Emulsifiers are surface active agents used to create a mixture of two or more immiscible liquids. They work by having a polar head group that is hydrophilic (water loving) and a non-polar tail that is lipophilic (oil loving). In most metalworking applications, an oil-in-water emulsion is desired. That emulsion requires a surface active agent (emulsifier) that has strong hydrophilic characteristics but yet is soluble in the oil needing to be dispersed. The number and size of the oil droplets that are subsequently formed, and how quickly they are formed, are largely dependent on the type of emulsifier used. For metalworking applications, this includes primarily anionic and nonionic emulsifiers. The efficiency and strength of the emulsifier system is controlled by how well the molecules line themselves up at the surface of the two immiscible liquids.
There are several laboratory methods for determining the emulsion stability of a fluid. A simple graduated cylinder test will provide you with a rough idea of the stability of your emulsion (typically to within 1ml). IP 263 Determination of stability of water mix metalworking fluids provides an even finer measurement of the same parameter for better comparisons of most emulsions.
For even greater detail as to how an emulsifier or emulsifier system will perform, the following more sophisticated test methods can be used:
The size of the oil droplet in a metalworking fluid is typically measured using one of a few methods: laser diffraction and dynamic light scattering, Coulter principle, or optical microscopy. In laser diffraction, a focused beam of light is passed though the emulsion sample and the various sizes of oil droplets scatter the beam at angles relative to the size. A volume distribution is obtained. Similarly, a dynamic light scattering method measures the incident light at a specific angle (or multiple angles simultaneously) when particles in an emulsion are illuminated by a laser. In the Coulter principle method, droplets from a microliter-sized emulsion sample are first dispersed in an isotonic medium. A pressure differential then pulls the oil particles through an orifice in the probe. An electric current passing through the orifice is momentarily interrupted, the duration of which is in proportion to the size and number of particles in the sample. In microscopy, the particles are simply viewed under magnification and their diameters determined. Oil droplet size can be related to an emulsion’s stability.
A common method for measuring the interfacial tension of a fluid is by pendant drop shape analysis. A droplet of liquid is formed on the tip of a hollow needle within a second, immiscible liquid. Gravity pulls and lengthens the drop while interfacial tension holds the drop up in spherical shape. Curvature of the drop is measured by digital camera and software. The curvature and the density of the liquids are used to calculate the interfacial tension. Interfacial tension is an indication of emulsion spontaneity since it relates the ability of the oil/surfactant blend to penetrate the water phase.
Surface tension is measured under two types of conditions: static and dynamic. A Wilhelmy Plate is used with static conditions. A solid plate of either platinum or glass is attached to a microbalance and placed into the liquid sample. As the plate is raised out of the liquid, the force required to break through the surface tension of the fluid is measured. The lower the force required, the lower the surface tension. Dynamic surface tension is measured using a bubble pressure tensiometer. An air bubble is created via a capillary in the liquid being measured. The pressure required to maintain the bubble is monitored to the point at which the bubble breaks away from the capillary opening. Both methods used together can give an indication of the metalworking fluid’s ability to wet on horizontal and vertical surfaces, foam and disperse fines.
Contact angle is the measure of how well a fluid spreads on and wets a surface. The angle is dependent on the solid surface because not all solids have the same surface energy. A drop of the test liquid is delivered to the surface using a hollow needle. The angle ? is measured over time as the droplet spreads on the surface. This method is a more direct measurement of surface wetting. Some surfaces are harder to wet than others (think Teflon® brand fluoropolymers and aluminum vs. steel).
Various analytical methods used in the field of surface science are applicable to the metalworking fluid industry. The methods described can help determine the degree of influence emulsifiers have on:
To learn more about the methods described here, emulsifiers, and their use in metalworking fluids, check out STLE’s webinar series Emulsifiers 101 and Emulsifiers 201.
Joe Schultz is a project manager for metal processing additives technical services for Lubrizol’s Industrial Metalworking Additives group in Wickliffe, OH. He provides application and troubleshooting support for customers and end-users along with finished product formulating guidance. He recently contributed to STLE’s webinar series and the Chicago Section’s Education Seminar, both on emulsifiers and their use in metalworking fluids.
Tom Oleksiak, Ph.D, research chemist at The Lubrizol Corporation contributed to this article. Coulter principle image freely licensed at Wikimedia Commons and created by Reimar Spohr. Teflon® is a registered trademark of DuPont.
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