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FORMULATION TECHNOLOGY

Steve Broadbent, regional director of Ensystex, looks in depth at formulation technology, explaining how and why certain ingredients are added to formulations used to control pests.

A pesticide formulation consists of both active and inert ingredients. The active ingredient (AI) functions as the pesticidal component; the inert ingredients include the carrier and adjuvants.

The active ingredient is always listed on the product label. The type of formulation is also usually given. The formulation type, and more particularly, the choice of the carrier(s) and adjuvants, will greatly affect the performance of the product.

Consequently two products with the same active ingredient(s) and the same formulation type on the label may perform very differently.

Pesticide active ingredients in their unformulated or technical grade state are not usually suitable for pest control. Manufacturers mix in other ingredients to ‘formulate’ the pesticide into a usable final product. Over the next few issues of Professional Pest Manager we will explore some of the more common formulations used in urban pest management.

A pesticide formulation is a homogeneous and stable mixture of active and inert ingredients that makes the final product simpler, safer, and more efficacious to apply to a target pest/situation.

Even though there are no chemical reactions involved, there are many aspects of chemistry present in developing a quality formulation. Some of the chemistry involved is the thermodynamics of mixing, phase equilibria, solutions chemistry, surface chemistry, colloids, emulsions and suspensions.

A pesticide formulation will typically consist of the active ingredient (ai), referred to as the ‘Active Constituent’ by the Australian Pesticides and Veterinary Medicines Authority (APVMA); a ‘carrier’, e.g. water, an organic solvent, mineral clay, etc.; adjuvants including surface-active ingredients, such as stickers and wetting agents; and/or other ingredients, such as stabilisers, dyes, and chemicals that improve or enhance the pesticidal activity of the product.

Pesticide active constituents have a range of solubilities. Some dissolve readily in water; others, only in oils; and some are relatively insoluble in either water or oil. The solubility characteristics of the active and the intended end use of the product will determine the formulations that will deliver the active to produce the best results.

Looking at liquid formulations, there are three concepts to recognise in order to understand what is available.

Solutions

A solution occurs when a substance is dissolved in a liquid. The components of a true solution cannot be mechanically separated. Thus a true solution does not require agitation to maintain its activity and stop it from settling out. Most solutions are clear in colour.

Suspensions

A suspension is a mixture of tiny (typically 5-20 microns) solid particles dispersed in a liquid, usually water. The solid particles are not dissolved in the water, which means the mixture requires agitation to keep the particles evenly distributed throughout the liquid phase. Most suspensions will have a cloudy appearance and the product label will direct you to shake well before using.

Emulsions

An emulsion is developed when one liquid is dispersed (as droplets) in another liquid. Each liquid retains its original identity. Some degree of agitation is required to form and maintain the emulsion, though less so than in the case of a suspension. Emulsions usually have a milky appearance. In emulsion concentrates, the active constituent is usually dissolved in an oil-based solvent, often forming a solution. When the emulsion concentrate is mixed with water, an emulsion of oil droplets in water is formed. An emulsifying agent formulated within the emulsion concentrate will stop the emulsion from separating out.

Within each formulation there are a range of adjuvants that are required to optimise the performance of the product. This is where companies can ‘cut corners’, perhaps omitting required adjuvants to save on costs, but at the expense of performance. Adjuvants are essential to optimise the performance of the active constituent. Two products may have the same formulation type, e.g. a suspension concentrate; and contain the exact same amount of the active constituent, from the same source; yet they will perform very differently, depending upon the type and quality of the adjuvants used within the product formulation.

Typical adjuvants used in urban pest management products include:

Surfactants

These are also referred to as wetting agents. Surfactants physically alter the surface tension of the water droplets allowing the product to spread out evenly over the target surfaces. The surfactant requirements of urban products are very different to those used in agricultural situations.

In agriculture there is often a requirement to penetrate the waxy cuticle of a leaf. (In the picture above, the leaf on the left is without surfactant, the leaf on the right is with surfactant.) Such a product in the urban environment is likely to run off vertical surfaces too easily, reducing efficacy due to an inadequate loading of the active on the treated surface.

Many years ago I launched a suspension concentrate product in Australia that was successful in the urban market. The company launched the ‘same’ product, at least the same brand name, in New Zealand. The product was a total failure. Investigations showed that, for reasons of production economy, the New Zealand subsidiary had used their animal health formulation. This was the same active and the same formulation type (suspension concentrate). However, it had a greater surfactant content since it had been designed to penetrate the fleece of sheep. Changing to the Australian pest control formulation resulted in similar market success to that achieved in Australia.

Surfactants are classified by the way they ionise or split apart into electrically charged atoms or molecules. A surfactant with a negative charge is ‘anionic’, while a surfactant with a positive charge is ‘cationic’, and one with no electrical charge is ‘non-ionic’. The performance of an active constituent formulated with a non-ionic surfactant will be very different from how it performs when formulated with a cationic or anionic surfactant. This is where the science, or art, of pesticide formulation really shows the difference between products. Selecting the wrong surfactant can significantly reduce the efficacy of a pesticide product and/ or damage treated surfaces, or minimise the penetration of the active through the soil in the case of a termiticide.

Stickers

Stickers are adjuvants that improve the adhesion of solid particles to target surfaces. These adjuvants will decrease the amount of active that washes off during rain or cleaning programs, and some will slow down the degradation of the active by sunlight. Thus the use of stickers can markedly improve the residual performance of the active.

Extenders

Extenders function like stickers, by retaining the active longer on the target area, slowing evaporation, and inhibiting degradation by sunlight.

pH modifiers

Sometimes referred to as buffers, pH modifiers are required to protect the active constituent from degradation in the formulation. Most actives are more stable, i.e. will last longer, meaning a longer shelf life in the bottle, when the pH is between 5.5 and 7.0 (slightly acidic to neutral). Above pH 7.0 (alkaline or basic), most actives start to break down. pH modifiers are used therefore to optimise the shelf life of the product.

In developing products for Australia, manufacturers have to prove the product will have a minimum shelf life of two years. The normal approach for showing this is to run an accelerated study. Two samples from the same batch are used. One is analysed for the active constituent content and the physical parameters of the product when it is freshly produced.

The other sample is stored for 14 days at 54 degrees Celsius’. It is then subjected to the same testing regime and a report, including HPLC scans is produced. The product must remain physically stable after the elevated storage period with the active constituent remaining within +/- 10% of the target rate.

De-foaming agents

These are used to prevent a frothy ‘head’ forming in the spray tank. This is often the result of the type of surfactant used, just like soap forms a froth.

Thickeners

They are used to increase the viscosity (thickness) of spray mixtures. These adjuvants help to control spray drift and slow evaporation after the spray has been applied.

Adjuvants will be discussed further when we look at the dynamics of each formulation type.

Formulation chemists need to consider many factors when selecting the adjuvants required to optimise the performance of an active in a given formulation. One of these of course is cost. Like everything in life, ‘you pay for what you get’. The best adjuvants cost more money. Consequently, lower priced products may not contain the best adjuvants, or may have them omitted all together!

Steve Broadbent, Regional Director, Ensystex