Impact of EO chain length of dodecanol ethoxylates (C12En) on the rheological properties and physical stability of pesticide suspension concentrate

Abstract

Rheological measurement can provide not only the shear-induced but also strain-induced microstructural changes, as well as transient network formation and subsequent recovery with time. It has been explored as a useful tool for probing the interparticle interactions which govern the physical stability of concentrated suspension. However, its application in pesticide formulation is rarely investigated. Herein, the rheological properties of 25% mancozeb suspension concentrate stabilized with nonionic surfactants dodecanol ethoxylates (C12En, n = 3, 7, 9, 15, 20) were studied. The steady-state shear measurement showed that all the 25% mancozeb suspension concentrate displayed shear-thinning flow. The experimental flow behavior followed prediction of Herschel–Bulkley model with the correlation coefficients greater than 0.99. Oscillatory shear measurements showed the viscoelastic response of 25% mancozeb suspension concentrate transformed from solid-like to gel-like viscoelasticity with the addition of C12En. As the EO chain length of C12En increased, the apparent viscosity and elastic modulus decreased initially, followed by an increase behavior. The thixotropic response of mancozeb formulations was evaluated by applying a three-step test. It turned out that the addition of C12En changed the mancozeb suspension concentrate from normal viscoelastic fluid to inelastic thixotropic material, and the recovery of viscosity gradually decreased as the EO chain length increased. Besides, the influence of C12En on suspension rate and particle size of 25% mancozeb suspension concentrate was investigated. The results showed that C12E9 performed best in improving the physical stability of the formulations before and after thermal storage. From SEM imagines of mancozeb particles, it was found that the aggregation was inhibited by C12E9, which was also responsible for the best dispersibility, minimum apparent viscosity and elastic modulus of 25% mancozeb suspension concentrate with the addition of C12E9.