In this method, emulsification is carried out in a closed vessel. It has many advantages over the wax to water route in that it offers:
- Greater uniformity and consistency of production.
- Production time / batch size is quicker / larger
- Higher solids content easier to produce
- Essential for some formulation types
In pressure emulsification there are two techniques which can be adopted, these being the “direct” and “indirect”methods. The latter is by far and away the better procedure but requires, however, two vessels rather than a single one as in the case of the first procedure.
The reactor should be of stainless steel and double jacketed for steam heating and water cooling. It should be fabricated sufficiently thick to withstand an internal pressure of 5 kgs / cm2 (approximately 65 psi ) and fitted with pressure and temperature recorders together with safety and drain valves. The stirrer can be of the propeller, turbine or anchor design and it is not necessary for high speed / shear.
a.) Direct
This method is suitable for the preparation of non-ionic emulsions but not, for good quality anionics or cationics.
Water is charged to the reactor and stirring is commenced. The other ingredients are charged and the order has no significance. Heating is begun and the filling port is closed. Heating is continued until a temperature of 125 -130ºC is attained. This condition is maintained for a period of 15 -30 minutes under continuous stirring. Heating is stopped and cooling is effected until ambient temperature is reached. The cooling is preferably done by evacuating the contents (whilst hot and under pressure) directly to atmospheric pressure via an external heat exchanger of a coil or plate design. This shock cooling step ensures a finer particle sized emulsion and also enables a wider margin of error to be built in to the procedure.
b.) Indirect
In this procedure the same means is followed except that a portion of the water charge is omitted such that a high solids concentrate is initially processed. Following a maintenance of the temperature for 15 – 30 minutes, the second water charge is injected at a controlled rate into the bulk of the concentrate, whilst it is still under pressure, at just below the boiling point of water. It is better if the temperature of the injected water is at the same temperature as the concentrate although, this necessitates a second pressure vessel. Following this, the mixture may be cooled immediately and the preferred means is via an external heat exchanger.
In general, this method allows greater lot to lot reproducibility and gives better results for all emulsion types but particularly for anionics and cationic.