LAUSR

Abstract In this study, poly(N-vinyl pyrrolidone) (PVP), a cheap, safe and non-toxic polymer, was explored using a range of analytical methods including fluorescence microscopy to gain insight into the role of polymer physicochemical properties on rainfastness, i.e. tenacity of foliar deposits against rain, of agrochemicals on plant surfaces. Three methods were approached to increase rainfastness of PVP, i.e. using high molecular weight grades of the polymer, pre- blending PVP with poly(acrylic acid) (PAA) and successively depositing drops of each polymer (PVP or PAA) on top of the other. Regarding the first method, from the different commercial grades of PVP studied, it was revealed that the polymer with highest molecular weight (1300 kDa) significantly improved the rainfastness of a model fungicide (azoxystrobin). The rainfastness results correlated with film dissolution in water. In the second method, rainfastness properties of PVP were improved by mixing it with PAA and it was shown that PVP-PAA mixtures at the 50:50 wt ratio retarded film dissolution by a factor of 2–3 compared to the PVP alone. In the third method, a novel approach was employed by placing drops of PAA solution on PVP drops on paraffin film and leaving to physically mix and dry down. In this proof-of-concept study, the washing-off profiles of the dry deposits revealed a striking rainfastness increase almost to the level of the insoluble controls. Methods employed in this study to increase rainfastness of agrochemical formulations can explain the previously reported effects of water-soluble polymers on rainfastness and allows the identification of improved rainfastness aids.