Abstract:
The invention relates to a film of plastics material stretched to present a privileged shrinkage orientation and used in particular for fabricating heat-shrink sleeves, the film including at least one line of mechanical weakness and at least one local region of extra thickness extending along the line of weakness so as to border it, the line of weakness and the adjacent local region of extra thickness together defining a tear line. According to the invention, the local region of extra thickness is constituted by applying liquid or semi-liquid material to the film, the applied material being applied to the film in a quantity that depends on the expected shrinkage ratio in the portion of the film that includes the tear line while a sleeve made using such a film is being heat-shrunk onto an article.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in French Patent Application No. 10 50283 filed on Jan. 15, 2010. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a film of single-oriented plastics material for fabricating a tearable heat-shrink sleeve, and to a heat-shrink sleeve made from such a film. 
       TECHNOLOGICAL BACKGROUND OF THE INVENTION 
       [0003]    Heat-shrink sleeves are known that are constituted by a film of single-oriented plastics material that has previously been stretched in a determined direction in order to give it a capacity for shrinkage under the effect of heat in the stretching direction. Such sleeves are used in particular for covering the walls of containers of all kinds. 
         [0004]    It would be advantageous to give such sleeves a capacity for tearing along a privileged tear line. For example, a sleeve of this type could be used for covering the wall of a container together with its stopper so as to guarantee that it has not been opened. It is then advantageous to provide controlled tear means in the sleeve so as to enable the sleeve to be subdivided into a portion that remains permanently on the container, and a portion that is removed by the user so as to enable the container to be opened. 
         [0005]    Naturally, proposals have been made to define a tear line in the sleeve by forming a line of mechanical weakness in the film, e.g. by making microperforations or by laser cutting to half-depth. Nevertheless, that method does not give results that are satisfactory. Sometimes the material of a sleeve made from such a film tears, not along that line, but in more or less random manner depending on the alignments of the molecules constituting the material of the film. 
         [0006]    It should be observed that that problem is specific to films that acquire a privileged orientation as a result of being stretched, in particular single-oriented films that are used in the fabrication of heat-shrink sleeves. In certain films that are constituted mainly by copolymers, a small quantity of the copolymer crystallizes so as to form crystallites in the film, i.e. so as to form ordered and aligned portions of adjacent polymer chains. The crystallites extend in the stretch direction of the film and the proportion of crystallite as formed in this way has a direct influence on the tearability of the film. Such crystallites constitute hard points that are likely to deflect the tearing of a heat-shrink sleeve made using such a film, giving rise to random tears. This problem of random tearability may also be encountered in sleeves made using stretch films of structure that is made strongly non-uniform by including molecules of large size that form obstacles to the tearability of the sleeve (e.g. an inclusion of polystyrene in a film constituted of styrene and butadiene copolymers, or indeed an inclusion of polyethylene copolymers in a film constituted of a polyethylene homopolymer). Naturally, this kind of problem is not encountered in the field of tearable adhesive labels since they are made of paper or of a film of plastics material that is amorphous or that presents two orientations. 
         [0007]    Proposals have also been made to incorporate a tear strip or “rip-cord” of plastics material in the film, as described for example in document FR 2 763 921. Nevertheless, such a rip-cord is far from presenting the high shrinkage ratios that can be achieved by the material of the film, such that the use of such a film cannot be envisaged for covering containers of shapes that vary greatly and that require large shrinkage ratios. 
         [0008]    Document GB 1 013 783 describes making tear lines that comprise a line of mechanical weakness bordered by a local region of extra thickness constituted by an applied strip. 
         [0009]    The local region of extra thickness guides tearing along the line of mechanical weakness and prevents bifurcation of the line. Nevertheless, such a solution is also poorly adapted to covering containers of shapes that vary greatly. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the invention is to propose a single-oriented film that enables heat-shrink sleeves to be made that are tearable along a predefined tear line. 
         [0011]    In order to achieve this object, the invention provides a film of plastics material stretched to present a privileged shrinkage orientation and used in particular for fabricating heat-shrink sleeves, the film including at least one line of mechanical weakness and at least one local region of extra thickness extending along the line of weakness so as to border it. According to the invention, the local region of extra thickness is constituted by applying liquid or semi-liquid material to the film, the applied material being applied to the film in a quantity that depends on the expected shrinkage ratio in the portion of the film that includes the tear line while a sleeve made using such a film is being heat-shrunk onto an article. 
         [0012]    Various experiments have demonstrated that the presence of a region of extra thickness along the line of weakness in a heat-shrink sleeve made using such a film prevents bifurcation of a tear that has been initiated along the tear line as defined in this way, such that the tear propagates along said line to its end, in particular when the orientation of the tear line is perpendicular or oblique relative to the privileged orientation of the film. 
         [0013]    Adapting the quantity of material that is applied enables the film to be stiffened locally in such a manner that the extra thickness applied in this way acts as a tearing guide, while nevertheless being capable of following heat-shrinkage of the film without countering such shrinkage to such an extent that the film forms waves in the shrunk sleeve. 
         [0014]    Advantageously, two parallel tear lines are formed in the film, each being defined by juxtaposing at least one line of weakness and at least one adjacent local region of extra thickness. Thus, in a sleeve fabricated using such a film, the two tear lines define between them a tear strip. It is then advantageous to provide a cup extending transversely to the two lines in the film so as to define a grip tab for the tear strip in the sleeve. 
         [0015]    The invention can be better understood in the light of the following detailed description of several embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]      FIG. 1  is a perspective view of a container covered in a heat-shrink sleeve made from a heat-shrink film with a tear line of the invention; 
           [0017]      FIG. 2A  is a section view of line II of  FIG. 4  showing a heat-shrink film in a first embodiment of the invention, the thicknesses of the film being exaggerated for greater clarity; 
           [0018]      FIG. 2B  is a section view analogous to  FIG. 2A  of a heat-shrink film in a second embodiment of the invention; 
           [0019]      FIG. 2C  is a section view analogous to  FIG. 2A  of a heat-shrink film in a third embodiment of the invention; 
           [0020]      FIG. 2D  is a section view on line III-III of  FIG. 5  showing a heat-shrink film in a fourth embodiment of the invention; 
           [0021]      FIG. 3  is a perspective view of a container covered in a heat-shrink sleeve made from a heat-shrink film having a tear line of the invention; 
           [0022]      FIG. 4  is a view of the film used for covering the container of  FIG. 1 , the film being shown flat; and 
           [0023]      FIG. 5  is a view of the film used for covering the container of  FIG. 3 , the film being shown flat with local magnification concerning the tear line. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    The technological field of heat-shrink films made of stretched thermoplastic material is well known, and it suffices here to recall that such a film may present a shrinkage ratio that is considerable. The invention applies in particular to films that are stretched, e.g. so as to give them a privileged orientation, and although not limited thereto, it applies to a film of polyethylene copolymer including a given percentage of crystallites, the majority of which are oriented in the stretch direction. 
         [0025]      FIG. 1  shows a container  1  covered in a heat-shrink sleeve  2  that is fabricated from a heat-shrink film presenting two parallel tear lines  10 ,  20  of the invention that define between them a tear strip  30 . A transverse cut  31  defines a grip tab  32  in the tear strip  30  to enable the user to initiate tearing of the tear strip  30 . In this example, the tear lines  10 ,  20  extend in the stretch direction of the film, and thus perpendicularly to the privileged orientation of the film. 
         [0026]    Tearing the tear strip  30  enables the bottom portion  3  of the sleeve, i.e. a portion that remains permanently on the container, to be separated from the top portion  4  of the sleeve that can be removed, thereby giving access to the stopper of the container. 
         [0027]    As can be seen in  FIG. 4 , the film used is a heat-shrink film obtained by stretching a film of plastics material, e.g. of polyethylene. In this example the tear lines  20  extend parallel to the stretch direction of the film (and thus to the main shrinkage direction of the sleeve). As the reader can see immediately, the tear lines  10 ,  20  are located on the container of  FIG. 1  in a region that is of greatly-varying shape, and it is therefore subjected to large shrinkage ratios during heat-shrinking of the sleeve made from the film of  FIG. 4 . 
         [0028]    The way in which the tear lines are made in the film is explained below in detail with reference to  FIGS. 2A to 2C , showing various embodiments. 
         [0029]    In a first embodiment shown in  FIG. 2A , two lines of weakness  11 ,  21  are made in the film, in this example by laser cutting to half-depth in the thickness of the film (for reasons of simplification, the film is shown as comprising a single layer, but that is not limiting, and the film could comprise a plurality of layers). In the figure, there can be seen the furrows left by the cutting. Then, according to the invention, a layer of polymer varnish  25  is applied to the surface of the film so as to form a strip that extends between the lines of weakness  11 ,  21  up to the vicinity thereof so as to form local extra thicknesses  12 ,  22  bordering the lines of weakness  11 ,  21 . Associating a line of weakness with an adjacent local region of extra thickness defines a tear line along which the sleeve can be torn. The two tear lines as defined in this way define between them the tear strip  30 . 
         [0030]    When making a sleeve, the film as prepared in this way is subjected to heat-shrinking, including in the portions of the film that constitute the tear lines as made in this way. It should be observed that the varnish applied on the heat-shrink film stiffens the film locally, and although it shrinks with the film, it locally reduces the ability of the film to shrink. In particular, it should be observed that the tear lines  10 ,  20  extend in this example over a portion of the container  1  that presents a small diameter and in which the amount of shrinkage of the sleeve during heat-shrinking is particularly great. It is thus appropriate for the extra thickness as applied in this way not to oppose the shrinkage capacity of the film forming the sleeve. 
         [0031]    In this respect, it is therefore appropriate to select varnish characteristics and to meter application of the varnish on the film in such a manner that the extra thickness as applied in this way stiffens the film locally and acts as a tearing guide, while nevertheless being capable of following the heat-shrinking of the film without opposing it to such an extent that the film ends up forming waves in the sleeve once it has been heat-shrunk. In particular, it is appropriate to adapt the thickness of the layer of varnish and to ensure that it is thinner if the region in question is to be subjected to a large amount of shrinkage. 
         [0032]    Thus, if it is desired to make two parallel tear lines in a single film, which lines occupy portions of the film that will be subjected to different shrinkage ratios during heat-shrinking, it is appropriate to adapt the application of varnish to each of the tear lines so as to take account of this difference in shrinkage ratio. To a first approximation, it appears to be advantageous to vary the thickness of the layer of varnish that is applied in this way in a manner that is inversely proportional to the shrinkage ratio expected in the region in which the tear line in question extends. 
         [0033]    In a variant, rather than varying the thickness of the layer of varnish, it is also possible to vary the width of the applied layer of varnish, or indeed to apply varnish in non-continuous manner, i.e. with interruptions, so as to form a dashed line of varnish with gaps between the dashes that are caused to vary as a function of the expected shrinkage ratio. Naturally, it is possible to combine the various options so as to vary the quantity of varnish applied as a function of the expected shrinkage ratio. 
         [0034]    In a second embodiment shown in  FIG. 2B , tear lines  110 ,  120  are formed by making lines of weakness  111 ,  121  in the film that are bordered by local regions  112 ,  112 ′ of extra thickness that extend on either side of the line of weakness  111 , and local regions  122 ,  122 ′ of extra thickness that extend on either side of the line of weakness  121 . Between them, the two lines of weakness  110 ,  120  define a tear strip  130 . In this embodiment, as in the above-described embodiment, the lines of weakness are made by laser cutting to half-depth, while the regions of local extra thickness are made by applying polymer varnish. 
         [0035]    In a third embodiment shown in  FIG. 2C , the tear lines  210 ,  220  comprise lines of weakness  211 ,  221  bordered on the outside of the tear strip  230  by local regions  212 ,  222  of extra thickness. 
         [0036]    Although in these examples, the tear line extends parallel to the stretch direction of the film, this configuration is not essential. As shown in  FIG. 5 , it is possible to make a film with a tear line  310  that extends obliquely relative to the stretch direction of the film. As shown in  FIG. 3 , such a film enables sleeves to be made in which the tear line extends along a spiral, thus making it possible, on initiating tearing at the top of the sleeve, to separate the sleeve entirely from the container. 
         [0037]    As shown in  FIG. 2D , the tear line  310  includes both a weakness line  311 , constituted by a sequence of microperforations in alignment, and also a local region  312  of extra thickness bordering the line of weakness  311 . 
         [0038]    Thus, making a local region of extra thickness on the stretched film makes it possible to define a tear line in the heat-shrink sleeve that enables tearing of the sleeve to be guided along said line. 
         [0039]    The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims. 
         [0040]    In particular, the local extra thickness may be obtained as shown above by applying a varnish, and more generally by applying a liquid or semi-liquid material of controlled viscosity. 
         [0041]    The material may be applied by contact (pad, brush, syringe, . . . ) or indeed without contact (controlled flow rate nozzle, coated cylinder, . . . ). The material is then preferably dried in controlled manner (blowing hot air, ultraviolet (UV) radiation, . . . ). 
         [0042]    The material used may be transparent, or indeed it may be colored, or it may include inclusions such as pigments or flakes, thereby providing an appearance effect, or making the tear line easier to see. 
         [0043]    Finally, although in the examples shown the tear lines are straight, these lines could be curved, or they could indeed extend around a closed outline so as to define a portion that is removable from the sleeve and that can be taken off by tearing along the closed tear line. 
         [0044]    While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present.