The performance and life expectancy of a vacuum insulation panel is determined by a number of factors. Following the major influencing criteria:

Properties of the core materials

Current commercial VIP materials include polystyrene and polyurethane foams, precipitated silica and fumed silica. All vacuum insulation panels rely on high vacuum to provide their low thermal conductivity values. Nevertheless, fumed silica outperform any core type, even at higher pressure levels. As the level of vacuum in the envelope decreases the thermal conductivity rises. Still the relationship between internal pressure rise and increasing thermal conductivity varies tremendously with different core materials.

The graph on the right side indicates the effect of rising internal pressure on Porextherm’s® Vacupor®-based VIPs in comparison to panels made from other core materials. Note that while all materials offer comparable performance at the highest evacuation levels, there are significant differences between them with only slight increases in internal pressure. Deterioration in performance is most pronounced with foams that can limit their use in many cases to shorter lifetime applications (<5 years).

 

Initial Vacuum Level

VIPs do not maintain a “perfect vacuum”. Porextherm’s® Vacupor® panels are evacuated to 1 torr (1.3mbar) whereas other core materials are initially evacuated to an internal pressure of about 0.05 torr (0.067mbar).
Creating vacuum levels lower than 1 torr would add significantly to the production cost and, in most cases, do not result in a higher insulation values. Panels that start out with a higher internal pressure will have a proportionately shorter effective lifetime than will an, otherwise identical, panel that is more thoroughly evacuated. Highest quality in the evacuation as well as in the sealing process is therefore most important.

 

Membrane and Seal Permeation Rates

The membrane film is the material that forms the walls of the VIP. All membrane films in use today permit some molecules of gas and moisture to pass through over time. The amount of permeation through a particular membrane film will depend on the material of its construction and the resistance of this material to degradation during handling in the production process. Some films contain of a very thin metal film (usually aluminum) which is reinforced by laminating a plastic film to each side. These films can have excellent barrier properties but can conduct significant heat around the edges. These “edge effects” can significantly reduce the effective performance of a VIP (for a more detailed analysis refer to “Thermal Edge Effects”). In order to reduce the unwanted “Thermal Edge Effects” to a minimum, some films are based on a thin film deposition technique which builds the metal layer even thinner. There are many films commercially available today and their properties and advice on selection are covered more fully in the chapter “Barrier Material selection”. The membrane films are sealed at the edges to form an envelope for the core material. A thin layer of low temperature plastic is laminated to the inside of the film so than it can be sealed using heat and pressure. These layers of heat-sealing plastic do not have the same resistance to gas and moisture permeation as does the rest of the film. To minimize the negative impact of permeation of the sealing layer, manufacturers use as thin a film layer as possible combined with a wide seal lip.

 

Getters and Desiccants

Getters are chemicals that absorb gases; desiccants are chemicals that absorb moisture. Getters and desiccants are used to extend the life of VIPs by absorbing unwanted gases and moisture that promote heat transfer within the vacuated space. To be effective, the getters and desiccants must be carefully matched to the kind and quantity of gas/moisture they will be expected to absorb. Besides that getters and desiccants must also be capable of effectively absorbing and holding the gasses and moisture at the low pressures inside the VIP. It is, therefore, important that the quantity and type used be selected in accordance to the core material, membrane film and required life expectancy. Foam-based panels have no absorbent capacity at all.

It is, therefore, necessary to add these chemicals into the VIP envelope. Vacupor® panels are natural desiccants itself. In addition getters are not required, even for long lifetimes (10 – 20 years) as long as a suitable barrier film is used. Getters can add significant cost to a panel and because of their heavy metal composition create major safety and environmental concerns.

 

Outgassing

Most materials release gases (outgas) when placed in a low pressure environment. The kind and quantity of gas released, as well as the length of time the outgassing will continue, varies from material to material. The released gases can contribute substantially to the rise in internal pressure (i. e. loss of vacuum) of a VIP. In some cases, the rate at which gas released from the core and membrane materials exceeds that at which it permeates through the membrane.

A few materials, such as Porextherm’s® Vacupor® do not outgas at all, while other materials never stop outgassing. The core and membrane materials used by a particular manufacturer will determine what, if any, impact outgassing will have on the life of their product.

 

Size and Thickness

Gas molecules can enter through the barrier film and the sealant material that bonds the film plies together. The larger the VIP the greater the film surface area vs. seal area and the smaller the VIP the greater the seal area vs. film surface area. Therefore, selecting a suitable barrier material requires that both the barrier film and sealant properties are appropriate for the type and size of panel. Thickness has a much greater effect on panel performance. Halving the thickness of a panel will halve the lifetime of a panel because the surface and seal areas remain almost constant whereas the insulation volume is halved. So although the transfer rates through the seal and barrier will be almost the same the gas pressure will be doubled because of the smaller volume.

Operating conditions

Operating conditions are important for both usability and lifetime. Usability refers to a panel's suitability for a given operating environment. Foams being plastics have a limited temperature range over which they can be used.

Outside of this range shrinkage and deformation occur which can render a panel practically useless. For example the upper limit for polystyrene foams is 88°C (190°F) which rules out their use in applications such as hot water heaters and hot food delivery systems. Porextherm's Vacupor® core material can be used at temperatures up to 950°C (1742°F) with appropriate barrier films like e.g. stainless steel envelope.

Operating conditions effect lifetime because the transfer rates of water vapor and gases through the barrier film and seals change with temperature. Higher temperatures promote increased transfer rates and lower temperatures slow down molecular movement. In addition, the higher the concentration of a gas surrounding the panel the higher will be it's concentration in the panel and consequently the greater it's effect on heat transfer. In general the smaller the gas molecule the faster it will penetrate into the panel and greater will be it's effect on thermal conductivity. So for example encasing a panel in polyurethane foam, the preferred method of application in refrigerators helps to prolong panel life because the heavy gas molecule of the foam blowing agent take longer to penetrate into the panel and when inside are not as good conductors of heat as nitrogen or oxygen because of their larger molecular sizes. Similarly for water vapor; the higher the humidity of the air around the panel the faster the transfer into the panel and the higher the final water concentration in the panel when equilibrium is reached.

 

Summary

Vacuum Insulation panels are an established technology in a number of applications including temperature controlled transportation, domestic and commercial refrigeration as well as construction insulation. Preferably, they be applied in any system where extra volume, better temperature control, longer shipping times, reduced shipping volume and weight or increased energy efficiency are desired. VIPs made from Vacupor® can be supplied in many different shapes (flat, curved, round) thus enhancing their design flexibility. However, panels are usually not a “drop-in” solution and may require some redesign of your system to achieve the maximum benefits. Porextherm® has extensive experience and technical resources available to help find the optimum use of panels in your application.