The words you are searching are inside this book. To get more targeted content, please make full-text search by clicking here.

## WORKSHOP ON THERMAL ANALYSIS ANDCALORIMETRY OF MATERIALS COMPOSITES-METAL/POLYMER

Related Publications

Discover the best professional documents and content resources in AnyFlip Document Base.

# P.L. Teh- Thermal Analysis

### Keywords: Thermal Analysis

SCHOOL OF MATERIAL
ENGINEERING

THERMAL ANALYSIS AND
CALORIMETRY OF MATERIALS

COMPOSITES

Presented by: PL Teh

Introduction

Thermal analysis:
• A group of techniques in which a porperty of the

material is monitored against time or temperature
of the material on a specified atmosphere, which
is programmed.
Programme:
• May involve heating or cooling at a fixed rate of
temperature change, or holding the temperature
constant, or any sequence of these.

Contents

Technique Abbreviation Measurement Applications

Thermogravimetry TG Mass Decomposition
(Thermogravimetric TGA Dehydration
analysis) Oxidation

Differential Scanning DSC Power (heat Phase transition
Calorimetry flow) Heat capacity
difference Reaction kinetics

Thermomechanical TMA Expansion Expansion,
Analysis volume change
during transition
Dynamic Mechanical DMA Moduli
Analysis Mechanical Phase
Loss change, internal
friction

Thermogravimetric analysis

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 1000°C.

The technique can characterize materials that exhibit weight
loss or gain due to decomposition, oxidation, or dehydration.

TGA
Balance

• As sample changes weight, its tendency to rise or fall is detected by the LVDT.
• A linear variable differential transformer (LVDT) is used as a sensor of the balance

movement to register changes in the sample mass.
• A current through the coil on the conterbalance side exerts a force on the

magnetic core which acts to return the balance pan to a null position.
• This current is directly proportional to the sample mass change.

TGA can give informations:

❑ Thermal Stability of Materials
❑ Oxidative Stability of Materials
❑ Composition of Multi-component Systems
❑ The Effect of Reactive or Corrosive

Atmospheres on Materials
❑ Determine of moisture, volatiles and ash content

Typical TG curves

1. no change
2. desorption/drying (rerun)
3. single stage decomposition
4. multi-stage decomposition
5. as 4, but no intermediates or heating rate

too fast
6. atmospheric reaction
7. as 6, but product decomposes at higher

temperature

Composition of
Multi-component
Systems

Determine of
moisture, volatiles
and ash content

Differential Scanning Calorimetry

Introduction

• DSC is a thermal analysis method where differences in heat
flow into a substance and a reference are measured as a
function of sample temperature, while both are subjected to
a controlled temperature program.

• As chemical reactions (such as curing, burning, etc) and many
physical transitions (such as melting, crystallization, and many
other structural transitions in solids) are connected with the
generation or consumption of heat, calorimetry is a widely
used technique for investigating such process.

• The objective of calorimetry is to measure the amount of heat
and the transition temperature associated with various
chemical and physical transitions.

Heat Capacity

• Heat capacity/specific heat: ability of a material to

store thermal energy. When an amount of heat Q is

supplied to a material, its temperature will be raised

by ∆T ; Q = mC ∆T

m: sample mass. C is a material property, called heat
capacity or specific heat.

The heat capacity of the system is the quantity of heat needed to raise
the temperature of the system of 1oC. To obtain the heat capacity we
have to know the equation (1).

Heat Capacity: Examples

Examples of heat capacity:

Al : 0.215 cal/g K
Cu: 0.093
Ag: 0.056
Liquid water: 1.00

This is why it is much easier to heat up
copper than trying to boil the same quantity
of water.

Latent heat:

When a material changes from solid form to liquid (melting),
or liquid to gas (vaporization), a certain amount of energy is
involved.

• A chunk of ice kept at 0 o C won’t change into 0o C water if
heat is called the latent heat

• Latent heat of melting (heat of fusion):

Ice: 80 cal/g
Silver: 21 cal/g

For a multi-component system, the temperature may or may
not change during such phase transitions

DSC: DEFINITION

Calorimetry: measurement of heat energy associated with
changes of state (such as melting or freezing) and chemical
reactions

Differential: Comparison. Difference between the sample and a
standard material which has no chemical reaction or changes of
state during the temperature range of consideration.

Scanning: temperature is usually scanned at a constant heating or
cooling rate (0 to 200°C/min).

Exothermic reaction: sample releases heat during the reaction
(such as crystallization, curing, or freezing).

Endothermic reaction: sample needs extra amount of heat energy
to complete the transition (such as melting).

Hest Flux vs Power Compensation

The Glass Transition Temperature, Tg

Exo

Heat Tg
Flow

Endo

Temperature

• This means heat is being absorbed by the sample. It also means that we
have a change (increase) in its heat capacity.

• This happened because the polymer has just gone through the glass
transition temperature.

• Polymer have a higher heat capacity above the glass transition
temperature than they do below it.

Example Tg

Start
53.49°C

Mid
point

93.01°C

125.49°C

End

Effect of Heating Rate on the Tg

Effect of Molecular Weight on the Tg

Effect of Hydrogen Bonding on the Tg

Effect of Plasticizer on the Tg

Effect of Bulky Substituent on the Tg

Effect of Flexible
Substituents on the

Tg

Effect of Intermolecular Interactions

Crystallization, Tc

• Above the glass transition, the polymers have
a lot of mobility.

• They wiggle and squirm, and never stay in one
position for very long.

• When they reach the right temperature, they
will have gained enough energy to move into
very ordered arrangement, which called
crystals.

Crystallization

• When polymers fall into these crystalline arrangements, they give off
heat.

• You can see this as a big peak in the plot of heat flow vs temperature.

Exo • The area under the melting
curve is proportional to
T amount of heat required to
c melt the sample such as the
heat of fusion.
Endo
• If we know the theoretical
Temperature heat of fusion of pure
polymer, we can calculate the
crystallinity (%) of the sample
from its measurement by
equation

Measurement of Crystallization

Melting

• This means that when you reach the melting
temperature, the polymer’s temperature
won’t rise until all the crystals have melted.

• This also means that the furnace is going to
have to put additional heat into the polymer
in order to melt both the crystal and keep the
temperature rising at the same rate as that of
the reference pan.

Measurement of Melting

Extrapolated
onset

221.83°C 271.52°C
Start End

Effect of Aromaticity on Melting
Effect of Polymer Type on Melting

Effect of Molecular Weight on Melting
Effect of Hydrogen Bonding on Melting

Effect of Heating Rate on Nylon 66
Melting Behavior

DSC plot

• If you look at the DSC plot you can see a big difference
between the glass transition and the other two thermal
transitions,c rystallization and melting.

• For the Tg, there is no peak, and there is no dip either.
This is because there is no latent heat given off, or
absorbed.

• Both melting and crystallization involve absorbing or
giving off heat. The only thing we do see at the Tg is a
change in the heat capacity of the polymer.

• Because there is a change in heat capacity, but
there is no latent heat involved with the Tg,
we call the Tg second order transition.

• Transition like melting and crystallization
which do have latent heats are called first
order transition.

DSC plot output

• To put them all together, a whole plot will
often look something like this :-

Examples of Tg
measurement

DSC Thermoset Cure: First and
Second Heat

• Curve 1 is the

measurement result of

base compound, Tg at
-17.50C.

• Curve 2 is the

measurement result for

the curing agent, Tg at -
43.40C.

• Curve 3 shows the

results when the base

compound and curing

agent were mixed.

• Curve 4 shows the

results for the 2nd

heating, Tg observed at
18.5°C and 105.5°C

• The mixture left to
curing at room
temperature for
different lengths of
time, then
measured.

• The longer the
curing time at
room temperature,
the higher the Tg
shifted.

• The shape and
temperature of the
exothermix curing
peaks changed,
change the degree
of polymerization.

Cure kinetics of resins

❖ Cure kinetics of resins systems are studied using non-isothermal
and isothermal methods.

❖ Ozawa and Kissinger are the two kinetic analysis models that
extensively used to understand and predict the cure behaviour
of the resin systems.

❖ Basic assumption: the rate of kinetic process (dα/dt) is
proportional to the measured heat flow Φ and Δ the enthalpy
of the reaction.

❖ The rate of the kinetic process in kinetic analysis can be
described by:

Where ( ) is heat flow from kinetic
process,

Where:
A is the pre exponential factor (frequency
factor).
is the activation energy.
R is the gas constant (8.314 J/mol/K) and
T is the absolute temperature

❖ The slope of ln dα/dt versus 1/T gives the value of activation
energy.

Ozawa Method:

where
is the heating rate,
is the activation energy, and
R = 8.314 J/°K-mol is the gas constant.
Tp = The peak of exotherm temperature
Activation energy can be was calculated from the slope of the plot of (ln )against (1/ )

Kissinger Method:

where
is the heating rate,
is the activation energy and
R = 8.314 J/°K-mol is the gas constant.
The activation energy can be obtained from the slope of the plot of ( / 2) against (1/ )

Non-isothermal DSC:

Non-isothermal DSC
traces for thermoset at
different heating rate.
The shifting of the DSC
peak as a function of
heating rate can provide
kinetics information
reaction.
To estimate the
activation energy,
Kissinger plot
was used in which ln (θ/
Tp2) is plotted against
1/Tp,

where is the heating rate, Tp is the peak temperature (in K) of the curing
reaction.
The slope of this plot is -E/R (R = 8.314 J/mol, K is the Universal Gas constant).

ln (θ/Tp2)

Isothermal DSC:

Experimentally, we observed
that the onset of cure time
has an Arrhenius temperature
dependence:

t = A exp(E/RT)

Where:
E: activation energy;
R: Universal Gas Constant;
T: absolute temperature;
A: pre-factor

Based on the slope of the linear fit, the activation energy E is calculated to be 89.1
kJ/mol, this can be used to predict the curing behavior at other temperatures.
For example, at 25 o C, it would take 67 days to reach the onset of cure.
The information allows us to have some idea about the material pot life at various
temperatures.

Dynamic Mechanical Analysis

Dynamic Mechanical Analysis

• A Dynamic Mechanical Analyzer (DMA) is used to measure the
stiffness and damping properties of a material.

• DMA is a technique where a small deformation is applied to a
sample in a cyclic manner. This allows the materials response
to stress, temperature, frequency and other values to be
studied.

• The DMA determines changes in sample properties resulting
from changes in five experimental variables: temperature,
time, frequency, force, and strain.

Dynamic mechanical analysis

• In viscous systems: all the work done by the system is
dissipated as heat

• In elastic systems: all the work is stored as potential energy
• Polymer = viscoelastic behavior (dual manner)
• DMA is an excellent technique for extracting information on

the dynamic material properties which relate to these two
regimes of behavior.
• DMA measurements can give the following properties as a
function of temperature:
• the elastic modulus (E'):

represents how much energy the polymer stores
• the viscous modulus (E"):

indicates the polymer's ability to dissipate the
energy as heat.

When the strain is in phase with stress, i.e δ is
0º , the sample is classed as elastic. An
example of an elastic material might be a
rubber band or a metal spring.

When the strain is 90º out of phase with the
stress, i.e δ is 90º, the sample is classed as
viscous. Viscous materials such as Glycerine
exhibit large damping properties.