In order to ensure that MWFs perform well in their cooling, lubrication and transport functions, they recirculate at speeds that can range from 100 gallons per minute (378 liters per minute) to more than 300 gpm (1,100 L/min). Directed jet nozzles are used to deliver MWFs to the tool-workpiece contact zone. Centrifugal pumps propel MWF through the recirculating system. These processes tend to entrain air and impart energy into the fluid.

When the resulting foam bubbles are unstable, they burst quickly-a process that contributes to mist generation, which can be harmful to workers health. (See ASTM E2889, Standard Practice for Control of Respiratory Hazards in the Metal Removal Fluid Environment.) Stable bubbles accumulate as foam, which is great for bubble baths but problematic for metalworking operations.

Because foam is mostly entrained air, and air is a poor heat conductor, foam interferes with metalworking fluids cooling performance. Moreover, air is a poor lubricant, so foam also interferes with the fluids lubrication performance. Uncontrolled foaming can build up sufficiently to overflow MWF systems, cause centrifugal pumps to cavitate (when collapsing bubbles can damage pump impellers) and block machinists views of the metal­working operation.

To control foam, MWF compounders use antifoaming agents as formulation components. Antifoaming agents are also used tankside to control foaming after it has developed in recirculating fluids.

Why does foaming occur?

Emulsifiable oil (often erroneously called soluble oils) and semi-synthetic MWF formulations contain at least 50 percent oil. The balance of these formulations includes water and functional additives. Emulsifiers are used to maintain emulsion stability. Various chemistries-including amides, ether carboxylates, ethoxylated alcohols and sulfonates-are used as emulsifiers.

Emulsifier selection is typically dictated by the MWFs hydrophilic-lipophilic balance
-the ratio of water-loving (hydrophilic) to oil-loving (lipophilic) molecules. Oil-soluble emulsifiers typically have HLB values in the 1 to 6 range. Water-soluble emulsifier HLBs range from 6 to greater than 13. Typically, two or more emulsifiers are used in combination to achieve HLBs in the 6 to 12 range. (Higher HLBs are needed as the formulations ratio of water to oil increases).

Stable emulsions are essential to MWF performance-until they arent. Water hardness (i.e. the combined calcium and magnesium concentration, reported as calcium carbonate as tested by ASTM D1126) is the primary factor affecting foaming in end-use diluted MWF. When MWFs are diluted in soft water (less than 50 milligrams of CaCO3 per liter), they are substantially more likely to form stable bubbles than MWFs diluted in harder water (CaCO3 between 50 milligrams per liter and 150 mg/L). Water harder than 150 mg/L is likely to have sufficient chloride and sulfate concentrations to cause corrosion problems.

Other factors contributing to MWF foam accumulation include:

MWF concentration too high. An emulsifier designed for a specified end-use dilution becomes too concentrated.

Unintentional air intake. MWF level in the reservoir is too low or pipe cracks cause air to be sucked into the recirculating MWF.

Excessive agitation/turbulence is created at MWF application nozzles, sharp bends are present in the system or waterfalls exist in MWF return sluices.

Contamination. Floor cleaners, washing agents and other detergent fluids can create stable MWF foam.

Historical Test Methods

Before 2013 when they were balloted to be withdrawn from the ASTM Annual Book of Standards, two test methods were available for evaluating MWF foaming tendency.

ASTM D3601 was a shake test, in which 200 milliliters of end-use diluted MWF was dispensed into a 0.5 L, wide-mouthed glass jar and allowed to come to about 25 degrees Celsius (77 degrees Fahrenheit). The jar was shaken vigorously for 40 shakes in 10 seconds, and the total height of MWF and foam was measured. This height was then remeasured at 30 second intervals until the foam had dissipated. The shake test was developed to simulate low shear conditions. However, users found that the test did not reliably predict foaming tendency under machine shop conditions.

While ASTM D3601 provided insufficient sheer to predict in-service MWF foaming tendency, ASTM D3519 created too much shear. In this method, 200 mL of end-use diluted MWF at 25 C was poured into a blender jar and blended at 4,000 to 13,000 rpm for 30 seconds. Defoamers exist as droplets dispersed in the fluid, and the high shear created by the blending step reduced the droplet size of the defoamers, leading to a loss of performance.

Neither of these methods did a very good job of predicting MWF foaming tendency in metalworking systems. In 2013, the members of ASTM subcommittee D02.L0.01 on Metal Removal Fluids and Lubricants recommended that both methods be withdrawn. The main Committee D02 on Petroleum Products, Liquid Fuels and Lubricants supported the recommendation. Although historical versions of the two methods are still available as PDF files, there are currently no consensus methods for evaluating MWF foaming tendency.

ASTM E34.50 Steps In

ASTM Subcommittee E34.50 was formed in 1992 to address a gap in consensus standards focused on improving the health and safety of the metalworking environment. The subcommittee, chaired by Fred Passman, an ASTM Fellow, currently has 13 active standards under its jurisdiction plus three new standards under development.

Members of the committee recognized that MWF foam contained microbes, biomolecules, dissolved metals and other potentially hazardous chemicals. Consequently, they concluded that foam control was as much a health and safety issue as an operational one. The committees members also recognized that no individual foaming test was likely to be universally appropriate.

They agreed that a guide explaining the considerations that needed to be taken into account when selecting a foaming tendency test, coupled with explanations of the commonly available test methods, would provide value to the metalworking stakeholder community. Committee member Justin Mykietyn volunteered to lead the effort as Technical Contact for ASTM Work Item 64558. A subcommittee ballot of the proposed new guides first draft is planned for 2020.

In development since August 2018, the proposed guide will be organized into eight sections. Following ASTMs form and style, the document will open with an introduction to MWF foaming issues. The next section will define the guides scope: primarily to provide an overview of foaming tendency evaluation protocols and their appropriate use. The scope statement will be followed by a list of references and a terminology section. The bulk of the proposed new standard will have sections that discuss the primary foaming test categories: shake, blender, recirculation (above) and air sparging test protocols.

These approaches to foam testing vary by how accurately they reflect actual operating conditions and by ease of use. For example, recirculation tests are the best model of end-use applications but require more elaborate equipment and time to perform. Shake, blender and air sparge tests are more convenient, easier, more cost effective to set up and require less test material.

For research and development projects, a combination of tests ending with recirculation is most appropriate. For quality control or condition monitoring, shake and blender testing are favored for their high throughput.

The tests also measure different aspects of foaming tendency. The shake test simulates low shear, while the blender test simulates high shear; both indicate how quickly foam will collapse after it forms. Recirculation testing measures the persistence of foam in a fluid formulation, or the foam profile over lengthy exposure to shear. Air sparge testing can also provide some insight into persistence under shear, but it is not typically run for long periods of time, and the agitation takes place in bulk liquid rather than at the surface as is common in the field.

Each test categorys section in the guide will include a discussion of the variant protocols that fall within the category, the objectives that are best achieved by each variant, and logistical, practical and technical considerations.

Industry members who are interested in MWF foaming tendency are welcome to participate in this effort. To volunteer, contact Justin Mykietyn at JMykietyn@
munzing.us. A person does not need to be an ASTM member to be part of the task force.

Once the Guide for Evaluating Water-Miscible MWF Foaming Tendency has been approved as an ASTM standard, the members of E34.50 will determine whether standard test methods are needed for one or more of the protocols reviewed in the guide.

Justin Mykietyn is the industrial fluids application manager for Munzing. He earned his Masters degree in chemistry from Seton Hall University and is an active member of STLE and ASTM. Contact him at JMykietyn@munzing.us.

Frederick J. Passman, Ph.D., is founder and president of Biodeterioration Control Associates Inc. He is chair of ASTM Committee E34 Occupational Health and Safety, and of Subcommittee E.34.50 Health and Safety of Metalworking Fluids. Contact him at fredp@biodeterioration-control.com.