Continued from page 18

The rate of creaming can change during the life of the MWF, and by repeated testing; the MWF performance can be continuously monitored.

The reasons for the change in Creaming Stability are numerous, but certainly include contamination from tramp oils, as well as the hardness of the water used to make up the emulsion.

The “Interfacial properties” determine the behaviour, at the surface of the droplets of oil in the water phase. Changes in the interfacial behaviour over time can reduce the MWF’s stability and performance. Colloidal Systems change in the following ways.

1) Droplet size Increase (Coalescence) 2) Emulsion splitting (Creaming)

The traditional method for testing MWF emulsions is to visually observe changes in a static sample over time, however this method is subjective and very time consuming.

There are analytical methods, such as Laser Diffraction Particle Sizing, Zetapotential, but they require the MWF emulsion to be diluted prior to testing. The dilution of the sample can affect its behaviour, and could mask an underlying problem with the emulsion.

There is now a much improved way to measure the MWF stability under “real” conditions which is critical to obtaining meaningful information, and this can be achieved using Static Multiple Light Scattering (SMLS) instruments.

By scanning the complete sample from the bottom to the top, repeated every 30 seconds, any colloidal changes in the sample can be measured and quantified.

Testing the sample at working concentration and in real-time allows any destabilisation to be immediately detected and quantified.

Size Increase An increase in the average droplet size of the emulsion is very hard to detect using visual observations and is often missed. As the increase in size continues over time the density contrast between the oil phase and the water phase become larger – which eventually will lead to the start of migration (Creaming) and the eventual splitting of the emulsion.

Figure 3: Size Increase (Coalescence)

Tramp oils can contaminate the emulsion system and “leach” additives into the continuous water phase. This can then interfere with the surfactant shells (Micelles) around the water droplets and significantly impact the stability of the MWF emulsion.

Metal Cations (i.e. sodium and magnesium), which are found in hard water, may interfere with the electrostatic repulsions of the surfactants (emulsifiers) and allow the formation of “lime soap”. This “lime soap” reduces the concentration of surfactants in the continuous water phase and thus prevents the surfactants carrying out their tasks of protecting and stabilising the emulsion. As the stabilisation breaks down, initially the droplets or micelles will increase in size, eventually leading to creaming and the splitting of the MWF.

Figure 2: Example of a SMLS Tubiscan Reading Head with a near infrared (880 nm) LED light source

Repeatedly testing the MWF throughout the course of its life means that the MWF can continue to be used until it is absolutely necessary to conduct a complete change of fluid. This in turn means a reduction in cost (as full fluid changes will become less frequent), downtime and lost production.

Continued on page 22 20 LUBE MAGAZINE NO.156 APRIL 2020

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