165
b). This monomer leads to an improvement in the thermal stability and resistance of
the resulting polymer against hydrolysis. Terephthalic acid compared to isophthalic
acid has exhibited better thermal resistance and mechanical properties of polyester
(Zeno et al. 1998). However, its reactivity is slower than isophthalic acid and also
due to greater crystallinity, the resulting product shows less solubility (Zeno et al.
1998). All above-mentioned aliphatic dicarboxylic acids impart softness to the final
product (Gao et al. 2015). Since adipic acid has four linear methylene groups in its
structure, the use of adipic acid in the polyester structure leads to a decrease in T g
and viscosity compared to the aromatic dicarboxylic acid (Edlund and
Albertsson 2003).
8.1.2.3 Production
Saturated polyesters are produced by direct polyesterification of glycols (or polyols)
with dicarboxylic acids (or anhydrides) at elevated temperatures about 210–250 °C
(De Hoe et al. 2018). Polyesterification is a step-growth equilibrium reaction with
water as the by-product. The industrial production of saturated polyesters is similar
to UPs via batch method. The concentration of raw materials affects the Mw and the
final functionality of the resulting polyester (Ionescu 2005). The use of the excess
equivalents of glycols leads to the hydroxyl functionality of end-group and the
excess equivalents of dicarboxylic acids result in the carboxylic acid functionality
of end-group (Hadavand et al. 2015). Ideally, the use of equal concentrations of
glycols and dicarboxylic acid leads to the formation of polyester with unlimited Mw
(Ionescu 2005). In order to reduce reaction time, a small amount of catalyst is
charged into the reaction vessel. Catalysts such as dibutyltinoxide (DBTO), monobutyl tin oxide, p-toluene sulfonic acid, tetrabutyltitanate, zinc acetate or even sulfuric acid are used on an industrial scale (Ionescu 2005). It has been shown that
mineral acids lead to deterioration of polyester quality and color (Deligny and Tuck
2000). Therefore, the best option is to use catalysts based on tin and titanate compounds (Oldring and Tuck 2000).
Polyesterification reaction takes place in 5 to 30 m
3
steel reactors. First, liquid
raw materials are charged into the reactor at room temperature. The reactor temperature is then increased to about 100 °C and solid raw materials are added to the
mixture. The reactor temperature is increased again to about 160 °C and is maintained at this temperature until the distillation of water begins. The reaction temperature is gradually increased to about 220 °C. The reaction continues at 220 °C
until 90% of the theoretical water is removed from the reaction mixture. At this level
of the reaction, catalyst is added, and the reaction temperature is increased, and
vacuum is introduced to remove the remaining water (or water is removed using a
compound which forms an azeotrope with water such as toluene or xylene). The
reaction temperature in the final esterification stage can reaches up to 250 °C. The
completion of the reaction is controlled by acid value or viscosity of the polyester.
Finally, the reaction mixture is cooled, and depending on the end use, is diluted with
solvent or solvent-free is packed (Ullmann et al. 1985).
8 Reactive and Functional Polyesters and Polyurethanes
b). This monomer leads to an improvement in the thermal stability and resistance of
the resulting polymer against hydrolysis. Terephthalic acid compared to isophthalic
acid has exhibited better thermal resistance and mechanical properties of polyester
(Zeno et al. 1998). However, its reactivity is slower than isophthalic acid and also
due to greater crystallinity, the resulting product shows less solubility (Zeno et al.
1998). All above-mentioned aliphatic dicarboxylic acids impart softness to the final
product (Gao et al. 2015). Since adipic acid has four linear methylene groups in its
structure, the use of adipic acid in the polyester structure leads to a decrease in T g
and viscosity compared to the aromatic dicarboxylic acid (Edlund and
Albertsson 2003).
8.1.2.3 Production
Saturated polyesters are produced by direct polyesterification of glycols (or polyols)
with dicarboxylic acids (or anhydrides) at elevated temperatures about 210–250 °C
(De Hoe et al. 2018). Polyesterification is a step-growth equilibrium reaction with
water as the by-product. The industrial production of saturated polyesters is similar
to UPs via batch method. The concentration of raw materials affects the Mw and the
final functionality of the resulting polyester (Ionescu 2005). The use of the excess
equivalents of glycols leads to the hydroxyl functionality of end-group and the
excess equivalents of dicarboxylic acids result in the carboxylic acid functionality
of end-group (Hadavand et al. 2015). Ideally, the use of equal concentrations of
glycols and dicarboxylic acid leads to the formation of polyester with unlimited Mw
(Ionescu 2005). In order to reduce reaction time, a small amount of catalyst is
charged into the reaction vessel. Catalysts such as dibutyltinoxide (DBTO), monobutyl tin oxide, p-toluene sulfonic acid, tetrabutyltitanate, zinc acetate or even sulfuric acid are used on an industrial scale (Ionescu 2005). It has been shown that
mineral acids lead to deterioration of polyester quality and color (Deligny and Tuck
2000). Therefore, the best option is to use catalysts based on tin and titanate compounds (Oldring and Tuck 2000).
Polyesterification reaction takes place in 5 to 30 m
3
steel reactors. First, liquid
raw materials are charged into the reactor at room temperature. The reactor temperature is then increased to about 100 °C and solid raw materials are added to the
mixture. The reactor temperature is increased again to about 160 °C and is maintained at this temperature until the distillation of water begins. The reaction temperature is gradually increased to about 220 °C. The reaction continues at 220 °C
until 90% of the theoretical water is removed from the reaction mixture. At this level
of the reaction, catalyst is added, and the reaction temperature is increased, and
vacuum is introduced to remove the remaining water (or water is removed using a
compound which forms an azeotrope with water such as toluene or xylene). The
reaction temperature in the final esterification stage can reaches up to 250 °C. The
completion of the reaction is controlled by acid value or viscosity of the polyester.
Finally, the reaction mixture is cooled, and depending on the end use, is diluted with
solvent or solvent-free is packed (Ullmann et al. 1985).
8 Reactive and Functional Polyesters and Polyurethanes