Special Polyether Polyols – Polymer Polyols

Vinyl polymer-grafted polyether polyols, commonly known as "polymer polyols" or POP, are produced using conventional polyether polyols (typically standard soft foam polyether triols and high-activity polyethers) as the base polyether. By introducing vinyl monomers such as acrylonitrile, styrene, methyl methacrylate, vinyl acetate, and vinyl chloride, along with an initiator, free radical graft polymerization is performed under nitrogen protection at around 100°C. POP is an essential type of polyol used in molded and block soft and semi-rigid polyurethane foams.

Polymer polyols are generally white or light milky yellow in appearance and contain organic fillers, which can replace inorganic fillers. This substitution enhances the polyurethane foam's load-bearing capacity, resilience, and improves its cell structure and physical-mechanical properties.

Research indicates that the -CH₂-CH₂-O- segment in polyether molecules achieves a higher grafting effect than the -CH₂-CH(CH₃)-O- segment. Thus, base polyether polyols are generally propylene oxide-ethylene oxide copolyethers, selected with varying end EO contents and molecular weights (3000-6000) based on the end use. The base polyether may be a single polyether polyol or a mixed polyether polyol. In practice, POP commonly uses polyether triol as the base polyether and styrene-acrylonitrile as the graft monomers.

In industry, commonly used initiators are azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO). Systems using BPO as the initiator have low monomer conversion rates, and BPO decomposition products increase the acid value, extend foaming time, and reduce foam strength, limiting its applicability. Although AIBN has disadvantages like toxicity, low solubility in ordinary solvents, slow decomposition rate, and high decomposition temperature, it offers several advantages:

1. An appropriate half-life within the 100-125°C graft polymerization temperature range;

2. High initiation efficiency, with styrene and acrylonitrile yields of 80% and 100%, respectively;

3. Produces only one radical upon decomposition;

4. Generates no acid during decomposition, leaving the acid value unaffected and avoiding unpleasant odors, resulting in a low-viscosity polymer polyol;

5. High monomer conversion rate with yields up to 98%;

6. Readily available and cost-effective.

For these reasons, AIBN is the preferred initiator for POP preparation.

Polymer polyols are mixtures containing three types of polymers: unreacted polyether (base polyether), grafted polyether of vinyl copolymer, and particulate vinyl polymers (or copolymers) dispersed within the polyether. The amount of vinyl copolymer grafted onto the polyether is typically minimal. Although the grafted polymer content in polymer polyols is low, it is a crucial component, enhancing the compatibility between the solid and liquid phases in the dispersion system, reducing particle aggregation and adhesion, and ensuring uniformity and stability in particle size and system viscosity. The dispersion remains stable due to partial grafting within the polymer.