aging of polymeric materials

Aging of polymeric materials: principlesEric David

Mechanical engineering department

cole de technologie suprieure

1 http://www.kb.nl/cons/leathers/

1

1

Aging: Slow and irreversible alteration of a material chemical or physical structure. This alteration has normally a detrimental effect on the material properties. It leads to gradual loss of the design function and ultimate failure or unacceptable loss of efficiency.

Chemical aging:Thermochemical aging

Anaerobic agingThermoxidation

Photochemical aging Aging in the presence of reactive agents (hydrolysis, ozone, air pollution..)

Physical aging: Free volume relaxation (polymers cooled from the melt below Tg) Solvant absorption and migration of additives

Aging models and prediction of life time

Chemical aging Random chain scission

The most important mechanism responsible for the aging of polymeric material is chain scission. It will leads to a significant deterioration of the mechanical properties.Ex.: Anaerobic thermolyse of PE and PP

2 J. Verdu, Techniques de lIngnieur, AM 3-152

non MtMtn 1

)(1)( =

2

2

It can be quantified by molecular mass measurement

Random chain scission: effect on the mechanical propertiesThe reduction of the molecular mass induced by chain scission lowers the mechanical strength and lead to embrittlement.

c

c

c

tLMM

tt

=

3

3 J. Verdu, Techniques de lIngnieur, AM 3-1514 A. Meyer et al, Macromolecules, 35, pp. 2784-2798, 2002

4

Thermo-chemical aging: case of PP

Thermo-oxidative aging (?) of PP in air at 90oC4

5 B. Fayolle et al, Polymer Degradation and Stability, 70, pp. 333-340, 2000

Good correlation Not a good correlation

Evolution molecular weight (round symbol), ultimate elongation (diamonds), carbonyl content (square) and OH groups (triangle).

01

2

3

4

5

6

7

8

9

0 100 200 300 400 500 600 700 800 900 1000Extension (mm)

F

o

r

c

e

(

N

)

LDPE - 72h at 105oC

LDPE - unaged

C H

H

H

Thermo-chemical aging: case of LDPE

No evidence of increase of carbonyl content

C O

P

P

C O

P

H

C O

P

OH

Thermo-chemical aging Thermo-oxidation

Another very important mechanism related to the degradation of polymeric material is oxidation. The inception involves the reaction between a free radical on a polymer chain and an oxygen molecule. It is followed by a chain reaction until inactive products are produced.

Inception:

Propagation:

The inception and propagation involve the extraction of and hydrogen atom and accordingly polymers with weak C-H bonds are the most vulnerable to oxidation.

products Inactive22

2

2

2

2

+++

+

POPO

PHPOPHPO

POOP

Termination:


3

An important parameter controlling oxidation is the diffusion of oxygen. Crystalline parts of polymers are more impermeable than amorphous parts. Accordingly, the amorphous parts of a semi-crystalline polymer (PE, PA, PP..) are preferentially attacked by oxidation. Polymers with rubbery amorphous phases (PP, PE, rubbers) has to be stabilized.


Rate of oxidation as a function of the partial pressure of oxygen3. For P < Pc, the oxidation process is controlled by the diffusion; for P > Pc the reaction process is controlled by the rate of production of the initial free radical.

Example of oxidation:

HDPE composites6


6 R. Yang, Polymer Degradation and Stability, 91, pp. 1651-1657, 2006

Formation of oxidative products in HDPE at 110 oC (FTIR measurements) a) HDPE g) HDPE/mica (presence of an induction time)

Induction time

Example of oxidation:

1) Field aging of XLPE Cable7


7 N. Amyot at al, Internal report HQ8 E.L. Leguenza et al, IEEE Trans. on Dielectrics and Electrical Insulation, 11, pp. 406-417, 2004

Expertise on a failed XLPE caramelized cable after overheating (Montreal, downtown): 0.1 C=O/1000 C

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Photo-chemical agingPhoto-chemical aging is also thermo activated and is related to both the chemical and the optical properties of the polymers

=RTEIo exp

The general steps for photo-chemical aging are usually the following steps: Creation of a radical by a UV photon Oxidation: creation of a carbonyl group Photolyse of the ketone group

Aging of LDPE under a Xenon lamp at different temperature3

Photo-chemical aging

Since the light intensity is absorbed as it goes through the material, photo-chemical aging is a surface phenomena. The intensity of the UV radiation deceases according to the Beer-Lambert law:

( ) [ ]CxIxI o = expAddition of TiO2 pigments or carbon black increases the absorptivity and limits the effect of photo-chemical aging

9

9 J.-P. Bailon et al., Des matriaux, Presses internationales Polytechniques, 2000

Hydrolysis and depolymerisation

Hydrolysis affect polymers generated by polycondensation. The cleavage of the bounds between monomer units leads to a decrease of molar mass and of the mechanical properties. Polyester, polyamide (nylon) and naturally occurring polymers such as paper and leather are particularly vulnerable to hydrolysis. Hydrolysis directly leads to a decrease of molecular mass and a deterioration of the mechanical properties. Although they involve different reaction mechanisms, hydrolysis and oxidation are closely related: both processes often act simultaneously and may reinforce one another.

10 R. Bernstein et al, Polymer Degradation and Stability, pp. 480-488, 2005

Hydrolysis10


The chemical bonds of biodegradable polymers such as the material used in drug delivery systems are cleaved by hydrolysis (passively of actively via enzymatic reaction). Reactivity can change tremendously upon catalysis. It can be either acid or base catalysed (the depolymerisation of paper for example is acid catalysed).

11 A. Gopferich, Biomaterials, 17, pp. 103-114, 1996

11


Ex.: The case of paper12:

OHP OH POH2 ++

12 L. Lungaard et al, Doble conference, 2002

Hydrolysis and depolymerisationEx.: The case of PA (fibres used in parachute)10:

1) Thermo-oxidative aging from 37oC to 138oC (measurement of the mechanical properties)

To construct the master curve the times at a T temperature are transform into times at a reference temperature by multiplying them by a shift factor aT. If a single mechanism is involved the product aTt(T) has a similar effect at any temperature

Hydrolysis and depolymerisationEx.: The case of PA10:

1) Thermo-oxidative aging from 37oC to 138oC (measurement of oxygen consumption)

The tensile strength loss is not directly connected to oxidation and must involve a mechanism that does not correlate directly to the total consumption of oxygen


2) Under 100% RH + Argon from 80oC to 138oC ( in a sealed container)


3) Comparison between oxidation and hydrolysis

O2: rate = 1

H2O: rate = 7

O2 + H2O: rate = 23

Physical aging

Evolution of the mechanical properties of PVC with physical aging13

13 L.C.E. Struik, Physical Aging in Amorphous Polymers and Other Materials, Elsevier, 1978

v

TTg

i) FVR

( ) ttJ =)(

Physical aging

ii) Loss of plastifiant14

14 J. Verdu, Techniques de lIngnieur, AM 3 150

Lifetime and enduranceThe basis background for the lifetime models are the reaction kinetics equations: Lets take the following process and a simple first order rate law:

[ ] [ ][ ] [ ][ ]HCONHkOHNHCOkdt

NHCOdph 222 =

If we assume that the recombination plays a negligible role and that the concentration of water does not change:

[ ]( ) [ ] tko heNHCOtNHCO =Where the rate constant can be written in the form of an Arrhenius law

=RTEkk o exp

The shift factor also follows an Arrhenius law when a single processes is involved

Lifetime and enduranceWe can distinguish two cases of aging:

1) Pyrolysis: this is a case of aging at very high temperature and it is not encounter in the use of polymeric material for practical application except in the case of an accident (fire). The pyrolysis is used to determined the onset of the thermal decomposition of a polymeric material and consequently the ultimate service temperature.

2) Aging during normal service conditions: this is the most difficult case to treat because of the very low rate of the degradation processes in real life. In order to establish aging model and to predict life time in real usage condition, one needs to rely on accelerated aging test. A compromise between being representative of the reality and the duration of the test most be careful adressed when using accelerated aging test.

Lifetime and enduranceA critical concentration of chain scission is reach for a critical value of kt, thus defining a life time

=RTELL o exp

Ex.: Example of accelerated aging test and endurance calculation (EPR15)

10

100

1000

10000

100000

1000000

10000000

100000000

1000000000

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

1000/T

L

(

h

o

u

r

s

)

DataRegression (Arrhenius)TI line 23 oC

L = 24000 y !

TI = 111 oC

15 G. Mazzanti et al, J. Phys. D, 1994

Lifetime and enduranceLoss of mass, % of elongation, tensile strength are the usual end of life criteria but any other relevant physical parameters can be used. For example, the electrical breakdown strength15:

Lifetime and enduranceMulti-stress agingIf an active stress such as a mechanical or an electrical stress is applied, the height of the energy barrier is reduced. It could be expressed by a modified Arrhenius law (many other empirical model are available)16:

=

=

kTGAk

NkNkdtdN

221

2211122

exp

=RTVELL oexp

16 T.J Lewis, IEEE Elect. Ins. Mag., 17, no 4, pp. 6-16, 2001

Lifetime and enduranceMulti-stress aging

Example: Electrical + thermal stress17

=RT

EqELL oexp

17 P. Cygan et al, IEEE Trans. Electrical Insulation, 25, pp. 923-934, 1990

Conclusions

Aging of polymeric material under normal service conditions is a difficult case to treat. Most of the studies are based on accelerated aging test and required extrapolation to normal service conditions.

Many factors can significantly affect the durability of a polymeric material, such as temperature, irradiation, moisture, chemicals, presence of an active physical stress..

Synergism in the global aging often occurs when the simultaneous action of several stresses results in an aging effect that differs from that which would be observed if the individual stresses were applied sequentially.

Aging models have a limited used for practical applications because real life aging conditions are usually far more complex than what can be simulated in laboratory. When assessing the conditions of a polymeric material is critical, it is usually better to conduct periodic diagnostic tests in the field (visual inspection, chemical measurements, physical measurements, ) than to rely on existing models.

aging of polymeric materials

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