ConferenceNantes aug. 2012. Humidageingof polymersand ......ConferenceNantes aug. 2012....
Transcript of ConferenceNantes aug. 2012. Humidageingof polymersand ......ConferenceNantes aug. 2012....
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!