Natural fats and oils are a class of renewables with an astonishingly uniform triacyl glycerol structure, with saturated and unsaturated fatty acids. There are a few exceptions to this rule. One important one is castor oil, the triacyl glycerol of ricinoleic acid, which offers additional possibilities as an intermediate due to its reactive hydroxyl groups. Various techniques that activate the double bonds open up further possibilities for drying and semi-drying oils, and most of these involve oxidation of the C–C double bond to introduce hydroxyl groups onto the carbon chain of the fatty acids. Other techniques use olefin cleavage or addition reactions. That way natural fats and oils become useful oleochemical intermediates and building blocks for polyesters, polyamides, polyurethanes, or other polymers, which qualify as biobased products, as defined by the United States Secretary of Agriculture in the Farm Security and Rural Investment Act of 2002.