182
8.2.5 Reactive PUs
Reactive PUs are categorized into two groups: One-component PUs and twocomponent PUs.
8.2.5.1 One-Component Reactive PUs
One-component reactive PUs are subcategorized into air-drying urethanes (Wicks
and Wicks Jr. 2005), blocked urethanes (Delebecq et al. 2012; Pilch-Pitera et al.
2017), moisture curable urethanes (Chattopadhyay et al. 2006) and UV cure hybrid
urethanes (Dzunuzovic et al. 2005; Li et al. 2019).
Air-drying urethanes which are known as uralkyds (or oil-modified urethanes)
are based on alkyds such as polyol for PU synthesis. This type of reactive PUs has
double bond in its structure. The uralkyds react with atmospheric oxygen to form a
crosslinked polymer. This reaction is catalyzed in the presence of cobalt, magnesium or zirconium compounds as catalysts at room temperatures (Xu et al. 2002).
The scratch resistance and tensile strength of uralkyds are superior to the alkyds, but
cannot compete with pure PUs (Chiou and Schoen 2002; Prashantha et al. 2015),
and also show better solvent resistance than alkyds.
Moisture curable PUs are urethanes with isocyanate end groups. For producing
moisture curable PUs, the isocyanates/polyol (NCO/OH) ratio is more than one
(Karak 2016). After applying on a substrate, the reactive isocyanate groups react
with atmospheric water even at the melting point of water to form the final product.
After reacting the water with isocyanates, an amine end group is formed and then
the amine reacts with isocyanate to form urea groups. Aliphatic isocyanates react
slower with atmospheric water and higher temperatures are required. The reaction
rate depends on the atmospheric moisture content, temperature and type of isocyanate (Hofacker et al. 2004).
In the blocked PU, the isocyanate groups are blocked by a compound, which
after being applied to the polymer is unblocked by means of a trigger such as temperature (Guo et al. 2016). The blocking agents protect the isocyanates group in
storage. Blocking agents such as 2-butanone oxime, 2-ethylhexyl alcohol,
e- caprolactam and uretdiones are used commercially (Rolph et al. 2016). The reaction of the blocking agents with isocyanates is an equilibrium reaction. The blocking reaction is exothermic, and the unblocking reaction is endothermic (Rolph et al.
2016). So, for activating the blocked polymer, the elevated temperature is necessary.
The blocking agents must be evaporated from the polymer because when the temperature drops it can block the isocyanate again. The curing rate depends on several
factors, such as catalyst, polymer thickness, steric hindrance of blocking agents,
temperature and volatility of blocking agents (Gnanarajan et al. 2000; Iyer
et al. 2002).
Another type of one-component reactive PU is UV curable PU. UV curable PUs
are prepared by a prepolymer method with the exception that instead of the chain
M. Akbari and R. Najjar
8.2.5 Reactive PUs
Reactive PUs are categorized into two groups: One-component PUs and twocomponent PUs.
8.2.5.1 One-Component Reactive PUs
One-component reactive PUs are subcategorized into air-drying urethanes (Wicks
and Wicks Jr. 2005), blocked urethanes (Delebecq et al. 2012; Pilch-Pitera et al.
2017), moisture curable urethanes (Chattopadhyay et al. 2006) and UV cure hybrid
urethanes (Dzunuzovic et al. 2005; Li et al. 2019).
Air-drying urethanes which are known as uralkyds (or oil-modified urethanes)
are based on alkyds such as polyol for PU synthesis. This type of reactive PUs has
double bond in its structure. The uralkyds react with atmospheric oxygen to form a
crosslinked polymer. This reaction is catalyzed in the presence of cobalt, magnesium or zirconium compounds as catalysts at room temperatures (Xu et al. 2002).
The scratch resistance and tensile strength of uralkyds are superior to the alkyds, but
cannot compete with pure PUs (Chiou and Schoen 2002; Prashantha et al. 2015),
and also show better solvent resistance than alkyds.
Moisture curable PUs are urethanes with isocyanate end groups. For producing
moisture curable PUs, the isocyanates/polyol (NCO/OH) ratio is more than one
(Karak 2016). After applying on a substrate, the reactive isocyanate groups react
with atmospheric water even at the melting point of water to form the final product.
After reacting the water with isocyanates, an amine end group is formed and then
the amine reacts with isocyanate to form urea groups. Aliphatic isocyanates react
slower with atmospheric water and higher temperatures are required. The reaction
rate depends on the atmospheric moisture content, temperature and type of isocyanate (Hofacker et al. 2004).
In the blocked PU, the isocyanate groups are blocked by a compound, which
after being applied to the polymer is unblocked by means of a trigger such as temperature (Guo et al. 2016). The blocking agents protect the isocyanates group in
storage. Blocking agents such as 2-butanone oxime, 2-ethylhexyl alcohol,
e- caprolactam and uretdiones are used commercially (Rolph et al. 2016). The reaction of the blocking agents with isocyanates is an equilibrium reaction. The blocking reaction is exothermic, and the unblocking reaction is endothermic (Rolph et al.
2016). So, for activating the blocked polymer, the elevated temperature is necessary.
The blocking agents must be evaporated from the polymer because when the temperature drops it can block the isocyanate again. The curing rate depends on several
factors, such as catalyst, polymer thickness, steric hindrance of blocking agents,
temperature and volatility of blocking agents (Gnanarajan et al. 2000; Iyer
et al. 2002).
Another type of one-component reactive PU is UV curable PU. UV curable PUs
are prepared by a prepolymer method with the exception that instead of the chain
M. Akbari and R. Najjar