EP4334381A1

EP4334381A1 - Method for forming a syntactically foamed polymer layer

Info

Publication number: EP4334381A1
Application number: EP22730067.0A
Authority: EP
Inventors: Karsten Kuss; Anika PETERSEN; Axel Burmeister
Original assignee: Tesa SE
Current assignee: Tesa SE
Priority date: 2021-05-06
Filing date: 2022-05-06
Publication date: 2024-03-13
Also published as: WO2022234133A1; DE102021204631A1

Abstract

The invention relates to a method for forming a syntactically foamed polymer layer, comprising the steps of: a) producing or providing a polymer mass comprising a plurality of at least partially expanded microballoons, b) introducing the polymer mass between two carrier layers in order to produce a layer system, and c) calendering the layer system between two calender rolls of a calender in order to form the syntactically foamed polymer layer between the carrier layers, wherein at least one of the calender rolls has a roll diameter in the range of 200 to 400 mm.

Description

Process for forming a syntactically foamed polymer sheet

description

The invention relates to a method of forming a syntactically foamed polymer layer. Also disclosed is a multilayer adhesive tape and system for forming a syntactically foamed polymer layer.

The subject of the invention is defined in the appended claims.

The joining of separate elements is one of the central processes in manufacturing technology. In addition to other methods, such as welding and soldering, bonding, i.e. joining using an adhesive, is of particular importance today. An alternative to the use of shapeless adhesives, which are applied from a tube, for example, are so-called adhesive tapes.

Pressure-sensitive adhesive tapes in which the adhesive effect is provided by a pressure-sensitive adhesive which is permanently tacky and adhesive under normal ambient conditions are particularly relevant for numerous technical applications. Corresponding pressure-sensitive adhesive tapes can be applied to a substrate by pressure and stick there, but can later be removed again more or less without leaving any residue.

In particular in the field of vehicle production, however, components that have so-called low-energy surfaces (LSE surfaces = Low Surface Energy surfaces) or medium-energy surfaces also have to be joined regularly Surfaces (MSE) have. On such MSE/LSE surfaces, many pressure-sensitive adhesive tapes show only an inadequate adhesive effect, so that the correct selection of the pressure-sensitive adhesive tapes is of particular importance. An improvement in the adhesion properties on untreated MSE/LSE surfaces can be achieved, for example, by using PSAs based on synthetic rubbers and/or poly(meth)acrylates. However, there is a problem here in that tacky or intrinsically tacky properties are not typical per se, at least for pure synthetic rubbers and the adhesives obtained therefrom, or are not sufficiently pronounced in the case of some poly(meth)acrylates. The effect of the pressure-sensitive tack in these adhesives is therefore often produced or reinforced in a targeted manner by the addition of so-called tackifying resins. Owing to the sometimes high tackifying resin concentration, however, corresponding pressure-sensitive adhesives regularly have a comparatively high Tg, ie a high glass transition temperature. In many cases, this limits the performance of corresponding pressure-sensitive adhesives and the pressure-sensitive adhesive tapes produced therefrom, for example the resistance to shock stress, at low temperatures.

To improve this problem, in particular to increase the low-temperature impact strength, adhesive tapes have recently been developed, among other things, which have a foamed or partially foamed backing layer and/or a foamed or partially foamed adhesive layer. For example, reference is made to the documents DE 19730854 A1, EP 2235098 A1 and WO 2019/076652 A1. The so-called syntactically foamed layers, which can be produced in particular through the use of expandable microballoons, are of particular importance here.

The implementation of corresponding adhesive tapes with foamed layers not only places high demands on the optimization of the physico-chemical properties of the polymer masses used, but also requires continuous improvement in process technology. Two methods in particular have proven fundamentally suitable for the production of foamed polymer layers. In the first method, the syntactically foamed layer is produced directly from the melt of the polymer mass, this method also being referred to as “hotmelt” by those skilled in the art. The polymer layer is shaped by a suitable calender, whereby, depending on the process, the microballoons contained in the polymer mass may expand as early as in the melt, during extrusion and/or during the shaping of the layers.

In the second variant of the method, syntactically foamed layers are produced from the solution, with a corresponding polymer mass which is at least partially dissolved in solvent being used, into which the expandable microballoons can be stirred. This solution can then be applied to a liner using standard coating systems, where it is dried to remove the solvent. The expansion of the expandable microballoons usually only takes place in a subsequent step.

Among the hot-melt processes, processes in which the syntactically foamed polymer layer can be shaped directly between two liners by two rolls of a rolling mill following extrusion are particularly advantageous from the point of view of process technology. Compared to other processes that use a molten polymer mass, this process is particularly time- and cost-efficient and also particularly robust due to its comparatively simple structure. In addition, this method regularly produces syntactically foamed polymer layers which have particularly good properties, with particularly smooth surfaces being able to be obtained in particular. There are many advantages, especially in comparison with solvent-based technology, in particular the lower costs, the higher level of occupational safety due to the fact that solvents are not required, the higher energy efficiency due to the elimination of drying steps, the easier implementation of a continuous process control, the simpler implementation in terms of equipment and lower consumption of chemicals. Despite the numerous advantages described, this hotmelt process is also perceived as disadvantageous for some applications. In fact, it is not possible with the structure known from the prior art to produce syntactically foamed polymer layers with very small layer thicknesses, at least not those that meet the customary industry requirements for the quality of the layer produced. The person skilled in the art knows that these hotmelt processes, which are advantageous per se, have a minimum layer thickness that can be achieved in practice, depending on their specific structure and the polymer masses used, which are usually in the range from about 170 to 190 gm, and that correspondingly thinner Layers in these systems cannot be achieved by increasing the line pressure. This is perceived as disadvantageous because adhesive tapes with layer thicknesses of less than 150 gm are regularly requested for many areas of application, in particular in the field of electrical engineering. So far, such adhesive tapes can only be produced using solvent-based technology.

The object of the present invention was therefore to eliminate or at least reduce the disadvantages of the prior art and, in particular, to provide a method for shaping a syntactically foamed polymer layer, with which particularly thin syntactically foamed polymer layers can be formed by directly shaping the polymer mass between two support layers can be obtained which have excellent physicochemical properties, in particular low surface roughness. It was an object of the present invention to provide a method which is suitable for a large number of polymer compositions, in particular for the production of carrier layers and adhesive layers, with the method being particularly suitable for adhesive layers, since the surface properties of these are regularly particularly important are. In this respect, it was an additional object of the present invention that the method should preferably be carried out in a particularly simple and efficient manner and if possible without the need for additional drying steps or the use of solvents.

The method to be specified should preferably be able to be carried out using materials which have already been used before in the production of adhesive tapes. In this respect, it was also desirable that the method to be specified should be executable with comparatively small modifications to existing production machines.

It was an additional object of the present invention that the process to be specified should be very flexible with regard to the degree of foaming that can be achieved in the polymer layer.

In addition, it was desirable if the method to be specified would ideally also lead to more homogeneous layers than the methods known from the prior art.

The inventors of the present invention have now found that the objects defined above can surprisingly be achieved if the roll diameter of the calender rolls used for calendering is reduced compared to the methods known from the prior art.

Without wanting to be bound by this theory, the experiments and calculations of the inventors indicate that the change in the size of the mass bead in the nip and its geometry, i.e. the volume, the liquor size and the liquor geometry, caused by the reduction in the roll diameter, is responsible for this According to the inventors, the reduced residence time of the polymer mass in the nip is responsible for the surprisingly advantageous results or at least contributes to them.

Ultimately, the adhesive mass is shaped in particular at the narrowest point in the nip. The volume of the adhesive mass that is in the nip and is to be shaped is determined, inter alia, by the diameter of the calender rolls and the size of the bead of mass. In this case, the mass bead size remains the same with an increase in the roll diameter Among other things, an increase in the mass volume to be shaped in the roller gap can be observed. The roll nip angle, which is set by the roll geometry (ie roll diameter), is flatter as the roll diameter increases. Due to the wetting surface on the roll, this leads to larger volumes in the mass bead compared to smaller calender rolls with steeper roll nip angles. According to the inventors, the volume of the adhesive mass in the mass bead has a constant

Coating speed has a direct influence on the residence time of the adhesive in the mass bead of the calender, so that with a constant liquor size diameter, longer residence times are obtained for larger calender rolls than for smaller calender rolls.

The above objects are thus achieved by a method of forming a syntactically foamed polymer sheet as defined in the claims. Preferred configurations according to the invention result from the dependent claims and the following statements.

Such features of methods according to the invention, which are referred to below as preferred, are combined in particularly preferred embodiments with other features referred to as preferred. Combinations of two or more of the embodiments described below as particularly preferred are therefore very particularly preferred. Also preferred are embodiments in which a feature identified as preferred to some extent is combined with one or more other features identified as preferred to some extent. Features of multilayer adhesive tapes and systems result from the features of preferred processes.

The invention relates to a method for shaping a syntactically foamed polymer layer, comprising the steps: a) producing or providing a polymer mass comprising a multiplicity of at least partially expanded microballoons, b) introducing the polymer mass between two carrier layers to produce a layer system, and c) calendering the layer system between two calender rolls of a calender for shaping the syntactically foamed polymer layer between the carrier layers, wherein at least one of the calender rolls has a roll diameter in the range from 200 to 400 mm.

A foamed polymer layer, e.g. a plastic or adhesive layer, is understood to be a structure made of gas-filled, three-dimensional cells, which are characterized by liquid, semi-liquid, higher-viscosity or solid

Cell webs of the polymer layer are limited and which are present in such a proportion that the density of the foamed polymer layer is reduced compared to the density of the matrix material, ie the totality of the non-gaseous materials from which the material is composed. With syntactic foamed polymer layers are in the matrix material

Hollow bodies, for example expandable microballoons, are integrated, whereby the cavities created are separated from one another and from the matrix material by a membrane. In the method according to the invention, the syntactic foaming is achieved through the use of at least partially expanded microballoons, it being preferred that the

Polymer layer is exclusively syntactically foamed by at least partially expanded microballoons, so that the polymer layer is a syntactically foamed polymer layer with microballoons.

"Microballoons" are understood to mean elastic hollow microspheres, which are expandable in their basic state and have a thermoplastic

Have polymer shell. These spheres are usually filled with low-boiling liquids or liquefied gas. In particular, polyacrylonitrile, PVDC, PVC or poly(meth)acrylates are used as the shell material. Short-chain hydrocarbons, for example isobutane or isopentane, are particularly common as the low-boiling liquid for example as a liquefied gas under pressure in the polymer shell are enclosed.

When the microballoons are heated, for example during processing in the extruder, the outer polymer shell softens. At the same time, the propellant arranged inside expands. In doing so, the microballoons expand essentially irreversibly and expand three-dimensionally. Expansion is complete when the internal and external pressures equalize. Since the polymeric shell is retained, a closed-cell, syntactically foamed foam is achieved. Microballoons are available in a large number of designs, which can essentially be characterized by their size (usually 6 to 45 μm in diameter d50 in the unexpanded state) and their starting temperatures (e.g. 75 to 220° C.) required for expansion. Unexpanded microballoons are available, for example, as an aqueous dispersion with a microballoon mass fraction of about 40 to 45% or as polymer-bound products, for example in ethylene vinyl acetate, with a microballoon mass fraction of about 65%, and as a dry material.

Microballoons which have not yet been thermally activated and which accordingly still have their original expansion are referred to by those skilled in the art as unexpanded microballoons and, in accordance with the understanding of the art, are not regarded as at least partially expanded microballoons.

Due to the inherent dependence of the properties and the morphology of microballoons on the temperature used for expansion and the ambient pressure, or the deformability of the matrix material, at least partially expanded microballoons are spoken of in the context of the present invention. This is to be understood in such a way that the microballoons, compared to the unexpanded microballoons, were treated at a temperature greater than or equal to the respective starting temperature for at least so long that a volume expansion occurs, preferably a volume expansion by more than 25%, preferably more than 50%. , more preferably more than 100%, whole more preferably more than 150%. However, this also means that the expanded microballoons do not necessarily have to be fully expanded.

Even if the precise definition of the expanded microballoons can be challenging, as is the case with all amorphous materials whose properties are largely determined by the process used for production, the distinction between at least partially expanded and unexpanded microballoons is fortunately reliable in practice for the person skilled in the art, with an industry acceptable fault tolerance.

Preference is given to a method according to the invention, wherein the polymer mass comprises at least partially expanded microballoons in a mass fraction of 0.1 to 10%, preferably 0.2 to 5%, particularly preferably 0.5 to 2.5%, based on the total mass of the polymer mass.

In the method according to the invention, the polymer mass, which comprises a large number of at least partially expanded microballoons, can either be provided, for example because it is obtained as a mixture from a supplier, or it can be produced beforehand, which is expediently done by mixing the polymer mass with unexpanded and/or at least partially expanded microballoons, takes place, in the case of unexpanded microballoons, these must be at least partially expanded, which can be done, for example, by thermal treatment in the extruder.

In the method according to the invention, the polymer mass is introduced between two carrier layers, which are also referred to as liners by those skilled in the art, in order to thereby produce a layer system consisting of a carrier layer, the polymer mass and a further carrier layer. In principle, the corresponding layer system can be generated separately from the subsequent processing steps in terms of time and space. For reasons of efficiency and process reliability, however, it has proven to be particularly advantageous if the layer system is produced by introducing the polymer mass between the two carrier layers directly in the nip of the calender rolls. This can advantageously be realized in that the carrier layers are each guided over one of the two calender rolls, so that when Entering the polymer mass into the nip automatically results in the layer system between the two carrier layers.

Preference is therefore given to a method according to the invention, in which the polymer mass is introduced in step b) between two carrier layers, which are each guided over one of the calender rolls of the calender through the nip, so that the layer system is produced in the region of the nip of the calender rolls, the polymer mass bead in the region of the nip of the calender rolls, preferably in the cross section viewed orthogonally to the running direction, has an area of less than 800 mm ² , preferably less than 600 mm ² , particularly preferably less than 400 mm ² .

In step c), the previously produced layer system is calendered. This means that the layer system is guided between two calender rolls whose nip is smaller than the height of the layer system. As a result, the calender rollers exert linear pressure on the layer system, which is thereby formed into a thinner polymer layer, with any previously existing inhomogeneities in the layer thickness or the density of the polymer mass in the layer system being compensated for.

As explained above, the inventors have recognized that the objects defined above can be achieved in that one of the calender rolls has a roll diameter in the range of 200 - 400 mm, ie compared to the prior art, in which mostly rolls with a diameter of more than 500 mm are used, have a reduced roller diameter. In the experiments of the inventors, it became clear that the required layer thicknesses of 150 μm and less cannot be realized or cannot be realized in a sufficient quality with larger roller diameters. At the same time, however, the inventors have also recognized that the roll diameter, at least in industrially relevant systems, cannot be reduced arbitrarily far. A roll diameter that is too small can lead to reduced stability of the calender rolls with the relevant roll widths, which can consequently experience unwanted deformation in the process and the necessary line pressure cannot be transferred, or not evenly enough, to the layer system to be calendered.

The method according to the invention has proven to be particularly efficient for the production of particularly thin syntactically foamed polymer layers. At the same time, it cannot be denied that larger roll diameters also have advantages, especially for the industrially relevant process control, in particular with regard to their stability and lower deformability. Even if the method according to the invention is suitable for basically producing syntactically foamed polymer layers with a greater thickness, this can, however, in some cases be less efficient than the method known from the prior art. In this respect, it is particularly advantageous if the method according to the invention is used to produce particularly thin polymer layers, since the advantages are particularly evident here. A method according to the invention is therefore preferred in which the syntactically foamed polymer layer has an average thickness in the range from 20 to 180 gm, preferably in the range from 30 to 150 gm, particularly preferably in the range from 40 to 120 gm, with the syntactically foamed polymer layer being particularly preferably has an average thickness of less than 100 gm. The great advantage of the method according to the invention is that the polymer layer calendered between two calender rolls of the calender already has the positive properties described above after passing through one calender gap and that the method according to the invention can be carried out in such a way that further calendering is not necessary. It goes without saying, however, that the syntactically foamed one produced is not ruled out

Continue to calender polymer layer after going through step c). However, with a view to the efficiency of the method and to economic aspects, this is explicitly not preferred.

An essential quality criterion for the evaluation of the syntactically foamed ones that can be produced with the method according to the invention

polymer layers is the mean surface roughness Ra, which is used in the processes of the prior art is particularly impaired by the at least partially expanded microballoons when they protrude from the polymer mass. If the surface roughness Ra is too great, the correspondingly produced polymer layers are not suitable for many applications. With the method according to the invention, it is possible to obtain particularly smooth and thin polymer layers in the hotmelt process, which then also have the advantage that their intrinsic material properties correspond to those inherent in extruded polymers and in some respects to those of solvent-based technology are superior, in particular inclusions of solvents can be avoided. A method according to the invention is therefore preferred in which the syntactically foamed polymer layer has an average surface roughness Ra of less than 10 μm, preferably less than 6 μm, particularly preferably less than 3 μm. Within the scope of the present invention, the surface roughness is determined by means of laser triangulation. The PRIMOS system used for this consists of an illumination unit and a recording unit. The lighting unit uses a digital micro-mirror projector to project lines onto the surface of the examined material. These projected, parallel lines are deflected or modulated by the surface structure. A CCD camera arranged at a certain angle, the so-called triangulation angle, is used to register the modulated lines. The measuring field size is 14.5×23.4 mm ² , the profile length is 20.0 mm, the surface roughness was determined 1.0 mm from the edge and a third-order polynomial filter was used for filtering. The surface roughness Ra thus represents the average height of the roughness, in particular the average absolute distance from the center line (regression line) of the roughness profile within the evaluation range. In other words, Ra is the arithmetic mean roughness, ie the arithmetic mean of all profile values of the roughness profile.

Those skilled in the art know that the density of syntactically foamed polymer layers or the reduction in density compared to that unfoamed starting material is a measure of the foaming of the polymer layer. In the inventors' experiments, it was possible to identify density ranges in which the method according to the invention works particularly well, it being shown in particular that a particularly low standard deviation in the density distribution can be achieved if the densities are selected in these ranges. A method according to the invention is preferred in this regard, in which the syntactically foamed polymer layer has a density in the range from 600 to 1000 kg/m ³ , preferably in the range from 700 to 900 kg/m ³ , particularly preferably in the range from 750 to 850 kg/m ³ , having.

In addition to what has been said above, it should be noted that the advantages of the method according to the invention come into play above all when at least a certain degree of foaming, i. H. some density reduction occurs. For smaller density reductions, it is for some, especially with regard to industrially relevant processes

Configurations expedient to use the larger calender rolls known from the prior art in order to benefit from the greater robustness. A method according to the invention is therefore preferred in which the syntactically foamed polymer layer has a density reduced by at least 30 kg/m ³ , preferably at least 50 kg/m ³ , particularly preferably at least 70 kg/m ³ , compared to the unfoamed polymer mass.

As explained above, surprisingly not only thinner layers can be achieved with the method according to the invention, but also those polymer layers which have a more homogeneous density profile, i. H. where the standard deviation of the density is lower. Such

Methods are particularly preferred since the polymer layers produced in this way have particularly homogeneous physicochemical properties and can be converted, for example, into adhesive tapes with particularly uniform adhesive properties, which is preferred for numerous applications. Against this background, a method according to the invention is preferred, in which the syntactically foamed polymer layer has a standard deviation of over the width of the polymer layer Density of less than 30 kg/m ² , preferably less than 20 kg/m ² , the syntactically foamed polymer layer particularly preferably having a standard deviation of the density of less than 8%, preferably less than 6, across the width of the polymer layer %, more preferably less than 4%, of the average density.

In principle, the polymer mass can be provided or produced. However, it has proven to be particularly efficient to produce the polymer mass immediately before the further processing steps. This is preferably done in a mixing unit in which the expandable microballoons, i. H. unexpanded microballoons or microballoons that have not yet fully expanded, expand through the application of temperature during the mixing process. A method according to the invention is therefore preferred in which the polymer mass is produced in step a) by mixing expandable microballoons with the other components of the polymer mass in a mixing unit and then expanding the expandable microballoons.

Even if the method according to the invention is flexible with regard to the mixing units to be used, continuously operating mixing units have proven particularly useful because they can be used to exploit the great advantage of the method according to the invention that it can also be carried out particularly efficiently continuously. In the inventors' experiments, it was possible to identify particularly suitable extruders for this, with which particularly good process results could be achieved very reliably. Namely, a method according to the invention is preferred, wherein the mixing unit is preferably a continuous mixing unit, particularly preferably a single- or multi-screw extruder, preferably a

Planetary roller extruder, a twin screw extruder or a ring extruder.

The only requirement for the method according to the invention is that the layer system produced in step b) is shaped between two calender rolls of a calender. Those skilled in the art know that there are calenders with a large number of different numbers of rolls. In principle, it is possible to the extent that the method according to the invention, for example, with a Execute four-roll calender, in which only two of the four rolls are used. With a view to the apparatus structure, the robustness of the process and the energy requirement, however, it is very particularly preferred to use only a two-roll calender from the outset and thus to implement the simplest possible process design. A method according to the invention is therefore clearly preferred in which the calender is a two-roll calender.

When experimenting with the roll diameters of the calender rolls, the inventors realized that there was a certain trade-off. Smaller roll diameters make thinner layer thicknesses more accessible overall and/or increase the quality of the thin layers produced. At the same time, the stability decreases as the roll diameter decreases, which means that the calender rolls deform more easily during operation and uneven application of the line pressure can occur. In addition, it cannot be ruled out that material fatigue occurs more quickly with thin rolls and correspondingly higher maintenance and service costs maintenance costs arise. As a result of this conflict of objectives, the inventors were able to identify sub-areas in which a particularly advantageous relationship between the aspects mentioned above is established. According to the estimates of the inventors, the local maximum in terms of performance with conventional roll widths is probably around 300 mm. Accordingly, a method according to the invention is preferred in which at least one of the calender rolls has a roll diameter in the range from 250 to 350 mm, preferably in the range from 280 to 320 mm. As explained above, the roll width of the calender rolls also plays a role, particularly in industrially relevant processes, particularly with regard to the stability of the rolls used. To this extent, the inventors were able to identify optimized areas with which particularly good results can be achieved despite the reduced roll diameter, with no indications being found that the service life of the corresponding rolls could be adversely affected. In this respect, a method according to the invention is preferred, in which at least one of the calender rolls, preferably both calender rolls, has a roll width in the range from 600 to 2000 mm, preferably in the range from 1600 to 1800 mm.

As explained above, the advantages of the method according to the invention already arise when at least one of the calender rolls has a reduced roll diameter. Without wishing to be bound by this theory, it is assumed that the change in the liquor geometry and the residence time in the nip that can already be achieved as a result is sufficient as a contribution. In fact, for some applications it is even particularly advantageous to design only one of the calender rolls with a reduced roll diameter, since this allows the properties of the calender to be adjusted specifically to the substances to be processed, and it is possible to combine the advantages of the stable, thicker rolls with the to combine smaller rollers to be used according to the present invention. According to the inventors, however, it is preferred for the majority of applications to design both calender rolls with a reduced diameter, since this makes them particularly advantageous

Fleet geometries can be achieved in the nip, which has a positive effect on the achievable layer thicknesses. The inventors consider it particularly preferred if the two rolls have essentially the same roll diameter. The consequence of this is that the resulting liquor geometry is very symmetrical, which regularly leads to particularly uniform polymer layers. In the light of these findings, a method according to the invention is preferred, the two

Calender rolls have a roll diameter in the range from 200 to 400 mm, preferably in the range from 250 to 350 mm, particularly preferably in the range from 280 to 320 mm, with the two calender rolls very particularly preferably having essentially the same roll diameter.

As explained above, the method according to the invention is particularly advantageous when it is operated as a continuous method, since in this case the advantages with regard to the large production quantities that can be achieved can be utilized particularly efficiently. A process according to the invention is therefore preferred in which the process is operated as a continuous process. The inventors of the present invention were able to identify suitable coating speeds, ie amounts of polymer layer produced per time, at which polymer layers of high quality could be obtained particularly reliably in the operation of the method according to the invention. In this regard, a method according to the invention is preferred, the method being carried out at a coating speed of 30 m/min or more, preferably 40 m/min or more, particularly preferably 50 m/min or more, very particularly preferably in the range from 100 to 150 m/min , is operated, based on the length of the polymer layer formed per minute. In addition, the inventors of the present invention have also identified suitable line pressures with which excellent results, in particular particularly smooth polymer layers, can be obtained with the calender rolls to be used according to the invention. Namely, a method according to the invention is preferred, wherein the calendering in step c) is carried out with a line pressure in the range from 100 to 300 N/mm, preferably in the range from 120 to 270 N/mm, particularly preferably in the range from 140 to 240 N/mm, he follows.

Although it is fundamentally conceivable to operate the calender rolls with at least slightly different tangential speeds, it is preferred for the majority of applications if the calender rolls are operated at substantially the same speed. Accordingly, a method according to the invention is preferred, in which the two calender rolls are operated in such a way that they have essentially the same tangential speed on their surface.

For the processing of numerous polymer masses, it has proven to be important to ensure that the polymer masses arranged in the layer system have sufficient viscosity in order to obtain thin layers in order to be shaped efficiently during calendering. In order to support this step, it has proven particularly advantageous to temper the calender rolls themselves. Namely, a method according to the invention is preferred, in which the two calender rolls have a temperature in the range from 80 to 190°C, preferably in the range from 120 to 160°C.

In the light of the above disclosure, the inventors have also identified suitable temperature ranges in which good processing properties can be achieved with the usual polymer compositions, in particular with the polymer compositions mentioned below as preferred, with these temperatures being selected in particular so that a suitable Degree of expansion of the at least partially expanded microballoons results. A process according to the invention is preferred in which the polymer mass in step b) has a temperature in the range from 120 to 190° C., preferably in the range from 130 to 160° C.

As explained above, the inventors have recognized that the average residence time of the polymer mass in the area of the nip probably plays a decisive role or an important contribution to the ability to form particularly thin syntactically foamed polymer layers. To this extent, the inventors were able to identify particularly suitable ranges of the mean distribution time, which have led to good results in their own experiments. While longer dwell times can result in the quality of the thin layers produced declining, shorter dwell times associated with a small amount of polymer mass in the nip can result in not being sufficient at all times and/or at every location in the nip Material is present in order to be able to form the polymer layer in the desired thickness, so that the risk of defects in the polymer layer increases. A method according to the invention is therefore preferred, in which the method is operated in such a way that the polymer mass has an average residence time in the region of the nip of the calender rolls in the range from 0.5 to 50 s, preferably 1 to 30 s, particularly preferably 2 to 10 s , lies.

In the course of developing the present invention, the inventors made another remarkable observation. In accordance with expert understanding, those carriers whose thickness is in the range of 50-100 μm are regularly used for the carrier layers. To that extent were the inventors are surprised to find that the use of thicker supports in the method according to the invention results in even smaller layer thicknesses of the polymer layer being able to be realized without the physico-chemical properties of this polymer layer being adversely affected. This was not to be expected, since the gap opening of the calender actually takes place independently of the carrier due to the gap load that occurs. In addition to the further reduction in the layer thickness, the use of thicker carrier layers also stabilized the coating process with regard to the higher tensile loads on the material. As a result, particularly thin and homogeneous syntactically foamed polymer layers could be obtained in this preferred embodiment. A method according to the invention is preferred in which the carrier layers have an average thickness in the range from 50 to 100 μm or in the range from 100 to 150 μm, preferably in the range from 100 to 150 μm, particularly preferably in the range from 125 to 150 μm.

Advantageously, the method according to the invention is very flexible with regard to the material of the carrier layers. With a view to the availability of the materials, however, it has proven to be advantageous to use in particular those materials that are regularly used as process liners in the field of adhesive technology. However, the inventors have recognized that some materials are better suited for the carrier layers, namely precisely those plastics from which carrier layers with particularly homogeneous and smooth surfaces can be produced. This prevents the carrier layers from embossing unwanted structures in the polymer layer to be produced. In the case of these materials, it was also found to be advantageous that they regularly offer sufficient resistance to the expanded microballoons striving out of the polymer layer. A method according to the invention is preferred in which the carrier layers independently consist of one or more materials selected from the group consisting of polyethylene, polypropylene, polyvinyl chloride, polyester, polyethylene terephthalate, paper, woven fabric and nonwoven fabric, preferably selected from the group consisting of polyethylene, polypropylene, polyvinyl chloride, polyester, and polyethylene terephthalate, the support layers preferably being made of the same materials.

As explained above, the process according to the invention can advantageously be used for a large number of polymer compositions, in particular for those which are used to produce carrier layers for multilayer adhesive tapes and/or adhesive layers. On the basis of the requirements placed on the corresponding layers in practice, however, it can be established that the process of the invention is particularly suitable for adhesives and the corresponding production of adhesive layers. In this respect, pressure-sensitive adhesives are particularly relevant for the process of the invention because syntactic foaming is particularly relevant for them and the advantages of the process of the invention are particularly evident. Preference is given to a process according to the invention in which the polymer composition is an adhesive composition, preferably a pressure-sensitive adhesive composition. Accordingly, preference is given to a method according to the invention in which the polymer layer is an adhesive layer, preferably a pressure-sensitive adhesive layer.

According to the understanding of the art, a pressure-sensitive adhesive is an adhesive which has pressure-sensitive adhesive properties, i.e. the property of entering into a permanent connection to an adhesion substrate even under relatively slight pressure. The pressure-sensitive adhesive tapes that can be produced in this way can usually be removed from the substrate again after use, essentially without leaving any residue, and are generally permanently intrinsically tacky even at room temperature, which means that they have a certain viscosity and tack, so that they wet the surface of a substrate even with little pressure.

The pressure-sensitive tack of a pressure-sensitive adhesive tape results from the fact that a pressure-sensitive adhesive is used as the adhesive. Without wishing to be bound by this theory, it is often assumed that a pressure-sensitive adhesive can be regarded as an extremely highly viscous liquid with an elastic portion, which consequently has characteristic viscoelastic properties that are permanent to the one described above Inherent tack and adhesiveness. It is assumed that in the case of corresponding pressure-sensitive adhesives, mechanical deformation leads both to viscous flow processes and to the build-up of elastic restoring forces. The proportionate viscous flow serves to achieve adhesion, while the proportionate elastic restoring forces are necessary in particular to achieve cohesion. The relationships between rheology and pressure-sensitive tack 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. The storage modulus (G') and the loss modulus (G") are usually used to characterize the degree of elastic and viscous content, which can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, as is described, for example, in WO 2015/189323 is disclosed. In the context of the present invention, an adhesive is preferably understood as pressure-sensitive adhesive and thus as pressure-sensitive adhesive if at a temperature of 23 ° C in the deformation frequency range of 10 ° to 10 ¹ rad / sec G 'and G "are each at least partially in the range of 10 ³ to 10 ⁷ Pa, the storage modulus particularly preferably being in the range from 2 ^* 10 ⁵ to 4 ^* 10 ⁵ Pa at a frequency of 0.1 rad/sec (0.017 Hz).

The fundamental suitability of the method according to the invention for a large number of polymer compositions also results from the fact that the polymers used for carrier layers and adhesive layers, which usually determine or at least significantly influence the processing properties of the polymer composition, are mostly comparable. The poly(meth)acrylates in particular play a particularly prominent role both in adhesive layers and in backing layers, since these regularly have particularly advantageous physicochemical properties for both applications. The inventors of the present invention have come to the conclusion on the basis of the tests carried out that the method according to the invention can basically be used with essentially all Polymer masses can be carried out, which are commonly used in the field of adhesive technology. However, the inventors were able to identify particularly suitable polymers here, which are particularly suitable for use in the method according to the invention. Accordingly, a method according to the invention is preferred, wherein the polymer composition comprises one or more, preferably two or more, polymers selected from the group consisting of poly(meth)acrylates, polyurethanes and synthetic rubbers, wherein the polymer composition preferably comprises one or more poly(meth ) acrylates. Of these polymers, the synthetic rubbers and the poly(meth)acrylate-based system are particularly relevant polymer masses, and in particular also mixtures of these two polymers, so-called blends, since good results can be achieved with these in the process according to the invention. A process according to the invention is preferred in which the polymer mass comprises synthetic rubbers with a mass fraction of 20% or more, preferably 40% or more, particularly preferably 60% or more and/or the polymer mass comprises poly(meth)acrylate with a mass fraction of 40% or more, preferably 60% or more, particularly preferably 90% or more, the polymer mass particularly preferably consisting essentially of poly(meth)acrylate apart from the at least partially expanded microballoons.

With a view to the achievable quality of the polymer layers, it has proven to be particularly advantageous if the microballoons are distributed as uniformly as possible in the polymer mass produced or provided, with the inventors at least partially expanding suitable diameters

Microballoons were able to be identified that, with regard to the desired thicknesses of the polymer layer, lead to particularly smooth surfaces and can be processed regularly in such a way that they do not push out through the surface of the polymer layer produced. Therefore, a method according to the invention is preferred, wherein the at least partially expanded microballoons in

Substantially homogeneously distributed in the polymer mass, the at least partially expanded microballoons particularly preferably one Diameter d50 in the range from 10 to 120 gm, preferably in the range from 15 to 80 gm, particularly preferably in the range from 20 to 40 gm.

For many applications it is advantageous if the polymer composition, which in this case is an adhesive composition, comprises adhesive resins with which the adhesive properties can be adjusted in a targeted manner, it being possible for certain adhesive resins to be identified as being particularly suitable. A method according to the invention is therefore preferred in which the polymer mass comprises at least one adhesive resin. Preference is given to a method according to the invention in which the adhesive resin is selected from the group consisting of non-hydrogenated, partially or fully hydrogenated resins based on rosin or rosin derivatives, terpene-phenolic resins, resins based on methacrylates, hydrogenated polymers of dicyclopentadiene, non-hydrogenated, partially selectively or completely hydrogenated hydrocarbon resins based on C5, C5/C9 and C9 monomer streams, and polyterpene resins based on α-pinene and/or β-pinene and/or δ-limonene.

In addition to adhesive resins, other additives can also be provided in the polymer mass, with which the properties of the polymer mass can be adjusted in a targeted manner. A method according to the invention is preferred in which the polymer mass additionally comprises one or more additives selected from the list consisting of aging inhibitors (antioxidants), light stabilizers, UV absorbers, rheological additives and fillers.

Also disclosed is a multilayer adhesive tape, preferably a pressure-sensitive adhesive tape, comprising a syntactically foamed polymer layer, preferably a syntactically foamed adhesive layer, produced or producible using the method according to the invention, the syntactically foamed polymer layer having a thickness of less than 150 gm, preferably less than 100 gm having.

The term adhesive tape is clear to those skilled in the field of adhesive technology. In the context of the present invention, the term tape designates all thin, flat structures, ie structures with a predominant extent in two dimensions, in particular foils, foil sections and labels, preferably Tapes of extended length and limited width, and corresponding tape sections. An adhesive tape can, for example, be offered in rolled-up form as an adhesive tape roll. Adhesive tapes regularly comprise a carrier layer on which an adhesive layer is arranged on one or both sides. The multilayer adhesive tapes disclosed above are preferred because they can be provided with particularly thin layers and, in addition to a particularly smooth surface, also have the intrinsic properties of polymer layers produced from the melt, so that they have no solvent residues, for example. Also disclosed is a system for forming a syntactically foamed polymer layer, preferably in a method according to the invention, comprising:

A mixing unit for mixing a polymer mass, the mixing unit preferably being a continuous mixing unit, particularly preferably a single- or multi-screw extruder, preferably a planetary roller extruder, a twin-screw extruder or a ring extruder, and a calender with at least two calender rolls, preferably a two-roll calender, with at least one of the calender rolls has a roll diameter in the range from 200 to 400 mm.

The invention is further explained below with reference to figures and exemplary embodiments. In the figures show:

1 shows a schematic representation of step c) of the method according to the invention; 2 shows a schematic comparison of the cross-sectional view of the bulk bead formed during calendering in comparison between two different-sized calender rolls with different diameters of the bulk bead; 3 shows an illustration of six exemplary micrographs of sections of syntactically foamed polymer layers.

4 shows an exemplary micrograph of a syntactically foamed polymer layer in section, which was produced by a method not according to the invention.

5 is an exemplary cross-sectional micrograph of a syntactically foamed polymer sheet made by a method of the present invention.

6 shows an exemplary micrograph of a syntactically foamed polymer layer in section, which was produced by a method not according to the invention.

7 is an exemplary cross-sectional micrograph of a syntactically foamed polymer sheet made by a method of the present invention. 8 is an exemplary cross-sectional micrograph of a syntactically foamed polymer sheet made by a method of the present invention.

1 shows a schematic representation of step c) of the method according to the invention in a preferred embodiment. The two calender rolls 14a, 14b can be seen, over each of which a carrier layer 12a, 12b is guided. The mass bead of the introduced polymer mass 10 is shown in the nip and thus between the two carrier layers 12a, 12b. A layer system is thus produced in the region of the calender gap, which is then shaped between the two calender rolls 14a, 14b of the calender to form a syntactically foamed polymer layer.

2 shows a schematic comparison of the cross-sectional views of the mass bead formed during calendering in comparison for two calender rolls 14a, 14b of different sizes with different ones diameters of the mass bead. It can be clearly seen here that with the same diameter of the mass bead, small cross-sectional areas of the mass bead result in the case of the smaller calender rolls 14a, 14b. The calenders shown in comparison in FIG. 2 correspond in their size ratio approximately to the comparison of a roll with a roll diameter of 300 mm to a roll diameter of about 520 mm.

3 shows micrographs of syntactically foamed polymer layers in section (each on the same scale), the two left and the two middle photographs being taken of syntactically foamed polymer layers which were produced using a method according to the invention. In contrast, the two images on the right correspond to syntactically foamed polymer layers that were produced using the solvent-based technology. From this comparison it can be seen that with the method according to the invention, advantageously and surprisingly, thin layers can be obtained with surface roughnesses which are comparable to the surface roughnesses of layers produced using the solvent-based technology (Ra of less than 1 to 3 gm).

4 to 8 show exemplary micrographs of syntactically foamed polymer layers in section, the size scale shown corresponding to 20 gm in each case. The syntactically foamed layers shown in FIGS. 4 and 6 were not produced using a method according to the invention, since only Klander rolls with a larger diameter (520 mm) were used, whereas the layers shown in FIGS. 5, 7 and 8 were produced using the method according to the invention were manufactured. It can be clearly seen that syntactically foamed layers are obtained with the conventional method, the surface roughness of which is significantly increased by the emerging at least partially expanded microballoons. Corresponding syntactically foamed polymer layers are regularly found to be inadequate for use in the field of adhesive technology. In contrast to this, those produced with the method according to the invention show Polymer layers are each of excellent quality, with almost no microballoons protruding from the surface in particular, since they remain almost completely inside the syntactically foamed polymer layer.

The measured values entered from the microscope in FIG. 7 are, from left to right, 26.9 μm, 41.86 μm, 55.96 μm and 60.71 μm. The readings plotted in Figure 8 are, from left to right, 26.31 pm, 38.74 pm, 48.24 pm, 33.11 pm, 22.43 pm, 63.88 pm, 15.41 pm, 19.27 pm and 32.10 pm.

1 and 2 thus illustrate the functional principle of the method according to the invention, whereas FIGS. 3 to 8 show impressions of the performance of the method according to the invention in comparison with the prior art.

While the basic feasibility studies and random samples were carried out with various polymer masses and expandable microballoons of different sizes, the systematic experiments to investigate the relationships were carried out in the manner customary in the industry with a polymer mass established for corresponding measurement series, which is particularly suitable for corresponding tests due to its processing properties.

The exemplary polymer composition used in the experiments disclosed below includes:

49% polyacrylate, 25% vinyl aromatic block copolymer (Kraton 1118), terpene phenpol resin (Dertophene T105), 0.3% Irganox 1726, 1.6% Levanyl N-FL, 0.1% Uvacure. and at least partially expandable microballoons of the Expancel 920DU40 variety with a share of 1.5%.

The corresponding polymer mass was produced in a continuous mixing unit by mixing the polymer mass with the unexpanded microballoons. First, the polymer mass was used in three different test setups (TA), which differ in terms of the roller diameter and the line pressure applied. The results are compiled in Table 1.

Table 1

It can be seen from Table 1 that with methods known from the prior art it is not possible to achieve layer thicknesses of less than 175 gm even at high line pressures, at least not in a quality that is acceptable for further applications. In clear contrast to this, it can be seen that the use of a 300 mm diameter roller makes it possible to achieve minimal layer thicknesses of just 50 gm, even with a line pressure that is reduced compared to the other test setups.

Experiments were then carried out for different roller diameters and different temperatures of the roller and the polymer mass. The results are compiled in Table 2.

Table 2

Experiments 1 to 6 were carried out with a roll diameter of 300 mm. However, as can be seen from the density, it is not a question of syntactically foamed polymer layers, so that experiments 1 to 6 do not represent the method according to the invention, but merely illustrate the basic feasibility of small layer thicknesses, even for non-foamed systems. The data compiled in Table 2 for Comparative Experiments 1 to 6 also show, according to the inventors, that the use of reduced roller diameters can possibly also be advantageous for non-foamed systems and can also have an advantageous effect on the production of thin layer thicknesses in high quality in non-foamed systems. Experiments 7 to 12 are experiments according to the invention in which, at different temperatures of the roller and different densities, ie different degrees of foaming, excellently thin layer thicknesses could be realized which are of the same order of magnitude as in comparative experiments 1 to 6. In clear contrast to the advantageous layer thicknesses that can be realized with the method according to the invention, experiments 13 and 14 clearly show that significantly greater layer thicknesses are obtained with the method according to the prior art under otherwise comparable conditions. In order to examine the aspect of density distribution, the inventors changed the size of the mass bead occurring in the calender gap (at the same coating speed) in the otherwise identical structure and using the otherwise identical conditions and use of a roller with a diameter of 300 mm, and in doing so large and a small mass bulge set. It was observed that the standard deviation of the density determined along the coating width decreases for smaller bulk beads, namely from about 39 kg/m ³ for the large bulk bulk to a standard deviation of only about 15 kg/m ³ for the small bulk bulk. In these experiments, the inventors realized how large the difference in the residence time in the calender gap is for different liquor diameters of the masses in the calender gap. Assuming a roll with a diameter of 300 mm, a liquor diameter of 10 mm means a residence time of about 5 seconds, whereas a liquor diameter of the mass in the calender gap of 50 mm already corresponds to a residence time of just under 70 seconds.

Further experiments by the inventors have confirmed that the foaming of the polymer mass and the foaming process are all the more homogeneous the lower the mass bead and the residence time of the adhesive mass in the nip, which can be influenced via the roll diameter. About the aspect of good surface properties and thin middle Beyond layer thicknesses, the method according to the invention thus proves to be particularly advantageous in this respect as well. In experiments with a constant coating speed and a constant line pressure, a syntactically foamed polymer layer was produced, the density of which was determined at various points along the syntactically foamed polymer layer, at a distance of 4 cm from one another. To this extent, two passes were carried out for a conventional method according to the prior art, which has a roll diameter of 520 mm for both rolls and an example according to the invention, in which two calender rolls with a diameter of 300 mm were used. In Comparative Example 1, the determined density was between 906 and 825 kg/m ³ . The standard deviation of the density was about 15 kg/m ³ . In the second comparative example, the determined density was between 818 and 898 kg/m ³ , the standard deviation being about 17 kg/m ³ . In clear contrast to this, the range of the density determined in the method according to the invention was 781 to 801 kg/m ³ and the standard deviation of the density was only about 7 kg/m ³ .

From these data it can be seen that the standard deviation in layer density could be halved by using smaller calender rolls. In the course of these experiments, the inventors also recognized that the short dwell time of the polymer mass in the calender gap is particularly advantageous for temperature-sensitive polymer masses, for example those that are still post-crosslinking, for which the mass temperature cannot be chosen arbitrarily high without undesirable changes in properties occurring is.

Another advantage that has proven to be that due to the lower mass volume in the calender gap, coating can be carried out with less back pressure that arises in the calender due to the polymer mass to be coated. This is in particular with regard to the process stability of the microballoons, the actual foaming process, the process stability of the whole Coating process and the mechanical wear on the coating calender advantageous.

Reference sign

10 polymer mass 12a-b support layers

14a-b calender rolls

Claims

Expectations

1. A method for shaping a syntactically foamed polymer layer, comprising the steps: a) producing or providing a polymer mass (10) comprising a multiplicity of at least partially expanded microballoons, b) introducing the polymer mass (10) between two carrier layers (12a, 12b) for Creating a layer system, and c) calendering the layer system between two calender rolls (14a, 14b) of a calender for shaping the syntactically foamed polymer layer between the carrier layers (12a, 12b), with at least one of the calender rolls (14a, 14b) having a

Has roll diameter in the range of 200 to 400 mm.

2. The method according to claim 1, wherein the syntactically foamed polymer layer has an average surface roughness Ra of less than 10 μm, preferably less than 6 μm, particularly preferably less than 3 μm.

3. The method according to any one of claims 1 or 2, wherein the calender is a two-roll calender.

4. The method according to any one of claims 1 to 3, wherein the two calender rolls (14a, 14b) have a roll diameter in the range from 200 to 400 mm, preferably in the range from 250 to 350 mm, particularly preferably in the range from 280 to 320 mm , wherein the two calender rolls (14a, 14b) very particularly preferably have essentially the same roll diameter.

5. The method according to any one of claims 1 to 4, wherein the carrier layers (12a, 12b) have an average thickness in the range from 50 to 100 μm or in the range from 100 to 150 μm, preferably in the range from 100 to 150 μm.

6. The method according to any one of claims 1 to 5, wherein the method is operated in such a way that the polymer mass (10) has an average residence time in the region of the nip of the calender rolls (14a, 14b) in the range from 0.5 to 50 s, preferably 1 to 30 s, particularly preferably 2 to 10 s

7. The method according to any one of claims 1 to 6, wherein the polymer composition (10) is an adhesive, preferably a pressure-sensitive adhesive.

8. The method according to any one of claims 1 to 7, wherein the polymer mass (10) at least partially expanded microballoons in a mass fraction of 0.1 to 10%, preferably 0.2 to 5%, particularly preferably 0.5 to 2.5

%, based on the total mass of the polymer mass (10).

9. The method according to any one of claims 1 to 8, wherein the syntactically foamed polymer layer has an average thickness in the range from 20 to 180 gm, preferably in the range from 30 to 150 gm, particularly preferably in the range from 40 to 120 gm, wherein the syntactically foamed

Polymer layer particularly preferably has an average thickness of less than 100 pm.

10. The method according to any one of claims 1 to 9, wherein the polymer mass (10) in step b) between two carrier layers (12a, 12b) is introduced, which are each guided over one of the calender rolls (14a, 14b) of the calender through the nip , so that the layer system is produced in the area of the nip of the calender rolls (14a, 14b), the polymer mass bead in the area of the nip of the calender rolls (14a, 14b) in the cross section viewed orthogonally to the running direction preferably having an area of less than 800 mm ² , preferably less than 600 mm ² , particularly preferably less than 400 mm ² .