Natural Product Polyols – Rosin Ester Polyols

Rosin ester polyols, derived from natural rosin resin from forest products, can serve as raw materials for rigid polyurethane foam. The performance of these foams is comparable to those made with polyether polyols and is also cost-competitive, finding some application in rigid foam production.

Rosin is a complex mixture of fused polycyclic natural compounds. Most rosin molecules contain a carboxylic acid group. Only a small amount of levopimaric acid can react with maleic anhydride at room temperature, forming maleopimaric acid. At temperatures above 100°C, abietic-type rosins (including abietic acid, palustric acid, and neoabietic acid) undergo isomerization, yielding levopimaric acid.

The resulting levopimaric acid reacts with maleic anhydride via a Diels-Alder addition, producing maleic anhydride-modified rosin (maleopimaric rosin). The maleopimaric acid molecule contains three carboxyl functional groups, which react with excess diols to produce rosin ester polyols, sometimes referred to as "rosin polyester polyols."

When the molar ratio of rosin to maleic anhydride is 1:(0.75-1), and the reaction occurs at 180°C for 3 hours, equilibrium is reached. The esterification of maleopimaric rosin and polyols is carried out under nitrogen protection. The reaction halts when the acid value drops to 5 mg KOH/g. By adjusting the ratio of maleopimaric rosin to polyols and the polyol composition, maleopimaric rosin ester polyols with different hydroxyl values can be obtained.

The reactions of rosin with maleic anhydride and subsequent esterification can be conducted continuously. For example, molten gum rosin is reacted with a specific amount of maleic anhydride under nitrogen protection at 160–200°C. Upon completion, excess diethylene glycol and a catalyst are added, and the reaction proceeds at around 200°C. Once the acid value falls below 2, excess diethylene glycol is recovered under reduced pressure, yielding maleopimaric rosin diethylene glycol ester polyols. The final product is a complex mixture, primarily comprising maleopimaric rosin diethylene glycol ester polyols, with small amounts of tricyclic diterpene monoacid esters and neutral substances.

Due to its fused polycyclic structure, maleopimaric rosin ester polyol exhibits excellent heat resistance, maintaining weight stability at 200°C.

Rigid polyurethane foams and polyisocyanurate (PIR) foams made from this polyol, either alone or blended with conventional 635 sucrose polyether polyols, display superior heat resistance. Maleopimaric rosin ester polyols improve the heat resistance of rigid foams derived from polyether polyols. Studies show that the smaller the molecular weight of the diol used in synthesizing maleopimaric rosin ester polyols, the better the heat resistance of the resulting polyesters and polyurethane foams.

A series of rigid polyisocyanurate foams (PIR, PAPI index 3) were prepared by reacting maleopimaric acid-derived polyols (based on ethylene glycol, diethylene glycol, triethylene glycol, and N210 polyether) with PAPI.

Rigid foams made from maleopimaric acid ethylene glycol ester polyols exhibited the best thermal stability. In contrast, foams derived from maleopimaric acid N210 polyether ester polyols showed the poorest thermal stability due to the higher content of thermally unstable ether bonds in their molecular structure.