Release liner with partial silicone coating

Description

[0001] The invention relates to the technical field of release liners, which are frequently used to protect pressure-sensitive adhesives. More specifically, the invention relates to a release liner that is fully siliconized on one side and partially siliconized on the other. This allows, for example, the application of a tab to the partially siliconized side with minimal impairment of the release action.

[0002] Adhesive tapes coated with adhesive on one or both sides are usually wound into a roll in the form of an Archimedean spiral at the end of the manufacturing process. To prevent the adhesive layers of double-sided tapes from coming into contact with each other, or to ensure easier unwinding of single-sided tapes, the adhesive is covered with a release liner (also known as a release agent) before the tape is wound. Such release liners are known to those skilled in the art as release liners or simply liners. Besides covering single- or double-sided tapes, liners are also used to cover labels.

[0003] Release liners also prevent contamination of the adhesive before application. Additionally, the type and composition of the release materials used in release liners can be adjusted to allow the tape to be unwound with the desired force (easy or difficult). For double-sided adhesive tapes, the release liners further ensure that the correct adhesive side is exposed first during unwinding.

[0004] A liner or release liner is not a component of an adhesive tape or label, but merely an aid in their manufacture, storage, or further processing. Furthermore, unlike an adhesive tape backing, a liner is not permanently bonded to an adhesive layer.

[0005] Industrially, release liners are paper or film substrates coated with an adhesive coating (also known as a deadhesive or anti-adhesive compound) to reduce the adhesion of adhesive products to these surfaces (release function). A variety of substances can be used as adhesive coatings, also called release coatings: waxes, fluorinated or partially fluorinated compounds, and especially silicones and various copolymers containing silicone. In recent years, silicones have become widely established as release materials in adhesive tape applications due to their good processability, low cost, and broad property profile.

[0006] In addition, liners with polyolefin separating layers have also come into focus.

[0007] EP 2 354 203 A1 describes an adhesive tape that can be used at temperatures of at least 90 °C and comprises an adhesive layer with a release liner applied to it. The release liner itself comprises a base layer containing a polyolefin resin and a release layer containing LDPE, which is in contact with the adhesive layer.

[0008] EP 2 025 507 A1 describes a release agent containing an ethylene multi-block copolymer, wherein the ethylene multi-block copolymer consists of hard segment blocks containing at least 95% (w / w) ethylene and a comonomer, and of soft segment blocks containing ethylene and a comonomer; and wherein the proportion of comonomer in the soft segment blocks is between 10 and 20 mol% and the proportion of hard segment blocks in the ethylene multi-block copolymer is at most 45% (w / w).

[0009] EP 2 298 844 A1 describes an acrylate adhesive tape with a release liner comprising an LDPE surface layer; a resin mixture containing LDPE and HDPE as resin components as a second surface layer; and an HDPE intermediate layer. EP 3 278 984 A1 discloses a release liner for use on pressure-sensitive adhesives, comprising: an outer, continuous silicone release layer (SR1); a film (BF) containing at least 80% by weight, based on the total weight of the film, a polyolefin or polyethylene terephthalate; and a layer (PER) containing at least 80% by weight, based on the total weight of the layer (PER), polyethylene.

[0010] EP 2 851 405 A1 discloses a release liner for pressure-sensitive adhesives, e.g., based on silicone, comprising an outer, continuous silicone release layer; a single-layer carrier layer based on PET, PE or PP; a further silicone release layer located on the other outer surface, which has defects formed by local inhibition of the silicone crosslinking reaction, which can take the form of circles, lamellae, etc.

[0011] To facilitate the removal of a release liner from the adhesive tape, which usually takes place immediately before application, the liners are sometimes provided with gripping aids, so-called "tabs," on their reverse side (the unwind side). These facilitate peeling the liner because it is not necessary to first penetrate between the liner and the adhesive to grasp a section of the liner and then continue peeling it off; rather, simply gripping the tab is sufficient to remove the liner without difficulty. For this purpose, the tabs are welded or glued to the reverse side of the liner in such a way that a grippable part of the tab is not connected to the liner but protrudes from its surface or rests loosely on it. Such gripping aids are described, for example, in EP 2 426 185 A1.

[0012] DE 38 44 250 A1 describes a method for coating a substrate surface with a release agent in such a way that partial areas with different release force values ​​are formed in the form of a grid. For this purpose, release layers are formed on partial areas of the substrate surface by printing the release material in a defined geometric manner, while other partial areas are left in their original state.

[0013] Similar release items are the subject of US 5,061,535.

[0014] DE 2016 213 661 A1 describes a thermally stable release liner that can be equipped with a handle. This release liner comprises an outer silicone release layer, a layer of several polyolefin-based layers, and an outer polyethylene-based layer bonded to the next layer in the liner structure by an adhesive. As has been shown, using the liner described in DE 2016 213 661 A1 presents problems with regard to the required pull-off forces when using adhesives that also provide good adhesion to low-energy substrates.

[0015] As has been shown, the ability to easily attach a tab to a liner and good release properties against strong adhesives are often conflicting requirements. Good release properties often make it difficult to apply a tab to the liner, and vice versa.

[0016] The object of the invention is to remedy this situation and to provide a release liner that has a sufficient separating effect on both sides, especially against pressure-sensitive adhesives with good adhesive strength even on low-energy substrates, and is also provided with a gripping aid and can be easily peeled off the adhesive by means of this aid.

[0017] The solution to the problem is based on the fundamental idea of ​​the invention, namely to provide a liner with a special layer structure and varying siliconization. A first and general object of the invention is a release liner for use on pressure-sensitive adhesives, which comprising an external, continuous silicone release layer (SR1); a film (BF) containing at least 80% by weight, based on the total weight of the film, of a polyolefin or polyethylene terephthalate; a layer (PER) containing at least 80% by weight, based on the total weight of the layer (PER), of polyethylene; and an external silicone release layer (SR2) lying on the layer (PER), wherein the layer (SR2) has defects such that 46 to 95% of the surface of the layer (PER) is covered by the layer (SR2).

[0018] The liners according to the invention exhibit good release forces, particularly against pressure-sensitive adhesives with relatively high adhesive strength on low-energy substrates, and are therefore easy to handle with these substrates. This applies both to removing the liner from the pressure-sensitive adhesive – the actual "peeling" – and to "unwinding" a composite of pressure-sensitive adhesive and release liner wound onto a disc or spool, whereby the pressure-sensitive adhesive, in the unwound state, rests particularly on the continuously siliconized side of the liner. On the side of the liners according to the invention coated with the silicone layer (SR2), one or more tabs, e.g., made of a PET / PE or an aluminum / PET / PE composite, could be thermally welded or bonded with a silicone-based adhesive.

[0019] According to the invention, a "pressure-sensitive adhesive" is understood to be, as is generally accepted, a substance that is permanently sticky and adhesive, particularly at room temperature. A characteristic of a pressure-sensitive adhesive is that it can be applied to a substrate by pressure and adheres there, whereby the pressure to be applied and the duration of this pressure are not defined in detail. In some cases, depending on the exact type of pressure-sensitive adhesive, the temperature and humidity, and the substrate, the application of a short-term, minimal pressure, not exceeding a light touch for a brief moment, is sufficient to achieve the adhesive effect; in other cases, a longer duration of high pressure may be necessary.

[0020] Pressure-sensitive adhesives possess special, characteristic viscoelastic properties that result in their permanent tackiness and bonding strength. A defining characteristic is that when mechanically deformed, they undergo both viscous flow processes and the development of elastic restoring forces. The relative proportions of these two processes depend on the precise composition, structure, and degree of cross-linking of the pressure-sensitive adhesive, as well as the rate and duration of deformation and the temperature.

[0021] The proportion of viscous flow is necessary to achieve adhesion. Only the viscous components, caused by macromolecules with relatively high mobility, enable good wetting and flow onto the substrate to be bonded. A high proportion of viscous flow leads to high tack (also known as surface tack) and thus often also to high adhesive strength. Highly cross-linked systems, crystalline or glassy polymers, are generally not tacky or at least only slightly tacky due to a lack of flowable components.

[0022] The elastic restoring forces are necessary to achieve cohesion. They are generated, for example, by very long-chain and highly entangled macromolecules, as well as by physically or chemically cross-linked macromolecules, and enable the transmission of forces acting on an adhesive bond. This allows an adhesive bond to withstand a sustained load, such as continuous shear stress, to a sufficient degree over an extended period.

[0023] To describe and quantify the degree of elastic and viscous components, as well as their ratio, more precisely, the storage modulus (G') and loss modulus (G"), which can be determined using Dynamic Mechanical Analysis (DMA), can be used. G' is a measure of the elastic component, and G'' is a measure of the viscous component of a material. Both quantities depend on the deformation frequency and the temperature.

[0024] The parameters can be determined using a rheometer. The material under investigation is subjected, for example, to a sinusoidally oscillating shear stress in a plate-plate arrangement. In shear-stress controlled devices, the deformation is measured as a function of time, along with the time lag of this deformation relative to the application of the shear stress. This time lag is called the phase angle δ.

[0025] The storage modulus G' is defined as follows: G' = (τ / γ) • cos(δ) (τ = shear stress, γ = deformation, δ = phase angle = phase shift between the shear stress and deformation vectors). The definition of the loss modulus G" is: G" = (τ / γ) • sin(δ) (τ = shear stress, γ = deformation, δ = phase angle = phase shift between the shear stress and deformation vectors).

[0026] A material is considered to be adhesive, and is defined as adhesive within the meaning of the invention, if at room temperature (here, by definition, at 23°C), in the deformation frequency range from 10°C to 10⁻¹ rad / sec, G' lies at least partially within the range of 10⁻³ to 10⁻⁷ Pa, and if G" also lies at least partially within this range. "Partially" means that at least a section of the G' curve lies within the window defined by the deformation frequency range from 10⁻¹ to 10⁻¹ rad / sec (abscissa) and the range of G' values ​​from 10⁻³ to 10⁻⁷ Pa (ordinate). The same applies to G" accordingly.

[0027] An "outer layer" is understood to mean that the layer in question forms one of the two layers that outwardly define the layer structure of the liner, i.e., there is no further layer belonging to the liner above this layer.

[0028] A "continuous layer" is understood to be one that has no defects but completely covers the layer beneath it. Thus, a continuous layer contains no area where its substrate is exposed. In this sense, a continuous layer can also be described as a "continuous layer".

[0029] The silicone release layer (SR1) is preferably based on a crosslinkable silicone system. These crosslinkable silicone systems include mixtures of crosslinking catalysts or initiators and so-called thermally curable, condensation- or addition-curing polysiloxanes, or radiation-induced crosslinking polysiloxanes. The silicone release layer (SR1) is preferably based on a radiation- (UV or electron beam), condensation-, or addition-curing system, particularly preferably on an addition-curing system.

[0030] The silicone release layer can be traced back to solvent-containing and / or solvent-free systems; preferably, it can be traced back to a solvent-containing system.

[0031] Silicone-based release agents with an addition-curing mechanism can generally be cured by hydrosilylation. The formulations for producing these release agents typically include the following components: a linear or branched polydiorganosiloxane containing alkenyl groups, a polyorganohydrogen siloxane crosslinking agent, and a hydrosilylation catalyst.

[0032] Platinum or platinum compounds, such as the Karstedt catalyst (a Pt(0) complex compound), have proven particularly effective as catalysts for addition-curing silicon systems (hydrosilylation catalysts).

[0033] More specifically, such addition-curing release coatings can include the following components: a) A linear or branched dimethylpolysiloxane consisting of approximately 80 to 200 dimethylpolysiloxane units, terminated at the chain ends by vinyldimethylsiloxy units. Typical examples include solvent-free, addition-curing silicone oils with terminal vinyl groups, such as Dehesive® < 921 or 610, both commercially available from Wacker-Chemie GmbH; b) A linear or branched crosslinker, which either has only methylhydrogensiloxy units in the chain (homopolymer crosslinker) or is composed of methylhydrogensiloxy and dimethylsiloxy units (copolymer crosslinker), with the chain ends saturated with either trimethylsiloxy or dimethylhydrogensiloxy groups.Typical representatives of this product class are, for example, hydrogen polysiloxanes with a high content of reactive Si-H, such as the crosslinkers V24, V90, or V06, which are commercially available from Wacker-Chemie GmbH; c) a silicone MQ resin which, in addition to the commonly used trimethylsiloxy units, also has vinyldimethylsiloxy units as its M-unit. Typical representatives of this group are, for example, the release force regulators CRA®< 17 or CRA®< 42, commercially available from Wacker-Chemie GmbH; d) a silicone-soluble platinum catalyst, such as a platinum divinyltetramethyldisiloxane complex, which is commonly referred to as a Karstedt complex and is commercially available from Wacker-Chemie GmbH, for example, under the name catalyst OL.

[0034] Silicone-based systems for the production of release coatings can be purchased commercially, for example from Dow Corning, Wacker or Momentive.

[0035] One example is "Dehesive®< 914", which contains a vinyl polydimethylsiloxane, the crosslinker "Crosslinker V24", a methyl hydrogen polysiloxane, and the catalyst "Catalyst Ol", a platinum catalyst in polydimethylsiloxane. This system is available from Wacker-Chemie GmbH.

[0036] Furthermore, the addition-curing silicone release system "Dehesive ®< 940A" from Wacker Chemie with an associated catalyst system can be used, for example.

[0037] Typically, a silicone release system is applied in its uncrosslinked state and subsequently crosslinked.

[0038] Of the silicones mentioned, addition-curing silicones are the most economically important. However, an undesirable property of these systems is their sensitivity to catalyst poisons, such as heavy metal, sulfur, and nitrogen compounds (see "Chemical Engineering, Processes and Products" by R. Dittmeyer et al., Volume 5, 5th edition, Wiley-VCH, Weinheim, Germany, 2005, Chapter 6-5.3.2, page 1142). Generally, electron donors can be considered platinum poisons (A. Colas, Silicone Chemistry Overview, Technical Paper, Dow Corning). Accordingly, phosphorus compounds such as phosphines and phosphites are also considered platinum poisons. The presence of catalyst poisons leads to the crosslinking reaction between the different components of a silicone release agent no longer occurring or only occurring to a limited extent.Therefore, the presence of catalyst poisons, especially platinum poisons, is usually strictly avoided in the production of anti-adhesive silicone coatings.

[0039] Special versions of the silicone systems include polysiloxane block copolymers, for example with a urea block, as offered by Wacker under the trade name "Geniomer", or separation systems made of fluorosilicones, which are used particularly in adhesive tapes with silicone adhesives.

[0040] Furthermore, photoactive catalysts, so-called photoinitiators, can also be used in combination with UV-curable, cationically crosslinking siloxanes based on epoxy and / or vinyl ethers, or with UV-curable, radically crosslinking siloxanes such as acrylate-modified siloxanes. The use of electron-beam curable silicone acrylates is also possible.

[0041] Photopolymerizable organopolysiloxane compounds can also be used. Examples include compounds that are crosslinked by the reaction between organopolysiloxanes containing hydrocarbon residues directly bonded to silicon atoms and substituted with (meth)acrylate groups, in the presence of a photosensitizer (see, e.g., EP 0 168 713 B1 or DE 38 20 294 C1). Compounds in which the crosslinking reaction occurs between organopolysiloxanes containing hydrocarbon residues directly bonded to silicon atoms and organopolysiloxanes with vinyl groups directly bonded to silicon atoms, also in the presence of a photosensitizer, are also suitable. Such compounds are described, for example, in US 4,725,630.

[0042] When using organopolysiloxane masses, such as those described in DE 33 16 166 C1, which have hydrocarbon residues directly bonded to silicon atoms and substituted with epoxy groups, the crosslinking reaction is induced by the release of a catalytic acid obtained through the photodecomposition of added onium salt catalysts. Other organopolysiloxane masses that can be cured by a cationic mechanism are materials that, for example, have propenyloxysiloxane end groups.

[0043] Depending on the intended use, the silicone systems may also contain other additives, such as stabilizers or flow aids.

[0044] The release liner according to the invention comprises a film (BF). Preferably, the film (BF) is selected from the group consisting of polyamide, polyimide, polyurethane, polymethyl methacrylate, polylactic acid, polystyrene, polyolefin, and polyester films. Particularly preferred is the film (BF) a polyolefin film or a polyester film, in particular a polyethylene film, a polypropylene film, or a polyethylene terephthalate film, or a film made of a blend of two or more polyolefins, for example, a film made of a polyethylene-polypropylene blend.

[0045] Preferably the film (BF) contains at least 80% by weight of polyethylene and polypropylene.

[0046] In one embodiment, the film (BF) is a multilayer film, preferably a coextrudate of two or more layers.

[0047] The film (BF) is particularly preferably a three-layer film, the central layer of which contains at least 60 wt.%, more preferably at least 65 wt.%, and in particular at least 70 wt.%, for example at least 75 wt.%, polypropylene, in each case based on the total weight of the layer. It has been shown that such polypropylene contents have an advantageous effect on the thermal stability of the central layer, the three-layer film, and ultimately the release liner according to the invention. The central layer particularly preferably consists of polypropylene mixed with rubber to a maximum of 15 wt.%, more preferably at a maximum of 12 wt.%, and with colorants to a maximum of 15 wt.%, more preferably at a maximum of 10 wt.%. The central layer of the film (BF) can be colored by means of pigments or organic dyes added in the form of a color masterbatch.

[0048] In particular, the central layer of the film (BF) consists of a heterophasic polypropylene copolymer with a rubber content of at most 15 wt.% and a colorant content of at most 15 wt.%, more preferably with a rubber content of at most 12 wt.% and a colorant content of at most 10 wt.%. The rubber is preferably an EPM rubber.

[0049] The central layer is preferably a polypropylene film with a melting point of at least 160 °C. The polypropylene of the central layer is preferably a so-called heterophasic PP copolymer (HECO PP, Impact PP). Such a copolymer exhibits particularly high temperature stability. The temperature stability resulting from the melting point is comparable to that of pure homo-PP. However, a heterophasic PP copolymer is characterized by better flexibility, lower strength, and reduced brittleness. This is achieved by copolymerizing homo-PP with an epoxy rubber.

[0050] The film (BF), particularly in the embodiment as a three-layer film, preferably comprises two outer layers of a polyethylene-polypropylene blend. The two outer layers preferably each contain at least 25 wt.%, more preferably at least 30 wt.%, in particular at least 35 wt.% polyethylene, and at least 20 wt.%, more preferably at least 30 wt.%, in particular at least 50 wt.% polypropylene, each based on the total weight of the layer. Most preferably, the two outer layers each contain at least 70 wt.%, more preferably at least 80 wt.%, in particular at least 90 wt.%, each based on the total weight of the layer, a mixture of polyethylene and polypropylene in a PP : PE weight ratio of 40 : 60 to 80 : 20, in particular from 50 : 50 to 70 : 30, for example from 55 : 45 to 65 : 35.Particularly preferably, the two outer layers each consist of such a mixture of polypropylene and polyethylene.

[0051] Particularly preferred is the polypropylene of the outer layers of the film (BF) in the embodiment as a multilayer, and especially as a three-layer, film, a heterophasic PP copolymer, e.g., an impact polypropylene. The polyethylene of the outer layers of the film (BF) in the embodiment as a multilayer, and especially as a three-layer, film is preferably an mPE, particularly an LLDPE. The mechanical properties of the multilayer film, especially its strength, can advantageously be fine-tuned by adjusting the blend ratio. The mechanical properties of this composite are particularly affected when the composite is coated with the silicone release compound provided for forming layer (SR1) and / or the compound provided for forming layer (PER).This is particularly advantageous when applying the release liner according to the invention, especially during its machine processing and when using it to bond adhesive tape around curves. The release liner, or at least the described composite material, advantageously exhibits good tensile strength, thus counteracting stretching or overstretching of the adhesive tape during its application and processing. It is noteworthy that the film (BF) achieves a particularly good balance between the two opposing properties of elongation and tensile strength.

[0052] The film (BF) is preferably available as a co-extruded three-layer film. In particular, the composite is produced as blown film, but can also be manufactured as flat film using the flat film process.

[0053] In particular, in the embodiments of the film (BF) as a multilayer structure, the outermost layers preferably have an identical thickness, whereby manufacturing- or process-related deviations are negligible. Such symmetrical structures, unlike asymmetrical structures, do not exhibit the tendency to curl that is frequently observed in the latter, especially under temperature stress.

[0054] Preferably, the raw materials for the production of the film (BF) contain less than 1000 ppm by weight of catalyst poisons with respect to the hydrosilylation catalyst of the layer (SR1). Otherwise, disruptions in the silicone crosslinking can occur, which can impair the release properties of the layer (SR1). Suitable catalyst poisons include, in particular, phosphorus- and nitrogen-containing compounds, e.g., phosphite stabilizers such as Irgafos 168 or Irgafos TNPP, lubricants, or antistatic agents with amide or amine functionalities, e.g., erucamide. Particularly preferred are the raw materials for the production of the film (BF) that are free of catalyst poisons with respect to the hydrosilylation catalyst of the layer (SR).

[0055] In the embodiment of the film (BF) as a three-layer film, the thickness of the central layer is preferably 40 to 80 µm, more preferably 50 to 70 µm, particularly 55 to 65 µm, and most preferably 57 to 63 µm. The thickness of each of the two outer layers is preferably 10 to 30 µm, particularly 15 to 25 µm, and most preferably 17 to 23 µm. The ratio of the thickness of the central layer to that of one of the outer layers is preferably 10:3 to 10:1.

[0056] The liner according to the invention further comprises at least one layer (PER) containing at least 80% by weight of polyethylene. Preferably, the layer (PER) contains at least 90% by weight, more preferably at least 95% by weight, in particular at least 98% by weight, for example at least 99% by weight, in each case based on the total weight of the layer (PER). Most preferably, the layer (PER) consists of polyethylene. In particular, the layer (PER) is a polyethylene film. The layer (PER) can also consist of a blend or a mixture of different polymers.

[0057] Preferably, the polyethylene of the layer (PER) is a low-density polyethylene (LDPE). Particularly preferably, the layer (PER) consists of LDPE, and in particular, the layer (PER) is an LDPE film. In particular, the layer (PER) is an LDPE film with a thickness of 10 to 45 µm, most preferably with a thickness of 20 to 40 µm, for example, 24 to 33 µm.

[0058] Preferably, the polyethylene film (PER) of the layer is a blown film. The polyethylene film (PER) can also be a multi-layer film, e.g., a three-layer film. In the latter case, the film consists of three PE layers.

[0059] The LDPE of the layer (PER) preferably has a density of less than 0.925 g / cm³, and particularly less than 0.92 g / cm³. A layer (PER) with this properties exhibits particularly low release forces and peel forces compared to many pressure-sensitive adhesives. The layer (PER) therefore also exhibits particularly high heat resistance. This, in turn, advantageously allows for the application of a gripping aid to the layer (PER) by an efficient heat-sealing process. Preferably, a gripping aid is applied to the outward-facing surface of the composite of layers (PER) and (SR2) such that at least part of the gripping aid rests on the layer (PER). Furthermore, it has been shown that the partially present silicone layer also permits the application of the gripping aid using a silicone-based adhesive.It is therefore also preferred that a gripping aid be applied to the outwardly facing surface of the composite layer (PER) and (SR2) such that at least part of the gripping aid rests on layer (SR2). The layer (PER), which is partially covered with the silicone layer (SR2), thus advantageously allows the attachment of a gripping tab in two alternative ways: via a heat-sealing process or via a silicone-based adhesive.

[0060] In one embodiment, the PER layer of the release liner according to the invention is bonded to the film (BF) by an adhesive. This has been shown to result in significantly improved bond strength. Alternative methods for constructing the film (BF) - PER layer sequence include, for example, co-extrusion of the PER layer with the film (BF) or extrusion coating.

[0061] The adhesive can be considered a laminating or bonding adhesive and is not fundamentally limited in the range of adhesives that can be used, as long as the layer (PER) and the film (BF) can be bonded as required.

[0062] In principle, all solvent-based, solvent-free, and water-based adhesives known in the prior art, with different polymer bases such as polyurethane, polyester, polyethylene, or ethylene vinyl acetate, are suitable. A polyurethane-based adhesive is preferred. "Polyurethane-based" here means that a polyurethane or a combination of several polyurethanes forms the main component of the polymer composition of this adhesive, i.e., it constitutes the largest proportion of the polymer composition.

[0063] Solvent-free polyurethane adhesives are available as one- or two-component systems. Further differences can arise from the structure of the polyurethane and the type of crosslinking. The following polymers are frequently preferred: 1-K system: Low molecular weight prepolymers, NCO-terminated, moisture-curing; 2-K system: Prepolymers with NCO end groups + polyols.

[0064] Aromatic isocyanates are frequently used; however, these are problematic in food contact applications because they can lead to the formation of primary aromatic amines. Therefore, aliphatic isocyanates are sometimes used, particularly when UV stability is required. Generally, aromatic isocyanates offer better adhesion and faster curing.

[0065] Polyether polyurethanes generally have higher temperature stability than polyester polyurethanes. However, the polyol component often consists of a mixture of polyester and polyether polyols. Tri- and higher-functionalized polyols are also frequently used to generate additional crosslinking effects, which in turn often results in higher temperature stability.

[0066] The adhesive in the embodiment described here is preferably a polyether-polyurethane-based adhesive, which is based on a solvent-free two-component system. It has proven advantageous to allow a composite of the film (BF) and the layer (PER) produced using the adhesive to rest for several days in order to allow the full bond strength to be reached. It has also proven advantageous to subject the surfaces to be bonded to corona pretreatment before bonding.

[0067] A particular advantage of bonding the film (BF) to the layer (PER) lies in the higher bond strength compared to coextrusion. Coextrusion of these layers has proven possible, but resulted in lower bond strengths. Some improvement in bond strength can be achieved by modifying one or both layers with so-called adhesion promoters, e.g., maleic acid-modified polymers, or by using an additional adhesion promoter layer, the so-called tie layer. Despite these measures, the bond strength is insufficient for the application in some cases, such that delamination of the layer composite (PER)-(SR2) has been observed when attempting to peel the liner off the adhesive tape using a tab.

[0068] The release liner according to the invention further comprises an outer silicone release layer (SR2) lying on the layer (PER). This layer has defects such that 46 to 95%, preferably 60 to 92%, particularly preferably 80 to 90% of the surface of the layer (PER) is covered by the layer (SR2).

[0069] "Defects" are understood to mean that the layer in question is partially perforated and thus has areas completely surrounded by the layer material, or—in the marginal area—extending to the edge of the liner, where the layer material (SR2) is absent. At these points, the outer surface of the layer (PER) is exposed. In this sense, the layer (SR2) can also be described as a "discontinuous layer".

[0070] The proportion of the layer (PER) covered by the layer (SR2) is determined by microscopy (100x magnification), manual 3-point area measurement and automatic area calculation.

[0071] The material of the silicone release layer (SR2) is essentially the same as that of the silicone release layer (SR1), and the statements made therein apply accordingly to the silicone release layer (SR2). The layers (SR1) and (SR2) can be identical or different in their composition; preferably, they are different. The silicone release layer (SR2) is preferably based on a UV-curing silicone system. Such systems comprise, as already described herein, photoactive catalysts, so-called photoinitiators, in combination with UV-curable cationically crosslinking polysiloxanes based on epoxide and / or vinyl ethers, or with UV-curable, radically crosslinking polysiloxanes such as acrylate-modified polysiloxanes. Preferably, the silicone release layer (SR2) is based on an epoxide-functionalized polydimethylsiloxane in combination with a photoinitiator.The photoinitiator is preferably a photoactive lodonium salt solution.

[0072] The thickness of the silicone release layer (SR2) is preferably 200 nm to 1,200 nm, particularly preferably 300 nm to 1,000 nm, and especially 400 nm to 800 nm.

[0073] Preferably, the pull-off force of a release liner according to the invention from a synthetic rubber-based adhesive compound lying on its layer composite PER-SR2 is 20 cN / cm to 12.5 N / cm, in particular 50 cN / cm to 8.5 N / cm.

[0074] The invention also relates to a method for producing a release liner according to the invention, characterized in that the release liner is first produced without the layer (SR2) and then the layer (SR2) is applied to the layer (PER) by A UV-curable silicone compound is applied to the layer (PER) using flexographic printing, and the silicone compound is then cured.

[0075] Methods for constructing the release liner without layer (SR2) are generally known. In one embodiment, the film (BF) is provided; in a first step, this is laminated to layer (PER) using an adhesive. The silicone compound for layer (SR1) is then applied to the remaining side of the film (BF) and subsequently cured.

[0076] Preferably, both free surfaces of the composite consisting of the film (BF) and the layer (PER) are physically pretreated before siliconization, in particular by means of corona.

[0077] According to the invention, the silicone release layer (SR2) is applied to the layer (PER) by means of flexographic printing.

[0078] In one possible embodiment, the uncured silicone compound is applied to the printing plate on the roller via an anilox roller. The final application weight of the silicone compound can be determined by the cavity volume of the anilox roller. The printing plate then transfers the silicone to the layer (PER) on a counter roller, which is bonded to the film (BF) and the release layer (SR1). The final application weight of the layer (SR2) can also be influenced by the distance between the printing plate roller and the counter roller.

[0079] A further aspect of the invention is the use of a release liner according to the invention for covering or protecting an adhesive, wherein the adhesive rests on layer (SR1). The excellent release properties of the partially siliconized side of the liner (composite (SR2) / (PER)) are particularly advantageous when an adhesive wound onto a roll or an adhesive tape provided with the adhesive is unwound or unwound in this way. The release liner according to the invention is generally suitable for use on all known adhesives. Preferably, the release liner according to the invention is used to cover or protect an adhesive containing synthetic rubber. The adhesive containing synthetic rubber preferably contains at least one synthetic rubber, more preferably at least two synthetic rubbers, in a total of 30 to 70 wt.%, and in particular in a total of 40 to 60 wt.%.-%, and at least one adhesive-strengthening resin at 30 to 70 wt.%, in particular at 40 to 60 wt.%, each based on the total weight of the pressure-sensitive adhesive. Naturally, the pressure-sensitive adhesive may contain further additives such as fillers, antioxidants, crosslinking agents, etc.

[0080] The pressure-sensitive adhesive particularly preferably contains at least one, and preferably at least two, vinyl aromatic block copolymers. Most preferably, these vinyl aromatic block copolymers contain at least one polymer block A formed predominantly by polymerization of vinyl aromatics and at least one block B formed predominantly by polymerization of conjugated dienes. The vinyl aromatic block copolymer used is preferably a block copolymer with a structure AB, ABA, (AB)n, (AB)nX, or (ABA)nX, wherein The blocks A independently represent a polymer formed by polymerization of at least one vinyl aromatic, the blocks B independently represent a polymer formed by polymerization of conjugated dienes with 4 to 18 C atoms, X represents the remainder of a coupling reagent or initiator, and n represents an integer ≥ 2.

[0081] The vinyl aromatics for the construction of block A preferably comprise styrene, α-methylstyrene and / or other styrene derivatives, in particular styrene.

[0082] The conjugated dienes for the construction of block B are preferably selected from the group consisting of butadiene, isoprene, ethylbutadiene, phenylbutadiene, pentadiene, hexadiene, ethylhexadiene and dimethylbutadiene as well as any mixture of these monomers, with butadiene being preferred.

[0083] The adhesive-enhancing resin preferably comprises at least 75 wt.%, based on the total resin content, a resin with a DACP (diacetone alcohol cloud point) greater than 0 °C, preferably greater than 10 °C, and a softening temperature (ring and ball) of 70 °C or higher, preferably greater than or higher than 100 °C. Particularly preferably, the adhesive-enhancing resin contains at least 75 wt.% of a hydrocarbon resin, in particular a terpene resin.

[0084] In one embodiment, the pressure-sensitive adhesive is foamed. Foamed pressure-sensitive adhesive systems have been known for some time and are described in the prior art. They have, for example, lower densities than comparable non-foamed systems and are typically characterized by non-destructive removability and repositionability. Polymer foams can generally be produced in two ways. Firstly, by the action of a blowing agent, either added as such or resulting from a chemical reaction; secondly, by incorporating hollow spheres into the material matrix. Foams produced in the latter way are called syntactic foams. In a syntactic foam, hollow spheres such as glass spheres or ceramic hollow spheres (microspheres) or microballoons are embedded in a polymer matrix.In syntactic foams, the cavities are separated from one another, and the substances (gas, air) within these cavities are separated from the surrounding matrix by a membrane. Foams made with microhollow spheres are characterized by a defined cell structure with a uniform size distribution of the foam cells. Microhollow spheres produce closed-cell foams without cavities, which, compared to open-cell variants, offer, among other things, better sealing against dust and liquids. Furthermore, chemically or physically foamed materials are more susceptible to irreversible collapse under pressure and temperature and often exhibit lower cohesive strength. Particularly advantageous properties can be achieved when expandable microspheres (also known as "microballoons") are used for foaming.Due to their flexible, thermoplastic polymer shell, such foams possess a higher adaptability than those filled with non-expandable, non-polymeric microspheres (e.g., glass spheres). They are better suited to compensating for manufacturing tolerances, which are common in injection-molded parts, and, due to their foam-like nature, can also better compensate for thermal stresses.

[0085] Preferably, the foamed pressure-sensitive adhesive is foamed with microballoons; that is, it contains expanded microballoons. "Microballoons" are defined as elastic, and therefore expandable in their ground state, hollow microspheres with a thermoplastic polymer shell. These spheres are filled with low-boiling liquids or liquefied gas. Polyacrylonitrile, PVDC, PVC, or polyacrylates are particularly suitable as shell materials. Suitable low-boiling liquids include hydrocarbons of the lower alkanes, such as isobutane or isopentane, which are enclosed as a liquefied gas under pressure within the polymer shell.

[0086] When the microballoons are subjected to stress, particularly heat, the outer polymer shell softens. Simultaneously, the liquid propellant gas inside the shell transitions into a gaseous state. This causes the microballoons to expand irreversibly and three-dimensionally. The expansion ceases when the internal and external pressures equalize. Because the polymer shell remains intact, this process results in a closed-cell foam.

[0087] A wide variety of unexpanded microballoon types are commercially available, differing primarily in their size and the initial expansion temperatures required (75 to 220 °C). An example of commercially available unexpanded microballoons are the Expancel® DU types (DU = dry unexpanded) from Akzo Nobel. In the type designation Expancel xxx DU yy (Dry unexpanded), "xxx" represents the composition of the microballoon mixture, and "yy" represents the size of the microballoons in their expanded state.

[0088] Unexpanded microballoon types are also available as aqueous dispersions with a solids or microballoon content of approximately 40 to 45 wt.%, and furthermore as polymer-bound microballoons (masterbatches), for example in ethyl vinyl acetate with a microballoon concentration of approximately 65 wt.%. Both the microballoon dispersions and the masterbatches, like the DU types, are suitable for producing a foamed self-adhesive compound according to the invention.

[0089] A foamed self-adhesive compound SK1 according to the invention can also be produced using so-called pre-expanded microballoons. With this group, expansion takes place before the microballoons are mixed into the polymer matrix. Pre-expanded microballoons are commercially available, for example, from Akzo Nobel under the name Dualite® or with the type designation Expancel xxx DE yy (Dry Expanded). "xxx" represents the composition of the microballoon mixture, and "yy" represents the size of the microballoons in the expanded state.

[0090] The proportion of microballoons in the foamed pressure-sensitive adhesive is preferably up to 12 wt.%, more preferably 0.25 wt.% to 5 wt.%, even more preferably 0.5 wt.% to 4 wt.%, and particularly 1 wt.% to 3.5 wt.%, for example 2.0 wt.% to 3.0 wt.%, in each case based on the total composition of the pressure-sensitive adhesive. Pressure-sensitive adhesives with a particularly good balance between adhesion and cohesion can be provided within these ranges.

[0091] The foamed adhesive may also contain non-expandable microhollow spheres. The crucial factor is that virtually all gas-containing cavities are sealed by a permanently airtight membrane, regardless of whether this membrane consists of an elastic and thermoplastically stretchable polymer mixture or, for example, of elastic and – within the temperature range possible in plastics processing – non-thermoplastic glass.

[0092] The foamed adhesive compound can – independently of other additives – also contain polymer solid spheres, hollow glass spheres, solid glass spheres, hollow ceramic spheres, solid ceramic spheres and / or carbon solid spheres ("Carbon Micro Balloons").

[0093] The absolute density of the foamed adhesive compound is preferably 400 to 990 kg / m 3< , more preferably 450 to 800 kg / m 3< , even more preferably 500 to 700 kg / m 3< and in particular 500 to 600 kg / m 3< .

[0094] The pressure-sensitive adhesives intended for use with the release liner according to the invention can, of course, be used without (so-called transfer tape) or with a carrier. All materials known for this purpose are suitable as carriers, in particular foamed, poly(meth)acrylate-based pressure-sensitive adhesives.

[0095] Preferably, the release liner according to the invention is provided with a gripping aid to simplify peeling the release liner from the adhesive. The gripping aid is applied to the outwardly facing surface of the composite layer (PER) and (SR2) such that at least part of the gripping aid rests on the (PER) layer. The combination of partial polyethylene and silicone release coatings enables both the formation of a sufficient and adjustable release force of the liner, even on special adhesives such as those intended for use on low-energy surfaces, and the heat-sealability of the release coating, which is necessary for welding the gripping aid in place. This combination of properties could not be achieved with a polyethylene release coating alone or with a continuous silicone release coating. Examples Release liner composition (unless otherwise specified: values ​​in wt.%) Liner 1:

[0096] It was assumed that the film was a three-layer blown film with the following structure: 20 µm blend of 40% LLDPE (Eltex PF6130, INEOS) and 60% PP (BA110CF, Borealis) 60 µm 92% PP (BA110CF, Borealis) and 8% color batch 20 µm blend of 40% LLDPE (Eltex PF6130, INEOS) and 60% PP (BA110CF, Borealis).

[0097] The aforementioned three-layer film was corona-treated on one side. A two-component polyurethane laminating adhesive system (Liofol UR 7780 / UR6080, Henkel) was then applied to the pre-treated side using anilox rollers at 40 °C with a basis weight of 2 g / m². A 28 µm thick blown LDPE film (Purell 1840H, LyondellBasell) was then laminated onto this adhesive. The bond was cured for 10 days at room temperature.

[0098] The remaining side of the aforementioned three-layer film was fully coated with an addition-curing silicone system according to the instructions below with a basis weight of 1 g / m²; the silicone system was then cured.

[0099] The layered composite obtained up to this point was placed on a roller in such a way that the LDPE blown film pointed away from the roller surface.

[0100] An uncrosslinked, UV-curable silicone system was applied to an anilox roller with a defined application volume; excess silicone was removed with a doctor blade. This material was then transferred from the anilox roller to a flexographic printing plate roller featuring a pattern as specified in Table 1.

[0101] The silicone system used was SilForce UV9300 together with the photoinitiator UV9380C (both from Momentive). The mixing ratio of SilForce UV9300 : UV9380C was 100 : 3. After transfer from the printing plate roller to the LDPE layer of the composite, the compound was activated and cured with a mercury lamp. The distance between the printing plate roller and the composite was constant in all examples shown here.

[0102] The partially siliconized side of the liners produced in this way (see table) was laminated with the test adhesive compound based on synthetic rubber described below (standard climate, 20 mm wide strips); the composite was then stored for 1, 3 and 7 days at room temperature and 50% relative humidity. Production of the test tape Raw materials used:

[0103] Kraton®< D1101: Styrene-butadiene-styrene triblock copolymer from Kraton Polymers with 16 wt% diblock, block polystyrene content: 31 wt%, proportion of 1,2-linked conjugated diene in the butadiene block: 10 wt% Kraton®< D1118: Styrene-butadiene-styrene triblock copolymer from Kraton Polymers with 78 wt% diblock, block polystyrene content: 33 wt%, proportion of 1,2-linked conjugated diene in the butadiene block: 10 wt% Dercolyte A115: solid α-pinene adhesive resin with a ring and ball softening temperature of 115 °C and a DACP of 35 °C Piccolyte®< A25: polyterpene resin based on α-pinene with a ring and Ball softening temperature of 22 to 28 °C. Expancel 920 DU40: unexpanded microballoons. Recipe:

[0104] Kraton ®< D1101 25 parts by weight Kraton ®< D1118 25 parts by weight Dercolyte A115: 48 parts by weight Piccolyte A25: 2 parts by weight Expandel 920DU40: 3 parts by weight Procedure:

[0105] For this purpose, a 40 wt% adhesive solution of the specified formula was first prepared in gasoline / toluene / acetone. The weight percentages of the dissolved components refer to the dry weight of the resulting solution.

[0106] The solution was then mixed with the unexpanded microballoons, which were used as a slurry in gasoline. The weight fraction of the microballoons refers to the dry weight of the solution to which they were added (i.e., the dry weight of the solution is set to 100%). The resulting mixture was then spread onto a PET liner coated with a separating silicone using a brush; the solvent was then evaporated at 100 °C for 15 minutes, thus drying the adhesive layer. Subsequently, a second PET liner of the same type was laminated onto the exposed surface of the prepared and dried adhesive layer, and the adhesive layer was then foamed between the two liners in an oven at 150 °C for 5 minutes, resulting in an approximately 100 µm thick adhesive strip according to the invention.The target thickness of the test tape, 1,000 µm, was achieved through multiple laminations of the resulting layers. The dimensions of 500 mm in length and 20 mm in width were obtained by die-cutting. Testing methods - Density

[0107] The density of an adhesive layer is determined by calculating the ratio of the amount of adhesive applied to the thickness of the adhesive layer applied to a substrate or liner.

[0108] The mass application can be determined by determining the mass of a section of such an adhesive mass layer applied to a substrate or liner, defined in terms of its length and width, minus the (known or separately determinable) mass of a section of the same dimensions of the substrate or liner used.

[0109] The thickness of an adhesive layer can be determined by measuring the thickness of a section of the adhesive layer applied to a substrate or liner, defined in terms of its length and width, and subtracting the (known or separately determinable) thickness of a section of the substrate or liner of the same dimensions. The thickness of the adhesive layer can be determined using commercially available thickness gauges (probe gauges) with accuracies of less than 1 µm deviation. Within the scope of the present invention, the precision thickness gauge Mod. 2000 F was used, which has a circular probe with a diameter of 10 mm (flat). The measuring force is 4 N. The value is read 1 s after the load is applied. If thickness variations are detected, the mean value of measurements at at least three representative locations is given, specifically excluding measurements taken at creases, folds, spots, and the like. - Thickness

[0110] Just as the thickness of an adhesive layer can be determined, the thickness of a pressure-sensitive adhesive strip, a film carrier layer, or a liner can also be determined using commercially available thickness gauges (probe gauges) with accuracies of less than 1 µm deviation. Within the scope of the present invention, the precision thickness gauge Mod. 2000 F was used, which has a circular probe with a diameter of 10 mm (flat). The measuring force is 4 N. The value is read 1 s after the load is applied. If thickness variations are detected, the mean value of measurements at at least three representative locations is given, specifically excluding measurements taken at creases, folds, spots, and the like. - Application-related testing (tabbing test)

[0111] Ten strips of the liner-coated test tape are applied to a PP plate. A tab is welded onto each liner at different positions to serve as a handle. The tabs are welded to the partially siliconized side of the liner.

[0112] The welding process is carried out at 190 °C, 3 bar pressure, and a welding time of 3 seconds using a metal stamp. A PET / PE laminate is used as the tab.

[0113] The liner should be removed from the test tape in one step using the handle. Evaluation scheme: "+": The liner can be completely removed from the test tape in one step using the tab, regardless of the tab's position on the test tape. "-": The liner cannot be removed from the test tape. The tab either tears away from the liner or the individual layers of the liner split. - Liner Removal Force:

[0114] This test is used to determine the force required to peel the liner away from an adhesive applied to it. It essentially concerns the interaction between the partially siliconized layer and the adhesive layer on top of it.

[0115] The pull-off force of the liners was determined using the test adhesive tape described above. Measurement:

[0116] The sample was fixed to a PE backing plate with the adhesive layer exposed. It was fixed in such a way that a short section of the liner under test protruded beyond the plate. The test plate was then clamped into the lower jaw of a tensile testing machine (BZ2.5 / TN1S, Zwick) with the extended end of the liner under test pointing downwards. The extended end of the liner under test was clamped into the upper jaw and pulled off at a machine speed of 300 mm / min at an angle of 180°. The force required for this was measured.

[0117] Good, highly practical release forces are considered to be between 50 and 1,250 cN / cm. Release forces exceeding 13 N / cm are no longer practical because the force required to remove the release liner becomes too great. Results of the exams

[0118] Table 1: Results Pullback force (cN / cm) after ... days Nr. Cliché design Silicone coating (%) 1 3 7 Tabbing test 1 (See below) 10 L / cm 35 1.325 -- -- + 2 (See below) d = 1 mm 45 -- -- -- + 3 50 L / cm 46 1.036 1.107 1.070 + 4 d = 0.2 mm 47 1.189 1.180 1.203 + 5 d=1mm 89 177 331 393 + 6 10 L / cm 89 72 84 78 + 7 (See below) d = 0.2 mm 97 8 12 9 - 8 (See below) 50 L / cm 100 6 9 7 - L / cm - lines / cm d - diameter of applied points made of silicone

[0119] In examples 1 and 2, the adhesive forces between the liner and the pressure-sensitive adhesive were sometimes so strong that separation with reasonable effort was no longer possible.

Claims

1. Release liner for use on pressure-sensitive adhesives, comprising - an outer, continuous silicone release layer (SR1); - a film (BF) containing at least 80% by weight, based on the total weight of the film, of a polyolefin or polyethylene terephthalate; - a layer (PER) containing at least 80% by weight, based on the total weight of the layer (PER), of polyethylene; and - an outer silicone release layer (SR2) on top of the layer (PER); wherein the layer (SR2) has vacancies such that 46% to 95% of the surface of the layer (PER) is covered with the layer (SR2).

2. Release liner according to Claim 1, characterized in that the polyethylene of the layer (PER) is a low-density polyethylene (LDPE).

3. Release liner according to at least one of the preceding claims, characterized in that the layer (PER) consists of an LDPE film having a thickness of 10 to 45 µm.

4. Release liner according to at least one of the preceding claims, characterized in that the outward-facing surface of the assembly composed of the layers (PER) and (SR2) bears a grip aid applied such that at least part of the grip aid is on top of the layer (PER).

5. Release liner according to at least one of the preceding claims, characterized in that the film (BF) contains a total of at least 80% by weight, based on the total weight of the film, of polyethylene and polypropylene.

6. Release liner according to at least one of the preceding claims, characterized in that the film (BF) is a three-layer film whose central layer contains at least 60% by weight, based on the total weight of the layer, of polypropylene.

7. Release liner according to Claim 6, characterized in that the film (BF) comprises two outside layers of a polyethylene-polypropylene blend.

8. Process for producing a release liner according to at least one of the preceding claims, characterized in that first the release liner is produced without the layer (SR2) and then the layer (SR2) is applied to the layer (PER), by - applying a UV-curable silicone compound to the layer (PER) by flexographic printing and - then curing the silicone compound.

9. Use of a release liner according to at least one of Claims 1 to 7 or produced by a process according to Claim 8 for covering a synthetic rubber-based pressure-sensitive adhesive, wherein the pressure-sensitive adhesive is on top of the silicone release layer (SR1).