Dry-film lubrication with PTFE

Aug. 22, 2002
Custom compounding of engineered polymers lets designers tailor materials to meet tough mechanical, thermal, and chemical challenges.

By Todd Grimm
Ellison Custom Coatings
Wartburg, Tenn.

Edited by Jean M. Hoffman

PTFE coatings are used on rubber parts, such as O-rings, to ease assembly, and with the addition of color, to aid in part identification.

Polytetrafluoroethylene (PTFE) or Teflon is synonymous with "nonstick." Although commonly used on everything from frying pans to carpet fibers, it's often overlooked as a solution for design and manufacturing challenges.

PTFE combined with various binders forms dry-film lubricants with low coefficients of friction and good corrosion and chemical resistance. These lubricants serve in applications ranging from small, rubber O-rings to room-sized, metal vessels. They can be applied with hand or electrostatic spraying and by dip spinning.

PTFE is a dry, solid polymer that by itself doesn't adhere well to substrates. It must be combined in a dispersion form with binders such as water, acrylic, epoxy, urethane, phenolic, polyamideimide. Other additives help adjust physical properties or initiate curing.

The key to a successful coating, however, is a clean, rough surface. Dirty or contaminated surfaces will compromise coating adhesion. For metals and plastics, grit blasting, phosphating, anodizing, pickling, or chemical etching work well for surface preparation. If the substrate is self-lubricated with silicone, for example, the silicone may leach out and degrade the coating's bond.

Any process used to coat the part involves the same basic steps. A thin film of material is applied to the part surface, followed by a heat curing cycle. The process repeats until the desired coating thickness is reached, typically between 0.0002 and 0.0015 in. There is little need to change the design to accommodate the coating for anything but the tightest tolerances.

Like other dry film lubricants, a key advantage of PTFE coatings is that they remain clean throughout their service life. During the manufacturing and assembly process, they eliminate problems associated with handling, conveying, and assembling parts coated with oils, greases, or silicones. Perhaps more importantly, the solid, dry film resists contamination from an accumulation of dirt and debris. This clean, lubricated surface often helps extend product life and makes service and repair efforts more manageable.

The primary benefit of applying a PTFE coating is lubricity. Depending on the material selected, the coefficient of friction ranges from 0.05 to 0.20. Applied as a primary or secondary lubricant, this low coefficient of friction is beneficial in a number of ways. As you'd expect, it cuts wear and stops squeals and rattles. But it also supplies interim lubrication during break-in, reduces binding during assembly, and provides release for tooling or parts in a manufacturing line. PTFE coatings also provide corrosion, abrasion, and chemical resistance.

An often-overlooked benefit of PTFE coatings is that they can be formulated to produce a variety of colors. A black rubber O-ring can be transformed to royal blue, and a lug nut can be a highly visible safety yellow. These colored coatings offer quick identification of parts that are similar in size, compound, or alloy. Colored parts also help ensure proper insertion and placement and give a visual appeal for decorative purposes.

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