Recent research has shown that a simple “convection” barrier, as shown in Figure 2, which does not extend through the edge of the insulating glass unit, is as effective an insulator as traditional designs. Advantages include a simpler and more reliable edge design and potentially reduced manufacturing costs.
We have investigated various plastics and edge constraint designs and are now focused on the use of an acrylic layer, between 1mm and 3mm thick. Bent edges will help keep the layer in place (see Figure 3a,b,c). In order to commercialize such a product, several issues remain to be addressed. These issues, as they relate to acrylic, are defined below. Other plastics and designs can still be explored.
Stiffness
The acrylic center layer has to be stiff enough to prevent sagging and bending, assuming a vertical position. Windows can be as large as 1800 mm * 1500 mm (h*w), and doors can be around 2000 mm * 1200 mm (h*w). The gaps on either side of the center layer can be between 6-12 mm with the most common gaps expected to be 9 mm. It is important the center layer remain parallel to the glass layers in either side; i.e. the gap with should not deviate by more than 1-2mm from the edge to the center (see Figure 4).
Weight
One of the reasons to investigate the use of plastics as a center layer instead of glass is to reduce the weight of the triple glazing unit. The weight of glass is 2579 kg/m3. Any plastic layer should be lighter then glass.
Width
The overall width of the insulating glass units should be as narrow as possible. Therefore, the narrower the center layer is (i.e. 1-2 mm instead of 3mm), the more effective the final product will be.
Thermal expansion
The thermal expansion of the plastic center layer needs to be considered. The maximum height of the sheet is around 2000 mm. The center layer does not need to maintain a hermetic seal between the two gaps on either side. Some evidence of thermally induced bowing of the acrylic out of the intended plane has been observed when prototype window assemblies have been exposed to a large temperature difference (-18C to 21C exterior). This behavior needs to be better understood, including any relation to production induced mechanical stresses in the material.
Optical clarity
Windows have very high requirements on optical clarity. Any plastic layer should maintain this optical clarity. Anti-reflection coatings to maximize solar transmittance through this center layer are being considered.
Moisture and organics out gassing
The plastic center layer will be in a sealed glazing cavity, with a desiccant in the spacer to absorb some moisture. If the plastic contains moisture at production time, this moisture will most likely be released during hot periods and might condense on the glass during cooler periods. Out gassing of the plastic is a concern for the durability of the low-e coatings, the desiccant and the sealants.
Cost
The cost of the added plastic layer is an important consideration. Windows are a commodity product.
Ease of forming
The suggested manufacturing method for the center sheet is to bend the edges over to provide spacing and rigidity. The plastic layer should be easily cut and bend to allow this manufacturing process.
UV stability
Windows are inherently going to receive a lot of solar exposure. The outer glass pane might be highly transparent (low-iron glass). This will require the plastic center layer to be resistant to UV radiation. Potential effects could be cracking, hazing, discoloration.
Long term stability (creep)
Windows are traditionally installed for very long terms; 20-40 years is not uncommon. The plastic should retain is functionality over that life time.
Production issues
Dust and scratches during manufacturing should be easily avoided. The window industry uses high quality washing machines for glass. A similar process may be needed for the plastic center layer. Understanding and potentially relieving any production induced mechanical stresses in the acrylic sheets will be important to maintain proper dimensional tolerances (acrylic sheet in plane for uniform air cavities).
Environmental impact
The production of the plastic should have the least environmental impact as possible. Concerns include embodied energy, waste materials, and source materials.
Last update: 08/11/2006
Figure 1 - Traditional triple glazing
Figure 2 - Alternative triple glazing
Figure 3a - Corner of folded edge plastic insert
Figure 3b - Edge of folded edge plastic insert
Figure 3c - close up of a folded edge insert in a window
Figure 4 - undesirable center layer deflection