Preparation of Low Unsaturation Polyether—EO-Capped Polyether Polyols

To achieve high resilience in polyurethane flexible foam, the molecular structure of polyether polyols must contain a significant amount of ethylene oxide (EO), with primary hydroxyl groups reaching 70%. This requires capping polyether, which has been polymerized with propylene oxide (PO), with EO to produce high-activity, primary-hydroxyl-terminated polyether polyols. To meet the EO requirements for high-resilience polyurethane flexible foam polyether polyols, the current approach often involves capping or block polymerizing with potassium hydroxide (KOH) following the DMC-catalyzed reaction. After DMC-catalyzed synthesis, it is unnecessary to remove DMC from the reaction mixture; simply adding a certain amount of KOH can passivate DMC, allowing the EO to undergo ring-opening reactions under standard KOH-catalysis conditions, resulting in EO-capped polyether polyols. However, polyether polyols containing EO made with the KOH catalyst passivation method exhibit increased unsaturation, potentially exceeding 0.01 mmol/g, though this level is still much lower than that of polyether polyols produced solely with KOH catalysis.

According to literature, at certain temperatures, treating polyether polyols containing a bimetallic catalyst with a specific amount of metallic sodium, metallic potassium, sodium hydride, or potassium hydroxide can convert the catalyst into an insoluble salt, transforming the secondary hydroxyl ends of the polyether into alkoxide-metal compounds. Subsequent reaction with EO converts the secondary hydroxyl groups into primary hydroxyl groups, with primary hydroxyl molar fractions reaching 77%. When using ammonium hydroxide, boron trifluoride, lithium methoxide, or sodium methoxide, the primary hydroxyl molar fraction can reach up to 89%.

Other studies report treating polyether polyols containing DMC catalysts with a certain amount of peroxide (30% H2O2 by mass) to convert the DMC catalyst into an insoluble form and remove it. The polyol is then treated with potassium hydroxide (0.15% by mass in the polyether) and subsequently reacts with EO (10% by mass), yielding EO-capped polyether polyols with a primary hydroxyl molar fraction of 75%.

The process for producing low-unsaturation, high-activity polyether polyols involves the following: preparing low-unsaturation PO polyether using the above methods, then adding a small amount of KOH (or another alkali metal catalyst) to the low-unsaturation polyether at 40–120°C, rendering the bimetallic catalyst inactive. This converts the hydroxyl groups of the polyether into alkoxides, enabling EO capping. Pressure is kept below 0.4 MPa, and temperature around 105°C. After the required amount of EO is added, an "aging" reaction is conducted, followed by purification.

Purification Process: Through post-treatment, including acid neutralization, adsorption, and filtration, high-activity polyether polyol products are obtained. The final product has a primary hydroxyl molar fraction of 65%–95% and an unsaturation level below 0.01 mmol/g.

Researchers at Liming Chemical Research Institute employed a ternary composite alkali metal compound to inactivate the bimetallic catalyst in polyether produced by the DMC process. After EO reaction, the crude polyether is adjusted to a pH of approximately 7 with a neutralizing agent, followed by adsorption with an adsorbent and filtration to remove solid residues, resulting in a refined, high-activity polyether. The final product appears as a pale yellow transparent liquid, remains clear upon prolonged storage, and has a Zn²⁺ and Co²⁺ ion content below 10 μg/g (<0.001%), an acid value of 0.02–0.04 mg KOH/g, and unsaturation below 0.008 mmol/g.

Increasing the EO content not only raises the primary hydroxyl content but also the molecular weight and hydrophilicity of the polyether, potentially increasing the hygroscopicity of the final foam product. Thus, the EO proportion must be carefully controlled.

At Jinling Petrochemical Company’s Research Institute, researchers developed a high-activity polyether polyol preparation method using polyether blending. This involves adding 25%–45% crude polyether prepared with traditional KOH catalyst to a low-unsaturation polyether polyol with secondary hydroxyl groups. The residual alkali in the crude polyether catalyzes the inactivation of DMC, followed by EO reaction to reach the target EO amount, and removal of unreacted monomers to obtain crude polyether.

Refinement of Crude Polyether:

1. Add H₂O₂ (1% of polyether mass) to decolorize at 100°C.

2. Gradually add 50% H₄P₂O₇ at 110°C for neutralization (pH=6) over 30 minutes.

3. Dehydrate under vacuum at 110°C (120 minutes).

4. Add 1% adsorbent to remove metal ions at 110°C (120 minutes).

5. Filter using plate-frame equipment to obtain high-primary hydroxyl, low-unsaturation polyether.

The resulting high-activity polyether diol (hydroxyl value ~28 mg KOH/g) and triol (hydroxyl value ~35 mg KOH/g) have primary hydroxyl molar fractions exceeding 65% and unsaturation levels below 0.010 mmol/g.

Using KOH to inactivate the bimetallic complex catalyst in low-unsaturation polyethers may produce traces of water, which, if not removed or fully dehydrated before EO capping, could lead to EO homopolymerization, forming minor amounts of polyethylene glycol and resulting in cloudiness and reduced primary hydroxyl content in the polyether product.