Conference Nantes aug. 2012.

Humid ageing of polymers and compositesby Xavier Colin and Jacques Verdu

Arts et métiers ParisTech. ParisArts et métiers ParisTech. Paris

Summary

• Introduction

• Equilibrium characteristics• Effect of activity, sorption mechanisms

• Effect of temperature and stresses.

• Structure-hydrophilicity relationships

• Consequences of water absorption on physical propertie• Consequences of water absorption on physical propertie

• Hydrolysis• Non equilibrated hydrolysis seen as a molecular process

• Non-equilibrated hydrolysis. Ion catalysis, autocatalysis

• Hydrolysis processes in the interfacial zone

• Consequences on mechanical properties

• Osmotic cracking

• Conclusions

Introduction

Some problems of polymer-water interaction

1. Packaging. Importance of transport processes in polymeric membranes.

2. Textile. Hydrolytic degradation of cotton (cellulosis), polyamides, PET, etc.

3. Aviation. Degradation of epoxy-carbon parts in wet tropical environment

4. Boat hulls (swimming pools, etc.). Blistering of glass fiber-polyester composites.

Two kinds of problemsTwo kinds of problems

1. In packaging and epoxy-carbon parts. No chemical reaction between the polymer and water.

Only physical interactions (generally failure induced by

swelling

2. In textiles and polyester composites. Water reacts chemically (hydrolysis) with the polymer.

Failure results from the embrittlement induced by

chain scission or by osmotic cracking induced by the release

of small molecules.

In both cases, the first step of ageing is the penetration of water in the material. Importance of the knowledge of water solubility and diffusivity in the polymer.

Equilibrium concentrationInfluence of water activity

• Main types of sorption isotherms

• H: Henry

• FH: Flory – Huggins

• HC: Henry + clustering

• LH: Langmuir + Henry

Equilibrium ConcentrationInfluence of water activity

Isotherm equations

The problem:

differentiate FH and

HC (both have a HC (both have a

positive curvature).

At low

hydrophilicities, FH

isotherms are

almost linear. The

existence of a

positive curvature

indicates clustering

Equilibrium concentrationHenry’s solubility. Influence of structure

How to predict Henry’s solubility from the polymer structure?

Many approaches of structure – hydrophilicity relationships:

- Free volume

- Molar additive function

- Solubility and interaction parameters

- Doubly bonded water

- The free volume approach can be rejected: free volume rich substances, e.g. Hydrocarbon or silicone

elastomers, liquid hydrocarbons, etc. have very low water absorptions, for instance about 20 ppm for pure

aliphatic hydrocarbons.

- The solubility parameter theories (Hildebrand, Hansen, etc. ) allow to qualitatively predict the trends, i.e.

the fact that, globally, the hydrophilicity increases with polymer polarity, but quantitative predictions are not

possible.

Equilibrium concentrationThe Henry’s component

Influence of temperature

Equilibrium concentrationHenry’s solubility vs polymer structure

The molar additive approach

From comparisons of water absorptions at saturation, three main categories of elementary groups depending

on their contribution to hydrophilicity:

1) Groups of very low contribution, ex: Polyethylene, rubbers, polypropylene, polytetrafluorethylene….

>CH-; -CH2-; -CH3; -C6H4-;-CF2-; -CHCl-; -S-; Si(CH3)2….

2) Groups of medium contribution, ex: polyoxymethylene, polyesters, polyurethanes…. 2) Groups of medium contribution, ex: polyoxymethylene, polyesters, polyurethanes….

-O-; -CO-; -O-CO-; -O-CO-NH- …..

3) Groups of high contribution, ex: Poly(vinyl alcohol), poly(acrylic acid), polyamides 4-6, 6 or 6-6…..

-OH; -CO-OH; -CO-NH-

The hydrophilicity depends also of the concentration of the most polar groups, for instance –SO2- in

polysulphones; -OH in amine crosslinked epoxies, -CO-NH- in polyamides, etc.

Equilibrium concentration Henry’s solubility vs structure

the molar additive approach

Equilibrium concentrationHenry’s solubility vs structure

Validity of the molar additive approach

This approach can work on limited structural series where group concentrations don’t vary strongly.

Example: Amine cured epoxies differing by the amine structure or by the epoxide structure (Bellenger et al.

1989)

But it don’t works when group concentrations display strong differences, for instance -OH in epoxies

(Tcharkhtchi et al. 2000), or –SO2- in polysulphones (Gaudichet et al. 2008, Marque et al. 2010).

Remark: in the literature, group contributions were calculated from whole concentrations, not from Henry’s

solubility only (Van Krevelen 2000).

Equilibrium concentrationHenry’s solubility

The approach of doubly bonded water

Equilibrium concentrationHenry’s solubility

Doubly bonded water

This theory explains:

- The non linear dependence of water equilibrium concentration with the

concentration of polar groups.

- The fact that, for a given type of polar group, the apparent activation energy

of the solubility is an increasing function of the group concentration

Evolution of the ideas:

- A hydrophilic site is a single group. Validity of the molar additive law (Barrie

1968, Van Krevelen 1983). Solubility is expected to obey Arrhenius law.1968, Van Krevelen 1983). Solubility is expected to obey Arrhenius law.

- Two kinds of hydrophilic sites coexist: the (apolar) matrix and (single) polar groups. The solubility is the sum of two arrhenian terms (Mc Call et al. 1984)

- There is an infinity of hydrophilic sites differing by the H bond distance and thus by the activation energy(Gaudichet et al. 2008).

Problem:

Determination of the distribution of pair distances by molecular dynamics simulation (DMS).

- One attempt for polysulphones (Marque et al. 2008): Nsites(DMS) > Nsites(experim.).

But how to fix the limits of H bond distances?

Equilibrium concentrationclustering

Two possible explanations for clustering:

1- Water fills pores. Macroscopic pores can be filled only at saturation.

In nanoscopic pores, confinment effects, clusters can appear at activities <1

2- Clusters result from double H bonding.2- Clusters result from double H bonding.

Equilibrium concentrationclustering

Main characteristics of clusters:

- Generally small: average size 2 or 3. favors the doubly H bonded hypothesis.

- Large size clusters can be recognized by the presence of crystallization exothermof water near 0°C.

- Coexistence of two kinds of water molecules with distinct relaxation times (NMR, dielectric spectrum).

- Water diffusivity is slightly decreasing function of water activity.- Water diffusivity is slightly decreasing function of water activity.

- Very important: water in clusters is inactive in plasticization, hydrolysis, etc.

Problems:

- According to sorption isotherms and Zimm-Lundberg theory, clusters must

appear only at high activities. They are presumably isotropic.

- According to DMS, clusters appear even at low activities. They are anisotropic.

- How to explain this discrepancy?

- How to predict clustering characteristics?

Equilibrium ConcentrationLangmuir sorption

Equilibrium concentrationLangmuir sorption

Shape of a sorption curve displaying Langmuir process. Case of amine cured epoxy

with unreacted epoxide groups:

Here, the first plateau corresponds

to Henry’s process involvingto Henry’s process involving

double H bonding. The second

plateau corresponds to the

Langmuir process which results

from the water (chemical) addition

to unreacted epoxide groups

(Tcharkhtchi et al. 2000)

Equilibrium concentrationremaining problems

- Quantitative prediction of the number of hydrophilic sites? Development of

MDS.

- If water establishes strong bonds with the hydrophilic sites, why Henry’s law is

obeyed? Why not Langmuir? Development of experimental methods to

determine free water concentration and lifetime of polymer-water complexes.

- Prediction of clustering characteristics? Resolution of the conflict between Zimm-

Lundberg and MDS.

- It is difficult to imagine physical bonds stronger than H ones working as Henry’s

sites. Are chemical processes always responsible of Langmuir type sorption?

Consequences of water absorption on physical propertiesGlass transition temperature

Consequences of water absorptionGlass transition temperature

Consequences of water absorptionswelling

Water absorption induces swelling but the volume increase is lower than the one

resulting from volume additivity.

Example: styrene crosslinked polyesters based on maleate-phthalate copolymer (A

and B) or from maleate homopolymer (C and D). 50°C, 100% RH. Belan et al.1997)

Sample A B C DSample A B C D

density 1.2000 1.184 1.221 1.153

m (%) 1.55 1.52 5.00 2.80

v (%) 0.20 0.27 0.68 0.65

v/m 0.13 0.18 0.14 0.23

Consequences of water absorptionswelling

Example 2 Aromatic polysulphones at 100°C (Marque et al. 2009)

Example of polysulphones (Marque et al. 2009)

Consequences of water absorptionswelling

- Swelling very important because differential swelling in transient regime of

diffusion → strains → « swelling stresses » → damagement.

- Swelling of glassy polymers not well understood. Mechanical aspects? Polymer- Swelling of glassy polymers not well understood. Mechanical aspects? Polymer

structure? Volumetric parameters? Strength of water-polymer H bonds?

Non-equilibrium character of polymer glasses?

- Available literature data too scarce, need for more experimental studies and

thermodynamical approaches of swelling.

HydrolysisA chain scission process

Hydrolysis reaction can be written: -X-Y- + H2O → -X-OH + H-Y-

Most common hydrolyzable groups in polymers: esters, imides and amides.

Example for an ester: R-CO-O-R’ + H2O → R-CO-OH + H-O-R’

For an amide: R-CO-NH-R’ + H20 → R-CO-OH + H2-N-R’

Important! When the hydrolyzable group belongs to the polymer skeleton, hydrolysisImportant! When the hydrolyzable group belongs to the polymer skeleton, hydrolysisleads to chain scission.

Linear polyesters (PET), polycarbonate, polyamides, crosslinked polyesters, anhydride crosslinked epoxies, polyurethanes based on polyesters, certain polyimides, are reactivewith water. Always:

Chain scission → decrease of molar mass or crosslink dansity → embrittlement

Hydrolysis can be monitored by molar mass measurments (linear polymers) or crosslinkdensity measurments (networks)

Hydrolysischain scission

HydrolysisKinetics

Hydrolysis as a non-equilibrated, molecular process

HydrolysisKinetics

Hydrolysis as an equilibrated process

HydrolysisCatalysis by acids or bases

N.B: Polymers are impermeable to ions.

Non-dissociated acids or bases first

penetrate in the polymer, then dissociate

into ions but at lower extent than in water

HydrolysisHydrolysis process in the interfacial zone

Example1: Glass fiber thermoplastic composites with 30% by weight glass

(Theberge 1970). Stress reduction after 100 hours and 1000 hours in boiling water

Polymer Glass fiber (%) SR (%) after 100h SR (%) after 1000h

PC 30 51 28PC 30 51 28

PC 0 100 28

POM 30 71 57

POM 0 100 98

PPO 30 84 65

PPO 0 100 100

In the case of PC, matrix is the « weakest point ».

In the case of POM and PPO, interphase is the « weakest point »

HydrolysisHydrolytic processes in the interfacial zone

Stabilizing role of coupling agent

Example2: Hydrolysis rate in boiling water of coupled and uncoupled polyester-glass

microspheres vs mass fraction of microspheres (Mortaigne 1989)

Here, the « weakest point » is

the glass-polymer interface,

then coupling agents can play

a stabilizing role

HydrolysisBehavior of coupling agents

HydrolysisConsequences on mechanical properties

case of linear polymers

Shape of the molar mass dependence of toughness (Kausch and coll. 2001)

Existence of a critical molar mass

separating the ductile/tough

where entanglements are active

and the brittle domain where

chains are too short to be

entangled

HydrolysisConsequences on mechanical properties

case of linear polymers

HydrolysisConsequences on mechanical properties

case of networks

Ulttimate stress against ultimate strain for polyester samples 0.7 mm thick

hydrolyzed at 70°C and 100°C (Mortaigne 1989). Same behavior observed for

anhydride cured epoxies (Lehuy et al.1993)

Degradation doesn’t modify the Degradation doesn’t modify the

tensile behavior except for the

ultimate strain which

decreases.

Relationship between fracture

properties and network

structure not well established

HydrolysisOsmotic cracking

Blistering of polyester-glass fiber laminates (boat hulls, swimming pools, tanks, etc.)

Blisters = subcutaneous cracks propagating parallel to surface. The driving force: osmotic pressure (Ashbee et al. 1967).

Hydrolysisosmotic cracking

Evidence from gravimetric study of polyester samples 0.7 mm thick at 100°C

(Mortaigne 1989)

1. Sorption equilibrium reached

3. Onset of cracking

Maxi. Coalescence of cracks

4. Loss of organic molecules in

the bath.

Hydrolysisosmotic cracking

crack propagation

Hydrolysisosmotic cracking

crack initiation

According to Gautier et al. (1999)

1) Hydrolysis generates organic molecules by chain scission.

2) The diffusivity of these molecules in the polymer is << water diffusivity

3) The molecules accumulate in the polymer until their solubility limit3) The molecules accumulate in the polymer until their solubility limit

4) The, they demix to form micropockets.

5) Since hydrolysis products are hydrophilic, the micropockets absorb water.

6) Since water diffuses rapidly, the layer separating the micropocket from surface works as a semi-permeable membrane → osmotic pressure → cracking

Conclusion

No one problem has been totally elucidated:

- Prediction of Henry’s solubility from structure

- Prediction of clustering conditions for a given polymer

- Discrepancy Zimm - MDS

- Prediction of Fick’s coefficient with structure, link with solubility- Prediction of Fick’s coefficient with structure, link with solubility

- Lack of data on stress effects

- Structure – swelling relationships

- Auto-catalyzed and catalyzed hydrolysis, kinetic modelling

- Embrittlement by chain scission in thermosets

- Critical conditions for osmotic cracking

Welcome to young research workers!