Machine Design

Ensuring Plastic and Lubricant Compatibility

George B. Mock III
Executive Vice President
Nye Lubricants
New Bedford, Mass.

Synthetic lubricants for plastic parts can be formulated to reduce wear, noise, and backlash. They can lower power consumption for robust applications and accommodate the low torque of micropowered gear boxes. And they can even withstand temperatures from –70 to 200°C. But when selecting a lubricant for plastic or elastomer parts, chemical compatibility is a must.

For instance, a thermostat manufacturer found its products malfunctioned because the ABS gear trains failed. The problem was not the design but the lubricant — a light, ester-based grease. Ester molecules are so similar to ABS resins that they act as slow solvents; in this case, the lubricant destroyed the gear train.

In another situation, a manufacturer looking to reduce costs on a hinge chose a less-rigid grade of plastic and altered the molding process. The lubricant, which worked well before the change, was not altered or retested. While synthetic hydrocarbons usually are compatible with most plastics, the new plastic had less cross-linking and it physically absorbed some of the lubricating oil. The oil absorption, coupled with internal stresses common to most spring-loaded hinges, caused the new design to crack within the warranty period.

There are some general guidelines to follow for selecting a synthetic base oil for plastics or elastomers. Fluoroethers and silicones are compatible with most plastics. Synthetic hydrocarbons generally can be used with all plastics but not elastomers. And esters should be used with extreme caution with polycarbonate, polyester, polyphenylene oxide, polystyrene, polysulfone, polyvinyl chloride, Buna S, Butyl, natural rubber, and neoprene. Ultimately, however, the only way to ensure compatibility is through rigorous testing — especially under high stress, high temperature, and poor elastomer/plastic conditions.

The accompanying reference chart rates the compatibility of synthetic base oils with more than 20 plastics and elastomers, serving as a guide for lubricant selection.

For the synthetic base oils listed, polyphenylether (PPE) has a viscosity index (VI) of 40 to 60 and features good lubricity and low volatility. Fluoroether or perfluoropolyether (PFPE) has VI = 100 to 350 along with excellent inertness, lubricity, and temperature range.

Halocarbon, including chlorotrifluor, has good chemical resistance, load-carrying capability, and low-temperature performance. Silicone, including dimethyl, phenyl, and halogenated versions, has VI = 200 to 650 and low volatility.

Esters, along with diesters and polyolesters, has VI = 120 to 150 and feature excellent lubricity and load-carrying capabilities. Polyglycol or polyether has VI = 160 to 220 and a low cost per pound. Synthetic hydrocarbons, including polyalphaolefin (PAO) has VI = 125 to 250 and good lubricity.

© 2010 Penton Media, Inc.

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