Thermo-oxidation and degradation
of polymers
Jozef Rychlý
Polymer Institute
Dúbravská cesta 9, 843 42 Bratislava, Slovak Republic
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Polymers utilised practically usually have unknown residual stability and the
unknown concentration of additives – trajectory of the service life of the material
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Possible harmful effect on polymer products
• Oxygen in air
• Heat
• Hydrolysis associated with humide atmosphere
• Light of wavelength >300 nm
• High energy radiation
• Mechanical stress
• Biological attack
• Contacting liquids
• Leaching of additives
•
Presence of different reagents
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Acid hydrolysis of cellulose chains – example of combination of cross
reaction of hydrolysis and free radical oxidation on terminal groups
formed subsequently from hydrolytic attack
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Chemical degradation is accompanied by the
reduction of the molar mass, increase of the
molar mass due to crosslinking, or it occurs as
polymer analogous reaction typical by unzipping
of side groups of the macromolecular chain.
Degradation (physical) may involve also the
physical processes like recrystallisation,
denaturation (proteins). Ageing, (corrosion) is
related to the long term degradation due to
weathering and involves both.
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Chemical degradation
Polymers are organic and inorganic
materials composed of structural units –
mers – kept together by chemical bonds.
Their stability properties are detemined by
long entangled chains and by free volume.
Small change such as disruption of the
chain may change the properties
significantly.
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Physical degradation
Loss of properties due the change in
position of macromolecular chains and
additives in the volume without necessary
chemical change.
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Foil from low density polyethylene in advanced
stage of its degradation
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Dissociation energies of bonds A-B in kJ/mol that may form
the skeleton of the macromolecular structure in polymers.
A\B
C
N
O
S
Si
C
348
292
352
259
290
N
292
160
222
O
352
222
139
S
259
Si
290
C=C 615
369
213
369
N=N 418
C=N 615
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Degradation starts by the formation of active sites (radicals, ions,
excited states) on the macromolecular chain.
initiation
+
+
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Tests of oxidation stability
Oxidation stability tests follow from Bolland Gee
scheme:
Time or temperature evolution of concentration of
hydroperoxides, DSC, thermogravimetry,
chemiluminiscence, analytical determination of
carbonyls, mechanical properties changes, etc.
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Bolland-Gee scheme for free radical mechanism of polymer degradation valid
to temperatures ca 250 oC, (P and Z denote macromolecular chains of the
different length, InH is chain breaking inhibitor, D peroxide decomposer, P., Z.
are polymer radicals
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Induction time, the easiest way of the
characterisation of the polymer stability
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concentration of hydroperoxides, rel.u.
more stabilized
sample
1.0
0.8
unstabilized sample
0.6
less stabilized
0.4
sample
0.2
induction period
0.0
1000
10000
time, s
Schématické znázornenie merania indukčnej periódy v stabilizovanom
polypropyléne.
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100000
0.010
20000
0.008
0.006
0.004
0.002
0.000
0.008
0.010
[D]0, mol/l
16000
induction time, s
1
12000
8000
2
4000
0
0.000
0.002
0.004
0.006
[InH]0, mol/l
Teoretická závislosť indukčnej periódy oxidácie určená pre wi=0, (nulová iniciačná
rýchlosť podľa rov. 1, Schéma 1) od zloženia zmesi inhibítorov InH (“chain breaking”
antioxidant) a D (rozkladač hydroperoxidov), u ktorej je suma koncentrácií 0.01 mol/l).
Spodná krivka 2 zobrazuje závislosť indukčnej periódy pre tie isté hodnoty parametrov
ako pre čiaru 1 ale wi=5 10-8 mol/l/. Počiatočná koncentrácia hydroperoxidov bola 0.001
mol/l
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Examples
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1. Original and aged samples of polyether
and polyester urethanes
daylight (0-15 days), 1000 Wm-2,
25°C/50% RH
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Structural segments of polyurethanes
CH3
CH3
CH3
O
CH CH2
n
O
C
NH
NH
C O CH2
CH
O
m
O
O
Polyether urethane 1
CH3
O
(CH2)6
O
C
CH2 CH2 2 CH2 C O
(CH2)6 O C NH
O
O
m
NH C
O
O
Polyester urethane 2
CH3
( O
CH CH2 )
CH3
n
O
C
NH
CH2
NH
C O ( CH2
O
O
Polyether urethane 3
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CH O )
m
The comparison of non-isothermal thermogravimetry for
reference sample polyether urethane 1 and polyester
urethane 2 in nitrogen and oxygen, the rate of heating
5oC/min. Points denote the theoretical fit.
100
% of the mass
80
60
sample 2 nitrogen
sample 1 oxygen
40
sample 2 oxygen
sample 1 nitrogen
20
0
50
100
150
200
250
300
350
400
o
temperature, C
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450
500
550
Comparison of nonisothermal thermogravimetry and DSC
records for polyether (sample 1) and polyester (sample 2)
polyurethane foams, oxygen, the rate of heating 5 oC/min
12
100
1O
10
80
1O
6
2O
60
4
40
2
0
20
2O
-2
100
200
300
o
temperature, C
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0
400
% of the mass
DSC, mW
8
Chemiluminescence and DSC measurements in oxygen for
non-aged samples 1 and 2. The rate of heating 5 oC/min.
300000
10
1
DSC, mW
8
1
200000
6
4
2
100000
0
2
-2
2
100
0
200
300
o
temperature, C
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400
chemiluminescence intensity, counts/s/1 mg
12
Obvious conclusion!
Polyester urethane are hydrolytically less
stable than polyether urethane while
polyether urethanes are less stable
towards light induced degradation.
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Characterisation of aged
polyurethanes
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The chemiluminescence intensity runs for aged polyether and
polyester urethanes samples 1a-3a (15 days-red), oxygen, 1-3 are
original non-aged samples, the rate of heating 5 oC/min.
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The non-isothermal thermogravimetry runs for aged
polyether and polyester urethanes samples 1a-2a (15 daysred), nitrogen, 1-2 are original non-aged samples, the rate
of heating 5 oC/min
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Conclusion
Degradation is inevitable symptom of the
polymer service life. It can be slowed
down, with much more pronounced
induction time, but it cannot be avoided.
The detailed knowledge on the kinetics is
always necessary!
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5, 2013, morning