Based on their formulation characteristics, metalworking fluids (MWFs) can be classified into three major categories: straight oils, emulsified types, and fully synthetic types. Accordingly, they are referred to as straight metalworking fluids, emulsified metalworking fluids, and fully synthetic metalworking fluids.
Among these, emulsified metalworking fluids can be viewed as emulsified straight oil products. Based on the size of their emulsion droplets, they are further divided into soluble oils (>0.1 μm) and micro-emulsions (<0.1 μm). Compared to the relatively simple structures of straight oils and fully synthetic fluids, emulsified metalworking fluids have much more complex compositions. The diluted working fluid is an O/W (Oil-in-Water) system, which means that the perspective for analyzing its formulation structure is also much richer.
To date, there are three known mixed systems containing oil, water, and surfactants: emulsions, microemulsions, and solubilized micellar/reverse micellar solutions (swollen micellar solutions). The vast majority of MWF products belong to the first two categories, while a small number of low-oil products fall into the third.
1. Droplet Size and Optical Properties of Emulsions
In traditional soluble oils, the water content is generally less than 10%, while the oil content can reach up to 80%. When static, the concentrate often presents as a W/O (Water-in-Oil) type or a reverse micelle state. However, to ensure that phase inversion occurs smoothly upon dilution with water—transforming from W/O to a stable O/W working fluid—the comprehensive HLB value of the surfactant package must be carefully balanced (usually between 8 and 12). It cannot be too low; otherwise, severe oil separation will occur during on-site dilution.
The appearance of the diluted soluble oil is usually a milky-white, opaque liquid. The diameter of the dispersed phase oil droplets roughly ranges from 0.1 to 10 μm. Since the wavelength of visible light is 0.4 to 0.8 μm, light reflection in the emulsion is highly significant. Additionally, the lower limit of general soluble oil droplet size (about 0.1 μm) is very close to the resolution limit of an ordinary optical microscope (about 0.2 μm).
For micro-emulsified oils, the water content can be as high as 30% to 60%, and the oil content is generally ≤30%. Its concentrate is an O/W type, and the diluted working fluid is mostly a transparent liquid. The oil droplet diameter is usually between 0.05 and 0.1 μm.
Interestingly, some micro-emulsions with an oil content above 50% may also have a water content below 20%. In this scenario, the concentrate is a W/O type, and the dilution is a blue-white, semi-transparent liquid. This represents a transitional emulsion (fine emulsion) with droplet sizes ranging from 0.1 to 0.4 μm, situated between a standard emulsion and a true microemulsion.
2. Winsor Phase Behavior in Microemulsion Systems
When studying the phase composition of microemulsion systems, scientist P.A. Winsor discovered three possible phase behaviors:
(1) Winsor I Type (“Lower Phase Microemulsion”): In an O/W microemulsion system, a two-phase system consisting of a microemulsion and excess oil may occur. Because the oil has a lower density, it sits at the top of the container, while the microemulsion phase is located at the bottom.
(2) Winsor II Type (“Upper Phase Microemulsion”): In a W/O microemulsion system, a two-phase system consisting of a microemulsion and excess water may appear. Here, the microemulsion phase is positioned at the top of the container.
(3) Winsor III Type (“Middle Phase Microemulsion”): This is a phenomenon where three phases coexist in the system (excess water, excess oil, and the middle microemulsion). The top layer is oil, the middle is the microemulsion, and the bottom layer is water. There are clear interfaces between each layer.
3. The True Driver of Phase Transitions
The mutual transformation among the three Winsor types mentioned above is fundamentally not just a simple matter of oil-to-water ratios. Instead, it depends on the changes in the interfacial affinity (the Hydrophilic-Lipophilic Balance, or HLB) of the surfactants between the oil and water phases.
For the exact same “oil-water-surfactant” system, you can shift the phase composition among Type I, III, and II by adjusting physical and chemical parameters. The most effective methods include changing the temperature, altering the electrolyte concentration, or introducing co-surfactants. These adjustments alter the spontaneous curvature of the interfacial film, thereby triggering a phase transition.
Of course, during actual formulation development, if the goal is to obtain a stable, single-phase microemulsion (Winsor IV type) at a specific “oil-water ratio,” optimizing the surfactant combination and ensuring a sufficient dosage remains the ultimate secret for formulators.
4. Achieving Formulation Balance: The Role of Alcohol Ether Carboxylates (AEC)
As mentioned, optimizing the surfactant combination is the key to maintaining a stable single-phase microemulsion. In practical formulation, finding a surfactant that can stably adjust the interfacial curvature while resisting complex water conditions is crucial.
This is exactly where Alcohol Ether Carboxylates (AEC) demonstrate their unique industrial value. As a specialty surfactant combining both non-ionic and anionic properties, AEC helps formulators precisely tune the HLB value of the system to ensure smooth phase inversion.
More importantly, it provides exceptional emulsion stability and hard water tolerance. For R&D technicians striving to develop premium micro-emulsions (Winsor IV type), incorporating AEC surfactants into the formulation is an ideal strategy to simplify the additive package, lock in the stability window, and prevent phase separation in harsh machining environments.
(If you are looking to upgrade your emulsification system and achieve ultimate stability, feel free to contact our technical team to request AEC samples and formulation guidance.)
