Environmental resistance of composites

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Environmental resistance of composites

• John Summerscales

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Outline of lecture

• glass transition temperature
• moisture
• osmosis and blistering
• cavitation erosion
• galvanic corrosion
• marine coatings
• antifouling paints
• flame, smoke and toxicity (FST)

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Glass transition temperature (Tg)

• Tg is a function of
• molecular structure
• ratio of chain ends to backbone polymer
• loading rate
• moisture content can reduce Tg
• A key design parameter inaerospace applications
  is hot wet Tg

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Moisture (Fickian diffusion)
equilibrium/saturation
Moisture content
v(time)
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Osmosis ...

• Osmosis can be defined (Clegg, 1996) as the
  equalisation of solution strengthby passage of a
  liquid (usually water) through a semi-permeable
  membrane

membrane
Weak solution
Strong solution
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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
• soluble binder on CSM
• PVA release agents, etc
• see pp 231-233 of Searle and Summerscalesfor a
  more complete list!
• these materials will dissolve in the solvent

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Osmosis and blistering

• a little solvent and a lot of solute-gt 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 fromhttp//www.wessex-resins.com/westsyst
  em/wsosmosis.html

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http//www.insightmarinesurveyors.co.uk/osmois20r
inged.jpg
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Osmosis and blistering

• For marine applications, consider
• changing from orthophthalicto isophthalic
  polyester resin
• and to improve iso resin further,use NPG (neo
  pentyl glygol)
• HO-CH2-C(CH3)2-CH2-OH
• 2,2-dimethyl-1, 3-propanediol
• Durability
• ortho lt iso lt NPG
• Chemical structure from http//chemicalland21.co
  m/specialtychem/perchem/NEOPENTYL20GLYCOL.htm

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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

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Cavitation erosion

• Collapsing bubble

Solid surfacemodel from Lauterborn and Bolle
(1975)
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Cavitation erosion

• Collapsing bubble

Solid surfacemodel from Lauterborn and Bolle
(1975)
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Cavitation erosion

• Collapsing bubble

Solid surfacemodel from Lauterborn and Bolle
(1975)
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Cavitation erosion

• Collapsing bubble

Solid surfacemodel from Lauterborn and Bolle
(1975)
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Cavitation erosion

• Collapsing bubblecreates jet towards a hard
  surfacewhich loosens material structureand
  removes material

Solid surfacemodel from Lauterborn and Bolle
(1975)
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Cavitation erosion

• Very limited data in public domain
• composites may perform better than metals because
  fibre gt grain size
• student projects suggested CFRP proportional
  loss in weightonly 40 of that for Al under
  identical conditions
• but difficult experiment
• CFRP absorbs some water
• may have low initial - but accelerating - loss
  rate
• Handley ..and.. Ladds (1995)

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Galvanic corrosion 1

• corrosion involves flow of an electric current
• most constituents of fibre-composites are
  insulators and henceelectrochemical corrosion is
  not an issue
• However, carbon (graphite) acts as a noble metal,
  lying between platinum and titanium in the
  galvanic series. 

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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.

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Marine coatings

• Surface coatings may be for
• provide aesthetic finish
• improve resistance to corrosion
• 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
  isprint-through
• surface echoes topology of reinforcement

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Antifouling paints

• Toxic compositions
• cuprous oxide increasing concern
• 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
• surface microstructure, Rz 76 µm
• soft silicone material (shore A 28)
• low surface energy (25 mN/m)

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Flame, Smoke and Toxicity (FST)

• especially important for
• submarines,
• underground railways, and
• 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 CO2in the
  presence of a good supply of air

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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 1150C
• throughout the fire test, the temperature
  inside never exceeded 27C.

Image from the front cover of International
Reinforced Plastics IndustryMay/June 1983, 2(5),
1