Method for sealing a fiber-based material

The invention relates to a method for sealing a fiber-based material (1, 4) to a counter-surface to be bonded to it by melting polymer present at the sealing point. In accordance with the invention, the sealing is performed by directing a laser beam (8) through the fiber layer (4) of the material to a radiation-absorbing pigment disposed in the sealing area (9), with the absorption heat melting the polymer (5) and generating the sealing. The method allows sealing of plastic-coated paper or board to itself, to another plastic counter-surface or also to paper or board without any plastic coating. It is also possible to seal uncoated paper or board to paper or board containing plastic, such as plastic-coated paper or board, or to a plastic film. The absorptive pigment, e.g. carbon black, may be admixed in the plastic coating of the board or paper or the plastic film formed on the counter-surface, or the pigment may be located on the surface of the material to be sealed or its fiber layer, or on the counter-surface to which the material is sealed, e.g. as a component of printing ink applied to the counter-surface.

This invention relates to a method for sealing a fibre-based material to a counter-surface to be bonded to the material by melting polymer present at the sealing point.

Heat sealing is a commonly used technique in bonding polymer-coated fibre-based materials, such as coated papers and boards. The sealing may be performed by pressing the surfaces to be sealed between heatable sealing elements, so that the heat melts the coating plastic, thus causing the surfaces to seal together, or a coating plastic can be melted by hot-air blowing before the surfaces to be sealed are joined. These heat-sealing techniques are widely used in the packaging technology for the manufacture of bags, containers, casings and boxes and in techniques for closing product packages.

It is also previously known to seal plastic-coated paper or board by means of a laser beam that melts polymer. Such sealing methods have been depicted i.a. in EP 1069984 B1, U.S. Pat. No. 4,540,392 and U.S. Pat. No. 5,690,775. The laser beam is directed to the plastic coating of the paper or board, and then the surfaces to be sealed are pressed against each other, the molten plastic bonding the surfaces to each other at the sealing point when cooling and solidifying.

Laser sealing has principally been performed by means of a CO2laser, the polymers used in the coatings and films absorbing the wavelength generated by this so that the beam immediately melts the polymers when hitting them. However, WO 02/00144 discloses a laser-sealing method for plastic films that is based on the use of a pigment that absorbs radiation and is heated under the effect of the absorption heat. This publication uses Nd:YAG or diode laser as the laser source, the wavelengths of these penetrating through the plastic layers without being absorbed. The method aims at directing the sealing to the interfaces between plastic films placed on top of each other, without the films melting, with consequent fusion to each other over their entire thickness. The pigment heated under the effect of the beam and melting the surrounding polymer may be disposed at the interface between the plastic films, or the sealed film may be totally dispersed with absorbing pigment.

In known methods for sealing polymer-coated paper or board using CO2laser, the laser beam is directed to the coating from an opposite laser source. It has not been possible to use a method in which the laser beam melting the polymer would be brought to the seal point through the fibre layer of the paper or board, because the laser beam would burn the paper or board. As far as the applicant knows, this is the reason why laser sealing has not been applied to the sealing of coated papers or boards that have been previously placed facing each other, but one has only used a process, in which the coating is first melted with laser and the surfaces are subsequently pressed against each other in order to bond them to each other.

The purpose of the present invention is thus to find a solution to the problem mentioned above, by means of which a fibre-based material, such as paper or board, could be sealed by a laser beam in a situation where the surfaces to be sealed have been disposed adjacent each other before the laser beam is directed to them. The solution of the invention is characterised by the sealing taking place with the laser beam directed through the fibrous layer of the material to a radiation-absorbing pigment disposed in the sealing area, so that the absorption heat melts the polymer present at the sealing point and thus produces the sealing.

The invention is based on the inventors' findings regarding the effects of laser radiation on fibre materials, such as paper or board. As mentioned above, CO2laser that melts and seals polymers is known to burn paper or board, and this process has been utilised for making identifying marks in these, among other things. Diode, Nd:YAG or fibre laser can generate wavelengths that do not burn paper, but then strong radiation scattering will occur in the fibre layer. Laser beams acting in the wavelength range 500-1,500 nm are suitable. According to the prevailing idea, the laser beam cannot be directed through a fibre material such as paper or board. The measurements conducted by the inventors also confirmed the scattering of the radiation; a measurement sensor placed opposite the laser source on the opposite side of the fibre layer detected low radiation intensities, which were but a fraction of the original ones.

Despite the facts above, the trials led to the following unexpected observation: when the laser beam was directed to a board that had been coated with a polymer layer containing an absorbing pigment on the side opposite to the incidence direction of the beam, the radiation resulted in melting of the coating layer. This observation was made both with a diode and an Nd:YAG laser, which do not have an effect that burns the fibre layer or melts the polymer directly, i.e. without the contribution of the pigment.

It has further been observed in connection with the invention that the pigment does not necessarily have to be in direct contact with the fibre layer, but the laser beam reaches it also through a transparent polymer layer placed adjacent the fibre layer, and also that the melting effect of the pigment reaches the adjacent transparent polymer layer, so that this layer becomes sealable also on its side opposite to the pigment.

Owing to the premises above, the invention offers the possibility of numerous different applications. The pigment may be included in the fibre-based material to be sealed, or optionally, the pigment may be contained in a member to which the fibre-based material is sealed. The pigment may be dispersed in the polymer coating of the fibre-based material or in a polymer film to which the fibre-based material is sealed. Optionally, the pigment may be disposed on the surface of the material to be sealed or a material layer of this, and then the pigment can also be interposed between the material layers, e.g. adjacent the fibre layer under the transparent polymer layer.

We may cite as an example of preferred application objects of the invention the sealing of a packaging board coated with a grey-pigmented polymer layer to a counter-surface, e.g. to itself. Such packaging boards, whose polymer coating imitates an aluminium foil, have been described in WO patent specification 01/76976 and FI patent application 20021359. The light-shielding grey coating layer is disposed on the inner surface of the board in the product packages, the invention allowing closing of the packages by sealing, with the laser beam melting the pigmented polymer layer being directed to the seal point through the board from the outside of the package. In addition to ordinary linear seals, we may mention another object of application comprising shaped seals, such as e.g. the seals of the lids of yoghurt containers.

FIGS. 1 and 2are schematic views of the sealing of a double-folded sheet1of plastic-coated board into a tubular shape. The opposite edges2,3of the sheet1are brought facing each other with the plastic coating layers5,5′ of the board4,4′ adjacent each other and pressed between elongated holders6in order to retain the sheet in position during the sealing. The sealing takes place by means of a laser head7moving in alignment with the holders6, the laser head being e.g. a diode laser generating a 940 nm wavelength. The laser head7directs a laser beam8to the seal point9from the side of the joined edge portions of the sheet1, the sealing being performed by moving the laser head from one end to the other of the sealing line in the direction of the arrow inFIG. 1.

The sealing of the invention is based on the effect of the pigment disposed at the seal point9, resulting in radiation absorption and fusion of the coating plastic. In the embodiment illustrated inFIG. 2, the plastic coating5of the board, constituting the inner surface of the tube thus formed, contains absorptive pigment evenly dispersed, such as e.g. carbon black. When board is sealed for package applications, pigmenting of the plastic coating may provide a light shield for the packaged product, and by combining black and white pigment, such as e.g. carbon black and titanium dioxide, the coating can be imparted a grey colour shade similar to that of an aluminium foil. The laser beam8directed from the laser head7to the seal point9, shown in the figure with a single arrow, hits the board layer4, where it is scattered into different directions without damage to the board. A sufficient portion of the beam8penetrates the board layer and is absorbed into the pigment contained in the plastic coating layers5,5′, the pigment being heated and the coating plastic at the seal point9melting under this heat. The coating plastic may be e.g. polyethene (PE), polypropene (PP) or polyethylene terephtalate (PET) commonly used for extruded heat-seal coatings. A variety of other plastics are applicable, e.g. ABS, CA, COC, EVA, HIPS, PA, PC, PEEK, PEI, PEN, PETG, PMMA, PPO, PS, PVC, PVDF, POM, PSU and PU. The melted plastic layers5,5′ are fused together, forming a tight seal between the edges of the sheet1when solidifying.

In the embodiment ofFIG. 3, the board4is equipped with an inner transparent coating layer10and an outer pigmented coating layer5. The transparent layer10may consist e.g. of ethylene vinyl alcohol polymer (EVOH) or polyamide (PA), which allow for an oxygen, aromatic and/or water vapour barrier shielding the packaged product, and the pigmented layer5may be similar to the one illustrated inFIG. 2. When the transparent layer10is pervious to the wavelength of the diode laser mentioned above, the pigmented layers5,5′ will be sealed together in the manner described above.

The coated board inFIG. 4differs from the one ofFIG. 2in that the pigmented plastic forms an inner coating layer5, onto which a thin transparent coating layer11has been applied. The pigmented layer5and the transparent layer11may both contain polyethene as the coating plastic. The laser beam8penetrating through the board4is mainly absorbed into the inner pigmented coating layer5, however, the absorption heat thus generated suffices for melting also the transparent plastic layers11,11′ so that they seal together.

InFIG. 5the edges2,3of a board sheet, which is a board4equipped with a pigmented plastic coating5corresponding toFIG. 2, have been disposed overlapping with a view to sealing. This sealing manner is applicable e.g. in the manufacture of casing or container packages. The laser beam8is directed to the seal point from the outside of the package, and the sealing is based on the effect of the laser beam penetrating through the board4, resulting in heating of the pigmented layer5and melting of the coating plastic. When solidified, the coating plastic, e.g. polyethene, adheres to the uncoated board surface, generating a seal that closes the side of the package. If desired, the board4can be equipped with a transparent plastic layer made of e.g. polyethene, which will form the outer surface of the package, and then the sealing takes place between the two coating layers containing polyethene.

In the embodiment ofFIG. 6, sealing is performed on a board4equipped with a transparent plastic coating layer12. With a view to sealing, a pigment13absorbing radiation has been disposed between the facing coating layers12,12′. The pigment may be contained e.g. in printing ink applied onto the transparent layer12. The laser beam8penetrates through the transparent layer12, thus heating the pigment13, resulting in the coating layers12,12′ melting and being sealed together.

FIGS. 7 and 8are examples of the sealing of a plastic-coated board and a plastic film14placed against this. InFIG. 7, the plastic coating5of the board4is pigmented, while the plastic film14to be sealed to the pigmented layer is transparent. The sealing is based on the effect of the laser beam8having penetrated through the board4, resulting in heating of the pigment and fusion of the plastic layers5,14to each other. InFIG. 8, the plastic coating12of the board4is also transparent, and to achieve sealing, an absorptive pigment, e.g. a component in printing ink, has been disposed between the board4and the transparent coating layer12. The effect of the pigment13heated under the laser beam8, which results in melting of the plastic in the transparent layers12,14, suffices for fusion of the layers to each other.

The embodiments shown inFIGS. 9 and 10relate to the sealing of uncoated board4to a plastic film disposed adjacent the board. InFIG. 9, the absorptive pigment generating the sealing is dispersed in the plastic film15, and inFIG. 10the pigment13is disposed between the board4and the transparent plastic film14to be sealed to the board. The pigment13may be contained in printing ink applied in advance either onto the board4or the plastic film14.

EXAMPLES

The invention was tested by sealing plastic-coated packaging board sheets to themselves using the technique ofFIG. 1, with the plastic-coated sides of the sheet adjacent each other. The laser head consisted of a diode laser having a wavelength of 940±10 nm, a power of 90 W, a focus point distance of 5 mm from the board surface and a speed of movement along the sealing line of 20-75 mm/s. The following is a list of the test materials and their layer weights together with the laser head speeds for the materials with which the sealing was successful:1. Board Natura Barr Silver (Stora Enso), board (300 g/m2)+PE (58 g/m2), containing 12% of TiO2and 0.15% of carbon black,laser head speed 60-75 mm/s,2. Board Performa (Stora Enso) (210 g/m2)+PET (40 g/m2), containing 2% of carbon black,laser head speed 70 mm/s,3. Board Trayforma (Stora Enso) (350 g/m2)+PET (40 g/m2), containing 12% of TiO2,4. Trayforma (315 g/m2)+PE (40 g/m2),5. Trayforma (250 g/m2)+PET (40 g/m2),6. Trayforma (250 g/m2)+PP (20 g/m2),7. Trayforma (315 g/m2)+PE (40 g/m2), having black Indian ink on the surface,laser head speed 20-25 mm/s,8. Trayforma (250 g/m2)+PET (40 g/m2), having black Indian ink on the surface,laser head speed 50-80 mm/s,9. Trayforma (250 g/m2)+PP (20 g/m2), having black Indian ink on the surface,laser head speed 50 mm/s,10. Trayforma (350 g/m2), having black printing ink on the surface+PET (40 g/m2),laser head speed 60-70 mm/s,11. Trayforma (420 g/m2), having red printing ink on the surface+PE (20 g/m2),laser head speed 70 mm/s,12. Trayforma (420 g/m2), having blue printing ink on the surface+PE (20 g/m2),laser head speed 70 mm/s,13. Trayforma (410 g/m2), having orange printing ink on the surface+PE (20 g/m2).

In the trials, sealing occurred with samples 1-2 and 7-12. With samples 3-6 and 13, no seal was formed or adhesion was inadequate. No burning of the board during sealing was observed.

The results indicate that sealing by means of a laser beam penetrating through the fibre layer of the board is feasible if the pigment is admixed with the coating plastic, if the pigment is applied onto the surface of the plastic layer or if the pigment is disposed on the surface of the board under the plastic coating. In addition to black pigment, a successful result was achieved also with red and blue pigment. The result is nevertheless dependent of the test conditions, such as the wavelength of the laser beam used.

It is obvious to those skilled in the art that the applications of the invention are not restricted to the examples given above, but may vary within the scope of the accompanying claims. Thus, for instance, the diode laser head can be replaced with an Nd:YAG laser on a wavelength of 1064 nm.