Physical stability of formulations is an important parameter during formulation development, especially when testing multiple phase systems (suspensions or emulsions). A broad temperature range needs to be observed in particular for agrochemical formulations. The phase transformation from a suspension to an emulsion can be easily detected with the particle view technology of the Crystalline instrument.
The importance of stability tests right from the first experiments is shown for an example of two active ingredients active 1 (AI1) and active 2 (AI2) by Dr. Martin Viertelhaus and his team from BASF. A binary system A1 – A2 was investigated. The temperature dependence of the suspensions in different solvents was followed with the Crystalline instrument. Individual temperature ramps were performed and pictures of individual experiments taken. The pictures were used for optical analysis.
A physically stable suspension needs to show neither crystal growth nor re-crystallization, dissolution or other phase transitions. Crystal growth or re-crystallization are not acceptable in a formulation. In general, a formulation is milled to obtain fine particles that could pass through fine nozzles when sprayed on the field. Dissolution due to elevated temperature would mean re-crystallization upon cooling with the same effect. Phase transition from suspension to emulsion can be observed in the discussed example. Pictures were taken simultaneously every second during the experiment. The transformation of the suspension (solid in liquid) into an emulsion (liquid in liquid) can be observed for the above mentioned system.
The physical stability of such a suspension prepared at room temperature cannot be guaranteed at 54°C when even a binary solvent mixture with water decreases the melting point. Crystal growth, agglomeration and/or sedimentation have to be expected and actually can be observed in first “full formulation experiments”. As a conclusion, the development of a suspension concentrate (SC) formulation with these two actives can only be done with the limitation to mostly avoid formulation additives. Therefore other formulation types such as emulsion concentrate (EC), wettable granule (WG), and others might be preferred.
If you want to find out more about this subject and learn how to test your formulations, do not miss out our webinar on the 13rd of June 2017.