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A GRP/Fibreglassing Glossary

As in any other field, the range of different fibreglass words and terms can seem daunting to the composites or fibreglass beginner. We’ve compiled this handy alphabetical list of GRP terms to help you get to grips with all the terminology:

Accelerator-

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.

Acetone

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.

Aerosil

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 Inhibition

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.

Binder

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.

Blank

An unformed, or only partially formed shape, often moulded in a foamed plastic-e.g. a surfboard ‘blank’.

Brush Cleaner

A Solvent for uncured resin, usually acetone. It is highly inflammable. 

Carbon Fibre

An extremely strong reinforcement which can be used in conjunction with glassfibre, and resin.

Catalyst

(or hardener) – This is the chemical added to resin to start the hardening reaction (cure). An organic peroxide, or similar compound which, together with the accelerator, initiates 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 into 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.

Catalyst Dispenser

A Purpose-designed instrument for measuring and dispensing liquid catalyst without splashing.

Chopped Strands

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 (CSM)

This is a “cloth” made up of thousands of glass strands bound together by a binder – either PVA emulsion or polyester powder – The binder is dissolved by the resin, leaving just the glass strands embedded in the plastic.

Clear Casting Resin

A clear resin used for embedding items to make transparent paperweights and other ornaments.

Cobalt Naphthanate

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.

Cold Curing

Able to cure to a hardened state at room temperature, usually when activated by a catalyst.

Compressive Strength

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 compressive strength.

Contact Moulding

Any method of moulding glass reinforced plastics without external pressure, as is used for injection moulding. The most common contact methods are hand lay-up and spray moulding.

Consolidating

Using a metal roller on a glass fibre/resin layer to force out air bubbles, thus consolidating the resin with the reinforcement.

Cure

Normal term for the polymerisation process by which polyester resins harden. The process of turning from a liquid resin into a solid plastic.

Curing Agents

Chemicals used to initiate the polymerisation process in resins- e.g. catalyst, accelerator, hardener

Curing Time

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 some time after it is apparently completely hard.

Ester

The compound, which, together with water, results from the chemical reaction between any organic acid and any alcohol. See ‘Polyesters’

Epoxy

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 component manufacture.
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.

Exotherm

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.

Female Mould

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 ‘Moulds’

Filler

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.

Finishing

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.   

Flowcoat

(or Topcoat) is a gelcoat with a wax additive, enabling it to cure in contact with air.

Former

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.

Furane

Resin often used on a plug (especially a wooden plug) to give a highly glazed surface.

Gel-Stage

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.

Gel Coat

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 a Flowcoat.

Gel Time

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’.

Glass Finish

Treatment of glassfibres, during manufacture, to improve adhesion to plastic resins.

Glassfibre

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.

GRP

Glassfibre Reinforced Plastic, or fibreglass, is what fibreglass is! A plastic resin strengthened with strands of glass. There are various other composite materials made the same way, with differing reinforcements eg. Carbon fibre etc. The plastic is a resin sometimes epoxy but usually polyester.

Green Stage

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.

Hand Lay-up

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.

Hardener

See Catalyst.

Inhibitor

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.

Laminate

Any material in which separate layers of material are bonded together. In GRP work, the layers are resin and glassfibre.

Latex Rubber

Liquid compound which air dries to a flexible rubber-popularly used for making small moulds for craftwork.

Lay-up

The process of applying the resin/glass laminates in the mould. See ‘Hand Lay-up’ and ‘Spray Lay-up’.

Male Mould

A mould having external working surface on which the laminate is laid-up.

Maturing Time

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 styrene.

MEKP

Methyl Ethyl Ketone Peroxide, an organic peroxide-the main constituent of a widely-used catalyst for polyester resin.

Melinex

A plastic polyester film which does not adhere to resin, and therefore, can be used for self-releasing formers, etc.

Metal Powers

Powdered metals used as a filler, giving a realistic metallic finish and texture to resin castings.

Metallic Jewels

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.

Monomer

A substance which is capable of being polymerised.

Moulds

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 are 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 mould).

Paddle Roller

Type of metal roller used in laminating.

Parting Agent

See ‘Release Agent’.

Paper Rope

Paper wound on a wire core. Widely used as a former over which to laminate stiffening ribs, etc.

Pattern

See ‘Plug’.

Pigments

Conventional polyester resins are translucent cloudy. They can be coloured with the addition of a wide variety pigments,-opaque, translucent, metallic.

Plasticiser

An additive which increases the flexibility of resins.

Plug

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.

Polyester

Substances produced by reacting certain glycols (alcohols) with anhydrides (organic acids). Conventional GRP resins are polyesters dissolved in styrene.

Polyethylene

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.

Polypropylene

In general, the remarks regarding polyethylene also apply to polypropylene, which is widely used for expensive heavy duty kitchenware etc.

Polystyrene

Polyester resins attack and dissolve polystyrene, which cannot therefore be used for storage containers, moulds etc

Polyurethane (PU)

A very versatile material which is used in many different materials such as paints, adhesives, varnishes, resins and foams often used in conjunction with polyester resin based fibreglass. Also used for various casting resins which are stronger and produce less odour than their polyester counterparts.

Post cure

The application of heat to accelerate the complete curing process and shorten maturing time.

Pot life

The working time of a resin – the period between the resin being catalysed and becoming gelled.

PVA

(Polyvinyl Alcohol): Used as the basis of some release agents.

Reinforced Plastic

Any plastic reinforced with additional material. The term is now virtually synonymous with ‘glass-reinforced plastic’ due to the universal popularity of glass as the reinforcement.

Release Agent

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 Melinex film. 

Resins

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

Respirator

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).

Rollers

Either Aluminium paddle, PTFE, nylon ect,  are used for rolling resin onto working area or consolidating laminate.

Rovings

Long fibres of glass, held together by dressing, supplied in a ‘cheese’.

Saturated Compound

A chemical compound incapable of polymerisation.

Setting Time

See ‘Gel Time’.

Sheathing

The Process by which an item is given a protective skin of resin impregnated glassfibre. Often applied to boat hull and roofs.

Shelf Life

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.

Silicone Rubber

A Synthetic rubber used for flexible moulding compounds, sealants etc. It is usually cold-curing. Por-a-mold is a typical example.

Spray Lay-Up

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.

Styrene

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.

Tensile Strength

Ultimate strength of a material measured under tension, normally expressed in MN/m2.

Thermoplastic

Plastic which can be softened (by heating) and hardened (by cooling) and still retain its properties – e.g. polythene, PVC, celluloid.

Thermosetting

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.

Undercure

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.

Viscosity

The ability of a liquid to resist flow – a thick, treacly liquid has high viscosity, a thin, runny liquid has low viscosity.

Vinamold

A proprietary, hot-melt, vinyl compound used for flexible ‘rubber’ moulds. It can be re-melted and used several times.

Vinylester

Vinylester resins are produced by reacting an epoxy with an unsaturated monocarboxylic acid – they are often thought of as being halfway between a polyester and an epoxy resin – stronger than a polyester, but not as strong as an epoxy etc.

Wetting Out

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 impregnated.

Xylene

Petroleum based compound from which are derived the organic acids used in polyester resin manufacture.
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