Polyurethanes, mainly synthesized by addition reactions of polyols with diisocyanates, have been considered as the most widely used polymers in various industries due to the variety of raw materials, synthetic methods and properties. The high demand, the significant share of the world market and the abundant use of this category of polymers have involved the major consumption of petroleum-based polyols and diisocyanates. Nonetheless, environmental challenges and the crude oil crisis have led to greater interest in renewable resources and systems based on green chemistry. Today, with the synthesis of waterborne polyurethanes) WPUs) and the use of water as a safe solvent, the release of volatile organic compounds and the production of solvent-based systems have been prevented to a considerable extent. The non-toxicity, non-flammability, environmentally friendly, and the wide applications of WPUs in plastics, paints, adhesives, printing ink and biomaterials are among the reasons for further development of these water-based systems. Because there are only a few commercially available diisocyanates in the synthesis of WPUs, it is the choice of polyol which may determine the WPUs properties. The most convenient renewable raw materials are natural oils, polysaccharides, wood and proteins, and vegetable oils which are the most beneficial options and have been widely studied. Today bio-polyols obtained from different vegetable oils have become vital in the synthesis of WPUs. However, castor oil has attracted special attention, as an excellent substitute for petroleum-based polyols and has been used in polyurethane compounds due to its availability, biodegradability and inherent hydroxyl groups. Considering the importance of bio-polyols and their role in the development of vegetable oil-based WPUs, in this article, while briefly introducing WPUs and their synthetic methods, the properties of WPUs based on castor oil, as the only natural polyol, are reviewed. Some green approaches to acquire a clear picture of the current and potential future applications of WPUs based bio-polyols in various fields are introduced.
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