A method for labelling a container and a labelled container

The present invention relates to a method for labelling a container and a labelled container.

More precisely, the present invention relates to roll-applied shrink- labels. The roll-applied shrink-labels are cut from a roll and wrapped around a container so that the leading edge and the trailing edge of the label overlap. The leading edge and the trailing edge are joined together, and the labelled container is led to a tunnel in which an elevated temperature prevails. The label shrinks in the tunnel so that its shape conforms to the shape of the container.

The roll-applied shrink-labels based on BOPP (biaxially oriented polypropylene) films are well documented in literature. However, they suffer from one major disadvantage in that under practical conditions their shrinkage performance is limited to a maximum of approximately 20%. This greatly limits the choice of containers for which these films can be applied.

Other polyolefin films, in particular mono-axially oriented polypropylene films, have been developed with a maximum shrink- performance of 30 - 40%. However, these suffer from the major disadvantage in that these shrinkage properties are only realised when using hot-air/infra-red shrinkage tunnels operating at temperatures of 130° - 135 ⁰ C, and therefore such films are not suitable for the commonly used steam-shrink tunnels operating at temperatures of approximately 95 ⁰ C.

Secondly, it is extremely difficult to hold the seams together in contrast to shrink-sleeves, when using adhesives to bond these seams combined with the use of shrink-tunnels at temperatures above 100 ⁰ C. This invariably results in wrinkles or creases in the seams or even in that they come apart partially or even completely.

Roll-applied PVC (polyvinyl chloride) films are available with shrinkage properties up to 40 - 45%. However, these suffer from the major disadvantage that they have a density, being PVC, of greater than 1 and cannot therefore be separated from polyester (PET) bottles during a flotation - washing and recycling process. Further, PVC is a less attractive alternative for environmental reasons.

Only labels with a density of less than one can be separated from the PET bottles in such a process.

With traditional sleeve materials, the seams are not joined by adhesives, but rather by a process of solvent-welding where the polymer material is softened by a solvent and then the softened or "melted" films are bonded together to form one integral film without the weakness of an adhesive-join. Such solvent-welded shrink-sleeve seams do not suffer from any problems in withstanding the temperature of heat tunnels even at very high shrink-performances of 70 - 80%.

However, such a process of solvent-welding with roll-applied shrink- labels is technically not feasible.

Some of these problems have been overcome with the introduction of cyclic-olefin-copolymer ( COC )-modified polyolefin films which have a high shrink of up to 70-80 %, as well as a density of less than one which allows separation from PET in the bottle-recycling process . However, the major disadvantage remains that also the seams in this case cannot be held adequately together with the use of adhesive systems.

The method of the invention overcomes the problems of prior art. In addition to the above-mentioned technical problems, the present invention also offers enhancements to the energy efficiency, high productivity with low costs and ease of automated assembly line production.

In the method of the invention, labels are cut from a roll and wrapped around a container so that the leading edge and the trailing edge of the label overlap. The leading edge and the trailing edge are joined together, and the labelled container is led to a tunnel in which an elevated temperature prevails. The label shrinks in the tunnel so that its shape conforms to the shape of the container. The containers may be, for example, bottles or jars. The container may contain, for example, drinks, food, or medicines. The label may cover the container substantially completely, or partially. It is possible that the label comprises perforations so that a part of the label, for example a part surrounding a cap, is easier to remove.

When the leading edge and the trailing edge are joined together, a process of ultra-sonic or high-frequency welding is used to cause the surface of the label film to soften or melt in a similar fashion to that of solvent-welding. The edges of the film are joined so that the label is uniform, and there is not weaknesses in the joint. These high- frequency welded films can withstand the high temperatures of the shrink tunnels even with very high-shrink performance films.

Ultrasonic welding involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency between 15 and 70 kHz. Since ultrasonic welding is very fast (weld times are typically less than 1 second), its use for seaming labels is well founded.

An ultrasonic welding machine consists of four main components: a power supply, a transducer, an amplitude modifying device (commonly called a Booster) and an acoustic tool called a sonotrode (commonly called a horn). The power supply changes mains electricity at a frequency of 50-60Hz into a high frequency electrical supply operating at 20, 30 or 40kHz. This electrical energy is supplied to the transducer. Within the transducer, discs of piezoelectric material are sandwiched

between two metal sections. The transducer changes the electrical energy into mechanical vibratory energy at ultrasonic frequencies.

The vibratory energy is then transmitted through the amplitude modifying device, which increases the amplitude of the sound wave. The sound waves are then transmitted to the sonotrode. The sonotrode is an acoustic tool that transfers the vibratory energy directly to the parts being assembled, and it also applies a welding pressure. The vibrations are transmitted through the workpiece to the joint area. Here the vibratory energy is converted to heat through friction - this then softens or melts the thermoplastic, and joins the parts together.

The materials to be welded together must be thermoplastic. It is also beneficial that the amorphous portion of the thermoplastic material is considerable because crystalline materials tend to absorb vibration energy. This means more power is needed to weld them, with far-field welding being a particular problem. Further, amorphous materials melt gradually over a range of temperature, whereas crystalline materials have a sharper melting temperature. This makes it harder to achieve a good quality weld. A copolymer of acrylonitrile, butadiene and styrene (ABS), polyacryl, polycarbonate, polyvinyl chloride and cyclic polyolefin copolymers (COC) are adequately amorphous so that they are ideal for ultrasonic welding. Polyethylene, polypropylene, polyester and polyamide are semi-crystalline, and thus their ultrasonic welding is more difficult.

Preferred materials are materials which comprise cyclic polyolefin polymers and copolymers. The COC is a copolymer that may be formed by polymerization of cyclic-olefin and alpha-olefin. A cyclic olefin is a compound containing a polymerizable carbon-carbon double bond that is either within an alicyclic ring (e.g., as in norbornene) or is linked to an alicyclic ring (e.g., as in vinyl cyclohexane). The COC may have a cyclic ring as part of the polymer backbone (e.g., ethylene/cyclopentene copolymer and ethylene/norbornene copolymer). The COC may have a cyclic ring pendant to the polymer backbone (e.g., ethylene/vinyl cyclohexane copolymer).

The COC may comprise, for example, (polymerized) cyclic-olefin content derived from one or more of cyclopentene, substituted cyclopentene, norbornene, substituted norbornene, cyclobutene, cyclopentene, methylcyclopentene, 5-vinylnorbornene, 5- methylnorbornene, 5-ethylidenorbornene, dicyclopentadiene, tetracyclododecene, and cyclododecatriene.

The material to be welded must also have adequate shrinking properties as a function of temperature. Typically the material should shrink at least from 30 to 40 % at a temperature range of from 90 to 14O ⁰ C.

In the following, the invention will be described with reference to a drawing.

Fig. 1 shows a schematic view of how a container is labelled.

A container 4 is labelled in an apparatus, which comprises a welding device 8 and a holder 3 for the container 4. A label 5 comprises a leading edge 6 and a trailing edge 7. The label 5 is wrapped around the container 4 so that the leading edge and the trailing edge 7 overlap. The label 5 comprises thermoplastic material. The thermoplastic material may be, for example, a cyclic polyolefin copolymer. The thermoplastic material may comprise one or more layers. There is printing and/or images on the surface of the label 5. It is also possible that the label is reverse printed, and the printing/images can be seen through the transparent thermoplastic material.

The welding device 8 comprises a power supply (not shown), a transducer 9, a booster 1 , a sonotrode 2, and a welding head 10. The power supply changes mains electricity at a frequency of 50 - 60 Hz into a high frequency electrical supply operating at 20, 30, or 40 kHz.

The electrical energy is supplied from the power supply to the transducer 9. The transducer 9 comprises discs of piezoelectric material sandwiched between two metallic sections. The transducer 9 changes the electrical energy into mechanical vibratory energy having ultrasonic frequency in the manner described above. The mechanical vibratory energy is transmitted to an amplitude modifying device 1 which increases the amplitudes of the sound waves. The modified sound waves are transmitted to the sonotrode 2. The sonotrode 2 is an acoustic tool which transfers the vibratory energy through its welding head 10 to the target point, i.e. to a seam of a label 5. The welding head 10 also brings to the process the required pressure when it contacts with the material to be joined (In Fig. 1 , the welding head 10 and the label 5 are illustrated apart from each other. However, during the seaming process the welding head 10 is in contact with the material to be welded, and the welding head exerts pressure to the material.). The vibratory energy is converted to heat so that the heat softens or melts the thermoplastic material of the label 5 at the seam. The seam joins the overlapping leading edge 6 and the trailing edge 7 of the label 5. After the seam has been formed by the welding device 8, the container 4 is transferred to a heated tunnel where the label 5 is allowed to shrink in order to conform with the shape of the container 4.

The construction of the apparatus may vary remarkably from the schematic drawing shown in Fig. 1 .