WO2018002055A1

WO2018002055A1 - Pressure sensitive adhesive tape

Info

Publication number: WO2018002055A1
Application number: PCT/EP2017/065862
Authority: WO
Inventors: Ko Hermans; Bart Kranz; Onno VAN KNIPPENBERG
Original assignee: Olympic Holding B.V.
Priority date: 2016-06-28
Filing date: 2017-06-27
Publication date: 2018-01-04

Abstract

A pressure sensitive adhesive tape with improved peel strength and static shear values containing a polymeric adhesive that comprises 71.5 - 85.5 wt.-% of an alkyl acrylate, 0.6 - 5 wt.-% of a copolymerizable monomer with at least one carboxylic acid moiety and 15 - 28 wt.-% of a cyclic polar copolymerizable monomer, comprising less than 1 wt.-% of any tackifier(s) and/or plasticizer(s).

Description

Pressure sensitive adhesive tape

Description:

Pressure sensitive adhesive tapes are used in a wide variety of applications and industries. In particular pressure sensitive adhesive tapes made via ultra-violet (UV) light initiated polymerization of acrylic monomers have excellent properties and are often used for structural bonding applications in automotive, electronic, building & construction and aerospace industry. Various processes that are suitable for making pressure sensitive adhesive tapes via UV initiated polymerization of acrylic monomers are known from prior art.

Usually these processes comprise a first step in which a mixture of low viscosity (acrylic) monomers is transformed into a high viscosity syrup. This can be done by partially polymerizing the mixture of low viscosity (acrylic) monomers or by adding polymer to the mixture of low viscosity (acrylic) monomers. In a second step, other components are added to the high viscosity syrup to make a coatable syrup that can be coated onto a carrier film or extruded to form a layer of coatable syrup. Said layer of coatable syrup is exposed to UV light, initiating a polymerization reaction that transforms the layer of coatable syrup into a pressure sensitive adhesive tape.

For example, patent BE 675420 teaches a process for making a pressure sensitive adhesive tape, which comprises coating a flexible support with acrylic monomers or a mixture of copolymerizable monomers and polymerizing the monomers in-situ by using UV light. US 4,181 ,752 teaches a process for making a pressure sensitive adhesive tape via exposing a solvent-less radiation sensitive acrylic containing mass to UV light. It specifically teaches the UV light irradiation conditions (0.1 -7 milliwatts per square centimeter) which should be used to make an acrylic pressure sensitive adhesive tape with optimal adhesive properties. US 4,223,067 discloses a process for making a pressure sensitive adhesive tape. The process involves making a coatable syrup by mixing, already partially reacted, acrylic monomers with hollow glass spheres, a crosslinker and photoinitator. The coatable syrup is coated onto low adhesion backing and cured with UV light under an inert nitrogen atmosphere.

US 4,415,615 teaches a process for making pressure sensitive adhesive tape comprising the steps of preparing a composition which has a viscosity in the range of about 1000-4000 cps and is polymerizable to a pressure sensitive adhesive state, frothing the composition, coating the froth onto a backing and polymerizing the froth in-situ to form a pressure sensitive adhesive. The polymerization may be carried out by exposure to UV light. It mentions that the syrup might be formed by partially polymerizing a mixture of monomers via thermal or UV polymerization.

US 4,710,536 discloses a process for making a pressure sensitive adhesive tape via UV light initiated polymerization of acrylic monomers. The process involves a first step in which a syrup is made by partially reacting a mixture of monomers via exposure to UV light or thermally. This process might involve dissolving the monomers in a solvent and/or excluding oxygen via purging with nitrogen. Next, a coatable syrup is made by mixing the obtained syrup with a crosslinker,

photoinitator and hydrophobic fumed silica. The coatable syrup is covered by two silicone coated poly-ester release liners and cured with UV light.

US 4,818,610 discloses a process for making a pressure sensitive adhesive tape comprising a plurality of superimposed layers which have different acrylic compositions. In this process separate layers are coated, which are laminated into a stack of layers and are simultaneously photopolymerized by using UV light. US 2002/0004130 teaches a process for making a pressure sensitive adhesive tape via UV light initiated polymerization of acrylic monomers that comprises a first step in which a mixture of (acrylic) monomers is excluded from oxygen and partially polymerized by using UV light. In a next step, a coatable syrup is formed by adding hollow glass spheres, crosslinker, photoinitiator and rheology modifier to the mixture of partially polymerized (acrylic) monomers. The coatable syrup is subjected to a vacuum to remove air bubbles and oxygen. Next, the coatable syrup is coated onto siliconized BOPET film, and subsequently covered by a second siliconized BOPET film. The layer of coatable syrup, which is sandwiched between two siliconized BOPET films, is then polymerized into a pressure sensitive adhesive tape. The polymerization is preferably done by exposure to UV light.

Also, chemical formulations that are suitable for making pressure sensitive adhesive tape via UV light initiation polymerization of acrylic monomers are known from prior art. The syrup usually comprises combinations of acrylic base

monomers (70-99%), modifying monomers (1 -30%) and a photo/thermal initiator (<0.5%). Sometimes, especially when a thermal initiator is used, the monomers and initiators might be dissolved in a solvent. A coatable syrup is formed by adding various other compounds to the syrup. For example, photo-initiators, crosslinkers, additives, fillers, pigments, tackifiers, wetting agents, flame retarders, rheology modifiers and foams of various nature can be added to form a coatable syrup.

For example, US 4,223,067 teaches a pressure sensitive adhesive tape that comprises hollow glass microbubbles. It further mentions that a syrup can be formed by using 60-90% of iso-octyl acrylate (i-OA) as a base monomer, 10-20% of acrylic acid (AA) as a modifying monomer. Photoinitiator (benzoin ethyl ether), fumed silica, tackifier, wettings agent and glass microbubbles can be added to the syrup to form a coatable syrup. US 4,415,615 teaches an acrylic pressure sensitive adhesive tape that comprises voids that do not collapse after being briefly compressed, and which tape has remarkable good adhesion on contact with rough surfaces and remarkably good flexibility and conformability at sub- freezing temperatures. It also mentions various syrup compositions. These compositions contain 50-100% of an alkyl acrylate (e.g. iso-octyl acrylate), 0-50% of a copolymerizable monoethylenically substituted monomer (e.g. acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide and n-vinyl pyrolidone)

US 4,612,242 teaches a pressure sensitive adhesive tape that has a dark appearance by virtue of glass micro bubbles that have a thin coating. US 4,710,536 teaches the use of hydrophobic fumed silica for improving the shear strength at elevated temperature, while maintaining or improving other adhesion and physical properties, of acrylic pressure sensitive adhesive tape. It mentions that a syrup can be made from 89 to 90% of iso-octyl acrylate (i-OA) as a base monomer and 9-10% of acrylic acid (AA) as a modifying monomer. It also mentions that a syrup can be made from 76% iso-octyl acrylate as a base monomer, 20% n-vinyl pyrrolidone as a first modifying monomer and 4% acryl amid as a second modifying monomer. Hydrophobic fumed silica, photo-initiator (Irgacure 651 ), cross-linker (hexanedioldiacrylate) and hollow glass microbubbles can be added to the syrup form a coatable syrup.

EP 0 257 984 teaches a pressure sensitive adhesive tape that comprises thermoplastic polymeric spheres.

US 2002/0004130 further describes a formulation that contains 35-45% of a first alkyl acrylate monomer, 30-40% of a second alkyl acrylate monomer, 6-10% of a first monoethylenically unsaturated polar copolymerizable monomer, 1 -2% of a second monoethylenically unsaturated polar copolymerizable monomer, 0.3-0.5% photo-initiator, 1 -2% filler, 0,05-0,07% crosslinker/chain extender and 8-12% hollow glass microspheres. Preferably 2- iso-octyl acrylate and ethyl hexylhexyl acrylate are used respectively as a first and second alkyl acrylate (C2-C12) monomer, acrylic acid and acryl amide are used respectively as a first and second monomethylenically unsaturated polymer copolymerizable monomer, benzoin ethyl ether is used as a photoinitiator, 1 ,4-butanediol diacrylate is used a crosslinker/chainextender, surface modified silica is used as a filler and the hollow glass spheres are based on borosilicate glass. It also teaches that first a syrup (referred to as an oligomer) can be made from 45-55% of a first alkyl acrylate,

34%-45% of a second alkyl acrylate, 3-4% of a first monoethylenically unsaturated polar copolymerizable monomer and 0.04 - 0.06% of a photoinitiator. A coatable syrup can be formed that comprises 75-80% of a syrup (oligomer) and adding 6- 9% of a first and second monoethylenically unsaturated polar copolymerizable monomer, 0.3-0.5% of at least one photoinitiator, 1 -2% of filler, 0.05-0.07% of crosslinker/chain extender and 8-12% of hollow glass spheres.

US 4,946,742 and US 4,985,488 teach a pressure sensitive adhesive tape that comprises a copolymer that is based on 60-88 parts of acrylic acid ester of non- tertiary (C4-C14) carbon alcohol, 2-30 parts of polar nitrogen-containing vinyl monomer, and 0-12 parts of vinyl carboxylic acid, and said copolymer being intimately blended with 2-35% of plasticizer.

WO 2013/019492 teaches a pressure sensitive adhesive tape that comprises a crosslinked (meth)acrylate polymer comprising repeating units derivable from a monomer composition comprising 55 - 97.4 wt.-% of alkyl acrylate, 2.5 - 40 wt.-% of one or more ethylenically unsaturated monomers comprising a nitrogen- or oxygen containing heterocyclic ring and 0.1 - 5 wt.-% of one or more polar comonomers. Said adhesive further comprises 1 -60 parts by weight of one or more hydrogenated resin tackifier per 100 parts by weight of cross-linked

(meth)acrylate polymer and 1 -20 part by weight microspheres per 100 parts by weight of the crosslinked (meth)acrylate polymer.

It can be concluded that a variety of processes and formulations for making a pressure sensitive adhesive tape via UV initiated polymerization of acrylic monomers are known from prior art. The present invention relates to a pressure sensitive adhesive tape that is made by UV initiated polymerization of acrylic monomers and which has surprisingly good adhesive properties. The invention also relates to a process for the manufacturing thereof.

This is achieved by a pressure sensitive adhesive tape containing a polymeric adhesive which is formed by UV/EB radiation induced polymerization of a mixture of monomer(s) and polymer(s) and which comprises 71 .5 - 85.5 wt.-% of an alkyl acrylate, 0.6 - 5 wt.-% of a copolymerizable monomer with at least one carboxylic acid moiety and 15 - 28 wt.-% of a cyclic polar copolymerizable monomer, wherein the polymeric adhesive contains less than 1 wt.-% of any tackifier(s) and/or plasticizer(s).

Remarkably - and as will be outlined in the course of the descriptions - it turned out that if in these concentration ranges of the monomers the amount of

(hydrogenated resin) tackifiers and/or plasticizers(s) is reduced to below 1 wt.-%, preferably below 0.5 wt.-% and even more preferably to less than 0.1 wt.-%, the peel strength and in particular the static shear improves considerably compared to the prior art where the addition of tackifiers and/or plasticizers are described or even prescribed. These findings are quite to the contrary to the teaching of e.g. WO 2013/019492 where it is disclosed that hydrogenated resin tackifier must be present in amounts of 1 to 60 parts on 100 parts by weight of the monomer composition. Further, it can be seen in from Table 2 of WO 2013/019492 that - if there is no tackifier present as in Comparative Example 4 both the 90°-peel strength values to both PE- and PP-foil decrease drastically. Thus, (hydrogenated resin) tackifiers in a considerable amount have to be added.

The invention relates to a UV/EB induced polymerization and not to thermal induced polymerizations. An example of a thermal induced polymerization of a pressure sensitive adhesive composition is given in US 5,252,662. The document teaches that a pressure sensitive adhesive can be made via a low viscosity hot- melt that is obtained by thermal bulk polymerization of a monomeric mixture. A low viscosity hot-melt would consist of low Mw polymers which would limited the adhesive properties of such a pressure sensitive adhesive. It should also be noted that US 5,252,662 teaches compositions which are substantially different from the current invention since the compositions contain max. 6% of cyclic polar copolymerizable monomer (i.e. maleic anhydride).

Another example of thermal induced polymerization of a pressure sensitive adhesive composition is given in WO 2000/016976. This document teaches, and also refers to US 4,812,541 for, the manufacturing a pressure sensitive adhesive composition via solvent based chemistry. The monomers are polymerized by mixing them with a proper solvent and thermal photo-initiator and subsequently boiling the system and keeping the reactor under constant reflux until most monomer is converted into polymer. The obtained polymer/solvent mixture is subsequently mixed a crosslinking agent and coated onto a web. It should be noted that the coated substrate might be exposed to EB, but not for polymerizing but for crosslinking the composition. Solvent chemistry is well known as an environmentally unfriendly process. It can also only be used to create pressure sensitive adhesive layer with limited thickness of no more than 100 micron.

Pressure sensitive adhesive tapes for structural bonding are typically at least 150 microns thick and usually between 250-3000, preferably between 400 and 3000 microns, to obtain good adhesion between large surfaces and compensate for thermal expansion.

The pressure sensitive adhesive tape according to the invention has optimal adhesion the high, medium and low surface energy substrates as well as excellent mechanical and static/dynamic shear properties. This will be further illustrated by the examples.

Preferably, the pressure sensitive adhesive tape according to the invention comprises 72-79 wt.-% of alkyl acrylate, 1 -4 wt.-% of a copolymerizable monomer with at least one carboxylic acid moiety and 17.5 -25 wt.-% of a copolymerizable monomer with at least one amide or ester moiety.

In a further preferred embodiment the pressure sensitive adhesive tape of present invention consists of a single layer only.

Often, in prior art, high performance adhesive tapes are created by stacking multiple layers of material on top of each other. Each layer has it is own unique function. Separately, these layers would not perform very well and the high performance of the tape is only created by the stack as a whole. In the present invention, a high performance adhesive tape is created by using only a single layer of material. The pressure sensitive adhesive tape according to the invention preferably has a thickness between 0.15 mm and 3 mm. Most preferably between 0.4 and 3 mm.

The pressure sensitive adhesive tape according to the invention comprises alkyl- acrylates. It might comprise of a single or a mixture of multiple alkyl acrylates. Alkyl-acrylates can both be alkyl-acrylate monomers and alkyl-methacrylate monomers, which are hereinafter collectively referred to as "alkyl acrylates".

Examples of alkyl acrylates are: dodecyl-acrylate, decyl -acrylate, iso-nonyl- acrylate, iso-octyl-acrylate, 2-ethylhexyl acrylate, hexyl acrylate, butyl-acrylate, ethyl-acrylate and their methacrylic counterparts. The preferred alkyl acrylates for the invention are iso-octyl acrylate and 2-ethyl hexyl acrylate.

The pressure sensitive adhesive tape according to the invention comprises copolymerizable monomer with at least one carboxylic acid moiety. It might comprise a single copolymerizable monomer with at least one carboxylic acid moiety or a mixture thereof. Examples of copolymerizable monomers with at least one carboxylic acid moiety are: acrylic acid, methacrylic acid and itaconic acid. The preferred copolymerizable monomer with at least one carboxylic acid moiety is acrylic acid.

The pressure sensitive adhesive tape according to the invention comprises cyclic polar copolymerizable monomer. It might comprise a single copolymerizable monomer or a mixture thereof. Examples of cyclic polar copolymerizable monomers are: n-vinyl pyrrolidone (nVP), n-vinyl caprolactam (nVC), acryloyl morpholine (ACMO) and cyclic trimethylol-propane formal acrylate (CTFA). The preferred copolymerizable monomer with at least one amide or ester moiety is n- vinyl caprolactam. The pressure sensitive adhesive tape according to the invention may further comprise additives and fillers. Examples of additives are photo-initiators, crosslinkers, rheology modifiers, levelling agents, pigments, dispersing agent, anti- foaming agents, foaming agents, flame retarders.

Examples of fillers are ceramic fillers or foams of various natures such as hollow glass spheres and hollow plastic spheres. In a preferred embodiment, a pressure sensitive adhesive tape according to the invention further comprises 0 - 1 wt.-% of a crosslinker, 0 - 1 wt.-% of a photo- initiator. The crosslinker can for example be multi-functional acrylic monomer such 1 ,4-butanedioldiacrylate or 1 ,6-hexanedioldiacrylate. It might be a single crosslinker or a mixture thereof. The photo-initiator can for example be 1 - hydroxycyclohexylphenyl ketone or 2,2-dimethoxy-1 ,2-diphenylethan-1 -one. It might be a single photo-initiator or a mixture thereof.

The static shear is measured according to ASTM D3654. Stainless steel substrates were used during measurement. A pressure sensitive adhesive tape with a surface area of 0.5 inch² (3.23 cm²) was used. The dwell time was 24 hours at room temperature. The tape will hold listed weight in table 1 for 10.000 minutes (approximately 7 days) at room temperature.

The 90° peel strength was measured according to ASTM D3330. The

measurements were performed on stainless steel or polypropylene substrates and an aluminum backing was used. The dwell time was 72 hours at room

temperature. The value listed in tables 1 and 2 is the average value force to remove tape at room temperature.

Table 1 Overview of results at different concentrations

2-ethylhexyl acrylate (2-EHA), acrylic acid (AA), and n-vinyl caprolactam (nVC) copolymerized in syrup to form a copolymer in the presence of 0.025 wt.-% of chain transfer agent (n-dodecyl mercaptan)

Composition further contains: 9 wt.-% hollow glass spheres (6014k), 2 wt.-% fumed silica, 0.356 wt.-% Irgacure 651 , 0,09 wt.-% HDDA Rokracure

VP6839 and Dercolite LTG are tackifiers (or plasticizers)

PETG = Polyethylene terephthalate

PMMA = Poly(methyl methacrylate)

PP = Polypropylene The results in Table 1 show that the pressure sensitive adhesive tapes according to the invention - exemplified by Sample 3 in the Table 1 - in particular when containing no tackifiers and/or plasticizers - show better peel strength to PETG and steel and an improved static shear. It is preferred that the pressures sensitive adhesive tape according to the invention also comprises a rheology modifier like for example hydrophilic fumed silica or hydrophobic fumed silica. Preferably, a pressure sensitive adhesive tape according to the invention comprises 0 - 6 wt.-% of rheology modifier.

The process for making the pressure sensitive adhesive tape according to the invention comprises the steps of making a syrup (also referred to as pre- polymerized mixture) by partially polymerizing a mixture of alkyl monomers(s), copolymerizable monomer(s) with at least one carboxylic acid moiety and copolymerizable monomer(s) with at least one amide or ester moiety. The syrup is made by adding a small amount (<0.5%) of initiator to said mixture, optionally degassing said mixture, and exposing said mixture to UV light or temperature. The polymerization is stopped once the proper viscosity is obtained. Next, a coatable syrup is formed by adding other components such as for example additives and fillers to the syrup. The coatable syrup is coated onto a siliconized biaxially oriented polyethylene terephthalate (BOPET) film and covered by a second siliconized BOPET film. A pressure sensitive adhesive tape is formed by exposing the coatable syrup, sandwiched between two layers of BOPET, to UV light. This process can be carried out in a continuous fashion.

Preferably, hollow glass and/or hollow polymeric spheres are added during the process. The amount of hollow glass spheres to be added is preferably 1 -20 wt.-% and more preferably between 5-15 wt.-%. The hollow glass spheres preferably have a density of 100-500 kg/m³ and more preferably a density of 100-300 kg/m³. The diameter of the hollow glass spheres is preferably between 10-250 micron and more preferably between 20-125 micron. The amount of hollow polymeric spheres to be added is preferably 0.1 - 5 wt.-% and more preferably between 0.5 - 3 wt.-%. The hollow polymeric spheres preferably have a density of 10-70 kg/m³ and more preferably a density of 15-50 kg/m³. The diameter of the hollow glass spheres is preferably between 10-250 micron and more preferably between 20-125 micron.

In contrast to the hollow glass spheres, hollow polymeric spheres lead to an increase of both the static shear and peel strength values of the resulting pressure sensitive adhesive tape.

Finally, it has been found out by the inventors that addition of chain transfer agents during the (pre-) polymerization process is a very convenient tool to adjust the viscosity during the process, also if no or only little solvent is applied. Prior art teaches how to make syrups with a relatively high viscosity of 1500-5000 mPas. In particular when using high amounts of filler (such as for example hollow plastic spheres), the viscosity of the syrup limits the amount of filler which can be incorporated. A lower viscosity syrup would therefore be desirable when high filler contents are required. As prior art teaches, it is possible to achieve a low viscosity by reducing the intensity of the UV light and/or reducing the exposure time.

However, radical initiated polymerizations of acrylates are extremely rapid and difficult to control. The viscosity of monomer mixture increases rapidly, even at low exposure dose, making it difficult to control the viscosity of the syrup. Therefore, the inventors found that by the addition of a chain transfer agent the viscosity increase of the monomer mixture during pre-polymerization is much slower. As a result, the viscosity of the syrup can be easily controlled especially when a low viscosity syrup is required. This happens without affecting the final performance of the pressure sensitive adhesive tape. The amount of chain transfer agent, such as n-dodecyl-mercaptan, is in approximately 0.025 wt.-percent preferably. Comparative example 1 (CE1):

A mixture of low viscosity monomers is formed by mixing 90 wt.-% 2-ethylhexyl acrylate, 10 wt.-% acrylic acid and 0.04 wt.-% 2,2-dimethoxy-2-phenyl

acetophenone. In total 600 grams of said mixture is placed in a glass container and said mixture is degassed for 5 minutes by flushing nitrogen at a rate of 3 liter/minute. After 5 minutes, said mixture is exposed to UV light. The UV light consists for >80% of UVA light (300 - 400 nm) and has an intensity of 10 mJ/cm². The exposure to UV light is stopped when a monomer conversion of approx. 10% is achieved and a syrup is formed which has a viscosity of approx. 1500 mPas. Next, a coatable syrup is formed by mixing 89.45 wt.-% of syrup with 0.089% of 1 ,6-hexanedioldiacrylate, 0.356 wt.-% of 2,2-dimethoxy-2-phenyl acetophenone, 0.1 wt.-% of black pigment paste, 1 wt.-% of hydrophilic fumed silica and 9 wt.-% of hollow glass spheres. A 1 mm thick layer of coatable syrup is coated onto a siliconized BOPET film, cover with a second siliconized BOPET film and exposed to UV light for 2.5 minutes. The UV light consists for >80% of UVA light and has an intensity of 10mJ/cm². During the exposure to UV light, the heat generated by the UV initiated polymerization of the coatable syrup is removed by blowing air over the outer surfaces of the siliconized BOPET films. After removal of the siliconized BOPET films a pressure sensitive adhesive tape is obtained. Comparative example 2 (CE2):

A pressure sensitive adhesive tape is formed according to comparative example 1 , except that a coatable syrup is formed by mixing 95.6 wt.-% of syrup with 0.096% of 1 ,6-hexanedioldiacrylate, 0.382 wt.-% of 2,2-dimethoxy-2-phenyl acetophenone, 1 .5 wt.-% of hydrophilic fumed silica and 2.4 wt.-% of hollow polymeric spheres (pre-expanded Expancel, with an average diameter of 40 micron and density of 25 kg/m³). Example 1 (E1):

A pressure sensitive adhesive tape is formed according to comparative example 1 , except that a mixture of low viscosity monomers is formed by mixing 75 wt.-% 2- ethylhexyl acrylate, 22.5 wt.-% n-vinyl caprolactam, 2.5 wt.-% of acrylic acid and 0.04 wt.-% 2,2-dimethoxy-2-phenyl acetophenone. In total 600 grams of said mixture is placed in a glass container and said mixture is degassed for 5 minutes by flushing nitrogen at a rate of 3 liter/minute. After 5 minutes, said mixture is exposed to UV light. The UV light consists for >80% of UVA light (300 - 400 nm) and has an intensity of 10 mJ/cm². The exposure to UV light is stopped when a monomer conversion of approx. 5% is achieved and a syrup is formed which has a viscosity of approx. 1500 mPas. Said syrup is processed further is described in comparative example 1 .

Example 2 (E2):

A pressure sensitive adhesive tape is formed according comparative example 2, except that 0.025 wt.-% of n-dodecyl mercaptan is added to the mixture of monomer and a syrup is formed as described in example 1 .

Example 3 (E3):

A pressure sensitive adhesive tape is formed according to comparative example 1 , except that a mixture of low viscosity monomers is formed by mixing 86 wt.-% 2- ethylhexyl acrylate, 22.5 wt.-% n-vinyl caprolactam, 1 .5 wt.-% of acrylic acid and 0.04 wt.-% 2,2-dimethoxy-2-phenyl acetophenone. In total 600 grams of said mixture is placed in a glass container and said mixture is degassed for 5 minutes by flushing nitrogen at a rate of 3 liter/minute. After 5 minutes, said mixture is exposed to UV light. The UV light consists for >80% of UVA light (300 - 400 nm) and has an intensity of 10 mJ/cm². The exposure to UV light is stopped when a monomer conversion of approx. 5% is achieved and a syrup is formed which has a viscosity of approx. 1500 mPas. Said syrup is processed further is described in comparative example 1 . The pressure sensitive adhesive tapes from comparative example 1 , comparative example 2, example 1 , example 2 and example 3 are evaluated by testing the static shear and 90-degree peel strength on stainless steel substrates and polypropylene substrates. The results are given in Table 2.

Table 2: Overview results of comparative examples (CE) and examples (E).

The results in Table 2 show that the pressure sensitive adhesive tapes according to the invention have much better properties than the pressure sensitive adhesive tapes that are known from prior art.

Surprisingly, a pressure sensitive adhesive tape based on known (triblock copolymer) compositions in the specific range of the invention has a very high static shear value. Example 4-34: (E4-E34)

A pressure sensitive adhesive tape is formed according to comparative example 1 , except that a mixture of low viscosity monomers is formed by mixing 2-ethylhexyl acrylate, acrylic acid and n-vinyl caprolactam in ratio according to the table 3 and 0.04 wt.-% 2,2-dimethoxy-2-phenyl acetophenone. In total 600 grams of said mixture is placed in a glass container and said mixture is degassed for 5 minutes by flushing nitrogen at a rate of 3 liter/minute. After 5 minutes, said mixture is exposed to UV light. The UV light consists for >80% of UVA light (300 - 400 nm) and has an intensity of 10 mJ/cm². The exposure to UV light is stopped when a monomer conversion of approx. 5% is achieved and a syrup is formed which has a viscosity of approx. 1500 mPas. Said syrup is processed further is described in comparative example 1 .

Table 3: Overview monomer composition of E4 to E34

The 90° peel strength was determined of sample E4-E34 was determined on stainless steel, poly(methyl methacrylate) (PMMA) and polypropylene (PP). The various substrates were used because each has different surface energy:

stainless steel (high surface energy), PMMA (medium surface energy) and PP (low surface energy) The results of the peel strength measurements are given in table 4, 5 and 6 respectively. Table 4: 90° peel strength on stainless steel for sample E4 to E34

Table 5: 90° peel strength on PMMA for sample E4 to E34

Table 5: 90° peel strength on PP for sample E4 to E34

The mechanical properties of the tapes were assessed by measuring the elongation at break and the tensile strength. The results are given Table 6 and Table 7 respectively.

Table 6: Elongation at break for sample E4 to E34

Table 7: Tensile strength for sample E4 to E34

The shear adhesion of the tapes was assessed by the static shear at room temperature (in steps of 0.5 kg, max. tested 2 kg) and the dynamic shear. The results are given Table 8 and Table 9 respectively. Table 8: Static shear at room temperature for sample E4 to E34

Table 9: Dynamic shear for sample E4 to E34

The temperature resistance of the tapes was assessed by measuring the static shear at 90 °C (in steps of 0.25 kg, max. tested 1 kg) and the dynamic shear. The results are given in Table 10.

Table 10: Static shear for sample E4 to E34

The test results from sample E4 to E34 show that an optimal composition is obtained when the acrylic acid concentration is kept between 0.6 and 5 wt.-% and the concentration of n-vinyl caprolactam is kept between 15 to 28 wt.-%. Within this regime an optimal balance between adhesion to various substrates, good mechanical properties, shear resistance and temperature stability is obtained.

Lower concentrations of acrylic acid result in poor adhesion to high surface energy substrates and low resistance to static shear. Higher concentrations of acrylic acid result in poor adhesion to medium and low surface energy substrates.

Both higher and lower concentration of n-vinyl caprolactam result in poor mechanical properties (too weak / too stiff) and poor overall adhesive properties.

Claims

1 . A pressure sensitive adhesive tape containing a polymeric adhesive which is formed by UV or EB radiation induced polymerization of a mixture of monomer(s) and polymer(s) and which comprises 71 .5 - 85.5 wt.-% of an alkyl acrylate, 0.6 - 5 wt.-% of a copolymerizable monomer with at least one carboxylic acid moiety and 15 - 28 wt.-% of a cyclic polar copolymerizable monomer, characterized in that the polymeric adhesive contains less than 1 wt.-% of any tackifier(s) and/or plasticizer(s).

2. The pressure sensitive adhesive tape of claim 1 , characterized in that the

polymeric adhesive comprises 72-79 wt.-% of alkyl acrylate, 1 -4 wt.-% of a copolymerizable monomer with at least one carboxylic acid moiety and 17.5 - 25 wt.-% of a copolymerizable monomer with at least one amide or ester moiety.

3. The pressure sensitive adhesive tape of any of the preceding claims,

characterized in that the tape consists of a single layer.

4. The pressure adhesive sensitive tape of claim 3, characterized in that the layer has a thickness of at least 150 microns, preferably that the layer is in a range from 400 to 3000 microns.

5. The pressure sensitive adhesive tape of any of the preceding claims,

characterized in that the alkyl acrylate is iso-octyl acrylate or 2-ethyl hexyl acrylate.

6. The pressure sensitive adhesive tape of any of the preceding claims, characterized in that the copolymenzable monomer with at least one carboxylic acid moiety is acrylic acid.

7. The pressure sensitive adhesive tape of any of the preceding claims,

characterized in that the copolymenzable monomer with at least one amide or ester moiety is n-vinyl caprolactam.

8. The pressure sensitive adhesive tape of any of the preceding claims,

characterized in that it further comprises 0 - 1 wt.-% of a crosslinker, 0 - 1 wt- % of a photo-initiator.

9. A pressure sensitive adhesive tape according to claim 8, characterized in that the crosslinker is 1 ,6-hexanedioldiacrylate.

10. The pressure sensitive adhesive tape of any of the preceding claims,

characterized in that the tape further comprises hollow glass spheres.

1 1 . The pressure sensitive adhesive tape of any of the preceding claims,

characterized in that the tape further comprises hollow polymeric spheres.

12. A process of manufacturing a pressure sensitive adhesive tape according to claims 1 to 1 1 comprising the steps of making a pre-polymerized mixture by partially polymerizing a mixture of alkyl monomers(s), copolymenzable monomer(s) with at least one carboxylic acid moiety and copolymenzable monomer(s) with at least one amide or ester moiety, adding an initiator in an amount of less than 0.5 wt.-% to said mixture, optionally degassing said mixture and exposing said mixture to UV light and/or temperature to obtain a pre-polymerized mixture having a viscosity that allows to coat said pre- polymerized mixture, optionally adding further components as additives and fillers to the pre-polymerized mixture, coating said pre-polymerized mixture onto a foil, followed by exposing the pre-polymerized mixture, preferably while covered by a second foil, to UV light, , characterized in that less than 1 wt.-% of any tackifier(s) and/or plasticizer(s) are added during the process.

13. The process of claim 12, characterized in that hollow polymeric spheres are added during the process.

14. The process according to any of the preceding claim 12 or 13, characterized in that a chain transfer agent, such as n-dodecyl mercaptan is added during the process.