Reactors Don’t Get Osmosis – Yeah Right! Part 1
This article is a cautionary tale for those who people who own a Reactor or an old polyester
boat and is based on my own experience. I have heard it said a number of times that
“Reactors don’t get osmosis”, sorry folks, this isn’t correct. I have given up on my Reactor
due to hydrolysis, osmosis and other issues. Osmosis appears to be patchily understood
and more critically many people appear not to understand hydrolysis; which is the
precursor to osmotic blistering and may start as soon as the boat put in the water!
Although it is a slow process, hydrolysis in some hull types can be structurally more serious
than osmotic blisters [A]. As Bumgarner states; “… not all [boat] bottoms with hydrolysis
damage have [osmotic] blisters but all bottoms with [osmotic] blisters have some degree of
hydrolysis damage” [A].
While fibreglass hulls offer a significant maintenance advantage over wooden hulls they are
not maintenance free. They still need periodic inspection and maintenance. If you have any
doubts about your boat I urge you to do your own research and engage an experienced
and INDEPENDENT consultant (I did) who does not have a vested interest in extracting
money to fix your boat. I used ‘Mike Menzies Marine Inspection’ and found Mike to be
helpful and pragmatic. Mike has kindly vetted this article to check that I have not made any
glaring mistakes or errors.
This article gives an introduction to hydrolysis and osmotic blistering in older polyester
boats. In researching I have made extensive use of other peoples’ work; significant points
are referenced and there is a reference list at the end of Part 2 of the article. I found a
variety of terminology and conflicting information and opinion. Without resolving these
conflicts this article attempts to give a generalised introduction to the topic although there
are some omissions for brevity and because the subjects are complex. The following three
terms will be used quite specifically; ‘osmosis’, ‘hydrolysis’ and ‘osmotic blistering’. In this
section alone the way some of the other practitioners use these terms will be {shown in
parenthesis}.
Figure 1: A small hole approximately 1.5mm
across. At the bottom of this hole glass fibres
can be seen that are depleted of resin, this is
a single point where hydrolysis is visible.
Osmosis: A physics definition of
osmosis is the process of “movement of
a solvent [such as water] through a
semi-permeable membrane … into a
solution of higher solute concentration
that tends to equalize the
concentrations of solute on the two
sides of the membrane” [B]. In the case
of osmotic blistering it is suggested that
the water moves through the gelcoat (NZ English spelling) and resin into the osmotic blister
cells.
{Some practitioners state that there is actually no osmosis taking place but it is a process of
hydrolysis and the creation of water soluble hydrolysis by-products that create the
characteristic blister cells [C]. Some practitioners use the term ‘osmosis’ to describe the
process of the physical degradation of a GRP (Glass Reinforced Plastic) hull from new all
the way through to the appearance of large destructive ‘osmotic blisters’.}
Hydrolysis: As a chemical phenomenon hydrolysis is the breakdown of a substance by
water [D], the hydrolysis of polyester results in the breakdown of the resin and the creation
of a series of waste products, some of which are harmful to the remaining resin [C]. In this
article, ‘hydrolysis’ will also be used to describe the physical process by which the resin is
successively lost from the polyester GRP system.
{By contrast some practitioners use the term “stage one” and “stage two” osmosis to
describe early and advanced hydrolysis and “stage three” osmosis is used to describes the
appearance of the osmotic blisters [E].}
Osmotic Blistering: This term will be used to describe the fluid filled blisters within the
laminate structure either at a shallow depth or deeper with the hull; they are the physical
sign that hydrolysis of the polyester has affected at least one laminate layer [C]. However
surface blisters can occur that are not associated with the GRP resin system or the hull.
Instead these may be a cosmetic or a maintenance problem such as antifoul blisters within
or under the layers of the antifouling or gelcoat blisters.
Figure 2: A hole approximately 4mm across with resin cracking about it. At the bottom of this hole, glass
fibres can be seen that are depleted of
resin. Before this photo the hull surface
was sanded with 40 grit paper on a random
orbital sander. The original gelcoat was the
blue-green colour, at some stage a white
resin was laid over the top of the gelcoat.
Approximately 8 years ago the grey resin
was laid up by a commercial operator. The
areas of black are where the grey resin has
been sanded back, revealing black
antifouling that appears to be filling a hole
or a crater within the hull surface - ie, there
was poor quality preparation. There is
obviously a hydrolysis problem at the whole
location. When gently tap-tested, say with
a car key, each one of the black spots
sounds hollow or will yield inwards to gentle
pressure. These are all areas of hydrolysis.
Polyester GRP Systems
Boat GRP systems are a matrix of resin that holds glass fibres that reinforce the resin. If
you were to manufacture a boat hull of wood or steel, you are getting a readymade quality
controlled materials; when you manufacture a GRP boat hull, you have to take
responsibility for the manufacture and quality control of the material from which the hull is
made.
The strength, resistance to hydrolysis and osmotic blistering and longevity of the final GRP
material is dependent on many things; the quality of the resins, the mixing ratios, mixing,
the glass to resin ratio is critical, cleanliness, curing conditions, fibreglass sizing (coatings
or primers on the glass fibres) [F & G] and compatibility of these with the resin system and
water, product layup, density, air voids, environment, temperature, waterproofing, damage
and maintenance. There are many different resin systems, of which polyester resin is one.
Polyester is favoured among other reasons, as it is significantly cheaper than many other
resin types such as epoxy and vinylester resins.
Polyester is an umbrella term for differing types of resin system; poly means many, so
polyester is many ester molecules joined in a long chemical chain. These polyester
molecules form a pale coloured liquid and are usually suspended in a monomer (individual
molecules) of styrene and other chemicals. This system is ‘cured’ by the addition of
catalysts and or accelerators; this curing or 'polymerisation' cross links the ester chains
together with the styrene as a bridge turning the liquid into a solid that is relatively
chemically resistant [D & J]. Often a significant amount of the styrene and other chemicals
remain unreacted within the cured resin (up to 5% is normal) [A] and are often referred to
as Water Soluble Materials (WSMs) [A] and later play a part in the hydrolysis or osmotic
blistering processes.
Added to the problem of uncured resin chemicals is that some of the binder coatings used
on the glass fibres before they were laminated into the GRP matrix were/are water soluble
[I & J]. Therefore the water that is slowly permeating the hull can wick along the glass
fibres through or even deep into the laminate and migrate into voids and areas of uncured
materials and WSMs within the laminate matrix. It can be seen that there are some basic
compromises with some of the materials before the laminate is even constructed.
Figure 3: An osmotic
blister that has been cut
open with a chisel.
The fibreglass fibres
are exposed with very
little resin adhered to
them.
Hydrolysis of
Polyester Resins
The chemistry of
hydrolysis is the
slow breakdown of
the resin in the
presence of water
[C]. The resulting
chemicals are a
mixture of organic
acids, water, and
(depending on the
original resin)
alcohols and glycols among others [A & K], these are also Water Soluble Materials
(WSMs). This chemical cocktail will accelerate the hydrolysis breakdown of the remaining
resin [K & I] and depending on the gelcoat and paint system, may or may not permeate out
of the hull. Hochgraf states “After hydrolysis, the new [created] molecules have some
mobility and also occupy a greater volume than the polyester molecules from which they
came …. the natural mobility of the molecules, causes them to fill any voids in the
fiberglass, including the pinhole porosity that is always present” [K] ….and migrate to areas
of uncured resin” [L]. The chemical process of hydrolysis therefore is the breakdown of the
polyester resin in the presence of water and the formation of waste products that create
further problems and occupy more volume.
Michael Hollis – Out’n’About. Checked by Mike Menzies.
With apologies to the author, the final three paragraphs of this section,
together with the rest of the article and the References, will be published next month.
Reactors Don’t Get Osmosis – Yeah Right! Part 2
The first three paragraphs below conclude the section on the
Hydrolysis of Polyester Resins started in Part 1.
On a GRP hull “Hydrolysis can be recognized by resin ‘wash out’ between the fibreglass
strands leaving a laminate [fibreglass] that appears to be ‘dry’, lacking resin [but wet when
tested with a moisture meter [M]]. The resin remaining will be soft … and if the condition is
severe, the bottom laminate may be easily deflected by hand pressure and will sound dull
when percussion tested” [L]. “…[O]ld[er] …. gelkotes (American spelling within the quote)
are so porous that ‘flow out’ equals ‘flow in’. The gelkote is not a semi-permeable
membrane, rather is a fully permeable membrane. The result is severe hydrolysis but no
blisters” [A]…. as the waste products don’t accumulate. Some modern boats have tighter
gelcoats” …which the boat builders had hoped would stop blistering and hydrolysis.
Though hydrolysis is greatly reduced, blistering occurs sooner and more dramatically [A] as
the waste products do accumulate. The beginning of accumulation of significant quantities
of these waste products and or water within the laminate is the nominal crossover point
between hydrolysis and osmotic blistering.
Not all practitioners agree on the terminology or the mechanisms, however they all agree
that the GRP matrix becomes successively depleted of resin; it starts and is most dramatic
at the surface and can involve complete “wash out” of the resin through to mildly depleted
and compromised resin deeper in the GRP matrix. Some boats may have significant
hydrolysis and possibly even deep-set minor delaminations (osmotic blisters) without
showing external signs of osmotic blistering [A & C]. The experts do agree that the
hydrolysis process is slow and Hartoft & Browning indicate that [polyester] resin has a life
of 30-35 years [L], however this is a difficult figure to nail down as it is dependent on so
many different conditions, including the quality of the original lay-up of the hull and the
waterproofing systems.
Figure 4: Osmotic blisters that
have been partially ground out
with coarse sanding disk. The
milky coloured area about the
outside of each blister site is
hydrolysis damaged laminate.
The bottom of the main blister
shows some good laminate but
some hydrolysis is still evident.
To effect a proper repair, the
blister really needs to be
ground out deeper and the
hydrolysis damaged laminate
about each blister site should
be removed.
Osmotic Blistering
In relation to Osmotic
blistering Bumgarner
states that, “ …[it]
becomes obvious that hydrolysis is the real problem and that [osmotic] blisters are an
unsightly and destructive by-product of the hydrolysis of the polyester resin in the gelkote
and laminate…” [A]. Bumgarner explains that osmotic blistering at depth within the hull
laminate below the hydrolysis by “…[t]he glass fibres assist by acting as capillary tunnels to
transport the water molecules into the laminate...” and that random laid fibres such as from
‘chopper guns’ may be in contact with the gelcoat and may also run deeper into the
laminate [A].
Further, both Menzies and Bumgarner suggest that the binder coatings on the glass fibres
play a role in hydrolysis [A & M]. These two clues suggest a mechanism for water getting
into some laminates. This could also explain why the woven fabric portions of boat layups
generally show less hydrolysis and osmotic blistering [A]. This however is a simplification;
hydrolysis and osmotic blistering are complex chemical and physical processes with many
variables and differing manifestations. Clegg cautions that osmotic blistering may take up
to 30 years to become visible [E].
The practitioners agree that hydrolysis begins as a surface effect due to permeability of the
surface coatings and works into the laminate while the osmotic blistering of polyester GRP
systems occurs within the GRP laminate and below the surface. Bumgarner’s statement is
worth repeating; “… not all [boat] bottoms with hydrolysis damage have [osmotic] blisters
but all bottoms with [osmotic] blisters have some degree of hydrolysis damage” [A].
Osmotic blisters may be a near-surface effect where the gelcoat and paint systems are
semi-permeable, or may occur deeper with the laminate after extensive hydrolysis has
already damaged the hull [A]. Hartoft & Browning warn that hydrolysis often prevents
blisters from forming in deeper layers due to the porosity caused by resin depletion and
further notes that for blisters to form there must be a membrane capable of holding
pressure [L]. Conversely, Clegg notes that he has been seeing an increase in older heavily
laid up boats with osmotic blistering [E].
Figure 5: Welcome to my
nightmare. Some osmotic
blisters have been opened,
some ground out, some
still need excising, i.e. at
the location of the black felt
pen crosses.
Almost all of the little black
spots represent areas of
exposed antifoul that was
under the outer resin coat
and are hydrolysis
damaged.
Gelcoats and
Sealing Hulls
It becomes obvious
that if hydrolysis is the
problem then sealing
the hull is all important;
although history has shown this has been difficult to achieve [M]. Sealing was usually
accomplished by a gelcoat system, the earlier coating systems are now sometimes referred
to as semi-permeable or even permeable membranes [A]. Later systems added epoxy
resins on top of the gelcoats which improved the water tightness, these moderns systems
are far from perfect although are better [M]. Later “tighter” [A] systems were not enough to
stop water migration; these systems tended to blister earlier and more dramatically
although the overall damage was less [A].
Bumgarner states that all continually immersed conventional polyester and gelcoat resin
boats show visible hydrolysis damage after five to ten years [A]. Clegg states that a
correctly applied epoxy system will provide a better moisture barrier than virtually any
gelcoat, reducing moisture levels to an almost insignificant amount and that a good system
should have a lifespan of at least ten to twenty years [E].
Vinylester resins have increasingly become the standard barrier coat for a number of
reasons, including that they are more compatible with polyester than are the epoxy resin
systems [A]. However to achieve the same level of waterproofing as an epoxy, a greater
thickness of the vinylester has to be laid up by applying many more coats.
Remedies
This article is not about repair but the following brief and generic descriptions of some
remedies are given. Osmotic blisters, where there are not many and they are small, can be
ground out and filled. Hydrolysis is not recoverable, much of the resin has gone from the
surface and the underlying hull is compromised. The hydrolysis damaged laminate can be
removed and reinstated but this is expensive. If the hydrolysis is not too bad it may not
significantly affect the hull strength and can be lived with, remedial repair may be
appropriate a season or two later [E].
There is much debate about moisture levels in GRP systems and about “drying out” the hull
before making a diagnosis or performing repairs. Many practitioners advocate washing and
or drying out of hulls. While the washing may get rid of surface Water Soluble Materials
(WSMs), it doesn’t work below the laminate’s surface. Bumgarner believes drying out is a
misnomer; even if the hull is “dried”, the dehydrated hydrolysis fluid will remain [A] and will
be available to cause further slow creeping damage to the hull or repairs.
Any resins used in repair or waterproofing must be carefully matched to the existing system
and problems. “…[E]poxies are only marginally tolerant of polyester substrates and seem
to reject acidic [hydrolysis damaged] laminates over time” [A]. This is demonstrated by the
experience that some hulls with extensive blistering have had a “full osmosis job done
previously” [M]. Bumgarner notes that a popular but less than successful approach is to
remove the existing coatings and some of the laminate and apply a barrier coat; he notes
that these often only last 2 to 5 years and then blister as the chemistry of the damaged
resin containing the WSMs underlying the new coating has not changed [A].
Summary
There is much talk about osmosis, or more correctly osmotic blistering. However this
damage in polyester hulls is often a development after significant longer term hydrolysis
damage and is traceable to inadequate waterproofing. Significantly, severe hydrolysis hull
damage can accumulate over time in polyester hulls without the appearance of osmotic
blistering; the hydrolysis is not obvious unless you look for it.
Earlier polyester boats were coated with gelcoat systems that are now considered
inadequate; better waterproofing systems are now available and are being effectively used
where applied to a dry and properly prepared surface; however, they need careful matching
to the hull, the problems, and to be properly maintained.
It should be noted that where repair and preservation of a hull is attempted, care is needed
in the selection of an appropriate system as they must be tailored to:
 the resin system(s)
 the damage type
 the hull ‘wetness’
 the cost and expected longevity of the repair, or the longevity of the boat.
Like all things within boating, these systems will have a limited life. Older polyester boats
are especially prone to hydrolysis damage; their owners should be aware of these issues
and should be inspecting for this and maintaining adequate underwater waterproofing
systems.
Michael Hollis – Out’n’About. Checked by Mike Menzies.
************************************
References
A Bumgarner, C. Blisters & Laminate Hydrolysis, Zahnister’s Yachting Centre, Dated 9 April 2003, retrieved from
http://www.zahnisers.com/blisters_hydrolysis.htm
B Merriam-Webster online dictionary, Retrieved from http://www.merriamwebster.com/dictionary/osmosis
August 2013.
C HOTVAC, Retrieved from http://www.hotvac.com/osmosis/ August 2013.
D Martin, B. Personal communication with a chemist, September 2013.
E Clegg, N. A short guide to osmosis & its treatment, revised edition, Retrieved from
www.passionforpaint.co.uk/pdf/osmosis3.pdf August 2013.
F http://en.wikipedia.org/wiki/Fiberglass#Sizing
retrieved August 2013
G
http://www.michelman.com/userfiles/File/Reinforce_Plastics_Michelma_sizing_stability_
article11.29.12.pdf retrieved August 2013.
H Cripps, D. Published courtesy of Gurit (http://www.gurit.com), retrieved from
http://www.netcomposites.com/guide/polyester-resins/ 8 September 2013
I Yachtnet. Osmosis and yacht hulls, Retrieved from www.yachtnet.co.uk/osmosis.htm
September 2013.
J Feloy, R. What is osmosis and how is it treated?, Retrieved from
www.yachtsurveys.co.uk/faq_osmosis.htm
September 2013.
K Hochgraf, F. Hydrolysis Failures of Fiberglass: Delamination, blistering & bleeding, 5 January 2006, retrieved
from http://www.nhml.com/hydrolysis-of-fiberglass.cfm
L Hartoft P. & Browning G. Hartoft Marine Survey Ltd, Retrieved from
http://www.hartoftmarinesurvey.com/guide.html
dated 1995.
M Menzies, M. The facts about osmosis, retrieved from www.mikemenzies-marine.co.nz/new_page_3.htm
August 2013.
Text For Each Figure In Article
Figure 1.
A small hole approximately 1.5mm across. At the bottom of this hole glass fibres can be seen that are
depleted of resin, this is a single point were hydrolysis is visible.
Figure 2.
A hole approximately 4mm across with resin cracking about it. At the bottom of this hole glass fibres can
be seen that are depleted of resin. Before this photo the hull surface was sanded with 40 grit paper on a
random orbital sander. The original gelcoat was the-blue green colour, at some stage a white resin was
laid over the top of the gelcoat. Approximately 8 years ago the grey resin was laid up by a commercial
operator. The areas of black are where the grey resin has been sanded back revealing black antifouling
that appears to be filling a hole or a crater within the hull surface. i.e. There was poor quality preparation.
There is obviously a hydrolysis problem at the hole location. When gently tap tested, say with a car key,
each one of the black spots sounds hollow or will yield inwards to gentle pressure. These are all areas of
hydrolysis.
Figure 3.
An osmotic blister that has been cut open with a chisel. The fibreglass fibres are exposed with very little
resin adhered to them.
Figure 4.
Osmotic blisters that have been partially ground out with coarse sanding disk. The milky coloured area
about the outside of each blister site is hydrolysis damaged laminate. The bottom of the main blister
shows some good laminate but some hydrolysis is still evident. To effect a proper repair, the blister really
needs to be ground out deeper and the hydrolysis damaged laminate about each blister site should be
removed.
Figure 5.
Welcome to my nightmare. Some osmotic blisters have been opened, some ground out, some still need
excising, i.e. at the location of the black felt pen crosses. Almost all of the little black spots represent
areas of exposed antifoul that was under the outer resin coat and are hydrolysis damaged.