Soft Coat Glass

Soft Coat Glass, otherwise known as vacuum coated or off-line coated glasses, are manufactured by a process which is entirely different from hard coat glass (discussed in last post). The name soft coat is given because of the susceptible nature of the coating to get peeled off (in single glazing/ monolithic application) when compared to hard coat. However, soft coat glasses can offer a very low solar factor when compared to hard coat glasses.

Manufacturing process involves metal particles being deposited on the glass surface inside a vacuum chamber. The process, otherwise known as Magnetron Sputtering Vapor Deposition (MSVD) is sometimes referred to as Cathodic Vapor Deposition. Some glass manufacturers mention it as CVD coating, just to create a confusion with actual Atmospheric Pressure Chemical Vapor Deposition, mis-interpret it and mis-sell it as hard coat.

During the process, the material to be sputtered is loaded in a high voltage electric circuit, which is followed by the feeding of process gas into vacuum chamber, where plasma is formed. An ion discharge takes place inside the chamber, these positive charged ions gets attracted and collide with the material to be sputtered. This process happens at a very high speed and atoms of the material sputtered gets ejected, which gets accumulated on the glass below. Most widely used metals for sputtering are Silver and Titanium.

Soft coat glasses are generally used in double glazed units, with the coated surface at position 2 or 3, so that the coating is kept protected from peeling off. With the advance in technology, soft coat glasses are now made which can also be used in monolithic form (single glazed) with much improved life for the coating, but still the life of the coating cannot match with that of hard coat glass in monolithic applications.

Soft coat glass also has problems while tempering when compared to hard coat glass. It tends to show up a problem called lensing, which happens because the coated surface of the glass reflects Infra Red radiation and heats up differently than the lower surface (which is heated).