A label for removable attachment to an article

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DESCRIPTION

Label for removable fixation on an article.

The invention relates to labels that are removably affixed to articles. More particularly, although not exclusively, the invention relates to labels for attachment to articles in the form of containers, such as glass bottles.

Traditionally, beverage bottle labels have been made from paper and affixed to the bottles using a liquid glue process. In this process, the labels are supplied free of adhesive; The adhesive is applied to the label just before the label is applied to the bottle. The labeling operation usually takes place on the bottling line, after the bottle has been filled and closed.

More recently, labels made from thermoplastic film have become popular. These labels are usually supplied with a pressure sensitive adhesive already on one side of the base material, where the adhesive layer is covered by a release layer to prevent premature bonding. Labels made of thermoplastic film allow a greater range of decorative effects to be achieved. Since thermoplastic films can be transparent, the backing material of the label can be nearly invisible, so that the material printed on the label appears as printed directly on the bottle. In addition, the use of a pressure sensitive adhesive avoids problems that can arise when handling liquid adhesives near the bottling line.

It is common for the bottles to be returnable for reuse. When the bottles are reused, they can be filled with a different beverage from the one that was previously filled, and consequently it is necessary to remove all the labels from the returned bottles. Label removal systems were developed with paper / liquid glue labels. In general, such systems include washing the bottles in a hot solution of caustic soda. The caustic solution penetrates the paper material of the label and interacts with the adhesive, for example by dissolving it, so that the label separates from the bottle. Mechanical means such as brushes or sprays can be used to aid label removal, or the bottles can simply be shaken in the caustic solution.

There is a resistance, in economic environments, to making major alterations in bottling lines. Consequently, bottles are reluctant to invest in brand-new bottle-washing equipment to manage the various types of labels. This puts pressure on label suppliers to develop labels using thermoplastic films, which can be adequately removed by conventional hot caustic wash systems. A problem arises because thermoplastic films are not significantly penetrated by the hot caustic solution, which means that the solution may initially contact the adhesive only at the edges of the label. As a result, tag removal can be unacceptably slow.

Various measures have been proposed to accelerate label removal based on thermoplastic film materials. For example, our British patent application number 0419398.3 teaches the use of a pressure and heat sensitive adhesive that loses peel strength when heated to temperatures found in hot caustic scrubbing systems. The same document also discloses the provision of microchannels at the interface between the adhesive and the bottle in order to assist the penetration of the washing liquid into the adhesive. Micro-channels can be produced either by embossing the label, or by applying an adhesive cushioning material to parts of the adhesive surface.

US 6680097 discloses a label made from a stretched film that shrinks on heating. The shrinkage effect is intended to cause the label to peel back along the bottle, in order to degrade the adhesion between the label and the bottle.

A problem with shrink-based systems such as that disclosed in US 6680097 is that stretched thermoplastic films shrink into a rigid structure in the form of a "stick", which can be difficult to remove from the washing machine. and subsequently being able to be caught in the operation of the washing equipment. In addition, materials that demonstrate adequate shrinkage properties at temperatures found in commercial washing machines tend to be expensive or environmentally undesirable.

In accordance with the present invention, there is provided a label that can be removed from an article upon heating, wherein the label comprises a back layer which is a polymeric film and an adhesive layer for attaching the label to the article, characterized in that the layer Backing is a multilayer structure comprising a polypropylene film and a polyester film, where the material of the films is such that the backing layer expands when subjected to heat, there being a differential expansion between the polypropylene film and the polyester film, such that the label tends to roll into a curved configuration with the adhesive layer on the outside of the curved area.

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Said label, when attached to an article such as a beverage bottle and placed in a caustic soda bath, will generate forces, in the form of a spring, tending to raise the areas of the label, in order to accelerate the process. of elimination.

The back layer comprises a sheet of two different materials that have different coefficients of thermal expansion. Thus, the material adjacent to the adhesive layer will have a higher coefficient of thermal expansion than the other material of the back layer, so that the label, when heated, will tend to bend away from the article to which it is attached.

According to the invention, the two sheet materials that form the back layer are a polypropylene film and a polyester layer. One or both layers may be oriented, and preferably bi-axially oriented, by an elongation process. In a preferred embodiment of a label in which the back layer comprises a polypropylene / polyester sheet, the polypropylene film is positioned adjacent to the adhesive layer.

The films of the sheet can be of different thickness from each other. For example, one layer may have a thickness that is 20% to 75% of the thickness of the other layer. The thickest layer can have a thickness in the range of 15 to 60 pm.

In the back layer comprising a sheet of polypropylene and polyester films, the polypropylene film may have a thickness in the range of 15 to 60 pm, more preferably 50 pm, and the polyester film may have a thickness in the range from 12 to 30 pm, and preferably from 19 to 23 pm.

Bi-axially oriented films have a "machine direction", which is the direction in which the film travels during the manufacturing process, and a "transverse direction" which is the direction in the plane of the film extending. perpendicular to the machine direction. The properties of the film are usually not the same in both directions. In order to obtain good results (i.e. optimal curvature) on a label according to the present invention, having a back layer comprising a polypropylene / polyester sheet, it is believed that the modulus of elasticity of the film of polypropylene is important. It is currently believed that the modulus of elasticity of polypropylene film in the machine direction is preferably 2000 to 2500 N / mm²2100 to 2300 N / mm more preferable², and in the transverse direction 3000 to 4000 N / mm is preferable², and more preferably 3250 to 3750 N / mm². Currently it is believed that a polyester layer having a modulus of elasticity of 4000 to 5000 N / mm²4500 N / mm preferable², in machine direction and 4500 to 5500 N / mm², preferably 5000 N / mm², in the transverse direction provides excellent results.

For a better understanding of the present invention, and to show more clearly how it can be carried out, reference will now be made by way of example to the accompanying drawings where:

Figure 1 shows schematically the construction of a label for a bottle;

Figure 2 shows a bottle provided with the label of Figure 1, during a label removal process;

Figures 3a, 3b and 3c show different degrees of curvature of a label; and Figures 4a, 4b and 4c show the application of a label to a bottle.

With reference to Figure 1, the label is shown schematically at an enlarged scale for the sake of clarity. In general terms, the label comprises a back layer 2, provided with a printing layer 4 on one side and an adhesive layer 6 on the opposite side. The adhesive layer 6 is provided with a release layer 10. Optionally, the adhesive buffer layer 8 is applied to the adhesive layer 6.

Print 4 is shown as a consistent layer, although in practice only parts of the layer can be printed. Similarly, adhesive buffer layer 8 is applied only over part of adhesive layer 6, for example, as a set of dots as described in more detail in our patent application 0419398.3, which is incorporated herein. as reference.

The back layer 2 has a laminated structure and comprises an outer film layer 12 and an inner film layer 14, laminated together by means of a laminated adhesive 16. Films 12 and 14 may be of either of two materials having different coefficients of thermal expansion. Preferably, the outer film 12 has a lower coefficient of thermal expansion than the inner layer 14, so that upon heating, the label will curve in the direction away from the adhesive layer 8. In other words, the label will curve around of an axis located above figure 1.

In a specific embodiment of a label as shown in the figure, the outer layer 12 of the sheet 2 is a biaxially oriented polyester, and is preferably a polyethylene terephthalate (PET) film. Suitable film is available under the designation Melinex® 313. PET material has a linear coefficient of thermal expansion in the range of 20 to 80 x 10^-6 K^-1.

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The bottom film 14 of sheet 2 is a polypropylene film and may be the film available from ExxonMobil under the designation Label-Lyte 50LL536. Polypropylene has a linear coefficient of thermal expansion in the range of 100 to 180 x 10⁶ K^-1. It will be appreciated that the coefficient of thermal expansion of polypropylene is approximately at least twice that of PET, and that it can be five or more times greater.

The adhesive layer 6 preferably comprises a pressure and heat sensitive adhesive. In the context of the present invention, the term "heat sensitive" means that the adhesive properties of the adhesive layer degrade or decrease upon heating to a temperature in the range of 40 ° C to 120 ° C, whether or not it is in the presence of a washing fluid. Preferably, the decrease in adhesive properties takes place at temperatures in the range of 40 ° C to 90 ° C, and more preferably at temperatures in the range of 50 ° C to 85 ° C. In preferred embodiments, the adhesive properties of the adhesive layer decrease at a temperature in the range of 60 ° C to 80 ° C, and more preferably 65 ° C to 75 ° C.

The term "heat sensitive" should not be construed as indicating that the properties of the adhesive layer decrease instantly when subjected to a sufficiently high temperature, but within a period of time consistent with the requirements of a washing process. . Thus, the decrease in the properties of the adhesive preferably takes place after a period of time not exceeding 3 minutes after the article incorporating the label is subjected to the appropriate temperature. However, it will be appreciated that long periods, for example not exceeding 10, 8 or 5 minutes after the article is subjected to an elevated temperature, will be considered as falling within the broad scope of the present invention.

The property of the adhesive that decreases when the article is subjected to elevated temperature is the adhesion, or peel strength, of the adhesive layer. Preferably, the peel strength decreases by at least 30%, and in some cases by at least 40% or even 50%, when the article is subjected to the elevated temperature. In especially preferred embodiments, the peel strength is reduced to about 20% of the peel strength under ambient conditions, that is, a reduction of 80%. In general, a reduction in peel strength will be sufficient for the purposes of the present invention if the residual peel strength of the adhesive layer is inadequate to retain the label on the article when the article is subjected to the mechanical action of a high temperature washing liquid in a normal commercial washing process. Reference is made to an industry recognized test for peel adhesion, which is described in the FINAT 8 Technical Manual^to edition, and that corresponds to the Peel Adhesion FTM1 (180 °) at 300 mm per minute. The peel test describes the permanence of adhesion or the ease of peeling of pressure sensitive adhesives. Test strips 25 mm wide are prepared, and the test plate (glass or metal) is adhered and tested after a specified period (usually 20 minutes and 24 hours). The force required to peel the label material from the breadboard is recorded in Newtons and observations of adhesive transfer are noted. For the purposes of the present invention, it is most important that the peel adhesion on the article under the wash conditions is low, for example not greater than 5N / 25mm. Peel adhesion under ambient conditions depends on the heat sensitivity of the adhesive, and also on the weight of the coating, but could be as high as 25N / 25mm.

The peel strength to be considered in the context of the present invention is the peel strength of the adhesive layer when the label is applied to the intended article, which may be made of glass, plastics, metal or ceramic, for example.

The pressure sensitive adhesive may be a hot melt, waterborne adhesive in a solvent medium having a combination of a polymer component and a surfactant component, which allows the label to be removed from an article to which the label was attached. label, preferably within 10 minutes using an aqueous wash between 60 ° C and 90 ° C. The adhesive material can be based on rubber, based on an acrylic material, or based on a modified acrylic material. In a preferred embodiment, adhesives based on acrylic elements are used, and modified in order to enhance their heat sensitive properties to meet the aforementioned requirements. Modifications of the base adhesive material can comprise the addition of a surfactant, which can be of a polymeric, nonionic, cationic, amphoteric, anionic type or of a polymeric surfactant type. If an anionic surfactant is used, it can comprise any of the organic sulfates, sulfonates, sulfonic acids, salts and mixtures thereof, or sulfosuccinates. If a nonionic surfactant is used, it may comprise an ethoxylate, an alcohol ethoxylate, or an alkoxylate. The surfactant is preferably present in the adhesive material in an amount of not more than 5%, preferably not more than 3%, and more preferably not more than 2%. The surfactant is preferably reactive so that it is attached to functional groups on the adhesive polymer. This avoids the possibility that the surfactant could wash off the adhesive layer, for example if the article incorporating the label is immersed in water below the temperature at which the adhesive layer loses its adhesion. For example, if the item is a beer or wine bottle, it can be immersed in ice water to cool its contents before consumption.

The adhesive material is preferably non-resin, ie lacks any tackifier or other added resin.

In achieving the functionality of the label as described herein, it is preferable that the pressure sensitive adhesive is coated on one side of the film label with a weight of between 10 gsm and 20 gsm. In a preferred embodiment the thickness of the adhesive layer may be less than is conventionally used for

ES 2 346 559 T3 pressure sensitive adhesive layers on labels. Preferably, the thickness of the adhesive layer is not more than 15 g / m², and can be no more than 14 g / m², although it is anticipated that in most applications, the adhesive layer will have a thickness of no less than 10 g / m².

Optionally, and as shown in Figure 1, the outer surface of the adhesive layer, prior to its application to an article, is provided with a coating of an adhesive modifying agent to provide the cushioning layer 8 of the adhesive. adhesive. An example of an adhesive modifying agent is an ultraviolet vulcanized adhesive buffer varnish.

In preferred embodiments incorporating an adhesive modifying agent, the adhesive modifying agent serves to reduce the adhesive strength of the adhesive layer in the area or areas where the adhesive modifying agent has been applied. However, the term "adhesive modifying agent" is used broadly to mean any agent that has an effect on the chemical, physical or mechanical properties of the adhesive layer.

The adhesive modifying agent can be applied to the surface of the adhesive through direct printing by a conventional printing process, that is, by flexography, letterpress printing or engraving. For its realization, the face material and the adhesive part of the construction is first laminated from the release liner, to allow the printing of the adhesive modifying agent on the face of the adhesive exposed at that time and being re-laminated after the printing by means of a structure of rollers tangent to each other. Such release coatings are invariably provided on labels with a pressure sensitive adhesive, to cover the adhesive during shipping and handling, to prevent unintentional sticking of the adhesive to themselves or to other items. The release layer is removed just before the label is attached to the target item.

The adhesive modifying agent can be applied to the entire surface or in patterns appropriate to the profile of the label.

The adhesive modifying agent can be applied as a series of round dots with the appropriate percentage coating to the functional requirements of the individual label.

Percentages in the range of 3% to 10% have been found to be preferable, based on the artwork produced by the printing rollers. The printing structure can also take the form of square dots or parallel intersecting lines, or other structures known to those skilled in the art.

The mechanisms are facilitated by the washes by the preferred adhesive, is that when the label is heated above 50 ° C, more preferably above 70 ° C, as would be typically found by the labeled container within a wash environment , the adhesion (ie, peel strength) will be lost to the container by 50%, that is, the article can be peeled off more easily, although the adhesive will still retain some tack. Loss of adhesion occurs rapidly, preferably after 1 minute of immersion at elevated temperature. This loss of adhesion makes it easier for the edges of the label to rise from the container through the bending action of the label, with the upward bending force of the label being greater than the adhesion, allowing washout to take place. without any mechanical means other than the turbulence created by the currents of water into which the edges of the label enter as they move away from the surface of the article. In addition, as the edges of the label rise from the container, through the bending action of the label, the wash solution is then able to penetrate behind the label, between the adhesive layer and the article, and completing the total removal of the label through a combination of physical and chemical means.

The physical removal of the label is carried out by the turbulence created within the washing machine due to its movement and more specifically by the high liquid flow generated by the pumps at the label removal points of the machine. The force of the water movement is greater than the adhesion on the container, which breaks the container label.

Chemical removal of the label is done by the adhesive coming into contact with the hot wash fluid, allowing chemical interactions to take place between the wash solution and additives in the adhesive, such as surfactants that have the effect to remove tack and thus prevent the label from re-adhering to the surface of the container or to any other parts within the machine system.

It will therefore be appreciated that a fundamental property of the adhesive material is its sensitivity to heat. This is in stark contrast to the known pressure sensitive adhesives used for labels on items such as bottles, which have to be removed in a washing process. Known adhesives have been developed on the basis that conventional washing processes use an alkaline washing fluid, such as an aqueous solution of NaOH, and that it has been developed to dissolve or otherwise be ineffective in the presence of said alkaline fluid. Known adhesives are not sensitive to heat, and consequently their properties are not affected, or are affected insignificantly, by the application of heat only (eg, by immersion in hot water).

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To fix the label shown in figure 1 on a bottle, the release layer is removed by exposing the adhesive 6 with its adhesive buffer layer 8, which is then applied to the bottle under pressure, for example, from a roller.

In its unstressed state at normal ambient temperatures, the label shown in Figure 1 is generally flat. Thus, when applied to a bottle that has a cylindrical profile, the label curves around the bottle, and acts as an effortless spring, trying to regain its flat configuration, and therefore trying to overcome the adhesive and recover. its flat configuration. Under normal conditions of bottle use, the adhesive strength of adhesive 6 is sufficient to retain the label in full face-to-face contact with the bottle.

However, on immersion in a hot washing liquid, such as dilute caustic soda, heated to between 60 ° C and 90 ° C, for example 60 ° C, the heat sensitive adhesive 6 suffers from a reduction in resistance to heat. peeled as described above. In addition, not only is the label forced into its flat configuration, but under the heat of the wash solution, the different expansion rates of the PET and polypropylene films 12 and 14 cause the label to cut away from the label. bottle, as shown in figure 2. Initially, while the label is in substantially face-to-face contact, the bending effect is eliminated by the adhesive 6, but nevertheless the force tending to lift the edges away from the bottle is substantially increased, thus accelerating the penetration of the wash solution between the label and the bottle. Eventually, the label is lifted substantially in its entirety from the bottle, and is released under its own force or under the action of mechanical brushes, sprays or with the agitation of the wash solution.

In order to assess the effectiveness of a label according to the present invention, it is possible to measure the level of curvature of a label of specific dimensions (50 mm x 70 mm), when subjected to heat, while it is not attached to an article. The labels were tested by immersion in a 1 liter beaker of washing solution at different temperatures. Preferred labels according to the present invention show curvature by exceeding 180 ° at a temperature below 60 ° C. An example is shown in Figures 3a, 3b and 3c, showing the curvature of a label at the different temperatures indicated. Specific examples of label materials according to the present invention were tested, but comprising biaxially oriented polypropylene and PET film sheets of different thicknesses according to the above procedure at 80 ° C, and the curvature result was measured. The results are shown in the table below:

TABLE

<td>Polypropylene film thickness pm (TO)</td><td>PET film thickness pm (B)</td><td>Angle of curvature</td><td>B / A%</td>

<td> 50</td><td> 23</td><td> 360°</td><td> 0,46%</td>

<td> 35</td><td> 23</td><td> 360°</td><td> 0,66%</td>

<td> 50</td><td> 19</td><td> 405°</td><td> 0,38%</td>

<td> 35</td><td> 19</td><td> 420°</td><td> 0,54%</td>

<td> 50</td><td> 15</td><td> 405°</td><td> 0,30%</td>

<td> 35</td><td> 15</td><td> 420°</td><td> 0,43%</td>

<td> 50</td><td> 12</td><td> 360°</td><td> 0,24%</td>

<td> 35</td><td> 12</td><td> 420°</td><td> 0,34%</td>

As mentioned above, PET film 12 and polypropylene film 14 are bi-axially oriented. The properties of bi-axially oriented films are different between the machine direction and the transverse direction of the film. In practice it has been found that when laminated together as indicated in Figure 1, the axis around which the curvature occurs varies according to the method of manufacturing the film 14.

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Oriented polypropylene films can be manufactured by the blown film method or by the tensioner process. In the blown film process, the polymer is forced through an extrusion head, which forms the polymer into a tube that is inflated between the extrusion head and a pair of contact rollers. The bubble stretches the film in the transverse direction while the contact rollers elongate the bubble in the machine direction by stretching the film at a speed faster than the discharge speed of the extrusion head.

In the tensioner process, the polymer is extruded through a flat die like a sheet. The sheet passes through rollers that rotate at different speeds, which lengthen the film in the machine direction, while the orientation in the transverse direction is achieved by a tensioner where the film is fixed at its edges and lengthening the herself.

It has been found that unless special measures are taken, polypropylene film made by the blown film method tends to bend in a transverse direction, that is, the curvature takes place around an axis extending parallel in the direction. from the film machine, so that the label tends to curve into a cylinder that extends in the machine direction. Normal practice is for reel labels to be applied to bottles with the machine direction of the label extending around the bottle, as shown in Figure 4a. This is because the web runs parallel to the direction of travel of the bottles as the labels are applied. However, if a label with this property is applied to a cylindrical item such as a bottle, it will bend around the bottle so that the machine direction of the film will bend. This is depicted in Figure 4b. The resulting curved label will resist any tendency to curl about an axis parallel to the plane in which the curved machine direction of the film lies. In other words, the label will resist bending away from the bottle at its top and bottom edges, and this resistance may overcome any tendency to bend in response to differential expansion of the label layers. Consequently, it is desirable for the label to have a tendency to curve about an axis perpendicular to the machine direction, such that the label curves away from the cylindrical article about an axis that is parallel to the longitudinal axis of the article, such as as shown in figure 4c.

By taking special measures, for example by selecting appropriate grades of oriented polypropylene films (for example, with those that are known to be suitable for use in label manufacturing), and by use of the tensioning process, it has been possible to manufacture labels that tend to curve about an axis parallel to the transverse direction. Such labels will therefore tend to curve away from the bottle or with another container curved by lifting the bottle at its side edges.