Methods for the production of sustainable adhesive elements

Abstract

The invention relates to a method for producing an adhesive element, comprising the process steps of: a) producing 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 of 4 g / (10 min) or more, and ii) one or more virgin polypropylene materials, wherein 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, and 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 to obtain 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 elements is one of the central processes in manufacturing technology. Along with other methods such as welding and soldering, bonding, i.e., joining using an adhesive, is becoming increasingly important. An alternative to the use of formless adhesives, such as those applied from a tube, is so-called adhesive tapes, whose adhesive effect is based on the adhesive masses used. The use of adhesive tapes is associated with many advantages that make this technology ideal for use in many industries, such as the electronics or automotive industries.

[0003] In most cases, such adhesive tapes are multi-component materials that, in addition to the adhesive, also include a carrier layer, usually made of a plastic. The plastic used in the carrier layers is often based on fossil resources. In light of growing awareness of the need for more sustainable business practices and more careful use of fossil resources, this is often seen 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 of the approaches being used here is the use of bio-based plastics. Such bio-based plastics are not—or not entirely—made from fossil-based raw materials, but rather 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 fields of technology. These include, for example, polylactide (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, in order to reduce dependence on fossil raw materials.

[0007] In principle, the use of such plastic recyclates is desirable because they can open up additional material sources and, instead of fossil raw materials, materials can be used that would otherwise be disposed of as waste and would often only be used for thermal recycling.

[0008] When it comes to plastic recyclates, experts distinguish between plastic waste that has arisen directly during the manufacturing process of plastic products and plastic waste that has previously been used for its intended purpose. The latter type of plastic waste is often collected near households and recycled. Waste from industrial production is also referred to as "post-industrial recyclate" (PIR). PIR plastic recyclates generally have the advantage of being relatively pure and not yet contaminated with foreign substances, or only to a small extent. Waste arising after intended use is also referred to as "post-consumer recyclate" (PCR) and includes, for example, food or cleaning product packaging.

[0009] PCR plastic recyclates are generally less pure and have to contend with the problem that, for example, waste collected near households is regularly contaminated with a multitude of other substances during production. As a result, the production of high-performance PCR plastic recyclates, which would come close to the properties of PIR plastic recyclates or even to new plastic materials, is very complex and involves high expenditures of time and / or money.

[0010] At the same time, however, many industries are very interested in using PCR plastic recyclates as secondary raw materials for the production of new products, as the underlying post-consumer waste accounts for a very large proportion of the total waste volume, and the use of PCR recyclates would provide a significantly broader raw material base. Further background information on PCR or PIR plastic recyclates is available, for example, from the standard DIN EN ISO 14021:2016-07.

[0011] Modern adhesive elements, such as adhesive tapes or die-cuts, are high-performance products whose production often relies on the use of high-performance raw materials. For this reason, the otherwise desirable use of PCR plastic recyclates in adhesive tape production has not yet become widespread.

[0012] In many cases, this is due to the fact that very high demands are often placed on the backing layers of adhesive tapes and their properties. In addition to the physical and chemical properties, for example with regard to chemical resistance, this particularly applies to the mechanical properties of the backing layers, such as tear strength, impact strength, flexural rigidity, and tensile strength. These properties are of particular relevance for the production of high-performance adhesive tapes, since the adhesive layer arranged on the backing layers, which imparts the adhesiveness to the adhesive tapes, usually makes such a small contribution to these properties that the overall properties are significantly influenced by the backing material of the backing layer.In addition, especially with pressure-sensitive adhesive tapes, it is necessary that the adhesive layers used show advantageous adhesion to the plastic used as the carrier material, so that when the pressure-sensitive adhesive tape is removed, there is no undesired tearing of the bond at the interface between the adhesive layer and the carrier layer.

[0013] The above requirements are often made even more stringent by the fact that the respective plastics of the carrier layer are intended for use only as very thin carrier films for many high-performance applications and must also often be suitable for applications in which they are exposed to elevated temperatures for long periods of time. 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 during the necessary coating steps, for example, during extrusion coating or during drying processes in the case of solvent-based coatings.

[0014] Polypropylene is an important raw material for the production of carrier layers for adhesive elements. In principle, the use of PCR polypropylene recyclate for the production of carrier layers for adhesive tapes, or the necessary film production, would be particularly desirable in order to be able to use this plastic, which is particularly advantageous for adhesive element production due to its physicochemical and mechanical properties, more sustainably and with a more favorable CO2 footprint.

[0015] However, the above-described problem of the poor availability of PCR plastic recyclates is particularly serious for PCR polypropylene recyclates that would be suitable for use in the production of polypropylene film. According to the inventors' assessment, no high-performance PCR polypropylene plastic recyclate of film quality is currently commercially available, at least not on the industrial scale that would be desirable for use in adhesive element production.

[0016] According to the inventors, this problem results in particular from the fact that polypropylene film waste, which contains a polypropylene that would naturally be more suitable for film production, is not processed as a separate recycling stream in most countries, or at least in most European countries, for example in Germany, or only in insignificant parts of it, as part of the usual plastics recycling of plastic waste collected near households. For example, the German recycling industry has defined and standardized recycling streams in order to make the recycling and sorting processes as efficient and cost-effective as possible. For example, there areFraction 329 Polyethylene, which consists of HDPE originating from bottles and containers, Fraction 310 Plastic film, which consists exclusively of LDPE, Fraction 324 Polypropylene, the so-called "PP rigid" fraction, which consists of PP injection-molded parts such as bottles and containers (e.g. yogurt cups). However, there is generally no separate collection fraction for PP film waste. In the inventor's opinion, the recycling industry does not intend to introduce such a PP film fraction in the near future, as demand and the recyclate quantities are too low. This means that corresponding PCR polypropylene film waste is generally not available in sufficient quantities in isolation, as isolated processing is not economically viable for most recycling companies.Instead, the polypropylene plastic film waste, often previously separated in an air sifter, is mixed with other batches, particularly fraction 324 "PP rigid," or directly thermally recycled. In Germany, fraction 324 includes, for example, used, completely emptied, dimensionally stable, system-compatible plastic articles made of PP with a volume of 5 L or less, such as bottles, bowls, and cups, including secondary components such as closures, labels, etc.

[0017] 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 products.This PCR polypropylene, which is obtained in Germany, for example, as fraction 324 polypropylene, would be available in sufficient quantities and could be used in an economically viable manner, but is, in accordance with expert understanding, essentially referred to as "injection molding quality" due to the material properties of the corresponding polypropylene, which indicates that the corresponding PCR polypropylene is generally not suitable for film production, which would be required for the production of carrier layers, but is rather used in injection molding processes, which in turn are used to produce typical packaging.

[0018] The primary object 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 with which sustainable adhesive element production is possible while reducing the need for fossil raw materials.

[0020] One objective of the present invention was to ensure that the carrier layers required for the production of adhesive elements could be produced as largely as possible from PCR polypropylene recyclate. Therefore, it was important that the proposed process be able to draw on a broad raw material base in order to ensure sufficient PCR polypropylene recyclate for large-scale adhesive element production, even in light of the existing recycling routines of typical recycling companies.

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

[0022] It was a supplementary object of the present invention that the method to be specified should not be dependent on additional substances harmful to the environment or health compared to conventional manufacturing methods and should ideally be feasible with such systems and devices that are already available to the person skilled in the art in the field of adhesive element production.

[0023] It was a further object of the present invention to provide a high-performance, sustainable adhesive element which can be produced by the method to be specified.

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

[0025] What was particularly surprising here was that, contrary to existing prejudice, such high quantities of polypropylene recyclates, and in turn such high quantities of injection-molding-quality PCR polypropylene recyclates, can be used. In addition to the formulation of the appropriate polymer composition, the inventors believe that monoaxial stretching is particularly important. Compared to unstretched or biaxially stretched films, this method appears to be surprisingly suitable for the use of PCR polypropylene recyclates originally intended for injection molding. In Germany, for example, this corresponds to fraction 324 polypropylene.

[0026] The process according to the invention produces 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 resulting PCR-MOPP films surprisingly sufficiently meet the specific requirements for carrier layers of adhesive elements, especially when produced by flat-film extrusion (casting process), thus advantageously meeting the existing conflicting objectives of a raw material base that is as sustainable as possible 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 also proves to be advantageous for demanding applications such as "strapping tape" or "tear tape", which require carrier layers with high tear and tensile strengths and, at the same time, stability against "splicing" and "shredding", which, according to the prevailing opinion, requires the use of PP polymers with high molecular weights (long chain densities, low melt flow rate), which PCR polypropylene with "injection molding grade", which is obtained in Germany, for example, as fraction 324 polypropylene, does not have.

[0027] The above-mentioned objects are thus achieved by the subject matter of the invention as defined in the claims. Preferred embodiments of the invention emerge from the subclaims and the following statements.

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

[0029] To the extent that both specific amounts or proportions of an element and preferred configurations of the element are disclosed below for an element, for example, for the PCR polypropylene recyclate with a high melt flow rate (hSFR PCR polypropylene recyclate) or the virgin polypropylene material, the specific amounts or proportions of the preferably configured elements are also disclosed in particular. Furthermore, it is disclosed that, with the corresponding specific total amounts or total proportions of the elements, at least some of the elements can be configured in a preferred manner and, in particular, that preferably configured elements can in turn be present in the specific amounts or proportions within the specific total amounts or total proportions.

[0030] The invention particularly relates to a method for producing an adhesive element, comprising the method steps: a) producing 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) or more, and (ii) one or more virgin polypropylene materials, where the combined mass fraction of all polypropylene recyclates and polypropylene virgin 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 to obtain an adhesive element.

[0031] The process according to the invention serves to produce an adhesive element. In principle, a wide range of adhesive elements can be produced using the process, including die-cuts or adhesive labels. The process according to the invention is particularly suitable for the production of adhesive tapes. Accordingly, a process according to the invention is preferred in which the adhesive element is a single-sided or double-sided adhesive tape.

[0032] In process step a), a polymer composition is initially introduced, 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, with the mixing preferably taking place 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] Central components of the polymer composition are one or more polypropylene recyclates and virgin polypropylene material.

[0034] These components defined above are each used as "one or more" in accordance with the expert understanding. The term "one or more" refers, in accordance with industry practice, to the chemical nature of the corresponding compounds and not to their amount. For example, the polymer composition may comprise only one type of virgin polypropylene material, which would mean that the polymer composition comprises a plurality of the corresponding molecules. Typically, the mass fractions are given as the combined mass fractions of the one or more components, thereby expressing that the mass fraction of the correspondingly formed components taken together meets the corresponding criteria.

[0035] The process according to the invention uses polypropylene recyclates and virgin polypropylene materials. Polypropylene is a polyolefin that is comprehensively familiar to the person skilled in the art based on their general technical knowledge. As is common for polymers, different polypropylenes can differ in their physicochemical properties, for example, in terms of average molecular weight and melt flow rate. These properties are usually determined significantly by the processes used to produce the polypropylenes.

[0036] Those skilled in the art will understand that many of the polypropylenes available on the market, which are referred to as such, are not, strictly speaking, pure polypropylenes, but rather copolymers obtained during production by polymerizing a monomer composition, which may include other comonomers in addition to propylene. These comonomers in the copolymers serve to optimize the physicochemical properties of the polypropylene for the respective application and, in practice, do not lead to a person skilled in the art no longer considering a corresponding material to be polypropylene.In accordance with this expert understanding, in the context of the present invention, a (co)polymer is referred to as polypropylene if it can be prepared by polymerizing 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, most preferably 95% or more, based on the mass of the monomer composition.This can be expressed to a good approximation and for essentially all cases relevant in practice also via the mass fraction of the polymer building blocks derived from propylene in the polymer chain, so that the (co)polymers referred to as polypropylene in the context of the inventors comprise repeating units derived from polypropylene in a mass fraction of 70% or more, preferably 80% or more, particularly preferably 90% or more, very particularly preferably 95% or more, particularly preferably essentially 100%, based on the mass of the polymer chain.

[0037] In accordance with the expert's understanding, there may be minimal differences between the two above definitions, particularly depending on the polymerization process, although these differences are irrelevant for the practical identification of polypropylene. If, contrary to expectations, a polypropylene lies on the borderline such that it would only be considered polypropylene according to one of the above definitions, the definition based on the proportions in the monomer composition used for production is given priority, particularly since this is easier for the person skilled in the art to understand in practice and requires less complex analysis.

[0038] It is clear to those skilled in the art that polypropylene recyclates can also comprise other additives in addition to polypropylene, for example, lubricants, fillers, pigments, anti-aging agents, or UV stabilizers. Accordingly, within the scope of the present invention, recyclates are understood to be those containing polypropylene in a mass fraction of 85% or more, preferably 90% or more, 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 still-unstretched PP plastic film. In accordance with this desired goal, it is defined that the combined mass fraction of polypropylene materials, i.e., polypropylene recyclates and virgin polypropylene materials, in the polymer composition should be 90% or more. According to the inventors' assessment, it is preferable to provide the highest possible basic polypropylene content, with it being particularly advantageous to reduce the content of polymers other than polypropylene.

[0040] Accordingly, a process according to the invention is preferred wherein 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, especially preferably 99% or more, based on the mass of the polymer composition. Additionally or alternatively, a process according to the invention is preferred wherein 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, very particularly 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 process according to the invention is particularly preferred for demanding applications, wherein the polymer composition additionally comprises, based on the mass of the polymer composition: 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%, most preferably in the range of 0.4 to 1%. A process according to the invention is particularly preferred, wherein the one or more additives are selected from the group consisting of anti-aging agents, UV stabilizers, dyes, plasticizers, antistatic agents, antiblocking agents, and processing aids.

[0042] The polymer composition includes polypropylene recyclates. The term "recyclate" is clear to those skilled in the art and is defined, for example, in DIN EN ISO 14021:2016-07. These are polypropylene materials that have not been immediately produced by polymerization and the usual subsequent steps of polymer production, but have already been used to manufacture products. Typically, this means that polypropylene recyclates have been extruded at least once and formed into a shape not intended for bulk storage, such as a film or packaging, using a forming process.In order to achieve a sufficient effect with regard to the sustainability of the adhesive elements produced, it is defined above that the combined mass fraction of the 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. Preference is given to a process according to the invention wherein 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, preference is given to a process according to the invention wherein the combined mass fraction of all polypropylene recyclates is in the range from 50% to 90%, preferably in the range from 55% to 85%, particularly preferably in the range from 60% to 80%, based on the mass of the polymer composition.

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

[0044] In the vast majority of cases, it will be a process according to the invention, wherein the one or more hSFR-PCR polypropylene recyclates are at least partially, preferably predominantly, particularly preferably substantially completely, produced 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 have recognized that hSFR-PCR polypropylene recyclates with even higher melt flow rates can advantageously be used without the quality of the adhesive tapes produced suffering too much, thereby advantageously making a particularly broad base of available materials accessible. At the same time, the inventors were also able to identify ranges in their own experiments outside of which the melt flow rate of the hSFR-PCR polypropylene recyclates can be associated with limitations in the resulting product quality, particularly with high contents of the corresponding injection molding materials. A process according to the invention is preferred, 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, preferably 8 g / (10 min) or more.Additionally or alternatively, a process according to the invention is preferred, 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 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 process according to the invention is preferred, 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 from 600 to 1800 MPa, preferably in the range from 700 to 1500 MPa, particularly preferably in the range from 800 to 1200 MPa.

[0047] In order to adequately exploit the advantages that the process according to the invention surprisingly also makes "injection molding grades" accessible for film production in the context of adhesive element production, a minimum proportion of hSFR-PCR polypropylene recyclates in the total mass of polypropylene recyclates is defined above. However, the proportion of these hSFR-PCR polypropylene recyclates, which according to prevailing prejudice are not actually suitable for film production, can advantageously also be selected higher, especially with lower total proportions of polypropylene recyclates in the polymer composition. The inventors have succeeded in identifying particularly suitable mass proportions for the content of hSFR-PCR polypropylene recyclate.Preferred is a process according to the invention wherein 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. Additionally or alternatively, preferred is a process according to the invention wherein 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%, 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 the addition of virgin polypropylene material, which is used as a "booster", so to speak, 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 molded into a form other than for bulk storage, such as films or packaging. Such virgin polypropylene materials typically originate directly from polymer production, i.e., the polymerization of monomers, along with the subsequent customary purification and processing steps that occur during polymer production. Corresponding virgin polypropylene materials are offered commercially as such by various suppliers.

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

[0051] The inventors have recognized that particularly those virgin polypropylene materials should advantageously be used that have been optimized by the manufacturers for the production of films, which thus themselves have "film-grade," which is expressed by low melt flow rates. A process according to the invention is preferred, 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, preferably of 1.5 g / (10 min) or less, particularly preferably of 1.0 g / (10 min) or less. Additionally or alternatively, a process according to the invention is preferred, 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 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 polypropylene recyclates whose melt flow rate would be more suitable for film production are usually not available in sufficient quantities due to the typical processes involved in plastics recycling. However, the inventors propose that, should corresponding PCR polypropylene recyclate from PP film waste be available, it can be advantageously admixed with the "rigid PP," thereby achieving a particularly high overall proportion of PCR recyclate and enabling a reduction in the mass fraction of virgin polypropylene material, which in turn can reduce the need for fossil starting materials. This generally involves a process according to the invention, wherein the one or more nSFR PCR polypropylene recyclates are at least partially, preferably predominantly, particularly preferably substantially completely, produced by recycling plastic films.Preference is given to a process according to the invention 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 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), very particularly preferably less than 2.5 g / (10 min), especially preferably less than 2 g / (10 min). Additionally or alternatively, preference is given to a process according to the invention 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.Additionally or alternatively, a process according to the invention is also preferred, wherein the combined mass fraction of all nSFR-PCR polypropylene recyclates is in the range from 20% to 70%, preferably in the range from 30% to 65%, particularly preferably in the range from 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 have great potential for selecting a particularly high total proportion of PP recyclate. Due to their high purity and availability with low melt flow rates, corresponding PIR polypropylene recyclates can also be used, in particular, to further reduce the proportion of virgin polypropylene material. Against this background, a process 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 production, preferably from the film production sector.Additionally or alternatively, a process according to the invention is preferred, 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 from 0.1 to 50 g / (10 min), preferably in the range from 0.1 to 35 g / (10 min), particularly preferably in the range from 0.1 to 20 g / (10 min).

[0054] In process step c), the polymer composition prepared 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' estimation, it is expedient in the vast majority of cases to dispense with 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, in the opinion of the inventors, it may be preferable to prepare the polymer composition prior to film extrusion, thereby advantageously obtaining particularly homogeneous starting materials for film extrusion. Accordingly, a process according to the invention is particularly preferred, additionally comprising between process steps a) and process step c) the process step: b) Extruding the polymer composition to obtain plastic granules.

[0056] In this case, too, it is preferable to process polymer compositions that are as pure as possible. Accordingly, a process according to the invention is preferred, wherein the plastic granulate consists of the polymer composition to a mass fraction of 95% or more, preferably 98% or more, particularly preferably 99% or more, and most particularly preferably essentially 100%.

[0057] For the extrusion of the polymer composition, which is more demanding than pure polypropylene virgin material, the inventors have succeeded in developing the present invention in identifying particularly advantageous processing temperatures that are advantageous both for film extrusion and any prior granulation. A process according to the invention is preferred, wherein the extrusion of the polymer composition to obtain plastic granules and / or the extrusion to form the plastic film, preferably the extrusion of the polymer composition to obtain plastic granules and the extrusion to form the plastic film, takes place at a temperature T Ein the range of 170 to 240 °C, preferably in the range of 180 to 220 °C. According to the inventors, the use of the lowest possible processing temperature is particularly advantageous when using recyclates, as this minimizes further damage (in the case of PP, chain scission) to the polymer chains caused by the processing temperature. Accordingly, these preferred processes produce particularly high-performance films, especially with high recyclate contents, especially hSFR-PCR polypropylene recyclate.

[0058] Due to their advantageous mixing effect, multi-screw extruders have proven suitable for extrusion to produce the plastic granules, whereas extrusion for film production can be carried out efficiently using a single-screw extruder, in which case granulation is preferably carried out beforehand. Accordingly, a process according to the invention is preferred, wherein 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 process according to the invention is also preferred, wherein the extrusion of the polymer composition for forming the plastic film in process step c) takes place in an extruder with one extruder screw.

[0059] In film extrusion, typical film production technologies can advantageously be used, with the production of multilayer carrier films also being possible. A process according to the invention is preferred in which the plastic film is a cast film or a blown film. Additionally or alternatively, a process according to the invention is preferred in which the plastic film is a single- or multi-layer carrier film, preferably a single-layer one. When producing multilayer films, it is advantageous if the outer layers each consist of a thin layer of virgin PP to ensure a homogeneous, smooth film surface.

[0060] According to the inventor's assessment, it is particularly preferred that the plastic film is formed by means of flat film extrusion (cast process), since this process is significantly less susceptible to the deficiencies in the material resulting from the use of "rigid PP" than the blown film process, which is generally regarded as the standard process in film production, so that cast films are particularly preferred.

[0061] An important part of the process according to the invention is that the produced plastic film is stretched monoaxially, resulting in a monoaxially oriented film. The term "monoaxial stretching" is understandable 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 tunnel to a temperature above the glass transition temperature T g , especially more than 20 °C above Tg, but below the melting temperature T mof the carrier material. The stretching takes place, for example, between two or more rollers rotating at increasing speeds. The temperature control during the stretching process should be very constant and homogeneous. Overall, with regard to the stretching and the processes required to obtain the appropriately oriented carrier layer, reference can be made to the prior art processes for stretching plastic films, such as those used for PET. In practice, the vast majority of cases will involve a process according to the invention, with the stretching of the plastic film taking place in the machine direction.

[0062] According to the inventors' assessment, particularly advantageous adhesive elements are obtained when comparatively high stretching factors are used, for which the material obtained from the polymer composition is surprisingly particularly well suited. Accordingly, a process according to the invention is preferred in which the plastic film is stretched with a stretching factor of 1:5 or more, preferably 1:6 or more, and very particularly preferably with a stretching factor in the range from 1:6 to 1:7.

[0063] In the course of optimizing the material properties of the film serving as the carrier layer, which contains a high proportion of "injection-molding-quality" polypropylene recyclate, the inventors identified that, especially at high contents, which are desirable in principle, the impact strength can be reduced too much to meet the requirements placed on the adhesive elements. In this respect, the inventors found that impact modifiers should be added to the corresponding material to achieve a particularly positive result, particularly at high proportions of hSFR-PCR polypropylene recyclate in the total polymer composition, for example, a mass fraction of 40% or more, in particular a mass fraction of 50% or more, based on the mass of the polymer composition. The inventors also succeeded in identifying preferred representatives of these "impact modifiers."Preferred is a process according to the invention, wherein the polymer composition and / or the extruded polymer composition additionally comprises:. 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, whereby the plastic film can be surface-treated beforehand to improve adhesion, with corona treatment in particular having proven to be particularly effective.

[0065] Accordingly, a method according to the invention is preferred, wherein the monoaxially stretched plastic film is subjected to a surface treatment, preferably a corona treatment, before coating.

[0066] 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 the high application relevance, a process 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 widely known to those skilled in the art in the field of adhesive technology. Corresponding pressure-sensitive adhesive tapes can be applied to a substrate by pressure and remain adhered there, but can later be removed again more or less residue-free.

[0067] According to the expert's understanding, a pressure-sensitive adhesive is an adhesive that possesses pressure-sensitive adhesive properties, i.e., the ability to form a permanent bond with a substrate even under relatively light pressure. Such pressure-sensitive adhesive tapes can usually be removed from the substrate after use without leaving any residue and are generally permanently tacky even at room temperature. This means that they exhibit a certain viscosity and tack (also referred to by experts as "tack"), allowing them to wet the surface of a substrate even under light pressure.Without wishing to be bound by this theory, it is often assumed that a pressure-sensitive adhesive can be viewed as an extremely viscous liquid with an elastic component, which consequently exhibits characteristic viscoelastic properties that lead to the permanent inherent tack and pressure-sensitive adhesive capacity described above. It is assumed that with corresponding pressure-sensitive adhesives, mechanical deformation leads to both viscous flow processes and the build-up of elastic restoring forces. The partial viscous flow serves to achieve adhesion, while the partial elastic restoring forces are necessary in particular to achieve cohesion. The relationships between rheology and pressure-sensitive tack are known in the 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 typically used. These can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, as disclosed, for example, in WO 2015 / 189323. For the purposes of the present invention, an adhesive is preferably understood as pressure-sensitive adhesive and thus as 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 partly in the range of 10 3 up to 10 7 Pa lie.

[0068] The inventors' experiments have shown that the sustainable polypropylene plastic films produced by the process according to the invention exhibit particularly advantageous compatibility with specific adhesives. A process according to the invention is preferred, wherein the adhesive 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, particularly preferably selected from the group consisting of natural rubber and styrene block copolymers.

[0069] Particularly for achieving the desired pressure-sensitive adhesive, it is advantageous if the adhesive also comprises one or more tackifier resins. The tackifier resin can, in principle, be any tackifier resin compatible with the adhesive and, in particular, with the (co)polymer(s) of the pressure-sensitive adhesive. In one embodiment, the tackifier 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. The tackifier resin is preferably selected from the group consisting of pinene, indene, and rosin resins, as well as their disproportionated, hydrogenated, polymerized, and esterified derivatives and salts; aliphatic and aromatic hydrocarbon resins, terpene resins and terpene-phenolic resins, and C5, C9, and other hydrocarbon resins.The adhesive resin is particularly preferably selected from the group consisting of rosin resins and polyterpene-based resins. These adhesive resins are advantageous because they can be largely, and especially entirely, produced or obtained from renewable raw materials.

[0070] For environmental reasons, the trend in tack coating materials is toward solvent-free coating. The preferred approach is to use water-based dispersion pressure-sensitive adhesives, e.g., based on polyacrylates or natural rubber, or extrusion "hotmelt" coatings, e.g., synthetic or natural rubber. However, there is also the technology of coating with solvent-based pressure-sensitive adhesives, in which over 80% of the solvents are regenerated and reused.

[0071] The inventors propose that a careful pre-selection should be carried out when selecting the starting materials for the polypropylene recyclates in the polymer composition, as plastic waste collected from households, in particular, can be contaminated with substances that can adversely affect the visual or olfactory perception of the produced films to such an extent that, despite having adequate mechanical properties, they would exhibit too low end-user acceptance. In practice, this pre-selection is usually carried out by the recycler or the supplier of the PCR polypropylene recyclates. According to the inventors, it is particularly useful to pay particular attention to the following aspects when selecting materials: - no or very low proportion of particles / spots / gel particles, and / or - little foreign material such as other polymers (PET, PA), paper or fabric, and / or - pleasant smell - low inherent colouring, in particular no intense colour such as black, so that a slight colouring of the produced MOPP film is possible, and / or - no or very low proportion of migratable additives such as lubricants (erucamide).

[0072] Overall, it is advisable to purchase PCR polypropylene recyclate from suppliers who use modern recycling processes with very good sorting, in particular including 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.

[0073] The invention also relates to an adhesive element produced or producible by the method according to the invention.

[0074] In the following, the invention and preferred embodiments of the invention are further explained and described with reference to experiments. A. Measurement method:

[0075] The following measurement procedures are carried out. Mechanical investigations:

[0076] The mechanical properties are determined in a tensile / elongation test. The tensile strain behavior is determined on Type 2 specimens (rectangular, 150 mm long and, if possible, 15 mm wide test strips) according to DIN EN ISO 527-3:2019-02 at a test speed of 300 mm / min, a clamping length of 100 mm, and a preload of 0.3 N / cm. Samples are cut with sharp blades to obtain the data.

[0077] Unless otherwise stated, tensile elongation behavior is tested in machine direction (MD). Tensile strength is measured in N / mm 2 and the elongation at break is expressed in %. 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:

[0078] Tear strength is the force in N / cm required to tear a specimen according to a specified method. Measurement is performed according to DIN EN ISO 6383-2:2004 (Part 2: Elmendorf method (ISO 6383-2:1983)). A rectangular specimen is used. The test results are also statistically validated by a sufficient number of measurements. Tensile impact strength:

[0079] The tensile impact strength of the film is determined according to DIN EN ISO 8256:2005-05 on specimens measuring 15 mm wide and 27 mm long. For longitudinal tensile impact strength, the specimens are cut parallel to the machine direction, so the crack propagation is perpendicular to the machine direction. The arithmetic mean of 10 measurements is reported in each case. B. Materials used:

[0080] 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 / 10 min (230 °C, 2.16 kg); modulus according to DIN EN ISO 178:2019-08 of 800 MPa (trade name: PreProp F299, company: WIS Kunststoff). Virgin PP Virgin polypropylene material, heterophasic PP copolymer with approximately 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 ISO 1133-1:2012-03 of 2.5 g / 10 min (230 °C, 2.16 kg); modulus according to DIN EN ISO 178:2019-08 of 1500 MPa (trade name: PP 1002, company: Ineos). Impact Modifier Olefinic block copolymer, melt flow rate according to DIN EN ISO 1133-1:2012-03 of 0.5 g / 10 min (190 °C, 2.16 kg); modulus according to DIN EN ISO 178:2019-08 of 1210 MPa, density 0.869 g / m 3 (Trade name: Infuse 9077, company: Dow). C. Production of samples:

[0081] The samples were manufactured from the materials listed in Table 1 according to the following manufacturing instructions. Table 1 - Composition of the plastic film (all data 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 - -

[0082] To produce the plastic films, the recycled granules are prepared from the polymer compositions listed in Table 1 and then extruded into plastic films using a Collin E30 single-screw extruder. For this purpose, the plastic mixtures were melted and formed into a flat film via a slot die on a rolling mill with a chill roll (trade name: TeachLine; company: Collin). This film is then immediately stretched monoaxially in the machine direction in a stretching unit (trade name: TeachLine; company: Collin) with a stretch ratio of 1:6.

[0083] For the production of the adhesive tape, 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) is used. These components are processed in gasoline to a 30% solution, which is then coated onto the plastic film using a hand-held coating machine. The plastic film is previously treated with corona (30 kW / m 2 ). The material is then dried in a drying cabinet at 100 °C for 10 minutes, resulting in a mass application of 22 g / m 2 results. D. Results:

[0084] The results of the tests carried out are summarized in the table below. Table 2 - Results of the tests carried out E1 E2 E3 E4 V1 V2 Film 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 Tensile impact strength / (kJ / mm 2 ) 92 73 143 128 165 20

[0085] The mechanical properties of the flat films of samples E1 and E2 demonstrate significantly better properties than the plastic film V2 made from 100% hSFR-PCR-PP recycled material, with many of the relevant properties advantageously close to those of the plastic film made from virgin polypropylene material. This is particularly evident in the significantly improved tear resistance and, in particular, the improved tensile impact strength. With increasing proportion of hSFR-PCR-PP recycled material, the tensile impact strength deteriorates without the addition of an impact modifier. However, both MOPP films E1 and E2 are still suitable for use as carrier films in adhesive tape. Examples E3 and E4 demonstrate that the additional addition of an impact modifier further positively influences the tensile impact strength. However, the tensile impact strength of the film made from virgin PP cannot be achieved.

[0086] In summary, the process according to the invention produces high-performance, sustainable adhesive elements that can be produced while reducing the need for fossil raw materials. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2015 / 189323

[0067] Cited non-patent literature

[0000] DIN EN ISO 14021:2016-07 [0010, 0042, 0043] DIN EN ISO 1133-1:2012-03 [0043, 0045, 0051, 0052, 0053, 0080] DIN EN ISO 178:2019-08 [0046, 0080] ISO 1133-1:2012-03

[0053] Satas, Handbook of Pressure Sensitive Adhesives Technology“, Third Edition, (1999), Pages 153 to 203

[0067] DIN EN ISO 527-3:2019-02

[0076] DIN EN ISO 6383-2:2004

[0078] ISO 6383-2:1983

[0078] DIN EN ISO 8256:2005-05

[0079]

Claims

[1] A method for producing an adhesive element, comprising the steps of: a) producing 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) or more, and (ii) one or more virgin polypropylene materials, 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 to obtain an adhesive element. [2] Method according to claim 1, wherein the forming of the plastic film is carried out by means of flat film extrusion. [3] The method of any 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] The process 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] A process according to any one of claims 1 to 4, 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. [6] A process according to any one of claims 1 to 5, 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 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). [7] Method according to one of claims 1 to 5, additionally comprising between method step a) and method step c) the method step: b) Extruding the polymer composition to obtain plastic granules. [8] Method according to one of claims 1 to 7, wherein the stretching of the plastic film is carried out with a stretching factor of 1:5 or more. [9] Method according to one of claims 1 to 8, wherein the adhesive comprises one or more (co)polymers selected from the group consisting of diene rubbers, styrene block copolymers and poly(meth)acrylates. [10] Adhesive element, produced or producible by a method according to one of claims 1 to 9.