Polycaprolactone Polyol

In addition to adipic acid-based polyester polyols, high-performance microcellular polyurethane and polyurethane flexible foam also use polycaprolactone polyols (PCL). PCL is synthesized through catalytic ring-opening polymerization of ε-caprolactone in the presence of diol (polyol) initiators. Its functionality depends on the functionality of the polyol initiator used.

Similar to the preparation of polyether polyols, the molecular weight and functionality of PCL can be tailored by adjusting the type and amount of initiators, enabling the synthesis of PCL suitable for flexible, semi-rigid, and rigid polyurethane foams. For example, medium molecular weight PCL derived from ethylene glycol, glycerol, or trimethylolpropane is used for elastomers and flexible foams. Polycaprolactone triol with a molecular weight of approximately 3000 can be employed in the synthesis of flexible polyurethane foams.

For instance, reacting 169 parts of dimethyl ε-caprolactone, 4.8 parts of ethylene glycol, and 0.09 parts of dibutyltin oxide catalyst under nitrogen at 170°C for 4.5 hours yields a colorless viscous liquid PCL with a hydroxyl value of 48 mgKOH/g and an acid value of 2.1 mgKOH/g. Polyurethane products synthesized from PCL exhibit the high mechanical strength, excellent wear resistance, and oil resistance of traditional polyester-based polyurethanes while also possessing the superior hydrolysis resistance and low-temperature flexibility of polyether-based polyurethanes.

The viscosity of polycaprolactone diols is significantly lower than that of adipic acid-based polyesters, making them easier to use and improving the low-temperature resistance, hydrolysis resistance, and dimensional stability of foams. However, PCL is relatively expensive and is now rarely used alone for the production of polyurethane foams. It is mainly used for polyurethane flexible foams and microcellular polyurethane elastomers with special requirements. It can also be blended with adipic acid-based polyesters or used to modify the latter with caprolactone to impart good low-temperature and hydrolysis resistance to the polyester.

An example of a modified mixed polyester synthesis involves heating 730 parts of adipic acid, 570 parts of ε-caprolactone, and 357 parts of ethylene glycol under a nitrogen atmosphere to 160°C while gradually removing condensation water. When the temperature rises to 180–190°C, the reaction is continued under reduced pressure for a period to produce a light brown viscous polyester with a hydroxyl value of 40 mgKOH/g and an acid value of 1.4 mgKOH/g.

Low-hydroxyl-value, high-functionality polycaprolactone polyols synthesized using pentaerythritol or sorbitol as initiators can be used to prepare rigid polyurethane foams. Since most of these polyhydroxy initiators are solids, polymerization is challenging and requires the addition of solvents. For example, mixing 1160 parts of pentaerythritol, 2580 parts of ε-caprolactone, and 200 parts of benzene under nitrogen and stirring while heating, followed by adding 2 parts of benzoyl chloride and maintaining the temperature at about 190°C until the acid value is less than 2 mgKOH/g, yields PCL polyols after removing the solvent and unreacted ε-caprolactone under reduced pressure.

Rigid polyurethane foams made from PCL exhibit excellent thermal stability. For example, polyurethane foam synthesized from PCL using TMP as the initiator and PAPI as the isocyanate exhibits only 3% weight loss after 24 hours at 204°C and 17% after 312 hours. Its compressive strength is 1.49 MPa at 149°C and 0.63 MPa at 204°C.