Investigation of gas permeation through Al-metallized film for vacuum insulation panels

Recently, vacuum insulation panels (VIPs) become a key issue to save the building energy. VIPs must have a good insulation performance and a long service life. Al-metallized film can meet these requirements by suppressing both of thermal bridging and gas permeation. In this work, gas permeabilities through an Al-metallized film for various gases are measured using a newly-developed experimental apparatus based on a pressure difference method. Its reliability is validated by measuring a few well-known gas permeabilities through uncoated 12 μm polyethylene terephthalate (PET) film. Then, a 12 μm PET film coated with a 33 nm aluminum layer is chosen as the sample metallized film. It is confirmed by residual gas analyzer (RGA) that nitrogen, oxygen, carbon dioxide and water vapor are the major gases permeating through the Al-metallized film, which are then taken as the test gases. Permeabilities of these gases through the Al-metallized film are measured to be only about 4.1% of those through uncoated PET film. Nevertheless, it is still higher than those of metal sheets because of the abundant pin-holes. Pin-holes are investigated for the distribution and the diameter by an optical microscope. An effective gas permeability including the pin-hole effect is calculated to be 3.6-4.0% of uncoated PET film permeability, agreeing well with the experimental results. Multiple Al-metallized layers must be thus employed to be effective gas barriers. A correlation for the permeation through a layer of Al-metallized film is also derived. These research results and methods can be usefully applied to actual VIP envelope materials with multiple Al-layers.