Development of an ASTM Method for Measurement of the Extensional Viscosity of Agricultural Spray Solutions

Polymeric drift control agents are used to modify the droplet size distribution of pesticide sprays. These agents function by increasing the extensional viscosity of the spray liquid, enabling it to resist disruption as it emerges from the spray nozzle, usually leading to an increase in the median droplet diameter of the spray droplets. Extensional viscosity is often the most important physical property controlling the spray droplet size, especially when soluble polymers are present, but methods available for measurement of extensional viscosity of dilute solutions are complicated and are not generally available. This paper describes a simple, portable apparatus that can detect and quantify a relative extensional viscosity (screen factor) that is useful in evaluating the properties of drift control agents. The apparatus has been used to determine the screen factors of test substances used by the Spray Drift Task Force and to correlate them with independent measurements of extensional viscosity obtained using a Rheometrics RFX Extensional Rheometer. The method has been adopted as ASTM Method E 2408-04, "Relative Extensional Viscosity of Agricultural Spray Tank Mixes."