A paper substrate containing glyoxal-crosslinked pva

Abstract

The invention relates to a method for preparing cross-linkable composition that can be used at a paper machine for coating a paper, such as glassine, SCK or a high-density paper suitable for packaging of a food product or another oxygen-sensitive product, wherein glyoxal is grafted into the structure of polyvinyl alcohol in an aqueous acetalization reaction prior to coating the reaction product on a paper, whereby modified PVA which comprises a controlled amount of glyoxal-based branching points ending into aldehyde groups is obtained and the amount of free glyoxal in the reaction product and on a primer layer of a paper substrate may be significantly reduced.

Description

[0001] A paper substrate containing glyoxal-crosslinked PVA

[0002] Field of invention

[0003] The invention relates to a paper substrate comprising a glyoxal-crosslinked PVA coating and to a method for manufacturing such paper. The invention further relates to a method for manufacturing a cross-linkable composition and product thereof which comprises a controlled amount of glyoxal-based branching points extending from the molecular chain of PVA and ending into aldehyde groups.

[0004] Paper types that are produced of bleached chemical pulp are typically high- end products that are calendered and coated, to improve their surface characteristic. Of particular interest in this context are high-density papers suitable for release liner applications, such as glassine paper, super calendered kraft paper and other high-density papers suitable for packaging of a food product or another oxygen-sensitive product, which typically comprise a primer layer that contains PVA, which is crosslinked with glyoxal.

[0005] Release liner REL1 , as disclosed herein with reference to Figure 1 , refers to a paper substrate SUBST1 which contains a release layer SIL1 , i.e. a cured silicone-based release coating provided on at least one side of the paper substrate SUBST1. Release liners comprising a cellulose fiber-based support layer PAP1 as part of the paper substrate SUBST1 are widely used as nonblocking backing material for self-adhesive products, such as self-adhesive labels. A typical cellulose fiber-based support layer PAP1 suitable for use on a release liner is a calendered industrial paper, such as glassine paper or a super calendered kraft paper, which is manufactured from a pulp mixture that contains chemical pulp, such as bleached Kraft pulp, on a paper machine and calendered to obtain a product which has a density and transparency in a desired range that is suitable for clients which use the paper as a substrate layer of a release liner. Prior to coating at least one side of the paper substrate SUBST1 with a silicone-based release coating, the cellulose fiber-based support layer PAP1 is coated from the same side with a film-forming polymer composition, which is dried on place on a paper machine, thereby forming a paper substrate SUBST1. The primer layer PRIM1 is intended to improve surface closeness, thereby reducing the porosity of the paper surface and enhancing the tightness and smoothness of the formed paper substrate surface, before the application of the release coating. This facilitates the spreading of uncured silicone and reduces the amount of release coating required to provide a functional release layer SIL1. The purpose of a paper substrate SUBST1 is to provide a dimensionally stable and dense surface, on which a release coating may be applied.

[0006] In high-density papers suitable for packaging of a food product or another oxygen-sensitive product, a primer layer may be used for improving barrier characteristics of the paper, such that the paper substrate comprising the cellulose fiber-based support layer and the primer layer may be used as part of a multi-layered material for packaging of a food product or another oxygensensitive product.

[0007] The primer layer PRIM1 typically contains water-soluble polyvinyl alcohol, hereafter abbreviated as PVA, which has a film-forming nature and which is widely used to provide barrier properties between the release layer SIL1 and the cellulose fiber-based support layer PAP1 surface. The primer layer PRIM1 is typically applied on the cellulose fiber-based support layer PAP1 surface as a coating composition which has been dissolved into water, when manufacturing the paper on a paper machine, prior to calendering.

[0008] Typically, prior to calendering, the paper substrate SUBST1 needs to be remoisturized, for example by using steam, to provide a moisture content on the paper substrate which is as even as possible. The addition of moisture prior to calendering enables to release stresses formed into the paper and improve the dimensional stability of the paper substrate, which is important upon curing the silicone-based release coating. The moisture content of the paper substrate SUBST1 before calendering is thus typically relatively high, often equal to or higher than 10 wt.%, such as in the range of 10 to 25 wt.%. The moisture content of an off-line paper sample is determinable from a test piece by oven-drying method according to ISO standard 287:2017(en).

[0009] The increased moisture content on the paper substrate SUBST1 imbues hydrolysis of the primer layer, since due to the presence of water-soluble PVA that contains multiple hydroxyl groups, it is sensitive to moisture. This is a challenge, as the PVA may absorb the moisture and swell, which also causes the primer layer to become more adhesive. This can lead to sticking problems on the calender and cause a production break. Furthermore, when the calendering is performed as an off-line operation, the paper substrate SUBST1 may be winded and stored on a roll before the calendering, whereby the opposite sides of the paper substrate SUBST1 that face each other on the winded roll can become attached together due to the stickiness of the moisturized PVA, thereby ruining the product.

[0010] One way of solving such problem is the addition of small cross-linking molecules that can react with the hydroxyl groups of the PVA. An example of such molecule is glyoxal, which is a dialdehyde, and which is widely used as a cross-linker chemical to enhance the wet strength of papers. Glyoxal is commercially available as a 40% aqueous solution, which can be applied on a paper substrate and then dried, whereby it cross-links with hydroxyl groups present in the primer layer. This may be used to reduce the swelling of the PVA.

[0011] The problem with commercially available aqueous solutions containing glyoxal is that unbound glyoxal has been determined to be a harmful substance, the acute toxicity depending on the active ingredient concentration on a product. The commercially available glyoxal solutions are typically applied on a paper surface where the glyoxal ties together polymer molecules, cellulosic fibers and filler particles once water has been evaporated from the paper surface in the drying section of a paper machine. The reactions are thus often unspecific and hence not highly efficient, whereby considerable amount of a cross-linker agent is often applied on the paper. Glyoxal as a small molecule may migrate and penetrate the cellulose fiber-based support layer beneath the primer layer. Therefore free, unbound glyoxal molecules remain on the paper substrate, often in abundance. The maximum concentration of free glyoxal on a substrate has already been restricted by legislation in several countries. As an example, the European Union has stated that tissue papers made from recycled fibres or mixtures of recycled and virgin fibres shall not contain glyoxal more than 1 .5 mg / dm2, determinable according to test DIN 54603. Therefore, there is a need to better control the concentration of free glyoxal molecules on a paper substrate.

[0012] The invention solves problems described above by providing a method and products thereof, wherein the method PVA that contains hydroxyl groups is reacted with glyoxal in an aqueous solution by means of an acid-catalyzed acetalization reaction at an elevated temperature such that a reaction product is formed, which is a polymer containing covalently bound, glyoxal-based branching points ending into aldehyde groups that are available for a further cross-linking reaction. The grafting of glyoxal with PVA before coating the aqueous solution on a cellulose fiber-based support layer offers a tool for determining an average number of glyoxal-based branching points n which will be formed per PVA chain during the acid-catalyzed acetalization reaction. By means of defining the weight average molecular weight Mwof the PVA and the amount of glyoxal used as a reagent beforehand, the concentration of free glyoxal molecules in the aqueous solution after the acetalization reaction may be controlled.

[0013] The main chain of the polymer, also denoted as the polymer backbone, in a PVA chain may further be arranged to function as a spacer. As the main chain of PVA is substantially linear, the weight average molecular weight Mw of the PVA provides means to control the density of glyoxal-based branching points that become grafted into a single PVA chain. The weight average molecular weight Mw of the PVA and the amount of glyoxal molecules may thus also be used for predetermining an average distance between two neighboring glyoxal-based branching points grafted into a PVA chain during the acid- catalyzed acetalization reaction in the aqueous solution. When on average at least 2 glyoxal-based branching points n per PVA chain end into aldehyde groups, such as in the range of 2 to 20, preferably in the range of 3 to 10, said aldehyde groups in a single PVA chain can later interact with hydroxyl groups present in other PVA chains. Advantageously, in a subsequent cross-linking reaction, each PVA chain that contains at least 2 glyoxal-based branching points n reacts with at least two other PVA chains. The efficiency of a subsequent cross-linking reaction can be optimized better when the average number of glyoxal-based branching points n formed per PVA chain is controlled and predetermined before the cross-linking reaction takes place. In a cross-linkable composition that comprises glyoxal that has been grafted into the structure of polyvinyl alcohol, the density of the glyoxal-based branching points intended to be cross-linked can thus be controlled. This is a clear improvement to what can be achieved by commercially available aqueous solutions containing glyoxal, wherein the distance between two aldehyde groups in a single glyoxal molecule facilitates the cross-linking reaction.

[0014] To increase the conversion into acetalized reaction product, the acid-catalyzed acetalization reaction between glyoxal and PVA in the aqueous solution is allowed to proceed until it reaches an equilibrium state. Further, the acetalization reaction is not stopped by neutralizing the solution pH, e.g. with a caustic substance. Instead, the aqueous solution which thus contains a cross-linkable PVA composition is used for coating a surface of a paper for a release liner. A coating composition that contains the aqueous solution should therefore not have a pH higher than 7. Advantageously, the pH of the coating composition remains below 7, such as between 3.5 and 7.0. A pH lower than 3.5 can also cause problems with dispersion of pigments, such as clay. When a cellulose fiber-based support layer is coated with the reaction product prior to or at a drying section of a paper machine, the elevated temperature at the drying section of a paper machine causes water to evaporate, whereby the reaction equilibrium in the aqueous reaction product shifts towards acetal formation. This causes the remaining aldehyde groups to undergo a further acetalization reaction. Due to a relatively rapid evaporation rate at the drying section of the paper machine, the further acetalization reaction takes place primarily between the aldehyde groups and hydroxyl groups of the PVA in the primer layer, which are in close proximity to each other. The method therefore enables to obtain a paper substrate comprising a primer layer that contains a cross-linked composition, wherein majority of the glyoxal molecules used as reactants have been covalently bonded by means of both aldehyde groups with the PVA polymer. This is a significant improvement over conventional glyoxal crosslinker products, wherein the extent of cross-linking density and the amount of free glyoxal in the primer layer of a paper substrate cannot be controlled to the same extent. Thus, an aqueous solution comprising a cross-linkable composition that is suitable for coating a surface of a paper is provided, wherein the cross-linkable composition is based on glyoxal that has been grafted into the structure of polyvinyl alcohol before coating, and wherein the concentration of free glyoxal is determinable in the aqueous solution by means of GC-FID. The concentration of free glyoxal may further be determined from a primer layer of a paper substrate on which the cross-linkable composition has been applied.

[0015] In the context of this description, unless otherwise indicated, a cross-linkable composition refers to a reaction product of an acetalization reaction in aqueous solution, wherein the reaction product glyoxal molecules that have been used as reactant are covalently bonded into the PVA, such that glyoxal-based branching points have been formed. A branching point, in this respect, refers to an acetal formed by a single aldehyde group of the glyoxal molecule which has in the presence of an acid catalyst reacted with two adjacent hydroxyl groups of a PVA polymer, the acetal having two ether groups on one carbon. A glyoxal molecule, as a reactant, refers to a molecule that contains two aldehyde groups. Therefore, the reaction product, which is an acetalized PVA, also contains aldehyde groups originating from the glyoxal reactant molecules, which aldehyde groups are available for a further cross-linking reaction. An acetalization reaction is reversible and has an equilibrium state, whereby the aqueous solution contains also a certain amount of unbound glyoxal molecules, in addition to the cross-linkable composition.

[0016] In the context of this description, unless otherwise indicated, the expression “glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction” therefore refers to the reaction product of the acetalization reaction, as disclosed above, wherein acetals have been formed by the PVA and glyoxal used as reagents. Such reaction product may also be referred to as “glyoxal grafted into the structure of the polyvinyl alcohol by means of acetal linkages”, the acetal linkages in this context denoting the glyoxal-based branching points in the PVA polymer that end into an aldehyde group available for a further cross-linking reaction.

[0017] In the context of this description, unless otherwise indicated, a cross-linked composition refers to an acetalized PVA which is typically on a paper surface, and wherein both aldehyde groups of the glyoxal molecules used as reactants have to a substantial degree formed acetals with the PVA. This occurs when a primer layer is dried, whereby majority of the water molecules evaporate from the aqueous solution and the reaction equilibrium is configured to shift towards acetal formation. The remaining aldehyde groups in the aqueous solution undergo further acetalization, whereby majority of the glyoxal molecules used as reactants can be covalently bonded with PVA by means of both aldehyde groups of the glyoxal molecule, provided that a sufficient amount of available hydroxyl groups are present in the PVA.

[0018] In the context of this description, unless otherwise indicated, the expression “glyoxal”, “free glyoxal” and “glyoxal as an active ingredient” further refer to a small reactive molecule, which is in the form of a dialdehyde and which thereby is a harmful substance. The small reactive molecule may be in the form of a hydrate or an oligomer, which behaves equivalently to the dialdehyde.

[0019] Referring to above, the efficiency of the acetalization reaction between PVA and glyoxal is affected by the properties of PVA and the relative mass shares of PVA and glyoxal participating in the reaction. Of particular importance is the average number of glyoxal branches per PVA chain n to be formed, which may be determined from the degree of polymerization DP and the degree of substitution DS, according to equation 1 below:

[0020] Eq.1 : n = DS ■ DP

[0021] , wherein the degree of polymerization DP refers to the number of monomeric units in a polymer, and the degree of substitution DS can be described according to equation 2:

[0022] , wherein

[0023] Rm=mglyoxal / mPVA mgiyoxai = mass of glyoxal, mpvA = mass of PVA,

[0024] Mgiyoxai = molecular mass of glyoxal = 58.04 g / mol, and Mo, PVA = molecular mass of the repeating unit of PVA = 44.05 g / mol.

[0025] When substituting equation 2 into equation 1 , the average number of glyoxal- based branching points per PVA chain n may be expressed according to equation 3:

[0026] As a practical example, when using a commercial polyvinyl alcohol Poval® I Q- 98, which has a weight average molecular weight Mwof approximately 30000 g / mol, the degree of polymerization DP = 30000 1 44.05 « 680. When using 2 % of glyoxal (relative to the mass of PVA), the average number of glyoxal branches per PVA chain n would be as follows:

[0027] 0.02 n = - T X 680 = 10.33 « 10

[0028] 58.04 g / mol

[0029] 44.05 g / mol

[0030] The number of glyoxal-based branching points per PVA chain n in the cross- linkable composition indicates the extent of PVA cross-linking, when the aqueous solution is applied on a paper surface and allowed to dry. A higher amount of glyoxal-based branching points per PVA chain n prior to drying will result into a more densely cross-linked composition on a paper surface. Unmodified PVA, denoting commercial polyvinyl alcohol having a degree of hydrolysis in the range of 70 to 99%, may be added and dissolved into the aqueous solution after the acetalization reaction, prior to using the reaction product as a cross-linkable coating composition for a paper surface. The addition of unmodified PVA into the reaction product enables to adjust the share of PVA chains comprising glyoxal-based branching points in the formed cross-linkable composition, provided that the reaction product is still an aqueous solution. If the average number of glyoxal-based branching points per PVA chain is too large, the cross-linking density may become excessive, resulting into a highly viscous composition that no longer stays as a solution and is not suitable for coating a surface of a paper. The density of the glyoxal- based branching points per PVA chain n also correlates with the adhesiveness of the acetalized PVA. Based on experimental results, to address the sticking problem on the calender as disclosed above, the average number of glyoxal- based branching points per PVA chain n is at least 2, preferably at least 3, most preferably at least 5, such as in the range of 2 to 20, preferably in the range of 3 to 10, most preferably in the range of 5 to 10. This facilitates the acetalization reaction efficiency and produces an aqueous solution comprising a cross-linkable composition, wherein the amount of free glyoxal is low, such as equal to or less than 1000 mg / kg, advantageously equal to or less than 700 mg / kg, preferably equal to or less than 500 mg / kg most preferably equal to or less than 100 mg / kg, while binding the PVA molecules together sufficiently to address the sticking problem.

[0031] The molecular weight of the PVA has also an effect to the viscosity of the water-based acetalization reaction. Commercial PVA grades having relatively low molecular weight Mwin the range of 25000 g / mol, typically have a viscosity in the range of 5±7 mPa s, when determined from 4 wt.% aqueous solution at 20°C (DIN 53015). However, PVA grades having a higher molecular weight Mw have a higher viscosity. For instance, PVA grades having molecular weight in the range of 100000 g / mol have a viscosity, which is typically in the range of 55±65 mPa s. Such PVA viscosity is too high to enable a controlled acetalization reaction of glyoxal, as disclosed above. Viscosity in this context refers to the property of a fluid that resists a force tending to cause the fluid to flow.

[0032] A lower Mw of the PVA has been observed to suppress the rise of viscosity of a solution during an acetalization reaction with glyoxal. The molecular weight Mw of the PVA may therefore be selected to adjust the efficiency of the acetalization reaction, to optimize the amount of glyoxal molecules to be acetalized with the hydroxyl groups of the PVA. Advantageously, the Mw of the PVA has a weight average molecular weight Mw equal to or less than 60000 g / mol, preferably equal to or less than 55000 g / mol, most preferably equal to or less than 50000 g / mol, such as in the range of 20000 to 60000 g / mol, preferably in the range of 25000 to 55000 g / mol, most preferably in the range of 30000 to 50000 g / mol. The weight average molecular weight Mw of commercial PVA grades is routinely determined by the manufacturer with known methods, such as by means of size exclusion chromatography. Reference is made in this respect to the Handbook of Size Exclusion Chromatography and Related Techniques (Wu, C. (2004), Marcel Dekker Inc., New York, Chapter 10). The flow behavior of PVA in water may be further controlled by selecting the degree of hydrolysis of the PVA grade. The viscosity r| of the water-based solution containing PVA and glyoxal may be configured low, such as less than 10000 mPa s, during the acetalization reaction in aqueous solution.

[0033] For practical reason, the viscosity of a solution comprising cross-linkable composition comprising glyoxal that has been grafted into the structure of polyvinyl alcohol should be less than 8000 mPa s, when determined as a Brookfield viscosity at 100 rpm and at a temperature of 25 °C. In practice, in on-line coating processes suitable for paper mills, a solution comprising cross- linkable composition having a viscosity higher than 8000 mPa s does not have sufficient flow characteristics for pumping on a coating machine, the viscosity thereby limiting the use of PVA modified with glyoxal in an acetalization reaction as a coating composition of a cellulose fiber-based support layer.

[0034] The amount of glyoxal to be grafted into the structure of the PVA by means of an acetal linkage may thus be adjusted, based on the properties of the PVA. The dry matter content of the solution containing PVA is advantageously equal to or higher than 18 wt.%, preferably in the range in 18 to 25 wt.%, most preferably in the range of 20 to 22 wt.%, determinable according to SCAN-P 39:80. Advantageously, to optimize the average number of glyoxal-based branching points per PVA chain and to prevent the rise of viscosity during the acetalization reaction, the amount of glyoxal is maintained relatively low, such as equal to or less than 5 wt.% of the amount of PVA. Typically, when the Mwof the PVA is in the range of 20000 to 60000 g / mol, an amount of glyoxal in the range of 0.4 to 4 wt.% of the amount of polyvinyl alcohol may be used. Advantageously, an amount of glyoxal in the range of 0.8 to 3 wt.% of the amount of polyvinyl alcohol may be used, in particular when the Mw of the PVA is in the range of 25000 to 55000 g / mol. Most advantageously, an amount of glyoxal in the range of 1 .0 to 2.5 wt.% of the amount of polyvinyl alcohol may be used, in particular when the Mw of the PVA is in the range of 30000 to 50000 g / mol.

[0035] Reference is made to Figures 2 and 3. Advantageously, the polyvinyl alcohol PVOH1 is reacted with glyoxal GL1 in an acid-catalyzed aqueous solution, wherein the pH of the solution is less than 7. Advantageously, the pH of the solution is equal to or less than 4, preferably in the range of 1.5 to 4, most preferably in the range of 1.5 to 3. Sulphuric acid may be used to adjust the pH of the solution. The temperature of the solution may be in the range of 25°C to 99°C during the acetalization reaction, advantageously in the range of 50°C to 99°C. The time period for the acid-catalyzed acetalization reaction in the solution is typically at least 20 minutes, such as in the range of 20 to 240 minutes or in the range of 20 to 120 minutes. For fully hydrolyzed PVA grades having a Mw in the range of 20000 to 60000 g / mol, a reaction time in the range 20 to 60 minutes is typically sufficient to reach an equilibrium state. An elevated temperature improves the solubility of the polyvinyl alcohol PVOH1 and reduces the time required to reach an equilibrium state during the acetalization reaction. The reaction produces cross-linkable composition POL1 , wherein the average number of glyoxal-based branching points per PVA chain n is at least 2, as disclosed above. Upon evaporating water molecules from the aqueous solution the reaction equilibrium may be configured to shift towards acetal formation, whereby a cross-linked composition CRL1 may be formed. This is desirable when the cross-linkable composition POL1 on at least one side of the paper prior to or at a drying section of a paper machine, to address the sticking problem of the PVA and to control the concentration of free glyoxal molecules on the formed paper substrate.

[0036] Thus, as disclosed above, there is provided a method for manufacturing cross- linkable composition that is suitable for coating a surface of a paper, the method comprising

[0037] - providing a solution containing polyvinyl alcohol that has been dissolved into water, the polyvinyl alcohol having a weight average molecular weight Mw equal to or less than 60000 g / mol and a degree of hydrolysis in the range of 70 to 99%,

[0038] - adding glyoxal into the solution in an amount in the range of 0.4 to 4 wt.% with respect to the amount of polyvinyl alcohol, thereby providing an aqueous solution suitable for grafting polyvinyl alcohol with glyoxal,

[0039] - providing reaction conditions suitable for an acetalization reaction, wherein the water-based mixture has an acidic pH and a temperature which is in the range of 25°C to 99°C, and

[0040] - reacting the aqueous solution in the reaction conditions for a time period of at least 20 minutes or until an equilibrium state has been reached, the method thereby producing cross-linkable composition comprising glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction such that the amount of free glyoxal in the aqueous solution is equal to or less than 1000 mg / kg.

[0041] Subsequently, there is provided an aqueous solution suitable for coating a surface of a paper, the aqueous solution containing cross-linkable composition comprising polyvinyl alcohol having a degree of hydrolysis in the range of 70 to 99% and wherein glyoxal has been grafted into the structure of the polyvinyl alcohol by means of acetal linkages in an amount of 0.4 to 4 wt.% of the amount of polyvinyl alcohol, and wherein the aqueous solution contains free glyoxal in an amount equal to or less than 1000 mg / kg. Advantageously, the amount of glyoxal is in the range of 0.8 to 3 wt.%, most preferably in an amount of 1 .0 to 2.5 wt.% with respect to the amount of polyvinyl alcohol, such that the average number of glyoxal-based branching points per PVA chain n in the cross-linkable composition is at least 2, preferably at least 3, most preferably at least 5, such as in the range of 2 to 20, preferably in the range of 3 to 10, most preferably in the range of 5 to 10, which glyoxal-based branching points end into an aldehyde group available for a further cross-linking reaction.

[0042] As disclosed above, the aqueous solution comprising the cross-linkable composition may be applied on a cellulose fiber-based support layer, thereby obtaining a paper substrate having a primer layer. A typical example of a cellulose fiber-based support layer is an industrial high-density paper, such as glassine paper or a super calendered kraft paper, which is manufactured from a stock comprising bleached chemical pulp that contains hardwood and softwood fibers. Characteristics typical for a suitable industrial high-density paper include a density equal to or higher than 1.000 g / cm3, preferably equal to or higher than 1.050 g / cm3, such as in the range of 1.000 to 1.200 g / cm3, when determined according to ISO 534, and a transparency equal to or higher than 40%, preferably equal to or higher than 42%, such as in the range of 40 to 56% when determined according to ISO 2469. In release liner applications, a sufficiently high density and a closed surface enables a subsequent release coating to be applied on the dried and calendered paper substrate. Similarly, in high-density papers suitable for packaging of a food product or another oxygen-sensitive product, which typically comprise a density equal to or higher than 0.800 g / cm3, when determined according to ISO 534, and a similar primer layer as glassine and super calendered kraft paper, a closed surface and a primer layer is beneficial for improving the barrier properties of the paper substrate.

[0043] According to another aspect, there is provided a method for manufacturing a paper substrate, the method comprising

[0044] - forming a cellulose fiber-based support layer on a paper machine of a stock that comprises bleached chemical pulp,

[0045] - preparing an aqueous solution comprising a cross-linkable composition as disclosed above,

[0046] - applying said cross-linkable composition on at least one side of the cellulose fiber-based support layer at a paper machine, thereby obtaining a paper substrate having a primer layer, and

[0047] - drying and calendering the paper substrate, such that a density equal to or higher than 0.800 g / cm3, when determined according to ISO 534, is obtained, thereby obtaining a paper substrate wherein the primer layer comprises a cross-linked composition and wherein the primer layer contains free glyoxal in an amount equal to or less than 1000 mg / kg.

[0048] The method above thus enables obtaining a corresponding paper substrate comprising a primer layer that contains a cross-linked composition, wherein the primer layer contains free glyoxal in an amount equal to or less than 1000 mg / kg. Advantageously, the primer layer PRIM1 contains the cross-linked composition in the range of 25 to 1000 mg / m2per side, preferably in the range of 50 to 500 mg / m2per side, most preferably in the range of 75 to 300 mg / m2per side, such that the total amount of primer layer on the paper substrate is in the range of 1 to 5 g / m2per side. The primer layer may further contain nonmodified water-soluble polyvinyl alcohol. Advantageously, the cross-linkable composition constitutes at least 5 wt.%, such as in the range of 5 to 100 wt.% of the total amount of polyvinyl alcohol that has been applied on the paper substrate.

[0049] Subsequently, there is provided a coating composition comprising polyvinyl alcohol and the aqueous solution, as described above, wherein the coating composition has a pH below 7, preferably between 3.5 and 7.0, and wherein the cross-linkable composition constitutes at least 5 wt.% of the total amount of polyvinyl alcohol, advantageously in the range of 5 to 100 wt.%. Subsequently, there is also provided a paper substrate, the paper substrate comprising

[0050] - bleached chemical pulp that contains hardwood and softwood fibers,

[0051] - a density equal to or higher than 0.800 g / cm3, preferably equal to or higher than 1.000 g / cm3, when determined according to ISO 534, and

[0052] - wherein one or both sides of the paper substrate comprises a primer layer that contains a cross-linked composition as disclosed above, and wherein the primer layer contains free glyoxal in an amount equal to or less than 1000 mg / kg.

[0053] The grafting of glyoxal with PVA before coating the aqueous solution on a cellulose fiber-based support layer offers thus controls the efficiency of a subsequent cross-linking reaction, when a primer layer is formed on a paper substrate. This enables a paper substrate to be formed, wherein the glyoxal is bound to the organic fraction of the primer layer PRIM1 layer and the amount of free glyoxal in the paper substrate is significantly reduced. The amount of glyoxal used for providing a paper substrate is therefore better controlled and predetermined before applying the cross-linkable composition disclosed above on at least one side of the cellulose fiber-based support layer at a paper machine.

[0054] Advantageously, a paper substrate SUBST1 may be provided which contains free glyoxal in an amount equal to or less than 100 mg / kg, preferably less than 50 mg / kg, most preferably less than 30 mg / kg, such as in the range of 5 to 100 mg / kg, preferably in the range of 10 to 50 mg / kg, most preferably in the range of 15 to 30 mg / kg, determinable according to ISO 1762:2019.

[0055] Advantageously, the amount of free glyoxal in the organic fraction of the primer layer PRIM1 of the paper substrate SUBST1 is equal to or less than 1000 mg / kg, preferably less than 500 mg / kg, most preferably less than 100 mg / kg, such as in the range of 100 to 1000 mg / kg, preferably in the range of 150 to 500 mg / kg, most preferably in the range of 200 to 400 mg / kg, determinable according to ISO 1762:2019.

[0056] Preferably, the paper formed on the paper machine is a glassine paper, a super calendered kraft paper having - a density equal to or higher than 1.000 g / cm3, preferably equal to or higher than 1.050 g / cm3, such as in the range of 1.000 to 1.200 g / cm3, when determined according to ISO 534, and

[0057] - a transparency equal to or higher than 40%, preferably equal to or higher than 42%, such as in the range of 40 to 56% when determined according to ISO 2469, or wherein the paper is a high-density paper suitable for flexible packaging, such as for packaging of a food product or another oxygen-sensitive product.

[0058] Hence, there is further provided a use of an aqueous solution comprising a cross-linkable composition POL1 as described above, either as such or as part of a coating composition, for coating a paper for a release liner, as also described above, wherein the aqueous solution the content of glyoxal as an active ingredient is equal to or less than 1000 mg / kg, preferably equal to or less than 700 mg / kg, most preferably less than 500 mg / kg, such as in the range of 300 to 1000 mg / kg, and wherein the cross-linkable composition POL1 constitutes at least 5 wt.% of the total amount of polyvinyl alcohol that has been applied on the paper substrate SUBST1 , advantageously in the range of 5 to 100 wt.%.

[0059] Objects and embodiments of the invention are described in the independent and dependent claims.

[0060] The symbols Sx and Sz, as used herein, refer to coordinate directions orthogonal to each other.

[0061] Figure 1 shows, by way of an example, a cross-dimensional view of a release liner comprising a paper substrate and a release layer,

[0062] Figure 2 shows a schematic illustration of glyoxal, as an active ingredient, which is reacted in an aqueous acid-catalyzed acetalization reaction with PVA, such that a reaction product is formed, wherein the reaction product is acetalized PVA wherein each PVA chain contains at least two glyoxal-based branching points that end into functional aldehyde groups that are available for a further reaction,

[0063] Figure 3 shows, by way of an example, a schematic illustration, wherein the reaction product which is acetalized PVA has been dried, thereby forming a composition wherein PVA chains have been cross-linked with glyoxal by means of acetal linkages.

[0064] Detailed

[0065] Release liner

[0066] As disclosed above with reference to Fig.1 , a release liner REL1 in the context herein refers to a thin multilayer structure having width, length and thickness dimensions. A release liner REL1 having a multilayer structure comprises at least a paper substrate SUBST1 and a release layer SIL1 , such as a silicone- based release layer, applied on at least one or both sides of the paper substrate SUBST1 .

[0067] A paper substrate

[0068] A paper substrate SUBST1 denotes a coated cellulose fiber-based support layer PAP1 , wherein the cellulose fiber-based support layer PAP1 is a paper that has been coated from at least one side or both sides with at least one primer layer PRIM1.

[0069] The cellulose fiber-based support layer is manufactured of bleached chemical pulp, denoted as BCP. A blend of bleached chemical pulps may be used, such as 30-70 % by dry weight of BCP of hardwood, the rest consisting of BCP of softwood. BCP of hardwood facilitates formation of density, while BCP of softwood pulp may be used to improve runnability in the paper machine and provide strength into the resulting paper. Typically, cellulose fiber-based support layer as disclosed herein benefits of BCP that has a Schopper-Riegler (°SR) number of less than 60, such as in the range of 30-60, when measured according to ISO 5267-1 :1999, which facilitates the draining of water and enables to provide a paper having a high density. A method for manufacturing a paper substrate suitable for purposes disclosed herein comprises forming a cellulose fiber-based support layer on a paper machine of a stock that comprises bleached chemical pulp, applying a film-forming polymer composition on one or both sides of the cellulose fiber-based support layer at a paper machine, thereby obtaining a paper substrate having a primer layer, and drying and calendering the paper substrate, such that sufficient characteristics, such as density and transparency, are obtained.

[0070] A paper substrate SUBST1 for a release liner further indicates that the coated side of the paper substrate SUBST 1 is suitable for binding silicone in a catalytic hydrosilation reaction. A catalytic hydrosilation reaction herein denotes an addition curing reaction. A paper substrate suitable for packaging of a food product or another oxygen-sensitive product denotes a paper substrate that has been calendered, has a high density and a similar primer layer as used in glassine and super calendered kraft papers. A high density, in this respect, denotes a density equal to or higher than 0.800 g / cm3(ISO 534), preferably equal to or higher than 0.850 g / cm3, such as in the range of 0.800 to 1.200 g / cm3, preferably in the range of 0.850 to 1 .200 g / cm3.

[0071] Glassine and SCK papers denote specific paper types which both have high density and are further suitable for use as a substrate for a release liner. Glassine and SCK paper both are conventionally prepared from a stock that contains highly refined bleached chemical pulp. Typically the bleached chemical pulp contains both hardwood and softwood fibers.

[0072] A paper substrate comprising a glassine paper is supercalendered, whereby it possesses an exceptional combination of high density, strength and transparency, which are beneficial characteristics for a paper substrate that is to be used for as a substrate layer of a release liner. SCK paper is also conventionally prepared from highly refined bleached chemical pulp that contains hardwood and softwood fibers that has been supercalendered, but to a lesser extent than glassine.

[0073] A calendered glassine or SCK paper suitable for release liner typically has

[0074] - smoothness of at least 900 sec / min (ISO 5627), - grammage equal to or less than 120 g / m2(ISO 536), such as in the range of 30 to 120 g / m2,

[0075] - density equal to or higher than 1 ,000 g / cm3(ISO 534), preferably equal to or higher than 1.050 g / cm3, such as in the range of 1.000 to 1.200 g / cm3, wherein the density refers to grammage (ISO 536) per thickness (ISO 534:2011 ),

[0076] - porosity equal to or less than 15000 pm / Pas (ISO 11004), and

[0077] - transparency of equal to or higher than 40%, preferably equal to or higher than 42%, such as in the range of 40 to 56% (ISO 2469), the parameter values corresponding to ISO standards referred in parentheses.

[0078] A high-density paper suitable for packaging of a food product or another oxygen-sensitive product is conventionally also prepared from a stock that contains highly refined bleached chemical pulp, but may have a relatively low grammage, when compared to the range typical for glassine or SCK papers, such as in the range of 35-70 g / m2and a density equal to or higher than 0.800 g / cm3(ISO 534), preferably equal to or higher than 0.850 g / cm3, such as in the range of 0.800 to 1.200 g / cm3, wherein the density refers to grammage (ISO 536) per thickness (ISO 534:2011 ). A relatively low grammage requires less polymer for a primer layer, which in a packaging application facilitates tight and durable seals to be formed, when transforming a multi-layer material into filled and sealed, cuboid-shaped packages.

[0079] Primer layer

[0080] A primer layer PRIM1 in this context refers to a polymer containing layer coated on a cellulose fiber-based support layer PAP1 . A cellulose fiber-based support layer PAP1 may contain one or more primer layers PRIM1. A primer layer PRIM1 is typically configured to reduce the porosity of the cellulose fiber-based support layer PAP1 surface, thereby improving the smoothness of the surface. Polyvinyl alcohol, starch, and / or carboxymethyl cellulose are polymers having a film-forming nature which are widely used to provide barrier properties between the release layer SIL1 and the cellulose fiber-based support layer PAP1 surface. The primer layer PRIM1 is typically applied as a coating composition on at least one side of the cellulose fiber-based support layer PAP1 surface prior to or at a drying section of a paper machine, when manufacturing the paper substrate SUBST1 , as disclosed above. The content of inorganic and organic fraction in the primer layer may be assessed by means of determining paper substrate residue ash content on ignition at 525°C (ISO 1762:2019). The residue on ignition ash content refers to a ratio of the mass of the residue remaining after a test specimen of paper is ignited at 525 °C ± 25 °C to the oven-dry mass of the test specimen before ignition. The organic fraction is ignited, whereas the inorganic fraction remains as residue.

[0081] Polyvinyl alcohol (PVA)

[0082] Polyvinyl alcohol is commercially manufactured from polyvinyl acetate via hydrolysis. The main structure and degree of polymerization of polyvinyl alcohol is established already when the vinyl acetate monomers are polymerized. Polymerization in this context refers to the rapid chain extension reaction connecting the used individual monomer units together into a compound having a high Mw, which consists of many monomer units covalently bound together in the polymerization reaction. A polymer hence refers to a product directly obtainable by a polymerization reaction. In polymer science, the backbone chain of a polymer is the longest series of covalently bonded atoms that together create the continuous chain of the molecule.

[0083] PVA may be obtained by controlled polymerization and subsequent hydrolysis of polyvinyl acetate, as explained above. Low molecular weight PVA is particularly useful as a reagent in grafting reaction of an aqueous solution, wherein low viscosity degree is needed. The PVA compounds that are believed to be useful in the practice of this invention have flow and viscosity characteristics which permit use of the formed acetalized polyvinyl alcohol as part of a coating composition applicable on a cellulose fiber-based support layer.

[0084] Within context of this disclosure, a weight average molecular weight Mw of the polyvinyl alcohol may be equal to or less than 60000 g / mol, preferably equal to or less than 55000 g / mol, most preferably equal to or less than 50000 g / mol, such as in the range of 20000 to 60000 g / mol, preferably in the range of 25000 to 55000 g / mol, most preferably in the range of 30000 to 50000 g / mol. The Mw of PVA can be determined by gel permeation chromatography GPC combined with static light scattering. The average molecular weight is measured from re- acetylated specimens by methods known from the literature, for example in a pyridine / acetic anhydride mixture.

[0085] Method of producing a cross-linkable composition from PVA with glyoxal

[0086] Reference is made to Figure 2, which is illustrates a synthesis, wherein glyoxal GL1 , as an active ingredient, is reacted in an aqueous acid-catalyzed acetalization reaction with PVA PVOH1 , such that a cross-linkable composition POL1 is formed, which is acetalized PVA wherein each PVA chain contains at least two glyoxal-based branching points that end into functional aldehyde groups that are available for a further reaction. Preferably, the average number of glyoxal-based branching points per PVA chain n is at least 3, most preferably at least 5, such as in the range of 2 to 20, preferably in the range of 3 to 10, most preferably in the range of 5 to 10.

[0087] For producing a cross-linkable composition POL1 suitable for coating a surface of a cellulose fiber based support layer, a solution containing polyvinyl alcohol having a degree of hydrolysis in the range of 70 to 99% is provided as a reagent. When the degree of hydrolysis of the polyvinyl alcohol is below 70%, the solubility of the polymer to water diminishes drastically. The degree of hydrolysis of the polyvinyl alcohol further has an effect on the affinity of the primer layer containing the acetal product towards the cellulose fiber-based support layer, whereby the adhesiveness of the polyvinyl alcohol towards the cellulose fiber-based support layer may be controlled by selecting the degree of hydrolysis of the polyvinyl alcohol. The polyvinyl alcohol may be arranged to be fully hydrolyzed, referring to a degree of hydrolysis in the range of 97 to 100%. Alternatively, the polyvinyl alcohol may be arranged to be partially hydrolyzed, referring to a degree of hydrolysis wherein the amount of hydroxyl groups is predominant, such that the whole compound is water-soluble. A partially hydrolyzed polyvinyl alcohol may have a degree of hydrolysis of less than 97%. The meaning of the degree of hydrolysis, also known as the saponification degree, in the context of polyvinyl alcohol is well known in the field, and can be expressed as follows:

[0088] Degree of hydrolysis (mol-%) = m / (n + m) x 100, wherein the m and n refer to the repeating units, m being a vinyl alcohol unit and n being a vinyl acetate unit. Degree of hydrolysis is therefore a value with which the proportion of the vinyl alcohol unit in all the repeating units is indicated in mol%, determinable according to JIS K 6726.

[0089] Implicitly, the degree of hydrolysis has an effect on the amount of acetyl groups in the compound. Fully hydrolyzed PVA has a high crystallization tendency and a reduced cold-solubility in water. This effect is reduced in partially hydrolyzed PVA, which contains more acetyl groups. However, the hydrophobicity increases, when the solubility to water diminishes. Preferably, to improve the efficiency of the later acetalization reaction at a paper substrate, the degree of hydrolysis of the polyvinyl alcohol used in an aqueous acid-catalyzed acetalization reaction is relatively high, such as in the range of 85 to 99%, most preferably in the range of 90 to 99%.

[0090] Dry matter content of the solution containing polyvinyl alcohol in the synthesis may be equal to or higher than 18 wt.%, preferably in the range in 18 to 25 wt.%, most preferably in the range of 20 to 22 wt.%, determinable according to SCAN-P 39:80. The dry matter content of the solution containing polyvinyl alcohol has an effect to the reaction rate in the aqueous acid-catalyzed acetalization reaction. A higher dry matter content provides a reaction at a higher concentration and at a high conversion efficiency. Moreover, a higher dry matter content in the formed reaction product is advantageous, since this enables coating of the reaction product containing the cross-linkable composition POL1 on a cellulose fiber-based support layer as a primer layer at high concentration. A high dry matter content in the coating composition requires less drying.

[0091] Glyoxal is typically provided as a 40 wt.% aqueous solution. In NTP condition (22°C, 1 atm) glyoxal has a relatively high solubility to water of >100 mg / ml, whereby an aqueous solution suitable for grafting polyvinyl alcohol with glyoxal can be provided by mixing the two solutions at room temperature. To obtain acetalized PVA wherein each PVA chain contains at least two glyoxal-based branching points that end into functional aldehyde groups that are available for a further reaction, the amount of glyoxal added into the aqueous solution can be in the range of 0.4 to 4 wt.% with respect to the amount of the polyvinyl alcohol (0.4 to 4 grams of glyoxal per 100 grams of polyvinyl alcohol). To initiate the acetalization reaction in an aqueous solution containing glyoxal and PVA, the pH of the solution may be adjusted with a suitable acid catalyst such as sulphuric acid, to provide acidic condition. Advantageously, a pH equal to or less than 4, preferably in the range of 1.5 to 4, most preferably in the range of 1 .5 to 3 is used in the synthesis to provide conditions for the aqueous solution wherein two hydroxyl groups of the polyvinyl alcohol PVOH1 react with the aldehyde group of the glyoxal GL1 to form an acetal. An elevated temperature facilitates the acetalization reaction. The temperature of the aqueous solution during the acetalization reaction may be in the range of 25°C to 99°C. Preferably the reaction temperature is equal to or above 50°C, such as in the range of 50°C to 99°C, as a higher reaction temperature correlates with accelerated reaction kinetics. Acetalization formation in an aqueous solution is a reaction wherein every step is reversible. To improve the amount of the reaction product POL1 , the reaction is advantageously allowed to proceed at least for a time period of 20 minutes or until an equilibrium state has been reached, the method thereby producing cross-linkable composition POL1 comprising glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction. When providing acidic conditions and an aqueous solution containing relatively low molar amount of water compared to a higher amount of PVA, the balance of the reaction is on the acetal side, the reaction thereby providing acetalized polyvinyl alcohol POL1 wherein two hydroxyl groups of the polyvinyl alcohol PVOH1 have been covalently bonded to the carbonyl group of the aldehyde. However, some of the glyoxal remains in the aqueous solution as a reagent, the amount of which may be determined from the aqueous solution by gas chromatography.

[0092] Analytical method to evaluate the free glyoxal content of an aqueous solution in the synthesis as disclosed above may be performed by means of gas chromatography from a sample obtained from the aqueous solution, which has first been adjusted to comprise a 5 wt.% dry matter content, and from which PVA, as well as glyoxal covalently bound to PVA, have then been phase separated by precipitation. The dry matter content of an aqueous solution as disclosed above can be determined according to the procedure disclosed in SCAN-P 39:80, with the specification that the sample is dried in the oven at 105 ± 3 °C for 16 h. The precipitation of PVA and glyoxal covalently bound to PVA from a solution having a 5 wt.% dry matter content can be performed by means of cold ethanol precipitation, since unbound glyoxal has significantly better miscibility with cold ethanol than PVA. Therefore, PVA as well as glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction is separated into a solid phase, while unbound glyoxal remains in the supernatant (ethanol). By comparing the gas chromatography result obtained from a sample of the supernatant after the precipitation to a calibration curve obtained from comparable results of a dilution series of pure glyoxal, the free glyoxal content of the aqueous solution may thus be determined. The analytical method may be performed, for example, with gas chromatography equipped with flame-ionization detector, abbreviated as GC-FID, which senses carbon ions and can be used to identify organic compounds based on their interaction with the flame ionization detector.

[0093] A practical example of synthetizing cross-linkable composition of PVA with glyoxal in aqueous solution and analysis of the free glyoxal amount therein by means of GC-FID is disclosed hereafter.

[0094] Example 1. Synthesis of a cross-linkable composition containing PVA that has been grafted with glyoxal in an aqueous solution

[0095] An aqueous solution was prepared by dissolving 18 g of fully hydrolyzed PVA having a Mw in the range of 48000 to 50000 g / mol, a DP in the range of 550 to 750 and a viscosity in the range of 9 to 11 mPa s (DIN 53015), into an amount of 100 ml of hot water (93 °C), thus obtaining an aqueous solution for a synthesis containing 18 wt.% of polyvinyl alcohol. Subsequently, solution was cooled to 80 °C and glyoxal was added as 40% aqueous solution into the aqueous PVA solution in an amount of 2.0 wt.%, with respect to the amount of polyvinyl alcohol, and the pH of the solution was adjusted to 3 with 98 wt.% sulphuric acid. The temperature of the aqueous solution during the synthesis was maintained at 80°C for 30 minutes, whereby an equilibrium state was reached, wherein acetal forming and dissolving occurred at substantially the same rate.

[0096] A sample of the aqueous solution from the synthesis was then taken and diluted with hot water having the same temperature as the aqueous solution (80°C), such that 40 g of diluted aqueous solution was obtained, the diluted solution having a 5 wt.% dry matter content. The weight of the aqueous solution needed for the dilution may be determined based on the dry matter content of the aqueous solution. For example, the mass mpvAi of 18 wt.% PVA solution needed for 40 g of diluted solution having a 5 wt.% dry matter content is calculated according to equation 4 below:

[0097] Hence, an amount of 11 .11 g of a solution having 18 wt.% of polyvinyl alcohol needs to be weighted and diluted with hot water such that 40 g of diluted aqueous solution having a 5 wt.% dry matter content is obtained.

[0098] The 40 g of diluted aqueous solution having a 5 wt.% dry matter content was then precipitated in a beaker with cold ethanol having a temperature of 5 °C. A volume ratio of 1 part of diluted aqueous solution (10 ml) and 3 parts of cold ethanol (30 ml) was used for the precipitation, which was performed in a beaker closed with a plastic film to prevent evaporation. The formed water- ethanol mixture was stirred in the beaker for 10 minutes. A plastic film was used to seal the beaker to avoid evaporation during the precipitation. The stirred, precipitated water-ethanol mixture was then poured into a centrifugation tube and centrifuged (10 min, 4000 rpm, 23 °C) to settle the formed solid precipitate at the bottom of the centrifugation tube. An aliquot of clear supernatant was carefully taken from the centrifugation tube and filtered through a 0.45 pm syringe filter made of regenerated cellulose. The filtered aliquot was analyzed by GC-FID, and amount of glyoxal was quantified. The result was calculated in mg / kg, according to a calibration curve obtained from a dilution series prepared from pure glyoxal. The free glyoxal content of the original, undiluted, sample was calculated back from the GC-FID result, taking into account the dilutions that were made during the procedure.

[0099] The concentrations of free glyoxal in samples during different steps can be calculated according to equations 5 and 6 below:

[0100] , wherein - C2 refers to the free glyoxal concentration of the diluted sample having a 5 wt.% dry matter content,

[0101] - C3 refers to the free glyoxal concentration determined of the sample of the supernatant after the precipitation with GC-FID,

[0102] - the mass mpvA3 refers to the mass of PVA in the diluted aqueous solution used for the precipitation, determined according to the procedure disclosed in SCAN-P 39:80 (105 ± 3°C, 16 h), and

[0103] - the mass rriEtoH refers to the weighed mass of ethanol used for the precipitation.

[0104] Therefore,

[0105] Eq. 6 _ _ mPVA2

[0106] Cl — ^2 mPVAl

[0107] , wherein

[0108] - ci refers to the free glyoxal concentration of the aqueous solution from the synthesis,

[0109] - the mass mpvA2 refers to the weighed mass of the diluted aqueous solution used for the precipitation, and

[0110] - the mass mpvAi refers to the weighted mass of the aqueous solution used for obtaining diluted aqueous solution.

[0111] Advantages of the method are hereafter illustrated by means of comparative working examples.

[0112] EXAMPLE 2 A. Preparation of aqueous solution containing a cross-linkable composition and determination of free glyoxal content therein

[0113] The analytical methods and equations disclosed above enable to determine the amount of unbound glyoxal present in an aqueous solution by means of GC-FID, after phase separation. An experimental study was thus performed to evaluate the efficiency of the synthesis for binding glyoxal, before application on a cellulose fiber-based support layer.

[0114] In the experimental study, an aqueous solution having 18 wt.% of polyvinyl alcohol was used for preparing a set of samples, some of which were reacted with glyoxal into a cross-linkable composition, in conditions as disclosed in example 1 above, while others were merely admixed with a 40% aqueous solution of glyoxal, without performing an acetalization reaction, as disclosed in example 1 above. A sample comprising only PVA without glyoxal served as a negative control. Each sample was prepared as a duplicate, to improve the reliability of the results, which are presented as arithmetic mean value of duplicate measurements. In all samples, the same PVA grade (10-98) was used.

[0115] The following set of samples was prepared:

[0116] CO PVA 10-98 only, negative control

[0117] C1 PVA 10-98 admixed with 6 wt.% of glyoxal, without acetalization C2 PVA 10-98 admixed with 3 wt.% of glyoxal, without acetalization

[0118] 51 PVA 10-98 grafted with 2 wt.% glyoxal by means of acetalization into a cross-linkable composition

[0119] 52 PVA 10-98 grafted with 2 wt.% glyoxal by means of acetalization into a cross-linkable composition, the reaction product thereafter admixed with unmodified PVA 10-98 in a ratio of 1 :4

[0120] 53 PVA 10-98 grafted with 2 wt.% glyoxal by means of acetalization into a cross-linkable composition, the reaction product thereafter admixed with unmodified PVA 10-98 in a ratio of 1 :9

[0121] Table 1 (below) illustrates the sample compositions by dry mass components, and the amount of unbound glyoxal detected in each sample by GC-FID, in units of parts per million (ppm). The samples S2 and S3 were prepared by first acetalizing 2 wt.% of glyoxal with respect to the amount of the PVA, and then diluting the reaction product with unmodified PVA. In the comparative samples C1 and C2, a 40% aqueous solution of glyoxal was admixed with PVA to obtain an aqueous solution, which comprised either 3 wt.% or 6 wt.% of glyoxal with respect to the amount of the PVA.

[0122] Table 1. Results of an experimental study illustrating the efficiency of acetalization reaction in covalently binding glyoxal to PVA in an aqueous solution. The amount of PVA and glyoxal refer to mass percentages of the total dry matter content (SCAN-P 39:80) which was used as reagent or component. The amount of free glyoxal is expressed as parts per million, abbreviated as ppm, due to relatively small concentrations of free glyoxal detected in the samples (1 ppm is equal to 1 mg per kg).

[0123] The negative control sample CO did not contain any free glyoxal, which was expected. The comparative samples C1 and C2, which contained PVA 10-98 admixed with 6 wt.% and 3 wt.% of glyoxal, respectively, contained the highest amounts of free glyoxal. These amounts differed an order of magnitude from the samples, wherein PVA had been grafted with glyoxal by means of acetalization into a cross-linkable composition.

[0124] As evidenced by the results of the experimental study, acetalization of PVA with glyoxal is advantageous for reducing the amount of free glyoxal in the aqueous solution, prior to applying it on a cellulose fiber-based support layer. When providing reaction conditions suitable for an acetalization reaction and an aqueous solution S1 comprising PVA having a Mw < 60000 g / mol and a sufficiently high degree of hydrolysis and a relatively small amount of 2 wt.% of glyoxal, a reaction product was produced, which comprised less than 475 ppm of glyoxal. This corresponds to a composition wherein the amount of free glyoxal is equal to or less than 475 mg / kg. Thus, when compared to the content of glyoxal detected from the sample C2, the amount of free glyoxal in the aqueous solution of sample S1 is significantly less.

[0125] EXAMPLE 2B. Determination of wet abrasion from a paper surface

[0126] A second experimental study was performed to evaluate the efficiency of the reaction product from the synthesis in providing abrasion resistance, when the reaction product comprising the cross-linkable composition had been applied as part of a coating composition onto a cellulose fiber-based support layer manufactured of bleached chemical pulp using a laboratory blade coater and dried at 105°C for 1 minute, but not calendered. Thus, paper substrates were formed, which contained a primer layer having PVA with or without glyoxal. The coating composition pH was determined to be 4.0. During drying, remaining hydroxyl groups and aldehyde groups reacted to form acetal bonds, thus cross-linking the components in the primer layer.

[0127] The cellulose fiber-based support layer used in the study was uncoated and uncalendered paper that had

[0128] - a density of 0.680 g / cm3(ISO 534:2011 ),

[0129] - a Bendtsen roughness of 900 ml / min (ISO 8791-2:2013), and

[0130] - a Bendtsen air permeance of 260 ml / min (ISO 5636-3:2013).

[0131] In the study, a set of coating composition samples were thus prepared, each coating composition comprising

[0132] - 50 wt.% of kaolin and

[0133] - 50 wt.% of o PVA 10-98 o a mixture of PVA 10-98 and glyoxal, or o a mixture of PVA 10-98 and reaction product from sample S1 , comprising PVA that has been grafted with 2 wt.% of glyoxal.

[0134] A synthesis, wherein 2 wt.% of glyoxal was used as an active ingredient as disclosed in examples 1 and 2A, enabled to form a cross-linkable composition wherein each PVA chain contained in average 10 glyoxal-based branching points that end into functional aldehyde groups. The main chain of the polymer, also denoted as the polymer backbone, in a grafted PVA chain functions as a spacer. As the main chain of PVA is substantially linear, it enables aldehyde groups that have been grafted into the same PVA chain to be distributed along the whole length of said PVA chain. Thereby, each PVA chain in an aqueous solution that has been arranged to contain at least 2, such as in the range of 2 to 20, preferably in the range of 3 to 10, glyoxal-based branching points n per PVA chain, can interact with hydroxyl groups of other PVA chains that are located much further apart than what is the distance between two aldehyde groups of a single glyoxal molecule. This spacer effect was observed in the experimental study 2B, as demonstrated by the fact that sample CS1 containing grafted PVA could provide significantly more efficient cross-linking than sample CC2, which contained a higher amount of glyoxal directly admixed with PVA without acetalization before coating. The abrasion loss was determined according to a modified standard TAPPI T476 om-11 (Abrasion loss of paper and paperboard (Taber-type method)), from a cellulose fiber-based support layer coated with a 2 to 3 g / m2of a composition as a wet abrasion test. The test was conducted according to the standard on a Taber Abraser 5130 device using a round paper sample (diameter 105 mm). Sample surface was wetted with 2 ml of deionized water and rubbed with an S-32 abrasion disc for 15 rotations. After rubbing, the sample was rinsed with deionized water that was collected and diluted to 60 ml. The kaolin from the abraded coating composition provided turbidity to the water, which was then measured from the 60 ml sample volume, to determine the abrasion loss as nephelometric turbidity units, denoted as NTU, in accordance with ISO 7027-1 (2016). The amount of kaolin rubbed from the surface correlated with the amount of turbidity measured from the 60 ml sample volume.

[0135] Table 2 (below) presents the abrasion resistance results in the set of samples that were prepared. In all samples, the same kaolin and PVA grade (10-98) was used. The sample CC0 comprised only kaolin and PVA, without glyoxal, served as a negative control. The samples CC1 and CC2 comprised kaolin and PVA admixed with glyoxal, which had not been reacted by an acetalization reaction before coating. The samples CS1 and CS2 comprised kaolin and PVA, in addition to PVA which had been grafted with 2 wt.% glyoxal by means of an acetalization reaction into a cross-linkable composition before coating. Each sample was prepared as a duplicate, to improve the reliability of the results, which are presented as arithmetic mean value of duplicate measurements.

[0136] Table 2. Comparative results, wherein 2-3 g / m2of primer layer coating was applied on a paper surface and tested for abrasion resistance. The component amounts per sample have been indicated as mass ratios, based on the dry matter content (SCAN-P 39:80). The abbreviation GL-PVA refers to the cross- linkable composition of example 2A, sample S1. The wet abrasion result has been indicated as nephelometric turbidity units.

[0137] The result of sample CCO shows that a primer layer which contains only polyvinyl alcohol without glyoxal has the highest abrasion loss, which indicates the least abrasion resistance in the set of samples. The results of table 2 also show that a primer layer which contains a cross-linked composition formed of a cross-linkable composition of glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction before coating has a significantly better abrasion resistance than a primer layer, wherein this has not been performed. In samples CC1 and CC2 that contained glyoxal which had not been reacted by an acetalization reaction before coating, the abrasion loss is more than 2 times higher. The results thus indicate that an aqueous solution comprising a cross-linkable composition of glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction before coating, as disclosed in examples 1 and 2A, improves abrasion resistance of a primer layer or a surface size, when used as part of a coating composition. A better abrasion resistance result also speaks of good cross-inking density within the primer layer, whereby the hydroxyl groups therein are less prone to cause swelling due to moisture. Such paper substrate may be more easily stored on a roll before calendering, without opposite sides becoming attached together.

[0138] Moreover, due to the mass ratio of kaolin being the same as the mass of PVA, the sample mass ratio of glyoxal in samples CC1 and CC2 corresponds to the amount of glyoxal used in samples C1 and C2, respectively. A further advantage of a primer layer that contains a cross-linked composition formed of a cross-linkable composition of glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction before coating is therefore in the small amount of free glyoxal, which is significantly less also on the paper substrate. By controlling the average number of glyoxal-based branching points which will be formed per PVA chain during the acid-catalyzed acetalization reaction, the amount of glyoxal needed as a reagent can be optimized.

[0139] EXAMPLE 2C. Determination of silicone rub-off

[0140] A third experimental study was performed to evaluate the performance of the reaction product from the synthesis for anchoring a silicone coating applied on a calendered paper substrate, wherein the primer layer comprises a crosslinked composition formed of a cross-linkable composition of glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction before coating, as disclosed in the above examples. Anchorage is a term used in the field to describe the attachment of the release coating to the paper substrate. Anchorage may be measured as relative rub-off of the silicone from the paper substrate. A relative rub-off value of 1 refers to a perfect anchorage of the silicone, such that the release coating is fully anchored to the substrate. A relative rub-off value of 2 or 3 means that the release coating could not hold on to the paper substrate sufficiently.

[0141] The samples disclosed in example 2, which had been coated and dried as disclosed in table 2 (above) were re-moisturized in a humidity-controlled chamber for 4 h at a temperature of 23°C and relative humidity of 85%, and calendered directly thereafter. The re-moisturized paper substrates were calendered on a laboratory calender with a steel roll and a polymer coated roll. A nip surface temperature of 100°C and a linear load of 150 kN / m was used, and each paper sheet was passed three times through the nip, the coated side facing the steel roll. After calendering, the paper substrates had a PPS roughness of 1.5 pm (ISO 8791 -4:2021 ) and a density of 1.100 g / cm3(ISO 534:2011 ).

[0142] All paper substrates were subsequently subjected to siliconization, which refers to coating of a paper substrate with silicone resin prepared from SL 161 silicone polymer and SL 11 cross-linker, using DC 4000 catalyst (all components provided by Dow Corning). The silicone resin applied on the paper substrate was prepared by stirring 92.2 parts per weight of the SL 161 with 7.0 parts of the SL 11 cross-linker for 2 minutes, then adding 0.8 parts of the DC 4000 platinum catalyst and stirring for 5 minutes. The silicone resin thus prepared was then applied on top of the paper substrate by blade coater in an amount of 1.2 g / m2and cured for 1 minute at 105°C, thereby curing the silicone resin into a release layer and forming a release liner.

[0143] Silicone anchorage was tested by strongly rubbing an area of the release coated surface in cross direction with a piece of flexible silicone tube. The result was evaluated visually, using light reflection at low angle to reveal possible removal of silicone, i.e. ruboff.

[0144] The silicone adhesion was tested immediately after the siliconization from the formed release liner. This is referred to as the initial rub off level. The release liners were then stored at 50°C and 75% relative humidity for a period of 3 days, 1 week, 2 weeks and 4 weeks before determining the silicone adhesion level again.

[0145] Table 3. Silicone rub-off results. Degree of rub-off is expressed on a scale from 1 to 3: 1 = no rub-off, 2 = visible rub-off, 3 = significant or complete rub-off.

[0146] The results show that the prepared paper substrates worked excellently with a standard silicone system. The release liner samples containing a cross-linked reaction product formed of a cross-linkable composition as disclosed above presented excellent relative rub-off values, which remained consistent over time. In view of the test results, a paper substrate containing a primer layer, which comprises glyoxal that has been grafted into the structure of polyvinyl alcohol in an aqueous acetalization reaction prior to coating the reaction product on a paper, demonstrated excellent silicone adhesion levels, when relative rub-off was measured. Due to controlled amount of glyoxal-based branching points ending into aldehyde groups in the aqueous acetalization reaction prior to coating, the amount of free glyoxal in the reaction product and also on a primer layer of a paper substrate may be significantly reduced.

Claims

34Claims1. A method for manufacturing cross-linkable composition (POL1 ) that is suitable for coating a surface of a paper, the method comprising- providing a solution containing polyvinyl alcohol that has been dissolved into water, the polyvinyl alcohol having a weight average molecular weight (Mw) equal to or less than 60000 g / mol and a degree of hydrolysis in the range of 70 to 99%,- adding glyoxal into the solution in an amount in the range of 0.4 to 4 wt.% with respect to the amount of polyvinyl alcohol, thereby providing an aqueous solution suitable for grafting polyvinyl alcohol with glyoxal,- providing reaction conditions suitable for an acetalization reaction, wherein the aqueous solution has an acidic pH and a temperature which is in the range of 25°C to 99°C, and- reacting the aqueous solution in the reaction conditions for a time period of at least 20 minutes or until an equilibrium state has been reached, the method thereby producing cross-linkable composition (POL1 ) comprising glyoxal that has been grafted into the structure of polyvinyl alcohol in an acetalization reaction such that the amount of free glyoxal in the aqueous solution is equal to or less than 1000 mg / kg.

2. The method according to claim 1 , wherein the pH of the aqueous solution is equal to or less than 4, preferably in the range of 1 .5 to 4, most preferably in the range of 1 .5 to 3, and wherein the temperature of the aqueous solution is in the range of 50°C to 99°C.

3. The method according to claim 1 or 2, wherein the time period is in the range of 20 to 240 minutes, preferably in the range of 20 to 120 minutes, most preferably in the range of 20 to 60 minutes.

4. The method according to any of the previous claims, wherein the acetalization reaction is not stopped by neutralizing the solution pH.

5. The method according to any of the previous claims, wherein the dry matter content of the solution containing polyvinyl alcohol is equal to or higher than 18 wt.%, preferably in the range in 18 to 25 wt.%, most preferably in the range of 20 to 22 wt.%, determinable according to SCAN-P 39:80.

356. An aqueous solution comprising a cross-linkable composition (POL1 ) suitable for coating a surface of a paper, the cross-linkable composition (POL1 ) comprising polyvinyl alcohol having a weight average molecular weight (Mw) equal to or less than 60000 g / mol and a degree of hydrolysis in the range of 70 to 99% and wherein glyoxal in an amount of 0.4 to 4 wt.% with respect to the amount of polyvinyl alcohol has been grafted into the structure of the polyvinyl alcohol by means of acetal linkages such that the aqueous solution contains free glyoxal in an amount equal to or less than 1000 mg / kg.

7. The method according to any of claims 1 to 5 or the aqueous solution according to claim 6, the aqueous solution having a viscosity that is equal to or less than 8000 mPa s, when determined as a Brookfield viscosity at 100 rpm and at a temperature of 25 °C.

8. The method or the aqueous solution according to any of the previous claims, wherein the amount of free glyoxal in the aqueous solution is equal to or less than 700 mg / kg, preferably less than 500 mg / kg, such as in the range of 300 to 1000 mg / kg.

9. The method or the aqueous solution according to any of the previous claims, wherein the polyvinyl alcohol has a weight average molecular weight (Mw) equal to or less than 60000 g / mol, preferably equal to or less than 55000 g / mol, most preferably equal to or less than 50000 g / mol, such as in the range of 20000 to 60000 g / mol, preferably in the range of 25000 to 55000 g / mol, most preferably in the range of 30000 to 50000 g / mol.

10. The method or the aqueous solution according to any of the previous claims, wherein the amount of glyoxal is preferably in the range of 0.8 to 3 wt.%, most preferably in an amount of 1.0 to 2.5 wt.% with respect to the amount of polyvinyl alcohol, the cross-linkable composition (POL1 ) thereby having an average number of glyoxal-based branching points per PVA chain ( n) which is at least 2, preferably at least 3, most preferably at least 5, such as in the range of 2 to 20, preferably in the range of 3 to 10, most preferably in the range of 5 to 10, which glyoxal-based branching points end into an aldehyde group available for a further cross-linking reaction.

11. A coating composition comprising polyvinyl alcohol and the aqueous solution according to any of the claims 6 to 10, the coating composition having a pH below 7, preferably between 3.5 and 7.0.

12. The coating composition according to claim 11 , wherein the cross-linkable composition (POL1 ) constitutes at least 5 wt.% of the total amount of polyvinyl alcohol, advantageously in the range of 5 to 100 wt.%.

13. A method for manufacturing a paper substrate (SUBST1 ), the method comprising- forming a cellulose fiber-based support layer on a paper machine of a stock that comprises bleached chemical pulp,- preparing an aqueous solution comprising a cross-linkable composition (POL1 ) according to any of the claims 1 to 12,- applying said cross-linkable composition (POL1 ) on at least one side of the cellulose fiber-based support layer at a paper machine, thereby obtaining a paper substrate (SUBST1 ) having a primer layer (PRIM1 ), and- drying and calendering the paper substrate, such that o a density equal to or higher than 0.800 g / cm3, when determined according to ISO 534, is obtained, thereby obtaining a paper substrate (SUBST1 ) wherein the primer layer (PRIM1 ) comprises a cross-linked composition (CRL1 ) and wherein the primer layer contains free glyoxal in an amount equal to or less than 1000 mg / kg.

14. A paper substrate (SUBST1 ) comprising- a cellulose fiber-based support layer containing bleached chemical pulp,- a density equal to or higher than 0.800 g / cm3, when determined according to ISO 534, and- wherein one or both sides of the paper substrate (SUBST1 ) comprises a primer layer (PRIM1 ) that contains cross-linked composition (CRL1 ) that has been formed of a cross-linkable composition (POL1 ) according to any of the claims 6 to 12, the primer layer (PRIM1 ) containing free glyoxal in an amount equal to or less than 1000 mg / kg.

15. The method according to claim 13 or the paper substrate (SUBST1 ) according to claim 14, wherein the primer layer (PRIM1 ) contains the crosslinked composition (CRL1 ) in the range of 25 to 1000 mg / m2per side, preferably in the range of 50 to 500 mg / m2per side, most preferably in the range of 75 to 300 mg / m2per side, such that the total amount of primer layer (PRIM1 ) on the paper substrate (SUBST1 ) is in the range of 1 to 5 g / m2per side.

16. The method according to claim 13 or 15 or the paper substrate (SUBST1 ) according to claim 14 or 15, wherein the amount of free glyoxal is equal to or less than 100 mg / kg, preferably less than 50 mg / kg, most preferably less than 30 mg / kg, such as in the range of 5 to 100 mg / kg, preferably in the range of 10 to 50 mg / kg, most preferably in the range of 15 to 30 mg / kg.

17. The method according to any of claims 13, 15 or 16 or the paper substrate (SUBST1 ) according to any of claims 14 to 16, wherein the amount of free glyoxal in the organic fraction of the primer layer (PRIM1 ) is equal to or less than 1000 mg / kg, preferably less than 500 mg / kg, most preferably less than 100 mg / kg, such as in the range of 100 to 1000 mg / kg, preferably in the range of 150 to 500 mg / kg, most preferably in the range of 200 to 400 mg / kg, determinable according to ISO 1762:2019 from the residue ash content.

18. The method according to any of claims 13 or 15 to 17 or the paper substrate (SUBST1 ) according to any of claims 14 to 17, wherein the cross-linkable composition (POL1 ) constitutes at least 5 wt.% of the total amount of polyvinyl alcohol that has been applied on the paper substrate (SUBST1 ), advantageously in the range of 5 to 100 wt.%.

19. The method according to any of claims 13 or 15 to 18 or the paper substrate (SUBST1 ) according to any of claims 14 to 18, wherein the paper is a glassine paper or a super calendered kraft paper having- a density equal to or higher than 1.000 g / cm3, preferably equal to or higher than 1.050 g / cm3, such as in the range of 1.000 to 1.200 g / cm3, when determined according to ISO 534, and- a transparency equal to or higher than 40%, preferably equal to or higher than 42%, such as in the range of 40 to 56% when determined according to ISO 2469,38 or wherein the paper is a high-density paper suitable for flexible packaging, such as for packaging of a food product or another oxygen-sensitive product.

20. The method according to any of claims 13 or 15 to 19 or the paper substrate (SUBST1 ) according to any of claims 14 to 19, wherein the cross-linkable composition (POL1 ) is applied as part of a coating composition having a pH below 7, preferably between 3.5 and 7.0, most preferably 4.

21. Use of an aqueous solution comprising a cross-linkable composition (POL1 ) according to any of the claims 6 to 10 or a coating composition comprising the aqueous solution according to claim 11 or 12 for coating a paper for a release liner, such as a glassine paper or a super calendered kraft paper having- a density equal to or higher than 1.000 g / cm3, preferably equal to or higher than 1.050 g / cm3, such as in the range of 1.000 to 1.200 g / cm3, when determined according to ISO 534, and- a transparency equal to or higher than 40%, preferably equal to or higher than 42%, such as in the range of 40 to 56% when determined according to ISO 2469.

22. Use of an aqueous solution comprising a cross-linkable composition (POL1 ) according to any of the claims 6 to 10 or a coating composition comprising the aqueous solution according to claim 11 or 12 on a paper substrate (SUBST 1 ) for providing a release liner, wherein the aqueous solution the content of glyoxal as an active ingredient is equal to or less than 1000 mg / kg, preferably equal to or less than 700 mg / kg, most preferably less than 500 mg / kg, such as in the range of 300 to 1000 mg / kg.

23. The use of claim 22, wherein the cross-linkable composition (POL1 ) constitutes at least 5 wt.% of the total amount of polyvinyl alcohol that has been applied on the paper substrate (SUBST1 ), advantageously in the range of 5 to 100 wt.%.

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