Natural Polyols from Castor Oil and Its Derivatives

Castor oil, a natural agricultural byproduct, is a compound containing multiple hydroxyl groups and has been widely applied in the paint and coatings industry. In the early development of the polyurethane industry, castor oil and its alcoholysis derivatives were commonly used as polyol raw materials for polyurethane foam. However, castor oil is less frequently used in foam formulations today.

Castor oil is a triglyceride of fatty acids, consisting of approximately 70% glyceryl triricinoleate and 30% glyceryl diricinoleate mono-linoleate. The fatty acid content comprises about 90% ricinoleic acid (9-hydroxy-12-octadecenoic acid) and 10% oleic acid and linoleic acid, which lack hydroxyl groups. Ricinoleic acid contains one hydroxyl group.

From its chemical structure, castor oil is a multi-hydroxyl compound with a long-chain fatty group, which gives the resulting polyurethane products good water resistance and flexibility. Castor oil has a hydroxyl value of 163 mg KOH/g, a hydroxyl content of 4.94%, and an equivalent molecular weight-to-functionality ratio of 345. Based on its hydroxyl value, it is estimated that castor oil consists of 70% tri-hydroxy glycerides and 30% di-hydroxy glycerides, with an average hydroxyl functionality of 2.7. After bleaching with activated clay, castor oil becomes colorless or light yellow, transparent, with an acid value of 1–2 mg KOH/g.

Through alcoholysis and transesterification with additives such as ethylene glycol, glycerol, trimethylolpropane, pentaerythritol, sorbitol, or even low-molecular-weight polyether polyols, castor oil derivatives with various hydroxyl values, functionalities, and molecular weights can be obtained. Transesterification reactions are generally conducted at temperatures of 200–220°C with the aid of catalysts to promote alcoholysis.

For example, the alcoholysis (transesterification) reaction of castor oil with glycerol yields glyceryl mono-ricinoleate and glyceryl di-ricinoleate. The castor oil derivatives obtained through transesterification not only exhibit increased hydroxyl values and functionalities but also enhanced reactivity due to a higher primary hydroxyl group content.

These polyols derived from castor oil alcoholysis products have been widely applied in coatings and can be used for rigid and semi-rigid polyurethane foams. Pure castor oil reacts with isocyanates to typically produce semi-rigid foams; however, for use in rigid polyurethane foams, the functionality of castor oil derivatives needs to be increased to yield high hydroxyl-value products.