Summer 2002

A Coat of Many Colors

Understanding the Differences and Changes in Glass Coatings
by Peter Tausch

  Why are coatings applied to glass? What are a coatings? How do they work? How are they applied? These are all frequently asked questions about a growing segment of the architect’s tool kit. This article provides an overview for one of the many types of coatings available: thin, transparent films on glass.

Coatings on glass are a means of extending the usefulness of glass by adding an extra aesthetic dimension and enhancing its thermal performance. Uncoated glass is commonly available in eight basic tints and thicknesses from .085 inch to 1 inch. The largest share of the commercial market is ¼ inch (also called 6 mm). The need for more energy-efficient buildings has pushed this market into using insulating glass units and away from single glass lites in many geographical areas. Adding thin films to the inside of these units adds even better 
performance.

Different Coatings 
There are many types of coatings: bulk coatings such as frits and paints for spandrels, translucent lites and decorative panels; coated plastics that are applied to glass either by a fabricator or in the field, and some are even suspended in insulating glass units for improved thermal performance; vacuum and pyrolytically deposited coatings.

The term coating is used broadly and many of the processes are referred to as coating processes. The word film is also used broadly and refers to either the layers deposited by vacuum and pyrolytic processes or the coated and uncoated plastics applied to the glass surface or suspended within an insulating unit. While plastic films are occasionally confused with thin films, the difference is the thickness of the product applied on the glass. The plastic films are bulk materials. They may have a very thin film on one or both of their surfaces, and they can be handled like a delicate plastic wrap. The thin film, which is the topic of this article, is applied to the glass directly by the vacuum or pyrolytic processes.

How Thin is Thin?
Since thickness is the criteria used to differentiate coating types, we need some idea of the thickness range involved. A typical sheet of computer printer paper is .004-inch thick. The paper is from 250 times to 5,000 times thicker than the thin films. Plastic films range from .004 inch to .014 inch in thickness and are comparable to the printer paper. The frit and paint coatings are .0005 inch to .005 inch thick and are again comparable to the printer paper.

Because the thin films are so thin, layers of metals can be deposited, which can be seen through. Chrome, titanium and stainless steel can be used either alone or with materials like tin oxide, titanium dioxide and zinc oxide to make low-transmitting reflective films. Silver can be sandwiched between oxides or some special nitrides to make low-E films. The appearance of these multi-layer thin films can be tailored by changing the thicknesses of the layers and using different materials. By combining these thin films with the glass tints a large range of reflective colors, brightnesses and transmissions is available.

The oxides and nitrides used in these films are often referred to collectively as dielectrics. These have essentially no electrical conductivity and are present to tune the appearance of the coating. For the low-E films, they also protect the silver. Some of the pyrolytics use dielectrics with intentionally introduced impurities that make them conductive. The presence of the electrical conductivity gives these films their low-E properties.

Mini-Med (above) and ASM Lithography (right) offer examples of color possibilities available with different coatings.

Low-E Coatings
There are two basic classes of low-E films: the vacuum-deposited, silver-based films and the pyrolytically deposited doped semiconductors. Pyrolysis is a technique in which high temperature, usually above 900 F, is used to initiate the film-forming process. These two types are often referred to as hard coats and soft coats, with the pyrolytically deposited films usually falling into the hard-coat class and the vacuum-deposited coating in the soft coat-class. Since with either class the useful thermal properties occur when they are assembled with the film on the inside of an insulating glass unit, the hardness classification is of little importance. Both types have a high transmission and the choice of one over the other is a matter of building size, location and the thermal and optical properties required.

A new development in low-E films is a hybrid of the older, low-transmitting, reflective films and low-E coatings. Many architects use filmed glass for aesthetic as well as performance reasons. Low-E films were developed originally for the residential market where low color and low reflection with high transmission are valued, along with the improved thermal performance. In the commercial market, a distinctive appearance can be a positive attribute, and high transmission, especially in larger buildings, means higher heat loads. To address these concerns filming companies have modified low-E films to have lower transmission, higher reflection and very good solar properties: low U-values and low shading coefficients. This trend is likely to continue with the introduction of different color films that have light transmissions high enough to minimize internal lighting requirements giving the building a natural, open feel, but have the look on the outside of reflective coatings and good solar properties. 

Peter J. Tausch, Ph.D., serves as coating technology and development manager for Interpane Glass Co. in Clinton, N.C.

Architect's Guide to Glass & Metal