The present invention relates to evacuated jackets for thermal insulation, and in particular it relates to an evacuated jacket comprising an improved envelope, as well as to a process for manufacturing the same.
Evacuated jackets are being increasingly used in a number of fields wherein thermal insulation at temperatures lower than about 100° C. is required. As examples of application of these jackets, mention can be made of the walls of domestic and industrial refrigerators, of the beverage dispenser machines or of the containers for isotherm transportation, for example of drugs or cold or frozen foods. Further, applications of these jackets in the building field or in the car industry are being studied.
As it is known, an evacuated jacket is formed of an envelope inside which a filling material is provided. The envelope has the purpose of preventing (or reducing as much as possible) the entrance of atmospheric gases into the jacket, so as to maintain a vacuum level compatible with the degree of thermal insulation required by the application. The filling material has mainly the function of spacing apart the two opposite sides of the envelope when the jacket is evacuated, and must have a porous or discontinuous internal structure, so that its porosities or interstices can be evacuated in order to perform the thermally insulating function. This material can be inorganic, such as for example silica powder, glass fibers, aerogels, diatomaceous earths, etc.; or organic, such as rigid polyurethane or polystyrene foams, both in the form of boards and of powders.
The envelope is made with so-called “barrier” sheets, which are characterized by their gas permeability being as low as possible and can be made of a single component but more frequently are multi-layers of different components. In the case of the multi-layers the “barrier” effect is conferred by one of the component layers, or barrier layer, which can be formed of polymeric materials, such as ethylene-vinyl alcohol copolymers (known in the literature with the abbreviation EVOH); of polymeric layers on which a thin layer (generally less than 0.5 μm) of aluminum or of an inorganic oxide is deposited; or of a metal sheet, mainly aluminum, having a thickness generally comprised between 4 and 10 μm. The multi-layer barrier sheet comprises at least one support layer of a polymeric material having good mechanical features, particularly plasticity; said layer can be formed for example of polyacrylonitrile (PAN) or a polyolefine. On the opposite side with respect to said support layer, the barrier layer is covered with at least one protection layer, also polymeric. The polymeric protection layers are commonly made of polyesters (for example polyethylene terephtalate, normally abbreviated in PET) or polyamides (for example, NylonŽ). Multi-layers comprising five, six or even more superimposed layers are also common.
The envelope is generally formed of two barrier sheets having rectangular shape, reciprocally joined along the margins thereof by means of perimetrical weldings. The so joined margins of the barrier sheets form four flanges arranged at the sides of the resulting envelope. However, the main drawback of the envelopes of this kind consists exactly in the presence of these flanges, which are very fragile and their possible fracturing can easily propagate beyond the perimetrical weldings, causing the permeation of atmospheric gases into the jacket and thus compromising the thermal insulating features thereof.
In other technical fields, for example in food packaging, a general process for the preparation of an envelope starting with a single rectangular sheet of plastic material is known, which enables the reduction of the number of the flanges from four to two to be obtained. According to this process, whose steps are briefly illustrated in FIG. 5a-5 c, sheet S is rolled up on itself until two opposite margins M, M′, which belong to the same side of the sheet, meet together (FIG. 5a). Said margins are reciprocally joined by a longitudinal welding, thus forming a welding flange F which is then folded over the external surface of the sheet (FIG. 5b). Thus, an envelope is formed having two opened ends whose sealing, transversely to flange F, is carried out by inserting the edges thereof between welding bars. In the two areas wherein these weldings intersect the flange, the envelope takes on the conformation shown in FIG. 5c (which has an enlarged scale with respect to FIGS. 5a and 5 b).
However, this process cannot be applied to evacuated jackets. As a matter of fact, in the folded position of FIG. 5c, flange F causes a thickness which reduces the passage of heat from the welding bars to the underlying polymeric layers of the barrier sheet and therefore prevents a perfect reciprocal sealing. Further, because of its stiffness, along the folding lines the barrier sheet hardly forms sharp corners and can be only curved; a slot (indicated with L in FIG. 5c) remains in the intersection area between flange F and the seals transversal thereto, which enables the passage of atmospheric gases towards the inside of the jacket, although in a reduced quantity. The smallest gas infiltrations resulting from this imperfections, which would be acceptable in other technical fields, are not acceptable in the case of the envelopes for evacuated jackets.
Object of the present invention is therefore providing an evacuated jacket free from said drawbacks and a process for manufacturing the same. Said object is achieved by means of an evacuated jacket whose main features are specified in the first claim and other features are specified in the subsequent claims. The features of the process are specified in claim 7.
A first advantage of the evacuated jacket according to the present invention consists in that its envelope has a very good gas tightness also at the end seals, although it is made starting from a single barrier sheet. As a matter of fact, thanks to said support layer and said protecting layer being formed of mutually heat-sealable materials having a similar melting temperature, it is possible that the opposite layers which are joined together by means of said longitudinal welding belong to opposite sides of the sheet, so that the resulting envelope is flat in the welding area and does not comprise a longitudinal flange.
Consequently, when the edge of one end of the envelope is inserted between welding bars for the sealing thereof, the heat of said bars causes the melting of said support and protection layers, which become soft, thus allowing the welding bars to near each other so as to eliminate all the slots between the portions of said edges.
An advantage of the process for manufacturing the evacuated jacket according to the present invention consists in that, simultaneously to the sealing of the envelope ends, the thickness of the transversal flanges is made uniform by the welding bars. As a matter of fact, while said bars are nearing each other, the exceeding material is discharged form the sides because of the pressure of said bars and can be removed.
According to a particular aspect of the invention, said support layer and said protection layer are made of the same material.
FIG. 4 shows an enlarged partial view in cross-section of one of said lateral flanges 8. In particular, it shows the portion of said flange which comprises the linear zone 7 of the longitudinal welding. With reference to said drawing, there is shown that the thickness of each lateral flange 8 is uniform all over its length, in spite of the threefold superimposition of the barrier sheet 1, in the linear zone 7 of the longitudinal welding. This can be explained considering that in said linear zone 7 the lower layer 3 and upper layer 4 of the barrier sheet have become thinner. As a matter of fact, during the step of sealing the envelope ends, the heat of the welding bars causes said layers to be melted and to become soft, so that some of the polymeric material of which they are formed is discharged form the sides because of the pressure of said bars and can be removed. As it appears from the drawing, the edges of the envelope ends are perfectly sealed on themselves and no slot is present. On the contrary, by the process according to the state of the art which was previously described with reference to FIG. 5c, the seals in this area are not perfect, so that the entrance of air inside the jacket is allowed which compromises the thermal insulation properties thereof.