Special Polyether Polyols – Aromatic and Heterocyclic Polyether Polyols

Common polyethers are typically synthesized using saturated aliphatic polyols as initiators, resulting in aliphatic polyether polyols. In contrast, aromatic or heterocyclic polyether polyols are synthesized using aromatic or heterocyclic polyols as initiators. Polyurethane rigid foams based on aromatic and heterocyclic polyether polyols exhibit high thermal resistance, flame retardancy, dimensional stability, and compressive strength. High-temperature-resistant polyethers usually contain benzene rings or heterocycles. Initiators for these polyethers include compounds such as bisphenol A, bisphenol S, phenol-formaldehyde oligomer, toluene diamine, aniline-formaldehyde oligomer, and tris(2-hydroxyethyl) isocyanurate (THEIC), or mixtures of these with common initiators. When tertiary amine structures are present in the molecule, the polyether may also have self-catalytic properties, leading to polyurethane rigid foams with fine cell structures and lower thermal conductivity.

A polyether polyol synthesized using toluene diamine as the initiator has a hydroxyl value of 320-340 mg KOH/g and a viscosity (at 25°C) of 5640 mPa·s.

The heterocyclic polyether polyol synthesized using tris(2-hydroxyethyl) isocyanurate as the initiator not only exhibits high thermal resistance but also has certain flame-retardant properties due to the nitrogen-containing six-membered heterocyclic structure of isocyanurate. When combined with phosphorus-containing polyether, the resulting polyurethane rigid foam demonstrates even superior flame retardancy, withstanding a 1000°C propane flame for up to 120 minutes.

The synthesis method for isocyanurate-containing polyether is as follows: In a reaction flask equipped with a thermometer, dropping funnel, reflux condenser, and stirrer, add 261 g of tris(2-hydroxyethyl) isocyanurate, 260 g of toluene, and 1 g of boron trifluoride etherate. When the temperature reaches 50°C, begin adding 200 g of propylene oxide dropwise. The reaction temperature increases, and the mixture is maintained at 120°C for 1 hour. After removing toluene and other volatiles under reduced pressure, a yellow, transparent, viscous polyether (450 g) is obtained, with a hydroxyl value of approximately 374 mg KOH/g.

To enhance the compressive strength of soft foams, long-chain bisphenol A polyether or bisphenol S polyether polyols with low hydroxyl values may be used. Phenol-formaldehyde oligomers and bisphenol A-formaldehyde oligomers can also serve as initiators for synthesizing polyethers.

For example, 228 g of bisphenol A (1 mol), 244 g of 37% formaldehyde solution (3 mol), and 5 g of triethylamine react at 70°C for 3 hours. After adding 92 g of glycerol (1 mol) and reacting for an additional 30 minutes, water is removed under vacuum at 70°C until the moisture content is around 8%. The resulting polyhydroxyphenolic compound reacts with 522 g of propylene oxide at 80–100°C under pressure for 3 hours. After removing unreacted propylene oxide, a viscous aromatic ring-containing polyether with a moisture content below 0.01% and a hydroxyl value of 568 mg KOH/g is obtained.

The above high-hydroxyl-value polyether can react with 1590 g of propylene oxide at 110°C under the influence of KOH (5.2 g). After neutralization and purification of the crude polyether, a soft foam polyether with a hydroxyl value of around 48 mg KOH/g and a viscosity (at 25°C) of 700 mPa·s is obtained.