Environmental resistance of composites

John Summerscales

Outline of lecture

• • • • • • • • 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: o o o o molecular structure ratio of chain ends to backbone polymer loading rate moisture content can reduce Tg A key design parameter in aerospace applications is “hot wet Tg”

Moisture (Fickian diffusion)

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 Weak solution Strong solution

Osmosis ...

• • • normally the fluid will pass through the material without affecting it but, there may be soluble materials, e.g.

o o o residual glycol from UP resin soluble binder on CSM 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: http://www.wessex-resins.com/ westsystem/wsosmosis.html

http://www.insightmarinesurveyors.co.uk/osmois%20ringed.jpg

Osmosis and blistering

• • For marine applications, consider o o changing from ortho phthalic to iso phthalic polyester resin and to improve “iso” resin further, use NPG (neo pentyl glygol):   HO-CH 2 -C(CH 3 ) 2 -CH 2 -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: o o o o o o o o 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 o o student projects * suggested CFRP proportional loss in weight only 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)

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 o o o 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 is “print-through” o surface echoes topology of reinforcement

Antifouling paints

• Toxic compositions o o cuprous oxide – tri-butyl tin – increasing concern

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 o soft silicone material (shore A = 28) low surface energy (25 mN/m)

Flame, Smoke and Toxicity (FST)

• especially important for o o o submarines, underground railways, and underground mines • • • F = low spread of Flame S = minimal emission of Smoke T = no Toxic products of combustion o phenolic resins burn to just H 2 O and CO 2 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