Lecture 8 Application DTA & DSC01

8/14/2019 Lecture 8 Application DTA & DSC01

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Lecture 8 Application DTA&DSC01 1

Thermal Analysis SSK 4242/3

THE REACTION KINETICS STUDY

Chemical reactions take place at a certain rate and it is defined

as the rate of change of the amount of the reactant(s)/product(s)material(s) per unit time

Parameter(s): pressure; concentration; reaction fraction,

dn/dt = kf(n)

k : rate constant

f(n) : amount function, n

e.g.: Polymer decay

d /dt = k (1 - )

Arrhenius: rate of reaction is

influenced by temperaturek = A exp (-E/RT)

E : activation energy

A : pre-exponential factor, depends on

the orientation and the structure of the

reactants

The rate of reaction is influenced by:

(a) The amount of reactants, and

(b) The temperature

k = A-E/RT


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Three types of chemical reaction

studies by TG, DTA/DSC:(i) The extent of reaction,

intermediates, and products

(i) Energy or heat involved in the

various levels of reaction

(i) Mechanism and kinetics of

reaction

For reactions in solution :

- concentration, cB For solid state reactions:

- reaction fraction,

Solid state endothermic reaction

A (solid) = B (solid) + C (gas)

- heat is absorbed as the gas and mass

lost

The extent of reaction, is a

quantity which has the dimension of

the amount of material

nB = nB,0 + B

nB : the amount of material B

nB,0 : the selected amount of B,

at t= 0

B : the stoichiometric number for B(+ve if B is the reaction product and

ve if B is the reactant)

The Reaction Kinetics


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Despite being measured at a fixed

temperature, rate of reaction changes

with time and the value of

Rate = d /dt = kT f( )

kT : rate constant at temperature T

: mathematical expression for

For a reaction in solution or

radiochemical reaction, f( ) is thesame throughout the sample, but it

may change when the reaction

becomes f( ) due to the chemicalchange, geometrical change, or a

change in reaction mechanism.

(a) Diffusion controlled reaction

d /dt = kT /2 ( )

(b) Two dimensional nucleus growth

(Avrami equation)

d /dt = kT (1 - ) (-ln(1 - ))1/2

(c) First order reaction: random decay of

an active species

d /dt = kT

(1 - )

Examples of relationship betweensolid state reaction and :

Interpretation of reaction kinetics

equation takes into account the

following:- the way reaction begins by

nucleation process- the nucleus growth

- the interaction or interfacial

geometry involved in the reaction

- the reactant decay

The Rate of Reaction


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The Rate Constant, k, is strongly influenced

by temperature and sometimes written as

kT.

Arrhenius equation:

kT = A exp(-E/RT)

A = pre-exponential factor

E = activation energy (J/mole)

R = molar gas constant (8.314 J/(K mole)

The reaction mechanism may change

during the reaction which can influencethe value ofE.

Temperature Control

In a non-isothermal experiment,

temperature is controlled according

to a linear temperature rise, K/min, and at the time t:

Tt = T0 + t

However, in the real situation an

endothermic or exothermic process

will change the actual temperature

and the equation is modified as

follows

Tt = T0 + t + s(t)

wheres(t) is the difference between

the sample temperature and the

programmed temperature.

The Reaction Rate Constant


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Heat capacity change

- estimated by the change in the

baseline of the DSC curve

- independent of the rate of reaction

but dependent on the materials

present in the sample Control of temperature gradient in

the sample- use as small sample size as possible

- instrumental control system.

The rate of a chemical reaction and a

physical change can be influenced bytemperature and H.

Hence, a relationship exists between

- the H and the DSC peak area

- the rate of reaction and the rate of

heat flow, P

DTA/DSC Peak Thermograms

The peaks are divided into several

fractions, each of which represents the

reaction fraction that has takenplace, respectively.

Kinetics study of a single step

reaction, example:

a) Decomposition of free radical initiator

of azobisisobutyronitrile (AIBN) by

DSC method:

4 32 K/min will produce data that is

suitable for first order kinetics and the

activation energy of 125 kJ/mol.

Reaction Kinetics and DSC Curves


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The DSC curve for an exothermic reaction

showing the measurement of partial peak area

b) Measurement of the rate of

polymer crystallization by

using the partial area to

determine the percantage ofthe polymer crystallized.

The results are normally

compatible with the Avrami

equation

[-ln(1 - ]1/ n

= kt

where

: the final crystallinity at thetime t

n : depends on the crystal growth

mechanism, e.g. n = 3 forspherical growth

The Relationship of DSC

Curve and

Figure 8.1


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Evaluation of thermal

hazards by the kinetics data

is a standard method ASTM

E698-79.

The sample is repeatedly heated for several times at various heating rates, , andthe peak temperature of the thermogram is recorded, Tmax . The plot of ln( )

versus 1/ Tmax produces a straight line with the slope ofE/R.

An isothermal DSC curve for polymer

crystallization

ASTM E698-79 Standard Method for

The Evaluation of Thermal Hazards

Figure 8.2


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The DTA curve for calcium

oxalate monohydrate:

(a) in nitrogen,

(b) in the air.

Sample: 10 mg, 10 K/min

Hydration waterOxalatedecomposition

Decomposition

of calcium

carbonate

Calcium oxalate monohydrate (CaC2O4 H2O)

(a)

(b)

CO + O2 CO2

The Application of DSC/DTA On The

Organic and Complex Compounds

Figure 8.3


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(a)

(b)

The DTA curve for copper sulphate

pentahydrate:

(a) unsealed plate

(b) sealed plate and pin-holed

Sample: 6 mg of powdered crystal,10 K/min, air-flow.

I CuSO4 5H2O = CuSO4 3H2O + 2H2O H (373 K) = 100 kJ/mole

II CuSO4 3H2O = CuSO4 H2O + 2H2O H (400 K) = 104 kJ/mole

III CuSO4 H2O = CuSO4 + 2H2O H (510 K) = 72 kJ/mole

Sample decomposition:

CuSO4 = CuO + SO2 + O2

I IIIII

Copper Sulphate Pentahydrate (CuSO4 5H2O)

Figure 8.4


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OPC = Ordinary Portland Cement contains largely calcium sylicates

because it is made by heating clay (alumino-sylicate) and calciumcarbonate

When used, cement is mixed with water that will hydrate the sylicates,

and it is gradually transformed into a good cement material that produces

the final strength in several months.

Figure 8.5: DTA curve for a sample of Portland Cement concrete (50 mg, 20K/min, nitrogen) (Source: H_F 3.38)

Quartz transition: 573o

C

dehydration

Ordinary Portland Cement (OPC)


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The high alumina cement (HAC) is made from bauxite and limestone to produce a calcium aluminate mixture.

When mixed with water, HAC produces cement at a faster rate

than the OPC, hence speeds up the construction process, but

under certain circumstances the HAC concrete beams tend to

break and result in accidents.

During the hardening process CaO Al2O3 10H2O is formed, but this is not the

most stable hydrate and will slowly be converted to a more stable compounds

such as hexahydrate, hydrated alumina or gibbsite.

3CaO Al2O3 10H2O (or C3AH6) and Al2O3 3H2O (or AH3)

3(CaO Al2O3 10H2O) = 3CaO Al2O3 6H2O + Al2O3 3H2O + 18H2O

High Alumina Cement HAC


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Conversion reaction weakens the

concrete because the reactionproduct is denser than the original

concrete and the reacting structure

becomes porous, especially if the

conversion process takes place

rapidly.

Decahydrate, hexahydrate andgibbsite lost their water molecules

when heated as shown in Figure

8.6.

Sampling by drilling of the

concrete should be carried out

carefully

Figure 8.6: DTA curves for standard HAC that has

conversion of 50 % (a) and 70 % (b) (source: H_F 3.39)

Avoid gypsum and plaster, remove the drilling metal pieces 10 100 mg of sample is analysed at 10 30 K/min, nitrogen atmosphere The degree of conversion is determined from the height of the peak

The Degree of

conversion (Dc)(a)

(b)


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100Amount of AH3

Dc =(Amount of AH3 + Amount of CAH10 )

or

100a(Peak height AH3)

Dc =

(aPeak height AH3 + b Peak height CAH10 )

or

100(Peak Height AH3)

Dc =

(Peak height AH3 + Peak height CAH10 )

Where a and b are calibration constants andK= b/a Figure 8.6: DTA curves for standard HAC that has

conversion of 50 % (a) and 70 % (b) (source: H_F 3.39)

Avoid gypsum and plaster, remove the drilling metal pieces 10 100 mg of sample is analysed at 10 30 K/min, nitrogen atmosphere The degree of conversion is determined from the height of the peak

The Determination of Dc


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The calibration constantKis determined by analysing a standard material

that has 50 % dan 70 % conversion. For the 50 % conversion, the peak

height AH3 is 3.5 cm, while the peak height for CAH10 is 3.7 cm.

Hence, 50 % = 100 x 3.5/(3.5 + Kx 3.7)

and K = 0.95

For the unknown sample HAC, the peak height HAC3 is 4.4 cm and for

the peak CAH10 is 2.6 cm

Therefore Dc = 100 x 4.4/(4.4 + 0.95 x 2.6)

= 64 5 % conversion

Calculation for the Determination of Dc


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Figure 8.7: DTA curve for

several types of minerals

(source: H_F 3.40)

Peaks- Single mineral components

e.g. quartz, undergoes phase

transition- Hydrated minerals- Dehydration peaks indicate

hydroxyl

- Carbonate minerals lost CO2

The Application of

DSC/DTA to the Clays

and Minerals


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Figure 8.8: DTA curve for kaolinite

(Source: H_F 3.41)

Hydration water

Dehydroxylation

Formation of mullite crystal

3Al2O32SiO2

Kaolinite


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A complex mineral

Some contains anion that is bound to chained

bondong and not easily dissociated except athigher tempertatures (source: H_F 3.42(b))

M2B6O11 xH2O

Some contains discrete ions that losses water molecules at atemperature below 250 oC (Source: H_F 3.42(a))

Borate Figure 8.9


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a) BaCO3

b) Iron oxide: Fe2O3

c) BaCO3 : 6Fe2O3 mixture

Two levels of crystal transition

Barium hexaferite: BaFe12 O19

- Useful as a solid magnet in

the induction component

High temperature inorganic reactions

produce many new compounds useful for

the electronics industry

Example: heating of a

BaCO3 and Fe2O3 mixture

Synthesis of compounds at

higher temperature

Figure 8.10


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Polymer/oil decays when heated in an

oxidizing atmosphere

The sample is heated in N2 until 200oC

The atmosphere is changed to oxygen and

the temperature maintained at 200 oC

The time required for the sample to achieve

the oxidation process is recorded, or

The polymer is heated in O2 and the

temperature where the oxidation occurs is

recorded.

Figure 8.11: DSC curve for the oxidation ofpolyethylene (PE). The dotted line indicates thechange of atmosphere N2O2.

(a) DSC scan for the PE layer, 10 K/min, oxidation

onset temperature = 220 oC

(b) DSC isotherm for the PE layer at 200 oC; onset

time = 35 min. H_F 3.46

Oxidation Decay of Polymers


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Polymerization: reaction

between small molecules to

produce bigger molecules with

different properties

For unsaturated molecules, such

as styrene and vinyl chloride,

H = -100 kJ/mole (exothermic)

R-NH2 + CH2----CH-R R-NH-CH2-CH-R

R-N(CH2-CHOH-R)2

O OH

a) Reaction takes place at low temperaturebut becomes faster when heated above

100 oC. The initial reaction ofTg is

followed by an exothermic curing

reaction.

b) Re-heating of the cured sample may

produce higher temperature of Tg .

1st heating

2nd heating

Polymer Curing

Figure 8.12


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Proteins (e.g. colagen) can exist as longmolecules such as fibre, or round or

compact molecules such as insulin.

Protein structure:- folded- scrolled- sheet- helix and super-helix

(helix in helix)

The structures are destroyed

when heated or denaturized

under an extreme pH.These changes are endothermic.Complex samples

of animal muscle

protein

Colagen (single material)

800 mg, colagen solution 0.3

%, sealed container, 0.5 K/min

850 mg, sealed container, 0.2 K/min

Protein Denaturation

Figure 8.13


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Figure 8.14 shows DTA curves for the

decomposition of poly(vinyl chloride), PVCand polypropylene (PP) (Source: H_F 3.49 & 3. 50)

PVC

PP

PVC: PVC powder shows a small glasstransition at 80 oC, followed by a small

endothermic process at 300 oC, and

immediately followed by a large exothermicat 550 oC:

- decay process with a loss of HCl

- volatile material

- oxidation of the released carbon

PP: decomposed in a single step processand the product is easily oxidised. Hence, the

endothermic process of melting at 80 oC is

followed by a large exothermic oxidation

peak.

Polymer Decay

Lecture 8 Application DTA & DSC01

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