WO2025243124A1

WO2025243124A1 - Method for manufacturing a heat-sealable packaging material arranged with at least one latex barrier with high gel content

Info

Publication number: WO2025243124A1
Application number: PCT/IB2025/054719
Authority: WO
Inventors: Chris Bonnerup; Kaj Backfolk; Teemu Karhu
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2024-05-20
Filing date: 2025-05-06
Publication date: 2025-11-27
Anticipated expiration: 2026-11-20
Also published as: SE2430279A1

Abstract

The invention relates to a method for of manufacturing a dispersion barrier coated heat-sealable packaging material. The method comprises applying a dispersion barrier composition comprising a latex on a paperboard web. The dispersion barrier composition comprises a latex having a glass transition temperature of less than 20 ºC and a gel content of at least 50%. The invention enables the manufacturing of a dispersion barrier coated paperboard with improved barrier properties and an increased blocking resistance.

Description

METHOD FOR MANUFACTURING A HEAT-SEALABLE PACKAGING MATERIAL ARRANGED WITH AT LEAST ONE LATEX BARRIER WITH HIGH GEL CONTENT

FIELD OF USE

The present invention relates to the field of manufacturing heat-sealable packaging materials, specifically paperboard materials, with dispersion barrier layers.

BACKGROUND

Dispersion barrier-coated paper or paperboard provides an alternative solution to extrusion-coated or film-laminated packaging materials. Dispersion barrier-coated paper or paperboard can be made in-line, or in off-line operations. Typically, the dispersion barrier comprises a main polymer or copolymer, which is often a latex emulsion such as styrene/butadiene (SB), acrylates such as styrene/acrylate (SA), polyvinyl acetate (PVAc), ethyl vinyl acetate (EVA), poly(vinylidene chloride) (PVdC), polyhydroxyalkanoate (PHA) or a polyolefin-based dispersion such as polyethylene or polypropylene dispersions.

One important requirement for dispersion barrier coating is that the particles soften and coalesce to form a film when the carrier medium (solvent, water) is removed, and the wet coating is subjected to sufficient heat or drying energy. Besides good film formation, the dispersion coating layer should provide good adhesion to the substrate and cohesion within the dispersion coating layer to ensure a good and durable barrier.

One important advantage of using a dispersion barrier is the ability to integrate the in-line dispersion coating with product property development in a single production line. In order for the in-line dispersion coating to be effective, the dispersed coating should be easily repulpable with minimal or no rejects after disintegration.

Additionally, it should cause minimal or no pitch in the wet end of a paper or paperboard machine. However, when providing a latex emulsion with features that improve film formation and adhesion to the base substrate, it also increases the risk of having more delamination and blisters and higher blocking behavior.

Blocking refers to 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. Blocking is typically controlled or reduced by the use of cooling units or by the addition of fillers. However, these solutions also come with drawbacks that can impact cost efficiency and product performance. Blocking behavior is also important for post-converting processes, such as calendaring, where too high of a blocking efficiency may negatively affect the sealing performance.

US11578462B2 (WESTROCK) discloses a high barrier paperboard structure with anti-blocking behavior. The document teaches that a protective top coating is applied onto the dispersion barrier coating which hence reduces the blocking behavior.

EP4180572A1 (KLABIN) teaches the use of a ductile dispersion barrier coating in which a cross-linker can be optionally used as an additive. However, the post addition of cross-linkers are not used to control intraparticle behavior but rather interparticle interactions such as cross-linking of rheology modifiers.

EP3363947A1 (VALMET TECHNOLOGIES OY) discloses a treatment section of a production line for producing a barrier coated fiber web and treatment method for producing a barrier coated fiber web. This document teaches that the blocking problem with barrier coating can be avoided by applying a protective coating. The document also emphasize that blocking is dependent on latex properties and the recipe, whereas the document is silent on the drawbacks with a protective coating.

Therefore, there is a need to address the aforementioned problems associated with dispersion barrier coated paper or paperboard and minimize the requirement for protective anti-blocking coatings. Additionally, there is a need to ensure that blocking is less reliant on cooling before reeling and that the dispersion coated product exhibits excellent barrier properties, particularly with regards to grease and oil resistance. Furthermore, it is desirable that broke generated during the production of dispersion barrier board is repulpable and can be reused in the manufacturing process.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a method for manufacturing a heat-sealable packaging material that ensures excellent barrier properties and increased blocking resistance, while also being cost and energy efficient.

The above mentioned object, as well as other objects as will realized by the skilled person in light of the present disclosure, are achieved by the various aspects of the present disclosure.

According to a first aspect illustrated herein, there is provided a method for of manufacturing a heat-sealable packaging material on a paperboard machine, the method comprising the steps of:

- forming a paperboard web comprising a first major side and a second major side opposed from said first major side,

- providing a first dispersion barrier composition comprising a first latex having a glass transition temperature of less than 20°C, preferably less than 15°C, wherein the first latex is a styrene butadiene (SB) latex having a gel content of at least 50% according to standard ASTM D2765-16, or a styrene acrylate (SA) latex having a gel content of at least 50% according to ASTM D2765-16,

- applying the first dispersion barrier composition on the first major side, forming a first dispersion barrier layer, whereby the web and the first dispersion barrier layer forms a dispersion barrier coated web,

- drying the dispersion barrier coated web, and

- reeling the dried dispersion barrier coated web. It has been surprisingly discovered that incorporating a latex exhibiting a gel content according to the invention allows for the use of latex with a low glass transition temperature without encountering blocking issues. Consequently, this results in achieving both excellent film formation, leading to improved barrier properties, and an increase in blocking resistance.

A dispersion barrier layer refers to a coating layer that provides a barrier against at least one of water vapor, aroma, mineral oil migration, liquid, and optionally gas (oxygen). The first dispersion barrier layer preferably has a grammage in the range of 5 - 15 gsm, particularly in the range of 5 - 10 gsm.

If not specifically denoted otherwise, given % as used herein are weight%, and are calculated on the basis of a dry weight of 100 weight% of the respective object, such as a layer, a ply, a furnish or a composition.

A paperboard web refers to a web comprising cellulose fibers. A typical paperboard web comprises at least one ply, preferably several plies. The paperboard web is preferably a multilayer paperboard, comprising at least two layers; a back ply and a top ply. The paperboard web may further comprise one or several middle plies. The paperboard web for example may comprise a top ply and a back ply and a middle ply. The paperboard may be made from chemical, mechanical and/or chemi- thermomechanical pulp (CTMP), including HT-CTMP. The paperboard web may comprise recycled fibers. In embodiments, the paperboard web may comprise top- and back plies comprising bleached or unbleached chemical pulp (e.g. kraft pulp) and a middle ply comprising CTMP or HT-CTMP. The paperboard web may have a basis weight of 100 - 400 gsm , such as at least 120 gsm , or at least 150 gsm , preferably at least 200 gsm, e.g. 120 - 400 gsm, or 150 - 300 gsm. The board can be treated with surface sizing, such as starch, starch-pigment, starch-latex, or starch-pigment-latex, to ensure strong adhesion and protection. This prevents the dispersion coating from contaminating the fibers of the board. In embodiments, the glass transition temperature of the first latex is less than 10 °C, preferably in the range from > -10 °C to < 15 °C, more preferably in a range from > - 10 °C to < 10 °C,

The glass transition temperature (Tg), as referred to in the description and the claims, is determined by differential scanning calorimetry method [ASTM D7426 - 08(2013)].

The gel content of the first latex can be adjusted by cross-linking the latex particles, which depends on the selected monomers and process conditions. Examples of cross-linking within the particles include dienes, difunctional acrylates, divinylbenzene, or OH-containing monomers like hydroxyacrylates. Cross-linking within the particle can be introduced during emulsion polymerization or activated during film formation by using reactive monomers that contain a latently reactive group even after incorporation into the polymer, such as glycidylmethacrylate or N- methylol(meth)acrylamide. These monomers can form a network between various particles and polymer molecules after film formation.

Latex, as used herein, refers to an emulsion of polymer particles in an aqueous medium. In embodiments, the polymer particles of the first latex have a mean particle size of less than 180 nm, preferably less than 160 nm, such as in the range of 40 - 160 nm, as measured using ISO 22412:2017 standard for dynamic light scattering. The polymer particles may further be in the form of a core-shell structure, wherein the core and the shell can have different glass transition temperatures to optimize the properties of the latex. In preferred embodiments, the core may have lower glass transition temperatures than the shell. In addition, the shell may have a higher degree of cross-linking than the core.

The drying of the dispersion barrier coated web is preferably performed such that the moisture content of the dispersion barrier coated web is in the range of 4-12 wt%.

In embodiments, the method further comprises the step of cooling the dispersion barrier coated web to a surface temperature in the range of 35 - 75 °C, preferably in the range of 40 - 70 °C. The invention allows for drying to lower moisture contents and less cooling, while still maintaining low blocking tendency.

In embodiments, first dispersion barrier composition comprises a second latex having a gel content of less than 10wt%. In this embodiment, the composition comprises the first latex in an amount of 50 - 100wt%, preferably 50 - 99.9 wt%, based on the total amount of latex in the composition and the second latex in an amount of 0 - 50, preferably 0.1 - 50 wt%, based on the total dry weight of the latex. In preferred embodiments, the composition comprises the first latex in an amount of 50 - 70 wt% and the second latex in an amount of 50 - 30 wt%, as calculated on the total dry weight of latex in the composition. In this way, the properties of the end product can be optimized while blocking is efficiently reduced.

The step of forming the paperboard web and applying the first dispersion barrier composition on the first major side is performed in-line on the paperboard machine. The method of the invention enables the recycling of broke, although the dispersion barrier compositions is applied in-line on the paperboard machine. The method may further comprise the step of applying steam on the first major side of the paperboard web before applying the dispersion barrier composition on the same side. In order to prevent curling, moisture or steam can also be applied on the second major side.

In embodiments, the method further comprises the step of calendering the web in a calender comprising a soft or hard nip, which calendering is performed after the step of applying the first dispersion barrier composition on the first major side, after the step of drying the dispersion barrier coated web and after the step of cooling the dried dispersion barrier coated web but before the step of reeling the web. The calendering can comprise soft nip, soft-soft nip, soft-hard nip or hard-hard nip with one or several passes. The method of the invention enables in-line calendering without problems with blocking. The calendering process can thus be carried out inline on the same paperboard machine. This can be achieved using an extended calendering nip, such as a shoe nip. The calendering line load may preferably be of at least 20 kN/m such as 20-200 kN/m at a temperature above 50 °C such as 60-180 °C.

In embodiments, the method further comprises the step of providing a second dispersion coating composition, comprising binder and optionally pigments, and applying said second dispersion composition on the first major side of the paperboard substrate to form a second dispersion coating layer before the application of the first dispersion barrier composition. In this way, the first dispersion barrier composition is applied on the second dispersion coating layer.

The binder of the second dispersion barrier coating is preferably chosen from the group consisting of starch, carboxymethyl cellulose (CMC) and latex. The latex can be selected from the group comprising styrene-butadiene latex or styrene acrylate latex, or derivatives thereof including but not limited to acrylate latex, vinyl acetate latex, vinyl acetate-acrylate latex, styrene butadiene-acrylonitrile latex, styrene- acrylate-acrylonitrile latex, styrene-maleic anhydride latex, styrene-acrylate-maleic anhydride latex, or mixture of these latexes. In embodiments, the latex in the second dispersion coating layer can be the same, i.e. exhibiting the same properties with regard to glass transition temperature and gel content, as the latex in the first dispersion coating layer.

As used herein, “pigment” refers to solid particulate materials such as clay, e.g. kaolin, calcium carbonate or titanium dioxide used for papermaking as usually referred to in the paper industry. In preferred embodiment the pigment has a D90 below 2 pm and a surface area greater than 10 m²/g, preferably greater than 15 m²/g.

In addition to binder (e.g. latex) and optionally pigments, the first and optional second dispersion barrier compositions may comprise small amounts, e.g. 0 - 5 wt% of additives, including e.g. biocides, rheology modifiers, defoamers, lubricants, UV- blocking chemicals, softeners, acids and cross-linkers. In embodiments, the second dispersion composition may comprise binder and optionally pigment at a binder to pigment dry weight ratio of from 100:0 to 50: 100. This results in a paperboard that possesses exceptional barrier properties.

In embodiments, the method further comprises the step of providing a pigment coating composition comprising binder and pigments at a binder to pigment dry weight ratio of from 18: 100 to 10: 100, e.g. in a binder to pigment dry weight ratio of 15:100, and the step of applying said pigment coating composition on the first major side of the paperboard substrate to form a pigment coating layer before the application of the first and the optional second dispersion barrier composition. In this way, the first and optional second dispersion barrier composition is applied on the pigment coating layer. The application of a pigment coating composition between the paperboard web and the first and optional second dispersion barrier composition enhances the film-forming ability of the dispersion barrier layer, thereby improving the barrier properties.

The first dispersion barrier layer preferably forms an outermost barrier layer on the first side of the paperboard substrate.

In embodiments, the first dispersion barrier composition comprises the first latex in an amount of at least 15 wt% latex, preferably at least 20 wt% or at least 50 wt%, such as in the range of 15-99 wt%, or 20-99 wt% or 50-99 wt%, as calculated on the total dry weight of the first dispersion barrier composition. The remaining wt% may be additives as outlined above.

In embodiments, first dispersion barrier composition does not comprise any pigments. In these embodiments, the first dispersion barrier composition may comprise the latex in an amount of 95 - 100 wt% (dry weight). Latex in this context means the total amount of latex, including the first and optional second latex.

In embodiments, first dispersion barrier composition further comprises pigments. In these embodiments, the first dispersion barrier composition may comprise latex (first and optional second latex) and pigments at a latex to pigment dry weight ratio of from 20:100 to 100:100, preferably 50:100 to 100:100. In these embodiments, the first dispersion barrier composition may comprise latex and pigments at a total amount of 95 - 100 wt% calculated on the dry weight of the first dispersion barrier composition. The remaining 0-5 wt% may be additives as outlined above.

The method may further comprise applying third dispersion barrier composition on the second major side of the paperboard. Such a third dispersion barrier composition may comprise binder and optionally pigments, which binder and pigments can be selected from the same group of binders and pigments as the second dispersion barrier composition. The third dispersion barrier composition may e.g. comprise binder and pigment at a binder pigment dry weight ratio of from 100:0 to 100:100.

The first and optional second and third dispersion barrier composition is preferably applied using contact or non-contact method, e.g. using blade coating, gravure or reverse gravure coating, rod, film press coating or curtain coating. In preferred embodiments, the application of the first and the optional second and third dispersion barrier composition is performed using blade coating. Blade coating in this context includes the application of coating to the surface using a roll or jet, and then the blade is used to level and distribute the coating evenly across the surface.

According to a second aspect illustrated herein, there is provided a paperboard made by the method according to the first aspect.

The paperboard may exhibit at least one of:

- a water absorption rate (COBB 600), determined according to SCAN-test P 12:64 of < 10 g/m², preferably < 5 g/m²

- a KIT barrier, determined according to TAPPI method 559, 3M KIT, in a range from > 6 to < 12, preferably in a range from > 9 to < 12.

- a water vapor transmission rate (WVTR) of less than 100 g/m²/day, preferably less than 75 g/m²/day and most preferably less than 50 g/m²/day when determined at 23 °C and 50% RH according to ASTM F1249-20.

The paperboard of the invention is recyclable. In embodiments, the reject level determined in accordance with test method PTS RH021/97 (Category II method) received from the recycling of the paperboard of the invention is less than 12% by weight, preferably less than 10% by weight, or less than 5% by weight, based on a dry weight of the paperboard of 100% by weight.

The paperboard is especially suitable for packaging liquid and food products, such as fresh food, hot or cold beverages, dry food, frozen food, and bakery products.

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1 . A method of manufacturing a heat-sealable packaging material on a paperboard machine, the method comprising the steps of:

- providing a first dispersion barrier composition comprising a first latex having a glass transition temperature of less than 20°C, preferably less than 15, wherein the first latex is a styrene butadiene (SB) latex having a gel content of at least 50% according to standard ASTM D2765-16, or a styrene acrylate (SA) latex having a gel content of at least 50% according to ASTM D2765-16,

- drying the dispersion barrier coated web, and

- reeling the dried dispersion barrier coated web.

2. A Method according to claim 1 , wherein the polymer particles of the first latex have an average polymer particle size of less than 180 nm, preferably less than 160 nm, as measured using ISO 22412:2017 standard for dynamic light scattering.

3. A method according to anyone of the preceding claims, wherein the drying of the dispersion barrier coated web is performed such that the moisture content of the dispersion barrier coated web is in the range of 4-12 wt%.

4. A method according to anyone of the preceding claims, wherein the method further comprises the step of cooling the dispersion barrier coated web to a temperature in the range of 35 - 75 °C, preferably in the range of 40 - 70 °C.

5. A method according to anyone of the preceding claims, wherein the first dispersion barrier composition comprises a second latex having a gel content of less than 10 wt% and wherein the composition comprises the first latex in an amount of 50 - 100 wt%, preferably 50 - 99.9 wt%, based on the total dry weight of latex in the composition and the second latex in an amount of 0 - 50, preferably 0.1 - 50 wt%, based on the total dry weight of the latex.

6. A method according to anyone of the preceding claims, wherein the composition comprises the first latex in an amount of 50 - 70 wt% and the second latex in an amount of 50 - 30 wt%, as calculated on the total dry weight of latex in the composition.

7. A method according to anyone of the claims 4 - 6, wherein the method further comprises the step of calendering the web in a calender comprising a soft or hard nip, which calendering is performed after the step of applying the first dispersion barrier composition on the first major side, the step of drying the dispersion barrier coated web and the step of cooling the dried dispersion barrier coated web but before the step of reeling the web.

8. A method according to anyone of the preceding claims, wherein the method further comprises the step of providing a second dispersion coating composition, comprising binder and optionally pigments, and applying said second dispersion composition on the first major side of the paperboard substrate to form a second dispersion coating layer before the application of the first dispersion barrier composition, whereby the first dispersion barrier composition is applied on the second dispersion coating layer.

9. A method according to claim 8, wherein the second dispersion composition comprises binder and optionally pigments at a binder to pigment dry weight ratio of from 100:0 to 50: 100.

10. A method according to anyone of the preceding claims, wherein the method further comprises the step of providing a pigment coating composition comprising binder and pigments at a binder to pigment dry weight ratio of from 30:100 to 15: 100, and the step of applying said pigment coating composition on the first major side of the paperboard substrate to form a pigment coating layer before the application of the first and the optional second dispersion barrier composition.

11 . A method according to anyone of the preceding claims, wherein the first dispersion barrier layer forms an outermost barrier layer on the first side of the paperboard substrate.

12. A method according to anyone of the preceding claims, wherein the first dispersion barrier composition comprises the first latex in an amount of at least 15 wt% latex, preferably at least 20 wt% or at least 50 wt%, as calculated on the total dry weight of the first dispersion barrier composition.

13. A method according to anyone of the preceding claims, wherein the first dispersion barrier composition does not comprise any pigments.

14. A method according to anyone of the claims 1 - 12, wherein the first dispersion barrier composition further comprises pigments, and wherein the first dispersion barrier composition comprises latex and pigments at a latex to pigment dry weight ratio of from 20: 100 to 100: 100, preferably 50: 100 to 100:100

15. A method according to anyone of the preceding claims, wherein the method comprises applying a third dispersion barrier composition on the second major side of the paperboard.

16. A method according to anyone of the preceding claims, wherein the step of applying the first and the optional second and third dispersion barrier composition is performed using blade coating.

17. A paperboard made by the method according to anyone of the claims 1 - 16.

18. A paperboard according to claim 16, which paperboard exhibits at least one of a water absorption rate (COBB 600), determined according to SCAN-test P 12:64 of < 10 g/m² preferably < 5 g/m² and a KIT barrier, determined according to TAPPI method 559, 3M KIT, in a range from > 6 to < 12, preferably in a range from > 9 to < 12.

19. A paperboard according to anyone of the claims 16 and 17, wherein the reject level determined in accordance with test method PTS RH021/97 received from the recycling of the paperboard is less than 12% by weight, preferably less than 10% by weight, or less than 5% by weight, based on a dry weight of the paperboard of 100% by weight.