Vacuum panel

Rigid polyurethane foams used in refrigerators and freezers - Alternatives include hydrocarbons (pentane) and HCFC-141b long-term alternatives include HFCs (134a, -245, -356, -365). Vacuum panels may be used in the future. 

To meet the 2001 U.S. energy standards and the 2003 phase-out of HCFCs, there is a great incentive to develop a significantly better thermal insulation. The most dramatic approach would use vacuum panels for insulating the cabinet. A number of U.S. and Japanese manufacturers have developed such panels and placed these kinds of refrigerators in homes. The panels consist of multilayer plastic envelopes filled with precipitated (fumed) silica. The claimed thermal conductivity is one-fourth that of polyurethane foam. The two major obstacles are cost and the maintenance of vacuum for twenty years. 

The design of vacuum panels for a bottle is predicated on a certain liquid and gas shrinkage. Therefore, if the volume of the headspace increases, or the temperatures change, one may end up with a paneled bottle. Paneling may not affect the performance of the bottle as a package, but it is esthetically unpleasing and it makes the bottle hard to label. 

Polyurethanes continue to be one of the most versatile of all polymers, finding applications in foams (flexible, rigid, and in-between), elastomers, coatings, sealants, adhesives, paints, textiles, and films. This volume presents some of the major advances in polyurethanes, both from the materials and research side of things as well as processing and applications, and includes studies on foams (additives, vacuum panel applications, blowing and processing), elastomers, adhesion behaviour and new urethane raw materials. 

Vacuum panels have been stndied as a means to improve thermal insolation for a long time. Several insnlating fillers, snch as silica and perlite powders [1, 2, 3], fibre glass [4, 5], and aerogels [6, 7, 8] have been proposed as core materials for VIP, each of them... 

In this process, the mechanical properties of the foam cannot be neglected, being essential to ensure structural stability of the vacuum panel. 

In fact, after sealing, the vacuum panel is exposed to the hydrostatic load of the atmospheric pressure and has to withstand it for a long time, which can be 15-20 years or even more depending on the application. 

Dimensional stability tests carried out on open cell PU foam-filled vacuum panels showed that creep problems should not occur for properly prepared panels, provided the open cell foam preparation has been optimised [15]. 

The fluff obtained from recycling one single refrigerator can be used to produce enough vacuum panels to insulate a new appliance, thus generating a virtual recycling loop. 

Before being used in a vacuum panel, the open cell PU foam needs a preliminary heat treatment in air, generally carried ont at 120-150 °C for 10-60 minutes to remove water and other volatile species which otherwise would desorb and rapidly cause the vacuum to deteriorate. The result of a typical outgassing test carried out at 23 °C on a foam sample baked at 120 °C for 30 minutes is shown in Figure 4.4 for all desorbed gases but water. Water is difficult to quantify since it sticks to the walls of the system and only partially reaches the mass spectrometer. Water can be estimated as the difference between the total absolute pressure and the sum of the partial pressures of the other gas species, which can be accurately quantified with the mass spectrometer. 

Since desorption is a thermally activated process [18], the outgassing rates increase as the temperature increases. The outgassing contribution has therefore to be carefully evaluated in all those applications where the vacuum panels operate continuously at temperatures higher than room temperature, e.g., 60-80 C, or have to withstand high temperature peaks, for example 100 °C, even for a relatively short period of time. Examples of such applications are presented and discussed in Section 4.6. 

The barrier film plays an important role in vacuum panel technology since it has the task of minimising air and moisture penetration into the vacuum core. The barrier must be... 

Several types of barrier materials, having different structures and gas transmission rates, are commercially available from the food, packaging and electronic industry. Gas barrier requirements for vacuum panels are however much more demanding. 

Results of such an analysis are given in one specific example in Figure 4.9, where the average thermal conductivity of a vacuum panel using a PET 12 i,m/Aluminium 6 i,m/ HDPE 50 pm laminate is plotted against the panel area. 

In general, very little data have been published on the outgassing properties of the skins for vacuum panels, even though they can contribute, in some cases, in a non-negligible way to the deterioration of the pressure inside the VIP. This can be due to the outgassing properties of the materials used as barrier layers and/or the lamination process, which may introduce volatile substances or trap gases in between the various sheets. 

Figure 4.16 Measurement of the pressure in two vacuum panels (with and without getter)...

Sample cut from the flat surface of the vacuum panel 7... 

The rapid development of the open cell foam-filled vacuum panel technology has required parallel development and improvements of the analytical techniques necessary to assess VIP performance and reliability. This latter aspect is key for the widespread adoption of the technology. The selection of the best components, foam, bag and adsorbent, as well as the careful control of the manufacturing cycle, minimises the chance of having poorly performing insulating panels. However, since the potential risk of defective seals or microleaks cannot be completely ruled out, several techniques have been developed to either support and establish the VIP manufacturing cycle or to assess their quality after production. 

In the following sections, some techniques to check the insulating performance of vacuum panels are illustrated, a few of them also being able to be used as a tool for Quality Assurance (QA) and Quality Control (QC) for the manufacturing process. 

The direct measurement of the thermal conductivity or X factor is one of the most common ways to check the quality of a vacuum panel. Several test methods are available, such as the heat flow meter [45], guarded hot-plate and guarded-calibrated hot box [46, 47] procedures, which measure the heat transferred under steady state conditions through the sample whose surfaces are kept at two different given temperatures. 

They allow a pass/fail test on a vacuum panel to be completed within a few minutes, thus providing the possibility for an extensive quality check during the manufacturing cycle. 

The spinning rotor gauge (SRG) technique [51], which is based on the measurement of the deceleration rate caused by gas friction on a freely spinning spherical metallic ball, is particularly well suited to monitor the pressure in sealed-off devices and has been proposed in the recent years also for vacuum panel applications [5, 40, 52, 53]. 

Figure 4.22 Picture of the SAES Getters SpiroTorr SRG connected to a vacuum panel.

This approach allows measurement of both total and partial pressures in the panel, thus providing useful information on the gas ratios, this being useful as an R D tool or to monitor and improve the manufacturing process. However, since this is a destructive and relatively time-consuming test it can only be used for one-off samples. A nondestructive technique to measure total pressure in a vacuum panel has been proposed recently, which is based on the use of a laser beam source coupled to a detection system. 

Insulation Performances of Open Cell PU-Filled Vacuum Panels... 

The actual insulation efficiency of PU-based vacuum panels depends on the intrinsic core material conductivity, the vacuum level, the type of barrier envelope, the panel size and arrangement in the insulating structure. 

It is clear from Figure 4.24 that high ECR values can only be obtained by reducing the aluminium thickness as much as possible and using relatively large vacuum panels, i.e., > 0.5 m per side. 

The influence of the vacuum panel thickness to the total insulation thickness ratio as well as the change in performance, by increasing the foam X value, has been determined by Hamilton [54]. In general, results can be optimised by adjusting the VIP size and thickness to provide the most cost effective option. 

Adoption of PU-based vacuum panels in refrigerators and freezers requires the proper handling of the foaming process, which may influence the structural and vacuum properties of the panel. 

This is very special equipment designed to operate at very low temperatures, e.g., from -30 °C to -86 °C, to age samples or to store valuable and perishable goods, like organs and tissues, biological and medical samples or vaccines. Vacuum panels are used mainly to increase the internal storage volume without increasing the energy consumption. 

Ultra-low temperature freezer models using foam filled vacuum panels were successfully placed on the market by a leading company some years ago and other companies are expected to follow soon. 

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