Chlorinated paraffins are among those rare chemicals that perform as extreme pressure additives in the most demanding metalworking applications. As soon as mid-2017, however, the U.S. Environmental Protection Agency may outlaw most CP additives, leaving the metalworking industry searching for alternatives that wont sacrifice performance or tool protection.
End users rely on lubricating fluids to rapidly machine metal parts to closely defined tolerances and maximize the life of valuable, custom-made tools. To be effective, lubricants must form and maintain protective films between those parts and tools, even under extreme pressure. Otherwise, asperities – microscopic bumps – on the rubbing surfaces will come in contact and form localized hot spots where metal melts, welds and causes permanent damage. Extreme pressure additives prevent this damage.
Chlorinated paraffins excel as EP additives because these molecules fit between the surfaces of tools and parts, react with iron atoms at hot spots, and form sacrificial iron-chloride films that allow moving parts to glide instead of sticking together. Most chlorinated paraffins could vanish from the formulators toolbox next year if EPA decides to ban medium- and long-chain CPs, based on its ongoing assessment of environmental risks.
The heat is on metalworking fluid formulators to find replacements for medium- and long-chain CPs before the threatened ban goes into effect.
Consultant Neil Canter, Ph.D., of Chemical Solutions in Philadelphia, regards this effort as a work in progress. Blenders are leaning on additive suppliers for answers, he told LubesnGreases last month, and additive suppliers are investigating options based on nitrogen, sulfur, phosphorus and ester chemistries, as well as new, innovative additives that may be able to replace medium- and long-chain CPs. Canter expects that multiple companies will be undertaking this challenge now and into the future.
At the 5th International Metal Removal Fluids Conference, in Chicago in late September, Brett Wessler, research manager at Lubrizol in Wickliffe, Ohio, outlined a new approach to the puzzle. He suggested operational severity categories to classify the likely ease of replacing CPs in metalworking applications (Figure 1). Operational severity depends on characteristics of parts (metallurgy, size) and operating conditions (type of operation, tool pressure and speed), he explained.
Wessler went on to compare five alternative chemistries to CPs:
Sulfurized olefins, with high active sulfur
Sulfurized fats, with low active sulfur
Phosphorus compounds
Overbased sulfonates, colloidally dispersed in oil
Polymeric esters
Lubrizol evaluated all of these chemistries in 20/80 additive/oil blends using a Microtap Tapping Torque Test. For cut tapping and form tapping of AISI standard 1018 carbon steel and 316 stainless steel, the five additive blends were equivalent to long-chain CPs, and showed promise as replacements in Category 1 operations.
Wessler then described Twist Compression Tests for five fluids formulated with various additive blends, this time on 1008 steel under 50 MPa pressure. The coefficient of friction remained low and stable during the two-minute test for a CP control fluid and for a CP/sulfurized olefin blend. However, he pointed out, the friction started to increase strongly after 65 seconds for a sulfurized olefin/overbased sulfonate formulation; after 85 sec. for a mix of sulfurized olefins, polymeric esters and sulfurized fats; and 90 sec. for blends of sulfurized olefins, polymeric esters and phosphorus compounds. Even so, such additive blends could be good enough in some metalworking applications, he pointed out.
Increasing the twist compression to 69 MPa for 1008 steel, Lubrizol next tested an experimental synthetic fluid that contained a blend of polymeric ester and sulfurized olefin additives versus a CP emulsifiable oil stamping fluid. In this laboratory test, friction data for the experimental fluid were encouraging but higher than for the chlorinated product. In a field trial of this experimental fluid, a 3,000-ton press stamped 4,400 parts with no defects. Operation conditions were identical for the experimental fluid and the original chlorinated stamping fluid, i.e., Category 1 severity.
Moving along to Twist Compression Tests on 304 stainless steel, which is a harder metal with different surface chemistry compared to 1008 steel, Wessler said the chlorine-free fluids failed after just five to 10 sec. at 50 MPa, versus 40 sec. for the CP control. More encouragingly, when pressure was ratcheted up to 69 MPa, the coefficient of friction for fluids based on a new, experimental chlorine-containing EP additive was comparable to the CP control. From this, Wessler concluded that current additive chemistries may not be adequate to replace CPs for certain severe operations, and other chlorinated chemistries may be the best alternatives to CPs at the current time.
In another presentation, Thomas Rossrucker, global director of application technology at Rhein Chemie Additives in Mannheim, Germany, focused on sulfonated molecules as CP replacements. Sulfur atoms can adsorb to metal surfaces and react to form iron-sulfide films that are similar to iron-chloride films formed by CPs.
Rossrucker reviewed six tribology tests that are often used to compare extreme pressure additives, such as the four-ball weld test, four-ball wear test, FZG, Microtap and others. Each additive chemistry type, including CPs, performs better in some tests than others, he noted. For example, sulfurized-triglycerides with active sulfur enhanced four-ball weld test results, while polymeric esters improved four-ball wear results, relative to CPs.
Next, he evaluated various polysulfide/polymeric ester combinations (5 percent total load) in a paraffinic base oil. The top performer here was a 50/50 blend of polysulfide and polymeric ester that gave a weld load of 5,500 Newtons in four-ball EP tests and a 0.58 mm wear scar in four-ball wear tests.
Rossrucker also measured the effects of milling speed on tool life. In this case – using titanium nitride-coated steel tools, AISI 4140 steel parts and 15 percent CP fluid – tool life decreased as cutting speed increased. Rossrucker explained that this trend was due to higher temperatures and faster degradation of the CP. With a sulfurized EP additive, he stated, tool life did not decrease at higher speeds.
Finally, Rossrucker compared six additive blends in field trials using a grooved or comb-like broaching tool (titanium nitride-coated high strength steel). As shown in Figure 2, field data for wear did not correlate with four-ball results from the lab. In fact, as he showed, the fluid with the highest four-ball weld load and the smallest four-ball wear scar (Fluid C below) gave only modest protection against angular wear during the broaching trial. Conversely, a blend of sulfurized ester, olefin, polysulfide and ashless antiwear agent (Fluid F) had a lower weld load and larger wear scar – but superior protection against angular wear during broaching.
Rossrucker concluded that it is not a simple matter to replace CPs in most metalworking fluids. Nevertheless, blends of sulfur and phosphorus additives can be optimized to meet and sometimes exceed CP performance in cutting and forming fluids.
Some of the most challenging metalworking applications involve titanium and other hard metals. Steve Griffiths, senior marketing technical specialist for metalworking at Afton Chemical in Manchester, U.K., discussed an innovative laboratory technique for evaluating additives for hard-metal machining. For decades, the aviation industry has struggled to mill titanium parts for aircraft such as the Lockheed A12 Blackbird, he said. Chlorinated EP additives are essential because titanium has very poor heat transfer properties and temperatures can be excessive at tool-to-part contacts.
Afton is one of 30-plus sponsors of the Advanced Manufacturing Research Centre affiliated with Sheffield University, U.K. In Chicago, Griffiths reported that work is underway at AMRC to develop a robust test method to screen alternatives to medium-chain CPs for machining titanium and other hard metals. The basis for their approach is the Taylor Equation for Tool Life Expectancy, a technique originally developed to evaluate tool materials. A Taylor curve is a log-log plot of cutting speed vs. tool life where the slope characterizes the wear mechanism.
The group at AMRC defined tool life as the time it takes to accumulate 0.2 mm of tool wear in a controlled milling experiment. First, they designed and validated a lab-scale milling apparatus and measured Taylor curves for a base fluid. Then they compared several additives at a 5 percent treat rate at several milling speeds.
So far, a certain phosphate ester shows particular promise to extend tool life while milling titanium, Griffiths revealed. Afton continues to develop new chemicals for AMRC to evaluate as replacements for medium-chain chlorinated paraffins in fluids for machining titanium.
With these and other developments, the stage is set for breakthroughs in EP additives for the next generation of metalworking fluids.
Mary Moon, Ph.D., is a physical chemist with hands-on R&D, management and problem-solving experience in the lubricating oil and grease and specialty chemicals industries. Contact her at marymoonphd@gmail.com or (267) 567-7234.