What is GRP?
GRP - meaning Glass Reinforced Plastic, is
plastic resins (usually polyester or epoxy) strengthened
by glass-fibre mat or fabric. The term is often used
(rather inaccurately) to include plastics reinforced with
materials other than glass; carbon fibres, for example,
or aramids such as Kevlar.
Polyester Resin is a thick sticky liquid which,
when activated by a suitable catalyst (hardener), sets to
a hard, rigid plastic. As a liquid, it naturally adopts
the shape of its container - hence it can be poured,
painted or injected into a mould, and, once cured, will
reproduce the shape of the mould.
The hardened plastic is fairly brittle so
glass-fibre materials are added to give considerably
increased strength, just as steel rods are used in
reinforced concrete.
The combination of polyester resin and
glass-fibre produces an incredibly versatile material –
strong, durable, weatherproof, waterproof, and
non-rusting. Easily moulded to virtually any shape,
highly adhesive to a wide range of materials, and capable
of making structures of almost any size.
The resin can also be used alone, or with
fillers, to make castings which accurately reproduce the
finest detail of the mould.
There are numerous instances where GRP can be
used as a viable alternative to traditional materials,
such as steel or wood, and in many cases, GRP is a more
appropriate choice.
Marine GRP products
The range of items which can be made with these
materials is almost unlimited... boats, catamarans,
canoes, marine grp components, yacht shower trays, tanks,
panels.
Items produced
include:
Hulls
Decks
Wheelhouses
Fairings
Battery Boxes
Anchor Hatches
Intake Ducts
Water Jets
Bulkheads
Bulwarks
Glass fibre reinforced plastic (GRP) … how to
use grp.
Few materials can match the versatility and
utility of glass fibre reinforced plastic, commonly
called GRP for short or simply fibreglass.
Strong, lightweight and completely waterproof,
it can be moulded into free form shapes, such as any of
the above, or laminated to make decorative wall panels,
sills or roofing.
Formed of woven or felted glass fabric
embedded in a plastic resin, the material combines the best
attributes of both. The strength of glass fibres in the plastic
makes the material rigid and resin gives it a smooth
impermeable surface.
The glass fibre fabrics that are used in
laminating are mostly made up of rovings, bunched strands of
glass filaments.
The rovings are pressed into mat or woven
into fabrics of varying densities; density is measured in terms
of the weight, in ounces per square foot, or grams per square
metre of fabric. The fabric weight as well as the forming
method to be used, determines the quantity of resin that will
be needed.
Generally, a chopped strand mat takes two and a
half times its weight in resin; the ratio of resin to
fabric for a woven cloth depending on type can be up to
two times.
Fabric choice depends on
the amount of reinforcement needed and on the type of resin
being used.
Woven fabrics which are stronger, will not bond
as well to each other as they will with mat fabrics, it
is best to alternate layers to ensure even strength
throughout the laminate, and to produce a smooth surface,
a fine glass fibre tissue is often used for the final
layer, alternatively coat over the final layer with
‘Flowcoat’ a gelcoat type resin that cures
touch-dry.
Although many resins are
suitable for making fibreglass, polyester is both the simplest
and most economical; it provides excellent strength and
moisture resistance.
When it is laminated, polyester’s normal high
shrinkage during curing is reduced to 2 per cent or less, and
this slight shrinkage is often an advantage. In moulded
laminations it assists easy release from the
mould.
With moulds it helps enormously if unobtrusive
holes are drilled and taped over prior to applying PVA,
into which compressed air can be directed later when
curing has sufficiently occurred for the removal of the
casting. A bicycle hand pump is often enough.
When you are laminating fibreglass with a
mould or form from which it will later be removed, you will
need to apply release agents to stop the fibreglass from
sticking to the mould, for simple moulds you can coat the
surface with PVA (polyvinyl alcohol) release liquids, for
larger or complex moulds, a two stage parting compound of non
silicone wax and PVA is generally used.
Apply the wax first and let it dry completely,
polish up, repeat twice more, then apply the PVA using a
sponge or brush to lay on a thin even coat, protect the
surface of the mould from dust whilst the parting
compound is drying
Polyester resins
Polyester resins are usually available as a
two-component system, with resin in one container and the
hardener often known as the catalyst in the
other.
Be sure to follow the manufacturers mixing
instructions exactly, combining the components carefully
to avoid mixing air bubbles, which weaken the cured
resin.
Usually the resin contains an accelerator, also
called a promoter, which speeds the cure time, but
sometimes this component needs to be added separately, if
you are adding an accelerator, be absolutely sure you mix
it into the resin before you add the catalyst.
The catalyst must be handled with great care at
all times; it is a corrosive substance and can ignite
spontaneously when in contact with materials like paper
and rags.
Gelcoats.
Special resins called gelcoats are always used
as the outermost layer in fibreglass
lamination.
This un-reinforced resin provides a smooth,
glossy, protective layer between the glassfibre and
outside moisture.
It is applied as the first layer in the mould,
by using a pigmented gelcoat, either pre-mixed or mixed
on the job with up to 10 per cent of a suitable polyester
pigment paste, you can impart the surface colour of your
choice to the fibreglass.
LAYING UP or FIBRE GLASSING using a
MOULD (female).
Typically a brushing gelcoat will be either
clear or pre-pigmented and have thixotropic (non-drip)
properties.
When catalysed it is applied to the prepared
mould surface ensuring complete coverage to a minimum
thickness of 0.5mm.
The spread rate is around two ounces per square
foot.
It now needs some time to cure, a minimum of
four hours should be allowed, otherwise the solvent in
the laminating resin next applied could attack the
gelcoat causing wrinkling.
Cut all the glass fabric to the approximate size
before mixing the resin, this can be done with scissors
or a trimming knife; glass matting can also be
torn.
If the matting edges are to be overlapped, torn
edges are most suitable as they will intermingle within
the resin and make the joint without a visible
seam.
The next step is to paint over the cured gelcoat
with lay-up resins, then apply your first layer of
matting, to impregnate the glass fibre with resin, use a
paint roller or brush, stippling the brush over the
fabric so as not to dislocate the glass strands with the
sticky, resin coated bristles.
Never apply more than the amount of resin
recommended by the manufacturer of the fabric; the heat
generated by the resin as it cures can adversely affect
the laminate if applied in quantities too
great.
Some polyester resins cure completely only in
the absence of air. Left exposed, they remain tacky
indefinitely.
This can be an advantage in multi layer
laminations over a large area, where new coats are
usually applied before the previous coat has cured, a
forming process called wet on wet.
As the fibreglass layers begin to cure (called
the “green” stage) trimming can be done easily (using a
sharp blade) but you have to be quick as this stage may
only last three to four minutes.
The work can be interrupted for several hours
without harming the laminate.
When using such resins, however, the final coat
must be sealed with flowcoat or covered with an airtight
polyethylene film until the resin cures, after which the
film can be peeled off.
The male moulding now needs to cure for several
hours but can be shortened somewhat by post curing
methods, involving warm air, still or fan
applied.
When sufficiently cured, the moulding will be
wanting to shrink away from the sides of the mould, this
can be helped along by dissolving the blue release agent
with water poured along the edges.
Keep tools soaking in solvent so you will be
able to clean them when the job is completed, before the
resin hardens.
A supply of clean rags is essential for removing
spilled resin, and the floor beneath the work should be
covered with newspaper to catch the drips, which are
difficult to remove once they have hardened.
In addition, the laminating process also
requires you to use several specialised tools.
One of these tools is a paddle roller or split
washer type for consolidating resin and glass and at the
same time pushing away any air bubbles that form between
the layers, these rollers come in several forms, with
washer or paddle shaped blades, and in sizes ranging from
6 to 300mmm wide.
For use on contoured surfaces there is a
flexible roller, with a head resembling a coiled
spring.
To trim and finish the completed lamination, if
you didn’t do this during the “green stage”, you will
need a metal cutting saw, a forming tool such as a file
or rasp, and a supply of wet and dry paper from 240 to
600 grit.
Glass fibre work is quite an easy skill to
master, providing you have the right tools and materials,
and approach it in a methodical manner, taking the
required safety precautions.
The basic material components include, release
agents, gelcoats, resins, glass mat, glass cloth, glass
tissue, and flowcoats.
Chopped strand mat
(csm).
The most common fibreglass reinforcement, is
made up of randomly arranged glass fibre strands, pressed
and held together with a binding agent.
Woven cloth.
Glass fibre filaments, spun into a yarn then
woven to make cloth, provides great strength with minimal
thickness, is usually used as an alternate layer between
CSM, giving added strength without substantially
increasing laminate thickness.
Glass fibre tissue.
A very fine mat weighing approx one ounce per
square yard, used as the final layer, to conceal the
coarser texture of CSM, and give a smoother
finish.
HOW DURABLE IS GRP?
The oldest boats made from glassfibre are now
over 50 years' old and no deterioration in the glassfibre
can be found. Properly constructed glass fibre mouldings
will last in excess of 50 years.
Where the glassfibre has been kept immersed in
water, as in boat hulls, or exposed continuously to the
elements, as in boat decks, there is no evidence of
deterioration of the laminate in either
application.
The surface finish and colour of a "GRP"
laminate can be modified to simulate almost any
appearance: shiny or matt, textured or smooth, any colour
- including translucent finishes or metallic. Resins are
already used commercially to manufacture synthetic
marble, onyx, granite and any stone imaginable. "GRP" is
also used for translucent roof lights and can be tinted
to give the appearance of stained glass.
A water tank or boat hole may be repaired by
forming a glassfibre patch over the damaged
area.
From levelling up the hull, selecting the tools
and timbers, to fitting grp bulkheads, grp bulwarks,
tanks, engine compartments, steering, deck fittings,
bunks, galley and much else.... all these are the next
stage of boat construction.
Osmosis
treatment.
Despite most people’s assumptions to the
contrary, fibreglass mouldings (yacht hulls for example),
no matter what resins are used, are not actually totally
waterproof. Individual water molecules are so small that
they can actually find their way into and ultimately
right through the layers of glass and resin forming a GRP
boat hull.
Problems start to occur when the water molecules
migrating into the GRP encounter other chemicals inside
the laminate, primarily water-soluble materials (WSMs)
such as the the emulsion binders used to hold the glass
mat together before it is moulded, or pockets of uncured
or only partly cured resins in the moulding.
The water molecules can then have a chemical
reaction with these substances, forming larger molecules
of a new chemical, often acidic - which unlike the
original small water molecules, cannot carry on passing
through the GRP. These larger molecules are then trapped.
This is the point at which osmosis actually
starts.
Pressure builds up inside the laminate. If
this process takes place in a solid part of the laminate, there
is usually no problem as the structure is strong enough to
contain the pressure. If however it takes place on the boundary
of a small air-bubble in the moulding, or at a point where
layers of GRP are poorly bonded, the resultant new chemical
compound or compounds slowly fill up the bubbles or the minute
gaps between layers with liquid.
Almost all mouldings have these air bubbles
and small areas of poor bonding, although they should not.
Ideally the resin should totally fill the gaps between the
glass strands, and every layer should perfectly bond to the
next. In practice, however, this is extremely difficult to
achieve with conventional moulding
techniques.
The process of osmosis in GRP is however very
slow, unless the moulding is appallingly badly made, and
no matter how long it remains in water a typical GRP
laminate cannot absorb more than about 2-3% of it’s own
weight of water.
Surveyors and boatyards (and some brokers) put
moisture meters on yachts hulls to check the moisture
content, on the basis, often but not always correct, that
high moisture levels in the GRP are a precursor to the
development of blisters.
If this osmosis (using the term in it’s
correct manner for once) was all that happened, it would be a
very minor problem. Even completely saturated with water
molecules, a GRP laminate still retains most of it’s strength,
although it does become slightly more
flexible.
Racers who want stiff hulls with the absolute
minimum weight already mostly keep their boats ashore when not
sailing, and for any properly built cruising boat small extra
weight and a trace more flexibility in the structure should not
be problems.
If the air bubble simply filled with this
acidic compound, the problem would still be relatively minor.
However the nature of the osmosis process is that water
molecules keep osmosing through the laminate, and join the
chemicals in the bubble, steadily building up pressure.
Eventually this causes the surface of the moulding to
blister.
These blisters are the typical sign of what
boat-owners usually refer to as ‘osmosis’. When pierced
these blisters will give off a small amount of
chemical-smelling (usually vinegary) liquid - which is
the juice built up inside the pressure-raised
blisters.
The term ‘blister juice’ is often used. This
‘blister juice’, which is usually acid, can break down
the polyester. This breakdown process is known as
hydrolysis, and causes a reduction in strength of the
laminate.
Once blisters in the gelcoat have appeared, a
period of storage ashore, particularly in warm dry
weather, may cause them to apparently disappear, as most
of the water in the blisters dries out. What is left
behind, though, is a highly concentrated solution of the
'blister juice', which will usually rapidly re-absorb
water once the hull is put back into the
water.
The process of osmosis is advanced by the
time visible blisters start to appear on the bottom of a hull.
It is the case, however, that the process starts the minute a
new yacht is craned into the water, or even when it’s hull and
deck first gets rained on as it is wheeled out of the
factory.
Another facet of water absorption into a hull
is known as ‘wicking’. This refers to the ability of water
molecules to creep along the boundary of the individual strands
of glass within the moulding.
A totally dry moulding, if moulded with clear
resin, will be virtually transparent. If you can see individual
strands of glass as whitish threads, what you are seeing is not
the glass strands themselves, but water around the
strands.
This ‘wicking’ is an indication that there is
a significant amount of moisture in the resin, and is often a
precursor to or accompanies
blistering.
All GRP yachts, from the day they are built,
suffer from osmosis. Manufacturers now typically offer
five year hull warranties, and it has been said -
cynically but probably accurately - that their main
concern is to do just enough that they don’t get visible
blisters within the warranty period, which for most
yachts is 5 years.
Several US manufacturers of small power boats,
however, now state in their warranty conditions that the
boats are not to be left afloat for more than 2 weeks, or
the warranty against blisters is invalid! At least one US
builder will not offer any warranty at all against
blisters.
The fact remains that in practice some yachts
‘get osmosis’ - ie blisters, and some don’t. It is known that
several factors increase the likelihood of blistering. These
are Long periods afloat
without layups
Warm tropical
waters. Fresh water is worse than salt
water
Coloured resins (including white - the most
common) are worse than clear
resins.
Some builders, including those who produce some
very expensive boats, have had runs of boats prone to
blistering, they have also turned out apparently
identical boats that have not blistered. Current thinking
is that cleanliness, temperature and humidity control in
the moulding shop, and precision of the mix of resins,
are the key to building boats that will not blister.
No-one really knows.
While all GRP boats slowly absorb some water, it
should not be fast. Visible blisters or wicking are an
indication of a well developed absorption of water, and
if they occur in the first few years of a boat's life are
an indication of a moulding problem of some sort, whether
it be poor materials, poor workmanship by the laminators,
or any other quality control problem ranging from sawdust
getting into the moulding to a prolonged delay between
laminating up the various layers that form the
hull.
Osmosis treatment involves stripping off the
external gelcoat, drying out, and recoating with epoxy
fillers.
As this new external coating is essentially
‘glued on’, and not chemically part of the original moulding
like the original gelcoat, you could argue that the boat is
substantially devalued by this
repair.
After ten to fifteen years,it is common to find
that yacht hulls have a moderate to high moisture
content.
Some may also have developed a few blisters.
This is normal, and not necessarily a sign that there is
anything terribly wrong with the boat.
If a yacht reaches twenty years of age
without high moisture content or visible blisters it is
actually a surprise.
These timescales assume a standard mass
production yacht, given average use of perhaps seven or eight
months afloat a year, with just antifouling paint on the
bottom. Painting with polyurethanes or epoxy coating the bottom
can considerably, but not totally, slow down the rate of water
absorption, and some builders do this from new. Opinions vary
as to the effectiveness of epoxying or hull painting once there
is already some moisture in the
moulding.
On boats which have been painted or epoxied, it
is not uncommon to find, after a few years, blisters in
the interface between the epoxy/paint coating and the
gelcoat. This is obviously less of a problem than
blisters under the gelcoat itself. Some experts believe
that paint or epoxy coatings should be renewed regularly
to maintain effectiveness.
Osmosis treatment can be local treatment,
cutting or grinding open individual blisters, repeatedly
washing out with hot water or steam, to remove the
‘blister juice’ from any blisters, drying thoroughly and
filling with epoxy paste.
Osmosis Treatment Centres doing a whole hull
remove all the gelcoat, wash and dry out, and recoat the
hull with epoxy. The smaller and older the boat the less
cost-effective this is.
If you are trying to sell the boat, buyers
almost always prefer boats without blisters.
Boatyards like doing the work... it is
profitable, and can be scheduled in to when staff have free
time. Some surveyors like to recommend it, as it means they’ve
‘covered their backs’ against a later claim that they didn’t
pick up a defect.
It is certainly easier to sell a boat with no
blisters and a low moisture content.
If you are buying a boat, it is perhaps
preferable to have one with no blisters and a dry hull
(low moisture levels on the magic meter).
If this condition is achieved by the original
hull surface, with no repairs, it is better than a
similar boat that is also dry and with no blisters, but
achieved by having recently had an ‘osmosis
treatment’.
The treatments are not cures - they simply
'restart the clock' on a progressive absorption of water
again, as even epoxy coatings are not totally
waterproof.
Occasionally buyers will happily accept a boat
with high moisture content or blisters - on the grounds
that they can haggle down the price as a
result.
They then may or may not get some form of
treatment done - perhaps just before they sell it on a
few years later. The “game” is
perpetual and ongoing.
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