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Today, there are many types of commercially available polyesters. Two types of
reactive polyesters based on the functional group are known in the art: unsaturated
and saturated polyesters.
8.1.1 Unsaturated Polyesters (UPs)
8.1.1.1 Introduction
UPs have double reactive bonds in their structure. UPs are synthesized by a polycondensation of dicarboxylic acids (or anhydrides) with glycols and usually water
as the by-product is removed from the reaction medium. In order to produce UPs,
which can be crosslinked in the final stage by vinylic monomer (e.g. methyl methacrylate -MMA, styrene), there must be a double bond in the polymer backbone.
This double bond is almost always introduced by dicarboxylic acids or anhydrides
which have a double bond in their structure by using of compounds such as fumaric
acid or maleic anhydride (MA). By varying the ratio of unsaturated dicarboxylic
acid (or anhydride) to glycols, a wide range of UPs can be produced from brittle to
soft. In addition, the saturated and unsaturated dicarboxylic acids (or anhydrides)
such as isophthalic acid or phthalic acid must be present in the chain. Otherwise, the
crosslink density would be too much, which can result in a brittle material (Penczek
et al. 2005; Albdiry and Yousif 2019).
The final properties of the UP are strongly related to the selection of monomers.
A polyester with a completely aliphatic chain results in soft material (Schneiderman
and Hillmyer 2016). On the other hand, polyester consisting aromatic moieties in its
structure shows rigid properties (Schneiderman and Hillmyer 2016). In addition, the
more aromatic moieties cause more hydrophobic properties, this problem is very
important for UPs because they have an ester bond in the structure which is prone
to hydrolysis. This can be improved by using of bulky monomers which prevent
water and other active species to reach the ester bond (Saxena et al. 2014). Likewise,
the rigidity of the final polyester can also be adjusted by ratio of the MA (as a double bond carrier) to other monomers (Suh et al. 2000; Raheem et al. 2019).
8.1.1.2 Monomers
The monomers used to obtain UPs can be classified into three groups:
1. Glycols
2. Dicarboxylic acids (or anhydrides)
3. Reactive monomers
Two main types of anhydrides or dicarboxylic acids are present in the UP backbone: unsaturated and saturated dicarboxylic acid (or anhydrides). The reactive
monomer not only acts as a crosslinking agent to cure UP, but also acts as a solvent
to dilute the polymer.
M. Akbari and R. Najjar
Today, there are many types of commercially available polyesters. Two types of
reactive polyesters based on the functional group are known in the art: unsaturated
and saturated polyesters.
8.1.1 Unsaturated Polyesters (UPs)
8.1.1.1 Introduction
UPs have double reactive bonds in their structure. UPs are synthesized by a polycondensation of dicarboxylic acids (or anhydrides) with glycols and usually water
as the by-product is removed from the reaction medium. In order to produce UPs,
which can be crosslinked in the final stage by vinylic monomer (e.g. methyl methacrylate -MMA, styrene), there must be a double bond in the polymer backbone.
This double bond is almost always introduced by dicarboxylic acids or anhydrides
which have a double bond in their structure by using of compounds such as fumaric
acid or maleic anhydride (MA). By varying the ratio of unsaturated dicarboxylic
acid (or anhydride) to glycols, a wide range of UPs can be produced from brittle to
soft. In addition, the saturated and unsaturated dicarboxylic acids (or anhydrides)
such as isophthalic acid or phthalic acid must be present in the chain. Otherwise, the
crosslink density would be too much, which can result in a brittle material (Penczek
et al. 2005; Albdiry and Yousif 2019).
The final properties of the UP are strongly related to the selection of monomers.
A polyester with a completely aliphatic chain results in soft material (Schneiderman
and Hillmyer 2016). On the other hand, polyester consisting aromatic moieties in its
structure shows rigid properties (Schneiderman and Hillmyer 2016). In addition, the
more aromatic moieties cause more hydrophobic properties, this problem is very
important for UPs because they have an ester bond in the structure which is prone
to hydrolysis. This can be improved by using of bulky monomers which prevent
water and other active species to reach the ester bond (Saxena et al. 2014). Likewise,
the rigidity of the final polyester can also be adjusted by ratio of the MA (as a double bond carrier) to other monomers (Suh et al. 2000; Raheem et al. 2019).
8.1.1.2 Monomers
The monomers used to obtain UPs can be classified into three groups:
1. Glycols
2. Dicarboxylic acids (or anhydrides)
3. Reactive monomers
Two main types of anhydrides or dicarboxylic acids are present in the UP backbone: unsaturated and saturated dicarboxylic acid (or anhydrides). The reactive
monomer not only acts as a crosslinking agent to cure UP, but also acts as a solvent
to dilute the polymer.
M. Akbari and R. Najjar