# Performance of Solar Control Films on Building Glazing: A Literature Review
## 2 Window Films
### Typical Layout of Window Films  
Window films are made of several membranes of different intercalated materials that can reach up to eight different layers and undergo seven different manufacturing procedures [19]. Figure 1 shows a typical layout of a window film [19,20] installed on the internal surface of the glass, constituted by the following layers:  
1. Scratch resistant coating: a hard acrylic coating finishing layer that is in contact with the indoor environment and aims to protect the film from abrasion and scratching.  
2. and 5. Polyester: polyester layers that introduce good mechanical, thermal, optical, physical and chemical characteristics to the film, and that are very durable, flexible, resistant and have the capacity to withstand high and low temperatures. The lamination of several layers of polyester using adhesives increases the film's absorption and solar front reflectance. This layer can have different finishing types, such as UV resin or adhesives.  
3. Metal: oxide metals are incorporated into the polyester layers of window films and have the purpose of reducing solar heat gains through the glazing. Usually, aluminum is used to reduce solar heat gains and visible transmittance by about 80% and 15-70%, respectively. Modern films based on nanotechnology are manufactured without metal oxides, resulting in thin window films with high visible transmittance and low solar front reflectance.  
4. Lamination adhesive: combines several membranes of polyester through lamination processes and can sometimes be embedded directly in the polyester layers.  
6. High performance UV resin: can be incorporated into the adhesives or the polyester membranes to block UV radiation, protect the polyester layers and lamination adhesives, and protect the indoor environment, occupants and materials from exposure to UV radiation.  
7. Adhesive: window films can be applied to glazing using pressure-sensitive or water-activated adhesives. Pressure-sensitive adhesives adhere to the surface of the glass by applying pressure forces without the need to use any type of solvent, water, or heat. Water-activated adhesives need water to form a correct adherence to the glass through chemical bonds, which guarantees higher durability and a more transparent appearance. However, using water-activated adhesives can make the removal or replacement of the film difficult.  
8. Protective release liner: a removable polyester film that shields the adhesive from contamination. The protective liner should only be removed right before applying the film.  
The production of window films usually requires the following high-quality manufacturing procedures [18]:  
* Coating: the material is transferred from a container to a large roller and then onto the surface of the polyester film. This process is typically used to apply adhesives and scratch resistant layers.
* Laminating: a film that is coated with adhesive is adhered to an uncoated film by using a roller system that presses the two films together.
* Metallizing: under vacuum conditions, a polyester roll is wound round a water-cooled roller in a large metal chamber while a metal, typically aluminum, is evaporated onto the cold film surface.
* Sputtering: the equipment needed for this process is very similar to that used for metallizing, but in the sputtering process, a metal or alloy target is bombarded with positive ions to sputter atoms of metal onto the cold film surface. Some metals used in sputtering, such as nickel, can give extra corrosion resistance to films. This process is more time consuming than metallizing.
* Coloring: color can be added to adhesives before coating, or the polyester film can be deep dyed through this process. The resulting color is usually a mixture of different dyes and, generally, a higher durability is obtained using deep dyeing processes.