High density paper production

Abstract

There is provided a method for producing a high-density paper comprising the steps of: a) providing or producing a paper substrate; b) coating the paper substrate with an aqueous coating composition using a blade coater to obtain a coated paper; c) drying the coated paper to obtain a dried coated paper; and d) supercalendering the dried coated paper at a moisture content of 4-9 %, such as 4-8%.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of paper coatings and in particular to the production of a coated high-density paper.BACKGROUND

[0002] Packaging materials used in food packaging have high demands in regard to barrier properties. These materials often need to provide a barrier against oxygen, moisture and in some cases light.

[0003] Cellulose-based packaging materials are typically provided as laminates in order to provide a packaging material having barrier properties. Paper or board laminated with a polyethylene film renders the paper moisture proof while lamination with an aluminium film renders the paper gas-tight, in particular oxygen tight.

[0004] However, the use of aluminium and polyethylene in paper laminates provide disadvantages such as poor carbon footprint and issues with recyclability.SUMMARY

[0005] There is a desire to provide a paper packaging material that is easily recyclable and has a reduced carbon footprint while having good oxygen barrier properties.

[0006] Accordingly, a method for producing a high-density (HD) paper is provided. The method comprises the steps of: a) providing or producing a paper substrate; b) coating the paper substrate with an aqueous coating composition using a blade coater to obtain a coated paper; c) drying the coated paper to obtain a dried coated paper; and d) supercalendering the dried coated paper at a moisture content of 4-9 %, such as 4-8%.

[0007] The method according to the first present disclosure results in an HD paper having satisfactory oxygen barrier properties. The combination of blade coating and supercalendering at a moisture content of 4-9 % has a synergistic effect on the oxygen barrier properties while the degree of blocking during supercalendering is reduced or even eliminated. In addition, the method according to the present disclosure enables supercalendering at a reduced total nip impulse while still providing the satisfactory oxygen barrier properties. Furthermore, the recyclability of the high-density paper produced according to the present disclosure is improved compared to existing packaging material comprising e.g. aluminium.

[0008] Preferably, both sides of the paper substrate are coated with the aqueous coating composition and dried prior to supercalendering in step d). This is associated with a particularly good oxygen barrier as the aqueous coating composition may penetrate the paper substrate from two sides and thereby more efficiently fill the pores in the paper substrate.

[0009] The aqueous coating composition may comprise polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), starch or starch derivatives. When these polymers form a film, they provide good oxygen barrier properties to the high-density paper. Preferably, the aqueous coating composition comprises PVOH. In another preferred embodiment, the aqueous coating composition comprises starch.

[0010] The aqueous coating composition may comprise a cross-linking agent. The cross-linking agent decreases the solubility of the polymer in the dried coated paper and may thus reduce blocking during supercalendering.

[0011] The aqueous coating composition may have a dry weight of 7-20 wt. %, such as 9-15 wt.%. Further, it may have a coating viscosity of 150-600 mPas, such as 200-600 mPas when measured using spindle 5 at 100 rpm and the temperature at which it is applied (i.e., the temperature that the coating composition has in step b)). This makes the aqueous coating composition particularly suitable for blade coating.

[0012] The coat weight of the aqueous coating on each coated side of the HD paper may be 0.3-4.0 g / m 2< , such as 0.5-3.0 g / m 2< .

[0013] During the supercalendering step, the total nip impulse may be 120-400 kPa*s, such as 130-250 kPa*s and / or the number of nips may be 6-10, such as 8-10.

[0014] The density of the HD paper is preferably at least 900 kg / m 3< , such as at least 950 kg / m 3< as determined according to ISO 534:2011. This density is associated with a high degree of fibre-to-fibre bonding and a reduced the number of pores in the HD paper and thus improved oxygen barrier properties.

[0015] The grammage of the paper substrate may be 30- 70 g / m 2< , such as 35-60 g / m 2< as determined according to ISO 536:2012. This grammage may facilitate a subsequent metallization of the produced HD paper.

[0016] At least one side of the HD paper may have a Bendtsen roughness of less than 80 ml / min as determined according to ISO 8791-2:2013.

[0017] The HD paper may further have an air permeance of less than 0.005 µm / Pas as determined according to ISO 5636-3:2013.

[0018] Preferably, the paper substrate is unsized, i.e., the paper substrate does not comprise hydrophobic sizing agents such as rosin size, alkyl ketene dimer (AKD) or alkyl succinic aldehyde (ASA). This improves the recyclability of the HD paper.DETAILED DESCRIPTION

[0019] The present disclosure provides a production method of an HD paper wherein the method combines blade coating and supercalendering at a moisture content of 4-9 %. This combination has a synergistic effect on oxygen barrier properties while the degree of blocking during supercalendering is reduced or even eliminated. The HD paper produced by the disclosed method has furthermore a good recyclability and a low carbon footprint especially compared to paper-based packaging comprising aluminium.

[0020] The method according to the present disclosure comprises the steps of: a) providing or producing a paper substrate; b) coating the paper substrate with an aqueous coating composition using a blade coater to obtain a coated paper; c) drying the coated paper to obtain a dried coated paper; and d) supercalendering the dried coated paper at a moisture content of 4-9 %, such as 4-8%.

[0021] The paper substate may comprise hardwood and / or softwood, preferably the paper substrate comprises both hardwood and softwood. Preferably, the paper substrate comprises 20-65 % by dry weight, such as 30-60 % by dry weight, hardwood pulp. Preferably, the paper substrate comprises chemical pulp such as kraft pulp, more preferably it comprises at least 50 wt.% by dry weight of chemical pulp. This pulp composition may allow for a paper substrate having particularly good properties to be used in the production of the HD paper.

[0022] The paper substrate is, preferably, unsized, i.e., the paper substrate does not comprise hydrophobic sizing agents such as rosin size, alkyl ketene dimer (AKD) or alkyl succinic aldehyde (ASA). This improves the recyclability of the produced HD paper.

[0023] The grammage of the paper substrate may be 30- 70 g / m 2< , such as 35-60 g / m 2< as determined according to ISO 536:2012.

[0024] The paper substrate is coated with an aqueous coating composition using a blade coater. The blade coater may be a bent blade coater. It may further comprise jet applicators.

[0025] Preferably, both sides of the paper substrate are coated and dried prior to supercalendering in step d). In such case, the sides may be coated with the same aqueous coating composition or different aqueous coating compositions. In the latter case, both aqueous coating compositions preferably comprise the same polymer (e.g. PVOH), but not necessarily in the same amount.

[0026] Coating both sides is associated with a particularly good oxygen barrier as the aqueous coating composition may penetrate the paper substrate from two sides and thereby more efficiently fill the pores present in the paper substrate. Coating both sides of the paper may be performed by first coating a first side of the paper substrate with the aqueous coating composition using a blade coater, followed by drying of the coated paper. After said drying, a second side of the paper may be coated with the aqueous coating composition using a blade coater, followed by drying. The drying after the respective coating steps may be performed using IR driers and / or air driers. Accordingly, in a preferred embodiment, coating is performed in a coating section comprising a first and a second blade coater arranged in series. The coating section may be an in-line coating section, which forms part of a paper machine used to form the paper substrate. An alternative coating procedure may be simultaneously coating the first and second side of the paper with the aqueous coating composition, followed by drying. The drying in this procedure may also comprise IR drying and / or air drying.

[0027] During the coating step, the polymer in the aqueous coating composition preferably penetrates the paper substrate to a substantial degree. However, it is not necessary that the paper substrate, across its thickness direction, has been completely saturated with the polymer. Consequently, the HD paper may comprise unfilled pores, especially in the middle.

[0028] To improve the applicability of the aqueous coating composition by a blade coater and thus further improve the oxygen barrier properties a dry weight of the aqueous coating composition may be 7-20 wt. %, such as 9-15 wt.%. Further, the coating viscosity of the aqueous coating composition may be 150-600 mPas, such as 200-600 mPas, when measured using spindle 5 at 100 rpm and the temperature at which the aqueous coating composition is applied (i.e., the temperature that the aqueous coating composition has in step b)).

[0029] The aqueous coating composition preferably comprises a polymer such as polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxylmethyl cellulose (CMC), starch or starch derivatives, preferably the coating composition comprises PVOH or starch. These polymers comprise polar groups that may decrease the solubility of oxygen.

[0030] Most preferably, the aqueous coating composition comprises PVOH. The PVOH may be medium hydrolyzed or fully hydrolyzed. In one embodiment, the degree of hydrolysis is 96-100 %, such as 97-100 %, such as 97-99 %. A PVOH having a higher degree of hydrolysis is less prone to dissolution in water and is preferred, both in production and in use. The weight average molecular weight (M w ) of the PVOH may be below 100,000 g / mol, such as 10,000-90,000 g / mol, such as 30,000-80,000 g / mol. This molecular weight enables good penetration of the PVOH into the paper substrate while obtaining good oxygen barrier properties of the final HD paper. Furthermore, the PVOH may have a degree of polymerization (DP) of below 3000, such as 1000-2000. The DP can be determined from the viscosity-average degree of polymerization derived from the viscosity of the PVOH in water. In such case, the viscosity may be measured on a 4 wt.% aqueous solution at 20 °C and determined by a Brookfield synchronized-motor rotary type viscometer.

[0031] The viscosity of the PVOH when measured according to DIN 53015 is preferably below 20 mPa*s, such as 5-16 mPa*s, such as 6-13 mPa*s.

[0032] A commercial example of a suitable PVOH is Poval 10 / 98 from Kuraray, having a viscosity of 10 mPa*s according to DIN 530150, a degree of hydrolysis of 98 %, a DP of about 1400 and a M w of about 61,000 g / mol. Another example is Poval 6 / 98 also from Kuraray, having a viscosity of 6 mPa*s and a degree of hydrolysis of 98 %.

[0033] The aqueous coating composition may further comprise a cross-linking agent, such as an aldehyde-functional cross-linking agent. The cross-linking agent is preferably a glyoxal. The cross-linking agent decreases the solubility of the polymer in the dried coated paper and may reduce the degree of blocking or eliminate it during supercalendering. The dry weight ratio of cross-linking agent to PVOH may be between 3:100 and 12:100, preferably between 4:100 and 9:100, more preferably between 5:100 and 8:100.

[0034] In addition, inorganic filler such as calcium carbonate, clay and / or talc may be added to the aqueous coating composition.

[0035] In one embodiment, the maximum amount of inorganic filler in the aqueous coating composition is 30 parts per 100 parts of polymer (by weight).

[0036] The coat weight on each coated side of the HD paper is preferably 0.3-4.0 g / m 2< , such as 0.5-3.0 g / m 2< .

[0037] After the paper substrate has been coated and dried, it is subjected to supercalendering. The supercalendering densifies the coated paper and produces the HD paper. The supercalendering is performed when the coated paper has a moisture content of 4-9 %, preferably 4-8 %. That is the coated paper has a moisture content of 4-9 % at the beginning of the supercalendering. It has been shown in this disclosure that the combination of the blade coating and supercalendering the coated paper at a moisture content of 4-9 % has a synergistic effect on the oxygen barrier properties.

[0038] During the supercalendering step, the total nip impulse of the supercalendering step may be 120-400 kPa*s, such as 130-250 kPa*s and / or the number of nips of the supercalendering step may be 6-10, such as 8-10. The method according to the present disclosure allows for a reduction in total nip impulse and number of nips and thus a reduction in cost and energy consumption while still obtaining good oxygen barrier properties.

[0039] The total nip impulse may be calculated according to Equation (I): Total nip impulse = # nips × line load speed × 60 kPas

[0040] For example, if the line load is 200 kN / m, speed of the supercalender is 709 m / min and number of nips is 9, the total nip impulse is 152 kPas.

[0041] The HD paper obtained by the method of this disclosure preferably has a density of at least 900 kg / m 3< , such as at least 950 kg / m 3< as determined according to ISO 534:2011. This density is associated with a high degree of fibre-to-fibre bonding, a reduced number of pores (i.e., reduced free volume) in the HD paper and thus improved the oxygen barrier properties. An upper limit for the density may be 1300 kg / m 3< , such as 1150 kg / m 3< .

[0042] Furthermore, at least one side of the HD paper may have a Bendtsen roughness of less than 80 ml / min as determined according to ISO 8791-2:2013. Preferably, the Bendtsen roughness is 7-80 ml / min, such as 8-70 ml / min, such as 10-60 ml / min, such as 30-60 ml / min as determined according to ISO 8791-2:2013.

[0043] The HD paper may further have an air permeance of less than 0.005 µm / Pas as determined according to ISO 5636-3:2013.

[0044] The HD paper of the present disclosure may further be laminated with a polymer such as polyethylene or polypropylene.

[0045] In one embodiment, the HD paper has an oxygen transmission rate (OTR) of less than 10 cm 3< / m 2< / 24h when measured at 23°C and 50% RH according to ASTM F1927-14 after lamination with 20 g / m 2< LDPE on the top side of the paper. A lower limit of the OTR may be 5 cm 3< / m 2< / 24h.

[0046] The HD paper produced by the method according to the present disclosure may be used in packaging materials, e.g. for packaging food products or other oxygen-sensitive products. Preferably, the high-density paper is part of a multilayered packaging material.EXAMPLES Example 1

[0047] A paper having a grammage of ~45 g / m 2< and consisting of 60 wt. % softwood fibers and 40 wt.% hardwood fibers is used as the paper substrate in this example. The paper substrate was unsized.

[0048] Three different aqueous coating compositions comprising PVOH were prepared, see Table 1. Poval 10 / 98 from Kuraray was used as the PVOH and Cartabond TSI, a glyoxal, was used as the cross-linking agent. Table 1 shows the different aqueous coating compositions used in example 1.Coating compositionPVOH8X*PVOH10X*PVOH8*PVOH (wt.% based on dry solids)9494100Cross-linking agent (wt.% based on dry solids)66-Solid content of the composition (wt.%)8108Coating viscosity (mPas)100200100*X denotes that the composition comprises a cross-linking agent and the number denotes the solid content of the compositions.

[0049] The PVOH was cold-mixed in water at the desired solid content. The temperature was raised to 95 °C and stirred continuous for one hour. The solution was cooled while maintaining a slow stir to avoid skin formation. The cross-linking agent was added to the PVOH composition, and the composition was stirred to obtain a homogeneous mixture.

[0050] Paper substrates were coated with the different aqueous coating compositions. Both sides of the paper substrate were coated. A first side of the paper substrate was first coated and the coated paper was dried using IR drying and air drying. Following the drying of the first side, the second side was coated with the same aqueous coating composition and dried using IR drying and air drying. A coat weight of 1 g / m 2< on each side was targeted.

[0051] In this example both blade coating and gravure coating were performed, and the results were compared. The different coated papers produced in this example can be seen in Table 2. Aqueous coating compositions PVOH8X and PVOH8 were applied with a gravure coater and PVOH10X was applied with a blade coater. An InvoCoat gravure was used for gravure coating and a Jagenberg Bent Blade with jet applicators was used for blade coating. For the blade coater, a steel blade 0381 / straight was used.

[0052] After the coated papers were dried, they were subjected to supercalendering for densification and for smoothing the surface, see Table 2 for the specifics. Prior to supercalendering some of the coated papers were re-moisturized to a moisture content of 13 %. The coated papers that were not subjected to re-moisturizing had a moisture content of 6 %. The surface temperature of the thermorolls was 100 °C when using 9 nips and 140 °C when using 12 nips. Table 2 shows the specific of the coating and supercalendering procedure.PaperCoaterCoating composition#nips*Speed (m / min)*Line Load (kN / m)*Impulse (kPas)*Moisture content** (%)405GravurePVOH8x1240045081013%408BladePVOH10x1240045081013%410GravurePVOH8x1240045081013%411GravurePVOH81240045081013%412GravurePVOH8x99002541526%413GravurePVOH8x912002541146%414GravurePVOH8x96502542116%415GravurePVOH8x94002543436%416BladePVOH10x99992821526%417BladePVOH10x99002541526%418BladePVOH10x97092001526%419BladePVOH10x95311501536%421GravurePVOH8x990025415213%422GravurePVOH8x990031619013%*During supercalendering. **The moisture content presented is the moisture content at which the coated papers were supercalendered and is the target moisture content.

[0053] The paper properties of the obtained HD papers were determined, see Table 3. Table 3 shows the properties of the HD papers produced in example 1. The oxygen transmission rate (OTR) is measured at 23°C / 50% RH and has the unit cm 3< / m 2< / 24h.IDCoaterMoisture content (%)Grammage (g / m 2< )Density (kg / m 3< )PPS (µm)Bendtsen (ml / min)Permeance (µm / Pas)OTR405Gravure13%46.310652.05109.3408Blade13%46.510471.3440.0057517.8410Gravure13%46.110381.842014.7411Gravure13%47.410561.3410.006511.7412Gravure6%46.29822.873018.2413Gravure6%46.69862.977023.9414Gravure6%46.99972.668026.5415Gravure6%46.19652.765021.5416Blade6%45.79952.7570.00377.8417Blade6%46.010112.75907.5418Blade6%45.510192.64609.1419Blade6%45.89702.77408.9421Gravure13%46.410332.3720.0015516.1422Gravure13%46.110262.151012.9

[0054] The density was measured according to ISO 534:2011 and for the gravure coated HD papers a reduction in density was observed when the moisture content was decreased, see Table 3. A slight decrease in density could be observed for the blade coated HD papers when the moisture content and the total nip impulse was reduced.

[0055] The Bendtsen roughness was measured according to ISO 8791-2:2013 and ranged from 44-74 ml / min for the blade coated high-density papers. For the gravure coated high-density papers the Bendtsen roughness varied from 41-77 ml / min. The Bendtsen roughness was not influenced by the coating technique nor the settings during supercalendering.

[0056] The Parker-Print Surf (PPS) roughness was measured according to ISO 8791-4:2013 increased from 1.3 µm to ~2.7 µm for the blade coated HD papers when reducing the moisture content from 6 % to 12 % simultaneously as reducing the total nip impulse. This indicates that the surface roughness increases when decreasing the total nip impulse and the moisture content when supercalendering.

[0057] The PPS roughness of the gravure coated HD papers varied more than the PPS roughness of the blade coated papers. However, the same trend was observed for both coating techniques.

[0058] The air permeance was measured according to ISO 5636-3:2013. This test was performed on the HD papers without a PE-layer. All the tested HD papers, both blade coated and gravure coated, had a low permeance, see Table 3.

[0059] The Oxygen Transmission Rate (OTR) has the unit cm 3< / m 2< / 24h, 0.2 atm (21%) oxygen. It was measured according to ASTM F1927-14 after lamination with 20 g / m 2< LDPE on the top side of the paper. The OTR was measured at 23 °C and 50 % relative humidity (RH). Comparing the gravure coated HD-paper (paper 405) to the blade coated HD paper (paper 408) at high moisture content and high total nip impulse, it can be observed that the blade coated HD paper has a higher OTR value than the gravure coated HD-paper, 9.3 cm 3< / m 2< / 24h compared to 17.8 cm 3< / m 2< / 24h. When reducing the moisture content and the total nip impulse, a decrease in OTR is observed for the blade coated HD papers (papers 146-149). For the gravure coated HD papers an increase in OTR is observed when decreasing the moisture content and the total nip impulse (papers 412-415). The gravure coated HD papers also show an increase in OTR when the total nip impulse is reduced but the moisture content is high (papers 421-422). This indicates that blade coating and a reduced moisture content in the supercalendering step have a synergistic effect on the OTR.

[0060] It is also noted that the blade coated substrates that were supercalendered at 6% moisture (i.e., papers 416-419) have lower OTR than all other HD papers.

[0061] Furthermore, the repulpability / recyclability of three of the produced HD papers were tested (405 - gravure coated and supercalendered at 13 % moisture content, 408 - blade coated and supercalendered at 13 % moisture content and 417 - blade coated and supercalendered at 6 % moisture content). Repulpability / recyclability was measured according to the method PTS-RH:021 / 97 Category II "PfR that are predominantly used in the manufacture of packaging papers" and all three of the tested HD papers were classified as recyclable with minimum reject, no adhesive impurities and no optical inhomogeneities, see table 4. Table 4 shows the recyclability data provided for papers 405, 408 and 417.ID405408417DisintegratabilityTotal reject0.34 %0.31 %0.26 %Recyclability percentage99.66 %99.69 %99.74 %Sheet formation (from accept)Adhesive impuritiesNoneNoneNoneOptical inhomogeneitiesNoneNoneNoneOverall recyclability ratingRecyclableRecyclableRecyclable

[0062] In conclusion, the HD papers that were blade coated and supercalendered at a moisture content of 6 % (papers 416-419) showed a significant improvement of the oxygen barrier properties while gravure coated papers supercalendered at a moisture content of 6 % showed a deterioration of the oxygen barrier properties. In addition, the results showed that using blade coating, a reduction in total nip impulse is possible without jeopardizing the oxygen barrier properties of the papers. The HD papers according to the present disclosure were also shown to be recyclable (see paper 417 in table 4).

Claims

1. A method for producing a high-density paper comprising the steps of: a) providing or producing a paper substrate; b) coating the paper substrate with an aqueous coating composition using a blade coater to obtain a coated paper; c) drying the coated paper to obtain a dried coated paper; and d) supercalendering the dried coated paper at a moisture content of 4-9 %, such as 4-8%.

2. The method according to claim 1, wherein both sides of the paper substrate are coated with the aqueous coating composition and dried prior to supercalendering in step d).

3. The method according to any one of the preceding claims, wherein the aqueous coating composition comprises polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), starch or starch derivative, preferably the coating composition comprises PVOH.

4. The method according to any one of the preceding claims, wherein the aqueous coating composition comprises a cross-linking agent.

5. The method according to any one of the preceding claims, wherein the dry weight of the aqueous coating composition is 7-20 wt. %, such as 9-15 wt. %.

6. The method according to any one of the preceding claims, wherein the aqueous coating composition has a coating viscosity of 150-600 mPas, such as 200-600 mPas, when measured using spindle 5 at 100 rpm and the temperature at which the aqueous coating composition is applied.

7. The method according to any one of the preceding claims, wherein the coat weight on each coated side of the high-density paper is 0.3-4.0 g / m2, such as 0.5-3.0 g / m2.

8. The method according to any one of the preceding claims, wherein the total nip impulse of the supercalendering step is 120-400 kPa*s, such as 130-250 kPa*s.

9. The method according to any one of the preceding claims, wherein the number of nips of the supercalendering step is 6-10, such as 8-10.

10. The method of any one of the preceding claims, wherein the density of the high-density paper is at least 900 kg / m3, such as at least 950 kg / m3 as determined according to ISO 534:2011.

11. The method of any one of the preceding claims, wherein the grammage according to ISO 536:2012 of the paper substrate is 30- 70 g / m2, such as 35-60 g / m2 as determined according to ISO 536:2012.

12. The method according to any one of the preceding claims, wherein at least one side of the high-density paper has a Bendtsen roughness of less than 80 ml / min as determined according to ISO 8791-2:2013.

13. The method of any one of the preceding claims, wherein the paper substrate is unsized.

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