DE69705935T2 - LABEL FOR TRANSFER WITH INKY LAYERS, CONTAINER WITH TRANSFER LAYER AND METHOD FOR CLEANING SUCH A CONTAINER - Google Patents

Description

Background of the invention

The invention relates to a sticker comprising a carrier layer and a transfer layer which is removably attached to the carrier layer, wherein the transfer layer comprises an ink layer.

The invention also relates to a container equipped with a transfer layer according to the invention and a method for removing the transfer layer from such a container.

As is well known in packaging technology, containers such as plastic boxes are labelled by providing a non-removable permanent image through a screen printing process. Such labels provide a highly curable surface, with two or three colours available. However, this technique offers limited colours, lacks the enhanced graphic characters offered by other labelling techniques, is not flexible in its ability to change the graphic characters to suit market strategies, resulting in large inventories of redundant units, and is prone to wear and tear after about 4 passes.

When removable inks are applied to reusable plastic crates using a screen or pad printing process, the inks must be applied at the bottling plant, such as a brewery, which can cause problems in terms of approval. When the crates are removed using crate washing machines, the ink dissolves in the washing liquid and contaminates the crate washing machine. In addition, the application speed is limited and the curing of the inks requires a lot of space and long storage times before delivery.

A second option for labelling containers involves sticking printed paper labels onto containers such as plastic crates or bottles during filling or sealing. However, this type of label is less resistant to label damage during use and exposure to moisture (wrinkling). In addition, paper labels are difficult to remove from crates and tend to clog currently available crate washing machines. When paper labels are removed from plastic crates, an adhesive residue may be left on the crates.

A third technique for labelling containers, particularly glass bottles, is based on the principles described in WO 90/05088. This publication describes a method for labelling bottles which provides a durable high impact label and yet allows for a very specific label printing. A label is provided which has a removable carrier layer, the carrier layer being reverse printed with a vinyl or acrylic ink which is cured and overprinted with adhesive. The label is applied to the container with the adhesive surface in contact with it. The carrier layer is separated from the transfer layer of the label, for example by applying heat to either the container, the label or both. The labelled container is then provided with a coating which is then cured. The cured coating provides the required level of impact resistance and durability. The disadvantage of permanently applied labels is that these labels cannot be easily removed from the bottles if they are scratched or otherwise damaged. Furthermore, it is not possible to provide the same containers with new and/or different labels, which is desirable for promotional purposes.

An object of the invention is to provide a transfer layer that can be easily attached to a container and removed in an environmentally friendly manner.

Another object of the invention is to provide such a label which can be removed in a washing process using a washing liquid without the inks from the label contaminating the washing liquid.

A further object of the invention is to provide such a label which has good adhesion during storage and during use of the container, but which can be very quickly and economically removed from the container by replacing the label with new and/or different labels.

Another object of the invention is to provide such a label that uses water-soluble inks as a printing substance, such inks being environmentally friendly and widely used in food technology.

Yet another object of the invention is to provide a deposit crate system that can be provided with attractive labels that can be easily and economically removed and reapplied. The labels should be applied and removed at a relatively high speed.

Summary of the invention

Thus, the sticker according to the invention is characterized in that the transfer layer comprises an upper and lower containment layer on each side of the ink layer, the upper and lower containment layers contacting each other outside the outer boundary of the ink layer to form a closed envelope around the ink layer.

By enclosing the ink in the shell between the containment layers, the removal of the transfer layer from the container to which it has been applied is possible by a wet removal process such as a soaking process or a process using high pressure water jets. In such a process, the ink is prevented from running out of the shell so that no contamination of the wash water occurs. During the wet removal process, no more than 10% by weight of the ink of a transfer layer dissolves in an alkaline wash solution. This prevents the containers from being discolored by the inks. In addition, the ink concentrations in the wash solution remain low enough so that no aerobic and anaerobic treatment takes place in the waste water treatment plants. The low concentrations of inks in the wash water prevent the accumulation of metals in the sludge of the waste water treatment plants so that this sludge does not have to be treated as chemical waste under legal requirements. By simply collecting the removed labels from the washing liquid, a very economical washing process can be achieved.

Preferably, the ink layer comprises separate zones of dimensions between 0.5 mm² and 500 cm², the upper and lower containment layers contacting each other outside the separate zones to form separate envelopes around each zone of the ink layer. The areas of the transfer layer connecting the separate zones of the ink layer have a reduced thickness compared to the zones where an ink layer is present between the containment layers. After transferring the transfer layer to a container, it is possible that no label material is present outside the separate ink zones. These areas of reduced thickness or open areas of the label outside the envelopes represent natural points of attack for the washing solution so that the label can be removed in separate parts. Since the washing solution has access to the label-container interface via these areas outside the envelopes around the print patterns, a very rapid removal of the transfer layer from the container with removal of the label in separate pieces is possible. The pieces can be sieved out of the washing solution using conventional sieves with openings of between 0.1 mm and 10 mm, preferably about 2 mm.

Although the use of the transfer layer according to the invention on recyclable plastic boxes is preferred, the label can also be used in combination with plastic bottles, such as PET bottles, plastic plates, glass bottles and the like.

A preferred sticker according to the invention comprises a transfer layer permeable to the soaking liquids.

"Permeable" means that a transfer layer has a water absorption value of between 0.0 and 100 g/m², preferably of about 5 g/m² in water at room temperature after 3 hours. Such labels have a water vapor transfer rate of between 50 and 750 g/m², preferably of about 600 g/m², for water at room temperature after 24 hours.

The transfer layer may have a top layer overlying the ink pattern, the top layer forming the outwardly facing surface when the transfer layer is adhered to a container. The top layer may be formed, for example, from an acrylic wax layer. The top layer may be a continuous layer or may be printed discontinuously and congruently with the ink pattern. Very conveniently, the acrylic wax top layer may be penetrated, for example, by a 0.5% NaOH solution, while providing a sufficient barrier to moisture penetration during the storage and use conditions of the label on a container. The labels of the invention, which combine sufficient durability with rapid and economical removal, have a pencil hardness of between 1 N and 7 N when dry. and after a soaking time of between 1 min and 15 min in water at 20ºC, a pencil hardness of less than 0.5 N.

The transfer layer preferably has a configuration such that it breaks up into at least 4 pieces under turbulent soaking conditions in an aqueous liquid at a temperature of below 100°C, preferably below 70°C, within a soaking time of not more than 20 minutes, preferably not more than 10 seconds. When the transfer layer is released from the container, the size of the majority of the pieces formed during the breakup of the transfer layer is preferably not smaller than the dimensions of the separated zones of the ink pattern. Although some of the shells may rupture during the washing process, this results in relatively little leakage of the ink contained in these shells since these inks are still surrounded by a significant portion of the containment layer.

Using the containment layers of the invention, water-soluble inks can be used. In a preferred embodiment, the uppermost containment layer comprises a pigment-free ink that is compatible with the underlying printed zone. The lower containment layer preferably comprises an adhesive layer or an intermediate layer that is compatible with an underlying adhesive layer and with the overlying ink zone layer.

The upper containment layer can be discontinuous and printed in register with the ink pattern. In this way, the lower containment layer can be directly attacked by the wash liquid during removal of the transfer layer. In a preferred embodiment, both containment layers and an underlying adhesive layer are discontinuous and all printed in register.

After applying the transfer layer to the container, a topcoat may be applied over the transfer layer, the topcoat containing an acrylic wax. The acrylic wax is relatively impermeable to water, such that good resistance of the label to scratching and removal of the label during use of the container is provided. The acrylic wax topcoat, however, is sufficiently permeable to an aqueous alkaline solution that the transfer layer can be easily removed with, for example, a 0.5% NaOH solution.

Preferably, the transfer layer is heat treated after application to a container to cause shrinkage of at least parts of the transfer layer. The heat treatment causes coalescence of the various layers of the sticker.

A label according to the invention, which combines sufficient durability during storage and use with rapid and economical removal, is preferably heat-treated after application to the container at a temperature between 40°C and 100°C, more preferably between 50 and 90°C.

By carefully choosing the composition of the container, the use of a protective layer and the type of post-treatment (heat treatment), it is possible to control the properties of the transfer layer, especially with regard to its behavior during washing.

The choice of adhesive used to adhere the label image to the container surface will affect the wash-off properties. The adhesive must be activated before or during application of the transfer layer to the container. An easy and generally preferred method of applying the image is to use heat-activated adhesives that are applied to the image in the form of a reverse label print. Other methods include the use of adhesive that can be activated by irradiation, chemicals, electro-beams, microwaves, UV and the like. The use of adhesives that can be activated by photo-start, moisture, enzyme action, pressure or ultrasonic treatment is also possible.

A preferred adhesive has a low tack temperature, preferably between 60º and 90º, and more preferably between 80º and 90º. Instead of a separate adhesive layer, it is also possible to use an ink, which itself has adhesive properties when activated, in the transfer layer.

Preferably, the application surface of the container for receiving the transfer layer has a surface tension of at least 60 dynes (60·10⁻⁵ N)/cm. The washing process for the container having a transfer layer according to the invention comprises the steps:

- placing the container in a soaking solution for a soaking time of not more than 20 minutes, preferably not more than 1 minute, the temperature of the soaking solution being below 100ºC, preferably below 70ºC, while agitating the soaking solution in such a way that the label breaks up into at least 4 parts, each part being not smaller than 5 µm and is released from the container, most of the remaining ink being contained inside the sleeves,

- Pumping the soaking solution through a sieve and collecting the label pieces on the sieve,

- Regularly clean the screen by collecting and removing the transfer layer pieces.

A transfer layer according to the invention can be removed using the conventional crate washing system, whereby the detached label pieces can be removed from the soaking solution by means of sieves. Since no parts of the label dissolve in the soaking solution, no special processing equipment is required to clean the washing solution.

Short description of the drawings

The embodiments of a sticker according to the invention and a washing method according to the invention are described in detail below with reference to the attached drawings. They show:

Fig. 1 shows a thermal sticker according to the invention, wherein separate ink patterns are present in each individual envelope.

Fig. 2 shows a washing system for removing a transfer layer according to the invention from a container, in particular from a plastic box.

Fig. 3 shows a cross section of the washing system according to the invention from Fig. 2 along the line III-III.

Fig. 4-7 show various embodiments of the transfer layer of a sticker according to the invention.

Fig. 8 schematically shows a top view of an embodiment of a sticker according to the invention having differently sized envelopes around the ink pattern.

Fig. 9 schematically shows a method for applying the transfer layer according to the invention to a reusable box; and

Fig. 10 and 11 graphically show the release time for a transfer layer at different heat post-treatment temperatures without and with a wax cover layer.

Detailed description of the invention

Fig. 1 shows an embodiment of a sticker 1 according to the invention, which has a carrier or a carrier layer 2, which is formed for example from a polypropylene film of 2 mil thickness (51 µm). On the carrier or the carrier layer 2 there is a silicone layer 3. On the silicone layer 3 there is a transfer layer 4 which consists of an upper containment layer 5,5', an ink layer 7,7', a lower containment layer 6,6' and an adhesive layer 8,8'.

When applying the transfer layer to a container, the backing layer 2 and the silicone layer 3 are removed using heat and pressure. The adhesive layer 8 bonds the transfer layer 4 to an underlying container surface and the outward-facing layer is formed by the upper containment layer 5,5'.

The label carrier 2 provided with the electron beam cured silicone layer 3 can be, for example, a polypropylene film of 1 to 3 mil thickness, as supplied by Mobil Chemical, Films Division, Rochester, New York. Before printing the upper containment layer 5,5' onto the silicone layer 3, the silicone surface must be subjected to a corona treatment. Corona treatment allows uniform wetting of the printing materials while still allowing removal of the transfer layer 4. Preferably, corona treatment is applied shortly before the first printing of the upper containment layer 5 onto the carrier layer 2 and the silicone layer 3. A target treatment concentration should be approximately 30% of 3.5 kW.

When handling the silicone-coated carrier layer 2, care must be taken not to scratch the silicone layer 3. Scratching the silicone layer 3 would prevent the upper containment layer 5 from contacting the underlying polypropylene film 2 and it would adhere thereto, which would adversely affect the transfer of the transfer layer 4 during application.

For example, the upper containment layer 5,5' is made of pigment-free ink and has several functions. First, it slows down or prevents the penetration of water into the underlying ink layer 7,7'. When the layer 5,5' is printed wider than the underlying ink pattern 7,7', it forms part of a shell that completely surrounds the colored ink layers 7,7'. In addition, the upper containment layer 5,5' provides a consistent medium between the inks and the silicone release surface 3. The layer 5,5' is very important for the overall transferability of the label and should be applied at a weight of at least 1.4 g/m². It is important that this layer is free of air bubbles and pinholes when the upper containment layer 5,5' is applied. In addition, the upper containment layer must be dry before the subsequent ink layer 7,7' is printed on top of it.

After printing the upper containment layer 5,5' onto the release layer 3, an optimum peel force of 100 g or less should be measured during a standard peel test. 5 hours after application, the peel force of the upper containment layer is about 60% less, or 40 g. The containment layer 5 should be completely removed with the specified peel force. A suitable material for the upper containment layer 51 is available from Environmental Inks and Coatings, Morganton, North Carolina, under Type No. 1304.

Examples of a preferred ink for the ink layer 7,7' include a water-based organic substance such as available from Environmental Inks and Coatings, Morganton, North Carolina under type number Aqua BW EH-31721, EH 53016, EH 90967. These inks have high stability even at temperatures above 200ºC without discoloration or loss of adhesion.

The lower containment layer 6,6' provides a strong interface between the adhesive layer 8,8' and the colored ink layers 7,7'. It is formulated to chemically anchor the ink and provides excellent cross-linking and bonding of the adhesive layer. The lower containment layer 6,6' adheres to the upper containment layer 5,5' outside the ink layer 7,7' so that a closed shell is formed around the separate ink patterns 7,7'. A suitable material for the lower containment layer 6,6' is available from Environmental Inks and Coatings under the type number XP 11358.

The adhesive layer 8,8' may be formed from a water-based organic material which is printed in a number of consecutive flexographic printing stations, such as three stations, or may be applied in a single station under flooding conditions. The adhesive layer 8,8' may also be applied in a single gravure printing station. Preferably, the adhesive 8,8' is heat activated and has a low tack temperature of 80ºC up to 107ºC. The preferred weight of the adhesive is about 3.5 g/m².

The layers of the transfer layer 4 can be applied in a flexographic printing press with up to 10 printing stations. Five stations can be used to print the layers 5,5', 6,6' and the adhesive layer 8,8', which can consist of three separate adhesive layers. Using the five remaining flexographic printing stations, five types of colored ink 7,7' can be applied.

Instead of a flexographic printing process, a gravure printing press equipped with a corona treatment device can also be used. Since the material deposition is then heavier than with the flexographic printing process, only three gravure printing stations may be necessary to apply the inclusion layers 5.5' and 6.6' and the adhesive layer 8.8'.

Rotary screen printing techniques can also be used to apply layers 5,5', 6,6' and 8,8'. The lower containment layer 6,6' should be printed carefully so that it extends beyond the outer boundary of the ink patterns 7,7', but remains within the outer boundary of the upper containment layer 5,5'. It is preferred that the adhesive layer 8,8' extends beyond the outer boundary of the lower containment layer and fits onto the outer boundary of the upper containment layer 5,5'.

Fig. 2 shows a schematic side view of a crate washing system for removing the transfer layers according to the invention from crates 12 fed to the crate washing system 10 via a conveyor belt 11. The crates 12 are first fed to the pre-rinse station 13 and sprayed with a pre-rinse solution which is applied from a number of nozzles 14 arranged above and below the conveyor belt 11. The speed of the belt 11 is such that the residence time of the crate 12 in the pre-rinse station is between 6 and 8 s. The temperature of the pre-rinse solution is 60°C. The pre-rinse solution preferably comprises a 0.5% NaOH solution.

After passing the pre-rinse station 13, the crates are guided through a soaking station 15 via a downwardly inclined section 16 of the belt 11. The dwell time of the crate in the soaking station is between 10 and 40 s. In the soaking station, the crate is completely immersed and in the soaking station 15 a soaking solution is recirculated by means of the nozzles 35, creating turbulent soaking conditions. The turbulent soaking conditions may, for example, comprise the recirculation of the liquid from the soaking station 15 via the nozzles 35 at a rate of 60 m³/h for a total volume of soaking solution of 5 m³. It is important that the transfer layers are completely removed from the crates 12 in the soaking station 15 without leaving any pieces on the crates. Such remaining Pieces, when dry, would stick firmly to the boxes and create unwanted contamination on the box surface.

From the soaking station 15, the crates are transported via the upwardly inclined belt track 17 to a rinsing station 18. The rinsing solution can comprise water at a temperature of 30ºC. The dwell time of the crates in the rinsing station is between 6 and 13 s.

Connected to each rinsing station 13, 18 and to the soaking station 15 are screen sections 20, 21 and 22. Each screen section comprises a rotating belt screen 23, 24, 25, which are driven by the motors 26, 27 and 28 respectively. The pumps 29, 30 and 31 draw the rinsing liquid and the soaking liquid at a speed of, for example, 60 m³/h from the corresponding station through the rotating belt screens 23, 24, 25. The screened liquids are returned to the nozzles 14 and 19 and to the soaking station 15 in the pre-rinsing and post-rinsing stations 13, 18.

Fig. 3 shows a cross section along the lines III-III of Fig. 2. It can be seen that the screen belt 24 rotates around the two rollers 37, 38. The upper end of the screen belt 24 runs above the level of the soaking liquid in the soaking station 15. The screen belt 24 comprises a double belt-like screen element with a mesh size of 2 mm. During operation, a continuous rotation of the screen belt 24 is important to prevent the screen belt from being blocked by the label pieces from the transfer layers which break into pieces in the soaking station 15. A spray nozzle 39 cleans the surface of the belt-like screen elements by high pressure water or air jets. The removed label elements are collected in a collecting compartment 40.

It has been found that using a 0.5% NaOH solution in the pre-rinse station 13 and the soaking station 15 achieves a very effective label removal from the crates 12. However, it is also possible to A pretreatment material can be applied to the labels before entering the crate washer 10, which causes the label to soften before entering the crate washer. For example, a surface-active component can be sprayed onto the crates 12 as they pass through the crate washer 10. It is also possible to apply a gel-like material of a chemical composition that starts the attack on the label before entering the crate washer 10. In such a case, only the use of water instead of the alkaline solution may be possible in the crate washer 10.

Fig. 4 shows an alternative embodiment of an adhesive according to the invention, which has a carrier layer 48, a silicone release layer 49 and a transfer layer 50. The ink layer 52 of the transfer layer 50 is a continuous layer, which can have dimensions of 10 by 10 cm, for example. The upper containment layer 51 and the lower containment layer 53 frame the ink layer 52 and interlock around the outer boundary of the ink layer. This forms a single shell around the ink layer 52. During removal of the transfer layer 50 from a container to which it has been applied, the transfer layer 50 can break into several pieces. This breaks up the shell formed by the upper and lower containment layers 51, 53. However, it has been found that sufficient entrapment still occurs to prevent dissolution of the ink layer 52 in the washing solution.

In the embodiment of Fig. 5, the ink layer 52 is formed on separate zones 52, 52'. Each zone of the ink layer can be formed, for example, by individual letters, individual sentences or individual word blocks. The individual zones 52, 52' can also be formed by other graphic objects. The upper containment layer 51 is shown to adhere to the lower containment layer 53 around the outer boundary of each individual ink zone 52, 52'. This forms envelopes around each individual ink zone and enables effective containment.

In the embodiment of Fig. 6, the upper containment layer 51 is formed by separated zones 51, 51'. The washing solution can easily penetrate through the open areas between the separated zones of the upper containment layer 51, 51' and easily attack the underlying containment layers 53 and the adhesive layer 54.

As shown in Fig. 7, the adhesive layer 54, the lower containment layer 53, the ink layer 52 and the upper containment layer 51 are each printed in register and form the separate zones 51,51', 52,52', 53,53' and 54,54'. Such a transfer layer has a very attractive appearance, and the container surface is clearly visible between each individual ink zone 52,52'. With this special construction, a very fast washability is achieved, since the washing liquid can very quickly attack the adhesive layer 54,54' by penetrating through the open areas between each ink zone 52,52'.

As shown in Fig. 8, a transfer layer according to the invention can consist of several parts. For example, a graphic object 55, such as an image, can consist of a single layer of ink that is framed around its perimeter 55' between an upper and lower containment layer by a structure as shown in Fig. 4. Instead of the graphic object 55, separate lines of text 56 can be framed between an upper and lower containment layer, for example by a structure according to Fig. 5, Fig. 6 or Fig. 7. As indicated at 58, individual letters in a sentence can each be individually enclosed between the upper and lower containment layers.

Fig. 9 shows a schematic view of the application process for a transfer layer from a sticker according to the invention to a deposit box 59.

The label application process will now be described in sequence. Station 60 shows the step of surface treatment and temperature stabilization by means of a preheating treatment using a flame heater or burner 60'. In order to achieve adhesion between two polymer materials, many factors must be taken into account, such as purity, pressure, temperature, contact time, surface roughness, agitation during bonding, and adhesive film thickness. An additional important consideration is critical surface tension. The currently generally accepted method for measuring critical surface tension is a dyne solution, which is well known. For most bonding applications, the critical surface tension of polyethylene is 31 dynes (31 x 10-5 N/)/cm. A series of tests were carried out which showed that a treatment level of 60 to 70 dynes/cm (60 x 10⁻⁵ to 70 x 10⁻⁵ N/cm) was required to achieve the best adhesion of the previously described adhesive to the polyethylene surface. A further test of commercially available equipment showed that flame treatment optimized both the financial and operational costs and the time required to achieve the critical surface treatment required.

In order for the adhesive to quickly achieve and maintain tack, heating of the polyethylene box 59 in station 61 is necessary before the label adhesive contacts it. To avoid deformation of the container, it is desirable not to heat the surface above 200ºF (93ºC). If the surface treatment temperature at the time of flame treatment is about 125ºF (52ºC), heating the surface to about 75ºF (24ºC) in station 61 is necessary. Again, many options for heating are available. Hot air, additional flame heaters, gas-fired infrared panels and electric ceramic panels have been tested and found to be either too slow or too difficult to control. It has been found that an electrically heated flat quartz glass emission plate 61' with zonal band control for local label transfer provides maximum free air transfer of infrared energy without the effects of surrounding environmental factors.

With an emissivity of 0.9 for polyethylene, a desired emission plate temperature between 1652ºF (900ºC) to 1725ºF (940ºC) emits the most effective wavelength (2.5 to 3.2 um) of infrared energy for maximum absorption. The unit tested was rated at 60 W/in.2. The time to heat the polyethylene surface to the necessary 75ºF (24ºC) was 4.5 s at a distance of 2.5 cm from the emission plate.

Station 62 explains the process of label application, whereby the printing ink materials are transferred from the polypropylene film substrate to the polyethylene surface using the tactile properties of the heat activated adhesive to eliminate the bond of the transfer layer to the corona treated silicone layer. The factors that affect the transfer are contact time, temperature and pressure applied during contact and film tension during contact, particularly the tension of the film after ink release. The diameter of the printing roller 63 is also a factor, but not a variable one. For this application, the roller diameter is 38 mm. The roller 63 is made of silicone rubber over a steel core, with the rubber durometer ranging from 50 Shore A to 80 Shore A. It should be noted that deformation (flattening) of the rubber roller is less with higher durometer and consequently the contact area is smaller and the transfer pressure is greater. This is important at the higher belt speeds when contact time is minimized. Thus, a box moving at 18.3 m/min (60 ft/min) over a 38 mm diameter roller will have a contact time of 1 ms/1º of roller rotation, with no roller deformation occurring.

The roller pressure is provided by an air cylinder 64 which is activated by a conventional solenoid valve which in turn is operated by two (2) adjacent switches, one switch for roller advancement and the other switch for roller retraction. Other devices such as mechanical linkages are clear and are not listed here. The Pressure is distributed over the length of the cylinder and for this particular label the transfer ranges from 12 to 17 kg/cm of cylinder length and is desired.

Thus, the invention causes the film to advance over the roller at exactly the same speed as the box due to the heat activated adhesive adhering to the high energy surface of the box. The pressure roller 63, which rotates freely, maintains the same tangential speed as the linear speed of the film and box. Thus, the ink is transferred completely and undistorted.

For rapid and complete adhesion, the pressure roller 63 is formed into a hollow core. A resistance heater is suspended in the hollow core and operated by a controller. The heating element, set at 500 watts, maintains the roller surface at a predetermined temperature. For the purposes of the invention, the roller surface temperature is in the range of between 250ºF and 370ºF (120ºC and 190ºC).

Many silicone coated polymer films can be used for the printing substrate. High temperature films such as polyester can be operated in continuous contact with the heat roller. Low temperature films such as polypropylene must be prevented from contacting the heat roller during pauses in the labeling process. To accomplish this, film guides 65 are used to support the film as the roller is retracted. The guides 65 are mounted so that a free space of approximately 13 mm is maintained between the guide and the surface being labeled. At the same time, the roller is retracted approximately 13 mm behind the film. By maintaining these free spaces, stretching and deformation of the film such as polypropylene is avoided. High temperature films do not require these guides.

It was also found that the film tension, especially on the film exit side of the roller, is important for complete ink transfer.

Through experimentation it has been found that a continuous tension of approximately 2.5 kg is useful. This is achieved by a spring-loaded swing arm and a roller.

Conventional nip rollers and stepper motors are used to advance the film to the next label and position it precisely using a pressure signal to trigger an optical scanner.

Protection of the ink against scratching during careless handling and ensuring its weather resistance during outdoor storage is achieved by applying an acrylic-based wax water emulsion in station 66. This is applied via a roller applicator 68 fed by a wet roller with a controlled degree of coating. Control is achieved by a doctor blade. The coating extends sufficiently beyond the edges of the ink pattern and seals the edges against moisture penetration.

The final stage of the process is a coalescence of the layers of coating, label ink and adhesive in station 67 by means of a flame heater 67' and also an interdiffusion of the adhesive layer with the polyethylene substrate formed by the box 59. This determination is made through extensive testing with many heating systems. It has been found that a flame treatment is the best technique that provides the required surface energy for label adhesion; thus, it has been found that a flame treatment of the label and the layer composite is the best technique for developing the required water immersion resistance without compromising the mechanical properties or changing the visual characteristics of the applied label or deforming the polypropylene box 59.

To clarify the various properties that explain the adhesion and washability of the preferred transfer layer of the present invention, the following tests were conducted including a wash test, a pencil scratch test, a water absorption/release test and a water vapor transmission rate test as described below.

Washing test

To determine the optimum washing conditions for the labels of the invention, a transfer layer 50 having the configuration shown in Figure 4 was applied to a polyethylene crate. The dimensions of the label were approximately 10 by 10 cm and the adhesive layer 54 was 100% urethane adhesive with a tack temperature of 79°C. The labels were applied to the crate at a roller temperature 63 in Figure 9 of 155°C at a roller pressure of 2.5 bar (2.5 x 10⁵ Pa). The preheat temperature of the crate (in stations 60 and 61 of Figure 9) was 75°C. The speed of the crates 59 through the label applicator was 40 crates/min. To determine the influence of the post-treatment temperature at which the boxes were heated after label application in station 67 of Fig. 9, the post-treatment temperatures of 40ºC, 65ºC and 90ºC were used. After label application, the boxes were stored for at least 24 h at a temperature of 20ºC. The boxes to which a label had been applied were subsequently soaked in a 0.5% NaOH solution at temperatures of 20ºC, 50ºC and 70ºC.

The soaking of the boxes was carried out in a 20 l soaking bath without turbulence for such a soaking time (10-50 s) that the label was completely removed after spraying the soaked box with a shower head at a speed of 6 l/min in 2 s.

A second series of boxes was prepared, with a top coat of wax applied after labeling, as in station 66 of Fig. 9.

The results of the soak times required for label removal within 2 seconds are again given in Tables I and II with the post-treatment temperature. From Table I, the results of which are shown graphically in Fig. 10, it can be seen that the soak time for the labels to which no wax coating was applied decreased dramatically at soak solution temperatures above 20ºC. For post-heat temperatures of 90ºC, the label durability was increased and the soak times remained above 5 seconds. Table I Crate Wash Test (Without Wax Coating Applied)

It was found that an optimal heat treatment temperature was between 65ºC and 90ºC. At a heat treatment temperature below 65ºC, insufficient coalescence of the applied transfer layers was observed. such that the applied transfer layers had insufficient durability and were too easily removed during storage and use. At a post-heat treatment temperature of over 90ºC, the durability of the transfer layer became too great and the short removal times could not be achieved in an economically feasible manner. During the spray period with the shower head, it was observed that the labels detached from the box after soaking and disintegrated into several (2 to 4) pieces.

If a wax coating was applied at station 66 prior to the flame treatment step at station 67 in Fig. 9, the durability of the labels is increased and the soaking times are increased. From Table II it can be seen that for a 0.5% caustic solution the wax coating resulted in longer soaking times. The results of Table II are presented graphically in Fig. 11. Table II Crate washing test (with wax coating applied)

It was found that when the labels were tested as in the washing test described above, only with high pressure water jets at 20ºC and at a pressure of 120 bar, at a belt speed of 15 m/min and a spray angle of 90º at a distance of 10 cm, no label removal was achieved. Even for labels without a wax layer and without post-heat treatment, no removal was possible using high-pressure water jets.

Pencil scratch test

The purpose of the pencil scratch test is to identify the minimum and maximum durability of a label that can be obtained by implementing various measures such as using a wax topcoat and heat treatment to induce coalescence of the label layers. Cases of labels applied at different heat treatment temperatures with and without wax were tested.

The labels were the same as those used in the washing test described above and were applied to the boxes under the same conditions.

The pencil scratch tests were carried out using a "Scratch Resistance Test Model 435" from Erichsen (PO Box 720, D-5870 Hemer Germany).

During the scratch test, a pen with a plastic insert was used to score the label at an angle of 90º horizontally in the middle of it.

After label application, the boxes were stored for at least 24 hours at a temperature of 20ºC. Before scratching, the boxes were soaked in water at 20ºC without agitation. The results of the scratch test are given in Table III and Table IV, where the scratch results are given in N. Table III Pencil scratch test (in N) Label without wax layer Table IV Pencil scratch test (in N) Label without wax layer

From Tables III and IV it can be seen that flame heat post-treatment does not appear to significantly affect the scratch resistance of the transfer layers in the dry state. The durability of the transfer layer is, however, increased by flame heat post-treatment as evidenced by the higher pencil hardness after soaking. From Table IV it can be seen that the application of a wax layer covering the label significantly improves the scratch resistance of the dry label. It was found that for high flame heat post-treatment temperatures of 110ºC in conjunction with a wax layer a scratch force of 8 Newtons was achieved. Labels with a pencil hardness of 8 Newtons are considered semi-permanent labels which cannot be removed in an economically feasible manner.

Furthermore, at post-heat temperatures above 90ºC, problems occurred during labeling, as the polyethylene crates became brittle after a few uses at these temperatures and it was observed that the crate pigments discolored and deformation of the softened crates occurred on the conveyor belt and pelletizer.

At a post-heat treatment temperature below 65ºC, it was found that the strength of the labels was insufficient for labels that did not have a wax coating. For labels without a wax coating, the target pencil scratch hardness in the dry state should be around 1.2 N and the soaking time until the scratch force drops below 0.3 N should be less than 3 minutes. For a wax-coated label, the target scratch strength in the dry state should be around 5 N and the soaking time until the scratch force drops below 0.3 N should be less than 10 minutes. The transfer layers with the above properties were found to provide an optimal combination of durability and washability.

Water absorption test

The labels of the invention can be easily removed from a container, especially a plastic crate, due to their special water permeability which allows the soaking solution to penetrate the label and then disintegrate the label into pieces and be detached from the container. It was found that the preferred labels had a water absorption of around 5 g/m² after 3 hours in a water absorption test as described below. The labels of the invention have a water absorption value of over 0 and less than 100 g/m² in 3 hours. The water release of a preferred label was 4.5 g/m² within 30 minutes in the water release test as described below. Preferred labels of the invention have a water release value of over 0 (complete barrier) and less than 100 g/m² after 3 hours.

Two samples were prepared, each sample comprising 2 labels of 12.7 microns thickness, each at 22.4ºC and 48% relative humidity, each sample having a specific surface area of 85.8 cm2. For each sample, two labels were applied to a single piece of pure glass measuring 3 in (7.62 cm) x 9 in (22.03 cm) x 0.02 in (0.05 cm). Due to the extremely low weight of the labels, it was necessary to apply two labels per piece of glass to obtain a weight that would be within two decimal places of the electronic gram scale.

The samples were prepared as follows: The glass slides were carefully cleaned and placed in a heating oven until an approximate temperature of 250ºF (121ºC) was reached on the glass surface. The glass was then removed from the oven and placed on a silicone rubber mat. A label was immediately applied to the glass and secured to the surface using a silicone roller. Rolling pressure was continuously applied along the full length of the label until all trapped air was removed (approximately 5-6 strokes forward and backward). After the glass had cooled, the carrier film was removed. The opposite side of the glass plates was then labeled by heating a clean aluminum plate (slightly larger than the glass plate) to approximately 250ºF (121ºC) in a convection oven and then placing the glass on the surface of the aluminum plate (label side down), allowing the top of the glass to heat. The label was then applied and secured in place by the silicone roller as described above. Again, after the glass plate had cooled, the carrier film was removed. Next, a layer of wax with a dry weight of 0.043 g was applied to the surface of both labels. The final step was flame treatment using an oxidizing propane gas flame on both labels by rapidly passing the flame across the entire surface of the label sample. After the samples had cooled, the labels were ready for the water absorption test.

A stainless steel dip tank 13.25 inches in diameter and 1.65 inches high was filled with deionized water. Care was taken to ensure the water level was deep enough to allow the sample to be completely submerged. The sample was placed with the short dimension perpendicular to the bottom of the tank. The glass slides were placed in the dip tank on a thin wire frame. A thermocouple was placed inside the water dip tank. After each time interval as indicated in Table V, the samples were removed from the tank, excess surface water was blotted off, the samples were weighed and returned to the tank. This procedure was continued for the duration of the test. The results are shown in Table V. With regard to Sample 1, this sample reached maximum absorbance of 0.04 g at the 3rd hour and maintained this level until the 5th hour before losing the ability to hold water at this level. After a 5-hour period, the label lost its ability to hold water. We believe that this phenomenon was caused by the degradation of the label structure. For sample 2, this sample also reached its maximum absorption of 0.04 g after the 3rd hour. After the 5th hour, no further tests were performed on this sample in preparation for the water release tests described below.

From the water absorption test it can be deduced that a preferred label of 12.7 micron thickness had a water absorption value of 0.04 g/85.8 cm² or about 5 g/m² after 3 hours at room temperature. Table V Water Absorption Test

To calculate the individual label weights in grams from the data in Table V, proceed as follows:

Each sample used two labels. To calculate the weight of sample 1 at 1:00 p.m., subtract the 8:00 a.m. reading from the 1:00 p.m. reading and divide by 2.

Example:

1pm value 59.85

8 o'clock value 59.77 -

0.08/2 = 0.04g

Water release test

Immediately after completion of the above water uptake test, Sample 2, prepared as above, was subjected to the water release test. The sample was blotted dry to remove excess water, weighed, and the data recorded. The sample was first exposed to ambient temperature for half an hour and weighed. Half an hour after weighing the sample, it was placed in a preheated (53ºC) test oven (small electrically heated oven, Quieny Lab Inc., Model 20 Laboratory Oven or equivalent). The sample was left in the preheated oven for over 1 hour and weighed. The sample was then returned to the test oven and remained there for 3.5 hours.

From Table VI it can be concluded that the water absorbed by Sample 2 was released during 30 min. exposure to ambient room temperature and humidity (48%). In fact, the sample was found to lose 0.01 g of its original weight, which seems to indicate that the label was not carefully dried during application. Thus, a preferred label of 85.8 cm² in size and 12.7 microns thick exhibits a water release under these parameters of greater than 0 and less than 0.10 g/24 h with an average release of 0.045 g within 30 min. Table VI Water Release Test

Water vapor transmission rate test

The optimum combination of durability and washability of the labels of the invention relies at least in part on the permeability of the label to the soaking solution. A sample of the transfer layer of the same type as tested in the water absorption/release test, 12.7 microns thick, was tested for water vapor transfer. A 25 ml glass container, 15.9 mm diameter, round opening, was cleaned with acetone and filled with approximately 10 ml of deionized water. The opening area of the container was heated to approximately 118ºF (47.8ºC) and a circular segment of the transfer layer was firmly applied using a small piece of silicone rubber as a pressure pad. After the container/label had cooled, the carrier film was carefully removed. The sample preparation was completed by adding a layer of wax (0.001 g over the 1.99 cm2 surface area) and allowed to air dry. A second glass container of the same dimension as described above was carefully cleaned with acetone and filled with 10 ml of deionized water. The orifice area of the sample was also heated. This sample was used as a control sample. The completed samples were then weighed at various time intervals over a period of 26.6 h. The water vapor transfer rate during the entire time of the experiment was equal to 568.75 g/m2 during a 24-hour time interval at 22.2ºC at 46% relative humidity. It was found that a "steady state" water vapor transfer rate was not reached until approximately 28 min. from time 0. Using the "equilibrium" data at 28 min from time 0, it was determined that the water vapor transmission rate was approximately 526.93 g/m2 in 24 h.

For the control sample without label, a water vapor transfer rate of 1085.7 g/m² in 24 h. The water vapor transmission rate of the preferred label according to the invention is between 50 g/m² and 750 g/m² (22.2 ºC, 44% relative humidity) after 24 h, preferably around 500 g/m² after 24 h.

Claims (22)

1. A sticker comprising a carrier layer and a transfer layer removably attached to the carrier layer, the transfer layer comprising an ink layer, characterized in that the transfer layer comprises an upper and lower containment layer on each side of the ink layer, the upper and lower containment layers contacting each other outside the outer boundary of the ink layer to form a closed envelope around the ink layer. 2. A sticker according to claim 1, wherein the ink layer comprises discrete zones with dimensions between 0.5 mm² and 500 mm², the upper and lower containment layers contacting each other outside the discrete zones to form individual envelopes around each zone of the ink layer. 3. A sticker according to any one of the preceding claims, wherein the ink is water-soluble. 4. A sticker according to any preceding claim, wherein the upper containment layer contains a pigment-free ink and the lower containment layer contains an adhesive. 5. A sticker according to any preceding claim, wherein the upper containment layer is arranged discontinuously and congruently with the ink pattern. 6. A sticker according to any preceding claim, wherein the lower containment layer is discontinuous. 7. A sticker according to any preceding claim, wherein the transfer layer is permeable to the soaking liquid outside the separate zones of the ink pattern. 8. Sticker according to one of the preceding claims, wherein the transfer layer has an adhesive layer whose stickiness is reduced at least by contact with the soaking liquid, preferably by dissolving in the soaking liquid. 9. A sticker according to claim 8, wherein the adhesive layer is discontinuous and is arranged congruently with the ink pattern. 10. A sticker according to claim 3 or 4, wherein the soaking solution is an aqueous alkaline solution. 11. Sticker according to one of the preceding claims with a thickness of less than 30 µm, preferably less than 20 µm, and a weight of the enclosure layers between 1 g/m² and 10 g/m². 12. Sticker according to one of the preceding claims, which has an adhesive layer with a weight between 1 and 10 g/m², preferably between 3 and 7 g/m². 13. A sticker according to any one of the preceding claims, wherein the adhesive layer has at least two sub-layers, wherein the tackiness of the sub-layer closest to a container when attached to the container is lower than the tackiness of the adhesive layer further away from the container. 14. A container having a transfer layer applied using a label according to any preceding claim. 15. A container according to claim 14, wherein a topcoat is applied over the transfer layer, the topcoat containing an acrylic wax. 16. A container according to any one of claims 14 to 15, which has an application surface for receiving the transfer layer, the application surface having a surface tension of at least 60 dy (60 x 10-5 N)/cm. 17. A container according to any one of claims 14 to 16, wherein the label on the container has a pencil hardness of between 1 N and 7 N in the dry state and a pencil hardness of less than 0.5 N after a soaking time of between 1 and 15 minutes in water as a soaking solution at 20°C. 18. Container according to one of claims 14 to 17, wherein the label on the container has a water absorption value after 3 hours of greater than 0 and less than 100 g/m², preferably of about 5 g/m². 19. A method for washing a container according to any one of claims 14 to 18, comprising the steps of: - placing the container in an aqueous soaking solution for a soaking time of not longer than 20 minutes, preferably not longer than 1 minute, the temperature of the soaking solution being below 100°C, preferably below 70°C, while agitating the soaking solution such that the transfer layer breaks up into at least 4 parts, each of which has dimensions of no less than 5 µm, and is detached from the container, the majority of the remaining ink being contained inside the shells, - Pumping the soaking liquid through a sieve and collecting the pieces of the transfer layer on the sieve, - regular, preferably continuous, cleaning of the screen by collecting and removing the transfer layer pieces. 20. The method according to claim 19, wherein the size of the sieve openings is between 0.1 mm and 10 mm, preferably about 2 mm. 21. A method according to claim 19 or 20, comprising the step of impinging water jets on the container before and/or after placing the container in the soaking solution. 22. The method of claim 19, 20 or 21, wherein the soaking solution contains between 0.1 and 5 wt.%, preferably 0.5 wt.% NaOH.