Thermogravimetric Analysis for Material Characterization

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23/09/19 Sciences Reference this

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Techniques of Material Characterization

 

 

 

 

 

 

Thermogravimetric Analysis (TGA)

 

 

 

 

 

 

 

 

 

 

 

TABLE OF CONTENTS

 

 

  1. INTRODUCTION
  1. STATE OF THE ART
  1. THE PHYSICS
  1. Operational Mode
  1. Atmosphere Control
  1. Curve Interpretation
  1. APPLICATIONS
  1. PROS V. CONS
  1. SUMMARY
  1. ILLUSTRATIONS & SCHEMATICS
  1. REFERENCES

  1. INTRODUCTION

TGA is a branch of thermal analysis that is a destructive technique by default of the nature in which it analyzes samples. It measures a sample’s mass as it is heated, cooled, or held at a constant temperature in a defined atmosphere. As the sample is heated, volatile gaseous products are released first during the heating process. So the original sample’s mass gradually decreases to ash, or an non-combustion residual component (assuming this residual’s combustion temperature exceeds the instrument’s operational temperature range). This process is output from the TGA instrument as a percent weight loss curve vs  temperature, as the derivative weight percent loss vs temperature. A typical mass loss curve for the former data output is shown in Figure 1 of the Appendix.

TGA analyzes these mass changes to a material’s physical and chemical/compositional properties  by one of two/three? operational modes. These are (1) the scanning mode, (2) the isothermal mode and (3) Quasi-static mode. In scanning mode, the sample’s mass change is analyzed as a function of increasing temperature (at a constant heating rate). In isothermal mode, the sample’s mass change is analyzed as a function of time (at constant temperature and/or constant mass loss). In quasi-static mode, the sample is analyzed over a series of increasing temperatures, where it is heated to a constant weight at each temperature. Consequently, this makes TGA ideal for studying materials that are susceptible to mass loss (or gain) from thermally or kinetically driven  processes like decomposition, absorption, sublimation, vaporization, oxidation, reduction, decomposition, combustion, magnetic transitions, solid-gas reactions, solid state reactions, and volatility (e.g. moisture, outgassing). This also makes TGA particularly ideal to study the thermal stabilities for polymer-based materials like elastomers, thermoplastics, thermosets, composites, fibers, films, coatings, paints. Additionally, its ease of application allows it to be readily integrated with other thermal analysis techniques (particularly DSC). The information TGA provides help facilitate material design selection, predict performance, improve product quality, and optimization of end use products.

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TGA can be coupled with other thermal and spectroscopic techniques (called hyphenated techniques) to provide superior, more detailed quantitative information for a material sample. So for example, it can be combined with powerful spectroscopic techniques like TGA-FTIR and TGA-MS. These provide the capability of a more complex analysis that make Evolved Gas Analysis (EGA) possible with high degree of accuracy. TGA-GC provides quantitative material identification. This coupling ability gives TGA a broad utilization across a range of sectors. Pharmaceuticals is a challenging industry where well characterized drug compounds with a high degree of purity are desired. TGA provides quantitative information about drug desolvation, decomposition, and compatibility properties. As an example, TGA facilitates analysis of pharma materials. Through its capability to apply appropriate heating rates (up to 750C/min) TGA can be easily used to determine the polymorphic purity of a drug compound. It can also be used to determine the extent of solubility of pharma materials in solvents. So this analysis capability is significant, because pharma material analysis is the largest area of application for thermal analysis in the pharmaceutical industry. In the food science industry, TGA can be used in conjunction with other thermal methods (e.g. DSC) to calculate food moisture content. Moisture not only affects food molecular structure, but is critical to the food’s texture/visual appeal and literally its taste as well. In the industrial sector, TGA is widely used to characterize polymer quality and characteristics during and after processing and manufacturing. It is often coupled with other spectroscopic methods for QA/QC determination.

  1. STATE OF THE ART – 1pp

State of the art advancements in R&D include

In industry

In commercial sector

 How instrumentation improved? Latest R&D and commercial R&D associated with improving instruments, data measurement

 

  1. THE PHYSICS

The TGA setup is straightforward. The TGA assembly consists of two functional units: a high precision thermobalance and a programmable temperature furnace.  The thermobalance is attached to a sample pan which is typically housed inside the furnace unit. The thermobalance unit. The thermobalance can be one of two configurations:-

Vertical. This configuration is based on a null-balance principle. The optically active servo loop maintains the balance arm in a horizontal (null) position by current regulation in the transducer coil. An infrared LED and photodiode pair are used to detect beam movement. A flag attached to the balance arms control the amount of light reaching each photodiode. As the TGA analysis is run, changes in sample weight  disturb and unbalance the beam from its equilibrium position. The net photodiode output signal is sent to the control program (shown by computer assembly in Figure 1). The control program uses the negative feedback from the photodiode to electrically adjust the current. This keeps the beam in balance and maintains its equilibrium position. This feedback current is significant, because its magnitude is  proportional to the sample’s weight change.

Horizontal. This configuration is like the vertical but with a primary difference. It is a dual-balance that uses both a sample (S) and reference (R) balances.  The S balance monitors the actual sample weight. The R balance corrects for any deviational changes in the sample and reference beams. The schematic diagram for the horizontal configuration is in Figure 2.5.

These two configurations result in thermobalance assemblies of Type I or II (vertical) or Type III (horizontal) configuration as shown in Figure 2.6.

Operation. Before data collection, the sample and reference pans are weighed by individually placing these on the sample platform. The automated system is used to load each pan onto the hang down wire, and the resulting tared weight is recorded by the computer. The sample/reference pans are returned to the platform. At this point, each pan is loaded with a sample which size is typically within 10mg-20mg. These sample size dimensions need to be kept small to avoid heat transfer or mass diffusion effects.

The TGA instrument then loads the sample pan, ignites the furnace[1], and records the sample weight for the test run.

During data collection, the technique measures the sample’s mass and/or derivative mass as a function of temperature under the constraint of a controlled atmosphere (active or inert). So descending TGA curve indicates a weight loss. Conversely an ascending curve indicates a weight gain (likely due to the existence of an ongoing reaction in the sample and/or the atmosphere is not set to be inert. Data output is displayed as time or temperature (abscissa) v. mass or mass percent (ordinate).

A. Operational Modes

1. Scanning mode. Uses air, N2 or O2 (or other inert gas like argon) as the default atmosphere

2. Isothermal mode. Similar to scan mode, air or oxygen is also used as the default environment.

3. Quasi Static mode

B. Atmospheric Controls. In TGA analysis operations that require inert atmospheric/purge gas conditions, gases such as air, oxygen or nitrogen are typically used. Nitrogen is particularly the best purge gas to use, as it is a relatively poor thermal conductor. So it is more likely to increase sample sensitivity. In contrast, an inert gas like helium which-being a good heat conductor-is better at increasing resolution. However, if an atmospheric sensitive analysis like TOS requires a reactive atmosphere, then air, oxygen, CO2, or some other specialty gas (besides hydrogen[2]) should be considered. This is regardless of which operational mode the analysis will be conducted for.

Gas products can be further analyzed using hyphenated techniques. This is done by coupling a MS or FTIR to the TGA thermal balance.

C. Curve Interpretation. TGA curve shape provides both qualitative and quantitative information (Hatakeyama & Quinn, 1999). A list and explanation of TGA curve profiles is below. curves are categorized according to seven types based on their shape.

• Curve Type 1:  Constant slope indicating constant mass or no change in mass change rate  over the temperature range. This implies the existing residual component mass has a decomposition temperature that exceeds the TGA instrument’s temperature limits.

Curve Type 2: Hyperbolic shape is characteristic of desorption and drying. The convex minimum with asymptotic tail with increasing temperature indicates the occurrence of a significant mass loss. This curve is characteristic of a reaction phase change. Or from high volatility losses due to evaporation of volatile gas product(s) when processes like desorption, drying or polymerization occur. Also if an inert atmosphere is present in the chamber, then curve 2 becomes curve 1.

Curve Type 3: Step shape down is characteristic of a single stage decomposition bounded by initial and final decomposition temperatures

• Curve Type 4: Descending staircase step shape is characteristic of a multistage decomposition reaction or process.

• Curve Type 5: Similar staircase shape as in type 4. But the lower resolution implies more heat transfer into the sample from a higher temperature gradient /heating rate. Or could also be due to short lived/lack intermediate species in the ongoing reaction process.

• Curve Type 6: Ascending staircase step shape is characteristic of a reactive atmosphere given the resulting increase in mass. This may be due to the reactions such as surface oxidation reactions in the presence of an interacting atmosphere.

Curve Type 7: Similar to type 6, but maxima indicate product degradation at high temperatures. e.g. surface oxidation reaction followed by subsequent decomposition of reaction product(s).

  1. APPLICATIONS 3 pp single  Use Ashby diagram to guide which matls fit criteria TGA is the best technique for analyzing

Due to the ease of utilization, TGA is a diverse tool which can be used for a variety of analytical applications where changes in a material mass from temporal or energy environmental variables are important from a material’s operational/sustainability perspective. Some applications

  • Materials Characterization – use shape TGA curves to fingerprint materials for identification

    • Identification of Two Similar Materials https://en.wikipedia.org/wiki/Thermogravimetric_analysis

  • QA/QC tool in manufacture CNTs. CNTs are classified by their percent purity i.e. purity = 100% – CNT residue. So TGA characterizes the amount of metallic catalytic residue remaining on the CNT. CNT also used to characterize products that contain nanoparticles (NP) and/or CNTs as part of the end product.
  • Process Control QC: Fiberglass reinforced printed circuit board. TGA determines % fiber v %resin

  • Compositional Analysis: This analysis is done by TGA or hyphenated TGA techniques to determine components like percent volatiles, filler, and moisture. As an example, the following combined DTG-TGA was used to determine the weight loss of hydrozincite. The first weight loss (less than 5%) is slight between 200C-250C. It represents sample mass loss from moisture. The second major mass loss is ~10% drop from 97% to 87% between 250C-280C. This represents the loss of 3 hydrated water molecules. The final loss is is ~15% drop from 87% to 72% between 350C-450C. This is the loss of CO2 molecules which leave an unreactive residual component mass behind.

  • Material Filler Content – see MAT dr. K class (e.g. composite fiber v. matrix  wt%)

*put dr k pic here*

  • Compositional analysis of multi-component materials or blends – find a block copolymer TGA – see Dr. K
  • Melting Point accompanied by a mass/phase change

  • Decomposition Kinetics – TGA curve shape #4

References: Perkins Elmeyer (see ref) collaborates with wheelite composition source  http://www.webmineral.com/data/Whewellite.shtml#.XALbMGhKhhE

 

Example 2:

Source: https://www.ipfdd.de/en/departments/institute-of-macromolecular-chemistry/analytics/analytical-methods/thermal-analysis/thermogravimetric-analysis-tga/

  • Thermal stability and/or Oxidative stability – TOS to study leading edge air craft

The effect of heating rate on TGA curve for powdered PVC. – p 1/6 Hatakeyma ref.

  • Corrosion studies/Reactive Atmospheric Effects on Material Integrity – TGA for sample that outgasses &reacts

    • Effect inert v reactive atmospheres

http://oaktrust.library.tamu.edu/bitstream/handle/1969.1/ETD-TAMU-2011-12-10551/ESELTINE-THESIS.pdf?sequence=2

    

Illustration of the effect of a reactive TGA atmosphere. Above 220C, the sample’s remaining weight percent is clearly less for CO2 vs the inert N2 atmosphere. (fig 4-5 p32 PAGE 46/408 AT LINK ABOVE)

  • Product Lifespan Estimation – TOS to estimate space craft sustainability
  • Reaction Kinetics: Example Adsorption/Desorption

Sample R&D paper using it

https://www.researchgate.net/publication/281365656_Experimental_and_computational_investigation_of_adsorption_performance_of_TC-5A_and_PSA-5A_for_manned_spacecraft


PROS v CONS 1 pp 

Pro v con http://www.flemingptc.co.uk/our-services/dsc-tga/

Advantage

Disadvantage

Comment

Easily/readily coupled with other thermal techniques like TGA-MS or TGA-FTIR for advanced analysis like EGA

None

Coupling provides complimentary &supplementary information to other thermal techniques like DSC & MS

Can implicitly detect energy shifts from heat capacity changes implicit in quantitative mass changes

Cannot determine direction of said implicit energy shifts (i.e. whether shifts are exothermic or endothermic)

TGA can implicitly detect energy shifts by directional mass change. But need to use other thermal techniques like DSC to establish if it’s energy loss/gain

Temperature limit is up to 2000C; allows detection coupling with MS, GC and FTIR

Good for tracking phase changes in chemical & physical reactions

Limited to phase changes where mass loss occurs.

Good for measuring volatility fraction. But useless for phase changes like glass transition, melting and  crystallization

 

  1. SUMMARY

TGA is a popular, cost effective and intuitive thermal analysis technique. It uses heat to induce mass changes through chemical reactions and physical changes within the material. As gaseous products are released from these changes, the original sample mass gradually decreases to ash or a residual component (assuming  the combustion temperature exceeds the instrument’s operational temperature range). This process is output from the TGA instrument as a mass loss (weight percent) curve vs  temperature or time (the derivative mass loss).

TGA uses two operational modes to provide quantitative measurement of a sample’s thermal degradation (by means of a mass/mass percent change) as a function of time and temperature. The material’s thermal degradation output is in the form of a declining  hyperbolic curve (a TGA curve). The complimentary mass derivative TGA curve, is a set of peaks that correspond to component weight loss from the sample. This curve is typically plotted on the same graph to illustrate where the maxima in sample weight loss is occurring. Each mass derivative’s peak typically coincides with the temperature at which the inflection point occurs on the mass/mass percent TGA curve. A sample’s output TGA curve can show one or more curve types. Some of the more popular TGA curves are  Type ___ and _____ which coincide with component losses like moisture, component amounts such as resin v. fiber, and chemical kinetics which degrade molecules into smaller, quantifiable, molecular fragments.

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The technique’s ease of use allows it to be used in conjunction with a wide variety of other techniques such as DSC, GC, and MS. For example, when used in combination with FTIR, the resulting TGA-FTIR analysis system has an improved quantitative and accurate measurement analysis capability. This coupling of analysis techniques permit more complex analyses of evolved gas products resulting from TGA like EGA.

Finally, TGA ease of utilization allows it to be implemented on a wide range materials, and across a diverse range of sectors (e.g. academia (R&D), food science, pharmaceutical, petrochemical, environmental, industrial manufacturing/processing).

  1. ILLUSTRATIONS & SCHEMATICS

TGA VERTICAL THERMOBALANCE  SETUP

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

HORIZONTAL TGA THERMOBALANCE  SETUP
 

 

 

TGA THERMOBALANCE  CONFIGURATIONS

 

 

  1. REFERENCES

 

 

 

Endnotes


[1] Typical heating/thermal ramp rates are 10C/min to 20 C/min and the atmosphere is typically air or inert (i.e. Ar, N2) unless a reactive atmosphere is required

[2] While Hydrogen is efficient for redox analyses where it reduces oxides to metals, it is not ad vised due to safety concerns

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