Suspension concentrates

The suspension concentrates (SC), also known as "flowables", consist of insoluble solid active ingredients dispersed (normally at high concentration) in water.

Suspension concentrates have shown a rapid development in the past, mainly due to their several benefits when compared to other formulations:

•     no dust
•     no problem of toxicity or flammability due to solvents
•     good efficiency due to the smaller particle size
•     low packaging volume

The following properties are required for this type of formulation:

•     long-term storage stability (no settlement, stable rheological behaviour, etc.)
•     fairly low viscosity (easily pourable and readily dispersible in water)
•     good stability of the diluted dispersion at the application rate of use

The choice of surfactants (wetting, dispersing, compatibility agents) is essential to meet these requirements.

General method of preparation

Defining suitable active ingredients

Following characteristics are necessary to formulate solid active ingredients in a SC form:

• Very low solubility in water (crystal growth can occur, especially at high temperatures).
• Excellent chemical stability to hydrolysis. Stabilizers, buffers, reducing agents etc. can be added when necessary to improve the chemical stability of active ingredients.
• Melting point higher than 50°C in order to allow easy wet grinding and storage stability.

Effect of surfactants

Surfactants are basic components in suspension concentrates. They play several key roles in the physical-chemical characteristics and properties of such water-based formulations:

• Wetting of the solid particles of active ingredients
• Wet milling aid
• Dispersing properties of solid particles in the continuous phase
• Long-term stabilisation of the micronised particles
• Stability of the micronised particles at a diluted state during application on field

Stabilization

After milling, the dispersed particles have an inherent tendency to flocculate irreversibly as a result of the attractive forces namely "Van der Waals forces" which depend on the active ingredient and on its particle size distribution. The adsorption of a surfactant onto the solid particles generates repulsive forces which contrast the attractive ones.

There are two types of these repulsive forces, which can be involved either singularly or in combination:

Electrostatic repulsion

The electric charge given by ionic surfactants (anionic, cationic or zwitterionic) absorbed onto particles causes electrostatic repulsion between the dispersed particles. 

The strength of this force depends on:

• the dielectric constant and the chemical composition of the continuous phase
• the particle size
• the electrolyte concentration

If the ionic strength (K) of the continuous phase increases, the electrostatic strength decreases: flocculation becomes possible (suspensions lose their stability). The strength of the electrostatic force can be evaluated by Zeta potential measurement vs pH, ionic strength or temperature.

Steric repulsion

This type of stabilisation is achieved by using a non-ionic surfactant with a long hydrophilic chain adsorbed onto solid particles. When two particles approach, the chains interact with each other, providing a repulsive force.

The strength of this force depends on:

• the quantity of surfactant adsorbed onto the particles
• the interaction between the hydrophilic chains of the surfactant and the continuous phase (solubility, compatibility vs temperature etc.)
• the length of the chains
• the temperature

Steric repulsion is a strong force but sensitive to temperature (for ethoxylated surfactants: the higher the temperature the weaker the force). The strength of this force can be evaluated by measuring the quantity of surfactant adsorbed onto the particles.

Choosing a surfactant is difficult and is the key step in the development of an SC formulation. An optimum choice can only be made by experimentation in order to find surfactants strongly adsorbed onto the formulated active ingredients and able to generate the highest repulsive forces between the solid particles in the chosen continuous phase.

Furthermore, to obtain a long-term stability of suspension concentrates in various climatic conditions, it is recommended to use the two types of stabilisation (electrostatic and steric) combined with a gravitational stabilisation given by thickeners.

Gravitational stabilisation

Besides the stabilisation of the dispersed solid active ingredient by modifying the interfaces (with surfactant or polymers), the use of a thickener (xanthan gum, guar gum, bentonites, etc.) is necessary to obtain a good long-term stability and to avoid settlement in various climatic conditions.

In the case of RHODOPOL (xanthan gum), a pseudo-cross-linked structure is obtained in water due to the form of the xanthan macromolecule and reversible interactions between these molecules even at low concentration. This phenomenon, called gravitational stabilisation by continuous phase structuration, should avoid sedimentation and caking during long-term storage of suspension concentrates. 
 

Selection of the components

Selection of wetting agents

Most of the dispersing agents themselves show wetting properties. Thus for some actives which are easy to wet it is not necessary to add a specific wetting agent, while for the others the addition of a wetting agent is a must.

Selection of dispersing agents

For the majority of the active ingredients, phosphoric esters and sulphated derivatives of ethoxylated adducts give the best results due to the ethoxylated chain and its anionic termination, which generates both steric and electrostatic repulsion. In some cases, it may be necessary to use only nonionic surfactants for compatibility problems or non-ethoxylated surfactants for specific formulations, crystal growth problems, etc.

Selection of the antifreeze agent

Monopropylene glycol is the most common antifreeze agent used. The recommended rate is around 10% of the total water content in the formulation.

Selection of the thickener

The main role of this ingredient is to keep active materials in suspension and to prevent sedimentation during long-term storage: it must therefore have a very strong suspending capability even at low concentration. Moreover, it must have good chemical compatibility with the active materials, good chemical stability and should cause the minimum increase in dynamic viscosity. The RHODOPOL (xanthan gum) range of thickener is suitable to provide long-term storage stability in the majority of cases. RHEOZAN, non-ionic thickener having similar rheological properties as Rhodopol, shall be used in acidic formulations at pH lower than 5,0.

Selection of the antifoam

The role of this additive is to avoid foam formation during the preparation of the suspension and also during water dilution and field application. It must be chemically inert and efficient even at low concentration. Non-ionic silicone antifoams perfectly fulfil such specifications, being efficient at low concentrations at any pH.

Selection of the preservative

Many different bactericides may be used to protect the products against bacteria.

Control methods

Dispersibility: the blooming effect

The blooming effect is checked by pouring a few mL of the suspension concentrate with a pipette into a test tube containing 250 mL of standard hardness water (CIPAC) at room temperature. By simply inverting the test tube, the formulation should disperse.

Suspensibility

The CIPAC test referred to wettable powders should be applied. Generally, the suspensibility will exceed 85% in various conditions (temperature, water hardness or concentration).

Fineness

Several types of equipment can be used:

• optical microscope
• particle size analyser

The average particle size should be about 4 µm with a maximum of 5% over 10 µm for active materials of density lower than 1.5. For active ingredients of higher density, it is preferable to go down to 3 µm with a maximum of 5% between 5 and 8 µm. It is also important to bear in mind that for certain active ingredients, an excessive fineness (< 1.5 µm) can greatly increase phytotoxicity and crystal growth. For some active ingredients, this fineness can improve the biological efficacy. This must to be checked in any case.

Viscosity and rheological behaviour

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

Different types of equipment can be used to measure viscosity or to evaluate the rheological behaviour of suspension concentrates:

• Ford cup or equivalent
• Brookfield (RVT or LVT)
• rheometers
• dynamical rheometers

They can be used either singularly or in combination. For example, a Ford cup or a Brookfield is convenient for control measurement. More sophisticated rheometers during formulation development are necessary in order to have a better prediction of the long-term stability of the formulation.

Content of active materials

This needs to be checked with appropriate methods for each active ingredient. The concentration is usually expressed in g/l.

Storage stability

Commercial formulations should be stable for at least 2 years without any significant change of viscosity and without sedimentation of the active. However, the presence of a supernatant water layer on the top (which can limit the formation of a film or crust on the surface) is acceptable, provided only a slight agitation is needed for the mixture to re-disperse.

Some accelerated ageing tests give an accurate indication of the long-term stability of SC formulations.

•     tropical test: 2 weeks at +54°C (CIPAC 1 -MT 46.1.3)
•     cold stability test: 1 week at 0° (CIPAC 1 -MT 39)
•     stability at "high" temperature for two months at +45°C
•     stability at thermal shock: 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 two-month period.

Tests can either be carried out alone or in combination, according to the long-term stability requirements. Aged samples have to be checked: no sedimentation (soft cake or claying of active ingredient at the bottom of the bottle) and no chemical degradation of active ingredients should occur. It is also important to check that viscosity has not changed by more than 10% compared to the initial value. Particle size must also be checked to ensure that no crystal growth or flocculation occurred during ageing tests, especially in high or cyclic temperature tests.

Industrial production

On an industrial scale, it is possible to make suspension concentrates using:

•     either pre-milled powdered active material (down to 100 µm)
•     or the still-damp filter-cake of the active directly from the last step of the synthesis.

This second method is much superior from both an economic and technical point of view:

•     gains in energy (drying and pre-milling are avoided)
•     grinding is easier: the active materials are not caked as they are after drying
•     less foam formation: there is no incorporation of air, as is the case when powders are used.

N.B.In order to be able to use this second method, it is necessary to have a filter cake with an active material content at least 15% higher than the desired final concentration. We have found that adding 0.1 to 0.2% of dispersing agents to the slurry prior to filtration facilitates this operation and increases the content of active materials in the cake by 15 to 20%. Moreover, the presence of dispersing agents facilitates subsequent redispersion.