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TECH TALK

Tinting Films: A Technical Overview

Although all tinting films start with a 2 to 7mm sheet of polyester, the way the sheet is coated makes for a different product with specific characteristics, uses and appearances.

Quite often, several thin layers of film are bonded together. One side is coated with either a pressure-sensitive or water-activated Clear Distortion Free adhesive. Unique to LLumar® films, the CDF adhesive system forms a molecular bond between film and glass, producing an installation of optical clarity and durability unattainable with alternative "tacky" adhesives. The CDF adhesive system incorporates a clear release liner which protects both film and adhesive from moisture and dirt until the moment of installation.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 purpose is to provide only 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 one half mph indoors--the ratio is 30:1 or better in favor of outdoor heat dissipation. Because double-glazed windows don't allow air movement between panes, interior-dyed films should not be used on thermal glass.

The other two processes, called deposition technology (vacuum coating/metallizing) and sputtering technology (advanced metallizing), 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. Metallized films reject heat by reflecting it 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, which is then flooded with argon gas and the ingots are heated. The heat causes the metal to give up particles that 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 metallized coating must be fairly thick, as the particles are comparitively large. What this means at a practical level is that a darker, more highly mirrored surface is produced. And secondly, 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 metallizing 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 metallized 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.

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 because of the mass and color of the metals involved.

To further complicate the issue, 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 good example is gray dye and titanium coating. If used alone, dye would darken the film significantly, while the titanium would produce a highly mirrored surface. When paired, 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.

Portions of tintcondition.com/techtalk.html excerpted from "Window Film: New high-tech glass films claim energy savings for your home." by Merle Henkenius. Popular Mechanics, October, 1996.

 
 

 

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