POLYMER COATED PAPER AND PAPERBOARD

Abstract

The present invention relates to a paper or paperboard comprising a polymeric coating, said polymeric coating comprising: a first coating layer attached to the paper or paperboard surface, said first coating layer comprising a blend of: a high density polyethylene (HDPE), medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE), or a mixture thereof, and a low density polyethylene (LDPE); and a second coating layer attached to the first coating layer, said second coating layer consisting essentially of a low density polyethylene (LDPE); wherein the first and second coating layers have a combined grammage of less than 12 g/m.sup.2. The present invention relates to a method for manufacturing a polyethylene (PE) coated paper or paperboard substrate.

Claims

1. A paper or paperboard comprising a polymeric coating, said polymeric coating comprising: a first coating layer attached to the paper or paperboard surface, said first coating layer comprising a blend of: a high density polyethylene (HDPE), medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE), or a mixture thereof, and a low density polyethylene (LDPE); and a second coating layer attached to the first coating layer, said second coating layer consisting essentially of a low density polyethylene (LDPE); wherein the first and second coating layers have a combined grammage of less than 12 g/m.sup.2.

2. The paper or paperboard according to claim 1, said first coating layer consisting essentially of a blend of: 1-49% by weight of a high density polyethylene (HDPE), medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE), or a mixture thereof, and 51-99% by weight of a low density polyethylene (LDPE).

3. The paper or paperboard according to claim 1, wherein the first coating layer is formed by extrusion coating onto the paper or paperboard surface.

4. The paper or paperboard according to claim 1, wherein the second coating layer is formed by extrusion coating onto the first coating layer.

5. The paper or paperboard according to claim 1, wherein the first and second coating layers have a combined grammage of less than 10 g/m.sup.2.

6. The paper or paperboard according to claim 1, wherein the first coating layer has a grammage of less than 5 g/m.sup.2.

7. The paper or paperboard according to claim 1, wherein the second coating layer has a grammage of less than 10 g/m.sup.2.

8. The paper or paperboard according to claim 1, wherein the HDPE has a density in the range of 0.930-0.970 g/cm.sup.3, the MDPE has a density in the range of 0.926-0.940 g/cm.sup.3, the LLDPE has a density in the range of 0.918-0.940 g/cm.sup.3, and the LDPE has a density in the range of 0.910-0.940 g/cm.sup.3.

9. The paper or paperboard according to claim 1, wherein said first coating layer comprises a blend of MDPE and LDPE.

10. The paper or paperboard according to claim 1, wherein the second coating layer has a lower density than the first coating layer.

11. The paper or paperboard according to claim 1, wherein the second coating layer is the top layer of the polymeric coating.

12. The paper or paperboard according to claim 1, wherein said polymeric coating has better adhesion to the paper or paperboard surface than an LDPE coating with the same total grammage.

13. A heat sealed paper or paperboard product comprising the paper or paperboard according to claim 1.

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

15. A method for manufacturing a polyethylene (PE) coated paper or paperboard substrate, the method comprising: a) providing a paper or paperboard substrate, b) applying at least one layer of a molten first polymeric resin to a surface of said paper or paperboard substrate by extrusion coating to form a first polymeric coating layer, said first polymeric resin comprising a blend of: a high density polyethylene (HDPE), medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE), or a mixture thereof, and a low density polyethylene (LDPE), c) applying at least one layer of a molten second polymeric resin to a surface of said first polymeric coating layer by extrusion coating to form a second polymeric coating layer, said second polymeric resin consisting essentially of a low density polyethylene (LDPE), d) allowing the first and second coating layers to cool down and solidify, and e) recovering the PE coated paper or paperboard substrate.

16. The method according to claim 15, wherein the first coating layer consists essentially of a blend of: 1-49% by weight of a high density polyethylene (HDPE), medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE), or a mixture thereof, and 51-99% by weight of a low density polyethylene (LDPE).

17. The method according to claim 15, wherein the first and second coating layers are formed simultaneously by coextrusion coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] FIG. 1 is an optical micrograph illustrating coating layer thickness of the coating structure 1 at the pinhole limit of about 11 g/m.sup.2. Magnification in the optical micrograph is 400×.

[0069] FIG. 2 is a diagram showing adhesion properties of the coating structure 1 as function of the decreasing coating weight.

[0070] FIG. 3 is a diagram showing pinhole properties of the coating structure 1 as function of the decreasing coating weight.

[0071] FIG. 4 is an optical micrograph (400×) illustrating coating layer thickness of the coating structure 2 at the pinhole limit of the coating weight about 10 g/m.sup.2.

[0072] FIG. 5 is a diagram showing adhesion properties of the coating structure 2 as function of the decreasing coating weight.

[0073] FIG. 6 is a diagram showing pinhole properties of the coating structure 2 as function of the decreasing coating weight.

[0074] FIG. 7 is an optical micrograph (400×) illustrating coating layer thickness of the coating structure 3 at the pinhole limit of the coating weight about 5.5 g/m.sup.2.

[0075] FIG. 8 is a diagram showing adhesion properties of the coating structure 3 as function of the decreasing coating weight.

[0076] FIG. 9 is a diagram showing pinhole properties of the coating structure 3 as function of the decreasing coating weight.

[0077] FIG. 10 is an optical micrograph (400×) illustrating coating layer thickness of the coating structure 4 at the pinhole limit of the coating weight about 9 g/m.sup.2.

[0078] FIG. 11 is a diagram showing adhesion properties of the coating structure 4 as function of the decreasing coating weight.

[0079] FIG. 12 is a diagram showing pinhole properties of the coating structure 4 as function of the decreasing coating weight.

[0080] FIG. 13 is an optical micrograph (400×) illustrating coating layer thickness of the coating structure 5 at the pinhole limit of the coating weight about 12 g/m.sup.2.

[0081] FIG. 14 is a diagram showing adhesion properties of the coating structure 5 as function of the decreasing coating weight.

[0082] FIG. 15 is a diagram showing pinhole properties of the coating structure 5 as function of the decreasing coating weight.

EXAMPLES

[0083] The invention will now be explained with the aid of five different low density polyethylene (LDPE) based coating structures, which were extrusion coated onto the same type of paperboard (Cupforma Natura 195 gsm, Stora Enso Oy) using the same extrusion coating equipment and the same optimized processing parameter set-up. The effect of the addition of MDPE/DOWLEX 2062GC, density 939 kg/m.sup.3) on the coating properties of a branched LDPE grade (Borealis CA7230, density 923 kg/m.sup.3) was studied by blending experiments. Draw down properties of the PE based coating structures were assessed extrusion coating with increasing line speed until polymer curtain break-up. Coating properties were measured as a function of layer thickness (coating weight, i.e. grammage).

[0084] A pilot line configuration having two single-screw extruders (1 and 2) and having a typical chill and nip roll arrangement was used in the extrusion coating procedures of examples 1-5 below. A conventional wide taper land die with lip heaters, inner deckles and encapsulation systems was used. The coating weight (grammage) of the extrusion coated structures was measured according to the standard EN ISO 536. Five (5) parallel measurements were done at each line speed. The actual film layer thicknesses on the coated paperboard samples were determined on an Axioskop 40 polarizing microscope (Carl Zeiss Light Microscopy, Germany).

[0085] The adhesion of the coated polymer layer to the paperboard substrate was assessed using the manual coating peeling evaluation method. An X-figure is cut in the coated film layer on the substrate and then the coating film is peeled off in the machine and transverse directions. If fibers are torn from the substrate, then the adhesion can be evaluated by determining the amount of the torn fibers. The size of the coating surface area of the peeled film covered by torn fibers is the visual measure of the adhesion value. When there are no fibers attached on the coating peeled, the coating is not adhered onto the substrate i.e., the adhesion value is one (1). When only few substrate fibers are covering the peeled coating surface, the adhesion value is two (2). When less than 50% of the peeled coating area is covered by torn substrate fibers, the adhesion value is three (3). When more than 50% of the peeled coating area is covered by torn substrate fibers, the adhesion value is four (4). When the peeled coating is totally (100%) covered by the torn fibers, the adhesion is five (5). When the coating is not all adhering to the substrate, i.e. it is loose, the adhesion value is zero (0).

[0086] The amount of pinholes in the coating structures was measured using the colored turpentine oil solution penetration method as follows: [0087] Pinhole solution ingredients: 1) Turpentine oil (L-Turpentine) as a solvent, 2) Sudan III (Sudan G) as a red colorant (1%), 3) Anhydrous calcium chloride (5%). [0088] The colored turpentine oil solution was applied by brush on the polymer coated cardboard. [0089] Solution was kept on the surface for 10 min allowing it to penetrate through possible pinholes in coating and to dry out. [0090] The number of pinholes on a surface area of 100 cm.sup.2 of the opposite side of coated structure were calculated and marked as the result. [0091] Three (3) parallel measurements were made, all of which had to show no pinholes at the gram mage in question to qualify as pinhole free.

Example 1—LDPE as Top and Adhesion Layer (Comparative Example)

[0092] The coating structure 1 consisting of low density polyethylene (LDPE, Borealis CA7230) as the first coating layer (1) and of the same low density polyethylene (LDPE) as the second coating layer (2) was co-extrusion coated onto the paperboard (Cupforma Natura 195 gsm, Stora Enso Oy) using the fixed processing parameter set-up. The second coating layer (2) was the top layer in the coating structure.

[0093] The lowest coating weight obtainable with coating structure 1 was 7 g/m.sup.2 (see FIG. 2).

[0094] The adhesion strength was perfect (the value of 5) down to the lowest coating weight of 7 g/m.sup.2.

[0095] Below a coating weight of about 11 g/m.sup.2 pinholes started to appear in the coating structure 1 (see FIG. 3). The thickness of the first and second coating layer was then 6.2 and 4.1 μm, respectively (see FIG. 1). The draw down ratio (DDR) at the pinhole limit was 54.

Example 2—MDPE/LDPE Blend as Top Layer and LDPE as Adhesion Layer

[0096] The coating structure 2 consisting of low density polyethylene (LDPE, Borealis CA7230) as the first coating layer (1) and of a blend of a medium density polyethylene (MDPE, DOWLEX 2062GC) and the same low density polyethylene (LDPE) as the second coating layer (2) was co-extrusion coated onto the paperboard (Cupforma Natura 195 gsm, Stora Enso Oy) using the fixed processing parameter set-up. The blend consisted of 80% by weight of the LDPE and 20% by weight of the MDPE. The second coating layer (2) was the top layer in the coating structure.

[0097] The lowest coating weight obtainable with coating structure 2 was 6.5 g/m.sup.2 (see FIG. 5).

[0098] The adhesion strength was perfect (the value of 5) down to the lowest coating weight of 6.5 g/m.sup.2.

[0099] Below a coating weight of about 10 g/m.sup.2 pinholes started to appear in the coating structure 2 (see FIG. 6). The thickness of the first and second coating layer was then 6.0 and 4.3 μm, respectively (see FIG. 4). The draw down ratio (DDR) at the pinhole limit was 57.

Example 3—LDPE as Top Layer and MDPE/LDPE Blend as Adhesion Layer

[0100] The coating structure 3 consisting of a blend of medium density polyethylene (MDPE, DOWLEX 2062GC) and low density polyethylene (LDPE, Borealis CA7230) as the first coating layer (1) and of and the same low density polyethylene (LDPE) as the second coating layer (2) was co-extrusion coated onto the paperboard (Cupforma Natura 195 gsm, Stora Enso Oy) using the fixed processing parameter set-up. The blend consisted of 80% by weight of the LDPE and 20% by weight of the MDPE. The second coating layer (2) was the top layer in the coating structure.

[0101] The lowest coating weight obtainable with coating structure 2 was 3.8 g/m.sup.2 (see FIG. 8).

[0102] The adhesion strength was perfect (the value of 5) down to the lowest coating weight of 3.8 g/m.sup.2.

[0103] Below a coating weight of about 5.5 g/m.sup.2 pinholes started to appear in the coating structure 3 (see FIG. 9). The thickness of the first and second coating layer was then 3.4 and 2.8 μm, respectively (see FIG. 7). The draw down ratio (DDR) at the pinhole limit was 103.

Example 4—MDPE/LDPE Blend as Top and Adhesion Layer

[0104] The coating structure 4 consisting of a blend of medium density polyethylene (MDPE, DOWLEX 2062GC) and low density polyethylene (LDPE, Borealis CA7230) as the first coating layer (1) and of and the same blend of medium density polyethylene (MDPE) and low density polyethylene (LDPE) as the second coating layer (2) was co-extrusion coated onto the paperboard (Cupforma Natura 195 gsm, Stora Enso Oy) using the fixed processing parameter set-up. The blend consisted of 80% by weight of the LDPE and 20% by weight of the MDPE. The second coating layer (2) was the top layer in the coating structure.

[0105] The lowest coating weight obtainable with coating structure 2 was 4.8 g/m.sup.2 (see FIG. 11).

[0106] The adhesion strength was perfect (the value of 5) down to the lowest coating weight of 4.8 g/m.sup.2.

[0107] Below a coating weight of about 9.0 g/m.sup.2 pinholes started to appear in the coating structure 4 (see FIG. 12). The thickness of the first and second coating layer was then 5.2 and 4.7 μm, respectively (see FIG. 10). The draw down ratio (DDR) at the pinhole limit was 65.

Example 5—LDPE Monolayer (Comparative Example)

[0108] The coating structure 5 consisting of the low density polyethylene (LDPE, Borealis CA7230) only as a single coating layer was extrusion coated onto the paperboard (Cupforma Natura 195 gsm, Stora Enso Oy) using the fixed processing parameter set-up.

[0109] The lowest coating weight obtainable with coating structure 1 was 3.6 g/m.sup.2 (see FIG. 14).

[0110] The adhesion strength was perfect (the value of 5) down to the lowest coating weight of 3.6 g/m.sup.2.

[0111] Below a coating weight of about 12 g/m.sup.2 pinholes started to appear in the coating structure 1 (see FIG. 15). The thickness of the first and second coating layer was then 11.9 μm (see FIG. 13). The draw down ratio (DDR) at the pinhole limit was 49.