Osmosis and blistering

Environmental resistance
of composites
John Summerscales
Outline of lecture
glass transition temperature
osmosis and blistering
cavitation erosion
galvanic corrosion
marine coatings
antifouling paints
flame, smoke and toxicity (FST)
Glass transition temperature (Tg)
• Tg is a function of:
molecular structure
o ratio of chain ends to backbone polymer
o loading rate
o moisture content can reduce Tg
• A key design parameter in
aerospace applications is “hot wet Tg”
Moisture (Fickian diffusion)
Moisture content
Osmosis ...
• Osmosis can be defined (Clegg, 1996) as
“the equalisation of solution strength
by passage of a liquid (usually water)
through a semi-permeable membrane
Weak solution
Strong solution
Osmosis ...
• normally the fluid will pass
through the material without affecting it
• but, there may be soluble materials, e.g.
residual glycol from UP resin
o soluble binder on CSM
o PVA release agents, etc
see pp 231-233 of Searle and Summerscales
for a more complete list!
• these materials will dissolve in the solvent
Osmosis and blistering
• a little solvent and a lot of solute
-> a strong solution
• strong driving force for osmotic cell
• high pressures generated cause/expand void
containing strong solution
• swelling leads to blisters with
associated surface undulation
• Image from:
Osmosis and blistering
• For marine applications, consider
changing from orthophthalic
to isophthalic polyester resin
o and to improve “iso” resin further,
use NPG (neo pentyl glygol):
 2,2-dimethyl-1, 3-propanediol
• Durability:
ortho < iso < NPG
Chemical structure from: http://chemicalland21.com/specialtychem/perchem/NEOPENTYL%20GLYCOL.htm
Osmosis and blistering
• To avoid osmosis leading to blistering:
no soluble components in resin system
avoid moisture on mould and reinforcement
completely wet-out the fibres
consolidate to minimise voids
gel coat of lower permeability than laminate
use a light glass scrim in the gelcoat
use primer (tie-coat) between gel coat and
structural laminate
control gelcoat thickness and quality
Cavitation erosion
• Collapsing bubble:
Solid surface
model from Lauterborn and Bolle (1975)
Cavitation erosion
• Collapsing bubble:
Solid surface
model from Lauterborn and Bolle (1975)
Cavitation erosion
• Collapsing bubble:
Solid surface
model from Lauterborn and Bolle (1975)
Cavitation erosion
• Collapsing bubble:
Solid surface
model from Lauterborn and Bolle (1975)
Cavitation erosion
• Collapsing bubble
creates jet towards a hard surface
which loosens material structure
and removes material:
Solid surface
model from Lauterborn and Bolle (1975)
Cavitation erosion
• Very limited data in public domain
• composites may perform better than metals
because fibre > grain size
student projects* suggested
CFRP proportional loss in weight
only 40% of that for Al under identical conditions
o but difficult experiment
CFRP absorbs some water
 may have low initial - but accelerating - loss rate
* Handley ..and.. Ladds (1995)
Galvanic corrosion 1
• corrosion involves flow of an electric current
• most constituents of fibre-composites are
insulators and hence
electrochemical corrosion is not an issue
• However, carbon (graphite) acts as a noble
metal, lying between platinum and titanium in
the galvanic series.
Galvanic corrosion 2
• Carbon fibres should
not come into contact
with structural metals
(especially Al or Mg)
in the presence of a conducting fluid
(eg sea-water).
• A thin glass fibre surface layer may be
sufficient to prevent the formation of such a
galvanic corrosion cell.
Marine coatings
• Surface coatings may be for
provide aesthetic finish
o improve resistance to corrosion
o protect against fouling
especially for marine or process plant applications
• gel-coat is normally applied to the mould
before the laminate is laid-up/injected
• a major issue in the marine industry is
surface echoes topology of reinforcement
Antifouling paints
• Toxic compositions
cuprous oxide – increasing concern
o tri-butyl tin – now banned worldwide
• Exfoliating/self-polishing surfaces
– increasing concern
• Non-toxic low surface energy compositions
• Bio-inspired approach (biomimetics)
Liedert and Kesel: shark skin as the analogue
o surface microstructure, Rz = 76 μm
o soft silicone material (shore A = 28)
o low surface energy (25 mN/m)
Flame, Smoke and Toxicity (FST)
• especially important for
o underground railways, and
o underground mines
• F = low spread of Flame
• S = minimal emission of Smoke
• T = no Toxic products of combustion
phenolic resins burn to just H2O and CO2
in the presence of a good supply of air
Balmoral offshore lifeboat
• glass reinforced plastic
fire-retardant resins
carries 21-66 people
• certification required to
withstand 30 m high
kerosene flames and
temperatures of 1150°C
• throughout the fire test,
the temperature inside
never exceeded 27°C.
Image from the front cover of
International Reinforced Plastics Industry
May/June 1983, 2(5), 1