Initial Design

4.3.3.1 Factors

When formulating an emulsion, there are a wide range of formulation and process

variables available and the Formax is designed to allow a systematic exploration of

them. As a result initial trials were designed in which a number of the potentially

variable factors were kept constant. The factors chosen to remain constant consisted

of:

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fold as discussed previously. The scrapers prevent viscous fluid from

sticking to the sides of the reaction vessel and improve the heat transfer so

that temperature can be controlled more accurately (when rotating they do so

in an opposite direction to the impeller) as well as preventing the entrainment

of air. The separate motor can drive them within the range of 20rpm to

200rpm.

 The total volume of reagents within each reaction vessel was kept constant at 60mL. This value was chosen, partly because is it recommended by

Chemspeed not to exceed this value when operating at the greatest impeller

speeds, and partly because preliminary trials indicated that some foaming

can be expected and some free space was required to prevent the reaction

vessel from overflowing.

 The temperature of each reaction vessel and its contents was kept at a constant 25°C using a combination of a heater/chiller unit and a livetime

feedback system. If the temperature dropped below the required value, warm

oil was circulated around the vessel to raise the temperature appropriately,

should the temperature exceed the required value coolant was circulated

around the vessel to lower the temperature until the target value was reached.  At the start of addition of the reagents the impeller was set initially at

200rpm. The slowest impeller rotation speed is 100rpm but the current

required to turn at this speed continuously is so small that the rotor speed

fluctuates around the target value. As a result an impeller speed of 200rpm

was chosen as this could be maintained consistently.

 The surfactant is added first to the reaction vessel and then the water is added. The reagents are mixed for a minimum of 10 minutes to ensure the

119 surfactant is evenly distributed throughout the water phase. In its

concentrated form SLES is very thick and exists as a lamella phase

surfactant. Upon dilution the lamella phase forms a hexagonal phase which is

considerably more difficult to disperse. At concentrations below about 25%

(by volume) the hexagonal phase reverts to a lamella phase again which is

relatively easy to disperse. As a consequence it is essential to allow

sufficient mixing time to fully disperse the SLES. Once the SLES was fully

dispersed to form a dilute surfactant solution the oil was added to the vessel.  Once all reagents were in the reactor vessel, then the impeller would begin

rotating at the required speed.

 The vessel contents were mixed for 80 minutes. Droplet size reduction during emulsification is typically very rapid initially, but the rate tends to

drop with time eventually approaching a steady state. In practice droplet size

continues to reduce but the rate is so slow that it has effectively reached

steady state.

 Once mixing was completed the contents of the vessel was decanted into a sample bottle

 Particle sizing was done using a Malvern Mastersizer X as described in section 2.6.

4.3.3.2 Variables

The impact of altering the following factors was investigated:

 Oil viscosity – three different viscosities of silicone oil were used in order to observe whether the Formax was capable of dealing with all of them. The

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 The phase volume of oil within the emulsion ranging from 5 – 60%.  The phase volume of surfactant ranged from 0.25 – 8%. The highest

surfactant percentage of 8% ensures that at the lowest volume of water, the

concentration would not exceed 25% in the water and thus would avoid the

hexagonal phase.

 A range of impeller speeds; 1000rpm, 2000rpm, 4000rpm & 6000rpm. It would have been ideal to continue increasing the impeller speed

logarithmically, however, the impellers within the formulation vessels are

limited to a speed of 6000rpm.

 Impeller type; rotor-stator & dissolver disk impeller.

4.3.3.3 Design of Experiments

After deciding upon which factors were to be varied and which were to be kept

constant, the information was placed in the appropriate fields within MODDE –

Design of Experiments software. Trials were selected using a minimised D-

efficiency value to ensure that residuals were even across the experimental space.

The software suggested a list of 22 experiments for each impeller. This method is

known as screening as it allows the researcher to ascertain which factors generate the

greatest response by taking every combination of highest and lowest variable and

generating a matrix of experiments, along with three centre points consisting of

experiments with the middle value for each variable used.

4.3.3.4 Particle Sizing

Once formulated the emulsions were analysed using a Malvern Mastersizer X with a

45mm lens with the focus of the results produced being primarily the D[3,2] value

121 To analyse samples using the Mastersizer, the sample is added to a small volume

dispersion unit and the red light is scattered by the particles and droplets within the

sample onto the detectors and this then calculates the size of them, as discussed in

section 2.6.

The presentation code interprets the readings that are received by the detector

dependent on the fluid being analysed. It interprets the data appropriately and

produces a result of what the size of the sample is based on the refractive index (RI)

of the sample and the location on which the detector was hit with the laser light.

When analysing the samples using the Mastersizer X the presentation code used

consisted of a refractive index of 1.403 (RI of silicone oil), absorption of 0.001

(translucent) and as the samples being made were polydispersed oil droplets in water

the refractive index of water made up the last part of the presentation code; 1.330.