TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a one-way valve designed to be mounted on a package and arranged to open in response to an excess pressure inside said package, said valve comprising an aperture formed in the package material, on top of which aperture an adhesive layer is arranged. 
     TECHNICAL BACKGROUND 
     A one-way valve of the kind outlined above is already known from patent application Ser. No. WO 02/087993. A valve of the kind referred to is used in particular in conjunction with cooking and packaging of ready-to-eat dishes, in a manner ensuring that the valve functions both when the food is being cooked and when it is finally heated preparatory to consumption. When the heating process in conjunction with the cooking of the food is stopped, the valve should close, and this could be achieved for example purely mechanically by means outside the package, pressing said layer against the package material. Alternatively, valve closing could be achieved by choosing an adhesive layer of a kind that resumes its position on top of the aperture in the package material automatically and in doing so closes the valve. On account of the considerable flow of hot vapour through the valve bubbles may form between the adhesive layer and the package material in the areas where said adhesive layer has separated from the package material (see  FIG. 1 ) when the valve closes following the initial heating in conjunction with the packaging of the food. In some cases, a series of bubbles may form in this manner, and these bubbles may form a channel running from the aperture formed in the package material and up to one of the edges of the adhesive layer. 
     SUMMARY OF THE INVENTION 
     The object of the present invention therefore is to provide a one-way valve designed to solve the problem outlined above. 
     This object is achieved in accordance with the invention by means of a one-way valve possessing the characteristics defined in the appended claim  1 . Preferred embodiments of the one-way valve are defined in the dependent claims. 
     By the expression “adhesive layer” as used herein should be understood a front/cover material coated with an adhesive. 
     The inventive one-way valve comprises an aperture formed in the package material and above which an adhesive layer is provided. The adhesive layer is arranged in such a manner that the valve, when opening, exhibits a pre-defined channel allowing passage-through. The advantage of a valve of this kind thus is that it opens in a more controlled manner in that it becomes possible to determine in advance the valve opening direction. A consequence of the feasibility to control the valve opening direction is that it becomes possible to produce a valve the channel length of which is sufficient to ensure that any bubbles that may generate underneath the adhesive layer as the valve closes do not form a chain running all the way from the aperture in the package material up to the edge of the adhesive layer. 
     The valve preferably is formed with two short sides and two long sides, i.e. it has an essentially rectangular shape. Owing to this configuration the manufacturing costs may be comparatively low, since it allows the valves to be manufactured in a manner identical to that of a long strip of labels. 
     In accordance with one preferred embodiment of the present invention a first adhesive is applied on one face of the adhesive layer along the long sides of the latter while a second adhesive, the adhesive force of which is less strong than that of the first adhesive, is applied between those two parts along the long sides of the adhesive layer on which said first adhesive is applied. Consequently, vapour from inside the package may escape at the short sides of the adhesive layer. 
     In accordance with one variety of the embodiment described above a first adhesive is applied on the entire surface of one face of the adhesive layer and a second adhesive, the adhesive force of which is less strong than that of the first adhesive, is applied on top of said first adhesive at a distance from the long sides of the adhesive layer. In this manner, the same function is obtained as in accordance with the embodiment described above. 
     In accordance with yet another embodiment only one of the short sides is arranged to open, i.e. the opposite short end is provided with the same adhesive as that applied along the long sides. This arrangement makes it possible for a channel to form, the length of which exceeds half the length of the adhesive layer, with consequential less risk that rows of touching bubbles form a channel extending all the way from the aperture in the package material to the openable short side. 
     In accordance with another preferred embodiment the adhesive layer is formed with an aperture the edges of which surrounds said aperture formed in the package material while yet a further adhesive layer is arranged on top of the first adhesive layer and the two adhesive layers are joined together along their long sides. One of the advantages of this kind of valve is that it makes it possible to use an identical adhesive over the entire surface (of one face) of the adhesive layers, when the long sides are joined together, for instance by means of welding. Also this embodiment makes it possible to ensure that valve opening may occur only at one of the short sides in that the short sides are joined together and then preferably in the same way as the long sides. 
     According to one variety of the above-described embodiment, the aperture formed in the package material consists of a plurality of parallel cuts and the aperture formed in the adhesive layer positioned closest to the package of a plurality of parallel cuts, said adhesive layer being applied on the package in such a manner that the parallel cuts in the adhesive layer extend at an angle relative to the parallel cuts formed in the package material. One advantage offered by this type of valve is that it reduces the risk that the adhesive in the upper adhesive layer will come into contact with the contents inside the package, which in turn increases the freedom of choice of adhesive to be used. From a purely handling point of view this valve offers a further advantage, when it is provided in a prefabrication step with a protective film. When the aperture in the adhesive layer closest to the package is a hole, this hole preferably is produced by punching with the protected film applied. As a result, when the valve is to be applied on a package, the protective film in the area of this hole, together with the part of the adhesive layer that is removed to form the hole, must be stripped off in a separate step. In a valve in accordance with the arrangement mentioned above comprising a plurality of cuts in one of the adhesive layers, the cuts are made in the adhesive layer only and there is no need for any manipulation of the protective layer. Accordingly, several valves may be manufactured in succession on a strip, which in turn may for example be wound into a roll of valves. 
     In accordance with yet another embodiment of the present invention a pattern may be printed on the package material, alternatively on the adhesive layer closest to the package material on the face that is turned away from the package. The advantage of providing some kind of print lies in the possibility it offers of lessening the adhesive force. In some embodiments in accordance with the present invention, exhibiting a longer distance from the aperture formed in the package material to the edge of the adhesive layer, i.e. the length of the ventilation channel, the force or the excess pressure required to open the valve preferably should not be too strong. If it is, there is a risk, in cases when the package consists for example of a tray covered by plastic foil, that the foil separates from the tray before the valve opens. By printing a pattern as indicated above using for example conventional printer&#39;s ink or silicone, a method commonly referred to as “release coating”, the adhesive force may be adjusted to a degree sufficient to avoid that the plastic foil separates from the tray in response to an excess pressure inside the package. In most adhesives the bonding force is at its peak some time after their application and depending on the nature of the adhesive this is a question of seconds, minutes, hours or days. The adhesives most suitable for use in conjunction with the invention are those reaching their bonding peak after a couple of days. In cases when the package is not intended to be subjected to the final heating within a couple of days, calculated from the instance of packaging, alternatively in cases when the valve is prefabricated and is not applied on the package within a couple of days, the adhesive force of the adhesive has reached its maximum. For this reason it is suitable to print a pattern on the intended area of application of the adhesive layer in order to reduce the maximum adhesive force of the adhesive layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention will be described in the following by means of some embodiments thereof with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a valve in accordance with the prior art as applied on a package. 
         FIGS. 2   a - 2   c  show a first embodiment of a valve in accordance with the present invention. 
         FIGS. 3   a - 3   c  show a second embodiment of a valve in accordance with the present invention. 
         FIGS. 4   a - 4   e  show a third embodiment of a valve in accordance with the present invention. 
         FIGS. 5   a - 5   c  show a fourth embodiment of a valve in accordance with the present invention. 
         FIGS. 6   a - 6   d  show a fifth embodiment of a valve in accordance with the present invention. 
         FIGS. 7   a - 7   d  show a sixth embodiment of a valve in accordance with the present invention. 
         FIGS. 8A and 8B  show a seventh embodiment of a valve in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a valve  11  applied on a package  12  in accordance with the prior art. The dotted line  13  illustrates the border line between the part of the adhesive layer  14  that separates from the package  12  and the part of the adhesive layer  14  that still sticks to the package when the valve  11  opens. Following opening of the valve  11  and its automatic closing, bubbles  15  may generate, as illustrated, within the arcuate shape defined by the dotted line  13 . 
       FIGS. 2   a - 2   c  illustrate an embodiment of a valve  21  in accordance with the present invention. Along the long sides of the adhesive layer  22  a first adhesive  23  is applied and intermediate these adhesive-coated areas a second adhesive  24  is applied. Upon generation of an excess pressure inside the package  25 , vapour tries to escape through the aperture  26  (which in this case consists of an arcuate cut) formed in the package  25 . By suitable choice of the adhesives  23 ,  24  to ensure that the adhesive  23  extending along the long sides will possess the stronger adhesive force, flow-through passages or channels form, leading up to the short sides of the adhesive layer  22  from the aperture  26  in the package  25 , as the excess pressure inside the package  25  rises to a level above a minimum opening value. Upon cease of heating of the package contents, pressure equalisation occurs, and the valve  21  closes as a result of the adhesive layer  22 , on account of its resiliency, resuming its original position in contact with the package  25 . 
       FIGS. 3   a - 3   c  show an alternative embodiment of a valve  31  in accordance with the present invention, which is distinguished from the embodiment of  FIGS. 2   a  and  2   b  in the manner of application of the various adhesives on the adhesive layer  32 . Adhesive  33 , possessing the stronger adhesive force, is applied over the entire surface of one face of the adhesive layer  32  whereas the second adhesive  34  is applied on the adhesive layer  32 , except along the long sides of the adhesive layer  32 . Owing to this arrangement, valve  31  operates in the same manner as valve  21  in  FIGS. 2   a - 2   c.    
       FIGS. 4   a - 4   e  show a further embodiment of a valve  41  in accordance with the present invention. Like in accordance with the embodiments in drawing  FIGS. 2 and 3 , an adhesive layer  42  is arranged in a manner ensuring that channels may form. The difference in this case is that a second adhesive layer  47  is interposed between the package  45  and the first adhesive layer  42 . Preferably, the adhesive  43  of the second adhesive layer  47  possesses an adhesive force exceeding that of the adhesive  44  of the first adhesive layer  42 . A hole  48  is punched in the second adhesive layer  47  in such a manner that it encloses the aperture  46  formed in the package  45 . Instead of employing only one adhesive layer  22 ,  32 , like in the two preceding embodiments, wherein two different adhesives  23 ,  24 ,  33 ,  34  are used in order to ensure the formation of a passage or channel when the valve opens, the two adhesive layers  42 ,  47  are in this case joined together along their long sides by means of a weld seam  49 . When the excess pressure inside the package  45  rises above a certain value, a channel consequently forms between the two adhesive layers  42 ,  47 . 
       FIGS. 5   a - 5   d  illustrate an additional embodiment of a valve  51  in accordance with the present invention. The difference from the embodiment of  FIG. 4  is that the hole  48  has been replaced by a plurality of parallel cuts  58 . Also the aperture  56  formed in the package  55  consists of a plurality of parallel cuts, the cuts in the package extending at an angle to the cuts formed in the adhesive layer  57 . When the valve  51  opens several small apertures form in the crossover points between the cuts  58  in the adhesive layer  57  and the cuts  56  in the package. When the valve  51  closes, these apertures shut and thus the contents inside the package will never come into contact with the adhesive  54  applied on the first adhesive layer  52 . 
       FIGS. 6   a - 6   d  show an alternative embodiment of a valve  61  in accordance with the present invention. A pattern  70  (known as a “release coating”) is printed on the second adhesive layer  67  by means of printer&#39;s ink or silicone in order to facilitate the opening of the valve  61 . In addition, one of the short sides of each one of the two adhesive layers  62 ,  67  also have been welded together in order to thus form a longer channel without prolonging the adhesive layers  62 ,  67 . 
     Yet another alternative embodiment of a valve  71  is shown in  FIGS. 7   a - 7   d . According to this embodiment the outer adhesive layer  72  has been shortened/cut at its short sides for the purpose of reducing the risk of formation of “bubbles” after the valve has been open. Under some circumstances, the outer adhesive layer may deform plastically on account of the heat, and shortening/cutting the short sides reduces the effect of such potential deformation. Furthermore, the cut  76  formed in the package material  75  preferably is arcuate and the aperture  78  formed in the adhesive layer  77  closest to the package material  75  preferably consists of a plurality of straight cuts placed crosswise. In accordance with this embodiment, the “release coating” is applied only between the aperture  78  in the adhesive layer  77  closest to the package material  75  and one of the short sides. When the valve assumes its open position over a longer period, for example during the cooking of the food, the valve opens more and more towards the short side devoid of “release coating”, and for this reason it is suitable that the aperture is spaced equally from both short sides in order to prevent the valve from opening at both short sides, i.e. that the distance between the aperture  78  and the short side devoid of “release coating” is sufficiently long. This valve designed without a weld at one of the short sides is easier to produce manufacture—wise than the valve in  FIG. 6  and the production thereof therefore cheaper. 
     Should nonetheless a long vapour passage be desired in the valve having welds or being provided with an adhesive possessing a high adhesive force applied in one direction only on an adhesive layer, an arrangement of this kind could be achieved for example as shown in  FIGS. 8   a  and  8   b . A first adhesive layer  81 , provided with an aperture  83 , is applied closest to the package material  82 , and said adhesive layer  81  is arranged in such a manner that its aperture  83  will be located on top of an aperture  84  formed in the package material  83 . A second adhesive layer  85  is arranged on top of said first adhesive layer  81  and this second layer  85  is adhesive-free along three parallel lines  86 . Adhesive-free areas of the adhesive layer often are easier to weld than areas provided with an adhesive. A third adhesive layer  87  is applied on top of the two first adhesive layers  81 ,  85 , and also this third adhesive layer is formed with adhesive-free lines  86 . The third adhesive layer  87  is placed at an angle of ninety degrees relative to the second adhesive layer  85 , and the short sides of the second adhesive layer  85  are sealed by welding together the third adhesive layer  87  with the first adhesive layer  81  and to some extent also with the second adhesive layer  85 . The second adhesive layer  85  is joined to the first adhesive layer  81  by welding in the areas marked by numeral  88  in  FIG. 8   b , and the third adhesive layer  87  is joined to the first adhesive layer  81  by means of welding along the areas indicated by numeral  89  in  FIG. 8   b . The second adhesive layer  85  thus is somewhat smaller than the other adhesive layers  81 ,  87 . When the valve is open, vapour will flow between the first adhesive layer  81  and the second adhesive layer  85 . In the valve according to this embodiment, a U-shaped vapour passage  90  is formed. 
     It will be appreciated that numerous modifications of the embodiments described above are possible within the scope of protection as defined in the appended claims. As described above, it is for instance possible to use two different adhesives for the outer adhesive layers  42 ,  52 ,  62 ,  72  instead of welding them together. In addition, the printed pattern may be effected by other means than printer&#39;s ink and silicone, provided the substitute reduces the adhesive force of the adhesive. The use of silicone makes it possible to make the valve practically transparent. An alternative to the printed pattern on the support is to print it instead on the adhesive layer. Owing to the formation of channels or passageways in the valve as defined above also control of the width of the vapour outlet is achieved, i.e. the width of the valve less the area of the adhesive layer having the stronger adhesive, alternatively the joined-together areas of the valve. Under some circumstances, the valve emits a sound signal owing to oscillation of the short side/short sides of the adhesive layer. The frequency of this sound signal may thus be controlled by varying the width of the short side. In addition, the shape of the cuts in the package may be varied. The cuts shown herein have an arcuate or straight configuration but they could also have a zigzag or wavy configuration. In addition, the above description of the invention is essentially directed to a valve formed with short sides and long sides, in order to facilitate the understanding of the description. However, the adhesive layer could have a different shape from the rectangular one, i.e. shapes without short and long sides, such as for example circular or star-shaped valves.
A one-way valve is disclosed, particularly for foodstuff packages, the valve designed to be mounted on a package and arranged to open in response to an excess pressure inside said package. The valve includes an aperture formed in the package material. On top of the aperture an adhesive layer is arranged. The adhesive layer is arranged in such a manner that the valve, upon opening, exhibits a pre-defined channel allowing passage-through.