EP0915013B1 - Method of lidding containers, cover film for carrying out this method and product thus obtained - Google Patents

Abstract

To seal a container, with its contents, a continuous film of at least two layers (1,2) is used, with perforations (3) in the inner film in a density of 100-10000 perforations/dm<2>, and an adhesive bond (5) between them. The adhesive bond leaves a grid (6,7) of zones free of adhesive, to give at least one longitudinal channel to the film edge after cutting. The film is placed over the filled container (A), and is adhesive (4) bonded all round its edge (C). The sealing adhesive is activated through the outer layer (2) without damage to the inner layer (1), to leave the edge channel after bonding to the container edge and after cutting. The continuous sealing adhesive is applied to the inner layer before perforation. The two layers are assembled together in a material which can be wound in a roll as a feed at the sealing station. The adhesive bond (5) between the layers and the sealing adhesive are activated together, when the film material is placed over the filled container. The sealing adhesive has a lower fusion point than the bonding adhesive, and the two adhesives are of different types, where the bonding adhesive is produced by condensation and the sealing adhesive by polymerization. A sealing adhesive is applied to at least one of the layers at the intersections of at least one of the open channels (6,7) at a temperature over ambient levels, but below a temperature where the container contents are heated. The perforated layer can be formed by at least two inner layers, bonded together, where the innermost gives a secure bond with the edge of the container and its covering layer gives a firm bond with the outer layer through the bonding adhesive. The perforations (3) can be in a density of 200-2000 perforations/dm<2> or 500-1200 perforations/dm<2>, equidistant or at different intervals on both plan axes. The open channels have a width of 2-5 mm or 3.0-3.5 mm, in a channel density of 5-15 per dm. Some legs of at least one channel (6,7) carry sealing adhesive (4).

Description

We have known for a long time containers with a rim on which a sheet lidding is sealed so that the sheet is multilayer or not. Such a structure presents no major difficulty when the container is filled with a produced at constant temperature.

If, on the contrary, the filled container is subjected to temperature variations, the result is pressure differences that can be large enough to destroy it or, at all less to make him unfit for his job.

Temperature constraints of this magnitude are unavoidable when the content must be sterilized or cooked after packing.

• la cuisson de l'aliment avant son conditionnement dans un récipient hermétiquement operculé,
• la cuisson après conditionnement dans le récipient.

In the case of food products, the cooking of a food can be carried out according to two fundamental processes which are:

• cooking the food before its packaging in a hermetically sealed container,
• cooking after conditioning in the container.

The cooking of the food made before the final packaging poses problems complex technical and economic Cooking must be done under conditions hygienic standards because it is essential to avoid any bacterial contamination after cooking, and if the skilled person knows how to perform a conditioning aseptic, it is at the cost of very elaborate and very expensive facilities.

When conditioning before cooking, the container and its lid are sterilized at the same time as the food, by the cooking operation itself, which has the advantage of simplicity.

But, this imposes precautions either in the design of the container itself, or in the implementation of cooking because it causes temperature variations and therefore pressure inside the closed container, variations which must necessarily be compensated to prevent rupture of the containers.

In addition to cans, which in themselves have a very high resistance to internal overpressures, there are many relatively flexible packages that do not could not withstand a rise in temperature, itself generating a overpressure much higher than the resistance of the materials used which are usually in synthetic materials and which risk irreversible deformations, or even complete destruction.

For these, compensation systems must be used in cooking, such as steam ovens.

In practice, not all furnaces are equipped with such equipment and even when exist, they do not always give satisfaction, in particular because of difficulties in setting.

In view of all these difficulties, it was thought to provide the receptacles themselves with valves, or valves, as simple as possible which ensure on the one hand the exit to the atmosphere of the gases under pressure inside the container itself, and secondly the return to a filling complete as soon as the internal pressure returns to normal.

To illustrate these known arrangements, there may be mentioned trays equipped with a valve thermofusible which opens during a temperature increase and then closes during cooling, by reconstituting the tightness of the tray and the protection antibacterial that this seal ensures.

Unfortunately, these trays are expensive and require a discontinuity of the line of manufacture, so that they do not constitute a really advantageous solution.

It was then thought to provide a valve not the actual container but its operculum, formed of a plastic sheet more or less complex, especially multilayer.

A solution of this type is described in US Pat. No. 2,870,954 which relates to a sachet of which the edges are hermetically welded and which has a central valve. Although this valve is presented as allowing the vacuum to be maintained inside the container and not to evacuate an internal overpressure, the structure is of the type also used for this application.

This solution is not satisfactory because the part which contributes to realize this valve must welded on the inner side of the bag and its small dimensions make its manipulations are very inconvenient. Moreover, it is necessary to respect a marking of positioning so that the two offset openings are properly arranged.

Another analogous solution is described in DE-A-2 331 862 and in the patent DE-A-3521373.

Patent FR-A-2,629,060, meanwhile, proposes to provide trays with a real flapper antiretour movable between two positions respectively corresponding to the opening to the atmosphere of the interior of the tray and its hermetic closure.

This very "mechanical" solution has the disadvantage of being complex to implement and, therefore, Dear.

There are also solutions applicable exclusively to bags obtained by folding a sheet on itself and welding in three lines, and therefore do not relate to sealing of containers but rather the bags known internationally under the name of "boil in bags", intended to be immersed in a hot fluid (water or steam) for the purpose of reheating foods they contain.

By way of example, mention may be made of the patent application EP-A-0 531 176 which relates to such a device. bag for a product called "boil-in-the-bag", and which describes a container of the general type recalled above, stating that one of the faces of the bag is formed by a sheet having two films, one of which has holes that may or may not be facing channels rectilinear and parallel. No precise indication is given as to the density of the holes or channels, which does not allow the craftsman to make a suitable article, because experience shows that very vague, and indeed very few, indications of the description lead to a container that bursts when its contents are warmed, as a result the internal pressure of vapors that can not be properly removed, reason for which, probably, this patent application has been abandoned.

DE 0 160 978 discloses a sheet of synthetic material formed of two superimposed films. A joining adhesive is interposed in the form of a large number of small areas that are not in contact with each other. This sheet presents a static-type operation with unglued areas that remain open for allow "a gas exchange".

US 4,884,694 discloses a sheet of synthetic material formed of two superimposed films, the upper film being made of paper and the lower film being thermoplastic material and provided with perforations. This sheet helps to guarantee the gas permeability required for gas sterilization, and in particular hot steam.

The present invention provides a novel solution to the problem of lidding containers to subject them to differences in temperature and pressure, while using simple, proven and economical manufacturing methods.

To this end, the subject of the invention is a process as defined in claim 1.

The subject of the invention is also a sheet of synthetic material as defined in the claim 11.

The invention also relates to an industrial product constituted by a container filled with a content, having a flange and closed by means of a seal which is sealed to the entire surface of the rim and which consists of a sheet of synthetic material conforming the above definition, with areas left free and constituting a network of several parallel channels having at least one outlet to the atmosphere by the edge, or one of the edges, of the closed container.

La figure 1 est une vue schématique en perspective qui illustre la mise en oeuvre générale du procédé conforme à l'invention.

La figure 2 est une vue schématique en perspective montrant la confection d'un produit intermédiaire conforme à l'invention, destiné à l'operculage de récipients.

La figure 3 est une vue schématique partielle en coupe illustrant l'évacuation à l'ambiance des gaz sous pression contenu dans le récipient, après operculage.

Les figures 4, 5 et 6 sont des vues schématiques en élévation montrant trois phases de operculage d'un récipient au moyen du produit intermédiaire.

Les figures 7, 8 et 9 sont des vues schématiques en plan, correspondant aux vues en élévation des figures 4, 5 et 6.

Les figures 10, 11 et 14 sont des vues schématiques montrant différentes dispositions possibles pour les zones démunies d'adhésif de jonction et formant canal d'évacuation de gaz sous pression.

La figure 16 est une vue schématique représentant un fragment d'une feuille conforme à l'invention, sensiblement en vraie grandeur, et montrant des dimensions réelles pour les ouvertures et leur densité, ainsi que pour les canaux.

La figure 18 est une vue schématique illustrant un mode de réalisation de l'invention selon lequel l'une des deux pellicules est formée de deux couches de compositions différentes.

The invention will be better understood from the detailed description given below with reference to the accompanying drawing. Of course, the description and the drawing are given only by way of indicative and nonlimiting example.

Figure 1 is a schematic perspective view which illustrates the general implementation of the method according to the invention.

Figure 2 is a schematic perspective view showing the manufacture of an intermediate product according to the invention for sealing containers.

Figure 3 is a partial schematic sectional view illustrating the evacuation to the atmosphere of the pressurized gas contained in the container, after sealing.

Figures 4, 5 and 6 are schematic elevational views showing three phases of sealing a container by means of the intermediate product.

Figures 7, 8 and 9 are schematic plan views corresponding to the elevational views of Figures 4, 5 and 6.

Figures 10, 11 and 14 are schematic views showing various possible arrangements for areas of poor jointing adhesive and forming a channel for discharging gas under pressure.

Figure 16 is a schematic view showing a fragment of a sheet according to the invention, substantially in full size, and showing real dimensions for the openings and their density, and for the channels.

Fig. 18 is a schematic view illustrating an embodiment of the invention wherein one of the two films is formed of two layers of different compositions.

Referring to FIG. 1, it can be seen that the process according to the invention is intended for the lidding of containers A of the known type comprising a body B and a flange continuous device C. Here, the container A is an oblong tray but it can be containers of other shapes, in particular pots whose body has a circular section and a frustoconical profile, the edge itself being circular. Another example of operculum containers are pots having flat faces, connected by curves, and whose rim has a substantially square contour with rounded corners.

The lidding must, of course, be carried out after filling the container with a D content that will be supposed here to be a food.

The cover must be applied over the entire surface of the edge C and be attached to it by means of a adhesive.

This operculum consists of a sheet composed of two films respectively internal 1 and external 2.

The inner film 1 is previously perforated to be crossed by multiple small perforations or "microperforations" 3, in order to be permeable to gases.

The outer film 2 is continuous and waterproof.

Both films 1 and 2 must be assembled together, which can be at the same time that the inner film 1 is fixed on the flange C or previously, but anyway, this assembly must leave free areas, so that an adhesive inserted between them, called "junction", must be present only on a part only from the surface of films 1 and 2.

When the two films 1 and 2 are superimposed together on the flange C, the fastening is continuously sealed over the entire surface of the rim C, because the microperforations 3 are considered to be negligible, on the one hand because of their very small size, and on the other hand because of the spreading of the synthetic material that results from the temperature and the pressure implemented to apply very strongly this film 1 against the rim C.

In contrast, the two films 1 and 2 are joined to each other only by the places where junction adhesive, to determine a network of channels that open up on the edges of the leaf, between the two films 1 and 2.

Thanks to the very large number of microperforations 3, we are certain that some of they will be in the channels, without the need for any tracking but also because of their high density, these microperforations present, together, a large section of total passage, while being distributed over all the surface of the film 1.

As a result, the atmosphere inside the tray A can communicate with external atmosphere, provided that it is brought to sufficient pressure to exclude films 1 and 2 of each other according to the channels determined by the absence of adhesive junction.

It is therefore understood that if, after lidding, the entire container A is carried and its contents D at a temperature of sterilization, cooking or reheating, gases and vapors will be evacuated easily to the outside instead of inflating the lid until the burst, although the attachment of the lid on the rim C is carried out in a manner tight and continuous, and despite the presence of the outer sheet 2 itself continues and waterproof.

During attachment of the bicellular sheet to the rim C of container A (or after this fixation), this sheet is cut, in particular by a cutting blade symbolized in E in FIG. 3, as close as possible to the contour of said flange C, in particular by leaving a tongue (not shown) facilitating its subsequent uprooting by the user, so that he can access content D.

The above description with reference to FIG. 1 implicitly assumes that both films 1 and 2 are distinct before being stretched together above the container A, the joining adhesive can, indifferently, itself be separate from the films 1 and 2, and be formed, in a manner known per se, of a web unwound from a reel (not shown), or be previously applied on the face of any of these films in front of to stand next to the other film.

Similarly, the fixing of the sheet on the flange C can be achieved by any means known, in particular by means of a lidding adhesive, it can also be separate from film 1 and edge C, or may have been previously applied to one or the other.

It is also possible to fix the sheet on the edge C by welding, when the synthetic materials constituting the rim C on the one hand and the sheet 1 on the other hand are provided for this purpose.

FIG. 2 shows the case where the sheet 1 bears on its underside an adhesive 4 and on its upper side a bonding adhesive 5.

Since this sheet 1 must be permeable to gases, it is necessary to avoid that the adhesive 4 closes the microperforations 3.

The method for producing sheet 1 then advantageously provides for the introduction of the adhesive 4 on the film 1 first, then the simultaneous perforation of the sheet 1 and the adhesive 4, then the fixing of the films 1 and 2 together by the bonding adhesive 5, the latter not necessarily perforated because it is discontinuous.

Indeed, it can be seen in FIG. 2 that the joining adhesive 5 is arranged according to squares aligned and separated from each other by zones 6 and 7 without adhesive and disposed in two perpendicular directions and forming a relatively dense network.

The film 2 is then superimposed on the sheet 1 provided with the adhesive 4.

As mentioned above, it is possible to superimpose sheets 1 and 2 just at the moment of their placement above the container A, so that the attachment of the sheet to the rim This will be done at the same time as the two films 1 and 2 are fixed to each other.

In this case, the two adhesives 4 and 5 may have the same physico-chemical characteristics, since the same single heating operation must have the same effects on both adhesives.

One difference, however, is to be underlined: while the lidding adhesive 4 is applied in a continuous surface, the bonding adhesive 5 is applied discontinuously.

Areas 6 and 7 lacking junction adhesive 5, create as many channels 8 and 9 (Figure 3) between the two films 1 and 2.

At room temperature, the two films 1 and 2 are strictly applied one on the other and the D content is perfectly preserved from any contamination from the external atmosphere, because the outer film 2 is waterproof and continuous and because the inner film 1 is itself fixed to the flange C in a sealed and continuous manner.

When, on the contrary, the entire container A is filled and sealed temperature rise (especially with a view to sterilizing the contents D), the gases and vapors that are necessarily released inside the container A, cause a overpressure and cross the film 1 by the multiple microperforations 3 that are found next to the channels 8 and 9, then, because of this pressure, separate the two films 1 and 2 where they are not fixed (zones 6 and 7) and use channels 8 and 9 whose outlet is on the four edges of container A.

Indeed, the zones 6 and 7 extend continuously to the edges of the film 1 as soon as the application of the bonding adhesive 5 and remain after cutting the entire sheet, both in the transverse direction (zones 6) and in the longitudinal direction (zones 7).

After reheating, the user can remove the lid to access the content D, without fear a violent effect due to the internal overpressure, this being automatically eliminated by evacuation of gases through channels 8 and 9.

Figures 4 to 9 illustrate another embodiment of the method according to the invention. Indeed, here we proceed first to the complete development of the bicellar sheet, in associating the two films 1 and 2 by the joining adhesive 5, in order to obtain a set unit forming a complete industrial product, capable of being sold, delivered, stored and used until closed, without prior operation for the undertaking which carries out the filling container A and sealing it.

The lidding adhesive 4 may or may not be associated with the sheet thus produced, according to the respective characteristics of the rim C and the film 1.

According to one variant, this lidding adhesive 4 is effectively applied to the face outer of the sheet, that is to say the outer face of the film 1 opposite the film 2, so that the complete industrial product is immediately available for the lidding by gluing of the sheet on the rim C, independently of the material used to make container A.

FIGS. 4 and 7 show a coil 10 formed by the winding of numerous turns of a sheet comprising on the one hand the two films 1 and 2 and the joining adhesive 5 after activation so that the two films are fixed to one another, and secondly the lidding adhesive 4.

This sheet is unwound from the spool 10 and is applied stretched over a container A filled with its content D, then this set is placed next to a known mechanism in comprising on the one hand an upper punch 11, bearing a peripheral failure 12 of shapes and dimensions adapted to those of the edge C and comprising means for heating and secondly an anvil 13.

The container A is placed between the punch 11 and the anvil 13, which are then approached one another, in order to strongly pinch the sheet between the upper punch 11 and the flange C supported by the anvil 13, which has the effect of activating the lidding adhesive 4 and to cause the sheet to be attached to the rim C.

Simultaneously, the sheet is cut as close as possible to the contour of the edge C, thanks to a cutting element 14 provided at the periphery of the fault 12.

Then, the punch 11 and the anvil 13 are spaced apart (FIGS. 5 and 8) and the sealed container is evacuated to another position in the packaging line.

The cutting of the leaf around the edge C, leaves a waste sometimes called "sail" F (Figures 6 and 9) which is evacuated and eliminated.

Using a previously prepared sheet, the activation of the lidding adhesive 4 intervenes while the joining adhesive 5 has already been activated since the two films 1 and 2 are fixed to each other.

However, the activation of the lidding adhesive 4 is done through the entire sheet, because that the fault 12 and the anvil 13 are located on either side of the rim C covered with the sheet, it is necessary to avoid that the heating due to the action of the failure 12 modifies the adhesive of junction 5 which is at the origin of the existence of the channels 8 and 9.

According to one characteristic of the invention, the two adhesives 4 and 5 have characteristics physicochemical, so that during sealing, the adhesive 4 is activated and that the junction adhesive 5, already used, remains intact.

For example, two different adhesives can be chosen, the one intended for lidding being active at a temperature significantly lower than that of the bonding adhesive 5.

It is also possible to adopt for the joining adhesive 5 a material obtained by polycondensation, whereas the lidding adhesive 4 is a material obtained by polymerization.

The arrangement of the bonding adhesive 5 can be in a number of variants, such as examples are shown in Figures 10, 11, 14 and 16.

Since the sheet must fulfill its function of evacuating the internal gases under pressure, it is necessary to it can be applied "per kilometer" above the containers to be sealed in series, it is necessary and sufficient that the inner sheet 1 is crossed by multiple perforations and that the outer sheet 2 is adhered to the sheet 1 only partially, in order to obtain not a single isolated channel (and therefore to locate) but a network of several channels, or a network of several branches of a single channel.

The interest of the diagrams of FIGS. 10, 11, 14 and 16 is to show different dispositions of the zones 6-7 lack of bonding adhesive 5 on the film 1, the latter being crossed multiple pinholes. These microperforations are not shown in FIGS. 10, 11 and 14, only to keep them as clear as possible, but in reality these microperforations are necessarily present, as shown schematically in the figure 16.

Similarly, film 1 has been given a circular shape, not that it constitutes a limit of the application of the invention to containers or pots having this section, but as representing any portion considered on a film 1.

In FIG. 10, the joining adhesive 5 leaves parallel straight regions 61 remaining, oriented longitudinally with respect to the film 1.

It is observed that the channels resulting from the existence of the zones 61 open necessarily at the ends of the film 1 perpendicular to its axis, and perhaps on one or both edges parallel to this axis, if, as shown at 62 on the part right of Figure 10, the sheet is cut along the rim of the container, in line with one channels.

In FIG. 11, the zones 63 are still rectilinear and parallel, but oriented obliquely to the axis of the film 1.

In FIG. 14, zones 68 are arranged in parallel but have a wavy course and no longer rectilinear. They are represented as extending parallel to the axis of the film 1, but they could also be oblique.

It is clear from the above description that channels 8 and 9 allow evacuation automatic gas and vapor pressure existing inside the container A sealed. When this pressure decreases or even disappears completely, the channels close again automatically thanks to the intrinsic elasticity of films 1 and 2 or, in any case, the top 2 film. When the internal pressure drops below the pressure Atmosphere of the atmosphere, the channels are even hermetically closed and oppose at any entrance of outside air towards the interior of the container, as a consequence of the depression which reigns in container A.

It is therefore possible, after filling and sealing, to subject the containers to several heating operations. A first operation can be planned at the place of packaging for the sterilization of containers and their contents. Then, a second operation can take place at the user to warm the content, especially when it comes to food.

But one can also wish a hermetic closure after submission of containers filled and capped at a temperature rise, for example after sterilization, in which case it is possible to shut off channels 8 and 9 after they have been played their venting role and the internal overpressure ceased.

For this purpose, it is possible to provide a sealing adhesive (not shown) on zones 6 and 7 of which the action, ie the adhesive power, vanishes at a significantly higher temperature than that of the bonding adhesive 5, so that it is not activated during the fixing of the films 1 and 2 between them or when lidding with the adhesive 4.

It is during a subsequent operation, especially when applying to the sheet capped a relatively high temperature, that the sealing adhesive becomes active, this which causes the hermetic closing of the channels 8 and 9.

The sealing adhesive ensures that there will be no communication between the interior container A and outside, during various handling and storage.

It may be an adhesive sensitive to temperature and / or humidity. For example, we may adopt a long-acting adhesive of the type applied to articles and sheets of paper repositionable, which allows the two films 1 and 2 to move easily apart when the container A is subjected to heating and / or immersion and which, in any case, is in contact with the gases and vapors released by the content D, while reconstituting the collage of these films 1 and 2 when the internal pressure allows them to put back in touch. This effect is even more marked, and the watertightness is ensured, when the internal pressure becomes lower than the atmospheric pressure because the depression which in This results in true plating of the films 1 and 2 against each other.

The sealing adhesive may also be chosen to melt at a certain temperature. In this case, it must be chosen to be neutralized easily during the elevation of temperature from the sterilization and / or reheating of the content D, so that the exhaust channels for gases and vapors are quickly and largely released before the internal overpressure reaches a dangerous value for the integrity of the container.

In addition, the sealing adhesive has the advantage of compensating, if necessary, the small spacing and / or lack of adhesion of films 1 and 2, especially from tiny irregularities created during the formation of microperforations.

Indeed, when these microperforations are created by means of heated needles, the rise in temperature can cause, by local softening, the formation of a small bead around each microperforation, and the thickness of this bead, imperceptible to the naked eye has the effect of spreading the two films 1 and 2, and consequently to create a small cavity decreasing the contact surface between these films, thus adversely affecting the sealing of the lidding.

This phenomenon is all the more formidable as the number of perforations is large.

Now, precisely, it is a characteristic of the invention to provide a high density perforations and, correspondingly, very small dimensions for each of them, in order to distribute over the entire surface of the lid countless gas and vapor outlets to channels themselves relatively numerous and well distributed, with outlets advantageously radiating all around the lid.

Microperforations can also be obtained with cold needles, but then acts in a way by tearing, which leaves minute irregularities but very visible under the microscope, having effects equivalent to those of the bead described above.

It is therefore important to create microperforations that are as regular as possible and whose edges are frank, that is to say, without overthicknesses.

The density of the microperforations, between 100 and 10 000 perforations per decimeter square, has been found to provide excellent results between 200 and 2000 perforations per square decimetre in some cases, and between 500 and 1,200 perforations per square decimetre in other cases. However, as noted above, current research lead to an increase in the density of perforations up to 10,000 perforations per square decimeter and miniaturization of the perforations, as this leads to an excellent distribution of the evacuation vents of gases and vapors under pressure.

It even seems that the density of 10,000 perforations per decimeter can be exceeded square, by means of apparatus being improved.

We understand that perforations so small and so many give birth to a different from those currently known and which have at most a few dozen holes.

It appears, however, that this density must be coordinated with the nature of the content D, because a liquid content is homogeneous in the interior space of the container A and requires a density of perforations incomparably less than that of a foil capping a container containing a food preparation consisting of bulky solid parts, preventing a free flow of gases and vapors. In this case, the microperforations must be numerous to be everywhere above the solid parts like liquid parts.

FIG. 16 is an almost full size view illustrating in a very schematic the appearance of a fraction of the film 1 according to the invention, being specified that in reality the film is transparent and microperforations virtually invisible because they correspond to the material removed.

In Figure 16 the channels have a wavy course and are parallel, having outlets located only at the transverse ends of the sheet, and therefore of the container capped.

The microperforations can be spaced from each other at equal distances in orthogonal directions, or on the contrary be separated from different distances.

Satisfactory tests have been carried out with intervals of between 2 millimeters and 10 millimeters.

Examples of equal intervals made: 2.2 X 2.2 millimeters; 3.5 X 3.5 millimeters, 6 X 6 millimeters.

For the same reasons as those concerning the density of microperforations, the canals should be narrow and numerous than broad and rare, especially when the content is heterogeneous.

Good results have been obtained with channels whose width is between 2 and 5 millimeters and preferably between 3 and 3.5 millimeters.

The density of the channels, that is to say the number of channels per linear decimeter considered transversely to said channels, is advantageously between 5 and 15.

The invention provides for a very large number of perforations of very small diameter as well that a significant number of narrow channels, sealing the fixation of both film 1 and 2 to each other is crucial, as well as, of course, the sealing of the lidding of film 1 on the c-flange

Therefore, according to a feature of the invention illustrated by Figure 18, itself the film 1 is formed of two layers 1a and 1b fixed to each other along their entire surface and of different but compatible composition.

One can then choose a composition of the lower layer 1 is coordinated with the composition of the constituent material of the rim C, while choosing the composition of the upper layer 1 b coordinated with that of the film 2.

This gives a double seal due to a particularly well-adapted bonding of a part of the lidding of the sheet on the container A, and secondly films 1 and 2 between they.

When the constituent material of the container A (and thus its rim C) is polypropylene, is chosen to constitute the lower layer 1a, cast polypropylene, that is to say of extruded polypropylene - calendered, non-oriented, then that the layer 1b is polyethylene terephthalate ("PET"), the film 2 is also PET.

The welding on the flange C on the one hand, and the bonding of the film 2 with the layer 1b on the other hand are excellent and provide a bonding seal of very high quality. This solution amounts to using for the lid of the container A no longer a biplex 1-2 but a triplex 1 to -1 b -2.

Here are some embodiments of a sealing foil according to the invention:

Example 1

A film of polyethylene terephthalate thickness μ is coated on one of its faces of a universal heat-sealing lacquer and constitutes the inner film 1. This film is pierced with microperforations 3 using a microperforator cold needles, in a configuration where the microperforations are 5 mm apart in the direction of the sheet and 5 mm in the longitudinal direction, shifted by a distance of 2.5 mm per row to give a pattern in squares.

A film also made of polyethylene terephthalate but only 12μ thick constitutes the outer film 2.

These two films 1 and 2 are laminated on a lamination line in a solvent medium with interposition of a two-component glue. A screen cylinder allowing the deposit of the glue comprises peripheral grooves of 2.5 mm width, equidistant from 5 mm.

The sheet thus "complexed" in width of 600 mm is used on a machine of packaging of the type known as "Fill Seal" for sealing trays crystallized polyethylene terephthalate ("CPET"), with a circular periphery of 95 mm diameter.

The equipment is used to seal two rows of trays over a width of 190 mm. Filling, sealing and cutting operations are performed on the machine.

The trays thus sealed are subjected to a cooking test:

Test No 1 :

The trays trays are placed in a steam oven where they undergo a temperature of 90 ° C for 35 minutes.

The packaged products are cooked properly and the container has not undergone any deformation.

Test No 2 :

The trays trays are placed in a traditional oven where they undergo a temperature of 150 ° C for 90 minutes.

The packaged products are cooked properly and the container has not undergone any deformation.

Example No. 2

The sheet is formed by the same films, as in Example 1. The screen cylinder which allows the deposition of glue has grooves in the "machine direction" (longitudinally with respect to the scrolling of the films) and in the cross direction, the grooves having a width of 2.5 mm and spaced 5 mm apart.

Example No. 3

The sheet is formed by the same films, as in Examples 1 and 2. screened cylinder which allows glue deposition has spiral grooves.

Example 4

The sheet is formed by the same films, as in Examples 1, 2 and 3. A screened cylinder which allows glue deposition has sinuous grooves.

The use of cold needles to create microperforations has the advantage of perforating the film without forming beads, the latter having the negative effect of limiting the contact between the faces facing the films, being reminded that the inner film 1 can itself be formed of several layers assembled ..

The outer film 2 is full, that is to say, continuous and waterproof. It can be simple, as described, or formed of several assembled layers.

Claims (24)

1. Method of lidding containers such as pots, trays and the like which are to contain a product that is intended to be heated in the container itself, according to which method a container which has a rim is manufactured, then it is filled with its contents and then a sheet of synthetic material is placed on the rim, said sheet comprising at least two layers, one of which is pierced with holes, said layers being assembled together over just a fraction of their surface so as to create at least one channel between them, which sheet is stretched over the filled container and then this sheet is fixed to the rim in a sealed manner, then the sheet is cut as close as possible to the outer contour of said rim so as thus to form a sealed lid, characterized in that a sheet of indefinite length is produced beforehand by means of at least two sealed films, respectively referred to as the "inner" film (1) and the "outer" film (2), and in that, in any order:

   the inner film (1) is pierced with a number of small perforations (3) in a density of between 100 and 10 000 perforations (3) per square decimetre,

   a "joining" adhesive (5) is deposited on either one of the two facing surfaces of the films (1-2) in a geometrically regular manner,

   the two films (1 and 2) are superposed,

   these two films (1 and 2) are assembled by means of the joining adhesive (5) located between the two films (1 and 2) and deposited just on part of the surface of the films, so as to leave clearly determined continuous geometric zones (6-7) with no joining adhesive (5), which zones form a network of several parallel channels, the opening of which is located on at least one edge of the sheet, which edge, if only one exists, is either longitudinal prior to cutting of the sheet or transverse after cutting, the channels being able to open and close automatically under the effect of pressure so as to allow the evacuation of gases,

   this stretched sheet is placed on top of the filled container (A) by bringing the inner film (1) into contact with the rim (C), with the interposition of a continuous lidding adhesive (4),

   the sheet is fixed to the rim (C) by activating the lidding adhesive (4) through the outer film (2) by means which act only on said inner film so as to leave the passage of each channel free even after fixing to the rim (C) and after cutting.
2. Method according to Claim 1, characterized in that the continuous lidding adhesive (4) is deposited on the inner film (1), prior to piercing of the latter.
3. Method according to Claim 1, characterized in that the two films (1 and 2) are assembled together before the resulting sheet is fixed to the rim (C) of a container (A), so as to form a unitary assembly which can be stored, in particular on a reel (10), and supplied as such with a view to a subsequent lidding operation.
4. Method according to Claim 1, characterized in that the two films (1 and 2) are superposed, one of said films bearing the joining adhesive (5), in that these two superposed films (1 and 2) are placed on top of the filled container (A), and in that the joining adhesive (5) and the lidding adhesive (4) are activated at the same time so as to simultaneously achieve assembly of the two films (1 and 2) to one another and the sealed fixing of the sheet as a whole to the rim (C).
5. Method according to Claim 1, characterized in that the lidding adhesive (4) has a melting point which is considerably below that of the joining adhesive (5).
6. Method according to Claim 1, characterized in that the joining adhesive (5) is of a different nature from that of the lidding adhesive (4) in order to be insensitive to the activation of the latter.
7. Method according to Claim 6, characterized in that the joining adhesive (5) is obtained by polycondensation whereas the lidding adhesive (4) is obtained by polymerization.
8. Method according to Claim 1, characterized in that a sealing adhesive is applied to at least one of the films (1 and 2) in at least some of the parallel channels (6, 7, 61, 63, 64-65, 66-67, 68, 69), the action of which sealing adhesive is cancelled at a temperature above room temperature and below the temperature at which the contents are to be heated.
9. Method according to Claim 1, characterized in that the perforated film (1) is formed by means of two layers (1a and 1b) which are bonded together over their entire facing surfaces, the layer (1a) which is to be in contact with the rim (C) of the container (A) having a composition which makes it particularly able to be fixed in a sealed manner to said rim (C), whereas the layer (1b) which is to be in contact with the sealed film (2) has a composition which makes it particularly able to be fixed to said sealed film (2) by means of the joining adhesive (5).
10. Method according to Claim 9, characterized in that, with the rim (C) of the container (A) being made of polypropylene, the layer (1a) which is to be in contact therewith is made of cast propylene whereas the layer (1b) which is fixed to the aforementioned layer (1a) is made of polyethylene terephthalate, along with the sealed film (2).
11. Sheet of synthetic material for lidding containers (A) such as pots, trays and the like which have a rim (C), said sheet being formed of two superposed films (1 and 2), one (1) of the films (1-2) having a number of small perforations (3) in a density of between 100 and 10 000 perforations (3) per square decimetre, the other (2) being sealed, characterized in that a "joining" adhesive (5) is interposed in a geometrically regular manner between the two films (1 and 2) over just part of their surface and leaves clearly determined continuous geometric zones (6, 7, 61, 63, 64-65, 66-67, 68, 69) with no joining adhesive (5), which zones form a network of several parallel channels (6, 7, 61, 63, 64-65, 66-67, 68, 69), the opening of which is located either on at least one of the two longitudinal edges of the films (1 and 2) or at at least one of their transverse ends, the channels being able to open and close automatically under the effect of pressure so as to allow the evacuation of gases.
12. Sheet according to Claim 11, characterized in that the density of the perforations (3) is 200 to 2000 perforations (3) per square decimetre.
13. Sheet according to Claim 12, characterized in that the density of the perforations (3) is 500 to 1200 perforations (3) per square decimetre.
14. Sheet according to Claim 11, characterized in that the distance which separates two perforations (3) is the same in two orthogonal directions of the surface.
15. Sheet according to Claim 11, characterized in that the distance which separates two perforations (3) is different in two orthogonal directions of the surface.
16. Sheet according to Claim 11, characterized in that the width of each parallel channel (6, 7, 61, 63, 64-65, 66-67, 68, 69) is between 2 and 5 millimetres.
17. Sheet according to Claim 16, characterized in that the width of each of the parallel channels (6, 7, 61, 63, 64-65, 66-67, 68, 69) is between 3 and 3.5 millimetres.
18. Sheet according to Claim 11, characterized in that the number of parallel channels (6, 7, 61, 63, 64-65, 66-67, 68, 69) is 5 to 15 per decimetre.
19. Sheet according to Claim 11, characterized in that at least some of the parallel channels (6, 7, 61, 63, 64-65, 66-67, 68, 69) contain a sealing adhesive.
20. Sheet according to Claim 11, characterized in that the perforated film (1) is of the two-layer type, that is to say is formed by bonding two layers (1a and 1b) together over their entire surface.
21. Sheet according to Claim 11, characterized in that the two films (1 and 2) are assembled by the joining adhesive (5) so as to form a unitary assembly which can be stored, in particular on a reel (10), and supplied as such with a view to a subsequent lidding operation.
22. Sheet according to Claim 11, characterized in that the film (1) which has a number of small perforations (3) bears a continuous adhesive (4) on its outer face, that is to say on its face which is opposite the other film (2).
23. Sheet according to Claim 10, characterized in that the film (1) which bears a continuous adhesive (4) and the other film (2) are assembled by the joining adhesive (5) so as to form a unitary assembly which can be stored, in particular on a reel (10), and supplied as such with a view to a subsequent lidding operation.
24. Industrial product formed by a container (A) filled with contents (D) and having a rim (C) and being closed by means of a lid which is sealed over the entire surface of the rim (C) and which consists of a sheet of synthetic material according to Claim 11, having zones (6, 7, 61, 63, 64-65, 66-67, 68, 69) which are left free and which form a network of several parallel channels which open at least once into the atmosphere at the edge, or at one of the edges, of the closed container (A).

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