AT526659B1 - Process for producing a base paper - Google Patents

Abstract

In a process for producing a base paper, in which a pulp containing essentially both softwood and hardwood components and optionally additives is subjected to a grinding step and at least one compaction step is carried out, a possibly bleached pulp is provided, consisting of 55 to 80 parts by weight, in particular 60 to 80 parts by weight of hardwood primary pulp and 25 to 45 parts, in particular 30 to 40 parts by weight of softwood primary pulp and up to 3 parts by weight of filler, the pulp is obtained by mixing one pulp made from hardwood and one from softwood, either each pulp forming the pulp is subjected to separate grinding to a degree of grinding of 39 to 45 °SR according to ISO 52671:1999 for the pulp made from hardwood or 35 to 41 °SR according to ISO 5267-1:1999 for the pulp from softwood or the pulp obtained by mixing the pulp from hardwood and the pulp from softwood is subjected to grinding to a degree of freeness of 39 to 42 °SR, the mixed pulp is subjected to dewatering, a formed sheet is calendered on at least one side and optionally at least one impregnation is applied to at least one side, and a base paper produced by the process.

Description

Description

[0001] The present invention relates to a process for producing a base paper, in which a pulp containing essentially both softwood and hardwood components and optionally additives is subjected to a grinding step and at least one compaction step is carried out, as well as a base paper produced according to this process.

[0002] Papers, especially for packaging purposes, are used for a wide variety of materials and objects and in recent times there have been increasing attempts to replace plastic packaging with paper packaging. In order to meet the respective requirements and in particular to achieve the material properties of plastic packaging, it is necessary that the papers used have appropriate surface properties, strengths and also barrier properties against moisture, grease and/or gases and are also able to hold the packaged goods safely and reliably without the paper tearing, being punctured, becoming soaked in humid environments that it no longer has any stability or tear resistance and the like. To date, the papers used for this purpose have mostly been provided with relatively strong coatings consisting of polymer materials such as polyethylene, polylactic acid, polypropylene and the like in order to be able to provide a combination of the positive properties of the paper used and the positive properties of plastic packaging and at the same time to be able to reduce the amount of plastic required for the respective packaging. However, for environmental reasons, particularly recycling reasons, such packaging is no longer up to date and attempts are increasingly being made to provide packaging or packaging materials that essentially do not require the addition of non-recyclable plastics or that contain only such small amounts of plastic, and in particular biodegradable plastic, that sustainable and in particular compostable or fully recyclable packaging can be provided.

[0003] Irrespective of the requirements placed on such papers, such as a certain degree of stretchability, air permeability or air resistance, in order to be able to fill in powdered materials quickly and reliably, as well as a certain degree of resistance to tearing of the paper or a product made from it when dropped and the like, it is an essential requirement for many papers that they have at least temporary resistance to penetration by moisture and/or grease, which resistance has so far been achieved mainly by applying a plastic coating to one or both sides of the paper. The more pronounced or stronger or denser such a coating is, the less suitable such paper is for recycling or, in particular, composting, and must be disposed of in the residual waste, as it is no longer suitable for the production of recycled paper.

[0004] Another way of making a paper resistant to moisture and/or grease is to seal at least one paper surface by pressure, by smoothing the surface by applying pressure using rollers, in which process, particularly when a relatively high pressure is applied, the surface pores present in the paper are closed and the paper surface is thus perfectly prepared for the application of grease and/or moisture barrier layers. Such surface smoothing can be achieved, for example, by using a so-called supercalender, which is a calender that has a large number of nips arranged one behind the other. The disadvantage of using a supercalender is that treating a raw paper in such a system weakens the paper so much that it can no longer be used for packaging purposes. Another possibility or a supplementary possibility to prevent such penetration of moisture and/or fat into the paper or through the paper is to produce a so-called filled paper, which contains a relatively large amount of additives such as carboxymethyl cellulose (CMC), microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC) or fillers, which additives

substances fill the pores inside the paper and thus close them, which is why penetration of moisture or fats, for example, is prevented or greatly reduced. Furthermore, such papers are often made exclusively from hardwood pulp, as their surface can be smoothed more easily and thus have better conditions for the subsequent application of a coating. However, such papers are significantly worse in terms of their mechanical properties, such as the tensile strength index, than kraft papers made from softwood, for example, so that due to these poorer mechanical properties they are only poorly suited for use in the packaging sector, especially when other special properties, such as resistance to moisture or the like, are required.

[0005] Finally, there are papers used in the packaging sector that can provide a certain moisture and grease barrier due to their relatively high basis weight, their multiple layers or the barrier materials used, such as coatings or impregnations. The disadvantage of these papers is that a high level of raw material is required for their production and additional coatings or surface treatment processes must be used, so that such papers are disadvantageous overall due to the high material consumption and the resulting high energy consumption during their production.

[0006] In the course of investigations, the applicant has found that the above-mentioned problems in the production of papers with certain barrier properties, such as moisture-resistant and/or grease-resistant papers, essentially arise from the fact that the raw papers used in the production of such papers are not optimally suited to achieving a corresponding barrier property, which is why not all properties of the desired end product, such as filler content, basis weight, mechanical properties and/or barrier properties, can be optimized.

[0007] The present invention now aims to provide a process for producing a base paper which, on the one hand, can be produced in a particularly energy-efficient manner and, on the other hand, is particularly suitable as a base paper for the subsequent application of moisture-resistant and/or grease-repellent coatings or surface treatment steps.

[0008] To achieve this object, the method according to the invention for producing a base paper is essentially characterized in that an optionally bleached cellulose pulp is provided, consisting of 55 to 80 parts by weight, in particular 60 to 80 parts by weight, of hardwood primary cellulose and 25 to 45 parts, in particular 30 to 40 parts by weight, of softwood primary cellulose and up to 3 parts by weight of filler, that the cellulose pulp is obtained by mixing one pulp made from hardwood and one from softwood, that either each pulp forming the cellulose pulp is subjected to separate grinding to a degree of grinding of 39 to 45 °SR according to ISO 5267-1:1999 for the pulp made from hardwood primary cellulose or 35 to 41 °SR according to ISO 5267-1:1999 for the pulp made from softwood primary cellulose or which is subjected to grinding to a degree of grinding of 39 to 42 °SR by mixing the pulp obtained from hardwood primary pulp and the pulp obtained from softwood primary pulp, that the mixed pulp is subjected to dewatering, that a sheet formed is calendered on at least one side and, if appropriate, at least one impregnation is applied to at least one side. By providing an optionally bleached cellulose pulp consisting of 55 to 80 parts by weight, in particular 60 to 80 parts by weight of hardwood primary cellulose and 25 to 45 parts, in particular 30 to 40 parts by weight of softwood primary cellulose and up to 3 parts by weight of filler, it is possible to make optimal use of the advantageous properties of hardwood primary cellulose and softwood primary cellulose, such as good strength properties and good printability, without having to use common process chemicals such as cationic polymers and the like or using them in above-average quantities. By using only small quantities of filler, namely up to 3 parts by weight, it is possible to save drying energy and, in addition, the non-swellable filler also contributes to either the compaction of the sheet or the surface.

smoothness, which is why a base paper produced in this way can not only serve as a basis for packaging papers that are suitable for use in the food sector, but can also be produced with significantly lower energy consumption than conventional base or raw papers. On the one hand, a base paper produced from a pulp with such a composition has good strength properties, and at the same time, due to the large proportion of hardwood primary pulp, it is possible to produce extremely thin base papers, i.e. papers with a low basis weight while at the same time being easy to print and smooth. State-of-the-art base papers used to produce grease and/or moisture barrier papers usually have significantly higher basis weights and lower mechanical strengths. Surprisingly, it is irrelevant in this context whether the pulp is bleached or unbleached; it has only been found to be important that the base paper produced according to the present process contains a higher proportion by weight of hardwood primary pulp than of softwood primary pulp. In this context, primary pulp means that only fresh fibers or reject fibers coming directly from the paper machine are used and no recycled paper is used.

[0009] In the context of the present text, a raw paper or base paper is understood to mean a finished and directly usable paper, which paper can additionally be subjected to a further treatment, such as impregnation and/or coating. After such a further treatment, a packaging paper is produced from this raw paper in accordance with the definition of the present application.

[0010] According to the invention, the process is carried out in such a way that the cellulose pulp is obtained by mixing one pulp each consisting of hardwood and one pulp each consisting of softwood. Therefore, according to the invention, one pulp each consisting of hardwood and one pulp each consisting of softwood are produced separately, which are mixed after their production or after a separate grinding step for each of the two pulps.

[0011] By conducting the process in such a way that either each pulp forming the cellulose pulp is subjected to separate grinding to a degree of 39 to 45 °SR for the pulp made from hardwood or 35 to 41 °SR for the pulp made from softwood, or the pulp obtained by mixing the hardwood and softwood pulp is subjected to grinding to a degree of 39 to 42 °SR, it is possible to ensure that a compaction of a sheet made from this cellulose pulp that is particularly suitable for subsequent coating of the base paper can be achieved and at the same time the degree of grinding is set in such a way that a shortening of the fibers contained in the cellulose pulp does not occur and thus the mechanical properties of a base paper made from it would not be impaired. Furthermore, the grinding degrees mentioned ensure that the base paper can be produced in an energy-saving manner due to the still moderate grinding energy and drying energy required. If, as is possible according to the invention, the softwood pulp and the hardwood pulp are ground separately, the respective pulp fibers can be taken into account even more precisely by adjusting the degree of grinding and, in particular, shortening of the fibers and destruction of the fiber sheath can be avoided even more reliably.

[0012] By continuing to conduct the process in such a way that the pulp is subjected to grinding to a degree of grinding in the range of 42 °SR, and then the mixed pulp is subjected to dewatering, for example on a Fourdrinier screen, the dewatering step is slowed down somewhat. On the other hand, however, it is possible in this way to optimize the density and in particular the homogeneous product properties of a paper produced in this way without having to use an excessively large amount of drying energy during the drying process, just as an excessively high pressure does not have to be used in a subsequent calendering step. Thus, surprisingly, it is possible to achieve cloudiness or sheet formation of the paper through the softwood components used and at the same time to fully retain the good mechanical properties. By conducting the process in this way, not only the product properties

Not only can the mechanical properties and density be optimized, but the process can also be carried out with significantly reduced energy consumption, both in terms of electrical energy and steam consumption.

[0013] By further conducting the process in such a way that a sheet formed after dewatering the pulp is calendered on at least one side and, if necessary, at least one impregnation is applied to at least one side, a surface that is smooth and compacted on at least one side is achieved, but which still has sufficient porosity to, for example, prevent objects packaged in the base paper from being completely sealed off from air. With such a process, for example, a base paper with an air resistance (Gurely), measured according to ISO 5636-5:2013, in the range of about 150 to 300 s can be achieved, whereby a base paper produced in this way is optimally suited for subsequent coating despite an air permeability that has a positive effect on the energy requirement in the production process.

[0014] If necessary, such a packaging paper can be impregnated and/or coated at least once on at least one side, whereby the impregnations and/or coatings can be selected and applied depending on the intended use. Surprisingly, this procedure makes it possible for any impregnations or coatings applied to penetrate or be pressed into the paper to a certain extent, whereby not only is a packaging paper with outstanding mechanical properties achieved from the raw paper, but also such good grease resistance and/or moisture resistance of the paper can be ensured with a low consumption of chemicals for such coatings and/or impregnations. The low amount of chemicals required also enables the chemicals to be anchored well to the paper surface, which is why coatings designed in this way surprisingly have a less tendency to flake off. Due to the structure of the paper surface of the raw paper produced using the method according to the invention, it is possible to use small amounts of coating chemicals, which ensures that, unlike grease and/or moisture-resistant packaging papers according to the state of the art, the maximum amounts of coating chemicals specified by law are not exceeded and that the use of the raw paper and also of a coated packaging paper based on it is possible and permitted in the food sector. Paper produced in this way can be used as packaging for vegetables, meat, ice cream or the like without it becoming soft or fat from the packaged food penetrating the paper.

[0015] According to a further development of the method according to the invention, this is carried out in such a way that the softwood pulp and the hardwood pulp are each subjected to a separate grinding process and subsequently mixed and that a fiber length of a mixture of hardwood pulp and softwood pulp is set to an average length-weighted fiber length according to ISO 16065-2:2014 of 1.00 mm to 1.35 mm. With such a process, the fiber length of the hardwood pulp fiber is not shortened during grinding and that of the softwood portion also remains essentially unchanged, so that overall a homogeneous pulp is achieved with an average length-weighted fiber length that is slightly longer than pulps made exclusively from hardwood. A packaging paper made from such a base paper still has the positive mechanical properties of the long fiber or softwood pulp fibers and, surprisingly, due to the larger proportion of hardwood in the pulp, it is also possible to produce a mechanically strong packaging paper with good printability and favorable surface properties from the base paper without having to use excessive amounts of coating chemicals, which nevertheless has ventilation or air permeability, e.g. when filled with powdery or granular goods. It is irrelevant here whether the base paper according to the invention or a packaging paper made from the base paper with a surface treatment such as calendering or impregnation is used.

[0016] In the context of the present invention, pulps are defined as defined by the Kraft

process.

[0017] By carrying out the dewatering of the mixed cellulose pulps at a paper machine speed of 700 m/s to 850 m/s, as corresponds to a further development of the method according to the invention, it is ensured on the one hand that the cellulose pulp is dewatered sufficiently quickly and on the other hand, taking into account the selected high degree of refining, the machine speed only needs to be reduced to a justifiably small extent compared to that used in the production of packaging papers that have not been or only slightly refined, with refining degrees in the range of 22 °SR. What is surprising in this context is that the machine speed can be kept relatively high compared to pulps that have only been slightly refined and good dewatering and homogeneous sheet formation can still be achieved. Surprisingly, it has been shown that due to the use of softwood fibers, which allows, for example, a reduction in the basis weight of the base paper according to the invention while at the same time maintaining the required mechanical strengths, the dewatering speed of the paper machine can even be increased by around 5% when using the mixed cellulose pulps, despite the relatively high degree of refining. The filler content of up to 3% used according to the invention also contributes to this, which promotes dewatering or drying but nevertheless does not have a negative effect on the mechanical strengths.

[0018] By carrying out the method according to a further development thereof in such a way that the at least one-sided calendering of a sheet of the base paper formed after dewatering, for example on a Fourdinier or any other wire section with a bottom and top former, is carried out with a hardnip calender, preferably a hardnip calender heated to a maximum of 150 °C, more preferably a maximum of 90 °C, at a line load of 50 to 200 kN/m, more preferably 70 to 120 kN/m, it is possible on the one hand to ensure good smoothness and thus printability of a packaging paper made from the base paper or also of the base paper itself, and in particular to achieve a mechanical surface refinement, in particular surface compaction of the paper, which reduces the porosity on the surface of the same to such an extent that a coating or impregnation applied subsequently, if necessary, cannot penetrate too deeply but at least to a small extent into the paper structure and thus with a compared to non-calendered papers with significantly reduced chemical consumption. At the same time, the surface is easy to print on, and here too, the consumption of printer inks and colors can be reduced due to the small number of surface pores.

[0019] According to a further development of the invention, the method is carried out in such a way that the base paper is calendered on both sides. With calendering on both sides, it is possible to smooth both surfaces of the paper so much that the base paper produced in this way can be used for packaging even without the additional application of a coating and/or impregnation. Calendering thus makes it possible to reduce the Bendtsen roughness of the paper (measured according to ISO 8791-2:2013) to values in the range of 60 to 150 ml/min, preferably around 120 ml/min, compared to non-calendered paper, which has a Bendtsen roughness in the range of up to 900 ml/min.

[0020] As is the case with a further development of the present invention, the process is carried out in such a way that before or after calendering, at least one side of the base paper is coated with surface starch and sizing agent, in particular in a total application amount of 0.8 to 2 g/m*, such a process not only makes it possible to give the paper produced in this way additional strength and dimensional stability, but above all it also makes it possible to significantly improve its printability, surprisingly without there being any risk of bleeding or running of the printing inks used when using the base paper itself or a packaging paper made from it.

[0021] According to a further development of the method according to the invention, both sides of the

Base paper, in particular with different coating materials, selected from polyolefins, such as polyethylene or polypropylene, starches and modified starches, acrylates, polyalcohols, such as polyvinyl alcohol, ethylene vinyl alcohol or polylactic acid, each coated with at least one layer. By applying at least one such additional coating to both sides of the base paper, a further improvement in the grease and/or moisture resistance and optionally also a gas barrier property against oxygen and aromas of a packaging paper made from the base paper is achieved.

[0022] To further improve the moisture and/or grease resistance, the process is essentially carried out in such a way that before or after calendering and before coating, an impregnation, preferably two-sided impregnation of the base paper is carried out by applying and pressing in a dispersion or solution selected from starches or modified starches, acrylates, polyalcohols, such as polyvinyl alcohol with at least 1.5 g/m2, preferably at least 2.0 g/m2. With such further impregnation of the surface of the paper, it is possible to further improve the surface properties of such a paper and in particular to further seal the pores of the packaging paper made from the base paper. Such additional impregnation is applied in small amounts of at least 1.5 g/m2 according to the invention. This can be an impregnation with a starch solution, which also improves the mechanical strength of the base paper. This type of process makes it possible to provide the paper with a better barrier effect across its entire cross-section. Surprisingly, it has been shown that if a surface coating is also applied, a synergistic effect can be achieved. A synergistic effect in that the paper is overall more robust during processing than if only the improvements in the properties of two individual surface treatment processes are added together. It has been shown that the barrier effect achieved is less damaged by folding, bending, gluing, etc. during processing or use than if only a surface coating had been applied.

[0023] The process according to the invention thus surprisingly succeeds in providing a base paper which has a smooth surface, is easy to print on and has surprisingly good mechanical properties. In terms of its surface smoothness, the paper is comparable to conventional translucent papers, such as glassine papers, and can replace them, which is surprising due to the high proportion of softwood in the paper. At the same time, the base paper can be produced without the addition of process additives, such as cationic retention polymers and the like, which further facilitates the use of either the base paper directly or a packaging paper made from it in the food sector. Due to the large amount or the large proportion by weight of up to 45 parts by weight of softwood pulp fibers, it is surprisingly possible to give the base paper mechanical properties which almost correspond to the mechanical properties of kraft papers made exclusively from softwood. By intensively refining the softwood pulp as well as by medium-intensively refining the hardwood pulp to °SR values in the range of 40, not only is maximum compaction in the subsequent calendering step to values in the range of up to 900 kg/m® of the resulting base paper achieved, but surprisingly the good mechanical properties achieved by the addition of the long-fibered softwood can be maintained at the same time.

[0024] A base paper produced using the method according to the invention is essentially characterized in that the base paper has an ash content of less than 4% and a content of fines with a size of < 0.2 mm of less than 36% and a specific tear resistance index according to ISO 1974:2012 of > 8 mN.m%g in the machine direction and > 9 mN.m2/g in the transverse direction. The extremely low ash content in the base paper ensures that it is essentially free of production aids such as cationic retention polymers. Furthermore, by reducing the content of fines with a size of

< 0.2 mm is less than 36%, it is ensured that the paper is, on the one hand, sufficiently refined to be able to be strongly compacted, which is reflected in a final density of the base paper according to the invention of approximately 900 kg/m?, and, on the other hand, this fines content ensures that excessive energy expenditure is not required during a paper manufacturing process and that excessive slowing of the paper machine at the mill to, for example, values of less than 800 m/min can be avoided, which higher fines contents would undoubtedly cause. By providing a base paper that is manufactured according to the method according to the invention, it has surprisingly been shown that a specific tear resistance index of this paper according to ISO 1974:2012 can be set at > 8 mN.m?/g in the machine direction and > 9 mN.m?/g in the transverse direction. Such a paper is surprisingly tear-resistant despite its low basis weight, particularly in relation to papers made from hardwood pulp, and can therefore be used in applications in which it is important that a package made from it does not tear accidentally or does not tear further if a hole or the like has been formed in it. This property, which is also retained in packaging papers made from it, is particularly important and advantageous, since known packaging papers, which are made, for example, only from hardwood pulp, only show very low tear resistance.

[0025] As a further development of the invention, the base paper is designed in such a way that it has a basis weight of between 50 g/m2 and 135 g/m2 and a tensile strength index according to ISO 1924-3:2005 which is at least 80 Nm2/g for a basis weight of 50 g/m2, it is possible to provide a paper with a high tensile strength index while at the same time using a low amount of material, which overall results in a saving of a significant amount of energy and material during production. Basis weights in the range of 50 g/m2 to 135 g/m2 are to be described as low or very low for the packaging of moist or greasy objects and, in particular when such objects are stored or kept in a frozen state, for example, are usually not sufficient to ensure that the paper does not soften or tear over time.

[0026] Such a tensile strength index ensures that the paper can withstand a wide variety of stresses, such as the storage of packaged goods in a stacked state, when a sack is dropped together with packaged goods, etc., without tearing. A further improved stretchability of the base paper can be achieved by subjecting the softwood pulp to high-consistency grinding, which grinding has a surprisingly positive effect on the stretchability of the base paper in the transverse direction, but at the same time has only an extremely small effect on the degree of grinding of the base paper. Finally, an improved stretchability in the machine direction of the base paper can also be achieved by micro-creping it on a Clupak system, for example. The TEA index according to ISO 1924-3:2005 of such a base paper is 1.1 to 2.3 J/g in the machine direction and 1.5 to 2.0 J/g in the transverse direction.

[0027] By further developing the base paper in such a way that the air resistance value (Gurley) of the uncoated base paper according to ISO 5636-5:2013 is between 150 and 300 s, it is possible to maintain a certain low air permeability of the base or base paper.

[0028] By developing the packaging paper in such a way that it is coated on both sides, in particular by providing both surfaces with at least one layer of an identical and/or different coating, the raw paper and also a packaging paper made from it can be given further advantageous properties, such as excellent printability, different resistance to moisture and grease on both sides of the paper, smoothness and the like.

[0029] These tailor-made properties of the base paper according to the invention can be further refined by, as corresponds to a further development of the invention,

the paper is designed in such a way that the coatings on both surfaces differ from one another, in particular with regard to the amount, composition and physical/chemical properties of the coating agent.

[0030] As is the case with a further development of the invention, if a further coating consisting of surface starch and sizing agent is applied to at least one side, it is possible not only to significantly improve the printability of the paper, but also to further increase the strength and dimensional stability of the paper. The coating with starch and sizing agent can be applied before or after calendering, but if further coatings or several layers of coatings are provided in order to produce packaging paper from the raw paper, the coating that gives the paper grease or moisture resistance should be provided as the outermost layer.

[0031] The invention is explained in more detail below using exemplary embodiments (or a schematic drawing of the method according to the invention). In these:

[0032] EXAMPLE 1: Process for producing the base paper in two grammages according to the invention (Paper 1 and Paper 2)

Process description:

[0033] A bleached pulp consisting of 42% primary pulp from softwood was ground with a grinding capacity of 297 kWh/t, whereby a degree of grinding of the softwood pulp of 41 °SR was obtained. In addition, 58% of a bleached primary pulp from hardwood was used, which was subjected to a grinding capacity of 97 kWh/t, whereby a degree of grinding of the hardwood pulp was 44 °SR. The auxiliary materials were added in the approach part of the paper machine. The pH value was set to 8.1, cationic starch with a degree of cationization DS of 0.05 was added in an amount of 11 kg/t of dry paper and alkenylsuccinic anhydrides were used as a sizing agent in an amount of 2.35 kg/t of dry paper. Calcium carbonate was also added as a filler in an amount of 25 kg/t of dry paper. No cationic retention polymer was added. Surprisingly, the two different types of pulp could be mixed together without any problems and the process additives could also be added in the conventional way. Surprisingly, no deterioration in the effect of the process additives was observed. The consistency of the pulp at the headbox was 0.315%. Dewatering took place on the wire section with a top and bottom former with a wire speed of 820 m/min, and in a press section with four nips, with the line pressure in the three nips being 48 kN/m, 95 kN/m and 115 kN/m and in the shoe press nip 600 kN/m. Despite the high softwood content, the sheet was well formed and the sheet formed in this way had extremely homogeneous properties and, above all, a uniform surface, despite the use of two different types of fiber. The paper was pre-dried and treated in a coating unit with a surface thickness of 1 g/m? The resulting coating is also surprisingly very homogeneous. The coated paper was smoothed in a single hardnip calender at 60 kN/m and 85°C immediately before winding and then wound up with a final residual moisture content of 7%. The speed on the pope was 865 m/min.

[0034] Under the conditions mentioned, base papers were produced in two grammages: paper 1 with 77 and paper 2 with 101 g/m*. The data on screen speed and speed at the pope are average values from the operation with both grammages.

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[0035] The raw papers produced in this way were coated on both sides in the next process steps. The raw papers were coated on both sides using a rotogravure process with a water-based varnish from the Michelman company from the Michem® Coat series, each with an application quantity of 6 g/m2 and 4 g/m2. The varnish recipe used creates a grease barrier. The kit values achieved according to Tappi T 559 were 10-11 for 4 g/m2 application and 1112 for 6 g/m2.

[0036] In a further test, the same base papers were also coated using a rotogravure process on one side with 4 g/m* of a Michem® Coat product and on the other side with 4 g/m? of a VaporCoat® product. The grease barriers achieved were again Kit 10-11 for the side treated with Michem® Coat. The water vapor transfer rate at 23 °C and 50% relative humidity according to ISO 2528:2017 was 12 g/m*.d for the other side of the paper, which was coated with VaporCoat®. The same test was carried out with Gervalux 100gsm paper. The grease barrier achieved was Kit 11 and the water vapor transfer rate was 10 g/m?.d. Taking into account the fluctuation range of the measurement methods, the results can be considered equivalent. However, with a packaging paper produced according to the invention, better mechanical properties such as longitudinal tensile strength index or tear resistance index can be provided while using approximately 20% less paper.

Raw paper 3 according to the invention Process description:

[0037] A bleached pulp consisting of 40% softwood primary pulp was subjected to grinding at a grinding capacity of 243 kWh/t, whereby a degree of grinding of the softwood pulp of 35 °SR was obtained. In addition, 60% of a bleached primary pulp from hardwood was used, which was subjected to a grinding capacity of 78 kWh/t, whereby a degree of grinding of the hardwood pulp was 39 °SR. The auxiliary materials were added in the approach part of the paper machine. The pH was set to 8.1, cationic starch with a degree of cationization DS of 0.05 was added in an amount of 10.4 kg/t paper atro and alkenylsuccinic anhydrides were used as sizing agents in an amount of 2.67 kg/t paper atro. Calcium carbonate was added as fillers in an amount of 10 kg/t and bentonite at 1.7 kg/t of dry paper. No cationic retention polymer was added. The consistency of the pulp at the headbox was 0.39%. Dewatering took place on the wire section with a top and bottom former with a wire speed of 770 m/min and in a press section with four nips, with the line pressure in the three nips being 48 kN/m, 95 kN/m and 115 kN/m and in the shoe press nip 600 kN/m. The kraft paper was pre-dried and treated in a coating unit with a surface thickness of 2 g/m? and smoothed immediately before winding in a single hardnip calender at 100 kN/m and 85 °C and then finally wound up with a final residual moisture content of 7%. The speed at the Pope was 811 m/min.

[0038] The paper thus obtained had a grammage of 80 g/m* and had the following properties:

Table 2:

Paper property Standard Unit Direction Paper 3 Grammage ISO 536:2019 g/m? 80 Thickness ISO 534:2011 um 87 Density kg/m? 920

Tensile strength ISO 1924-3:2005 kN/m MD 6.65

Tensile strength index ISO 1924-3:2005 Nm/g MD 83.1 Tensile strength ISO 1924-3:2005 kN/m CD 2.57 Tensile strength index ISO 1924-3:2005 Nm/g CD 32.1 Elongation at break ISO 1924-3:2005 % MD 1.85 Elongation at break ISO 1924-3:2005 % CD 4.51 Air permeability Gurley ISO 5636-5:2013 Ss 122 Bendtsen roughness ISO 8791-2:2013 ml/min smooth side |68 Bendtsen roughness ISO 8791-2:2013 ml/min rough side 78 Burst strength ISO 2758:2014 kPa 237 Burst index ISO 2758:2014 kPa.m?/g 2.96 Tear resistance index ISO 1974:2012 mN.g/m? MD 8.39 Tear resistance index ISO 1974:2012 mN. g/m? CD 10.2 Ash content index ISO 1762:2019 % 1.6 Cobb 60 ISO 535:2014 g/m? Smooth side |20.9 ISO 535:2014 g/m? Rough side |22.4 Fibre length ISO 16065-2:2014 mm 1.2 Fines with a size < % 28 0.2 mm Reaction to starch*) positive

Raw paper 4 according to the invention Process description:

[0039] A bleached pulp consisting of 27% primary pulp from softwood was subjected to grinding together with a 73% primary pulp consisting of hardwood at a grinding capacity of 141 - 174 kWh/t, whereby a pulp mixture with a grinding degree of 40 - 41 °SR was obtained. The auxiliary materials were added in the approach part of the paper machine. The pH was set to 8.1, cationic starch with a degree of cationization DS of 0.05 was added in an amount of 8.1 kg/t of dry paper and alkenylsuccinic anhydrides were used as a sizing agent in an amount of 3.8 kg/t of dry paper. Bentonite was added as a filler at 2.4 kg/t of dry paper. No cationic retention polymer was added. The consistency of the pulp at the headbox was 0.15%. Dewatering took place on the wire section with a top and bottom former with a wire speed of 770 m/min and in a press section with four nips, whereby the line pressure in the three nips was 52 kN/m, 98 kN/m and 120 kN/m and in the shoe press nip 600 kN/m. The kraft paper was pre-dried and treated in a coating unit with a surface thickness of 2 g/m?, containing 1.6 kg/t alkenyl succinic anhydride as a sizing agent, and smoothed directly before winding in a single hardnip calender with 100 kN/m and 85 °C and then finally wound up with a final residual moisture content of 7 %. The speed on the pope was

815m/min.

[0040] The paper thus obtained had a grammage of 61 g/m* and had the following properties:

Table 3: Paper properties Standard Unit Direction Paper 4 Grammage ISO 536:2019 g/m? 61 Thickness ISO 534:2011 um 67 Density kg/m? 913 Tensile strength ISO 1924-3:2005 kN/m MD 6.13 Tensile strength index ISO 1924-3:2005 Nm/g MD 97.2 Tensile strength ISO 1924-3:2005 kN/m CD 2.31 Tensile strength index ISO 1924-3:2005 Nm/g CD 35.4 Elongation at break ISO 1924-3:2005 % MD 1.89 Elongation at break ISO 1924-3:2005 % CD 5.33 Air permeability Gurley |ISO 5636-5:2013 Ss 197 Bendtsen roughness ISO 8791-2:2013 ml/min smooth side |51 Bendtsen roughness ISO 8791-2:2013 ml/min rough side |62 Bursting strength ISO 2758:2014 kPa 210 Burst Index ISO 2758:2014 kPa.m?/g 3,44 Tear Resistance Index ISO 1974:2012 mN.g/m? MD 8,18 Tear Resistance Index ISO 1974:2012 mN.g/m? CD 9,43 Ash Content Index ISO 1762:2019 % 0,8 Cobb 60 ISO 535:2014 g/m? Smooth Side |20,1 ISO 535:2014 g/m? Rough Side |22,8 Fibre Length ISO 16065-2:2014 mm 1,1 Fines with a size % 34 < 0,2 mm Reaction to Starch*) positive

[0041] In summary, it can be seen that in the production of the base papers according to the invention only about 32 to 55% CO2 E/T are generated compared to common base papers available on the market.

Claims (14)

Patent claims 1. A process for producing a base paper, in which a pulp containing essentially both softwood and hardwood components and optionally additives is subjected to a grinding step and at least one compaction step is carried out, characterized in that a possibly bleached pulp consisting of 55 to 80 parts by weight, in particular 60 to 80 parts by weight of hardwood primary pulp and 25 to 45 parts, in particular 30 to 40 parts by weight of softwood primary pulp and up to 3 parts by weight of filler is provided, that the pulp is obtained by mixing one pulp made from hardwood and one from softwood, that either each pulp forming the pulp is subjected to separate grinding to a degree of grinding of 39 to 45 °SR according to ISO 5267-1:1999 for the pulp made from hardwood or 35 to 41 °SR according to ISO 5267-1:1999 for which pulp from softwood or which is obtained by mixing pulp from hardwood and pulp from softwood is subjected to grinding to a degree of freeness of 39 to 42 °SR, that the mixed pulp is subjected to dewatering, that a sheet formed is calendered on at least one side and, optionally, at least one impregnation is applied to at least one side. 2, Method according to claim 1, characterized in that the softwood pulp and the hardwood pulp are each subjected to a separate grinding process and subsequently mixed and that a fiber length of a mixture of hardwood pulp and softwood pulp is adjusted to an average length-weighted fiber length according to ISO 16065-2:2014 of 1.00 mm to 1.35 mm. 3. Process according to claim 1 or 2, characterized in that the dewatering of the mixed pulps is carried out at a screening speed of 700 to 900 m/s. 4. Process according to claim 1, 2 or 3, characterized in that the at least one-sided calendering of a sheet of the base paper formed after dewatering on the screen is carried out with a hardnip calender, preferably a hardnip calender heated to a maximum of 150 °C, more preferably a maximum of 90 °C, at a line load of 50 to 200 kN/m, more preferably 70 to 120 kN/m. 5. Process according to claim 4, characterized in that the base paper is calendered on both sides. 6. Process according to one of claims 1 to 5, characterized in that before or after calendering, a coating of at least one side of the base paper with surface thickness, in particular in an amount of 0.8 to 2 g/m?, is carried out. 7. Method according to one of claims 1 to 6, characterized in that both sides of the base paper are coated, in particular with different coating materials, selected from polyolefins, such as polyethylene or polypropylene, starches and modified starches, acrylates, polyalcohols, such as polyvinyl alcohol, ethylene vinyl alcohol or polylactic acid, with at least one layer each. 8. Process according to one of claims 1 to 7, characterized in that before or after calendering and before coating, an impregnation, preferably two-sided impregnation of the base paper is carried out by applying and pressing in a dispersion, solution or solution selected from starches or modified starches, acrylates, polyalcohols, such as polyvinyl alcohol with at least 1.5 g/m®, preferably at least 2.0 g/m?. 9. Raw paper produced by a process according to one of claims 1 to 8, characterized in that the raw paper has an ash content of less than 4% and a content of fines with a size of < 0.2 mm of less than 36% and a specific tear resistance index according to ISO 1974:2012 of > 8 mN.m?/g in the machine direction and > 9 mN.m?/g in the transverse direction. 10. Raw paper according to claim 9, characterized in that the raw paper has a basis weight according to ISO 536:2019 between 50 g/m? and 135 g/m? and a tensile strength index according to ISO 1924-3:2005 of 80 to 105 kN/m in the machine direction and of 30 to 40 kN/m in the transverse direction. 11. Base paper according to claim 9 or 10, characterized in that the air resistance value (Gurley) of the uncoated base paper according to ISO 5636-5:2013 is between 100 and 300 s. 12. Raw paper according to claim 8, 9 or 10, characterized in that it is coated on both sides, in particular on both surfaces it is provided with at least one layer of an identical or different coating. 13. Raw paper according to claim 12, characterized in that the coatings on both surfaces differ from one another, in particular with regard to the amount, composition and physical/chemical properties of the coating agent. 14. Raw paper according to claim 12, characterized in that a further coating of surface strength is additionally applied to at least one side. No drawings for this