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EBB 427 Application and Technology of

Engineering Polymers (Second Half)

Dr. Hazizan Md Akil

School of Materials and Mineral Resources Engineering

Engineering Campus, USM.

THERMOSETTING POLYMERS:

Preparation, Properties, Processing,

Application and Future Direction

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Unsaturated Polyesters resin

Cross-linking of linear unsaturated polyester

Involve the reaction between the unsaturated sites in

polymer chain and vinyl-type monomer

As carried commercially, cross-linking of unsaturated

polyester is invariably a free radical reaction

Two types of initiating systems are commonly employed

for this reaction, namely those effective at elevated

temperature and those effective at room temperature.

The most important initiators used at elevated

temperatures are peroxides, which liberate free radicals as

a result of thermal decomposition.

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A peroxide which is widely used in this way is benzoyl

peroxide.

Other peroxides include 2,4-chlorobenzoyl peroxide, di-

tert-butyl peroxide and lauroyl peroxide

Mixtures of polyester resin and this type of peroxide are

comparatively stable at room temperature but rapidly

cross-link at temperature ranging from about 70 150C.

Such peroxides are used principally in processes

employing moulding compositions, when short curing

times are required

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Initiating systems which are effective at room temperature

normally consist of mixtures of a peroxy compound and an

activator (accelerator)

In the presence of accelerator, the peroxy compound

rapidly decomposes without the application of heat into

free radicals

The two most important peroxy materials now used for the

cold curing of polyester resin are methyl ethyl ketone

peroxide (MEKP) and cyclohexane peroxide.

In reality, MEKP and cyclohexane peroroxides arehydroperoxides

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The most common accelerators for MEKP and

cyclohexane peroxide are salts of metals which exhibit

more than one valency.

The most widely used metal of this kind is cobalt, although

salts of cerium, iron, manganase, tin, vanadium also find

some application.

In order to be effective as an accelerator a metal salt must

be soluble in the polyester resin

The most commonly used salts are naphthenates, which

are readily soluble, octoate may also be used

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The decomposition of a hydroperoxide (ROOH) by a metal

salt such as cobalt napthenate to give free radicals

proceeds according to the following:

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The cycle is repeated until all hydroperoxide has been

decomposed.

Cobalt napthenate-methyl-ethyl-ketone peroxide or

cyclohexane peroxide systems are very extensively used

in the production of large glass-fibre laminates by hand

lay-up and cured at room temperature.

It may be noted that the foregoing metal-based

accelerators, which are highly reactive towards

hydroperoxides, have little influence on the decomposition

of peroxide especially those used for curing polyester atelevated temperature

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However, peroxide do decompose rapidly at room

temperature into free radicals in the presence of tertiary

amines.

Amines such as dimethylaniline, diethylaniline, and

dimethyl-p-toluidine react violently with benzoyl peroxide

or tertiary amine.

Polyester resin cured with benzoyl peroxide-tertiary amine

tend to discolour and craze on ageing

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The reaction between benzoyl peroxide and tertiary amine

is thought to proceed via a one-electron transfer from

nitrogen, as shown in the following example involving

dimethylaniline:

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The cross-linking of an unsaturated polyester by means of

a vinyl monomer such as styrene may be represented as:

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The average value of n in the structure may be deduced

from both spectroscopic and degradation studies.

Both the spectroscopic and degradation studies indicate

that in a typical cured general purpose polyester resin the

cross-links contain 1-3 styrene units.

Determination of the molecular weight of the polymer

enables estimation of the average of chains linked

together by a continuous cross-link

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Typically such copolymers have Mn ~2500 which indicates

that each continuous styrene-fumarate copolymer chains

passes through 6-10 cross-link sites in the un-degraded cured

polyester.

It will be noted that the cross-linking of linear unsaturated

polyester by vinyl monomers does not involve the eliminationof any volatile by-products.

Hence, it is possible to cure the resin without the application of

pressure.

It is also possible to cure the resins without the application ofheat, they are very useful in the manufacture of large structure

such as boat hulls

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Properties of cross-linked polymers

Cross-linked unsaturated polyesters are rigid, infusible and

insoluble.

There are so many varieties of polyester resins now

commercially available that it is difficult to give typical

values for physical properties of cured materials.

Furthermore, polyester resin are mostly used in conjunction

with glass fibre and the physical properties of the final

product greatly depend on the type and quantity of glass

fibre incorporated.

Cross-linked polyesters have good heat stability, showinglittle weight loss up to about 200C.

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The mechanical strength of general purpose materials

begins to decline at about 100C and the maximum servicetemperature of glass-fibre laminates and mouldings is

about 150C; for heat resistance grades of resins these

temperature are of the order of 160C and 200C.

The electrical insulating properties of cured polyester aresatisfactory for many purposes but the polar nature of the

ester group results in a relatively high power factor and

dielectric constant and so the use of the resins in high

frequency applications is limited.

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Cross-linked polyesters are resistant to a wide range of

organic solvents but they are attacked by chlorinatedhydrocarbons (e.g. chloroform, ethylene dichloride and

trichloroethylene), esters (e.g. ethyl acetate) and ketones

(e.g. acetone and MEK).

The ester groups in the polymer provide sites for hydrolyticattack and strong alkalis cause appreciable degradation.

The polymer is, however, resistant to most inorganic and

organic acids, with exception of strong oxodising acids.

ebb 427 (3) hazizan

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