In this two-part series, Steve Broadbent, Regional Director of Ensystex, takes a closer look at suspension concentrate technology, explaining the sophisticated chemistry behind popular SC products.
Suspension concentrates are one of the more preferred formulations used by professional pest managers. They offer performance benefits as both insecticides and termiticides, and allow manufacturers to develop sophisticated formulations with the significant performance benefits provided from top quality products; thus allowing for market/price differentiation.
In Part 1 we saw the importance of milling the crystals of the active to a uniform size of less than 5µ.
Here in Part 2 we discuss the impact of the formulation to ensure optimal performance – the product needs to remain evenly dispersed in the tank and apply evenly to surfaces.
In Part 1 of this two-part series we looked at the importance of particle size of active crystals in the production of a suspension concentrate. The next issue to address is the suspension of those particles and the protection of the formulation.
This is achieved through the use of adjuvants. The use of the correct adjuvants will have a significant impact on the final product presentation. Adjuvants are used to:
- Maximise the uniformity of the suspension and ensure that the milled particles of active ingredient are homogeneously suspended and dispersed throughout the liquid. This ensures the correct amount of active is applied to every tank mix.
- Optimise the formulation viscosity to ensure the product is easy to pour, and all the contents of the container can be used.
- Ensure that the active(s), once the product is diluted with water, remains evenly dispersed and suspended in the tank mix. If the suspension ‘drops out’ when diluted, a professional pest manager might at times be only spraying water.
- Ensure the diluted product has the required ‘wetting’ properties to allow even coating of the surfaces to which it is applied.
- Protect the product from ‘breakdown’ in the container. Products need to be ‘stable’ to ensure they remain optimally effective for at least two years from the date of manufacture.
Thus, in addition to the active ingredient, we typically find the following components in a quality suspension concentrate:
- Stabilisers to suspend the active ingredient particles;
- Anionic dispersants to ensure the active ingredient particles do not agglomerate together;
- Non-ionic surfactants (wetting agents) to ensure the product evenly coats a surface;
- Anti-microbials to protect against biological degradation of the water-based product and the natural stabilisers that are used;
- Buffering agents to protect the pH of the product and improve stability; and
- Water.
Stabilisers
The stabiliser system is one of the more critical aspects of the formulation. The stabiliser(s) keep the milled active ingredient particles in suspension and ensure they do not sink to the bottom of the bottle; or to the bottom of the spray tank, once the product is diluted with water.
They also address the rheology (viscosity/flow) of the product to ensure it is easy to pour.
The most commonly used stabiliser is xanthan gum. Xanthan gum is a polysaccharide (carbohydrate) secreted by the bacterium Xanthomonas campestris. Xanthan gum is also commonly used as a food additive, and is produced by fermentation of sugars. After fermentation, it is precipitated, dried, and ground into a fine powder.
There are many different grades of xanthan gum available. The higher quality grades will produce a more stable suspension and lead to improved product performance. Of course these improved gums are more expensive, so there exists a trade-off between improved product performance and product cost.
Dispersants
The dispersants ensure that the milled particles of active ingredients do not agglomerate together. Particle agglomeration is a natural phenomenon of small particles and refers to the formation of clusters in a suspension. Small particles dispersed in the liquid phase stick to each other and spontaneously form irregular particle clusters, flocs or aggregates. This phenomenon is also referred to as coagulation or flocculation and creates an unstable suspension.
Once particle agglomerations have formed, they will not easily break apart, so prevention is vital. In the course of agglomeration, the agglomerates will grow in size. As a consequence, they settle to the bottom of the container, which is referred to as sedimentation. Alternatively, a colloidal gel may form in concentrated suspensions, which changes its rheological properties, usually making it too viscous (thick).
Ideally the milled active particles should remain dispersed (and suspended) for long periods of time, in both the product container and in the spray tank after dilution. It is the combination of dispersants and stabilisers in the formulation that deliver these properties.
Surfactants
To understand the need for surfactants, it helps to understand how water works. Water molecules have a negative and a positive charge, like a magnet. If you put several water molecules together, the positive and negative forces attract each other. Consequently, the molecules on the surface of a water droplet are held with more force than those within a water droplet, which creates surface tension.
It is the surface tension properties that can prevent the wetting of surfaces and soils being penetrated by the liquid. Surfactants are used to overcome this surface tension. Most surfactants have a water-loving head (hydrophilic head) and a water-hating tail (hydrophobic tail). Therefore, a surfactant contains both a water-insoluble (or oil-soluble) component and a water-soluble component.
These components of a surfactant reduce the surface tension, allowing the formulation to be more evenly dispersed on a surface and the pesticide to better reach its target. However, if too much surfactant is present, the water will run-off without depositing the active correctly.
Non-ionic surfactants do not have a charge and are the most commonly used surfactants for professional pest control products. When used properly they do not harm plants, remain stable, and effectively break water surface tension.
The dynamics of the surfactant(s) used are also of great importance for preventing foaming and they play an important role in the stability of the diluted chemical emulsion.
Other adjuvants
Antimicrobials are required to stop degradation of the product. Various bacteria and fungi can grow within a suspension concentrate due to the presence of water and food (xanthan gum). This can cause strong odours and discolouration of the product. Ideally the microbial blend should protect against both gram-negative bacteria and gram-positive bacteria. However, often this is an area where some formulators save costs by not adding any antimicrobial, or choosing to only control gram-positive bacteria.
A further important additive is propylene glycol (anti- freeze), which is added to prevent the water component freezing in cold weather conditions.
Buffers are used to correct imbalances in the pH of the water. Buffers usually contain phosphoric acid or one of its salts, which will stabilise the pH during storage.
Steve Broadbent, Regional Director, Ensystex
