POLYPROPYLENE COATED PAPER AND PAPERBOARD

Abstract

The present invention relates to paper or paperboard comprising cellulose fibers, said paper or paperboard comprising at least one coating layer formed by extrusion coating of a PP (polypropylene) resin, characterized in that the PP resin comprises at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133), and wherein the PP resin is applied to the paper or paperboard at a grammage of less than 30 g/m.sup.2.

Claims

1. A paper or paperboard comprising: cellulose fibers, and at least one coating layer formed by extrusion coating of a PP (polypropylene) resin, wherein the PP resin comprises at least 50% by weight of a branched PP copolymer having a density in a range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in a range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in a range of 10-15 g/10 min (as determined according to ISO 1133), and wherein the PP resin is applied to the paper or paperboard at a grammage of less than 30 g/m.sup.2.

2. The paper or paperboard according to claim 1, wherein said PP copolymer has a density in a range of 895-905 kg/m.sup.3.

3. The paper or paperboard according to claim 1, wherein said PP copolymer has a melting temperature in a range of 158-166° C.

4. The paper or paperboard according to claim 1, wherein said PP copolymer has a melt flow rate (230° C./2.16 kg) in a range of 12-14 g/10 mm.

5. The paper or paperboard according to claim 1, wherein said PP copolymer is a long chain branched polypropylene (LCB-PP).

6. The paper or paperboard according to claim 1, wherein said PP copolymer has been subjected to visbreaking.

7. The paper or paperboard according to claim 1, wherein said PP resin comprises at least 70% by weight of said PP copolymer.

8. The paper or paperboard according to claim 1, wherein the PP resin further comprises a PP having a density outside the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature outside the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) outside the range of 10-15 g/10 min (as determined according to ISO 1133).

9. The paper or paperboard according claim 1, wherein the PP resin is applied to the substrate at a grammage of less than 28 g/m.sup.2.

10. The paper or paperboard according to claim 1, wherein the PP resin comprises at least one additional component selected from a group consisting of: a polymer other than a PP, a pigment, a dye, and a filler.

11. The paper or paperboard according to claim 1, further comprising: at least one additional polymeric coating layer disposed on top of the extrusion coated PP resin, wherein the additional polymeric coating layer has a composition different from the extrusion coated PP resin.

12. The paper or paperboard according to claim 11, wherein said at least one additional coating layer comprises polypropylene (PP).

13. The paper or paperboard according to claim 11, wherein said at least one additional coating layer is formed by extrusion coating or by extrusion film lamination.

14. A heat resistant receptacle comprising: the paper or paperboard according to claim 1.

15. (canceled)

16. (canceled)

17. A method for manufacturing a PP resin coated paper or paperboard substrate comprising cellulose fibers, said method comprising: a) providing paper or paperboard substrate, b) applying at least one layer of a molten PP resin to a surface of said substrate by extrusion coating, c) allowing the molten PP resin to cool down and solidify, and d) recovering the PP resin coated substrate, wherein the PP resin comprises at least 50% by weight of a branched PP copolymer having a density in a range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in a range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in a range of 10-15 g/10 min (as determined according to ISO 1133), and wherein the PP resin is applied to the substrate at a grammage of less than 30 g/m.sup.2.

18. The method according to claim 17, wherein the PP copolymer has a density in a range of 895-905 kg/m.sup.3; or the PP copolymer has a melting temperature in a range of 158-166° C.; or the PP copolymer has a melt flow rate (230° C./2.16 kg) in a range of 12-14 g/10 min; or the PP copolymer is a long chain branched polypropylene (LCB-PP); or, the PP copolymer has been subjected to visbreaking; or the PP resin comprises at least 70% by weight of said PP copolymer; or the PP resin further comprises a PP having a density outside the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature outside the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) outside the range of 10-15 g/10 min as determined according to ISO 1133); or the PP resin is applied to the substrate at a grammage of less than 28 g/m.sup.2; or the PP resin comprises at least one additional component selected from a group consisting of: a polymer other than a PP, a pigment, a dye, and a filler; a combination thereof.

19. The method according to claim 17, further comprising: applying at least one additional polymeric coating layer on top of the extrusion coated PP resin, wherein the additional polymeric coating layer has a composition different from the extrusion coated PP resin.

20. The method according to claim 19, wherein said at least one additional coating layer comprises polypropylene (PP).

21. The method according to claim 19, wherein said at least one additional coating layer is formed by extrusion coating or by extrusion film lamination.

Description

DESCRIPTION OF THE INVENTION

[0011] It an object of the present disclosure to reduce the problem of pinhole formation in extrusion coating of paper and paperboard with PP at low grammages, particularly at grammages of less than 30 g/m.sup.2.

[0012] It is a further object of the present disclosure to reduce the minimum grammage of a PP resin required to achieve a useful coating, particularly in terms of moisture and CO.sub.2 barrier properties, in extrusion coating of paper and paperboard.

[0013] It is a further object of the present disclosure to provide a method for manufacturing PP resin coated paper or paperboard, which allows for reduced grammage of the PP resin, such as a grammage of less than 30 g/m.sup.2, while maintaining good moisture and CO.sub.2 barrier properties.

[0014] The above mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure are achieved by the various aspects of the present disclosure.

[0015] According to a first aspect illustrated herein, there is provided a paper or paperboard comprising cellulose fibers, said paper or paperboard comprising at least one coating layer formed by extrusion coating of a PP (polypropylene) resin, characterized in that the PP resin comprises at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133), and wherein the PP resin is applied to the paper or paperboard at a grammage of less than 30 g/m.sup.2.

[0016] Paper generally refers to a material manufactured in thin sheets from the pulp of wood or other fibrous substances comprising cellulose fibers, used for writing, drawing, or printing on, or as packaging material.

[0017] Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for boxes and other types of packaging. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end use requirements.

[0018] The term coating, as used herein, refers to an operation in which the surface of a substrate is covered with a composition to impart desired properties, finish or texture to the substrate. The coating can be a single or multilayer coating wherein the PP coating resin can be used in one or several layers. The coating can be applied to one side or both sides of the paper or paperboard.

[0019] The problems with pinhole formation in PP-coatings at lower grammages is especially pronounced in the coating of paper and paperboard. The fiber based substrate and its natural voids and surface roughness likely plays a significant role here. PP extrusion coatings for paper or paperboard based microwavable food containers today typically require a coating grammage of at least 30-40 g/m.sup.2, and typically even higher grammages are used. Lower coating grammages result in a large increase of the number of pinholes in the coatings.

[0020] The present inventors have now found that using a specific type of PP resin, the minimum grammage of PP resin required to achieve a pinhole free, or almost pinhole free coating in extrusion coating of paper and paperboard can be significantly reduced.

[0021] Reduction of pinhole formation is very important for product safety in high heat end uses i.e. in microwave ovenable applications, in which pinholes as such possess a risk. The inventive solution allows for adjusting moisture and CO.sub.2 barrier to sufficient and desired level at lower grammages.

[0022] Additionally, the specific type of PP resin used in the present invention has been found to offer further advantages compared to standard extrusion coating grade PP, including improved adhesion to the paper or paperboard substrate at low grammages, better runnability allowing for higher line speed in the extrusion coating process, as well as improved heat sealability.

[0023] The PP resin used in the present invention comprises at least comprises at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133).

[0024] The PP copolymer used in the present invention is a branched PP copolymer. Branching can be achieved by using specific catalysts, i.e. specific single-site catalysts, or by chemical modification. A branched polypropylene obtained by chemical modification is referred to as a high melt strength polypropylene. Preferably the branched PP copolymer of the present invention is obtained by chemical modification and thus is a high melt strength polypropylene (HMS-PP).

[0025] In some embodiments, the PP copolymer is a long chain branched polypropylene (LCB-PP). Long chain branched polypropylene generally refers to polypropylene modified by the addition of long-chain branches. The modification can be achieved by a range of different methods, e.g. by a reactive extrusion process with the addition of a multifunctional monomer and a peroxide.

[0026] In some embodiments, the PP copolymer has a density in the range of 895-905 kg/m.sup.3, preferably in the range of 898-902 kg/m.sup.3.

[0027] In some embodiments, the PP copolymer has a melting temperature in the range of 158-166° C., preferably in the range of 160-164° C.

[0028] In some embodiments, the PP copolymer has a melt flow rate (230° C./2.16 kg) in the range of 12-14 g/10 min, preferably about 13 g/10 min.

[0029] In some embodiments, the PP copolymer has been subjected to visbreaking. Visbreaking means that the molecular weight of the polypropylene is reduced by mixing the molten polymer in the extruder with a free radical generator, such as a peroxide. Visbreaking is well known to the person skilled in the art documented in the literature.

[0030] In a specific embodiment the PP resin used in the present invention comprises at least 50% by weight of a branched PP copolymer having a density in the range of about 900 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of about 162° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of about 13 g/10 min (as determined according to ISO 1133). In a further specific embodiment the PP resin used in the present invention consists, or substantially consists, of a branched PP copolymer having a density in the range of about 900 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of about 162° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of about 13 g/10 min (as determined according to ISO 1133).

[0031] One example of a branched PP copolymer having these preferred properties is Daploy™ SF313HMS (Borealis AG). Daploy™ SF313HMS is a polypropylene copolymer with high melt strength, good heat resistance and excellent heat sealing performance.

[0032] The branched PP copolymer used in the present invention is generally more expensive than conventional extrusion coating grade polypropylenes. However, since the coating grammage can be significantly reduced, the branched PP copolymer can still provide a cost effective alternative.

[0033] The PP resin of the invention comprises at least 50% by weight of the PP copolymer. In some embodiments, the PP resin comprises at least 70% by weight, preferably at least 90% by weight, of the PP copolymer. The PP resin of the invention may also consist, or substantially consist, of the PP copolymer. An embodiment wherein the PP resin consists, or substantially consists, of the PP copolymer is of particular interest since such a monomaterial coating offers obvious recycling advantages.

[0034] The remainder of the PP resin can be made up of other polymeric or non-polymeric coating constituents. In other words, the PP resin can be a blend of the PP copolymer and another polymer. In some embodiments, the remainder of the PP resin is comprised of a PP having density outside the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature outside the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) outside the range of 10-15 g/10 min (as determined according to ISO 1133).

[0035] 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 maintained at an acceptable level. The PP resin used in the present invention allows for the preparation of thin pinhole free PP coatings on paper and paperboard. Particularly, the PP resin used in the present invention allows for the preparation of thin pinhole free PP coatings on paper and paperboard with a coating grammage of less than 30 g/m.sup.2. In some embodiments, the PP resin is applied to the substrate at a grammage of less than 28 g/m.sup.2, preferably less than 26 g/m.sup.2, more preferably less than 24 g/m.sup.2.

[0036] The formulation of a coating resin may vary greatly depending on the intended use of the coating and the coated paper or paperboard. Coating compositions may include a wide range of ingredients in varying quantities to improve the end performance of the product or processing of the coating. In some embodiments, the PP resin comprises at least one additional component selected from the group consisting of a polymer other than a PP, a pigment, a dye, and a filler.

[0037] The paper or paperboard of the present invention may be coated only with the PP resin comprising at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133). Alternatively, the paper or paperboard of the present invention may include the PP resin coating layer as one of two or more polymeric coating layers. As the PP resin of the invention is capable of providing a thin and pinhole free PP coating on paper or paperboard it may preferably be used as the first (or innermost) coating layer applied directly onto the paper or paperboard surface. The extrusion coated PP resin coating layer may then serve to promote adhesion of subsequently applied or coextruded PP coating layers having a composition different from the first PP resin coating layer.

[0038] In some embodiments, the paper or paperboard further comprises at least one additional polymeric coating layer disposed on top of the extrusion coated PP resin, wherein the additional polymeric coating layer has a composition different from the extrusion coated PP resin. In a preferred embodiment, the at least one additional coating layer comprises polypropylene (PP).

[0039] In some embodiments, the at least one additional coating layer is formed by extrusion coating or by extrusion film lamination.

[0040] The inventive paper or paperboard is particularly useful in the manufacture of heat resistant receptacles, such as food containers or trays. According to a second aspect illustrated herein, there is provided a heat resistant receptacle, preferably a food container for microwave heating, comprising paper or paperboard defined as set out above with reference to the first aspect.

[0041] According to a third aspect illustrated herein, there is provided the use of a PP resin comprising at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133), for coating a paper or paperboard substrate comprising cellulose fibers, wherein the PP resin is applied to the substrate at a grammage of less than 30 g/m.sup.2. The present inventors have found that using this specific type of PP resin, the minimum grammage of PP resin required to achieve pinhole free film formation in extrusion coating of paper and paperboard can be significantly reduced.

[0042] The PP resin of the third aspect may be further defined as set out above with reference to the first aspect.

[0043] According to a fourth aspect illustrated herein, there is provided a method for manufacturing a PP resin coated paper or paperboard substrate comprising cellulose fibers, said method comprising: [0044] a) providing paper or paperboard substrate, [0045] b) applying at least one layer of molten PP resin to a surface of said substrate by extrusion coating, [0046] c) allowing the PP resin to cool down and solidify, and [0047] d) recovering the PP resin coated substrate, [0048] characterized in that the PP resin comprises at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133), and [0049] wherein the PP resin is applied to the substrate at a grammage of less than 30 g/m.sup.2.

[0050] The PP resin of the fourth aspect may be further defined as set out above with reference to the first aspect.

[0051] The paper or paperboard of the present invention may be coated only with the PP resin comprising at least 50% by weight of a branched PP copolymer having a density in the range of 890-910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature in the range of 155-170° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) in the range of 10-15 g/10 min (as determined according to ISO 1133). Alternatively, the paper or paperboard of the present invention may include the PP resin coating layer as one of two or more polymeric coating layers. As the PP resin of the invention is capable of providing a thin and pinhole free PP coating on paper or paperboard it may preferably be used as the first (or innermost) coating layer applied directly onto the paper or paperboard surface. The extrusion coated PP resin coating layer may then serve to promote adhesion of subsequently applied or coextruded PP coating layers having a composition different from the first PP resin coating layers.

[0052] In some embodiments, the method further comprises applying at least one additional polymeric coating layer disposed on top of the extrusion coated PP resin, wherein the additional polymeric coating layer has a composition different from the extrusion coated PP resin. In a preferred embodiment, the at least one additional coating layer comprises polypropylene (PP).

[0053] In some embodiments, the at least one additional coating layer is formed by extrusion coating or by extrusion film lamination.

[0054] 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.

Example—Pinhole Formation as a Function of Coating Grammage for Different PP Grades

[0055] Paperboard (Cupforma Natura, Stora Enso Oy) having a grammage of 180-190 g/m.sup.2 was extrusion coated (one sided coating, top extruder temperature 290-300° C.) with a single layer of polypropylene.

[0056] In a first run, the paperboard was coated with a standard extrusion coating grade polypropylene (Std PP). The standard extrusion coating grade polypropylene had a density of 910 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature of 161° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) of 25 g/10 min (as determined according to ISO 1133).

[0057] In a second run, the paperboard was coated with a branched PP copolymer (New PP) having a density of 900 kg/m.sup.3 (as determined according to ISO 1183), a melting temperature of 162° C. (as determined according to ISO 11357-3), and a melt flow rate (230° C./2.16 kg) of 13 g/10 min (as determined according to ISO 1133).

[0058] For each PP coated paperboard with two different coating weights were prepared, a first coating weight of about 30 g/m.sup.2 and a second coating weight of about 20 g/m.sup.2.

[0059] The four PP coated paperboard samples were analyzed for the presence of pinholes in the PP coatings. Pinhole testing was done by spreading a liquid consisting of 99.5% ethanol dyed with Erythrosin B (10 g/5 l ethanol) on the coated surface, allowing the liquid to dry, and counting the pinholes in a defined area using an optical microscope. Results are obtained as number of pinholes per m.sup.2. The results are presented in Table 1. From the measurements, it is evident that using the branched PP copolymer, the pinhole formation can be significantly reduced

TABLE-US-00001 TABLE 1 Target Measured Coating weight Coating weight Pinholes Sample (g/m.sup.2) (g/m.sup.2) (1/m.sup.2) Std PP 30 30.0 170 Std PP 20 22.7 395 New PP 30 31.8 2 New PP 20 21.0 6

[0060] In addition to the reduced pinhole formation it was also noticed that when using the branched PP copolymer instead of the standard extrusion coating grade PP, the line speed of the extrusion coating line could be significantly increased. Using the branched PP copolymer, the line speed could be increased by at least 30% compared to the standard grade while maintaining an acceptable standard deviation of the coating grammage.