Osmosis (a technical article)

Even with a background in chemistry, it is not easy to find an explanation for the scare of many second-hand boat buyers: OSMOSIS.
One of the reasons is, that 'osmosis' on polyester boats cannot be osmosis. Scientifically, osmosis is the diffusion of a liquid through a semi-permeable membrane. And while the argument, that water penetrates the polyester is true, this can only be part of the problem. Anyone who's ever seen an affected boat and opened its blisters, knows that the ugly slimey liquid inside smells. An acidic smell with a touch of vinegar. So there's organic acid involved; where does that come from? All polyester boat owners know their pride doesn't usually smell 'acidic'. Some of them know - those that had to repair hull damage - that polyester resin doesn't smell of vinegar either.
So osmosis doesn't just involve well... osmosis. There is a chemisty involved too.


Polyester is a polymer made from two components (monomers) that link up with a chemical ester bond. The result looks like this, where n is a large number of repetitions:

Now in theory, you would like to make the chains very long (n in the picture above is a large number) for greater material strength. But there is a practical problem with that. The material becomes more and more solid and therefore hard to process or mould. So while long chained polyester is nice for woven fabric in clothing, it's not feasible in boat building.
The solution to that is to use polyester resin. Polyester made with molecule chains, sufficiently short to yield a viscous liquid. The polyester is mixed with a solvent, that will act as a 'cross-linking agent' during a curing process. This mixture is easy to work with, can be used so soak fibreglass in and laminate the inside of a hull's mould. The curing agent usually is Styrene (see picture).
So what is required to give 'glass reinforced polyester' its strength? Curing! During the curing process, the cross-linking agent (styrene) reacts with the unsaturated (c=c) bonds in the polyester to create a huge netted molecular structure. To start this reaction, an initiator is required. This is usually a peroxide. The curing is done at elevated temperatures. The polymerisation reaction generates heat and to prevent overheating of the polyester, laminate thickness is limited. Excess styrene is supposed to evaporate. The chemical end result is this:


So how, when and where does osmosis start? The answer really is: "From the moment you launch a polyester boat." And initially it's no osmosis at all, it's diffusion. Plain and simple, every polymer immersed in liquid, will absorb some of that liquid at a rate determined by the "diffusion coefficient". It not easy to find good estimates for the diffusion coefficient, but assuming it is of the order of magnitude 5 × 10-11 [m2/s], it will take about 20 days to diffuse through a hull and about one season in the water to saturate a polyester hull completely.
If there are pockets with water-soluble material trapped in the polyester, these can attract water according to an osmotic principle. And don't be mistaken about the internal pressure once a pocket attracts water; that may easily be 50 bar. Enough to cause local delamination or blisters. But this doesn't explain two things:
- Why do blisters appear after considerable time?
- Why 'osmosis' can progress (rapidy), spreading from one location to the other; rather than appearing here and there
If osmosis was just 'osmosis' blisters would appear here and there, depending on the number of inclusions. It would also start from the moment the polyester starts to absorb water. Reality seems to be that blisters appear after years, the affected areas can grow and the polyester itself can become affected.
Incomplete curing in combination with a small portion of bad luck, may start degradation of the polyester. Traces of styrene, polymerisation accelerators, metals and salts can all help to start an hydrolysis reaction. Not the cross links that were made during the curing, but the polyester's bond itself is then attacked:

Unfortunately, this reaction releases organic acid and solvents. The latter explains why the liquid from 'osmosis' blisters, has the vinegar smell. And the reaction products almost ensure that other pockets of impurities hydrolyse too. Acid, styrene and water break down all barriers in the material. As such, it's a process that generates its own initiators.....
So can it happen in any boat? Yes. It certainly looks like it. Though it is clear that carefully laminated boats are less affected. Resin, cross-linking agent and initiator mixed properly in the right stoichiometry is a good start to avoid 'osmosis initiating conditions'. The fewer additives seems the better. And the best way to avoid it, is a boat that has been post-cured shortly after the initial curing.

The cure

Is there a cure? Yes. But the only reliable method, is to remove at least one of the reactive components. Particularly water, styrene, uncured polyester and acid. Water in itself is not an option, it will return; such is the nature of a boat.... Though storage 'on shore' for a few months a year, may decrease water levels in the polyester. But from the chemical point of view, the only way, is heat treatment. If possible with a vaccuum method to remove as many 'volatiles' as possible; the latter particularly if repairs are part of the process.