Influence of Various Groups in Polyurethane on Foam Properties

Previous studies have indicated that polyurethane is a polymer composed of various functional groups. The effect of these groups on the intermolecular forces within the polymer can be expressed through the cohesive energy of different groups in small molecules. Higher values correspond to stronger attractive forces.

As seen in Table 1, the cohesive energy of aliphatic hydrocarbons and ether groups is the lowest, while that of urethane and amide groups is higher. Though urea groups are not listed in the table, their cohesive energy surpasses that of urethane groups. For example, the cohesive energy of different groups in synthetic fibers can be inferred from the melting points of the fibers (Figure 2). Generally, higher cohesive energy correlates with higher melting points and greater crystallinity. The data in Figure 2 show that clustered fibers have the highest melting points, indicating that urea groups have higher intramolecular cohesive energy compared to urethane and amide groups.

Table 1: Cohesive Energy of Various Organic Groups

The significant cohesive energy of these groups is primarily due to the effect of hydrogen bonding.

From the polyurethane, polyamide, and polyurea fiber systems in Figure 2, it is evident that the melting points decrease with the reduction in the content of highly polar groups. However, polyesters are different; the ester group's content has a minor impact on the melting point. This characteristic is mainly because of the presence of C-O-C linkages in ester groups, which are relatively flexible, counteracting the moderate cohesive energy of ester groups. Consequently, polyurethane has a lower melting point than polyamide in equivalent structures due to the flexibility of C-O-C linkages.

Figure 2: Melting Point Trends of Various Polymers

Although the concentration of ester groups has a relatively minor impact on the melting point of polyesters, different results are observed in polyester-type polyurethane or polyurea. In these mixed polymers, the presence of strong hydrogen donor groups leads to more extensive hydrogen bonding than in pure polyester. Increasing the concentration of ester groups in polyester-type polyurethane enhances the polymer's strength.

The flexibility of the C-O-C linkage is also evident in the melting point of polyethylene oxide (polyethylene glycol). Despite its cohesive energy (1.00) being higher than that of methylene (0.68), the melting point of polyether is only 55-70°C, while polyethylene can reach 110°C or higher. This difference in chain flexibility is mainly due to the lower rotational barrier of the C-O-C bond compared to the C-C bond. For instance, the rotational energy of the C-C bond in ethane is 12.6 kJ/mol, mainly due to the repulsion between hydrogen atoms on adjacent methyl groups. When these methyl groups are separated by an ether oxygen bond, the distance between the hydrogen atoms increases, making chain rotation easier.

Table 3: Impact of Various Ether Groups on Melting Points

The presence of aromatic rings, in contrast to ether groups, significantly affects the rigidity of polymer chains, resulting in higher melting points, increased hardness, and dimensional stability.

Although the above data are derived from non-foamed linear polyurethanes, similar trends are observed in foam materials. For example, rigid polyurethane foams made from aromatic polyether polyols or aromatic polyisocyanates generally exhibit higher temperature resistance and dimensional stability. Furthermore, the softening temperatures of polyurethane foams made from trifunctional polyethers initiated by trimethylolpropane (aliphatic-3 type), hexafunctional polyethers initiated by sorbitol (aliphatic-6 type), and trifunctional polyethers containing benzene rings (aromatic-3 type) with IDI and MDI differ. For polyethers with the same hydroxyl equivalent and functionality, foams made from aromatic polyethers have higher softening points than those made from aliphatic polyethers. Additionally, foams made from MDI have higher softening temperatures than those made from TDI, primarily due to the higher density of benzene rings in the foams made from MDI.

Foams made from heterocyclic polyethers such as methyl glucoside-oxirane polyether polyol exhibit significantly higher softening points than those made from the same tetrol functionality, such as pentaerythritol polyether polyol, under the same isocyanate index. This also indicates that the rigidity of heterocyclic groups is much higher than that of aliphatic hydrocarbon groups.