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Architectural window film / Important points when handling architectural film

Architectural window film / Important points when handling architectural film

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Important points when handling architectural film

One of the major differences between architectural film and automotive film is that architectural film does not move after installation. That lack of movement causes the thermal cracking that is unique to architectural film. When selecting a film for installation, a necessary final step is to objectively determine whether or not the film is suitable for the installation environment by performing a thermal stress calculation. This section explains thermal cracking and related points.

We will introduce the phenomenon of thermal cracking, which is unique to architectural film and does not occur in automotive film.

Thermal cracking and its symptoms

Unlike the tempered glass used in automobiles, the center portion of architectural window glass normally becomes hot when subjected to solar radiation due to heat absorption. The edges of the pane, on the other hand, are hidden inside the sash — not only shielding the edges from solar radiation, but also transferring heat from the edges to the sash and frame. This causes the edges to stay cool and not expand. For this reason, the edges restrict the thermal expansion of the center, generating tensile stress in the edges. When the stress overcomes the strength of the edges, the glass cracks. This phenomenon is called “thermal cracking.” Thermal cracking begins from the edges. Typically, the crack starts at a right angle on an edge and then branches into further cracks. However, there are also cases with only one crack or where a separate crack forms midway through. This thermal cracking often occurs when eastward-facing glass is heated by the sun on winter mornings after being chilled during the night.

Thermal stress calculation

Installing a film on window glass increases the rate of solar absorption, which makes the glass more susceptible to thermal cracking. By calculating an estimate of the thermal stress generated in the glass, and comparing that to the stress tolerance of the edges of the glass, it can be determined whether or not thermal cracking is likely to occur. This is called a thermal stress calculation. The calculation method is based on the calculation method performed by the glass manufacturer.

Symptoms of thermal cracking

Causes of thermal cracking

1 Types of glass

Heat-reflective/heat-absorptive glass reaches high temperatures when exposed to solar radiation, which makes it prone to developing temperatures differences between its edges and center. In wired glass and mesh glass, because the wires are exposed at the edges, the edges have less strength than normal glass and thermal cracking occurs easily. The thickness of the glass also affects the strength of the edges. * See the bottom of the page for details about glass.

2 Shadow type

Depending on the type of shadow cast on the window, a single piece of glass can be divided into high temperature and low temperature parts. This complicates the thermal expansion, affecting the likelihood of thermal cracking.

Shadow type<

3 Curtains & Blinds

If there are curtains or blinds in the room, the glass exposed to sunlight will reach high temperatures, increasing the risk of thermal cracking.

4 Glass surface area

The larger the surface area, the more heat a piece of glass will receive and the more it will expand, increasing the risk of thermal cracking.

5 Edge fixing method

The temperature at the edges of the glass is influenced by the edge fixing method used to fix the window glass in place.

6 Other

For wired glass or glass with poorly manufactured edges, if the glass has not been made watertight and rustproofed, rust may form, reducing the strength at the edges. Since thermal cracking starts at the edges of the glass, when the strength of those edges is reduced, thermal cracking may occur with a smaller temperature difference than normal.

In addition to the above, other factors like the region and the window's orientation may also influence thermal cracking. Thermal cracking occurs when a complex interaction of these factors leads to thermal stress which exceeds the strength of the glass’s edges.

Exterior window film

Although standard architectural film is interior, exterior films are also available for situations where installing the film on the inside would be difficult. External film has half the weather resistance of internal film and lacks a hard coat. As shown in the illustration on the right, hard coats deteriorate under exposure to ultraviolet rays. For this reason, window film is usually made from weather-resistant PET. Exterior films produced by Madico, however, use hard coats that are specially processed for high durability, which is uncommon for this type of film.

Exterior window film

Thermal stress calculation example

As mentioned above, performing a thermal stress calculation is a key step when installing architectural film. However, it is difficult to understand how all the different factors interact. Here, we will provide an example using an actual UNIGLOBE R&C film.

Glass type and thickness

The first and most important factors are the glass type and thickness. Here, we will determine the tolerable level of thermal stress, which will be the basis of our final judgment on whether or not the film is likely to cause thermal cracking.
Thickness 6.8mm
Transmittance 0.734
Reflectance 0.073
Absorptance 0.193

Film to be installed

The next factor is the film to be installed. The adaptability of a piece of glass can vary depending on the film applied to it.

Mounting type

The next factor is the glass mounting type. Here, the coefficient varies depending on whether it is a fixed window, a sliding window, etc.

+

In addition to the above, a number of other factors play a role, such as the location of the building and the indoor environment.

1.Shape of shadow

Example: A divided shadow

1.Shape of shadow

Coefficient 1.7

2.Use of curtains/blinds

Example: Thin curtains (lace)

2.Use of curtains/blinds

Coefficient 1.3

3.Glass surface area

Example: 1.62 square meters

3.Glass surface area

Coefficient 1.07

4.Glass fixing method

Example: Standard fixing

4.Glass fixing method

Coefficient 0.50

We will use the above representative factors in a thermal stress calculation to derive
the generated thermal stress and determine whether installation is possible.

* For actual calculations, more detailed conditions are necessary, and a complex calculation including all of those conditions would be performed.

Generated thermal stress = K × f0 × f1 × f2 × f3(θg-θs)

K = Basic stress factor (0.47MPa/℃)
f0 = area factor
f1 = curtain factor
f2 = shadow factor
f3 = edge temperature factor
θg = Temperature at the center of the plate glass (℃)
θs = Temperature at the sash (℃)

For this example, the calculation results are as follows.

Tolerable thermal stress 9.8 > Generated thermal stress 6.1

Result: The desired film can be installed on this glass

As these results indicate, the generated thermal stress is a lower value than the tolerable thermal stress. Therefore, we judge that the film can be installed without risk of thermal cracking.

* The thermal stress calculation itself is only a guideline for making judgments. It does not guarantee safety after installation.

Thermal stress calculations like this are an important task for determining whether or not a client's requests can be met during installation.

We can perform thermal stress calculations for every kind of film, so please do not hesitate to make a request.

Click here for the thermal stress calculation request form

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Product lineup

UNIGLOBE Film R&C

ユニグローブ・レジデンシャル&コマーシャル

UNIGLOBE Residential & Commercial is a comprehensive brand of architectural window films, a new category in UNIGLOBE's lineup. In cooperation with SKC, one of the world's leading polyurethane manufacturers, we offer a range of sophisticated and high-quality products to meet your needs.

WINCOS Architectural Window Film

WINCOS Architectural Window Film

This architectural window film is from Lintec, a leader in the Japanese market for automotive film. You can select from a wide lineup of films to suit your application and purpose.

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