The term “easy-to-clean coat” characterises coats with ultra-hydrophobic and in particular in addition to this, lipophobic properties. Sometimes, these surfaces are also called “self-cleaning”. Easy-to-clean means that surfaces can be cleaned by simply wiping them down. As a precondition, it must be ensured that dirt – in particular organic substances based on grease and oil components – does not adhere to the surface. This presupposes a lipophobic surface with a surface energy of under 20 mJ/m² and a contact angle against water of > 100o . Typically, surfaces of this type will also have low friction values. There are various processes for applying this type of coat, but plasma processes are used particularly often. Suitable processes are HMDSO coats or PTFE-like coats, generated by plasma polymerisation. Applications:
- Optical glasses
- Other optical components (camera lenses etc.)
- Surfaces with high decorative requirements (mobile phones)
As yet, no approved test procedures or standardised regulatory requirements have been determined for easy-to-clean surfaces. For durable easy-to-clean effects, the coats must be wipe-proof. For testing, this not yet standardised test process has been proven to be useful: A piece of cotton fabric backed with rubber and loaded with approx. 2.5 kg is pulled over the coated object in an oscillating movement in several thousand strokes. A good standard is that the wetting angle does not decrease by more than 5° after 5000 strokes. Generally, this is no problem with optical glasses, provided that these are made of mineral glass: In this case, antireflection coatings can be applied at higher temperatures, making them quite compact and stable. Hard-lacquered spectacle lenses made of plastics that do not allow high temperatures pose greater problems. The coats vapour-deposited on them are relatively rough. If they are insufficiently coated or the easy-to-clean coat is rubbed off during the wipe test, particles are torn out of the rough surface of the anti-reflective coating, which will later in the wipe test lead to a dramatic collapse of the anti-reflective coatings and even damage to the paint.