FRP Terminology

ABS (Acrylonitrile-Butadiene-Styrene) – A group of tough, rigid thermoplastics derived from the reaction of
acrylonitrile, styrene, and butadiene gas. These materials are polymerized together in a variety of ratios to
produce ABS resins.

Accelerator – A highly active oxidizing material suspended in a liquid carrier used to accelerate the
decomposition of peroxide catalysts into highly reactive free radicals. These free radicals react readily with
polymer and monomer molecules to cure a thermoset resin. Examples are diethylaniline, dimethylaniline,
cobalt naphthanate, and cobalt octoate. A cleaning fluid used to remove uncured plastic resin from brushes
and clothing.

Air-Inhibited Resin – A resin in which surface cure will be inhibited or stopped by the presence of air.

BMC (Bulk Molding Compound) – A combination of resin paste and chopped glass combined with a “sigma”
blade mixer under conditions of very high mechanical “working” stress. The compound is delivered to the
press in the form of a ball, slab or an extruded log and dropped into the bottom of a mold; the material is
flowed outward until it assumes the shape of the mold.

Catalyst – A substance (usually a peroxide) which readily forms free-radicals. These free radicals react with
polymer and monomer molecules to speed up the curing of thermoset resins. Catalyst content can vary from
0.2% to 2.0% with higher catalyst levels giving faster cure times. Examples are methyl ethyl ketone peroxide
and benzoyl peroxide.

Color Pigments – Ground coloring materials supported in a thick liquid. Added to the resin, they give it color.

Crazing – Hairline cracks either within or on the surface of a laminate, caused by stresses generated during
cure, removal from a mold, impact or flexing.

Crosslinking – Chain-reaction polymerization which results in chemical links (bonds) between individual
polymer chains. This occurs in all thermosetting resins. Styrene monomer and methyl methacrylate monomer
are the most common crosslinking agents used in polyester resins.

Cure – The total crosslinking or polymerization of resin molecules which permanently alters the properties of
the resin changing it from a liquid to a solid.

Cure Time – The time required for the liquid resin to reach a cured or fully polymerized state after the catalyst
has been added.

Delamination – Failure of internal bending between layers of the laminate.

Dimensional Stability – Ability to retain constant shape and size under various environmental conditions,
such as temperature and humidity.

End – As applied to fibrous reinforcements, a bundle of essentially parallel (i.e., entwined) fibers, usually

Exotherm Curve – A graph of temperature plotted against time during the curing cycle. Peak exotherm is the
highest temperature reached during the curing reaction.

Exothermic Heat – Heat given off during a polymerization reaction by the chemical ingredients as they react
and the resin cures.

FBVF (Fiberglass Backed Vacuum Forming) – Combining a thermoformed thermoplastic sheet with a
fiberglass mat or roving using the spray-up or hand lay-up process.

Filament – A single, hair-like fiber of glass characterized by extreme length, which permits its use in yarn with
little or no twist and usually without the required spinning operation.

Fill or Sanding Resin – A general purpose polyester resin used to soak and fill reinforcing material in the
initial lay-up of a surfacing application; usually contains wax.

Fillers – Any one of a number of inexpensive substances which are added to plastic resins to extend volume,
improve properties and lower the cost of the article being produced. Examples are calcium carbonate, alumina
trihydrate, feldspar, and calcium sulfate.

Fire Retardancy – Reduction in the ability of a plastic to ignite and burn. This is accomplished by using
compounds (resins or additives) that contain halogens (bromine or chlorine) or phosphorous. Usually alumina
trihydrate filler is also used because of its ability to release water when exposed to high heat.

Foams, Urethane – Polyurethane resins are produced by reacting diisocyanates with polyols to form polymers
having free isocyanate groups. These groups, under the influence of heat or certain catalysts, will react with
each other, or with water, glycols, etc., to form a foam.

Foams, Flexible – A thermoplastic urethane foam which is adaptable and often used for cushioning in the
furniture and automotive industries.

Foams, Rigid – A thermoset urethane foam which has a higher density, higher modulus, and harder surface
than flexible urethane foams.

Gel – A partial cure of plastic resins; a semi-solid, jelly-like state similar to gelatin in consistency.

Gelcoat – A thin surface coat, either colored or clear, of non-reinforced plastic resin. It is occasionally used for
decorative purposes but also provides a protective coating for the underlying laminate.
Gel Time – Time required to change a flowable liquid resin into a non-flowing gel.

HDPE (High Density Polyethylene) – A thermoplastic material composed of polymers of ethylene. It has
relatively high rigidity and can be modified, e.g. with isoprene, to have high impact strength.

Hand Lay-Up – The oldest and simplest molding technique in which reinforcing materials and catalyzed resin
are laid into or over a mold by hand. These materials are then compressed with a roller to eliminate entrapped

Hardener – See catalyst.

Inhibitor – A substance that retards polymerization, thus extending the shelf life of polymers and monomers.
Also used to extend the gel time and cure time of a thermoset resin.

Laminate – A material composed of successive layers of resin and fiberglass bonded together.

Lamination – The compilation of layers of glass matte and resin, and eventual bonding of these layers

Fiberglass Mat – A flat, coarse fabric composed of glass fibers. There are three types: chopped-strand mat,
continuous strand mat, and surfacing veil.

Monomer – A single molecule capable of polymerizing.

Non-Air-Inhibited Resin – A polyester resin using phthalic anhydride as the starting point. A surfacing agent
is added to exclude air from the surface of the resin.

Orthophthalic Resin – A polyester resin using phthalic anhydride as the starting point. Most thermoset
polyester resins use two types of anhydrides in their production: phthalic anhydride and maleic anhydride. A
higher percentage of phthalic anhydride yields a less reactive resin.

PE (Polyethylene) – A thermoplastic material composed of polymers from ethylene. It is normally a
translucent, tough, waxy solid which is unaffected by water or by a large range of chemicals.

PVC (Polyvinyl Chloride) – A thermoplastic material composed of copolymers of vinyl chloride. A colorless
solid resistant to water, concentrated acids and alkalis.

Polyester Resin – The term generally used for unsaturated polyesters. Formed by the reaction of a dibasic
organic acid or anhydride and a polyhydric alcohol to form a series of ester linkages.

Polymer – The end product, usually a solid, produced from monomers.

Porosity – The formation of undesirable clusters of air bubbles in the surface or body of the laminate.

Pot Life – The length of time that a catalyzed resin remains workable.

Preform Fiber – Glass formed over a screen shaped like the mold in which the preform will be used. It
eliminates the need for over-lapping or mitering the corners in molding. Used primarily to form deep draws or
complex parts.

Prepreg – Glass roving or cloth loaded with B-stage resin, catalyst, and pigment ready for placement in a

Promoter – See accelerator.

Release Agent – A lubricant, often wax, is used to prevent the adhesion of the molded part to the mold. An
internal lubricant such as zinc stearate is used in high temperature molding to obtain release where wax would
melt or be absorbed.

Resin – A liquid plastic substance used as a matrix for glass fibers. It is cured by crosslinking.

Roving – Continuous strands of glass fibers which are grouped together and wound on a tube like untwisted

Shelf Life – The length of time a non-catalyzed resin maintains specified working properties while stored in a
tightly sealed opaque container.

Sizing – The treatment applied to the glass fiber to allow the plastic resins to flow freely around and bond to

SMC (Sheet Molding Compound) – An integrated, ready-to-mold fiberglass reinforced polyester material. The compound is composed of a filled thermosetting resin and a chopped or continuous strand reinforcement.  The primary use is in matched die molding.

Spray-Up – Covers a number of techniques in which a spray gun is used to simultaneously deposit fiberglass
and catalyzed resin on a mold.

Stage (of Resin) – The condition of a partially cured resin polymer when it is only partially soluble in monomer
or acetone but still plastic and still heat fusible.

Staple Fiber – A glass fiber of short length formed by blowing molten glass through holes.

Styrene Monomer – A water-thin liquid monomer used to thin polyester resins and act as the crosslinking

Substrate – Any material which provides a support surface for other materials.

Tack – The stickiness of an adhesive measurable as the force required to separate an adherent from it by
viscous or plastic flow of the adhesion.

Thermoplastic – A plastic material that can be readily softened and reformed by heating and be re-hardened
by cooling.

Thermoset – A plastic material that will undergo or has undergone a chemical reaction caused by heat,
catalyst, ultraviolet light, etc., leading to the formation of a solid. Once it becomes a solid, it cannot be

Thickeners – Material added to the resin to thicken it so that it will not flow as readily.

Thinners – Material added to plastic resins to thin it. They may also be crosslinking agents.

Thixotropic – The property of becoming a gel at rest, but liquefying again on agitation.

Viscosity – A measure of the resistance of liquid to flow.

Wet-out –The ability of a resin to saturate fiberglass reinforcement.

Yarn – A twisted strand or strands of glass fibers which can be woven, braided, served, and processed.