172
PU foams, coatings, adhesives and elastomer are important sections for reactive
polyesters. Polyisocyanates play the role of hardener or crosslinker for reactive
polyesters (Septevani et al. 2015). Polyesters cured with aliphatic polyisocyanates
show excellent stability and flexibility outdoors (Sahoo et al. 2018). They can be
used for aircraft finishes. High Mw polyesters (3000–5000 g/mol) are used for PU
elastomers as high quality shoe soles (Clemitson 2015). Two-component PU adhesives are another application of polyesters cured with polyisocyanates, which will
be discussed in Sect. 8.2.5.2.
Powder coating is a promising method to replace solvent-based methods that are
harmful to the environment (Knudsen and Forsgren 2017). Reactive polyesters
cured by epoxy resins or hydroxyalkylamides are widely used for coating appliances such as cooling systems and refrigerators, steel furniture and some agricultural machinery. The resulting coating shows good adhesion to metals, good outdoor
stability and high impact strength (Knudsen and Forsgren 2017).
8.2 Polyurethanes (PUs)
8.2.1 Introduction
PUs were first introduced by Otto Bayer in 1937 in Germany as a polymer to compete with polyamides, whose patents were held by DuPont Company in the United
States (Bayer 1947). At first, polyester polyol was used to synthesize PUs on a large
scale. But then the polyethers became commercially available and replaced the
polyesters to a large extent. Since then, thousands of patents have been registered in
PUs by different companies worldwide.
PUs are among the most versatile polymers which are synthesized by polyaddition reactions (Vaidya and Chaudhury 2002; Jeong et al. 2006; Tan et al. 2011;
Kreye et al. 2013). These polymers are mainly produced by reaction of diisocyanates and polyols at temperatures below 100 °C with formation of urethane group.
PUs can be produced with completely different properties (Finnigan et al. 2004;
Sonnenschein et al. 2005). For example, elastomers, and soft and hard foams can be
produced from almost the same starting materials. The final properties of PUs
depends on many parameters, such as atmospheric humid, crosslinking, degree of
branching, Mw, NCO/OH ratio, type of monomer used (e.g. aromatic or aliphatic,
linear or branched, hydrophile or hydrophobe), distance between urethane groups
(hard segments) in a single chain, etc. (Crawford et al. 1998; Gorna et al. 2002;
Zlatanić et al. 2004; Pattanayak and Jana 2005; Corcuera et al. 2011; Saralegi
et al. 2013).
Mw of polyol affects the hardness and elasticity of the final PU (Gorna et al.
2002). For example, a high Mw of polyol leads to the formation of an elastomer and
a low Mw of polyol gives a hard PU. The PUs resulting from the reaction of polyols
M. Akbari and R. Najjar
PU foams, coatings, adhesives and elastomer are important sections for reactive
polyesters. Polyisocyanates play the role of hardener or crosslinker for reactive
polyesters (Septevani et al. 2015). Polyesters cured with aliphatic polyisocyanates
show excellent stability and flexibility outdoors (Sahoo et al. 2018). They can be
used for aircraft finishes. High Mw polyesters (3000–5000 g/mol) are used for PU
elastomers as high quality shoe soles (Clemitson 2015). Two-component PU adhesives are another application of polyesters cured with polyisocyanates, which will
be discussed in Sect. 8.2.5.2.
Powder coating is a promising method to replace solvent-based methods that are
harmful to the environment (Knudsen and Forsgren 2017). Reactive polyesters
cured by epoxy resins or hydroxyalkylamides are widely used for coating appliances such as cooling systems and refrigerators, steel furniture and some agricultural machinery. The resulting coating shows good adhesion to metals, good outdoor
stability and high impact strength (Knudsen and Forsgren 2017).
8.2 Polyurethanes (PUs)
8.2.1 Introduction
PUs were first introduced by Otto Bayer in 1937 in Germany as a polymer to compete with polyamides, whose patents were held by DuPont Company in the United
States (Bayer 1947). At first, polyester polyol was used to synthesize PUs on a large
scale. But then the polyethers became commercially available and replaced the
polyesters to a large extent. Since then, thousands of patents have been registered in
PUs by different companies worldwide.
PUs are among the most versatile polymers which are synthesized by polyaddition reactions (Vaidya and Chaudhury 2002; Jeong et al. 2006; Tan et al. 2011;
Kreye et al. 2013). These polymers are mainly produced by reaction of diisocyanates and polyols at temperatures below 100 °C with formation of urethane group.
PUs can be produced with completely different properties (Finnigan et al. 2004;
Sonnenschein et al. 2005). For example, elastomers, and soft and hard foams can be
produced from almost the same starting materials. The final properties of PUs
depends on many parameters, such as atmospheric humid, crosslinking, degree of
branching, Mw, NCO/OH ratio, type of monomer used (e.g. aromatic or aliphatic,
linear or branched, hydrophile or hydrophobe), distance between urethane groups
(hard segments) in a single chain, etc. (Crawford et al. 1998; Gorna et al. 2002;
Zlatanić et al. 2004; Pattanayak and Jana 2005; Corcuera et al. 2011; Saralegi
et al. 2013).
Mw of polyol affects the hardness and elasticity of the final PU (Gorna et al.
2002). For example, a high Mw of polyol leads to the formation of an elastomer and
a low Mw of polyol gives a hard PU. The PUs resulting from the reaction of polyols
M. Akbari and R. Najjar