A METHOD FOR SEALING PAPERBOARD

Abstract

Disclosed are methods for sealing paperboard, wherein the sealing is performed by using water-based barrier coating materials applied and dried on the paperboard. According to the method, the heating is performed on uncoated side of the paperboard. Also disclosed is a sealed paperboard product wherein the seal is obtainable by the disclosed method.

Claims

1. A method for sealing paperboard, the method comprising: a) providing a first paperboard comprising a first surface and a second surface, wherein the second surface is coated with a barrier composition comprising 0-60% by weight platy particles, 0-60% by weight precipitated calcium carbonate (PCC), and 40-100% by weight a binder comprising 20-95% by weight one or more synthetic polymers and 5-80% by weight one or more biopolymers, b) subjecting a first part of the first surface of the first paperboard to temperature of 120-450° C., and c) pressing part of the second surface of the first paperboard opposing the first part of the first surface against a paperboard surface, wherein said paperboard surface is not coated with the binder composition.

2. The method according to claim 1, wherein said paperboard surface is a second part of the first surface of the first paperboard.

3. The method according to claim 1, wherein said paperboard surface is a first part of a first surface of a second paperboard.

4. The method according to claim 1, comprising moistening the barrier composition prior to the pressing.

5. The method according to claim 1, wherein the platy particles are selected from talc, kaolin, bentonite, and combinations thereof.

6. The method according to claim 1, wherein the one or more synthetic polymers are selected from polymer latexes and polyvinyl alcohol.

7. The method according to claim 6 wherein the polymer latexes are selected from styrene butadiene latex, styrene acrylate latex, vinyl acetate acrylate latex, ethylene vinyl acetate latex, and vinyl acetate latex.

8. The method according to claim 1, wherein the one or more biopolymers are selected from starch, proteins, and carboxy methyl cellulose.

9. The method according to claim 1, wherein the subjecting comprises applying flow of air on the first part of the first surface of the first paperboard, wherein temperature of the air is 250-450° C.

10. The method according to claim 1, wherein the subjecting comprises contacting the first part of the first surface of the first paperboard with bar of metal.

11. The method according to claim 1, wherein the subjecting is for 0.25-1 s.

12. The method according to claim 1, wherein the pressing is for 0.25-1 s.

13. The method according to claim 1, wherein the first surface and/or the second surface of the first paperboard comprises a coating layer comprising inorganic filler particles.

14. The method according to claim 13, wherein coating layer of the first surface of the first paperboard further comprises a binder selected from polymer latex, polyvinyl alcohol, protein and starch.

15. The method according to claim 1, wherein the PCC has an average diameter 30-60 nm.

16. A sealed paperboard product comprising a seal obtainable by a method according to claim 1.

17. A paperboard product obtainable by a method according to claim 1.

18. The method according to claim 2, comprising moistening the barrier composition prior to the pressing.

19. The method according to claim 3, comprising moistening the barrier composition prior to the pressing.

20. The method according to claim 2, wherein the platy particles are selected from talc, kaolin, bentonite, and combinations thereof.

Description

BRIEF DESCRIPTION OF FIGURES

[0021] FIG. 1 shows an exemplary method for sealing paperboard according to prior art,

[0022] FIGS. 2-4 show exemplary methods for sealing paperboards according to exemplary non-limiting embodiments of the preset invention, and

[0023] FIG. 5 shows a top view of exemplary sealed paperboard product manufactured according to the method of the present invention.

DESCRIPTION

[0024] The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

[0025] The principle of heat sealing of paperboards using hot air according to prior art is shown in FIG. 1. The paperboard has a first surface 101, and a second surface 102 which is coated with a sealing agent such as polymer like PE. For sealing, the polymer is melted by applying hot air (A) to the PE on the first part 102a of the second surface. Finally, the melted polymer layer is pressed (B) against the second part 101b of the first surface by using an external force. A seal is formed upon cooling.

[0026] When the sealing is done by using PE, a proper sealing can be achieved by using sealing temperatures like 350° C. However, when barrier compositions based on WBBCs comprising biopolymers such as those disclosed in WO2016/174309 are used instead of PE, significantly higher sealing temperatures must be used. The heat, in turn, at least partially destroys the barrier material, and a low-quality sealing is achieved.

[0027] Exemplary methods for sealing a paperboard according to the present invention is shown in FIGS. 2-4. In the figures, only representative portions of the paperboard are shown for clarity. A top view of an exemplary paperboard product wherein the seal is prepared according to the method of the present invention is shown in FIG. 5.

[0028] FIG. 2 shows an exemplary method according to present invention for sealing two ends of a paperboard together. The method includes the following steps: [0029] providing a paperboard comprising a first surface 201 and a WBBC applied and dried on a second surface 202 to form a barrier composition comprising [0030] 0-60% by weight platy particles; [0031] 0-60% by weight precipitated calcium carbonate preferably having an average diameter 30-60 nm and [0032] 40-100% by weight a binder comprising 20-95% by weight one or more synthetic polymers and 5-80% by weight one or more biopolymers; [0033] subjecting a first end 201a of the first surface to temperature of 250-450° C., preferably of 300-400° C. by using hot air, and [0034] pressing the barrier composition on the first end 202a of the second surface against the second end 201b of the first surface by subjecting an external force to the first end of 201a the first surface and/or to the second end of the second surface 202b.

[0035] In FIG. 2, the subjecting with hot air is marked with an arrow (A), and the pressing is marked with the two arrows (B), respectively.

[0036] According to the present disclosure, the first end of the first surface of the paperboard, the second end of the first surface of the paperboard, the first end of the second surface of the paperboard, and the second end of the second surface of the paperboard are to be understood as parts of the paperboard which are used for the sealing, i.e. parts of the paperboard which are subjected to heating, parts of the paperboard comprising the barrier composition to be used for sealing, and parts of the paperboard used for pressing. It is obvious for a skilled person that the areas and shapes of the parts used for sealing depend on the application, and that they can be modified if needed.

[0037] FIG. 3 shows another exemplary embodiment of the present invention wherein the heating is performed by using a hot plate 303. According to this embodiment, the first end 301a of the first surface is subjected to temperature of 120-450° C., preferably 300-400° C., more preferably 350-400° C. A particular temperature is 350° C. The heat is subjected preferably for 0.5-1 s followed by pressing the ends to the paperboard towards each other by an external force. A typical pressing time is 1 s. According to a particular embodiment the pressing is performed by using two plates 303, 304, wherein the plate 303 is heated and the plate 304 is not heated. When a hot plate heating was used instead of hot air, a sufficient sealing could be obtained even at 125° C. In FIG. 3, the heating and pressing is marked with arrows (A,B).

[0038] The subjecting to heat can be performed by using hot air or hot plates as shown in FIGS. 2 and 3, respectively. Also, other heating methods such as infrared heating or ultrasound heating can be used.

[0039] According to a further embodiment the heating is performed by using two or more hot plates, such as two hot plates 303 and 304 shown in FIG. 3. The heat of the plate 304 preferably reduced. An exemplary temperature of the plate 304 is 45° C.

[0040] According to a particular embodiment the method comprises moistening the barrier composition on the first end of the second surface prior or during subjecting to heat. The moistening can be done e.g. by spraying water to the barrier composition.

[0041] The methods shown in FIGS. 2 and 3 are suitable e.g. for sealing side walls of a paperboard cup.

[0042] FIG. 4 shows a further embodiment of the method of the present invention wherein two pieces of paperboard are sealed together. According to this embodiment at least one of the two paperboards, i.e. the first paperboard, includes a first surface 401 which is not covered by the barrier composition, and a second surface 402 which is covered by the barrier composition. According to this embodiment, the first part 401a of a first surface of the first paperboard is heated using the method of the present disclosure followed by pressing part of the second side 401b of this paperboard opposing the heated part 401a against a surface 403 of a second paperboard. The surface 403 does not include the barrier composition. The seal is formed upon cooling. Further parts of the two paperboards, such as 402b and 403 can be sealed accordingly. This method is suitable e.g. for preparing handles for paperboard cups. The pressing and heating of the parts of the first surface 401a,b of the first paperboard are represented in figure with two arrows.

[0043] Naturally, the method of the present invention is applicable for sealing plurality of paperboards together.

[0044] A variety of paperboards may be used in the present disclosure. These include, but are not limited to, coated natural kraft board (CNK board), solid bleached sulphate board (SBS), solid unbleached sulphate board (SUS), coated recycled board (CRB), coated white lined chipboard (WLC), folding boxboard (FBB), and other paperboard grades suitable for cup formation. A particular paper board has a rough surface, which is difficult to seal with WBBCs by using the methods of prior art.

[0045] According to a particular embodiment the first surface and or the second surface of the paperboard comprises a coating layer comprising inorganic filler particles preferably selected from calcium carbonate, kaolin and titanium dioxide. According to this embodiment the barrier composition is applied on the coating layer of the paperboard.

[0046] The water-based barrier coating layer can be produced as disclosed e.g. in WO2016/174309. The WBBC can be applied on the paperboard and dried using techniques known in the art. The dried WBBC comprises 40-100% by weight binder, 0-60% by weight platy particles and 0-60% by weight precipitated calcium carbonate (PCC). Diameter of the PCC is preferably 30-60 nm. The binder comprises 20-95% by weight synthetic polymers and 5-80% by weight biopolymers. According to an exemplary embodiment the binder comprises 10% by weight biopolymers.

[0047] According to one embodiment the synthetic polymer is selected from synthetic latexes and polyvinyl alcohol. Exemplary synthetic latexes are styrene butadiene latex, styrene acrylate latex, vinyl acetate acrylate latex, ethylene vinyl acetate latex, and vinyl acetate latex. The biopolymers are preferably selected from starches, proteins, carboxy methyl cellulose and other cellulose derivatives. A particular biopolymer is starch. A particular synthetic polymer is styrene acrylate latex.

[0048] According to a particular embodiment the synthetic polymer is styrene acrylate latex and the biopolymer is starch. According to this embodiment, the barrier composition comprises 90% by weight styrene acrylate and 10% by weight starch.

[0049] According to one embodiment the platy particles are platy pigments. According to another embodiment the platy particles are selected from talc, kaolin, bentonite and combinations thereof. According to an exemplary embodiment the barrier composition comprises 10-40% by weight platy particles, preferably talc, 0-20% by weight precipitated calcium carbonate, and 40-90% by weight binder. The binder comprises 5-80%, preferably 10-40% by weight one or more biopolymers, in particular starch. According one embodiment the composition does not comprise PCC.

[0050] According to a particular embodiment the barrier composition comprises 50-65% by weight styrene acrylate latex, 10% by weight biopolymer, 0-5% by weight PCC, and 20-40% platy particles, such as platy pigments.

[0051] According to one embodiment the method comprises subjecting a first end 201a of the first surface to temperature of 120-450° C., preferably 250-450° C., more preferably of 300-400° C., even more preferably of 350-400° C. A particular temperature is 350° C. The heat is subjected preferably for 0.5-1 s followed by pressing the ends to the paperboard towards each other by an external force. A typical pressing time is 1 s. An exemplary pressure used for pressing is 4 bar.

[0052] According to one embodiment the present invention concerns a paperboard product comprising a seal obtainable by a method of the present invention. An exemplary paperboard product is a cup. The cup comprising the seal according to the present invention can be manufactured by using cup forming machines. A top view of a cup 400 wherein the paperboard is prepared according to the present method is show in FIG. 4. The shape of the seal is exaggerated for clarity.

[0053] A typical cup forming machine consists of a blank feeding system, heating elements, carousal forming station, bottom stock web feeding and die cutting, rim curl station, transport system, and a packaging/inspection station. The heating elements provide a controlled heating process to activate the water-based barrier as described above. The heating elements are applied for a side seam of the container body component, as well as for the sealing between the container body component and bottom component of the cup. The heating unit of the machine is modified such that the can be directed to the side of the paperboard opposite to the water-based barrier. The typical heating method is by blowing hot air through multiple nozzles to the selected areas of the structure. Nonetheless, other heating methods may be used to activate the sealant such as infrared heating, ultrasonic heating and hot bar heating.

Experiments

[0054] The water-based barrier compositions were prepared as disclosed in WO 2016/174309, applied on a paperboard and dried. Also, WBBCs not including PCC were prepared. Hot air and hot bar heat sealability of barrier dispersion coated boards were tested with a OAF heat sealing machine. In hot air sealing test, the coated side or the non-coated side of the material was heated under air flow of 200 L/min at a given temperature for 0.5-3 seconds. Heated side of the sample was then pressed against an unheated side of the material for one second at pressure of 4 bars. In hot bar sealing tests, the temperature of a lower sealing bar was 45° C. and the temperature of the upper sealing bar was adjusted. Samples were placed between the bars coating against coating and bars were clamped for 1 second with a pressure of 4 bars. After cooling formed seals were torn open by hands and sealability was evaluated visually using following scale: 0=no seal, 1=weak adhesion, 2=adhered but no fibre tear, 3=under 50% fibre tear, 4=over 50% fibre tear and 5=100% fibre tear.

[0055] In few experiments the WBBC was moistened by subjecting the coating layer with spray of water during and/or before heating.

[0056] The results are shown in Table 1.

TABLE-US-00001 sealing when sealing when heating pressure heating the heating the Dispersion Temp/° C. time/s time/s moisture coated side uncoated side 1.sup.a 525 .sup.e 0.5 1 no 3 1 350 .sup.e 0.5 1 no 5 1 500 .sup.e 1 1 no 1 1 450 .sup.e 3 1 no 1 2.sup.b 525 .sup.e 0.5 1 no 3 2 350 .sup.e 0.5 1 no 5 2 500 .sup.e 1 1 no 1 3.sup.c 525 .sup.e 0.5 1 no 3 3 350 .sup.e 0.5 1 no 5 3 500 .sup.e 1 1 no 3 3 450 .sup.e 0.5 1 yes 3 3 450 .sup.e 2 1 no 4 3 300 .sup.e 0.5 1 no 0 3 3 400 .sup.e 0.5 1 no 5 3 300 .sup.e 0.5 1 yes 5 3 125.sup.f  0.5 1 no 5 PE.sup.d 350 .sup.e 0.5 1 no 5 PE 350 .sup.e 0.5 1 no 1 .sup.a-cpaperboard coated with composition comprising 50-65% by weight styrene acrylate latex; 10% by weight biopolymer, 20-40% platy pigments, and 0-5% by weight PCC; .sup.dpaperboard coated with polyethylene; .sup.e heating using hot air; .sup.fheating using hot plate.

[0057] The results can be summarized as follows. [0058] Paperboard can be sealed with WBBCs as effectively as with PE when the heat is not subjected directly to the WBBCs. [0059] The heating temperature can be reduced by moistening the WBBC and by using hot plate instead of hot air [0060] The method of the present invention is not suitable for sealing paperboards coated with PE.