Abstract/Summary

Rainfastness is the ability of agrochemical deposits to resist wash-off by rain and other related environmental phenomena. This work reports studies of the rainfastness of selected water-soluble polymers, including poly(vinyl alcohol) (PVA) and chitosan, on Vicia faba leaf surfaces. This was achieved using a novel method involving fluorescent microscopy combined with image analysis. The retention of polymer deposits was analysed via lab-scale washing to simulate rain. PVA over a threshold molecular weight and chitosan were shown to be excellent rainfastness aids. The washing method was scaled up with the use of a raintower and it was shown that the lab-scale washing method was representative of low-to-medium intensity rain (10 mm/h). Physical characterisation indicated that rainfastness correlated with polymer film dissolution, swelling and crystalline properties. It was established that the rainfastness of PVA scaled with increasing molecular weight and crystallinity. Chitosan proved the most effective of the polymers analysed and even samples of moderate molecular weight were able to resist the highest intensity simulated rain. Those polymers which exhibited rainfastness were only soluble in water with a stimulus, such as heating for PVA or decreased pH for chitosan. The microscopy analysis was expanded to assess the rainfastening effect of these polymers on a model agrochemical. This was achieved by following the retention of azoxystrobin – a fluorescently active fungicide. Those polymers which showed retention alone also improved the retention of azoxystrobin. A spot and wash‘ method using mass spectrometry to quantify rainfastness performed alongside fluorescent microscopy analysis further validated the findings.