← All articles

Epilame

Robin Israel ·

An oil drop placed correctly during a service should stay there for years. This does not happen on its own: on clean steel, brass and ruby, watch oil spreads out and creeps away. Epilame prevents this. With good epilame treatment, an oil drop remains in place for decades.

An epilame is an extremely thin layer on a component that prevents oil and grease from spreading. The name comes from Greek and means “little skin”. Treating the parts is called epilame treatment. Epilame is not a lubricant: it changes the surface, not the oil.

Incidentally, the idea is almost a hundred years old: Woog presented the first epilame in 1925, based on stearic acid. Fluorinated epilames have been used since the 1970s. Their solvents had to be replaced after the CFC ban in the 1990s; today they are dissolved in fluorinated solvents that do not attack the ozone layer.

Why oil spreads

Oil spreads over any surface whose surface energy is higher than its own surface tension. Watch oils are around 30 mN/m, while clean steel, brass and ruby are far higher. On freshly cleaned parts, a drop therefore spreads out until only an extremely thin film remains. Grooves and roughness accelerate this because they act like tiny channels. The lubrication point then lacks oil, although the oil is still inside the movement.

Comparison: on the left, an oil drop spread over an untreated surface; on the right, a compact drop on an epilame-treated surface
On the left, the oil spreads over the untreated surface; on the right, it remains as a drop on the epilame-treated surface. The epilame layer is shown greatly magnified.

The underlying physics, including contact angle, capillarity and equations, is explained in the article Lubricants in watches.

What epilame does

Epilames consist of fluorinated polymers, chemically related to the non-stick coating of a frying pan. They lower the component's surface energy to around 20 mN/m. A watch oil with 32 mN/m can no longer spread on it and remains as a drop, with a contact angle of 55° or more.

In physical terms, this can be pictured as gravitational potential energy: water flows downhill until it reaches the lowest point, and in the same way oil adopts the state with the lowest energy level. Here, cohesion, adhesion and surface tension take the place of height. Cohesion holds the oil molecules together, while adhesion makes them stick to the component. On bare parts, adhesion releases more energy than the larger oil surface costs, so spreading takes the oil downhill. On epilame, any further spreading would take it uphill.

Model of an epilame layer on a base material with an oil drop above it
Model of epilame treatment: the layer lowers the material's surface energy and the oil drop remains in place. The layer and molecules are shown greatly magnified. Illustration: Dr. Tillwich GmbH Werner Stehr, training notes of 14 October 2020, p. 50.

The layer is only a few nanometres thick: Tillwich specifies 10 to 40 nm, while Moebius gives 3 to 5 nm for Fixodrop ES/BS. It therefore does not alter fits or bearing clearance. The bath itself contains only 0.01 to 0.05% active ingredient; the remainder is solvent. Modern cleaning and rinsing solutions themselves leave a beneficial layer.

Application in the workshop

Epilame treatment follows cleaning and precedes oiling. The parts must be clean and dry, because the epilame should coat the metal rather than a film of dirt. The bath must also remain clean; a fibre carried into it will later become an oil bridge. Immersion is usual, spraying less common. Concentration and solvent vary by product: Tillwich's Antispread E2/50, for example, is four times as concentrated as E2/200.

For Fixodrop ES/BS, Moebius specifies at least 30 seconds of immersion with gentle movement, followed immediately by drying, preferably with warm air at around 60 °C. These instructions apply to this product, but are also a useful guide for applying other epilames. Epilame concentrates are adjusted only with the specified solvent. The bath should be closed promptly after use so that evaporation does not alter its composition uncontrollably.
There are substantial differences in evaporation rates: Episurf Neo epilame, for example, evaporates considerably more slowly and can therefore be considerably less expensive to use.

Above approximately 65% humidity, the solvent dries more slowly and leaves spots. Warm air lowers the relative humidity and thereby prevents spotting.

Which parts are treated also depends on the design and the required paths of the oil. In our workshop, the movement undergoes extensive epilame treatment with a few specific exceptions. According to most manufacturers' recommendations, the hairspring, regulator pins, automatic-rotor ball bearings and the shock-setting housing with its hole jewel should not be treated.

Take care with plastic parts, however: some epilames and solvents are unsuitable for them. The Lemania 5100 with its plastic components is a well-known example.

The special case of the escapement

At the pallet, the escape-wheel tooth slides over a tiny impulse face. The oil should remain there rather than spreading over the whole pallet. The solution is a short dry run: the tooth rubs the epilame off its contact track while it remains intact around it. The oil spreads within the bare track and is confined there by the epilame.

Cross-sectional diagram of a pallet jewel with an epilame layer, the worn running track of the escape-wheel tooth, and oil confined within it
Dry running at the pallet jewel: the tooth wears the epilame off its running track. The oil remains there, stopped by the surrounding epilame.

For pallet jewels treated with Fixodrop ES/BS, Moebius specifies, for example, an initial one to two minutes of dry running followed only then by lubrication. Anyone who does not generally use epilame should at least consider treating the escapement and pallet jewels, because this can considerably improve the life of the escapement lubrication.

The same confinement effect applies wherever a functional area where the epilame has been rubbed off is surrounded by intact epilame: the oil spreads over it and remains confined there.

In the potential-energy analogy, each bare contact track is a hollow in the plateau of the epilame. Oil touching the track can spread a little within it and thus reach a lower overall energy level; moving out onto the epilame would take it uphill. Even gravity cannot pull the oil out. For an oil film 20 µm thick, the track edge corresponds mathematically to a rise of around 8 cm (see Lubricants in Watches), whereas the entire ETA 2824-2 movement is only 25.6 mm in diameter.

Diagram: at the top, a cross-section of a pallet impulse face with an epilame layer, an oil-filled contact track and a drop on the epilame; below, the oil's energy across the impulse face, with the epilame as a plateau and the contact track as a hollow
Potential energy at the pallet: for the oil, the bare contact track is a hollow, while the surrounding epilame forms a plateau.

But take care: more oil than the hollow can hold will overflow the track edge onto the epilame. From there it can reach the next edge or a gap and drain away.

Checking the layer: test oil

A test oil shows whether the layer is present. Moebius offers six of them. Test oil 1 (9701) is fluorinated and indicates epilame positively: it spreads over the treated surface. Test oils 2 to 6 (9702 to 9706) indicate it in the opposite way: they spread over untreated surfaces and remain as drops on epilame. Their surface tension increases from test oil 2 to 6. Test oil 2 therefore spreads most readily and provides the strictest test.

Assess the test drop after 20 to 40 minutes, preferably in comparison with an untreated surface.

Limits, cleaning and maintenance

A good epilame layer survives five to ten cleaning cycles with organic solvents. It can be removed only mechanically or with strongly alkaline cleaners. An exception is water-based Fixodrop W-10, which can be washed off again with an aqueous or alcoholic solution. At ETA and Sellita, reused parts receive fresh epilame treatment during servicing; Fixodrop BS-10 (8981) is specified.

Epilame has one weakness: any contamination lays a new, thin film over the surface, and the oil creeps away over that film. Typical causes are wear particles from unlubricated or dry-running teeth, dirt entering through the crown, and moisture and condensation. Even a tiny dirt bridge is enough. On heavily contaminated parts, epilame is no miracle cure, and it cannot compensate for a worn pivot or an incorrect amount of oil.

If the movement is clean and sealed, however, the statement from Tillwich's training notes applies: “Epilame treatment ensures lifetime lubrication.”

What about a watch winder?

In the past, the recommendation became established that a watch should be moved and worn regularly. This was based on slightly better oil retention while the movement is running.

At rest, only the forces drawing the oil away act, continuously in the same direction. These include spreading, creeping along grooves and gravity. Movement repeatedly brings the oil back to the friction contact.

Today, however, epilame treatment considerably reduces this problem, so after a high-quality service the recommendation is outdated. A watch can safely be set aside for weeks or months. There can still be a slight redistribution of oil in the first few days after winding (“resting position” after prolonged storage versus “working position” after prolonged running), with a minimal effect on rate and amplitude. However, there is virtually no remaining risk of oil spreading away, provided that the oil reservoirs are not excessively overfilled.

Sources

All articles