The Resins most commonly used in
GRP are unsaturated polyesters dissolved in styrene. The polyesters are
produced by reacting various organic acids (usually phthalic or maleic
anhydrides) with an alcohol such as propylene glycol or ethylene
glycol. Depending on the particular alcohol or acid used, various types
of resin can be produced-indeed, since a wide variety of both alcohols
and acids are available, it is possible to have polyesters tailor-made
to specific requirements.
The Polyester/styrene solution sets
to a hard, rigid substance, a co-polymer of polyester and styrene, by
the cross linking of molecules (i.e. polymerisation). The hardening
process is commonly referred to as ‘curing’ Partial cross-linking
occurs spontaneously, thereby limiting the storage life of the resins,
but, for the process to take place quickly and completely it has to be
activated by two additives. One is a catalyst, which triggers the
process and the other is an accelerator, which- as the name
implies-speeds it up. Typically, the catalyst is an organic peroxide
(e.g. methyl ethyl ketone peroxide) and the accelerator is normally
cobalt naphanate. These two substances, if mixed directly together will react violently , even explosively.
It is therefore , essential to ad the accelerator to the resin first (
stirring in thoroughly) and then mix the catalyst. In practice, many
resins are supplied ‘pre-accelerated’- with the accelerator added by
the manufacturer- so that only catalyst is needed to activate the
Internal heat (’exotherm’)is
generated within the resin during curing, and can reach relatively high
temperatures about 170oC is typical.
The amount of heat generated
normally varies according the quantity of catalyst used, the volume of
resin and other factors such as the presence of fillers and reactivity
of the resin.
The curing of polyester resins
takes place at room temperature, preferably about 18°C-20°C, and this
is one of the great advantages of the material. Most other plastics
require heat or pressure (or both) so that their use in any
manufacturing process demands expensive and complex machinery. They are
therefore only economically viable for producing high volume runs of
relatively small articles. As polyester resins can be cast at room
temperatures with the simplest equipment , they are practicable for
low-volume production, and even for one-off articles with virtually no
size limit. Although the cured resin is very hard, it is also quite
brittle, which could prevent its use for large articles but this
problem is obviated by the use if reinforcements. A large number of
materials can be used to reinforce the resin, but in practice, the one
offering the best combination of strength, versatility and economy is
Specification of A typical cured Polyester resin, Without Reinforcement
Specific Gravity: 1.28
Tensile Strength: 55 MN/M2
Compressive Strength: 140 MN/m2
Youngs Modulus: 3.5 Gn/M2
Elongation at Break: 2%
Specific Heat: 2.3kJ/kg0C
Thermal Conductivity: 0.3 W/moC
Coefficient of linear expansion: 100 x 10 -6/oC
Water absorption: (24 hr at 20oC): 0.15%
Voltage breakdown (0.2mm sample): 22kV/mm
The Glass used commonly for GRP is
a calcium-alumina borosilicate with an alkali content of less than one
per cent. It is commonly known as ‘E’ type glass, since it was
originally developed for use in electrical insulation systems.
Glassfibres are produced by running
molten glass from a direct melt furnace into a platinum alloy bushing
containing a large number of small holes, from each of which a glass
filament is drawn. Filaments for commercial use are normally between 9
and 15 microns in diameter. The filaments are “dressed” with an
emulsion before being gathered into fibres. The fibres are remarkably
strong-the tensile strength being particularly high. They also exhibit
good chemical and moisture resistance, have excellent electrical
properties, are not subject to biological attack and are
non-combustible with a melting point around 1500oC-all excellent
qualities in a plastic reinforcement.
Polyester resin (unreinforced)
Chopped Strand Mat Laminate 30% glass
Woven Rovings Laminate 45% glass
Satin Weave Cloth Laminate 55% glass
Continuous Rovings Laminate 70% glass
High Density Polythene
The fibres can be used in a variety
of ways-chopped into short lengths(“chopped strands”); gathered
together into loosely bound ropes (“rovings”); woven into a variety of
fabrics, produced from yarn made by twisting and doubling continuous
strands. In the UK, the most widely used glassfibre material is chopped
strand mat, which consists of glass strands chopped together in short
lengths (approx. 50mm) and held together in mat form by a polyvinyl
acetate or polyester binder. The mat is available in a range of
weights, from 225gm2 to 1200gm2, and is a useful general purpose
Provided the glass reinforcement
has been thoroughly impregnated with resin, the result, after curing,
is a cohesive completely integrated matrix of resin and fibres. The
matrix can have a surprising range of properties, depending on the type
of glass material and the formulation of the resin. In general, the GRP
laminate will display excellent tensile and compressive strength,
acceptable thermal conductivity, a low coefficient of linear expansion,
reasonable chemical resistance and good dielectric properties. Compared
to other materials of equivalent strength, it will be light durable,
moisture-resistant, non-rusting and economic.
A Glossary of Materials and Terms
One of the two compounds (the other
is catalyst) required to initiate the polymerisation process. See
Preaccelerated. Mixed directly with catalyst, the accelerator reacts
explosively it is therefore usually added to the resin in manufacture
so only catalyst need be added later.
Solvent for cleaning uncured resin
from brushes and tools. It is HIGHLY FLAMMABLE. It is a powerful grease
solvent and should NOT be used for removing resin from the skin-it
destroys the natural oils and can lead to dermatitis.
An extremely lightweight filler
powder (it is so light it is likely to become airborne if not dispensed
with care). It is used to thicken resins and make them thixotropic.
Air inhibits the curing process in
some resins, with the result that the exposed resin, with the result
that the exposed resin surface tends to remain tacky. This effect is
used deliberately in gel coats, but can be a problem with some resins
additives which prevent air inhibition discolour the resin and
therefore cannot be used for some applications, e.g. clear casting.
In chopped strand mat (the most
commonly used glassfibre material), the strands are held together in a
random pattern by a binder, either a PVA emulsion or a polyester
powder. Powder bound mat gives faster ‘wet-out’ but emulsion-bound
gives greater ease of handling.
An unformed, or only partially formed shape, often moulded in a foamed plastic-e.g. a surfboard ‘blank’.
A Solvent for uncured resin, usually acetone. It is highly inflammable.
An extremely strong, (but expensive) reinforcement which can be used in conjunction with glassfibre, and resin.
Hardener. An organic peroxide, or
similar compound which, together with the accelerator, initites the
polymerisation process of polyester and other resins. It should NEVER
be mixed directly with an accelerator-this can cause an explosion.
Catalyst is available as a liquid or paste.
Catalyst is an organic peroxide (a
powerful corrosive) and should not come contact with eyes, mouth or
skin. Should it do so, wash from the skin immediately under a running
tap. If it is splashed in the eyes, flush them with running water for
at least fifteen, minutes, and call a doctor.
A Purpose-designed instrument for measuring and dispensing liquid catalyst without splashing.
Short (6mm or 12mm) lengths of
glassfibre. Can be used to make a resin dough, stronger than that made
by mixing resin with filler powder.
Chopped Strand Mat
A popular and economical form of glass reinforcement for polyester resins.
Short strands of glass are bonded with a powder or emulsion into a mat available in a variety of thickness.
Clear Casting Resin
A clear resin used for embedding items to make transparent paperweights and other ornaments.
Used in a solution with styrene as
an accelerator for polyester resins, it should NEVER be mixed directly
with catalyst as the two substances react explosively.
Able to cure to a hardened state at room temperature, usually when activated by a catalyst.
The ability of a material to
withstand being crushed. It is found by testing a sample to failure-the
load applied, divided by the cross-section of the sample, gives the
Any method of moulding glass
reinforced plastics without external pressure, as is used for injection
moulding. The commonest contact methods are hand lay-up and spray
Using a metal roller on a glass fibre/resin layer to force out air bubbles.
Normal term for the polymerisation process by which polyester resins harden.
Chemicals used to initiate the polymerisation process in resins- e.g. catalyst, accelerator.
The period required for a polyester
resin to cure fully. In practice, it is taken as the time from the
addition of catalyst to the point of full hardening. A resin may
actually continue to cure for sometime after it is apparently
The compound, which, together with
water, results from the chemical reaction between any organic acid and
any alcohol. See ‘Polyesters’
Epoxy laminating resin boasts
higher adhesive properties and resistance to water, ideal for use in
applications such as boat building. Also used extensively in aircraft
Epoxies are widely used as a
primary construction material for high-performance boats or as a
secondary application to sheath a hull or replace water-degraded
polyester resins and gel coats.
The internal heat generated within
a resin by the polymerisation process. As the resin cures it becomes
noticeably hotter. This can create problems in resin casting, since the
temperature can be high enough to crack the casting.
A mould in which the internal
surface decides the form of the casting or laminate takes from it. A
child making sandcastles with a bucket is using a female mould! See
Any Substance added to a resin to
extend it. A typical filler is an inert calcite (talc) which increases
the bulk of the resin without affecting its chemical properties.
Fillers can also be used to alter the texture of a cast piece by
creating realistic metallic or stone effects. Almost any dry substances
can be used as a filler- stone or slate powder, metal powder, sawdust,
sand, gravel etc. Most fillers have the advantage of reducing exotherm.
Glassfibre materials once hardened
can be polished, sanded, drilled, sawn or filed. Diamond carborundum or
metal finishing tools generally are required. Since the dust produced
can be extremely hazardous to eyes and lungs protective goggles and
breathing masks should be worn at all times when machining hardened
resins, with or without glassfibre reinforcement.
Anything round which a GRP
lamination can be laid –e.g. a cardboard tube can be used as a former
for a laminated stiffening rib. The term could be applied to such items
as a surfboard blanks, and is also used for the ‘pattern’ or ‘plug’
from which a mould is taken.
Resin often used on a plug (especially a wooden plug) to give a highly glazed surface.
Before hardening completely, a
catalysed resin first reaches a thick jelly like consistency known as
the ‘gel’ state. Once it reaches this stage, the resin is impossible to
spray, paint or pour. Stored resin which has passed its shelf life may
gel without being catalysed.
A thixotropic resin invariably used
as the first coat (applied without glass reinforcement) on the mould
surface. It forms the hard, smooth shiny surface of the finished
article and is usually pigmented. It paints on easily but does not
drain from vertical surfaces. When 2% wax solution is added it becomes
The Period between a resin
catalysing and reaching gel state- in effect, the time in which it is
still workable. Gel time varies from one type of resin to another. It
is also known as ‘setting time’.
Treatment of glassfibres, during manufacture, to improve adhesion to plastic resins.
Glass Filaments drawn together into
fibres and used to reinforce polyester resins, to produce a strong,
lightweight, versatile material. The fibres can be woven into a variety
of fabric types or used as a random matrix of short (‘chopped’) strands
held together by a powder or a emulsion binder.
Abbreviation for Glassfibre Reinforced Plastic. The plastic is a resin sometimes epoxy but usually polyester.
A point reached by a GRP laminate
after the gel-time but before it is fully hardened. When ‘green’, the
laminate is fairly firm but can be cut with a knife- it is therefore
easy to trim at this stage.
The process of applying the
resin/glass laminates to the mould manually with brushes and rollers-
an economical but effective method, requiring no specialised equipment,
and therefore the most popular DIY method.
Any substances which slows, or
stops the curing process. Air is an inhibitor to the surface of some
resins, oil or water will inhibit most.
A proprietary cleansing cream which removes resin, etc., from the skin- it is rubbed in prior to washing in soap and water.
Any material in which separate layers of material are bonded together. In GRP work, the layers are resin and glassfibre.
Liquid compound which air dries to a flexible rubber-popularly used for making small moulds for craftwork.
The process of applying the resin/glass laminates in the mould. See ‘Hand Lay-up’ and ‘Spray Lay-up’.
A mould having external working surface on which the laminate is laid-up.
The time taken for an apparently
hardened resin to become completely cured and stable. This is only
important in mould-making and in certain specialised applications-e.g.
the construction of fish ponds or of food and chemical containers,
where a not fully cured resin may release traces of chemicals, such as
Methyl Ethyl Ketone Peroxide, an organic peroxide-the main constituent of a widely-used catalyst for polyester resin.
A plastic polyester film which does not adhere to resin, and therefore, can be used for self-releasing formers, etc.
Powdered metals used as a filler, giving a realistic metallic finish and texture to resin castings.
Tiny flakes of epoxy-coated metal
foil, available in a wide range of colours. Can be mixed with resin to
produce an attractive sparkling effect.
A substance which is capable of being polymerised.
Many glassfibres projects require a
mould in which to cast the resin, or lay-up the laminations. A mould
can be made in almost any material as long as it is sufficiently rigid,
has a smooth surface, and will not adhere to the resin (or can be
treated so that it will not do so). The most usual mould materials for
laminating wood or GRP. Various flexible rubber compounds are popularly
used for small castings. Moulds can be male or female (the lamination
is laid up on the outside of a male mould and the inside of a female
Type of metal roller used in laminating.
See ‘Release Agent’.
Paper wound on a wire core. Widely used as a former over which to laminate stiffening ribs, etc.
Conventional polyester resins are
translucent cloudy. They can be coloured with the addition of a wide
variety pigments,-opaque, translucent, metallic.
An additive which increases the flexibility of resins.
Also known as a pattern or former-a
full size model or mock up from which a mould is taken. The mould is
then used to produce the finished glassfibre article.
Substances produced by reacting
certain glycols (alcohols) with anhydrides (organic acids).
Conventional GRP resins are polyesters dissolved in styrene.
Commonly called polythene. As it is
completely resistant to polyester resins it can be used for buckets,
mixing containers, and small casting moulds. Polythene sheet is a
useful release agent in some circumstances.
In general, the remarks regarding
polyethylene also apply to polypropylene, which is widely used for
expensive heavy duty kitchenware etc.
Polyester resins attack and dissolve polystyrene, which cannot therefore be used for storage containers, moulds etc.
The application of heat to accelerate the complete curing process and shorten maturing time.
The working time of a resin – the period between the resin being catalysed and becoming gelled.
(Polyvinyl Alcohol): Used as the basis of some release agents.
Any plastic reinforced with
additional material. The term is now virtually synonymous with
‘glass-reinforced plastic’ due to the universal popularity of glass as
Since polyester resins adhere to
most substances to some extent (with the exception of a few plastics),
moulds invariably have to be treated to enable the laminate to be
released easily. The treatment usually takes the form of polishing with
purpose designed waxes called release agents. Moulds made of
polypropylene, polythene or silicone rubber will sometimes be
self-releasing, as will those protected by a sheet if polythene or
Occur in nature as organic
compounds, soluble in organic solvents but not in water – e.g. amber,
shellac. Synthetic resins have similar properties and are normally
produced by polymerisation. The resins most used in GRP are polyesters
Breathing Mask incorporating filter
designed to protect the lungs from minute dust particles and/or harmful
fumes. Essential when machining glass fibre laminates. Make sure you
are using the correct OEL ( occupational exposure limit).
Either Aluminium paddle, PTFE, nylon ect, are used for rolling resin onto working area or consolidating laminate.
Long fibres of glass, held together by dressing, supplied in a ‘cheese’.
A chemical compound incapable of polymerisation.
See ‘Gel Time’.
The Process by which an item is given a protective skin of resin impregnated glassfibre. Often applied to boat hull and roofs.
Catalysts and accelerators speed up
the polymerisation process-even without these additives, resins will
tend to polymerise slowly by themselves. Hence, all resins have a
limited shelf life, from three months to a year or so depending on the
type of resin and storage conditions.
A Synthetic rubber used for
flexible moulding compounds, sealants etc. It is usually cold-curing.
Por-a-mold is a typical example.
Mainly used in industrial
workshops, spray lay up involves spraying the resin, catalyst and
finely chopped glass fibres into the mould form a special spray gun.
Benzene feed stock product
(phylethylene) used as a solvent and thinner for polyester resins. It
is highly inflammable and produces fumes which can be dangerous in
excess. During curing, the styrene molecules cross link with polyester
to produce a co-polymer.
Ultimate strength of a material measured under tension, normally expressed in MN/m2.
Plastic which can be softened (by
heating) and hardened (by cooling) and still retain its properties –
e.g. polythene, PVC, celluloid.
Plastic, which hardens by a
non-reversible chemical reaction initiated by heat and / or curing
agents. Once hardened, it cannot be melted without being altered or
destroyed – e.g. polyester, epoxies.
The condition of a resin which has
failed to harden completely due to very low temperature, insufficient
catalyst, or the presence of inhibitors. If any of these factors are
excessive, the resin can be permanently under-cured and never harden.
The ability of a liquid to resist flow – a thick, treacly liquid has high viscosity, a thin, runny liquid has low viscosity.
A proprietary, hot-melt, vinyl compound used for flexible ‘rubber’ moulds. It can be re-melted and used several times.
To achieve full strength, the
glassfibre materials in a laminate should be thoroughly impregnated
with resin – the glass is said to be wetted out when it is so
Petroleum based compound from which are derived the organic acids used in polyester resin manufacture.
information is, to the best of our knowledge, true and accurate.
Recommendations are made without warranty or guarantee. Users are
advised to make their own tests to determine the suitability of
specific materials or methods.