.

Fig. 3: Structure of a vacuum insulation panel: The core made of fumed silica is processed within a fleece, which is then sealed with a high barrier laminate.
© FIW München

Fig. 6 : The comparison with a 10-μm-thick fibre shows just how finely structured fumed silica is.
© ZAE Bayern

Fig. 7a: The largest cavities in the panels made of fumed silica are only around 200 nm in size.
© ZAE Bayern

Fig. 7a: The largest cavities in the panels made of fumed silica are only around 200 nm in size.
© ZAE Bayern

Fig. 8 : Thermal conductivity of various filler materials optimised for use in VIPs relative to the (nitrogen) gas pressure. The pressure axis is depicted in a logarithmic scale.
© ZAE Bayern

Fig. 9 Semi-automatic VIP production. The device in the centre of the image is the vacuum chamber.
© the vac company
3 / 17

Vacuum insulation: Material and manufacture

Vacuum insulation panels improve the thermal insulation effect not by using thicker materials but through further reducing the thermal conductivity. If the gas pressure inside the panel is sufficiently reduced, the heat transferred by the gas is almost entirely eliminated. This means that even very thin structures can achieve excellent performances. However, the technology poses considerable challenges in terms of the materials and processing.

According to DIN 28400 Part 1: “A vacuum is the state of a gas when the pressure of the gas in a container and thus the particle density is lower than outside or when the pressure of the gas is less than 300 mbar, i.e. lower than the lowest atmospheric pressure on the earth’s surface.” The principle that an “air-free” volume provides good insulation is utilised by the thermos flask, which was already invented at the end of the 18th century. This is because lowering the gas pressure within a volume (evacuation) suppresses gaseous thermal conduction. A sufficiently vacuum-tight envelope– whereby a thermos flask uses stainless steel, aluminium or glass – prevents the gas pressure from exceeding a critical amount.
Cylindrical containers, such as those used for thermos flasks, are by their very nature highly resistant to pressure. In order, however, to transfer the highly efficient insulation technology to flat panels, as is required for insulation in the construction industry, special filler materials or structures for the cavities are required. These must be able to withstand the external atmospheric load pressure of 1 bar, which corresponds to a weight load of 10 t/m². A VIP therefore mainly consists of a panel-shaped, pressure-resistant core material and a sufficiently vacuum-tight envelope.

Criteria for filler materials

For a material to be considered suitable as a core for vacuum insulation, it must be possible to evacuate it and its overall thermal conductivity should be as low as possible. A completely open structure is required. Since the filler materialmust be able to withstand the mechanical pressure load, a higher density is usually required than would be the case if the same type ofmaterial were used in conventional, non-evacuated insulation. In addition to having a low solid-state thermal conductivity, the infrared radiative heat transfer is also reduced by as much as possible. Given the low overall thermal conductivity of the evacuated system, this component has a relatively greater impact. The addition of infrared opacifiers such as carbon black, iron oxide and silicon carbide helps here. Because the gas is largely evacuated, the gas thermal conductivity is insignificant. The extent to which the gas pressure has to be lowered in the VIP elements substantially depends on the size of the pores. The finer they are, the lower the requirements placed on the quality of the vacuum. Depending on the core material, this lies between 0.1 and around 20 mbar (rough vacuum). Not only does this vacuum pressure have to be achieved during the manufacture of the elements but it also has to be maintained during the entire lifespan. Depending on the filler material, this places different demands on the impermeability of the envelope.

Choice of materials for the filling

Possible core materials include open-cell polymer foams (e.g. special polyurethane or polystyrene foams), glass fibres, loose powder or powder pellets (e.g. from silica), and aerogels. Foams and glass fibres require a high quality vacuum with a gas pressure less than 1 mbar. With the particularly finely structured fumed silica and aerogels, a gas pressure of just 10 to 50 mbar is sufficient to substantially suppress the gaseous thermal conductivity. The overall thermal conduction only doubles when there is a residual gas pressure of typically around 100 mbar (Fig. 8). Since there is no single filler material that combines all advantages, the choice of VIP filling depends in particular on the physical properties of the envelope and the type of application.

The thermal conductivity of nano-structured fumed silica is only roughly half that of conventional thermal insulation materials, even in non-evacuated, atmospheric condition with 0.018 W/(m K). In addition to the comparatively low requirements regarding the impermeability of the envelope, the fact that the vacuum envelope still provides a minimum of thermal insulation for preventing mould formation in the case of complete failure predestines this material for manufacturing durable vacuum insulation elements for the construction industry.

Silica ismade from artificiallymanufactured silicon dioxide (SiO2), which in its natural form is most commonly found as sand. Nano-structured fumed silica is mostly produced as a by-product in silicon wafer production. Silica is non-toxic, easy to recycle, incombustible and does not cause harmful emissions. In addition to its low sensitivity to increasing gas pressure, a further advantage of its fine structure is its considerable ability to absorb water vapour that can permeate through the envelope in small amounts. In contrast to coarser materials, nanostructured powders can be easily compressed to form panels and can therefore be easily handled in the VIP manufacturing process.

Criteria for the envelope materials

Whether a material is suitable as an envelope principally depends on its gas impermeability. In terms of the life expectancy of a VIP, its impermeability to water vapour is also important. In addition, a low thermal conductivity is also decisive in order to keep the thermal bridge effect as low as possible along the edges of the panels. Because of the mechanical loads that occur during construction, the envelope materials should also have sufficient puncture resistance.

Choice of materials for the envelope

The envelope materials used for thermos flasks, namely stainless steel, aluminium and glass, are also generally suitable for flat vacuum insulation panels. In combination with filler materials such as foams and fibres, it is actually only these materials that achieve the required high gas impermeability that is required for durable products. In practice, manufacturersmostly use aluminium-metallised high barrier plastic laminates, aluminium composite films and stainless steel films or sheets to provide the VIP envelopes.

  • Three times metallised high barrier laminates

This rather inelegant name actually provides a rather good description of the multilayer structure of these films. They consist of several layers of polymers that are characterised by their low thermal conductivity and high durability. Because the barrier effect provided by pure polymer films is nowhere near sufficient for use in VIPs – in particular to protect against water vapour – additional layers made of aluminium, aluminium oxide or silicon oxide increase the impermeability. Because it is not possible to produce perfect and faultless barrier layers in the vapour deposition process, several of these are required – typically three in building construction.
With the high barrier laminates, it is solely the inner layers that are metallised. The outermost layer has to protect against environmental and ambient influences, whereas the innermost layer acts as a sealing material. Depending on the underlying conditions and the requirements for the VIP, polyester, polyamide, polypropylene and polyethylene are used for the individual layers. Polyethylene is mostly used for the sealing material and occasionally polypropylene.

The greatest advantage of thismulti-coated, multi-layered plastic laminate is its low thermal conductivity. Because the overall thickness of the aluminium layer in the entire assembly is only around 100 nm, the metal does not significantly influence the effective thermal conductivity of the VIP. However, if aluminium is deployed as the barrier layer, the vapour-deposited films are susceptible to corrosion under certain conditions.

  • Aluminium composite films

Aluminium composite films for VIPs consist of an aluminium film with a layer thickness between 6 and 12 μm in the middle and two plastic films on the outer surfaces. The advantage of such a metallic envelope is its high gas impermeability. The disadvantage is the strong flow of heat through the edge of the panel, which reduces the effective thermal insulation of the VIP. The thicker the metal layer and the smaller the panel, the greater the edge effect.

The manufacture of VIPs

The manufacture of vacuum insulation panels substantially corresponds to the vacuum packaging technology used in the food industry. The main differences relate to the dimensions of the VIPs and the increased demands regarding the permitted residual gas pressure (vacuum). Depending on the production process, the core is first of all cut to any desired format from pre-pressed panels or a special press mould is required for each panel size, which reduces the flexibility in the production process. The core is then encapsulated in a correspondingly sized envelope. The VIP is generally evacuated in a vacuum chamber. The size of the chamber determines the maximum possible dimensions of the panels (e.g. 2 m x 1 m).

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