A METHOD OF PRODUCING A FIBER-BASED PAPER COMPRISING AT LEAST ONE TRANSPARENT AREA

Abstract

 The present invention relates to fiber-based transparent papers. More particularly, the invention relates to a method of producing at least one transparent area on a fiber-based paper and to a fiber-based paper obtained by said method. The invention also relates to the use of the obtained fiber-based paper in packaging applications.

Description

FIELD

[0001] The present invention relates to fiber-based transparent papers. More particularly, the invention relates to a method of producing at least one transparent area on a fiber-based paper and to a fiber-based paper obtained by said method. The invention also relates to the use of the obtained fiber-based paper in packaging applications.

BACKGROUND

[0002] Previously, most transparent materials were formed from plastic-based materials. In an attempt to reduce the use of non-renewable, petroleum-based resources, renewable materials such as cellulose fibers were later combined with the petroleum-based chemicals or synthesized resins. Further, in order to improve transparent properties of papers comprising petroleum-based chemicals and cellulose fibers, the obtained papers were typically supercalendered off-line, i.e. passed through several sets of metal cylinders or rollers to achieve desired transparency, thickness and density levels. This supercalendering step is highly energy consuming and adds complexity to the manufacturing process of such papers. Furthermore, this supercalendering step is also limited by the paper machine speed and therefore it is performed offline. Accordingly, this supercalendering step is expensive in time and energy.

[0003] Moreover, some of the existing transparent papers have a very low grammage (typically of less than 25 g/m²) and therefore exhibit weak mechanical properties. Such current transparent papers are encountering difficulties in achieving a balance between transparency and mechanical properties. Typically, the technologies that are currently accessible include a highly transparent paper with low mechanical properties or a paper with excellent mechanical properties but low transparency. The technology may also include the use of a high amount coating material compared to the weight of the fibers to attain the balance between the mechanical and transparency properties, but simultaneously utilizing high costs and energy.

[0004] Another option for producing transparent papers is the use of highly refined cellulosic fibers, such as for example of at least 70°SR, and optionally a supercalendering step. However, this refining step is also energy consuming and requires high drying energy of the transparent paper.

[0005] As apparent from above, it is challenging to balance between transparency and mechanical properties in order to create satisfying finished products. Highly transparent papers typically are not strong enough (because of their low grammages for example) for certain applications in terms of properties such as tensile strength, tear resistance and burst. On the other hand, the technical features may be in line with the requirements but the process is either too expensive or does not fulfill sustainability requirements. For example, highly refined transparent paper, such as tracing paper or Cristal paper, which are very transparent ones (at least 70% transparency roughly) consume a lot of energy for their productions because of the required very high refining levels (around 70-90°SR) and an off-line supercalendering step in order to achieve the level of transparencies of such kinds of papers.

[0006] Some attempts to find further solutions to prepare transparent papers have been made. WO 2024024896 A1 relates to a base paper having a basis weight of 40-75 g/m², impregnated with a transparent material such as acrylic resin, vegetable oil, or wax. CN 109706792 A discloses papers, wherein transparency is imparted by coating the paper with a transparent release coating composition consisting of modified nanocellulose, carboxymethyl cellulose, and styrene-acrylate copolymer composition, followed by a calendering process after the coated paper has been dried. EP 3483337 A1 relates to a highly refined translucent or transparent paper coated with a material conferring moisture barrier properties and transparentizing properties, typically a material selected from styrene or vinylidene copolymers and paraffin waxes. EP 4471207 A1 discloses a low opacity and heat-sealable paper made from highly refined cellulose fibers, coated first with a wax and then with a coating composition providing heat-sealable properties, while EP 4471210 A1 discloses a similar paper without a heat-seal layer.

[0007] However, the solutions proposed in the prior art still suffer from several shortcomings or do not fully solve the problem how to find a balance between transparency, mechanical properties, cost-effectiveness, and sustainability.

[0008] The present invention enables the production of a highly transparent paper with less energy consumption by providing a new process to obtain highly transparent paper with sufficient mechanical properties and without the need to use highly refined fibers. The present invention is also intended to produce transparent papers with higher grammages than the ones known from the prior art. The present invention further provides an online process that yields a fiber-based paper, which can be repulpable, recyclable and/or compostable, thus further reducing the environmental impact of the process and the obtained paper.

SUMMARY OF THE INVENTION

[0009] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0010] The present invention provides a new process to obtain a highly transparent paper with high mechanical properties. The invention provides an on-line treatment method that involves the application of a low amount (typically ranging from 0.05 to 10 g/m²) of chemical treatment onto a fiber-based substrate, and then submitting the coated fiber-based substrate to a calendering step. The fiber-based substrate can be calendered or not, preferably the fiber-based substrate is a non-calendered fiber-based substrate as it will have a higher porosity enabling a better penetration of the coating composition into it. In particular, the implementation of a calendering step after the coating step enables in the present method to decrease the amount of the applied transparentizing composition to reach a high transparency of at least 60% when measured according to DIN 53147:1993-01 standard. Calendering further improves the transparency values achieved by coating the fiber-based substrate with the transparentizing composition. However, no supercalendering off-line is needed.

[0011] According to a first aspect of the present invention, there is provided a method of producing at least one transparent area on a fiber-based paper, wherein the method comprises the steps of:

• providing or producing a fiber-based substrate, said fiber-based substrate having a first side and a second side opposite to the first side;
• applying a coating composition comprising a transparentizing agent on at least one area of at least one side of the fiber-based substrate to form a coated fiber-based substrate; and
• calendering the coated fiber-based substrate to obtain the at least one transparent area on the fiber-based paper;

wherein the at least one transparent area on the fiber-based paper has a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard. The calendering step of the coated fiber-based substrate enables a good penetration of the coating composition in the fiber-based substrate and also enables a good repartition of the coating composition on the surface of the fiber-based substrate to avoid discrete diffractive points at the surface of the transparent area on the fiber-based paper. Furthermore, the calendaring step enables the smoothness of the coating composition applied on the fiber-based substrate enabling a further improvement of the transparency of the transparent area on the fiber-based paper. This calendaring step further allows to ensure a perfect contact between the transparentizing agent and the fibers, lowering the risks of having imperfections such as small pockets of air between the transparentizing agent and the fibers diffracting the light.

[0012] According to a second aspect of the present invention, there is provided a fiber-based paper comprising at least one transparent area obtained by the present method, wherein the at least one transparent area has a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard.

[0013] Embodiments of the invention comprise the use of the fiber-based paper obtained by the present method in packaging applications.

[0014] Considerable advantages are obtained by the invention. The present method allows to reach a very high transparency with fiber-based substrate having heavier basis weights than the ones know in the art (up to 150 g/m²). By using fiber-based substrate having heavier basis weight, it is possible to have transparent fiber-based papers with higher mechanical properties or higher physical properties (tensile, tearing) than the low basis weight transparent papers known from the prior art. Furthermore, the present method enables the use of low coat weights of the coating composition to reach high transparency levels as the calendering step is increasing the transparentizing performances of the coating composition. Furthermore, this method enables to produce transparent fiber-based papers with a lower refining level than the ones known in the art which enable to lower the energy input for preparing the fiber-based substrate. The process allows to manufacture the product online and to avoid the substrate curling, preserve its strength, and bring higher transparency while also maintaining the normal speed of the machine.

[0015] Further, if desired, by the present method it is possible to fill all or as much porosity as possible with the transparentizing agent, thus minimizing the refraction of light on and within the transparent area on the fiber-based paper.

[0016] Still further, the present fiber-based papers may be recyclable, repulpable, and/or compostable. In particular, they can be suitable for recycling in the standard paper and board recycling streams according to EN13430 and/or CEPI method of 2022.

[0017] Further features and advantages of the present technology will appear from the following description of some embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 

FIGURE 1 is a flow graph showing the different steps of a method (1) for producing at least one transparent area on a fiber-based paper. Step 3 indicates a step of providing or producing a fiber-based substrate, while the other reference numbers indicate the following: step 5 = applying a coating composition → forming a coated fiber-based substrate; step 7 = at least partial drying the coated fiber-based substrate (optional step); step 9 = calendering; step 11 = drying; step 13 = rolling.

FIGURE 2 is a photography of a fiber-based paper having transparent areas in the form of lines.

FIGURE 3 is a photography of a fiber-based paper having transparent areas in the form of pyramids.

EMBODIMENTS

DEFINITIONS

[0019] In the present context, the term "transparent" in connection with a material or an area of a material, such as a fiber-based paper, means that said material or an area thereof has a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard using a test equipment model BCMTS M Type 40605, with Touchscreen M software of the company Frank PTI. This equipment uses a Konica Minolta CM-3630 Spectrophotometer. In order to measure the transparency of the transparent area, said transparent area should cover at least an area comprising a circle of 9 mm of diameter to allow testing.

[0020] A "fiber-based paper" includes in particular a paper based on cellulosic fibers or regenerated cellulosic fibers. Cellulosic fibers or cellulose fibers refer to fibers composed of or derived from cellulose. The cellulosic fibers may be selected from the group consisting of kraft pulp fibers, sulfite pulp fibers, chemically treated fibers, mechanically treated pulp fibers, chemi-thermo mechanically treated pulp fibers, regenerated cellulosic fibers and combinations thereof. Advantageously, the use of cellulosic fibers or regenerated cellulosic fibers enables the repulpability, recyclability, and/or compostability of the fiber-based paper.

[0021] In the present invention it has been found that by applying a coating composition comprising a transparentizing agent on at least one area of at least one side of a fiber-based substrate and calendering the thus obtained coated fiber-based substrate it is possible to achieve a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard.

[0022] Within the present context, "calendering" refers to a finishing process in which the fiber-based substrate, coated with a coating composition comprising a transparentizing agent, is passed through calender rollers, typically at elevated temperature and pressure to smoothen and increase the apparent density of the substrate and to obtain at least one transparent area on the fiber-based paper. "Calendering" does not comprise and is different from supercalendering, wherein an additional calendering is performed off-line, i.e. on a special machine not connected to the main papermaking machine.

[0023] With reference to Figure 1, this one shows a flow chart illustrating a method of producing at least one transparent area 1 on a fiber-based paper. First, the method of the invention for producing at least one transparent area on a fiber-based paper comprises a step of providing or producing a fiber-based substrate 3. The fiber-based substrate has a first side and a second side opposite to the first side. A coating composition comprising a transparentizing agent is applied on at least one area of at least one side of the fiber-based substrate to form a coated fiber-based substrate. Accordingly, the method 1 comprises a step of forming a coated fiber-based substrate 5. The coated fiber-based substrate is calendered 9 to obtain the fiber-based paper with the at least one transparent area having a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard. If needed, the method can further implement a rolling step 13 of the fiber-based paper.

[0024] The fiber-based substrate can be a calendered or an uncalendered fiber-based substrate. Preferably, the fiber-based substrate is an uncalendered fiber-based substrate to facilitate the penetration of the coating composition inside it and also to decrease the energy consumptions of the overall method. According to another embodiment the coating composition can be applied on at least one side of the fiber-based substrate during the manufacturing of same. Furthermore, the fiber-based substrate has a basis weight of at least 25 g/m², preferably at least 30 g/m², more preferably at least 35 g/m², still more preferably at least 40 g/m², and less than 150 g/m², preferably less than 120 g/m², more preferably less than 90 g/m², more preferably less than 80 g/m², still more preferably less than 70 g/m². Any combinations of these end points are also possible (e.g. greater than or equal to 25 g/m² and less than or equal to 150 g/m², greater than or equal to 30 g/m² and less than or equal to 120 g/m², greater than or equal to 35 g/m² and less than or equal to 90 g/m², greater than or equal to 40 g/m² and less than or equal to 120 g/m², or also greater than or equal to 40 g/m² and less than or equal to 80 g/m²). Other ranges are also possible.

[0025] The fiber-based substrate comprises at least 60 wt%, preferably at least 70 wt%, more preferably at least 80 wt%, still more preferably at least 95 wt% cellulosic fibers and/or regenerated cellulose fibers based on the total weight of the fibers forming the fiber-based substrate. According to a preferred embodiment, the cellulosic fibers and/or the regenerated cellulose fibers have a freeness of at least 25°SR, preferably of at least 30°SR, more preferably of at least 35°SR, and below 95°SR, preferably below 80°SR, more preferably below 70°SR using the Schopper-Riegler method. Any combinations of these end points are also possible. Advantageously, the present method works also on fibers with lower refining such as between 40 and 75°SR. Other ranges are also possible.

[0026] With reference to step 5 of Figure 1, the coating composition may be applied by a technique selected from roll-to-roll coating, blade coating, spray coating, Meyer rod coating, air knife coating, direct gravure, offset gravure, reverse gravure, deep coating, smooth roll coating, curtain coating, bead coating, slot coating, twin HSM coating, film press coating, size press coating, and/or transfer film method. Printing technologies, for example flexography, heliography, or inkjet printing, may also be used to deposit the coating. The choice of the coating method can be adjusted by a person skilled in the art depending on the viscosity of the coating composition and/or on the type of the fiber-based substrate, including its basis weight, porosity and thickness. In some embodiments, the coating composition may be applied by size press coating, which allows to use a minimum amount of coating while ensuring that the penetration of the coating in the depth of the paper is high. In other embodiments, the coating composition can be applied by direct gravure, reverse gravure or blade coating.

[0027] The method 1 of the invention may also comprise an at least partial drying step 7 of the coated fiber-based substrate before the calendering step 9. This at least partial drying step 7 can be performed by gravity or heating. In case this partial drying is performed by heat, it can decrease the viscosity of the coating composition and facilitate the penetration of the coating composition in the fiber-based substrate.

[0028] According to an embodiment, if the coated fiber-based substrate is still wet after the calendaring step 9, the method can implement a drying step 11 in order to dry the fiber-based paper comprising the at least one transparent area before a rolling step 13.

[0029] According to a preferred embodiment, the coated fiber-based substrate after the at least partial drying step 7 contains at most 20 wt% water, preferably at most 10 wt% water, most preferably at most 7 wt%; before the calendering step 9. In other words, the coated fiber-based substrate can be quite dried before the calendaring step 9. The inventors surprisingly found that having a water content of at most 20 wt% based on the total weight of the coated fiber-based substrate prevents breaks during the calendaring step 9. According to this preferred embodiment, there is no need to implement the drying step 11 before the rolling step 13.

[0030] The coating composition is applied on at least one area of at least one side of the fiber-based substrate. The coating composition may also be applied on the entire area of at least one side of the fiber-based substrate. The coating composition may also be applied on at least one area of both sides (the first and second sides) of the fiber-based substrate. In some embodiments the coating composition may be applied on the entire area of both sides of the fiber-based substrate. However, applying a coating composition on one side of the fiber-based substrate, either on at least one area or on the entire area of the fiber-based substrate, already provides good results in terms of transparency. The at least one area on which the coating composition is applied covers a surface of the fiber-based substrate from at least 1%, preferably from at least 5%, more preferably from at least 10% of the surface of the fiber-based substrate, to the entire area of the fiber-based substrate to form an at least one transparent area on the fiber-based paper.

[0031] Applying the coating composition before the calendering step 9 allows the coating composition to penetrate into the fibers and to cover them before the calendering step 9 blocks up the pores and densifies the paper. By proceeding in this order, the coating composition fills the voids more efficiently than would be the case if the calendering step 9 preceded the step of applying the coating composition 5. Furthermore, the calendering step 9 enables to obtain a very smooth surface of the coating composition on the fiber-based substrate and therefore prevents some diffraction phenomenon at the surface of the fiber-based paper enabling an enhancement of the overall transparency of the fiber-based paper.

[0032] Preferably, the method is performed on-line on a paper machine. In some embodiments, the method is performed on-line directly after the production of a paper web on the paper machine. According to another embodiment, the coating composition can be provided on an already produced fiber-based substrate to enhance its transparency.

[0033] In some embodiments, the at least one transparent area on the fiber-based paper may have a transparency level of at least 70%, preferably of at least 80%, more preferably of at least 85%, when measured according to DIN 53147:1993-01 standard.

[0034] The coating composition is applied on the fiber-based substrate in a dry coat weight of at least 0.05 g/m², preferably at least 0.30 g/m, preferably at least 0.50 g/m², preferably at least 0.70 g/m², preferably at least 1 g/m², preferably at least 1.30 g/m², more preferably at least 1.50 g/m² and in a dry coat weight of less than 10 g/m², preferably less than 7 g/m², more preferably less than 5 g/m², still more preferably less than 4 g/m². Any combinations of these end points are also possible, for example in some embodiments, the coating composition is on the fiber-based substrate in a dry coat weight of between 0.05 and 10 g/m²; of between 0.05 and 7 g/m², typically between 0.50 and 5 g/m², preferably between 1 and 3 g/m². Other ranges for the coating composition on the fiber-based substrate are also possible. The amount of the coating composition may thus be reduced compared to the prior art processes due to the calendering step performed after the coating step, wherein depending on the basis weight and structure of the paper, dry coat weight of over 5 g/m² and up to 15 g/m² or even more can be recommended.

[0035] The coating composition may comprise the transparentizing agent and optionally additives. If present, the additives are present in an amount of less than 20%, preferably less than 10%, preferably less than 5%, more preferably less than 3% based on the total dry weight of the coating composition. The additives may comprise rheology modifiers, anti-foam additives, wetting agents, coloring agents such as pigments dyes or oxides, binders, dispersants, lubricants, optical brightening agents, preservatives, or UV stabilizers. According to an embodiment, the coating composition comprises a coloring agent in addition to the transparentizing agent in order to produce a colored transparentized area on the fiber-based substrate.

Transparentizing agents

[0036] The coating compositions used in the present process may include various transparentizing agents or a mixture of different transparentizing agents. In some embodiments, the transparentizing agent is selected from silanes and/or siloxanes, waxes, oils, typically oils with a siccative agent, resins, water soluble polymers and/or cellulose derivatives, and combinations thereof.

[0037] The transparentizing agent may comprise silanes and/or siloxanes. In some embodiments, silanes may be selected from trimethylethoxysilane, trimethoxymethylsilane, methyltriethoxysilane, methyltrimethoxysilane, trimethoxysilane, and triethoxysilane, to name a few. Siloxanes may comprise for example polydimethylsiloxane.

[0038] When the transparentizing agent comprise waxes, the waxes are typically selected from fully bio-based waxes and/or partially bio-based waxes, preferably from fully bio-based waxes. Examples of preferred waxes include waxes having a melting point of above 45 °C, preferably between 45 and 130°C, more preferably between 65°C and 120°C, still more preferably between 70°C and 100°C. Waxes having such melting points enable the transparentizing agent to remain hard at room temperature and also prevents some dusting of the paper machine or roll pollution on machine parts. According to some embodiments, the transparentizing agent can comprise synthetic or petroleum-derived waxes like paraffin for example.

[0039] In some embodiments, the transparentizing agent may comprise oils, typically oils with a siccative agent. Examples of oils include but are not limited to vegetable oils such as linseed oil, rapeseed oil, and soybean oil. The siccative agent can be chosen among cobalt salts, manganese salts, lead salts, zirconium salt, calcium salts, zinc salts and iron salts

[0040] If the transparentizing agent comprises resins, said resins are typically selected from acrylic resins, polyethylene resins, polyester resins, urethane resins, nitrocellulose resin, shellac, rosin, preferably from acrylic resins and rosin.

[0041] In some embodiments, the transparentizing agent may comprise water soluble polymers and/or cellulose derivatives.

[0042] The transparentizing agent may comprise or consist of a mixture of different transparentizing agents.

[0043] The transparentizing agent may have a refractive index in the range of 1.3-1.7, preferably of 1.4-1.6, when measured at 589 nm. The indicated refractive index values provide a range wherein the reflection of light on and inside the material is minimized. Indeed, the transparentizing agent should have a refractive index close to the one of the material forming the fiber-based substrate in order to limit the distorsion of the light when passing through the fiber-based paper and therefore preventing the loss of light passing through the at least one transparent area on the fiber-based paper.

[0044] The transparentizing agent may be applied on the fiber-based substrate in a dry coat weight of at least 0.05 g/m², preferably at least 0.30 g/m², preferably at least 0.50 g/m², preferably at least 0.70 g/m², preferably at least 1 g/m², preferably at least 1.30 g/m², more preferably at least 1.50 g/m², and in a dry coat weight of less than 10 g/m², preferably less than 7 g/m², more preferably less than 5 g/m², still more preferably less than 4 g/m². Any combinations of these end points are also possible such as between 0.05 and 10 g/m², or between 0.05 and 7 g/m², typically between 0.50 and 5 g/m², preferably between 1 and 4 g/m². Other ranges are also possible.

[0045] The fiber-based paper obtained by the present method, comprising or consisting essentially of the fiber-based substrate and the coating composition, typically has a basis weight of at least 25.05 g/m², preferably of at least 30 g/m², more preferably of at least 35 g/m² and less than 160 g/m², preferably less than 140 g/m², preferably less than 120 g/m², preferably less than 100 g/m², more preferably less than 75 g/m². Any combinations of these end points are also possible, the fiber-based paper can have a basis weight of between 25.05 g/m² and 160 g/m², of between 30 g/m² and 140 g/m², of between 35 g/m² and 120 g/m², of between 40 g/m² and 100 g/m², more preferably of between 40 g/m² and 80 g/m². Other ranges for the fiber-based paper are also possible.

[0046] In some embodiments, the coating composition represents at most 30 wt% of the dry weight of the fiber-based paper, preferably at most 20 wt%, more preferably at most 10 wt% and at least 1.5 wt%, preferably at least 3 wt%, more preferably at least 5 wt% of the dry weight of the fiber-based paper. Any combinations of these end points are also possible, the coating composition can represent from 1.5 to 30 wt%, more preferably from 3 to 20 wt%, still more preferably from 3 to 10 wt%, or from 5 to 10 wt% of the dry weight of the fiber-based paper. Other ranges are also possible. As indicated previously, in the case of high grammages of the fiber-based substrate, it is possible to have more than 15 g/m² of coating composition applied on the fiber-based substrate.

[0047] The fiber-based paper obtained by the present method comprises at least one transparent area, wherein the at least one transparent area has a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard. Typically, the at least one transparent area extends on a surface of the fiber-based paper from at least 1%; preferably from at least 5%, more preferably from at least 10%, of the surface of the fiber-based paper to the entire area of the fiber-based paper. According to some specific embodiments, the at least one transparent area has a transparency of at least 70%, preferably at least 80%, more preferably at least 85% when measured according to DIN 53147:1993-01 standard.

[0048] In some embodiments, the fiber-based paper has a thickness of at least 20 µm, preferably of at least 25 µm, more preferably of at least 30 µm, still more preferably of at least 40 µm and of less than 150 µm, preferably less than 120 µm, preferably of less than 90 µm, more preferably of less than 80 µm, still more preferably of less than 70 µm, when measured according to ISO 534 standard. Any combinations of these end points are also possible, for example the fiber-based paper can have a thickness of 20 to 150 µm, of 25 to 120 µm, of 30 µm to 120 µm, of 35 µm to 90 µm, or also of 40 µm to 120 µm. Other ranges are also possible.

[0049] In some embodiments, the transparent paper is advantageously recyclable and/or repulpable. In some embodiments, the transparent paper may be compostable. In addition, the transparent paper provides sufficient mechanical properties for a wide range of applications in terms of for example tearing and burst. Indeed, the fiber-based paper disclosed here-above can have a basis weight sufficiently high to present some mechanical properties (like tear and burst for example) required for its use in several applications as well as the level of transparency needed to enable the end user to see through this fiber-based paper.

[0050] With reference to Figures 2 and 3, there are represented two pictures showing a fiber-based paper 20 comprising at least one transparent area 21 and at least one non-transparent area 23. The fiber-based substrate used for the preparation of these samples of fiber-based paper 20 had an original transparency of 59%, corresponding also to the transparency of the non-transparent areas 23 as well. The coating composition comprising the transparency agent has been applied to the fiber-based substrate by a direct roll gravure technique in order to enable the formation of several transparent areas 21 having different shapes on the fiber-based substrate before the calendaring step is performed.

[0051] More particularly, Figure 2 shows a picture of a fiber-based paper 20 holding transparent areas 21 in the form of lines. In order to form these transparent areas 21, the coating composition containing a biobased wax (made of a blend of soybean wax and rice bran wax) having a melting point of about 70°C and a refractive index of 1.53 as a transparentizing agent has been applied on the fiber-based substrate. According to this specific embodiment, the coating composition contains only the transparentizing agent and no additive. The coating composition has been applied on the fiber-based substrate in an amount such that it is present in the transparentized areas 21 of the fiber-based paper 20 in an amount of 1.9 g/m². After the calendaring step, the transparency of the transparent area 21 is of 78% when measured according to DIN 53147:1993-01 standard. Each line forming a transparent area 21 extends on a surface of approximately 5% of the total surface of the fiber-based substrate.

[0052] Figure 3 shows a picture of a fiber-based paper 20 holding transparent areas 21 in the form of pyramids. In order to form these transparent areas 21, the coating composition containing a biobased wax (made of a blend of soybean wax and rice bran wax) having a melting point of about 70°C and a refractive index of 1.53 as a transparentizing agent has been applied on the fiber-based substrate. According to this specific embodiment, the coating composition contains only the transparentizing agent and no additive. The coating composition has been applied on the fiber-based substrate in an amount such that the transparentizing agent is present in the transparentized area 21 of the fiber-based paper 20 in an amount of 1.5 g/m². After the calendaring step, the transparency of the transparent area 21 is of 77% when measured according to DIN 53147:1993-01 standard. Each pyramid forming a transparent area 21 extends on a surface of approximately 1% of the total surface of the fiber-based substrate.

[0053] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0054] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0055] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0056] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

EXPERIMENTAL

Measurement methods

[0057] 

Grammage or basis weight: ISO 536 standard

Thickness: ISO 534 standard

Transparency: DIN 53147:1993-01 standard using a test equipment model BCMTS M Type 40605, with Touchscreen M software of the company Frank PTI. This test equipment uses a Konica Minolta CM-3630 Spectrophotometer. In order to measure the transparency of the transparent area, said transparent area should cover at least an area comprising a circle of 9 mm of diameter to allow testing.

Opacity: ISO 2471:2008 standard

Pulp freeness: Schopper-Riegler method and Schopper-Riegler (SR) freeness tester

Refractive index: ISO 489:2022: Plastics - Determination of refractive index

[0058] In the following examples, different fiber-based substrates have been used, said fiber-based substrate comprising cellulosic fibers and/or regenerated cellulose fibers. More particularly, the following fiber-based substrate have been used:

• A corresponds to a cellulosic based substrate comprising 83% softwood fibers and 17% hardwood fibers refined at a level of 70°SR, having a basis weight of 40 g/m² and having an initial level of transparency of 59% when measured according to DIN 53147:1993-01 standard
• B corresponds to a cellulosic based substrate comprising 85% softwood and 15% hardwood fibers refined at a level of 85°SR, having a basis weight of 40 g/m² and having a transparency of 82% when measured according to DIN 53147:1993-01 standard
• C corresponds to a cellulosic based substrate comprising 100% softwood fibers refined at a level of 72°SR and which is further supercalendered. This fiber-based has a basis weight of 64 g/m² and an initial transparency of this fiber-based substrate is of 51.4% when measured according to DIN 53147:1993-01
• D corresponds to a cellulose based substrate comprising 100% softwood fibers refined at a level of about 85°SR, having a basis weight of 35 g/m² and having an initial of transparency of 85% when measured according to DIN 53147:1993-01 standard.

[0059] Furthermore, for these different paper-based substrate, (C) (in brackets) refers to a calendered fiber-based substrate (before the transparentizing composition is coated on same) and (NC) refers to a non-calendered fiber-based substrate.

[0060] Also in the following examples, different coating compositions have been used:

• S corresponds to a rosin ester aqueous dispersion (sold under the name Dermulsene DP1904 by Les Dérivés Résiniques & Terpéniques) having a refractive index of 1.54.
• T corresponds to an acrylic polymer (sold under the name PFT6010 by CAP-SIRA) having a refractive index of 1.50
• U corresponds to a flaxseed oil without any siccative agent (sold under the reference number Linseed Oil 430021 by Sigma Aldrich) and having a refractive index of 1.48
• W corresponds to a natural oil composition containing a siccative agent and having a refractive index of between 1.54 and 1.55, disclosed for example in patent application WO 2021/215981.
• Y corresponds to a natural wax (blend of soybean wax and rice bran wax) having a melting temperature of 70°C and a refractive index of 1.53;
• Z corresponds to a silane composition having a refractive index of 1.48, disclosed for example in patent application WO 2022/112566.

[0061] For the following specific examples, the coating composition has been applied on the entire area of the fiber-based substrate.

Example 1. Grammage and composition of base paper

[0062] In some embodiments, where the effect of change in grammage, fiber composition and calendering of the fiber-based substrate was tested in the present method, the at least one transparent area on the fiber-based paper may have transparency levels indicated in Table 1.

[0063] After coating, the fiber-based substrates were calendered at 140°C, pressure 100 kN/m, speed 5 m/min on a laboratory calender.

[0064] The results in Table 1 show that fiber-based substrates having a relatively high grammage (such as 64 g/m²) when coated with a low amount of a coating composition (such as 1.7 g/m²) and subsequently calendered, resulted in transparent areas having a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard. More particularly, Table 1 shows an increase of the transparency of about 30% even with paper-base substrate having a grammage of more than 60 g/m².

[0065] Furthermore, Table 1 shows that it is also possible to further increase the transparency of an already transparent paper (transparency of more than 80%) to reach a transparency close to 90%.

Table 1. Grammage and composition of base paper

Substrate	Grammage substrate (g/m2)	Transparency substrate (%)	Chemistry	Dry amount of coating composition (g/m2)	Coating technique	Transparency coated substrate (%)	Transparency fiber-based paper (after calandering) (%)	Transparency evolution between coated substrate and fiber-based paper (%)	Transparency evolution between substrate and fiber-based paper (%)
A (NC)	40	59	NA	NA	NA	NA	NA	NA	NA
C (NC)	64	51.4	NA	NA	NA	NA	NA	NA	NA
C (NC)	64	51.4	z	1.7	Size Press	59.1	63.1	7	23
A (NC)	40	59	Y	2,8	Size Press	71,4	82,8	16	40
A (NC)	40	59	Y	2,6	Hand-Coater	74,8	86,9	16	47
B (C)	40	81.8	Y	4.6	Hand-Coater	85.98	88.6	3	8
D (C)	35	85.8	Y	3.9	Hand-Coater	87.3	89.1	2	4

Example 2. Coating technique

[0066] Different coating techniques were tested in the method of the invention using the fiber-base substrate A as defined in Example 1.

[0067] The results are shown in Table 2. As can be seen from the results, various coating compositions can be applied in low amounts (1.5 to 3.1 g/m²) with different coating techniques and then calendered to achieve a high transparency of above 60%, even above 70% or above 80%, when measured according to DIN 53147:1993-01 standard. For this example 2, the coated paper substrate were calendered with a laboratory calendered using a pressure of 300 kN/m at a temperature of 120°C.

Example 3. Coating compositions

[0068] Different coating compositions were tested in the method of the invention using a base paper having a grammage of 35-40 g/m².

[0069] The results are shown in Table 3. As can be seen from the results, different coating compositions result in transparency values above 60%, even above 70%, according to DIN 53147:1993-01 standard when applied in low amounts and when the coated fiber-based substrate is subsequently calendered with a laboratory calender using a pressure of 300 kN/m and a temperature of 120°C.

Table 3. Change in chemistry of the coating composition. Coating technique: Size press.

Substrate	Grammage substrate (g/m2)	Transparency substrate (%)	Chemistry	Dry amount of coating composition (g/m2)	Transparency coated substrate (%)	Transparency evolution between base substrate and coated substrate (%)	Transparency fiber-based paper (after calandering) (%)	Transparency evolution between coated paper and fiber-based paper (%)	Transparency evolution between substrate and fiber-based paper (%)
A (NC)	40	59	NA	NA	NA	NA	NA	NA	NA	Starting material
A (NC)	40	59	U	9,8	89,3	41	NA (whitening of the paper)	NA	NA	Comparative example
A (NC)	40	59	W	1,5	63,9	8	72,9	14	24	Working examples
A (NC)	40	59	Y	2,8	71,4	13	82,8	16	40
A (NC)	35,3	59	Z	1,5	69,9	14	75,2	8	27
A (NC)	40	59	S	1,3	NA	NA	74,8	NA	27

Example 4. Change in coatweight

[0070] The influence of the coat weight of the coating composition has also been investigated and the results are presented in Table 4.

[0071] This table shows that at least with coating composition Y the coat weight can be very low (as low as 2.3 g/m²) in order to reach a transparency of the fiber-based substrate of at least 80% when measured according to DIN 53147:1993-01 standard. For these examples, the coated paper-base substrate have been calendered at a pressure of 300 kN/m and at a temperature of 120°C.

Table 4. Change in coat weight

Substrate	Grammage substrate (g/m2)	Transparency substrate (%)	Chemistry	Dry amount of coating composition (g/m2)	Transparency coated substrate (%)	Transparency evolution between substrate and coated substrate (%)	Transparency fiber-based paper (after calandering) (%)	Transparency evolution between coated substrate and fiber-based paper (%)	Transparency evolution between substrate and fiber-based paper (%)
A (NC)	40	59	NA	NA	NA	NA	NA	NA	NA	Starting material
A (NC)	40	59	Y	9,6	78,5	33	86,6	10	47	Experimental
A (NC)	40	59	Y	5,9	77,1	31	87,3	13	48
A (NC)	40	59	Y	4,8	75,2	27	87,7	17	49
A (NC)	40	59	Y	2,6	74,8	27	86,9	16	48
A (NC)	40	59	Y	2,6 + 2,6	79,6	35	87,7	10	49
A (NC)	40	59	Y	2,3	71,9	22	84,05	17	42

[0072] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0073] The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0074] At least some embodiments of the present invention find industrial application in packaging applications, including food packaging and non-food packaging, as well as in office tape base papers, transparent release liners, cosmetic wraps, cloth bags, protective sheets, lids, envelope windows and more.

CITATION LIST

Patent Literature

[0075] 

CN 109706792 A

EP 3483337 A1

EP 4471207 A1

EP 4471210 A1

WO 2024024896 A1

Claims

1. A method of producing at least one transparent area on a fiber-based paper, comprising the steps of:

- providing or producing a fiber-based substrate, said fiber-based substrate having a first side and a second side opposite to the first side;

- applying a coating composition comprising a transparentizing agent on at least one area of at least one side of the fiber-based substrate to form a coated fiber-based substrate; and

- calendering the coated fiber-based substrate to obtain the at least one transparent area on the fiber-based paper;

wherein the at least one transparent area on the fiber-based paper has a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard.

2. The method according to claim 1, wherein the coating composition is applied on the fiber-based substrate in a dry coat weight of at least 0.05 g/m², preferably at least 0.30 g/m², preferably at least 0.50 g/m², preferably at least 0.70 g/m², preferably at least 1 g/m², preferably at least 1.30 g/m², more preferably at least 1.50 g/m² and in a dry coat weight of less than 10 g/m², preferably less than 7 g/m², more preferably less than 5 g/m², still more preferably less than 4 g/m².

3. The method according to any one of the preceding claims, wherein the coating composition comprises the transparentizing agent and optionally additives, said additives being present in an amount of less than 20%, preferably less than 10%, preferably less than 5%, more preferably less than 3% based on the total dry weight of the coating composition.

4. The method according to any one of the preceding claims, wherein the fiber-based substrate has a basis weight of at least 25 g/m², preferably at least 30 g/m², more preferably at least 35 g/m², still more preferably at least 40 g/m², and less than 150 g/m², preferably less than 120 g/m², more preferably less than 90 g/m², more preferably less than 80 g/m², still more preferably less than 70 g/m²

5. The method according to any one of the preceding claims, wherein the at least one transparent area on the fiber-based paper has a transparency level of at least 70%, preferably of at least 80%, more preferably of at least 85%, when measured according to DIN 53147:1993-01 standard.

6. The method according to any one of the preceding claims, wherein the fiber-based substrate comprises at least 60 wt%, preferably at least 70 wt%, more preferably at least 80 wt%, still more preferably at least 95 wt% cellulosic fibers and/or regenerated cellulose fibers based on the total weight of the fibers forming the fiber-based substrate.

7. The method according to claim 6, wherein the cellulosic fibers and/or regenerated cellulose fibers have a freeness of at least 25°SR, preferably of at least 30°SR, more preferably of at least 35°SR, and below 95°SR, preferably below 80°SR, more preferably below 70°SR using the Schopper-Riegler method.

8. The method according to any one of the preceding claims, wherein the transparentizing agent is selected from silanes and/or siloxanes, waxes, oils with a siccative agent, resins, water soluble polymers and/or cellulose derivatives, and combinations thereof.

9. The method according to any one of the preceding claims, wherein the transparentizing agent has a refractive index in the range of 1.3-1.7, preferably of 1.4-1.6, when measured at 589 nm.

10. The method according to any one of the preceding claims, wherein the at least one transparent area extends on a surface of the fiber-based substrate from at least 1%; preferably from at least 5%, more preferably from at least 10%, of the surface of the fiber-based substrate to the entire area of the fiber-based substrate.

11. The method according to any one of the preceding claims, wherein the coating composition is applied by a technique selected from roll-to-roll coating, blade coating, spray coating, Meyer rod coating, air knife coating, direct gravure, offset gravure, reverse gravure, deep coating, smooth roll coating, curtain coating, bead coating, slot coating, twin HSM coating, film press coating, size press coating, and/or transfer film method.

12. The method according to any one of the preceding claims, wherein it implements an at least partial drying step of the coated fiber-based substrate before the calendering step.

13. The method according to any one of the preceding claims, wherein the method is performed on-line on a paper machine.

14. A fiber-based paper comprising at least one transparent area, obtained by the method according to any one of the preceding claims, wherein the at least one transparent area has a transparency level of at least 60% when measured according to DIN 53147:1993-01 standard.

15. The fiber-based paper according to claim 14, wherein the at least one transparent area has a transparency of at least 70%, preferably at least 80%, more preferably at least 85%, measured according to DIN 53147:1993-01 standard.

16. The fiber-based paper according to any one of claims 14 or 15, wherein it has a basis weight of at least 25.05 g/m², preferably of at least 30 g/m², more preferably of at least 35 g/m² and less than 160 g/m², preferably less than 140 g/m², preferably less than 120 g/m², preferably less than 100 g/m², more preferably less than 75 g/m².

17. The fiber-based paper according to any one of claims 14 to 16, wherein the at least one transparent area extends on a surface of from at least 1%; preferably from at least 5%, more preferably from at least 10%, of the surface of the fiber-based paper to the entire area of the fiber-based paper.

18. The fiber-based paper according to any one of claims 14 to 17, wherein the coating composition represents at most 30 wt% of the dry weight of the fiber-based paper, preferably at most 20 wt%, more preferably at most 10 wt% and at least 1.5 wt%, preferably at least 3 wt%, more preferably at least 5 wt% of the dry weight of the fiber-based paper.

19. Use of a fiber-based paper according to any one of claims 14 to 18 in packaging applications.

Drawing