Field of invention

The invention concerns a pressure sensitive adhesive tape, in particular for mounting of components in an electronic device, comprising a foamed film, a method for producing such a pressure sensitive adhesive tape and a foamed film for such a pressure sensitive adhesive tape.

Background of the invention

Adhesive tapes, in particular pressure sensitive adhesive tapes, generally comprise a backing layer which is provided with a pressure sensitive adhesive layer on one or, in the case of a double-sided pressure sensitive adhesive tape, on both sides. The backing layers can be made of films, metal foils, papers, woven or nonwoven cloth or other materials. Many applications of such tapes require a shock absorbance of the pressure sensitive adhesive tape. In this case, the backing layer often comprises a crosslinked polyurethane (PU) or polyethylene (PE) foam, i.e. a sheet or ribbon of a PU or PE base material including cavities (bubbles) that allow the foam to be, usually elastically, compressed. As such, the backing layer can provide a shock-absorbing or damping function. Such pressure sensitive adhesive tapes are often required in mobile electronic appliances where sensitive electronic parts need to be mounted in a shock- absorbing manner. In particular, such pressure sensitive adhesive tapes have proven advantageous for the mounting of e.g. accumulators, batteries and/or display screens in mobile devices, such as e.g. mobile phones or tablet computers, due to easy mounting, easy removal and good damping properties. Also stationary appliances, such as e.g. computer or TV screens, can benefit from such pressure sensitive adhesive tapes in order to reduce the likelihood of damages e.g. during shipment and transport.

Known polyolefin foams are usually produced in a step-by-step production method. The first production step includes the extrusion of an unfoamed sheet or ribbon. Before or during the extrusion, all components are mixed and the inorganic or organic components, such as e.g. pigments or a chemical foaming agent, are homogeneously distributed in a polymeric base material, i.e. a polymer matrix. Thereby, the extrusion temperatures are held below the activation temperatures of the chemical foaming agent to avoid early activation of the foaming agent. After forming the sheet or ribbon through an extrusion die, the sheet or ribbon is cooled down.

The unfoamed sheet or ribbon is subsequently crosslinked in a separate, second step using e.g. electron beam curing (EBC). The unfoamed sheet or ribbon is thereby exposed to a high-energy electron beam. The electron beam generates radicals within the polymer chains. These radicals can form new covalent bondings between polymer chains which result in a higher internal strength and are also necessary for the following foaming procedure to ensure a proper embedding of the generated gas bubbles. The internal strength can be adjusted by the power of the electron beam and/or by the exposure time to the electron beam. In order to achieve high internal strengths, the speed of the EBC process is generally relatively low.

The thus crosslinked and still unfoamed sheet or ribbon is foamed in another, third production step in order to yield the foam. In the third production step, the crosslinked and unfoamed sheet or ribbon is usually conveyed through a foaming furnace at sufficiently high temperatures to activate the chemical foaming agent. The degree of foaming can be adjusted by e.g. the exposure time to the heat, i.e. for example the speed at which the sheet or ribbon is conveyed through the furnace, and/or the temperature in the furnace. After foaming is completed, the foam is wound into a roll. The foam can be unwound e.g. for cutting or further processing as needed. In between the first, second and third step, the film is usually wound into rolls and later unwound. This is necessary since the different process steps have significantly differing processing speeds with respect to each other. A continuous production process is therefore not possible. As a result, the length of the foam produced is limited and intermediate storage of the foam between process steps is required. The whole production process is thus cost-intensive and requires special machinery such as e.g. an electron beam curing apparatus.

In contrast to crosslinked foams as described in the above, foamed films can be produced in one-step processes. The term "foamed film" in contrast to "foam" herein refers to films that are foamed without crosslinking of the base material. In addition, in contrast to foams as described in the above, the foaming step in the production of foamed films occurs at the time of forming the film, e.g. at the extrusion through an extrusion die. Throughout the present application, the terms "foamed film" and "foam" are clearly distinguished.

During production of a foamed film, a chemical foaming agent is premixed with a polymer base material and, as the case may be, with other additives. This mixture is typically fed into an extruder. In the extruder, the polymer material is heated and becomes soft to form a melt. The heat generated to melt the polymer is high enough to decompose the chemical foaming agent resulting in gas being liberated. Due to the pressure in the extruder, the liberated gas remains dissolved in the melt. At the extrusion die, the mixture is formed into a film. Once the molten mixture exits the extrusion die, the gas of the chemical foaming agent can expand due to the pressure drop at the die and can form bubbles inside the polymer to form the foamed film. The foamed film can e.g. be formed as a foamed film tube whereby the tube's diameter can be rapidly expanded via e.g. air pressure blown from the inside of the tube. The extruded film is thereby stretched in transverse and/or, e.g. by drawing, in longitudinal direction. The drawing and blowing cause the foamed film to thin out and also leads to an elongation of the bubbles in the stretching directions, yielding anisotropically shaped cells.

Whereas easy and cost-effective to manufacture, in particular in a continuous process, such foamed films are generally considered unsuitable for pressure sensitive adhesive tapes due to their poor mechanical properties in terms of e.g. softness or strength and/or possibly anisotropic damping and compression properties.

It is therefore an object of the invention to provide a pressure sensitive adhesive tape and a method for producing the same which overcomes the disadvantages of the prior art. Furthermore, it is the object of the invention to provide a pressure sensitive adhesive tape and a method for producing the same that has good damping and compression properties and which can be produced in a simple and cost-efficient manner, in particular in a continuous process.

Description of the invention The objects of the invention are achieved by a pressure sensitive adhesive tape according to the invention including a foamed film, a method for producing the same as well as a foamed film as described herein.

According to the invention, a pressure sensitive adhesive tape comprises a foamed film, wherein the foamed film includes a thermoplastic polymeric base material with therein embedded cavities. The foamed film has two opposing surfaces with a thickness between them, wherein a pressure sensitive adhesive layer is provided on at least one, preferably both, of the surfaces of the foamed film. According to a first aspect of the invention, the pressure sensitive adhesive layer is made from a pressure sensitive adhesive material comprising a base material with an embedded elastic material phase in the form of discrete elastic material domains in order to improve shock resistance of the pressure sensitive adhesive tape.

The polymeric base material of the foamed film, i.e. the polymer matrix, is a thermoplastic material and can comprise e.g. Polyolefin, Polyester, Polyurethan, Rubber or Polyamide or mixtures thereof. Suitable polyolefin based materials in the context of conventional crosslinked foams are e.g. described in WO 2013/174482 A1. The foamed film according to the invention, in contrast to known foams, is not crosslinked and, as such, can be produced in a simple and cost-efficient manner.

The foamed film used in the pressure sensitive adhesive tape according to the invention is formed in a foaming step at the time of forming the film, e.g. at the extrusion through an extrusion die. As such, the foamed film and, as a consequence, also the pressure sensitive adhesive tape according to the invention can be formed in a continuous process. Intermediate curing times, crosslinking times as well as a subsequent separate foaming step can be omitted and intermediate storage is not necessary.

By the additional elastic material phase in the pressure sensitive adhesive layer, the elastic compressibility of the pressure sensitive adhesive tape, in particular the perpendicular elastic compressibility, can be improved (perpendicular hereby refers to a direction with respect to the surfaces of the foamed film). In particular, it has surprisingly been found that the pressure sensitive adhesive tape comprising a pressure sensitive adhesive layer including the elastic material phase in combination with the foamed film can provide good shock-absorbing or damping performances in all directions that can even surpass corresponding properties of conventional pressure sensitive adhesive tapes including a crosslinked foam. The invention therefore provides an easy and cost- efficient pressure sensitive adhesive tape which has at least equal or even better mechanical properties as conventional pressure sensitive adhesive tapes.

According to a preferred embodiment, the elastic material domains of the pressure sensitive adhesive layer have dimensions in the range of 1 to 100 pm and preferably form essentially spherical domains. As such, it can be ensured that the pressure sensitive adhesive layer exhibits homogeneous and isotropic damping properties. It is to be understood that the dimensions of the elastic material domains are adapted to the thickness of the pressure sensitive adhesive layer, i.e. the average dimensions of the domains are typically at most equal and preferably less than the thickness of the pressure sensitive adhesive layer.

The elastic material domains of the pressure sensitive adhesive layer are obtained by mixing the base material, which is formed by a polymer component with tackifier / resin, with an incompatible polymer / elastomer component to form an inhomogeneous mixture. In a preferred embodiment, the base material of the pressure sensitive adhesive layer preferably comprises a polyacrylate component. The polyacrylate component on its own is preferably a homogeneous phase. The elastomer component can be homogeneous in itself or can be a multiphase system, as is known of microphase-separating block copolymers. The polyacrylate component and the elastomer component are here so chosen that - after intimate mixing - they are substantially not miscible at 23°C (that is to say the conventional use temperature for adhesives). "Substantially not miscible" means either that the components are not miscible homogeneously with one another at all, so that none of the phases contains a portion of the second component homogeneously mixed therein, or that the components are partially compatible to such a small degree - that is to say that one or both of the components are able to homogeneously take up only such a small portion of the other component - that the partial compatibility is unimportant for the performance. The corresponding components are then regarded within the meaning of this specification as being "substantially free" of the other component.

In a preferred embodiment, the polyacrylate components amount to from 60 % to 90 % by weight, preferably from 65 % to 80 % by weight of the pressure sensitive adhesive material, and the elastic material phase is an elastomer-based polymer component which is substantially not miscible with the base material, in particular a synthetic rubber, in an amount from 10 % to 40 % by weight, preferably from 15 % to 30 % by weight of the pressure sensitive adhesive material.

Polyacrylate component The polyacrylate component comprises one or more polyacrylate-based polymers, which constitute the polyacrylate base polymer of the polyacrylate component, and optionally one or more crosslinkers. Resins, accelerators and/or further additives can further be present in the polyacrylate component. In addition to the polyacrylate base polymer and the resins, a certain proportion of non-acrylic polymers which are compatible with the polyacrylate base polymer can theoretically be mixed in, but such non-acrylic polymers are preferably not present. Polyacrylate-based polymers are, in particular, polymers that are based, at least predominantly, in particular to the extent of more than 60% by weight, on acrylic acid esters and/or methacrylic acid and optionally the free acids thereof, as monomers (referred to as "acrylic monomers" hereinbelow). Polyacrylates are preferably obtainable by free radical polymerization. Polyacrylates may optionally comprise further, copolymerizable monomers.

The polyacrylates can be homopolymers and/or in particular copolymers. The term "copolymer" includes both, copolymers in which the comonomers used in the polymerization are incorporated purely randomly, and those in which gradients in the comonomer composition and/or local concentrations of individual comonomer types as well as entire blocks of a monomer occur in the polymer chains. Alternating comonomer sequences are also conceivable.

The polyacrylates can be of linear, branched, star-shaped or grafted structure, for example, and they can be homopolymers or copolymers. The mean molar mass (weight-average Mw) of at least one of the polyacrylates of the polyacrylate base polymer, where a plurality of polyacrylates are present advantageously of the predominant portion by weight of the polyacrylates, in particular of all the polyacrylates present, is advantageously in the range of from 250,000 g/mol to 10,000,000 g/mol, preferably in the range of from 500,000 g/mol to 5,000,000 g/mol. The composition of the polyacrylate component is particularly preferably so chosen that the polyacrylate component has a glass transition temperature (DSC, see below) of not more than 0°C, preferably of not more than -20°C, particularly preferably of not more than -40°C. The glass transition temperature of copolymers can advantageously be so chosen, by the choice and composition in terms of amount of the components used, that, in analogy to the Fox equation according to the following equation G1 a suitable glass transition point TG for the polymer is obtained; where n = consecutive number over the monomers used, w _n = amount by mass of the respective monomer n (% by weight) and Ts _,h = glass transition temperature of the homopolymer of the respective monomers n in K. Glass transition temperatures of homopolymers can depend up to a certain upper molar mass limit on the molar mass of the homopolymer; the reference to glass transition temperatures of homopolymers in this specification takes place in relation to polymers whose molar masses lie above that molar mass limit, that is to say in the constant glass transition temperature range. Determination of the TG is carried out after removal of the solvent in the uncrosslinked state (in the absence of crosslinkers).

Equation G1 can also be used analogously to determine and predict the glass transition temperature of polymer mixtures. In that case, provided the mixtures are homogeneous, n = consecutive number over the polymers used, w _n = amount by mass of the respective polymer n (% by weight) and Ts _,h = glass transition temperature of the polymer n in K.

The static glass transition temperature generally increases as a result of mixing with adhesive resins.

Random copolymers can be used advantageously. At least one polymer type of the polyacrylate component is advantageously based on unfunctionalized a,b-unsaturated esters. If these are used for the at least one polymer in the polyacrylate component of copolymer nature, it is possible to use as monomers in the preparation of this at least one polymer type in principle any compounds known to the person skilled in the art that are suitable for the synthesis of (meth)acrylate (co)polymers. There are preferably used a,b-unsaturated alkyl esters of the general structure

CH2=C(R ¹ )(COOR ² ) wherein R ¹ = H or CH3 and R ² = H or linear, branched or cyclic, saturated or unsaturated alkyl radicals having from 1 to 30 carbon atoms, in particular having from 4 to 18 carbon atoms.

At least one type of monomers for the polyacrylates of the polyacrylate component are those whose homopolymer has a glass transition temperature TG of not more than 0°C, particularly preferably not more than -20°C. These are in particular esters of acrylic acid with linear alcohols having up to 10 carbon atoms or branched alcohols having at least 4 carbon atoms, and esters of methacrylic acid with linear alcohols having from 8 to 10 carbon atoms or branched alcohols having at least 10 carbon atoms. Furthermore, monomers whose homopolymer has a glass transition temperature TG of more than 0°C can additionally be used. As specific examples according to the invention, there are preferably used one or more members chosen from the group comprising methyl acrylate, methyl methacrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, n-butyl methacrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n- octyl methacrylate, n-nonyl acrylate, n-nonyl methacrylate, n-decyl acrylate, n-decyl methacrylate, isobutyl acrylate, isopentyl acrylate, isooctyl acrylate, isooctyl methacrylate, as well as the branched isomers of the above-mentioned compounds, such as, for example, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-propylheptyl acrylate.

Monomers having the tendency to form semicrystalline regions in the polymer can further be chosen. This behavior is found for acrylic acid esters and methacrylic acid esters with a linear alkyl radical having at least 12 carbon atoms in the alcohol moiety, preferably having at least 14 carbon atoms in the alcohol moiety. Stearyl acrylate and/or stearyl methacrylate, for example, can particularly advantageously be used here according to the invention.

Further monomers which can advantageously be used are monofunctional acrylates and/or methacrylates of bridged cycloalkyl alcohols having at least 6 carbon atoms in the cycloalkyl alcohol moiety. The cycloalkyl alcohols can also be substituted, for example by C1 - to C6-alkyl groups, halogen atoms or cyano groups. Specific examples are cyclohexyl methacrylates, isobornyl acrylate, isobornyl methacrylate and 3,5-dimethyladamantyl acrylate.

In order to vary the glass transition temperature, it is also possible to use a portion of comonomers, whose homopolymers have a high static glass transition temperature, for the preparation of the polyacrylates. Suitable components are aromatic vinyl compounds, such as, for example, styrene, wherein the aromatic nuclei preferably comprise C4 to C18 structural units and can also contain heteroatoms. Particularly preferred examples are 4-vinylpyridine, N-vinylphthalimide, methylstyrene, 3,4-dimethoxystyrene, 4-vinylbenzoic acid, benzyl acrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate, tert-butylphenyl acrylate, tert-butyl phenyl methacrylate, 4-biphenyl acrylate and methacrylate, 2-naphthyl acrylate and methacrylate, as well as mixtures of such monomers, whereby this list is not exhaustive.

As comonomers to the acrylic monomers there can also be used further monomers which are copolymerizable with acrylic monomers, for example in an amount of up to 40% by weight. Such comonomers can in principle be any compounds having copolymerizable double bonds which are compatible with the acrylates, such as, for example, vinyl compounds. Such vinyl compounds can be chosen wholly or partially from the group comprising vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, vinyl compounds with aromatic rings and heterocyclic rings, in particular in the a- position relative to the double bond. Comonomers that are particularly preferably suitable are, for example, vinyl acetate, vinyl formamide, vinylpyridine, ethyl vinyl ether, vinyl chloride, vinylidene chloride, and acrylonitrile.

However, other compounds copolymerizable with acrylic monomers can also be used here.

There are particularly advantageously added to the polyacrylate component one or more crosslinkers for a chemical and/or physical crosslinking. However, since radiation crosslinking of the polyacrylate component is also possible in principle, crosslinkers are not necessarily present. Crosslinkers are compounds, in particular bi- or poly-functional, mostly low molecular weight compounds, which are able to react under the chosen crosslinking conditions with suitable, in particular functional, groups of the polymers to be crosslinked, thus linking two or more polymers or polymer sites with one another to form "bridges" and accordingly create a network from the polymer or polymers to be crosslinked. This generally results in increased cohesion. The degree of crosslinking depends on the number of bridges that are formed.

Suitable crosslinkers for the present polymer are in principle any crosslinker systems known to the person skilled in the art for the formation of, in particular, covalent, coordinative or associative bond systems with correspondingly equipped (meth)acrylate monomers, according to the nature of the chosen polymers and their functional groups. Examples of chemical crosslinking systems are di- or poly-functional isocyanates or di- or poly-functional epoxides or di- or poly-functional hydroxides or di- or poly-functional amines or di- or poly-functional acid anhydrides. Combinations of different crosslinkers are likewise conceivable.

Further suitable crosslinkers which may be mentioned are chelate formers, which, in combination with acid functionalities in polymer chains, form complexes which act as crosslinking points.

It is particularly advantageous for effective crosslinking if at least a portion of the polyacrylates contain functional groups with which the crosslinkers in question are able to react. There are preferably used for this purpose monomers with functional groups selected from the group comprising: hydroxy, carboxy, and sulfonic acid or phosphonic acid groups, acid anhydrides, epoxides, and amines. Particularly preferred examples of monomers for polyacrylates are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, aconitic acid, dimethylacrylic acid, b-acryloyloxypropionic acid, trichloroacrylic acid, vinylacetic acid, vinylphosphonic acid, maleic anhydride, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate, allyl alcohol, glycidyl acrylate, and glycidyl methacrylate.

It has been found to be particularly advantageous to use as a crosslinker from 0.03 to 0.2 part by weight, in particular from 0.04 to 0.15 part by weight, N,N,N',N'-tetrakis(2,3- epoxypropyl)-m-xylene-a,a'-diannine (tetraglycidyl-meta-xylenediamine; CAS 63738-22- 7), based on 100 parts by weight of polyacrylate base polymer.

Alternatively or in addition, it can be advantageous to crosslink the adhesive by means of radiation. Suitable as the radiation for this purpose are ultraviolet light (especially when suitable photoinitiators are added to the formulation or at least one polymer in the acrylate component comprises comonomers having units with photoinitiating functionality) and/or electron beams.

It can be advantageous for radiation-induced crosslinking if a portion of the monomers used comprises functional groups which promote subsequent radiation crosslinking. Suitable copolymerizable photoinitiators are, for example, benzoin acrylate and acrylate-functionalized benzophenone derivatives. Monomers which promote crosslinking by electron irradiation are, for example, tetrahydrofurfuryl acrylate, N-tert- butylacrylamide and allyl acrylate.

For the chemical and/or physical and/or radiation-induced crosslinking, reference is made in particular to the relevant prior art.

In an advantageous embodiment, there are added to the polyacrylate component one or more polyacrylate-compatible adhesive resins which are substantially compatible with the polyacrylate. The adhesive resins known to be suitable therefor can in principle be used here. Terpene-phenol resins are particularly preferably used. However, it is also possible to use, for example, colophony derivatives, in particular colophony esters.

The polyacrylate-compatible resins preferably have a DACP value of less than 0°C, particularly preferably of not more than -20°C, and/or preferably an MMAP value of less than 40°C, particularly preferably of not more than 20°C. For the determination of MMAP and DACP values, reference is made to C. Donker, PSTC Annual Technical Seminar, Proceedings, p. 149-164, May 2001.

The polyacrylate component can further comprise additives, such as initiators, activators, and accelerators for the crosslinking and the like.

Polyacrylate-compatible (adhesive) resins are particularly preferably used in such an amount that the ratio of the polyacrylate base polymer to polyacrylate-compatible resins is in the range of from 100:0 (threshold 100:0 means the absence of polyacrylate- compatible resins) to 50:50, particularly preferably in the range of from 80:20 to 60:40.

For the polyacrylate component, polyacrylate compositions as described in particular in WO 2012/062589 A have been found to be very suitable.

Elastomer component

The elastomer component, which is substantially not compatible with the polyacrylate component, advantageously comprises a synthetic rubber or a plurality of synthetic rubbers chosen independently of one another as the base polymer component, as well as optionally resins and/or other additives.

Block copolymers are preferred for the elastomer component. According to the invention, the synthetic rubbers are advantageously in particular those in the form of thermoplastic block copolymers, the structure of which can be represented by one of the following formulae

A-B

A-B-X(A'-B')n wherein

- A or A' is a polymer formed by polymerization of a vinyl aromatic compound, such as, for example, styrene or a-methylstyrene,

- B or B' is a polymer of an isoprene, butadiene, a farnesene isomer or a mixture of butadiene and isoprene or a mixture of butadiene and styrene, or comprising wholly or partially of ethylene, propylene, butylene and/or isobutylene,

- X is an optional linking group (e.g. a radical of a coupling reagent or initiator),

- n is an integer from 1 to 4,

- (A'-B') _n can be linked to X or to (A-B) via A' (structure Ilia) or B' (structure lllb), preferably via B', and - A can be = A' in terms of composition and/or molar mass, and B can be = B' in terms of composition and/or molar mass.

Suitable vinyl aromatic block copolymers comprise one or more rubber-like blocks B or B' (soft blocks, elastomer blocks) and one or more glassy blocks A or A'. In some embodiments, the block copolymer comprises at least one glassy block. In some further embodiments according to the invention, the block copolymer comprises from one to five glassy blocks

In some advantageous embodiments, there is used in addition to the structures II, Ilia and/or Nib or exclusively a block copolymer which is a multiarm block copolymer. This is described by the general formula

Qm-Y wherein Q represents an arm of the multiarm block copolymer and m in turn represents the number of arms, wherein m is an integer of at least 3. Y is the radical of a multifunctional linking reagent which originates, for example, from a coupling reagent or from a multifunctional initiator. In particular, each arm Q independently has the formula A*-B*, wherein A* and B* are each chosen independently of the other arms according to the above definitions for A or A' and B or B', so that, analogously to structures II, Ilia and lllb, A ^* in each case represents a glassy block and B ^* represents a soft block. It is also possible, of course, to choose identical A ^* s and/or identical B ^* s for a plurality of arms Q or for all the arms Q.

The blocks A, A' and A ^* are together referred to as A blocks hereinbelow. Correspondingly, the blocks B, B' and B ^* are together referred to as B blocks hereinbelow.

A blocks are generally glassy blocks having a glass transition temperature (DSC, see below) which is above room temperature (room temperature is understood within the context of this invention as being 23°C). In some advantageous embodiments, the Ts qί the glassy block is at least 40°C, preferably at least 60°C, even more preferably at least 80°C or most preferably at least 100°C. The vinyl aromatic block copolymer further generally comprises one or more rubber-like B blocks [or soft blocks or elastomer blocks] having a TG below room temperature. In some embodiments, the TG of the soft block is below -30°C or even below -60°C.

In addition to the particularly preferred monomers for formulae II, llla/lllb, and IV for the B blocks, further advantageous embodiments comprise a polymerized conjugated diene, a hydrogenated derivative of a polymerized conjugated diene, or a combination thereof. In some embodiments, the conjugated dienes comprise from 4 to 18 carbon atoms. Examples of further advantageous conjugated dienes for the rubber-like B blocks which may be mentioned are additionally ethylbutadiene, phenylbutadiene, piperylene, pentadiene, hexadiene, ethylhexadiene and dimethylbutadiene, wherein the polymerized conjugated dienes can be present as a homopolymer or as a copolymer.

The content of A blocks, based on the total block copolymers, is on average preferably from 10 to 40% by weight, more preferably from 15 to 33% by weight.

Polystyrene is preferred as the polymer for A blocks. Preferred polymers for B blocks are polybutadiene, polyisoprene, polyfarnesene and partially or completely hydrogenated derivatives thereof, such as polyethylenebutylene, polyethylenepropylene, polyethylene-ethylenepropylene, polybutylenebutadiene or polyisobutylene. Polybutadiene is particularly preferred.

Mixtures of different block copolymers can be used. Preference is given to the use of triblock copolymers ABA and/or diblock copolymers AB.

Block copolymers can be linear, radial or star-shaped (multiarm), also independently of the structures II and III.

Hydrocarbon resins can particularly advantageously be used as elastomer-compatible resins. Suitable adhesive resins for this class of resins are, inter alia, preferably hydrogenated polymers of dicyclopentadiene, non-hydrogenated, partially, selectively or completely hydrogenated hydrocarbon resins based on C5, C5/C9 or C9 monomer streams, or particularly preferably polyterpene resins based on a-pinene and/or b- pinene and/or d-limonene. The above mentioned adhesive resins can be used both alone and in a mixture. Ideally, it is substantially not compatible with the acrylate polymers. The aromatic portion should therefore not be chosen to be too high. Suitable adhesive resins of this class of resins are in particular compatible with the soft block or soft blocks of the elastomer component. The hydrocarbon resins of the pressure sensitive adhesive according to the invention that are compatible with the synthetic rubbers preferably have a DACP value of at least 0°C, particularly preferably of at least 20°C, and/or preferably an MMAP value of at least 40°C, particularly preferably of at least 60°C. For the determination of MMAP and DACP values, reference is made to C. Donker, PSTC Annual Technical Seminar, Proceedings, p. 149-164, May 2001.

The hydrocarbon resins which can optionally be used within the meaning of this specification, are also oligomeric and polymeric compounds having a number-average molar mass Mn of typically not more than 5000 g/mol. It is also possible to use hydrocarbon resin mixtures. In particular, the major portion of the hydrocarbon resins (based on the portion by weight in the total amount of hydrocarbon resin), preferably all the hydrocarbon resins, have a softening point of at least 80°C and not more than 150°C (ring and ball method analogously to DIN EN 1427:2007, see below).

It is also conceivable to use aromatic hydrocarbon resins which are compatible with the A blocks. In particular, such adhesive resins 2 can also be (partially) compatible with the polyacrylate component.

In an advantageous variant of the invention, in addition to polyacrylate-compatible adhesive resin(s) and/or in addition to elastomer-compatible adhesive resin(s), or alternatively to those adhesive resins, there are used one or more adhesive resins which are compatible with both components or which are compatible with one component and partially compatible with the other component.

For example, it is possible to use one or more polyacrylate-compatible adhesive resins which are at least partially compatible or completely miscible with the elastomer component. If thermoplastic block copolymers are used as the elastomer component, as described above, the polyacrylate-compatible adhesive resins used can be at least partially compatible or completely miscible with the A blocks and/or the B blocks of the elastomer component.

For example, it is also possible to use one or more adhesive resins which are compatible with the elastomer component and at least partially compatible with the polyacrylate component. If thermoplastic block copolymers are used as the elastomer component, as described above, the polyacrylate-compatible adhesive resins used can be miscible with the A blocks and/or the B blocks of the elastomer component.

References

The stated number-average molar mass Mn and weight-average molar mass Mw in this application relate to determination by gel permeation chromatography (GPC).

The determination is carried out on a 100 pi clear-filtered sample (sample concentration

4 g/l). Tetrahydrofuran with 0.1 vol. % trifluoroacetic acid is used as the eluent. The measurement is performed at 25°C. The precolumn used is a column of type PSS-SDV,

5 pm, 10 ³ A, 8.0 mm ^* 50 mm (details here and below are in the following order: type, particle size, porosity, inside diameter*length; wherein 1 A=10 ¹ ° m). For separation there is used a combination of columns of type PSS-SDV, 5 pm, 10 ³ A, 10 ⁵ A and 10 ⁶ A, with in each case 8.0 mm ^* 300 mm (columns from Polymer Standards Service; detection by means of a Shodex R171 differential refractometer). The throughput is 1.0 ml per minute. Calibration is carried out in the case of polyacrylates against PMMA standards (polymethyl methacrylate calibration) and otherwise (resins, elastomers) against PS standards (polystyrene calibration).

Data relating to the softening point - also referred to synonymously as the softening temperature - of oligomeric and polymeric compounds, such as, for example, of the resins, relate to the ring and ball method according to DIN EN 1427:2007 with appropriate application of the provisions (testing of the oligomer or polymer sample instead of bitumen, with the procedure otherwise being retained).

Glass transition points - referred to synonymously as glass transition temperatures - are indicated as the result of measurements by means of dynamic scanning calorimetry (DSC) according to DIN 53 765; in particular sections 7.1 and 8.1 , but with uniform heating and cooling rates of 10 K/min in all the heating and cooling steps (see DIN 53 765; section 7.1 ; note 1 ). The original weighed amount of the sample is 20 mg. Pretreatment of the pressure sensitive adhesive is carried out (see section 7.1 , first run). Temperature limits: -140°C (instead of TG -50°C)/+200°C (instead of TG +50°C.). The indicated glass transition temperature TG is the sample temperature in the heating operation of the second run at which half of the change in specific heat capacity has been reached. Further preferred materials for the base material as well as the elastic material phase of the pressure sensitive adhesive material are e.g. described in US 2016/0167339 or DE 10 2014 207 974, the corresponding contents of which are hereby incorporated by reference.

In a further embodiment, the pressure sensitive adhesive layer has a thickness in the range from 1 to 500 pm, preferably from 1 to 300 pm.

In a second aspect of the invention, which can be advantageous as an independent aspect of the invention or in combination with the elastic material domains of the pressure sensitive adhesive tape according to the first aspect of the invention, the pressure sensitive adhesive tape comprises a foamed film, wherein the foamed film comprises a thermoplastic polymeric base material with therein embedded cavities, the foamed film having two opposing surfaces with a thickness between them, and wherein a pressure sensitive adhesive layer is provided on at least one, preferably both, of the surfaces of the foamed film. According to the second aspect of the invention, the thermoplastic polymeric base material of the foamed film comprises 1 % to 10 % vinylacetate (VA) by weight.

Foamed film

Whereas the polymeric base material of the foamed film can comprise a variety of thermoplastic polymeric materials or mixtures thereof, it has surprisingly been found that the presence of vinylacetate in the foamed film can significantly improve the mechanical properties of the foamed film and render it suitable for the requirements of the pressure sensitive adhesive tape according to the invention. In particular, an amount in the range from 1 % to 10 % vinylacetate by weight has proven to strongly enhance the softness and damping properties of the foamed film while the necessary strength for the desired applications of the pressure sensitive adhesive tape can still be maintained. If the amount of vinylacetate is larger, the structural properties of the foamed film render its use for pressure sensitive adhesive tapes unfavorable since the foamed film e.g. becomes too soft and/or the mechanical strength is not sufficient. In addition, e.g. extrusion parameters would change significantly and production of the foamed film would become unfavorable. If the amount is smaller, the softness is too low and e.g. the compression and damping properties cannot fulfill the requirements. According to the second aspect of the invention, the pressure sensitive adhesive tape does not need to, but can include an elastic material phase in the pressure sensitive adhesive layer.

In particular, it has been found that an amount of 1 % to 5 %, and more preferably 3 % to 4 %, vinylacetate (VA) by weight in the thermoplastic polymeric base material yields most preferable mechanical properties while still being easy and reliable to manufacture. In a particularly advantageous embodiment, the vinylacetate is present in the form of a copolymer of polyethylene (PE) and vinylacetate (VA) as ethylene-vinylacetate (EVA) copolymer, preferably with a ratio of about 75 % polyethylene (PE) and about 25 % vinylacetate (VA) by weight. Thereby, it has surprisingly been found that the thermoplastic polymeric base material comprising about 84 % of a low density polyethylene (LDPE) and about 16 % ethylene-vinylacetate (EVA) by weight results in optimal mechanical properties while being easy and reliable to manufacture. The thermoplastic polymeric base material can additionally comprise small amounts of at least one component of the group of extrusion aids and lubricants, for example zinc stearate, antistatics, for example aliphatic amines, and pigments, for example carbon black or titanium dioxide.

The applicant has surprisingly found that isotropically shaped cavities in the foamed film are not required to provide a pressure sensitive adhesive tape with sufficient compression and damping properties for the desired applications. In particular, it has been found that damping and compression properties in a direction perpendicular to the surface of the foamed film can be comparatively low and still yield good results e.g. when mounting electronic components. In particular, it has been found by statistical analysis of drop tests of e.g. mobile phones that the majority of impacts have mixed components in perpendicular and parallel directions and, generally, much of the drop energy can be absorbed in a direction parallel to the surface of the foamed film. In a preferred embodiment, in particular for the sake of simple manufacturing, an average dimension of the embedded cavities in at least one direction parallel to the surfaces of the foamed film, preferably all directions parallel to the surfaces of the foamed film, is larger than an average dimension in a direction perpendicular to the surfaces of the foamed film. In other words, the cavities can have e.g. a flattened, elongate, ovoid or disk-like shape. Such a foamed film with sufficient shock-absorbing or damping properties can be easily produced by stretching an initial foamed film after extrusion. As a further advantage, due to the anisotropic shape, the elastic compressibility resulting from the cavities can be designed in an anisotropic way in order to e.g. have increased damping properties in a parallel direction rather than in a perpendicular direction. The larger dimension of the cavities in parallel direction can thereby surpass the thickness of the foamed film and, as such, can be larger than it would be the case with e.g. spherically shaped cavities. The corresponding parallel damping properties can thereby be greatly improved. The foamed film as used in the present invention is therefore versatile for adaptation to the specific requirements. Furthermore, possible deficiencies of the damping properties in a purely perpendicular direction can be compensated by addition of the elastic material phase in the pressure sensitive adhesive layer according to the first aspect of the invention. It is to be understood that, dependent on the desired application, the cavities of the foamed film can also be essentially spherical and have similar dimensions in all directions. Providing such cavities in a foamed film, however, can render the productions process comparatively more tedious and may even be difficult to achieve during production.

In a preferred embodiment, the average dimension of the cavities in the direction parallel to the surface is larger by a factor 5 or more, preferably larger by a factor of 10 or more, more preferably larger by a factor of 20 or more, than the dimension of the cavities in direction perpendicular to the surfaces of the film. As such, the foamed film has a higher compressibility in a direction parallel to the surfaces than perpendicular to the surfaces, resulting in an increased ability for absorbing shocks or damping resulting from e.g. shear forces when used in the pressure sensitive adhesive tape.

An average dimension of the cavities in a direction perpendicular to the surfaces of the film is preferably smaller than or equal to 60 micrometers. As such, sufficient densities of the cavities can be achieved at the usually required thicknesses of the foamed film in pressure sensitive adhesive tapes.

It has thereby proven advantageous that the density of the cavities in the foamed film is in the range from 50 to 1 ,000,000 per cm ³ . The wide range allows for adapting the foamed film to the requirements of a wide variety of specific applications. The foamed film preferably has a thickness in the range from 10 to 10,000 pm, preferably from 50 to 500 pm, in order to allow for various different applications. Potential applications of the pressure sensitive tape according to the invention comprise, but are not limited to:

- Cameras, digital cameras, photographic accessories (such as exposure meters, flashguns, diaphragms, camera casings, lenses, etc.), film cameras, video cameras;

- microcomputers (portable computers, hand-held computers, hand-held calculators), laptops, notebooks, netbooks, ultrabooks, tablet computers, handhelds, electronic diaries and organizers (so-called "electronic organizers" or "personal digital assistants", PDA, palmtops), modems;

- computer accessories and operating units for electronic devices, such as mice, drawing pads, graphics tablets, microphones, loudspeakers, games consoles, gamepads, remote controls, remote operating devices, touchpads;

- monitors, displays, screens, touch-sensitive screens (sensor screens, touchscreen devices), projectors;

- reading devices for electronic books ("E-books");

- mini TVs, pocket TVs, devices for playing films, video players;

- radios (including mini and pocket radios), Walkmans, Discmans, music players for e.g. CD, DVD, Blu-ray, cassettes, USB, MP3, headphones;

- cordless telephones, mobile telephones, smart phones, two-way radios, hands-free telephones, devices for summoning people (pagers, bleepers);

- mobile defibrillators, blood sugar meters, blood pressure monitors, step counters, pulse meters;

- torches, laser pointers;

- mobile detectors, optical magnifiers, binoculars, night vision devices;

- GPS devices, navigation devices, portable interface devices for satellite communications;

- data storage devices (USB sticks, external hard drives, memory cards); - wristwatches, digital watches, pocket watches, chain watches, and stopwatches.

The invention also concerns a method for producing a pressure sensitive adhesive tape, in particular a pressure sensitive adhesive tape as described herein. In a first aspect, the method includes providing a pressure sensitive adhesive tape comprising a foamed film including a thermoplastic polymeric base material with therein embedded cavities, the foamed film having two opposing surfaces with a thickness in between them, applying a pressure sensitive adhesive layer on at least one, preferably both, of the surfaces of the foamed film. The method according to the first aspect is characterized in that a pressure sensitive adhesive material is applied as the pressure sensitive adhesive layer comprising a base material with an embedded elastic material phase in the form of discrete elastic material domains in order to improve shock resistance of the pressure sensitive adhesive tape.

In a second aspect, which can be advantageous independent from the first aspect of the method or in combination with it, the method includes providing a foamed film comprising a thermoplastic polymeric base material with therein embedded cavities, the foamed film having two opposing surfaces with a thickness in between them, and applying a pressure sensitive adhesive layer on at least one, preferably both, of the surfaces of the foamed film. The method according to the second aspect is characterized in that the thermoplastic polymeric base material of the foamed film comprises 1 % to 10 % vinylacetate (VA). Preferably, the thermoplastic polymeric base material comprises 1 % to 5 %, in particular 3 % to 4 %, vinylacetate (VA). In a preferred embodiment of the method, the vinylacetate (VA) comprised in the polymeric base material is present in the form of a copolymer of polyethylene (PE) and vinylacetate (VA) as ethylene-vinylacetate (EVA), preferably with a ratio of about 75 % polyethylene (PE) and about 25 % vinylacetate (VA). In particular, the method includes providing the thermoplastic polymeric base material comprising about 84 % of a low density polyethylene (LDPE) and about 16 % ethylene-vinylacetate (EVA). Related advantages are described herein in relation with the pressure sensitive adhesive tape according to the invention.

The invention also concerns a foamed film for the pressure sensitive adhesive tape as described herein. The foamed film as used in the invention is preferably formed in a continuous process and includes the step of providing a mixture comprising a polymeric thermoplastic material and a chemical foaming agent which, upon activation, liberates gas, wherein the chemical foaming agent can be activated by heating to or above an activation temperature. Chemical foaming agents are well known in the art and can be provided in the raw materials mixture in the form of e.g. particulate material or other forms that are suitable. The polymeric base material can further comprise additives like pigments such as carbon black, titanium dioxide or barium sulfate, or antixodants as well as e.g. lubricants.

The mixture is heated to a processing temperature at which the polymeric thermoplastic material forms a melt, the processing temperature being equal to or higher than the activation temperature of the foaming agent such that the chemical foaming agent is activated. The heating of the mixture to form a melt is preferably achieved in an extruder which, during heating and forming the melt, homogenizes the components of the mixture, i.e. ensures that the distribution of the chemical foaming agent and, as the case may be, further additives in the polymeric base material is homogeneous. A pressure in the melt is maintained such that the gas liberated by the chemical foaming agent remains dissolved in the melt. The melt is formed into an elongate, in particular continuous, film in a longitudinal direction, preferably by extrusion through an extrusion die. The film is subjected to a sudden pressure drop such that the dissolved gas of the chemical foaming agent is released and forms cavities within the film in order to form the foamed film.

As such, the foamed film can be produced in a continuous operation such that endless foamed films can be provided in one single production line. Thereby, it is possible to supply the foamed film directly to further processing stations for e.g. production of the pressure sensitive adhesive tape according to the invention without the need for intermediate storage or other interruptions of the processing line. Furthermore, standard machinery can be used and operated at high speeds at a comparatively high yield, resulting in a cost-efficient method for producing the pressure sensitive adhesive tape according to the invention.

The foamed film can be stretched in the longitudinal direction and/or in a transverse direction after extrusion. The stretching has several advantages: On the one hand, the foamed film can be thinned by the stretching to a thickness required for the specific application. On the other hand, the cavities formed in the foamed film are deformed and receive an elongate or e.g. ovoid-like shape in the direction of the stretching. As such, the elastic compressibility of the foamed film that allows for the damping or shock- absorbing properties can be adjusted to have a directional dependence. The foamed film can e.g. be adjusted to have particularly good properties for receiving and absorbing shear forces in a direction essentially parallel to the surfaces of the foamed film.

Another benefit of the stretching in the longitudinal and/or the transverse direction, in particular when crystalline or semi-crystalline polymers are used, results from a preferred orientation of the polymer molecular chains in the stretching direction. Depending on the stretching direction, the resulting orientation of the molecular chains leads to a higher strength in the longitudinal or the transverse direction, allowing for adaptation of the foamed film to the specific requirements. Preferably, the stretching of the film in the longitudinal direction includes drawing the film in the processing direction. Thereby, it has proven advantageous that the stretching of the foamed film in the longitudinal direction is achieved by drawing the film upwards, against a direction of gravity. The e.g. extrusion die for forming the film can thereby be directed upwards and the foamed film is drawn via rollers against the foamed film's own weight.

The foamed film is preferably formed as a foamed film tube. In other words, the foamed film is formed in a circumferentially closed shape, continuously extending in longitudinal direction. In case the foamed film is formed as a foamed film tube, the stretching in the transverse direction preferably includes increasing the diameter of the film tube by e.g. blowing the foamed film tube to a desired diameter. The blowing can be combined with e.g. upwards drawing of the foamed film in order to achieve a simultaneous stretching in both the longitudinal and the transverse direction. By adjusting the relative amount of stretching, the foamed film can be adapted to the specific requirements such as e.g. direction and amount of cavity elongation or orientation of the molecular chains of the polymeric base material.

Once the foamed film has been initially stretched as desired, in particular as the foamed film continues to cool, the foamed film can be guided via one or more nip rollers in order to further draw and/or flatten the film. In particular in the case of a foamed film tube, the foamed film tube can be collapsed for easier further processing or storing, i.e. can be collapsed into a double layer structure which can be stored as is or cut into single layer foamed films as needed.

The foamed film can be wound into a roll for storage and/or further processing. In particular in the case of a foamed film tube, the foamed film tube can be flattened and then directly, as a double layer, be wound into a roll. Alternatively or additionally, the foamed film can also be cut into strips or ribbons as needed or, preferably, can be directly conveyed to further processing for producing the pressure sensitive adhesive tape according to the invention.

The invention is described at hand of examples and drawings in the following. Brief description of the drawings

The exemplary figures used for illustration of the invention schematically show:

Fig. 1 : a cross-sectional view of a pressure sensitive adhesive tape according to the invention;

Fig. 2: the image on the left shows a cross-sectional microscopic view of a PE foam according to the prior art, whereas the right-hand image shows an electron-microscopic cross-sectional view of a foamed film according to the invention;

Fig. 3: a process line for producing a foamed film as used in the present invention;

Fig. 4a: a polycarbonate frame for sample preparation for testing shock absorbance or damping performance of a pressure sensitive adhesive tape;

Fig. 4b: a polycarbonate plate for sample preparation for testing shock absorbance or damping performance of a pressure sensitive adhesive tape;

Fig. 4c: a die-cut of a pressure sensitive adhesive tape for sample preparation for testing shock absorbance or damping performance of a pressure sensitive adhesive tape; Fig. 5a: an assembled sample for testing shock absorbance or damping performance of a pressure sensitive adhesive tape in a plan view;

Fig. 5b: an assembled sample for testing shock absorbance or damping performance of a pressure sensitive adhesive tape in a side view. The dimensions and aspect ratios in the figures are not to scale and have been oversized in part for better visualization. Corresponding elements in the figures are generally referred to by the same reference numerals.

Detailed description of figures and examples

Figure 1 shows a schematic cross-sectional view of a pressure sensitive adhesive tape 1 according to the invention. The pressure sensitive adhesive tape 1 comprises three layers: A central layer constitutes a backing layer of the adhesive tape and is formed by a foamed film 2 according to the invention. The foamed film 2 has two opposing surfaces 3 and 4 and a thickness t _f between them. A direction perpendicular to the surfaces 3 and 4 is referred to as z-direction, whereas directions in parallel to the surfaces 3 and 4 are referred to as x- or y-direction, respectively.

The foamed film 2 comprises a base material 5 which is a thermoplastic polymeric material. Further, the foamed film 2 comprises cavities 6, i.e. bubbles or voids that have average dimensions in x- and/or y-direction which are larger than a dimension in z- direction, indicating that the foamed film 2 has been stretched. The cavities 4 are flattened and have an essentially ovoid or disk-like shape due to stretching in the x- and/or y-direction.

Pressure sensitive adhesive layers 7 and 8 are applied to each of the surfaces 3 and 4. The pressure sensitive adhesive layers 7 and 8 each have a thickness t _a . The pressure sensitive adhesive layers 7 and 8 comprise an elastic material phase 10 of presently essentially spherical rubber domains embedded in a base material 9 of the pressure sensitive adhesive layers 7, 8. It is to be understood, however, that the pressure sensitive adhesive layers 7, 8 including the elastic material phase 10 form an embodiment according to the first aspect of the invention. The pressure sensitive adhesive tape 1 according to the second aspect of the invention, i.e. including a foamed film 2 comprising 1 % to 10 % vinylacetate (VA), can be embodied without the elastic material phase 10. Both aspects of the invention can also advantageously be combined.

Outer surfaces 11 and 12 of the pressure sensitive adhesive layers 7 and 8 form adhesive surfaces which allow attachment of the pressure sensitive adhesive tape 1 to surfaces of objects or components.

Figure 2 shows a comparison of a prior art PE foam 2' on the left-hand side and a foamed film layer 2 according to the invention on the right-hand side in an electron- microscopic cross-sectional view. Whereas the cavities 6' of the prior art foam V have a rather spherical shape, i.e. dimensions in z-direction and in xy-direction are essentially comparable, the cavities 6 of the foamed film layer 2 according to the invention are flat and stretched out in x- and/or y-direction. It can be seen that the cavities 6 of the foamed film layer 2 according to the invention thus form an interwoven layer system in the base material 7 of the foamed film, resulting in an entirely different internal structure than the foamed film 2' of the prior art.

Figure 3 shows a schematic view of a process line for producing a foamed film as used in the present invention. The raw materials (not shown) for the foamed film 2 are fed to an extruder 50 by dosing units 51 and 52. Depending on the desired mixture, dosing units for each component can be provided. Alternatively, the mixture can be formed before being fed to the extruder 50 such that only one dosing unit would suffice. The raw materials comprise at least a thermoplastic polymeric base material and a chemical foaming agent that can be activated by heat.

In the extruder 50, the material mixture is homogenized and heated under pressure to form a melt. The mixture is thereby heated to a temperature at which the chemical foaming agent is activated and liberates gas. The pressure in the extruder is sufficient to prevent outgassing of the gas liberated by the chemical foaming agent such that the gas remains dissolved in the base material.

The melt formed in the extruder 50 is provided to a die or an extrusion nozzle 53 via a corresponding duct system 54. Thereby, the pressure is maintained and the gas remains dissolved in the melt in the duct system. The extrusion nozzle 53 has an annular shape such that the melt is extruded into a film tube 55. The nozzle 53 is directed upwards such that an extrusion direction is directed upwards, against a direction of gravity. As the case may be, it may be necessary to cool the melt slightly prior to extrusion through the extrusion nozzle 53 in order to increase viscosity such that the film tube is semi-solid. A cooling system 56 can be provided, e.g. in the duct system 54, to this end.

At the extrusion nozzle 53, the melt is suddenly exposed to ambient pressure, e.g. atmospheric pressure. As such, the gas dissolved in the melt can outgas, ultimately forming bubbles or cavities in the polymeric base material such that that the film tube 55 forms a foamed film tube 55.

Within the film tube 55, a blow device 57 is arranged that produces an overpressure inside the film tube 55. As such, the foamed film tube 55 is blown to a larger diameter whereby the foamed film tube 55 is stretched in a direction perpendicular to the extrusion direction. At the same time, since the processing direction defined by the extrusion direction is directed upwards, the foamed film tube 55 is stretched in the extrusion direction, i.e. in the longitudinal direction due to its own weight under the influence of gravity. An additional upward drawing force can be applied in longitudinal direction in order to stretch the foamed film 55 further in this direction.

After having been conveyed a certain distance upwards, after sufficient cooling, the foamed film tube 55 is collapsed by a collapsing frame 58. In processing direction downstream of the collapsing frame 58, the foamed film tube 55 is further conveyed as a double layer by rollers 59. The foamed film tube 55 is then guided over a set of nip- rollers 60 for flattening and, if required, further stretching of the foamed film tube 55.

The foamed film tube 55 is subsequently further conveyed by rollers 61 to e.g. a processing unit 62 for cutting the double-layer flattened film tube 55 into two single layer films 55.1 an 55.2. The films 55.1 and 55.2 can e.g. be wound into rolls for storage or for transport to further processing stations. However, it is to be understood that the foamed film 55 or the single-layer foamed films 55.1 and 55.2 can also directly be conveyed by the rollers 61 to a further processing station that e.g. provides the foamed films 55.1 and 55.2 with pressure sensitive adhesive layers 7, 8 to form the pressure sensitive adhesive tape 1.

The exemplary process line as shown in Fig. 3 thus allows for production of the foamed films 55 or 55.1 and 55.2 in a single, continuous process line. As such, the advantages with respect to the stepwise production system of the prior art as described in the above become immediately obvious.

Pressure sensitive adhesive tape preparation

The following is a characterization of the commercially available chemicals used: Crosslinker:

Erisys GA 240: N,N,N'N'-tetrakis(2,3-epoxypropyl)-m-xylene-a,a'-diamine Emerald Performance Materials.

Rubbers:

Block copolymer mixtures based on styrene and butadiene; styrene-butadiene diblock copolymer (SB), styrene-butadiene-styrene triblock copolymer (SBS): styrene- ethylene/butylene-styrene triblock copolymer (SEBS).

Kraton D 1118: diblock/triblock 78/22; polystyrene content approx. 33%: Brookfield viscosity (25°C, 25% in toluene) ~0.6 Pa s; triblock linear SBS.

Kraton D 1102: triblock/diblock 83/17, polystyrene content approx. 29.5%, Brookfield viscosity (25°C, 25% in toluene) -1.2 Pa s; triblock linear SBS.

Kraton D 1101 : triblock/diblock 84/16, polystyrene content approx. 31 %, Brookfield viscosity (25°C, 25% in toluene) -4 Pa s; triblock linear SBS.

Kraton G 1675: triblock/diblock 71/29, polystyrene content approx. 13%, triblock linear SEBS (middle block hydrogenated). Kraton D 0243 ET: triblock/diblock 25/75, polystyrene content approx. 33% styrene, Brookfield viscosity (25°C, 25% in toluene) 0.3 Pa s.

All Kraton: Kraton Polymers

Resins:

Sylvares ® TP95: terpene-phenol resin: softening point approx. 95°C; Mw ~900 g/mol; hydroxyl value: 40 mgKOH/g.

Arizona Dertophene T: terpene-phenol resin, softening point approx. 95°C; Mw -500-800 g/mol; hydroxyl value 20-50 mgKOH/g.

Dertophene T 110: terpene-phenol resin, softening point approx. 110°C; Mw -500-800 g/mol; hydroxyl value 40-60 mgKOH/g.

All Dertophene: DRT

Unless specifically indicated otherwise, all percentages below are percent by weight. Indicated amounts relating to the adhesive are based on polyacrylate + rubbers + resin = 100% by weight, crosslinker (amounts based on 100% by weight polyacrylate) additive to 100 parts by weight adhesive.

Preparation of the Polyacrylate

A conventional 2 liter glass reactor suitable for radical polymerizations with boiling- cooling was filled with 300 g of a monomer mixture comprising 142.5 g of butyl acrylate, 142.5 g of ethylhexyl acrylate, 15 g of acrylic acid, and 200 g of acetone: special boiling- point spirit 60/95 (1 :1 ). After nitrogen gas had been passed through for 45 minutes, with stirring, the reactor was heated to 58°C and 0.15 g of 2,2'-azodi(2-methylbutyronitrile) (Vazo 67®, DuPont), dissolved in 6 g of acetone, was added. The external heating bath was then heated to 75°C and the reaction was carried out constantly at that external temperature. After a reaction time of 1 hour, a further 0.15 g of VAZO 67®, dissolved in 6 g of acetone, was added. After 3 hours, the mixture was diluted with 90 g of special boiling-point spirit 60/95.

After a reaction time of 5.5 hours, 0.45 g of bis-(4-tert-butylcyclohexanyl) peroxydicarbonate (Perkadox 16®, Akzo Nobel), dissolved in 9 g of acetone, was added. After a reaction time of 7 hours, a further 0.45 g of bis-(4-tert-butylcyclohexanyl) peroxydicarbonate (Perkadox 16®, Akzo Nobel), dissolved in 9 g of acetone, was added. After a reaction time of 10 hours, the mixture was diluted with 90 g of special boiling-point spirit 60/95. The reaction was terminated after a reaction time of 24 hours and cooled to room temperature.

The at least two-phase adhesive was prepared as follows: A stock solution of the synthetic rubber was first prepared. The solids content was 35% by weight, special boiling-point spirit 60/95:acetone 70:30 was used as the solvent mixture (special boiling- point spirit 60/ 95 is referred to simply as "spirit" below). The desired amount of stock solution was added to a polyacrylate solution. The desired amount of resin was added to the polyacrylate solution obtained as described above (polyacrylate: 47.5% 2- ethylhexyl acrylate, 47.5% n-butyl acrylate, 5% acrylic acid, Mn 98,000 g/mol, Mw: 1 ,100,000 g/mol), and the mixture was diluted with a solvent mixture spiritacetone 70:30 in such a manner that a final solids content of 35% by weight was obtained and dissolved for 12 hours on a roller bench. Finally, the crosslinker solution (3% by weight in acetone) was added and the mixture was coated on a siliconized release paper by means of a doctor blade on a laboratory coating table. The coatings were then dried at 120°C for 15 minutes. The adhesive layers with a layer thickness of 46 pm were laminated onto a foamed film having a thickness of 100 pm and 150 pm for batches 2 and 3, respectively, so that a double-sided adhesive tape sample was obtained. The sample was conditioned for one week in a standard atmosphere (23°C, 50% relative humidity).

The acrylate content was 52% and crosslinking was carried out by means of Erisys GA 240 (0.075%, based on the polyacrylate). Kraton D1118 in an amount of 20% was used as the second polymer component. The resin component used was Sylvares® TP95 in an amount of 28%.

Further descriptions of exemplary sample preparations of the pressure sensitive adhesive material can be found in US 2016/0167339 A1 , the corresponding content of which is hereby incorporated by reference. The same acrylic base polymers and tackifier / resin as mentioned above and in US 2016/0167339 A1 can also be used for the comparative examples, i.e. batches 1 , and 4 to 5 (see below). In the case of the comparative examples, however, no additional elastomer is mixed into the adhesive. In the case of batches 4 and 5 (see below), the adhesive layers were laminated onto a conventional foam and not a foamed film according to the invention.

Comparative examples

Comparative tests were performed. A well-established test method for evaluating shock- absorbing properties of a double-sided pressure sensitive adhesive tape is the so-called DuPont test. By this method, the impact resistance (shock) properties of double-sided pressure sensitive adhesive tapes for impacts in z- and xy-direction can be quantified.

Principle Die-cuts 86 (see Fig. 4c) of double-sided tapes are used to create batches of five test samples 83 (see Figs. 5a and 5b) by adhering a polycarbonate plate 82 (PC plate; see Fig. 4b) to a polycarbonate frame 80 (PC frame; see Fig 4a). The PC frame 80 has an essentially square shape with an outer edge length wi of 45 mm. A square window 81 with rounded inner corners having an edge length W2 of 25 mm is arranged in the PC frame 80. The PC plate 82 also has an essentially square shape with an edge length w ₃ of 35 mm. The samples 83 are prepared as described below by attaching the PC plate 82 to the PC frame 80 via the interposed frame-shaped die-cut 86 of the pressure sensitive adhesive tape as shown in Fig. 4c and Fig. 5a. After 24 h storage in a climate room (23 ± 1 °C, 50 ± 5 % rel. humidity), the impact resistance for the specified test direction, z or xy, is tested with a 150 g weight which is repeatedly dropped onto an impactor 84, 85 positioned on the PC plate 82 from increasing heights. For the z- direction test, the samples 83 are laid flat, i.e. with the plane, in which the die-cut 86 is arranged, oriented perpendicular to the impact direction, the PC frame 80 thereby facing the impact weight. The impactor 84 is positioned in the middle of the PC plate 82. For the xy-direction test, the samples 83 are positioned upright, i.e. the plane of the pressure sensitive adhesive tape is oriented in parallel to the impact direction and the impactor 85 is positioned in the middle of an edge of the PC plate 82. The samples 83 are held or clamped on the PC frames 80 in a jig of the testing device. The positions of the impactors 84 and 85 with respect to the sample 83 are indicated in Fig. 5b. The force F indicates the direction of the impact force of the weight (not shown) onto the impactor 84, 85.

The initial drop height is 5 cm and increases in 5 cm steps until the sample 83 shows full failure and the connection between PC frame 80 and PC plate 82 is lost. The height where full failure occurs is recorded. The impact energy is calculated via E = m-g-h where m is the impact mass, h is the height from which the impact mass was dropped and g is the gravitational acceleration. The resulting energy is usually stated in J.

Sample Preparation

The double-sided foamed film pressure sensitive adhesive tape is laminated onto liners to protect the adhesive surfaces against contaminations. With a Laser cutter, the frame- shaped die-cuts 86 of the pressure sensitive adhesive tapes as shown in Fig. 4c are prepared. The outer width w ₄ is 33 mm and the inner width ws is 29 mm (the resulting adhesive contact area is 248 mm ² ). The outer and inner squares are cut with a kiss cut. The PC frames 80 and PC plates 82 as seen in Fig. 4a and 4b are cleaned with ethanol und dried in a fume hood for at least two hours prior to assembly of the samples 83.

In order to prepare the samples 83, the liner is removed to expose one of the adhesive surfaces and the die-cut 86 is placed in a holder for application. The holder forms a template in order to ensure that all samples 83 are prepared according to the same specifications. The exposed adhesive side of the die-cut 86 is facing upwards. Secondly, the PC plate 82 is attached to the die-cut 86. Thereby, air inclusion is minimized by contacting the PC plate 82 with the adhesive at one side of the PC plate 82 at first and subsequently lowering the PC plate 82 onto the die-cut 86 by a tilting motion. Subsequently, the PC plate 82 is flipped and the backside liner from the die-cut 86 is removed. After that, the PC frame 80 is attached in the same manner to the die-cut 86 as the PC plate 82. The thusly created test piece is pressed with a force of 248 N for 5 s in automated press. Finally, the test pieces are left to dwell for 24 hours at room temperature in order to yield the samples 83 for testing. Figure 5a shows a plan view of the samples 83, whereas Fig. 5b shows a side view.

Test

The sample is mounted in a jig of the DuPont tester and the impactor 84 or 85 is positioned onto the plate according to the required testing conditions as described in the above, i.e. in z-direction or in xy-direction. The 150 g weight is lifted to a starting height of 5 cm and is dropped onto the impactor 84 or 85. If the sample 83 does not show full failure, the height is increased in 5 cm steps until the sample 83 shows full failure. This procedure is repeated for all 5 samples of each test batch.

Results

The results of several comparative test batches are shown in Table 1 below. Batch 1 refers to samples including double-sided pressure sensitive adhesive tapes according to the invention comprising a foamed film including between 3% and 4% vinylacetate in the thermoplastic polymeric base material, wherein the pressure sensitive adhesive does not include an elastic material phase. Batch 2 and batch 3 refer to samples including double-sided pressure sensitive adhesive tapes comprising a foamed film with different thicknesses of 100 pm (batch 2) and 150 pm (batch 3) according to the invention, wherein the pressure sensitive adhesive is provided with an additional elastic material phase. Batches 4 and 5 refer to comparative samples made from conventional double-sided adhesive foam tapes using a conventional PE foam as well as conventional pressure sensitive adhesive layers as described in the prior art section of this application. The base materials of the pressure sensitive adhesive of batches 1 to 3 and batches 4 to 5 are identical and the thicknesses of the foamed film backing layers corresponds to each other.

Table 1 : Average impact energy at failure under the DuPont test of batches 1 to 5 in comparison.

As can be seen from the test results in Table 1 , the pressure sensitive adhesive tape according to the invention of batch 1 without elastic material phase in the pressure sensitive adhesive layers has a slightly lower performance in the z-direction tests as compared to the batches 4 to 5 of the pressure sensitive adhesives tapes of the prior art. This is not surprising since the cavities in the foamed film according to the invention have average dimensions in z-direction that are smaller than in xy-direction. As such, it is expected that the shock-absorbing performance in z-direction is lower than in prior art pressure sensitive adhesive tapes with essentially spherical cavities. However, as can be seen from the xy-direction test results, there are virtually no differences in these directions and the pressure sensitive adhesive tapes of batches 1 and 4 to 5 have comparable performances. As such, the pressure sensitive adhesive tape according to the invention including a foamed film provides damping properties that are comparable with conventional prior art PE-foam-based pressure sensitive adhesive tapes.

Surprisingly, however, batches 2 to 3 of the pressure sensitive adhesive tapes according to the invention which include an elastic material phase in the pressure sensitive adhesive layers have a significantly better performance in all tests, in particular also in the z-direction test, than the pressure sensitive adhesive tapes of the prior art of batches 4 and 5. In particular, the addition of the elastic material phase allows for compensating a lower shock absorbance or damping performance in z- direction of the foamed film. As such, the pressure sensitive adhesive tape of the invention exhibits a comparable or even better performance as compared to pressure sensitive adhesive tapes of the prior art, but can be produced according to the method of the invention with all its advantages, such as e.g. continuous production line, low cost, standard machinery etc.