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In prepolymer method, a diisocyanate reacts with a polyol with specific Mw in
the temperature range of 60 to 90 °C (Rogers and Long 2003). This reaction can
occur in the presence of a catalyst for aliphatic diisocyanates or without using any
catalyst for aromatic diisocyanates (Szycher 2012). Tin compounds such as dibutyl
tin dilaurate (DBTDL) and amine compounds such as triethylene diamine (under
DABCO trade name) can be used to catalyze the urethane reaction (Ashida 2006).
The prepolymer method takes place in two stages: first, the molar ratio of diisocyanate to hydroxyl NCO/OH is maintained about 2 (Ashida 2006). A prepolymer is
formed with a terminated isocyanate group, and second, a chain extender compound
such as BD, EG or ethylene diamine is added to produce the mature PU and a high
Mw PU is formed (Krol 2007).
The quasiprepolymer method is similar to the prepolymer method, except that
the NCO/OH ratio is greater than 2 (Szycher 2012). In this method, the free diisocyanate is present in the reaction medium together with the prepolymer end-capped
with isocyanate (Ding et al. 2015). In the second stage, a chain extender is added to
obtain the high Mw PU. The final NCO/OH ratio for quasi-prepolymer and prepolymer method is 1 to 1.1 (Szycher 2012). In regions with high humidity, this ratio is
about 1.1 due to the reaction of some isocyanate groups with water, and in dry
regions, this ratio is about 1 (Wiser-Halladay 1990).
One-shot method is the simplest method. All reactants are mixed at the same
time. In this method, efficient mixing is very important and should be done as
quickly as possible before reaching gel-point (Szycher 2012). Although this method
is simple, it needs a careful calculation of the necessary materials and temperature
control must be taken into account. This method may not be applicable for bulky
materials, because the large amount of heat released can cause distortion in the final
product (Rausch Jr. and Mcclellan 1972; Rivera-Armenta et al. 2004).
8.2.4.2 Waterborne PU (WPU) Synthesis
WPU also called PU dispersion (PUD) is produced in a different way compared to
solventborne PUs. For producing WPU some internal surfactants are used such as
dimethylolpropionic acid (DMPA). In a typical synthetic reaction, an aliphatic
diisocyanate such as IPDI, a polyol with a Mw between 1000 and 2000 g/mol, an
internal surfactant and a catalyst are charged to the reactor. The aromatic diisocyanates will hardly be used due to extremely rapid reactions with water. The temperature is raised to about 70 °C and maintained at that temperature until the required
NCO number is reached. Whenever necessary, acetone is added to decrease the
viscosity. Temperature is then lowered to 45 °C and a neutralizing agent such triethylamine (TEA) is added to neutralize the carboxylic acid group of surfactants. Then,
the temperature is lowered to room temperature and water is added to disperse the
PU in the water medium. The chain extender can be added after adding water or
before adding TEA. After adding water, only diamine chain extenders are usable.
Once the reaction is complete, acetone is removed by lowering the reactor pressure
(Najjar et al. 2018; Mirmohseni et al. 2019).
8 Reactive and Functional Polyesters and Polyurethanes
In prepolymer method, a diisocyanate reacts with a polyol with specific Mw in
the temperature range of 60 to 90 °C (Rogers and Long 2003). This reaction can
occur in the presence of a catalyst for aliphatic diisocyanates or without using any
catalyst for aromatic diisocyanates (Szycher 2012). Tin compounds such as dibutyl
tin dilaurate (DBTDL) and amine compounds such as triethylene diamine (under
DABCO trade name) can be used to catalyze the urethane reaction (Ashida 2006).
The prepolymer method takes place in two stages: first, the molar ratio of diisocyanate to hydroxyl NCO/OH is maintained about 2 (Ashida 2006). A prepolymer is
formed with a terminated isocyanate group, and second, a chain extender compound
such as BD, EG or ethylene diamine is added to produce the mature PU and a high
Mw PU is formed (Krol 2007).
The quasiprepolymer method is similar to the prepolymer method, except that
the NCO/OH ratio is greater than 2 (Szycher 2012). In this method, the free diisocyanate is present in the reaction medium together with the prepolymer end-capped
with isocyanate (Ding et al. 2015). In the second stage, a chain extender is added to
obtain the high Mw PU. The final NCO/OH ratio for quasi-prepolymer and prepolymer method is 1 to 1.1 (Szycher 2012). In regions with high humidity, this ratio is
about 1.1 due to the reaction of some isocyanate groups with water, and in dry
regions, this ratio is about 1 (Wiser-Halladay 1990).
One-shot method is the simplest method. All reactants are mixed at the same
time. In this method, efficient mixing is very important and should be done as
quickly as possible before reaching gel-point (Szycher 2012). Although this method
is simple, it needs a careful calculation of the necessary materials and temperature
control must be taken into account. This method may not be applicable for bulky
materials, because the large amount of heat released can cause distortion in the final
product (Rausch Jr. and Mcclellan 1972; Rivera-Armenta et al. 2004).
8.2.4.2 Waterborne PU (WPU) Synthesis
WPU also called PU dispersion (PUD) is produced in a different way compared to
solventborne PUs. For producing WPU some internal surfactants are used such as
dimethylolpropionic acid (DMPA). In a typical synthetic reaction, an aliphatic
diisocyanate such as IPDI, a polyol with a Mw between 1000 and 2000 g/mol, an
internal surfactant and a catalyst are charged to the reactor. The aromatic diisocyanates will hardly be used due to extremely rapid reactions with water. The temperature is raised to about 70 °C and maintained at that temperature until the required
NCO number is reached. Whenever necessary, acetone is added to decrease the
viscosity. Temperature is then lowered to 45 °C and a neutralizing agent such triethylamine (TEA) is added to neutralize the carboxylic acid group of surfactants. Then,
the temperature is lowered to room temperature and water is added to disperse the
PU in the water medium. The chain extender can be added after adding water or
before adding TEA. After adding water, only diamine chain extenders are usable.
Once the reaction is complete, acetone is removed by lowering the reactor pressure
(Najjar et al. 2018; Mirmohseni et al. 2019).
8 Reactive and Functional Polyesters and Polyurethanes