FIELD OF THE INVENTION

The present invention generally relates to a method and a system for manufacturing a paper packaging material or paperboard, and particularly a coated paper or coated paperboard having water, heat and grease resistance properties while being recyclable and repulpable.

BACKGROUND OF THE INVENTION

Fibre based products, such as paper and paperboard, are widely used in different packaging applications such as disposable cups, paper straws, single use containers, food packaging, non-food packaging, etc.

For example, paperboard may be used to provide disposable, single use cups for a variety of beverage products. These disposable, single use cups may be used for "hot" beverages such as coffee, lattes, mochas, hot chocolate, etc., as well as for "cold" beverages such soft drinks, milk, slushes, smoothies, etc. Packaging such as these disposable, single use cups may also be used as containers for other moisture-containing food products such as ice cream, other frozen foods, soup, etc., as well as other wet and dry food packages, non-food packaging such as soap boxes, etc.

Paperboards applied in liquid packages or food packages, must be able to withstand the influence of the packed items such as the influence of moisture on the paperboard.

Moisture usually penetrates normal paperboard, thereby causing the board to become soggy thus, losing its structural integrity. In the case of "cold" beverage or food products, moisture also gets condensed on the side or surface away from the beverage or food, thus also penetrating the cupboard.

A usual method to inhibit such moisture penetration in the paperboard is by coating the paperboard on one or both sides/surfaces with a water-repellent material or barrier.

Barriers are usually applied by coating the fibre-based substrate with compositions which provide the requisite barrier properties. Different levels of coatings can be applied to a cardboard based on the degree of barrier, that a wet-proof cardboard require in relation to its usage. Further, the number of coatings is usually limited by costs and the number of coating stations on a given machine. Commonly used materials when forming a barrier on a fiber-based product are polyethylene (PE), polypropylene (PP), polyethylene therephthalate (PET), ethylene vinyl alcohol (EVOH) or ethylene vinyl acetate (EVA). The polymers can for example be laminated or extrusion coated on the fiber based product. Currently, most of the barrier coatings are manufactured with extrusion coating techniques and hence made off-line in a separate coating unit. Several patents that corroborate the point are stated below.

JP 2006028697 A discloses a greaseproof paper with undercoat and overcoat layer formed by coating acrylic-type synthetic resin emulsion on a paper base material. However, the greaseproof paper is not heat-sealable.

EP 3 559 344 Bl discloses a method for manufacturing a packaging material. However, the packaging material requires off-line coating which presents the challenge of being expensive, since it requires extra handling of the reels and an extra converting step.

WO 2016/170229 discloses a method and system for manufacturing a coated paperboard. It relates to multi-layer coating whereby the second and third coating layers are applied simultaneously as aqueous dispersions without intermediate drying between the coating layers. However, the method still faces the challenges related to blocking and practical sealability mentioned above. It has the disadvantage of weak sealing which seems not to be particularly strong and open easily.

US 2018 / 0094165 Al discloses a waterproof and oil proof paper. However, the waterproof and oil proof paper still has a so-called blocking issue.

The off-line coating technique is considered expensive since it requires extra handling of the reels and an extra converting step.

Besides, off-line processing includes handling and transportation of the paperboard rolls which exposes the paperboard web to defects such as tears, point defects, moisture damages, possible temperature issues, and causes yield loss.

To take care of the aforesaid deficiency, a new technique, e.g., dispersion barrier coating techniques has emerged. The technique has edges with respect to recyclability, enabling value chain integration with offline or online coating utilizing the existing coating equipment on paper or paperboard machine and ensuring optimal repulpability.

However, in the dispersion coating techniques, too low coat weight does not create the required heat sealing properties.

One significant problem related to dispersion coating is the tendency of the coating to stick to hot guide or carrier rolls or stick to itself at the rewinder stage when the paperboard is rolled up. This phenomenon is called blocking. If blocking occurs, the product is damaged and cannot be used.

Another problem associated with the dispersion coated paperboards on the market is heat-sealability and/or leakage issues. In practice, when such paperboards are formed as food packaging, particularly in form of containers, straws or disposable cups, a considerable number of products seem have heavy smell and/or leakage issues and soften after coming into contact with food.

The present invention relates to a method and system of applying dispersion barrier coating technique to cardboards with known deficiencies of said process scuttled to its minimum.

SUMMARY OF THE INVENTION

An aspect of the invention is to provide a manufacturing method which overcomes the shortcomings of the prior art.

An aspect of the invention is to provide a manufacturing method for production of paperboard and paper webs having properties of blocking resistance and optionally also heat-sealability.

An aspect of the invention is to provide a manufacturing method for production of paperboard and paper webs which are recyclable and repulpable.

An aspect of the invention is to provide a manufacturing method for production of paperboard and paper web which are not only recyclable and repulpable but also printable and less bulky thereby being more effective in saving raw material.

An aspect of the invention is to provide a manufacturing process which is cost effective.

An aspect of the invention is to provide a paperboard manufacturing process, where the total thickness of the coatings, and thereby also the total thickness of the coated paperboard, can be minimized. The above aspects have been addressed by a method of manufacturing paperboard comprising :

- a coating step; and

- a post-treatment step wherein, said coating step includes coating a paper substrate with multiple layers of coating, the multiple layers including: a base barrier coating; a barrier coating; an optional heat-sealing coating; and a shield coating wherein, the coating step includes: applying a base barrier coating on the paper substrate; reducing the moisture content of the paper substrate by drying the base barrier coating by means of a drying unit; compressing and smoothening of the paper substrate; applying a barrier coating layer; compressing and smoothening of the paper substrate; optionally applying a heat-sealing coating; applying a shield coating; moisturizing the resulting paper substrate after coating the paper substrate with multiple layers; and wherein, the post-treatment step includes compressing and smoothening the paper substrate being the result of the coating step; and drying the resulting paper substrate.

The paper substrate may also be referred to as a paperboard substrate, a paper web or a paperboard web depending upon which properties one would like to emphasise; e.g., the fact that it is paper and/or the fact that it e.g., is a web, and/or the fact that it e.g., forms a substrate for the coatings.

The method may further comprise a web-forming step and a pretreatment step before the coating step, wherein the web forming step includes forming a web-shaped paper substrate of paper fibres; and wherein the pre-treatment step includes: drying and reducing the moisture content of paper pulp forming the paper substrate; cooling the resulting paper substrate; compressing and smoothening of the paper substrate; and sealing surface fibres by means of surface sizer.

The web forming step may include adding an internally sizing agent.

The surface sizer may be any one of film size press, a spray sizer, pond size press and a nip sizer, preferably with one or more curtain applicators and/or one or more foam applicators .

The weight of the base barrier coating preferably ranges between 1— 17 g/m ² , more preferably between 3— 11 g/m ² , and most preferably between 3— 8 g/m ² .

The weight of the barrier coating preferably ranges between 1— 14 g/m ² , more preferably between 2— 8 g/m ² , and most preferably between 2— 4 g/m ² . The weight of the heat sealing coating, if present, preferably ranges between 1— 11 g/m ² , more preferably between 2-11 g/m ² , yet more preferably between 2— 5 g/m ² , and most preferably between 2— 4 g/m ² .

The weight of the shield coating layer preferably ranges between 1— 10 g/m ² , more preferably between 2 - 8 g/m ² , yet more preferably between 2— 6 g/m ² , and most preferably between 2— 4 g/m ² .

It may in this context be noted that the weight of the various coatings denoted as g/m2 in this application refers, unless otherwise specifically specified, to the weight of the resulting coating, i.e., after water or other solvents of e.g., a dispersion has been removed.

As indicated above, the method may further comprise applying a heat sealing coating.

The composition of the heat sealing coating is preferably selected from copolymers of ethylene and methacrylic acid or acrylic acid.

The action of moisturizing the resulting paperboard web after coating the multiple layers in the coating step may be performed by means of a spraying unit spraying a moistening agent onto the paperboard web.

The moistening agent is, preferably mostly, more preferably at least 99% by weight, and most preferably only, water.

The spraying unit may spray the moistening agent from nozzles with a spraying pressure which preferably is between 1 - 100 MPa, more preferably between 1 - 17MPa, and most preferably between 1 - 12MPa, as measured at the nozzles.

The paperboard web may be moved at a web speed which preferably is below 500 m/min while the action of moisturizing the resulting paperboard web after coating the multiple layers in the coating step is performed. The action of drying the resulting paperboard web in the posttreatment step may include drying the paperboard web with a drying unit which is maintained at a temperature range of 95 -117 °C.

The barrier coating composition is in the form of aqueous dispersions, preferably aqueous polymer dispersions. An example can be polyvinyl alcohol dispersion.

The polymers of the barrier coating composition are preferably biodegradable/renewable polymers. the method further comprises applying a heat sealing coating (C), wherein preferably a composition of the heat sealing coating (C) is selected from copolymers of ethylene and methacrylic acid or acrylic acid.

The method may further comprise a finishing step after the posttreatment step, wherein the finishing step includes cooling the paper substrate being the result of the posttreatment step; and winding the coated paper substrate into rolls.

The above aspects have also been addressed by a system of manufacturing paper board, the system comprising: an internal sizer; a surface sizer; a spraying unit; at least one coating unit configured to apply a base barrier coating; a barrier coating; a heat-sealing coating; and a shield coating; at least one drying unit; at least one cooling unit; at least one calendaring unit; and a paperboard roll winding unit.

It may be noted that the present method and system for production may be used for both one-side coated, and two-side coated, paperboard products.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanying drawings, in which:

Figure 1 is a flowchart to schematically illustrate the method according to embodiment of the present invention

Figure 2 is the sample product (disposable paper cup) produced from a paperboard with the method of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to Figure 1, the manufacturing process of the present invention comprises:

1. a web forming section;

2. a pre-treatment section;

3. a coating section;

4. a post-treatment section; and

5. a finishing section.

It may in this context be noted that this specific separation into these specific different sections with the specific naming of the different sections is as such not important for the invention. One may e.g., alternatively refer to a manufacturing process comprising a web forming section followed by a coating section, i.e., without any specifically identified pre-treatment section, and in such a manufacturing process the steps that in the following are identified as being part of the pre-treatment section may e.g., be considered to form a late part of the web forming section, or alternatively, to form an early part of the coating section. The specific separation chosen in the detailed description is chosen since it is considered to provide clear overview of the various steps included in the manufacturing process. It may in this context also be noted that it is preferred that the different steps starting with the web-forming section and ending with the finishing section are all performed in line, i.e., without the web being reeled up on a reel until it is finally reeled up at the end of the finishing section. However, it is also possible to have set-up where the web is reeled up onto a reel after the web-forming section or alternatively after the pre-treatment section and that the pre-treatment section or the coating section begins with unreeling the web.

When referring to a manufacturing process, or manufacturing site, the different actions or parts may be referred to as different sections of the process or different sections of the site. When referring to a method the different actions may be referred to as different steps. Thus, the words sections and steps are used as interchangeable words merely depending on if reference is made a manufacturing process having different sections or if reference is made to a method having different steps. Solid lines of the boxes indicate steps that are considered of greater importance to include compared to the more optional steps indicated by boxes of dashed lines. However, it should be noted that just because a step is indicated within a solid line box, it is not a mandatory step, although it is preferred that the step is included in the method.

1. The web forming section: In the web forming section, or web forming step, paper fibres 1 are prepared and transformed into a web or paperboard substrate. The web forming section may provide an internally sized paperboard substrate 2 formed by combining said paper fibres 1 and one or more internal paper sizing agents 3.

The internal paper sizing agents 3 refers to sizing agents which are included, added, etc., during the papermaking process. These agents generally resist penetration of water or other liquids into the paper substrate by making the paper substrate more hydrophobic. Suitable internal paper sizing agents may include nonreactive sizing agents and/or reactive sizing agents. Examples of nonreactive sizing agents may include rosin-based sizing agents e.g., sizing agents formed from rosin acids isolated from "tall oil" produced e.g., during kraft pulping of softwood species. Examples of reactive sizing agents may include one or more of: alkyl ketene dimers (AKDs); alkenyl succinic acid anhydrides (ASAs), etc.

2. The pre-treatment section: This section, or step, preferably comprises a first drying unit 4, a first cooling unit 5, a first calendaring unit 6 and a surface sizer 7.

The first drying unit 4 functions as a pre-dryer and reduces the moisture content by removing water from the web. The material forming the web may also be referred to as the pulp. The first drying unit 4 preferably comprises a plurality of drying cylinders. The drying cylinders are typically temperature controlled and are configured to preliminarily remove water from the moist web before the application of the multiple coating layers.

The first cooling unit 5 reduces the "bulkiness" of the web by cooling the paper substrate before calendaring. Once the bulkiness of the web is reduced by the first cooling unit 5, the web is passed through the first calendaring unit 6 comprising of a plurality of heated rolls configured to compress and smooth the paper substrate. The process termed as calendaring as is known in the art.

The next step in the pre-treatment section involves surface sizing. Surface sizing is performed in order to seal the surface fibres and increase the sheet strength and resistance to stresses. Surface sizing also reduces fuzz, or loose paper fibres, which otherwise may cause blanket piling on offset presses. The surface sizer 7 may preferably be a film size press, a spray sizer, pond size press or a nip sizer. More preferably the surface sizer 7 is a nip sizer, and most preferably the surface sizer 7 is a nip sizer with curtain applicator(s) or a foam applicator(s).

Suitable surface sizing agents may include one or more natural starches. It can be for example, unmodified starches, modified natural starches or a combination thereof.

Unmodified starch refers to starch obtained from plant sources (such as maize, potato, wheat, rice, tapioca, etc.) or cereal starches (such as corn starch, potato starch, wheat starch, rice starch, oat starch, rye starch, barley starch, millet sorghum starch, etc.) or non-cereal starches (such as tapioca starch, etc.).

Examples of modified natural starch are ethylated starches, oxidized starches, such as oxidized corn starch, etc.). 3. Coating section

The pre-treatment section is followed by a coating section, or coating step. The coating section is configured to apply a plurality of coating layers onto one or both sides of the paper web leaving the pre-treatment section. The plurality of coating layers may include: a) a base barrier coating-Layer A; b) a barrier coating- layer B; c) a heat sealing coating- layer C; and d) a shield coating- layer D.

Further details of the plurality of coating layers are presented below. a) Base Barrier Coating: The first coater unit 8 applies the base barrier coating onto the paper web. The base barrier coating may include mineral pigments such calcium carbonate or talcum. This coating optimizes the properties such as increasing smoothness and opacity while reducing the materials' cost.

The base barrier coating and shield coating layers, discussed in more detail below, may include binders such as starch or polyvinyl alcohol (PVOH).The weight of the base barrier coating ranges preferably between 3— 8 g/m ² . This layer plays improves the runnability and smoothness of the paperboard web. b) The Barrier Coating: The first coating unit 8 is followed by a second drying unit 9 which is configured to decrease the moisture content of the paperboard web. This is then followed by a second calendaring unit 10 which further helps in compressing and smoothening of the paper substrate. Thereafter, a barrier coating layer, also referred to as a second coating, is applied by a second coater unit 11. The weight of the barrier coating ranges preferably between 2— 4 g/m ² . The barrier coating composition is in an aqueous polymer dispersion form, which helps in optimizing the quantity of the coating and its homogenous distribution on the surface of the paper web. The second coating composition, i.e., the composition of the barrier coating, may include polyvinyl alcohol dispersion or aqueous dispersions. It may also include Biodegradable/Renewable Polymers. Such Biodegradable/Renewable Polymers are commercially available. c) Heat Sealing Barrier coating: The barrier coating step is in turn followed by a calendaring process undertaken by a third calendaring unit 12 and thereafter a heat-sealable barrier coating layer is applied by a third coating unit 13, which preferably is a Curtain coater C 13. It may in this context be noted that the process works both with and without a drying between steps 11 and 12, especially given an online continuous process. If an optional drying step (11a) is performed between steps 11 and 12, it has been found to be beneficial when it comes to improving compression and smoothness of the paper substrate. However, it has also been found that by omitting such optional drying step between steps 11 and 12 it is made possible to save energy, costs and also to increase the production speed.

The composition of the third coating, also referred to as the heat- sealable barrier coating, is preferably selected from copolymers of ethylene and methacrylic acid or acrylic acid. These are highly functional copolymers that offer flexibility and excellent adhesion to the substrate. It may also include aqueous dispersions comprising a polymer based component which is selected from polyvinyl acetate as the main dispersed component. The composition of the third coating, or heat-sealable barrier coating, may be exactly the same as the second coating composition. The weight of the barrier coating weight preferably ranges between 2 — 4 g/m ² . It may be noted that in cases where there is no need for heat sealability properties, this heat sealing barrier coating may be omitted. d) Shield barrier coating: The heat sealing step is directly, preferably immediately, followed by another coating application. A shield coating application is undertaken by a fourth coating unit 14, preferably a curtain coater D. The base barrier coating and shield coating layers may include binders such as starch or polyvinyl alcohol (PVOH).The weight of the shield coating weight preferably ranges between 2 — 4 g/m ² .

In the context of the above disclosed method including the steps a), b), c) and d), some additional details and general comments are mentioned below.

Each coating layer may comprise additional pigments, dispersing aids, anti-blocking agents and additives.

The shield coating and base barrier coating layers function as support layers which minimizes the need for more expensive substances in the barrier and/or heat sealing layers, which gives cost-savings.

Additionally, the base barrier coating A and shield coating D layers reduces the risk of reactions between coating layers B and C and reduces the risk of migration or blending of coating layers B and C.

By dividing the coating into several substages, it becomes possible to control and optimize the total thickness of the coated paperboard.

The multilayer coating, preferably multilayer curtain coating, enables more precise control on the amount of coating in order to minimize the thickness while still achieving the desired properties. This gives material savings and subsequently cost savings. The shield coating and base barrier coating layers function as support layers which minimizes the need for more expensive substances in the barrier and/or heat-sealable layers, which gives cost-savings.

The shield coating D acts also as a supporting layer to reduce any leakage issues and minimizes the risk for mechanical damages to coating layers B and C.

The multiple coating applications is undertaken preferably through online application, which increases the manufacturing speed, thereby increasing the output, giving reduced costs.

After the multiple layer coating process, the paperboard is moistened through spraying with water by means of a spraying unit 15 .

In this application, water denotes a liquid which essentially contains water, preferably consists exclusively of water.

Water is applied coherently in the form of only a single phase, preferably liquid. Spraying with water can be done, for example, in such a way that small amounts of liquid are continuously or intermittently sprayed onto the surface of the paperboard web by means of spray nozzles. This step improves the sealability and smoothness of the finished paperboard.

For spraying water, conventional spraying equipment are preferably employed for applying water through high-pressure spraying nozzles. The spraying pressure at the nozzles is preferably between 1 - 100 MPa, more preferably 1 - 17 MPa, most preferably 1 - 12 MPa. For spraying water, the paperboard web is moved at a web speed past the spraying unit 15, the web speed preferably being below 500 m/min. 4. Post-treatment section

The post-treatment section, or post-treatment step, comprises a calendaring unit 16, which also referred to as a fourth calendaring unit, and a drying unit 17 , which also may be referred to as a third drying unit.

The coating section is followed by the post-treatment section comprising a calendaring process, preferably a final calendaring process, and drying of the resulting paperboard web. The paperboard web is dried, typically by contact drying by leading the paperboard web to run in contact over heated surfaces of drying cylinders. The drying unit runs most preferably in a temperature range of 95 -117 °C.

5. Finishing Section

The finishing section, or finishing step, comprises a cooling unit 18, which also may be referred to as a second cooling unit, and a paper rolling unit 19 for rolling the paperboard web into rolls.

The post-treatment section is followed by the finishing section, in which the paperboard web is cooled and finally the coated paperboard is reeled by winding to rolls.

Cooling of the paperboard following the post-treatment step is especially relevant where the drying temperatures in the previous step is above 130 °C in order to reduce the risks of bubble formation in the coated layer, sticky paperboard, or inhomogeneous surface.

Example and Analysis

The recyclability of a paper based material which has been manufactured in accordance with the method above has been analysed. The specific example involved paperboard having a weight of 270 g/m ² , a base barrier coating having a weight of 7 g/m ² , a barrier coating having a weight of 3 g/m ² , a heat sealing coating having a weight of 4 g/m ² , and a shield coating having a weight of 3 g/m ² .

The analysis was carried out in accordance with the PTS method PTS- RH 021:2012 "Identification of the recyclability of paper and board packages and of graphic print products".

It was found that a one-sided barrier coated sample of the specific sample above gave in the above analysis under the heading "disintegrability" a total reject of 0,8% and that 99,2% was recyclable. Under the heading "sheet formation" it was found that "percentage adhesive impurities" was "none" and "optical inhomogeneities" was "none".

Thus, as verified through testing, the product is considered to be recyclable and repulpable in standard conventional paper recycling streams.

For the purpose of said test method, 'recyclability' is a post-consumer or pre-consumer paper or board product's ability to be treated in a recovered paper treatment plant according to recognised rules of engineering so as to ensure that the secondary fibre furnish allows the undisturbed and cost-effective manufacture of a recycled fibre-based new paper of acceptable quality.

The resulting paperboard may be used e.g., to produce paper-based cups. One example thereof is shown in figure 2. The heat sealing coating provides heat sealability such that the bottom circular piece may be heat sealed to the piece forming the walls of the cup, and such that the ends of the piece forming the walls of the cup may be sealed along the overlap seam extending upwardly from the bottom of the cup. The paperboard may be coated on both sides or only on the side being intended to form an inside of the cup. Although the present invention has been fully described in conjunction with several embodiments thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.