Chemistry Reference
In-Depth Information
The saturated dicarboxylic acids act as modifiers. While aliphatic dicarboxylic acids can be used,
the most common one is
phthalic acid (added to the reaction mixture as an anhydride). The acid
improves compatibility with styrene that is polymerized in the presence of the polyester to form hard,
rigid, cross-linked materials. Other modifiers are used to obtain special properties. When a flexible
product is needed, adipic or sebacic acids may be used instead. For better heat resistance,
ortho
endo-
methylene tetrahydrophthalic anhydride (nadic anhydride) may be utilized. Flame retardency is
achieved by using chlorinated dicarboxylic acids, like tetrachlorophthalic.
Styrene is the most common monomer used in cross-linking unsaturated polyesters. When special
properties are required, other monomers like methyl methacrylate may be employed. Sometimes this
is done in combination with styrene. Diallyl phthalate and triallyl cyanurate form better heat-resistant
products.
An example of a typical batch preparation of a polyester is one where 1.2 moles of propylene
glycol, 0.67 moles of maleic anhydride, and 0.33 moles of phthalic anhydride are combined.
Propylene glycol is used in excess to compensate for loss during the reaction. The condensation at
150-200
C lasts for 6-16 h, with constant removal of water, the by-product. An aromatic solvent, like
toluene or xylene, is often added to the reaction mixtures to facilitate water removal by azeotropic
distillation. Esterification catalysts, like toluene sulfonic acid, reduce the reaction time. In addition,
the reactions are blanketed by inert gases, like nitrogen or carbon dioxide, to prevent discoloration
from oxygen at high temperatures. When molecular weights of 1,000-2,000 are reached, the products
are cooled to 90
C and blended with vinyl monomers. Often the blends are mixtures of equal weights
of the polyesters and the monomers. The structure of the above-described unsaturated polyester can
be illustrated as follows:
O
O
O
O
O
n
O
O
O
It is interesting that unsaturated polyesters can actually be prepared by anionic alternating
copolymerization of an epoxide and bicyclic bis(
g
-butyrolactone) bearing an isopropenyl group
[
34
]. The reaction was illustrated as follows:
O
O
O
O
O
O
PPh
3
+
O
O
O
R
O
n
R
If the reaction is carried for a prolonged period of time, an insoluble, cross-linked product results.
7.2.3 Network Polyesters for Surface Coatings
The original polyesters for coatings were prepared from phthalic anhydride and glycerol and were
referred to as
glyptals
or
glyptal resins
:
