Method and system for manufacturing a coated paperboard and a coated paperboard

Abstract

The present invention relates to a method for manufacturing coated paperboard suitable for packaging paperboard applications. The method comprises coating a first surface of the paperboard web by applying a) a first coating composition in a precoating unit for forming a precoating layer comprising inorganic mineral pigment particles and least one binder for sealing the first surface of the paperboard web; and b) a second coating composition in a curtain coating unit for forming at least one barrier coating layer on the first surface of the paperboard web; and c) a third coating composition comprising a polymer dispersion in the curtain coating unit for forming at least one heat sealable coating layer on the first surface of the paperboard web. The temperature of the coated paperboard web is controlled and adjusted, and the paperboard web is cooled in at least one cooling unit.

Claims

1. A method for manufacturing coated paperboard by providing a repulpable multilayer coating on a first surface of a paperboard web in a paper manufacturing system comprising a precoating unit, a curtain coating unit, an adjustable heating unit, a cooling unit, and a jumbo reel onto which resulting multilayer coated paperboard is reeled, the method comprising a) forming a precoating layer directly onto the first surface by applying 4-20 g/m.sup.2 of a first coating composition in a precoater which is a size press or a metering size press, said first coating composition comprising inorganic mineral pigment particles selected from particles of calcium carbonates, and kaolin, and at least one binder selected from synthetic polymer latex, polyvinyl alcohol and starch for sealing the first surface of the paperboard web, b) forming at least one barrier coating layer directly on the precoating layer by applying a second coating composition in form of an aqueous polymer dispersion in the curtain coating unit, wherein the aqueous polymer dispersion is selected from latex dispersions, polyvinyl alcohol dispersions and polyolefin dispersions and wherein the second coating composition is applied in an amount of less than 20 g/m.sup.2, and c) forming at least one heat sealable coating layer directly on the at least one barrier coating layer by applying a third coating composition consisting of a polymer dispersion in the curtain coating unit, wherein the second and third coating compositions are applied simultaneously as aqueous dispersions without intermediate drying in between application of the coating compositions, and the third coating composition forms the outermost surface of the coated paperboard, d) allowing the third coating composition to partially or fully melt in the heat sealable coating layer to form a film surface by controlling and adjusting the temperature of the coated paperboard web to a temperature of 90-150° C., and e) cooling the paperboard web in at least one cooling unit to reduce tackiness of the coated paperboard web before reeling to the jumbo reel.

2. The method according to claim 1, wherein the temperature of the paperboard web after the temperature adjustment is cooled down to a temperature below 50° C.

3. The method according to claim 1, wherein the paperboard web is calendered after forming the precoating in step a) and/or after cooling in step e).

4. The method according to claim 1, wherein the precoating is also formed on a second surface of the paperboard web.

5. The method according to claim 1, wherein at least the first surface of the paperboard is exposed to electrical surface modification treatment before winding the coated paperboard web onto jumbo reel.

6. The method according to claim 5, wherein the surface energy of the first surface of the coated paperboard web is charged by the electrical surface modification treatment.

7. The method according to claim 5, wherein the electrical power used in the electrical surface modification treatment is less than 50 W/m.sup.2/min.

Description

(1) Some embodiments of the present invention are described in more detail below with reference to the enclosed schematical figures, in which

(2) FIG. 1 depicts schematically an online system according to one embodiment of the present invention,

(3) FIG. 2A-2B depict schematically coated paperboards obtained according to some embodiments of the present invention,

(4) FIG. 3A depicts measured temperature from the paperboard surfaces, and

(5) FIG. 3B depicts the predetermined temperature setting profile according to the present invention.

(6) An exemplary embodiment depicting an online system according one embodiment of the present invention is depicted in FIG. 1 schematically. In FIG. 1 the following abbreviations are used: A=wire section; B=press section; C=drying section; D=coating section; and E=winding to jumbo reel.

(7) In an exemplary embodiment of the present invention the coating section comprises a precoating unit, where the web enters in between the rolls of the precoater, and subsequently the web is guided through the post drying section.

(8) The coating unit comprises a curtain coater. A preferred curtain coater may coat two or three layers simultaneously on a base paper web. The curtain coating unit is followed by a heating unit.

(9) The sections of FIG. 1 are further explained in example 1.

(10) FIGS. 2A and 2B depict the some embodiments of the preferred coated paperboard structures. In FIG. 5A three layers of coating are on the both sides and in FIG. 5B three layers of coating are only on one side.

(11) The resulting multilayer structure is depicted in FIGS. 2A and 2B wherein 1=base paperboard, 2=precoating layer, 3=barrier layer and 4=heat sealable coating layer.

(12) In one embodiment the heat sealable coating layer on top of the precoating and barrier layers if heated according to the heating profile of FIG. 3B. The three heating units whereby the web is passing after application of the third coating composition for forming the heat sealable coating layer are adjusted to 130° C., 60° C. and 40° C. The actual measured temperature experienced by the web is disclosed in FIG. 3A.

(13) The present invention is further illustrated by the following non-limiting examples.

EXAMPLES

Example 1

(14) FIG. 1 shows schematically the sections of one paperboard system according to the present invention. The sequence of the sections is the following:

(15) A Wire section wherein water is removed from stock with gravity and suction through a wire. Dry content about 0.5% to about 20%

(16) B Press section wherein water is removed by pressing web. Dry content about 20% to about 50%.

(17) C Drying section wherein water is removed by heating paperboard with the help of steam in drying cylinders. Dry content about 50% to about 95%.

(18) D Coating section wherein web is coated and dried several times. Final dry content is about 90%. The coating section further comprises the following units:

(19) precoating calendering 1st curtain 2nd curtain calendering cooling optional surface charging
E Winding to jumbo reel

Example 2

(20) The paperboard production system according to the invention was used to produce cups for hot beverages, such as coffee.

(21) Final caliper of the paperboard product aimed at was 300 μm. The following production parameters and sequence were applied: Base paper thickness was 200 g/m.sup.2 Precoating layer thickness was 10 g/m.sup.2 on each side of the paperboard Calendering was performed after precoating to close the surfaces Curtain coating was used to coat the inside surface of the cup paperboard by 6 g/m.sup.2 barrier coating layer and 8 g/m.sup.2 heat sealable coating layer, the dispersions were applied accordingly based on flow meter measurement results. Temperature was controlled and adjusted for the paperboard web for 150° C. and then the web was cooled down to 50° C. Final calendering was performed to reach the required target caliper of paper before directing the paperboards to winding.

(22) A structure according to FIG. 2A was thus obtained.

(23) This on-line produced product was used to successfully to make cups by the cup producer. The cups were rigid and their sealing properties were excellent (heat sealable function). Barrier properties were good as hot water or coffee did not penetrate into the paperboard product within a time period of 12 h.

(24) Possible waste in production was reused as the paperboard product produced by the present method is recyclable without requiring any intermittent processing or treatment.

Example 3

(25) The paperboard production system according to the invention was used to produce cups for cold beverages.

(26) Final caliper of the paperboard product aimed at was 330 μm. The following production parameters and sequence were applied: Base paper 200 g/m.sup.2. Precoating 10 g/m.sup.2 on each side. Calendering to close the surface. Curtain coating on inside surface: 6 g/m.sup.2 for barrier coating layer, 8 g/m.sup.2 for heat sealable coating layer. Temperature control, web 150 C and then cooled down to 50 C. Curtain coating on outside surface: 3 g/m.sup.2 for barrier coating layer, 3 g/m.sup.2 for heat sealable coating layer. Temperature control for web 150° C. and then cooled down to 40° C. Final calendering to reach needed caliper of paper. Outside surface of the paperboard for the cup was treated electrically using 25 W/m.sup.2/min to reach 60 dyn/cm surface tension.

(27) A structure according to FIG. 2B was thus obtained.

(28) This on-line produced paperboard product was used to successfully make cups by the cup producer. The cups were rigid and sealing properties were excellent (heat sealable function). Barrier properties were good as liquids did not penetrate into the product within a time period of 12 h. Outside of cup provided enough barrier against condensate water.

(29) Possible waste in production was reused as the paperboard product produced by the present method is recyclable without requiring any intermittent processing or treatment.

Example 4

(30) Fast food company used two different waste cups: a dispersion coated cup according to the present invention and a PE(polyethylene)-extruded cup. The traditional PE-extruded cups had to be treated as waste and therefore the fast food company had to pay an external party to collect the used cups and either burn or process them in a PE eliminating process. Dispersion coated cups were treated as recyclable paperboard and they could be sold accordingly.