Petroleum-derived spray oils: chemistry, history, refining and formulation

Modern use of petroleum-derived oils as agricultural crop protectants dates back to the 1800s, but it was not until the latter half of the 20th century that advances in petroleum chemistry allowed substantial modification and diversification in their commercial applicability. In the 1880s, a kerosene–soap emulsion was used with some success against scales and aphids in citrus trees. The search for something more effective led to the use of emulsified crude petroleum sprays, but these were too phytotoxic. Eventually, researchers concentrated on distillates in the range between kerosene and lubricating oils. Three basic classes of carbon structures present in petroleum oils are paraffins, aromatics and naphthenes. In 1915, E. de Ong showed that tree injury was caused by substances (aromatics and other unsaturated components) in oils that were removable with sulfuric acid; the remainder is the unsulfonated residue, or UR. In 1929, this principle became the basis for W. Volck’s patents on low-unsaturate oils having a high level of nonreactivity to sulfuric acid. A few years later, R. Smith noted the relationship between pesticidal efficiency, phytotoxicity and oil heaviness; he recommended distillation as a preferred basis for grading spray oils for heaviness in terms of average molecular weight. Viscosity and boiling-range measurements ultimately became the most useful data for indicating performance rankings among oils of common origin and manufacture. The importance of the emulsifier used with the oil was determined to be in its influence on spray deposit, which affects its pest control efficacy. Modern refining improvements after distillation progressed from solvent extraction (1950s) to hydrogenation and dewaxing to increase the UR value (1960s), and hydrocracking, which Petroleum-derived spray oils: chemistry, history, refining and formulation