POLYMER COATED PAPER AND PAPERBOARD

Abstract

The present invention relates to paper or paperboard comprising at least one coating layer formed by extrusion coating of a PET (polyethylene terephthalate) resin, characterized in that the PET resin comprises at least 50% by weight of a PET copolymer having an intrinsic viscosity of less than 0.7 dl/g, preferably less than 0.65 dl/g, as determined according to ISO 1628.

Claims

1. A paper or paperboard comprising: at least one coating layer formed by extrusion coating of a PET (polyethylene terephthalate) resin, wherein the PET resin comprises at least 50% by weight of a PET copolymer having an intrinsic viscosity of less than 0.7 dl/g, as determined according to ISO 1628.

2. The paper or paperboard according to claim 1, wherein said PET copolymer has an intrinsic viscosity in the range of from 0.55 to 0.63 dl/g, as determined according to ISO 1628.

3. The paper or paperboard according to claim 1, wherein said PET copolymer is a PET copolymer comprising repeating units of terephthalic acid, isophthalic acid, monoethylene glycol, and diethylene glycol.

4. The paper or paperboard according to claim 3, wherein a molar ratio of terephthalic acid:isophthalic acid in said PET copolymer is in a range of from 99:1 to 50:50.

5. The paper or paperboard according to claim 3, wherein a molar ratio of monoethylene glycol:diethylene glycol in said PET copolymer is in a range of from 99:1 to 50:50.

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

7. The paper or paperboard according to claim 1, wherein the remainder of the PET resin is comprised of a PET having an intrinsic viscosity 0.7 dl/g or higher, as determined according to ISO 1628.

8. The paper or paperboard according to claim 1, wherein the PET resin is applied to a substrate at a grammage of less than 50 g/m.sup.2.

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

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

11. The paper or paperboard according to claim 10, wherein said at least one additional coating layer comprises polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polylactic acid (PLA), or a combination thereof.

12. The paper or paperboard according to claim 10, wherein said at least one additional coating layer is formed by extrusion coating or by extrusion film lamination.

13. The paper or paperboard according to claim 10, further comprising: at least two additional polymeric coating layers disposed on top of the extrusion coated PET resin, wherein at least one of the at least two additional polymeric coating layers has a composition different from the extrusion coated PET resin, and wherein an outermost layer of the at least two additional polymeric coating layer comprises at least 50% by weight of a PET copolymer having an intrinsic viscosity of less than 0.7 dl/g, as determined according to ISO 1628.

14. An ovenable tray comprising paper or paperboard according to claim 1.

15. A method for coating a paper or paperboard substrate, the method comprising: containing a PET resin comprising at least 50% by weight of a PET copolymer having an intrinsic viscosity (IV) of less than 0.7 dl/g, as determined according to ISO 1628, on a paper or paperboard substrate.

16. The method according to claim 15, wherein the PET copolymer has an intrinsic viscosity in the range of from 0.55 to 0.63 dl/g, as determined according to ISO 1628.

17. A method for manufacturing a PET resin coated paper or paperboard substrate, comprising: a) providing paper or paperboard substrate, b) applying at least one layer of molten PET resin to a surface of said substrate by extrusion coating, c) allowing the PET resin to cool down and solidify, and d) recovering the PET resin coated substrate, wherein the PET resin comprises at least 50% by weight of a PET copolymer having an intrinsic viscosity (IV) of less than 0.7 dl/g, as determined according to ISO 1628.

18. The method according to claim 17, wherein the PET copolymer has an intrinsic viscosity in the range of from 0.55 to 0.63 dl/g, as determined according to ISO 1628.

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

20. The method according to claim 19, wherein said at least one additional coating layer comprises polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polylactic acid (PLA) or a combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] FIG. 1 is a diagram illustrating adhesion to a paperboard substrate as a function of coating grammage of a PET resin according to the invention.

[0078] FIG. 2 contains two diagrams illustrating the runnability for a coating of a PET resin according to the invention.

[0079] FIG. 3 is a diagram illustrating adhesion as a function of the grammage in extrusion coating (monolayer) for different PET grades.

[0080] FIG. 4 is a diagram illustrating grammage as a function of the line speed in extrusion coating (monolayer) for different PET grades.

[0081] FIG. 5 is a diagram illustrating the runnability (the standard deviation in the grammage) as a function of the line speed in extrusion coating (monolayer) for different PET grades.

EXAMPLES

Example 1—Coextrusion Coating of Paperboard with PET Copolymer and Standard PET Resin

[0082] A cardboard substrate (Trayforma 350 Natura, Stora Enso Oy) having a grammage of about 350 g/m.sup.2 was co extrusion coated in an extrusion coating pilot line with a standard PET grade A and a PET copolymer resin B. The PET copolymer resin B was extruded against the cardboard substrate from the extruder B and the plug was BBAAA-DD. Coextrusion was done with screw speeds of 140 and 100 rpm in the extruder A and B, respectively.

[0083] The PET copolymer B was a PET copolymer comprising repeat units from terephthalic acid, isophthalic acid, monoethylene glycol, and diethylene glycol, had an intrinsic viscosity (IV) of 0.59 dl/g, as determined according to ISO 1628. The standard PET grade A had an intrinsic viscosity (IV) of 0.82-0.85 dl/g, as determined according to ISO 1628, a glass transition temperature (T.sub.g) of 75-80° C. and a melting temperature (T.sub.m) of 245-250° C.

[0084] The coextruded substrates (A+B) were analyzed for adhesion, runnability and MAP tightness and compared to corresponding substrates prepared with only the standard PET grade (A).

[0085] Adhesion

[0086] Adhesion testing was done by visual evaluation based on X-figure cutting method and the evaluation criteria in Table 1. Coating was peeled off manually from the fiber substrate.

TABLE-US-00001 TABLE 1 Adhesion testing. Value Evaluation Criteria 0 Coating does not adhere 1 Peeling off without any torn fibers 2 Peeling off with some torn fibers 3 Peeling off with area <50% covered by torn fibers 4 Peeling off with area <50% covered by torn fibers 5 Peeling off with area totally covered by torn fibers (100%)

[0087] The adhesion testing showed that the coextruded substrates maintained an adhesion value of 5 down to a coating grammage of 20 g/m.sup.2 (see FIG. 1) The corresponding substrates prepared with only the standard PET grade A maintained an adhesion value of 5 down to a coating gram mage of 30 g/m.sup.2.

[0088] Runnability

[0089] Runnability was evaluated based on standard deviation of coating grammage in cross- and machine direction. Deviation level below 3 is acceptable.

[0090] The runnability test showed that the coextruded substrates maintained runnability down to a coating grammage of 15 g/m.sup.2 (see FIG. 2) The corresponding substrates prepared with only the standard PET grade A maintained runnability down to a coating grammage of 20 g/m.sup.2.

[0091] MAP-Testing

[0092] The oxygen permeability of the PET coated substrates was evaluated using so called modified atmosphere packaging (MAP). The coated substrates were used in tray packaging, closed tightly with lid. MAP tightness was evaluated based on oxygen level in packaging after 7, 14 and 28 days. An oxygen level of less than 1 after 14 days is considered acceptable. The results are presented in Table 2.

TABLE-US-00002 TABLE 2 MAP-testing. Std PET coated Coextruded substrate substrate (40 g/m.sup.2) (30 g/m.sup.2) Time O.sub.2 amount (%) O.sub.2 amount (%)  7 d 0.45 0.12 14 d 0.93 0.37 28 d 1.93 0.94

[0093] The results in Table 2 show that even with a slightly thinner coating (30 g/m.sup.2 as compared to 40 g/m.sup.2) the MAP tightness of the coextruded substrate is significantly higher than the substrates prepared with only the standard PET grade A. The standard PET grade has a value of 0.93% oxygen after 14 days. This value, although it is below 1%, would not be considered as acceptable, due to measurement uncertainty. The value 0.37% for the coextruded substrate is clearly within the acceptable range, even with the thinner coating.

Example 2—Comparison of Different PET Copolymers

[0094] Five different poly(ethylene terephthalate)s were tested in mono-extrusion coating on the extrusion coating pilot line at Tampere University of Technology (TUT). The tested PET grades are shown in Table 3.

[0095] The PET grades differ in chemical structure and intrinsic viscosity. The web material used was Stora Enso Trayforma 190.

[0096] The adhesion as a function of the grammage in extrusion coating (monolayer, Extruder A, Plug AAAAA-DD, Air gap 130 mm, corona treatment 3.4 kW) for the five PET grades is shown in FIG. 3. The grammage as a function of the line speed in extrusion coating (monolayer) for the five PET grades is shown in FIG. 4. The runnability (the standard deviation in the grammage) as a function of the line speed in extrusion coating (monolayer) for the five PET grades is shown in FIG. 5.

[0097] The extrusion coating parameters were similar in the case of the five PET grades tested (see FIG. 3). The screw speed of the extruder A was 140 rpm resulting in the PET grade dependent throughput of 103-115 kg/h. The target melt temperature was about 300-310° C. (Comparison 1, 2 and 4) or 280° C. (Invention and Comparison 3).

[0098] Clearly the best overall properties were achieved with the low viscosity copolymer made from terephthalic acid, isophthalic acid, mono-ethylene glycol, and diethylene glycol (Invention).

TABLE-US-00003 TABLE 3 Properties of five different poly(ethylene terephthalate) grades tested in extrusion coating. Crystalline Crystallinity Intrinsic density solid state viscosity solid pellets Example Composition [dl/g] [g/cm.sup.3] [%] Comparison 1 Copolymer made 0.82 ± 1.39-1.40 50 ± 5  from terephthalic 0.02 acid, isophthalic acid, and mono-ethylene glycol Comparison 2 Copolymer made 0.84 ± 1.40 ≥50 from terephthalic 0.02 acid and mono- ethylene glycol Invention Copolymer made 0.59 ± >1.37 >30 from terephthalic 0.02 acid, isophthalic acid, mono-ethylene glycol, and diethylene glycol Comparison 3 Copolymer made 0.58 ± 1.39-1.40 50 ± 5  from terephthalic 0.02 acid, isophthalic acid, and mono-ethylene glycol Comparison 4 Copolymer made 0.71 ± 1.39-1.40 50 ± 5  from terephthalic 0.02 acid, isophthalic acid, and mono-ethylene glycol