What is the Application of Polyether Polyols in Foam Plastics?

Polyether polyols, a large class of polyols with ether linkages in the molecular backbone, are used for synthesizing polyurethane products, with functionalities ranging from 2 to 8 and molecular weights from 200 to 8000. Various functionalities and molecular weights can be achieved by selecting different initiators, either individually or in combination, and polymerizing propylene oxide alone or copolymerizing it with ethylene oxide in different ratios.

The properties of polyether polyols are closely related to the initiator and the length and structure of the alkylene oxide chains. The functionality of polyether polyols depends on the type of initiator and the number of active hydrogens. In practical formulations for polyurethane foam production, several polyether polyols with different functionalities and molecular weights are often blended to prepare foams with different physical properties.

Polyether polyols synthesized by anionic polymerization of propylene oxide mainly have secondary hydroxyl groups as terminal hydroxyl groups. Introducing ethylene oxide segments during alkylene oxide polymerization enhances the hydrophilicity and compatibility with water and polyisocyanates, especially when some EO is added at the molecular chain ends, increasing primary hydroxyl content and reactivity with isocyanates. This allows for lower curing temperatures for polyurethane products, enabling foam molding and curing even at room temperature.

Different polyurethane foam types have different requirements for polyether polyols. Polyether polyols used in flexible foams are typically long-chain, low-functionality polyethers. For flexible foams, the functionality usually ranges from 2 to 3, with molecular weights between 2000 and 6500. The most common polyether for flexible foams is polyether triol, usually synthesized using glycerol as the initiator and propylene oxide, with or without a small amount of ethylene oxide.

Early flexible foam polyethers were mostly propylene oxide-based triols with molecular weights around 3000 and mainly secondary hydroxyl groups. Later, propylene oxide/ethylene oxide copolymers became common, providing different molecular weights and reactivities.

High-functionality, high-hydroxyl polyether polyols are used in rigid foams to ensure sufficient cross-linking and rigidity. Rigid foam polyols typically have hydroxyl values ranging from 350 to 650 mgKOH/g, with functionalities above 3. The formulations often include multiple polyether polyols for an average hydroxyl value around 400 mgKOH/g.

Polyether polyols initiated with glycerol provide good flowability, while those initiated with amines exhibit self-catalytic properties, reducing the need for amine catalysts. Aromatic diamine-based polyether polyols produce foams with high strength and low thermal conductivity.