Osmosis
| Everything (you'd probably rather not know) about osmosis ...
Aegis Yachts has considerable experience in diagnosis and advice on treatment of the condition known as osmosis,
which still causes concern, especially for people contemplating the purchase of a used GRP or FRP boat.
The problem has become clouded and complicated by myths, misunderstandings, doubtful theories and misused
terminology. Many people yearn for a black-and-white statement of the problem, and what to do about
it. Unfortunately, the reality is less straightforward.
This page offers a summary of the current understanding of the condition with a minimal amount of technical detail and
then looks at the problems of diagnosis, and describes possible courses of action and treatment.
This is not a comprehensive statement of the issues such a paper would be too long for the purpose here, which is to
provide a straightforward overview. If you would like more information or to discuss specific issues, please get in touch.
Osmosis what is it?
Osmosis is a natural process whereby a solvent (such as water) will flow through a semi-permeable membrane dividing
two solutions of different concentration. Its what makes a wilted plant stand up when you water it, and enables
your body to extract excess liquid from your digestive tract. The key element in this process is the semi-permeable
membrane, which allows the solvent to pass through, but not a substance dissolved in it (the solute).
In the 1960s, when glass reinforced plastic (GRP) was still a relatively new material for boat construction, boat owners
were horrified to discover that GRP was not quite the perfect, maintenance-free material that had been thought. Hulls were
developing a number of problems, including blisters ranging from the size of a pinhead to a hand, sometimes appearing in rashes.
Investigation suggested that the problem was connected with water penetration, and that osmosis lay behind the formation
of blisters. The conclusion was not wholly wrong, but it failed to take account of other important factors.
In order to understand what is really going on, one must first look at GRP as material how it is made, both
physically and chemically and then examine the reality of boat building. Finally one must consider what happens when a
hull made from GRP is left afloat.
GRP is a composite material, made from layers of glass fibres bound in a matrix of polyester resin. FRP (fibre reinforced
plastic) is similar, except that the fibres may be some other material such as Kevlar or carbon fibre. Some boat hulls are now being
built with epoxy resin, rather than polyester. So far as older boats and the problem of osmosis are concerned, however,
polyester-glass is the most important material to consider.
Polyester resin is made from organic acids and alcohols, which are first reacted with each other to form esters. These
esters are then joined together, along with further organic acids, to form very long chain molecules.
The long chain molecules are then mixed with styrene (which gives polyester resin its characteristic smell), together
with various other chemicals, to form a liquid resin. It can be kept in this form for some time, although it does have
a definite shelf life.
Finally, a chemical known as an initiator is added (often incorrectly referred to as a catalyst), which starts the curing
process. Curing creates molecular links between the long polyester chains, and the resin starts to set into a hard plastic.
The creation of a boat hull involves first mixing the resin and initiator in precise quantities, and then proceeding with
construction, or lay up. The first step is to brush or spray a thin layer of polyester resin onto a mould. This is the
gel coat, which usually contains pigments to give the finished hull an opaque, coloured finish.
Construction then proceeds by laying on alternate layers of glass fibre and resin, squeezing the two together with metal
rollers, until the desired thickness is reached. A final coat of finishing resin is then brushed onto the inside, and the
whole mass left to cure. The new hull can usually be removed from the mould after about 24 hours, although curing continues
for a number of days before the resin reaches its full hardness.
The finished material has excellent properties for a boat it is relatively light and strong, it is (of course) easily
moulded into shape, with no unsightly joins, and it comes with a ready-made, high gloss finish. It is not as stiff as an
engineer might wish, but this can easily be overcome by good design. Above all, it is remarkably durable.
It is not, however, completely maintenance-free. GRP suffers from wear and tear like anything else, and is susceptible to
knocks and abrasion. Coloured gel coats can fade, white gel coats can be stained, and all gel coats can develop fine crazing
under stress and the influence of the weather. These problems are essentially cosmetic, however, and can ultimately be
fixed by painting.
Leaving aside physical issues like overloading and fatigue, almost all of the remaining problems can be bundled
together under the general heading of "osmosis" however inadequate the term may be. And nearly all of these problems
can ultimately be traced to some fault in manufacturing.
There are many things that can go wrong with a GRP boat during the manufacturing process, starting first with the production of
the resin.
The volume and purity of the reactants has to be carefully controlled, as does the production process. Even so, some of the
ingredients may remain unreacted. One of the by-products of esterification is water. Most is removed, but some
is inevitably left behind. This can cause problems later.
The amount of styrene added is critical to controlling viscosity. Styrene is not, however, a simple solvent: it later
becomes incorporated into the molecular structure of the resin when it is cured. If there is too much, the excess can end up
physically trapped in the cured resin. Finally, various chemicals may be added to extend the shelf-life of the resin.
Some of these can cause problems in the long term.
Fortunately, modern production processes have good quality control, and bad batches of resin are rare. But it wasn't
always so, and the problem for the boat owner (and surveyor) is that it is impossible to know enough about the resin
used in the vessel's construction.
More problems can appear when the resin arrives at the boat builder. All being well, it will be fresh and ready to use.
Resin that has sat on the shelf, however, may be too viscous. The builder may attempt to fix the problem by adding extra
styrene. Although this thins the resin, it can result in excess styrene being trapped in the cured laminate.
The next step is to mix the resin with the initiator. Various problems can arise here getting the proportions exactly
right takes care; thorough mixing of the two is harder still. The mixing process sometimes injects millions of tiny air
bubbles into the resin. Extra chemicals are sometimes added either to speed up the curing process, to or extend the "pot
life" the time during which the resin remains liquid and again, these chemicals end up trapped in the cured resin.
The resin is then used in the lay-up. The short pot-life of the resin means that the work has to be done quickly. And
yet each layer has to be thoroughly consolidated, if the glass fibres are to be thoroughly saturated with resin.
Inevitably, these conflicting demands cannot always be met, with the result that some patches of the lay-up are poorly
consolidated.
The timing between layers is also critical. The gel coat must be left to cure slightly before starting the glass fibre
lamination, otherwise the glass fibres can be pushed right through the gel coat. But it mustn't be left too long, otherwise
the bond between the gel coat and the laminate will be imperfect. The same applies between individual laminations of glass
fibre; any departure from the ideal might create pockets of poor lamination within the finished hull.
The glass fibres used come in a range of different forms. It can be woven into a fine cloth. Or bundles of glass fibres can
be coarsely woven together in a form known as woven rovings. One of the most common materials is a mat made from chopped
strands (chopped strand mat or CSM). A typical hull normally employs a mixture of these. Chopped strand mat, however, is held
together with a binding agent, usually polyvinyl alcohol (PVA) yet another chemical additive that can cause problems,
as we shall see.
The final factor concerns the working environment. Cleanliness is important, but small quantities of dirt, dust and rubbish
inevitably get consolidated into the resin along with the glass fibres. The temperature and humidity must also be right if the
resin is to cure properly. Modern builders tend to use environmentally controlled facilities, but in the past many boats were
built in unheated sheds, or even in the open. Even the best made hulls of today are frequently found to be only partially
cured, in laboratory tests. Hulls laid up in very warm or cold weather, or on extremely humid days, are likely to suffer from
serious undercure.
In summary, and bearing in mind all the potential problems described above, it is probably true to say that all boats will
have some defects that result from imperfections in the manufacturing process. The next section examines the consequences.
GRP afloat what can go wrong
When a GRP boat is launched, water starts to soak into the hull. This may seem surprising, but polyester is not completely
impermeable. At a molecular level, polyester is made from many millions of long chain molecules, cross linked to each other,
forming a crystalline lattice. The lattice has microscopic holes in it, big enough to hold water molecules. So, under
hydraulic pressure, water molecules make their way in to the gel coat.
Once through the gel coat, the water molecules meet the glass fibre laminate. This is even more permeable, not because of the
molecular structure, but because imperfections in manufacturing provide millions of tiny physical pathways for the water: bubble
inclusions in the resin; voids where the resin and glass are not fully consolidated; patches of poor bonding between laminates;
capillary pipes where resin has not fully bonded to the glass fibres.
In time, the hull will become saturated with water. The amount is small no more than a few litres for a typical hull.
There is no concern about weight gain, or loss of buoyancy. Most of the water passes harmlessly right through the hull, and
evaporates in the bilges.
Some of the water, however, will come into contact with the various chemicals, left over from the manufacturing
process, discussed above. These will include some or all of:
Some of these materials are water soluble, creating concentrated aqueous solutions. Furthermore, the dissolved molecules are
all much larger than water.
This creates the conditions for classic osmosis: the gel coat can act as a semi-permeable membrane, dividing the concentrated
solutions trapped in the hull from the water outside. The result is that water tends to flow towards the concentrated solution,
causing a build-up of pressure which, in time, can produce a blister.
This, however, is not the whole story. Water continues to penetrate the hull under the influence of hydraulic pressure,
especially through physical defects. Once inside, the water can attack some of the chemicals mentioned above, and
start breaking them down through a process generally described as hydrolysis. So, for example, the PVA binder used on
chopped strand mat hydrolyses into acetic acid, with a characteristic vinegary smell. Styrene is hydrolysed to form benzaldehyde
which has a heavy smell like almonds.
Over a long period of time it can also hydrolyse incompletely-cured polyester resin, which can break cross-links between the long chain molecules,
and can even reverse the process of esterification, liberating the organic acids and alcohols from which the resin was made.
The result is a softening of the resin, and gradual loss of strength, as the resin deteriorates and the bond between the
resin and the glass fibres begins to fail. The break-down products of these reactions sometimes occupy more space than the
original material, with the result that blisters can form although these have nothing to do with osmosis.
How do you know if a boat has a problem? If a boat is seen to have blisters then there is clearly a call for further
investigation.
There are some forms of blistering that have nothing to do with water penetration or osmosis. It is not unknown, for
example, for bubbles of air to expand under the influence of heat, and cause raised blisters. Although unsightly, such
blisters are really only a cosmetic problem.
If a blister is found to contain a liquid, however, then it is almost certainly related to water penetration and the
problems associated with it. Acids (accompanied by the smell of vinegar) usually indicate hydrolysis of PVA. Alkaline
blister fluids normally indicate the presence of a particular type of accelerator a chemical sometimes used to speed
up the curing process during construction. A greasy or sticky fluid usually indicates the presence of an alcohol,
possibly a break-down product from the laminate itself.
If the boat is not blistered, any problems become harder to detect. Water penetration along the lines of glass fibres (often
referred to as "wicking") can sometimes be seen if the gel coat is clear, or if it has been removed. Voids
and poorly consolidated laminate can also sometimes be seen, even through a pigmented gel coat. The problem here is that
the underside of a boat is almost always coated in antifouling, which makes close inspection difficult.
An experienced surveyor can tell a great deal by tapping the hull with a light hammer. Areas of delamination and softened resin can
produce their own distinctive sounds; softening can sometimes be detected through the use of a standard industrial hardness test.
Finally, it is possible to detect water and other chemicals in the laminate with a moisture meter although the presence of
water alone is not a certain indication of trouble. A better test is to monitor the levels of moisture in the hull over a long
period of time, and compare the results with theoretical predictions.
Ultimately it may be helpful to take a core sample, which can then be closely examined, or to have the gel coat removed to allow
thorough inspection of the laminate.
In summary, most hulls have faults, but not all faults turn into problems. And even an apparently sound hull can harbour
deterioration. Correct diagnosis is not straightforward, and is best left to an experienced surveyor.
If diagnosis is tricky, deciding on a course of action is, perhaps, harder still. Brokers love to point out that "no boat
has ever sunk because of osmosis". However, cases are sometimes seen where the laminate is so severely deteriorated not by
osmosis, but by hydrolysis of undercured resin that there is real concern for the structural integrity of the hull.
The only course of action in such severe (and happily rare) cases is to grind or peel away the damaged material and relaminate,
preferably using epoxy resin (which is less water permeable) and new glass fibre (usually woven rovings).
When a boat has blistered, it is important to break open the blisters to check on their cause and to take the opportunity to
examine the laminate beneath. Provided nothing too serious is discovered, it may be possible simply to fill the holes
and make good, grinding out and relaminating locally if necessary. It must be understood, however, that new blisters may
yet appear elsewhere, if the treated blisters are a symptom of a more general problem.
Widespread blistering almost certainly calls for the gel coat to be removed, by mechanical planing or grit blasting, to
allow a thorough inspection. The great majority of blisters occur between the gel coat and the structural laminate, but
it is important to check that there is no deeper damage within the structure before proceeding.
The next step is usually to apply an epoxy coating, which has excellent water resistance, which will take over from the
gel coat that has been removed. Before doing so, however, the hull must be degreased, washed and dried to ensure a good
bond for the coating.
The problem comes with drying the hull. In fact, drying is often advocated as part of the process of "curing" osmosis. This
is something of a fallacy. It is impossible to keep water out of GRP completely problems only arise when it enters voids,
comes into contact with areas of undercured resin and chemical residues in the laminate.
And here lies the difficulty: the chemical residues are all large molecules, and they are trapped within the material.
Some of these large molecules are hygroscopic, which means they attract and retain water molecules.
Consequently it can be very difficult to dry the hull by conventional means.
Indeed, it is not unknown for boats to be left standing for months, waiting for them to dry out. Sometimes they never do.
Hot water washing can sometimes help, as can the application of heat, usually using infrared lamps.
Unfortunately, however, these large molecules have high boiling points at atmospheric pressure. Simply warming the hull
with a lamp or hot water is insufficient to vaporise them. The only readily available process that seems able to remove
these undesirable chemicals is the HotVac system, invented by Suffolk engineer and surveyor
Terry Davey.
HotVac uses heater pads and a vacuum pump to apply heat and simultaneously reduce the pressure, allowing the large
molecules to evaporate and pass out through physical pathways in the laminate. There is some evidence that the controlled
heating also has a beneficial effect on the level of cure in the resin: the laminate is usually found to be harder
and stiffer after treatment. Using this process it is possible to dry a complete hull in no more than a few days.
Once dried, the hull can be finished with a solvent-free epoxy system, starting with a coat of paint, then filler to restore the profile
of the hull if necessary, and finally three or more coats of epoxy paint. The hull can then be primed, anti-fouled, and
put back into service.
Prevention is it worthwhile?
What steps can be taken to guard against osmosis, and the other problems associated with water penetration?
Many owners and surveyors advocate laying up ashore out of season, or if the boat is out of commission and this will
certainly help to reduce water penetration into the hull. It will also allow absorbed water to evaporate, drying the hull,
provided that there are no hygroscopic breakdown products present. Similarly, many owners apply epoxy coatings, even
though no problem has yet appeared. Again, this will probably reduce water penetration.
Fresh water and warm water are also more problematic than salt water and cold water. Boats in the tropics, or moored in
rivers, are more prone to blistering. If your boat experiences these conditions, precautionary measures such as regular
lay-up is likely to have a greater benefit.
The key point is that water will get into the hull. Modern gel coat materials such as isophthalic polyester
and vinylester resins have superior water resistance compared with traditional polyesters, but water still gets in.
If the laminate is stable it is likely that little or no harm will result. But this cannot easily be predicted with certainty.
It is always prudent to take precautions, but if the pre-conditions for a problem exist in the
hull, preventative measures will delay the onset but they probably won't stop it completely.
If you are concerned about osmosis in your boat, or are buying a used boat, why not contact us for an expert opinion?
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