Starch–borate complexes for EPTC encapsulation

Abstract

A modification of the starch–borate technique for encapsulating the herbicide S-ethyl dipropylthiocarbamate (EPTC) was explored. It consists of mixing the herbicide into a neutral paste of pregelatinized starch or flour followed by addition of ammonia or an amine. The paste is treated with boric acid or a borate salt to form a gel, which is coated with additional starch or flour to facilitate breaking down the gel to particles in the 10–35-mesh range suitable for drying. An important advantage of this new technique is the moderate operating pH range (9.0–11.0) and final pH range (7.2–8.5) of aqueous suspensions of the dried product, which may allow encapsulation of acid and alkali labile substances. Addition of a surfactant in the encapsulation mixture improves the recovery of the volatile EPTC. Also, about 15% less water is required than in the original technique. In the presence of water, these particles show greater swelling than particles encapsulated by the starch xanthate or starch calcium adduct techniques. However, swelling is decreased by increasing the borate content. EPTC encapsulated by this new technique shows no loss of active ingredient when dry and exposed to the atmosphere, but shows loss when placed in water. The rate of release of active ingredient through several wetting and drying cycles compares favorably with that of products made by other starch-based encapsulation methods.