Methods for the production of sustainable adhesive elements and adhesive elements

Abstract

Method for manufacturing an adhesive element, comprising the following process steps: a) Manufacturing or providing a polymer composition, comprising: i) one or more polypropylene recyclates, wherein at least one of the polypropylene recyclates is an hSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and having a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 4 g / (10 min) to 15 g / (10 min), ii) wherein at least one of the polypropylene recyclates is an nSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and has a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230°C of less than 4 g / (10 min). iii) one or more virgin polypropylene materials, wherein the one or more virgin polypropylene materials have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 2.0 g / (10 min) or less, where the combined mass fraction of all polypropylene recyclates and virgin polypropylene materials is 90% or more, based on the mass of the polymer composition, wherein the combined mass fraction of all polypropylene recyclates is 50% or more, based on the mass of the polymer composition, wherein the combined mass fraction of all hSFR-PCR polypropylene recyclates is 20% or more, based on the combined mass of all polypropylene recyclates in the polymer composition, c) Extruding the polymer composition to form a plastic film, d) Stretching the plastic film to obtain a monoaxially stretched plastic film, and e) Coating the monoaxially stretched plastic film with an adhesive compound to create an adhesive element.

Description

[0001] The invention relates to a method for producing an adhesive element and an adhesive element produced by the method.

[0002] Joining separate components is a key process in manufacturing technology. Alongside other methods, such as welding and soldering, bonding—that is, joining using an adhesive—is of particular importance today. An alternative to using formless adhesives, such as those applied from a tube, are adhesive tapes, whose adhesive effect is based on the adhesive compounds used. The use of adhesive tapes offers numerous advantages, making this technology ideal for use in many industries, such as electronics and automotive.

[0003] In most cases, such adhesive tapes are multi-component materials, comprising not only the adhesive but also a backing layer, usually made of a plastic. The plastic used in these backing layers is often derived from fossil resources. Given the growing awareness of the need for more sustainable economic practices and a more careful use of fossil resources, this is regularly perceived as a disadvantage.

[0004] Against this background, there has been an effort in the field of adhesive technology for several years to increase the sustainability of adhesive tapes and, in particular, to reduce the need for fossil resources in their production.

[0005] One approach used here is the application of bio-based plastics. These bio-based plastics are not produced—or at least not entirely—from fossil raw materials, but contain building blocks derived from biomass. In recent years, a wide range of bio-based plastics and processes for their production have been developed, which are now used in various technical fields. These include, for example, polylactic acid (PLA), cellulose acetate (CA), and thermoplastic starch (TPS).

[0006] A promising alternative to the use of bio-based plastics is the use of recycled plastic materials, so-called recyclates, to reduce dependence on fossil raw materials.

[0007] In principle, the use of such recycled plastics is desirable because it opens up additional material sources and allows the use of materials that would otherwise be waste and often only be subject to thermal recycling, instead of fossil raw materials.

[0008] Experts distinguish between plastic recyclates, in particular, between those plastic wastes that arise directly from the manufacturing processes of plastic products and those that have previously been used for their intended purpose. The latter are often collected from households and recycled. Waste from industrial production is also referred to as "post-industrial recyclate" (PIR). PIR plastic recyclates typically have the advantage of being relatively pure and uncontaminated, or only minimally contaminated, with foreign substances. Waste generated after intended use is also referred to as "post-consumer recyclate" (PCR) and includes, for example, food or cleaning product packaging.PCR plastic recyclates are regularly less pure and have to contend with the problem in their production that, for example, household waste is regularly contaminated with a variety of other substances, so that the production of high-performance PCR plastic recyclates, which would approach the properties of PIR plastic recyclates or even virgin plastic materials, is very complex and involves a high expenditure of time and / or money.

[0009] At the same time, many industries have a strong interest in using PCR-recycled plastics as a secondary raw material for the production of new products, since the underlying post-consumer waste constitutes a very large proportion of the total waste volume, and the use of PCR-recycled materials would provide a significantly broader raw material base. Further background information on PCR and PIR-recycled plastics is available, for example, in the standard DIN EN ISO 14021:2016-07.

[0010] Modern adhesive products, such as adhesive tapes or die-cuts, are high-performance products whose manufacture regularly relies on the use of high-performance raw materials. For this reason, the use of PCR plastic recyclates in adhesive tape production, while desirable in principle, has not yet become widespread.

[0011] In many cases, this is due to the fact that very high demands are placed on the backing layers of adhesive tapes and their properties. Besides the physicochemical properties, such as chemical resistance, this particularly concerns the mechanical properties of the backing layers, for example, tear strength, impact strength, flexural stiffness, and tensile strength. These properties are of particular relevance for the production of high-performance adhesive tapes, since the adhesive layer applied to the backing layers, which gives the tapes their stickiness, typically contributes so little to these properties that the overall properties are significantly influenced by the backing material.Furthermore, especially with adhesive tapes, it is necessary that the adhesive layers used exhibit advantageous adhesion to the plastic used as a carrier material, so that when the adhesive tape is peeled off, there is no undesirable tearing of the bond at the interface between the adhesive layer and the carrier layer.

[0012] The aforementioned requirements are regularly further tightened by the fact that the respective plastics of the carrier layer are often intended for use as very thin carrier films in many high-performance applications and must also frequently be suitable for applications where they are exposed to elevated temperatures over the long term. Therefore, the specifications for the mechanical properties must be guaranteed, especially at these elevated temperatures. Advantageous temperature resistance is also relevant in the production of the adhesive tapes, since elevated temperatures are also used in the necessary coating steps, for example, during extrusion coating, or in drying processes in the case of solvent-based coatings.

[0013] Polypropylene is an important raw material for the production of backing layers for adhesive elements. In principle, the use of PCR-recycled polypropylene for the production of adhesive tape backing layers, or for the necessary film production, would be particularly desirable in order to utilize this plastic, which is especially advantageous for adhesive element production due to its physicochemical and mechanical properties, more sustainably and with a more favorable CO2 balance.

[0014] The aforementioned problem of the poor availability of PCR plastic recyclates is particularly problematic for PCR polypropylene recyclates, which would be suitable for use in the production of polypropylene film. According to the inventors, no high-performance PCR polypropylene plastic recyclate with film quality is currently commercially available, at least not on the large-scale industrial scale that would be desirable for use in adhesive element production.

[0015] According to the inventors, this problem arises primarily from the fact that polypropylene film waste, which contains a polypropylene that would naturally be better suited for film production, is not processed as a separate recycling stream in most countries, or at least in most European countries, such as Germany, within the framework of standard household plastics recycling. For example, the German recycling industry has defined and standardized recycling streams to make recycling and sorting processes as efficient and cost-effective as possible. For instance, there are...Fraction 329, polyethylene, consisting of HDPE originating from bottles and containers; fraction 310, plastic films, consisting exclusively of LDPE; and fraction 324, polypropylene, the so-called "PP rigid" fraction, consisting of PP injection-molded parts such as bottles and containers (e.g., yogurt cups). However, a separate collection fraction for PP film waste does not regularly exist. According to the inventor, the recycling industry does not intend to introduce such a PP film fraction in the near future, as demand and the quantities of recyclate are too low. Consequently, sufficient quantities of PCR-recycled polypropylene film waste are not regularly available in isolation, as isolated processing is not economically viable for most recycling companies.Rather, the polypropylene plastic film waste, often previously separated in an air classifier, is mixed with other batches, particularly fraction 324 "PP rigid", or directly thermally recycled. In Germany, fraction 324, for example, includes used, completely emptied, dimensionally stable, system-compatible plastic articles made of PP with a volume of 5 liters or less, such as bottles, trays, and cups, including ancillary components like closures, labels, etc.

[0016] The recycled polypropylene streams obtained from typical recycling processes focus on a material which, as explained above, is often referred to as "PP rigid", which alludes to the fact that it is polypropylene from mostly injection-molded products with a certain wall thickness, for example yogurt cups or packaging for cleaning agents.This PCR polypropylene, which is obtained in Germany, for example, as fraction 324 polypropylene, would be available in sufficient quantities and could be used economically. However, in accordance with expert understanding, due to the material properties of the corresponding polypropylene, it is essentially referred to as "injection molding quality," which indicates that the corresponding PCR polypropylene is generally considered unsuitable for film production, which would be needed for the manufacture of carrier layers, but rather is used in injection molding processes, which in turn serve the production of typical packaging.

[0017] German patent DE 10 2012 215 133 A1 discloses a strapping tape designed for high mechanical loads, particularly for bundling, strapping, or reinforcing packaging. The tape is based on a monoaxially oriented polypropylene carrier film, which exhibits high tensile strength and low elongation. A pressure-sensitive adhesive compound based on natural or synthetic elastomers, resins, plasticizers, and fillers is applied to this film. The carrier film is a monoaxially oriented polypropylene film containing at least 80% polypropylene by weight. Multiple recycled polypropylene can also be used.

[0018] The primary objective of the present invention was to eliminate or at least reduce the disadvantages of the prior art.

[0019] In particular, it was an object of the present invention to provide a method for producing an adhesive element which enables sustainable adhesive element production while reducing the need for fossil raw materials.

[0020] One objective of the present invention was to enable the carrier layers required for the production of adhesive elements to be manufactured as far as possible from PCR-recycled polypropylene. In this respect, it was important that the described process be able to draw on a broad raw material base in order to ensure that sufficient PCR-recycled polypropylene is available for large-scale adhesive element production, even considering the existing recycling routines of typical recycling companies.

[0021] It was an object of the present invention that the specified method should be suitable for the production of particularly high-performance adhesive elements which, with regard to their physicochemical and mechanical properties, should not be, or at least only insignificantly, deteriorated compared to adhesive elements made of virgin polypropylene materials, so that a high level of user acceptance for the sustainable products can be achieved.

[0022] It was a complementary objective of the present invention that the specified method should not rely on additional environmentally or health-damaging substances compared to conventional manufacturing methods and should ideally be feasible with such plants and devices that are already available to the person skilled in the art in the field of adhesive element production.

[0023] A further objective of the present invention was to provide a high-performance, sustainable adhesive element which can be manufactured using the specified method.

[0024] The inventors of the present invention have now found that the problems described above can surprisingly be solved if, contrary to existing prejudice, PCR-recycled polypropylene is used in the film production preceding the manufacture of the adhesive element, which is actually not intended or suitable for film production, namely "PP rigid" or "injection molding grade", which in Germany, for example, corresponds to fraction 324 polypropylene, if this is optimized by clever formulation, namely by mixing with virgin polypropylene material, and is subsequently subjected to monoaxial stretching after film production, as defined in the claims.

[0025] What was particularly surprising was that, contrary to existing prejudices, such high quantities of polypropylene recyclates, and in turn such high quantities of PCR-recycled polypropylene recyclates with injection molding quality, can be used. According to the inventors, in addition to the formulation of the appropriate polymer composition, monoaxial stretching is of particular importance, as it apparently exhibits a surprisingly high suitability for the use of PCR-recycled polypropylene intended for injection molding, compared to non-stretched or biaxially stretched films. In Germany, this corresponds, for example, to fraction 324 polypropylene.

[0026] The inventive process yields a polypropylene film which, in the inventors' estimation, would likely be considered inadequate for some other high-performance applications outside of adhesive technology. However, the specific requirements for carrier layers of adhesive elements are surprisingly well met by the resulting PCR-MOPP films, particularly when produced by flat film extrusion (cast process), thus advantageously reconciling the existing conflicts of objectives between a sustainable raw material base and the high demands placed on the carrier layers.This is particularly surprising because the advantageous suitability of the monoaxially stretched MOPP films produced in this way is also advantageously demonstrated for demanding applications such as "strapping tape" or "tear tape", for which carrier layers with high tensile and elongation strengths with simultaneous stability against "splicing" and "shredding" are required, which according to prevailing opinion necessitates the use of PP polymers with high molecular weights (long chain densities, low melt flow rate), which PCR polypropylene with "injection molding grade", which in Germany, for example, is obtained as fraction 324 polypropylene, does not exhibit.

[0027] The aforementioned problems are thus solved by the subject matter of the invention as defined in the claims. Preferred embodiments of the invention are described in the dependent claims and the following descriptions.

[0028] Such embodiments, which are hereinafter referred to as preferred, are combined in particularly preferred embodiments with features of other embodiments referred to as preferred. Combinations of two or more of the embodiments referred to below as particularly preferred are therefore especially preferred. Also preferred are embodiments in which a feature of one embodiment, referred to as preferred to any extent, is combined with one or more further features of other embodiments, which are referred to as preferred to any extent. Features of preferred adhesive elements result from the features of preferred methods.

[0029] Insofar as specific amounts or proportions of an element, for example, high-melt-flow PCR-PCR polypropylene recyclate (hSFR-PCR polypropylene recyclate) or virgin polypropylene material, as well as preferred embodiments of the element, are disclosed below, the specific amounts or proportions of the preferably embodiments are also disclosed. Furthermore, it is disclosed that, within the corresponding specific total amounts or proportions of the elements, at least some of the elements may be preferably embodiments, and in particular, that preferably embodiments may, within the specific total amounts or proportions, again be present in specific amounts or proportions.

[0030] The invention relates in particular to a method for producing an adhesive element, comprising the following process steps: a) Manufacturing or providing a polymer composition, comprising: i) one or more polypropylene recyclates, wherein at least one of the polypropylene recyclates is an hSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and having a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 4 g / (10 min) to 15 g / (10 min), ii) wherein at least one of the polypropylene recyclates is an nSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and has a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of less than 4 g / (10 min), and iii) one or more virgin polypropylene materials, wherein the one or more virgin polypropylene materials have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 2.0 g / (10 min) or less, where the combined mass fraction of all polypropylene recyclates and virgin polypropylene materials is 90% or more, based on the mass of the polymer composition, wherein the combined mass fraction of all polypropylene recyclates is 50% or more, based on the mass of the polymer composition, wherein the combined mass fraction of all hSFR-PCR polypropylene recyclates is 20% or more, based on the combined mass of all polypropylene recyclates in the polymer composition, c) Extruding the polymer composition to form a plastic film, d) Stretching the plastic film to obtain a monoaxially stretched plastic film, and e) Coating the monoaxially stretched plastic film with an adhesive compound to create an adhesive element.

[0031] The method according to the invention serves to produce an adhesive element, and in principle, a wide range of adhesive elements can be produced using the method, for example, die-cut parts or adhesive labels. The method according to the invention is particularly suitable for the production of adhesive tapes. A preferred method according to the invention is one in which the adhesive element is a single-sided or double-sided adhesive tape.

[0032] In process step a), a polymer composition is initially provided, which in the subsequent processing steps is to be extruded, granulated, and formed into a plastic film, which is then stretched and coated with an adhesive. The polymer composition can be produced by mixing the components, preferably in the solid state, for example, by mixing the corresponding starting granules. Accordingly, a process according to the invention is preferred, wherein the polymer composition is produced in process step a), preferably by mixing the polypropylene recyclates with the virgin polypropylene materials.

[0033] The central components of the polymer composition are one or more recycled polypropylene materials as well as virgin polypropylene material.

[0034] These components, defined above, are each referred to as "one or more" in accordance with industry practice. The term "one or more" refers, in the usual way, to the chemical nature of the respective compounds and not to their quantity. For example, the polymer composition of virgin polypropylene may consist exclusively of one type of virgin polypropylene, which would mean that the polymer composition comprises a multitude of the corresponding molecules. The mass fractions are usually expressed as the combined mass fractions of the one or more components, thus indicating that the mass fraction of the respective components, taken together, meets the relevant criteria.

[0035] In the process according to the invention, recycled polypropylene and virgin polypropylene materials are used. Polypropylene is a polyolefin that is thoroughly familiar to those skilled in the art based on their general technical knowledge. Different polypropylenes can differ—as is typical for polymers—in their physicochemical properties, for example, in their average molecular weights and melt flow rates, with these properties being largely determined by the processes used to produce the polypropylenes.

[0036] A person skilled in the art understands that many of the polypropylenes available on the market, which are marketed as such, are not strictly speaking pure polypropylenes, but rather copolymers. These copolymers are produced by polymerizing a monomer composition that may include other copolymers besides propylene. These copolymers serve to optimize the physicochemical properties of the polypropylene for the specific application and, in practice, do not lead a person skilled in the art to believe that the material is no longer polypropylene.In accordance with this skilled person's understanding, within the scope of the present invention a (co-)polymer is then referred to as polypropylene if it can be produced by polymerization of a monomer composition, wherein the mass fraction of propylene in the monomer composition comprises 70% or more, preferably 80% or more, particularly preferably 90% or more, and most particularly preferably 95% or more, based on the mass of the monomer composition.This can also be expressed to a good approximation and for essentially all practically relevant cases via the mass fraction of the polymer building blocks derived from propylene in the polymer chain, such that the (co-)polymers referred to as polypropylene in the scope of the invention comprise repeating units derived from polypropylene in a mass fraction of 70% or more, preferably 80% or more, particularly preferably 90% or more, very preferably 95% or more, and particularly preferably essentially 100%, based on the mass of the polymer chain.

[0037] In accordance with expert understanding, there may be minimal differences between the two preceding definitions, particularly depending on the polymerization process, but these differences are negligible for the practical identification of polypropylene. If, contrary to expectations, a polypropylene were to fall so close to the boundary that it could only be considered polypropylene according to one of the preceding definitions, the definition based on the proportions in the monomer composition used for its manufacture shall prevail, especially since this is easier for the skilled person to understand in practice and requires less complex analysis.

[0038] It is clear to those skilled in the art that polypropylene recyclates can include not only polypropylene but also other additives, such as lubricants, fillers, pigments, aging agents, or UV protectants. Accordingly, within the scope of the present invention, such recyclates are understood to be polypropylene recyclates which contain polypropylene in a mass fraction of 85% or more, preferably 90% or more, and particularly preferably 95% or more, based on the mass of the polypropylene recyclate.

[0039] An important intermediate product of the process according to the invention is the unstretched PP plastic film. In accordance with this objective, it is defined that the combined mass fraction of polypropylene materials, i.e., recycled polypropylene and virgin polypropylene, in the polymer composition should be 90% or more, whereby, in the inventor's opinion, it is preferable to provide the highest possible basic polypropylene content, and it is particularly advantageous to reduce the content of polymers other than polypropylene.

[0040] A preferred method according to the invention is one in which the combined mass fraction of all polypropylene recyclates and virgin polypropylene materials is 92.5% or more, preferably 95% or more, particularly preferably 97% or more, and especially preferably 99% or more, based on the mass of the polymer composition. A preferred additional or alternative method according to the invention is one in which the combined mass fraction of all polymers other than polypropylene in the polymer composition is 10% or less, preferably 7.5% or less, particularly preferably 5% or less, and most preferably 2.5% or less, based on the mass of the polymer composition.

[0041] While the content of foreign polymers should ideally be kept low, it is at least conceivable to add further additives to the polymer composition, for example to optimize its aging and / or processing properties. A method according to the invention is particularly preferred for demanding applications, wherein the polymer composition, based on the mass of the polymer composition, additionally comprises: iii) one or more additives in a combined mass fraction in the range of 0.1 to 5%, preferably in the range of 0.2 to 3%, particularly preferably in the range of 0.3 to 2%, and most preferably in the range of 0.4 to 1%. A method according to the invention is particularly preferred, wherein the one or more additives are selected from the group consisting of oxidizing agents, UV protectants, dyes, plasticizers, antistatic agents, antiblocking agents, and processing aids.

[0042] The polymer composition comprises recycled polypropylene. The term "recycled" is clearly defined for those skilled in the art and is specified, for example, in DIN EN ISO 14021:2016-07. This refers to polypropylene materials that were not produced immediately beforehand through polymerization and the usual subsequent steps of polymer production, but rather were previously used in the manufacture of products. Typically, this means that recycled polypropylene has been extruded at least once and shaped in a forming process not intended for storage as bulk material, such as into the form of a film or packaging.To achieve a sufficient sustainability effect in the manufactured adhesive elements, it is defined above that the combined mass fraction of polypropylene recyclates in the polymer composition must meet a certain minimum value, which is advantageously possible in the process according to the invention. In this respect, the inventors have succeeded in specifying particularly preferred mass fractions for the polypropylene recyclates. A preferred method according to the invention is one in which the combined mass fraction of all polypropylene recyclates is 60% or more, preferably 70% or more, based on the mass of the polymer composition. Additionally or alternatively, a preferred method according to the invention is one in which the combined mass fraction of all polypropylene recyclates is in the range of 50% to 90%, preferably in the range of 55% to 85%, and particularly preferably in the range of 60% to 80%, based on the mass of the polymer composition.

[0043] According to the invention, the polymer composition comprises a specific type of polypropylene recyclate, which, for clear identification within the scope of the present invention, is referred to as hSFR-PCR polypropylene recyclate. The name identifies it as a polypropylene recyclate, with the suffix PCR indicating that it is a "post-consumer recyclate" as defined in DIN EN ISO 14021:2016-07. In accordance with this definition, hSFR-PCR polypropylene recyclate is produced by recycling plastic waste from plastic products after their intended use, for example, household waste.The suffix "hSFR" indicates that these polypropylene recyclates exhibit a high melt flow rate, typical for materials made from rigid PP and typically intended for injection molding. In Germany, for example, such materials are obtained from recycling as fraction 324 polypropylene. Within the scope of the present invention, the melt flow rate is determined according to DIN EN ISO 1133-1:2012-03, with the determination being carried out for 2.16 kg at a temperature of 230 °C. Generally, the process will be according to the invention, wherein one or more hSFR-PCR polypropylene recyclates are produced at least partially, preferably predominantly, and particularly preferably substantially entirely, by recycling plastic waste that is not film waste.

[0044] In the vast majority of cases, the process will be according to the invention, wherein one or more hSFR-PCR polypropylene recyclates are produced at least partially, preferably predominantly, and particularly preferably substantially completely, by recycling plastic waste from plastic products selected from the group consisting of rigid polypropylene packaging, in particular bottles and rigid food packaging, for example yogurt cups.

[0045] The inventors recognized that hSFR-PCR polypropylene recyclates with even higher melt flow rates can advantageously be used without significantly compromising the quality of the manufactured adhesive tapes, thus advantageously opening up a particularly broad range of available materials. At the same time, the inventors were able to identify, through their own experiments, areas outside which the melt flow rate of the hSFR-PCR polypropylene recyclates may be associated with limitations in the resulting product quality, particularly at high concentrations of the corresponding injection molding materials. A preferred method according to the invention is one in which the one or more hSFR-PCR polypropylene recyclates have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 6 g / (10 min) to 15 g / (10 min), preferably 8 g / (10 min) to 15 g / (10 min).According to the invention, one or more hSFR-PCR polypropylene recyclates have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C in the range of 4 to 15 g / (10 min), preferably in the range of 6 to 14 g / (10 min), particularly preferably in the range of 8 to 13 g / (10 min).

[0046] A preferred method according to the invention is wherein the one or more hSFR-PCR polypropylene recyclates have a modulus / tensile modulus according to DIN EN ISO 178:2019-08 in the range of 600 to 1800 MPa, preferably in the range of 700 to 1500 MPa, particularly preferably in the range of 800 to 1200 MPa.

[0047] To fully exploit the advantages that the inventive process surprisingly makes "injection-moldable grades" accessible for film production in the context of adhesive element manufacturing, a minimum proportion of hSFR-PCR polypropylene recyclates in the total mass of polypropylene recyclates is defined above. Advantageously, the proportion of these hSFR-PCR polypropylene recyclates, which are generally considered unsuitable for film production, can also be chosen to be higher, particularly when the total proportion of polypropylene recyclates in the polymer composition is lower. The inventors have succeeded in identifying particularly suitable mass fractions for the hSFR-PCR polypropylene recyclate content.A preferred method according to the invention is one in which the combined mass fraction of all hSFR-PCR polypropylene recyclates is 30% or more, preferably 40% or more, based on the combined mass of all polypropylene recyclates in the polymer composition. A further or alternative preferred method according to the invention is one in which the combined mass fraction of all hSFR-PCR polypropylene recyclates is in the range of 20% to 90%, preferably in the range of 30% to 80%, and particularly preferably in the range of 40% to 70%, based on the combined mass of all polypropylene recyclates in the polymer composition.

[0048] The suitability of the polymer composition for film production is achieved in the process according to the invention by adding virgin polypropylene material, which is used as a "booster", whereby there is a conflict of objectives between the most sustainable production possible and optimized material properties.

[0049] The term "virgin material" is clear to those skilled in the field of plastics processing and refers to materials that are not recycled, i.e., in particular, materials that have not previously been shaped by a forming process into a form other than that intended for storage as bulk material, such as films or packaging. Such virgin polypropylene materials typically originate directly from polymer production, i.e., the polymerization of monomers followed by the usual purification and processing steps involved in polymer production. These virgin polypropylene materials are commercially available from various suppliers.

[0050] In this respect, the inventors have succeeded in identifying advantageous mass fractions for the content of virgin polypropylene. A preferred method according to the invention is one in which the combined mass fraction of virgin polypropylene is 10% or more, preferably 15% or more, particularly preferably 20% or more, very preferably 25% or more, and particularly preferably 30% or more, based on the mass of the polymer composition.

[0051] The inventors recognized that it is advantageous to use, in particular, virgin polypropylene materials that have been optimized by manufacturers for the production of films, thus exhibiting "film-grade" properties, characterized by low melt flow rates. According to the invention, one or more virgin polypropylene materials have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 2.0 g / (10 min) or less, preferably 1.5 g / (10 min) or less, and most preferably 1.0 g / (10 min) or less. Preferably, or alternatively, a method according to the invention is used, wherein one or more virgin polypropylene materials have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C in the range of 0.2 to 2.0 g / (10 min), preferably in the range of 0.5 to 1.5 g / (10 min).

[0052] As explained above, PCR-recycled polypropylene recyclates, whose melt flow rate would be better suited for film production, are generally not available in sufficient quantities due to the typical processes involved in plastics recycling. In this respect, however, the inventors propose that, should suitable PCR-recycled polypropylene be available from PP film waste, it can advantageously be blended with the rigid PP. This would allow for a particularly high overall proportion of PCR-recycled material and a reduction in the mass fraction of virgin polypropylene, thereby reducing the need for fossil-based raw materials. This is generally a process according to the invention, wherein one or more nSFR-PCR-recycled polypropylene recyclates are produced at least partially, preferably predominantly, and most preferably substantially completely, by recycling plastic films.According to the invention, at least one of the polypropylene recyclates used is an nSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and having a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of less than 4 g / (10 min), preferably less than 3.5 g / (10 min), particularly preferably less than 3 g / (10 min), most preferably less than 2.5 g / (10 min), and most preferably less than 2 g / (10 min). A preferred or alternative method according to the invention is further preferred, wherein the combined mass fraction of all nSFR-PCR polypropylene recyclates is 30% or more, preferably 40% or more, based on the combined mass of all polypropylene recyclates in the polymer composition.A preferred or alternative method according to the invention is also preferred, wherein the combined mass fraction of all nSFR-PCR polypropylene recyclates is in the range of 20% to 70%, preferably in the range of 30% to 65%, particularly preferably in the range of 40% to 60%, based on the combined mass of all polypropylene recyclates in the polymer composition.

[0053] Similarly, PIR polypropylene recyclates, i.e., recyclates from industrial waste generated during film production, also offer great potential for achieving a particularly high overall proportion of PP recyclate. Due to their high purity and availability with low melt flow rates, such PIR polypropylene recyclates can also be used to further reduce the proportion of virgin polypropylene. In this context, a method according to the invention is preferred, wherein at least one of the polypropylene recyclates is a PIR polypropylene recyclate produced by recycling plastic waste from industrial manufacturing, preferably from the film production sector.Preferably, or alternatively, a method according to the invention is used, wherein the one or more PIR polypropylene recyclates have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 6.0 g / (10 min) or less, preferably of 5.0 g / (10 min) or less, particularly preferably of 4.0 g / (10 min) or less, and / or wherein the one or more PIR polypropylene recyclates have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C in the range of 0.1 to 50 g / (10 min), preferably in the range of 0.1 to 35 g / (10 min), particularly preferably in the range of 0.1 to 20 g / (10 min).

[0054] In process step c), the polymer composition assembled as above is extruded to form a plastic film. In principle, further additives or additional polymer material can be added in this step; however, in the inventors' opinion, it is advantageous in the vast majority of cases to forgo this for reasons of time and cost efficiency and to use the pure extruded polymer composition. A process according to the invention is conceivable in which the polymer composition is produced in the extruder used for extrusion, for example, by adding the virgin polypropylene material only in the extruder.

[0055] However, according to the inventors, it may be preferable to produce the polymer composition before film extrusion, thereby advantageously obtaining particularly homogeneous starting materials for film extrusion. A process according to the invention is particularly preferred, additionally comprising an intermediate process step. a) and process step c) the process step: b) Extruding the polymer composition to obtain a plastic granulate.

[0056] In this case as well, it is preferred to process polymer compositions that are as pure as possible. A preferred method according to the invention is one in which the plastic granules consist of the polymer composition to a mass fraction of 95% or more, preferably 98% or more, particularly preferably 99% or more, and most preferably essentially 100%.

[0057] For extruding the polymer composition, which is more demanding than pure virgin polypropylene, the inventors have succeeded in identifying particularly advantageous processing temperatures during the development of the present invention. These temperatures are beneficial for both film extrusion and any prior granulation. A preferred method according to the invention involves extruding the polymer composition to obtain plastic granules and / or extruding it to form the plastic film, preferably both extruding the polymer composition to obtain plastic granules and extruding it to form the plastic film, at a temperature T. EThe processing takes place in the range of 170 to 240 °C, preferably in the range of 180 to 220 °C. According to the inventors, using the lowest possible processing temperature is particularly advantageous when using recycled materials, as this minimizes further damage (in the case of PP chain severance) to the polymer chains caused by the processing temperature. Accordingly, these preferred processes yield particularly high-performance films, especially with high recycled content, particularly hSFR-PCR-polypropylene recycled material.

[0058] For extrusion to produce the plastic granules, multi-screw extruders have proven effective due to their advantageous mixing action, whereas extrusion for film production can be carried out efficiently with a single-screw extruder, preferably with prior granulation in this case. Accordingly, a method according to the invention is preferred in which the extrusion of the polymer composition in process step b) takes place in an extruder with two or more extruder screws. Additionally or alternatively, a method according to the invention is also preferred in which the extrusion of the polymer composition for forming the plastic film in process step c) takes place in an extruder with a single extruder screw.

[0059] Advantageously, the typical technologies for film production can be used in film extrusion, and in particular, the production of multilayer carrier films is also possible. A preferred method according to the invention is one in which the plastic film is a cast film or a blown film. A preferred additional or alternative method according to the invention is one in which the plastic film is a single-layer or multilayer carrier film, preferably a single-layer film. In the case of the production of multilayer films, it is advantageous if the outer layers each consist of a thin layer of virgin PP material in order to guarantee a homogeneous, smooth film surface.

[0060] The inventor considers it particularly preferable that the plastic film is formed by means of flat film extrusion (cast process), since this process, compared to blown film extrusion, which is generally considered the standard process in film production, exhibits a significantly lower susceptibility to the material defects caused by the use of "rigid PP", making cast films particularly preferred.

[0061] An important part of the inventive method consists in the monoaxial stretching of the produced plastic film, resulting in a monoaxially oriented film. The term monoaxial stretching is self-explanatory to those skilled in the art. In monoaxial stretching, a plastic film, for example a cast or blown film, preferably a cast film, is heated inline during film production or offline in a separate process by means of rollers or in a heat channel to a temperature above the glass transition temperature Tg. g , especially more than 20 °C above Tg, but below the melting temperature T mThe substrate material is temperature-controlled. The stretching takes place, for example, between two or more rollers running at increasing speeds. The temperature control during stretching should be very constant and homogeneous. Overall, with regard to the stretching and the process of maintaining the appropriately oriented substrate layer, reference can be made to the methods known in the art for stretching plastic films, such as those used for PET. In practice, in the vast majority of cases, a method according to the invention will be used, wherein the plastic film is stretched in the machine direction.

[0062] According to the inventors, particularly advantageous adhesive elements are obtained when comparatively high stretching factors are used, for which the material obtained from the polymer composition is surprisingly well suited. A preferred method according to the invention is therefore one in which the stretching of the plastic film is carried out with a stretching factor of 1:5 or more, preferably 1:6 or more, and most preferably with a stretching factor in the range of 1:6 to 1:7.

[0063] In the course of optimizing the material properties of the film serving as a carrier layer, which contains a high proportion of recycled polypropylene with "injection molding quality," the inventors identified that, particularly at high concentrations, which are generally desirable, the impact strength can be reduced too significantly to meet the requirements of the adhesive elements. Therefore, the inventors found that impact modifiers should be added to the material, especially at high proportions of hSFR-PCR recycled polypropylene in the total polymer composition, for example, a mass fraction of 40% or more, and particularly a mass fraction of 50% or more, based on the mass of the polymer composition, in order to achieve a particularly positive result. The inventors also succeeded in identifying preferred representatives of these impact modifiers.A preferred method according to the invention comprises the polymer composition and / or the extruded polymer composition additionally comprising:. iv) one or more impact modifiers, preferably with a combined mass fraction in the range of 0.5 to 10%, particularly preferably 1 to 7.5%, wherein the impact modifiers are preferably selected from the group consisting of ethylene-propylene copolymers, polypropylene block copolymers and styrene block copolymers.

[0064] In process step e), the previously produced monoaxially stretched plastic film is coated with an adhesive, wherein the plastic film can be surface-treated beforehand to improve adhesion, with corona treatment proving particularly effective. A preferred method according to the invention is one in which the monoaxially stretched plastic film is subjected to surface treatment before coating, preferably corona treatment.

[0065] In principle, all typical adhesives known to those skilled in the art can be used as adhesives, which are commercially available from various manufacturers and can be easily produced by typical suppliers of adhesive products themselves. Due to its high application relevance, a method according to the invention is preferred, wherein the adhesive is a pressure-sensitive adhesive. The resulting pressure-sensitive adhesive tapes, i.e., adhesive tapes in which a pressure-sensitive adhesive provides the adhesive effect and is permanently tacky and adhesive under normal ambient conditions, are well known to those skilled in the art in adhesive technology. Such pressure-sensitive adhesive tapes can be applied to a substrate by pressure and remain adhered there, but can later be removed more or less without leaving any residue.

[0066] In accordance with professional understanding, a pressure-sensitive adhesive is an adhesive that possesses tacky properties, meaning it forms a permanent bond to a substrate even under relatively light pressure. Such pressure-sensitive tapes are typically removable from the substrate after use, leaving virtually no residue, and are generally permanently tacky even at room temperature. This means they exhibit a certain viscosity and initial tackiness (also referred to as "tack" by professionals), allowing them to adhere to the surface of a substrate even with minimal pressure.Without being bound to this theory, it is often assumed that an adhesive compound can be considered an extremely highly viscous liquid with an elastic component, which consequently exhibits characteristic viscoelastic properties that lead to the permanent inherent tackiness and pressure-sensitive adhesion described above. It is assumed that, in such adhesive compounds, mechanical deformation results in both viscous flow processes and the generation of elastic restoring forces. The viscous flow component serves to achieve adhesion, while the elastic restoring forces component is particularly necessary for achieving cohesion. The relationships between rheology and pressure sensitivity are known in the prior art and are described, for example, in "Satas, Handbook of Pressure Sensitive Adhesives Technology", Third Edition, (1999), pages 153 to 203.To characterize the degree of elastic and viscous components, the storage modulus (G') and the loss modulus (G'') are usually used, which can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, as disclosed, for example, in WO 2015 / 189323. Within the scope of the present invention, an adhesive compound is preferably considered to have a pressure-sensitive adhesive properties and thus to be a pressure-sensitive adhesive if, at a temperature of 23 °C in the deformation frequency range of 10 0 up to 10 1 rad / sec G' and G'' each at least partially in the range of 10 3 up to 10 7 Pa lie.

[0067] The inventors' experiments have shown that the sustainable polypropylene plastic films produced using the inventive method exhibit particularly advantageous compatibility with specific adhesives. A preferred method according to the invention comprises one or more (co)polymers selected from the group consisting of diene rubbers, styrene block copolymers, and poly(meth)acrylates; preferably selected from the group consisting of diene rubbers and styrene block copolymers; and most preferably selected from the group consisting of natural rubber and styrene block copolymers.

[0068] Particularly for adjusting the desired tackiness, it is advantageous if the adhesive compound also comprises one or more adhesive resins. In principle, any adhesive resin compatible with the adhesive compound, and especially with the (co)polymer(s) of the adhesive compound, can be used. In one embodiment, the adhesive resin is selected from the group consisting of aliphatic, aromatic, and alkylaromatic hydrocarbon resins; hydrocarbon resins based on pure monomers; hydrogenated hydrocarbon resins; functional hydrocarbon resins; and optionally, derivatized natural resins. Preferably, the adhesive resin is selected from the group consisting of pinene, indene, and rosin resins, as well as their disproportionated, hydrogenated, polymerized, esterified derivatives and salts; aliphatic and aromatic hydrocarbon resins; terpene resins and terpenephenolic resins; and C5, C9, and other hydrocarbon resins.The adhesive resin selected from the group consisting of rosin resins and polyterpene-based resins is particularly preferred. These adhesive resins are advantageous because they can be largely, and in some cases entirely, produced or obtained from renewable raw materials.

[0069] From an environmental perspective, the trend in adhesive coatings is moving towards solvent-free coatings. The preferred application is waterborne dispersion adhesives, for example, based on polyacrylates or natural rubber, or extrusion "hot melt" coatings of, for example, synthetic or natural rubber. However, there is also the technology of coating with solvent-based adhesives, in which over 80% of the solvents are regenerated and reused.

[0070] The inventors propose that careful pre-selection should be carried out when choosing the starting materials for the polypropylene recyclates in the polymer composition, as household plastic waste in particular can be contaminated with substances that can adversely affect the visual or olfactory perception of the manufactured films to such an extent that, despite sufficient mechanical properties, they would exhibit insufficient end-user acceptance. In practice, this pre-selection will usually be carried out by the recycler or the supplier of the PCR polypropylene recyclates. According to the inventors, it is particularly beneficial to pay attention to the following aspects when selecting the materials: - no or very low proportion of particles / specks / gel particles, and / or - little foreign material such as other polymers (PET, PA), paper or fabric, and / or - pleasant smell - low inherent color, especially no intense color like black, so that easy dyeing of the manufactured MOPP film is possible, and / or - no or very low proportion of migrable additives such as lubricants (erucamide).

[0071] Overall, it is advisable to source PCR polypropylene recyclate from suppliers who use modern recycling processes with very good sorting, in particular comprehensive washing, vacuum extraction from the melt (DSE) to separate volatile components, and / or very good continuous melt filtration, e.g. using laser filters or belt filters.

[0072] The invention also relates to an adhesive element, manufactured or manufacturable using the method according to the invention.

[0073] The invention and preferred embodiments of the invention are further explained and described below with reference to experiments. A. Measurement method:

[0074] The following measurement procedures will be carried out. Mechanical investigations:

[0075] The mechanical properties are determined in a tensile / elongation test. The tensile elongation behavior is determined on type 2 test specimens (rectangular, 150 mm long and, if possible, 15 mm wide test strips) according to DIN EN ISO 527-3:2019-02 with a test speed of 300 mm / min, a clamping length of 100 mm, and a preload of 0.3 N / cm, whereby samples for data determination are cut with sharp blades.

[0076] Unless otherwise specified, tensile elongation is tested in the machine direction (MD, running direction). Tensile strength is measured in N / mm². 2 and the elongation at break expressed as a percentage. The modulus of elasticity is determined from the force-strain curve at low strain. The test results, especially the elongation at break, are statistically validated by a sufficient number of measurements. Tear resistance:

[0077] The tear strength is the force in N / cm required to tear a test specimen according to a defined procedure. The measurement is performed according to DIN EN ISO 6383-2:2004 (Part 2: Elmendorf method (ISO 6383-2:1983)). A rectangular test specimen is used. The test results are also statistically validated by a sufficient number of measurements. Impact strength:

[0078] The impact tensile strength of the film is determined according to DIN EN ISO 8256:2005-05 on samples with dimensions of 15 mm width and 27 mm length. For longitudinal impact tensile strength testing, the specimens are cut parallel to the machine direction, and the crack propagation therefore runs perpendicular to the machine direction. The arithmetic mean of 10 measurements is reported in each case. B. Materials used:

[0079] The following materials were used in the experiments: hSFR-PCR-PP hSFR-PCR-polypropylene recyclate with 100% PCR recyclate content, melt flow rate according to DIN EN ISO 1133-1:2012-03 of 5 g / 10min (230 °C, 2.16 kg); ModulDIN EN ISO 178:2019-08 of 800 MPa (trade name: PreProp F299, company: WIS Kunststoff). Virgin PP Virgin polypropylene material, hetrophasic PP copolymer with approx. 5% ethylene, melt flow rate according to DIN EN ISO 1133-1:2012-03 of 0.8 g / 10 min (230 °C, 2.16 kg); modulus according to DIN EN ISO 178:2019-08 of 1200 MPa (trade name: BA110CF, company: Borealis). nSFR-PCR-PP nSFR-PCR polypropylene recyclate with 70% PCR recyclate content, melt flow rate according to DIN EN ISO1133-1:2012-03 of 2.5 g / 10 min (230 °C, 2.16 kg); modulus DIN EN ISO 178:2019-08 of 1500 MPa (trade name: PP 1002, company: Ineos). Impact Modifier Olefinisches Blockcopolymer, Schmelzflussrate gemäßDIN EN ISO 1133-1:2012-03 von 0.5 g / 10min (190 °C, 2.16kg); Modul DIN EN ISO 178:2019-08 von 1210 MPa, Density0.869 g / m 3 (Trade names: Infuse 9077, Company: Dow). C. Production of the samples:

[0080] The samples were produced from the materials listed in Table 1 according to the following manufacturing instructions. Table 1 - Composition of the plastic film (all values ​​in mass fractions) E1 E2 E3 E4 V1 V2 hSFR-PCR-PP 20 40 20 40 - 100 Virgin PP 15 15 15 15 - - nSFR-PCR-PP 65 45 58 38 100 - Impact Modifier - - 7 7 - -

[0081] To produce the plastic films, recycled granules are manufactured from the polymer compositions listed in Table 1 and then extruded using a Collin E30 single-screw extruder and formed into plastic films. For this purpose, the plastic mixtures, after melting, were formed into a flat film via a slot die on a rolling mill with a cooling roller (trade name: TeachLine; company: Collin). This film is then immediately stretched monoaxially in the machine direction inline in a stretching unit (trade name: TeachLine; company: Collin) at a stretch ratio of 1:6.

[0082] The adhesive tape is manufactured using a solvent-based pressure-sensitive adhesive consisting of 54 wt% natural rubber (CV60), 11.5 wt% of a first adhesive resin (trade name: Escorez 1102, company: Exxon), 11.5 wt% of a second adhesive resin (trade name: Escorez 2184, company: Exxon), and 23 wt% of a third adhesive resin (trade name: Regalite R1125, company: Eastman). These components are processed in gasoline to a 30% solution, which is then applied to the plastic film using a hand-held coating table. The plastic film has been previously treated with corona discharge (30 kW / m²). 2 ) is pretreated. It is then dried in a drying oven at 100 °C for 10 minutes, resulting in a mass application of 22 g / m² after drying. 2 results. D. Results:

[0083] The results of the experiments carried out are summarized in the table below. Table 2 - Results of the experiments carried out E1 E2 E3 E4 V1 V2 Foil thickness / µm 83 81 85 82 80 90 Tensile strength (N / mm2) 290 263 232 218 197 225 Elongation at break / % 31 29 32 33 35 23 Tear strength (mN) 192 212 211 216 221 130 Impact tensile strength / (kJ / mm²) 2 ) 92 73 143 128 165 20

[0084] The mechanical properties of the flat films samples E1 and E2 show significantly better characteristics than the plastic film V2 made from 100% hSFR-PCR-PP recycled material, with many of the relevant properties being advantageously close to those of the plastic film made from virgin polypropylene. This is particularly noticeable in the significantly better tear resistance and, in particular, the improved impact strength. With an increasing proportion of hSFR-PCR-PP recycled material, the impact strength deteriorates without the addition of an impact modifier. However, both MOPP films E1 and E2 are still suitable for use as a backing film in an adhesive tape. Examples E3 and E4 demonstrate that the additional addition of an impact modifier further improves the impact strength. However, the impact strength of the film made from virgin PP cannot be achieved.

[0085] In summary, the inventive method yields high-performance, sustainable adhesive elements that can be produced while reducing the need for fossil raw materials.

Claims

[1] Method for producing an adhesive element, comprising the process steps: a) Manufacturing or providing a polymer composition, comprising: i) one or more polypropylene recyclates, wherein at least one of the polypropylene recyclates is an hSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and having a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 4 g / (10 min) to 15 g / (10 min), ii) wherein at least one of the polypropylene recyclates is an nSFR-PCR polypropylene recyclate produced by recycling plastic waste from plastic products after their intended use and has a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230°C of less than 4 g / (10 min). iii) one or more virgin polypropylene materials, wherein the one or more virgin polypropylene materials have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 2.0 g / (10 min) or less, where the combined mass fraction of all polypropylene recyclates and virgin polypropylene materials is 90% or more, based on the mass of the polymer composition, wherein the combined mass fraction of all polypropylene recyclates is 50% or more, based on the mass of the polymer composition, wherein the combined mass fraction of all hSFR-PCR polypropylene recyclates is 20% or more, based on the combined mass of all polypropylene recyclates in the polymer composition, c) Extruding the polymer composition to form a plastic film, d) Stretching the plastic film to obtain a monoaxially stretched plastic film, and e) Coating the monoaxially stretched plastic film with an adhesive compound to create an adhesive element. [2] Method according to claim 1, wherein the plastic film is formed by means of flat film extrusion. [3] Method according to one of claims 1 or 2, wherein the one or more hSFR-PCR polypropylene recyclates are at least partially produced by recycling plastic waste from plastic products selected from the group consisting of rigid polypropylene packaging. [4] Method according to any one of claims 1 to 3, wherein the one or more hSFR-PCR polypropylene recyclates have a melt flow rate according to DIN EN ISO 1133-1:2012-03 for 2.16 kg and 230 °C of 6 g / (10 min) or more. [5] Method according to any one of claims 1 to 4, further comprising, between process step a) and process step c), the following process step: b) Extruding the polymer composition to obtain a plastic granulate. [6] Method according to any one of claims 1 to 5, wherein the stretching of the plastic film is carried out with a stretching factor of 1 : 5 or more. [7] Method according to any one of claims 1 to 6, wherein the adhesive mass comprises one or more (co-)polymers selected from the group consisting of diene rubbers, styrene block copolymers and poly(meth)acrylates. [8] Adhesive element produced by a method according to any one of claims 1 to 7.