161
8.1.1.2.3 Reactive Monomers
The reactive monomers not only serve as a material to convert the immature product
to the mature one, but also act as solvents and dilute the prepolymer and decrease
the viscosity. Until now, styrene is the most common and attractive monomer as a
crosslinking agent. Unfortunately, this monomer can cause cancer in operators
(Christensen et al. 2017). 1-methylstyrene is an option for using as a curing agent
instead of styrene. Although it has a lower and safer vapor pressure than styrene, it
has a lower reactivity and a lower peak temperature (Spasojevic 2019). There are
some other crosslinking agents such as butyl acrylate (BA) and MMA which have a
lower reactivity towards UP and are more expensive but grant gloss to the final piece
(Gallacher and Bettelheim 1962; Shi and Rånby 1994).
8.1.1.3 Production
UPs are produced by polycondensation reaction between dicarboxylic acids or
anhydrides with glycols at elevated temperatures up to 230 °C (Cousinet et al.
2015). This polycondensation is an equilibrium reaction between raw materials with
UP and water (more often as a by-product). In order to obtain UP with high yield,
water must be removed from the reaction medium. This reaction is self-catalyzed
and dicarboxylic acids play the catalyst role (Ionescu 2005). However, it is better to
precede the reaction in the presence of an external catalyst such as tin, titanium or
zinc compounds because in the late stage of the reaction dicarboxylic concentration
becomes very low and cannot effectively play the role of catalyst (Brännström et al.
2018). Acidic compounds such as p-toluene sulfonic acid or sulfuric acid which are
usable in production of saturated polyesters are not a suitable choice due to interference with the double bonds (Dodiuk and Goodman 2013). High temperatures are
needed to produce UPs, generally up to 230 °C. The reaction takes place in the presence of an inert gas such as argon or nitrogen to exclude oxygen from the medium.
In addition, the inert gas helps remove water from the medium and precedes the
reaction to the polyester. The reaction can also be carried out in bulk polymerization
or in the azeotropic solvents such as toluene and xylene. In general, water removal
is done by three methods: azeotropic solvent, vacuum and purging the reaction
medium with an inert gas (Zetterlund et al. 2002; Dodiuk and Goodman 2013).
When the polymerization process is finished (usually by measuring acid value
and viscosity), the reaction temperature is lowered to about 100 °C and inhibitor and
styrene (to dilute the polymer) are added to the reactor and then is cooled down to
the room temperature.
New chemical compounds such as fatty acids have been used recently to modify
the mechanical properties of UPs (Fidanovski et al. 2018; Heredia-Guerrero et al.
2018) or using waterborne systems to overcome environmental problems related to
the pollution of UPs (Dai et al. 2015; Gao et al. 2018).
8 Reactive and Functional Polyesters and Polyurethanes
8.1.1.2.3 Reactive Monomers
The reactive monomers not only serve as a material to convert the immature product
to the mature one, but also act as solvents and dilute the prepolymer and decrease
the viscosity. Until now, styrene is the most common and attractive monomer as a
crosslinking agent. Unfortunately, this monomer can cause cancer in operators
(Christensen et al. 2017). 1-methylstyrene is an option for using as a curing agent
instead of styrene. Although it has a lower and safer vapor pressure than styrene, it
has a lower reactivity and a lower peak temperature (Spasojevic 2019). There are
some other crosslinking agents such as butyl acrylate (BA) and MMA which have a
lower reactivity towards UP and are more expensive but grant gloss to the final piece
(Gallacher and Bettelheim 1962; Shi and Rånby 1994).
8.1.1.3 Production
UPs are produced by polycondensation reaction between dicarboxylic acids or
anhydrides with glycols at elevated temperatures up to 230 °C (Cousinet et al.
2015). This polycondensation is an equilibrium reaction between raw materials with
UP and water (more often as a by-product). In order to obtain UP with high yield,
water must be removed from the reaction medium. This reaction is self-catalyzed
and dicarboxylic acids play the catalyst role (Ionescu 2005). However, it is better to
precede the reaction in the presence of an external catalyst such as tin, titanium or
zinc compounds because in the late stage of the reaction dicarboxylic concentration
becomes very low and cannot effectively play the role of catalyst (Brännström et al.
2018). Acidic compounds such as p-toluene sulfonic acid or sulfuric acid which are
usable in production of saturated polyesters are not a suitable choice due to interference with the double bonds (Dodiuk and Goodman 2013). High temperatures are
needed to produce UPs, generally up to 230 °C. The reaction takes place in the presence of an inert gas such as argon or nitrogen to exclude oxygen from the medium.
In addition, the inert gas helps remove water from the medium and precedes the
reaction to the polyester. The reaction can also be carried out in bulk polymerization
or in the azeotropic solvents such as toluene and xylene. In general, water removal
is done by three methods: azeotropic solvent, vacuum and purging the reaction
medium with an inert gas (Zetterlund et al. 2002; Dodiuk and Goodman 2013).
When the polymerization process is finished (usually by measuring acid value
and viscosity), the reaction temperature is lowered to about 100 °C and inhibitor and
styrene (to dilute the polymer) are added to the reactor and then is cooled down to
the room temperature.
New chemical compounds such as fatty acids have been used recently to modify
the mechanical properties of UPs (Fidanovski et al. 2018; Heredia-Guerrero et al.
2018) or using waterborne systems to overcome environmental problems related to
the pollution of UPs (Dai et al. 2015; Gao et al. 2018).
8 Reactive and Functional Polyesters and Polyurethanes