Method for producing a recyclable mono-material multi-layer system

Abstract

The invention relates to a method for producing a mono-material multi-layer system, in particular a recyclable mono-material multi-layer system, wherein the method comprises or consists of the following steps: I) providing at least two mono-material layers; II) applying a two-component adhesive indirectly or directly onto at least one of the two provided mono-material layers, wherein the two-component adhesive comprises at least one isocyanate-reactive component and at least one isocyanate component; III) bonding the mono-material layers by means of the two-component adhesive and curing the two-component adhesive to obtain the mono-material multi-layer system, characterised in that the material of the mono-material layers comprises polyolefins and the proportion of the polyolefin is from 80.0 to 99.30 wt.%, based on the total weight of the mono-material multi-layer system, the proportion of the two-component adhesive is from 0.70 to 20.0 wt.%, based on the total weight of the mono-material multi-layer system, the isocyanate-reactive component and / or the isocyanate component contains [CH2-O]n units, and the cured two-component adhesive comprises a proportion of from 7.0 to 25.0 wt.% [CH2-O]n units, based on the total weigh of the adhesive, where n is a number from 2 to 50.

Description

[0001] Process for producing a recyclable monomaterial multilayer system

[0002] The present invention relates to a process for producing a monomaterial multilayer system, in particular a recyclable monomaterial multilayer system, the monomaterial multilayer system obtained or obtainable by this process, and the use of the monomaterial multilayer system. Furthermore, the invention relates to the use of a two-component adhesive in monomaterial multilayer systems, in particular for improving their recyclability. Finally, the invention relates to a process for obtaining a recyclate from waste containing a monomaterial multilayer system, which recyclate contains a monomaterial multilayer system according to the invention, and to a molded part obtained or obtainable by processing a recyclate obtained or obtainable by the aforementioned recycling process.

[0003] State of the art

[0004] For packaging industrial and consumer goods, packaging often consists of film composites (multilayer systems) consisting of film layers with different properties. Such film composites can be produced, for example, by lamination with an adhesive or by extrusion, possibly with the use of an intermediate layer as an adhesive layer (tie layer). For example, such multilayer film composites contain layers of polyolefin, polyester, and aluminum foil. The properties necessary for use as packaging material, such as mechanical strength, sealability, and barrier effect, can be generated by combining different materials with very specific properties. Unfortunately, due to the different material properties, these composite materials cannot be processed into high-quality recyclates for mechanical recycling.However, recycling represents an attractive opportunity to conserve resources and reduce environmental impact. More and more companies are striving to reduce their emissions, often for the reasons mentioned above, as well as for cost reasons and regulatory requirements, making recycling concepts desirable in these areas as well. The industry is therefore looking for ways to improve the quality of the recyclates produced through mechanical packaging recycling so that they can be used in ever-increasing proportions in the production of films.

[0005] Recently, so-called monomaterial packaging consisting of monomaterial layer systems have been introduced which are particularly well suited for mechanical recycling after use. These are usually produced by extrusion lamination or by laminating two polyolefin film layers with an adhesive, in particular by lamination with a two-component (2K) polyurethane adhesive. The latter offers the advantage that a wider range of materials can be used, e.g. including pre-coated film layers. In so-called monomaterial packaging, film layers with different properties are also combined with one another, but these mostly consist of materials from the same material class, such as polyolefins. The different properties of the respective film layer can be achieved by choosing different base polymers, additives, pretreatment, and in particular coating.

[0006] However, it has been shown that the recyclates obtained from the mechanical recycling of such packaging consisting of mono-material layer systems are not suitable, or only suitable to a limited extent, for the production of high-quality films or other molded parts, and can only be added in minor quantities, if at all, in the production process of new polyolefin films from so-called virgin materials. A common problem is that the new films produced using recyclate exhibit optical defects and / or poorer mechanical properties (e.g., elongation at break, modulus) compared to films made from "virgin" materials. This is attributed to the 2-component polyurethane adhesives commonly used in film lamination, which are incompatible with polyolefins in their cured form and whose use is increasingly viewed with skepticism.The use of such adhesives is limited to small weight proportions within the framework of product specifications for recycling-friendly packaging, and alternatives are being sought.

[0007] CN 110861380 A describes a recyclable packaging using a polyurethane adhesive with barrier properties. The content of unspecified solvent-free polyurethane adhesive in the composite is limited to less than 1%. The influence of the adhesive on the quality of the resulting recyclates is not mentioned. The use of adhesive raw materials based on renewable resources is not described.

[0008] WO 2019 / 209337 A1 describes multilayer composite films with increased recyclability and produced using polylactides as an intermediate layer. Polylactides can be produced from renewable raw materials. The use of polyolefins as a film layer is not explicitly described. WO 2021 / 069668 A1 describes a recyclable polyethylene-based packaging; the possible use of solvent-free or solvent-based polyurethane-based adhesives is mentioned, but the exact composition of the adhesive is not discussed. The influence of the adhesive on the quality of the recyclate is not discussed.

[0009] WO 2019 / 027527 A1 describes a recyclable polyethylene-based packaging using a solvent-based polyurethane adhesive with barrier properties. The influence of the adhesive on the quality of the recycled material is not discussed.

[0010] Object of the invention

[0011] The object of the present invention was therefore to provide a process for producing a monomaterial multilayer system that is recyclable and, after recycling, results in a recyclate of good quality, in particular with good homogeneity. Furthermore, adhesives should be used in particular for the production of the monomaterial multilayer systems that can be produced at least partially from bio-based raw materials and that potentially lead to good bond strength of the monomaterial multilayer systems.

[0012] Solution to the task and detailed description

[0013] The problem was solved by a method according to claim 1.

[0014] In the course of the development work leading to the present invention, it was surprisingly discovered that the use of a specific two-component adhesive in the production of a monomaterial multilayer system, which primarily comprises polyolefins (80.0 to 99.30 wt. %, based on the total weight of the monomaterial multilayer system), results in recyclable monomaterial multilayer systems that, in particular, exhibit high bond strength. Recycling these monomaterial multilayer systems yields recyclates of improved quality, particularly with good homogeneity. Due to their good quality, recyclates are therefore suitable for further processing, particularly for molding.

[0015] In the context of the present invention, the term "recyclable" or "recyclability" is understood to mean the suitability of a monomaterial multilayer system to be processed into a recyclate with high homogeneity by means of a mechanical recycling process, even if the monomaterial multilayer system has a proportion of a two-component adhesive of > 1.0 wt.%, based on the total weight of the monomaterial multilayer system. The homogeneity of the recyclate is determined by compounding the monomaterial multilayer system at high temperatures in an extruder and producing a film with a thickness of 200 μm from the extrudate by pressing. This film is examined optically for inhomogeneities using a light microscope and, if necessary, supported by an evaluation program. A recyclate with high or good homogeneity is characterized by the number of defects per examined area of ​​1.1 cm 2, which are determined with the aid of a microscopic examination (light microscope) is < 3000, preferably < 2000, particularly preferably < 1500. With regard to the total area of ​​the defects, a recyclate is said to be homogeneous if the total area of ​​the defects in the pressed film pressed from the produced recyclate is < 3%, preferably < 2.5%. As an exception to this, the homogeneity can be determined on a film produced from an extrudate obtained by compounding a certain amount of the comminuted, cured adhesive with a defined amount of the other components of the monomaterial layer system (alternatively: “the monomaterial layers used”) at high temperatures in an extruder and producing a film from the extrudate by pressing. This method is particularly suitable for determining the maximum adhesive content at which a recyclable monomaterial layer system is still obtained.

[0016] The bond strength of the monomaterial multilayer system is determined according to the invention in a 180° peel test (180° peel strength), in which composite films are cut into 15 mm wide strips. The strips are then peeled off at 23°C in a Zwick universal peel strength testing machine (type 5 kN Allround Table-top BT2-FA005TH.A50.002) at a speed of 100 mm / min at an angle of 2 x 90° (180°), and the required force is measured in Newtons. Adequate bond strength is achieved if a value of at least 0.5 N / 15 mm is achieved.

[0017] According to the invention, a monomaterial multilayer system is understood to mean a layer system made from at least two monomaterial layers and a two-component adhesive. The production comprises or consists of the following steps:

[0018] I) Providing at least two monomaterial layers; II) Applying a two-component adhesive directly or indirectly to at least one of the two provided monomaterial layers, wherein the two-component adhesive comprises at least one isocyanate-reactive component and at least one isocyanate component;

[0019] III) Bonding the monomaterial layers using the two-component adhesive and curing the two-component adhesive to obtain the monomaterial multi-layer system.

[0020] The material of the monomaterial layer comprises polyolefins, the proportion of polyolefin being from 80.0 to 99.30 wt. %, preferably from 95.0 to 98.0 wt. %, based on the total weight of the monomaterial multilayer system. These are preferably polyolefin films, which may additionally be coated. The monomaterial layer may further contain additives known to those skilled in the art, in particular antiblocking agents, dyes, pigments, and stabilizers. The monomaterial layers, such as the polyolefin films, can be produced by various processes known to those skilled in the art, for example by blown film extrusion. These processes can be used to obtain monomaterial layers consisting of multiple layers of different or identical compositions. To achieve desired properties, the monomaterial layers can optionally be axially or biaxially stretched below the melting temperature and / or coated with a coating.In particular, a barrier layer, for example made of silicon oxide, aluminum oxide, EVOH, PVOH or acrylates and / or a printing ink and / or a primer can be applied.

[0021] The at least two monomaterial layers preferably differ in melting point by 5 to 90 K, more preferably 10 to 20 K. Furthermore, it is preferred that one layer of the at least two monomaterial layers has a melting point of 100 to 115 °C, more preferably 100 to 105 °C, and the other layer has a melting point of 140 to 170 °C, more preferably 140 to 155 °C. It is also possible to use polyolefins with an identical or almost identical melting point to produce a monomaterial multilayer system if a sealing layer is present on at least one side of the monomaterial multilayer system. Melting point and melting temperature are used synonymously here. The melting temperatures are determined by DSC (differential scanning calorimetry) at a heating rate of 20 K / min in accordance with DIN EN ISO 11357-1:2017 (2017-02) and standard part 11357-2.

[0022] It is preferred that the polyolefin of the monomaterial layers be selected from the group comprising or consisting of polyethylene, polypropylene, or mixtures thereof. The polyolefins can be produced, for example, from alkenes such as ethylene, propylene, 1-butene, and / or isobutene by chain polymerization. Ethylene and / or propylene are preferably used as monomers in the production of the polyolefins.

[0023] Monomaterial layers which consist essentially of polypropylene (for example BOPP (biaxially oriented polypropylene)) and / or polyethylene (for example BOPE (biaxially oriented polyethylene)) are preferably used. Monomaterial layers which consist of polyethylene, in particular BOPE (biaxially oriented polyethylene), are very particularly preferably used. The monomaterial layer systems preferably consist of two monomaterial layers which are largely made of polyethylene or of two monomaterial layers which are largely made of polypropylene. In a further preferred embodiment, the monomaterial layer systems each consist of a monomaterial layer which is largely made of polyethylene and a monomaterial layer which is largely made of polypropylene.

[0024] The two-component adhesive comprises at least one isocyanate-reactive component and at least one isocyanate component. The two-component adhesive is also referred to below as a 2K adhesive or 2K polyurethane adhesive. The proportion of the two-component adhesive is 0.70 to 20.0 wt.%, preferably 0.70 to 10.0 wt.%, more preferably 1.0 to 5.0 wt.%, based on the total weight of the monomaterial multilayer system. The two-component adhesive preferably also contains organic solvents, catalysts, fillers, processing aids, adhesion promoters, in particular silanes, stabilizers, in particular antioxidants, or mixtures thereof. It is further preferred that the two-component adhesive has a stoichiometric ratio of isocyanate groups to isocyanate-reactive groups between 0.8:1 to 2:1, preferably from 1.1:1 to 1.5:1, particularly preferably between 1.1:1 to 1.4, most preferably 1.1 to 1.2.The components of the two-component adhesive are preferably mixed or otherwise brought into intimate contact prior to application to a substrate. If necessary, one or both monomaterial layers that come into direct or indirect contact with the adhesive are subjected to a corona pretreatment to increase the surface energy before applying the adhesive and / or bringing it into contact with the applied adhesive layer.

[0025] The production of two-component polyurethane adhesives is generally known to those skilled in the art. The chemical structure and formulation, as well as the processing, particularly in the case of two-component polyurethane adhesives for the production of composite films or multilayer composite films, are described in Ulrich Meier-Westhues, "Polyurethanes - Paints, Adhesives and Sealants," Hanover: Vincenz Network 2007, Chapter 6, for example, on pages 247-253.

[0026] Unless otherwise stated, the average functionality of a compound is understood to mean the average number of isocyanate-functional groups or isocyanate groups per molecule. For undefined or uniformly composed compounds, such as oligomers or polymers, various methods for determining the average functionality are available to the skilled person. For example, an average functionality can be calculated from the isocyanate content and the number-average molecular weight determined, for example, by gel permeation chromatography (GPC) or vapor pressure osmosis. A similar procedure can be used for isocyanate-reactive compounds. The average OH functionality of a compound, for example, can be calculated based on the number-average molecular weight and the hydroxyl number.The hydroxyl number (OH number) of a compound is determined, unless otherwise stated, according to DIN EN ISO 4629-2, where N-methylpyrroledione is replaced by pyridine.

[0027] The isocyanate-reactive component and / or the isocyanate component of the two-component adhesive contains [CH2-O] n -units and the cured two-component adhesive has a content of 7.0 to 25.0 wt.% of [CH2-O] n- units, based on the total weight of the adhesive, where n is a number from 2 to 50, preferably from 8 to 20. It is preferred that either the isocyanate-reactive component or the isocyanate component has an average molecular weight Mn of < 1000 g / mol and an average functionality of > 2 (also called “crosslinker component”) and the respective other component has an average molecular weight Mn of > 1000 g / mol and a functionality of < 4 (also called “prepolymer component”). In other words, either the isocyanate-reactive component is preferably a crosslinker component and the isocyanate component is a prepolymer component, or the isocyanate-reactive component is preferably a prepolymer component and the isocyanate component is a crosslinker component.It is also possible to use a prepolymer component for both components, but this is less preferred because the molecular weight buildup generally occurs less rapidly and the buildup of bond strength is delayed. This leads to problems in the production of composite films, such as the well-known phenomenon of the film roll "telescoping" under tensile stress during winding. Unless otherwise stated, the number-average molecular weight Mn and the weight-average molecular weight Mw are determined according to DIN 55672-1 "Gel Permeation Chromatography, Part I — Tetrahydrofuran as Eluent."

[0028] In the context of the present invention, "isocyanate-reactive groups" are understood to mean functional groups that can react with isocyanate groups (-N=C=O) at temperatures up to 200°C and, optionally, with the addition of a catalyst, to form at least one covalent bond. Isocyanate-reactive compounds include, for example, compounds with active hydrogen atoms. Examples of such compounds are water, alcohols, primary and secondary amines, thiols, and carboxylic acids. Compounds that do not contain an active hydrogen atom but are nevertheless isocyanate-reactive include, for example, epoxides, oximes, carbon dioxide, and carboxylic anhydrides. Suitable reactive functional groups can be monofunctional in the sense of reacting with isocyanates, for example, OH groups or mercapto groups. However, they can also be difunctional with isocyanates, for example, amino groups.A molecule with an amino group therefore also has two functional groups that are reactive towards isocyanate groups. In this context, it is not necessary for a single molecule to have two separate functional groups that are reactive towards isocyanate groups – what is crucial is that the molecule can form a bond with two isocyanate groups, forming a covalent bond in each case. Examples of structural features that are created by the reaction of isocyanate groups with isocyanate-reactive compounds are urethane, allophanate, biuret, urea, thiourethane, amide, imide, acylurea, oxazolidione, carbamate, and oxadiazinetrion. Examples of structural features that are formed by the reaction of at least two isocyanate groups are uretdione, isocyanurate, and carbodiimide. Catalysts are generally used to form these groups. Examples of suitable catalysts are given in Chapter XII.Summary of Catalytic Effects High Polymers Vol.

[0029] It is preferred that the isocyanate-reactive component of the two-component adhesive comprises polyols, in particular polyester polyols, polyether polyols, hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers, hydroxyl-containing

[0030] Reaction products of hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers and diisocyanates or mixtures of the aforementioned compounds. A polyol is understood to be a polyfunctional alcohol, i.e., a compound with more than one OH group in the molecule. If the isocyanate-reactive component is [CH2-O] n-units, these are preferably polyols with an OH functionality of between 1 and 4 and an OH number of 5 mg KOH / g to 900 mg KOH / g, such as in particular hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers. Such polyols are accessible via various routes known to the person skilled in the art: for example, WO 2004 / 096746 A1 experimentally describes isocyanate-reactive diols with OH numbers of 685 and 868 mg KOH / g based on polyoxymethylene (POM), which can be converted by reaction with polyisocyanates to isocyanate-reactive groups with a lower OH number. Other possible starting compounds can be prepared according to EP 3 922 661 A1 by reacting a polymeric formaldehyde compound with an alkylene oxide and a polyol in the presence of a double metal cyanide (DMC) catalyst. US 3754053 describes polyoxymethylene-polyoxyalkylene copolymers with a molecular weight of >10,000 g / mol starting from trioxane.WO2021122402A1 teaches the preparation of polyoxymethylene-polyoxyalkylene copolymers comprising the reaction of a polymeric formaldehyde compound with an alkylene oxide in the presence of a double metal cyanide (DMC) catalyst, wherein the polymeric formaldehyde compound has at least one terminal hydroxyl group, and wherein the process comprises the steps of initially introducing a suspending agent into a reactor and subsequently gradually or continuously metering the polymeric formaldehyde compound into the reactor during the reaction. Further possible starting compounds are described in WO2015 / 155094A1, WO2012 / 091968, EP 1870425A1, JP 2007211082. [CH2-O-] containing urethane groups. n Polyols containing α-units with a molecular weight Mw of >1000 to 30,000 are also preferred. These can be prepared, for example, by reacting diisocyanates with [CH2-O-] nPolyols containing -units are obtained by using a stoichiometric excess of hydroxyl groups over isocyanate groups. Particularly preferred diisocyanates for this reaction are 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, I PDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI), and 2,4- and 2,6-diisocyanatotoluene (TDI). The stoichiometric ratio of isocyanate groups to hydroxyl groups is preferably between 1:10 and 9.5:10.

[0031] The isocyanate-reactive component may contain only one polyol, but a mixture of two or more polyols may also be used as the polyol component. Mixtures of [CH2-O-] n -units containing polyols, optionally also alongside other compounds with isocyanate-reactive groups which do not contain [CH2-O-]n units. In a particular embodiment of the invention, the isocyanate-reactive component is water (H2O), or the isocyanate-reactive component contains water. Although a water molecule contains only one hydroxyl group, it can react with two NCO groups because, after reaction with an isocyanate group, carbon dioxide is released, an amine group is formed which in turn can react with one or more further isocyanate groups. In a particular embodiment, the isocyanate-reactive component is therefore not mixed with the B component before application of the adhesive, but rather enters, e.g. as air humidity by diffusion after joining the two substrates through a substrate with corresponding water vapor permeability.

[0032] In the process according to the invention, the [CH2-O-] nThe compounds containing units are preferably polyoxymethylene-polyoxyalkylene copolymers and prepolymers derived therefrom and further derivatives. Polyoxymethylene-polyoxyalkylene copolymers in the sense of the invention refer to polymeric compounds which contain at least one polyoxymethylene block and at least one additional polyoxyalkylene block and preferably do not exceed a molecular weight in the four-digit range. The polyoxymethylene-polyoxyalkylene copolymers used in the process according to the invention are preferably prepared by catalytic addition of alkylene oxides and optionally further comonomers onto at least one polymeric formaldehyde starter compound which has at least one terminal hydroxyl group, in the presence of a double metal cyanide (DMC) catalyst, as described in WO2022049012 A1, page 5, line 28 to page 13, line 3.[CH2-O-] are particularly preferred as polyols. n Units, and as a constituent of the isocyanate-reactive component as such or as a building component for compounds which are used in the isocyanate-reactive component and / or in the isocyanate component, in the sense of this invention polyoxymethylene-polyoxyalkylene copolymers with a hydroxyl number according to DIN EN ISO 4629-2 of 5 mg KOH / g to 200 mg KOH / g and an average functionality of 1.6 to 4 are used.

[0033] With regard to the isocyanate component of the two-component adhesive, it is preferred that this

[0034] • monomeric polyisocyanates, in particular hexamethylene-1,6-diisocyanate, pentamethylene-1,5-diisocyanate, 1,4-butane diisocyanate, isophorone diisocyanate, toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, diphenylmethane-2,2'-diisocyanate, diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate or mixtures thereof; • oligomeric polyisocyanates which are produced from at least two monomeric polyisocyanates;

[0035] • Isocyanate-terminated prepolymers which are prepared by reacting monomeric and / or oligomeric polyisocyanates and isocyanate-reactive compounds having > 1 isocyanate-reactive group per molecule, wherein the isocyanate-reactive compound preferably comprises polyols, more preferably hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers,

[0036] comprises or consists of polyacrylates, polycarbonates, polyesters, polyurethanes and / or polyethers;

[0037] • or mixtures of the aforementioned compounds, wherein the isocyanate-terminated prepolymers are preferably [CH2-O] n -units as defined above.

[0038] If the isocyanate component [CH2-O] n -units, it is preferably prepared by methods known to the person skilled in the art, for example by reaction of compounds containing [CH2-O-] n -units and can be used in the isocyanate-reactive component of the two-component adhesive with an excess of polyisocyanate to form isocyanate-terminated compounds with [CH2-O] nUnits. For example, isocyanate-terminated prepolymers prepared from polyoxymethylene-polyoxyalkylene copolymers (described, for example, in EP3960783A1) can be used as the isocyanate component. The use of an amino-containing polyether is possible, but not mandatory. Furthermore, the isocyanate-reactive diols based on polyoxymethylene (POM) described in WO 2004 / 096746 A1, for example those with OH numbers of 685 and 868 mg KOH / g, which can be converted into isocyanate-containing compounds by reaction with polyisocyanates, can be used. The NCO-modified polyoxymethylene copolymers disclosed in WO 2014 / 095679 can also be used. All reactive compounds contained in the isocyanate-reactive compound of the two-component adhesive can be [CH2-O-] n units or in addition to the reactive compounds containing [CH2-O-] nUnits may also contain compounds that do not contain [CH2-O-] n units. However, the isocyanate-reactive compound can also consist exclusively of reactive compounds that do not contain [CH2-O-] n Contain units when compounds, [CH2-O-] n units present in the isocyanate-reactive compound.

[0039] The term "polyisocyanate," as used here, is a collective term for compounds containing two or more isocyanate groups (which, to those skilled in the art, means free isocyanate groups of the general structure -N=C=O) in the molecule. The simplest and most important representatives of these polyisocyanates are diisocyanates. These have the general structure O=C=NRN=C=O, where R typically represents aliphatic, cycloaliphatic, and / or aromatic radicals.

[0040] The term "polyisocyanates" in this application therefore refers equally to monomeric and oligomeric polyisocyanates. When reference is made to "oligomeric polyisocyanates" in this application, this refers to polyisocyanates composed of at least two monomeric polyisocyanate molecules, i.e., compounds that represent or contain a reaction product of at least two monomeric polyisocyanate molecules. Preferably, said monomeric polyisocyanates are diisocyanates, i.e., monomeric isocyanates with two isocyanate groups per molecule. In contrast to the isocyanate-terminated prepolymers defined further below in this application, oligomeric polyisocyanates are characterized by a molecular weight of at most 900 g / mol, preferably at most 800 g / mol, and particularly preferably at most 700 g / mol.

[0041] According to the invention, the oligomeric polyisocyanates can, in particular, have uretdione, isocyanurate, allophanate, biuret, iminooxadiazinedione, and / or oxadiazinetrione structures. According to one embodiment of the invention, the oligomeric polyisocyanates have at least one of the following oligomeric structure types or mixtures thereof:

[0042] Uretdione Isocyanurate Allophanate Biuret Iminooxadiazindione Oxadiazintrione

[0043] Preparation processes for the oligomeric polyisocyanates having a uretdione, isocyanurate, allophanate, biuret, iminooxadiazinedione and / or oxadiazinetrione structure to be used according to the invention are described, for example, in J. Prakt. Chem. 336 (1994) 185-200, in DE-A 1 670 666, DE-A 1 954 093, DE-A 2 414 413, DE-A 2 452 532, DE-A 2 641 380, DE-A 3 700 209, DE-A 3 900 053 and DE-A 3 928 503 or in EP-A 0 336 205, EP A 0 339 396 and EP-A 0 798 299.

[0044] In an isocyanate with aliphatically bound isocyanate groups, all isocyanate groups are bound to a sp 3 -hybridized carbon atom. Preferred polyisocyanates with aliphatically bound isocyanate groups are n-butyl isocyanate and all isomers thereof, n-pentyl isocyanate and all isomers thereof, n-hexyl isocyanate and all isomers thereof, 1,4-butyl diisocyanate, 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 2,2,4- or 2,4,4-trimethyl-1,6-diisocyanatohexane, 1,10-diisocyanatodecane, and triisocyanatonane.

[0045] In an isocyanate with cycloaliphatically bonded isocyanate groups, all isocyanate groups are bonded to carbon atoms that are part of a closed ring of carbon atoms. This ring can be unsaturated at one or more positions, as long as it does not acquire aromatic character due to the presence of double bonds. Preferred polyisocyanates with cycloaliphatically bonded isocyanate groups are cyclohexyl isocyanate, 1,3- and 1,4-diisocyanatocyclohexane, 1,4-diisocyanato-3,3,5-trimethylcyclohexane, 1,3-diisocyanato-2-methylcyclohexane, 1,3-diisocyanato-4-methylcyclohexane, and 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane.

[0046] Isophorone diisocyanate; (IPDI) 4,4'-Diisocyanato-3,3'-dimethyldicyclohexylmethane, 4,4'-Diisocyanato-3,3',5,5'-tetramethyl-dicyclohexylmethane, 4,4'-Diisocyanato-1,1'-bi(cyclohexyl), 4,4'-Diisocyanato-3,3'-dimethyl-1 ,1'-bi(cyclohexyl), 4,4'-diisocyanato-2,2',5,5'-tetra-methyl-1,1'-bi(cyclohexyl), 1,8-Diisocyanato-p-menthane,

[0047] 1,3-Diisocyanato-adamantane and 1,3-Dimethyl-5,7-diisocyanatoadamantane.

[0048] In an isocyanate with araliphatically bound isocyanate groups, all isocyanate groups are bonded to alkylene radicals, which in turn are bonded to an aromatic ring. Preferred polyisocyanates with araliphatically bound isocyanate groups are 1,3- and 1,4-bis(isocyanatomethyl)benzene (xylene diisocyanate; XDI), 1,3- and 1,4-bis(1-isocyanato-1-methylethyl)benzene (TMXDI), and bis(4-(1-isocyanato-1-methylethyl)phenyl) carbonate.

[0049] In an isocyanate with aromatically bound isocyanate groups, all isocyanate groups are directly bonded to carbon atoms that are part of an aromatic ring. Preferred isocyanates with aromatically bound isocyanate groups are

[0050] 2,4- and 2,6-diisocyanatotoluene (TDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI),

[0051] 1,5-Diisocyanatonaphthalene, tris-(p-isocyanatophenyl)-thiophosphate and triphenylmethane-4,4',4"-triisocyanate.

[0052] Isocyanate-terminated prepolymers are obtained by reacting monomeric or oligomeric polyisocyanates with compounds that contain, on average, more than one isocyanate-reactive group per molecule, with a molar excess of isocyanate groups over the isocyanate-reactive groups present in the reaction mixture. Preferably, the compounds that contain, on average, more than one isocyanate-reactive group per molecule are polyols and / or polyamines, with a molar excess of isocyanate groups over amino and hydroxyl groups present in the reaction mixture. Corresponding preparation processes are well known to those skilled in the art.

[0053] The 2K polyurethane adhesives used according to the invention can be produced to a significant extent from renewable raw materials (biomethanol). In this case, for example, diisocyanates with a proportion of bio-based carbon are used, for example, pentamethylene diisocyanate produced from bio-based cadaverine, hexamethylene diisocyanate based on bio-based hexamethylenediamine, methylene diisocyanate produced from bio-based aniline, and / or isophorone diisocyanate produced from bio-based acetone, or derivatives, oligomers, and / or prepolymers derived from these compounds.

[0054] The corresponding diamines mentioned are accessible via enzymatic conversions, for example from sugar or starch compounds,

[0055] In addition, [CH2-O-] n Units manufactured using formaldehyde produced from bio-methanol.

[0056] The invention further relates to the use of a two-component adhesive according to the invention in monomaterial multilayer systems, in particular for improving the recyclability of monomaterial multilayer systems, wherein the material of the monomaterial layers comprises polyolefins in a proportion of 80.0 to 99.3 wt.%, based on the total weight of the monomaterial multilayer system.

[0057] The invention further relates to monomaterial multilayer systems, in particular recyclable monomaterial multilayer systems, which are obtained or obtainable by the process according to the invention. The monomaterial multilayer systems according to the invention can be used, among other things, as preferably recyclable packaging material, in particular packaging material for food.

[0058] Since the monomaterial multilayer system according to the invention is suitable for producing high-quality recyclates, the invention consequently also relates to a process for obtaining a recyclate from waste containing a monomaterial multilayer system, which contains a monomaterial multilayer system according to the invention, wherein the extraction comprises the following steps: i. Comminution, washing, and optionally sterilization of the waste containing a monomaterial multilayer system to obtain a material mixture; ii. Optionally sorting the material mixture; iii. Optionally mixing the material mixture with further components, in particular with polyolefins; iv. Further processing, in particular melting and extruding, of the material mixture to obtain the recyclate.

[0059] This process is therefore a mechanical recycling process. In addition to the inventive monomaterial multilayer systems, non-inventive monomaterial multilayer systems, in particular recyclable monomaterial multilayer systems, as well as other additives (further components) such as synthetically produced or non-reused PE or PO granules (so-called "virgin" materials or "virgin polyolefins") can be used.

[0060] For the purposes of the invention, recyclate is understood to mean a product obtained at least after comminution and washing of the waste containing a monomaterial multilayer system and further processing. Preferably, the further processing involves an extrusion process, whereby the recyclate can then be obtained, for example, in granulate or strand form. The recyclate comprises at least one polyolefin and at least one 2-component polyurethane adhesive in cured form.

[0061] A homogeneous recyclate is produced, for example after melting and extrusion, in particular when from 7.0 to 25.0 wt.% [CH2-O-] n Units, preferably from 9.0 to 20.0 wt.% [CH2-O-] n Units, particularly preferably from 15.0 to 18.0 wt.% [CH2-O-] n units are contained in the cured two-component adhesive.

[0062] The recyclates according to the invention can be used to produce films or other molded parts, optionally with the addition of (further) virgin polyolefins. Consequently, the invention also relates to molded parts and films obtained or obtainable by processing a recyclate obtained or obtainable by the recycling process according to the invention.

[0063] Embodiments:

[0064] The invention particularly relates to the following embodiments:

[0065] According to a first embodiment, the invention relates to a method for producing a monomaterial multilayer system, in particular a recyclable monomaterial multilayer system, the method comprising or consisting of the following steps:

[0066] I) providing at least two monomaterial layers;

[0067] II) applying a two-component adhesive directly or indirectly to at least one of the two monomaterial layers provided, wherein the two-component adhesive comprises at least one isocyanate-reactive component and at least one isocyanate component;

[0068] III) Bonding the monomaterial layers using the two-component adhesive and curing the two-component adhesive to obtain the monomaterial multilayer system; characterized in that the material of the monomaterial layers comprises polyolefins and the proportion of the polyolefin is from 80.0 to 99.30 wt.%, based on the total weight of the monomaterial multilayer system, the proportion of the two-component adhesive is from 0.70 to 20.0 wt.%, based on the total weight of the monomaterial multilayer system, the isocyanate-reactive component and / or the isocyanate component [CH2-O] n - units and the cured two-component adhesive contains 7.0 to 25.0 wt.% of [CH2- O] n units, based on the total weight of the adhesive, where n is a number from 2 to 50.

[0069] According to a second embodiment, the invention relates to a method according to the first embodiment, characterized in that the isocyanate-reactive component of the two-component adhesive comprises or consists of polyols, in particular polyester polyols, polyether polyols, hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers, hydroxyl-containing reaction products of hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers and diisocyanates, or mixtures of the aforementioned compounds. According to a third embodiment, the invention relates to a method according to the first or second embodiment, characterized in that the isocyanate component of the two-component adhesive

[0070] • monomeric polyisocyanates, in particular hexamethylene-1,6-diisocyanate, pentamethylene-1,5-diisocyanate, 1,4-butane diisocyanate, isophorone diisocyanate, toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, diphenylmethane-2,2'-diisocyanate, diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate or mixtures thereof;

[0071] • Oligomeric polyisocyanates, which are made from at least two monomeric polyisocyanates;

[0072] • Isocyanate-terminated prepolymers which are prepared by reacting monomeric and / or oligomeric polyisocyanates and isocyanate-reactive compounds having > 1 isocyanate-reactive group per molecule, wherein the isocyanate-reactive compound preferably comprises polyols, more preferably hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers,

[0073] comprises or consists of polyacrylates, polycarbonates, polyesters, polyurethanes and / or polyethers;

[0074] • or mixtures of the aforementioned compounds, wherein the isocyanate-terminated prepolymers are preferably [CH2-O] n -units as defined in embodiment 1.

[0075] According to a fourth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the n of the [CH2-O] n - units is a number from 8 to 20.

[0076] According to a fifth embodiment, the invention relates to a method according to one of the preceding embodiments, characterized in that the two-component adhesive additionally contains organic solvents, catalysts, fillers, processing aids, adhesion promoters, in particular silanes, stabilizers, in particular antioxidants or mixtures thereof.

[0077] According to a sixth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that either the isocyanate-reactive component or the isocyanate component has an average molecular weight Mn of < 1000 g / mol and an average functionality of > 2 and the respective other component has an average molecular weight Mn of > 1000 g / mol and a functionality of < 4. According to a seventh embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the two-component adhesive has a stoichiometric ratio of isocyanate groups to isocyanate-reactive groups between 0.8:1 and 2:1.

[0078] According to an eighth embodiment, the invention relates to a method according to one of the preceding embodiments, characterized in that the polyolefin of the monomaterial layers is selected from the group comprising or consisting of polyethylene, polypropylene or mixtures thereof.

[0079] According to a ninth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the proportion of the polyolefin is from 95.0 to 99.3 wt.% and / or the proportion of the two-component adhesive is from 0.70 to 10.0 wt.%, preferably 1.0 to 5.0 wt.%, in each case based on the total weight of the monomaterial multi-layer system.

[0080] According to a tenth embodiment, the invention relates to a monomaterial multilayer system, in particular a recyclable monomaterial multilayer system, obtained or obtainable by a process according to one of embodiments 1 to 9.

[0081] According to an eleventh embodiment, the invention relates to the use of a monomaterial multilayer system according to embodiment 10 as a preferably recyclable packaging material, in particular packaging material for food.

[0082] According to a twelfth embodiment, the invention relates to the use of a two-component adhesive as defined in one of embodiments 1 to 7 in monomaterial multilayer systems, in particular for improving the recyclability of monomaterial multilayer systems, wherein the material of the monomaterial layers comprises polyolefins in a proportion of 80.0 to 99.3 wt.%, based on the total weight of the monomaterial multilayer system.

[0083] According to a thirteenth embodiment, the invention relates to a process for obtaining a recyclate from waste containing a monomaterial multilayer system, which contains a monomaterial multilayer system according to embodiment 10, wherein the recovery comprises the following steps: i. Comminution, washing, and optionally sterilization of the waste containing a monomaterial multilayer system to obtain a material mixture; ii. Optionally sorting the material mixture; iii. Optionally mixing the material mixture with further components, in particular with polyolefins; iv. Further processing, in particular melting and extruding, of the material mixture to obtain the recyclate. According to a fourteenth embodiment, the invention relates to molded parts obtained or obtainable by processing a recyclate obtained or obtainable according to the process according to embodiment 13.

[0084] The present invention is explained in more detail below using the following examples.

[0085] Measurement methods

[0086] • Gel permeation chromatography (GPC): The weight- and number-average molecular weights Mw and Mn of the resulting polymers were determined using gel permeation chromatography (GPC). The procedure was based on DIN 55672-1: "Gel permeation chromatography, Part I — Tetrahydrofuran as eluent." Polystyrene samples of known molecular weight were used for calibration. The polydispersity index (PDI) is calculated from the quotient of the weight-average and number-average molecular weights.

[0087] • The hydroxyl number (or OH number) of a compound is determined according to DIN EN ISO 4629-2, unless otherwise stated. N-methylpyrrolidone is replaced by pyridine in 5.1 and 5.5, and ethanol is used as the catalyst solvent instead of methanol in 5.2.

[0088] Materials:

[0089] The following raw materials and materials were used:

[0090] Raw materials for the production of polyoxymethylene-polyalkylene oxide copolymers

[0091] • Double metal cyanide (DMC) catalyst, prepared according to Example 6 in WQ2001 / 80994 A1 containing zinc hexacyanocobaltate, tert-butanol, and polypropylene glycol with a Mn of 1000 g / mol. Cyclic propylene carbonate (cPC), CAS No. 108-32-7, Merck

[0092] • Paraformaldehyde (pFA), Prefere Paraform GmbH&Co KG

[0093] • Propylene oxide (PO), Sigma-Aldrich

[0094] Raw material for the production of monomaterial layers

[0095] • BorShape® FX1001: Alpha-Olefin Terpolymer Polyethylene (Pellets)

[0096] Monomaterial layers

[0097] • Cast polypropylene (CPP) film: Cast polypropylene film without embossing PB81AB with 50 pm thickness; Manufacturer: Profol GmbH, Material number 15357005 Isocyanate-reactive component

[0098] • Glycerin: Glycerin is a trifunctional low molecular weight polyol.

[0099] • Baycoll® AD 2047: Baycoll® AD 2047 is a linear polyester polyol with OH functionality 2. The hydroxyl number is approximately 55.0 mg KOH / g.

[0100] • Acclaim® Polyol 18200 N: Acclaim® Polyol 18200 N is a linear polypropylene ether polyol with OH functionality 2. The hydroxyl number is approximately 6.0 mg KOH / g.

[0101] • Polyol 22: Linear polypropylene ether polyol with OH functionality 2. The hydroxyl number is approximately 4.9 mg KOH / g.

[0102] • Example - Preparation of a polyoxymethylene-polyalkylene oxide copolymer with DMC total catalyst loading of 1000 ppm and continuous pFA and PO addition:

[0103] In a 1.0 L pressure reactor, 10 g of pretreated pFA (pretreatment: 50 °C, 5 mbar, 1.5 h) and 300 mg of DMC catalyst were suspended in 150 g of cPC. At 60 °C, the mixture was inerted for 30 min with N2 (25 L / h) while stirring (500 rpm) at 30 mbar. The suspension was heated to 70 °C while stirring (1000 rpm). After the reactor temperature was reached, 20 g of propylene oxide (8.8 wt%) were quickly added to the suspension. The onset of the reaction was noticeable by a temperature peak ("hotspot") combined with a simultaneous pressure drop. The reactor temperature was then increased to 80°C, and 208 g of PO were added at a rate of 0.9 g / min and 62 g of paraformaldehyde (as a 20% suspension in cPC) at a rate of 1.7 g / min. After the addition was complete, stirring was continued until the exothermic reaction subsided and the pressure reached constant at 80°C. The product mixture was then removed and degassed on a rotary evaporator at 60°C and 10 mbar.The cPC suspension agent was removed under vacuum (thin-film evaporator, pre-evaporator: 150°C, main evaporator: 160°C, 2 mbar). This procedure was repeated several times, and the reaction products from five identical experiments were pooled.

[0104] The average molecular weight is approximately Mn(GPC) = 2605 g / mol and the PDI is 1.19.

[0105] • Example 2 - Production of OH prepolymer based on the raw material from Example 1

[0106] 1200 g of raw material from Example 1 are placed in a ground-glass vessel at 75°C. 0.23 g of 2-chloropropionic acid is added while stirring, followed by 107.5 g of Desmodur 44 M at 75°C. Stirring is continued at 75°C until the NCO% value drops to 0%. The viscosity of the prepolymer is 46,060 mPas at 23°C. The average molecular weight is approximately Mn (GPC) = 9600 g / mol. The OH number of the prepolymer is 14.0 mg KOH / g.

[0107] Isocyanate component

[0108] • Desmodur® 44 M: Desmodur® 44 M Liquid is a monomeric diisocyanate that is solid at room temperature and liquid above approximately 40 °C. The NCO content is approximately 33.6 wt.%.

[0109] • Desmodur® 2460 M: Desmodur® 2460 M is an isomer mixture of 2,4- and 4,4-diphenylmethane diisocyanate that is liquid above 20 °C. The NCO content is approximately 33.6 wt.%.

[0110] • Desmodur® ultra N 3300: Desmodur® ultra N 3300 is an aliphatic polyisocyanate (HDI trimer) with a low residual monomer content of < 0.1%. The NCO content is approximately 21.8% by weight.

[0111] • Desmodur® ultra N 3600: Aliphatic, low-viscosity polyisocyanate (HDI trimer) with a low residual monomer content of < 0.1%. The NCO content is approximately 23.0% by weight.

[0112] • Desmodur® E 23: Aromatic polyisocyanate prepolymer based on diphenylmethane diisocyanate (MDI). The NCO content is approximately 15.4 wt.%.

[0113] • Example 3: Production of NCO prepolymer based on the raw material from Example 1

[0114] At 75°C, 250 g of Desmodur 2460 M and 0.04 g of 2-chloropropionic acid are placed in a flat-bottomed vessel. Then, 750 g of the prepolymer from Example 2 are slowly added dropwise until a target NCO content of 7.3% is reached. The viscosity of the prepolymer is 36,300 mPas at 23°C. The average molecular weight is approximately Mn (GPC) = 7200 g / mol.

[0115] Production of two-component adhesives

[0116] The raw materials used to produce the two-component adhesive are heated at 60°C for 3 hours before formulation. The isocyanate-reactive and isocyanate components are placed in a beaker and mixed. Mixing is performed with a wooden spatula until a homogeneous mixture is achieved.

[0117] Inventive examples for the production of a two-component adhesive can be found in the following table. Table 1: Production of two-component polyurethane adhesives

[0118] Examples 8 and 9 are comparative examples (V).

[0119] Production of cured two-component adhesives

[0120] The polyurethane adhesives from Examples 4 to 9 were cured in Teflon trays (diameter: 15 cm / depth: 0.5 cm). The adhesive films were cured by storage at 23°C and 50% relative humidity for 7 days, followed by a subsequent storage at 60°C for 48 hours. The thickness of the respective adhesive films after drying was approximately 100 μm.

[0121] Production of monomaterial multilayer systems

[0122] A mono-material multi-layer system is produced using a handheld laminator, designated K Control Coater Model 623 / 624 (K101 / K202). A first film is clamped into the handheld laminator (cast polypropylene film, 50 μm thick; manufacturer: Profol GmbH). The films are DIN A4 size and were corona-treated before coating with the adhesive, ensuring a surface tension of > 50 dyn. The two-component adhesive is then applied to the top edge of the first clamped film in the form of a bead. The adhesive is then evenly distributed over the film using a squeegee. The two-component adhesive is heated to 100°C using a Steinei HL2002LE heat gun, resulting in a surface temperature of 60°C. In the next step, a second, identical film is laminated onto the coated first film.The resulting composite is then stored at 60°C for 24 hours. After storage and cooling to room temperature, the bond strength is determined in a 180°C peel test.

[0123] Determination of bond strength in the 180° peel test

[0124] To determine the peel strength, the composite films are cut into 15 mm wide strips. The strips are then peeled off at 23°C in a Zwick universal peel strength testing machine (type 5 kN Allround Table-top BT2-FA005TH.A50.002) at a speed of 100 mm / min at an angle of 2 x 90° (180°). The required force is measured in Newtons. The values ​​given are the averages of five individual measurements. The determined bond strength can be found in the table below.

[0125] Table 2: Bond strength

[0126] Sufficient bond strength is achieved when a value of at least 0.5 N / 15mm is achieved.

[0127] Process for obtaining a recyclate from raw materials for the production of monomaterial multilayer systems

[0128] The cured two-component adhesive films are cut into 0.5 cm x 0.5 cm pieces using scissors. In the extrusion process, 0.75 g of the respective two-component adhesives from Examples 4 to 9 are mixed with 14.25 g of BorShape® FX1001 PE granules and extruded. Extrusion is carried out in a laboratory extruder (Micro Compounder / DSM Xplore / Model 2005) at 200 °C and with a processing time of 6 minutes. The extrusion results in a homogeneous polymer strand.

[0129] Assessment of recyclate quality

[0130] A portion of the extruded polymer strand is pressed into thin polymer films with a thickness of 200 μm and a diameter of 5 cm using a hydraulic press (hand lever press / Vogt Maschinenbau GmbH / LaboPress P150H) at 200 °C, a processing time of 2 minutes, and a pressure of 30 kN. The resulting thin polymer films are cooled and used to test for homogeneity. Analogously, a recyclate can be produced directly from a monomaterial layer system or waste containing a monomaterial layer system. In this case, the adhesive and the PE granules are replaced by the monomaterial layer system (or the waste containing a monomaterial layer system). The test specimens or thin polymer films are placed under a Nikon Ni-E light microscope with a motorized stage, and optical micrographs of the sample surface are taken in transmitted light. The objective lens is used with a fourfold magnification.In order to obtain statistically reliable data, a total area of ​​11.84 x 10.47 cm is used. 2 of the sample surface using the automatic mapping

[0131] module that stitches 5 x 4 images. After the images are acquired, automatic image analysis is performed using Nikon NIS Elements BR version 5.4 software. Defects in the sample are identified by thresholding the intensity of areas within the sample from the rest of the sample. For defects with an optical density lower than that of the matrix, a threshold of 150 is applied. For defects with an optical density higher than that of the film, a threshold of 120 is applied. After separation, a further size restriction is applied to the results, excluding features with an equivalent diameter of less than 20 pm and a circularity of less than 0.1 to remove artifacts. The filtered results are then calculated, and the number of defects, the area fraction of the defects, and the mean equivalent diameter of the defects are reported.

[0132] Table 3: Evaluation of recycled material quality

[0133] Examples 10, 15 and 16 are comparative examples (V).

[0134] As can be seen from the results in Table 2, the recyclates according to the invention have a significantly higher homogeneity, expressed by the

[0135] Comparative examples 15 and 16 show a significantly lower number and total area of ​​defects.

Claims

1. A method for producing a monomaterial multilayer system, in particular a recyclable monomaterial multilayer system, the method comprising or consisting of the following steps: I) providing at least two monomaterial layers; II) applying a two-component adhesive directly or indirectly to at least one of the two monomaterial layers provided, wherein the two-component adhesive comprises at least one isocyanate-reactive component and at least one isocyanate component; III) Bonding the monomaterial layers using the two-component adhesive and curing the two-component adhesive to obtain the monomaterial multilayer system; characterized in that the material of the monomaterial layers comprises polyolefins and the proportion of the polyolefin is from 80.0 to 99.30 wt.%, based on the total weight of the monomaterial multilayer system, the proportion of the two-component adhesive is from 0.70 to 20.0 wt.%, based on the total weight of the monomaterial multilayer system, the isocyanate-reactive component and / or the isocyanate component [CH2-O] n - units and the cured two-component adhesive has a proportion of 7.0 to 25.0 wt.% of [CH2-O]n units, based on the total weight of the adhesive, where n is a number from 2 to 50.

2. The method according to claim 1, characterized in that the isocyanate-reactive component of the two-component adhesive comprises or consists of polyols, in particular polyester polyols, polyether polyols, hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers, hydroxyl-containing reaction products of hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers and diisocyanates or mixtures of the aforementioned compounds.

3. Method according to claim 1 or 2, characterized in that the isocyanate component of the two-component adhesive • monomeric polyisocyanates, in particular hexamethylene-1,6-diisocyanate, pentamethylene-1,5-diisocyanate, 1,4-butane diisocyanate, isophorone diisocyanate, isotoluene-2,4-diisocyanate, toluene-2,6-diisocyanate, diphenylmethane-2,2'-diisocyanate, diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate or mixtures thereof; • Oligomeric polyisocyanates, which are made from at least two monomeric polyisocyanates; • Isocyanate-terminated prepolymers which are prepared by reacting monomeric and / or oligomeric polyisocyanates and isocyanate-reactive compounds having > 1 isocyanate-reactive group per molecule, wherein the isocyanate-reactive compound preferably comprises polyols, more preferably hydroxyl-containing polyoxymethylene-polyoxyalkylene copolymers, comprises or consists of polyacrylates, polycarbonates, polyesters, polyurethanes and / or polyethers; • or mixtures of the aforementioned compounds, wherein the isocyanate-terminated prepolymers are preferably [CH2-O] n -units as defined in claim 1.

4. Process according to one of the preceding claims, characterized in that the n of the [CH2-O] n -units is a number from 8 to 20.

5. Process according to one of the preceding claims, characterized in that the two-component adhesive also contains organic solvents, catalysts, fillers, processing aids, adhesion promoters, in particular silanes, stabilizers, in particular antioxidants or mixtures thereof.

6. Process according to one of the preceding claims, characterized in that either the isocyanate-reactive component or the isocyanate component has an average molecular weight Mn of < 1000 g / mol and an average functionality of > 2 and the respective other component has an average molecular weight Mn of > 1000 g / mol and a functionality of < 4.

7. Method according to one of the preceding claims, characterized in that the two-component adhesive has a stoichiometric ratio of isocyanate groups to isocyanate-reactive groups between 0.8:1 and 2:

1.

8. Process according to one of the preceding claims, characterized in that the polyolefin of the monomaterial layers is selected from the group comprising or consisting of polyethylene, polypropylene or mixtures thereof.

9. Process according to one of the preceding claims, characterized in that the proportion of polyolefin is from 95.0 to 99.3 wt.% and / or the proportion of the two-component adhesive is from 0.70 to 10.0 wt.%, preferably 1.0 to 5.0 wt.%, in each case based on the total weight of the monomaterial multi-layer system.

10. A monomaterial multilayer system, in particular a recyclable monomaterial multilayer system, obtained or obtainable by a process according to any one of claims 1 to 9.

11. Use of a monomaterial multilayer system according to claim 10 as a preferably recyclable packaging material, in particular packaging material for food.

12. Use of a two-component adhesive as defined in any one of claims 1 to 7 in monomaterial multilayer systems, in particular for improving the recyclability of monomaterial multilayer systems, wherein the material of the monomaterial layers comprises polyolefins in a proportion of 80.0 to 99.3 wt.%, based on the total weight of the monomaterial multilayer system.

13. A process for obtaining a recyclate from waste containing a monomaterial multilayer system, which contains a monomaterial multilayer system according to claim 10, wherein the recovery comprises the following steps: i. Comminution, washing, and optionally sterilization of the waste containing a monomaterial multilayer system to obtain a material mixture; ii. Optionally sorting the material mixture; iii. Optionally mixing the material mixture with further components, in particular with polyolefins; iv. Further processing, in particular melting and extruding, of the material mixture to obtain the recyclate.

14. Molded parts obtained or obtainable by processing a recyclate obtained or obtainable by the process according to claim 13.

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