Catalysts and accelerators

In order to mould or laminate a polyester resin,

the resin must be cured. This is the name given to

the overall process of gelation and hardening,

which is achieved either by the use of a catalyst

and heating, or at normal room temperature

by using a catalyst and an accelerator. Catalysts

for polyester resins are usually organic peroxides.

Pure catalysts are chemically unstable and

liable to decompose with explosive violence.

They are supplied, therefore, as a paste or liquid

dispersion in a plasticizer, or as a powder in an

inert filler. Many chemical compounds act as

accelerators, making it possible for the resin-containing

catalyst to be cured without the use of

heat. Some accelerators have only limited or specific

uses, such as quaternary ammonium compounds,

vanadium, tin or zirconium salts. By far

the most important of all accelerators are those

based on a cobalt soap or those based on a tertiary

amine. It is essential to choose the correct

type of catalyst and accelerator, as well as to

use the correct amount, if the optimum properties

of the final cured resin or laminate are to be

obtained.

Pre-accelerated resins

Many resins are supplied with an in-built accelerator

system controlled to give the most suitable

gelling and hardening characteristics for

the fabricator. Pre-accelerated resins need only

Figure 16.12Production of unsaturated polyester resin (Scott Bader Co. Ltd )

Reinforced composite materials 541

the addition of a catalyst to start the curing reaction

at room temperature. Resins of this type are

ideal for production runs under controlled workshop

conditions.

The cure of a polyester resin will begin as soon

as a suitable catalyst is added. The speed of the

reactions will depend on the resin and the activity

of the catalyst. Without the addition of an accelerator,

heat or ultraviolet radiation, the resin will take

a considerable time to cure. In order to speed up

this reaction at room temperature it is usual to add

an accelerator. The quantity of accelerator added

will control the time of gelation and the rate of

hardening.

There are three distinct phases in the curing

reaction:

Gel time This is the period from the addition

of the accelerator to the setting of the resin to a

soft gel.

Hardening time This is the time from the setting

of the resin to the point when the resin is hard

enough to allow the moulding or laminate to be

withdrawn from the mould.

Maturing time This may be hours, several days

or even weeks depending on the resin and curing

system, and is the time taken for the moulding or

laminate to acquire its full hardness and chemical

resistance. The maturing process can be accelerated

by post-curing.

Fillers and pigments

Fillers are used in polyester resins to impart particular

properties. They will give opacity to castings

and laminates, produce dense gel coats, and

impart specific mechanical, electrical and fire

resisting properties. A particular property may

often be improved by the selection of a suitable

filler. Powdered mineral fillers usually increase

compressive strength; fibrous fillers improve tensile

and impact strength. Moulding properties can

also be modified by the use of fillers; for example,

shrinkage of the moulding during cure can be considerably

reduced. There is no doubt, also, that the

wet lay-up process on vertical surfaces would be

virtually impossible if thixotropic fillers were not

available.

Polyester resins can be coloured to any shade by

the addition of selected pigments and pigment

pastes, the main requirement being to ensure thorough

dispersion of colouring matter throughout the

resin to avoid patchy mouldings.

Both pigments and fillers can increase the

cure time of the resin by dilution effect, and the

adjusted catalyst and promoter are added to

compensate.

Releasing agents

Releasing agents used in the normal moulding

processes may be either water-soluble film-forming

compounds, or some type of wax compound.

The choice of releasing agent depends on the size

and complexity of the moulding and on the surface

finish of the mould. Small mouldings of simple

shape, taken from a suitable GRP mould,

should require only a film of polyvinyl alcohol

(PVAL) to be applied as a solution by cloth,

sponge or spray. Some mouldings are likely to

stick if only PVAL is used. PVAL is available as a

soultion in water or solvent, or as a concentrate

which has to be diluted, and it may be in either

coloured or colourless form.

Suitable wax emulsions are also available as a

releasing agent. They are supplied as surface finishing

pastes, liquid wax or wax polishes. The

recommended method of application can vary

depending upon the material to be finished. Hand

apply with a pad of damp, good quality mutton

cloth or equivalent, in straight even strokes.

Buff lightly to a shine with a clean, dry, good

quality mutton cloth. Machine at 1800 rpm using

a G-mop foam finishing head. Soak this head in

clean water before use and keep damp during

compounding. Apply the wax to the surface.

After compounding, remove residue and buff

lightly to a shine with a clean, dry, good quality

mutton cloth.

Wax polishes should be applied in small quantities

since they contain a high percentage of wax

solids. Application with a pad of clean, soft cloth

should be limited to an area of approximately 1

square metre. Polishing should be carried out

immediately, before the wax is allowed to dry. This

can be done either by hand or by machine with the

aid of a wool mop polishing bonnet.

Frekote is a semi-permanent, multi-release,

gloss finish, non-wax polymeric mould release

system specially designed for high-gloss polyester

mouldings. It will give a semi-permanent release

542Repair of Vehicle Bodies

interface when correctly applied to moulds from

ambient up to 135 °C. This multi-release interface

chemically bonds to the mould’s surface and

forms on it a microthin layer of a chemically

resistant coating. It does not build up on the mould

and will give a high-gloss finish to all polyester

resins, cultured marble and onyx. It can be used

on moulds made from polyester, epoxy, metal or

composite moulds. Care should be taken to avoid

contact with the skin, and the wearing of suitable

clothing, especially gloves, is highly recommended.

These products must be used in a well

ventilated area.

Adhesives used with GRP

Since polyester resin is highly adhesive, it is the

logical choice for bonding most materials to GRP

surfaces.

Suitable alternatives include the Sika Technique,

which is a heavy-duty, polyurethane-based joining

compound. It cures to a flexible rubber which

bonds firmly to wood, metal, glass and GRP. It is

ideal for such jobs as bonding glass to GRP or

bonding GRP and metal, as is often required on

vehicles with GRP bodywork. It is not affected by

vibration and is totally waterproof. The Araldite

range includes a number of industrial adhesives

which are highly recommended for use with GRP.

Most high-strength impact adhesives (superglues)

can be used on GRP laminates.

Most other adhesives will be incapable of bonding

strongly to GRP and should not be used when

maximum adhesion is essential.

Core materials

Core materials, usually polyurethane, are used in

sandwich construction, that is basically a laminate

consisting of a foam sheet between two or

more glass fibre layers (Figure 16.13). This gives

the laminate considerable added rigidity without

greatly increasing weight. Foam materials are

available which can be bent and folded to follow

curved surfaces such as vehicle bodies. Foam

sheet can be glued or stapled together, then laminated

over to produce a strong box structure,

without requiring a mould. Typical formers

and core materials are paper rope, polyurethane

rigid foam sheet, scoreboard contoured foam

sheet, Termanto PVC rigid foam sheet, Term PVC

contoured foam sheet, and Termino PVC contoured

foam sheet.

Formers

A former is anything which provides shape or form

to a GRP laminate. They are often used as a basis

for stiffening ribs or box sections. A popular material

for formers is a paper rope, made of paper

wound on flexible wire cord. This is laid on the

GRP surface and is laminated over to produce reinforcing

ribs, which give added stiffness with little

extra weight. The former itself provides none of

the extra stiffness; this results entirely from the box

section of the laminate rib. Wood, metal, or plastic

tubing and folded cardboard can all be used successfully

as formers. Another popular material is

polyurethane foam sheet, which can be cut and

shaped to any required form (Figure 16.14).

16.6 Moulding techniques for reinforced

composite laminates

Contact moulding

This is the oldest, simplest and most popular fabrication

technique for the automotive, reinforced plastic

body industry. It is normally used for relatively

short runs, but it has also been adapted successfully

for series production. It is the only production

method which takes full advantage of the two

Figure 16.13Typical sandwich construction (Scott

Bader Co. Ltd )

Reinforced composite materials 543

most important characteristics of polyester resin,

namely that it can be set without heat and without

pressure. A considerable industry has been built

around contact moulding, which has facilitated the

cost effective production of large one-piece mouldings,

particularly for low production runs. Contact

moulding advantages are that a minimum of equipment

is required, tooling is inexpensive, there are

practically no size restrictions, and design changes

are easily made. Disadvantages are that the labour

context is high and the quality of the moulding

depends on the skill of the operators. The lay-up and

curing times are comparatively slow, and only one

good surface finish is achieved.

The contact moulding process is carried out in

the following manner. A master pattern or model is

made, representing in all its dimensions the finished

product. This could be, for example, a full-size

wood model of the type of body shell or cab shell

required, or it might equally well be a steel or aluminium

panel-beaten structure of composite type,

or even a plaster model reinforced with wire mesh.

From this is made a master mould, which must be

female or concave for the most part, and this would

in all probability be made in reinforced plastics

similar to those used for the final product. It is

important to differentiate, as a matter terminology,

between ‘mould’ and ‘moulding’, one being the

production tool and the other the product itself.

An important aspect of the process is that the surface

of the mould will inevitably be faithfully reproduced

in the moulding, and accordingly if the mould

is bumpy or rough so will be the final article. It does

not usually matter if the unseen or partly hidden side

of the moulding is rough (indeed, it usually is), but

for the displayed surface to be unsightly is not normally

tolerable. This is why a female mould is usually

used. If the original pattern was very smooth, so

will be the inside of the mould and, therefore, the

outside of the moulding. This is why so much trouble

is taken over the pattern. If in wood it is tooled

and sanded to perfection, and if in metal it is panel

beaten with the greatest possible skill, then ground

and polished if necessary. It is of the greatest importance

that separation of the moulding from the

mould be easy. A special compound or a polish such

as carnauba wax and/or silicone lubricant can be

used; plastics film is occasionally used as a separating

membrane, as it will not adhere strongly to

either the mould or the moulding. For contact

moulding, the equipment is relatively simple and

inexpensive. Contact moulding can be further subdivided

into hand lay-up and spray lay-up.

The application of the release agent to the mould

is followed by brush or spray application of a gel

coat of resin. There has been a constant improvement

and development in polyester resins, and

among other things this has led to the introduction

of successful semi-flexible gel coats, which are of

particular interest to the motor industry. The gel coat

is a continuous skin on the working face of a moulding.

It is almost pure resin and its object is to give a

good finish as well as to protect the working surface

from corrosion and other damage. It also hides the

fibre pattern of the reinforcement. The gel coat can

be colour impregnated or otherwise specially formulated,

e.g. for extra abrasion or impact resistance. It

should be as even in thickness as possible, as thicker

areas are very susceptible to accidental damage,

while thin patches can lower the resistance of the

structure to moisture and to chemical attack.

Even in hand lay-up the spray method may be

used for this stage and for the application of the separating

agent, so that there is a small element of mechanization.

A fine surfacing tissue may be applied to

Figure 16.14Typical formers: (a) metal tube

(b) paper rope (c) cardboard tube (d) foam strip

(e) wood (f) folded cardboard (Scott Bader Co. Ltd )

544Repair of Vehicle Bodies

the gel coat while wet, or it may simply be allowed

to gel as it is. Further resin is sprayed on or brushed

on, and mat or woven cloth, which has been carefully

cut to patterns, is laid in position. Consolidation and

air removal are then effected by manual means. It is

customary to use rollers made up of split washers for

this operation, which is an extremely important one

if consistency and strength of the moulding are to be

obtained. More mat or cloth is added in order to

build up the requisite thickness of reinforced plastics,

and the moulding is then allowed to set or cure.

Curing normally takes place at room temperature,

but sometimes under a certain degree of heat if the

process is to be accelerated. It should be remembered

that curing is itself a heat-producing process. Contact

moulding can also be carried out by simple mechanical

means, but the general principle is always that of

bringing the materials into contact with the mould,

without the use of any dies.

The following is a summary of the contact

moulding process:

1 The master mould must be spotlessly clean.

2 A release agent is applied to the entire surface

of the mould face (Figure 16.15).

and the drawing-through process is repeated

until the required thickness is achieved. It is

critical that all air bubbles be removed by the

brushing and rolling (Figure 16.18).

Figure 16.15Application of release agent to mould

surface (Scott Bader Co. Ltd )

Figure 16.18Rolling impregnated mat (Scott Bader

Co. Ltd )

Figure 16.17Impregnation of glass mat with resin

(Scott Bader Co. Ltd )

Figure 16.16Application of gel coat covering (Scott

Bader Co. Ltd )

3 Gel coat covering is applied by brush or spray

(Figure 16.16).

4 Catalyst is added to the resin and the catalysed

resin is smoothed over the gel-coated mould.

5 Glass fibre mat, precut to exact size, is laid on

the mould and a further small quantity of resin

is poured on to the mat. With brushes and hand

roller, the resin is drawn through the mat

(Figure 16.17). A second layer of mat is applied

Reinforced composite materials 545

6 The mould is allowed to cure naturally or heat

is used to speed up the curing process.

7 After curing, the moulds are broken and the

completed sections are removed (Figure 16.19).

8 They are then trimmed and ready for use.

single-nozzle gun, but liquid catalyst is metered

into the resin in the gun itself. A glass rovings

chopper delivers the reinforcement to the mould

surface as in the former system.

Although much of the manual labour of the hand

lay-up is eliminated by using the spray process,

thorough rolling is still necessary, not only to consolidate

the deposited glass resin mixture, but also

Figure 16.19Final mould and moulding (Scott

Bader Co. Ltd )

Figure 16.20The spray-up technique (BP

Chemicals (UK) Ltd )

Spray-up technique

A development from the basic manual contact

process which is employed with increasing frequency

in the automotive body industry is known

as spray-up (Figure 16.20). In this method, rovings

are automatically fed through a chopping unit and

the resultant chopped strands are blown or carried

by the sprayed resin stream on to the mould. The

glass and resin mix applied in this way is consolidated,

and the air pockets or bubbles are removed

by manual rolling, as in simple hand lay-up.

There are several commercial spraying systems

available, where the glass fibre and resin are

deposited simultaneously on the mould face. They

consist of two principal types. In the twin-pot system

a twin-nozzle spray gun is used, and in order

to prevent gelation in the gun the resin is divided

into two parts, one of which is catalysed and the

other accelerated. The two streams of resin spray

converge near the surface of the mould simultaneously

with a stream of glass fibre ejected by a glass

rovings chopper. In the other type, the catalyst

injector system, accelerated resin is sprayed from a

(b)

546Repair of Vehicle Bodies

to ensure that the accelerated and catalysed portions

of the resin are adequately mixed. Considerable

skill is needed to control the thickness of the laminate

when using the resin glass spray gun. Spraying

reduces labour costs, especially when the volume of

production is large enough to keep the equipment in

constant use.

Hot press moulding

This process involves the use of chopped strand

glass mat, pre-impregnated with polyester resin,

which is then in general principle formed in presses

in a similar manner to that used for forming steel

sheet (Figure 16.21). In this case, however, the dies,

which are preheated, have to remain closed for the

curing cycle of the pre-impregnated mat, which may

involve a period from 15 to 30 seconds. Hot press

moulding using matched dies have good finish on

both surfaces. Further, this method enables high

glass content and uniform dimensional properties

and appearances to be achieved at lower cost than

by other methods for runs above 1000 units. Such a

process reduces the labour content of producing

panels, but much increases the initial tooling charge.

systems. The most common moulding processes are

as follows (Figure 16.22): hand lay-up, spray-up,

resin transfer moulding, compression moulding,

injection moulding thermoplastics, injection moulding

thermosets, pultrusion, and reinforced reaction

injection moulding.

The three techniques used in the production of

body panels are as follows:

Cold press mouldings This is used in the manufacture

of the boot lid. The boot lid is formed by

cold pressing a mineral reinforced resin-coated

glass fibre mat in a gel-coated mould, forming a

component which is very stiff in relation to its

weight (see Figure 16.23).

Reinforced reaction injection moulding The RRIM

technique is used for all the vertical body panels

such as the front and rear wings, front grille and

bumper assembly, and rear panel and bumper

assembly. RRIM polyurethane has the properties of

good recovery from deformation, outstanding resistance

to wear, impact and abrasion, and a fast cycle

time in manufacture. The use of this material for all

exposed corners of the car helps to reduce minor

body damage repair (see Figure 16.22h).

Pressure assisted resin injection moulding The

bonnet is a pressure assisted moulding of sandwich

construction with polyester resin exterior on either

side of a rigid urethane core. The underside of the

moulding is an intumescent fire barrier which is a

major safety factor for an engine compartment

cover (see Figure 16.22e).

Lotus Cars Ltd

Lotus have been producing reinforced composite

motor cars since 1957. In 1973 the company introduced

the vacuum assisted resin injection (VARI)

system for vehicle body manufacture. The first

VARI moulded production car was the Lotus Elite

introduced in 1974; since then developments have

continued in the processes, tooling and techniques

of producing composite vehicle bodies.

The Lotus VARI process provides a method of

moulding fibre reinforced composite panels from

matched tooling. The process can be used to manufacture

large body panels with integrated foam

structures and captive metal fixings, using relatively

low-cost tooling. As there is no dependency upon

platen size and press tonnage – an obvious limitation

of other processes – there are no panel size

Figure 16.21Hot press moulding