Rainfastness of agrochemical formulations based on N-vinyl pyrrolidone polymers and their interpolymer complexes with poly(acrylic acid)

Highlights

• Rainfastness properties of PVP were explored.

• Rainfastness correlates well with the molecular weight and film dissolution properties of PVP.

• Formation of PVP/PAA interpolymer complexes results in improved rainfastness.

• In situ interpolymer complexation approach by PAA-on-PVP drops results in a striking improvement in rainfastness.

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.

Introduction

Rain is one of the most important weather factors that adversely affect the performance of agrochemical compounds [1]. The extent of agrochemical rain wash-off is pertinent to the formulation, length of weathering and inherent tenacity of actives [2]. During rainfall as much as 90% of the initial agrochemical deposit applied on plants can be washed-off. This can reduce the effectiveness of applied crop protection agents and may lead to a requirement to reapply [1]. The environmental, economic and agronomic impact of the extra applications and off target movement of the active ingredient can be avoided by achieving improved rainfastness on leaves.

Specialized additives known as “stickers” are incorporated into the formulation or spray tank to address the issue of rain erosion [3]. Sticker adjuvants comprise polymeric materials or materials that polymerize and evaporate by air-drying leaving a flexible polymeric deposit on plant surfaces. This deposit protects agrochemical particles from rain washing and at the same time provides a controlled type delivery system thereby extending the residual activity of the active ingredient. The affinity-assisting properties of stickers for plant foliage are still poorly understood, but they can be tentatively described by various mechanisms like (1) surface tension reduction which assists droplet wetting and spreading; (2) physical retention by droplet infiltration into the plant stomata and surface microstructure; and (3) direct intermolecular adhesion via hydrophobic interactions, polar forces, electrostatic forces and hydrogen bonding [4], [5], [6]. In the past, simple sticker materials comprising fatty acids, mineral oils, glue, sugar, starches and natural resins were used, while more recently these have been replaced by latexes, resins and other synthetic polymers [7], [8], [9], [10], [11], [12]. However, there is continuing need for new and more effective sticker adjuvants [13] as a lot of those currently used are associated with issues like formulation instability [11], locking up effects in pesticide biokinetics [10] and toxicological effects [8]. Yet, there is limited information on the fundamental principles governing the behavior of polymeric materials as rainfastness aids on plant surfaces.

There are three main methods to control the rainfastness of an agrochemical formulation using polymeric-based sticker adjuvants, i.e. using semi-crystalline thermoplastics with temperature controlling effects, ionic polymers with pH-dependent dissolution and UV-polymerizable materials. Regarding the first two methods, previous studies [14], [15] have reported the effects of crystallinity, film dissolution and swelling degree of different grades of poly(vinyl alcohol) (PVOH) and chitosan that underlie rainfastness behavior of agrochemical sprays on plant surfaces. On the basis of the mechanism, temperature and pH can significantly affect the hardening process of the rainfast deposit and upon evaporation, these materials solidify or form a water repellent gel on foliage which encloses the active ingredient. Latex-based adjuvants (e.g. Bond®) comprise colloidal dispersions of particles; as droplets dry down on the plant surfaces, latex particles pack together and eventually coalesce forming a continuous hydrophobic film [16]. Swelling and dissolution of the polymer film are retarded provided that its glass transition temperature (Tg) is well above the ambient temperature in the field. Usually, there is a critical molecular weight of the polymer under which it is not rainfast. It has been concluded that polymers giving insoluble films, which can retard deposit erosion by water, gave enhanced rainfastness [14], [15]. Another type of controlled sticking mechanism is the in situ UV-polymerization by which terpene materials are used, which polymerize when exposed to UV light [10]. However, the role of other factors must not be overlooked such as surface and adhesion phenomena between the polymer films, the plant surfaces and the plant topography.

Poly(N-vinyl pyrrolidone) (PVP) is a water-soluble polymer with a universal solubility in various solvents, high versatility and adhesive capacity to many surfaces; it can also tolerate high concentrations of electrolytes in solutions [17]. Different grades of PVP (soluble and insoluble) and copolymers of vinyl pyrrolidone with vinyl acetate (PVPVA) are commercially available with different molecular weights exhibiting multifunctional properties, appropriate for numerous applications as binders, carriers, stabilizers, film forming and coating agents in both agrochemical and pharmaceutical fields. PVP has been claimed to be biodegradable and is biologically inert and exempt from the requirements of a tolerance concentration limits (EPA 40 CFR part 180.960) [17] and thus can be used in agri-food applications. Advantageously, in comparison to other polymers, PVP formulations improve residence and bioavailability of pesticidal agent by forming more homogenous and coherent deposits on the target surfaces [9], [12]. However, to the best of our knowledge, there is no systematic study examining the correlation of the fundamental properties of PVP deposits, films, solutions and its rainfastness. As PVP has a moisture absorbing capacity, newer more hydrophobic copolymers of PVP have been introduced into the market to produce less hydrophilic materials with increased surface activity [18]. Alternatively, PVP solubility properties can be tailored by blending PVP with other polymers like polymonoethyl itaconate and polyacrylic acid (PAA) [19], [20]. In particular, PVP-PAA mixtures are known to form insoluble interpolymer complexes (IPC) due to hydrogen bonding [20]. Several authors indicated that these interactions are influenced by critical factors such as molecular weight, salt concentration, pH, polymer concentration, solvent nature and polymer ratios [21], [22]. Nevertheless, there are no reports on the use of PVP-PAA blends or complexes as sticker adjuvants in agrochemical formulations.

This study addresses the evaluation of rainfastness activity of different grades of PVP in agrochemical formulations. A range of commercially available PVP grades were evaluated in detail by examining both their bulk polymer film properties and washing-off patterns using a range of lab-based established methods [14], [15] and theoretical models based on surface physical-chemistry. We also successfully demonstrated a new promising application technique for the sticker adjuvant inspired by the in situ polymer precipitation [23].