AgroPages-Formulating for efficacy-Agricultural news

Dr. Kathryn M. Knight

Research and Technology Manager, Crop Care business

One of the biggest barriers to the agrochemical industry today is posed by the regulators. In Europe, Regulation 1107/2009 continues to come into effect and poses significant risk to many agrochemical active ingredients (AIs). It is estimated that about 100 AIs could

be impacted, more than 25% of those currently registered, resulting in further losses to European farmers. When coupled with the decline in new AI discovery and the growth of resistance to all classes of agrochemicals, crop protection is now facing unprecedented challenges which directly impact national and international food security.¹

As such, there is now a real opportunity for new approaches to deliver innovation in the sector. Considerable time and money is necessary for the successful development of active ingredients (AIs) for effective crop protection. But even the best AI is of no commercial benefit without formulation.

High performance, speciality surfactants are increasingly relied on to guarantee efficacy. In particular, polymeric surfactants (Figure 1) have proved indispensable due to:

i) their multi-attachment to the designated surface, thereby minimizing desorption

ii) their size, giving a wide protective barrier around the dispersed liquid or solid which prevents the close approach of neighbouring particles that can lead to the breakdown of the formulation.

Figure 1. Illustration showing the advantages of polymeric surfactants.

Polymeric surfactants result in a wide formulation stability window, thus allowing rapid, cost-effective formulation development and modification. An additional advantage of polymeric surfactants is that in general they have a better safety and environmental profile than lower molecular weight surfactants, resulting in a more benign regulatory burden for formulation registration.

Formulation principally offers stability but arguably more important is the capability to enhance delivery of the AI. The addition of adjuvants offering wetting ability, spreading, sticking, extenders, or diluents can make the product easier to apply, more convenient to handle and importantly more effective. New AIs in the development pipeline come with a wealth of chemical and physical property obstacles to the formulator and, with the expected resurgence of older chemistry AIs, require innovative formulation expertise to ensure their best performance.

Building different types of adjuvant into a formulation can present a challenge from a materials and colloid chemistry perspective. New adjuvant development for the agrochemical industry is being realised as a requirement to maintain and enhance the toolbox to the industry. However, the actual usage volume from this commercial sector is relatively small.

Surfactant usage for agrochemicals is estimated at approximately 4% of US consumption making their bespoke design fitting to niche specialty chemical suppliers such as Croda. The design and application of such novel materials requires them to show performance in the field, backed up by scientific explanation.

For example, Figure 2 shows the increased mortality rate for a systemic insecticide, imidacloprid, with no adjuvant compared to a series of formulations containing a built-in adjuvant to maximize the enhancement of this AI. A significant increase in pest mortality can be seen by including an alkoxylated polyol ester adjuvant designed to enhance penetration of the systemic insecticide.

Figure 2. Increased mortality rate when imidacloprid is combined with an alkoxylated polyol ester (added at 100 g/L) as the adjuvant of choice. The level of loading is also relevant.

An effective adjuvant for penetration can also have the effect of forming an amorphous film to retain moisture and surfactant contact with the AI. Surfactants are much less volatile and become concentrated in the dried-down deposit. Screening of leaf deposits shows that the choice of surfactant can dramatically affect the deposit microstructure (see Figure 3). The surfactant phase produced on dry-down varies according to the shape of the surfactant molecule. Some surfactant combinations entrap water upon dry-down and thereby affect the propensity for the active to crystallise. Moreover it has been recently realised that the location of crystallisation within the droplet is vital and crystallization at the edges of the evaporating droplet are a current focus of study. Other surfactant dry-down phases are very viscous and so enhance the ‘rainfastness’ of the formulation.

Figure 3. Scanning electron microscopy images show the relationship between the deposit microstructure and a built-in surfactant adjuvant system. Azoxystrobin 250 g/L Suspension Concentrate formulation containing A) no adjuvant (LHS) compared to B) Alkoxylated polyol ester (middle) and C) an alcohol ethoxylate (RHS) applied as a 1/200 spray dilution to prepared apple leaf cuticles. The improved “icing layer effect” distribution in B, provided by the alkoxylated polyol ester upon droplet dry down can be extrapolated to improved field performance.

In summary, surfactants play an important role in agrochemical formulations and present great opportunities to deliver optimum performance. Chemistry that offers unique formulation properties ensures the best performance of the AI, minimizing use rates and environmental strains yet enabling farmers to get the best crop yields. A successful selection follows from a clear understanding of the complex interactions and compromises to give the best biological efficacy.

1. www.gov.uk/government/publications/uk-agriculturaltechnologies-strategy

Acknowledgement: The author is grateful to Dr Hannah Griffiths for experimental work and valuable comments.