Release liner, system, and method

Abstract

The invention relates to a release liner (9) comprising or consisting of: a carrier substrate (1); a release layer (2); and a thermoplastic polymer layer (3) arranged between the carrier substrate (1) and the release layer (2); wherein the carrier substrate (1) is paper formed from a suspension consisting of: water; a fibrous composition; optionally at least one, particularly non-fibrous, additive, wherein at least 90 wt.-%, such as at least 95 wt.-%, of the dry weight of the fibrous composition is hardwood pulp, wherein the carrier substrate (1) is machine glazed paper, wherein the glazed side (4) faces the release layer (2), and wherein the glazed side (4) has a coating (5). The invention also relates to a system containing a release liner and an adhesive article (8) was well as to a production method.

Description

[0001] 65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0002] Release liner, system, and method

[0003] The present invention relates to the technical field of release liner materials used for example in graphic arts laminates, specifically release liners incorporating paper substrates as carrier material. Apart from a release liner as such, the invention also relates to a system comprising a release liner and a method for its production.

[0004] Graphic arts laminates are used in a wide range of visually demanding applications, including vehicle wrapping, color replacement films, visual communication graphics, window films, and other decorative surfaces. These laminates typically consist of at least three layers, namely a facestock, for example made from soft polyvinyl chloride (PVC), polyurethane, or polyolefin films; a pressure-sensitive adhesive (PSA) layer, which may be solvent-based acrylic, rubber-based, silicone-based, hot-melt, or any other suitable type of adhesive; and a release liner. The release liner is removed prior to application of the graphic arts laminate onto the desired surface and plays a critical role in protecting the adhesive. Additionally, it influences the optical properties of the final product.

[0005] In such systems, poly coated Kraft (PCK) papers are widely used as release liner due to their superior dimensional stability, lay-flat characteristics, stiffness, and low moisture sensitivity. PCK is typically composed of Kraft, machine glazed (MG), or Fine paper that is extrusion-coated with polyolefins, most commonly polyethylene (PE), on one or both65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) sides. One extrusion-coated surface is then coated with a release material, such as silicone, to enable easy release of the PSA during application.

[0006] While PCK has become widely used for many release liners, it has certain limitations in optically sensitive use cases. Despite being coated with a solid PE layer and often processed against high-gloss or textured chrome-plated cylinders during cooling of the PE layer to enhance surface finish, the underlying fibrous structure of the Kraft paper carrier layer can still subtly influence the surface texture of the laminate. This influence may not be visible on the release liner itself, but becomes evident on the facestock material once applied to a final substrate. This phenomenon, often referred to as mottling, orange peel, or surface waviness, is especially noticeable when using softer or lower-caliper films, or facestocks with higher adhesive coat weights, where underlying irregularities are amplified.

[0007] To eliminate these visual defects, the market has increasingly adopted paperless filmbased release liners, particularly polyethylene-coated polyethylene terephthalate (PET / PE) liners. These materials provide a completely fiber-free and ultra-smooth surface, which eliminates the issue of surface structure transfer from the liner to the facestock. However, these solutions come with significant cost and sustainability drawbacks. PET / PE liners are non-recyclable, due to their non-mono-material composition, and cannot be effectively separated in standard plastic recycling processes. They also carry a significantly higher material and processing cost, making them impractical for widespread or cost-sensitive use.

[0008] As a paper-based alternative to PCK release liners, clay coated Kraft (CCK) release liners are known, which offer a smoother surface due to the presence of a mineral coating. However, they lack the dimensional and moisture stability of PCK release liners, making them unsuitable for high-performance applications where humidity and thermal stress are critical factors.

[0009] These limitations of prior art solutions are particularly relevant with adhesives used in premium sectors, such as automotive wraps, where high gloss and flawless finishes are of utmost importance. End customers expect the final product to have excellent and65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) flawless surface properties, such that the adhesive article should at least maintain or even improve the visual quality of the underlying surface, such as factory paintwork. Any surface waviness or imperfection transferred through the laminate can result in customer dissatisfaction and damage to brand reputation, especially when the adhesive articles are sold at a high premium.

[0010] As a result, manufacturers have increasingly shifted toward paperless release liner systems for their high-end products, accepting the cost and ecological tradeoffs in exchange for superior optical quality.

[0011] Thus, a conflict of interest arises from the available prior art solutions, which can be summarized as follows: On the one hand, paper-based release liners are typically recyclable and can be produced in a cost-effective manner, but they are in a lot of cases not suitable for high-end applications. On the other hand, paperless release liners are suitable for high-end applications, but typically cannot be recycled and have high production costs.

[0012] An object of the invention may be seen in overcoming this conflict of interest, by providing a release liner that is paper-based in combination with being suitable for high-end applications.

[0013] Other objects of the invention may be seen in providing a release liner, which: allows to obtain an optically smooth surface in the final applied adhesive article; and / or allows to maintain the functional benefits of PCK, including dimensional stability, flatness, and moisture resistance; and / or is cost-effective and recyclable, supporting lower environmental impact.

[0014] The present invention meets this object by providing a release liner with a specifically designed paper-based carrier layer.

[0015] In particular, it has been surprisingly found in the course of the present invention that a carrier layer with at least one of the following features can be used to obtain beneficial effects: it consists mostly or completely of short fibers ( / .e., hardwood fibers); it is a65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) machine glazed paper (MG; i.e., dried by guiding over a Yankee cylinder); it has a coating on the glazed side, which coating may contain an organic and / or inorganic filler, in particular a mineral filler. The coating may particularly contains an inorganic filler, a binder, and optionally one or more additives. The coating may be applied in one, two, or more layers or steps. Thus, the coating may for example be a one-layer coating, a two-layer coating, a three-layer coating, etc.

[0016] The filler used in the coating may be selected from one or more of the following: clay; kaolin; calcined kaolin; calcium carbonate, in particular precipitated and / or ground calcium carbonate; gypsum; talc; titanium dioxide.

[0017] It is preferable, if all three of the above-mentioned features, namely the choice of fiber composition, the machine glazing, and the coating, are provided in the release liner. This result is especially surprising, as it is known that conventional CCK release liners are typically not suitable for high-end applications. However, the combination of this specific paper type used as a carrier layer and coating has shown to exert extraordinary improvements of the final product’s surface evenness.

[0018] The release liner may further comprise a release layer, which particularly is an outer layer of the release liner. The release layer is generally designed to reduce adhesion between the liner and an adjacent layer of the adhesive article, typically an adhesive layer, such as a pressure-sensitive adhesive (PSA) layer. The release layer typically contains silicone, and its specific composition may be adapted to the characteristics of the adhesive.

[0019] A thermoplastic polymer layer, for example comprising or consisting of a polyolefin, such as polyethylene (PE) and / or polypropylene (PP), may be arranged between the release layer and the carrier layer.

[0020] An outer thermoplastic polymer layer, for example comprising or consisting of a polyolefin, such as polyethylene (PE) and / or polypropylene (PP), may be arranged on the side of the carrier layer, which opposes the release layer. The outer thermoplastic polymer layer may provide an outer surface of the release liner.65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0021] The release layer and the thermoplastic layer, if provided, may be structured or unstructured. Structured layers may be obtained by embossing the layers using a roll having a specific surface pattern, such as described in the published European Patent Application EP 3838591 A1. Such structure will transfer to the adhesive article and airegress functionality and / or repositionability.

[0022] As mentioned above, the carrier layer particularly consists mostly or completely of short fibers. Such short fibers are typically obtained from hardwood (deciduous tree species) with common hardwood sources include birch, aspen, maple, beech, eucalyptus, and acacia.

[0023] The short fibers contained in the carrier layer are in particular Kraft fibers, i.e., they are obtained from pulp made by the sulphate or Kraft process.

[0024] Paper is generally produced from a suspension containing water, a fibrous composition, and one or more additives, which are preferably non-fibrous, using techniques commonly known in the art of papermaking.

[0025] In the carrier layer of the present invention, the fibrous composition used for its preparation may thus consist of a mix between short fibers and long fibers (i.e., softwood fibers).

[0026] After forming a web from the suspension by de-watering, the web is dried to obtain the carrier layer. During drying, the web may be guided over a heated highly polished cylinder called a Yankee cylinder, which imparts a high-gloss surface to one side of the paper. Thus, a so-called machine glazed (MG) paper can be obtained, which has one glazed side and one unglazed side, the latter remaining relatively rough.

[0027] After machine glazing, the coating may be applied to the glazed side of the paper, which may particularly be inline, i.e., within the paper-making machine, or offline.65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) Before and / or after coating, the coated paper may be calendered. Optionally, the paper may be calendered without the machine glazing step.

[0028] Further aspects of the invention relate to a method for producing a release liner as described herein as well as to a system, which contains a release liner as described herein with an adhesive article attached thereto.

[0029] The invention particularly relates to a release liner comprising or consisting of: a carrier substrate; a release layer; and a thermoplastic polymer layer arranged between the carrier substrate and the release layer.

[0030] It is preferably provided that: the carrier substrate is paper formed from a suspension consisting of: water; a fibrous composition; optionally at least one, particularly non-fibrous, additive; wherein at least 20 wt.-%, such as at least 40 wt.-%, of the dry weight of the fibrous composition is hardwood pulp; the carrier substrate is machine glazed paper, wherein the glazed side faces the release layer; and that the glazed side has a coating, which coating contains an organic and / or inorganic filler, and in particular contains an inorganic filler, a binder, and optionally other additives.

[0031] It may be provided that the hardwood pulp is derived from one or more deciduous tree species and has a number average fiber length of below 1.2 mm determined in accordance with ISO 16065-2:2014.

[0032] It may be provided that the coating has a basis weight of from 2.0 g / m2to 20 g / m2, and / or wherein the filler of the coating is selected from one or more of the following: clay; kaolin; calcined kaolin; calcium carbonate, in particular precipitated and / or ground calcium carbonate; gypsum; talc; titanium dioxide.

[0033] It may be provided that the carrier substrate together with the coating is calendered.

[0034] It may be provided that the release layer comprises a silicone, and wherein the release layer has a basis weight of from 0.3 g / m2to 3.0 g / m2.65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) It may be provided that an outer thermoplastic polymer layer is arranged on unglazed side of the carrier substrate.

[0035] It may be provided that the thermoplastic polymer layer and / or the outer thermoplastic polymer layer, if provided, has a basis weight of from 5.0 g / m2to 50 g / m2.

[0036] It may be provided that the thermoplastic polymer layer and / or the outer thermoplastic polymer layer, if provided, comprises or consists of polyethylene and / or polypropylene.

[0037] It may be provided that the hardwood pulp in the fibrous composition is Kraft pulp, and / or wherein the carrier substrate has a basis weight of between 40 g / m2and 150 g / m2determined in accordance with ISO 536:2019.

[0038] It may be provided that the carrier substrate on the glazed side has a Bendtsen roughness of at most 20 mL / min, in particular at most 15 mL / min, determined in accordance with ISO 8791-2:2013, and / or wherein the carrier substrate on the glazed side has a Parker Print-Surf roughness of at most 3.0 pm, in particular at most 1.5 pm, determined in accordance with ISO 8791-4:2021.

[0039] The invention particularly also relates to a system comprising or consisting of: a release liner as described herein; and an adhesive article.

[0040] Preferably, it is provided that the adhesive article comprises or consists of a facestock and an adhesive layer, the adhesive layer being in direct contact with the release layer of the release liner.

[0041] It may be provided that the facestock is a polymer layer, comprising or consisting of for example polyvinylchloride, and / or wherein the adhesive layer comprises or consists of a pressure sensitive adhesive, such as an acrylic polymer.

[0042] The invention particularly also relates to a method for producing a release liner, in particular a release liner as described herein, the method comprising the following steps:65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) (a) providing a carrier substrate, the carrier substrate being paper made by: forming a web from a suspension consisting of: water; a fibrous composition; optionally at least one, particularly non-fibrous, additive; wherein at least 20 wt.-%, such as at least 40 wt.-%, of the dry weight of the fibrous composition is hardwood pulp; and guiding the web over a Yankee cylinder to form a glazed side of the web

[0043] (b) coating the glazed side of the carrier substrate with a coating, which coating contains an organic and / or inorganic filler, and is in particular a clay coating;

[0044] (c) applying a thermoplastic polymer layer on the side of the coating; and

[0045] (d) applying a release layer on the side of the coating, such that the thermoplastic polymer layer is arranged between the carrier substrate and the release layer.

[0046] The invention particularly also relates to a coating is applied to the carrier substrate using one of the following: blade coating; air knife coating; roll coating; curtain coating, and / or wherein the thermoplastic polymer layer is applied using extrusion coating.

[0047] The invention particularly also relates to a carrier substrate is calendered, in particular after having been coated with the coating.

[0048] Additional features of the invention will become apparent from the claims, the drawing, and the description of the exemplary embodiment.

[0049] In the drawing, the following features are shown, except otherwise mentioned or clearly apparent from the context: carrier substrate 1 ; release layer 2; thermoplastic polymer layer 3; glazed side 4 (of the carrier substrate 1 ); coating 5; unglazed side 6 (of the carrier substrate 1); outer thermoplastic polymer layer 7; adhesive article 8; release liner 9; facestock 10; adhesive layer 11.

[0050] Fig. 1 shows a schematic drawing of the layered structure of a system, containing a release liner 9 according to a first inventive embodiment and an adhesive article 8.

[0051] As can be seen in Fig. 1 , the release liner 9 contains the following layers: Outer thermoplastic polymer layer 7 - carrier layer 1 -coating 5 - thermoplastic polymer layer65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) 3 - release layer 2, wherein outer thermoplastic polymer layer 7 and release layer 2 form outer layers of the release liner 9.

[0052] The adhesive article 8 contains a facestock 10 and an adhesive layer 11 , wherein the adhesive layer 11 is in contact with the release layer 2.

[0053] Preparation of release liner and system according to inventive embodiments:

[0054] In a first exemplary embodiment, the carrier layer 1 is Kraft paper made from 60% hardwood fiber with a number average fiber length of approx. 0.7 mm determined in accordance with ISO 16065-2:2014. The remainder of the fiber to 100% is softwood fiber.

[0055] The Kraft paper is machine glazed, so to have a smooth glazed side 4 and a relatively rough unglazed side 6. The glazed side has a Bendtsen roughness of approx. 10 mL / min determined in accordance with ISO 8791-2:2013 and a Parker Print-Surf roughness of approx. 0.98 pm determined in accordance with ISO 8791-4:2021. The carrier layer 1 has a grammage of 78.5 g / m2determined in accordance with ISO 536:2019.

[0056] The coating 5 is applied to the carrier layer’s 1 glazed side 4 in two steps ( / .e., there are two coating layers, not shown in Fig. 1) and predominantly consists of kaolin clay in combination with a styrene acrylate as polymeric binder and has a basis weight of approx. 11.5 g / m2. The first coating layer has a basis weight of approx. 3.5 g / m2, while the second coating layer has a basis weight of approx. 8.0 g / m2. The first coating layer is applied inline directly to the paper web, while the second coating layer is applied offline.

[0057] The outer thermoplastic polymer layer 7 is applied to the unglazed side 6 of the carrier layer 1 , while the thermoplastic polymer layer 3 is applied to the coating 5. Both thermoplastic polymer layers 3, 7 are extrusion-coated PE layers with a basis weight of approx. 25-30 g / m2.65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) The release layer 2 is applied to the thermoplastic polymer layer 3 and contains silicone to provide release functionality. It has a basis weight of approx. 0.95 g / m2

[0058] The release layer 2 and the thermoplastic layer 3 are structured, wherein the structure has been produced by embossing the release liner 1 using a structured roll, as described in the published European Patent Application EP 3838591 A1. Such structuring imparts air-egress functionality as well as repositionability to the adhesive article.

[0059] After unwinding the release liner 9, this liner is coated with the material for the adhesive layer 11 and either dryed or cured. This layer is then laminated onto the facestock 10. Facestock 10 and adhesive layer 11 form the adhesive article 8 attached to the release liner 9. In this exemplary embodiment, the adhesive article 8 consists of a facestock 10 being black glossy PVC and an adhesive layer 11 being a solvent-based PSA. The specific facestock 10 of this embodiment was chosen for its sensitivity to surface distortion, making it ideal for visually detecting surface transfer effects.

[0060] Release liners 2 according to a second and third embodiment were prepared in a similar way as the first embodiment, with the only difference that other surface structures are applied.

[0061] Preparation of release liner and system according to non-inventive embodiments:

[0062] In addition to the inventive embodiments of the release liner and the system, non-inventive embodiments were prepared.

[0063] They were prepared in a similar way as the inventive first to third embodiment, with the only difference the carrier layer 1 is a standard Kraft paper made from softwood pulp, without machine glazing, and without coating. The surface structures correspond to those of the respective inventive embodiment, i.e., the first inventive embodiment has the same structuring as the first non-inventive embodiment etc.

[0064] Comparison of inventive and non-inventive embodiments65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0065] The adhesive articles 8 from the in total six embodiments (three inventive, three non-inventive) peeled from the release liners 2 and applied to flat glass panels, where they were allowed to rest for over 24 hours to enable adhesive flow and leveling. Surface quality was then assessed both visually and through instrumental analysis using a Wave-Scan device (BYK-Gardner), a standardized tool in the automotive industry for evaluating surface waviness and gloss uniformity.

[0066] The Wave-Scan device simulates human visual perception by optically scanning surface texture in a manner similar to the human eye. A laser point light source illuminates the test surface at a 60° angle, while a detector captures the reflected light at the corresponding opposite angle. As the instrument is moved across the surface, it performs a point-by-point measurement of the surface’s optical profile over a defined distance.

[0067] The device categorizes surface structures based on their wavelength or size, using mathematical filter functions to divide the signal into multiple ranges. These filters are designed to reflect the eye’s resolution at various viewing distances, enabling a quantified analysis of surface waviness and appearance, often referred to as orange peel.

[0068] In the present evaluation, Wc (short wave, wavelength 1 - 3mm) and LW (longwave, wavelength 1.2 - 12 mm) parameters were determined, with lower values indicating better surface appearance. The data was evaluated using Minitab statistical software. The results are given in Table 1 below.

[0069] Table 1: Wave-Scan results

[0070]

[0071] 65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0072]

[0073] Across all embodiments, the inventive release liner 2 consistently yielded better surface quality than the standard paper, both quantitatively via Wave-Scan as well as in subjective tester feedback. These results validate the effectiveness of the present invention in reducing visual surface defects, confirming its utility for optically demanding adhesive article applications.

Claims

65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) Claims1. A release liner (9) comprising or consisting of: a carrier substrate (1 ); a release layer (2); and a thermoplastic polymer layer (3) arranged between the carrier substrate (1) and the release layer (2); characterized- in that the carrier substrate (1 ) is paper formed from a suspension consisting of: water; a fibrous composition; optionally at least one, particularly non-fibrous, additive; wherein at least 20 wt.-%, such as at least 40 wt.-%, of the dry weight of the fibrous composition is hardwood pulp, - in that the carrier substrate (1 ) is machine glazed paper, wherein the glazed side (4) faces the release layer (2); and- in that the glazed side (4) has a coating (5), which coating (5) contains an organic and / or inorganic filler, and in particular contains an inorganic filler, a binder, and optionally other additives.

2. The release liner according to claim 1 , wherein the hardwood pulp is derived from one or more deciduous tree species and has a number average fiber length of below 1.2 mm determined in accordance with ISO 16065-2:2014.

3. The release liner according to claim 1 or 2, wherein the coating (5) has a basis weight of from 2.0 g / m2to 20 g / m2, and / or wherein the filler of the coating is selected from one or more of the following: clay; kaolin; calcined kaolin; calcium carbonate, in particular precipitated and / or ground calcium carbonate; gypsum; talc; titanium dioxide.

4. The release liner according to any of claims 1 to 3, wherein the carrier substrate (1) together with the coating (5) is calendered.

5. The release liner according to any of claims 1 to 4, wherein the release layer (2) comprises a silicone, and wherein the release layer (2) has a basis weight of from 0.3 g / m2to 3.0 g / m2.65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) 6. The release liner according to any of claims 1 to 5, wherein an outer thermoplastic polymer layer (7) is arranged on unglazed side (6) of the carrier substrate (1 ).

7. The release liner according to any of claims 1 to 6, wherein the thermoplastic polymer layer (3) and / or the outer thermoplastic polymer layer (7), if provided, has a basis weight of from 5.0 g / m2to 50 g / m2.

8. The release liner according to any of claims 1 to 7, wherein the thermoplastic polymer layer (3) and / or the outer thermoplastic polymer layer (7), if provided, comprises or consists of polyethylene and / or polypropylene.

9. The release liner according to any of claims 1 to 8, wherein the hardwood pulp in the fibrous composition is Kraft pulp, and / or wherein the carrier substrate (1) has a basis weight of between 40 g / m2and 150 g / m2determined in accordance with ISO 536:2019.

10. The release liner according to any of claims 1 to 9, wherein the carrier substrate (1) on the glazed side (4) has a Bendtsen roughness of at most 20 mL / min, in particular at most 15 mL / min, determined in accordance with ISO 8791-2:2013, and / or wherein the carrier substrate (1) on the glazed side (4) has a Parker Print- Surf roughness of at most 3.0 pm, in particular at most 1.5 pm, determined in accordance with ISO 8791-4:2021.

11. A system comprising or consisting of:- a release liner (9) according to any of claims 1 to 10, and- an adhesive article (8),the adhesive article (8) comprising or consisting of a facestock (10) and an adhesive layer (11 ), the adhesive layer (11 ) being in direct contact with the release layer (2) of the release liner (9).

12. The system according to claim 11, wherein the facestock (10) is a polymer layer, comprising or consisting of for example polyvinylchloride, and / or wherein the65489 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) adhesive layer (11 ) comprises or consists of a pressure sensitive adhesive, such as an acrylic polymer.

13. A method for producing a release liner (9), in particular a release liner (9) according to any of claims 1 to 10, the method comprising the following steps: a. providing a carrier substrate (1 ), the carrier substrate (1 ) being paper made by:- forming a web from a suspension consisting of: water; a fibrous composition; optionally at least one, particularly non-fibrous, additive; wherein at least 20 wt.-%, such as at least 40 wt.-%, of the dry weight of the fibrous composition is hardwood pulp; and- guiding the web over a Yankee cylinder to form a glazed side (4) of the webb. coating the glazed side (4) of the carrier substrate (1 ) with a coating (5), which coating (5) contains an organic and / or inorganic filler, and is in particular a clay coating;c. applying a thermoplastic polymer layer (3) on the side of the coating (5);andd. applying a release layer (2) on the side of the coating (5), such that the thermoplastic polymer layer (3) is arranged between the carrier substrate (1 ) and the release layer (2).

14. The method according to claim 13, wherein the coating (5) is applied to the carrier substrate (1) using one of the following: blade coating; air knife coating; roll coating; curtain coating, and / or wherein the thermoplastic polymer layer (3) is applied using extrusion coating.

15. The method according to claim 13 or 14, wherein the carrier substrate (1) is calendered, in particular after having been coated with the coating (5).

Images