Introduction

The lifetime and performance of vacuum insulation panels (VIP’s) depends upon the ability of the outer barrier or envelope material to prevent gases from penetrating into the panel during the panel’s operating lifetime. Low pressures are desirable for improved thermal performance because when the mean free path (the average distance a molecule will travel before hitting another molecule) of the gas approaches the pore size of the VIP insert, ”gas phase thermal conduction” is greatly reduced. Therefore, having very small and uniform pores in the VIP insert is desirable since that allows operation at moderate vacuum levels. The ”pore size effect” on the vacuum level required to eliminate gas phase conduction for various fillers is shown in Figure 1. For an insert such as an ”open cell foam”, which has pores in the 10 to 100micron size range, vacuum levels of 0.01 to 1mbar are required to achieve good thermal performance. In contrast, Vacupor® has pores in the 10 to 100 nanometer range (one thousand times smaller than foams) which means that only very moderate vacuum level are required. In fact, because of Vacupor’s® small pores, even the thermal performance at ambient pressure is superior leading to significantly improved performance compared to other inserts if the barrier ever fails completely.

Although the VIP inserts shown above require different vacuum levels to operate, they all require a barrier to minimize the permeation of gases into the panel for it’s application lifetime. These gases may be atmospheric such as nitrogen, oxygen and water vapor or they may be application specific such as cyclopentanes, carbon dioxide and/or HCFC’s (when the VIP is encased in foam). The major issue in the selection of the appropriate barrier material(s) for a particular application is the compromise between the permeability of the barrier material(s) and the cost and thermal edge performance effects associated with the particular barrier. Thermal edge effects (also known as thermal shunting or thermal short-circuiting) arise because the thermal performance of the highly porous insert is very high as compared to the dense barrier material. As a result, the effective thermal performance of a vacuum panel is always lower than the value measured at the centre of the panel. The magnitude of this difference depends upon the insert’s intrinsic thermal performance, the barrier thickness and composition, the boundary conditions around the VIP and most importantly, the VIP size. In general, thermal edge effects are negligible for plastic and metallized plastic barriers and are quite significant when metal foil barriers are used. The effect of barrier material and VIP geometry are discussed more completely further on.

Properties of barrier materials

In general, barrier materials for vacuum insulation panels can be selected from either plastics, metallized plastics (for example, produced by vapor depositions of metals such as aluminum), metal foil/plastic composites produced by lamination, or welded metal foils. In most cases the barrier film structure is typically multilayer produced by lamination in order to impart a range of functionality to the film e.g. water and gas permeability, heat sealing, mechanical properties, etc. For barriers using metal foil, aluminum foil is the metal of choice because of its ductility, availability and cost. However, aluminum has a very high thermal conductivity which is why it is also the material of choice for cooling fins on electronics, etc. In fact, the thermal conductivity of aluminum is approximately 1,000 times greater than that of common plastics used in barriers and 20,000 to 100,000 times greater than that of typical VIP filler materials. Therefore, from a thermal edge effects viewpoint, the use of plastics or metallized plastics is strongly preferred as compared to metal foils.

 

Water vapor transport rates/Oxygen transport rates

 

For barrier materials, manufacturers typically report two properties related to how fast gases and vapors will permeate through the barrier. The first is the Water Vapor Transport Rate (WVTR) which has units of grams per square meter per day (in the U.S., grams per 100 square inches per day). From the known surface area and internal volume as well as accounting for any water adsorption by the VIP insert, the water partial pressure and amount of water in the VIP as a function of service life can be calculated for a barrier with a given WVTR.

As with all barrier properties, the manufacturer’s WVTR represent a best case that can only be approached in a VIP. The second property often quoted by the barrier producer is the oxygen permeability or Oxygen Transport Rate (OTR) in units of cm3/m2 per day per atm (or cm3/100 in2 per day per atm in the U.S.). Although oxygen only represents ~21% of the atmosphere, the oxygen permeability is reported because of its effect on food degradation and the fact that oxygen transport through many plastics is quite high. For vacuum panels, the permeation of nitrogen is also of major concern since it represents the most plentiful atmospheric gas. For many plastics, the nitrogen permeability is four to five times lower than that of oxygen but this is offset by the pressure driving force which is four times larger than that of oxygen because of the higher concentration.
For VIP applications in which the panel will be surrounded by gases and vapors other than atmospheric gases, than the permeability of the barrier for those gases must be measured for accurate lifetime predictions.

For simple, single component plastics ranging from polyethylene to nylon to PVDC, the WVTR typically has values ranging from 1 to 300g/m2 · day and the OTR ranges from 0.3 to 4,000cm3/m2 · day · atm. For a plastic, the properties of being a good barrier for both water and for oxygen do not normally coincide.

This is the reason why composite films are normally employed. In addition to layers for water vapor and oxygen permeability, layers are often added to heat sealing and as an adhesive between layers. Depending upon the thickness and composition of a laminated structure, WVTR values of 0.02 to 0.2g/m2 · day and OTR of 0.05 to 0.5cm3/m2 · day · atm. are achievable but can be quite expensive. For lower values, one can use either metallized films, which have one or more thin layers of vapor-deposited metals such aluminum vapor or laminates which employ thin aluminum foils.

The advantages of metallized films are minimal ”Thermal Edge Effects”, safer processing and lower cost but they typically do not achieve as low a permeability as foils. Metallized films offer WVTR values in the 0.01 to 0.5g/m2 · day and OTR values of 0.01 to 0.5cm3/m2 · day · atm. In contrast, laminates containing aluminum foils have lower permeability but suffer from significant thermal edge effects and high cost.

Below are the predicted pressure rises over a 30 year lifetime in a 25mm (1" thick) Vacupor® VIP using barrier materials with a range of OTR values. The calculations include oxygen, nitrogen and water vapor permeation.

The calculations assume zero initial pressure and any residual pressure actually in the panel should simply be added to the calculated pressure. For thinner panels, the pressure rise occurs faster. For a panel which is 12.5mm thick, the time required to reach the same pressure will be 1/2. These calculations assume that the barriers have been correctly sealed.

Since the change in thermal performance with pressure is different for various VIP inserts (see Figure 1), the effect of the pressure change during the panel’s lifetime will be quite different.

Using the pressure versus lifetime results for the K = 0.1 barrier of Figure 2, the thermal conductivity as a function of lifetime can be calculated.

This is illustrated in Figure 3 for Vacupor® and an open cell polystyrene. Desiccants or getters are not used for either insert. In general, foams require barriers with permeability (OTR and WVTR) which is approximately 100 times lower than for Vacupor® because of the more stringent vacuum requirements associated with the larger pore sizes of foams. This is difficult to achieve without employing foilbased laminates with their associated thermal edge effects and high cost.

Water Vapor Transport

The transport of water vapor through barrier materials and into VIP’s is discussed separately from the gas permeation above for three main reasons. For insulation applications near ambient temperature or below, water is different from other atmospheric gases because the total pressure that it can be achieve during the lifetime of the panel is limited by the equilibrium vapor pressure. The second reason is that because of its low molecular weight and unique chemical structure, barrier materials which are excellent barriers for gases such as oxygen and nitrogen may not be good barriers for water vapor and vice-versa. Finally, some VIP inserts such as Vacupor® and precipitated silica can adsorb large quantities of water which mitigate the pressure rise associated with a given quantity of water permeating into the VIP.

This effect is indicated in Figure 4 which illustrates the pressure in the panel after different quantities of water have permeated into the panel. These results are for ambient temperature which means that the water vapor pressure would never exceed ~28mbar. The pressure in the opencell foam rises rapidly to the equilibrium water vapor pressure. Since they can only tolerate pressures of 1mbar before a large degradation in performance is observed, water sensitivity is a major problem. In order to ban this problem, barriers with very low water permeation rates must be utilized. Also, expensive desiccants and getters are often added to foam-based panels to keep the water partial pressure low. However, since the rate of water transport across the barrier depends upon the relative humidity inside and outside the panel, if the pressure is maintained low in the panel, the water transport rate is higher than if the water pressure inside is close to ambient. Therefore, getters and desiccants which strive to maintain the very low partial pressures necessary in foam panels actually serve to promote faster water transport.

Also, depending upon the nature of the getters/desiccants employed, they can involve significant recycling and safety problems.

In contrast to foams, Vacupor® vacuum panels do not suffer from this water problem since they can operate at higher pressures (on the same order as the saturation pressure for Vacupor® in many applications) and contain a large amount of inherent water adsorption capacity

In general, the loss in thermal performance with moisture uptake near ambient temperature is roughly proportional to the moisture content (i.e., 0.1 grams of water per gram of Vacupor® corresponds to a 10% decline in thermal performance).

Figure 5 shows the predicted amount of water which will enter a 25mm thick panel if surrounded by a 100% humidity atmosphere. If the panel is one half this thickness, the time required to achieve a given moisture content would be one-half of the designated value.

This implies that the water vapor transport rate (WVTR) of the barrier film for Vacupor®-based inserts needs to be only on the order of 0.1 grams/m2·day for product lifetimes of several years and 0.01 grams/m2 · day for lifetimes well in excess of 10 years. These values for WVTR are easily obtained with certain relatively inexpensive, metallized barrier films which are commercially available.