Overview of Various Polyethers—Polyether Triols and High-Activity Polyethers

Polyether triols are generally produced using glycerol (propane-1,2,3-triol), trimethylolpropane, and other initiators, serving as the basic raw materials for polyurethane flexible foams, semi-rigid foams, and rigid foams. The requirements for the molecular weight or hydroxyl value of polyethers vary between polyurethane flexible and rigid foams. Polyether polyols used in flexible foams are typically long-chain, low-functionality polyethers, with a relative molecular weight of around 3000 and a hydroxyl value of approximately 56 mg KOH/g. Rigid foams require polyethers with a relative molecular weight in the range of 300-400 and a hydroxyl value of about 450-550 mg KOH/g.

Compounds containing three active hydrogen atoms, such as glycerol, trimethylolpropane, 1,2,6-hexanetriol, and triethanolamine, can serve as initiators for polyether triols, with glycerol and trimethylolpropane being the most commonly used. The most widely used polyether triol in flexible foams is generally initiated with glycerol (propane-1,2,3-triol) through ring-opening polymerization of propylene oxide or copolymerization with ethylene oxide, with a relative molecular weight typically ranging from 3000 to 7000.

During the preparation of polyether triols, in a heated autoclave with a catalyst concentration of 0.5% KOH and a reaction temperature of 100°C, the average molecular weight of the synthesized polyoxypropylene triol decreases with an increasing molar ratio of the initiator to propylene oxide. When this molar ratio is around 1.21, the relative molecular weight of the synthesized polyether is approximately 3000. The actual molecular weight of synthesized polyethers is often lower than the theoretical molecular weight, primarily due to the influence of the catalyst and temperature. Initially, the catalyst KOH reacts with the initiator to produce potassium alcohol and water; the water acts as a bifunctional initiator and consumes some of the cyclic ether, resulting in a lower actual molecular weight for the polyether compared to the theoretical design value. Therefore, while increasing the amount of catalyst can speed up the reaction, it can also lower the molecular weight of the polyether and increase the deviation from the theoretical molecular weight.

The reaction temperature directly affects the unsaturation of the polyether products. Once unsaturated double bonds form at the polyether end groups during the reaction, they lose reactivity, halting chain growth and thus affecting the molecular weight of the polyether.

When propylene oxide polymerizes under the catalysis of basic catalysts like KOH, the hydroxyl groups in the resulting polyether molecules are predominantly secondary hydroxyls. When using polyethers to prepare polyurethane foams, the activity of the terminal hydroxyl groups is crucial as it directly affects the foaming rate and the amount of catalyst used. Since the activity of primary hydroxyls is approximately three times that of secondary hydroxyls, polyethers with primary hydroxyls can shorten the curing time during foaming and increase the turnover rate of machinery, while also providing better thermal aging performance for the resulting foam.

To enhance the primary hydroxyl content of polyoxypropylene triols, increase polyether reactivity, and improve the miscibility of polyethers with water or isocyanates, industrially, copolymerization of propylene oxide with 10%-15% ethylene oxide is primarily employed, either through random copolymerization or block copolymerization. Polyethers with a relative molecular weight of 3000-3500 and low ethylene oxide end-capping are used for standard flexible foams, while high-activity polyethers with a relative molecular weight of 5000-6500 and high ethylene oxide end-capping are used for high-resilience flexible foams. Random or block copolymerized polyether triols of ethylene oxide and propylene oxide have been widely applied in the production of flexible and semi-rigid polyurethane foams.

High-activity polyethers are a class of polyether polyols with high reactivity developed in the 1970s. The preparation principle involves adding the desired ethylene oxide monomer for continued reaction after the propylene oxide polymerization has ended, introducing ethylene oxide segments at the end of the polyoxypropylene chain and converting secondary hydroxyls into primary hydroxyls to enhance reactivity, enabling cold curing of flexible foams. Due to the structural composition of the semi-finished polyether, reaction conditions, and the homopolymerization of ethylene oxide, it is challenging to achieve a 100% conversion of secondary hydroxyls to primary hydroxyls. The most commonly used high-activity polyether polyols typically utilize trimethylolpropane as the initiator, falling within the polyether triol category, with a relative molecular weight generally between 4500-6500 and a primary hydroxyl content of 70%-90%. The total content of ethylene oxide is around 10%-20%. Classic high-activity polyethers include those with relative molecular weights of 5000 (hydroxyl value 36 mg KOH/g) and 6000 (hydroxyl value 28 mg KOH/g).

Since their development, high-activity polyether polyols have led to new process technologies such as cold curing and reaction injection molding (RIM), as well as foam products like integral skin foams and high-resilience foams. High-activity polyether polyols serve as the foundational materials for producing high-resilience, cold-cured foams, integral skin molded foams, and RIM microcellular elastomers. In recent years, their development has accelerated, with widespread applications and significant usage, gradually positioning them as a general category of polyethers. Because these polyethers incorporate polyethylene oxide groups within the polyoxypropylene chains during synthesis, their miscibility with water and diisocyanates has improved significantly. However, due to pronounced side reactions in the synthesis of high molecular weight polyethers catalyzed by KOH, resulting in the formation of diols and monols, the functionality is much less than 3. The use of bimetallic cyanide complexes as catalysts to synthesize low unsaturation, high molecular weight polyethers can increase the functionality of the polyethers and greatly enhance their performance.

Hard foam polyether polyols initiated with glycerol typically have lower functionality, resulting in slower formation of cross-linked networks compared to high-functionality polyether polyols, thereby providing better flowability for the hard foam materials.