Thermo-oxidation and degradation of polymers

Jozef RychlýPolymer 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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5, 2013, morning

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.

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A\B C N O S Si

C 348 292 352 259 290

N 292 160 222

O 352 222 139 369

S 259 213

Si 290 369

C=C 615 N=N 418

C=N 615

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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1000 10000 1000000.0

0.2

0.4

0.6

0.8

1.0

more stabilized

sample

less stabilized

sample

unstabilized sample

con

cen

tra

tion o

f hyd

rop

ero

xid

es,

rel.u

.

time, s

induction period

Schématické znázornenie merania indukčnej periódy v stabilizovanom polypropyléne.

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0.000 0.002 0.004 0.006 0.008 0.0100

4000

8000

12000

16000

20000

indu

ctio

n tim

e, s

[InH]0, mol/l

0.010 0.008 0.006 0.004 0.002 0.000

2

1

[D]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

O CH

CH3

CH2 n O C

O

NH

CH3

NH C

O

O CH2 OmCH

CH3

Polyether urethane 1

O C

O

CH2 2 CH2CH2 NH C

O

CH3

)6CH2(O C

O

O (CH2)6 O C

O

NH

m

Polyester urethane 2

O n O C

O

NH C

O

O CH2 CH

CH3

OCH2 NHCH

CH3

CH2( ) ( )m

Polyether urethane 3

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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.

50 100 150 200 250 300 350 400 450 500 550

0

20

40

60

80

100

sample 2 oxygen

sample 2 nitrogen

sample 1 nitrogen

sample 1 oxygen

% o

f th

e m

ass

temperature, oC

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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

100 200 300 400

-2

0

2

4

6

8

10

12

0

20

40

60

80

100

2O

DS

C,

mW

temperature, oC

2O1O

1O

% o

f th

e m

ass

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Chemiluminescence and DSC measurements in oxygen for non-aged samples 1 and 2. The rate of heating 5 oC/min.

100 200 300 400

-2

0

2

4

6

8

10

12

0

100000

200000

300000

2

1

2

1

DS

C, m

W

temperature, oC

che

milu

min

esc

en

ce in

ten

sity

, co

un

ts/s

/1 m

g

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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 days-red), 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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