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Thermal Analysis of

Rubbers and Elastomers

Mackenzie Geiger

Applications Scientist

September 6, 2017


What does it measure and what is it used for?

Thermal Analysis is a series of complementary techniques to measure various properties of materials as a function of temperature and time

▪Thermogravimetric Analysis (TGA) Weight loss, weight gain, compositional and thermal stability

of a material. [Sorption Analysis] Provides information to aid in the interpretation of DSC data.

Extremely complementary to DSC.

▪Differential Scanning Calorimetry (DSC) Measures Heat Flow into or out of a sample Modulated DSC – Separates Heat Flow into Heat Capacity,

Reversing and Non Reversing Heat Flow

▪Thermomechanical Analysis (TMA) Determines dimensional changes of a material, coefficient of

thermal expansion, glass transition

Thermal Analysis


What Materials?

•Iron and Steel

•Aluminum and other metals (Mg)

•Adhesives and Sealants

•Textiles / Leather

•Rubbers and elastomers

•Thermoplastic Polymers

•Composites

•Thermosets ans Resins

•Coatings and Paints

•Fluids and Lubricants

•Glass

•Other (electronics, fuel cell, gasoline, nano-materials, ceramics, sensors, etc.)


Agenda

1. Thermogravimetric Analysis

Standard and Hi-Res TGATM, TGA-MS, Decomposition and Lifetime Kinetics

2. Differential Scanning Calorimetry

Technique Overview, Conventional vs. Modulated DSC®, Curing Kinetics


Thermogravimetric Analysis

(TGA)


Thermogravimetric Analysis (TGA)

•TGA measures amount and rate of

weight change vs. temperature or

time in a controlled atmosphere

•Used to determine composition and

thermal stability up to 1000°C (Q50 &

Q500); 1200°C (Discovery TGA) &

1500°C (SDT)

•Characterizes materials that exhibit

weight loss or gain due to

decomposition, oxidation, or

dehydration


Mechanisms of Weight Change in TGA

• Weight Loss:

▪ Decomposition: The breaking apart of chemical bonds.

▪ Evaporation: The loss of volatiles with elevated

temperature.

▪ Reduction: Interaction of sample to a reducing atmosphere

(hydrogen, ammonia, etc).

▪ Desorption.

• Weight Gain:

▪ Oxidation: Interaction of the sample with an oxidizing

atmosphere.

▪ Absorption.

All of these are kinetic processes (i.e. there is a rate at

which they occur).


Decomposition of Elastomers in Nitrogen


Volatilization of Plasticizers/Oils


Styrene-Butadiene Rubber Analysis

0 200 400 600 800 1,0000

25

50

75

100

Temperature (°C)

Wt (%

)

8.4% Oil

50.4% Polymer

Air

36.2% Carbon Black

5% Inert Filler

Sample weight: 30 mg

Program rate : 20°C/min

Atmosphere : N , Air2


TGA of Tire Rubber

6.122% Oil(0.6654mg)

51.84% Polymer(5.634mg)

38.96% Carbon(4.234mg)

Residue:3.013%(0.3275mg)

TGA Analysis10.87 mg of Tire Rubber Compound20°C/min to 600 °C in N220°C/min to 1000°C in air

0

2

4

6

8

10

12

Deriv.

Weig

ht (%

/min

)

0

20

40

60

80

100

Weig

ht (%

)

0 200 400 600 800 1000

Temperature (°C)

Sample: Cured Tire Tread Rubber CompounSize: 10.8690 mg

File: CURED TIRE TREAD RUBBER COMPOUND.001Run Date: 18-Nov-2005 12:06

Universal V4.5A TA Instruments


TGA of Rubber in Nitrogen

424.01°C

487.73°C

141.62°C

1.202%(0.1205mg)

62.81%(6.297mg)

Residue:32.14%(3.223mg)

-1

1

De

riv.

Weig

ht (%

/°C

)

20

40

60

80

100

120

Weig

ht (%

)

200 400 600 800 1000

Temperature (°C)

Sample: Rubber 10C/min N2 - Green ColorantSize: 10.0264 mg


TGA Rubber in Air

420.58°C

477.91°C

1.567%(0.1551mg)

76.50%(7.569mg)

Residue:21.69%(2.146mg)

-2

0

2

4

6

Der

iv. W

eigh

t (%

/°C

)

20

40

60

80

100

120

Wei

ght (

%)

200 400 600 800 1000

Temperature (°C)

Sample: Rubber 10C/min Air - Green ColorantSize: 9.8941 mg


TGA Rubber in Air vs Nitrogen

1.567%(0.1551mg)

76.50%(7.569mg)

Residue:21.02%(2.080mg)

1.202%(0.1205mg) 62.81%

(6.297mg)

Residue:32.14%(3.223mg)

420.58°C

477.91°C

424.01°C 487.73°C

-2

0

2

4

6

Deriv. W

eig

ht (%

/°C

)

20

40

60

80

100

120

Weig

ht (%

)

200 400 600 800 1000

Temperature (°C)

Rubber 10Cmin Air - Green Colorant.UA––––––– Rubber 10Cmin N2 - Green Colorant.UA––– –––


Hi-ResTM TGA

• Faster heating rates during

periods of no weight loss, and

slowing down the heating rate

during a weight loss – therefore

not sacrificing as much time

• Hi-ResTM TGA can give better

resolution or faster run times,

and sometimes both

• In a Hi-ResTM TGA experiment the heating rate is controlled by the rate of

decomposition.


EPDM Rubber by TGA & Hi-Res™ TGA


Process Oil in a Rubber Compound by Hi-Res

TGA


TGA-Evolve Gas Analysis (EGA)

•TGA measures weight changes (quantitative)

•Difficult to separate, identify, and quantify individual

degradation products (off-gases)

•Direct coupling to identification techniques (Mass Spec,

FTIR) reduces this problem


TGA-MS of Styrene Butadiene Rubber (SBR)

Deri

v. W

eig

ht (%

/min

)

30

50

70

90

Weig

ht (%

)

0 100 200 300 400 500 600 700

Temperature (°C)

Sample: SBRSize: 1.8030 mg

Comment: Helium

TGAFile: C:...\Bnl\Rubber\tga\SBR\Sbr1.001Operator: Daniel ROEDOLFRun Date: 31-Dec-01 16:31Instrument: AutoTGA 2950HR V5.4A

Universal V3.2A TA Instruments

2mg sample

He purge,

40°C/min


Fingerprint of Natural Rubber vs SBR

Natural

Rubber

Styrene

Butadiene

Rubber

9167

67


Mass 45 to 170

Natural

Rubber

Styrene

Butadiene

Rubber

Nitrile

Rubber

N

N


Mass 100 to 200

Natural

Rubber

Styrene

Butadiene

Rubber

Nitrile

Rubber

N

N


Styrene Butadiene Rubber

SBR

250°C

400°C

500°C

Deri

v. W

eig

ht (%

/min

)

Weig

ht (%

)

0 100 200 300 400 500 600 700 800

Temperature (°C)

64

4878

250°C

400°C

500°C


Decomposition and Lifetime

Kinetics


Simple Polymer Decomposition

Constant Gas Universal R

(K) re temperatu T

mol) / (kJEnergy Activation E

lexponentia-pre A

min) / C( rate heating

where

ln

exp

RT

EA

RT

EA


Simple Polymer Decomposition

•Experimental

•Run TGA experiment on polymer at 4 different heating

rates. Use the same gas for each – typically nitrogen or

air.

•Obtain a temperature at an isoconversional point – for

example 2% weight loss for each heating rate

•Plot the ln of the heating rate () versus 1/T (temperature

units must be in Kelvin)

•Slope of the line is (-ΔE/R). Multiply the slope of the line by

–(8.314 x 10-3) to obtain the activation energy in kJ/mol.


Decomposition Kinetics


Decomposition Kinetics

1000/T (K)


TGA Kinetics – Isoconversion Plot


TGA Kinetics - Estimated Lifetime

TEMPERATURE (°C)

1.51.61.71.81.910

100

1000

10000

100000

1000000

1000/T (K)

ES

TIM

AT

ED

LIF

E (

hr.

)

260 280 300 320 340 360

1 century

1 decade

1 yr.

1 mo.

1 week

1 day

ES

TIM

AT

ED

LIF

E

TA Instruments Application Note TA125

Estimation of Polymer Lifetime by TGA Decomposition Kinetics


Differential Scanning Calorimetry

(DSC) and Modulated DSC® (MDSC)

Overview


Differential Scanning Calorimetry (DSC)

•Differential Scanning Calorimetry (DSC) is most popular

thermal analysis technique

•DSC measures endothermic and exothermic transitions as

a function of temperature

▪Endothermic heat flows into a sample

▪Exothermic heat flows out of the sample

•Used to characterize polymers (thermosets,

thermoplastics, elastomers)

•Also used with pharmaceuticals, foods/biologicals, organic

chemicals and inorganics

•Transitions measured include Tg, melting, crystallization,

curing and cure kinetics, onset of oxidation and heat

capacity


What is Differential Scanning Calorimetry?

•Calorimetry is a technique for determining the quantity of heat that is

either absorbed or released by a substance undergoing a physical or

chemical change.

•A DSC measures the difference in Heat Flow Rate between a sample

and inert reference as a function of time and temperature.


Simple Heat Flux DSC Cell Schematic

Reference Sensor

An empty pan on the

reference sensor should

react similarly to the pan

on the sample sensor, thus

canceling out any pan

contribution

Sample Sensor

Heat absorbed by the

sample gives an

endothermic response

Heat released by the

sample gives an

exothermic response

Sample


DSC Heat Flow

t)(T,dt

dT Cp

dt

dHf

Heat Capacity

Glass Transition

Kinetic

Crystallization

Cure reactions

Volatilization

Decomposition

Denaturation


Exo Up

Exo Up

Glass Transition, Tg(Half height) 23.08 °C

Endothermic Heat Flow

Heat Absorbed by Substance

Endothermic Events

•Glass transition

•Melting

•Evaporation/

volatilization

•Enthalpic recovery

•Polymorphic

transitions

•Some decompositions


Exothermic Heat Flow

Heat Released by Substance

Exothermic Events

•Crystallization

•Cure reactions

•Polymorphic transitions

•Oxidation

•Decomposition

Exo Up

Exo Up


Typical DSC Transitions

Temperature

Heat F

low

->

exoth

erm

ic

Glass

Transition

Crystallization

Melting

Cross-Linking

(Cure)

Oxidation

Or

Decomposition

Composite graph


Polyethylene Terephthalate (PET)

Second Heat

First Heat

Cool

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Heat F

low

(W

/g)

20 60 100 140 180 220 260

Temperature (°C)

Exo Up


What Does MDSC® Measure?

•MDSC separates the Total heat flow of DSC into two parts based on the heat flow that does and does not respond to a changing heating rate

•MDSC applies a changing heating rate on top of a linear heating rate in order measure the heat flow that responds to the changing heating rate

•In general, only heat capacity and melting respond to the changing heating rate

•The Reversing and Non-reversing signals of MDSC should never be interpreted as the measurement of reversible and nonreversible properties


Reversing Heat Flow and Heat Capacity

-6

-4

-2

0

2

Rev H

eat F

low

(m

W)

-12

-8

-4

0

4

Rev C

p (

J/(

g·°

C))

0

4

8

12

16

Deriv. M

odula

ted T

em

pera

ture

(°C

/min

)

-8

-6

-4

-2

0

2

Modula

ted H

eat F

low

(m

W)

-50 0 50 100 150 200 250 300 350

Temperature (°C)

Exo Up Universal V4.2E TA Instruments


•Enthalpic Recovery

•Evaporation

•Crystallization

•Thermoset Cure

•Denaturation

•Decomposition

•Some Melting

MDSC Heat Flow Signals

Total

Heat Flow

t)(T,dt

dT Cp

dt

dHf

Reversing

Heat Flow

Non-Reversing

Heat Flow

•Heat Capacity

•Glass Transition

•Most Melting

•All Transitions


Calculated MDSC Heat Flow Signals

Quenched PET – 8.99mg

.424/40@4


PET/ABS Blend - Conventional DSC

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

Temperature (°C)50 100 150 200 250

He

at F

low

(W

/g)

first heat on molded part

(Curve shifted on Y axis to avoid overlap)

second heat after 10°C/min cooling

120.92°C67.38°C

70.262°C (H)235.36°C

111.82°C9.016J/g

22.63J/g

249.75°C

9.22 mg sample, nitrogen purge 10°C/minute heating rate


PET/ABS Blend - MDSC

8.46mg sample

nitrogen purge

2°C/minute heating rate, ±1°C amplitude, 60 second period

first heat on molded part

PET Tg

ABS Tg

-0.10

-0.11

-0.12

-0.13

-0.14

-0.15

Temperature (°C)

40 60 100 120 140

He

at

Flo

w (

mW

)

20 80 160

-0.02

-0.03

-0.04

-0.05

-0.06

-0.04

-0.05

-0.06

-0.07

-0.08

-0.09

(

) N

on

rev.

He

at

Flo

w (

W/g

)

(

) R

ev.

He

at F

low

(W

/g)

67.00°C

+72.89°C (H)

104.45°C

107.25°C (H)

180 200


Elastomer Tg by DSC

-62.63°C(H)

Elastomer - 10.18mgDSC - 10°C/min

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Heat

Flo

w (

mW

)

-80 -60 -40 -20 0 20

Temperature (°C)

Exo Up Universal V4.3A TA Instruments


Elastomer Tg by DSC & MDSC

-61.08°C(H)

-64.57°C(H)Elastomer - 10.46mgMDSC - 0.48/60@3°C/min

-0.4

-0.2

0.0

0.2

Rev H

eat F

low

(m

W)

-0.6

-0.4

-0.2

0.0

Heat F

low

(m

W)

-80 -60 -40 -20 0 20

Temperature (°C)



Curing Kinetics


Typical properties of crosslinking reactions

• Crosslinking reactions are generally

exothermic. As the chemical reaction

takes place, it is almost always

accompanied by a release of heat.

• The reactions can be easily monitored

using a DSC.

▪Heat of reaction

▪Residual cure

▪Glass transition

▪Heat capacity

• Crosslinking reactions are generally accompanied by a sharp change in the material’s mechanical properties.

• Increase in modulus that may be accompanied by shrinkage.

• The reactions can thus be monitored using a Thermo-mechanical Analyzer (TMA)/Dynamic Mechanical Analyzer (DMA)/Rheometer.

▪Viscosity

▪Modulus

▪Glass transition

▪Dimension change (shrinkage)

These techniques give useful information about the impact of the polymerization

conditions on the end product’s thermo-mechanical properties.


Curing reactions are kinetic in nature

128.29°C0.5594W/g

122.26°C323.9J/g

137.04°C0.9506W/g

130.12°C315.5J/g

149.93°C1.972W/g

141.85°C315.1J/g

160.93°C3.431W/g

151.92°C320.0J/g

172.86°C5.792W/g

162.53°C320.5J/g

-2

0

2

4

6

He

at F

low

T4

(W

/g)

100 120 140 160 180 200 220 240

Temperature (°C)

1°C/min2°C/min5°C/min10°C/min20°C/min


Use of Kinetic Modeling for Characterization of Curing

Reactions

•Predict how long a reaction takes to go to completion

•Optimize polymerization, curing

•Quantify parameters that characterize time-temperature-dependent process behavior under conditions that may not always be experimentally feasible.


Fundamental equation for kinetics

)()(

fTfdt

d• Where:

= reaction rate

f() = a function of

f(T) = a function of Temp.

= fraction reacted

or converted

dt

d

dt

d

T

f(T) f() dt

d f(T)*f()dt

d

)()(

fTfdt

d• Where:

= reaction rate

f() = a function of


= fraction reacted

or converted

dt

d

)()(

fTfdt

d• Where:

= reaction rate

f() = a function of


= fraction reacted

or converted

dt

d

dt

d

T

f(T)dt

d

T

f(T)dt

d

T

f(T) f() dt

d f(T)*f()dt

df() dt

d f(T)*f()dt

d

Our Goal: Use DSC to solve for these functions


Fundamental equation for kinetics: the

temperature factor

•Fundamental equation for kinetics

•Arrhenius temperature dependence

•Derived from dilute gas

or solution, refined for solids

•Physical significance: Molecules

colliding with sufficient kinetic energy to

overcome Ea cause a reaction

•Pre-exponential factor, Z, “frequency factor”

accounts for steric effects

RTEaZeTf /)(

Where Ea is activation energy

Z is the “frequency factor”

R is the gas constant

T in kelvin

)()(

fTfdt

d•

E Ea


Selection of appropriate model – the “” factor

•Fundamental equation

•Many models, three simple ones

▪nth order reaction:

▪Modelling technique:

n = 1: ASTM E6981/Ozawa, Wall and Flynn method2

n ≠ 1: ASTM E20413/Borchardt and Daniels method4

▪Autocatalyzed reaction:

▪Modeling technique:

ASTM E20705/Sestak and Berggren method (Isothermal

kinetics)6

n and m are reaction orders

)()(

fTfdt

d•

nf )1()(

nmf )1()(

1ASTM E698, ASTM Annual Book of Standards 2005 volume 14.022Ozawa, T.J. J. Thermal Analysis, 1970, v2, p3013ASTM E2041, ASTM Annual Book of Standards 2005 volume 14.024Borchardt, H.J., and Daniels, F.J., Am. Chem. Soc. 1956, v79, pg 41

5ASTM E2070, ASTM Annual Book of Standards 2005 volume 14.026Sestak, J., and Berggren, G., Thermochim. Acta, 1971, vol 3, pg 1

n is reaction order


Obtaining kinetics information from a DSC

experiment

10

30

50

70

90

Are

a p

erc

en

t (%

)

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

He

at

Flo

w (

W/g

)

40 60 80 100 120 140 160 180 200

Temperature (°C)

Sample: ThermosetSize: 5.5360 mgMethod: RT to 250°C at 10°C/minComment: He Purge = 25ml/min

DSCFile: C:\TA\Data\DSC\DSC-CURE.001Operator: C. Jay Lundgren


DSC gives us and d/dt

vs time and temperature

Running integral option

provides as a direct function

of time/temperature


Kinetic analyses can provide valuable

information

Half-life of the reaction: Use as litmus test

for validity of kinetic model

Predict the cure temperature for one hour halflife and carry out the iso cure in the DSC. Coolthe sample after 1 hour and rerun. Was theresidual reaction ½ the total heat of reaction?


Case Study: DSC of Green Colorant Rubber


Case Study: Isothermal DSC Curing

Method:

1. Equilibrate to 150 °C (160, 170, or 180 °C)

2. Mark end of cycle

3. Isothermal for 30 minutes

4. Mark end of cycle

5. Equilibrate to -100°C

6. Ramp 10°C/min to 300°C


Case Study: DSC Isothermal at 150 °C


Case Study: Green Colorant Rubber Sample

Ramp After Iso Hold at 150C


Case Study: DSC Isothermal Treatments


Case Study: Summary

• Differential scanning calorimetry can be used to simulate

plant processing of thermosetting materials with limitations

•Common limitations are:

▪ Too high a curing temperature to get good DSC data

▪ Too low small or no curing exotherm

Examples: low level peroxide crosslinkers, vulcanization,

highly cured specimens

• DSC can be used both to thermally condition the

thermoset and then determine the extent of cure

• DSC kinetic model can be highly predictive

• Ongoing cutting edge academic studies


Section Summary

•Thermal analysis – both TGA and DSC – are widely used

in the rubber industry

▪ Material characterization (QC, R&D, etc.)

▪ Process optimization through cure kinetics

▪ Stability (thermal, oxidative)

•TA Instruments is a premier supplies of thermal analytical,

rheological, thermal physical and other instrumentation to

all technology based industries


Any questions?

Thermal Analysis of Rubbers and Elastomers - TA Instruments · Thermal Analysis of Rubbers and...

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