Epoxidized soybean Oil (ESO) is an additive to flexible Polyvinyl Chloride (PVC) that gains its name from the reaction that occurs with the unsaturated soybean oil. The epoxidation reaction takes place at a carbon to carbon double bond site. The oxygen atom (usually in the form of a peroxide or a peracid) comes in and attaches itself between the two carbons to create a single bonded triangular ring between the three molecules called an oxirane. This oxirane can be used as a way of separate well formed ESO molecules from those that will be less effective. Those ESO molecules with higher oxirane percentages will product better results than those with low oxirane percentages.
Because epoxidized soybean oil is synthesized from the bio-based, renewable resource of soybean crops, the level of saturation of the soybean oil and ultimately the oxirane value of the ESO created from it, is dependent on the growing conditions of the soybeans. Very hot, dry weather is known to inhibit the formation of the unsaturated C18-3 and C18-2 double bonds in the soybean oil and will therefore lead to lower oxirane values in the ESO produced from it.
The climate of American Midwest is very well suited to producing high oxirane value ESO. The United States is responsible for about 32% of the total soybeans grown worldwide, with Brazil and Argentina also significant producers of the oilseed.
Although ESO was first introduced into the plastics market over thirty years ago as a bio based plasticizer that could be a drop-in replacement to Dioctyl phthalate (DOP), it has found a strong niche as a secondary plasticizer due to its heat and light stabilization effects in PVC compounds. The epoxide group is more reactive than a double bond, thus providing a more energetically favorable site for reaction and making it a good hydrochloric acid scavenger and plasticizer.