Air bubbles or Osmosis?

[beneteau_305_553]

When laminating hulls the gell coat is painted onto the mould usually with a brush leaving an uneven surface. If air bubbles are trapped between the first laminate layer and this gel coat which is semi permiable, when the temperature in the void falls, a lower pressure exists in the void. Moisture then gets pulled into the void.

When the temperature rises, as air is easier to expell than water, air is expelled and any condensation in the void remains.

Repeat many times and the bubble ends up full of water.

Then when a temperature change occurs the pressure in the void becomes very high because water is non compressible.

This pressure then expands the void and results in a bubble of water between the gel coat and the laminate.

Perhaps we should be eliminating air pockets between the gel coat and the laminates during moulding as well as using isopthalic resins?

<hr width=100% size=1>Richard

 

[malcp]

Does this mean that when we lift the boats out of the water for A/F etc we might actually be doing harm at the same time? ie the greater temperature variation of the air around the hull (midday sun to night time frost), compared to when in the water, might cause pressure buildup between the gelcoat and laminate?

Perhaps if it's left out of the water long enough the water in the void would eventually evapourate out through the gelcoat.

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[Guest]

Just supposing ....

OK so the water has got in as moisture vapour in the air. It has condensed and now starts to absorb uncured agenst from the starta and density increases .... at some point the density is high enough that the water is now like a goo and cannot escape ........

Yes it is true that voids do occur, but are much reduced nowadays in the world of sprayed laminates where chopped strand is sprayed onto the gel along with the resin ......

The world does move on ....


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I came - cos they said was FREE Guinness !
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[beneteau_305_553]

Re: Just supposing ....

There may well be less air bubbles in the mouldings if they are rolled out well during laminating. But do surveyors inspect for air bubbles in new mouldings as these may well become the sites of water filled blisters in the future.

the ammount of increase in blister size is tiny for each temperature reversal so taking the boat out for painting etc makes little difference. leaving the boat out for a few months in a dryish atmosphere probably would allow the water in a blister to transpire out. so annual lift outs should reduce this effect.

I saw lines of blisters on a hull that seemed to repeat brush strokes of the gel coat. The ridges left in the gel on the inside make it very difficult to roll out the trapped air.

Beneteau apply a thin first laminate to the gelcoat to eliminate this problem. Is this common practice?



<hr width=100% size=1>Richard

 

[vyv_cox]

Assuming that the pockets are in fully-cured gelcoat and laminate, then there is little harm in their presence. Where osmosis occurs, the underlying laminate is not fully cured. Water permeates through the gelcoat, driven by osmotic pressure, "trying" to dilute the more concentrated solution inside the gelcoat to match the concentration outside. Osmotic pressure can reach considerable levels, far more than could be generated by temperature fluctuations.

One of the biggest improvements in hull construction has been due to building them in non-pigmented resins. It is now relatively easy for inspectors to see any defects, whereas previously they were virtually impossible to detect.

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[jamesjermain]

A tissue

The first, tissue, layer applied by Beneteau and others is mainly there to prevent 'print-through' of the pattern in woven rovings through the gel coat. It is particularly important if the gel is coloured (blue is worst) which shows every blemish.

I was slightly surprised by the original post because the elimination of trapped air is the thing all builders work hardest at achieving, and has been a well known (even obvious) cause of osmosis since the condition was first diagnosed. Spray gel coats, careful wetting out of laminates and, these days, the increased use of vacuum bagging, resin injection and SCRIMP processes make significant voids and bubbles rare. Also designers and mould makers are wiser and avoid sharp corners and deep recesses where voids often develope. Modern resins such as vinylesters and even epoxies are much more impermeable. Despite all this, osmosis still happens but it is less common.

Storing a boat ashore over winter is a great help is staving off blistering but a well-saturated hull, particularly an older one with a thick, mat based laminate, may take from 6 months to 18 months to dry out signifcantly with the gel coat still on. And during this time you should be washing the hull down with fresh water every few days to remove the salt deposits.

<hr width=100% size=1>JJ