2O (g)
• Calcium Oxalate Monohydrate Calcium Oxalate
• 2nd Step CaC2O4 (s) CaCO3 (s) CO (g)
• Calcium Oxalate Calcium Carbonate
• 3rd Step CaCO3 (s) CaO (s) CO2 (g)
• Calcium Carbonate Calcium Oxide

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Calcium Oxalate Repeatability
Overlay of 8 runs, same conditions
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Calcium Oxalate Repeatability
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General Considerations(Experimental Effects)
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TGA Curves are not Fingerprint Curves
Because most events that occur in a TGA are
kinetic in nature (meaning they are dependent on
absolute temperature and time spent at that
temperature), any experimental parameter that can
effect the reaction rate will change the shape /
transition temperatures of the curve. These
things include

• Pan material type, shape and size.
• Ramp rate.
• Purge gas.
• Sample mass, volume/form and morphology.

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Effect of Sample Size on Decomposition Temperature
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Effect of Heating Rate on Decomposition
Temperature
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Mass Effect Semi-crystalline PE
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Shift in Onset with Ramp Rate
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Typical Applications

• Thermal Stability
• Compositional Analysis
• Oxidative Stability

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Thermal Stability of Polymers
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TGA of an Adhesive
25.18mg of an adhesive _at_ 10C/min
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Inset View Shows Strange Result
Is this real?
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Use time based derivative of temperature to plot
the heating rate
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Aberration in Heating Rate
Usually means that the sample touched the
thermocouple
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Typical Applications

• Thermal Stability
• Compositional Analysis
• Oxidative Stability

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PET w/ Carbon Black Filler
How much Carbon Black was in this sample?
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PET
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Comparison of Filled Un-Filled PET
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Filled Polymer Analysis

Inert filler
Inert filler
Inert filler
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Kinetic Analysis

• The rate at which a kinetic process proceeds
  depends not only on the temperature the specimen
  is at, but also the time it has spent at that
  temperature.
• Typically kinetic analysis is concerned with
  obtaining parameters such as activation energy
  (Ea), reaction order (k), etc. and/or with
  generating predictive curves.

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Kinetic Analysis, cont.
Activation energy (Ea) can be defined as the
minimum amount of energy needed to initiate a
chemical process.
Ea
State 1
State 2
With Modulated TGA, Ea can be measured directly.
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TGA Kinetics

• 1st Order Kinetics based on Flynn and Wall method
• Lifetime Estimation based on Toops and Toops
  method
• PTFE tested at 1, 5, 10 and 20 deg/min
• Sample sizes constant
• Nitrogen purge
• Conversion levels selected at 1, 2.5, 5, 10 and
  20

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Common Thermogram with TGA Scans
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Log Heating Rate versus 1/T
Check for linearity
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Activation Energy by MTGA
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Sample of TGA Application Briefs

• H-16781 Thermogravimetry-Mass Spectrometry Using
  a Simple Capillary Interface
• TA023 Thermal Analysis Review High Resolution
  TGA - Theory and Applications
• TA075 High Resolution TGA Kinetics
• TA 122 Determination of Carbon Black Pigment in
  Nylon 66 by TGA
• TA 125 Estimation of Polymer Lifetime by TGA
  Decomposition Kinetics
• TA231 TGA Evaluation of Zeolite Catalysts
• TN6 Consideration of Subtle Experimental Effects
  (Simultaneous TGA-DTA)
• TN24 TGA Temperature Calibration Using Curie
  Point Standards
• TN40 Optimizing Stepwise Isothermal Experiments
  in Hi-Res? TGA
• TS13 Clarification of Inorganic Decomposition by
  TG-MS
• TS39 Characterization of Polyurethane by TGA and
  Hi-Res ? TGA

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Common TGA Parts Accessories
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Common TGA Parts Accessories
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