阅读说明：本技术 聚合物涂覆的纸和纸板 (Polymer coated paper and paperboard ) 是由 V.里布 K.内瓦莱宁 J.托里塞瓦 E.索卡斯 J.库西帕洛 于 2019-10-01 设计创作，主要内容包括：本发明涉及包括聚合物涂层的纸或纸板,所述聚合物涂层包括：附着至纸或纸板表面的第一涂层,所述第一涂层包含以下的共混物：高密度聚乙烯(HDPE)、中密度聚乙烯(MDPE)或线性低密度聚乙烯(LLDPE)或其混合物,以及低密度聚乙烯(LDPE)；以及附着至第一涂层的第二涂层,所述第二涂层基本上由低密度聚乙烯(LDPE)组成；其中第一和第二涂层的组合克重小于12g/m～2。本发明涉及制造聚乙烯(PE)涂覆的纸或纸板基底的方法。(The present invention relates to a paper or paperboard comprising a polymeric coating comprising: a first coating adhered to a surface of paper or paperboard, the first coating comprising a blend of: high Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and Low Density Polyethylene (LDPE); and a second coating layer adhered to the first coating layer, the second coating layer consisting essentially of Low Density Polyethylene (LDPE); wherein the first and second coating layersCombined grammage of less than 12g/m 2 . The present invention relates to a process for manufacturing a Polyethylene (PE) coated paper or paperboard substrate.)

1. Paper or paperboard comprising a polymeric coating comprising:

a first coating adhered to a surface of paper or paperboard, the first coating comprising a blend of:

high Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

low Density Polyethylene (LDPE); and

a second coating layer adhered to the first coating layer, the second coating layer consisting essentially of Low Density Polyethylene (LDPE);

wherein the combined grammage of the first and second coating layers is less than 12g/m²。

2. The paper or paperboard of claim 1, the first coating consisting essentially of a blend of:

1 to 49% by weight of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

51-99 wt% of Low Density Polyethylene (LDPE).

3. The paper or paperboard of any of the preceding claims, where the first coating is formed by extrusion coating onto a paper or paperboard surface.

4. The paper or paperboard of any of the preceding claims, where the second coating layer is formed by extrusion coating onto the first coating layer.

5. The paper or paperboard of any of the preceding claims, where the combined grammage of the first and second coating layers is less than 10g/m²Preferably less than 8g/m²。

6. The paper or paperboard of any of the preceding claims, where the grammage of the first coating is less than 5g/m²Preferably less than 4g/m²More preferably less than 3g/m²。

7. The paper or paperboard of any of the preceding claims, where the grammage of the second coating is less than 10g/m²Preferably less than 8g/m²More preferably less than 6g/m²。

8. The paper or paperboard of any of the preceding claims, where the HDPE has a density in the range of 0.930-0.970g/cm³In the range of 0.926 to 0.940g/cm of MDPE³In the range of 0.918 to 0.940g/cm³And/or the density of the LDPE is in the range of 0.910 to 0.940g/cm³Within the range of (1).

9. The paper or paperboard of any of the preceding claims, where the first coating comprises a blend of MDPE and LDPE.

10. The paper or paperboard of any of the preceding claims, where the second coating has a lower density than the first coating.

11. The paper or paperboard of any of the preceding claims, where the second coating is a top layer of a polymeric coating.

12. Paper or paperboard according to any of the preceding claims, whereby the polymer coating has a better adhesion to the paper or paperboard surface than an LDPE coating having the same total grammage.

13. Heat sealed paper or paperboard product comprising the paper or paperboard according to any one of the preceding claims.

14. A heat sealed paper or paperboard product according to claim 13 wherein the product is a paper cup.

15. A method of making a Polyethylene (PE) coated paper or paperboard substrate, the method comprising:

a) a paper or paperboard substrate is provided,

b) applying at least one layer of a molten first polymeric resin to a surface of the substrate by extrusion coating to form a first polymeric coating layer, the first polymeric resin comprising a blend of:

high Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

a Low Density Polyethylene (LDPE),

c) applying at least one layer of a molten second polymeric resin to a surface of the first polymeric coating layer by extrusion coating to form a second polymeric coating layer, the second polymeric resin consisting essentially of Low Density Polyethylene (LDPE),

d) allowing the first and second coatings to cool and solidify, an

e) Recovering the PE coated paper or paperboard substrate.

16. The method of claim 15, wherein the first and second coatings are as further defined as set forth in any one of claims 2-12.

17. The method of any of claims 15-16, wherein the first and second coating layers are formed simultaneously by co-extrusion coating.

Technical Field

The present disclosure relates to coated paper and paperboard comprising a polyethylene coating.

Background

Paper and paperboard are often coated with plastic for combining the mechanical properties of the paperboard with the barrier and sealing properties of plastic films. Even providing paperboard with relatively small amounts of suitable plastic materials can provide the properties required to make the paperboard suitable for many demanding applications.

The paper or paperboard itself is generally suitable for packaging of dry products. However, the use of untreated board in direct contact with moist or greasy products is limited, since moisture can affect the mechanical properties of the packaging and absorbed grease can lead to contamination of the paper. These effects can impair the protective function and the appearance of the package. Polyethylene (PE) coating of paper and paperboard is generally suitable for packaging applications where moisture barrier properties are important. Examples include packaging for fresh and frozen foods such as vegetables, meat, fish and ice cream. One important application of PE coated paperboard is in the manufacture of water-resistant paper cups.

Extrusion coating is a method of applying molten plastic material to a substrate (e.g., paper or paperboard) to form a very thin, smooth and uniform layer. The coating may be formed from the extruded plastic itself, or molten plastic may be used as the adhesive to laminate a solid plastic film to the substrate. Common plastic resins used in extrusion coating include Polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET).

Extrusion coating can be used to achieve, for example, moisture resistance, barrier properties to water vapor, oxygen, fragrance, and the like, dirt or grease resistance, heat sealability, and/or impart a desired finish (finish) or texture to the substrate surface.

Extrusion coating greatly expands the range of applications for paper and paperboard. The thin plastic layer imparts resistance to grease and moisture, and in some cases also heat resistance. Plastic coatings may also be used for heat sealing. Depending on the application, the paper or paperboard may be extrusion coated on one or both sides.

For environmental and economic reasons, it is generally desirable to keep the plastic coating as thin as possible, as long as the barrier and protective properties are kept at acceptable levels. However, in many cases, further reduction in the thickness (or grammage) of the plastic coating is limited by impairment of adhesion and film formation stability during extrusion, and the formation of pinholes. For example, PE is typically extrusion coated to 15-25g/m²Gram weight of (c). The PE resin conventionally used for manufacturing paper cups cannot be extrusion-coated on paper or paperboard to less than 12g/m²Without losing adhesion, reducing heat sealability, and increasing pinhole formation, resulting in defects in the coated product.

In extrusion coating and lamination of paper and paperboard with plastics, it is very important to obtain satisfactory adhesion of the plastic to the substrate. The adhesion of the plastic depends mainly on the surface properties of the substrate and the heat content of the plastic melt when applied to the paperboard. Insufficient adhesion between plastic coatings and paper or paperboard is a common and persistent problem.

Pinholes are tiny holes that can be formed in the plastic film during the coating process. The main causes of pinholes include irregularities in the substrate surface (e.g., high surface roughness or loose fibers), uneven coating distribution, or too low a coating grammage.

Adhesion can be improved by surface treatment of the substrate (e.g. with corona discharge or ozone), but there is still a need for improved solutions for reducing the grammage of plastic coatings in extrusion coating of PE, while maintaining good adhesion, heat sealability and film formation stability during extrusion.

Disclosure of Invention

It is an object of the present disclosure to reduce the minimum grammage of PE resin required to achieve sufficient adhesion, heat sealability, and/or film-forming stability in extrusion coating.

It is another object of the present disclosure to provide a PE resin coated paper or paperboard that allows for a reduced total grammage of the PE resin, e.g., less than 12g/m²While maintaining good adhesion of the PE resin to the paper or paperboard and avoiding the formation of pinholes.

It is another object of the present disclosure to provide a method of making PE resin coated paper or paperboard that allows for a reduced grammage of the PE resin, e.g., less than 12g/m²While maintaining good film forming stability during extrusion.

It is another object of the present disclosure to provide a method of making PE resin coated paper or paperboard that allows for improved film forming stability during extrusion with low grammage of PE resin.

The above objects, as well as other objects that will be recognized by those skilled in the art in light of the present disclosure, are achieved by various aspects of the present disclosure.

According to a first aspect illustrated herein, there is provided a paper or paperboard comprising a polymeric coating comprising:

a first coating adhered to a surface of paper or paperboard, the first coating comprising a blend of:

high Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

low Density Polyethylene (LDPE); and

a second coating layer adhered to the first coating layer, the second coating layer consisting essentially of Low Density Polyethylene (LDPE);

wherein the combined grammage of the first and second coating layers is less than 12g/m²。

Paper generally refers to a material made in the form of a sheet from wood pulp or other fibrous material containing cellulose fibers, for writing, drawing or printing thereon, or as a packaging material.

Paperboard generally refers to strong, thick paper or cardboard (cardboard) containing cellulosic fibers for boxes and other types of packaging. The paperboard can be bleached or unbleached, coated or uncoated, and can be produced in a variety of thicknesses, depending on the requirements of the end use.

As used herein, the term coating refers to an operation in which the surface of a substrate is covered with a composition to impart a desired property, finish, or texture to the substrate. The coating may be a multi-layer coating, wherein the PE coating resin may be applied in one or more layers. The coating may be applied to one or both sides of the paper or paperboard.

The problems of poor adhesion of PE coatings at lower grammage and pinhole formation are particularly evident in the coating of paper and board. The fiber-based substrate and its natural voids and surface roughness may play an important role here. The existing waterproof paper cup is made of polyolefin (with density of 0.910-0.940 g/cm)³Low Density Polyethylene (LDPE)) in the range of (a). The grammage of the coating is typically 12g/m²Or higher. Such conventional LDPE cannot be extrusion coated onto paper or paperboard to less than 12g/m²Without losing adhesion and film forming stability, resulting in defects in the coated product.

The inventors have now found that with a specific combination of different PE types, in particular a first coating comprising a blend of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof and Low Density Polyethylene (LDPE), the minimum grammage of the PE required to achieve sufficient adhesion and film stability in extrusion coating of paper and paperboard can be greatly reduced. In many converting operations (e.g., printing and heat sealing), adequate adhesion is important.

The inventors have surprisingly found that by the coating structure of the present invention having as an adhesion layer a first coating layer comprising a blend of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or a mixture thereof and Low Density Polyethylene (LDPE) and as a top layer a second coating layer consisting essentially of Low Density Polyethylene (LDPE), the total coating grammage can be further reduced compared to the total coating grammage possible with a coating structure comprising only said blend and further compared to the total coating grammage possible with a similar structure in which the order of layers is reversed, i.e. said blend as top layer and LDPE as adhesion layer. This shows that not only the blend composition, but also the order of the layers affects the coating process and the obtained coating.

The first coating comprises a blend of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE), or mixtures thereof, and Low Density Polyethylene (LDPE).

Low Density Polyethylene (LDPE) has rheological properties suitable for producing films by extrusion. LDPE has some long chain branches and many short chain branches. Typically, each molecule may have three long chain branches and 30 short chain branches. The molecular weight is relatively low and has a broad molecular weight distribution. The melt strength and shear thinning properties of LDPE enhance processability. LDPE films have relatively low tensile strength but good impact strength. LDPE films exhibit good clarity and gloss. Good clarity and gloss result from relatively low crystallinity. LDPE is obtained by a high pressure free radical polymerisation process, typically in an autoclave or tubular reactor. Autoclaves generally produce more branching and a broader molecular weight distribution. LDPE has a wide melting range (melting range) with a peak melting temperature of 110 ℃. The density of LDPE is typically in the range of 0.910 to 0.940g/cm³Within the range of (1).

High Density Polyethylene (HDPE) has a linear structure with little or no branching. HDPE is typically prepared by the Ziegler-Natta, Phillips or Unipol processes. These processes involve relatively low pressures and are catalyzed by organometallic complexes with transition metals. The polymerization is usually carried out in a slurry with a liquid, for example heptane, or in the gas phase in the form of a fluidized bed with the catalyst. The density of HDPE is typically in the range of 0.930 to 0.970g/cm³Within the range of (1).

Medium density polyethylene(MDPE) is a variant of HDPE in which some short chain branches are introduced by copolymerization with 1-olefins, such as 1-butene, 1-hexene or 1-octene. The density of MDPE is typically in the range of 0.926 to 0.940g/cm³Within the range of (1).

HDPE and MDPE show more newtonian rheology than LDPE and are therefore less suitable for extrusion processing. HDPE and MDPE have higher crystallinity and therefore higher tensile strength than LDPE, although their impact strength may be insufficient for many applications.

Linear Low Density Polyethylene (LLDPE) is a copolymer of ethylene and a 1-olefin (typically 1-butene, 1-hexene or 1-octene), although branched olefins (e.g. 4-methyl-1-pentene) are also used. The density of these polymers is in the range of 0.918 to 0.940g/cm³And they contain 2 to 7% by weight of 1-olefins. Like HDPE, they can be polymerized by gas phase or slurry processes using a multi-site catalyst (e.g., Ziegler-Natta). Comonomer compositions typically have a broad distribution, so some molecules have few branches, while others have many branches. This distribution is reflected in the broad melting temperature range of LLDPE. The properties of LLDPE tend to lie between those of LDPE and HDPE. They have short chain branches but no long chain branches and thus improve the mechanical properties depending on crystallization, but the processing rheology is inferior to that of LDPE.

The skilled person would expect that LDPE would show the lowest pinhole sensitivity due to its strain hardening behaviour during the extrusion coating process. This behavior is expected to protect the coating from pinhole formation due to defects in the film and non-uniformity of the fiber-based substrate. Surprisingly, the present inventors have now found that the incorporation of HDPE, MDPE or LLDPE into LDPE significantly reduces pinhole formation, thus minimising the amount of coating required in the first extrusion coating for paper or paperboard.

In some embodiments, the polymeric coating does not include any other coating in addition to the first coating and the second coating, i.e., the polymeric coating consists of the first coating and the second coating. In some embodiments, the polymeric coating includes one or more additional coatings in addition to the first coating and the second coating.

In some embodiments, the first coating consists essentially of a blend of:

1 to 49% by weight of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

51-99 wt% of Low Density Polyethylene (LDPE).

In some embodiments, the first coating consists essentially of a blend of:

1-39 wt%, such as 1-29 wt%, such as 1-19 wt%, of a High Density Polyethylene (HDPE), a Medium Density Polyethylene (MDPE) or a Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

61-99 wt%, such as 71-99 wt%, such as 81-99 wt% of a Low Density Polyethylene (LDPE).

As used herein, the phrase "consisting essentially of … …" means that the coating consists of at least 95% by weight, preferably at least 98% by weight, of the component in question. The remainder may be other polymers or additives.

The formulation of the coating resin can vary widely depending on the intended use of the coating and the coated paper or paperboard. The coating composition may include a wide range of ingredients in various amounts to improve the final properties of the product or the processing of the coating. In some embodiments, the PE coating comprises at least one additional component selected from the group consisting of: polymers other than PE, pigments (e.g. TiO)₂Or carbon black), dyes, and fillers (e.g., CaCO)₃Talc).

In some embodiments, the first coating is formed by extrusion coating onto a paper or paperboard surface. Extrusion coated PE blend coatings can be used to promote adhesion of subsequently applied or coextruded polymer coatings. The extrusion coated PE blend layer may, for example, be used to promote adhesion of a subsequently applied or coextruded second coating layer consisting essentially of Low Density Polyethylene (LDPE).

In some embodiments, the second coating layer is formed by extrusion coating onto the first coating layer. Preferably, the first and second coating layers are formed simultaneously by coextrusion coating.

The PE blend used in the first coating of the invention allows the production of coated paper or paperboard having a low total grammage (e.g. less than 12 g/m)²Grammage) of PE and adhesion of the PE coating to paper or paperboard.

The combined grammage of the first and second coating layers is less than 12g/m². Preferably, the combined grammage of the first and second coating layers is in the range of 5-12g/m²Within the range of (1). In some embodiments, the combined grammage of the first and second coating layers is less than 10g/m²E.g. in the range of 5-10g/m²In the range of (1), preferably less than 8g/m²E.g. in the range of 5-8g/m²Within the range of (1).

In some embodiments, the grammage of the first coating is less than 5g/m²E.g. in the range of 1-5g/m²In the range of (1), preferably less than 4g/m²E.g. in the range of 1-4g/m²More preferably less than 3g/m²E.g. in the range of 1-3g/m²Within the range of (1).

In some embodiments, the grammage of the second coating is less than 10g/m²E.g. in the range of 4-10g/m²In the range of (1), preferably less than 8g/m²E.g. in the range of 4-8g/m²More preferably less than 6g/m²E.g. in the range of 4-6g/m²Within the range of (1).

The first coating comprises a blend of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE), or mixtures thereof, and Low Density Polyethylene (LDPE). The density of HDPE is 0.930-0.970g/cm³In the range of 0.926 to 0.940g/cm of MDPE³In the range of 0.918 to 0.940g/cm³And the density of the LDPE is in the range of 0.910 to 0.940g/cm³Within the range of (1).

In some embodiments, the first coating comprises a blend of MDPE and LDPE. The MDPE preferably comprises higher alpha-olefin branches, preferably octene.

In some embodiments, the second coating has a lower density than the first coating.

In some embodiments, the second coating is a top layer of a polymeric coating.

In some embodiments, the polymer coating has better adhesion to the paper or paperboard surface than an LDPE coating having the same total grammage.

The paper or paperboard of the invention is particularly useful in the manufacture of sealed paper or paperboard products such as water-resistant paper cups. According to a second aspect shown herein, there is provided a sealed paper or paperboard product comprising a paper or paperboard according to the first aspect described herein. In a preferred embodiment, the product is a paper cup.

According to a third aspect shown herein, there is provided a method of manufacturing a Polyethylene (PE) -coated paper or paperboard substrate, the method comprising:

a) a paper or paperboard substrate is provided,

b) applying at least one layer of a molten first polymeric resin to a surface of the substrate by extrusion coating to form a first polymeric coating layer, the first polymeric resin comprising a blend of:

high Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE) or Linear Low Density Polyethylene (LLDPE) or mixtures thereof, and

a Low Density Polyethylene (LDPE),

c) applying at least one layer of a molten second polymeric resin to a surface of the first polymeric coating layer by extrusion coating to form a second polymeric coating layer, the second polymeric resin consisting essentially of Low Density Polyethylene (LDPE),

d) allowing the first and second coatings to cool and solidify, an

e) Recovering the PE coated paper or paperboard substrate.

The first and second coating layers of the third aspect may be further defined as described above with reference to the first aspect.

In some embodiments, the first and second coating layers are formed simultaneously by coextrusion coating.

In some embodiments, the method does not include applying any additional coating layers in addition to the first and second coating layers, i.e., the polymeric coating layer of the formed PE coated substrate consists of the first and second coating layers. In other embodiments, the method includes applying one or more additional coatings in addition to the first coating and the second coating.

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.

Drawings

FIG. 1 is a graph showing that the coating thickness of the coating structure 1 is about 11g/m²Optical micrograph at pinhole limit (pinhole limit). The magnification in the optical micrograph is 400 x.

Fig. 2 is a graph showing the change in adhesion properties of the coated structural body 1 with decreasing coating weight.

Fig. 3 is a graph showing the pinhole property of the coated structural body 1 as a function of the decreased coating weight.

FIG. 4 is a graph showing the coating thickness of the coated structural body 2 at a coating weight of about 10g/m²Optical micrograph at pinhole limit (400 x).

Fig. 5 is a graph showing the change in adhesion property of the coated structural body 2 with a decreased coating weight.

Fig. 6 is a graph showing the pinhole property of the coated structural body 2 as a function of the decreased coating weight.

FIG. 7 is a graph showing the coating thickness of the coating structure 3 at a coating weight of about 5.5g/m²Optical micrograph at pinhole limit (400 x).

Fig. 8 is a graph showing the change in adhesion property of the coating structural body 3 with a decreased coating weight.

Fig. 9 is a graph showing the pinhole property of the coating structure 3 as a function of decreasing coating weight.

FIG. 10 is a graph showing the coating thickness of the coating structure 4 at about 9g/m in coating weight²Optical micrograph at pinhole limit (400 x).

Fig. 11 is a graph showing the change in adhesion property of the coated structural body 4 with a decreased coating weight.

Fig. 12 is a graph showing the pinhole property of the coating structure 4 as a function of the decreased coating weight.

FIG. 13 is a graph showing the coating thickness of the coating structure 5 at a coating weight of about 12g/m²Optical micrograph at pinhole limit (400 x).

Fig. 14 is a graph showing the change in adhesion property of the coating structure 5 with decreasing coating weight.

Fig. 15 is a graph showing the pinhole property of the coating structure 5 as a function of decreasing coating weight.

Examples

The invention will now be explained with the aid of coating structures of five different Low Density Polyethylenes (LDPE) extrusion coated onto the same type of paperboard (Cupforma Natura 195gsm, Stora Enso Oy) using the same extrusion coating equipment and the same optimized process parameter settings. The addition of MDPE/DOWLEX 2062GC with a density of 939kg/m was investigated by blending experiments³) For branched LDPE grade (Borealis CA7230, density 923 kg/m)³) The coating properties of (a). The tensile (Draw down) properties of the PE-based coating structure were evaluated with increasing line speed extrusion coating until the polymer curtain broke. Coating properties were measured as a function of layer thickness (coating weight, i.e. grammage).

A test line configuration with two single screw extruders (1 and 2) and with typical cooling and nip roll arrangements was used in the extrusion coating procedure of examples 1-5 below. A conventional wide tapered ground die (land die) with edge heaters, inner deckles, and encapsulation system was used. The coat weight (gram weight) of the extrusion coated structures was measured according to standard EN ISO 536. Five (5) replicates were performed at each linear velocity. The actual film thickness on the coated paperboard samples was determined on an Axioskop 40 polarizing microscope (Carl Zeiss Light Microcopy, Germany).

The adhesion of the coated polymer layer to the paperboard substrate was evaluated using a manual coating peel evaluation method. The X-pattern is cut in the coated film layer on the substrate and the coated film is then peeled in the machine and cross directions. If the fibers tear from the substrate, the adhesion can be evaluated by determining the amount of fibers torn. The size of the coating surface area (surface area) of the peeled film covered by the torn fibers is a visual measure of the adhesion value. When no fibers are attached to the peeled coating, the coating is not adhered to the substrate, i.e., the adhesion value is one (1). When only a small amount of base fibers covered the surface of the peeled coating, the adhesion value was two (2). When less than 50% of the peeled coating area was covered by the torn base fiber, the adhesion value was three (3). When greater than 50% of the peeled coating area was covered by the torn base fiber, the adhesion value was four (4). When the peeled coating was completely (100%) covered by the torn fibers, the adhesion was five (5). When the coating does not adhere to the substrate at all (i.e., it is released), the adhesion value is zero (0).

The amount of pinholes in the coated structure was measured using the colored turpentine solution penetration method as follows:

pinhole solution composition: 1) turpentine (L-turpentine) as solvent, 2) Sudan III (Sudan G) as red colorant (1%), 3) anhydrous calcium chloride (5%).

-applying a colored turpentine solution on the polymer coated cardboard by brush.

The solution was kept on the surface for 10min, allowed to penetrate through possible pinholes in the coating and dried.

-calculating 100cm of the opposite side of the coated structure²The number of pinholes on the surface area of (a) and labeled as a result.

Three (3) parallel measurements were made, all of which must show no pinholes at the grammage in question, in order to be consistent with no pinholes.

Example 1 LDPE as Top layer and adhesive layer (comparative example)

A coating structure 1 consisting of Low Density Polyethylene (LDPE) as the first coating (1) and the same Low Density Polyethylene (LDPE) as the second coating (2) was co-extrusion coated onto a paperboard (Cupforma Natura 195gsm, Stora Enso Oy) using a fixed process parameter set. The second coating (2) is the top layer in the coated structure.

The minimum coating weight obtainable with the coated structure 1 is 7g/m²(see FIG. 2).

The adhesion strength is perfect (value 5) up to a minimum coating weight of 7g/m²。

Less than about 11g/m²The pinholes begin to appear in the coated structure 1 (see fig. 3). Then, the thicknesses of the first and second coating layers were 6.2 and 4.1 μm, respectively (see fig. 1). The draw ratio at pinhole limit (DDR) was 54.

Example 2 MDPE/LDPE blend as Top layer and LDPE as adhesion layer

A coating structure 2 consisting of a blend of low density polyethylene (LDPE, Borealis CA7230) as the first coating (1) and medium density polyethylene (MDPE, DOWLEX 2062GC) and the same Low Density Polyethylene (LDPE) as the second coating (2) was co-extrusion coated onto a paperboard (Cupforma Natura 195gsm, Stora Enso Oy) using a fixed process parameter set. The blend consisted of 80 wt% LDPE and 20 wt% MDPE. The second coating (2) is the top layer in the coated structure.

The minimum coating weight obtainable with the coated structure 2 is 6.5g/m²(see FIG. 5).

The adhesion strength is perfect (value 5) up to a minimum coating weight of 6.5g/m²。

Less than about 10g/m²The pinholes begin to appear in the coated structure 2 (see fig. 6). Then, the thicknesses of the first and second coating layers were 6.0 and 4.3 μm, respectively (see fig. 4). The draw ratio at pinhole limit (DDR) was 57.

Example 3 LDPE as Top layer and MDPE/LDPE blend as adhesion layer

A coating structure 3 consisting of a blend of medium density polyethylene (MDPE, DOWLEX 2062GC) and low density polyethylene (LDPE, Borealis CA7230) as a first coating (1) and the same Low Density Polyethylene (LDPE) as a second coating (2) was co-extrusion coated onto a paperboard (Cupforma Natura 195gsm, Stora Enso Oy) using a fixed process parameter set. The blend consisted of 80 wt% LDPE and 20 wt% MDPE. The second coating (2) is the top layer in the coated structure.

The minimum coating weight obtainable with the coated structure 2 is 3.8g/m²(see fig. 8).

The adhesion strength is perfect (value 5) up to a minimum coating weight of 3.8g/m²。

Less than about 5.5g/m²The pinholes begin to appear in the coated structure 3 (see fig. 9). Then, the thicknesses of the first and second coating layers were 3.4 and 2.8 μm, respectively (see fig. 7). The draw ratio at pinhole limit (DDR) was 103.

Example 4 MDPE/LDPE blend as Top layer and adhesion layer

A coating structure 4 consisting of a blend of medium density polyethylene (MDPE, DOWLEX 2062GC) and low density polyethylene (LDPE, Borealis CA7230) as a first coating (1) and the same blend of Medium Density Polyethylene (MDPE) and Low Density Polyethylene (LDPE) as a second coating (2) was co-extrusion coated onto a paperboard (Cupforma Natura 195gsm, Stora Enso Oy) using a fixed set of process parameters. The blend consisted of 80 wt% LDPE and 20 wt% MDPE. The second coating (2) is the top layer in the coated structure.

The minimum coating weight obtainable with the coated structure 2 is 4.8g/m²(see FIG. 11).

The adhesion strength is perfect (value 5) up to a minimum coating weight of 4.8g/m²。

Less than about 9.0g/m²The pinholes begin to appear in the coated structure 4 (see fig. 12). Then, the thicknesses of the first and second coating layers were 5.2 and 4.7 μm, respectively (see fig. 10). The draw ratio at pinhole limit (DDR) was 65.

Example 5 LDPE Single layer (comparative example)

A coated structure 5 consisting of low density polyethylene (LDPE, Borealis CA7230) as only a single coating was extrusion coated onto paperboard (Cupforma nature 195gsm, Stora Enso Oy) using a fixed set of process parameters.

The minimum coating weight obtainable with the coated structure 1 is 3.6g/m²(see fig. 14).

The adhesion strength is perfect (value 5) up to a minimum coating weight of 3.6g/m²。

Less than about 12g/m²The pinhole starts to appear in the coated structural body 1 (see fig. 15). Then, the thickness of the first and second coating layers was 11.9 μm (see fig. 13). The draw ratio at pinhole limit (DDR) was 49.