144
The main chains of polyether polyols are mostly of linear structure (-C-C-O-C-),
which is a soft structure. Due to the poor mechanical properties of PU foams
obtained by polyether polyols, crosslinking agents are often added in the foaming
system to improve the mechanical strength of these materials (Huo et al. 2019). The
crosslinking agents, also known as chain extenders within the PU industry, are generally organic compounds with two or more functional groups (such as diethanolamine and glycerol) in their chemical structure. These crosslinking agents react
with other foaming ingredients during the foaming process to generate PU foam
products with crosslinked network structures (Chen et al. 2014). The crosslinking
agent commonly has a relatively small molecular weight (Mw) and short chain
joints, so it can increase the content of hard segment in the PU foam polymer, thus
improving the density and mechanical strength (hardness, and compression and tensile strength) of the PU foams (Dounis and Wilkes 1997b; Kang et al. 2014). When
the isocyanate index (NCO/OH molar ratio) is kept constant, increasing the amount
of crosslinking agent would lead to a higher hard segment content in the synthesized
PU foam, and therefore, improves mechanical strength of the foam. For example, Li
et al. (2017b) reported that the compression strength increased from 90.9 kPa to
104.7 kPa with the addition of crosslinking agent from 10% to 15% under the same
conditions.
7.1.3 Methods for the Synthesis of PU Foams
The first step of the two-step foaming method (also known as the prepolymer
method) is to use excessive isocyanate to react with polyols to obtain the prepolymer with the -NCO group as the end group. The second step is to mix the prepolymer obtained with other foaming ingredients, such as catalysts, surfactants and
water, to finally obtain the PU foam materials (Ashida 2006). Due to the high viscosity of the prepolymer, the long period for PU preparation and the high storage
requirement, the two-step foaming method is rarely used for practical foam
preparation.
The semi-prepolymer method is also performed in two steps. Firstly, a small
amount of polyols is reacted with all isocyanates to obtain a prepolymer. Secondly,
the prepolymer obtained is mixed with the remaining polyols, water and other additives to produce PU foams. This method is mainly used in the preparation of rigid
PU foam materials (Fiore et al. 2018).
The one-shot foaming process has been the most popular method in PU foam
industry in recent years (Li et al. 2017b; Li et al. 2018; Huo et al. 2019). This
method consists in the preparation of PU foam material by directly mixing isocyanate, polyol and other foaming agents (such as catalysts, surfactants, water, etc.)
which have been strictly measured. In general, duo to the high reactivity of the isocyanate, the polyol and other agents are first mixed evenly, and then isocyanate is
virtued, which can shorten the reaction time and obtain PU foam with a uniform
internal structure.
H. Li et al.
The main chains of polyether polyols are mostly of linear structure (-C-C-O-C-),
which is a soft structure. Due to the poor mechanical properties of PU foams
obtained by polyether polyols, crosslinking agents are often added in the foaming
system to improve the mechanical strength of these materials (Huo et al. 2019). The
crosslinking agents, also known as chain extenders within the PU industry, are generally organic compounds with two or more functional groups (such as diethanolamine and glycerol) in their chemical structure. These crosslinking agents react
with other foaming ingredients during the foaming process to generate PU foam
products with crosslinked network structures (Chen et al. 2014). The crosslinking
agent commonly has a relatively small molecular weight (Mw) and short chain
joints, so it can increase the content of hard segment in the PU foam polymer, thus
improving the density and mechanical strength (hardness, and compression and tensile strength) of the PU foams (Dounis and Wilkes 1997b; Kang et al. 2014). When
the isocyanate index (NCO/OH molar ratio) is kept constant, increasing the amount
of crosslinking agent would lead to a higher hard segment content in the synthesized
PU foam, and therefore, improves mechanical strength of the foam. For example, Li
et al. (2017b) reported that the compression strength increased from 90.9 kPa to
104.7 kPa with the addition of crosslinking agent from 10% to 15% under the same
conditions.
7.1.3 Methods for the Synthesis of PU Foams
The first step of the two-step foaming method (also known as the prepolymer
method) is to use excessive isocyanate to react with polyols to obtain the prepolymer with the -NCO group as the end group. The second step is to mix the prepolymer obtained with other foaming ingredients, such as catalysts, surfactants and
water, to finally obtain the PU foam materials (Ashida 2006). Due to the high viscosity of the prepolymer, the long period for PU preparation and the high storage
requirement, the two-step foaming method is rarely used for practical foam
preparation.
The semi-prepolymer method is also performed in two steps. Firstly, a small
amount of polyols is reacted with all isocyanates to obtain a prepolymer. Secondly,
the prepolymer obtained is mixed with the remaining polyols, water and other additives to produce PU foams. This method is mainly used in the preparation of rigid
PU foam materials (Fiore et al. 2018).
The one-shot foaming process has been the most popular method in PU foam
industry in recent years (Li et al. 2017b; Li et al. 2018; Huo et al. 2019). This
method consists in the preparation of PU foam material by directly mixing isocyanate, polyol and other foaming agents (such as catalysts, surfactants, water, etc.)
which have been strictly measured. In general, duo to the high reactivity of the isocyanate, the polyol and other agents are first mixed evenly, and then isocyanate is
virtued, which can shorten the reaction time and obtain PU foam with a uniform
internal structure.
H. Li et al.