Stretchable high-density paper

Abstract

A paper having:-a density, measured according to ISO 534: 2011, higher than 1000 kg / m3; -the strain at break in the machine direction of the paper machine, measured according to ISO 1924-3: 2005, is higher than 3%; -a quantification, measured according to ISO 536: 2012, of less than 100 g / m2, such as less than 70 g / m2; and-a Benth surface roughness on at least one side of less than 150 ml / min when measured according to ISO 8791-2: 2013.

Description

Technical Field

[0001] The present disclosure relates to a paper substrate for use in a multi-layer material for packaging oxygen sensitive items, such as food. Background Art

[0002] Paperboard-based packaging laminates for liquid food packaging often include aluminum foil to make the packaging container airtight, in particular oxygen-tight.

[0003] To promote recycling and reduce carbon footprint, it would be beneficial to find alternatives to aluminum foil.

[0004] WO2011 / 003565 discloses a packaging laminate in which the aluminum foil is replaced by a paper or cellulose-based material ("substrate") which is pre-coated and then covered (i.e. metallized) with a metal vapor deposited layer. The basis weight (grammage) of the substrate is preferably 20 to 100 g / m 2 .

[0005] Later patent application WO2017 / 089508 disclosed further details about this substrate, namely that it is a dense surface barrier paper with a density of 800 kg / m 3 or higher, surface roughness value less than 450ml / min (Bendtsen, ISO 8791-2), thickness of 60μm or less, and grammage of 60g / m 2 A specific example of a dense surface barrier paper is "Super Perga WS" (Nordic Paper), which has a grammage of 32 g / m 2 And the surface roughness value is about 200ml / min.

[0006] EP3168362 discloses a paper for vertical form fill seal (VFFS). The aim is to provide a paper that will not break in the VFFS process and has satisfactory printing properties. Suitable flexibility is required, and the density is preferably 820-900 kg / m 3 In order to provide printing properties, the surface roughness is preferably 200-300 ml / min.

[0007] EP4036305 discloses a method for producing paper having high bending resistance and smoothness without increasing density too much, which method is not calendered to a certain density but adds cationic glyoxalated polyacrylamide (G-PAM) to the pulp.

[0008] EP3385445 discloses a highly stretchable paper having a stretchability of more than 9% in the machine direction, which is produced by micro-creping the paper in the machine direction. The micro-creping produces wrinkles in the paper, so that the obtained Bendtsen surface roughness is 460 ml / min. Summary of the invention

[0009] One object of the present disclosure is to provide a new cellulose-based substrate that improves the barrier properties of non-aluminum foil packaging. Another object is to provide a new cellulose-based substrate that facilitates the recycling of packaging materials containing the substrate. The ultimate overall purpose is to reduce the impact of packaging materials on the environment.

[0010] Therefore, the paper provided has:

[0011] - Density measured according to ISO 534:2011 higher than 1000 kg / m 3 ;

[0012] - a strain at break in the machine direction measured according to ISO 1924-3:2005 higher than 3%;

[0013] - Basis weight less than 100 g / m2 measured according to ISO 536:2012 2 , such as less than 70g / m 2 ;and

[0014] - The Bendtsen surface roughness on at least one side is less than 150 ml / min when measured according to ISO 8791-2:2013.

[0015] As shown in Table 1 below, such paper exhibits a high degree of recyclability. Additionally, Table 2 shows that the paper improves barrier properties for non-aluminum foil packaging applications.

[0016] In addition, a method for producing high-density paper is provided, wherein the high-density paper has a density higher than 1000 kg / m 3 , the breaking strain in the longitudinal direction of the paper machine is higher than 3%, and the basis weight is lower than 100g / m 2 , such as less than 70g / m 2 , the method comprises the following steps:

[0017] - forming a paper web;

[0018] - drying the paper web in a drying section to obtain paper, the drying section comprising a telescopic unit which compacts the paper web in the longitudinal direction of the paper machine; and

[0019] - Supercalendering said paper to obtain said high density paper. DETAILED DESCRIPTION

[0020] As a first aspect of the present disclosure, there is provided a paper having:

[0021] - Density higher than 1000kg / m 3 ;

[0022] - The breaking strain in the machine direction is higher than 3%;

[0023] - Basis weight less than 100g / m 2 ;and

[0024] - The Bendtsen surface roughness on at least one side is less than 150 ml / min.

[0025] This density is preferably obtained by supercalendering. Therefore, the paper of the present disclosure is preferably supercalendered. In an embodiment, the density is at least 1050 kg / m 3 , such as at least 1070 kg / m 3 The typical upper limit of density may be 1300 kg / m 3 In the present disclosure, density is measured according to ISO 534:2011.

[0026] The strain at break in the machine direction is preferably in the range of 3.5%-9.0%, such as 4.0%-8.0%. The strain at break in the cross direction is preferably higher than 5.0%, such as 5.5%-8.5%. In the present disclosure, the strain at break is measured according to ISO 1924-3:2005.

[0027] The basis weight of the paper is preferably less than 70 g / m 2 , such as 66g / m 2 or lower, such as 30-66g / m 2 , such as 35-66g / m 2 If the basis weight is too high, the paper may become too stiff to be effectively used as a barrier substrate for certain packaging materials. However, if the basis weight is too low, the strength and toughness of the paper may be insufficient. In this disclosure, the basis weight is measured according to ISO 536:2012.

[0028] The cellulose fibers preferably constitute at least 90% of the dry weight of the paper.

[0029] In one embodiment, the Bendtsen surface roughness on at least one side of the paper is less than 100 ml / min, such as less than 70 ml / min. The lower limit may be, for example, 10 ml / min. If the surface of the paper is too rough, it may be difficult to form a continuous coating thereon. In the present disclosure, the Bendtsen surface roughness is measured according to ISO 8791-2:2013.

[0030] In one embodiment, at least one side of the paper, such as both sides, is coated or impregnated with a polymer. The polymer is preferably selected from PVOH, EVOH, starch and carboxymethyl cellulose (CMC). Other examples are nanocrystalline cellulose (NCC) and carboxymethyl starch (CMS). More preferably, the polymer is PVOH or EVOH.

[0031] The most preferred polymer in this group is PVOH. The degree of hydrolysis of PVOH can be 96%-100%, such as 97%-100%, such as 97%-99%. PVOH with a high degree of hydrolysis is less sensitive to water and is preferred in production and use. The weight average molecular weight (M) of PVOH is w ) is preferably less than 100,000 g / mol, such as 10,000-90,000 g / mol, such as 30,000-80,000 g / mol. Such relatively low M w It is preferred in the impregnation process because it has a relatively low viscosity at a relatively high concentration. w The PVOH is more likely to penetrate into the fiber web rather than stay on the paper surface.

[0032] The viscosity of the PVOH is preferably below 20 mPa*s, such as 5-16 mPa*s, such as 6-13 mPa*s, when measured according to DIN 53015.

[0033] The degree of polymerization (DP) of PVOH is preferably below 3000, such as 1000-2000. DP can be determined from the viscosity average degree of polymerization derived from the viscosity in water. In this case, the viscosity is measured in a 4% aqueous solution at 20°C and determined by a Brookfield synchronous motor rotational viscometer.

[0034] An example of a suitable PVOH is Poval 10 / 98 from Kuraray, which has a viscosity of 10 mPa*s, a degree of hydrolysis of 98%, a DP of about 1400 and an M of about 61,000 g / mol. w Another example is Poval 6 / 98 from Kuraray, which has a viscosity of 6 mPa*s and a degree of hydrolysis of 98%.

[0035] PVOH or EVOH may contain a crosslinking agent such as glyoxal. The dry weight ratio of glyoxal 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.

[0036] In some applications, it may be beneficial to choose EVOH as the polymer. EVOH has high moisture resistance and excellent oxygen barrier properties. An example of EVOH is AQ-4104, which has low viscosity.

[0037] As mentioned above, the polymer may also be NCC, which is a form of nanocellulose, but is distinct from microfibrillar cellulose ("MFC") or nanofibrillar cellulose ("NFC").

[0038] NCC may be defined according to TAPPI draft specification WI3021.

[0039] The term "NCC" is used for shorter particles and "rod-like" particles, which have a width of 3-50 nm and a length of 100 up to 1000 nm, such as 100 to 900 nm, such as 100 to 500 nm, such as 100 to 200 nm. For the purpose of impregnation and filling the pores in the forming paper, the preferred size of the NCC may be a length of 100-500 nm, such as 100-200 nm and a width of 3-50 nm, which means that most of the NCC particles in the composition should have this size.

[0040] In an embodiment, the paper is formed from at least 50% by dry weight of chemical pulp, such as at least 75% by dry weight of chemical pulp, such as at least 85% by dry weight of chemical pulp, such as at least 95% by dry weight of chemical pulp. The chemical pulp is preferably kraft pulp, but may also be sulfite pulp (i.e. pulp obtained by a sulfite pulping process). Thus, the paper may be kraft paper.

[0041] In an embodiment, 20-65% by dry weight, such as 30-60% by dry weight, of the pulp used to form the paper is hardwood pulp. In an alternative or supplementary embodiment, 35-80% by dry weight, such as 40-70% by dry weight, of the pulp used to form the paper is softwood pulp. The benefit of including hardwood pulp is that it collapses relatively easily during the pulping process while still allowing effective dewatering in the wire section of the paper machine. The benefit of including softwood pulp is improved runnability of the paper machine and beneficial strength / toughness properties of the resulting paper. The latter properties can be improved by subjecting the softwood pulp to high consistency (HC) pulping. HC pulping can also increase the breaking strain value. HC pulping refers to pulping at a consistency of 20%-40%, preferably 25%-38%.

[0042] In an embodiment, the paper has a top sheet layer and a bottom sheet layer. In such a configuration, the properties of the top sheet layer can be tailored to receive another barrier layer, while the properties of the bottom sheet layer are tailored for strength / toughness. Alternatively, the top sheet layer can be tailored for printing, while the bottom sheet layer is coated with one or more additional layers.

[0043] In an embodiment the topsheet layer is formed from at least 50% by dry weight hardwood pulp, such as at least 65% by dry weight hardwood pulp, such as at least 75% by dry weight hardwood pulp.Hardwood pulp may provide an improved surface for printing or another barrier layer.

[0044] In an embodiment, the backsheet layer is formed of at least 50% softwood pulp by dry weight, such as at least 65% softwood pulp by dry weight, such as at least 75% softwood pulp by dry weight. As mentioned above, softwood is associated with better runnability in paper machines and provides beneficial strength / toughness properties to the resulting paper product.

[0045] In one embodiment, the backsheet layer side is impregnated with a greater amount of polymer than the topsheet layer.

[0046] Preferably, beneficial barrier and recycling properties are obtained without sacrificing strength, such as tensile strength.

[0047] The tensile index measured in the MD of the paper according to ISO 1924-3:2005 is preferably higher than 90 Nm / g, such as 90-150 Nm / g. The tensile index measured in the CD according to ISO 1924-3:2005 is preferably higher than 50 Nm / g, such as 55-90 Nm / g, such as 60-90 Nm / g.

[0048] Preferably, the density of the paper is obtained without adding any inorganic filler to the paper stock in significant amounts. Silica or bentonite used as retention aids (usually in amounts of less than 1 kg per ton of dry pulp) are not considered as inorganic fillers. Thus, the ash content of the paper measured according to ISO 2144:2015 is preferably less than 5% by dry weight, such as less than 3% by dry weight, such as less than 1% by dry weight.

[0049] The paper of the present disclosure can be obtained without extensive low-consistency (LC) beating, which improves the speed of the papermaking process (and reduces its energy consumption) and facilitates recycling. This relatively limited beating can be reflected by the drainability measured after repulping. In an embodiment, after repulping according to ISO 5263-1:2004, the paper exhibits a Schopper-Riegler (°SR) number measured according to ISO 5267-1:1999 of 30-50, such as 33-45. Another drainability value is the Canadian Standard Freeness (CSF). In an embodiment, after repulping in a Valmet repulper of HD400 according to the Valmet repulping method, the paper exhibits a CSF measured according to ISO 5267-2:2001 of at least 200 ml, such as 200-450 ml, such as 200-350 ml. The Valmet repulping method is described in the Examples section below.

[0050] As can be seen from the above, in a preferred embodiment of the first aspect, the paper has the following characteristics:

[0051] - Density higher than 1000kg / m 3 , but not higher than 1300kg / m 3 ,

[0052] - The breaking strain in the longitudinal direction of the paper machine is 3.5%-9.0%;

[0053] - Basis weight 30-66g / m 2 ;and

[0054] - The Bendtsen surface roughness on at least one side is lower than 150 ml / min, and optionally not lower than 10 ml / min.

[0055] As a second aspect of the present disclosure, there is provided a method for producing high-density paper having a density higher than 1000 kg / m 3 , the breaking strain in the longitudinal direction of the paper machine is higher than 3%, and the basis weight is lower than 100g / m 2 , such as less than 70g / m 2 , the method comprises the following steps:

[0056] - forming a paper web;

[0057] - drying the paper web in a drying section to obtain paper, the drying section comprising a telescopic unit which compacts the paper web in the longitudinal direction of the paper machine; and

[0058] - Supercalendering said paper to obtain said high density paper.

[0059] In an alternative configuration of the second aspect, no retractable unit is employed. Instead, the strain at break value is obtained by negative stretching in the dryer section.

[0060] Preferably, the web is dried in drying sections upstream and downstream of the telescopic unit.The moisture content of the web in the telescopic unit is preferably in the range of 25%-40%, such as 30%-40%, such as 30%-38%.

[0061] The expandable unit is preferably an Expanda unit or a Clupak unit. Such units are known to the skilled person.

[0062] In one embodiment, the method further comprises a step of coating or impregnating at least one side, such as both sides, of the paper with a composition comprising a polymer, said coating or impregnation step being performed between the drying step and the supercalendering step.

[0063] Suitable examples of polymers are described above in conjunction with the first aspect.

[0064] In one embodiment, the impregnation step comprises adding an aqueous composition comprising a polymer to one or both sides of the paper. The viscosity of the aqueous composition measured at 60° C. may be 55-90 mPa*s. This relatively low viscosity facilitates the penetration of the polymer into the fiber web. The concentration of the polymer in the aqueous composition is preferably 7.0%-13.0% (w / v), such as 8.0%-12.0% (w / v).

[0065] The 60°C viscosity measurement is preferably conducted using a Brookfield rotational viscometer equipped with a No. 3 spindle at 100 rpm.

[0066] In one embodiment, the polymer is PVOH or EVOH. In such an embodiment, the aqueous composition may further include a crosslinking agent, such as a glyoxal crosslinking agent. The dry weight ratio of PVOH or EVOH to the glyoxal crosslinking agent in the aqueous composition may be 100:3 to 100:8, such as 100:4 to 100:7.

[0067] The aqueous composition may further comprise inorganic particles, preferably in a small amount.

[0068] To promote densification and possibly also impregnation, the impregnated paper entering the supercalendering step preferably has a relatively high moisture content, such as 11.0%-20.0%. The moisture content may for example be 12.0%-19.0%, such as 13.5%-18.0%.

[0069] In one embodiment, the impregnated paper is dried after the impregnation step to a moisture content of less than 11%, such as less than 10%, such as less than 9%. It is then re-moistened before the supercalendering step, for example to a moisture content in the range of 11.0%-20.0%, 12.0%-19.5% or 13.5%-18.0%.

[0070] The impregnation step is preferably carried out with the aid of a size press or a film press. A film press is the most preferred equipment. The film press can be an OptiSizer Film (Valmet) or a SpeedSizer (Voith).

[0071] The number of nips in the super calendering step may be 7 to 19, preferably 11 to 17. The surface temperature of the heated rolls in the super calendering step may be 120 to 160°C.

[0072] The total nip pulse of the supercalendering step may be at least 600 kPa*s.

[0073] In an embodiment of the second aspect, the headbox concentration or the headbox concentrations in the case of multi-ply paper are in the range of 0.06%-0.60%, such as 0.10%-0.40%, such as 0.10%-0.30%. This relatively low concentration is conducive to the production of low porosity paper, which indirectly means high density.

[0074] In one embodiment, the paper web of the second aspect is formed by at least 50% by dry weight of chemical pulp, such as at least 75% by dry weight of chemical pulp, such as at least 85% by dry weight of chemical pulp, such as at least 95% by dry weight of chemical pulp. The chemical pulp is preferably kraft pulp, but may also be sulfite pulp.

[0075] In one embodiment the web of the second aspect is formed from 20-65% by dry weight hardwood pulp, such as 30-60% by dry weight hardwood pulp and / or 35-80% by dry weight softwood pulp, such as 40-70% by dry weight softwood pulp.

[0076] When softwood pulp is used, it may have been subjected to high consistency (HC) beating, i.e. beating at a consistency of 20%-40%, such as 25%-38%. The specific energy of the HC beating step may be at least 100 kWh / tonne, such as at least 150 kWh / tonne, such as 150-300 kWh / tonne. The "tonne" in the unit refers to tons of dry fiber.

[0077] The effects of one or more pulp selections are described above in conjunction with the first aspect.

[0078] In one embodiment, the paper of the second aspect has a first sheet and a second sheet. A first forming wire can be used to form a first web material that becomes the first sheet (top sheet), and a second forming wire can be used to form a second web material that becomes the second sheet (bottom sheet), and the first web material and the second web material are pressed together.

[0079] The first web may be formed from a first stock comprising at least 50% by dry weight hardwood pulp, such as at least 65% by dry weight hardwood pulp, such as at least 75% by dry weight hardwood pulp. The headbox consistency of the first stock may be 0.12%-0.60%, such as 0.18%-0.35%.

[0080] The first stock in the headbox may have a Schober-Rieger (°SR) number measured according to ISO 5267-1: 1999 of 33-50, such as 40-50. Such SR numbers may promote sufficiently high density without causing dewatering and / or recovery problems and may be obtained by adjusting the degree of low consistency (LC) beating.

[0081] The second web may be formed from a second stock comprising at least 50% by dry weight of softwood pulp, such as at least 65% by dry weight of softwood pulp, such as at least 75% by dry weight of softwood pulp. The softwood pulp has preferably been subjected to high consistency (HC) beating (suitable specific energies are discussed above). The headbox consistency of the second stock may be 0.06%-0.40%, such as 0.10%-0.25%.

[0082] In one embodiment, the headbox consistency of the second stock is lower than the headbox consistency of the first stock.

[0083] The second stock in the headbox may have a Schober-Rieger (°SR) number measured according to ISO 5267-1: 1999 of 23 to 35. Such an SR number may promote sufficiently high density without causing dewatering and / or recovery problems and may be obtained by adjusting the degree of low consistency (LC) beating.

[0084] Preferably, the paper stock comprises less than 2% by dry weight of inorganic filler, such as less than 1% by dry weight of inorganic filler, such as being substantially free of inorganic filler.

[0085] In one embodiment, the second ply side is impregnated with a greater amount of polymer than the first ply.

[0086] In addition, the embodiments of the first aspect discussed above apply mutatis mutandis to the second aspect.

[0087] As demonstrated in the Examples section below, the paper of the first aspect is an excellent substrate for coatings, especially oxygen barrier coatings. As a third aspect of the present disclosure, a coated paper is provided, which comprises the paper according to the first aspect, wherein the surface of the paper is provided with a barrier coating, for example comprising polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), starch or starch derivatives, nano- or microfibrillar cellulose, polyvinylidene chloride (PVDC) or polyamide. The preferred barrier coating comprises PVOH and / or EVOH, for example in an amount of 1-3 g / m 2 .

[0088] In one embodiment, the coated paper of the third aspect comprises several barrier layers, for example comprises a water vapor barrier layer.

[0089] The coated paper of the third aspect can be used for packaging oxygen-sensitive products, such as dry and / or fatty foods. Examples of fatty foods are cheese, butter and spreads. Such packaging can be a flow-wrap packaging or a forming, filling, sealing (FFS) packaging, for example in a bag. It can also be packaged in a jar, a tray, a covered smear container, a foldable tube, a clamshell packaging, a sleeve, an envelope or a wrapping paper. Another application is to be used as a packaging window. In these applications, the packaging material is usually subjected to folding or similar types of stress (e.g., wrinkling, stretching), which makes the packaging material based on the paper of the present disclosure particularly suitable.

[0090] As a fourth aspect of the present disclosure, there is provided a use of a multilayer material for packaging a food product or other oxygen sensitive product, wherein one layer of the multilayer material is a paper according to the first aspect, with the proviso that the food product is not a liquid, semi-liquid or viscous food product. In the context of the present disclosure, "liquid food" includes water.

[0091] The application of the fourth aspect is discussed above in conjunction with the third aspect.

[0092] The multilayer material of the fourth aspect may comprise a coated paper according to the third aspect.

[0093] Example

[0094] Embodiment 1 (present invention)

[0095] 1A: Paper substrate production

[0096] Two types of pulp were provided: i) ECF bleached kraft pulp from softwood, namely a mixture of pine and spruce; and ii) ECF bleached kraft pulp from hardwood, namely birch.

[0097] The softwood pulp was beaten using a high consistency (HC) beater at a specific energy of 200 kWh / ton. The HC beaten pulp was then mixed in a mixing chest with broke pulp, which consisted of a blend of bleached softwood pulp and hardwood pulp (most of the broke was obtained from the same paper production). The broke proportion in this softwood-based mixture was 10%. The softwood-based mixture was then beaten by low consistency (LC) beating at a specific energy of 80-85 kWh / ton. This LC beating resulted in a Schober-Rigler (°SR) of ~26°SR according to ISO 5267-1:1999.

[0098] Hardwood pulp was mixed with the same type of broke separately and then beaten by low consistency beating using a specific energy of 90 kWh / ton. The broke proportion in the hardwood based mixture was 20%. The Schober-Riegle (°SR) value obtained for the LC beaten hardwood based mixture was ˜44°SR.

[0099] Papermaking chemicals (4 kg / ton cationic starch, 0.2 kg / ton silica, 1.5 kg / ton rosin size and 2.2 kg / ton alum) were added to each of the two fiber streams. The softwood-based mixture was pumped to the bottom sheet headbox of a two-sheet Fourdrinier machine, while the hardwood-based mixture was pumped to the top sheet headbox of the same Fourdrinier machine. The dry mass flow through each headbox was similar and was adjusted to achieve a total grammage (before coating) of ~62 g / m 2 , divided into the bottom layer (~32g / m 2 ) and top sheet layer (~30g / m 2 ). The vertical slicing lip of the headbox for the backsheet layer is 33 mm and that for the topsheet layer is 18.5 mm, reflecting the relatively low headbox consistency (about 0.14% for the backsheet layer and about 0.21% for the topsheet layer). The forming wire speed is 608 m / min. In paper machines specifically adapted for this product, the forming wire speed can be quite high.

[0100] The two sheets formed on the Fourdrinier machine are pressed together at ~10% dryness and further dewatered to ~20% dryness using a vacuum suction box and then wet pressed in a press section with two single felt press zones, where the first press has the felt on the top side and the second press has the felt on the bottom side.

[0101] After wet pressing, the web is dried in the pre-drying section to a dryness level of ~65% and then enters the expandable unit. The expandable unit is a double roll compactor type, commonly called an "Expanda", which operates with an inlet web speed of ~638 m / min and an outlet web speed of ~581 m / min. As a result, the paper web is compacted in the machine direction (the web becomes "shorter" by about 9%), which increases the breaking strain value ("stretchability") of the final paper. The drive group after the expandable unit operates at a slightly increased speed, resulting in a decrease in the MD breaking strain value. Immediately after the expandable unit, the paper web is dried in the post-drying section using conventional steam heated cylinders to form a paper substrate with a moisture content of ~6%.

[0102] 1B: Dipping

[0103] The paper substrate from 1A was impregnated offline from both sides with an aqueous polyvinyl alcohol (PVOH) composition in a conventional film press. The type of PVOH was Poval 10 / 98 (Kuraray), and its concentration in the composition was 10% (in another test, the concentration was changed to 8%, which also worked). The composition further contained glyoxal (Cartabond TSI) in an amount of 6 wt.% compared to the amount of PVOH. Glyoxal served as a crosslinking agent. The viscosity of the composition was 74 mPa*s (measured at 60°C). The amount of PVOH applied was 1 g / m on the top side. 2 , and 2g / m on the reverse / bottom side 2 The reason for using a higher amount of PVOH on the back / bottom side is that the pulp used to form the backsheet layer has a lower SR number (thus, the back / bottom side has a lower surface density compared to the top side). The PVOH impregnated paper substrate was dried to about 8% moisture using hot air.

[0104] 1C: Super calendering

[0105] The impregnated paper substrate from 1B was remoistened to 15%. The remoistened paper was fed into an off-line multi-nip calender, also called super calender (number of nips was 12). Super calendering was performed on the hot rolls using a surface temperature of 140°C, which could be obtained with the aid of an external induction heater, to obtain a high density paper. The line load in each nip was 360 kN / m (simulating 400 kN / m in full-scale production) and the speed was 360 m / min (simulating 400 m / min in full-scale production). The total super calendering nip pulse was ~720 kPa·s [number of nips × line load / web speed]. The heat from the hot rolls dried the high density paper. The moisture content at the time of winding was ~5.5%. The properties of the high density paper are listed in Table 1 below.

[0106] Example 2 (Comparative Example)

[0107] This comparative example is not disclosed prior art, but is part of a co-pending patent application.

[0108] 2A: Paper substrate production

[0109] Two types of pulp were provided: i) ECF bleached kraft pulp from softwood, namely a mixture of pine and spruce; and ii) ECF bleached kraft pulp from hardwood, namely birch.

[0110] The softwood pulp was beaten using a high consistency (HC) beater at a specific energy of 225 kWh / ton. The HC beaten pulp was then mixed in a mixing chest with broke pulp, which consisted of a blend of bleached softwood pulp and hardwood pulp (most of the broke was obtained from the same paper production). The broke proportion in this softwood-based mixture was 30%. The softwood-based mixture was then beaten by low consistency (LC) beating at a specific energy of 75 kWh / ton. This LC beating resulted in a Schober-Rigler (°SR) of ~30°SR according to ISO 5267-1:1999.

[0111] Hardwood pulp was mixed separately with the same type of broke and then beaten by low consistency beating using a specific energy of 85 kWh / ton. The broke proportion in the hardwood based mixture was 20%. The Schober-Riegle (°SR) value obtained for the LC beaten hardwood based mixture was ˜38°SR.

[0112] Papermaking chemicals (4 kg / ton cationic starch, 0.2 kg / ton silica and 0.4 kg / ton AKD) were added to each of the two fiber streams. The softwood-based mixture was pumped to the bottom sheet headbox of a two-layer Fourdrinier machine, while the hardwood-based mixture was pumped to the top sheet headbox of the same Fourdrinier machine. The dry mass flow through each headbox was the same and was adjusted to achieve a total grammage of 60 g / m before coating. 2 (i.e. 30g / m per layer 2 ). The vertical slicing lip of the headbox for the backsheet layer is 34 mm and that for the topsheet layer is 16 mm, reflecting the relatively low headbox consistency (about 0.12% for the backsheet layer and about 0.25% for the topsheet layer). The forming wire speed is 600 m / min. In paper machines specifically adapted for this product, the forming wire speed can be quite high.

[0113] The two sheets formed on the Fourdrinier machine are pressed together at ~10% dryness and further dewatered to ~20% dryness using a vacuum suction box and then wet pressed in a press section with two single felt press zones, where the first press has the felt on the top side and the second press has the felt on the bottom side.

[0114] After wet pressing, the web was dried in a conventional multi-drum dryer to form a paper substrate with a moisture content of ~5%.Before winding, the paper substrate was calendered in a soft nip at a line load of 20 kN / m.

[0115] 2B: Dipping

[0116] The paper substrate from 2A was impregnated offline from both sides with an aqueous polyvinyl alcohol (PVOH) composition in a conventional film press. The type of PVOH was Poval 10 / 98 (Kuraray), and its concentration in the composition was 10% (in another test, the concentration was changed to 8%, which also worked). The composition further contained glyoxal (Cartabond TSI) in an amount of 6 wt.% compared to the amount of PVOH. Glyoxal acts as a crosslinking agent. The viscosity of the composition was 74 mPa*s (measured at 60°C). The amount of PVOH applied on the top side was 1 g / m 2 , and 2g / m on the reverse / bottom side 2 The reason for using a higher amount of PVOH on the back / bottom side is that the pulp used to form the backsheet layer has a lower SR number (thus, the back / bottom side has a lower surface density compared to the top side). The PVOH impregnated paper substrate was dried to about 8% moisture using hot air.

[0117] 2C: Super calendering

[0118] The impregnated paper substrate from 2B was remoistened to 15%. The remoistened paper was fed to an off-line multi-nip calender, also called super calender (number of nips was 12). Super calendering was performed on the hot rolls using a surface temperature of 140°C, which could be obtained with the aid of an external induction heater, to obtain a high density paper. The line load in each nip was 405 kN / m (simulating 450 m / min in full-scale production). The total super calender nip pulse was ~800 kPa·s [number of nips × line load / web speed]. The heat from the hot rolls dried the high density paper. The moisture content at winding was 8%. The properties of the high density paper are listed in Table 1 below.

[0119] Example 3 (Comparative Example)

[0120] This comparative example is not published prior art, but it is disclosed in a co-pending patent application.

[0121] A single-ply paper for a different purpose but having similar properties to the papers of Examples 1 and 2 is provided. The single-ply paper is made of a pulp mixture from kraft softwood pulp and kraft hardwood pulp and a small amount of CTMP pulp in a mixture ratio of 45:45:10. The single-ply paper is impregnated with polyvinyl alcohol from the top side and subsequently calendered to a density of about 1050 kg / m 3 The weight is 45g / m 2 The top side surface had a surface roughness of about 25 ml / min Bendtsen.

[0122] result

[0123] The paper properties of the materials produced in Examples 1-3 are listed in Table 1 below. Table 1 also includes the paper properties of Super Perga WS, which is a commercial greaseproof paper from Nordic Paper. Super Perga WS was used as the paper substrate in WO2017 / 089508.

[0124] For Table 1, the following applies:

[0125] Gram weight (i.e. basis weight) is measured according to ISO 536:2012 and is expressed in g / m 2 Density is measured according to ISO 534:2011 in kg / m 3 . Roughness refers to Bendtsen roughness and is measured according to ISO 8791-2:2013 with units of ml / min. Tensile index is measured according to ISO 1924-3:2005 in MD and CD with units of Nm / g. °SR is measured according to ISO 5267-1:1999 after repulping according to ISO 5263-1:2004. Canadian Standard Freeness ("CSF") has units of ml and is measured according to ISO 5267-2:2001 after repulping according to the Valmet Repulping Method using a Valmet Repulper model HD400. The Valmet Repulping Method is described in more detail below. Recyclability is measured according to PTS method PTS-RH 021 / 97. Oxygen Transmission Rate ("OTR") has units of cm 3 / m 2 / 24h, 0.2atm (21%) oxygen. Laminate 20g / m 2 After LDPE, the measurement was performed according to ASTM F1927-14.

[0126] Table 1

[0127]

[0128]

[0129] § According to the supplier's data sheet

[0130] ¤ Each side has been impregnated with 1.5 g / m 2 PVOH was tested on high density paper

[0131] *Weight: 38g / m 2 , not 32g / m 2

[0132] **After supercalendering according to Example 1C (without PVOH impregnation)

[0133] ***Weight is 45g / m 2 , not 32g / m 2

[0134] Valmet Repulping Method: Repulping was carried out in a Valmet repulper of the HD400 type, which is designed for stock preparation, i.e. fiber disintegration. Agitation was carried out with an impeller having three radial serrated blades of dimensions 30×40 mm, rotating at a speed of 3000 rpm. The material to be repulped was cut into 90×90 mm pieces. 0.5 kg of air-dried pieces were mixed with 10 liters of water, i.e. to a consistency of 5%, and repulped at a temperature of 57°C for 2.5 minutes. Then 5 liters of water were added, providing a consistency of 3.3%, and further repulping was carried out at a temperature of 57°C for another 17.5 minutes. The total repulping time was therefore 20 minutes.

[0135] Example 4

[0136] The high density paper prepared according to Examples 1-3 was subjected to two dispersion coatings, with drying operations performed in between and thereafter, to provide a 3 g / m 2 PVOH coating. The PVOH coated high density paper is then metallized to an optical density of about 2. The laminated packaging material is then produced according to the following layer structure: / LDPE 12 g / m 2 / Paperboard 80mN / LDPE 15 or 20g / m 2 / paper substrate+PVOH+metallization / adhesive EAA copolymer 6g / m 2 +19 or 29g / m 2 Blend LDPE+mLLDPE / .

[0137] Packaged in Tetra The products are produced in an E3 / CompactFlex filling machine. This type of filling machine is capable of filling partial packs at a rate of 9,000 packs / hour and has the flexibility to allow quick changeover between different packaging formats. The packaging is made of Tetra Form, volume is 200ml.

[0138] No major issues with package integrity (ie, tightness of the package relative to the surrounding environment) and seal performance were found during the test and were therefore considered successful.

[0139] Oxygen transmission rate of flat packaging materials is measured using a coulometric detector according to standard ASTM F1927-14. Moisture level is 50% or 80% relative humidity. Units are cm 3 / m 2 / 24h, with the option of using 0.2atm or 1atm oxygen pressure. In order to be able to compare the OTR value measured at 1atm with the OTR value measured at 0.2atm, the previous value can be multiplied by 0.2.

[0140] The oxygen transmission rate of packages (filled, empty and dry) was measured according to ASTM F1307-14 at 0.2 atm (ambient air contains 21% oxygen). The unit is cm 3 / package / 24h.

[0141] The package is mounted on a dedicated stand; the interior of the package is purged with nitrogen; the exterior of the package is exposed to the environment surrounding the instrument. As oxygen permeates through the package into the nitrogen carrier gas, it is transported to the coulometric sensor. The sensor reads how much oxygen has leaked into the nitrogen inside the package.

[0142] Use 32g / m2 from Nordic Paper (SuperPergaWS) 2 Further comparative examples (in the form of laminated packaging materials) were prepared using greaseproof paper. Further details of these laminates are provided in Table 2 below.

[0143] The properties of the laminated packaging material are listed in Table 2 below.

[0144] Table 2. OTR values ​​measured at 23°C and 50% RH.

[0145]

[0146]

[0147] * / LDPE 12g / m 2 / Paperboard 260mN / LDPE 20g / m 2 / Paper substrate+PVOH+metallization / LDPE 20g / m 2 / LDPE+mLLDPE 20g / m 2 /

[0148] ** / LDPE 12g / m 2 / Paperboard 80mN / LDPE 20g / m 2 / Paper substrate+PVOH+metallization / LDPE 40g / m 2 /

[0149] *** / LDPE 12g / m 2 / Paperboard 80mN / LDPE 20g / m 2 / paper substrate+PVOH+metallization / adhesive EAA copolymer 6g / m 2 +29g / m 2Blend LDPE+mLLDPE /

[0150] **** / LDPE 12g / m 2 / Paperboard 80mN / LDPE 15g / m 2 / paper substrate+PVOH+metallization / adhesive EAA copolymer 6g / m 2 +19g / m 2 Blend LDPE+mLLDPE /

[0151] Although the laminates in Table 2 differ in the amount of thermoplastic polyethylene-based polymer in the layer facing the interior of the package, this has little or no practical effect on the comparison of the OTR values. This is because polyethylene has poor oxygen barrier properties relative to coated and metallized high density paper. Typical oxygen transmission rates for LDPE at 23°C with a thickness of 40 μm are 600-900 cm 3 / m 2 / 24h / 0.2atm.

[0152] As shown in Table 2, the high density paper of the present invention (Example 1C) not only provides the lowest measured OTR for the laminate after forming the package, but also provides the lowest loss factor for the laminate, which demonstrates excellent robustness in packaging applications. For flat materials, their low OTR values ​​are of little value in packaging applications unless the OTR values ​​are still low after folding to form a three-dimensional package.

Claims

1. A paper having: - Density measured according to ISO 534:2011 higher than 1000 kg / m 3 ; - a strain at break in the machine direction measured according to ISO 1924-3:2005 higher than 3%; - Basis weight less than 100 g / m2 measured according to ISO 536:2012 2 , such as less than 70g / m 2 ;and - The Bendtsen surface roughness on at least one side is less than 150 ml / min when measured according to ISO 8791-2:2013.

2. The paper according to claim 1, in, The strain at break in the machine direction is in the range of 3.5% to 9.0%, such as 4.0% to 8.0%.

3. The paper according to claim 1 or 2, in, The strain at break in the transverse direction measured according to ISO 1924-3:2005 is higher than 5.0%.

4. Paper according to any one of the preceding claims, in, The basis weight of the paper is 66 g / m 2 or lower, such as 30-66g / m 2 , such as 35-66g / m 2 .

5. Paper according to any one of the preceding claims, in, The cellulose fibers comprise at least 90% of the dry weight of the paper.

6. Paper according to any one of the preceding claims, in, The Benttsen surface roughness on the at least one side is lower than 100 ml / min, such as lower than 70 ml / min.

7. Paper according to any one of the preceding claims, in, At least one side of the paper is coated or impregnated with a polymer, such as PVOH, EVOH, starch and / or CMC.

8. Paper according to any one of the preceding claims, in, Both sides of the paper are coated or impregnated with a polymer such as PVOH, EVOH, starch and / or CMC.

9. Paper according to any one of the preceding claims, in, The paper is kraft paper.

10. A method for producing high-density paper, wherein the high-density paper has a density higher than 1000 kg / m 3 , the breaking strain in the longitudinal direction of the paper machine is higher than 3%, and the basis weight is lower than 100g / m 2 , such as less than 70g / m 2 , the method The following steps are involved: - forming a paper web; - drying the paper web in a drying section to obtain paper, the drying section comprising a telescopic unit which compacts the paper web in the longitudinal direction of the paper machine; and - Supercalendering said paper to obtain said high density paper.

11. The method according to claim 10, in, The method further comprises a step of coating or impregnating at least one side of the paper with a composition comprising PVOH, EVOH, starch and / or CMC, the coating or impregnation step being performed between the drying step and the supercalendering step.

12. The method according to claim 11, in, Both sides of the paper are coated or impregnated with a composition comprising PVOH, EVOH, starch and / or CMC.

13. The method according to claim 10 or 11, in, The expandable unit is an Expanda unit or a Clupak unit.

14. A coated paper comprising the paper according to any one of claims 1 to 9, in, The surface of the paper is provided with a barrier coating, for example, the barrier coating comprises polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), starch or starch derivatives, nano- or microfibrillar cellulose, polyvinylidene chloride (PVDC) or polyamide.

15. Use of multilayer materials for packaging food products or other oxygen-sensitive products, in, One layer of the multi-layer material is a paper according to any one of claims 1 to 9, with the proviso that the food product is not a liquid, semi-liquid or viscous food product.

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