When you say the word wax, everyone tends to think of candles, shoe polish and car wax, says Peter Pyka of Germanys Keim Additec Surface GmbH. Lubricant additive may not spring to mind as quickly, he concedes, yet wax deserves a place of honor in the formulators chemistry set, especially for what it can bring to metal-forming lubricants.
Speaking in early January to the 21st International Colloquium Tribology, Pyka explained that waxes possess good release and lubrication properties as well as a friction-reducing profile that can shift the metalworking process towards hydrodynamic friction. In the hydrodynamic regime, friction is drastically reduced as moving parts are completely separated by a lubricant film. With less contact or friction between the tool and the workpiece, theres also less temperature buildup during the forming process, he said, resulting in much better surface quality of the finished part. And in terms of workplace safety, waxes exhibit few dermatological problems and no regulatory issues-another plus.
This makes waxes useful in many metalworking processes such as drawing, forming and punching. Which type to select, however, is the first hurdle to harnessing waxs frictional benefits for a drawing or stamping lubricant, said Pyka, the specialty chemical companys business development manager in Kirchberg, Germany. Its easy to divide waxes into two simple groups-natural and synthetic-but after that the choices can become quite confusing.
On the natural side are fossil waxes that are solvent-extracted from petroleum, coal, lignite and peat, as well as non-fossil waxes obtained from animals (beeswax, spermaceti from whales, lanolin from sheep, etc.) and vegetable waxes such as carnauba, candelilla and jojoba, he enumerated.
Synthetic waxes likewise have many permutations, starting with partially synthetic ones, which can be natural waxes modified by chemical reactions such as estearification or amidation. One such example is the fatty acid amide wax, bis-stearyl ethylenediamide.
The full-synthetic slate includes polyolefin waxes (essentially polyethylene or polypropylene); Fischer-Tropsch gas-to-liquids material; and waxes made via synthesis routes such as oxidation, polymerization and esterification. (The key difference between polyethylene plastics and polyethylene wax is molar mass, which for the wax version is adjusted to a much lower range by the addition of regulators during the high-pressure, high-temperature polymerization process.)
While theres no universally accepted definition of a wax, Pyka said the mostly comprehensive description-and the one adopted for the purpose of trade in the European Union-came in 1974 from the German Association for Fat Science:
Solid but kneadable at 20 degrees Celsius (room temperature);
Consistency can be soft and plastic, brittle or hard;
Coarse to finely crystalline;
Transparent to opaque, but not glassy;
Should be polishable under slight pressure;
Drop melting point must be above 40 C, and it should melt without decomposition;
Low viscosity-no more than 10 Pascalseconds at 10 degrees above the melting point;
Consistency and solubility are strongly temperature-dependent.
A material must fulfill all points of this definition-or its not a wax, Pyka emphasized to the audience at the Technische Akademie Esslingen in Ostfildern, Germany.
Over 200 field applications depend on wax additives: release agents, papermaking, can and coil coating, plastics, building materials, lacquers and paints, cosmetics, pigments and too many others to name. Some of these waxes have very high viscosity, and others have almost none, but the upshot is that most days we have something to do with wax in our lives, Pyka said, and that includes wax-additized lubricants, which can be either water or oil based.
In metalworking lubricants, he went on, wax additives are found principally in non-cutting machining operations, including wire drawing, extrusion molding, deep drawing, bending and punching operations. The waxes typically come in the form of flakes, granules, powders and micronized powder, especially the polymeric materials such as polypropylene wax and oxidized high-density polyethylene wax. Whatever the shape, he stressed, in order to achieve the ideal of hydrodynamic friction, a wax destined for use in additive amounts in water-based lubricants needs to have high melt viscosity.
High melt viscosity is what assures that when opposing surfaces are in motion, the lubricant at the metal surface will move at higher velocity than the lubricant at the center of the load-bearing film layer. The thickness of this interfacial layer is directly proportional to the viscosity of the lubricant film, Pyka explained. Because the parts are completely separated, no wear can occur.
Another crucial wax property to consider is high adsorption, or adhesion to the metal surface. To boost this property, formulators may reach for a more polar, short-chain wax such as ethoxylated ester wax. Even though they may show almost no melt viscosity (see table), these small molecules are highly functionalized, and help give good release-lubrication effects. They show big polarity at the surface, and in some cases can have high solubility in water, Pyka related. Another option is ethylene-bis-stearamide (EBS) wax, which is useful in wire drawing, deep drawing and aluminum rolling and extrusion processes.
Lubricant formulators usually find that a combination of the wax additive types-high viscosity and short-chain functionalized-yields the best results, he observed.
Another issue to be aware of: Because these wax additives are water-based dispersions, end users may have to adjust some of their metalworking processes and lubricant application habits. For one thing, users also must have a water-based, alkaline cleaning process, Pyka stated. On balance, though, theyll find that wax-additized metal-forming fluids are more skin-friendly, pose fewer regulatory problems and have no or very little labeling requirements. Customers will also see less wastewater disposal issues, versus additives like chlorinated paraffins, and get better surface quality on the workpieces.
To familiarize fluid suppliers with these additive options, Keim Additec has developed several guide formulations.
For deep drawing of stainless steel based on polyalkylene glycol, Pyka suggested starting with just three ingredients: 85 percent PAG (type: a 4:1 EO:PO based on pentaerythritol); 10 percent EBS wax micropowder (6.5 micron); and 5 percent extreme-pressure additive, such as neutralized phosphoric acid ester.
To create a deep drawing concentrate, formulators begin with fatty acids and fatty acid esters, oxidized HDPE, EP additive and PAG, and then amp up the wax content with EBS wax and modified ester in dispersion-to as much as 45 percent of the concentrate.
To serve an aluminum forming operation, the guide formulation also relies on several types of wax:
5 percent tallow fatty acid,
25 percent phosphoric acid ester (potassium-salt),
30 percent modified ester wax dispersion,
27 percent water,
13 percent EBS wax dispersion.
The above formulation, Pyka noted, would be suitable for a multi-stage process where storage is needed in between the process steps. An added bonus: After the process, this lubricant can be washed off easily with water.
For wire drawing-a multi-step process where wire thickness is reduced by pulling the wire strand through a series of draw plates-dry lubrication with calcium stearate has been a longtime favorite. In this case, EBS wax powder offers a way to further lower the coefficient of friction; however, it cannot be applied directly. Blenders first must mix the melted wax with a filler. A formulation for this process might incorporate 10 to 20 percent EBS wax powder, 20 to 40 percent calcium stearate and 40 to 60 percent calcium hydroxide as the lubricant carrier. As with any dry lubricant, Pyka added, the particle size distribution must be tightly controlled to get the optimum results.
Keim Additec has numerous other wax additive projects underway, including research on combining modified ester wax with a fatty acid ester, Pyka said. This experimental combination does not dry completely (like a mineral oil formulation), and it is easy to clean with water and alkaline cleaners. The solid content is about 50 percent and melting range is 70 C. Early test results are showing very high lubricity, he revealed.
Were also working on powder metallurgy and indirect lubricant applications, Pyka said. One of these is backward extrusion of aluminum, where the aluminum is mixed with solid lubricant, EBS wax micropowder or an EBS dispersion, then pressed in the extruder.
A final project is the companys first oil-soluble wax additive, to be launched later this year. Pyka said he looks forward to reporting more on that chemistry at the next International Colloquium Tribology, in January 2020.