Environmental resistance
of composites
John Summerscales
Outline of lecture
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glass transition temperature
moisture
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
o
• A key design parameter in
aerospace applications is “hot wet Tg”
Moisture (Fickian diffusion)
Moisture content
equilibrium/saturation
√(time)
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
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
o

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:
http://www.wessex-resins.com/
westsystem/wsosmosis.html
http://www.insightmarinesurveyors.co.uk/osmois%20ringed.jpg
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):
o
HO-CH2-C(CH3)2-CH2-OH
 2,2-dimethyl-1, 3-propanediol

• Durability:
o
ortho < iso < NPG
Chemical structure from: http://chemicalland21.com/specialtychem/perchem/NEOPENTYL%20GLYCOL.htm
Osmosis and blistering
• To avoid osmosis leading to blistering:
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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
o
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
o
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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
“print-through”
o
surface echoes topology of reinforcement
Antifouling paints
• Toxic compositions
cuprous oxide – increasing concern
o tri-butyl tin – now banned worldwide
o
• 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
submarines,
o underground railways, and
o underground mines
o
• F = low spread of Flame
• S = minimal emission of Smoke
• T = no Toxic products of combustion
o
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