APOSTILLA, Anandhan, Thermal Analysis

8/10/2019 APOSTILLA, Anandhan, Thermal Analysis

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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


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Thermogravimetric Analysis (TG

Thermogravimetry fundamental

Principle

Changes in the massof a s

programme.

The temperature programm

also be carried out, when th

TGA is inherently quantita

direct chemical information.

value

TGA + Mass Spectr

TGA + Infrared Spe

Processes that lead to weight gain

Instrumentation

Thermobalance/microbalanc

Balance sensitivity

milligrams

Furnace

Temperature programmer

A typical operatin

200C/min

) or Thermogravimetry

s

mple are studied while the sample is subjected

is most often a linear increase in temperature,

changes in sample mass with time are followed.

tive, and therefore an extremely powerful ther

The ability to analyze the volatile products du

metry: TGA-MS

ctroscopy: TGA-FTIR

r loss in TGA experiments

e

is usually around one microgram, with a total

range for the furnace is ambient to 1500

to a controlled temperature

but, isothermal studies can

al technique, but gives no

ing a weight loss is of great

capacity of a few hundred

, with heating rates up to


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Temperature sensor

thermocouple place

Sample holder/pan

An enclosure for establishin

Reactive or inert

Microcomputer/microproces

Instrument control

Data acquisition and

Balance/furnace configurations

close to the sample

the required atmosphere

sor

display


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Data Analysis

Thermogram is graph of mass versustemperature. 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 sampleTiand Tf depend on operating variables

Typical TG curves


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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.


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Calibration using curie point

Factors affecting TG Analysis

heating rate and sample size

increase in either of

resolution between

particle size and packing of

crucible shape

Gaseous atmosphere

Nature

flow rate

Effect of gaseous atmosphere

Polymer

which tends to increase the decomposition tem

uccessive mass losses

he sample

degrade at a lower temperature in presence of

erature, and to decrease the

2


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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 consistencyin experimental details normally results in good repeatability.

On the other hand, studying the effect of deliberate alterationsin such factors as the heating ratecan give

valuable insightinto 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.


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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


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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


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Ex.5. Effect of additives on therm

EX.6.Analysis of Chewing Gum w

Chewing gum is a complex

natural elastomers, glycer

colorants.

The correct combination of

including: stickiness, softnes

Sample is heated at a consta

Equipment automatically ho

that the given component ha

l stability of materials

th Auto Stepwise TGA

mixture of a number of components, including

in, softening agents, and carbonates, flavori

the gum formulation provides the end charact

s and chewability.

t rate until a significant weight loss event is enc

lds the sample under isothermal conditions until

s essentially completed its given degradation.

: PVAc (poly vinyl acetate),

g agents, sweeteners and

ristics to the chewing gum,

untered.

becomes small, meaning


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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

APOSTILLA, Anandhan, Thermal Analysis

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