Abstract:
A label including a substrate having a face side and a rear side. An adhesive layer is arranged on the rear side of the substrate. The adhesive layer is non-tacky but activatable to become tacky adhesive. The adhesive layer includes a polyurethane polymer composition that is reversibly changeable from the non-tacky state to the tacky adhesive state. A method for attaching a label to an item.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a label comprising a substrate having a face side and a rear side, and an adhesive layer arranged on the rear side of the substrate, the adhesive layer being non-tacky but activatable to become tacky. The invention also relates to a method for attaching the label to an item. 
       BACKGROUND OF THE INVENTION 
       [0002]    A number of known label decoration technologies are available and each of them possesses various advantages and disadvantages. 
         [0003]    Self-adhesive or pressure-sensitive labels are well-known and widely used in the industry. They suffer, however, from a number of disadvantages which include the fact that the production process is rather complex. Firstly a release liner is coated on one side with a release agent, typically silicone, and then on top of this cured silicone layer, a layer of pressure-sensitive tacky adhesive is applied which remains tacky for an unlimited time throughout the lifetime of the product. A paper or filmic print-carrier is then laminated to the adhesive coated release liner, at which point the adhesive is preferably transferred to the print carrier. Rolls of such a pressure-sensitive laminate are then supplied for printing the face side and die-cutting the labels to the required shape and finally removing the waste matrix of the face side. The labels are then ready to be applied to the item to be labelled, and at this point the removable liner becomes a waste product which is a major disposal issue for users of such labels and the industry at large as well as the whole environment. 
         [0004]    Another known alternative is the use of wet-glue labels in which pre-printed and die-cut paper labels are coated with a wet adhesive and applied to the substrate. Typically, such wet-glue label operations are very messy with a large amount of time being lost for clean-up, set-up and changing of label formats. In addition, such wet-glue labels almost always exhibit the unattractive gripper marks of the “label-box” on the finished labelled object (the label-box is a device which holds the paper labels prior to the application of the adhesive and their application to a surface). Furthermore, wet-glue labels are not available with clear films due to the technical problem that the water cannot evaporate and escape from such labels in a completely satisfactory manner resulting in unattractive bubbles in the label. 
         [0005]    Another decoration technology is that of shrink-sleeves, in which case the total applied cost is very high and the production process is complex. Shrink-sleeve films are normally printed on wide-web gravure or flexographic presses with solvent-based inks, which could be an environmental and safety hazard. In a separate offline process after printing, the film is formed into a tube and the seams are adhered together by the use of a solvent welding process. This tube is then cut into shorter lengths and dropped over the article to be labelled which is then fed through a shrink-tunnel at high temperature causing the film to shrink and fit the shape of the container. Although this technology has produced a number of eye-catching and appealing results for some target market segments, it has a number of disadvantages. These include the fact that by definition, a shrink-sleeve covers the whole surface of the object to be labelled, and therefore 180° decoration or a label covering only part of the container surface is not possible. The visibility of unattractive seams on shrink-sleeve labels is also a negative issue. It is not possible to use shrink-sleeves for containers with flat sides or with containers with handles. It is not possible to use paper labels with shrink-sleeves; nor is it possible to have a variety of textures or tactile effects. It is also not possible to have spot labels, multiple labels on a container or unique label shapes when using shrink-sleeves. Neither is it possible to achieve the so-called “no-label” look with shrink-sleeves, that is, it is not possible to have the label blend with the container colour and material as if the label was not there and that the container was pre-printed. 
         [0006]    Another widely-used label technology is that of wrap-around labels. Wrap-around labels can be produced from either paper or film and can be fed from stationary magazines or directly from reels. Typically, a hot-melt adhesive is applied to the leading edge of the label which is tightly wrapped around the container being rotated at a controlled speed, with the trailing edge being fixed in place by a second narrow strip of hot-melt adhesive. The major markets for such labels are soft drinks and mineral waters due to the lack of premium look and therefore the marketing appeal of the labels. It is not possible in this case to achieve the “no-label” look. The choice of materials is very limited and design variety is restricted to that of simple cylindrical shapes. 
         [0007]    Publication US 2007/0014985 discloses compositions having a structured morphology. When triggered or activated by a suitable action, the morphology changes, causing a corresponding predetermined change in the property of the composition. Examples range from non-tacky to tacky, from uncured to cured, including change in colour, change in intensity of presence of fragrance, odour or smell, ranging from non-reactive to reactive and from stable to non-stable. The publication also discloses non-tacky films which become tacky in consequence of heating. Such activation by heating thus results in change of the tackiness of the film, but this tackiness is preserved upon cooling and the end result is similar to a pressure sensitive adhesive that remains tacky for an unlimited time. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an aim of the invention to provide a novel label that eliminates the above-mentioned drawbacks of the labels. In detail, the label according to the invention, among other things, does not require a release liner, which not only saves vital resources and manufacturing complexity but also avoids the major drawback of pressure-sensitive labels which is the disposal of the release liner after the application of the labels. In this case, there is no release liner to dispose of, nor is there any silicone involved or applied to the facestock in order to prevent blocking in the rolls, which could interfere with the subsequent printability of the facestock. 
         [0009]    The aim of the present invention is also to provide a label which makes possible a firm joint between the label and the surface which the label has been attached. 
         [0010]    Further advantages which relate to a novel label according to the invention are:
       It is possible to label very sensitive materials that do not tolerate hot adhesives even for a short time,   labelling machines become simpler because the activation section of the adhesive may be placed further away from the section where the label is attached to a surface, and   manual labelling is also possible because labels may cool down before the attachment of the labels.       
 
         [0014]    To achieve the above-mentioned aims and advantages, the label according to the invention comprising a substrate having a face side and a rear side, and an adhesive layer arranged on the rear side of the substrate, the adhesive layer being non-tacky but activatable to become tacky, is characterized in that the adhesive layer comprises a polyurethane composition which is reversibly changeable from the non-tacky state to the tacky state. 
         [0015]    In the labels according to the invention, the temporal tackiness period of the adhesive is not directly linked with the duration of the activation period. For example, if the activation from non-tacky to tacky is performed with heating, the tackiness is preserved a certain time also outside the cooling period. In other words, the already cooled down adhesive maintains its tackiness over a certain period of time after the temperature has equalized with the ambient temperature. According to one embodiment of the invention, the adhesive coating may remain tacky for up to 15 minutes which is more than adequate for it to be applied to a target item after activation. 
         [0016]    The adhesive layer of the label can be altered from tacky to non-tacky, i.e. after the attachment of the label the adhesive changes to a non-tacky state. 
         [0017]    This happens after the label has been attached to a surface, and thus a firm joint is formed and it would be difficult to remove the label from its place. 
         [0018]    A characteristic feature of the adhesive is that it is non-tacky at room temperature. The adhesive becomes tacky when it is activated by external energy and becomes non-tacky after a certain period of time after the external energy source has been removed. Thus, following the above-mentioned principle, the adhesive becomes tacky when it is heated and it becomes non-tacky after a certain period after cooling. However, the adhesive can be re-heated to become tacky again. Therefore, the polymer composition is reversibly changeable from the non-tacky state to the tacky state. By the mechanism of the adhesive, the non-tacky highly crystalline polymer melts when heated and changes to a largely amorphous tacky state. 
         [0019]    Compared to the pressure-sensitive adhesives, the present adhesive has, for example, better bond strength, water resistance, solvent and heat resistance. 
         [0020]    The label according to the invention comprises a substrate and an adhesive layer. The substrate has a face side and a rear side. The adhesive layer is situated on the rear side of the substrate. However, it is possible that this particular adhesive layer is not the closest layer of the rear side of the substrate but other layers may exist between the rear side of the substrate and the adhesive layer. The substrate may consist of one layer, or it may comprise several layers which may be, for example, co-extruded or laminated layers. The substrate may be made of paper or plastic or a combination of them. The plastic layer comprises polymers or co-polymers, such as polyester, polyolefin, polystyrene, polyurethane, polylactic acid, cyclic olefin copolymer, polyamide or combinations of these. The face side may serve as a printing substrate but it is also possible that the face side remains unprinted. The face side may also be treated so that the surface properties become more suitable for printing. The substrate may be a plastic film which may be clear or opaque. Also “no label” look labels can be manufactured. It is possible that the substrate or at least one layer of the substrate is metallised, coloured, or textured. 
         [0021]    The adhesive of the adhesive layer is non-tacky in its initial state, i.e. the label may be manufactured without a release liner. However, the adhesive of the adhesive layer is activatable so that it becomes tacky when external energy is exerted on the adhesive layer. The adhesive comprises a polyurethane composition which is reversibly changeable from the non-tacky state to the tacky state. The adhesive may also comprise other polymers in addition to the polyurethane composition. The adhesive may include polymers, such as acrylic polymers, polyvinyl alcohol, ethylene/vinyl acetate copolymers or styrene/acrylate copolymers. The adhesive may also comprise inert fillers, tackifiers and/or plasticizers together with the polymer composition, such as the polyurethane composition. The external energy may comprise, for example, heat, infra-red radiation, microwaves, laser, or some other external energy which is capable of activating the adhesive layer. As one possibility, the adhesive layer is activated by heating it to a temperature between 50 to 90° C. 
         [0022]    The face side of the label may be printed by any type of printing process, such as UV-flexo, UV-letterpress, water-based-flexo, gravure, offset, litho, screenprocess, thermal-transfer, direct-thermal hot- or cold-foil stamping. After printing, the labels may be die-cut and supplied to the application point in pre-cut form in any shape or design-format required. Alternatively, the labels may be supplied in rolls to the application point, where they could be die-cut, for example, using laser and transferred to a vacuum drum, of the type typically used for wrap-around labels, where the non-tacky dry coating on the reverse side of the facestock would be activated by heat, IR radiation or another energy source to produce a coating with sufficient “hot-tack” in order for it to be applied to the container and remain firmly in place during or after cooling. 
         [0023]    The invention also relates to a method for attaching the label according to the invention to an item. The method is characterized in that the method comprises at least the steps of:
       activating the non-tacky adhesive layer of the label to become tacky, and   attaching the label to the item so that the tacky adhesive layer comes in contact with the surface of the item.       
 
         [0026]    The label of the invention may be attached to any type of item, container or surface which may be made of, for example, plastic, glass, metal, or cardboard. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0027]    In the following, the invention will be explained by an example and by referring to the appended drawings, in which 
           [0028]      FIG. 1  shows raw materials for the production of polyurethane dispersions, 
           [0029]      FIG. 2  shows the stabilization of dispersion particles by hydrate shells which are formed due to anionic groups, 
           [0030]      FIG. 3  shows the preparation of polyurethane dispersions by using the acetone process, 
           [0031]      FIG. 4  shows the preparation of polyurethane dispersions by using the melt dispersion process, 
           [0032]      FIG. 5  shows the drying and film formation of a polymer dispersion, and 
           [0033]      FIG. 6  shows the heat activation of polyurethane adhesives with crystalline polyester soft segments measured by TMA. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    In the following, the invention will be explained in more detail. 
       Preparation of the Adhesive Layer 
       [0035]    In the following, one possibility to prepare an adhesive for an adhesive layer will be explained. 
         [0036]    Polyurethane adhesives are generally produced by reacting long-chain polyols with aromatic or aliphatic isocyanates using the diisocyanate polyaddition process. Among the polyurethane heat-activatable adhesives, the polymer backbone is primarily based on crystalline polyester polyols, but amorphous polyester polyols have also been used for the production of heat-activatable adhesives (see  FIG. 1  which shows raw materials for the production of polyurethane dispersions). 
         [0037]    In the production of polyurethane dispersions by the acetone process or the melt-dispersion process, use is made of emulsifiers integrated in the polymer chain. Hydrophilic modification can be carried out through the covalent integration of ionic groups in the polymer chain, or through long polyether units at the ends of the chains. The dispersion particles are then surrounded by a stabilizing hydrate shell, with ionic and non-ionic hydrophilic groups generally acting synergistically (see  FIG. 2 ). 
         [0038]    Compared with the use of external emulsifiers, the incorporation of hydrophilic centers brings a number of advantages, such as self-dispersibility, high dispersion stability and good film-forming properties. Added to this is the fact that the covalent link prevents any diffusion of the emulsifier to the polymer surface. 
         [0039]    With the acetone process (illustrated in  FIG. 3 ), the first reaction stage—the so-called prepolymer-forming reaction—involves reacting the diisocyanate with the polyol in the melt to form a prepolymer terminated with isocyanate groups. In the second reaction stage—the chain extension reaction—after the addition of acetone, the molecular weight increases further through the addition of suitable chain extenders carrying ionic groups. Through the addition of water to the acetone solution, a fine dispersion is formed and, at the last stage, the acetone is distilled off. Because of the homogeneous structure of the polymer, the acetone process produces particularly high-quality dispersions. 
         [0040]    The melt dispersion process (illustrated in  FIG. 4 ) has become established as a simpler alternative production procedure. In the prepolymer-forming reaction, the polyols are reacted with the diisocyanates to form an isocyanate-terminated prepolymer. Following dispersion in water, during which a fine dispersion forms spontaneously due to the internal hydrophilic character of the prepolymer, a high molecular weight polyurethane is produced by means of a short-chain diamine in the chain extension reaction. Both processes result in dispersions with good storage life and solids contents of 40 to 50% by weight, in which the polymer is present in discrete particles of 100 to 200 nm diameter. Unlike solvent-based systems, this means that low application viscosities can also be achieved with high solids contents, and it is even possible to produce branched or crosslinked polyurethanes. 
         [0041]    The dispersion produces a homogeneous adhesive film on the substrate after the water has evaporated (see  FIG. 5 ). Although the drying process with adhesive dispersions is initially regarded as a rather problematic procedure, it is possible, with the correct choice of temperature, to achieve a drying rate for waterborne adhesives similar to that of solvent based systems. 
       Drying and Bonding of the Adhesive 
       [0042]    In the following, the drying and bonding behaviour of the adhesive described above will be explained. 
         [0043]    An important bonding process with polyurethane adhesives, both solvent-based and dispersion-based, is by heat activation. After applying the adhesive to the substrate, non-blocking films are obtained. The films only become tacky upon activation in a heat tunnel or under an infrared lamp through melting of the crystalline polyester segments at temperatures above the minimum activation temperature (see  FIG. 6 ). The period of time during which the polymer film has sufficient tackiness for bonding is called the hot-tack life. During this time, which can range from seconds to minutes depending on the polymer structure, the substrates can be joined. It should be noted that the hot-tack life is not directly related to the temperature of the adhesive. The hot-tack period, the period when the adhesive is tacky, may extend beyond the period after which the adhesive has reached ambient temperature. 
         [0044]    Through cooling of the adhesive film and crystallization of the polyester segments, a high initial bond strength is obtained. 
         [0045]    Another characteristic of heat-activatable polyurethane adhesives is that, because of their high molecular weight and segmented polymer structure, the heat stability is higher than the decrystallization temperature of the polyester segments. This means that the adhesives only begin to slow down the thermoplastic flow to a considerable extent at temperatures significantly higher than the minimum activation temperature. 
       Use of the Labelstock According to the Invention 
       [0046]    A labelstock according to the invention was manufactured. The reverse side of the facestock was coated with the polyurethane coating described above. The coating was dried at an elevated temperature. After drying, the coating was completely tack-free and non-sticky and, therefore, in comparison to pressure-sensitive labelstock, it did not require a release liner. 
         [0047]    After coating and drying of the polyurethane coating on the facestock, the labelstock was wound into reels and then supplied to printers for further processing. 
         [0048]    The facestock may be printed with any type of printing process such as UV-flexo, UV-letterpress, water-based-flexo, gravure, offset, litho, screenprocess, thermal-transfer, direct-thermal hot- or cold-foil stamping. 
         [0049]    After printing, the labels may be die-cut, for example, by using laser and supplied to the application point in pre-cut form in any shape or design-format required. Alternatively, the labels may be supplied in rolls to the application point, where they could be die-cut using laser and transferred to a vacuum drum, of the type typically used for wrap-around labels, where the non-tacky dry coating on the reverse side of the facestock would be activated by heat, IR radiation or another energy source to produce a coating with sufficient “hot-tack” in order for it to be applied to the container and remain firmly in place whilst cooling. Typically, the polyurethane coating could by activated by heating it to a temperature between 50 and 90° C. for only a few seconds. 
         [0050]    However, even due to the short heating period, the coating will typically remain tacky for up to 15 minutes which is more than adequate for it be applied to the container after activation. After that, the coating is no longer tacky and forms a permanent bond to the container and displays good heat-resistance. Using this technique, any paper or filmic facestock may be chosen together with any label shape and printed by any known printing process. Clear labels with a “no-label” look can also be achieved.