Suspo-emulsions

Definition

A suspo-emulsion (SE) is a formulation containing both solid and liquid (or low melting point solid + solvent) active ingredients dispersed in an aqueous phase. It is possible using this method to put together in the same water based formulation different active ingredients with different physical-chemical characteristics.

General Method of Preparation

Defining suitable active ingredients

For the development of such formulations, the solid and liquid ingredients must be insoluble in the aqueous phase.

Effects of emulsifiers

One of the best methods to obtain good suspo-emulsions is to separately prepare a suspension concentrate with the solid active and a concentrate emulsion with the liquid or low-melting-point active (EW/EC). These two formulations are then mixed together under normal agitation. In this way the characteristics of the formulation are controlled very well. It is also important to avoid a wet-milling of the mixture solid + liquid active for three main reasons:

• The liquid active could act as a lubricant and could disturb the milling process.
• The solid active content is usually too low for a good wet-milling. It is better to mill the solid alone at the highest possible concentration.
• The emulsifier ratio and the grade of obtained emulsifiability could be disturbed (wider particle size distribution, etc.)
  by the components used for the SC.

Surfactants (emulsifier, dispersing and wetting agents) are the basic components in water-based suspo-emulsions, they play several roles in the development, the properties and the long-term stability of these formulations:

Emulsifiers, dispersing and wetting agents provide.

• wetting and dispersion of the solid particles of the active ingredient into the water continuous phase by reduction of the oil/water interfacial tension
• steric and/or electrostatic stabilisation of the micronised droplets to avoid flocculation, aggregation or coalescence
• wet milling aid
• long-term stabilisation of micronised particles
• improvement of the compatibility of the suspension with the emulsion at a concentrated or diluted state

Surfactants also serve to avoid flocculation or aggregation between solid dispersed particles and liquid active droplets. They can have the double role of dispersing agent and emulsifier.

Selection of Components

Selection of dispersing agents and emulsifiers

Surfactants must be selected with the understanding that the two formulations will be mixed. In this case, the compatibility between the different components is important.

Selection of the antifreeze

Monopropylene glycol or glycerine, used at approx 10% on total water volume in the formulation, is the most common antifreeze. The dose sharing can be done between suspension and emulsion during their preparation.

Selection of the thickener

The RHODOPOL (xanthan gum) range of thickeners is perfectly suited to provide long-term storage stability in the vast majority of cases. It needs to be added (2% water solution) in the blending phase of the two pre-mixes. RHEOZAN, a non-ionic thickening agent having rheological properties similar to RHODOPOL, shall be used when formulations have a pH < 5,0.

Selection of the antifoam

The role of this additive is to prevent foam formation during formulation preparation, water dilution, and field application. It must be chemically inert and efficient even at low concentrations.

Laboratory Formulation

Prepare the suspension and the emulsion. Mix the two formulations by stirring to obtain a homogeneous suspo-emulsion. An antifoam can be used during the process.

After this step the viscosity should be between 300 and 500 mPa.s (Brookfield 20 rpm). The final viscosity should be between 750 and 1500 mPa.s (Brookfield 20 rpm). If necessary, add under slow stirring a RHODOPOL or RHEOZAN 2% water solution to increase viscosity and eventually water to adjust the active concentration.

Control

Dispersibility: blooming effect

The blooming effect is checked by adding a few ml of the flowable formulation, using a pipette, into a test tube containing 250 ml of water of standard hardness (CIPAC) at room temperature. Simply by inverting the test tube, the suspo-emulsion should disperse.

Suspensibility

Due to the nature of the formulation, the suspensibility will exceed 85% in various conditions (temperature, water hardness or concentration). However, it is necessary to check if the formulation is stable when diluted and, more specifically, that no phase separation or hetero-flocculation occurs.

Fineness

Several types of equipment can be used:

• optical microscope to obtain a general idea of the particle size and particle size distribution
• particle size analyser laser, Counter Coulter, when accurate information on particle size and particle size distribution is needed

The average fineness should vary between 2 and 8 µm depending on the physical characteristics of the active ingredient. With a specific gravity of a solid active higher than 1.5 kg/l. The best is to go down to 3 µm.

We recommend first separately measuring the particle size of the SC and of the EW, and then the particle size of the suspo-emulsion. This provides a lot of information on the compatibility and potential long-term stability of this type of formulation.

It is also important to bear in mind that for certain active ingredients, an excessive fineness (< 1.5 µm) can greatly increase phytotoxicity, crystal growth or Laplace ripening.

Rheological measurement

The correlation between rheological parameters and different quality criteria, such as physical stability, flowability, suspensibility or long-term stability, will help the formulator to choose the right ingredients and precisely check the long-term stability of suspo-emulsions.

Different types of equipment can be used to measure viscosity to evaluate the rheological behaviour of suspo-emulsion:

•     Brookfield (RVT or LVT)
•     rheometers such as Rheomoat 115 (Contraves) or equivalent
•     dynamical rheometers (such as Carimed or RFS types)

They can be used either singularly or in combination. Sophisticated rheometers provide a lot of information on the characteristics and on the long-term stability of suspo-emulsions.

The rheological measurement for suspo-emulsions is the same as that for suspension concentrates or emulsions in water.

Dynamic rheological studies (viscoelastic behaviour) can also be carried out to obtain further information concerning the stability and the general properties of suspo-emulsions.

Active ingredients content

To avoid crystal growth and ensure long-term stability, actives must be water-insoluble. As appropriate to each product, the concentration is usually expressed in g/l of the different active ingredients.

Storage stability

The commercial formulation should be stable for at least 2 years without any significant change of viscosity, phase separation or agglomeration between droplets and solid particles. The presence of a supernatant water layer on the surface is acceptable, provided only slight agitation is needed for the re-homogenization.

Some accelerated ageing tests give a clear indication of the long-term stability:

•     tropical test: 2 weeks at +54° (CIPAC 1 -MT 46.1.3)
•     cold stability test: 1 week at 0°C (CIPAC 1 -MT 39)
•     stability at "high" temperature for two months at +45°C
•     stability at thermal shocks: samples in sealed opaque glass bottles are submitted to
      temperature cycles (24 hours at -5°C and 24 hours at +45°C) for a one or
      a two-month period.

Aged samples must be checked for no sedimentation (soft cake or caking of the active ingredient at the bottom of the bottle) and for no chemical degradation of the active ingredient. It is also important to check that the viscosity has not changed by more than 10% from the initial value. Particle size must also be checked to ensure that no crystal growth, flocculation, or droplet size increase occurred during ageing tests, especially at hot and cyclic temperatures and cycles.