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Metalworking Fluids (MWF): Overcoming Stability Issues

aug 19 2019

Metalworking fluids (MWFs) are a family of industrial liquids used to reduce the temperature of and/or lubricate metal workpieces while they are being machined. Cutting and grinding processes can generate significant amounts of heat and friction, which can cause undesirable thermomechanical side-effects: burning, deformation, smoking, surface roughness, thermal warping, etc. If a workpiece burns it may be rendered unfit for downstream application, while off-gases and smoke represent a respiratory health risk for personnel. The thermomechanical drawbacks of conventional metalworking can represent logistical and health and safety issues if not addressed. 

Formulating Different Types of Metalworking Fluids (MWFs)

MWFs are complex formulations containing mineral (petroleum) or organic oils and a range of additives and stabilizers, which may be emulsified in as much as 50% water. The three main classes of MWFs are:

- Straight oils: Also known as neat oils, comprising either petroleum, animal, marine, or vegetable oils and are formulated without water dilution.

- Soluble oils: Comprising 30-85% severely refined – or hydrotreated – petroleum oils, with emulsifiers and water dilution.

- Semisynthetic MWFs: Comprising 5-30% petroleum oils in a 30-50% water dilution.

Alternative MWFs that contain neither mineral nor organic oils are known as synthetic fluids. These typically use detergents rather than oil to lubricate and cool workpieces during machining.
The compositions of different MWF classes are nominal, and there are many different components that may be introduced during formulation to improve the functionality and stability of the product. These include, but are not limited to antimist agents, antifoams, biocides, corrosion inhibitors, dispersants, stabilizers, etc. As many as 20 different ingredients may be required to achieve the desired end-use properties of the MWF, such as optimal lubricity. In many cases, multiple different additives from the same category may be required. This can create a plethora of compatibility problems between the base fluids and the additives, or between the additives themselves. 

Issues with Metalworking Fluid Stability

Conventionally, emulsified MWFs are prepared by mixing different compositions and individually assessing the effects of various additives. While this method is useful in determining the impact of individual ingredients on functionality and stability, it is time-consuming and costly. It also fails to offer true quantitative insights into multiple factor interactions between distinct ingredients or phases in the MWF under storage or use conditions. 

 
Chemical and physical instability issues in MWFs can reduce performance and detrimentally affect product quality, and there are myriad of destabilizing phenomena that may affect them: blooming, coalescence of oil droplets, emulsion breaking, foaming, phase separation, etc. Although emulsion composition and component affinity are the underlying factors of destabilization, the external force also plays a crucial role in colloidal breakdown. The force of pumping can accelerate instability mechanics, which means products will destabilize quicker in real-world use conditions. This property directly correlates to MWF efficiency. 

 

Typically, metalworking fluids would be tested for chemical or physical instability by leaving a sample on a test bench for an extended period (days, weeks, months) and examining it for physicochemical changes. Testing for instability in actual use conditions is more challenging. Centrifugation and general agitation have been used in the past to predict mechanically driven instability, such as destabilization pumping conditions. Alternative methods of instability testing have been developed to test for at-rest-stability and meet the demands of MWF development and quality control.  

Overcoming Metalworking Fluid Instability

Current technologies used for MWF quality testing (NMR, DLS) rely on measuring the droplet size distribution but can be costly, do not provide direct aging information and may be limited by droplet size range. Achieving a true measure of stability requires an analytical solution that offers a direct measure of occurring phenomena and does not rely on mechanical agitation.

TURBISCAN® technology is a huge step forward for measuring stability directly and allows to move away from test methods like salt titration or ionic conductivity. It is the reference stability analyzer, trusted worldwide to determine the stability of emulsions and dispersions at the earliest possible stage with a wide droplet size range available. This S-MLS based technology monitors all instability phenomena over time, providing mean droplet size and global stability rankings with a direct method of measurement and has already been proven a useful solution for Metalworking Fluid stability testing and efficiency analysis.

Turbiscan Analyzers from Formulaction

Formulaction supplies a comprehensive range of TURBISCAN® analyzers for stability analysis of concentrated dispersions, including oil-water emulsions for MWFs. These analyzers help in optimizing new product formulations and assessing the effects of additives on the emulsion physical stability thus providing formulators with a cost-effective R&D tool for productive and sensitive metalworking fluids performance enhancement. The result: safer and more efficient products that improve metalworking efficiency, worker safety, and sustainability. 

If you need any more information about performing MWF stability tests with Formulaction products, simply contact a member of the team today. 

References:
Design of Experiment Reduces Development Time for High-Performing Metal-Cutting Fluids

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