Clearing Up Mysteries Behind Window Film

Most homeowners are familiar with the benefits of window film to control sunlight, but few know the technical story behind this product.

The film is always polyester, 2 to 7 millimeters thick. Often, several thin layers of film are bonded. One side is coated with a pressure-sensitive or water-activated adhesive. The exposed surfaces of most films are also treated with a hard, scratch-resistant coating.

To filter out ultraviolet radiation, chemical UV blockers are incorporated. If the film’s only purpose is to provide UV protection and shatter resistance, no other materials need to be added.

From there, three separate technologies are applied to achieve different performance characteristics. The first is simply a dye, which absorbs heat. Because most films are applied to the inside surfaces of windows, it’s easy to imagine that the absorbed heat would disperse indoors. In fact, the heat rejected by the film is stored largely in the glass and is drawn away by external air movement.

A tiny percentage does bleed inward, but because the average speed of external air movement is so much greater--the daily average is 15 mph versus a half-mile per hour indoors--the ratio is 30 to 1 or better in favor of outdoor heat dissipation.

Because double-glazed windows don’t allow air movement between pains, interior dyed films should not be used on thermal glass.

The other two processes, deposition technology (vacuum coating/metalizing) and sputtering technology (advanced metalizing), deposit a layer of metallic particles on the film, giving it a reflective coating. In each case, a second layer of film protects the coating. Metalized films reject heat by reflection, bouncing it back before it can be transferred through the glass.

In deposition technology, the film is drawn through a tank containing metal ingots--usually aluminum or nickel-chrome and, occasionally, copper.

A vacuum is created by reducing the pressure in the tank, the tank is then flooded with argon gas and the ingots are heated. The heat causes the metal to give up particles, which migrate to the film’s surface. The density of the metal deposition is controlled by the speed of the film through the chamber.

While deposition technology works well and is relatively inexpensive, it has its limits. To be effective, the metalized coating must be fairly thick, as the particles are comparatively large. What this means at a practical level is a darker, more highly mirrored surface. And second, the list of metals that can be deposited evenly is fairly short, which means fewer product options.

Sputtering technology is more complicated. Sputtering is also done in a vacuum chamber, but the metalizing is achieved at the atomic level. In brief, electromagnetic fields direct streams of ions from a chemically inert gas--usually argon--toward the metal. This ion bombardment, which is often described as “atomic billiards” causes groups of atoms to dislodge in small bursts and scatter uniformly across the film.

The practical benefits of sputtering are that 25 to 30 different metals can be used, and the metalized coating is much lighter. It’s possible to sputter metal in a layer one-hundredth the thickness of a human hair.

Different metals are chosen to subtract specific bands of radiation from the solar spectrum. The result is a highly reflective layer with very little mirror effect, heat absorption or color shift. Because sputtering is more expensive, these films occupy the high end of the price range.

While the performance characteristics of dyed and metallic films are generally distinct, there is some overlap. Heat-absorbing dyed films are somewhat reflective, and metallic films do absorb some heat due to the mass and color of the metals involved.

Many films contain both dyes and reflective metals. By combining dyes and metals, the negative effects of each can be reduced without sacrificing performance.

A popular example is a gray dye and a titanium coating. If used alone, the amount of dye needed would significantly darken the window, while the titanium would produce a highly mirrored surface. Together, less of each can be used, resulting in a film that is relatively bright and nonreflective.

This point is significant, if only because it quells the notion that the darkest films reject the most heat. In most cases, dark films are chosen because they offer greater privacy.