Panel Open cell foam

Fig. 5. Scanning electron micrographs of hoUow fiber dialysis membranes. Membranes in left panels are prepared from regenerated cellulose (Cuprophan) and those on the right from a copolymer of polyacrylonitrile. The ceUulosic materials are hydrogels and the synthetic thermoplastic forms a microreticulated open cell foam with a tight skin on the inner wall. Pictures at top are membrane cross sections those below are of the wall region. Dimensions as indicated.

The newer open-cell foams, based on polyimides (qv), polybenzimidazoles, polypyrones, polyureas, polyphenylquinoxalines, and phenolic resins (qv), produce less smoke, are more fire resistant and can be used at higher temperatures. These materials are more expensive and used only for special applications including aircraft and marine vessels. Rigid poly (vinyl chloride) (PVC) foams are available in small quantities mainly for use in composite panels and piping applications (see Flame retardants Hrat-rrststantpot.ymf.rs). 

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]. 

A different approach to making open cell foam slabs has been proposed recently [11], which is based on the use of the PU fluff obtained from the recycling of used refrigerators (Recycled Urethane Fluff, or RUF panel). 

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. 

Mass production technologies to manufacture high quality open cell foam panels are on the market. Several techniques are also becoming available for the fast measurement of the thermal conductivity. This is a very important issue to build a QA/QC system able to ensure the quality and the reliability of the products, not only after production but also... 

Some General Properties of Open Cell PU Foams for Vacuum Insulated Panels... 

To fully exploit the insulating performances of the open cell PU foams, pressure in the panel has to be kept preferably below 1.0 Pa during its life. To achieve this demanding target, the foam must be 100% open celled and with a very low outgassing rate. 

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. 

An additional factor which may have an impact on the vacuum properties is the closed cell content of the PU foam. All the open cell PU foams so far presented in the literature for VIP applications are quoted to be 100% open cell based on pycnometric measurement. On the other hand, the accuracy of this test method is generally close to 0.2% [15], so that the possibility of a small fraction of closed cells cannot be completely ruled out and has to be considered as an additional potential source of pressure build-up in the panel. 

In general, for household appliances, depending on the surface coverage of the cabinet and the panel thickness, energy savings from 10% to 30% have been reported using open cell PU foam-filled panels packaged in a 6 pm aluminium foil-based barrier [12, 15, 52, 56]. 

Foams consist of open or closed cell materials made from many plastics and may be used in sandwich panel manufacture from slab stock or by foaming in situ. The latter presents more problems of attaining uniformity of properties. Commonly used core materials include polyurethanes and rigid polyvinyls and polymethacryUmides (PMl) available in lightweight and industrial grades. 

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