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Technology Notebook: Extreme-environment lubricant maintains extrusion cleanroomTechnology Notebook: Extreme-environment lubricant maintains extrusion cleanroom

March 8, 2005

5 Min Read
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The male portion of the cup bearing where electrostatic shock was oxidizing lubricants.

Cleanrooms for extrusion can be compromised by airborne contaminants emitted when electrostatic shock oxidizes grease on a key bearing. By applying synthetic grease designed for extreme environments, contamination and associated maintenance costs can be halted.

At its plant in Trenton, MI, Solutia Inc. extrudes the plastic inner layer used in making laminated safety glass in automobile windshields and architectural glass. The extruded product is wound into a roll on a winder arbor, at one end of which is a large cup-type bearing.

The bearing presented a persistent problem. Electrostatic shock from the winding process was oxidizing the grease on the bearing. Although the bearing would eventually be damaged from the lack of lubrication, the real problem was that black flakes of oxidized lubricant were contaminating the Class 10,000 cleanroom.

Engineers tried both petroleum-based and synthetic greases. Several greases, including some identified as high-performance greases, gave essentially identical results when exposed to electrostatic shock?they oxidized and generated black flakes.

The grease that solved the problem was designed for use in extreme environments. Its base stock is a polyphenyl ether.

Lubricant Classes

Most lubricants fall into one of two classes, those with a base stock that is refined directly from petroleum, and those with a synthetic base stock. Additives can tweak the performance of either type of lubricant by increasing lubricity, for example, or increasing corrosion resistance, but additives cannot greatly alter the inherent limitations of the base stock.

All of the greases tested had adequate lubricity, but none was built on a base stock that could withstand the high-energy electrostatic shock that the bearing was experiencing. The base stock in polyphenyl ether lubricants, however, is designed to withstand much greater stress without decomposing.

What puts polyphenyl ethers in a class by themselves is the extraordinarily high resonance energy of the polyphenyl ether molecules. It takes a great deal of energy to decompose the molecule, or to oxidize it. The numerous greases that engineers had tested all had much lower resonance energies?meaning that the electrostatic shock from the winding operation quickly oxidized the molecules of the base stock and created the black flakes that were dispersed into the cleanroom.

It makes no difference to a polyphenyl ether lubricant what form the energy takes. It can be electrostatic shock, heat, or even gamma rays; up to very high levels, the lubricant remains unaffected. This is why polyphenyl lubricants are used in environments such as nuclear reactors?as well as on earth-orbiting and deep-space satellites and spacecraft?where high levels of cosmic radiation are the extreme factor.

Polyphenyl lubricants have other useful features in addition to their robustness in extreme environments. They tend to give a high level of protection from rust and corrosion. They have a very low vapor pressure, meaning that they do not disperse into the environment. They are also resistant to chemical attack and to moisture.

A Cleaner Cleanroom

Before the use of the polyphenyl ether grease, the typical service life of the grease on the winder arbor was about two days. After that point, the grease would oxidize. It was not uncommon to have a reduction in lubricity that would interfere with the chucking of the arbor. In most cases, the chrome plating on the arbor end was damaged, and re-plating became necessary every six months. But the biggest problem was airborne contamination by the oxidized grease.When technicians first applied the polyphenyl ether grease known as Santovac 6 GB to the arbor ends, they did so without removing the lower-performance grease that was already there. This mixture, which was left in place for three weeks, provided a test of the real abilities of 6 GB. No loss of lubricity, and no oxidation happened during those three weeks. The 6 GB probably isolated the lower-performance grease, just as polyphenyl ether lubricants do with other contaminants and with moisture.

After three weeks, the arbor end was cleaned and lubricated with 6 GB alone. During four months of continuous operation, involving repeated exposure to the electrostatic shock that had oxidized all previous greases, the new grease remained unaltered and fully operational. The grease has now been used on the winder arbors for more than one year. Technicians who previously relubricated the arbor end every day or two have discovered that one machine can be lubricated for an entire year with about 100g of 6 GB.

According to Santovac, the manufacturer of 6 GB, the polyphenyl ether base stock has an upper-limit thermal stability of 250°C. While this temperature is well above the temperature at which most other synthetic lubricants decompose, it is probably an understatement of the real thermal limit, which is probably 300°C or higher.

It is the resistance not only to heat but to other forms of energy that sets polyphenyl ether lubricants apart. The extrusion application involves no extreme temperatures. The winder arbors operate at ambient temperature but subject the lubricant to a level of electrostatic shock that made previous lubricants ineffective. For 6 GB and other polyphenyl ether lubricants, this has little or no impact on performance.

Editor?s note: Rick Urban is a process engineering specialist at Solutia Inc., and Sibtain Hamid is a technology manager at Santovac Fluids Inc.

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