The Network Challenge: Oil Dispersion Development

Date:04-16-2015

The delivery of crop protection products is most commonly achieved by dilution of a formulated material into a spray tank. Selection of which formulation type is appropriate for the product is governed principally by the physical chemical properties of the active ingredient(s) it contains, in tandem with the delivery performance demand, and more often than not, strategic marketing requirements. For example, most contact pesticides are formulated as granules or suspension concentrates, whereas for systemic compounds, the preference would be to have them in a dissolved form to allow for increased bioavailability.

In recent years, more and more classical solvents have been subject to toxicological and regulatory pressure, examples such as DMF, xylene and naphthalene based solvents, have been removed them from the formulator’s toolbox. Concurrently, we understand that many of the new potential active ingredients coming through development pipelines with the major agrochemical companies suffer from relatively high melting points and unfavourable logP values, rendering them in solubility terms akin to “brick dust”. For these reasons, oil dispersion formulations have become increasingly popular. An oil-based suspension concentrate (OD) is a stable suspension of an active ingredient(s) in an organic fluid, which may contain other dissolved active ingredient(s), intended for dilution with water before use. [1]

An OD possesses all of the advantages of a liquid formulation and also allows the formulation of aqueous sensitive active ingredients. In addition, oils are often used as adjuvants to help to optimise the bioefficacy of an active ingredient sohaving the ability to formulate in oil maximises the built-in adjuvant potential.

Figure 1 illustrates the principle components of an OD in relation to their functional properties. The left-hand side illustrates the “in-can” formulation whereby the active ingredient is stabilised by a non-aqueous dispersant predominantly through steric stabilisation mechanisms. This dispersant has a high affinity for the active ingredient and is soluble in the oil phase. Other surfactants are typically included in the system, including wetting agents to assist in milling preparation, an aqueous dispersant and oil phase emulsifiersfor dilution. Additives, for example penetration enhancers to offer maximum biological efficiency can also be used. Upon dilution several processes are required to occur spontaneously. The oil continuous phase is diluted and emulsified plus the active ingredient is re-stabilised by the aqueous dispersant.

The performance of an OD is therefore best characterised by its shelf stability, dispersion stability and its ability to easily transfer from the container. Fitting with these requirements, the ability to structure the oil phase is fundamental. For this, a rheology modifier is used to provide physical stability by increasing the viscosity of the liquid phase to prevent sedimentation and syneresis.

For the formulator, selecting a rheology modifier is not a simple task. Several factors should be considered in relation to the design of an OD formulation, including the type of solvent, the interactions with the emulsifiers and other surfactants, the robustness in activation of the thickening and the temperature sensitivity of the final system. A typical OD would be expected to show shear thinning and thixotropic behaviour.

Common rheology modifiers for ODs include organically modified bentonite clays, fumed silica and associative thickeners such as polyurethane technology. All of these materials have limitations. The performance of clay can be affected by the polarity of the oil and interaction with surfactants and electrolytes. Other disadvantages include the formulation having a broad shear thinning range and issues with scalability because the structure development occurs during the milling process. Silica is a low density material, so a large volume is required for a relatively low weight percent and is an inhalation hazard. The structuring effect silica provides is dependent on the polarity of the system. Non-polar environments are preferred since polar materials introduce additional hydrogen bond interactions and disrupt the temporary 3D network. Hence, performance is affected by ionic surfactants.Polymer molecules are often adsorbed onto the silica surface and the mobility of the polymer chains are restricted. Polyurethanes often have the drawback of having a small effective window of activity, selective to the concentration.

With all of these factors in mind, effectively designing a stable yet pourable OD formulation can be particularly challenging. Atlox Rheostrux™ materials have been shown to give excellent performance in formulation. These materials are both polymeric structuring agents; Atlox Rheostrux 100 is a polyester block copolymer and Atlox Rheostrux 200 is a polyamide polymer. Both of these materials provide structuring through not only a hydrogen bonded network but secondary interactions with the oil in hydrophobic pockets created within the 3D framework. Figure 2 illustrates the rheological advantages of Atlox Rheostrux 100 and Atlox Rheostrux 200 in a 630g/L copper oxychloride OD formulation.

Figure 2. Rheology profiles of formulations containing Atlox Rheostrux products compared to the standard clay structured benchmarkformulation. Superior structuring is observed from Atlox Rheostrux 100 and 200.

The rheology profilesshow that similar rest viscosity can be achieved in formulations structured with Atlox Rheostrux 100 or 200 compared to a standard clay benchmark. This resulted in better storage stability and suspensibility performance (CIPAC MT46.1.3 and MT180, 14 days at 54 ⁰C respectively). Importantly, the formulations all yield under similar applied force but those containing the Rheostrux agents shear thin earlier rendering them easier to mill and process. For example, at a fixed torque of 200 µN.m the formulation structured with clay maintains a higher viscosity, 224.7 Pa.s. This limits the flexibility for inclusion of additional clay to assist poor storage stability and suspensibilty performance testing which was observed for this formulation.

In summary, fine tuning all of the components in an OD system is a real challenge requiring formulation expertise and a toolbox of complementary materials. Croda’s unmatched range of additives and adjuvants and unique formulation expertise help crop protection customers get the best performance out of their active ingredients, enabling farmers to get the best yields for their crops.

If you would like to hear more on this subject, Croda are hosting a live eSeminar on 20th April. The eSeminar will explore OD formulations further and there will also be a live Q&A session where you can directly ask Kathryn Knight your questions. To register for the event click here.


[1] Food and Agricultural Organization of the United Nations

Acknowledgement: The author is grateful to James Flavell for experimental work and valuable comments.