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Semi-Synthetic Metalworking Fluid Pros, Cons, and Options

In previous blogs, we’ve discussed advantages, disadvantages and best applications for the four main metalworking fluid types: straight oils, soluble oils, full synthetics, and semi-synthetics. In this edition, we’ll focus in on semi-synthetic fluids. Let’s ask – and answer – some questions about these products.

What Is a Semi-Synthetic, Exactly?

Semi-synthetics are essentially a hybrid of soluble oils (which are petroleum based) and full synthetics. They contain synthetic polymers, emulsifiers (to hold lubricants in suspension) and petroleum or mineral base oils. The amount of oil in them is usually less than 30%. When diluted with water, semi-synthetics create a micro-emulsion; particles of lubricant suspended in the fluid will have an average particle size of less than a micron.

Most semi-synthetics also contain corrosion preventatives and biocides. Some may contain extreme pressure additives for certain machining applications. They tend to have a milky white or bluish, near translucent appearance. 

Why Were Semi-Synthetics Developed?

In the 1950’s, crude oil prices began rising, making traditional oil-based MWFs more expensive. Synthetic lubricants were developed in response to this trend, and in the early 1960’s, semi-synthetics were created to bring customers the best of both worlds with the cooling benefits of a soluble oil and the low cost of a full synthetic.

What Are the Advantages of Using Semi-Synthetics?

As a hybrid, semi-synthetics offer excellent lubricity from the emulsified oil, but also deliver superior heat reduction because of their high water content. Despite that water, they usually have good rust prevention characteristics, and they’re also clean.

One other advantage is their versatility. A wide range of formulations is possible, and this, combined with the ability to change their concentration, makes them useful in almost any machining application. In addition, semi-synthetics are useful when machining non-ferrous metals; full synthetics are restricted to ferrous metals only.

Semi-synthetics are more stable than soluble oils, which makes them an excellent choice for facilities with large central fluid distribution systems.

What Are the Issues?

Notice we say “issues,” not disadvantages, because with proper care, nearly any vulnerability in a fluid can be accounted for and dealt with.

Semi-synthetics can foam, so it’s essential that shops eliminate sources of entrained air. You’ll need to monitor your water carefully as well, because soft water will promote foaming and hard water will tend to harm the fluid’s stability. Salts in hard water damage the emulsifiers in the MWF, causing lubricants to come out of the solution.

Tramp oils can affect these MWFs, because they are either caused to enter the solution (changing the characteristics of the fluid) or leach emulsifiers and other ingredients out of the solution.

Lastly, semi-synthetics are vulnerable to biological growth, because of their water mix and because emulsifiers and base oils are a food source for bacteria.

How Have Semi-Synthetics Improved?

A new generation of chemical engineering has further improved semi-synthetics. Previously, only thin viscosity oils and esters could be incorporated to form a stable and homogenous mixture. The NuSol® line from Chemtool incorporates advanced emulsifiers that allow high viscosity components, making these formulas better overall and especially well-suited to modern high-speed operations. Using pre-formed emulsions also better disperses dirt and maintains not only on the part, but in the entire machining operation.

In short, just as semi-synthetics offer the best of full synthetics and soluble oils, NuSol® products offer the advantages of semi-synthetics, full synthetics, and soluble oils: High lubricity, excellent cooling, biostability, ease of maintenance, low foam and no sticky residues.

What’s Next?

Interested in learning more about NuSol® or setting up a semi-synthetic trial in your shop? Speak with one of our experts.

Talk with a Metalworking Fluids Expert

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