1
An ester is a compound whose structure may be derived
by the replacement of the replaceable hydrogen of an
acid by an alkyl, aryl, alicyclic or heterocyclic group.
The most important esters are these derived from
carboxylic acids; the structural formula of these esters is
(R-CO-O-R’)
For the purpose of these chapter, polyesters are defined
as polymers containing -CO-O groups in the main chain.
It may be noted that this definition excludes polymers of
esters such as vinyl acetate and methyl methacrylate
since in these polymers the ester groups occur in the
side-chain and not in the main chain
The polymers making up this first group polyesters are linear
polyesters containing aliphatic unsaturation which provides
sites for subsequent cross-linking. A polymer of this type first
became available in the U.S.A. in 1946; the polymer was
prepared from diethylene glycol and maleic anhydride and
could be cross-linked by reaction with styrene. The polymer
was of interest for the preparation of glass-fibre laminates by
techniques which did not involve high pressures. This
application stemmed from the discovery of the reinforcing
properties of glass fibres by United states Ruber Co. (U.S.A.)
in 1942. The commercial production of glass-fibles reinforced
2
polyesters laminates was firmly established by about 1949 and
these composite materials have since become of great
importance, particularly in such applications as boat hulls,
lorry cabs, roofing panels and chemical plant. Unsaturated
linear polyesters also find in thermosetting moulding
compositions, casting resins and solventless lacquers.
Linear unsaturated polyesters are prepared
commercially by the reaction of a saturated diol with
a mixture of an unsaturated dibasic acid and a
“modifying”
dibasic
acid(or
corresponding
anhydrides).
Propylene glycol is the diol most widely used for the
manufacture of linear unsaturated polyesters; it is
prepared by the hydration of propylene oxide
O
CH3-CH-CH2 +H2O
CH3
HO-CH–CH2-OH
Commonly the reaction is carried out without a catalyst
at about 200C and 20 atmospheres.
3
Diethylene glycol (I) leads to greater flexibility
although water sensitivity is increased and
neopentyleneglycol (2,2-dimethylpropane-1,3diol)(II)
gives polymers with improved resistance to thermal
degradation. Polyesters with good alkali resistance are
prepared from the diether of propylene glycol and
bisfenol A (III)
CH3
HO-CH2-CH2-O-CH2-CH2-OH
HO-CH2-C-CH2-OH
CH3
(I)
(II)
(III)
Maleic anhydride is the most important unsaturated
component used in the manufacture of linear
unsaturated polyesters. It is generally obtained by the
oxidation of benzene:
4
Fumaric acid (IV), the trans isomer of maleic acid is
sometimes preferred to maleic anhydride as it is less
corrosive and gives lighter-coulured products with
slightly improved heat resistance. Chloromaleic acid
may be used in the production of self-extinguishing
resins.
It is possible to cross-link unsaturated linear polyester
chains directly one to another; however, reaction is slow
and a low degree cross-linking is achieved. The
limitations are overcome by the materials most
commonly used to cross-link unsaturated linear
polyesters in this way are vinyl monomers(X and XI).
The addition of a liquid vinyl monomer to the polymer
also leads to a reduction in viscosity and this facilitates
the impregnation of glass-fibre in the preparation of
laminates.
5
When polyester resins have been cross-linked they are
rigid, infusible and insoluble. There are so many
varieties of polyester resins now commercially available
that is difficult to give typical values for physical
properties of cured materials. Furthermore, polyester
resins are mostly used in conjunction with glass fibre
and the physical properties of the final products greatly
depend on the type and quantity of glass fibre
incorporated. This point is illustrated by Table given
below. In which are given comparative values of some
polyesters have good heat stability, showing little
weight loss up to about 200 C. The electric insulating
properties of cured polyesters are satisfactory 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 resins in high frequency applications is
limited.
6
Cross-linked polyesters are resistant to a wide range of
organic solvents but they are attacked by chlorinated
hydrocarbons(e.g., chloroform, ethylenedichroride),
esters(e.g. ethyl acetate) and ketones(e.g., acetone and
methyl ethyl ketone). The ester groups in the polymer
provide sites for hydrolytic attack and strong alkalis
cause appreciable degradation. The polymer is, however
resistant to most inorganic and organic acids, with the
exception of strong oxidizing acids.
Although many linear saturated polyesters of high
molecular weight (i.e. greater than 10 000) have been
investigated only poly(ethylene terephthalate) is of
significant importance.
Ethylene glycol is prepared by the hydration of ethylene
oxide
O
CH3-CH-CH2 +H2O
HO- CH2- CH2-OH
Ethylene glycol is a colorless liquid b.p. 197 C
7
Although poly(ethylene terephthalate) can be obtained
by direct esterificarion of ethylene glycol with
terephthalic acid, commertial processes are enviably
based on the ester interchange reaction of the glycol
with the dimethyl ester of the asid. The main reason for
this preference is that terephthalic acid is diffucult to
purify(since it sublimes at 300 C and is rather insoble)
whereas the ester is ready purified by distillation or
The preparation of poly(ethylene terephthalate) is based
on the reaction termed ester interchange or
alcoholysis). In this reaction, the alcohol residue of an
ester is replased by another alcohol residue by treating
the ester with an alcohol. A simple example of ester
interchange is as follows:
CH3COOCH3 + C2H5OH
CH3COC2H5 + CH3OH
If the liberated alcohol is distilled from the system as it
formed, the reaction may be taken to completion.
8
In the preparation of poly(ethylene terephthalate) two
successive ester interchange reaction are performed.
Ester interchange occurs, the principal product being
bis(2-hydroxyethyl terephthalate :
The methanol which is formed is distilled off
continuously. In addition of bis(2-hydroxyethyl
terephthalate, small amounts of oligomers up to about
the tetramer are formed; these have the general formula
In the second step, the polymerization reaction proper,
the product from the first stage is heated at 270-285 C
with continuous evacuation to pressures below 1 mm
Hg. Successive interchanges result in the formation of a
polyester, as represent in the following scheme :
9
Poly(ethylene terephthalete) is a colorless rigid
substance. Because of its structure regularity, the
polymer ready crystallized and the physical properties
of the bulk material are greatly affected by the extent of
crystallinity, which , in turn, is largely determined by
the previous history of the material. The filaments are
thus rapidly cooled and are largely amorphous and are
weak. The filaments are then drawn at a temperature
above the glass-transition temperature ( 80 C), whereby
molecular orientation and crystallinity are induced.
Finally, the filaments are heated at about 200C under
tension to five a dimensionally-stable material of
maximum crystallinity.
10
Poly(ethylene terephthalete) glades suitable for injection
moulding and extrusion are now avaible; the mouldings
may be either amorphous of crystalline.
The polymer intended to give amorphous moulding has
a high molecular weight and is extremely pure; it thus
has little tendency to crystallize, provided it is not
exposed to temperature above 50 C. The completely
amorphous material is transparent and has good impact
strength. The polymer indented to give crystalline
mouldings contains an added nucleating agent and so
leads to a very highly surface hardness.
The values of some properties of various forms of
poly(ethylene terephthalate) which are given table
below.