Tga

Thermal Analysis

Dr. S. Anandhan,

Asst. Professor,

Dept. of Met. and Mat. Engg., NITK

What is thermal analysis?

"A group of techniques in which a physical property of a substance and/or its reaction products is measured as a function of temperature whilst the substance is subjected to a controlled temperature program"

R.C.Mackenzie, Thermochim. Acta, 1979, 28, 1.

Heat flow into a substance induces many physical and chemical changes which can help to identify and characterize a sample

Thermal Analysis

A group of analytical techniques

Each technique defines a material property

TA Techniques TA Use

Thermal Analysis is widely used

For a wide variety of Applications

Over a dozen thermal methods can be recognized, which differ in the properties measured and the temperature programs

These are used for quality control and research applications on industrial products, such as polymers, pharmaceuticals, clays and minerals, metals and alloys

Thermogravimetric Analysis (TGA) or Thermogravimetry

Thermogravimetry fundamentals

Principle

Changes in the mass of a sample are studied while the sample is subjected to a programme.

The temperature programme is most often a linear increase in temperature, but, also be carried out, when the changes in sample mass with time are followed.

TGA is inherently quantitative, and therefore an extremely powerful thermal technique, but gives no direct chemical information. The ability to analyze the volatile products duvalue

TGA + Mass Spectrometry: TGA

TGA + Infrared Spectroscopy: TGA

Processes that lead to weight gain or loss in TGA experiments

Instrumentation

Thermobalance/microbalance

Balance sensitivity is usually around one microgram, with a total capacity of a few hundred milligrams

Furnace

Temperature programmer

A typical operating range for the furnace is ambient to 1500C, with heating rates up to 200C/min

Thermogravimetric Analysis (TGA) or Thermogravimetry

fundamentals

of a sample are studied while the sample is subjected to a

The temperature programme is most often a linear increase in temperature, but, also be carried out, when the changes in sample mass with time are followed.

TGA is inherently quantitative, and therefore an extremely powerful thermal technique, but gives no direct chemical information. The ability to analyze the volatile products during a weight loss is of great

TGA + Mass Spectrometry: TGA-MS

TGA + Infrared Spectroscopy: TGA-FTIR

Processes that lead to weight gain or loss in TGA experiments

Thermobalance/microbalance

sensitivity is usually around one microgram, with a total capacity of a few hundred

A typical operating range for the furnace is ambient to 1500C, with heating rates up to

of a sample are studied while the sample is subjected to a controlled temperature

The temperature programme is most often a linear increase in temperature, but, isothermal studies can also be carried out, when the changes in sample mass with time are followed.

TGA is inherently quantitative, and therefore an extremely powerful thermal technique, but gives no ring a weight loss is of great

sensitivity is usually around one microgram, with a total capacity of a few hundred

A typical operating range for the furnace is ambient to 1500C, with heating rates up to

Temperature sensor

thermocouple placed close to the sample

Sample holder/pan

An enclosure for establishing the required atmosphere

Reactive or inert

Microcomputer/microprocessor

Instrument control

Data acquisition and display

Balance/furnace configurations

thermocouple placed close to the sample

An enclosure for establishing the required atmosphere

Microcomputer/microprocessor

Data acquisition and display

Pans for TGA

aluminum, platinum, silica, and alumina

Data Analysis

Thermogram is graph of mass versus temperature. Sometimes given as % of original mass.

Draw tangents of the curve to find the onset and the offset points

mi, mf and m are fundamental properties ofthe sampleTi and Tf depend on operating variables

Typical TG curves

Derivative thermogram (DTG)

plots change in mass with temperature, dm/dt, and resolves changes more clearly.

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 curie point

Factors affecting TG Analysis

heating rate and sample size

increase in either of which tends to increase the decomposition temperature, and to decrease the resolution between successive mass losses

particle size and packing of the sample

crucible shape

Gaseous atmosphere

Nature

flow rate

Effect of gaseous atmosphere

Polymers degrade at a lower temperature in presence of O

heating rate and sample size

in either of which tends to increase the decomposition temperature, and to decrease the resolution between successive mass losses

particle size and packing of the sample

Polymers degrade at a lower temperature in presence of O

in either of which tends to increase the decomposition temperature, and to decrease the

Polymers degrade at a lower temperature in presence of O2

Effect of heating rate

10 mg samples of PTFE, heated at 2.5, 5, 10 and 20 C/min in nitrogen

Important note

Careful attention to consistency in experimental details normally results in good repeatability.

On the other hand, studying the effect of deliberate alterations in such factors as the heating rate can give valuable insight into the nature of the observed reactions.

Sources of error

A) MASS

Classical buoyancy

Effect temp. on balance

convection and/or turbulence

viscous drag on suspension

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.

NOISY OR ERRATIC RECORDS CAN ARISE FROM:

static

vibration

pressure pulses in lab.

uneven gas flow

Applications of TGA

Ability of TG to generate fundamental quantitative data from almost any class of materials, has led to its widespread use in every field of science and technology. Key application areas are listed below:

Thermal Stability: related materials can be compared at elevated temperatures under the required atmosphere. The TG curve can help to elucidate decomposition mechanisms.

Material characterization: TG and DTG curves can be used to "fingerprint" materials for identification or quality control.

Compositional analysis: by careful choice of temperature programming and gaseous environment, many complex materials or mixtures may be analyzed by selectively decomposing or removing their components. This approach is regularly used to analyze e.g. filler content in polymers; carbon black in oils; ash and carbon in coals, and the moisture content of many substances.

Simulation of industrial processes: the thermobalance furnace may be thought of as a mini-reactor, with the ability to mimic the conditions in some types of industrial reactor.

Kinetic Studies: a variety of methods exist for analyzing the kinetic features of all types of weight loss or gain, either with a view to predictive studies, or to understanding the controlling chemistry.

Corrosion studies: TG provides an excellent means of studying oxidation, or reaction with other reactive gases or vapors.

Ex.1.Comparison of thermal stability of materials

Ex.2.Quantitative analysis of materials - % composition of a rubber sample

Ex.3.Quantitative analysis of materials - % composition of a composite used in making doors

Ex.4.Mechanism of thermal reactions

Ex.5. Effect of additives on thermal stability of materials

EX.6.Analysis of Chewing Gum with Auto Stepwise TGA

Chewing gum is a complex mixture of a number of components, including: natural elastomers, glycerin, softening agents, and carbonates, flavoring agents, sweeteners and colorants.

The correct combination of the gum formulation proincluding: stickiness, softness and chewability.

Sample is heated at a constant rate until a significant weight loss event is encountered.

Equipment automatically holds the sample under isothermal conditiothat the given component has essentially completed its given degradation.

Ex.5. Effect of additives on thermal stability of materials

EX.6.Analysis of Chewing Gum with Auto Stepwise TGA

Chewing gum is a complex mixture of a number of components, including: natural elastomers, glycerin, softening agents, and carbonates, flavoring agents, sweeteners and

The correct combination of the gum formulation provides the end characteristics to the chewing gum, stickiness, softness and chewability.


Equipment automatically holds the sample under isothermal conditions until becomes small, meaning that the given component has essentially completed its given degradation.

Chewing gum is a complex mixture of a number of components, including: PVAc (poly vinyl acetate), natural elastomers, glycerin, softening agents, and carbonates, flavoring agents, sweeteners and

vides the end characteristics to the chewing gum,


ns until becomes small, meaning

Heating is automatically resumed, at a constant rate, until the next significant weight loss event. By this process, we are able to nicely resolve closely occurring decomposition events and provide better quantitative analysis of a sample.

Standard TGA results for Doublemint chewing gum sample

TGA auto stepwise results for Doublemint chewing gum sample

References

D. A. Skoog et al., Principles of instrumental analysis, fifth edition, Harcourt Publishers, 2001.

http://www.anasys.co.uk/library/macrota.htm

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