Spiral twists and turns and torques may be nothing new in engineering applications, but some recent developments are putting a new spin on metalworking fluid formulation and testing. Unexpected findings about effects of neutralizers on lubricity and the significance of data from twist compression tests were presented during the Society of Tribologists and Lubrication Engineers annual meeting in Atlanta.
Patrick Brutto, R&D scientist, global metalworking fluids at Angus Chemical Co. in Buffalo Grove, Illinois, revealed some surprising results from research on neutralizers during the May gathering. We looked but didnt find published work related to the effects of neutralizer chemistry on lubrication by water-dilutable metalworking fluids, so we started a project to compare specific neutralizer chemistries, he explained.
Brutto prepared model formulations from 0.5 percent tall oil fatty acid and 0.25 percent of a hydrolysis-resistant ethoxylated phosphate ester of C12-15 alcohol in deionized water. Eight bases (neutralizers) were used to adjust formulations to a target pH of 9.5. Three of these bases-potassium hydroxide, 3-amino-4-octanol (3A4O), and a blend of two primary amines (2-aminobutanol and 2-amino-2-ethyl-1,3-propanediol, or AB/AEPD)-were used to study effects of pH on lubricity in the 7.5 to 9.5 pH range, which is typical for metalworking fluids.
A tapping torque instrument equipped with high-speed steel forming taps with nitride surface treatment was used to drill holes in blocks of AA 6061, an aluminum alloy containing magnesium and silicon, at 500 revolutions per minute. Torque versus depth was measured, and the average torque was calculated from data collected between 5 and 10 millimeter tap depth. Micelle size was measured with a dynamic light-scattering instrument.
Brutto reported that the choice of neutralizer affected lubricity (average torque), and that the effect is pH dependent. For example, average torque was higher for the 3A4O formulation than the amine blend and potassium hydroxide formulations at pH 9.5. At lower pH values, lubricity improved for all three bases, but was only statistically significant for 3A4O. At pH 7.5, lubricity was comparable for all three formulations.
Brutto noted that the average micelle size (microscopic clusters of phosphate ester and tall oil faty acid molecules present in his test solutions) depended on the neutralizer and was smallest for the 3A4O formulation (27 nm) near the target pH 9.5. There was a tendency for average micelle size to decrease with an increase in pH. He reported a poor correlation between micelle size and lubricity, possibly affected by the time interval between lubricity and micelle size measurements.
Although this was only an initial foray into this area of research, we did find some interesting and important results. Both neutralizer choice and the pH of tall oil fatty acid/phosphate ester formulations can impact lubrication of 6061 aluminum alloy, Brutto summarized. The pH effect was greatest for 3A4O, which was the largest neutralizer molecule in this study. When pH was decreased from 9.7 to 7.6, lubricity improved by about 15 percent for our model formulation with 3A4O, while lubricity improved only 4-5 percent with the amine blend and potassium hydroxide.
Next, Bruttos colleague, Angus R&D Applications Technologist Soraya Kraszczyk, performed a microbial challenge test on formulations of tall oil fatty acid, phosphate ester and neutralizer. A single dose of microorganisms isolated from the field (1 million colony-forming units of bacteria per milliliter and 10,000 CFU/mL fungi) was added to biocide-free model formulations with a target pH of 9.5. These solutions were subjected to repeated cycles of five days shaking followed by two days resting. The test solutions were analyzed weekly for microbial counts, and samples were taken periodically for lubricity and particle size analyses.
Brutto reported that the 3A4O formulations maintained the most stable lubricity, measured as mean torque, for seven weeks after the formulations were inoculated with microorganisms. This is in comparison to fluids neutralized with diglycolamine, monoisopropanolamine, butylethanolamine, and the AB/AEPD blend, where torque (friction) increased substantially during this test.
According to Brutto, Maintenance of lubricity in the presence of microbiological contamination often correlates with microbial resistance. In our study, the TOFA/phosphate ester/3A4O model formulation maintained the best lubricity and microbial resistance. This is especially significant because these fluids were tested without a registered biocide, and because phosphate esters are known to support microbiological growth.
Doing the Twist
Formulators and metalworking shops alike use twist compression testing to evaluate metalworking fluids. In Atlanta, Joe Schultz, project manager, metalworking technical services, at Lubrizol Corp., shared a new approach to analyzing and interpreting the data from TCT.
Schultz explained that TCT is a lab-scale bench test that can be adapted to study lubrication in a variety of industrial applications. A small amount of lubricant is placed on the surface of a flat test specimen, and the flat end of a hollow cylinder tool is put in contact with the specimen. Pressure is applied to the cylinder as it is rotated against the specimen. Torsional force is measured and used to calculate the coefficient of friction as a function of time.
For this study, Schultz used a TCT instrument with tools made of D2 tool steel, test specimens of 1008 aluminum-killed drawing quality (AKDQ) steel, contact pressure of 138 megapascals (20,000 pounds per square inch) and tool rotation speed at 9 rpm.
Schultz explained the results: A sharp increase in coefficient of friction at the start of a test indicates rapid lube failure. More gradual changes in COF can correspond to break-in, followed by effective lubrication, depletion of lubricant in the contact and finally metal-to-metal contact and film failure.
Ted McClure of Sea-Land Chemical Co., who also presented at STLE, agreed: In many other bench tribotests, including four-ball and pin-and-vee block, lubricant replenishes the contact between surfaces undergoing relative motion. In TCT, the edges of the contact are sharp (not beveled or curved), so there is no replenishment. The small initial amount of lube in the contact is exposed to continuous sliding. As lube in the contact degrades and depletes, metal-to-metal contact occurs. Thus, TCT is particularly useful for evaluating boundary lubrication and extreme pressure performance.
Lube Breakdown According to Schultz, In the literature, there are claims that TBD [time for a lubricating fluid to break down during TCT] characterizes the ability of a fluid to lubricate and prevent adhesion. The traditional definition of TBD is the time when the coefficient of friction starts to increase rapidly and may become unstable.
But this definition is sometimes subjective and allows for variability and errors, he warned.
In some cases, it is easy to compare TCT data for fluids with similar coefficients of friction but different TBDs, he continued. However, this kind of comparison is not always clear. Statisticians at Lubrizol are analyzing coefficient of friction data during each rotation of a cylinder on a test specimen. The goal is to develop models to compare TCT data sets, using the least-significant-differences method to determine whether or not they are statistically unique.
Schultz explained, We were looking for a way to go beyond analyzing TBD for fluids. We needed to figure out a way to apply test variability to the data that the instrument provides. Rotational average was the simplest and most logical way for us to break down a coefficient of friction vs. time plot in order to incorporate test variation.
The least-significant-differences method was chosen because it is used in ASTM methods to establish repeatability. Schultz continued, Differences between two single results which exceed the least significant difference by more than what might be expected based on testing variation alone can truly be blamed on fluid performance.
The additive maker is confident about using TCT data for comparing fluids and their performance in real-world applications, Schultz reported. I think the most important aspect is that we show that time in the TCT is a useful surrogate for application severity. Not all metal deformation processes are the same, and different applications require different levels of lubrication.
Lubrizol has proposed three categories for metalworking fluids on the basis of suitability for less severe, moderately severe and most severe operating conditions in applications, he went on. In addition to using the TBD, by incorporating the concept of application severity when using and interpreting data from the TCT, we believe that we can dial in with more confidence on an appropriate product recommendation.
Rotating Out CPs
McClure, who is technical resources manager at Sea-Land in Westlake, Ohio, brought his audience up to speed on his efforts using TCT to identify possible replacements for chlorinated paraffin additives in metalworking fluids for 304 stainless steel, which is used widely in pipelines, tanks, pans and other equipment in the food processing industry, as well as in kitchen appliances.
CPs have been the gold standard extreme pressure additives for metalworking fluids used in high-performance forming, drawing and removal operations. Though it had threatened to ban the manufacture and import of medium-chain and long-chain CPs as early as 2016, the Environmental Protection Agency is currently adding these materials to the Toxic Substances Control Act Inventory prior to review. The Chlorinated Paraffins Industry Association is developing a testing program to address EPAs requirements, and the long-term availability of these additives remains in question.
McClure reminded the audience that EP additives typically are activated by pressure or temperature in order to react and form protective layers on metal surfaces. Alloys can differ significantly in their interactions with various additive chemistries.
He prepared a benchmark formulation with 10 percent of a very-long-chain chlorinated paraffin (C21 or higher, 48 percent chlorine) in oil. A second formulation contained 20 percent of a sulfur/ester/zinc dialkyl dithiophosphate additive package. TCT data showed similar coefficients of friction for both fluids on cold-rolled SAE 1008 steel. On 304 stainless steel, the benchmark performed well while the second formulation failed rapidly.
McClure presented extensive TCT results for model formulations based on a naphthenic base oil blend and a vegetable oil, both 38 centiStokes at 40 degrees Celsius. His COF and TBD results depended strongly on choices of base stocks and test metals.
For naphthenic mineral oil tested on cold-rolled steel, the best results were obtained using additive blends of a sulfurized olefin and an ethoxylated phosphate ester. This combination, as well as a blend of a calcium sulfonate and sulfurized olefin in mineral oil, gave promising results for 304 stainless steel. In general, COFs were lower and TBDs were longer for vegetable oil formulations compared with naphthenic oils on cold-rolled steel.
Mary Moon, Ph.D., is a chemist with hands-on R&D and management experience formulating, testing and manufacturing lubricating oils and greases and specialty chemicals. She is skilled in industrial applications of tribology, electrochemistry and spectroscopy. Contact her at email@example.com or (267) 567-7234.