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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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
The cross-linking of an unsaturated polyester by means of
a vinyl monomer such as styrene may be represented as:
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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
Cross-linking of linear unsaturated polyester
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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Unsaturated Polyesters resin
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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Unsaturated Polyesters resin
Properties of cross-linked polymers
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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Unsaturated Polyesters resin
Properties of cross-linked polymers
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.