Thermogravimetric Analysis Theory Operation Calibration and Data Interpretation

Thermogravimetric Analysis Theory, Operation, Calibration and Data Interpretation Prepared by Kadine Mohomed, Ph. D Thermal Applications Chemist TA Instruments

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

TGA: The Technique ØThermogravimetric 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. ØMeasurements are used primarily to determine the composition of materials and to predict their thermal stability at temperatures up to 1000°C. ØThe technique can characterize materials that exhibit weight loss or gain due to decomposition, oxidation, or dehydration.

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

Calcium Oxalate Example

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).

Features of the Q 500/ Q 50 TGA The Q 500 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 Q 500 perfect for the multi-user laboratory where a wide variety of TGA applications are conducted and where future expansion of analytical work is anticipated.

Features of the Q 500 TGA Photodiodes Infrared LED Meter movement Balance arm Tare pan Sample platform Thermocouple Sample pan Furnace assembly Purge gas outlet Heater Elevator base Purge gas inlet Sample pan holder 1. Q Series Two Point Mass Adjustment • 200 mg range • 1000 mg. 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 ØMaterials 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: ØTA Instruments is the exclusive worldwide distributor for a set of six certified and traceable Curie temperature materials developed by ICTAC

Q 50/Q 500 Features and Options Feature Q 500 Furnace – low mass Standard Furnace – EGA Option Temperature Range RT-1000°C MFC / GSA Standard Autosampler Option Hi-Res TGA™ Option Modulated™ TGA Option Touch-screen display Standard TGA / MS operation Option TGA / FTIR operation 3 rd Party NA = Not Available Q 50 Standard Option RT-1000°C Option NA NA Option 3 rd Party

TGA Furnaces • Standard Furnace – Low mass – Used for Hi-Res Runs – Cools down in <20 min • EGA Furnace – Higher Mass – Used for EGA runs due to quartz liner – Cools down in ~40 min

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

Standard Furnace

EGA Furnace Schematic Balance Purge Quartz Liner Off-Gases Sample Thermocouple Sample Pan Purge Gas In Low internal Volume ~15 ml Furnace Core

TGA: How the balance works ØThe balance operates on a null-balance principle. At the zero, or “null” position equal amounts of light shine on the 2 photodiodes. ØIf 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. ØThe amount of current applied is proportional to the weight loss or gain.

TGA: Q Series MFC and GSA standard on Q 500 and optional on Q 50

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.

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 2 xxx, after filling, continue starting a dummy run until error 119 (heat exchanger – no flow) goes away.

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

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

Unnormailzed Sample Mass Quantifying TGA Baseline Performance Drift Temperature or Time

Measuring Q 500 TGA Baseline Performance Drift ~19 mg Q 500, 20°C/min Ramp

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 MFC’s. These have been calibrated by TA Instruments and should not require further calibration. Contact TAI if a problem arises.

TGA: Mass Calibration Two point mass adjustment: 2050, 2950, Q 500 • 100 mg. (2 XXX modules) or 200 mg (Q series) range (use 100 mg. weight) • 1000 mg. range (use 1000 mg. weight) • Q 5000 IR – 100 mg • Run TGA weight calibration routine • Follow screen instructions to tare and mass calibrate using two calibration weights (if known, enter exact mass of calibration weights)

Mass Loss And Residue Validation P/N 952540. 901 TGA / SDT Mass Loss Reference Materials Kit $1, 760 2. 4 % 49. 7 % 99. 1 % 0. 017% Mass Loss Reference Materials: Materials with nominal 2%, 50% and 98% mass loss are available for verification of TGA weight calibration.

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

TGA: Temperature Calibration Vertical Balance Configuration - TGA 2050/2950/Q 500 Tare Sample % Offset Furnace Magnet temp Attraction of Sample to Magnet Results in Initial Weight Gain

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.

Standards Can Be Run Simultaneously Alumel 157. 00 C Nickel 368. 80 C

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 (~5 mg) 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

Calcium Oxalate Decomposition • 1 st Step Ca. C 2 O 4 • H 2 O (s) Ca. C 2 O 4 (s) + H 2 O (g) Calcium Oxalate Monohydrate • 2 nd Step Ca. C 2 O 4 (s) Calcium Oxalate • 3 rd Step Ca. CO 3 (s) Calcium Carbonate Calcium Oxalate Ca. CO 3 (s) + CO (g) Calcium Carbonate Ca. O (s) + CO 2 (g) Calcium Oxide

Calcium Oxalate Repeatability Overlay of 8 runs, same conditions

Calcium Oxalate Repeatability

General Considerations (Experimental Effects)

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.

Effect of Sample Size on Decomposition Temperature

Effect of Heating Rate on Decomposition Temperature

Mass Effect – Semi-crystalline PE

Shift in Onset with Ramp Rate

Typical Applications • Thermal Stability • Compositional Analysis • Oxidative Stability

Thermal Stability of Polymers

TGA of an Adhesive 25. 18 mg of an adhesive @ 10°C/min

Inset View Shows Strange Result Is this real?

Use time based derivative of temperature to plot the heating rate

Aberration in Heating Rate Usually means that the sample touched thermocouple

Typical Applications • Thermal Stability • Compositional Analysis • Oxidative Stability

PET w/ Carbon Black Filler How much Carbon Black was in this sample?

PET

Comparison of Filled & Un-Filled PET

Filled Polymer Analysis 100 Polymer (A) Air Carbon Black "Light" Oil Air Polymer (B) Carbon Black Inert filler 0 100 Polymer + "Heavy" Oil Air (C) Carbon Black Inert filler 0 TEMPERATURE (°C) \ \ WEIGHT (%) 0 100 \ \ Inert filler

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.

Kinetic Analysis, con’t. Activation energy (Ea) can be defined as the minimum amount of energy needed to initiate a chemical process. State 1 Ea State 2 With Modulated TGA, Ea can be measured directly.

TGA Kinetics • 1 st 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%

Common Thermogram with TGA Scans

Log Heating Rate versus 1/T Check for linearity

Activation Energy by MTGA

Sample of TGA Application Briefs • H-16781 “Thermogravimetry-Mass Spectrometry Using a Simple • • • Capillary Interface” TA 023 “Thermal Analysis Review: High Resolution TGA - Theory and Applications” TA 075 “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” TA 231 “TGA Evaluation of Zeolite Catalysts” TN 6 “Consideration of Subtle Experimental Effects (Simultaneous TGADTA)” TN 24 “TGA Temperature Calibration Using Curie Point Standards” TN 40 “Optimizing Stepwise Isothermal Experiments in Hi-Res TGA” TS 13 “Clarification of Inorganic Decomposition by TG-MS” TS 39 “Characterization of Polyurethane by TGA and Hi-Res TGA”

Common TGA Parts & Accessories

Common TGA Parts & Accessories

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