Regeneration of stripped lining glass paper and SCK into high quality pulp

Abstract

The present invention relates to a method, a system and a product thereof in which cellophane or super-calendered kraft (SCK) paper, which has been used as a release liner substrate, is regenerated into high quality pulp. When a highly specific feedstock is used for regeneration, the characteristics of the regenerated pulp may have been adjusted at the time of regeneration. The method and the system are configured to adjust the fibrillation of the pulp such that the recycled pulp can be used to make new cellophane or SCK paper without further refining. The compatibility of the recycled pulp can be further optimized for use in the production of cellophane or SCK paper, respectively. Advantageously, the renewable material is sorted based on paper color prior to regeneration. The recycled pulp that does not require bleaching during pulp regeneration can be provided prior to introduction into new cellophane or SCK paper production.

Description

Technical Field

[0001] The present invention relates to a method and system for producing high-quality recycled pulp, wherein glassine or supercalendered kraft paper that has been used as a release liner substrate is recycled into pulp that can be introduced into a papermaking process, where the same paper type is produced. A more closed-loop recycling is achieved. Background Art

[0002] The release liner market is experiencing significant growth. Release liners can be used to protect sensitive surfaces, such as the adhesive surface of self-adhesive labels, before use. Release liners are widely used in high-speed industrial labeling processes, where the number of products to be labeled can be very large. This requires large amounts of release liners as a carrier for the labels. High-speed processes require reliable die-cutting and separation of the self-adhesive labels from the release liners. Unexpected interruptions to the labeling process due to release liner defects are problematic. Therefore, release liners require strength and a uniform surface that exhibits stable release properties. Both can be provided, but the acquisition cost is relatively high. Therefore, the expectation of high quality extends to the paper used as the release liner substrate, which should have sufficient properties to withstand the stresses applied at high-speed processes.

[0003] Glassine and supercalendered kraft paper, hereinafter abbreviated as SCK, are different types of paper used as release liner substrates due to their outstanding characteristics. Glassine and SCK paper are expensive because they are typically produced by highly refined bleached chemical pulp (hereinafter referred to as BCP). The production of such paper is a complicated process, requiring skills, a large amount of virgin wood materials and energy. The BCP for producing glassine or SCK paper is typically a pulp mixture containing a BCP made from softwood and a BCP made from hardwood. Especially the original BCP made from softwood is very expensive, and most of the furnishings (up to 70 wt % to 80 wt %) of both glassine and SCK paper are typically the original BCP made from hardwood. However, the original BCP made from softwood is preferred because the fibers in the BCP made from softwood are longer. However, a part of the longer average fiber length of the BCP made from softwood is lost due to the refining of the pulp, which is carried out before the furnish is introduced on the paper machine.

[0004] Refining is a grinding operation performed on BCP before making glassine or SCK paper, in which the pulp fibers are subjected to high shear forces. This physically modifies the pulp fibers, for example by fibrillating them, making the fiber structure looser. The degree of pulp refining can be determined using the Schopper-Riegler test, which measures the water drainage of a pulp suspension in water based on the Schopper-Riegler number (known as the SR number or °SR). Refining further reduces the average fiber length of the pulp fibers. As a result, the specific volume of the formed glassine or SCK paper is also reduced because the shorter fibers can be packed more tightly together. This also enables the production of glassine or SCK paper with higher surface smoothness and density. When producing release liner, a smooth and dense paper surface helps reduce the consumption of subsequent release coatings. However, refining also increases the water absorption of the BCP, expressed as swelling, because the loose fiber structure of the refined BCP is more accessible to water molecules. Therefore, when making glassine paper on a paper machine, refining increases the amount of water removed from the formed paper web. When the paper web is dried on the paper machine, excess water removed from the fibers can cause shrinkage, which alters the dimensions of the glassine paper and also negatively impacts paper quality, such as strength. Therefore, pulp refining has multiple downstream effects on the glassine or SCK papermaking process. While some of the effects of refining are positive and improve paper quality, others are not.

[0005] In order to balance the effects of extensive refining and obtain final paper quality properties such as smoothness, thickness, density and transparency, glassine paper and SCK paper are typically surface sized and intensively calendered. For example, glassine paper is typically calendered with the aid of a multi-nip calender or a super calender.

[0006] Sustainability is driving paper manufacturers to develop products and manufacturing methods at paper mills. While paper has been collected for recycling for a long time, recycling recycled waste paper into specific paper production processes, such as glassine or SCK paper production, presents challenges. Pulp obtained from such materials has shown reduced quality and is used in products where quality is less important.

[0007] A large percentage of industrial paper grades, such as those used for printing and writing, use different types of furnishes than those typically used when making glassine paper for release liners. Many paper types primarily intended to convey information to consumers also contain relatively large amounts of various printing inks. This is a concern because dye-based inks and pigment-based inks have different de-inking properties.

[0008] Label waste poses another type of challenge, as the material to be recycled can often contain plastic and adhesive label residues. Release liners, on the other hand, contain cured silicone polymers that have adhered to the paper surface.

[0009] As an example, US Pat. No. 5,316,621 discloses that glassine paper, used as a release liner, is extremely difficult to defibrate because it is supercalendered, made from highly beaten pulp fibers, and contains release agents such as organosilicon compounds. This publication proposes an accelerated method involving the addition of acid and elevated temperature, followed by kneading, fine screening, and mechanical stirring of the thickened pulp at a temperature below 12°C. It is anticipated that mineral pigments will be added during the process to achieve even better results.

[0010] As another example, WO2020084188 A1 discloses a backing material layer for release liner paper, the backing material layer comprising desiliconized release liner pulp that is bleached prior to use in the backing material. The recycled release liner pulp is produced by a desiliconization process and by bleaching the desiliconized pulp with a solution of hydrogen peroxide, sodium dithionite, sodium bisulfite, or sodium borohydride to remove at least some siloxane particles.

[0011] Thus, in the past, considerable technical challenges have been disclosed when attempting to reuse release liner materials without ensuring the quality of the repulped material. Summary of the Invention

[0012] The growing demand for sustainability has led to the production of considerable quantities of release liners for the labeling industry, in which paper is used as the release liner substrate. The large-scale industrial use of such release liners enables the targeted collection and sorting of used release liners for recycling. Of particular interest is the collection and sorting of release liners in which the substrate is glassine or supercalendered kraft paper. Release liners in which the substrate is glassine are hereinafter referred to as release liners glassine and are abbreviated as RGP. Release liners in which the substrate is supercalendered kraft paper are hereinafter referred to as release liners supercalendered kraft paper and are abbreviated as RSCK.

[0013] Recycled plastic (RGP) and recycled plastic (RSCK) recycling offer more sustainable means for glassine and softwood paper production, respectively, while addressing the aforementioned challenges. Due to extensive keratinization, the fibers of RGP and RSCK show signs of damage and no longer possess the same properties as fibers from virgin softwood recycled plastic (BCP). However, separating RGP or RSCK from other wastepapers provides a specific and highly homogeneous material for recycling, which already enables better adjustment of the material's properties during the recycling process.

[0014] Hereinafter, a method for producing recycled pulp from release liner glassine paper or SCK paper is disclosed, the method comprising:

[0015] - Sorting of release liner, glassine or SCK paper for recycling;

[0016] - disintegration of the sorted release liner, glassine paper or SCK paper, and detachment of non-fibrous material from the fibers in a first process stage known as the caustic circuit; and

[0017] - the removal of non-fibrous material from the fibers in a second process stage known as the cleaning circuit,

[0018] wherein the process, the caustic circuit and the cleaning circuit are configured to regulate the fibrillation of the pulp suspension so that the recycled pulp obtained from the release liner, glassine paper or SCK paper has a pulp fibrillation and drainage within a range that enables the use of recycled pulp obtained from RGP or RSCK, respectively, in the process for making glassine paper or SCK paper. Advantageously, the caustic circuit and the cleaning circuit are configured to regulate the fibrillation of the pulp so that the recycled pulp obtained from RGP or RSCK can be used in the process for making glassine paper or SCK paper without further refining.

[0019] Calendered glassine or supercalendered kraft paper can be sorted based on paper type to obtain a sorted release liner substrate. For example, over-refining and / or bleaching of the recycled pulp can be avoided. Advantageously, the recycled pulp obtained from the sorted release liner glassine or SCK paper is not bleached during or after the caustic and / or cleaning circuits. In particular, pulp produced from RGP or RSCK can be used to replace non-recycled BCP in compositions of glassine or SCK paper, respectively.

[0020] Further advantages are obtained when the sorting is performed based on paper color. When the release liner substrate is white calendered glassine or white super calendered kraft, the method disclosed above can be arranged to provide recycled pulp that does not require bleaching during pulp regeneration before introducing the recycled pulp into new glassine or SCK paper production.

[0021] Two-step sorting can further be used to facilitate the recycling of non-white RGP grades, such as buff, yellow, brown or blue. Two-step sorting can be used to facilitate the introduction of such recycled pulp into the glassine papermaking process, where the same or similar paper color is produced without bleaching the papermaking fibers.

[0022] Therefore, sorting RGP and RSCK into white or non-white paper colors is advantageous because the compatibility of the recycled pulp can thus already be adapted and optimized during the recycling process for further production of a specific glassine or SCK paper grade. Thus, a more closed loop for papermaking fibers is possible.

[0023] Recycled pulp obtained from RGP or RSCK has a pH in the alkaline range, as measured from an aqueous pulp extract. An alkaline pH during the regeneration process softens the pulp, requiring less energy for refining. However, an alkaline pH can inhibit subsequent drying of the pulp. Therefore, the pulp pH can be adjusted as needed before mixing with other pulp components. Advantageously, when the recycled pulp obtained from RGP or RSCK is used in a process for making calendered glassine or SCK paper suitable as a substrate for release liner, the recycled pulp obtained from the release liner glassine or SCK paper has a pH in the range of 6.0 to 9.1. Preferably, the pH is in the range of 7.0 to 8.5, as a high alkaline pH can inhibit the operation of cationic UV-curable silicone systems. Preferably, the pH is in the range of 7.5 to 8.2, thereby optimizing the drying and compatibility of the recycled pulp for use in glassine or SCK paper production.

[0024] Recycled pulp obtained from RGP or RSCK is refined very quickly compared to non-recycled pulp components. Recycled pulp obtained from RGP or RSCK also has a relatively high SR number compared to non-recycled bleached chemical pulp that has not yet been refined. Therefore, the recycled pulp obtained from RGP or RSCK can be used for glassine or SCK paper production without further refining. When the fibrillation and / or drainage of the recycled pulp obtained from RGP or RSCK has been pre-adjusted to a suitable level, the recycled pulp obtained from RGP or RSCK can be directly mixed with other non-recycled pulp components in the method for making glassine or RSCK paper. Advantageously, the recycled pulp obtained from RGP or RSCK has an SR number equal to or higher than 25, such as in the range of 25 to 65, preferably in the range of 30 to 60, and most preferably in the range of 40 to 55, when measured according to ISO 5267-1.

[0025] Recycled pulp obtained from RGP or RSCK includes an average fiber length in the same range as non-recycled BCP made from hardwood. However, the average fiber length of recycled pulp obtained from RGP or RSCK is significantly less than the average fiber length of non-recycled BCP made from softwood or pulp mill broke used for glassine production. The amount of virgin fiber in pulp obtained from RGP or RSCK also differs from that in non-recycled BCP. Recycled pulp obtained from RGP or RSCK typically contains particles with a length of less than 200 microns, derived from recycled pulp, in an amount equal to or greater than 10% of the total amount of fibers in the recycled pulp, such as in the range of 10% to 30%, preferably in the range of 12% to 20%, and most preferably in the range of 15% to 17%, as measured as length-weighted average fiber length by automated optical analysis using unpolarized light in accordance with ISO 16065-2:2014. Fibers of recycled pulp obtained from RGP or RSCK typically have an average fiber width of less than 25 microns, preferably in the range of 19 to 25 microns, most preferably in the range of 19 to 21 microns, when determined by automated optical analysis using unpolarized light according to ISO 16065-2:2014.

[0026] Fiber furnish analysis according to ISO 9184-4 in conjunction with ISO 9184-1 can be used for fiber identification and to determine the fiber properties of a given pulp. Combined with pulp drainage analysis, such as the measurement of the pulp water retention value and / or SR number, these analyses differentiate between recycled pulps obtained from release liner paper, glassine, and SCK paper.

[0027] Empirical studies have shown that recycled pulp obtained from RGP or RSCK has very favorable properties for glassine production throughout the entire manufacturing process at the paper machine. The fibers of recycled pulp obtained from RGP or RSCK are less accessible to water molecules. Consequently, recycled pulp obtained from RGP or RSCK inhibits moisture absorption by the raw material. Recycled pulp obtained from RGP or RSCK has a low water retention value, typically lower than that of non-recycled BCP. Therefore, when forming the paper web, the amount of recycled pulp obtained from RGP or RSCK can be used to control the dry matter content of the raw material. The reduced ability of recycled pulp obtained from RGP or RSCK to absorb moisture has also led to enhanced dewatering of the paper web in the press section of the paper machine. Consequently, upon entering the drying section, the paper web contains less moisture that needs to be evaporated. Consequently, less steam pressure is required, which improves the energy efficiency of the drying section during paper production.

[0028] The combined effect of reduced refining, improved dewatering and more efficient drying can be observed by measuring the water retention and drying behavior of the paper web. For example, as the amount of recycled pulp obtained from RGP or RSCK in the raw material increases, the water retention value decreases. This indicates that less water needs to be removed in the press section during the production of glassine or RSCK paper. Pulp analysis from paper mills further shows that when using a McNett sieve as a F according to SCAN-CM 6:05 ＜200 When fractions were measured, replacing non-recycled BCP with recycled pulp derived from RGP or RSCK in the pulp mixture resulted in an increase in the fines content of the pulp mixture. This suggests that recycled pulp derived from RGP or RSCK can be used to adjust the quality of the paper web formed on the paper machine. The experimental results also indicate a positive impact downstream in the production of glassine and SCK paper. Drainage is related to the surface condition and swelling of the fibers and is an indicator of the amount of mechanical treatment the pulp has undergone. Paper webs containing recycled pulp derived from RGP or RSCK exhibited improved drainage on the paper machine. A higher amount of recycled pulp derived from RGP or RSCK in the raw material correlated with improved drainage, resulting in lower steam pressure requirements for drying. Surprisingly, when drying glassine paper, a 0.1 bar reduction in steam pressure was achieved at a 5% by weight ratio of recycled pulp derived from RGP or RSCK in the composition. When the amount of recycled pulp derived from RGP or RSCK in the composition was 15% by weight, steam pressures of less than 0.3 bar could be used to dry the glassine paper. Therefore, a lot of energy can be saved.

[0029] Furthermore, off-line analysis at the paper machine revealed that the produced paper exhibited less shrinkage and less variability in grammage in the cross-machine direction at the paper machine, which correlated with the amount of recycled pulp obtained from RGP or RSCK. The amount of shrinkage is an indicator of dimensional stability. Less variability in grammage in the cross-machine direction at the paper machine is an indicator of a more uniform product. Consequently, calendered glassine paper or supercalendered kraft paper containing recycled pulp obtained from RGP or RSCK, respectively, exhibited improved quality characteristics. The experimental results also demonstrated reduced curl in the paper samples containing recycled pulp obtained from RGP or RSCK. The improved properties of calendered glassine paper are important when considering the use of glassine paper as the substrate onto which the release coating is subsequently applied and cured.

[0030] Typically, calendered glass paper or SCK paper has a thickness equal to or less than 120 g / m 2 , such as at 40g / m 2 Up to 120g / m 2 When producing substrates for use as release liners, lower grammages may be preferred, such as 40 g / m 2 Up to 90g / m 2In the range of 45g / m 2 Up to 70g / m 2 The lower grammage can be calendered to produce glass paper or SCK paper with less thickness and higher transparency. The thickness of glass paper or SCK paper can be controlled by calendering and is therefore related to the grammage and density.

[0031] Recycled pulp obtained from RGP or RSCK enables the quality properties of calendered glass paper or SCK paper to be maintained at a sufficient level while enabling the spent end product, release liner glass paper or SCK paper to be recycled back into the manufacturing process. In this context, a sufficient level of quality properties means that the calendered glass paper has a 3 to 1.190g / cm 3 Advantageously, the density is 1.060 g / cm 3 to 1.190g / cm 3 In the range of 1.060 g / cm 3 to 1.180g / cm 3 %, which can be measured by standard ISO 534. Advantageously, the transparency is in the range of 42% to 54%, most preferably in the range of 44% to 54%, which can be measured by standard ISO 2469. Respectively, in this context, a sufficient level of quality properties of SCK paper means a value of 1.030 g / cm 3 to 1.190g / cm 3 Advantageously, the density is 1.050 g / cm 3 to 1.190g / cm 3 In the range of 1.060 g / cm 3 to 1.180g / cm 3 Advantageously, the transparency is in the range of 38% to 54%, most preferably in the range of 40% to 52%, as can be determined by standard ISO 2469.

[0032] The combination of density and transparency is relevant because it can be used as an indicator of the compressibility level of calendered glass paper or SCK paper. Calendered glass paper or SCK paper intended for use as a release liner substrate has a thickness S parallel to the surface normal of the paper. zAppropriately low compressibility is required because release liner is typically used as a backing material for a surface material including an adhesive layer. The surface material is formed into a label using a cutting die, which is pressed against the surface material with a predefined pressure. When the release liner substrate exhibits appropriately low compressibility, the blade cuts through the surface material to a predefined depth, allowing the surface material including the adhesive layer to be peeled off around the cut area without damaging the substrate. Therefore, the combination of density and transparency indicates the suitability of calendered glassine paper or SCK paper as a release liner substrate for self-adhesive labels.

[0033] According to a first aspect, there is provided a method for producing recycled pulp, the method comprising

[0034] - receiving a release liner for recycling, wherein the release liner comprises a substrate that is calendered glassine paper or supercalendered kraft paper, the release liner comprising non-fibrous material and fibers,

[0035] - disintegration and detachment of non-fibrous material from the fibers in a first process stage denoted as a caustic circuit, thereby forming a pulp suspension; and

[0036] - removing non-fibrous material from the fibers in a second process stage denoted as cleaning circuit,

[0037] wherein the method is configured to adjust the fibrillation of the fibers such that the recycled pulp obtained from the substrate being calendered glassine paper or supercalendered kraft paper has an SR number equal to or higher than 25 when measured according to ISO 5267-1.

[0038] According to a second aspect, there is provided a recycled pulp obtained from:

[0039] - Release liner comprising calendered glassine paper as substrate or

[0040] - a release liner comprising supercalendered kraft paper as a base material,

[0041] The recycled pulp comprises an SR number equal to or higher than 25, such as in the range of 25 to 65, when measured according to ISO 5267-1.

[0042] According to a third aspect, there is provided a system adapted as a cooperating device for producing recycled pulp, the system comprising: a sorting stage configured to separate a release liner comprising calendered glassine paper as substrate or a release liner comprising supercalendered kraft paper as substrate paper from other papers;

[0043] - a caustic circuit comprising

[0044] o a high consistency pulping unit configured to operate in batch mode and in which the consistency and temperature of the suspension, the operating time of the pulping and the addition of chemical additives can be adjusted; and

[0045] o a dewatering unit configured to remove process water from the pulp suspension and increase pulp consistency; and

[0046] - a cleaning circuit comprising

[0047] o a dispersion unit configured to regulate fibrillation of fibers in the pulp suspension; and

[0048] o a second dewatering unit configured to remove process water from the pulp suspension, thereby increasing the pulp consistency,

[0049] Such that the recycled pulp obtained from the substrate has an SR number equal to or higher than 25 when measured according to ISO 5267-1.

[0050] As mentioned above, when highly specific raw materials are used for regeneration in an adapted system of a cooperating device, the properties of the recycled pulp can be adjusted during regeneration. Advantageously, the recycled pulp obtained from release liner, glassine paper, or SCK paper can be used to manufacture new paper of the same type without further refining. Consequently, over-refining of the recycled pulp obtained from release liner, glassine paper, or SCK paper can be avoided. This further leads to positive effects in the subsequent paper manufacturing process, such as improved dewatering and energy-saving drying.

[0051] An advantage of sorting RGP or RSCK based on the paper's white or non-white hue is that recycled pulp from RGP or RSCK can be produced without bleaching. Furthermore, the compatibility of the recycled pulp can be optimized for glassine or SCK paper production, respectively. Thus, the recycled pulp disclosed above is advantageously obtained from a release liner comprising calendered glassine or SCK paper as a substrate, wherein the calendered glassine or SCK paper is white in color.

[0052] Two-step sorting based on both paper type and paper color can further facilitate the recycling of non-white RGP of a specific color into a different pulp type, which can then be introduced into the glassine paper manufacturing process to produce paper of a similar or identical color. Because the same type of colorant is typically used to provide a specific hue to glassine paper, such as buff, yellow, brown, or blue, combining the same paper type with paper of the same or similar color can be advantageously used to provide a less complex and more closed-loop recycling of non-white glassine paper. Thus, advantageously, recycled pulp as disclosed above can also be obtained from a release liner comprising a calendered glassine paper as a substrate, wherein the calendered glassine paper has a specific non-white color, such as buff, yellow, brown, or blue.

[0053] Recycled pulp obtained from RGP or RSCK can be used to replace non-recycled BCP made from hardwood and / or softwood. In this document, non-recycled BCP may also be referred to as virgin BCP. Similarly, when recycled pulp obtained from RGP or RSCK is used to replace non-recycled BCP, the refining of non-recycled BCP in the glassine paper manufacturing process can be reduced. Reducing the refining of non-recycled BCP preserves the quality of the fiber. In particular, non-recycled BCP made from softwood has a longer average fiber length than other components in the raw material and can be used to improve internal bonding strength when forming a paper web. Advantageously, when measured according to ISO 5267-1, the non-recycled BCP made from softwood has a Schopper-Riegler number equal to or less than 50, such as in the range of 25 to 50, preferably in the range of 25 to 45, and most preferably in the range of 25 to 40 before being mixed with recycled pulp obtained from RGP or RSCK. Advantageously, the composition of the calendered glassine or SCK paper contains non-recycled BCP produced from softwood in an amount equal to or higher than 10 wt.-%, preferably in the range of 10 wt.-% to 50 wt.-%, most preferably in the range of 10 wt.-% to 30 wt.-%, when measured as dry matter content according to SCAN-P 39:80.

[0054] When manufacturing glassine or SCK paper on a paper machine, the retained quality of BCP fibers can be used to compensate for the negative impact that damaged fibers in recycled pulp obtained from RGP or RSCK may have on paper formation. Advantageously, the retained quality of fibers in non-recycled BCP is used to increase the proportion of recycled pulp obtained from RGP or RSCK in the glassine paper composition. Therefore, when recycled pulp obtained from RGP or RSCK is used together with non-recycled BCP in a process for manufacturing glassine or SCK paper, a synergistic effect is felt. The composition of calendered glassine or SCK paper advantageously contains recycled pulp obtained from RGP or RSCK in an amount equal to or greater than 5% by weight, more preferably equal to or greater than 10% by weight, most preferably equal to or greater than 15% by weight, or equal to or greater than 30% by weight, such as in the range of 5% to 50% by weight, preferably in the range of 10% to 45% by weight, and most preferably in the range of 15% to 30% by weight, when measured as dry matter content according to SCAN-P 39:80.

[0055] When recycled pulp is produced from sorted RGP or RSCK substrates, where the RGP or RSCK is white in color, recycled pulp can be produced and advantageously incorporated into the subsequent manufacturing process of white glassine or SCK paper without bleaching. White RGP or RSCK does not contain colorants. Thus, calendered glassine or SCK paper suitable for use as a substrate for release liner may contain fibers from non-recycled bleached chemical pulp produced from hardwood and softwood, as well as recycled pulp obtained from release liner glassine or SCK paper, which has not been bleached.

[0056] In the context of calendered glass paper, paper whiteness and white refer to the CIE L*, a*, b* color space coordinate values, where

[0057] -L* is in the range of 92 to 98,

[0058] -a* is in the range of -4 to +2, and

[0059] -b* is in the range of +3 to +9,

[0060] These values can be determined by means of the diffuse reflectance method with specular gloss eliminated in accordance with ISO 5631:2022 using standard illuminant D65 and a 10° standard observer.

[0061] In the context of supercalendered kraft paper, paper whiteness and white refer to the CIE L*, a*, b* color space coordinate values, where

[0062] -L* is in the range of 92 to 98,

[0063] -a* is in the range of -4 to +2, and

[0064] -b* is in the range of +5 to +11,

[0065] These values can be determined by means of the diffuse reflectance method with specular gloss eliminated in accordance with ISO 11475:2017 using standard illuminant D65 and a 10° standard observer.

[0066] When recycled pulp is produced from sorted RGP substrates, where the RGP is non-white in color, recycled pulp can be produced and advantageously incorporated into the subsequent manufacture of glassine paper of the same or similar color. Non-white RGP typically contains colorants, where similar colorants are used to produce specific paper colors, such as light yellow, yellow, brown, or blue glassine paper. Thus, calendered glassine paper suitable for use as a substrate for release liner paper can contain fibers from non-recycled bleached chemical pulp produced from hardwood and softwood, as well as recycled pulp obtained from release liner glassine paper, which has been sorted based on paper type and paper color.

[0067] In the context of calendered glass paper herein, pale yellow, yellow, brown or blue paper colors refer to the CIEL*, a*, b* color space coordinate values, where

[0068] -Light yellow refers to the CIE L*, a*, b* color space coordinate values of the paper, where

[0069] o L* is in the range of 87 to 96,

[0070] oa* is in the range of -4 to +8, and

[0071] ob* is in the range of +24 to +43,

[0072] - Yellow refers to the CIE L*, a*, b* color space coordinates of the paper, where

[0073] o L* is in the range of 65 to 71,

[0074] oa* is in the range of +7 to +13, and

[0075] ob* is in the range of +50 to +56,

[0076] - Brown refers to the CIE L*, a*, b* color space coordinate values of the paper, where

[0077] o L* is in the range of 64 to 70,

[0078] oa* is in the range of +3 to +9, and

[0079] ob* is in the range of +17 to +23, and

[0080] - Blue refers to the CIE L*, a*, b* color space coordinates of the paper, where

[0081] o L* is in the range of 80 to 89,

[0082] oa* is in the range of -17 to 0, and

[0083] ob* is in the range of -15 to +9,

[0084] These values can be determined from a sample of paper by means of the diffuse reflectance method with specular gloss eliminated, in accordance with ISO 5631-1:2022, using standard illuminant C and a 2° standard observer.

[0085] Objects and embodiments of the invention are further described in the independent and dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] As used herein, the symbol S x 、Sz and S y Refers to coordinate directions that are orthogonal to each other.

[0087] Figure 1 By way of example, a cross-dimensional structure of a release liner is shown, which includes a surface-sized paper substrate and a release coating.

[0088] Figure 2 By way of example, a method for producing calendered glass paper or SCK paper is described, wherein the paper is formed from a raw material containing non-recycled bleached chemical pulp and recycled pulp obtained from release liner glass paper or SCK paper. Calendered glass paper or SCK paper can be used as a substrate for release liner. Therefore, the release liner glass paper or SCK paper can be recycled and reused.

[0089] Figure 3 By way of example, a method for producing recycled pulp from release liner, such as glassine or SCK paper, is described. The method comprises a sorting stage, a caustic circuit, and a cleaning circuit for disintegrating the fibers and for separating and removing non-fibrous material from the fibers. In addition to its primary function, the method is designed to improve the fiber properties so that the recycled pulp can be used as raw material for glassine or SCK paper production without further refining.

[0090] Figure 4 Shown are data comparing the average length (mm) of fibers in recycled pulp obtained from RGP and non-recycled pulp types measured using a Valmet Fiber Image Analyzer (Valmet FS5).

[0091] Figure 5 Shown are data comparing the average fiber width (micrometers) of fibers in recycled pulp obtained from RGP and non-recycled pulp types measured using a Valmet Fiber Image Analyzer (Valmet FS5).

[0092] Figure 6 Comparative data are shown for the average amount of hydrophobic particles in different pulp types when measured by means of flow cytometry.The particles have been further sorted based on the average diameter.

[0093] Figure 7 is when using McNett sieve as F according to SCAN-CM 6:05 <200 During fraction determination, the fines content at the machine chest of a paper machine is plotted as a function of the amount of recycled pulp obtained from RGP in the feedstock.

[0094] Figure 8This graph shows the trend of water retention at the machine chest of a paper machine as a function of pulp content. The amount of recycled pulp obtained from RGP is negatively correlated with the water retention value. As the amount of recycled pulp obtained from RGP increases, the water retention value decreases.

[0095] Figure 9 Is a graph showing the trend of water drainage as a function of pulp content when measured as the main steam group pressure level on a paper machine. Adding recycled pulp obtained from RGP reduces the steam demand in the pre-dryer.

[0096] Figure 10 Comparative data on the development of paper width (cm) at the reel of a paper machine when measured using the ABB Web Imaging System (WIS) are shown. y The paper shrinkage above is negatively correlated with the amount of recycled pulp obtained from RGP in the paper.

[0097] Figure 11 Shows that when in horizontal S y Comparative data on the induced curl of calendered glassine paper when measured from test pieces. The results show that the magnitude of the curl is negatively correlated with the amount of recycled pulp obtained from RGP in the furnish. Test pieces containing a higher amount of recycled pulp obtained from RGP showed less curl. DETAILED DESCRIPTION

[0098] Release liner cellophane

[0099] Release liner glass paper, abbreviated as RGP, is used to describe release liners whose substrate is calendered glass paper. In other words, RGP refers to a release liner whose substrate is calendered glass paper. Release liner SCK paper, abbreviated as RSCK, is used to describe release liners whose substrate is supercalendered kraft paper. In other words, RSCK refers to a release liner whose substrate is supercalendered kraft paper. Several aspects distinguish RGP and RSCK from other paper types collected for recycling.

[0100] Glassine and SCK paper refer to specific paper types that are both suitable for use as a substrate for release liners. Glassine is typically made from highly refined bleached chemical pulp that has been heavily calendered. This gives it a unique combination of high density, strength, and clarity—beneficial properties for a release liner substrate. SCK paper is also typically made from highly refined bleached chemical pulp that has been heavily calendered, but is not as strong as glassine.

[0101] Typical properties of defined calendered glass paper are

[0102] - a smoothness of at least 900 seconds per minute (ISO 5627),

[0103] -Equal to or less than 120g / m 2 Gram weight (ISO 536),

[0104] -Equal to or higher than 1.050g / cm 3 density (ISO 534), where density is the weight per grammage (ISO 536) per thickness (ISO 534:2011),

[0105] - a porosity equal to or less than 15,000 pm / Pas (ISO 11004) and

[0106] - a transparency equal to or higher than 40% (ISO 2469),

[0107] Parameter values correspond to the ISO standards cited in brackets.

[0108] Calendered glassine papers suitable for release liners typically have

[0109] - at 40g / m 2 Up to 120g / m 2 Gram weight within the range (ISO 536),

[0110] -at 1.050g / cm 3 to 1.190g / cm 3 Density within the range (ISO 534) and

[0111] - Transparency in the range of 40% to 56% (ISO 2469),

[0112] High transparency is preferred, such as in the range of 42% to 54%, most preferably in the range of 44% to 54% (ISO 2469).

[0113] The typical characteristics of SCK paper are

[0114] - a smoothness of at least 900 seconds per minute (ISO 5627),

[0115] -Equal to or less than 120g / m 2 Gram weight (ISO 536),

[0116] -Equal to or higher than 1.030 g / cm 3 density (ISO 534), where density is the weight per grammage (ISO 536) per thickness (ISO 534:2011),

[0117] - a porosity equal to or less than 15,000 pm / Pas (ISO 11004) and

[0118] - a transparency equal to or higher than 36% (ISO 2469),

[0119] Parameter values correspond to the ISO standards cited in brackets.

[0120] SCK paper suitable for release liner typically has

[0121] - at 50g / m 2 Up to 120g / m 2 Gram weight within the range (ISO 536),

[0122] -at 1.030g / cm 3 to 1.190g / cm 3 Density within the range (ISO 534) and

[0123] - Transparency in the range of 36% to 56% (ISO 2469),

[0124] High transparency is preferred, such as in the range of 38% to 54%, most preferably in the range of 40% to 52% (ISO 2469).

[0125] The thickness of calendered glassine or SCK paper indicates the thickness in microns after the calendering treatment before the release coating is applied. Unless otherwise stated, the thickness refers to the apparent thickness, measured as a single sheet thickness (ISO 534:2011). The glassine is calendered with a multi-nip calender or a super calender before or after the primer coating is applied. The SCK paper is calendered using a super calender before or after the primer coating is applied. Calendering enables the production of glassine or SCK paper with a high density surface and high transparency, but may result in a moderate reduction in the bursting strength, tensile strength and tear strength of the glassine. Calendering also reduces the thickness of the paper to a predefined target thickness. Glassine and SCK paper are typically surface sized with a primer coating that is chemically compatible with the silicone polymer release coating. The primer coating can be applied on one or both sides, typically at 1 g / m 2 Up to 5g / m 2 Within the range, preferably 1g / m 2 Up to 2g / m 2 Primer coatings for glassine or SCK paper typically contain water-soluble binders such as starch, polyvinyl alcohol and / or carboxymethyl cellulose.

[0126] refer to Figure 1, which discloses, by way of example, a cross-sectional view of the structure of a release liner REL1, wherein the substrate GLA1 is glassine paper. Herein, release liner REL1 refers to an industrially manufactured paper product comprising a release coating on at least one side of a calendered paper substrate GLA1. The release coating is often referred to as a release coating SIL1. The release coating can be used as a protective layer for self-adhesive labels comprising a surface material and an adhesive layer.

[0127] A method for producing a release liner REL1 comprises applying a release coating SIL1 to a paper substrate GLA1. The detackifying properties of the release coating SIL1 are typically obtained with the aid of an addition-curing silicone system in the presence of a suitable metal catalyst, such as platinum. The addition-curing silicone system comprises a reactive silicone polymer and a silane hydride crosslinker comprising functional vinyl groups, which are provided in fluid form and can be applied at a rate of about 1 g / m 2 The reactive silicone polymer is typically spread on the paper substrate GLA1. When the release coating on the paper is exposed to a crosslinking temperature (typically in the range of 65°C to 150°C), a chemical reaction is initiated that cures the release coating and anchors it to the substrate GLA1. This method makes it possible to obtain a release liner REL1 comprising a debonding and hydrophobic surface coating based on a cured silicone polymer.

[0128] When used as substrate GLA1 in release liner REL1, calendered glass paper typically comprises paper PAP1 as support layer and primer coating POL1. This also applies to SCK paper. Paper PAP1 is calendered on the paper machine in machine direction S x The machine direction refers to the direction in which the paper web and paper travel on the paper machine. The properties of paper can be determined in the machine direction as well as in the direction perpendicular to the machine direction S along the surface of the paper. x direction (called transverse S y ) is different. The paper is parallel to the surface normal of the paper in the direction S z Unlike many other paper types, the surface of glassine or SCK paper is typically not coated with mineral pigments, at least not in significant amounts. However, glassine and SCK paper often include a primer coating POL1, such as a surface sizing applied to at least one side of the paper. The surface sizing improves the paper's surface properties, such as barrier properties. An advantageous primer coating POL1 is a water-soluble polyvinyl alcohol containing hydroxyl groups. Some of the hydroxyl groups of the polyvinyl alcohol may have been modified to contain reactive groups, such as vinyl groups. This enables the polymer to participate in the crosslinking reaction of the addition-curing silicone system. The primer coating POL1 thus improves the anchoring of the debonding surface coating to the paper substrate GLA1.

[0129] Due to the high-quality hydrophobic silicone polymers used in release coatings for glassine and SCK paper today, RGP and RSCK typically have stable release values. Consequently, very little adhesive residue remains on the release liner surface after the self-adhesive label has been removed. Consequently, RGP or RSCK used as a carrier for self-adhesive labels contains very little adhesive residue.

[0130] A method for producing glass paper for release liner

[0131] refer to Figure 2 , which shows by way of example a method for producing calendered glass paper or SCK paper, the method comprising

[0132] - Refining 11a and 11b pulps PULP1 and PULP2;

[0133] - mixing 12 together pulps PULP1, PULP2, PULP3 and optionally broken BRK1 and white water WHT1 to obtain raw material MIX1;

[0134] - forming 13 a paper web at the headbox of a paper machine; and

[0135] - Forming 14 calendered glassine paper or SCK paper on a paper machine.

[0136] Calendered glass paper or SCK paper is suitable for use as substrate GLA1 in method 15 for producing release liner REL1.

[0137] In the process for producing calendered glassine or SCK paper, stock MIX 1 is obtained after mixing different pulps together 12 during stock preparation. Mixing can be performed, for example, by homogenizing stock MIX 1 in a mixer. Stock refers to the pulp mixture from which paper is made on a paper machine. Stock can also be referred to as furnish. During papermaking, stock is fed to the forming section of the paper machine. When the paper web 13 is formed in the headbox of the paper machine, a pulp suspension is required to adjust the loading during stock preparation and control fiber bonding. Therefore, the stock is typically first fed to a machine chest. The machine chest is a consistency leveling unit that provides holding time to smooth out any variations in consistency before the stock is pumped to the headbox, where the stock is evenly distributed onto the moving wire in the forming section of the paper machine. Consistency is expressed as the percentage of oven dry mass to total mass. Oven dry mass consistency is 100%. The machine chest contains a valve system unit arranged to receive feedback from an online scanner measuring basis weight, which enables adjustment of the basis weight of the paper to be formed.

[0138] Stock preparation may include loading and refining 11a, 11b pulp components PULP1, PULP2 to provide a pulp mixture with desired properties. Pulp components PULP1, PULP2 may be refined separately. Depending on the paper to be manufactured, stock MIX1 may further contain non-fiber additives, such as sizing agents.

[0139] When manufacturing glass paper containing recycled pulp obtained from RGP, the raw material includes two of non-recycled bleached chemical pulp PULP1 produced from hardwood, non-recycled bleached chemical pulp PULP2 produced from softwood, and recycled pulp PULP3 obtained from release liner glass paper. When manufacturing SCK paper containing recycled pulp obtained from RSCK, the raw material contains two of non-recycled bleached chemical pulp PULP1 produced from hardwood pulp, non-recycled bleached chemical pulp PULP2 produced from softwood pulp, and recycled pulp PULP3 obtained from release liner SCK paper. In this article, non-recycled pulp refers to the original pulp material introduced into the papermaking process for the first time. Non-recycled pulp can be bleached chemical pulp from the kraft process. Raw material MIX1 may contain broken BRK1, which refers to non-standard material produced on the papermaking machine, such as paper trimmings. Broken paper can be recycled back into the papermaking process. Broken paper can be refined before mixing 12. However, the broke has already been through at least a part of the paper making process on the paper machine and is therefore not considered as virgin pulp material when reintroduced into the paper making process.The broke is also not obtained from a release liner REL1.

[0140] When preparing the stock MIX1, white water WHT1 can also be used. White water is used to describe the slurry formed in the forming section of the paper machine when the fine particles present in the stock are discharged from the formed paper web WEB1 into a pit below the paper machine. The white water contains fine particles suspended in the stock. Fine particles are particles with a width in the range of 10 microns to 75 microns and a length of less than 0.2 mm. The white water can be recycled back to the stock preparation by means of a short circuit of the paper machine, or it can be processed and used elsewhere in the papermaking process. The amount of recycled fine particles defines the retention level, which describes the ability of the formed paper web to retain fine particles and therefore describes the balance between drainage and formability 13 of the paper web.

[0141] In the forming section of a paper machine, after a paper web WEB1 is formed 13 from a pulp suspension and dewatered, the web moves to a pressing section to further reduce its moisture content. The pressing section of a paper machine typically includes a plurality of rollers for guiding and / or pressing the web. The web then moves from the pressing section to the drying section of the paper machine. In the drying section, the web is heated to evaporate most of the remaining moisture in the web. After the drying section, the web can have a dry matter content level equal to or greater than 90% by weight, for example, in the range of 90% to 95% by weight, as measured according to SCAN-P 39:80. Thus, paper forming 14 comprises a step for reducing the moisture content of the paper web in a pressing section and a step for drying the paper web in a drying section, thereby forming paper from a raw material MIX1 containing non-recycled bleached chemical pulp PULP1 from hardwood, non-recycled bleached chemical pulp PULP2 from softwood and recycled pulp PULP3 obtained from release liner glassine or SCK paper.

[0142] Weight percent, abbreviated as wt%, is used to describe the weight fraction of a component in a composition. The weight percent of pulp is used to describe the weight fraction of pulp in a material. The weight percent of pulp in paper, when determined according to the SCANP-39:80 test method for dry matter content, represents the dry weight of the pulp in the dry paper. The dry weight of the sample is determined by weighing 20 grams of the sample on a weighing dish before and after oven drying at 105°C and eliminating the mass of the empty weighing dish from the measurement. The oven-dried pulp has been dried at 105°C until its mass is constant and then cooled in a cooler to an ambient temperature of 25°C before weighing.

[0143] As mentioned above, the raw material used to make glassine or SCK paper in this context is different because it mainly contains bleached chemical pulp made from softwood and hardwood. Recycled pulp obtained from RGP or RSCK, due to its origin, also contains bleached chemical pulp made mainly from softwood and hardwood. The surface of glassine and SCK paper is typically coated with a water-soluble polymer (such as polyvinyl alcohol) at 1 g / m 2 Up to 5g / m 2 When compared to other paper types (such as printing and writing papers), RGP or RSCK typically does not contain significant amounts of mineral fillers or coatings, such as kaolin (i.e., aluminosilicate dihydrate), clay pigments, or calcium carbonate. Therefore, the ash content of RGP or RSCK, which can be determined according to standard Tappi T 413 om-17, is usually very low, such as less than 3% by weight, and typically ranges from 1% to 3% by weight of the weight of the paper.

[0144] The properties of glassine and SCK paper are typically achieved through the use of highly refined BCP, supercalendering, and surface sizing agents. Supercalendering of glassine or SCK paper is typically carried out at temperatures ranging from 120°C to 200°C. The line pressure used for supercalendering glassine or SCK paper is typically in the range of 300 kN / m to 500 kN / m. Glassine and SCK paper are usually moistened before calendering to enhance the effect. This increases the transparency of calendered glassine or SCK paper. The transparency of calendered glassine or SCK paper is significantly higher than the typical transparency of other paper types with similar grammage. Calendering increases the surface density and transparency of the paper. Calendering also reduces the specific volume and thickness of the paper. Calendered glassine and SCK paper are very strong, with a very smooth and dense surface and excellent barrier properties. A smooth and dense surface that resists penetration by many fluids is beneficial when a release coating is applied to the paper surface.

[0145] As is evident from the characteristics disclosed above, calendered glass paper and SCK paper are not designed for printing or writing. Instead, calendered glass paper and SCK paper are typically used as a substrate GLA1 to form a release liner REL1, such as Figure 2 As indicated. Consequently, RGP or RSCK rarely contain significant amounts of printing ink. Generally speaking, RGP or RSCK is largely unprinted compared to other paper types, which facilitates its recycling 16 into pulp PULP3, which can be used to replace non-recycled bleached chemical pulp PULP2 made from softwood in the process for making calendered glassine paper or SCK paper. Consequently, RGP and RSCK possess a combination of desirable properties not available to the same degree in other paper types.

[0146] When considered from the perspective of the circular economy, RGP and RSCK are special materials. When glassine or SCK paper is produced from non-recycled BCP, the fibers are subjected to very harsh conditions. At the papermaking machine, the delignified hardwood and / or softwood fibers in the bleached chemical pulp undergo repeated drying and wetting cycles in the presence of chemicals, relatively high temperatures and high pressures. These treatments lead to irreversible changes in the fiber structure, in particular the pores formed between the cellulose fibrils. This results in a reduction in the swelling capacity of the fiber. When compared with other types of fibers (such as, for example, from unrecycled bleached chemical pulp or broke), the morphology and swelling capacity of the fibers are different. This phenomenon is unique to chemical pulping fibers. Due to this phenomenon, which is called keratinization, fibers derived from glassine or SCK paper show lower bonding ability. When producing release liner, the fibers are coated with a hydrophobic silicone polymer and heated, which exposes the fibers to further modification.

[0147] A method for producing recycled pulp from release liner glassine

[0148] refer to Figure 3. Release liner glassine has a common processing history. Separately, release liner SCK paper also has a common processing history. This enables the use of RGP or RSCK as raw material in a recycling process, which can be arranged to produce pulp with excellent properties. In order to obtain recycled pulp of sufficient quality for the method of manufacturing glassine or SCK paper, the raw material for the recycling process should contain at least 75% by weight, more preferably at least 85% by weight, most preferably at least 90% by weight of release liner glassine or SCK paper, respectively. Advantageously, the raw material for the recycling process consists essentially of release liner glassine or SCK paper. Most advantageously, with respect to release liner glassine, the raw material for the recycling process consists essentially of release liner glassine of the same or similar color, wherein the color can be white or non-white, such as light yellow, yellow, brown or blue. Therefore, the method for manufacturing recycled pulp from a release liner substrate can include a step for sorting the release liner substrate for regeneration before disintegration, thereby obtaining a sorted release liner substrate REL1, which can be a calendered glassine or a super calendered kraft paper.

[0149] A method for producing recycled pulp from a release liner substrate includes a sorting stage 20 for separating RGP or RSCK from other paper; a first process stage, designated as a caustic circuit CL1, which has the primary function of disintegrating the RGP or RSCK into pulp and separating non-fibrous material from the fibers; and a second process stage, designated as a cleaning circuit NL1, which has the primary function of separating the pulp fibers from non-fibrous material, particularly silicone particles derived from the release coating. The caustic circuit CL1 provides conditions under which the pulp fibers can swell and fibrillate. Cured silicon-based organic polymers, particularly polydimethylsiloxane, are generally water-resistant and relatively chemically inert. Therefore, under RGP or RSCK regeneration conditions, as disclosed herein, the release coating typically breaks down into fragments, hereinafter designated as silicone-based particles. In addition to their primary functions, the caustic circuit CL1 and the cleaning circuit NL1 are configured to regulate the fibrillation of the pulp suspension so that the recycled pulp PULP3 obtained from release liner glassine or SCK paper has a pulp drainage within a range that enables the use of recycled pulp obtained from RGP or RSCK, respectively, in the process for making glassine or SCK paper without further refining. The caustic circuit CL1 and the cleaning circuit NL1 provide a means of controlling the chemical load and temperature of the regeneration process, as well as a means of regulating the consistency of the suspension.

[0150] Due to their industrial use in high-speed labeling processes, RGP and RSCK can be collected in large quantities directly from industrial users. Therefore, advantageously, sorting of RGP or RSCK occurs at the location where, for example, the release liners REL1, REL2 are used during the labeling process and converted into recyclable release liner waste. For example, polyethylene-coated kraft paper can be separated and excluded from recycling at this point. Unlike water-soluble polymers or mineral coatings, polyethylene film does not dissolve into a suspension and is therefore challenging to recycle. Alternatively, sorting can be performed later at a sorting unit, for example by using visual inspection, so that the release liner REL1, glass paper, or SCK paper REL1 is separated from the other paper components REL2 and non-paper components. To the extent possible, non-paper components are rejected before entering the RGP or RSCK recycling process. Non-paper components refer to objects that typically become part of the paper recycling process unintentionally due to material handling. Non-paper components do not adhere to the paper and are rejected during the recycling process. Examples of non-paper components are plastic and film components, as well as fragments of metal, glass, or sand.

[0151] The RGP through sorting can be further separated based on the tone of paper.For example, light RGP tone (such as white and light yellow tone) can be separated with darker RGP tone (such as yellow, blue and brown RGP tone).Advantageously, wherein the paper furnish does not contain the white RGP grade of colorant and the non-white RGP grade (such as light yellow, yellow, blue and brown RGP grade) are separated.Advantageously, wherein the paper furnish does not contain the white RSCK of colorant and can also be separated with other paper grades.CIELAB color space can be used for determining color from the sample of RGP or RSCK paper.Color information can be used as the basis for accepting and / or rejecting paper with specific color (such as white, light yellow, yellow, blue or brown) or specific tone (such as non-white, non-light or dark) to carry out regeneration.

[0152] In the context of this article, white cellophane refers to the CIE L*, a*, b* color space coordinate values of the paper, where

[0153] -L* is in the range of 92 to 98,

[0154] -a* is in the range of -4 to +2, and

[0155] - b* is in the range of +3 to +9, preferably in the range of +5 to +7,

[0156] These values can be determined from a sample of paper by means of the diffuse reflectance method with specular gloss eliminated, in accordance with ISO 5631:2022, using standard illuminant D65 and a 10° standard observer.

[0157] In the context of this article, white SCK paper refers to the CIE L*, a*, b* color space coordinate values of the sheet, where

[0158] -L* is in the range of 92 to 98,

[0159] -a* is in the range of -4 to +2, and

[0160] -b* is in the range of +5 to +11,

[0161] These values can be determined from a sample of paper by means of the diffuse reflectance method with specular gloss eliminated, in accordance with ISO 5631:2022, using standard illuminant D65 and a 10° standard observer.

[0162] An advantage of sorting RGP or RSCK into white and non-white tones based on the paper's color tone is that recycled pulp obtained from RGP or RSCK can be produced without bleaching. Thus, calendered glassine paper or SCK paper suitable for use as a substrate for release liner paper can contain fibers from non-recycled bleached chemical pulp produced from hardwood and softwood, respectively, as well as recycled pulp obtained from release liner glassine paper or SCK paper, which has not been bleached.

[0163] The CIELAB color space can be further used to determine the color of non-white RGP grades, such as the pale yellow, yellow, brown, or blue hues of cellophane. Non-white RGP grades can be further sorted based on the color of the paper, so that the sorted RGP is pale yellow, yellow, brown, or blue. This enables a two-step sorting process at sorting stage 20, where the release liner is sorted first based on paper type and then based on the non-white color of the paper, as determined from the paper sample using a diffuse reflectance method with specular gloss eliminated using standard illuminant C and a 2° standard observer. This provides a highly uniform starting material for producing recycled pulp, where the paper is of the same type and further includes the same or similar colors. Typically, the same type of colorant is used to provide a specific hue, such as yellow, brown, or blue, to the cellophane. This combination of the same paper type and the same or similar color facilitates a more closed-loop recycling of the cellophane. This type of recycling process is also less complex. Therefore, a two-step sorting process can be used to facilitate the recycling of non-white RGP grades. Two-step sorting can further be used to facilitate the introduction of such recycled pulp into the glassine paper manufacturing process, where the same or similar paper color is produced.

[0164] In the context of this article, light yellow refers to the CIE L*, a*, b* color space coordinate values, where

[0165] -L* ranges from 87 to 96,

[0166] -a* is in the range of -4 to +8, and

[0167] -b* is in the range of +24 to +43,

[0168] These values can be determined from release liner glassine paper samples by means of the specular gloss-canceled diffuse reflectance method in accordance with ISO 5631-1:2022 using standard illuminant C and a 2° standard observer.

[0169] In the context of this article, yellow refers to the CIE L*, a*, b* color space coordinate values, where

[0170] -L* is in the range of 65 to 71,

[0171] -a* is in the range of +7 to +13, and

[0172] -b* is in the range of +50 to +56,

[0173] These values can be determined from release liner glassine paper samples by means of the specular gloss-canceled diffuse reflectance method in accordance with ISO 5631-1:2022 using standard illuminant C and a 2° standard observer.

[0174] In the context of this article, brown refers to the CIE L*, a*, b* color space coordinate values, where

[0175] -L* is in the range of 64 to 70,

[0176] -a* is in the range of +3 to +9, and

[0177] -b* is in the range of +17 to +23,

[0178] These values can be determined from release liner glassine paper samples by means of the specular gloss-canceled diffuse reflectance method in accordance with ISO 5631-1:2022 using standard illuminant C and a 2° standard observer.

[0179] In the context of this article, blue refers to the CIE L*, a*, b* color space coordinate values of calendered glass paper, where

[0180] -L* is in the range of 80 to 89,

[0181] -a* is in the range of -17 to 0, and

[0182] -b* is in the range of -15 to +9,

[0183] These values can be determined from release liner glassine paper samples by means of the specular gloss-canceled diffuse reflectance method in accordance with ISO 5631-1:2022 using standard illuminant C and a 2° standard observer.

[0184] Sorting can be performed mechanically, for example, using automated image analysis. The automated image analysis system may include, for example, a detection unit, a control unit, and a sorting unit configured to detect RGP or RSCK based on particle shape, size, and contrast, and to separate them from other paper products and non-paper products. The detection unit may include optical instruments capable of detecting wavelengths in the visible spectrum for detecting and identifying the color of the paper. This can be supplemented by instruments capable of detecting near-infrared light, which can provide further information about the properties of the paper material. Automated image analysis can be configured to assess paper quality based on multiple parameters, such as paper whiteness, brightness, color shading, transparency, or contrast. Compressed air and nozzles operating on a conveyor belt can be used to separate rejected and accepted materials. Advantageously, after sorting, the material contains RGP or RSCK in the range of 75% to 100% by weight, preferably 85% to 100% by weight, and most preferably 90% to 100% by weight, based on the weight of the recycled paper component. Ideally, the material sorted for recycling consists essentially of RGP or RSCK, which can be further sorted based on paper color.

[0185] The caustic circuit CL1 comprises a high-consistency pulping unit 21, a screening unit 22, a cleaning unit 23, and a dewatering unit 24. The high-consistency pulping unit 21 is arranged to operate in batch mode, which facilitates adjustment of pulping conditions. When RGP or RSCK and clean water F1 are fed to the high-consistency pulper, a pulp suspension is formed. The consistency of the pulp suspension can be adjusted by the amount of clean water F1, which can be obtained from another process. Clean water F1 can be fresh water. If desired, the consistency of the pulp suspension can be further adjusted by reusing process water F2, F3, and F4 downstream in the regeneration process. The process water circulating within the circuits CL1 and NL1 can be further used to improve fiber recovery within the circuits CL1 and NL1. To effectively disintegrate the RGP or RSCK, the consistency of the material during pulping should be above 15% by weight, preferably above 18% by weight, such as in the range of 20% to 30% by weight, and advantageously in the range of 20% to 25% by weight.

[0186] Pulping of RGP or RSCK is carried out under alkaline conditions to promote the disintegration of cellulose fibers from RGP or RSCK, as both RGP and RSCK include a dense surface, a polymer primer coating, and a release coating. Advantageously, during pulping, the pH is maintained in a range of 8.5 to 10. The pH can be adjusted by adding NaOH (known as caustic soda). Caustic soda reacts with the hydrogen groups of the fibers and promotes fiber swelling (known as alkali swelling), which will relax the fiber network of RGP and RSCK. Caustic soda also acts as an activator for hydrogen peroxide, which can be used to promote oxidative bleaching when the pulp suspension contains a colorant (e.g., a blue colorant from non-white grades of RGP or RSCK). Hydrogen peroxide is also used to prevent yellowing during pulping. Typically, hydrogen peroxide is added in a range of 0.5% to 2% by weight. Sodium silicate is typically added to buffer the pH of the pulp suspension and prevent the pH of the suspension from rising excessively at the beginning of pulping. Thus, sodium silicate contributes to the alkalinity of the pulp suspension, creating conditions suitable for caustic swelling. Sodium silicate can also serve as a stabilizer for hydrogen peroxide. Sodium silicate can further improve the detachment of the release liner from the fibers. Typically, sodium silicate is added in an amount ranging from 1% to 6% by weight. In addition to sodium silicate, a saponifying agent (typically a fatty acid such as palmitic acid or stearic acid) is used to promote the detachment of silicone-based particles and other hydrophobic impurities from the fibers. The fatty acid first reacts with the caustic soda and then reacts with the calcium ions present in the pulp suspension to form calcium soap, which is insoluble in water and finely dispersed in the aqueous phase. The highly hydrophobic soap particles help keep the pulping fibers in the pulp suspension and the detached hydrophobic particles (such as silicone-based particles) separated from each other. Typically, the fatty acid is used in an amount ranging from 0.1% to 1.5% by weight of the RGP or RSCK, depending on the composition of the sorted release liner substrate. The fatty acid dosage is advantageously matched to the water hardness so that the amount of fatty acid is substantially equal to the amount of calcium ions present in the suspension.

[0187] Depending on the HC pulper type, the pulping operation time can be adjusted. The total operation time (called pulping or residence time) is generally in the range of 30 minutes to 60 minutes, preferably at least 40 minutes, to ensure sufficient disintegration of the cellulose fibers. Typically, the temperature of the pulp suspension during pulping is at least 60°C, preferably at least 75°C, such as in the range of 60°C to 85°C. The primer coating of RGP and RSCK typically contains water-soluble polymers, such as partially or fully hydrolyzed polyvinyl alcohol, carboxymethyl cellulose and / or starch, which have a tendency to agglomerate at elevated temperatures. Although at least some water-soluble polymers can be dissolved during pulping and thus filtered out in subsequent dewatering operations, higher pulp suspension temperatures, preferably at least 75°C, promote agglomeration of any undissolved water-soluble polymers that have been detached from the fibers. Agglomerated polymer particles from the sizing agent or release coating are more easily removed in subsequent screening and cleaning operations.

[0188] Therefore, high consistency suspension, sufficient time, temperature and chemical additives such as hydrogen peroxide, sodium silicate (water glass) and caustic soda (NaOH) can be used to disintegrate and detach the fibers of RGP and RSCK and induce caustic swelling despite fiber keratinization.

[0189] A coarse screening unit 22, such as a disc screen having a hole size equal to or less than 4 mm, such as in the range of 2 mm to 4 mm, preferably in the range of 2.0 mm to 3.0 mm, and most preferably in the range of 2.2 mm to 2.5 mm, is used to separate the particles from the pulper based on their size, form, and shape. The screen operates under pressure, and particles that pass through the holes are accepted, while other particles are rejected. This enables the removal of solid contaminants and non-paper components, such as sand and metal objects, as well as larger particle agglomerates, from the pulp suspension.

[0190] A high consistency cleaning unit 23, such as a cleaner using a centrifugal field, is used to supplement the coarse screening to separate the pulp fibers from the contaminants based on specific gravity. A centrifugal cleaner can remove particles as small as 10 microns. In addition to heavy particles such as sand and metals, a centrifugal cleaner can also separate light particles present in RGP or RSCK, such as polymer particles, peel coating agglomerates, or residual adhesive stickies, when their density differs sufficiently from that of water. For example, PVA has a typical density of 1.19 g / cm at 25°C. 3 Up to 1.35g / cm 3 The density of the metal is in the range of 1.00 g / cm, which is significantly different from the density of water. 3The separation of high-density particles can be improved by increasing the pulp suspension temperature, which reduces the water density. The pulp suspension temperature during high-consistency cleaning is typically in the range of 30°C to 85°C, preferably in the range of 50°C to 85°C, to facilitate the cleaning of PVA. When using high-consistency detergents, pulp consistencies of 2% to 6% by weight are typically used. The consistency of the pulp suspension during cleaning can be adjusted by adjusting the pulping and screening conditions. If necessary, the consistency of the pulp suspension can be further adjusted by reusing process water F4 downstream in the regeneration process.

[0191] A dewatering unit 24, based on pressing or filtration, serves to mechanically remove process water F4 from the pulp suspension and increase the pulp consistency. Dewatering thus separates solids from the suspension. Preferably, a disc filter, a screw press, or a twin-wire press is used for effective circuit separation between the caustic circuit CL1 and the cleaning circuit NL1. High consistency enables effective dispersion in the cleaning circuit NL1, which can be used to adjust pulp fibrillation and drainability. Effective solids removal further enables the removal of dissolved sizing agents that have not yet been screened or removed from the pulp suspension. Pressing the pulp suspension at the dewatering unit 24 results in a thickened pulp suspension containing the fibers to be retained. Advantageously, at the end of the caustic circuit CL1, the pulp suspension is thickened to a consistency equal to or greater than 20% by weight, such as in the range of 20% to 50% by weight, preferably in the range of 25% to 40% by weight.

[0192] Cleaning circuit NL1 comprises a dispersion unit 25, a flotation unit 26, a second screening unit 27, a washing unit 28, and a dewatering unit 29. The dispersion unit is used to generate shear forces sufficient to detach remaining contaminants (such as silicone-based polymers) from the fibers and adjust the average size of the contaminant particles to below 100 microns, making them suitable for removal by flotation. The dispersion unit can be operated with a thickened pulp suspension received directly from the dewatering unit 24. The method may further include a dilution tank before the dispersion unit 25 for adjusting the consistency and / or temperature of the dewatered pulp suspension. Fresh water F1 and / or process waters F2 and F3 downstream of the regeneration process can be used to adjust the consistency of the dewatered pulp suspension. The process waters F2 and F3 downstream of the regeneration process can further be used to adjust the pH of the dewatered pulp suspension. The consistency of the dewatered pulp suspension provides a means for adjusting the amount of dispersing energy applied to the pulp suspension. Advantageously, a cone or disk disperser is used for dispersion instead of a kneader. Unlike kneaders, conical and disc dispersers operate under refining-like conditions. This enables efficient and simultaneous adjustment of pulp fiber properties, allowing at least some of the fiber properties of RGP fibers lost due to keratinization to be compensated during the RGP or RSCK regeneration process. Thus, recycled pulp properties such as drainage and bulk can be optimized for processes used to manufacture glassine or SCK paper, respectively. Conical and disc-type dispersers operate in such a way that there is a negative correlation between pulp fibrillation and temperature; a lower pulp suspension temperature at the inlet is associated with a higher reduction in fibrillation. Typically, when using a stock suspension with a consistency in the range of 25% to 40% by weight, the temperature of the stock suspension at the disperser inlet is in the range of 50°C to 130°C, preferably in the range of 50°C to 85°C. Therefore, in addition to the amount of specific energy consumed (SEC), the fibrillation and / or drainage of the pulp can also be adjusted during dispersion by controlling the pulp consistency and temperature. Typically, a SEC in the range of 30 kWh / t to 150 kWh / t, preferably in the range of 40 kWh / t to 100 kWh / t, most preferably in the range of 45 kWh / t to 90 kWh / t may be used during dispersion to obtain a pulp having an SR number, when measured according to ISO 5267-1, equal to or higher than 25, such as in the range of 30 to 55. Preferably, after the caustic circuit CL1, the degree of fibrillation of the pulp is adjusted by means of dispersion in a conical disperser or a disc disperser before flotation, such that the pulp suspension temperature at the inlet of the disperser is in the range of 50° C. to 130° C., preferably in the range of 50° C. to 85° C., and the pulp suspension has been thickened to a consistency equal to or higher than 20 wt.-%, preferably in the range of 20 wt.-% to 50 wt.-%, most preferably in the range of 25 wt.-% to 40 wt.-%.

[0193] The flotation cell 26 is used to remove hydrophobic particles from the pulp suspension with the aid of air bubbles, which collide and adhere to the particles. Clean water F1 and / or process waters F2, F3 downstream of the regeneration process are used to adjust the consistency of the pulp suspension for flotation. Typically, a pulp suspension with a consistency of less than 2% by weight, such as in the range of 0.5% to 1.5% by weight, is used for flotation. The temperature of the pulp suspension during flotation is typically in the range of 40°C to 70°C. Advantageously, during flotation, the pH is maintained alkaline, in the range of between 7 and 10, preferably equal to or higher than 8.5, such as in the range of 8.5 to 10. The pH can be adjusted and buffered by adding suitable alkaline agents, such as caustic soda and sodium silicate. Soaps, such as sodium soaps, or other surfactants containing hydrophilic and hydrophobic moieties are added to act as collectors. Collectors are used to promote agglomeration of the organosilicon particles and facilitate their loading and flotation. During flotation, a low water hardness in the range of 10 dH to 20 dH (determinable according to ISO / TS 15923-2:2017) is preferred to further promote agglomeration. The flotation unit 26 may comprise several flotation cells arranged in a series.

[0194] The second screening unit 27 is used for fine screening to separate debris from the fibers from the flotation, in particular silicone particles from the release coating. The fine screening can use a slotted screen with a mesh size equal to or less than 0.25 mm, such as in the range of 0.10 mm to 0.25 mm, preferably in the range of 0.10 mm to 0.20 mm. The screening is operated under pressure, and the pulp suspension that passes through the mesh is acceptable.

[0195] A washing unit 28, such as a belt filter type machine, is used to separate particles from the pulp suspension by size. Washing is typically performed using a set of two or more rollers under wire pressure, with the wire having a mesh size ranging from 36 to 60 microns, so that particles with a maximum size of less than 30 microns are removed. Typically, a pulp suspension with a consistency equal to or less than 2% by weight, such as in the range of 0.5 to 2% by weight, is used at the inlet of the washing unit. Clean water F1 is used to wash the filtered fiber mat and adjust the consistency of the suspension during washing. The filtrate is used to remove dissolved contaminants. The filtrate can be used as process water F3 upstream of the regeneration process.

[0196] After washing, a second dewatering unit 29, based on pressing or filtration, is used to mechanically remove process water F2 from the washed pulp suspension. Due to the relatively low consistency of the pulp after the washing unit, a twin-wire press is preferred so that the pulp consistency can be efficiently increased for transport or storage. Advantageously, at the end of cleaning loop NL1, the pulp suspension is thickened to a consistency equal to or higher than 30% by weight, preferably equal to or higher than 40% by weight, such as in the range of 30% to 50% by weight. Thus, the recycled pulp obtained from release liner glass paper PULP3 can then be used in a process for producing calendered glass paper.

[0197] As a transition to the above disclosure, and with reference to Figure 2 and Figure 3 The regeneration process 16 is arranged to include operations and conditions that optimize the separation of fibers from non-fiber components in the pulp suspension. At the same time, the caustic circuit and the cleaning circuit are configured to adjust the fibrillation of the pulp suspension so as to obtain a pulp drainage property that is within a range that allows the recycled pulp obtained from RGP or RSCK, respectively, to be used in a process for making glassine paper or SCK paper, preferably without further refining.

[0198] Thus, the regeneration process 16 is arranged to improve the fiber properties so that the recycled pulp PULP3 from RGP or RSCK can be used to prepare raw materials for glassine or SCK paper manufacturing, respectively, without further refining. These operations and conditions homogenize the pulp and develop properties such as pulp fibrillation, drainage, and pH, which improve the quality of the pulp used in the process of manufacturing glassine or SCK paper.

[0199] A pulp consistency in the range of 30% to 50% by weight is advantageous because the pulp fibers are not exposed to further drying treatments that could lead to further keratinization. A pulp consistency in the range of 30% to 50% by weight is also advantageous when the recycled pulp PULP3 is mixed with different pulps during stock preparation. However, when preparing recycled pulp for storage, the dewatering unit 29 can be supplemented with a drying unit (such as a fluffer) to increase the dryness of the pulp so that a pulp consistency equal to or higher than 80%, such as in the range of 80% to 90% by weight, is obtained.

[0200] Properties of recycled pulp obtained from release liner glassine

[0201] As mentioned above, recycled pulp obtained from RGP or RSCK typically has a neutral or alkaline pH, as measured from an aqueous pulp extract. An alkaline pH during regeneration is preferred because a higher pH softens the pulp and promotes flotation. Pulp alkalinity also contributes to changes in pulp fibrillation and drainage. Recycled pulp obtained from RGP or RSCK requires less energy for refining when it has an alkaline pH. However, if desired, the pH can be adjusted before using the recycled pulp.

[0202] Recycled pulp obtained from RGP or RSCK differs from non-recycled BCP due to the degree of fiber keratinization. This can be measured, for example, by the water retention value (abbreviated as WRV) according to ISO 23714:2014 (en). WRV is an empirical measure of the ability of a pulp sample to retain water. Typically, the WRV of recycled pulp obtained from RGP is low, such as in the range of 1.3 g / g to 1.6 g / g.

[0203] Recycled pulps obtained from RGP are also distinguished by their drainage resistance, which is a measure of the fibrillation of the pulp and can be determined by the Schopper-Riegler test. The SR number is a measure of the degree of fibrillation in the recycled pulp PULP3. Recycled pulps obtained from RGP or RSCK may have an SR number equal to or higher than 25, such as in the range of 25 to 65, when measured according to ISO 5267-1. Typically, recycled pulps obtained from RGP or RSCK have an SR number equal to or higher than 30 if the aqueous extract in which the measurement is made is process water containing electrolytes. When the drainage resistance of dry pulp is measured with standard water according to ISO 5267-1 in combination with ISO 14487, the SR number can be higher, such as equal to or higher than 40, because the concentration of electrolytes (salts) in the pulp suspension affects the drainability. Regardless of the initial SR number, the SR number of recycled pulps obtained from RGP and RSCK develops very rapidly upon refining. This is a characteristic of recycled pulp obtained from RGP, which can be used to distinguish recycled pulp obtained from RGP from other non-recycled pulp components used in glassine paper. This is also a characteristic of recycled pulp obtained from RSCK, which can be used to distinguish recycled pulp obtained therefrom from other non-recycled pulp components used in SCK paper. Table 1 (below) shows, by way of example, the development of the SR number (°SR) in recycled pulp obtained from RGP as a function of the specific energy consumption (SEC) in kWh / t. In this example, a specific edge load (SEL) of 0.3 J / m was applied using a Voith-Sulzer laboratory refiner with 40D hardwood boards. Before refining, the recycled pulp obtained from RGP exhibited an SR number of 32.

[0204] Table 1. Development of SR in recycled pulp obtained from RGP according to SEC (kWh / t).

[0205]

[0206]

[0207] Advantageously, the recycled pulp obtained from release liner glass paper has, before mixing in the process for making calendered glass paper, a °SR, when measured according to ISO 5267-1, equal to or higher than 25, such as in the range of 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55. Advantageously, also before mixing in the process for making calendered SCK paper, the recycled pulp obtained from release liner SCK paper has, when measured according to ISO 5267-1, a °SR, when measured according to ISO 5267-1, equal to or higher than 25, such as in the range of 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55.

[0208] Advantageously, when recycled pulp obtained from RGP or RSCK is used in a process for producing calendered glassine or SCK paper suitable as a substrate for release liner, the recycled pulp obtained from RGP or RSCK has a pH in the range of 6.0 to 9.1. Preferably, the pH is slightly alkaline, such as in the range of 7.0 to 8.5. Recycled pulp obtained from release liner glassine with an alkaline pH requires less energy to refine the fibers. A high alkaline pH can inhibit the operation of cationic UV-curable silicone systems. Preferably, the pH is in the range of 7.5 to 8.2, thereby optimizing the drying and compatibility of the recycled pulp for use in glassine or SCK paper production. When determining the pH of a dried pulp sample, standard ISO 6588-2 (2020) can be used. When determining the pH of a pulp suspension sample from a papermaking machine, the pH can be measured directly from the pulp sample (when the consistency is 5% by weight or less) or from the filtrate (when the consistency is higher than 5% by weight). As used herein, filtrate refers to an aqueous extract. When determining the pH of dry pulp, 2 grams of dry pulp are cut into pieces so that each piece has a maximum dimension of 1 cm. The cut pieces are mixed with 100 ml of deionized water to disperse the pulp, resulting in a suspension having a pulp concentration of 2% by weight of water. The sample thus obtained is heated to boiling point and boiled for 60 minutes. After boiling, the sample is cooled so that the sample temperature is within the range of 20°C to 25°C and filtered through a filter with a 200-mesh mesh, for example, using a Buchner funnel, to obtain a filtrate separated from the pulp. The pH is measured from the filtrate thus obtained.

[0209] The pulp pH is measured from the filtrate or aqueous extract at a temperature in the range of 20° C. to 25° C. using two buffer solutions, respectively having a pH of 4 and a pH of 7. Suitable pH meters are, for example, the pH meter CG 840 with electrode N 1042A, the Knick pH meter 766 Calimatic with electrode SE 103, or the Mettler-Toledo MP 120, which are used according to the manufacturer's instructions.

[0210] When recycled pulp is produced from release liners such as glassine or SCK, as disclosed above, the removal of silicone-based particles is incomplete. Recycled pulp from RGP and RSCK, respectively, still contains traces of cured release coating in the form of very small, chemically quite inert particles. The maximum particle size of the silicone-based particles is typically in the range of 100 to 150 microns and is limited by the mesh size of the fine screening used in the cleaning circuit NL1. Although detectable, the amount of silicone-based particles in recycled pulp from RGP or RSCK has not been observed to cause problems when producing calendered glassine or SCK paper on paper machines. The amount of silicone-based particles can be measured using energy-dispersive X-ray spectroscopy from test samples burned at 900°C, according to Tappi Standard T 413 for the detection of silicon oxides. Typically, calendered glassine paper comprising recycled pulp from RGP or SCK comprising recycled pulp from RSCK contains silicon in an amount equal to or less than 0.3 wt.-%, preferably equal to or less than 0.28 wt.-%, most preferably equal to or higher than 0.25 wt.-%, such as in an amount in the range of 0.01 wt.-% to 0.3 wt.-%, which can be determined as dry matter content from a paper sample burned at 900° C. using energy dispersive X-ray spectroscopy according to Tappi standard T 413.

[0211] Experimental studies

[0212] refer to Figures 4 to 11 An experimental study was prepared to evaluate the properties of recycled pulp obtained from RGP and to determine its effect in a process for manufacturing calendered glass paper.

[0213] Experimental Study 1

[0214] In the first experimental study, the pulp properties of recycled pulp obtained from RGP were measured and compared with the properties of non-recycled bleached chemical pulp used for glassine production at the paper mill and pulp mill broke. The pulp types and their abbreviations used in the experimental study are listed below:

[0215] BCP SW Northern Bleached Softwood Kraft Pulp (Coniferous)

[0216] BCP HW Bleached Hardwood Kraft Pulp (Eucalyptus)

[0217] BCP SW rf. Paper mill refined BCP SW (SEC 240kWh / t)

[0218] BCP HW rf. Paper mill refined BCP HW (SEC 135kWh / t)

[0219] Millbroke pulp mill broke from glassine production

[0220] PULP3 Recycled pulp obtained from RGP

[0221] The consistency of the pulp in the study was 4 wt%. The properties of non-recycled bleached chemical pulp were measured before and after refining in order to compare the properties of recycled RGP and non-recycled bleached chemical pulp.

[0222] Pulp analysis

[0223] The pH of the pulps disclosed above was measured from an aqueous pulp extract according to ISO 6588-2 (2020). The results are shown in Table 2 (below).

[0224] Table 2. Measured pH of pulp samples.

[0225] sample pH BCP SW 5.3 BCP HW 5.1 PULP3 6.8 Paper mill broke 5.3

[0226] The results represent the average value of measurements, during which the pH of recycled pulp obtained from RGP varied within the range of 6.8 to 7.3. The pH measured in recycled pulp obtained from RGP was significantly higher than that measured in non-recycled chemical pulp made from softwood or hardwood. The pH measured in recycled pulp obtained from RGP was also significantly higher than that in pulp mill broke.

[0227] According to ISO standards ISO 9184-1 and 9184-4:1990, the pulp as disclosed above is further analyzed by means of fiber furnish analysis. Fiber furnish analysis enables the identification of papermaking fibers from a sample. This analysis can further be used to quantify the average size of different fiber types detected in a sample. The wood species used in the pulp can be distinguished by a comparative method in which the sample fibers are compared with known reference fibers. The Valmet fiber image analyzer (Valmet FS5) is an example of an apparatus that can be used to perform fiber furnish analysis according to the manufacturer's instructions. For example, automated optical analysis (such as an ultra-high resolution (UHD) camera system equipped with image analysis software) can be used to obtain a grayscale image of the sample from which the properties of the fibers in the sample can be determined. According to the ISO 16505-2 standard, a grayscale image can be obtained from a sample placed in a transparent sample holder (such as a cuvette) using a 0.5 mm depth of focus. The Valmet Fiber Image Analyzer (Valmet FS5) can further be used to determine fiber dimensions such as fiber length and fiber width, and length-weighted distribution of pulp fibers by means of automated optical analysis using unpolarized light according to ISO 16065-2:2014.

[0228] refer to Figure 4 , which shows the average length (in millimeters) of fibers in recycled pulp obtained from RGP and other pulp types, measured as length-weighted average fiber length using a Valmet fiber image analyzer (Valmet FS5). The recycled pulp obtained from RGP contained an average fiber length of 0.94 mm. Non-recycled BCP made from hardwood contained an average fiber length of 0.86 mm, which was reduced to 0.84 mm during refining. Therefore, the average fiber length of the recycled pulp obtained from RGP was higher than the average fiber length of the non-recycled BCP made from hardwood. Non-recycled BCP made from softwood contained an average fiber length of 2.10 mm, which was reduced to 2.00 mm during refining. Therefore, the average fiber length of the recycled pulp obtained from RGP was significantly smaller than the average fiber length of the non-recycled BCP made from softwood. Pulp mill broke had an average fiber length of 1.04 mm.

[0229] Further references Figure 5, which shows comparative data of the average fiber width (micrometers) of fibers in recycled pulp obtained from RGP and other pulp types measured using a Valmet fiber image analyzer (Valmet FS5). The recycled pulp obtained from RGP comprises an average fiber width of 20 micrometers. Non-recycled BCP made from hardwood comprises an average fiber width of 18 micrometers, which increases to 19 micrometers during refining. Therefore, the average fiber width of the recycled pulp obtained from RGP is greater than the average fiber width of the non-recycled BCP made from hardwood. Non-recycled BCP made from softwood comprises an average fiber width of 28 micrometers, which increases to 29 micrometers during refining. Therefore, the average fiber width of the recycled pulp obtained from RGP is significantly less than the average fiber width of the non-recycled BCP made from softwood. Pulp mill broke has an average fiber width of 20 micrometers.

[0230] Therefore, the average fiber length and width of recycled pulp obtained from RGP are closer to the average fiber length of non-recycled BCP made from hardwood or broke, but are significantly different from the average fiber length of non-recycled BCP made from softwood.

[0231] According to the manufacturer's instructions, the length-weighted distribution of pulp fibers was further analyzed using a Valmet fiber image analyzer (Valmet FS5). In the analysis, fibers were defined as the pulp fraction comprising particles having a width in the range of 10 microns to 75 microns and a length in the range of 0.2 mm to 7.0 mm. Fines were defined as the pulp fraction comprising particles having a width in the range of 10 microns to 75 microns and a length less than 0.2 mm. Fibrils were defined as the pulp fraction comprising particles having a width less than 10 microns and a length longer than 0.2 mm. Flakes were defined as the pulp fraction comprising particles having a width less than 200 microns and a length less than 0.2 mm. Fibrils are typically particles generated from the secondary walls of the wood cell layer structure, which can improve the adhesive properties of the pulp due to their elongated shape. Flakes are typically particles generated from the middle glue layer and primary walls of the wood cell layer structure, which tend to reduce the adhesive properties of the pulp. The flakes scatter light and can therefore affect the optical properties of the pulp by increasing opacity and reducing transparency.

[0232] The fines content in bleached chemical pulps, such as bleached kraft pulp, naturally varies depending on the wood species used. The fines content in pulp also changes due to pulp processing, such as refining and recycling, as disclosed above. The length-weighted distribution of fines is a fundamental property of pulp that, among other things, influences the formability of the paper web during manufacturing. Pulp properties also have an impact on the tensile strength, bursting strength, folding endurance, and tear resistance of the paper.

[0233] The analysis results show that the amount of fines in the recycled pulp obtained from RGP, as measured as length-weighted average fiber length by automated optical analysis using unpolarized light in accordance with ISO 16065-2:2014, was 16.3% of the total amount of fibers in the recycled pulp. The amount of fines in the recycled pulp obtained from RGP is comparable to that in non-recycled bleached chemical pulp refined in paper mills made from hardwood. Surprisingly, the amount of virgin fiber in the recycled pulp obtained from RGP was much higher than that in non-recycled bleached chemical pulp refined in paper mills made from hardwood, but lower than that in non-recycled bleached chemical pulp refined in paper mills made from softwood. The results indicate that the recycled pulp obtained from release liner glassine contains particles of recycled pulp originating from recycled pulp having a length of less than 200 microns in an amount equal to or greater than 10%, such as in the range of 10% to 30%, preferably in the range of 12% to 20%, and most preferably in the range of 15% to 17%.

[0234] The Valmet Fiber Image Analyzer also provides results on the amount of fiber deformation in the pulp, such as fiber kinks and fiber curl. Fiber kinks and curl tend to reduce the tensile strength of the resulting paper due to a reduction in the bonding ability of the fibers in the fiber network. It is noteworthy that the number of kinks in the recycled pulp obtained from RGP is 3250 1 / m, which is significantly higher than the number of kinks in non-recycled bleached chemical pulp after refining or in pulp mill broke. The number of kinks in non-recycled bleached chemical pulp made from hardwood is 2880 1 / m before refining and 2310 1 / m after refining. The number of kinks in non-recycled bleached chemical pulp made from softwood is 3410 1 / m before refining and 2730 1 / m after refining.

[0235] In the experimental study, the results of measured fiber analyses of recycled pulp obtained from RGP, non-recycled bleached chemical pulp (before and after refining at the paper mill), and pulp mill broke used for glassine production at the paper mill are presented in Table 3 (below). Comparison of the samples shows that the fiber properties and the relative amounts of fiber fractions are different in the recycled pulp obtained from RGP.

[0236] Table 3. Fiber analysis results and properties of recycled pulp obtained from RGP (PULP3), non-recycled bleached chemical pulp (before and after refining at the paper mill), and pulp mill broke used for glassine production at the paper mill.

[0237]

[0238] refer to Figure 6. The pulp as disclosed above was further analyzed based on the hydrophobic properties of the pulp. The hydrophobicity of the particles in the pulp was measured by means of flow cytometry, which is a well-known analytical method for counting, identifying and sorting particles based on selected properties. The analysis was performed using a Sysmex CyFlow Cube 6 (V2m) benchtop flow cytometer. A 20 ml representative sample was collected from the paper machine and diluted 5 times with ultrapure water, and the diluted and well-mixed sample was then filtered through a 200 mesh sieve. A 50 ml aliquot of the filtrate was collected for further dilution. A series of dilutions (in the range of 10 to 1000 times) was prepared with ultrapure water so that a suitable dilution was obtained that contained particles in an amount that produced 700 to 1000 events per second when analyzed by flow cytometry. A certain volume of 20 ml of the dilution to be analyzed was mixed with 1 ml of Nile red stain, which was used as a fluorescent marker to selectively stain the hydrophobic parts of the sample. Before analyzing the samples, the flow cytometer was calibrated to a size standard using 3 μm commercially available polystyrene beads. Relative hydrophobicity (>10) was used to gate the particles. The particles in each sample were further sorted based on their size, so that hydrophobic particles with a diameter of 1 micron or less were denoted as small, while hydrophobic particles with a diameter of more than 1 micron were denoted as large. The results showed that the recycled pulp obtained from RGP contained 2 to 3 times more large and small hydrophobic particles than non-recycled BCP made from hardwood. The recycled pulp obtained from RGP contained nearly 10 times more large and small hydrophobic particles than non-recycled BCP made from softwood. The difference from pulp mill broke was also obvious. Although the majority of the hydrophobic particles in all analyzed samples belonged to the large particle group, i.e. with a diameter of more than 1 micron, the highest relative difference between recycled pulp obtained from RGP and other pulp types was measured in the small particle group. The amount of hydrophobic particles in the samples (in pieces per milliliter (pcs / ml)) and the total amount of particles measured in the samples (pcs) are shown in Table 4 (below).

[0239] Table 4. Amount of hydrophobic particles and total particles in samples measured by flow cytometry.

[0240]

[0241]

[0242] The increase in hydrophobic particles, especially small hydrophobic particles, observed in recycled pulp obtained from RGP was expected to be due to silicone polymer residues forming the release coating. However, despite the presence of hydrophobic particles in recycled pulp obtained from RGP, no detectable problems with runnability or paper quality were observed in the experimental glass paper production.

[0243] Experimental Study 2

[0244] In the second experimental study, a 53 g / m 2 Calendered glass paper with a grammage of 100 μm and a thickness of 48 μm was used, resulting in varying amounts of recycled pulp derived from RGP in the raw material. The amount of recycled pulp derived from RGP varied between 0% and 30% by weight, based on the dry matter content of the produced glass paper, according to SCAN-P 39:80. The ratio of non-recycled bleached chemical pulp produced from hardwood to non-recycled bleached chemical pulp produced from softwood remained constant. Thus, the non-recycled BCP contained 35% by weight of non-recycled BCP produced from softwood and 65% by weight of non-recycled BCP produced from hardwood. Thus, as the amount of recycled pulp derived from RGP in the raw material increased, the amount of BCP decreased, maintaining the same proportion of non-recycled BCP from hardwood to softwood. The amount of broke remained constant at 12% by weight in all experiments.

[0245] The samples were measured at various test points. The composition containing only non-recycled bleached chemical pulp and broke, but no recycled pulp obtained from RGP, was Figures 7 to 11 The reference point is marked in FIG and abbreviated as REF. The composition containing 15 wt.% recycled pulp obtained from RGP is Figures 7 to 11 The composition containing 30 wt.% of recycled pulp obtained from RGP was Figures 7 to 11 The raw material compositions for the reference point and test points 1 and 2 are described in Table 5 (below).

[0246] Table 5. Composition of the feedstock for the reference site and test sites 1 and 2 in the experimental study. The abbreviation "BCP total." refers to the total amount of non-recycled bleached chemical pulp in the feedstock, expressed in weight percent. Broke refers to the amount of paper mill-derived pulp in the feedstock, expressed in weight percent. PULP3 refers to the amount of recycled pulp obtained from RGP in the feedstock, expressed in weight percent. The last column on the right indicates the contribution of each component (BCP SW, BCP HW, broke, PULP3) to the feedstock, totaling 100 weight percent.

[0247]

[0248] Fines content at the pulp chest (BMN method)

[0249] refer to Figure 7The effect of recycled pulp obtained from RGP on the production of glass paper was evaluated by measuring the development of the fines content at the machine chest of the paper machine as a function of the amount of recycled pulp obtained from RGP in the raw material. In this context, the fines content refers to the fibrous material in the pulp, which was classified according to SCAN-CM 6:05 using a 20-minute classification time, a set of 16-mesh, 28-mesh, 48-mesh and 200-mesh screens and a weighed filter paper (Macherey-Nagel MN616, 125 mm diameter) for collecting the fiber fraction with a McNett sieve as F ＜200 Fraction determination. This method describes a fiber fractionation procedure in which the fibers in a pulp suspension are grouped into fractions of different average fiber sizes. The mass of fiber retained in the fractions is expressed as a percentage of the dry mass of the original sample. When the relative ratio of BCP SW and BCP SW remains constant, the retained F ＜200 The fractions were used as an indicator of how much the fines content in glassine paper production changes due to increasing amounts of pulp derived from RGP, while the amount of pulp mill broke remains the same. The results demonstrate that when the amount of recycled pulp derived from RGP in the glassine paper ranges from 0% to 10% by weight, the fines content remains relatively stable, ranging from 10.2% to 10.5% by weight. However, unexpectedly, when the amount of recycled pulp derived from RGP in the glassine paper is equal to or greater than 10% by weight, the fines content begins to increase more rapidly. In particular, when the amount of recycled pulp derived from RGP in the glassine paper is equal to or greater than 15% by weight, such as in the range of 15% to 30% by weight, the fines content in the fiber furnish of the glassine paper increases very rapidly. During the experiment, the fines content increased from 10.2% to 13.8% by weight when the amount of recycled pulp derived from RGP in the glassine paper ranged from 0% to 30% by weight. The fines content has an impact on paper properties. This effect can already be detected when forming a paper web. The results show that the amount of recycled pulp obtained from RGP in the stock can be used to adjust the retention level, which describes the ability of the formed paper web to retain fine particles on the paper web and therefore describes the balance between drainage and formability of the paper web.

[0250] Water retention value in the pulping tank

[0251] refer to Figure 8 The impact of recycled pulp obtained from RGP on glass paper production was further evaluated by measuring the water retention value (abbreviated as WRV) at the machine chest according to ISO 23714:2014 (en). WRV was determined as the average of two replicate samples, each consisting of 1 g of dry pulp diluted in 500 ml of water and having a temperature of 23 ± 3 °C. The following materials and methods were used:

[0252] Beckman Coulter Avanti J-30I Laboratory Centrifuge

[0253] Centrifugal force 3000g±50g, 30 minutes

[0254] JS 7,5 rotor (speed 5350; RPM 5289)

[0255] After centrifugation, the sample was weighed for the first time. The sample was then dried overnight (12 h) at 105 ± 2°C and cooled in a desiccator to room temperature (23 ± 3°C). The sample was then weighed a second time using a laboratory balance (0,0001 g precision).

[0256] Calculate the water retention value according to the following equation 1:

[0257] Equation 1:

[0258] in

[0259] m1 = mass of the sample after centrifugation, in grams

[0260] m2 = mass of the sample after drying, in grams.

[0261] The results demonstrate that replacing non-recycled BCP with recycled pulp derived from RGP leads to a steady decrease in water retention, which is inversely proportional to the amount of recycled pulp derived from RGP in the glassine paper. Each 10% by weight replacement of non-recycled BCP with recycled pulp derived from RGP resulted in a decrease in the WRV of the glassine paper over a range of 0.1 g / g. The WRV reduction was evident across the entire range. At the reference point, the WRV was 1.98 g / g. At test point 1, the WRV was 1.83 g / g. At test point 2, the WRV was 1.72 g / g. The results of the water retention level analysis support and validate the observations from the fines content analysis disclosed above. The correlation of WRV with the amount of recycled pulp derived from RGP in the feedstock indicates that recycled pulp derived from RGP in the feedstock can be used to adjust the water retention level. The lower WRV of fibers from recycled pulp derived from RGP, compared to fibers from non-recycled BCP, is beneficial during drying. The reduced amount of water absorbed into the fiber network at the machine chest indicates that the glassine paper has better dimensional stability during drying. Therefore, taking into account the fines content and drainage trends discussed below, the calendered glassine advantageously contains equal to or less than 50% by weight, such as in the range of 5% to 50% by weight, preferably in the range of 10% to 45% by weight, and most preferably in the range of 15% to 40% by weight, of recycled pulp obtained from release liner glassine, when measured as dry matter content according to SCAN-P 39:80. In addition, the stock at the machine chest of the paper machine has a water retention value in the range of 1.5 g / g to 1.9 g / g, preferably in the range of 1.55 to 1.85, and most preferably in the range of 1.6 to 1.8, which can be determined according to ISO 23714:2014 from a sample having a dry matter content of 1 gram.

[0262] Main steam group pressure in paper drainage-drying section

[0263] refer to Figure 9 . Next the effect of recycled pulp obtained from RGP on the production of glass paper was evaluated by measuring the main steam group pressure at the paper machine during the production of glass paper. The main steam group pressure is an indication of drainage and is also direct evidence of the amount of energy consumed when drying the paper. The results demonstrate that drainage improves when the amount of recycled pulp obtained from RGP in the glass paper increases. The formed glass paper has a higher dry matter content. Furthermore, glass paper comprising a higher amount of recycled pulp obtained from RGP requires less steam pressure for drying. Surprisingly, drainage appears to be most effective when the amount of recycled pulp obtained from RGP in the glass paper is equal to or less than 15 wt%, such as in the range of 5 wt% to 15 wt%, as measured by means of the main steam group pressure. As Figure 9As shown, an amount of 5 wt% recycled pulp obtained from RGP in the composition already requires a steam pressure of less than 0.1 bar to dry the glassine paper. An amount of 15 wt% recycled pulp obtained from RGP in the composition already requires a steam pressure of less than 0.3 bar to dry the glassine paper.

[0264] Paper transverse profiling at the winder

[0265] The effect of recycled pulp obtained from RGP on the production of glass paper was further evaluated in the drying section. The density of the calendered glass paper samples was 1100 g / m 3 ±11g / m 3 The results show that the properties of the samples produced according to the reference point and test point compositions are presented in Table 6 (below). For all compositions (REF, TP1, TP2), the test point was run at the same speed and settings, making it possible to evaluate the effect of recycled pulp obtained from RGP on calendered glass paper.

[0266] Table 6. Properties of calendered glassine paper samples.

[0267]

[0268] The results show that recycled pulp obtained from RGP makes it possible to maintain the quality properties of calendered glass paper, such as density and transparency, at a sufficient level. The combination of retained density and transparency serves as an indirect indicator of this.

[0269] refer to Figure 10 The paper width was measured from the calendered glass paper samples at the reference point, test point 1, and test point 2. The paper width (in centimeters) at the reel was measured with the aid of a Web Imaging System (WIS), an automated image analysis system supplied by ABB. The system was used according to the manufacturer's instructions. Figure 10 The width of the paper shown is perpendicular to the machine direction S x Horizontal S yThe WIS results are the average of 8 measurements taken along the surface of the paper. The results demonstrate that replacing non-recycled BCP with recycled pulp obtained from RGP leads to a reduction in the shrinkage of glassine paper, which is proportional to the amount of recycled pulp obtained from RGP in the glassine paper. Replacing 15 wt.% of the non-recycled BCP with recycled pulp obtained from RGP resulted in glassine paper that exhibited 3 cm less shrinkage than the reference value. Replacing 30 wt.% of the non-recycled BCP with recycled pulp obtained from RGP resulted in glassine paper that exhibited 4 cm less shrinkage than the reference value. The WIS results were verified in an independent test run, in which the paper was profiled off-line at the winder from 30 calendered and uncalendered paper samples with the aid of Tapio PMA (automated paper quality control system provided by Tapiotechnologies). The system was used according to the manufacturer's instructions. The results of the latter independent test run with Tapio PMA verified the paper width results of the first test run. In the samples without recycled pulp obtained from RGP (REF), in the transverse direction S y The shrinkage measured along the surface of the paper was 3.6%. In the sample (TP1) containing 15 wt.% of recycled pulp obtained from RGP, the shrinkage in the transverse direction S y The shrinkage measured along the surface of the paper was 3.0%. In the sample (TP2) containing 30 wt.% of recycled pulp obtained from RGP, the shrinkage in the transverse direction S y During a subsequent test run, the shrinkage of the uncalendered paper along the paper surface in the transverse direction S was also determined simultaneously from 30 paper samples according to the manufacturer's instructions using the Tapio PMA. y The variability analysis of grammage showed that the standard deviation of the samples at test points 1 and 2 containing recycled pulp obtained from RGP was 0.5 g / m 2 This is at the same level as the standard deviation of the samples at the reference point, which does not contain recycled pulp obtained from RGP (REF). In all the measured sample compositions (REF, TP1, TP2), the variability in grammage (maximum - minimum) is within 3.1 g / m 2 Up to 3.6g / m 2within the range of . The thickness variability analysis showed that the standard deviation of the samples containing recycled pulp obtained from RGP at test points 1 and 2 (TP1, TP2) was 0.4 μm, which was at the same level as the standard deviation of the samples at the reference point, which did not contain recycled pulp obtained from RGP (REF). However, when the amount of recycled pulp obtained from RGP was larger, the thickness variability (maximum-minimum) showed a reduced variability. In the reference sample (REF), the thickness variability (maximum-minimum) was 2.4 μm, while the thickness variability (maximum-minimum) of test points 1 and 2 (TP1, TP2) was 1.9 μm and 2.1 μm, respectively. In addition to the reduction in shrinkage, replacing non-recycled BCP with recycled pulp obtained from RGP resulted in a reduction in thickness variability, which correlated with the shrinkage results, while maintaining the grammage variability. Therefore, both shrinkage and thickness variability at the paper machine are related to the amount of recycled pulp obtained from RGP. Reduced shrinkage and stable grammage variability are indicators of improved dimensional stability.

[0270] Induced curl test of calendered paper

[0271] refer to Figure 11 The calendered glass paper samples produced in the industrial scale pilot run were subjected to the S y The induced curl of the paper was further evaluated and expressed as paper curl (CD). Curl was induced at 150°C (laboratory oven) for 1 minute and then measured immediately. The induced curl method was chosen because it provides an indication of the processability of calendered glass paper when used as a substrate for release coatings. Release coatings are typically cured under conditions similar to those used in this case.

[0272] Induced curl is measured using a modified version of the test method ISO 11556:2005 (en). A sheet with a length of 10 cm (lateral S of the paper) is cut from the middle of the paper which has been allowed to stabilize under NTP conditions (25°C, 1 bar) for 24 hours after production. y ) and has a width of 5 cm (machine direction S of paper x). The specimen was placed on a cylindrical holder with a diameter of 10 mm and a slot extending over 5 cm along the length of the cylindrical holder. When placed in the slot, the specimen was suspended from the center by the slot across its entire width, allowing each half of the specimen's length to extend freely for a distance of 4.5 cm in opposite directions. The cylindrical holder was attached to a curling template to measure the magnitude of the induced curl. Before induced curling, the test specimen was aligned parallel to a reference position. The reference plane was given a value of zero. Because the induced curl on the suspended specimen approximates a circular arc, a mark indicating the angle of curvature from the reference plane was imprinted on the template. Therefore, the magnitude of the curl was imprinted in the template as the curvature angle of the curled specimen relative to the reference plane, in degrees. The curl of the specimen was compared with the curvature angle imprinted on the curling template; the curvature on both sides was recorded. Two specimens were measured, and the four recorded values were averaged. The curl test result is therefore the average of the four values recorded. If the curl is recorded towards the wire side, the curl value is positive. If the curl is recorded towards the top side, the curl value is negative. In this context, the wire side refers to the side of the paper that has come into contact with the forming wire of the paper machine when forming the paper web. In this context, the top side refers to the back side of the paper.

[0273] The results demonstrate that replacing non-recycled BCP with recycled pulp obtained from RGP results in a steady reduction in curl values, which is proportional to the amount of recycled pulp obtained from RGP in the glassine paper. In the sample without recycled pulp obtained from RGP (REF), the measured curl was 61 mm. In the sample containing 15 wt.% recycled pulp obtained from RGP (TP1), the measured curl was 47 mm. In the sample containing 30 wt.% recycled pulp obtained from RGP (TP2), the measured curl was 32 mm. Thus, replacing 15 wt.% of non-recycled BCP with recycled pulp obtained from RGP results in a 23% reduction in curl of calendered glassine paper. Furthermore, replacing 30 wt.% of non-recycled BCP with recycled pulp obtained from RGP results in a 48% reduction in curl of calendered glassine paper. The reduction in curl was demonstrated in all measured samples. The induced curl results support and verify the observations disclosed above. Therefore, when taking into account the improved dimensional stability and drainage discussed above, the calendered glassine advantageously contains equal to or less than 50 wt. %, such as in the range of 5 wt. % to 50 wt. %, preferably in the range of 10 wt. % to 45 wt. %, most preferably in the range of 15 wt. % to 30 wt. % of recycled pulp obtained from release liner glassine, when measured as dry matter content according to SCAN-P 39:80.

[0274] Paper strength properties

[0275] The strength properties of calendered glass paper samples produced in industrial scale pilot runs were further evaluated. Machine direction (MD) S was measured according to ISO 1924-3 x and crosswise (CD)S y tensile strength in MD, strain at break in MD, and tensile energy absorption in MD.

[0276] When calendered glass paper is used as a substrate for release liners in labeling operations, tensile strength can be used as an indicator of the calendered glass paper's potential resistance to web breakage. Break strain can be used as an indicator of how well the paper will conform to irregular shapes and, along with tensile energy absorption, as an indicator of the paper's performance under dynamic strain and stress. Tensile energy absorption is a measure of the paper's ability to absorb energy. Thus, tensile energy absorption is an indication of the toughness of the paper sheet. Consequently, these parameters predict the performance of the paper, especially when the paper is subjected to uneven or dynamic stresses. Table 7 (below) indicates the results measured from a calendered glass paper sample containing no recycled pulp obtained from RGP (REF), a calendered glass paper sample containing 15% by weight of recycled pulp obtained from RGP (TP1), and a calendered glass paper sample containing 30% by weight of recycled pulp obtained from RGP (TP2).

[0277] Table 7. Comparative results from calendered glassine paper samples (MD and CD).

[0278]

[0279] The results show that despite the replacement of non-recycled BCP with recycled pulp obtained from RGP, the paper strength in the samples, as measured by tensile strength, strain at break and tensile energy absorption, remained at a sufficiently high level. During the test period, no significant changes in paper strength or orientation properties were observed.

[0280] In summary, the compatibility of recycled pulp produced from release-liner glassine for glassine production is excellent. Several different methods have been used to better measure the positive effects of the glassine manufacturing process, such as improved dewatering during web formation and in the pressing section, and improved drainage in the drying section, while maintaining the properties of the calendered glassine at a level sufficient for use as a release liner substrate. The improved manufacturing process is also noticeable in the produced glassine, which exhibits reduced shrinkage, better dimensional stability, and reduced curl.

Claims

1. A method for producing recycled pulp (PULP3), comprising - receiving a release liner for recycling (REL1), wherein the release liner comprises a substrate that is calendered glassine paper or supercalendered kraft paper, the release liner comprising non-fibrous material and fibers; - disintegration and detachment of non-fibrous material from the fibers in a first process stage denoted caustic circuit (CL1), thereby forming a pulp suspension; and - removing non-fibrous material from said fibers in a second process stage denoted cleaning loop (NL1), wherein the method has been configured to adjust the fibrillation of the fibers such that the recycled pulp (PULP3) obtained from the substrate being calendered glassine paper or supercalendered kraft paper has an SR number equal to or higher than 25 when measured according to ISO 5267-1.

2. The method according to claim 1 , wherein the recycled pulp (PULP3) has an SR number equal to or higher than 30, preferably equal to or higher than 40, such as in the range of 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55, when measured according to ISO 5267-1.

3. The method according to claim 1 or 2, further comprising sorting the release liner substrate for regeneration before disintegrating to obtain a sorted release liner substrate.

4. The method according to any of the preceding claims, wherein the recycled pulp (PULP3) obtained from a release liner paper substrate is not bleached during or after the caustic circuit (CL1) and / or cleaning circuit (NL1).

5. A method according to any one of the preceding claims, wherein - The color of the calendered glass paper is white, and the white color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 92 to 98, oa* is in the range of -4 to +2, and ob* is in the range of +3 to +9, preferably in the range of +5 to +7, Said value can be determined from a sample of said paper by means of the diffuse reflectance method with specular gloss eliminated, using standard illuminant D65 and a 10° standard observer, in accordance with ISO 5631:2022, or - The color of the super calendered kraft paper is white, and the white color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 92 to 98, oa* is in the range of -4 to +2, and ob* is in the range of +5 to +11, The values can be determined from samples of the paper by means of the diffuse reflectance method with specular gloss eliminated, using standard illuminant D65 and a 10° standard observer in accordance with ISO 11475:2017 (en).

6. The method according to any one of the preceding claims, wherein the color of the calendered glass paper is light yellow, yellow, brown or blue, - The light yellow refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 87 to 96, oa* is in the range of -4 to +8, and ob* is in the range of +24 to +43, - The yellow color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 65 to 71, oa* is in the range of +7 to +13, and ob* is in the range of +50 to +56, - The brown color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 64 to 70, oa* is in the range of +3 to +9, and ob* is in the range of +17 to +23, - the blue color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 80 to 89, oa* is in the range of -17 to 0, and ob* is in the range of -15 to +9, Said values can be determined from samples of the paper in accordance with ISO 5631-1:2022 using standard illuminant C and a 2° standard observer by means of the diffuse reflectance method with specular gloss eliminated.

7. The method according to any one of the preceding claims, wherein said disintegrating and detaching non-fibrous material from said fibers is pulping carried out under alkaline conditions at high consistency in the presence of chemical additives comprising hydrogen peroxide, sodium silicate, caustic soda and a saponifying agent.

8. The method according to any one of the preceding claims, wherein said disintegrating and detaching non-fibrous material from said fibers is pulping performed in a batch mode, wherein the total operation time of said pulping is in the range of 30 minutes to 60 minutes.

9. The method according to any one of the preceding claims, wherein the disintegrating and detaching non-fibrous material from the fibers is pulping, wherein the temperature of the pulp suspension during pulping is in the range of 60°C to 85°C.

10. The method according to any of the preceding claims, wherein at the end of the caustic circuit (CL1 ) process water (F4) is mechanically removed from the pulp suspension using pressing or filtration, so that at the end of the caustic circuit (CL1 ) the pulp suspension is thickened to a consistency equal to or higher than 20 wt.-%, preferably in the range of 20 to 50 wt.-%, most preferably in the range of 25 to 40 wt.-%.

11. The method according to any of the preceding claims, wherein the fibrillation of the fibers in the pulp suspension is adjusted after the caustic circuit (CL1) before flotation by means of dispersion in a cone disperser or a disk disperser.

12. The method according to claim 11, wherein the amount of specific energy consumed during dispersion is in the range of 30 kWh / t to 150 kWh / t, preferably in the range of 40 kWh / t to 100 kWh / t, most preferably in the range of 45 kWh / t to 90 kWh / t.

13. The method according to claim 11 or 12, wherein the temperature of the pulp suspension at the inlet of the disperser is in the range of 50 to 130°C, preferably in the range of 50 to 85°C.

14. The method according to any of the preceding claims, further comprising flotation of the pulp suspension by means of air bubbles during the cleaning circuit (NL1), the pulp suspension having a temperature in the range of 40°C to 70°C and a pH in the range of 7 to 10 during the flotation.

15. The method according to claim 14, wherein during the flotation, the water hardness of the pulp suspension is in the range of 10 dH to 20 dH, which can be measured according to ISO / TS 15923-2:2017, and a collector such as soap, sodium soap or other surfactants containing hydrophilic and hydrophobic parts is added to the pulp suspension.

16. The method according to any of the preceding claims, further comprising washing and dewatering the pulp suspension during the cleaning circuit (NL1) so as to form a recycled pulp (PULP3) obtained from a release liner paper substrate having a consistency equal to or higher than 30 wt.%, preferably equal to or higher than 40 wt.%, such as in the range of 30 to 50 wt.%.

17. The method according to any of the preceding claims, further comprising increasing the dryness of the pulp (PULP3) obtained from the release liner such that a pulp consistency equal to or higher than 80, such as in the range of 80 to 90 wt.-%, is obtained.

18. Recycled pulp (PULP3) obtained according to the method of any one of claims 1 to 17.

19. A recycled pulp (PULP3) obtained from: - Release liner comprising calendered glassine paper as substrate or - a release liner comprising supercalendered kraft paper as a base material, The recycled pulp (PULP3) comprises an SR number equal to or higher than 25, such as in the range of 25 to 65, when measured according to ISO 5267-1.

20. Recycled pulp (PULP3) according to claim 19, wherein the SR number is equal to or higher than 30, preferably equal to or higher than 40, such as in the range of 30 to 60, most preferably in the range of 40 to 55.

21. Recycled pulp (PULP3) according to claim 19 or 20, further comprising a water retention value in the range of 1.3 g / g to 1.6 g / g, measurable according to ISO 23714:2014 (en).

22. Recycled pulp (PULP3) according to any one of claims 19 to 21, further comprising a pH in the range of 6.0 to 9.1, preferably in the range of 7.0 to 8.5, most preferably in the range of 7.5 to 8.2, when the pH of a dry pulp sample is determined according to standard ISO 6588-2 (2020) and using an amount of 2 grams of dry pulp, as disclosed in the description.

23. Recycled pulp (PULP3) according to any of claims 19 to 22, further comprising particles having a length of less than 200 microns originating from the recycled pulp (PULP3), the amount of said particles being equal to or higher than 10% of the total amount of fibers in the recycled pulp, such as in the range of 10 to 30%, preferably in the range of 12 to 20%, most preferably in the range of 15 to 17%, when determined as length-weighted average fiber length according to ISO 16065-2:2014 by automated optical analysis using unpolarized light.

24. Recycled pulp (PULP3) according to any of claims 19 to 23, further comprising silicon in an amount equal to or less than 0.3 wt.-%, preferably equal to or less than 0.28 wt.-%, most preferably equal to or higher than 0.25 wt.-%, such as in the range of 0.01 wt.-% to 0.3 wt.-%, which can be determined as dry matter content from a paper sample burned at 900°C using energy dispersive X-ray spectroscopy according to Tappi standard T 413.

25. Recycled pulp (PULP3) according to any one of claims 19 to 24, wherein the recycled pulp has been obtained from a release liner paper substrate, - The color of the calendered glass paper is white, and the white color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 92 to 98, oa* is in the range of -4 to +2, and ob* is in the range of +3 to +9, preferably in the range of +5 to +7, Said value can be determined from a sample of said paper by means of the diffuse reflectance method with specular gloss eliminated, using standard illuminant D65 and a 10° standard observer, in accordance with ISO 5631:2022, or - The color of the super calendered kraft paper is white, and the white color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 92 to 98, oa* is in the range of -4 to +2, and ob* is in the range of +5 to +11, The values can be determined from samples of the paper by means of the diffuse reflectance method with specular gloss eliminated, using standard illuminant D65 and a 10° standard observer in accordance with ISO 11475:2017 (en).

26. Recycled pulp (PULP3) according to any one of claims 19 to 24, wherein the recycled pulp has been obtained from a release liner paper substrate, wherein the color of the calendered glassine paper is light yellow, yellow, brown or blue, - The light yellow refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 87 to 96, oa* is in the range of -4 to +8, and ob* is in the range of +24 to +43, - The yellow color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 65 to 96, oa* is in the range of -4 to +13, and ob* is in the range of +24 to +56, - The brown color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 64 to 70, oa* is in the range of +3 to +9, and ob* is in the range of +17 to +23, - the blue color refers to the CIE L*, a*, b* color space coordinate values of the paper, wherein o L* is in the range of 80 to 89, oa* is in the range of -17 to 0, and ob* is in the range of -15 to +9, Said values can be determined from samples of the paper in accordance with ISO 5631-1:2022 using standard illuminant C and a 2° standard observer by means of the diffuse reflectance method with specular gloss eliminated.

27. A system of cooperating devices suitable for producing recycled pulp, the system comprising - a sorting stage (20) configured to separate release liners comprising calendered glassine paper as a base paper or release liners comprising supercalendered kraft paper as a base paper from other papers; - a caustic circuit (CL1) comprising o a high consistency pulping unit (21) configured to operate in batch mode and in which the consistency and temperature of the suspension, the operating time of the pulping and the addition of chemical additives can be adjusted; and o a dewatering unit (24) configured to remove process water (F4) from the pulp suspension and to increase the pulp consistency; as well as - Cleaning circuit (NL1), which comprises o a dispersion unit (25) configured to regulate the fibrillation of fibers in the pulp suspension; and o a second dewatering unit (29) configured to remove process water (F2) from the pulp suspension, thereby increasing the pulp consistency, Such that the recycled pulp (PULP3) obtained from said substrate has an SR number equal to or higher than 25 when measured according to ISO 5267-1.

28. The system according to claim 27, wherein the caustic circuit (CL1) further comprises - a coarse screening unit (22) configured to remove solid contaminants and non-paper components such as sand and metal objects, as well as larger particle agglomerates from the pulp suspension.

29. The system according to claim 27 or 28, wherein the caustic circuit (CL1) further comprises - A cleaning unit (23), preferably a high consistency cleaning unit, such as a cleaner using a centrifugal field, configured to supplement the coarse screening unit (22) to separate pulp fibers from contaminants based on specific gravity.

30. The system according to any one of claims 27 to 29, wherein - The dispersion unit (25) is a cone or disc disperser configured to operate with the thickened pulp suspension received directly from the dewatering unit (24).

31. The system according to any one of claims 27 to 30, wherein the cleaning circuit (NL1) further comprises a flotation unit (26) configured to receive clean water (F1) and / or process water (F2, F3) downstream to adjust the consistency of the pulp suspension, the flotation unit (26) being configured to remove hydrophobic particles from the pulp suspension if necessary.

32. The system according to claim 31, wherein the cleaning circuit (NL1) further comprises - a second screening unit (27) configured to separate debris from the fibers coming from the flotation, in particular the silicone particles coming from the release coating.

33. The system according to any one of claims 27 to 32, wherein the cleaning circuit (NL1) further comprises - A washing unit 28, such as a belt filter type machine, configured to separate particles from the pulp suspension by size.

34. The system according to any one of claims 27 to 33, further comprising - a drying unit, such as a fluffer, configured to increase the dryness of the pulp.

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