Overview of Various Polyethers - High-Functionality Polyethers

High-functionality polyethers can be synthesized by using hexahydroxy compounds like sorbitol and mannitol as initiators, which are polymerized with propylene oxide under KOH catalysis at 100-110°C under pressure. This produces hexahydroxy polyethers. Due to their six hydroxyl groups, these polyethers have high functionality, which results in high cross-linking density in the polyurethane foam, improving the product's oil resistance, thermal oxidation stability, and dimensional stability. China has abundant resources of sorbitol and mannitol; particularly, mannitol (with a purity of 80-90%) is a by-product in the iodine extraction industry from seaweed. The synthesis method of hexahydroxy mannitol poly(propylene oxide) is the same as that of hexahydroxy sorbitol poly(propylene oxide).

High-quality mannitol polyether can yield rigid polyurethane foam with heat resistance up to 180°C. The viscosity of sorbitol poly(propylene oxide) hexols with hydroxyl values of 654 mg KOH/g and 488 mg KOH/g (theoretical average molecular weights of 514 and 691, respectively) is 134 Pa·s and 131 Pa·s, with an APHA color index of 240 and 200, a water content of 0.02%, and unsaturation of 0.02 mmol/g.

Due to the high functionality, hexahydroxy polyethers have high viscosity, which reduces their compatibility with other foaming components (isocyanates, foaming agents, catalysts, etc.), making foam production challenging. To reduce the viscosity of polyethers, industrial processes often use mixed initiators. For example, a sorbitol-glycerol mixed initiator can be used, and controlling the amount of glycerol allows for the synthesis of poly(propylene oxide) with various viscosities and compositions, yielding polyethers with effective functionalities between 3 and 6.

If sucrose alone is used as an initiator in propylene oxide ring-opening polymerization, an octafunctional sucrose polyether is obtained. This product is a high-viscosity, light brown liquid, and the resulting rigid polyurethane foam has good heat resistance, high compressive strength, and stable dimensions. During the synthesis of octahydroxy sucrose polyether, since sucrose is crystalline and not miscible with propylene oxide, and pure sucrose polyether has high functionality and viscosity with poor compatibility with other foaming components, mixed initiators are typically used in practical polymerization. For instance, glycerol can be mixed with sucrose as an initiator, or other lower-functionality polyhydroxy compounds may be combined with sucrose. The sucrose-glycerol polyether is effectively a blend of octahydroxy and trihydroxy poly(propylene oxide). The amount of glycerol in the mixed initiator significantly impacts the properties of the rigid polyurethane foam; for good dimensional stability and high heat resistance, the molar ratio of glycerol to sucrose in the sucrose-glycerol polyether is generally controlled at 0.5.

High-functionality polyether polyols are primarily used as raw materials for rigid polyurethane foams. However, it has also been reported that polyether polyols with an average functionality above 3 and relatively high molecular weight, synthesized using high-functionality initiators, can be used as raw materials for soft and semi-rigid polyurethane foams.