Application of these materials

Probably chopped strand mat is most commonly

used for the average moulding. It is available in

several different thicknesses and specified by

weight: 300, 450 and 600 g/m2. The 450 g/m2 is

the most frequently used, and is often supplemented

with the 300 g/m2. The 600 g/m2 density is

rather too bulky for many purposes, and may not

drape as easily, although all forms become very

pliable when wetted with the resin.

The woven glass fibre cloths are generally of

two kinds, made from continuous filaments or

from staple fibres. Obviously, most fabricators

use the woven variety of glass fibre for those

structures that are going to be the most highly

stressed. For example, a moulded glass fibre seat

pan and squab unit would be made with woven

material as reinforcement, but a detachable hard

top for a sports car body would more probably be

made with chopped strand mat as a basis.

However, it is quite customary to combine cloth

and mat, not only to obtain adequate thickness,

but because if the sandwich of resin, mat and

cloth is arranged so that the mat is nearest to the

surface of the final product, the appearance will

be better.

The top layer of resin is comparatively thin, and

the weave of cloth can show up underneath it,

especially if some areas have to be buffed subsequently.

Chopped fibres do not show up so prominently,

but some fabricators compromise by using

the thinnest possible cloth (surfacing tissue as it is

known) nearest the surface, on top of the chopped

strand mat. When moulding, these orders are of

course reversed, the tissue going on to the gel coat

on the inside of the mould, followed by the mat

and resin lay-up.

It is important to note that if glass cloths or

woven mat are used, it is possible to lay up the

materials so that the reinforcement is in the

direction of the greatest stresses, thus giving

extra strength to the entire fabrication. In plain

weave cloths, each warp and weft thread passes

over one yarn and under the next. In twill

weaves, the weft yarns pass over one warp and

under more than one warp yarn; in 2 _ 1 twill,

the weft yarns pass over one warp yarn and under

two warp yarns. Satin weaves may be of multishaft

types, when each warp and weft yarn goes

under one and over several yarns. Unidirectional

cloth is one in which the strength is higher in one

direction than the other, and balanced cloth is a

type with the warp and weft strength about equal.

Although relatively expensive the woven forms

have many excellent qualities, including high

dimensional stability, high tensile and impact

strength, good heat, weather and chemical resistance,

how moisture absorption, resistance to fire

and good thermo-electrical properties. A number

of different weaves and weights is available, and

Figure 16.11Staple fibres (surface mat)

(Owens-Corning Fiberglas)

Figure 16.10Chopped strands (Owens-Corning

Fiberglas)

Reinforced composite materials 539

thickness may range from 0.05 mm to 9.14 mm,

with weights from 30 g/m2 to 1 kg/m2, although

the grades mostly used in the automotive field

probably have weights of about 60 g/m2 and will

be of plain, twill or satin weave.

The advantages and disadvantages of glass

fibre reinforcement materials are indicated in

Table 16.2.

Carbon fibre

This is another reinforcing material. Carbon fibres

possess a very high modulus of elasticity, and have

been used successfully in conjunction with epoxy

resin to produce low-density composites possessing

high strength.

16.5 Resins used in reinforced composite

materials

The first man-made plastics were produced in

this country in 1862 by Alexander Parkes and

were the forerunner of celluloid. Since then a

large variety of plastics has been developed

commercially, particularly in the last twenty-five

years. They extend over a wide range of properties.

Phenol formaldehyde is a hard thermoset

material; polystyrene is a hard, brittle thermoplastic;

polythene and plasticized polyvinyl chloride

(PVC) are soft, tough thermoplastic materials;

and so on. Plastics also exist in various

physical forms. They can be bulk solid materials,

rigid or flexible foams, or in the form of

sheet or film. All plastics have one important

common property. They are composed of macromolecules,

which are large chain-like molecules

consisting of many simple repeating units.

The chemist calls these molecular chains polymers.

Not all polymers are used for making plastic

mouldings. Man-made polymers are called

synthetic resins until they have been moulded

in some way, when they are called plastics

(Figure 16.12).

Most synthetic resins are made from oil. The

resin is an essential component of glass fibre reinforced

plastic. The most widely used is unsaturated

polyester resin, which can be cured to a

solid state either by catalyst and heat or by catalyst

and accelerators at room temperature. The

ability of polyester resin to cure at room temperature

into a hard material is one of the main

reasons for the growth of the reinforced plastics

industry. It was this which led to development of

the room temperature contact moulding methods

which permit production of extremely large integral

units.

Table 16.2Advantages and disadvantages of glass fibre reinforcement

Advantages Uses Disadvantages

Rovings Unidirectional strength Spray-up Limited use in hand lay-up

Local longitudinal strength

Mechanical bond for bulkheads

Making tubes

Woven rovings Easy to handle To increase longitudinal and Poor interlaminar adhesion

Bidirectional strength transverse strength Traps air, causes voids

High glass content High cost

High impact resistance

Chopped strand mat Multidirectional strength Various

Low cost General-purpose

Good interlaminar bond reinforcement

Can be moulded into

complex shape

Woven cloth High strength Sheathing Very high cost difficult to

Smooth finish As a fire barrier ‘wet out’

Surface tissue Fine texture Reinforcing gelcoat Low strength

540Repair of Vehicle Bodies

Polyester resins are formulated by the reaction

of organic acids and alcohols which produces a

class of material called esters. When the acids are

polybasic and the alcohols are polyhydric they

can react to form a very complex ester which is

generally known as polyester. These are usually

called alkyds, and have long been important in

surface coating formulations because of their

toughness, chemical resistance, and endurance. If

the acid or alcohol used contains an unsaturated

carbon bond, the polyester formed can react further

with other unsaturated materials such as

styrene or diallyl phthalate. The result of this

reaction is to interconnect the different polyester

units to form the three-dimensional cross-linked

structure that is characteristic of thermosetting

resins. The available polyesters are solutions of

these alkyds in the cross-linking monomers. The

curing of the resin is the reaction of the monomer

and the alkyd to form the cross-linked structure.

An unsaturated polyester resin is one which is

capable of being cured from a liquid to a solid

state when subjected to the right conditions. It is

usually referred to as polyester.