WO2024141186A1 - Recycling of release liner glassine and sck into high quality pulp - Google Patents
Recycling of release liner glassine and sck into high quality pulpInfo
- Publication number
- WO2024141186A1 WO2024141186A1 PCT/EP2023/075765 EP2023075765W WO2024141186A1 WO 2024141186 A1 WO2024141186 A1 WO 2024141186A1 EP 2023075765 W EP2023075765 W EP 2023075765W WO 2024141186 A1 WO2024141186 A1 WO 2024141186A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- paper
- range
- pulp
- recycled
- glassine
- Prior art date
Links
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Abstract
The invention relates to a method, system and a product thereof, wherein glassine or supercalendered kraft (SCK) paper, which has been used as a release liner substrate, is recycled into a high-quality pulp. When highly specific raw material is used for recycling, the characteristics of the recycled pulp may be adjusted already upon recycling. The method and the system is configured to adjust the fibrillation of the pulp such that the recycled pulp can be used without further refining for manufacturing new glassine or SCK paper. The compatibility of the recycled pulp may further be optimized for glassine or SCK paper production, respectively. Advantageously, the recyclable material is sorted based on the paper colour, prior to recycling. A recycled pulp may be provided which does not need to be bleached during the pulp recycling, prior to introducing it into a new glassine or SCK paper production.
Description
Recycling of release liner glassine and SCK into high quality pulp
Field of invention
The invention relates to a method and a system for manufacturing high quality recycled pulp, wherein glassine or supercalendered Kraft paper, which has been used as a release liner substrate, is recycled into a pulp which may be introduced into a paper manufacturing process, wherein the same paper type is produced. A more closed-loop recycling is enabled.
Background
The release liner market is experiencing significant growth. A release liner may be used to protect sensitive surfaces prior to use, such as the adhesive surface of an adhesive label. Release liners are widely used in high-speed industrial labelling processes, wherein the number of products to be labelled can be very large. This necessitates large amount of release liners as carriers for the labels. High-speed processes require reliable die-cutting and detachment of adhesive labels from a release liner. Unintentional break to the labelling process due to a release liner defect is problematic. Therefore, a release liner needs strength and a homogeneous surface displaying stable release properties. Both may be provided but are expensive to obtain. The expectation of high quality thus extends to the paper used as a release liner substrate, which should have sufficient characteristics to withstand the stresses applied at high-speed processes.
Glassine and supercalendered Kraft, hereafter abbreviated as SCK, are distinguished types of paper that are used as a release liner substrate due to their outstanding characteristics. Glassine and SCK paper are expensive, as they are typically produced of bleached chemical pulp, hereafter abbreviated as BCP, that has been highly refined. The production of such paper is a complex process, which requires skills, large amounts of virgin wood-based material and energy. The BCP used for producing glassine or SCK paper is typically a pulp mixture that contains both BCP made of softwood and BCP made of hardwood. Virgin BCP made of softwood, in particular, is very expensive, whereby in general majority of the furnish, up to 70-80 % by weight, of both glassine and SCK paper is typically virgin BCP made of hardwood.
However, BCP made of softwood is preferred, due to the longer fibers in BCP made of softwood. Part of the longer average fiber length of BCP made of softwood, however, is lost due to the refining of the pulp, which is performed prior to introducing the furnish on the paper machine.
Refining is a mill operation performed on the BCP prior to manufacturing glassine or SCK paper, wherein the pulp fibers are subjected to high shear forces. This modifies the pulp fibers physically, for example by fibrillation, such that the fiber structures become looser. The extent of refining of a pulp may be determined by a Schopper-Riegler test, which measures the drainability of a pulp suspension in water in terms of the Schopper-Riegler number, referred to as the SR number or °SR. Refining further reduces the average fiber length of the pulp fibers. Consequently, the specific volume of the formed glassine or SCK paper is also reduced, since shorter fibers may be packed together closer. This also enables to manufacture glassine or SCK paper having higher surface smoothness and density. A smooth and dense paper surface is advantageous for reducing the consumption of a subsequent release coating, upon producing a release liner. However, refining also increases the moisture uptake of the BCP, denoted as swelling, since the loosened fiber structure of refined BCP is better accessible for water molecules. Thus, refining increases the amount of water to be removed from the formed paper web, when manufacturing glassine paper on a paper machine. Upon drying the paper web on a paper machine, the excess water to be removed from the fibers may cause shrinkage, which changes the dimensions of the glassine paper and is also detrimental for the paper quality, such as paper strength. Refining of the pulp thus causes multiple effects downstream on the glassine and SCK paper manufacturing process. While some effects of refining are positive and improve the paper quality, others are not.
To balance the effects of extensive refining and to obtain the final paper quality characteristics, such as smoothness, thickness, density and transparency, a glassine paper, as well as a SCK paper, is typically surface sized and strongly calendered. A glassine paper, for example, is typically calendered by means of a multi-nip calender or a supercalender.
Sustainability drives the paper manufacturers to develop the products and their manufacturing methods at the paper mill. While paper has been collected for recycling for a long time, the circulation of recycled paper waste into a specific paper production, such as glassine or SCK paper production, poses challenges. The pulp obtained from such material has presented reduced quality and is used in products where quality is of less importance.
A large proportion of the industrial paper grades, such as papers used for printing and writing, use different kind of furnish than what is typically used when manufacturing glassine paper for a release liner. Many paper types which are primarily meant for conveying information to consumers also contain relatively high amount of various printing inks. This needs to be considered, since dye-based and pigment-based inks have different deinkability properties. Label waste poses other types of challenges, as the material to be recycled may often contain plastic and adhesive label remnants. A release liner, on the other hand, contains cured silicone polymer that has been adhered on the paper surface.
As an example, US 5,316,621 discloses that glassine paper used as a material for release liner is very difficult to defiberize because it is supercalendered, made of highly beaten pulp fibers and comprises a release agent such as a silicone compound. The publication suggests an accelerated method, involving acid addition and elevated temperature, followed by kneading, fine screening and mechanically agitating a thickened slurry at a temperature below 12°C. Mineral pigments are desirably added to the process to obtain better effects.
As another example, W02020084188 A1 discloses a backing material layer for a release liner that comprises de-siliconized release liner pulp, which is bleached before it is used for the backing material. The recovered release liner pulp is produced by a de-siliconizing process wherein at least some silicone particles are removed and by bleaching the de-siliconized pulp with hydrogen peroxide, sodium dithionite, sodium bisulfite or sodium borohydride solution.
In the past, considerable technical challenges thus have been disclosed when attempting to reuse release liner material, without assurance of the quality of the repulped material.
Summary
The growing sustainability requirement leads to considerably larger quantities of release liners being produced for the labelling industry, where paper is used as a release liner substrate. The industrial use of such release liners in larger quantity enables a targeted collection and sorting of used release liners for recycling. Of particular interest is the collection and sorting of release liner, wherein the substrate is glassine paper or supercalendered Kraft paper. A release liner, wherein the substrate is glassine paper, is hereafter referred to as release liner glassine paper and abbreviated as RGP. A release liner,
wherein the substrate is supercalendered Kraft paper, is hereafter referred to as release liner supercalendered Kraft paper and abbreviated as RSCK.
RGP and RSCK recycling provides means for glassine and SCK paper production to be more sustainable, respectively, while solving challenges mentioned above. Fibers of RGP and RSCK display signs of damages due to extensive hornification and no longer have the same characteristics as fibers of virgin BCP made of softwood. However, sorting of RGP or RSCK apart from other paper waste provides specific and highly homogeneous material for recycling, which enables to better adjust characteristics of the material already during the recycling process.
Hereafter, a method is disclosed for manufacturing recycled pulp from a release liner glassine or SCK paper, the method comprising:
- sorting release liner glassine or SCK paper for recycling,
- disintegrating the sorted release liner glassine or SCK paper and detaching non-fibrous material from the fibers on a first process stage, referred to as a caustic loop, and
- removing non-fibrous material from the fibers on a second process stage, referred to as a cleaning loop, wherein the method, the caustic loop and the cleaning loop have been configured to adjust the fibrillation of the pulp suspension, such that the recycled pulp obtained from the release liner glassine or SCK paper has a pulp fibrillation and drainability in a range which enables the use of the recycled pulp obtained from the RGP or RSCK in a method for manufacturing glassine or SCK paper, respectively. Advantageously, the caustic loop and the cleaning loop have been configured to adjust the fibrillation of the pulp such that the recycled pulp obtained from the RGP or RSCK can be used in a method for manufacturing glassine or SCK paper without further refining.
The sorting may be performed based on the paper type to calendered glassine paper or supercalendered kraft paper, thereby obtaining sorted release liner substrate. For instance, excessive refining of the recycled pulp and/or bleaching may be avoided. Advantageously, the recycled pulp obtained from a sorted release liner glassine or SCK paper is not bleached during or after the caustic loop and/or cleaning loop. In particular, pulp produced from RGP or RSCK may be used to replace non-recycled BCP in the composition of the glassine or SCK paper, respectively.
When the sorting is performed based on the paper colour, further advantages are obtainable. When the release liner substrate is white calendered glassine paper or white supercalendered kraft paper, the method disclosed above may
be arranged provide recycled pulp, which does not need to be bleached during the pulp recycling, prior to introducing it into a new glassine or SCK paper production.
A two-step sorting may further be used to facilitate the recycling of non-white RGP grades, such as light yellow, yellow, brown or blue shades. A two-step sorting may be used to facilitate introduction of such recycled pulp into a glassine paper manufacturing process, wherein the same or similar paper colour is produced, without bleaching the papermaking fibers.
The sorting of RGP and RSCK to white or non-white paper colour is therefore advantageous, as the compatibility of the recycled pulp can thus be adapted and optimized already during the recycling process for a further production of a specific glassine or SCK paper grade. A more closed loop is therefore possible for the papermaking fibers.
Recycled pulp obtained from RGP or RSCK has a pH which is in an alkaline range, when determined from aqueous pulp extracts. An alkaline pH during the recycling process softens the pulp, which thereby requires less energy for refining. An alkaline pH, however, may inhibit the subsequent drying of the pulp. The pulp pH may thus be adjusted, as necessary, prior to mixing the pulp with other pulp components. Advantageously, when using the recycled pulp obtained from RGP or RSCK in a method for manufacturing calendered glassine or SCK paper suitable for use as a substrate of a release liner, the recycled pulp obtained from release liner glassine or SCK paper has a pH which is in the range of 6.0 to 9.1 . Preferably the pH is in the range of 7.0 to 8.5, since a highly alkaline pH may inhibit the functioning of cationic UV curing silicone systems. Most preferably, the pH in the range of 7.5 to 8.2, whereby the drying and the compatibility of the recycled pulp can be optimized for glassine or SCK paper production.
Recycled pulp obtained from RGP or RSCK is very quick to refine, compared to non-recycled pulp components. Recycled pulp obtained from RGP or RSCKalso has a relatively high SR number, compared to non-recycled bleached chemical pulps, which have not been refined. Hence, the recycled pulp obtained from RGP or RSCK may be used in glassine or SCK paper production without further refining. When the fibrillation and/or drainability of a recycled pulp obtained from RGP or RSCK has been adjusted to a suitable level already beforehand, the recycled pulp obtained from RGP or RSCK may be directly mixed with other non-recycled pulp components in a method for manufacturing glassine or RSCK paper. Advantageously, recycled pulp obtained from RGP or RSCK has a SR number equal to or higher than 25,
such as in a range from 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55, when determined according to ISO 5267- 1.
Recycled pulp obtained from RGP or RSCK comprises an average fiber length that is in the same range as in non-recycled BCP made of hardwood. The average fiber length of recycled pulp obtained from RGP or RSCK is, however, significantly less than in non-recycled BCP made of softwood or mill broke used for glassine paper production. The amount of fibrils in the pulp obtained from RGP or RSCK also differs from the amount of fibrils in the non-recycled BCP. The recycled pulp obtained from RGP or RSCK typically contains particles derived from the recycled pulp having a length less than 200 micrometers in an amount equal to or higher than 10 %, such as in a range from 10 to 30 %, preferably in the range of 12 to 20 %, most preferably in the range of 15 to 17 % of the total amount of fibers in the recycled pulp, when determined as length weighted average fiber length by automated optical analysis using unpolarized light according to ISO 16065-2: 2014. The fibers of the recycled pulp obtained RGP or RSCK typically have an average fiber width of less than 25 micrometers, preferably in the range of 19-25 micrometers, most preferably in the range of 19-21 micrometers, when determined by automated optical analysis using unpolarized light according to ISO 16065-2: 2014.
Fiber furnish analysis according to ISO 9184-4 in conjunction with ISO 9184-1 may be used for fiber identification and to determine the fiber properties of a given pulp. In combination with pulp drainage analyses, such as measurement of the pulp water retention value and/or the SR number, these analyses distinguish recycled pulp obtained from release liner glassine and SCK paper.
Empirical studies indicate that recycled pulp obtained from RGP or RSCK comprises very good characteristics for glassine paper production, throughout the manufacturing method, at a paper machine. The fibers of recycled pulp obtained from RGP and RSCK are less accessible for water molecules. Recycled pulp obtained from RGP or RSCK therefore inhibits the moisture uptake of the stock. The water retention value of recycled pulp obtained from RGP or RSCK is low, typically lower than in non-recycled BCP. The amount of recycled pulp obtained from RGP or RSCK may therefore be used to control the dry matter content of the stock, upon forming the paper web. The reduced ability of the recycled pulp obtained from RGP or RSCK to absorb moisture also leads to enhanced dewatering of the paper web, already at the press section of the paper machine. Upon entering the drying section, the paper web therefore contains less moisture which needs to be evaporated. Hence, less
steam pressure is needed, which improves the energy efficiency of the drying section during the paper production.
The compounded effects of reduced refining, improved dewatering and more efficient drying are observable by methods, which measure the water retention and drying behaviour of the paper web. For instance, when the amount of recycled pulp obtained from RGP or RSCK in the stock is increased, the water retention value decreases. This indicates that less water needs to be removed on the press section, during glassine or RSCK paper production. Pulp analyses from a paper mill further indicate that replacement of non-recycled BCP with recycled pulp obtained from RGP or RSCK in a pulp mixture results to an increase in the fines content in the pulp mixture, when determined as the F<2OO fraction with McNett classifier according to SCAN-CM 6:05. This indicates, that recycled pulp obtained from RGP or RSCK may be used to adjust the quality of the paper web formed on the paper machine. Experimental results indicate positive effects also downstream in the glassine and SCK paper production process. Drainability is related to the surface conditions and swelling of the fibres and is an indicator of the amount of mechanical treatment to which the pulp has been subjected. A paper web, which contains recycled pulp obtained from RGP or RSCK, demonstrates improved drainage on a paper machine. A higher amount of the recycled pulp obtained from RGP or RSCK in the stock correlates with the level of drainage, such that less steam pressure is needed for drying. Unexpectedly, a reduction of 0.1 bar in the steam pressure may be obtained already with an amount of 5 wt.% of recycled pulp obtained from RGP or RSCK in the composition, when drying the glassine paper. When the amount of recycled pulp obtained from RGP or RSCK in the composition is 15 wt.%, 0.3 bar less of steam pressure may be used for drying the glassine paper. A considerable amount of energy may thus be saved.
Further to this, paper machine off-line analyses demonstrate that the produced paper has less shrinkage and less variability of the grammage in the crossdirection at a paper machine, which correlates with the amount of the recycled pulp obtained from RGP or RSCK. The amount of shrinkage is an indicator of dimensional stability. The smaller variability of the grammage in the crossdirection at a paper machine is an indicator of more homogeneous product. A calendered glassine or supercalendered kraft paper which comprises recycled pulp obtained from RGP or RSCK, respectively, thus has improved quality characteristics. Experimental results also evidence of reduced curl in paper samples comprising recycled pulp obtained from RGP or RSCK . The improved properties of the calendered glassine paper are of importance, when considering the use of glassine paper as a substrate, on which a release coating is subsequently spread and cured.
Typically, a calendered glassine paper or a SCK paper has a grammage equal to or less than 120 g/m2, such as in the range of 40 to 120 g/m2 When produced for use as a substrate of a release liner, a lower grammage may be preferred, such as in the range of 40 to 90 g/m2, most preferably in the range of 45 to 70 g/m2. A lower grammage may be calendered into a glassine or SCK paper with less thickness and higher transparency. The thickness of a glassine or a SCK paper may be controlled by calendering and hence correlates with the grammage and density.
Recycled pulp obtained from RGP or RSCK enables to maintain quality characteristics of calendered glassine or SCK paper at a sufficient level, while enabling recycling of the used end product, a release liner glassine or SCK paper, back into the manufacturing process. A sufficient level of quality characteristics, in this context, refers to a calendered glassine paper having a density in the range of 1 .050 to 1 .190 g/cm3, and a transparency in the range of 40 to 56%. Advantageously the density is in the range of 1.060 to 1.190 g/cm3, most preferably in the range of 1.060 to 1.180 g/cm3, determinable by standard ISO 534. Advantageously the transparency is in the range of 42 to 54%, most preferably in the range of 44 to 54%, determinable by standard ISO 2469. Respectively, a sufficient level of quality characteristics for SCK paper, in this context, refers to a density in the range of 1 .030 to 1 .190 g/cm3, and a transparency in the range of 36 to 56%. Advantageously the density is in the range of 1 .050 to 1 .190 g/cm3, most preferably in the range of 1 .060 to 1 .180 g/cm3, determinable by standard ISO 534. Advantageously the transparency is in the range of 38 to 54%, most preferably in the range of 40 to 52%, determinable by standard ISO 2469.
The combination of density and transparency is of relevance, as it can be used as an index of the compressibility level of the calendered glassine or SCK paper. A calendered glassine or SCK paper, which is intended to be used as a release liner substrate, needs suitably low compressibility in the thickness direction Sz parallel to surface normal of the paper, as the release liner typically acts as backing material for face material comprising an adhesive layer. The face material is shaped into labels with cutting die that is pressed against the face material with a predefined pressure. When the release liner substrate exhibits suitably low compressibility, the blades cut through the face material into a predefined depth, such that the face material comprising the adhesive layer may be stripped away around the cut area without damaging the substrate. The combination of density and transparency therefore indicates the suitability of the calendered glassine or SCK paper to function as a release liner substrate for adhesive labels.
According to a first aspect, there is provided a method for manufacturing recycled pulp, the method comprising
- receiving release liner for recycling, wherein the release liner comprises a substrate which is calendered glassine paper or supercalendered kraft paper, the release liner comprising non-fibrous material and fibers,
- disintegrating and detaching non-fibrous material from the fibers on a first process stage, denoted as a caustic loop, thereby forming a pulp suspension, and
- removing non-fibrous material from the fibers on a second process stage, denoted as a cleaning loop, wherein the method has been configured to adjust the fibrillation of the fibers, such that recycled pulp obtained from the substrate which is calendered glassine paper or supercalendered kraft paper has a SR number equal to or higher than 25, when determined according to ISO 5267-1 .
According to a second aspect, there is provided recycled pulp obtained from
- release liner comprising calendered glassine paper as a substrate or
- release liner comprising supercalendered kraft paper as a substrate, the recycled pulp comprising a SR number equal to or higher than 25, such as in a range from 25 to 65, when determined according to ISO 5267-1 .
According to a third aspect, there is provided a system of co-operating apparatuses adapted for producing recycled pulp, the system comprising
- a sorting stage configured to separate release liner comprising calendered glassine paper as a substrate or release liner comprising supercalendered kraft paper as a substrate paper apart from other papers,
- a caustic loop comprising o a high consistency pulping unit configured to operate in a batch mode and wherein the consistency and temperature of the suspension, operating time of the pulping and addition of chemical additives may be adjusted, and o a dewatering unit, configured to remove process water from the pulp suspension and to increase the pulp consistency, and
- a cleaning loop comprising o a dispersion unit configured to adjust the fibrillation of the fibers in the pulp suspension, and o a second dewatering unit configured to remove process water from the pulp suspension, thereby increasing the pulp consistency, such that recycled pulp obtained from the substrate has a SR number equal to or higher than 25, when determined according to ISO 5267-1 .
Referring to the above, when highly specific raw material is used for recycling in an adapted system of co-operating apparatuses, the characteristics of the recycled pulp may be adjusted already upon recycling. Advantageously, recycled pulp obtained from release liner glassine or SCK paper may be used without further refining for manufacturing new paper of the same type. Excessive refining of the recycled pulp obtained from release liner glassine or SCK paper can thus be avoided. This further leads to positive effects in the subsequent paper manufacturing process, such as improved dewatering and energy efficient drying.
An advantage of sorting the RGP or the RSCK based on a white or a non-white color shade of the paper is that recycled pulp obtained from RGP or RSCK may be produced without bleaching. Further, the compatibility of the recycled pulp may be optimized for glassine or SCK paper production, respectively. Hence, advantageously, recycled pulp as disclosed above is obtained from release liner comprising calendered glassine paper or SCK paper as a substrate, wherein the colour of the calendered glassine paper or SCK paper is white.
A two-step sorting, based on both paper type and paper colour, may be further used to facilitate recycling of non-white RGP of specific colours into distinct pulp types, which may be later introduced into a glassine paper manufacturing process, wherein paper of similar or same colour is produced. As same kind of colourants are typically used to provide a specific colour shade for a glassine paper, such as light yellow, yellow, brown or blue, the combination of a same paper type together with the same or a similar colour of the paper is advantageous for providing a less complex and more closed-loop recycling of non-white glassine paper. Hence, advantageously, recycled pulp as disclosed above may also be obtained from release liner comprising calendered glassine paper as a substrate, wherein the calendered glassine paper has a specific non-white colour, such as light yellow, yellow, brown or blue.
The recycled pulp obtained from RGP or RSCK may be used to replace nonrecycled BCP made of hardwood and/or softwood. Non-recycled BCP, in this context, may also be referred to as virgin BCP. When recycled pulp obtained from RGP or RSCK is used to replace non-recycled BCP, the refining of nonrecycled BCP in the glassine paper manufacturing process may be reduced, as well. Reduced refining of the non-recycled BCP preserves the quality of the fibers. In particular, non-recycled BCP made of softwood has a longer average fiber length than other components in the stock and can be used to improve the internal bond strength upon formation of the paper web. Advantageously, the non-recycled BCP produced from softwood, prior to mixing with recycled
pulp obtained from RGP or RSCK, has a Schopper-Riegler number equal to or less than 50, such as in a range from 25 to 50, preferably in the range of 25 to 45, most preferably in the range of 25 to 40, when determined according to ISO 5267-1. Advantageously, the composition of a 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 to 50 wt.%, most preferably in the range of 10 to 30 wt.%, when determined as dry matter content according to SCAN-P 39:80.
The preserved quality of the BCP fibers may be used for compensating negative effects, which the damaged fibers in the recycled pulp obtained from RGP or RSCK may cause to paper formation, when manufacturing glassine or SCK paper at a paper machine. Advantageously, the preserved quality of the fibers in the non-recycled BCP is used to increase the proportion of recycled pulp obtained from RGP or RSCK in the composition of the glassine paper. Hence, a synergy is perceived, when using recycled pulp obtained from RGP or RSCK together with non-recycled BCP in a method for manufacturing glassine or SCK paper. The composition of a calendered glassine or SCK paper advantageously contains recycled pulp obtained from RGP or RSCK in an amount equal to or higher than 5 wt.%, more preferably in an amount equal to or higher than 10 wt.%, most preferably in an amount equal to or higher than 15 wt.% or in an amount equal to or higher than 30 wt.%, such as in the range of 5 to 50 wt.%, preferably in the range of 10 to 45 wt.%, most preferably in the range of 15 to 30 wt.%, when determined as dry matter content according to SCAN-P 39:80.
When the recycled pulp is produced of sorted RGP or RSCK substrate, wherein the colour of the RGP or RSCK is white, the recycled pulp may be produced and advantageously introduced into a subsequent manufacturing process of white glassine or SCK paper without bleaching. A white RGP or RSCK does not contain colorants. Thus, a calendered glassine or a SCK paper suitable for use as a substrate of a release liner may comprise 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 recycled pulp has not been bleached.
Paper whiteness and white colour, in the context of calendered glassine paper, refer to CIE L*, a*, b* colour space coordinate values, wherein
- L* is in the range of 92 to 98,
- a* is in the range of -4 to +2, and
- b* is in the range of +3 to +9
, the values determinable by means of diffuse reflectance method with the elimination of specular gloss, using standard illuminant D65 and 10° standard observer, in accordance with ISO 5631 :2022.
Paper whiteness and white colour, in the context of supercalendered kraft paper, refer to CIE L* a* b* colour space coordinate values, wherein
- L* is in the range of 92 to 98,
- a* is in the range of -4 to +2, and
- b* is in the range of +5 to +11
, the values determinable by means of diffuse reflectance method with the elimination of specular gloss, using standard illuminant D65 and 10° standard observer, in accordance with ISO 11475:2017(en).
When the recycled pulp is produced of sorted RGP substrate, wherein the colour of the RGP is non-white, the recycled pulp may be produced and advantageously introduced into a subsequent manufacturing process of glassine paper of the same or similar colour. A non-white RGP typically contains colorants, wherein similar colorants are used for producing a specific paper colour, such as light yellow, yellow, brown or blue glassine paper. Thus, a calendered glassine paper suitable for use as a substrate of a release liner may comprise 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 both based on the paper type and the paper colour.
A light yellow, yellow, brown or blue paper colour, in the context of calendered glassine paper in this text, refers to CIE L* a* b* colour space coordinate values, wherein
- the light yellow refers to CIE L*, a* b* colour space coordinate values of the paper, wherein o L* is in the range of 87 to 96, o a* is in the range of -4 to +8, and o b* is in the range of +24 to +43,
- the yellow refers to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 65 to 71 , o a* is in the range of +7 to +13, and o b* is in the range of +50 to +56,
- the brown refers to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 64 to 70, o a* is in the range of +3 to +9, and
o b* is in the range of +17 to +23, and
- the blue refers to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 80 to 89, o a* is in the range of -17 to 0, and o b* is in the range of -15 to +9,
, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard illuminant C and 2° standard observer, in accordance with ISO 5631-1 :2022.
Objects and embodiments of the invention are further described in the independent and dependent claims.
The symbols Sx, Sz and Sy, as used herein, refer to coordinate directions orthogonal to each other.
Figure 1 shows, by way of an example, a cross-dimensional structure of a release liner, which comprises a surface sized paper substrate and a release coating.
Figure 2 shows, by way of an example, a method for manufacturing calendered glassine or SCK paper, wherein the method a paper is formed from a stock that contains non-recycled bleached chemical pulp and recycled pulp obtained from release liner glassine paper or SCK paper, respectively. The calendered glassine or SCK paper may be used as a substrate for a release liner. The release liner glassine or SCK paper may thus be recycled and reused.
Figure 3 shows, by way of an example, a method for manufacturing recycled pulp from a release liner glassine or SCK paper, which contains a sorting stage, a caustic loop and a cleaning loop for disintegrating fibers and for detaching and removing non-fiber material from the fibers. In addition to the principal functions, the method has been arranged to improve the fiber characteristics such that the recycled pulp may be used without further refining for stock preparation in glassine or SCK paper manufacturing.
Figure 4 shows comparative data of average length in millimeters of fibers in recycled pulp obtained from RGP vs. non-recycled pulp types, measured using a Valmet Fiber Image Analyzer (Valmet FS5).
Figure 5 shows comparative data of average fiber width in micrometers of fibers in recycled pulp obtained from RGP vs. non-recycled pulp types, measured using the Valmet Fiber Image Analyzer (Valmet FS5).
Figure 6 shows comparative data of 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 an average diameter.
Figure 7 is a trend diagram representing the development of fines content at the machine chest of a paper machine as a function of the amount of recycled pulp obtained from RGP in the stock, when determined as the F<2oo fraction with McNett classifier according to SCAN-CM 6:05.
Figure 8 is a trend diagram representing the development of water retention value as a function of pulp content, at machine chest of a paper machine. The content of recycled pulp obtained from RGP correlates inversely with the water retention value. When the content of recycled pulp obtained from RGP increases, the water retention value decreases.
Figure 9 is a trend diagram representing the drainage as a function of pulp content, when measured as the main steam group pressure level on a paper machine. The addition of recycled pulp obtained from RGP reduces the need for steam in the pre-dryer.
Figure 10 shows comparative data of the development of paper width in centimetres at the reeler of a paper machine, when measured using ABB Web Imaging System (WIS). The results demonstrate that paper shrinkage in the cross-direction Sy correlates inversely with the amount of recycled pulp obtained from RGP in the paper.
Figure 11 shows comparative data of induced curl of calendered glassine paper, when measured from test pieces in the cross-direction Sy. The results demonstrate that the magnitude of curl correlates inversely with the amount of recycled pulp obtained from RGP in
the furnish. Test pieces which contained higher amount of recycled pulp obtained from RGP displayed less curl.
Detailed description
A release liner glassine paper
A release liner glassine paper, abbreviated as RGP, is used to describe a release liner, wherein the substrate is calendered glassine paper. In other words, RGP denotes a release liner that comprises a substrate which is calendered glassine paper. A release liner SCK paper, abbreviated as RSCK, is used to describe a release liner, wherein the substrate is supercalendered kraft paper. In other words, RSCK denotes a release liner that comprises a substrate which is supercalendered kraft paper. Multiple aspects distinguish RGP and RSCK from other paper types collected for recycling.
Glassine and SCK papers denote specific paper types which both are suitable for use as a substrate of a release liner. Glassine paper is conventionally prepared from highly refined bleached chemical pulp that has been strongly calendered, whereby it possesses an exceptional combination of high density, strength and transparency, which are beneficial characteristics for a release liner substrate. SCK paper is also conventionally prepared from highly refined bleached chemical pulp that has been calendered strongly, but less strongly than glassine.
Typical characteristics defining a calendered glassine paper are
- smoothness of at least 900 sec/min (ISO 5627),
- grammage equal to or less than 120 g/m2 (ISO 536),
- density equal to or higher than 1.050 g/cm3 (ISO 534), wherein the density refers to grammage (ISO 536) per thickness (ISO 534:2011 ),
- porosity equal to or less than 15000 pm/Pas (ISO 11004), and
- transparency of equal to or higher than 40% (ISO 2469), the parameter values corresponding to ISO standards referred in parentheses.
A calendered glassine paper suitable for release liner typically has
- a grammage in the range of 40 to 120 g/m2 (ISO 536),
- a density in the range of 1 .050 to 1 .190 g/cm3 (ISO 534), and
- a transparency in the range of 40 to 56%, (ISO 2469),
A high transparency is preferred, such as in the range of 42 to 54%, most preferably in the range of 44 to 54% (ISO 2469).
Typical characteristics defining a SCK paper are
- smoothness of at least 900 sec/min (ISO 5627),
- grammage equal to or less than 120 g/m2 (ISO 536),
- density equal to or higher than 1.030 g/cm3 (ISO 534), wherein the density refers to grammage (ISO 536) per thickness (ISO 534:2011 ),
- porosity equal to or less than 15000 pm/Pas (ISO 11004), and
- transparency of equal to or higher than 36% (ISO 2469), the parameter values corresponding to ISO standards referred in parentheses.
A SCK paper suitable for release liner typically has
- a grammage in the range of 50 to 120 g/m2 (ISO 536),
- a density in the range of 1 .030 to 1 .190 g/cm3 (ISO 534), and
- a transparency in the range of 36 to 56%, (ISO 2469),
A high transparency is preferred, such as in the range of 38 to 54%, most preferably in the range of 40 to 52% (ISO 2469).
The thickness of a calendered glassine or a SCK paper denotes thickness in micrometers after a calendering treatment, prior to applying a release coating. Thickness, unless otherwise stated, refers to the apparent thickness, determined as single sheet thickness (ISO 534:2011 ). Glassine paper is calendered with a multi-nip calender or a supercalender before or after applying a primer coating. SCK paper is calendered with a supercalender before or after applying a primer coating Calendering enables to produce a glassine or SCK paper having high density surface and high transparency, but may lead to moderate reduction in the burst, tensile, and tear strength of the glassine paper. Calendering also reduces the thickness of the paper to a predefined target thickness. Glassine and SCK papers are typically surface sized with a primer coating, which is chemically compatible with a silicone polymer release coating. The primer coating can be applied on one or both sides, typically in the range of 1 to 5 g/m2, preferably an amount in the range of 1 to 2 g/m2 per side is used. A primer coating for glassine or SCK paper generally comprises water soluble binders, such as starch, polyvinyl alcohol and/or carboxymethyl cellulose.
Reference is made to Figure 1 , which, by means of an example, discloses a structural cross-dimensional view of release liner REL1 , wherein the substrate GLA1 is a glassine paper. A release liner REL1 , in this context, refers to an industrially manufactured paper product, which comprises a dehesive surface coating on at least one side of a calendered paper substrate GLA1. The
dehesive surface coating is generally referred to as release coating SIL1 . The dehesive surface coating may be used as a protective layer for an adhesive label which contains a face material and an adhesive layer.
A method for manufacturing a release liner REL1 comprises applying a release coating SIL1 on a paper substrate GLA1. The dehesive properties of the release coating SIL1 are typically obtained by means of an addition-curing silicone system in the presence of a suitable metal catalyst, such as platinum. An addition-curing silicone system comprises a reactive silicone polymer and a silane hydride cross-linker comprising functional vinyl groups, which are provided in a fluid form and may be spread on the paper substrate GLA1 in an amount of ca. 1 g/m2 The reactive silicone polymer is typically a hydrophobic, silicon-based organic polymer, such as polydimethylsiloxane. When the release coating on the paper is exposed to a cross-linking temperature, typically in the range of 65-150°C, a chemical reaction initiates, which cures the release coating and anchors it on the substrate GLA1. This method enables to obtain a release liner REL1 which comprises a dehesive and hydrophobic surface coating layer based on a cured silicone polymer.
A calendered glassine paper, when used as substrate GLA1 in a release liner REL1 , typically comprises a paper PAP1 as support layer and a primer coating POL1 . This applies also to a SCK paper. The paper PAP1 is manufactured on a paper machine on a machine direction Sx, which refers to the travelling direction of a paper web and paper on the paper machine. The properties of the paper may be different in the machine direction and in a direction perpendicular to the machine direction Sx along the surface of the paper, referred to as the cross-direction Sy. The paper has a thickness in direction Sz parallel to surface normal of the paper. Unlike many other paper types, glassine or SCK paper surface is typically not coated with mineral pigments, at least not in significant amounts. A glassine paper and a SCK paper, however, in general comprises a primer coating POL1 , such as a surface sizing applied on at least one side of the paper. Surface sizing improves the paper surface characteristic, such as barrier properties. An advantageous primer coating POL1 is a water-soluble polyvinyl alcohol comprising hydroxyl groups. Some of the hydroxyl groups of the polyvinyl alcohol may have been modified to comprise reactive groups, such as vinyl groups. This enables the polymer to participate into the cross-linking reaction of the addition-curing silicone system. The primer coating POL1 thereby improves the anchorage of the dehesive surface coating layer to the paper substrate GLA1 .
Due to high quality hydrophobic silicone polymers used nowadays in the release coatings for glassine and SCK paper, RGP and RSCK typically have
a stable release value. Thus, after the adhesive labels have been removed, very low amount of adhesive residue remains on the release liner surface. A RGP or RSCK which has been used as a carrier for adhesive labels therefore contains very low amounts of adhesive residues.
A method for manufacturing glassine paper for a release liner
Reference is made to Figure 2, which, by way of an example, presents a method for manufacturing calendered glassine or SCK paper, which comprises
- refining 11 a, 11 b of pulps PULP1 , PULP2,
- mixing 12 together pulps PLILP1 , PLILP2, PLILP3, and, optionally, broke BRK1 and white water WHT 1 , to obtain a stock MIX1 ,
- forming 13 a paper web at a headbox of a paper machine, and
- forming 14 a calendered glassine or SCK paper on the paper machine.
The calendered glassine or SCK paper is suitable for use as a substrate GLA1 in a method 15 for manufacturing a release liner REL1 .
In the method for manufacturing calendered glassine or SCK paper, a stock MIX1 is obtained after mixing 12 together different pulps during stock preparation. The mixing may be performed, for example by homogenising the stock MIX1 in a mixer. Stock refers to a pulp mixture from which paper is manufactured on a paper machine. Stock may also be referred to as furnish. Stock is fed to the forming section of a paper machine when manufacturing paper. A pulp suspension is needed to adjust loading upon stock preparation and to control fiber bonding, when forming a paper web 13 at a headbox of a paper machine. Thus, the stock is typically first fed to a machine chest. A machine chest is a consistency levelling unit, which provides a retention time such that any variations in consistency can be levelled out, prior to pumping the stock to a headbox, where it is dispensed evenly on to a moving wire at the forming section of a paper machine. Consistency is used to describe the percentage of oven dry mass from the total mass. The consistency of oven dry mass is 100%. The machine chest contains a valve system unit arranged to receive feedback from an on-line scanner measuring basis weight, which enables to adjust the basis weight of the paper to be formed.
Stock preparation may comprise loading and refining 11a, 11 b of pulp components PULP1 , PLILP2 to provide a pulp mixture with desired characteristics. The pulp components PLILP1 , PLILP2 may be refined
separately. Depending on the paper to be manufactured, the stock MIX1 may further contain non-fibrous additives, such as sizing agents.
When manufacturing glassine paper comprising recycled pulp obtained from RGP, the stock contains both non-recycled bleached chemical pulp produced from hardwood PULP1 , non-recycled bleached chemical pulp produced from softwood PLILP2 and recycled pulp PLILP3 obtained from release liner glassine paper. When manufacturing SCK paper comprising recycled pulp obtained from RSCK, the stock, respectively, contains both non-recycled bleached chemical pulp produced from hardwood PLILP1 , non-recycled bleached chemical pulp produced from softwood PLILP2 and recycled pulp PLILP3 obtained from release liner SCK paper. Non-recycled pulp, in this context, refers to virgin pulp material which is introduced into a paper manufacturing process for the first time. The non-recycled pulp may be bleached chemical pulp from a Kraft process. The stock MIX1 may contain broke BRK1 , which refers to material produced on a paper machine, which is not up to specification, such as paper trimmings. Broke may be recycled back to the paper manufacturing process. Broke may be refined prior to mixing 12. However, broke has undergone at least part of a paper manufacturing process on a paper machine, and hence is not considered to be virgin pulp material, when introduced again into the paper manufacturing process. Broke is not obtained from a release liner REL1 , either.
White water WHT1 may also be used, when preparing the stock MIX1 . White water is used to describe slurry, which is formed at a forming section of a paper machine, when fine particles present in the stock drain from the formed paper web WEB1 into a pit below the paper machine. White water contains fines suspended in the stock. Fines refers to particles having a width in the range of 10 to 75 micrometers and a length less than 0.2 millimeters. White water may be circulated back into the stock preparation by means of a short circulation of the paper machine or treated and used elsewhere in the papermaking process. The19mountt of circulated fines defines a retention level, which describes the ability of the formed paper web to retain fines, and therefore the balance between drainage and formation 13 of the paper web.
On the forming section of the paper machine, after the paper web WEB1 is formed 13 from the pulp suspension and dewatered, the paper web is moved on a press section to reduce the moisture content of the paper web further. The press section of a paper machine typically comprises a number of rolls for guiding and/or pressing the paper web. The paper web is then moved from the press section to a drying section of a paper machine. In the drying section, the paper web is heated to evaporate most of the remaining moisture in the paper
web. After drying section, the paper web may have a dry matter content level equal to or more than 90 wt.-%, for example in the range of 90 to 95 wt.-%, when determined according to SCAN-P 39:80. The forming of paper 14 therefore comprises a step for reducing moisture content of the paper web in a press section, and a step for drying the paper web in a drying section, thereby forming paper from a stock MIX1 that contains non-recycled bleached chemical pulp from hardwood PULP1 , non-recycled bleached chemical pulp from softwood PLILP2 and recycled pulp PLILP3 obtained from release liner glassine or SCK paper.
A weight percentage, abbreviated as wt.%, is used to describe a weight fraction of component in a composition. A weight percentage of pulp is used to describe a weight fraction of a pulp in a material. A weight percentage of pulp in a paper denotes the dry weight of the pulp in a dry paper, when determined according to SCANP-39:80 test method for dry matter content. The dry weight of a sample is determined by weighing 20 grams of sample on a dish before and after oven drying at 105°C and eliminating the mass of the empty dish from the measurement. Oven dry pulp has been dried at 105°C until its mass is constant and cooled thereafter in an exicator to ambient temperature of 25°C, prior to weighing.
As explained above, the stock used for manufacturing glassine or SCK paper in this context is distinguished, as it contains mainly bleached chemical pulp made of softwood and hardwood. A recycled pulp obtained from RGP or RSCK, due to its origin, also contains mainly bleached chemical pulp made of softwood and hardwood. The surface of a glassine and SCK paper is typically sized with a water-soluble polymer, such as polyvinyl alcohol in an amount ranging from 1 to 5 g/m2 A RGP or RSCK typically does not contain mineral fillers or coatings in significant amounts, such as kaolin (i.e. aluminium silicate dihydrate), clay pigments or calcium carbonate, when compared to other paper types, such as printing and writing papers. Thus, the ash content of a RGP or RSCK, determinable according to standard Tappi T 413 om-17, is generally very low, such a less than 3 wt.%, typically in the range of 1 to 3 wt.% of the weight of the paper.
The characteristics of glassine and SCK paper are typically obtained by using highly refined BCP, supercalendering and surface sizing agents. The supercalendering of glassine or SCK paper is typically performed in a temperature in the range of 120 to 200°C. The line pressure used for supercalendering a glassine or SCK paper is generally in the range of 300 to 500 kN/m. The glassine and SCK paper is generally moistened prior to calendering, to enhance the effects. This increases the transparency of
calendered glassine paper. The transparency of calendered glassine or SCK paper is significantly higher than is typical for other paper types with similar gram mage. Calendering increases surface density and transparency of the paper. Calendering also reduces specific volume and thickness of the paper. Calendered glassine and SCK paper is very strong, has a very smooth and dense surface and excellent barrier properties. A smooth and dense surface, which resists the penetration of many fluids, is beneficial when spreading a release coating on the paper surface.
Calendered glassine and SCK paper, as evident from the characteristics disclosed above, has not been designed for printing or writing. Instead, the calendered glassine and SCK paper is often used as a substrate GLA1 to form 15 a release liner REL1 , as indicated in Figure 2. RGP or RSCK thus seldom contains printing inks in significant amounts. In general, RGP or RSCK is substantially unprinted, compared to other paper types, which facilitates the recycling 16 of the RGP or RSCK into pulp PLILP3, which may be used to replace non-recycled bleached chemical pulp made of softwood PLILP2 in the method for manufacturing calendered glassine or SCK paper. RGP and RSCK thus possesses a combination of desired characteristics not available in other paper types to the same extent.
RGP, as well as RSCK, is exceptional material, when considering it from a viewpoint of circular economy. When producing glassine or SCK paper from non-recycled BCP the fibers experience very harsh conditions. At a paper 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 pressure. These treatments cause irreversible changes to the fiber structure, in particular to the pores formed between the cellulose protofibrils. This leads to reduced swelling capability of the fibers. The morphology as well as the ability of the fibres to swell is different, when compared to other type of fibers, such as, for instance, fibers from non-recycled bleached chemical pulp or broke. The phenomenon is specific for chemically pulped fibers. Due to this phenomenon, referred to as hornification, fibers derived from glassine or SCK paper display less bonding ability. Upon producing a release liner, the fibers are coated with a hydrophobic silicone polymer and heated, which exposes the fibers to further modifications.
A method for manufacturing recycled pulp from a release liner glassine paper
Reference is made to Figure 3. Release liner glassine papers share a common history of treatments. Respectively, release liner SCK papers also share a common history of treatments. This enables to use RGP or RSCK as raw material in a recycling process, which may be arranged to produce pulp with exceptional characteristics. To obtain sufficient quality recycled pulp for a method for manufacturing glassine or SCK paper, the raw material used for the recycling process should contain at least 75 wt.%, more preferably at least 85 wt.%, most preferably at least 90 wt.% of release liner glassine or SCK paper, respectively. Advantageously the raw material used for the recycling process consists substantially of release liner glassine or SCK paper. Most advantageously, with respect to release liner glassine paper, the raw material used for the recycling process consists substantially of release liner glassine of the same or similar colour, wherein the colour may be white or a non-white colour, such as light yellow, yellow, brown or blue. A method for manufacturing recycled pulp from a release liner substrate may therefore contain a step for sorting the release liner substrate for recycling prior to disintegrating, thereby obtaining sorted release liner substrate REL1 , which may be calendered glassine paper or supercalendered kraft paper..
A method for manufacturing recycled pulp from a release liner substrate comprises a sorting stage 20 for separating RGP or RSCK apart from other papers, a first process stage, denoted as a caustic loop CL1 , having a principal function of disintegrating the RGP or RSCK into pulp and detaching non-fiber material from fibers, and a second process stage, denoted as a cleaning loop NL1 , having a principal function of separating pulp fibers from non-fiber material, in particular silicone particles originating from the release coating. Caustic loop CL1 provides conditions in which the pulp fibers are able to swell and fibrillate. Cured silicon-based organic polymers, polydimethylsiloxanes in particular, are generally water-resistant and relatively inert chemically. Hence, in RGP or RSCK recycling conditions, as disclosed herein, the release coating is typically fragmented into pieces, which are hereafter denoted as silicone- based particles. In addition to the principal functions, the caustic loop CL1 and the cleaning loop NL1 are configured to adjust the fibrillation of the pulp suspension, such that the recycled pulp PLILP3 obtained from the release liner glassine or SCK paper has a pulp drainability in a range which enables the use of the recycled pulp obtained from the RGP or RSCK in a method for manufacturing glassine or SCK paper, respectively, without further refining. The caustic loop CL1 and cleaning loop NL1 provide means to control the
chemical load and temperature of the recycling process, as well as a means to adjust the consistency of the suspension.
Due to industrial use in high-speed labelling processes, RGP and RSCK may be collected in large quantities directly from an industrial user. Therefore, advantageously, the sorting of the RGP or RSCK takes place at a site where release liner REL1 , REL2 is used and converted into recyclable release liner waste, for example during a labelling process. For example, polyethylene coated Kraft papers can at this point be separated and excluded from recycling. Unlike water-soluble polymers or mineral coatings, a polyethylene film does not dissolve into the suspension and is therefore challenging to recycle. Alternatively, the sorting can be performed later at a sorting unit, for instance by using visual inspection, such that release liner glassine or SCK paper REL1 is separated from other paper components REL2 and non-paper components. The non-paper components, to the extent possible, are rejected already prior to entering a RGP or RSCK recycling process. A non-paper component refers to an object which has typically become unintentionally part of a paper recycling process due to material handling. A non-paper component is not adhered to paper and is meant to be rejected during the recycling process. Examples of non-paper components are plastic and films components, as well as pieces of metal, glass or sand.
Sorted RGP may be further separated based on a color shade of the paper. For example, light RGP shades, such as white and light yellow shades, may be separated from darker RGP shades, such as yellow, blue and brown RGP shades. Advantageously, white RGP grades, wherein the paper furnish does not contain colorants, are separated apart from non-white RGP grades, such as light yellow, yellow, blue and brown RGP grades. Advantageously, white RSCK, wherein the paper furnish does not contain colorants, may also be separated apart from other paper grades. A CIELAB colour space may be used for determining the colour from a sample of the RGP or RSCK paper. The colour information may be used as a basis for accepting and/or rejecting paper having a specific colour, such as white, light yellow, yellow, blue or brown or a specific shade, such as non-white, non-light or dark, for recycling.
A white glassine paper, in the context of this text, refers to CIE L* a* b* colour space coordinate values of the paper, wherein
- L* is in the range of 92 to 98,
- a* is in the range of -4 to +2, and
- b* is in the range of +3 to +9, preferably in the range of +5 to +7
, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant D65 and 10° standard observer, in accordance with ISO 5631 :2022.
A white SCK paper, in the context of this text, refers to CIE L* a* b* colour space coordinate values of the paper, wherein
- L* is in the range of 92 to 98,
- a* is in the range of -4 to +2, and
- b* is in the range of +5 to +11 ,
, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant D65 and 10° standard observer, in accordance with ISO 5631 :2022.
An advantage of sorting the RGP or the RSCK based on a color shade of the paper to white and non-white grades is that recycled pulp obtained from RGP or RSCK may be produced without bleaching. Thus, a calendered glassine or SCK paper suitable for use as a substrate of a release liner may comprise 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, respectively, which recycled pulp has not been bleached.
A CIELAB colour space may further be used for determining the colour of the non-white RGP grades, such as light yellow, yellow, brown or blue shades of glassine paper. Non-white RGP grades may be further sorted based on the colour of the paper, such that the sorted RGP is light yellow, yellow, brown or blue. This enables a two-step sorting at the sorting stage 20, wherein the release liner is first sorted based on the paper type and then based on the non- white colour of the paper, which is determinable from a paper sample by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant C and 2° standard observer, in accordance with ISO 5631 -1 :2022. This provides highly homogeneous starting material for manufacturing of recycled pulp, wherein the paper is of the same type and further comprises the same or similar colour. Typically, same kind of colourants are used to provide a specific colour shade for a glassine paper, such as yellow, brown or blue, whereby the combination of the same paper type and the same or similar colour is advantageous for a more closed-loop recycling of glassine paper. Such recycling process is also less complex. A two-step sorting may thus be used to facilitate the recycling of non-white RGP grades. A two-step sorting may be further used to facilitate introduction of such recycled pulp into a glassine paper manufacturing process, wherein the same or similar paper colour is produced.
In the context of this text, light yellow refers to CIE L* a* b* colour space coordinate values, wherein
- L* is in the range of 87 to 96,
- a* is in the range of -4 to +8, and
- b* is in the range of +24 to +43,
, the values determinable from a sample of release liner glassine paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant C and 2° standard observer, in accordance with ISO 5631 -1 :2022.
In the context of this text, yellow refers to CIE L* a* b* colour space coordinate values, wherein
- L* is in the range of 65 to 71 ,
- a* is in the range of +7 to +13, and
- b* is in the range of +50 to +56,
, the values determinable from a sample of release liner glassine paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant C and 2° standard observer, in accordance with ISO 5631 -1 :2022.
In the context of this text, brown refers to CIE L* a* b* colour space coordinate values, wherein
- L* is in the range of 64 to 70,
- a* is in the range of +3 to +9, and
- b* is in the range of +17 to +23,
, the values determinable from a sample of release liner glassine paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant C and 2° standard observer, in accordance with ISO 5631 -1 :2022.
In the context of this text, blue refers to CIE L* a*, b* colour space coordinate values of a calendered glassine paper, wherein
- L* is in the range of 80 to 89,
- a* is in the range of -17 to 0, and
- b* is in the range of -15 to +9,
, the values determinable from a sample of release liner glassine paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant C and 2° standard observer, in accordance with ISO 5631 -1 :2022.
The sorting can be performed mechanically, for example by using automated image analysis. An automated image analysis system may comprise, for
example, a detection unit, a control unit, and sorting unit arranged to detect and separate RGP or RSCK apart from other paper products and non-paper products, based on particle shape, size and contrast. A detection unit may comprise optical instruments capable of identifying wavelengths of the visible light spectrum for detecting and identifying the colour of the paper. This may be complemented by instruments capable of identifying near infrared light, which are able to provide further information of the nature of the materials in the paper. The automated image analysis may be configured to assess paper quality based on multiple parameters, such as paper whiteness, brightness, colour shade, transparency or contrast. Pressurized air and nozzles operating on a conveyer belt may be used to separate rejected material and accepted material Advantageously, the material, after sorting, contains RGP or RSCK in the range of 75-100 wt.%, preferably in the range of 85-100 wt.%, most preferably in the range of 90-100 wt.% of the weight of the recyclable paper components. In an ideal case, the material sorted for recycling consists substantially of RGP or RSCK, which may be further sorted based on the paper colour.
The caustic loop 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 a batch mode, which facilitates the adjustment of the pulping conditions. When RGP or RSCK and clear water F1 are fed to a high consistency pulper, a pulp suspension is formed. The consistency of the pulp suspension may be adjusted by the amount of clear water F1 , which may be obtained from another process. The clear water F1 may be fresh water. The consistency of the pulp suspension may be further adjusted by reusing process water F2, F3, F4 downstream from the recycling process, as needed. Process water circulated within a loop CL1 , NL1 may be further used to improve the recovery of fibers within the loop CL1 , NL1. For efficient disintegration of the RGP or RSCK, the consistency of the material during the pulping may be higher than 15 wt.%, preferably higher than 18 wt.%, such as in the range of 20 to 30 wt.%, advantageously in the range of 20 to 25 wt.%.
The pulping of RGP or RSCK is performed in alkaline conditions to facilitate disintegration of the cellulose fibers from the RGP or RSCK, since RGP and RSCK both comprise a dense surface, a polymeric primer coating and a release coating. Advantageously, during pulping, the pH is maintained in a range between 8.5 to 10. The pH may be adjusted by addition of NaOH, referred to as caustic soda. Caustic soda reacts with the hydrogen groups of the fiber and promotes fiber swelling, referred to as caustic swelling, which will loosen the fiber network of the RGP and RSCK. Caustic soda also acts as an
activator for hydrogen peroxide, which may be used to facilitate oxidative bleaching, when the pulp suspension contains colourants, for example blue colorant from a non-white grade of RGP or RSCK. Hydrogen peroxide is also used to prevent yellowing during the pulping. Typically, hydrogen peroxide is added in the range of 0.5 - 2 wt.%. Sodium silicate is typically added to buffer the pH of the pulp suspension and to prevent the pH of the suspension from rising excessively at the beginning of pulping. Sodium silicate thus contributes to the alkalinity of the pulp suspension, such that the conditions are suitable for the caustic swelling. Sodium silicate may be also used as a stabilising agent for the hydrogen peroxide. Sodium silicate may further improve the detachment of release liner from the fibers. Typically, sodium silicate is added in the range of 1 - 6 wt.%. In addition to sodium silicate, a saponifying agent, typically a fatty acid such as palmitic acid or stearic acid, is used for facilitating the detachment of silicone-based particles and other hydrophobic impurities from the fibers. Fatty acids react first with caustic soda and then with calcium ions present in the pulp suspension and form calcium soap, which is waterinsoluble and finely dispersed in an aqueous phase. Soap particles, which are strongly hydrophobic, facilitate maintaining the pulped fibers and the detached hydrophobic particles, such as silicone-based particles, apart from each other in the pulp suspension. Typically, a fatty acid is used in a range of 0.1 to 1.5 wt.% of the RGP or RSCK, depending on composition the sorted release liner substrate. The fatty acid dose is advantageously matched with the water hardness, such that the amount of fatty acids is substantially equal with the amount of calcium ions present in the suspension.
Depending on the HC pulper type, the operating time of the pulping may be adjusted. The total operating time, referred to as slushing or dwell time, is generally in the range of 30 to 60 minutes, preferably at least 40 minutes, to ensure sufficient disintegration of the cellulose fibers. Typically, the temperature of a pulp suspension during pulping is at least 60°C, preferably at least 75°C, such as in the range of 60-85°C. The primer coating of the RGP and RSCK typically comprises water-soluble polymers, such as partially or fully hydrolysed polyvinyl alcohol, carboxymethyl cellulose and/or starch, which have a tendency to agglomerate at elevated temperature. While at least some of the water-soluble polymers may be dissolved during the pulping and hence filtered out during the subsequent dewatering operations, a higher pulp suspension temperature, preferably at least 75°C, promotes the agglomeration of any non-dissolved water-soluble polymers detached from the fibers. Agglomerated polymer particles from the sizing agents or release coating are easier to remove in subsequent screening and cleaning operations.
Thus, a high consistency suspension, sufficient time, temperature and chemical additives, such as hydrogen peroxide, sodium silicate (water glass) and caustic soda (NaOH), may be used to disintegrate and detach the fibers of the RGP and the RSCK and induce caustic swelling, despite the hornification of the fibers.
A coarse screening unit 22, such as a disc screen having aperture size equal to or less than 4 millimeters, such as in the range of 2 to 4 millimeters, preferably in the range of 2.0 to 3.0 millimeters, most preferably in the range of 2.2 to 2.5 millimeters, is used to separate particles coming from the pulper based on their size, form and shape. The screening operates under pressure and particles passing through the aperture are accepted, while others are rejected. This enables to remove solid contaminants and non-paper components from the pulp suspension, such as sand and metal objects, as well as larger particle agglomerates.
A high-consistency cleaning unit 23, such as a cleaner using centrifugal field, is used to complement the coarse screening to separate pulp fibers from contaminants based on specific gravity. A centrifugal cleaner can remove particles down to a dimension of 10 micrometers. In addition to heavy particles such as sand and metal, a centrifugal cleaner can separate also light-weight particles present in the RGP or RSCK, such as polymer particles, release coating agglomerates or residual adhesive stickies, when their density differs sufficiently from the density of water. PVA, for example, has a density typically in the range of 1.19-1.35 g/cm3 at 25°C, which differs significantly from the density of water. The separation of particles having a density closer to 1 .00 g/cm3, may be improved by raising the pulp suspension temperature, which decreases the density of the water. The pulp suspension temperature during the high-consistency cleaning is typically in the range of 30 to 85°C, preferably in the range of 50 to 85°C, to facilitate the cleaning of the PVA. When using a high-consistency cleaner, in general, a pulp consistency of 2-6 wt.% is used. The consistency of the pulp suspension during the cleaning may be adjusted by means of adjusting the pulping and screening conditions. The consistency of the pulp suspension may be further adjusted by reusing process water F4 downstream from the recycling process, as needed.
A dewatering unit 24 based on pressing or filtration is used to mechanically remove process water F4 from the pulp suspension and to increase the pulp consistency. Dewatering thus separates solids from a suspension. Preferably, a disk filter, a screw press or a twin-wire press is used, for efficient loop separation between the caustic loop CL1 and the cleaning loop NL1. A high consistency enables an efficient dispersion in the cleaning loop NL1 , which
can be used to adjust the pulp fibrillation and drainability. An efficient solid removal further enables to remove dissolved sizing agents which have not been screened or cleaned out from the pulp suspension. The pressing of the pulp suspension at the dewatering unit 24 results into a thickened pulp suspension, which comprises the fibers to be retained. Advantageously, at the end of the caustic loop CL1 , the pulp suspension is thickened into a consistency equal to or higher than 20 wt.%, such as in the range of 20 to 50 wt.%, preferably in the range of 25 to 40 wt.%.
The cleaning loop NL1 comprises a dispersion unit 25, a flotation unit 26, a second screening unit 27, a washing unit 28 and a dewatering unit 29. A dispersion unit is used for producing shear forces which are sufficient for detaching remaining contaminants, such as silicone-based polymer, from the fibers and to adjust the average size of the contaminant particles to below 100 micrometers, suitable for removal by means of flotation. The dispersion unit may operate with a thickened pulp suspension received directly from the dewatering unit 24. The method may further comprise a dilution chest prior to the dispersion unit 25, for adjusting the consistency and/or temperature of the dewatered pulp suspension. Clear water F1 and/or process water F2, F3 downstream from the recycling process may be used to adjust the consistency of the dewatered pulp suspension. The process water F2, F3 downstream from the recycling process may further be used to adjust the pH of the dewatered pulp suspension. Consistency of the dewatered pulp suspension provides a means for adjusting the amount of dispersion energy applied to the pulp suspension. Advantageously, the dispersion is performed with a conical or disc disperger instead of a kneader. Unlike a kneader, a conical disperger and a disc disperger operate in conditions similar to refining. This enables an efficient and simultaneous adjustment of pulp fiber properties such that at least some of the fiber properties of RGP or RSCK fibers lost due to hornification may be compensated already during the RGP recycling process. Thereby the recycled pulp characteristics, such as drainage and bulk, may be optimized for a method for manufacturing glassine or SCK paper, respectively. Conical and disc type dispergers operate in a manner where inverse correlation between pulp fibrillation and temperature exists; a lower pulp suspension temperature at the inlet correlates with a higher decrease in fibrillation. Typically, when a pulp suspension having a consistency in the range of 25 to 40 wt.% is used, the temperature of the pulp suspension at the inlet to the disperger is in the is range of 50 to 130°C, preferably in the range of 50 to 85°C. Thus, the fibrillation and/or drainability of the pulp can be adjusted during the dispersion by means of controlling the pulp consistency and temperature, in addition to the amount of specific energy consumed (SEC). In general, SEC in the range of 30-150 kWh/t, preferably in the range of 40-100 kWh/t, most preferably in the range of
45-90 kWh/t, may be used during the dispersion, to obtain pulp having a SR number equal to or higher than 25, such as in a range from 30 to 55, when determined according to ISO 5267-1. Preferably, the fibrillation of the pulp is adjusted after the caustic loop CL1 by means of dispersion in a conical disperger or a disc disperger, prior to flotation, such that the temperature of the pulp suspension at the inlet to the disperger is in the is range of 50 to 130°C, preferably in the range of 50 to 85°C and the pulp suspension has been thickened into 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.%.
A flotation unit 26 is used to remove hydrophobic particles from the pulp suspension by means of air bubbles, which collide and adhere to the particles. Clear water F1 and/or process water F2, F3 downstream from the recycling process is used for adjusting the consistency of the pulp suspension for flotation. Typically, a pulp suspension having a consistency less than 2 wt.%, such as in the range of 0.5 to 1.5 wt.%, is used for the flotation. The pulp suspension temperature during the flotation is typically in the range of 40 to 70°C. Advantageously, during flotation, the pH is maintained alkaline, in a range between 7 to 10, preferably equal to or higher than 8.5, such as in the range of 8.5 to 10. The pH may be adjusted and buffered by addition of suitable alkaline agents, such as caustic soda and sodium silicate. Soap, such as sodium soap, or other surfactant comprising a hydrophilic and a hydrophobic part, is added to act as a collector. A collector is used for promoting agglomeration of silicone particles and facilitate their charging and flotation. During flotation, a low water hardness in the range of 10-20 dH, determinable in accordance with ISO/TS 15923-2:2017, is preferred, for promoting the agglomeration further. The flotation unit 26 may contain several flotation cells arranged into a series.
A second screening unit 27 is used for fine screening to separate debris from the fibers coming from the flotation, in particular silicone particles originating from the release coating. The fine screening may use slot screens having a slot size equal to or less than 0.25 millimeters, such as in the range of 0.10 to 0.25 millimeters, preferably in the range of 0.10 to 0.20 millimeters. The screening operates under pressure and pulp suspension passing through the slots is accepted.
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 under wire pressure with a set of two or more rolls, wherein the wire has a mesh size in the range of 36 to 60 micrometers, such that particles with a maximum size less than 30 micrometers are removed. A pulp suspension
having a consistency equal to or less than 2 wt.%, such as in the range of 0.5 to 2 wt.% is typically used at the inlet of the washing unit. Clear water F1 is used to wash the filtered fiber mat and to adjust the consistency of the suspension during the washing. Dissolved contaminants are removed with the filtrate. The filtrate may be used as process water F3 upstream in the recycling process.
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, such that the pulp consistency may be increased efficiently for transport or storage. Advantageously, at the end of the cleaning loop NL1 , the pulp suspension is thickened into 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.%. The recycled pulp thus obtained from release liner glassine paper PULP3 may then be used in a method for manufacturing calendered glassine paper.
As an interim of what was disclosed above, and with reference to Figures 2 and 3, the recycling process 16 is arranged to contain operations and conditions, which optimize the separation of fibers from non-fiber components in the pulp suspension. Simultaneously, the caustic loop and the cleaning loop have been configured to adjust the fibrillation of the pulp suspension, such that a pulp drainability is obtained, which is in a range enabling the use of the recycled pulp obtained from the RGP or RSCK in a method for manufacturing glassine or SCK paper, respectively, preferably without further refining.
Hence, the recycling process 16 is arranged to improve the fiber characteristics such that the recycled pulp PULP3 from the RGP or RSCK may be used without further refining for preparing a stock for glassine or SCK paper manufacturing, respectively. The operations and conditions homogenize the pulp and develop characteristics such as pulp fibrillation, drainability and pH, which improve the quality of the pulp for a method for manufacturing glassine or SCK paper.
A pulp consistency in the range of 30 to 50 wt.% is advantageous in that the pulp fibers are not exposed to a further drying treatment, which may cause further hornification. A pulp consistency in the range of 30 to 50 wt.% is advantageous also when mixing the recycled pulp PULP3 together with different pulps, during stock preparation. However, when preparing recycled pulp for storage, the dewatering unit 29 may be supplemented with a drying unit, such as a fluffer, to increase the dryness of the pulp, such that a pulp
consistency equal to or higher than 80, such as in the range of 80 to 90 wt.% is obtained.
Properties of recycled pulp obtained from release liner glassine paper
Referring to above, recycled pulp obtained from RGP or RSCK has a pH which is typically neutral or alkaline, when determined from aqueous pulp extracts. An alkaline pH during the recycling is preferred, as a higher pH softens the pulp and facilitates the flotation. Alkalinity of the pulp also facilitates the modification of pulp fibrillation and drainability. Recycled pulp obtained from RGP or RSCK, when having alkaline pH, needs less energy for refining. The pH, however, may be adjusted, as necessary, prior to using the recycled pulp.
Recycled pulp obtained from RGP or RSCK is distinguished from non-recycled BCP due to the extent of hornification of the fibers. This can be measured, for instance, by water retention value, abbreviated as WRV, according to ISO 23714:2014(en). WRV is an empirical measure of the capacity of a pulp sample to hold water. Typically, the WRV of recycled pulp obtained from RGP is low, such as in the range of 1 .3 to 1 .6 g/g.
Recycled pulp obtained from RGP is also distinguished by its water drainage resistance, which is a measure of pulp fibrillation, and which may be determined by the Schopper-Riegler test. The SR number is a measure of the extent of fibrillation in the recycled pulp PLILP3. The recycled pulp obtained from RGP or RSCK may have a SR number equal to or higher than 25, such as in a range from 25 to 65, when determined according to ISO 5267-1. Typically, recycled pulp obtained from RGP or RSCK has a SR number equal to or higher than 30, if the aqueous extract, from which the measurement is performed, is process water that contains electrolytes. When measuring the water drainage resistance from dry pulp with standard water in accordance with ISO 5267-1 , in conjunction with ISO 14487, the SR number may be higher, such as equal to or higher than 40, since the concentration of electrolytes (salts) in a pulp suspension influences the drainability. Regardless of the initial SR number, upon refining the SR number of the recycled pulp obtained from RGP and RSCK develops very quickly. This is a feature of recycled pulp obtained from RGP, which may be used to distinguish it from other nonrecycled pulp components used in a glassine paper. This is also a feature of recycled pulp obtained from RSCK, which may be used to distinguish it from other non-recycled pulp components used in a SCK paper. Table 1 (below) demonstrates, by means of an example, the development of SR number (°SR)
in recycled pulp obtained from RGP, as a function of specific energy consumption (SEC) in kWh/t. In the example, a specific edge load (SEL) of 0.3 J/m was applied, using Voith-Sulzer laboratory refiner having 40D hardwood plates. Prior to refining, the recycled pulp obtained from RGP presented a SR number of 32.
Table 1. Development of SR in the recycled pulp obtained from RGP, as a function of SEC (kWh/t).
Advantageously, prior to the mixing in a method for manufacturing calendered glassine paper, the recycled pulp obtained from release liner glassine paper has a °SR equal to or higher than 25, such as in a range from 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55, when determined according to ISO 5267-1. Advantageously, also prior to the mixing in a method for manufacturing calendered SCK paper, the recycled pulp obtained from release liner SCK paper has a °SR equal to or higher than 25, such as in a range from 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55, when determined according to ISO 5267- 1.
Advantageously, when using the recycled pulp obtained from RGP or RSCK in a method for manufacturing calendered glassine paper, or SCK, respectively, suitable for use as a substrate of a release liner, the recycled pulp obtained from the RGP or RSCK has a pH which is 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. A recycled pulp obtained from release liner glassine paper having an alkaline pH requires less energy for refining of the fibers. A highly alkaline pH may inhibit the functioning of cationic UV curing silicone systems. Most preferably, the pH in the range of 7.5 to 8.2, whereby the drying and the compatibility of the recycled pulp can be optimized for glassine or SCK paper production. When determining the pH of dried pulp samples, standard ISO 6588-2 (2020) may be used. When determining the pH of pulp suspension samples from a paper machine, the pH may be measured either directly from the pulp sample (when
the consistency is 5 wt.% or less) or from a filtrate (when the consistency is higher than 5 wt.%). A filtrate, as used herein, refers to an aqueous extract. When determining the pH from dry pulp, an amount of 2 grams of dry pulp is cut into pieces, such that each piece has a maximum dimension of 1 centimetre. The cut pieces are mixed with 100 millilitres of deionised water to disperse the pulp with the water such that a suspension having a pulp concentration of 2 wt.-% of water is obtained. The sample thus obtained is heated to a boiling point and boiled for 60 minutes. After boiling, the sample is cooled down, such that the temperature of the sample is in the range of 20 to 25°C, and the sample is filtrated through a filter having a 200 mesh grid, for example by means of a Buchner-funnel, thereby obtaining a filtrate separated from the pulp. The pH is measured from the filtrate thus obtained.
The pulp pH is measured from the filtrate or an aqueous extract that has a temperature in the range of 20 to 25°C, by means of a pH meter, using two buffer solutions having pH 4 and pH 7, respectively. Suitable pH meters are, for example, pH-meter CG 840 with electrode N 1042A, Knick pH-meter 766 Calimatic with electrode SE 103 or Mettler-Toledo MP 120, used according to the manufacturer’s instructions.
When manufacturing recycled pulp from a release liner glassine or SCK paper as disclosed above, the removal of silicone-based particles is not complete. The recycled pulp obtained from RGP, and RSCK, respectively, still contains traces of the cured release coating, in very small size particles, which are chemically rather inert. The maximum particle size of the silicone-based particles is typically in the range of 100 to 150 micrometers and limited by the slot size used in the fine screening in the cleaning loop NL1 . While detectable, the amount of silicone-based particles in the recycled pulp obtained from RGP or in the recycled pulp obtained from RSCK has not been observed to cause difficulties, upon manufacturing calendered glassine or SCK paper on a paper machine. The amount of silicone-based particles may be measured with an Energy Dispersive X-ray Spectroscopy from a test specimen which is combusted at 900°C, in accordance with Tappi standard T 413, which detects the oxides of silicon. Typically, a calendered glassine paper comprising recycled pulp from RGP or SCK comprising recycled pulp from RSCK contains silicon in an amount of 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 to 0.3 wt.%, determinable as dry matter content from a paper specimen which is combusted at 900°C with an Energy Dispersive X- ray Spectroscopy, in accordance with Tappi standard T 413.
Experimental studies
Reference is made to Figures 4-11 . Experimental studies were prepared to assess the characteristics of the recycled pulp obtained from RGP and to determine its effects in a method for manufacturing calendered glassine paper.
Experimental study 1
In a first experimental study, pulp properties of recycled pulp obtained from RGP were measured and compared to properties of non-recycled bleached chemical pulps and mill broke used at a paper mill for glassine paper production. Below are listed the pulp types and their abbreviation in the experimental study:
BCP SW northern bleached softwood kraft pulp (coniferous trees) BCP HW bleached hardwood kraft pulp (eucalyptus) BCP SW rf. mill refined BCP SW (SEC 240 kWh/t) BCP HW rf. mill refined BCP HW (SEC 135 kWh/t) mill broke mill broke obtained from glassine paper production PLILP3 recycled pulp obtained from RGP
The consistency of the pulps in the study was 4 wt.%. The properties of the non-recycled bleached chemical pulps were measured before and after refining, to compare the properties of the recycled RGP and the non-recycled bleached chemical pulps.
Pulp analyses
The pH of the pulps disclosed above were measured from aqueous pulp extracts according to ISO 6588-2 (2020). The results are shown in Table 2 (below).
Table 2. Measured pH of pulp samples.
The results represent an average of measurements, during which the recycled pulp obtained from RGP varied in the range of 6.8 to 7.3. The measured pH in the recycled pulp obtained from RGP was clearly higher than in the nonrecycled chemical pulps made of softwood or hardwood. The measured pH in the recycled pulp obtained from RGP was clearly higher than in the mill broke, as well.
Pulps as disclosed above were further analysed by means of a fiber furnish analysis according to ISO standards ISO 9184-1 and 9184-4:1990. A fiber furnish analysis is capable to identify papermaking fibers from a sample. The analysis may further be used to quantify average dimensions of the different fiber types detected in a sample. The wood species used in a pulp may be distinguished by comparison method, wherein a sample fiber is compared against a known reference fiber. Valmet Fiber Image Analyzer (Valmet FS5) is an example of a device, which can be used according to the manufacturer’s instructions to perform the fiber furnish analysis. For example, automated optical analysis, such as an ultra high resolution (UHD) camera system equipped with image analysis software, may be used to acquire a greyscale image of a sample, of which image the properties of the fibers in the sample may be determined. The greyscale image may be acquired from a sample placed in a transparent sample holder, such as a cuvette, using a 0.5 millimetre depth of focus according to ISO 16505-2 standard. Valmet Fiber Image Analyzer (Valmet FS5) may further be used to determine fiber dimensions, such as fiber length and fiber width, as well as length weighted distribution of the pulp fibers, by means of automated optical analysis using unpolarized light, according to ISO 16065-2: 2014.
Reference is made 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 comprises an average fiber length of 0.94 millimeters. The non-recycled BCP made of hardwood comprises an average fiber length of 0.86 millimeters, which upon refining was reduced to 0.84 millimeters. Hence, the average fiber length of recycled pulp obtained from RGP is higher than the average fiber length of non-recycled BCP made of hardwood. The non-recycled BCP made of softwood comprises an average fiber length of 2.10 millimeters, which upon refining was reduced to 2.00 millimeters. Hence, the average fiber length of recycled pulp obtained from RGP is significantly less than the average fiber length of non-recycled BCP made of softwood. Mill broke had an average fiber length of 1.04 millimeters.
Reference is further made to Figure 5, which shows comparative data of average fiber width in micrometers of fibers in recycled pulp obtained from RGP and other pulp types, measured, using the Valmet Fiber Image Analyzer (Valmet FS5). The recycled pulp obtained from RGP comprises an average fiber width of 20 micrometers. The non-recycled BCP made of hardwood comprises an average fiber width of 18 micrometers, which upon refining was increased to 19 micrometers. Hence, the average fiber width of recycled pulp obtained from RGP is larger than the average fiber width of non-recycled BCP made of hardwood. The non-recycled BCP made of softwood comprises an average fiber width of 28 micrometers, which upon refining was increased to 29 micrometers. Hence, the average fiber width of recycled pulp obtained from RGP is significantly less than the average fiber width of non-recycled BCP made of softwood. Mill broke had an average fiber width of 20 micrometers.
Hence, the average fiber length and width of recycled pulp obtained from RGP is closer to the average fiber length of non-recycled BCP made of hardwood or broke, but clearly distinguished from the average fiber length of nonrecycled BCP made of softwood.
The length weighted distribution of the pulp fibers was further analysed using Valmet Fiber Image Analyzer (Valmet FS5), according to the manufacturer’s instructions. In the analysis, fibers were defined to be the fraction of the pulp that included particles having a width in the range of 10 to 75 micrometers and a length in the range of 0.2 to 7.0 millimeters. Fines were defined to be the fraction of the pulp that included particles having a width in the range of 10 to 75 micrometers and a length less than 0.2 millimeters. Fibrils were defined to be the fraction of the pulp that included particles having a width less than 10 micrometers and a length longer than 0.2 millimeters. Flakes were defined to be the fraction of the pulp that included particles having a width less than 200 micrometers and a length less than 0.2 millimeters. Fibrils are typically particles generated from the secondary wall of the wood cell layer structure, which due to their elongated shape may improve bonding properties of the pulp. Flakes are typically particles generated from the middle lamella and primary wall of the wood cell layer structure, which tend to decrease the bonding properties of the pulp. The flakes scatter light and may hence affect the optical properties of the pulp by increasing opacity and decreasing transparency.
The content of fines in a bleached chemical pulp, such as bleached kraft pulp, varies naturally depending on the used wood species. The content of the fines in a pulp varies also due to pulp treatments, such as refining and recycling, as disclosed above. The length weighted distribution of fines is a fundamental
property of pulp, which affects inter alia the formation of paper web during manufacturing. The pulp characteristics also have an effect on the tensile strength, the burst strength, the fold endurance and the tear resistance of a paper.
The results of the analysis was, that the amount of fines in the recycled pulp obtained from RGP was 16.3 % of the total amount of fibers in the recycled pulp, when determined as length weighted average fiber length, by means of an automated optical analysis using unpolarized light according to ISO 16065- 2: 2014. The amount of fines in the recycled pulp obtained from RGP was in the same level as in the mill refined non-recycled bleached chemical pulp made of hardwood. The amount of fibrils in the recycled pulp obtained from RGP, unexpectedly, was much higher than in the mill refined non-recycled bleached chemical pulp made of hardwood, but lower than in the mill refined non-recycled bleached chemical pulp made of softwood. The results demonstrate that the recycled pulp obtained from release liner glassine paper contains particles derived from the recycled pulp having a length less than 200 micrometers in an amount equal to or higher than 10 %, such as in a range from 10 to 30 %, preferably in the range of 12 to 20 %, most preferably in the range of 15 to 17 %.
The Valmet Fiber Image Analyzer also provided results of the amount fiber deformations, such as fiber kinks and fiber curl, in the pulps. Fiber kinks and curls tend to decrease the tensile strength of the formed paper, due to reduced bonding ability of the fibers in a fiber network. Of notice, the number of kinks in the recycled pulp obtained from RGP was 3250 1/m, which was considerably higher than in the non-recycled bleached chemical pulps after refining or in the mill broke. The number of kinks in the non-recycled bleached chemical pulp made of hardwood was 2880 1/m before refining and 2310 1/m after refining. The number of kinks in the non-recycled bleached chemical pulp made of softwood was 3410 1/m before refining and 2730 1/m after refining.
Measured fiber analysis results of recycled pulp obtained from RGP, nonrecycled bleached chemical pulps (before and after mill refining) and mill broke used at a paper mill for glassine paper production in the experimental study are presented in Table 3 (below). Comparison of the samples demonstrates that the fiber characteristics and the relative amount of fiber fractions is different in the recycled pulp obtained from RGP.
Table 3. Fiber analysis results and properties of recycled pulp obtained from RGP (PLILP3), non-recycled bleached chemical pulps (before and after mill refining) and mill broke used at a paper mill for glassine paper production.
Reference is made to Figure 6. The pulps as disclosed above were further analysed on the basis of the hydrophobic nature 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 characteristics. A Sysmex CyFlow Cube 6 (V2m) bench-top flow cytometer was used for the analysis. Representative samples of 20 ml were collected from the paper machine and diluted with ultrapure water 5-fold and the diluted and well mixed sample was then filtered through a 200 mesh screen. A 50 ml aliquot of the filtrate was collected for further dilution. A series of dilutions (in the range of 10-1000 fold) was prepared with ultrapure water such that a suitable dilution was obtained, which contained particles in an amount that resulted into 700-1000 events per second, when analysed with the flow cytometer. A volume on 20 ml of the dilution to be analysed was mixed with 1 ml of Nile red stain, which was used as a fluorescent marker to selectively stain hydrophobic moieties in the samples. Prior to analyzing the samples, the flow cytometry was calibrated to size standards with 3 pm commercial polystyrene beads. Relative hydrophobicity (>10) was used for gating the particles. The particles in each sample were further sorted based on their size, such that hydrophobic particles with a diameter of 1 micrometer or less was denoted as small, whereas hydrophobic particles with a diameter over 1 micrometer were denoted as large. The results indicate that the recycled pulp obtained from RGP contains in the range of 2-3 times higher amount of large and small hydrophobic particles than non-recycled BCP made of hardwood. The recycled pulp obtained from RGP contains close to 10 times higher amount of large and small hydrophobic particles than non-recycled BCP made of softwood. The difference to mill broke was also clear. While most of the hydrophobic particles in all of the analysed samples belonged to the group of large particles, that is, over 1 micrometer in diameter, the highest relative difference between the recycled pulp obtained from RGP and other pulp types was measured in the group of small particles. The amount of hydrophobic particles in a sample, in units of pieces per millilitre (pcs/ml) and the total measured particle amount in the samples (pcs) is shown in Table 4 (below).
Table 4. Amount of hydrophobic particles and total particle amount in samples measured by flow cytometry.
It was contemplated that the observed increase of hydrophobic particles, particularly small hydrophobic particles, in the recycled pulp obtained from RGP, would be due to silicone polymer residues form the release coating. However, despite the amount of hydrophobic particles in the recycled pulp obtained from RGP, no detectable problems were observed upon glassine paper production in the experiments, with respect to runnability or paper quality.
Experimental study 2
In a second experimental study, calendered glassine paper having grammage of 53 g/m2 and a thickness of 48 pm was produced, such that the amount of recycled pulp obtained from RGP in the stock was varied. The amount of recycled pulp obtained from RGP was varied from 0 to 30 wt.%, referring to the dry matter content of the produced glassine paper, according to SCAN-P 39:80. The ratio of non-recycled bleached chemical pulp produced from hardwood to the non-recycled bleached chemical pulp produced from softwood was maintained constant. Hence, the non-recycled BCP contained 35 wt.% of non-recycled BCP produced from softwood and 65 wt.% of nonrecycled BCP produced from hardwood. Thus, upon increasing the amount of recycled pulp obtained from RGP in the stock, the amount of BCP was decreased such that the share of non-recycled BCP produced from hardwood to softwood was maintained. The amount of broke was maintained the same, 12 wt.%, in all experiments.
Samples were measured at various trial points. A composition, which contained only non-recycled bleached chemical pulps and broke, but did not contain recycled pulp obtained from RGP, is marked in the Figures 7-11 as a reference point, and abbreviated as REF. A composition, which contained 15
wt.% of recycled pulp obtained from RGP, is marked in the Figures 7-11 as a trial point 1 , and abbreviated as TP1 . A composition, which contained 30 wt.% of recycled pulp obtained from RGP, is marked in the Figures 7-11 as a trial point 2, and abbreviated as TP2. The stock composition of the reference point and trial points 1 and 2, is described in Table 5 (below).
Table 5. Composition of stock at reference and trial points 1 and 2 in experimental studies. The abbreviation ‘BCP tot.’ refers to the total amount of non-recycled bleached chemical pulp in the stock, in wt.%. Broke refers to the amount of mill broke wt.% in the stock, in wt.%. PLILP3 refers to the amount of recycled pulp obtained from RGP in the stock, in wt.%. The last column on the right refers to the share of each component (BCP SW, BCP HW, broke, PLILP3) in the stock, which sums up to 100 wt.%.
Fines content at the machine chest (BMN method)
Reference is made to Figure 7. The effect of the recycled pulp obtained from RGP on glassine paper production was assessed by measuring the development of fines content at the machine chest of a paper machine as a function of the amount of recycled pulp obtained from RGP in the stock. The fines content, in this context, refers to fibrous material in the pulp that was determined as the F<2oo fraction with McNett classifier according to SCAN-CM 6:05, using a 20 minutes fractionation time, a set of 16, 28, 48 and 200 mesh wires, and weighed filter papers (Macherey-Nagel MN616, 125 mm diameter) for collecting the fibre fractions. The method describes a fibre-fractionation procedure, wherein the fibres in a pulp suspension are grouped into fractions of different average fibre size. The mass of the fibres retained in a fraction is expressed as a percentage of the dry mass of the original sample. The retained F<2OO fraction serves as an indication of how much the fines content changes in glassine paper production due to an increase in the amount of pulp obtained from RGP, when the relative ratio of the BCP SW and BCP SW is maintained, the amount of broke mill staying the same. The results evidence that when the amount of recycled pulp obtained from RGP in the glassine paper is in the range of 0 to 10 wt.%, the fines content remains relatively stable, in the range of 10.2 wt.% to 10.5 wt.%. However, unexpectedly, when the amount of recycled pulp obtained from RGP in the glassine paper is equal to or higher
than 10 wt.%, the fines content begins to increase more rapidly. In particular, when the amount of recycled pulp obtained from RGP in the glassine paper is equal to or higher than 15 wt.%, such as in the range of 15 to 30 wt.%, the fines content in the fiber furnish of the glassine paper increases very rapidly. During the experiment, when the amount of recycled pulp obtained from RGP in the glassine paper was in the range of 0 to 30 wt.%, the fines content increased from 10.2 wt.% to 13.8 wt.%. The fines content had an effect to the characteristics of the paper. The effect was detectable already upon forming the paper web. The results indicate that the amount of recycled pulp obtained from RGP in the stock may be used for adjusting the retention level, which describes the ability of the formed paper web to retain fine particles on the web, and therefore the balance between drainage and formation of the paper web.
Water retention value at the machine chest
Reference is made to Figure 8. The effect of the recycled pulp obtained from RGP on glassine paper production was further assessed by measuring the water retention value, abbreviated as WRV, at the machine chest according to ISO 23714:2014(en). The WRV was determined as an average of two parallel samples, each sample amount consisting of 1 g of dry pulp diluted into 500 ml of water and having a temperature of 23 ± 3 °C. Materials and methods as listed below were used:
Beckman Coulter Avanti J-30I laboratory centrifuge Centrifugal force of 3000 g ± 50 g, 30 minutes JS 7,5 rotor (speed 5350; RPM 5289)
The sample was weighed first time after the centrifugation. The sample was then dried overnight (12h) at 105 ± 2 °C and cooled down to a room temperature of 23 ± 3 °C in an excicator. The sample was then weighed a second time. A laboratory scale (0,0001 g precision) was used for the weighing.
The water retention value was calculated according to equation 1 below:
Equation ! :
, wherein ml = mass of sample after centrifugation, in grams m2 = mass of sample after drying, in grams.
The results evidence that a replacement of non-recycled BCP with recycled pulp obtained from RGP leads to a steady decrease in the water retention value, which is inversely proportional to the amount of the recycled pulp obtained from RGP in the glassine paper. Each replacement of 10 wt.% of nonrecycled BCP by recycled pulp obtained from RGP results into a WRV decrease in the range of 0.1 g/g in the glassine paper. The decrease in the WRV was evidenced over the whole range. At the reference point, the WRV was 1 .98 g/g. At the trial point 1 , the WRV was 1 .83 g/g. At the trial point 2, the WRV was 1.72 g/g. The water retention level analysis results support and validate the observations of the fines content analysis disclosed above. The correlation of WRV as a function of the amount of recycled pulp obtained from RGP in the stock demonstrates that recycled pulp obtained from RGP in the stock may be used for adjusting the water retention level. The lower WRV of the fibers in the recycled pulp obtained from RGP, compared to the fibers in the non-recycled BCP, is advantageous upon drying. A reduced amount of water absorbed into the fiber network at the machine chest indicates a better dimensional stability of the glassine paper upon drying. Thus, considering the trend of development of the fines content and the drainage discussed hereafter, the calendered glassine paper advantageously contains recycled pulp obtained from release liner glassine paper equal to or less than 50 wt.%, such as in the range of 5 to 50 wt.%, preferably in the range of 10 to 45 wt.%, most preferably in the range of 15 to 40 wt.% of the paper, when determined as dry matter content according to SCAN-P 39:80. Further, the stock at a machine chest of a paper machine has a water retention value which is in the range of 1 .5 to 1 .9 g/g, preferably in the range of 1 .55 to 1 .85, most preferably in the range of 1.6 to 1.8, determinable according to ISO 23714:2014 from a sample having a dry matter content of 1 gram.
Paper drainage - main steam group pressure at the drying section
Reference is made to Figure 9. The effect of the recycled pulp obtained from RGP on glassine paper production was next assessed by measuring the main steam group pressure at a paper machine during glassine paper production. The main steam group pressure is an indication of the drainage and direct evidence of the amount of energy consumed, when drying the paper. The results evidence that the drainage improves, when the amount of recycled pulp obtained from RGP in the glassine paper increases. The formed glassine paper had a higher dry matter content. Further, a glassine paper comprising a higher amount of recycled pulp obtained from RGP needed less steam pressure for drying. Unexpectedly, the drainage, when measured by means of the main steam group pressure, seemed to be most effective, when the amount of recycled pulp obtained from RGP in the glassine paper was equal
to or less than 15 wt.%, such as in the range of 5 to 15 wt.%. Already an amount of 5 wt.% of recycled pulp obtained from RGP in the composition required 0.1 bar less of steam pressure for drying the glassine paper, as evidenced by Figure 9. An amount of 15 wt.% of recycled pulp obtained from RGP in the composition required 0.3 bar less of steam pressure for drying the glassine paper.
Paper cross-directional profiling at the reeler
The effect of the recycled pulp obtained from RGP on glassine paper production was further evaluated at the drying section. The calendered glassine paper samples demonstrated a density in the range of 1100 ± 11 g/m3 and a transparency in the range of 50 ±1 %. The characteristics of samples produced according to the reference and trial points compositions are presented in Table 6 (below). The trial points were run with the same speed and settings for all the compositions (REF, TP1 , TP2), such that the effect of recycled pulp obtained from RGP to the calendered glassine paper could be evaluated.
Table 6. Characteristics of calendered glassine paper samples.
The results indicate that recycled pulp obtained from RGP enables to maintain quality characteristics of calendered glassine paper, such as density and transparency, at a sufficient level. The combination of preserved density and transparency serves as an indirect indicator of this.
Reference is made to Figure 10. The paper width was measured from calendered glassine paper samples at the reference point, trial point 1 and trial point 2. The paper width was measured at the reeler, in centimeters, by means of Web Imaging System, abbreviated as WIS, which is an automated image analysis system provided by ABB. The system was used according to manufacturer’s instructions. The width of the paper indicated in Figure 10 is an average value of 8 measurements along the surface of the paper in the crossdirection Sy, which is perpendicular to the machine direction Sx. The results evidence that a replacement of non-recycled BCP with recycled pulp obtained from RGP leads to reduced shrinkage of the glassine paper, which is directly proportional to the amount of the recycled pulp obtained from RGP in the
glassine paper. A replacement of 15 wt.% of non-recycled BCP by recycled pulp obtained from RGP resulted into a glassine paper, which demonstrated 3 cm less shrinkage than the reference. A replacement of 30 wt.% of nonrecycled BCP by recycled pulp obtained from RGP resulted into a glassine paper, which demonstrated 4 cm less shrinkage than the reference. The WIS results were validated in an independent trial run, wherein the paper was profiled off-line from 30 calendered and uncalendered paper samples at the reeler, by means of a Tapio PMA, which is an automated paper quality control system provided by Tapio technologies. The system was used according to manufacturer’s instructions. The results of the latter independent trial run with Tapio PMA validated the paper width results of the first trial run. In samples without recycled pulp obtained from RGP (REF), the measured shrinkage along the surface of the paper in the cross-direction Sy was 3.6%. In samples containing 15 wt.% of recycled pulp obtained from RGP (TP1 ), the measured shrinkage along the surface of the paper in the cross-direction Sy was 3.0 %. In samples containing 30 wt.% of recycled pulp obtained from RGP (TP2), the measured shrinkage along the surface of the paper in the cross-direction Sy was 2.9 %. During the latter trial run, also the grammage and thickness variability of the uncalendered paper along the surface of the paper in the cross-direction Sy was simultaneously determined from the 30 paper samples by means of the Tapio PMA, according to the manufacturer’s instructions. The grammage variability analysis indicated, that the standard deviation of samples at the trial points 1 and 2, containing recycled pulp obtained from RGP, was 0.5 g/m2 This was on the same level as the standard deviation of samples at the reference point, which did not contain recycled pulp obtained from RGP (REF). The grammage variability (max-min) was in the range of 3.1 to 3.6 g/m2, in all the measured sample compositions (REF, TP1 , TP2). The thickness variability analysis indicated that the standard deviation of samples at the trial points 1 and 2, containing recycled pulp obtained from RGP (TP1 , TP2) was 0.4 pm, which was on the same level as the standard deviation of samples at the reference point, which did not contain recycled pulp obtained from RGP (REF). The thickness variability (max-min), however, demonstrated a decrease in variability when the amount of recycled pulp obtained from RGP was larger. In the reference sample (REF), the thickness variability (max-min) was 2.4 pm, whereas the thickness variability (max-min) of the trial points 1 and 2 (TP1 , TP2) was 1.9 pm and 2.1 pm, respectively. In addition to reduced shrinkage, the replacement of non-recycled BCP with recycled pulp obtained from RGP lead into a decrease in the thickness variability, which correlated with the shrinkage results, while maintaining the grammage variability. Thus, both the shrinkage and thickness variability at a paper machine correlated with the amount of the recycled pulp obtained from RGP. Reduced shrinkage and stable grammage variability are indicators of improved dimensional stability.
Induced curl test on calendered paper
Reference is made to Figure 11. The calendered glassine paper samples produced in the industrial scale trial run were further evaluated for induced curl into the cross-direction Sy, denoted as paper curl (CD). A curl was induced in conditions of 1 minute at a temperature of 150°C (laboratory oven) and measured immediately afterwards. The induced curl method was selected, since it is indicative of the processability of a calendered glassine paper when used as a substrate for a release coating. A release coating is typically cured in conditions resembling the selected situation.
A modified version of a test method ISO 11556:2005(en) was used for measuring the induced curl. A rectangular test piece from the middle of a paper sheet that had been allowed to stabilize in NTP conditions (25°C, 1 bar) 24 hours after production was cut, having a shape with a length of 10 cm (in the cross-direction Sy of the paper) and a width of 5 cm (in the machine direction Sx of the paper). The test piece was set on a cylindrical holder having a diameter of 10 mm and a slot extending over 5 cm along the length of the holder, such that the test piece, when set into the slot, was suspended by the slot from its whole width from the middle, each half of the test piece length thus able to extend freely for a distance of 4.5 cm in opposite directions. The cylindrical holder was attached on a curl template for measuring the magnitude of the induced curl. Prior to inducing curl, the test specimen was aligned parallel with a reference place. The reference plane was given a value of zero. Since the induced curl on the suspended test piece approximates the arc of a circle, markings were imprinted on the template which indicated an angle of curvature deviating from the reference plane. The magnitude of curl was thus imprinted into the template as the angle of curvature of the curled test piece from a reference plane, in units of degree of angle. The curl of the test piece was compared to the angle of curvature imprinted on the curl template; the curvature on both sides was recorded. Two test pieces were measured and the four recorded values were averaged. The result of the curl test was thus an average value of the recorded four values. If the recorded curl was towards wire-side, the curl was positive. If the recorded curl was towards top-side, the curl was negative. The wire-side, in this context, refers to the side of the paper that upon forming the paper web has been in contact with the papermaking machine's forming wire. The top-side, in this context, refers to the opposite side of the paper.
The results evidence that a replacement of non-recycled BCP with recycled pulp obtained from RGP leads to a steady decrease in the curl value, which is
proportional to the amount of the recycled pulp obtained from RGP in the glassine paper. In samples without recycled pulp obtained from RGP (REF), the measured curl was 61 mm. In samples containing 15 wt.% of recycled pulp obtained from RGP (TP1 ), the measured curl was 47 mm. In samples containing 30 wt.% of recycled pulp obtained from RGP (TP2), the measured curl was 32 mm. Therefore, a replacement of 15 wt.% of non-recycled BCP by recycled pulp obtained from RGP resulted into a curl decrease of 23% in the calendered glassine paper. Moreover, a replacement of 30 wt.% of nonrecycled BCP by recycled pulp obtained from RGP resulted into a curl decrease of 48% in the calendered glassine paper. The decrease in the curl was evidenced in all measured samples. The induced curl results support and validate the observations disclosed above. Thus, when considering in light of the improved dimensional stability and the drainage discussed above, the calendered glassine paper advantageously contains recycled pulp obtained from release liner glassine paper equal to or less than 50 wt.%, such as in the range of 5 to 50 wt.%, preferably in the range of 10 to 45 wt.%, most preferably in the range of 15 to 30 wt.% of the paper, when determined as dry matter content according to SCAN-P 39:80.
Paper strength properties
The calendered glassine paper samples produced in the industrial scale trial run were further evaluated for strength properties. The tensile strength in the machine direction (MD) Sx and in the cross-direction (CD) Sy, the strain at break in the MD, and the tensile energy absorption in the MD were measured in accordance with ISO 1924-3.
Tensile strength can be used as an indication of the potential resistance of the calendered glassine paper to a web break, when the calendered glassine paper is used as a substrate of a release liner in a labelling operation. The strain at break can be used as an indication of how well the paper will conform to irregular shapes and, along with tensile energy absorption, as an indication of the paper’s performance under dynamic straining and stressing. Tensile energy absorption is a measure of the ability of a paper to absorb energy. Tensile energy absorption thus expresses the toughness of the sheet. The parameters thus predict the performance of paper, especially when that paper is subjected to an uneven stress or a dynamic stress. Table 7 (below) indicates the results measured from calendered glassine paper samples that did not contain recycled pulp obtained from RGP (REF), from calendered glassine paper samples that contained 15 wt.% of the recycled pulp obtained from RGP (TP1 ) and from calendered glassine paper samples that contained 30 wt.% of the recycled pulp obtained from RGP (TP2).
Table 7. Comparative results (MD and CD) from calendered glassine paper samples.
The results indicate that the paper strength, when determined as tensile strength, strain at break and tensile energy absorption, remained at sufficiently high level in the samples, despite the replacement of non-recycled BCP with recycled pulp obtained from RGP. No significant changes were observed in the paper strength or orientation properties during the trial.
As a summary of the results, the compatibility of recycled pulp produced from release liner glassine paper is excellent for glassine paper production. Positive effects in glassine paper manufacturing process, such as improved dewatering both when forming the paper web and at the press section, improved drainage at the drying section, better were measured with several different methods, while maintaining the properties of the calendered glassine paper at sufficient level for use as a substrate for a release liner. The improved manufacturing process was perceivable also in the produced glassine paper, which demonstrated reduced shrinkage, better dimensional stability and reduced curl.
Claims
1 . A method for manufacturing recycled pulp (PLILP3), the method comprising
- receiving release liner for recycling (REL1 ), wherein the release liner comprises a substrate which is calendered glassine paper or supercalendered kraft paper, the release liner comprising non-fibrous material and fibers,
- disintegrating and detaching non-fibrous material from the fibers on a first process stage, denoted as a caustic loop (CL1 ), thereby forming a pulp suspension, and
- removing non-fibrous material from the fibers on a second process stage, denoted as a cleaning loop (NL1 ),
, wherein the method has been configured to adjust the fibrillation of the fibers, such that recycled pulp (PLILP3) obtained from the substrate which is calendered glassine paper or supercalendered kraft paper has a SR number equal to or higher than 25, when determined according to ISO 5267-1 .
2. The method according to claim 1 , wherein the recycled pulp (PLILP3) has a SR number equal to or higher than 30, preferably equal to or higher than 40, such as in a range from 25 to 65, preferably in the range of 30 to 60, most preferably in the range of 40 to 55, when determined according to ISO 5267- 1.
3. The method according to claim 1 or 2, further comprising sorting the release liner substrate for recycling prior to disintegrating, thereby obtaining sorted release liner substrate.
4. The method according to any of the previous claims, wherein the recycled pulp (PULP3) obtained from a release liner substrate is not bleached during or after the caustic loop (CL1 ) and/or cleaning loop (NL1 ).
5. The method according to any of the previous claims, wherein
- the colour of the calendered glassine paper is white, the white referring to colour space coordinate values of the paper, wherein the range of 92 to 98, the range of -4 to +2, and
the range of +3 to +9, preferably in the range of +5 to +7, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant D65 and 10° standard observer, in accordance with ISO 5631 :2022
or
- the colour of the supercalendered kraft paper is white, the white referring b* colour space coordinate values of the paper, wherein the range of 92 to 98, the range of -4 to +2, and
the range of +5 to +11 , the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant D65 and 10° standard observer, in accordance with ISO 11475:2017(en).
6. The method according to any of the previous claims, wherein the colour of the calendered glassine paper is light yellow, yellow, brown or blue
- the light yellow referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 87 to 96, o a* is in the range of -4 to +8, and o b* is in the range of +24 to +43,
- the yellow referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 65 to 71 , o a* is in the range of +7 to +13, and o b* is in the range of +50 to +56,
- the brown referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 64 to 70, o a* is in the range of +3 to +9, and o b* is in the range of +17 to +23,
- the blue referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 80 to 89, o a* is in the range of -17 to 0, and o b* is in the range of -15 to +9,
, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard illuminant C and 2° standard observer, in accordance with ISO 5631-1 :2022.
7. The method according to any of the previous claims, wherein the disintegrating and detaching non-fibrous material from the fibers is a pulping performed at a high consistency in alkaline conditions in the presence of chemical additives comprising hydrogen peroxide, sodium silicate, caustic soda and a saponifying agent.
8. The method according to any of the previous claims, wherein the disintegrating and detaching non-fibrous material from the fibers is a pulping performed in a batch mode, wherein the total operating time of the pulping is in the range of 30 to 60 minutes.
9. The method according to any of the previous claims, wherein the disintegrating and detaching non-fibrous material from the fibers is a pulping, wherein the temperature of the pulp suspension during the pulping is in the range of 60-85°C.
10. The method according to any of the previous claims, wherein pressing or filtration is used at the end of the caustic loop (CL1 ) to mechanically remove process water (F4) from the pulp suspension, such that at the end of the caustic loop (CL1 ), the pulp suspension is thickened into 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 previous claims, wherein the fibrillation of the fibers in the pulp suspension is adjusted after the caustic loop (CL1 ) by means of dispersion in a conical disperger or a disc disperger, prior to flotation.
12. The method according to claim 11 , wherein the amount of specific energy consumed during dispersion is in the range of 30-150 kWh/t, preferably in the range of 40-100 kWh/t, most preferably in the range of 45-90 kWh/t.
13. The method according to claim 11 or 12, wherein the temperature of the pulp suspension at the inlet to the disperger is in the is range of 50 to 130°C, preferably in the range of 50 to 85°C.
14. The method according to any of the previous claims, further comprising flotating the pulp suspension during the cleaning loop (NL1 ) by means of air bubbles, the pulp suspension having a temperature in the range of 40 to 70°C and a pH in a range between 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 10-20 dH, determinable in accordance with ISO/TS 15923-2:2017, and a collector, such as soap, sodium soap, or other surfactant comprising a hydrophilic and a hydrophobic part, is added into the pulp suspension.
16. The method according to any of the previous claims, further comprising washing and dewatering the pulp suspension during the cleaning loop (NL1 ),
such that recycled pulp (PLILP3) obtained from a release liner substrate having 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.% is formed.
17. The method according to any of the previous claims, further comprising increase the dryness of the pulp (PLILP3) obtained from a 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. A recycled pulp (PLILP3) obtained according to the method of any of the claims 1-17.
19. A recycled pulp (PLILP3) obtained from
- release liner comprising calendered glassine paper as a substrate or
- release liner comprising supercalendered kraft paper as a substrate, the recycled pulp (PLILP3) comprising a SR number equal to or higher than 25, such as in a range from 25 to 65, when determined according to ISO 5267- 1.
20. The recycled pulp (PLILP3) 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 . The recycled pulp (PLILP3) according to claim 19 or 20, further comprising a water retention value in the range of 1.3 to 1.6 g/g, determinable according to ISO 23714:2014(en).
22. The recycled pulp (PLILP3) according to any of claims 19 to 21 , further comprising a pH which is 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 determining the pH of dry pulp sample according to standard ISO 6588-2 (2020), and using an amount of 2 grams of dry pulp, as disclosed in the description.
23. The recycled pulp (PLILP3) according to any of claims 19 to 22, further comprising particles derived from the recycled pulp (PLILP3) having a length less than 200 micrometers in an amount equal to or higher than 10 %, such as in a range from 10 to 30 %, preferably in the range of 12 to 20 %, most preferably in the range of 15 to 17 % of the total amount of fibers in the recycled pulp, when determined as length weighted average fiber length by automated optical analysis using unpolarized light according to ISO 16065-2: 2014.
24. The recycled pulp (PLILP3) according to any of claims 19 to 23, further comprising silicon in an amount of 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 to 0.3 wt.%, determinable as dry matter content from a paper specimen which is combusted at 900°C with an Energy Dispersive X-ray Spectroscopy, in accordance with Tappi standard T 413.
25. The recycled pulp (PLILP3) according to any of claims 19 to 24, wherein the recycled pulp has been obtained from release liner substrate,
- wherein the colour of the calendered glassine paper is white, the white referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 92 to 98, o a* is in the range of -4 to +2, and o b* is in the range of +3 to +9, preferably in the range of +5 to +7, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant D65 and 10° standard observer, in accordance with ISO 5631 :2022 or
- wherein the colour of the supercalendered kraft paper is white, the white referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 92 to 98, o a* is in the range of -4 to +2, and o b* is in the range of +5 to +11 , the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard ilium inant D65 and 10° standard observer, in accordance with ISO 11475:2017(en).
26. The recycled pulp (PULP3) according to any of claims 19 to 24, wherein the recycled pulp has been obtained from a release liner substrate, wherein the colour of the calendered glassine paper is light yellow, yellow, brown or blue,
- the light yellow referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 87 to 96, o a* is in the range of -4 to +8, and o b* is in the range of +24 to +43,
- the yellow referring to CIE L* a* b* colour space coordinate values of the paper, wherein
o L* is in the range of 65 to 96, o a* is in the range of -4 to +13, and o b* is in the range of +24 to +56,
- the brown referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 64 to 70, o a* is in the range of +3 to +9, and o b* is in the range of +17 to +23,
- the blue referring to CIE L* a* b* colour space coordinate values of the paper, wherein o L* is in the range of 80 to 89, o a* is in the range of -17 to 0, and o b* is in the range of -15 to +9,
, the values determinable from a sample of the paper by means of diffuse reflectance method with the elimination of specular gloss, using standard illuminant C and 2° standard observer, in accordance with ISO 5631-1 :2022.
27. A system of co-operating apparatuses adapted for producing recycled pulp, the system comprising
- a sorting stage (20) configured to separate release liner comprising calendered glassine paper as a substrate or release liner comprising supercalendered kraft paper as a substrate paper apart from other papers,
- a caustic loop (CL1 ) comprising o a high consistency pulping unit (21 ) configured to operate in a batch mode and wherein the consistency and temperature of the suspension, operating time of the pulping and addition of chemical additives may be adjusted, and o a dewatering unit (24), configured to remove process water (F4) from the pulp suspension and to increase the pulp consistency, and
- a cleaning loop (NL1 ) comprising o a dispersion unit (25) configured to adjust the fibrillation of the 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 recycled pulp (PULP3) obtained from the substrate has a SR number equal to or higher than 25, when determined according to ISO 5267-1 .
28. The system according to claim 27, the caustic loop (CL1 ) further comprising
a coarse screening unit (22) configured to remove solid contaminants and non-paper components from the pulp suspension, such as sand and metal objects, as well as larger particle agglomerates.
29. The system according to claim 27 or 28, the caustic loop (CL1 ) further comprising
- a cleaning unit (23), preferably a high-consistency cleaning unit, such as a cleaner using centrifugal field, configured to complement the coarse screening unit (22) to separate pulp fibers from contaminants based on specific gravity.
30. The system according to any of the claims 27 to 29, wherein
- the dispersion unit (25) is a conical or disc disperger configured operate with a thickened pulp suspension received directly from the dewatering unit (24).
31 . The system according to any of the claims 27 to 30, the cleaning loop (NL1 ) further comprising
- a flotation unit (26) configured to receive clear water (F1 ) and/or process water (F2, F3) downstream to adjust the consistency of the pulp suspension, when necessary, the flotation unit (26) configured to remove hydrophobic particles from the pulp suspension.
32. The system according to claim 31 , the cleaning loop (NL1 ) further comprising
- a second screening unit (27) configured to separate debris from the fibers coming from the flotation, in particular silicone particles originating from the release coating.
33. The system according to any of the claims 27 to 32, the cleaning loop (NL1 ) further comprising
- 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 of the claims 27 to 33, further comprising
- a drying unit, such as a fluffer, configured to to increase the dryness of the pulp.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI20226184 | 2022-12-30 | ||
| EP22217245.4 | 2022-12-30 | ||
| US18/091,505 | 2022-12-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO2024141186A1 true WO2024141186A1 (en) | 2024-07-04 |
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