WO2026008400A1

WO2026008400A1 - Aqueous pressure-sensitive adhesive composition comprising polyisobutene emulsion and emulsion polymerized acrylate polymer

Info

Publication number: WO2026008400A1
Application number: PCT/EP2025/067804
Authority: WO
Inventors: Holger Kern; Beatrice LANDHERR; Selina SPECK; Jasmin Tanja KIELMANN; Saskia Groeer; Michael Gross; Lena Samia FRIEDRICH
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2024-07-01
Filing date: 2025-06-24
Publication date: 2026-01-08
Anticipated expiration: 2027-01-01

Abstract

An aqueous pressure-sensitive adhesive composition is described. The adhesive composition comprises (A) a specific aqueous polyisobutene emulsion, and (B) a specific pressure-sensitive adhesive acrylate polymer dispersed in water with a glass transition temperature of -10 °C or less; and (C) optionally at least one crosslinking agent for the pressure-sensitive adhesive acrylate polymer.

Description

Aqueous pressure-sensitive adhesive composition comprising polyisobutene emulsion and emulsion polymerized acrylate polymer

Description

The invention relates to an aqueous pressure-sensitive adhesive composition comprising (A) a specific aqueous polyisobutene emulsion, and (B) a specific pressure-sensitive adhesive acrylate polymer dispersed in water; and (C) optionally at least one crosslinking agent for the pressure-sensitive adhesive acrylate polymer. Also described are self-adhesive articles comprising a pressure-sensitive adhesive layer made from the aqueous pressure-sensitive adhesive composition, methods of making the self-adhesive articles and the use of the aqueous pressure-sensitive adhesive compositions for making a removable protective film.

There are some important issues to consider when developing improved adhesives for self-adhesive articles in general and for removable self-adhesive articles like removable labels, removable tapes or removable protective films. Requirements for such adhesives are, for example:

1) A good adhesive should offer strong adhesion to the surfaces (in particular to rough surfaces such as brushed steel) to ensure reliable attachment of the protective film. At the same time, however, it should be easy to remove from the surface without leaving any residue or damage.

2) The adhesive should have optimum viscosity and flowability in order to spread well even on hydrophobic or rough surfaces. This ensures uniform adhesion and prevents air pockets or uneven adhesive layers that could impair adhesion.

3) The adhesive should have bonding properties similar to organic solvent-based adhesives (e.g. solvent based natural rubber, solvent based acrylate, etc.) but should be aqueous based without the use of organic solvents. This is important to reduce the environmental impact and minimize health risks.

4) The adhesive should have high tack. Tack refers to the ability of an adhesive to build up instant adhesion. An adhesive with high tack enables fast and strong adhesion, which is particularly important in applications with protective films to ensure immediate protection.

5) The aqueous-based adhesive should have viscoelastic properties, similar to organic solvent-based adhesives. This enables good adaptability to different surfaces and high resilience to temperature fluctuations and mechanical stress.

The development of adhesives that meet all these requirements requires extensive research to find the right compositions and formulations. It is important to investigate different materials and technologies in order to produce water-based adhesives that meet the specific requirements of self-adhesive articles, such as, self-adhesive, removable protective films.

The use of pressure-sensitive adhesives based on aqueous dispersions of polyacrylates for producing self-adhesive articles is generally known. For example, EP 2697323 B1 describes pressure sensitive adhesive composition based on aqueous (meth)acrylate emulsion polymers including (meth)acrylate monomers with ureido groups. WO 2014/012884 describes aqueous emulsions of polyisobutene and their use in chemical-technical applications, such as glue systems.

The problem on which the invention is based is that of providing an aqueous pressure-sensitive adhesive composition which enables the production of self-adhesive articles with the above-mentioned bonding properties similar to solvent-based adhesives, without the need for organic solvents.

The invention provides an aqueous pressure-sensitive adhesive composition comprising

(A) an aqueous polyisobutene emulsion, said polyisobutene having a particle size D50 not greater than 100 m, preferably from 250 nm to 50 pm, measured by laser diffraction, and having a viscosity average molecular weight Mv of at least 45000 g/mol, preferably at least 80 000 g/mol, measured as described in the examples;

(B) a pressure-sensitive adhesive acrylate polymer dispersed in water with a glass transition temperature of -10 °C or less, preferably from -60 to -15 °C, wherein the pressure-sensitive adhesive acrylate polymer is made by emulsion polymerization of

(a) from 70 to 99.6 wt.% of acrylic acid alkyl ester monomers with 2 to 12 carbon atoms in the alkyl group;

(b) at least 0.4 wt.%, preferably more than 1 and preferably up to 10 wt.% of at least one ethylenically unsaturated, copolymerizable monomer having at least one acid group;

(c) optionally one or more ethylenically unsaturated monomers different from monomers (a) and (b); wt.% amounts of the monomers are based on the total weight amount of all monomers of polymer (B) ; and

(C) at least one crosslinking agent for the pressure-sensitive adhesive acrylate polymer; wherein glass transition temperatures are measured by differential scanning calorimetry according to ASTM D 3418- 08 as the midpoint temperature when evaluating the second heating curve at a heating rate of 20°C/min.

The invention also provides a self-adhesive article, preferably a self-adhesive label, a self-adhesive tape or a self- adhesive film including graphic films and protective films, comprising a substrate, preferably selected from paper, plastic films and metal foils, and a pressure-sensitive adhesive layer made from an aqueous pressure-sensitive adhesive composition as described herein.

The invention also provides a method of making such a self-adhesive article, wherein

(1) a substrate is provided which is optionally pre-coated with a primer and its surface is optionally corona-treated., and

(2) an aqueous pressure-sensitive adhesive layer is applied to the substrate, wherein the pressure-sensitive adhesive layer is formed by coating the optionally pre-coated substrate with an aqueous pressure-sensitive adhesive composition as described herein, and drying. The invention also provides the use of an aqueous pressure-sensitive adhesive composition as described herein for making a removable protective film comprising a substrate made of polyolefin plastic film, preferably selected from polyethylene and polypropylene, coated with said aqueous pressure-sensitive adhesive composition.

The text below occasionally uses the designation "(meth)acryl..." and similar designations as an abbreviating notation for "acryl... or methacryl...". In the designation Cx alkyl (meth)acrylate and analogous designations, x denotes the number of carbon atoms in the alkyl group.

A pressure-sensitive adhesive (PSA) is a viscoelastic adhesive whose set film at room temperature (20°C) in the dry state remains permanently tacky and adhesive (self-adhesive). Bonding to substrates is accomplished instantaneously by gentle applied pressure. The loop tack of a dried film of the pressure-sensitive adhesive preferably is more than 1 .7 N/25 mm, also preferably at least 2 N/25 mm, more preferred at least 2.5 N/25 mm or more preferably at least 3 N/25 mm (adhesive applied at an application thickness of 20 pm on a 12 pm thick polyester film, measured on steel at room temperature (20°C) at a peeling speed of 300 mm/min).

The pressure-sensitive adhesive acrylate polymer (B) has a glass transition temperature of -10 °C or less, preferably from -60 to -20 °C. By a controlled variation of the nature and amount of the monomers it is possible for the skilled person to prepare adhesive polymer compositions whose polymers have a glass transition temperature within the desired range.

Glass transition temperatures are determined by differential scanning calorimetry (ASTM D 3418-08, midpoint temperature). The glass transition temperature of the polymer is the glass transition temperature obtained on evaluation of the second heating curve at a heating rate 20°C/min.

Particle size and particle size distribution can be measured as particle size D50 by laser diffraction using a Mastersizer 3000 instrument of the company Malvern. The particle size D50, also known as the median particle size, represents the particle diameter at which 50% of the cumulative volume of particles is larger and 50% is smaller. To determine the D50 value or median particle size, the sample is dispersed in a liquid medium. A laser diffraction instrument (Mastersizer 3000 of the company Malvern) is then used to measure the scattering pattern produced by the dispersed particles. The collected scattering data is analyzed using mathematical algorithms (according to Mie theory and Fraunhofer theory) to calculate the particle size distribution.

The viscosity average molecular weight Mv (given in g/mol) is measured based on the viscosity method as outlined below and described in the examples. Mv is calculated from the Staudinger Index Jo as follows:

Mv = (Jo x 100 / 3.06) ^1/065

The Staudinger Index Jo [cm³/g] is calculated from the flow time at 20°C through capillary I of an Ubbelohde viscometer via the Schulz-Blaschke equation: Jo = q_sp / (c x (1 + 0.31 q_sp)) [cm³/g] with the specific viscosity q_sp being q_sp = (t / to) - 1 , where t = flow time of the solution with Hagebach-Courette correction to = flow time of the solvent with Hagebach-Courette correction c = concentration of the solution in g/cm³

The terms "aqueous composition” and "aqueous polymer dispersion” refers to solvent systems primarily based on water, preferably containing no or less than 5%, less than 3% or less than 1% by weight of organic solvents (such as for example methanol, ethanol or tetrahydrofuran), based on the total composition. It is preferred not to use organic solvents.

Aqueous polyisobutene emulsion (A)

The aqueous pressure-sensitive adhesive composition comprises an aqueous polyisobutene emulsion (A). The polyisobutene of the present invention is a polymer of isobutene. It can be obtained from isobutene by a known method of polymerization, for example by using a Friedel-Craft catalyst, such as boron fluoride or aluminum chloride. Non-emulsified polyisobutene is commercially available under tradename Oppanol® by BASF. Aqueous polyisobutene emulsions are commercially available under tradename AquaPib® by Emulco or under the tradename Polybut™ EM by Kemat.

The polyisobutene is obtainable by polymerizing isobutene or an isobutene-containing monomer composition. Suitable isobutene sources are C4 cuts, preferably pure isobutene which generally comprises at most 0.5 % by volume of residual impurities such as 1-butene, 2-butenes, butane, water and/or C1- to C4-alcohols. It is preferred to use isobutene-containing technical C4 hydrocarbon streams, for example, C4 raffinates, C4 cuts from isobutane dehydrogenation, C4cuts from steamcrackers and from FCC crackers (fluid catalyzed cracking), provided that they have been substantially freed of 1 ,3-butadiene present therein. Suitable technical C4 hydrocarbon streams comprise generally less than 500 ppm, preferably less than 200 ppm, of butadiene. The isobutene from such technical C4 hydrocarbon streams can be polymerized substantially selectively to the desired isobutene homopolymer without incorporation of significant amounts of other C4 monomers into the polymer chain.

The viscosity average molecular weight Mv of the polyisobutene is preferably at least 40,000 g/mol, more preferably at least 45,000 g/mol, for example 45,000 g/mol to 90,000 g/mol, more preferably at least 80,000 g/mol, for example 80,000 g/mol to 90,000 g/mol. Suitable commercially available emulsified products are for example those of the AquaPib® series, such as AquaPib® AquaPib® 60-25-150 (viscosity average molecular weight Mv = 85,000 g/mol) or AquaPib® 60-25-120 (viscosity average molecular weight Mv = 55,000 g/mol); or those of the PolybutTM EM series such as PolybutTM EM MM 12 (viscosity average molecular weight Mv = 55,000 g/mol) or PolybutTM EM MM 15 (viscosity average molecular weight Mv = 85,000 g/mol).

The particle size (particle diameter) D50 of the polyisobutene preferably is not greater than 100 pm, preferably from 250 nm to 50 pm, more preferably from 250 nm to 25 pm, measured by laser diffraction. The glass transition temperature of the polyisobutene is preferably lower than -60 °C, for example -64 °C.

The content of polyisobutylene is preferably 10 to 80 wt.%, preferably from 40 to 80 wt.% in the aqueous polyisobutene emulsion (A). Polyisobutene can be emulsified in water by suitable methods, for example as described in WO 2014/012884, using aids such as surfactants, wax or oil. The aqueous polyisobutene emulsion (A) may contain surfactants, waxes, oils or other additives, e.g., for stabilizing or preservation of the emulsion. The aqueous polyisobutene emulsion may contain for example 1 to 5 wt.%, preferably 2 to 4 wt.%, based on the total weight of said emulsion, of surfactants such as anionic, cationic or non-ionic surfactants. The aqueous polyisobutene emulsion (A) may contain for example 5 to 50 wt.%. preferably 10 to 50 wt.%, based on the total weight of said emulsion, of oil and/or wax. Suitable wax may be selected from the group comprising animal waxes, vegetable waxes, mineral waxes, petroleum waxes, polyolefin waxes, amide waxes, chemically modified waxes and combinations thereof. Suitable oil may be selected from the group comprising natural and mineral oils and combinations thereof.

One or a combination of two or more types of polyisobutylene with different molecular weights can be used. In one embodiment the aqueous polyisobutene emulsion (A) comprises at least one first polyisobutene with a viscosity average molecular weight Mv of from 45,000 g/mol to 90,000 g/mol and at least one second polyisobutene with a high viscosity average molecular weight Mv of more than 90,000 g/mol, preferably more than 100,000 g/mol, more than 200,000 g/mol of more than 400,000 g/mol. Suitable commercially available high molecular weight polyisobutenes are for example those of the Oppanol® N series, such as Oppanol® N50 (viscosity average molecular weight 425,000 g/mol).

In a preferred embodiment, the polyisobutene is made at least partly from recycled or bio-based, renewable raw materials as alternatives to fossil resources at the beginning of its value chain. The alternative feedstocks are attributed to downstream products via the mass balance approach, which contributes to the substitution of fossil resources and may have a smaller product carbon footprint than their conventional equivalent.

The total amount of polyisobutene in the aqueous pressure-sensitive adhesive composition is preferably from 5 to 40 wt.%, more preferably from 5 to 30 wt.%, more preferably from 10 to 30 wt.% of the total composition (based on solid content of the aqueous polyisobutene emulsion and the total weight of polymers in the aqueous pressure-sensitive adhesive composition).

The total amount of polyisobutene emulsion (A) in the aqueous pressure-sensitive adhesive composition is preferably from 5 to 40 wt.%, more preferably from 5 to 30 wt.%, more preferably from 10 to 30 wt.% of the total composition (based on the aqueous polyisobutene emulsion and total amount of the aqueous pressure-sensitive adhesive composition). Pressure-sensitive adhesive acrylate polymer (B)

The aqueous pressure-sensitive adhesive composition comprises a pressure-sensitive adhesive acrylate polymer (B) dispersed in water. The aqueous adhesive polymer dispersion of the invention forms an adhesive coating after application to a substrate material and drying. The pressure-sensitive adhesive acrylate polymer is made by emulsion polymerization of

(c) optionally one or more ethylenically unsaturated monomers different from monomers (a) and (b); wt.% amounts of the monomers are based on the total weight amount of all monomers of polymer (B).

The pressure-sensitive adhesive acrylate polymer (B) dispersed in water may preferably comprise two or more different acrylate polymers as described having with a glass transition temperature of -10 °C or less, preferably from - 60 to -15 °C.

The total amount of pressure-sensitive adhesive acrylate polymers (B) in the aqueous pressure-sensitive adhesive composition is preferably from 20 to 80 wt.-%, also preferably 20 to 50 wt.%, more preferably from 25 to 45 wt.%, more preferably from 30 to 40 wt.% of the total composition (based on solid content of all aqueous adhesive acrylate polymer(s) (B) and the total weight of polymers in the aqueous pressure-sensitive adhesive composition).

Also preferably, the total amount of pressure-sensitive adhesive acrylate polymers (B) in the aqueous pressuresensitive adhesive composition is preferably from 20 to 90 wt.-%, also preferably 20 to 80 wt.%, more preferably from 25 to 75 wt.%, more preferably from 30 to 70 wt.% of the total composition (based on solid content of all aqueous adhesive acrylate polymer(s) (B) and the total weight of polymers in the aqueous pressure-sensitive adhesive composition).

The total amount of pressure-sensitive adhesive acrylate polymer(s) (B) in the aqueous pressure-sensitive adhesive composition is preferably from 20 to 90 wt.-%, also preferably 20 to 80 wt.%, more preferably from 25 to 75 wt.%, more preferably from 30 to 70 wt.% of the total composition (based on all aqueous adhesive acrylate polymer(s) (B) and total amount of the aqueous pressure-sensitive adhesive composition).

Monomers (a)

The monomer mixture for making the pressure-sensitive adhesive acrylate polymer comprises from 70 to 99.6 wt.%, preferably from 75 to 99 wt.%, from 80 to 98 wt.% or from 91 to 98 wt.%, based on the total amount of monomers, of acrylic acid alkyl ester monomers (a) with 2 to 12 carbon atoms in the alkyl group. Preferred monomers (a) are acrylic acid alkyl esters with 2 to 8 carbon atoms in the alkyl group. Preferred monomers (a) are ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, tert-butyl acrylate, cyclohexyl acrylate, n-hexyl acrylate, n-octyl acrylate, 2-octyl acrylate, isooctyl acrylate, 2-propy I heptyl acrylate and mixtures thereof. More preferred monomers (a) are one or more selected from ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, 2-octyl acrylate and isooctyl acrylate. Particularly preferred are one or more of ethyl acrylate, n-butyl acrylate, isobutyl acrylate and 2-ethylhexyl acrylate.

Monomers (b)

The monomer mixture for making the pressure-sensitive adhesive acrylate polymer comprises at least 0.4 wt.%, preferably more than 1 and preferably up to 10 wt.%, from more than 1 and up to less than 5 wt.% or from more than 1 and up to 4.5 wt.%, based on the total amount of monomers, of at least one ethylenically unsaturated, copolymerizable monomer having at least one acid group (acid monomer). The acid monomers (b) comprise monomers which contain at least one acid group, and also their anhydrides and salts thereof. The monomers (b) include alpha, beta-monoethylenically unsaturated monocarboxylic and dicarboxylic acids, monoesters of alpha, beta- monoethylenically unsaturated dicarboxylic acids, the anhydrides of the aforesaid alpha, beta-monoethylenically unsaturated carboxylic acids, and also ethylenically unsaturated sulfonic acids and their water-soluble salts, as for example their alkali metal salts. Examples thereof are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, and viny llactic acid. Examples of suitable ethylenically unsaturated sulfonic acids include vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropane sulfonic acid, sulfopropyl acrylate and sulfopropyl methacrylate. Preferred monomers (b) are alpha, beta-monoethylenically unsaturated carboxylic acids with 3 to 8 carbon atoms and dicarboxylic acids with 4 to 8 carbon atoms, e.g., acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrenesulfonic acid, acrylamidoglycolic acid, acrylamidomethyl propane sulfonic acid, sulfopropyl acrylate, sulfopropyl methacrylate, their respective anhydrides and mixtures of these monomers. Particularly preferred monomers (b) are itaconic acid, acrylic acid and methacrylic acid. Most preferred are acrylic acid and methacrylic acid and the combination of both.

Monomers (c)

The monomer mixture for making the pressure-sensitive adhesive acrylate polymer optionally comprises one or more monomers (c) selected from ethylenically unsaturated monomers different from monomers (a) and (b).

The amount of monomers (c) is for example from 0 to 20 wt.%, preferably 0 to 15 wt.%, based on the total amount of monomers. Preferably, the monomer mixture comprises no monomer (c) or the amount of monomers (c) is less than 10 wt.%, less than 5 wt.% or less than 1 wt.% of monomers (c).

Monomers (d)

Suitable monomers (c) are for example one or more monomers (d) selected from methyl acrylate, methyl methacrylate and methacrylic acid alkyl ester monomers with 2 to 12 carbon atoms in the alkyl group, for example methyl methacrylate, methyl acrylate, ethyl methacrylate, propyl methacrylate, tert-butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate and iso-decyl methacrylate. In particular, mixtures of the alkyl (meth)acrylates are also suitable. If present, preferred monomers (d) are methyl acrylate, methyl methacrylate or a mixture thereof. The amount of monomers (d) is for example from 0 to 20 wt.%, preferably 0 to 15 wt.%, more preferably 1 to 15 wt.%, based on the total amount of monomers.

Monomers (c2)

Suitable monomers (c) are also for example one or more monomers (c2) selected from hydroxyalkyl (meth)acrylates having from 1 to 10 C atoms, preferably from 1 to 4 C atoms in the hydroxyalkyl group, vinyl esters of carboxylic acids comprising up to 20 C atoms, vinylaromatics having up to 20 C atoms, ethylenically unsaturated nitriles, amides of ethylenically unsaturated carboxylic acids (preferably acrylamide or methacrylamide), N-alky lolamides of ethylenically unsaturated carboxylic acids (preferably N-methylol acrylamide and N-methylol methacrylamide), phenyloxyethyl glycol mono(meth)acrylates, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 C atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, ethylenically unsaturated monomers containing amino groups, bifunctional monomers which as well as an ethylenically unsaturated double bond have at least one glycidyl group (preferably glycidyl acrylate or glycidyl methacrylate), oxazoline group, ureido group, ureido-analogous group or carbonyl group, preferably diacetone acrylamide, and crosslinking monomers which have more than one free-radical ly polymerizable ethylenically unsaturated group, or mixtures of these monomers. Monomers (c2) are used in amounts of preferably 0 to 20 wt.%, more preferably 0.1 to 10 wt.%, based on the total amount of monomers,

Hydroxyalkyl (meth)acrylates having from 1 to 10 C atoms in the alkyl group are for example hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate and 4-hydroxybutyl acrylate. Vinyl esters of carboxylic acids having 1 to 20 carbons are, for example, vinyl laurate, vinyl stearate, vinyl propionate, Versatic acid vinyl esters, and vinyl acetate. Useful vinylaromatic compounds include vinyltoluene, alpha- and paramethylstyrene, alpha-butylstyrene, 4-n-buty Isty rene, 4-n-decylstyrene and, preferably, styrene. Monomers containing amino groups are for example the aminoalkyl esters of the aforesaid alpha, beta-monoethylenically unsaturated carboxylic acids, preferably C1-C10 aminoalkyl (meth)acrylates such as, for example, 2-ami noethyl (meth)acrylate or tert-butylaminoethyl methacrylate. Examples of nitriles are the nitriles of alpha, beta-monoethylenically unsaturated C3-C8 carboxylic acids, preferably acrylonitrile and methacrylonitrile. The vinyl halides are ethylenically unsaturated compounds substituted by chlorine, fluorine or bromine, preferably vinyl chloride and vinylidene chloride. Examples of vinyl ethers which may be mentioned are vinyl methyl ether or vinyl isobutyl ether. Preference is given to vinyl ethers of alcohols comprising 1 to 4 carbons. Hydrocarbons having 4 to 8 carbons and two olefinic double bonds include butadiene, isoprene and chloroprene. Monomers with an ureido group are monomers having a substituent of formula where X is NH or NR and R is an organic group such as for example alkyl, preferably alkyl with 1 to 4 C-atoms. The arrow at the N-atoms indicates the connection to the remaining part of the monomer, preferably a (meth)acrylate monomer. A preferred example is. Monomers with an ureido-analogous groups are monomers having a substituent of the above formula where X is 0 or CH2.

The amount of monomers (c2) is for example from 0 to 20 wt.%, based on the total amount of monomers. Preferably, the monomer mixture comprises no monomer (c2) or the amount of monomers (c2) is from 0.1 to 10 wt.%, from 0.1 to 5 wt.% or from 0.2 to 1 wt.%.

A preferred aqueous pressure-sensitive adhesive composition comprises a pressure-sensitive adhesive acrylate polymer (B) made by emulsion polymerization of

(b) at least 0.4 wt.%, preferably more than 1 wt.% and preferably up to 10 wt.% of at least one ethylenically unsaturated, copolymerizable monomer having at least one acid group;

(d) optionally one or more monomers selected from methyl acrylate, methyl methacrylate and methacrylic acid alkyl ester monomers with 2 to 12 carbon atoms in the alkyl group;

(c2) optionally one or more ethylenically unsaturated monomers different from monomers (a), (b) and (d); wherein wt.% amounts of the monomers are based on the total weight amount of all monomers.

Preferably and for sustainability reasons, bio-based materials are used for producing the pressure-sensitive adhesive polymer, which preferably is partly or completely made of partly or fully bio-based monomers. Bio-based materials are materials which are made from a renewable source and have a smaller impact on the environment. They do not require all the refining steps required for petroleum-based products, which are very expensive in terms of energy. The production of CO2 is reduced such that they contribute less to global warming. The term "bio-based” indicates that the material is of biological origin and comes from biomaterial/ renewable resources. A material of renewable origin or biomaterial is an organic material wherein the carbon comes from the CO2 fixed recently (on a human scale) by photosynthesis from the atmosphere. A biomaterial (carbon of 100% natural origin) has an isotopic ratio ¹⁴C/¹²C greater than 10^-12, typically about 1.2x10^-12, while a fossil material has a zero ratio. Indeed, the isotopic ¹⁴C is formed in the atmosphere and is then integrated via photosynthesis, according to a time scale of a few tens of years at most. The half-life of the ¹⁴C is 5730 years. Thus, the materials coming from photosynthesis, namely plants in general, necessarily have a maximum content in isotope ¹⁴C. The determination of the content of biomaterial or of bio-carbon can be carried out in accordance with the standards ASTM D 6866-12, the method B (ASTM D 6866-06) and ASTM D 7026 (ASTM D 7026-04). Preferably, the pressure-sensitive adhesive acrylate polymer (B) has a content of biocarbon of at least 10 mol-%, in particular at least 15 mol-% or at least 20 mol-% or higher, e.g. 30 mol-% or 40 mol-% or higher, based on the total amount of carbon atoms in the pressure-sensitive adhesive polymer. A polymer which is partly made of fully or partly biobased monomers is a polymer where not all monomers used in the polymerization are partly or fully biobased monomers. A partly biobased monomer is a monomer where not all C-atoms are biobased, for example (meth)acrylic acid esters where only the acid part or only the alcohol part is biobased.

Suitable bio-based materials for producing the pressure-sensitive adhesive acrylate polymer are for example (meth)acrylic esters, wherein the(meth)acrylic acid component or the alcohol component or both are bio-based. Various methods of producing bio-based acrylic acid from renewable plant materials are mentioned in EP 2626397 A1. Suitable bio-based alcohols are for example bio-based ethanol, bio-based n-butanol, bio-based isobutanol, biobased iso-pentanol (3-methylbutan-1 -ol), bio-based 2-octanol and bio-based n-heptanol. Preferably, at least 50% of the monomers are at partly or fully bio-based monomers. Preferred partly biobased monomers are esters of (methacrylic acid and bio-based alcohols, preferably bio-based ethanol, bio-based n-butanol, bio-based isobutanol (2- methylpropan-1 -ol), bio-based iso-pentanol (3-methylbutan-1-ol), bio-based 2-octanol and bio-based n-heptanol. Preferred fully biobased monomers are esters of bio-based acrylic acid and bio-based alcohols as mentioned above.

The pressure-sensitive adhesive acrylate polymer (B) has a glass transition temperature of -10 °C or less, preferably from -60 to -20 °C. In a preferred embodiment, the aqueous pressure-sensitive adhesive composition additionally comprising at least one further acrylate polymer (B1) dispersed in water with a glass transition temperature of more than -10 °C, preferably at least -7 °C. This further acrylate polymer (B1) may be comprised of the same monomers as described above for pressure-sensitive adhesive acrylate polymer (B), except that monomer type and quantity are selected such that the further acrylate polymer has said glass transition temperature.

The further acrylate polymer (B1) is preferably used in amounts of 0 to 30% by weight, more preferably 1 to 25% by weight, more preferably from 5 to 20% by weight, based on the total weight of all polymers of the adhesive composition.

Preferably, the inventive aqueous pressure-sensitive adhesive composition comprises:

(A) from 5.0 to 40.0 wt.%, preferably from 10.0 to 35.0 wt.%, more preferably from 15.0 to 30.0 wt.%, of one or more polyisobutene (A);

(B) from 20.0 to 90.0 wt.%, preferably from 30 to 85 wt.%, more preferably from 35.0 to 80 wt.%, of one or more pressure-sensitive adhesive acrylate polymer (B) with a glass transition temperature of - 10 °C or less, preferably from -60 to -15 °C,

(B1) from 1.0 to 30.0 wt.-%, preferably from 2.0 to 35.0 wt.-%, more preferably 5.0 to 25.0 wt.%, of one or more further acrylate polymer (B1) with a glass transition temperature of more than -10 °C, preferably at least -7 °C, more preferably in the range of -10 °C to 0 °C, more preferably in the range of -7 °C to 0 °C;

(C) from 0.1 to 10.0 wt.-%, preferably from 0.5 to 8.0 wt.-%, more preferably 1.0 to 5.0 wt.%, of at least one crosslinking agent (C) for the pressure-sensitive adhesive acrylate polymer; wherein all amounts are based on the total weight of all polymers in the adhesive composition and calculated based on solid content of the respective component (e.g. (A), (B), (B1) or (C)).

The polyisobutene (A), the acrylate polymers (B) and (B1) as well as the crosslinker (C) can be selected as described above.

By purposive variation of monomer type and quantity, those skilled in the art are able according to the invention to prepare aqueous polymeric compositions whose polymers have a glass transition temperature in the desired range. Orientation is possible by means of the Fox equation. According to Fox (T.G. Fox, Bull. Am. Phys. Soc. 1956 [Ser. II]

I , page 123, and according to Ullmann's Encyclopadie der technischen Chemie, Vol. 19, page 18, 4th edition, Verlag Chemie, Weinheim, 1980), the glass transition temperature of copolymers is given to a good approximation by:

1/Tg = X¹/Tg¹ + X²/Tg² + .... Xⁿ/Tgⁿ, where x¹, x², .... xⁿ are the mass fractions of the monomers 1 , 2, .... n and T_g ¹, T_g ², .... T_g ⁿ are the glass transition temperatures in degrees Kelvin of the polymers synthesized from only one of the monomers 1 , 2, .... n at a time. The T_g values for the homopolymers of the majority of monomers are known and are listed for example in Ullmann's Encyclopaedia of Industrial Chemistry, Vol. 5, Vol. A21 , page 169, VCH Weinheim, 1992; further sources for glass transition temperatures of homopolymers are, for example, J. Brandrup, E.H. Immergut, Polymer Handbook, 1^st Ed.,

J. Wiley, New York 1966, 2^nd Ed. J. Wiley, New York 1975, and 3^rd Ed. J. Wiley, New York 1989.

The pressure-sensitive adhesive acrylate polymer dispersion is prepared by emulsion polymerization. Emulsion polymerization comprises polymerizing ethylenically unsaturated compounds (monomers) in water using typically ionic and/or nonionic emulsifiers and/or protective colloids or stabilizers as surface-active compounds to stabilize the monomer droplets and the polymer particles subsequently formed from the monomers. A detailed description of suitable protective colloids is found in Houben-Weyl, Methoden der organischen Chemie, Volume XIV/1 , Makromolekulare Stoffe [Macromolecular Compounds], Georg-Thieme-Verlag, Stuttgart, 1961, pp. 411 to 420. Suitable emulsifiers include anionic, cationic, and nonionic emulsifiers. As accompanying surface-active substances, it is preferred to use exclusively emulsifiers, whose molecular weights, unlike those of the protective colloids, are usually below 2000 g/mol. Where mixtures of surface-active substances are used the individual components must of course be compatible with one another, something which in case of doubt can be checked by means of a few preliminary tests. Preference is given to using anionic and nonionic emulsifiers as surface-active substances.

Common accompanying emulsifiers are, for example, ethoxylated fatty alcohols (EC units: 3 to 50, alkyl radical: Cs to C36), ethoxylated mono-, di-, and tri-alky Iphenols (EC units: 3 to 50, alkyl radical: C4 to C9), alkali metal salts of dialkyl esters of sulfosuccinic acid and also alkali metal salts and ammonium salts of alkyl sulfates (alkyl radical: Cs to C12), of ethoxylated alkanols (EC units: 4 to 30, alkyl radical: C12 to G ), of ethoxylated alkylphenols (EC units: 3 to 50, alkyl radical: C4 to C9), of alkylsulfonic acids (alkyl radical: C12 to G ), and of alkylarylsulfonic acids (alkyl radical: C9 to G ). Further suitable emulsifiers are compounds of the general formula II in which R⁵ and R⁶ are hydrogen or C4 to C14 alkyl and are not simultaneously hydrogen, and X and Y can be alkali metal ions and/or ammonium ions. Preferably R⁵ and R⁶ are linear or branched alkyl radicals having 6 to 18 carbon atoms or hydrogen and in particular having 6, 12, and 16 carbon atoms, with R⁵ and R⁶ not both simultaneously being hydrogen. X and Y are preferably sodium, potassium or ammonium ions, with sodium being particularly preferred. Particularly advantageous compounds are compounds II in which X and Y are sodium, R⁵ is a branched alkyl radical having 12 carbon atoms, and R⁶ is hydrogen or R⁵. Frequently use is made of technical-grade mixtures which contain a fraction of from 50 to 90% by weight of the monoalkylated product, an example being Dowfax® 2A1 (trademark of the Dow Chemical Company). Suitable emulsifiers are also found in Houben-Weyl, Methoden der organischen Chemie, Volume 14/1, Makromolekulare Stoffe [Macromolecular Compounds], Georg Thieme Verlag, Stuttgart, 1961, pages 192 to 208. Examples of emulsifier trade names include Dowfax®2 A1, Emulan® NP 50, Dextrol®OC 50, Emulgator 825, Emulgator 825 S, Emulan® OG, Texapon® NSC, Nekanil® 904 S, Lumiten® l-RA, Lumiten® E 3065, Disponil® FES 77, Lutensol® AT 18, Steinapol VSL, and Emulphor NPS 25. For the present invention non-ionic emulsifiers, ionic emulsifiers or protective colloids may be used. The compounds in question may be ionic emulsifiers, especially salts and acids, such as carboxylic acids, sulfonic acids, and sulfates, sulfonates or carboxylates. In particular it is also possible to use mixtures of ionic and nonionic emulsifiers. The surface-active substance is used usually in amounts of from 0.1 to 10 parts by weight, preferably from 0.2 to 5 parts by weight per 100 parts by weight of the monomers to be polymerized.

Water-soluble initiators for the emulsion polymerization are for example ammonium salts and alkali metal salts of peroxodisulfuric acid, e.g., sodium peroxodisulfate, hydrogen peroxide or organic peroxides, e.g., tert-butyl hydroperoxide. Also suitable are what are called reduction-oxidation (Red-Ox) initiator systems. The Red-Ox initiator systems are composed of at least one, usually inorganic, reducing agent and one organic or inorganic oxidizing agent. The oxidizing component comprises, for example, the initiators already mentioned above for the emulsion polymerization. The reducing components comprise, for example, alkali metal salts of sulphurous acid, such as sodium sulphite, sodium hydrogen sulfate, alkali metal salts of disulfurous acid such as sodium disulfite, bisulfite addition compounds of aliphatic aldehydes and ketones, such as acetone bisulfite, or reducing agents such as hydroxymethanesulfinic acid and the salts thereof, or ascorbic acid. The red-ox initiator systems can be used together with soluble metal compounds whose metallic component is able to exist in a plurality of valence states. Customary Red-Ox initiator systems are, for example, ascorbic acid/iron(l I) sulfate/ sodium peroxodisulfate, tert-butyl hydroperoxide/sodium disulfite, tert-butyl hydroperoxide/Na hydroxymethanesulfinate. The individual components, the reducing component for example, can also be mixtures, an example being a mixture of the sodium salt of hydroxymethanesulfinic acid and sodium disulfite. The compounds stated are used generally in the form of aqueous solutions, the lower concentration being determined by the amount of water that is acceptable in the dispersion and the upper concentration by the solubility of the respective compound in water. In general, the concentration is from 0.1 to 30% by weight, preferably from 0.5 to 20% by weight, more preferably from 1.0 to 10% by weight, based on the solution. The amount of the initiators is generally from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, based on the monomers to be polymerized. It is also possible for two or more different initiators to be used in the emulsion polymerization.

For the polymerization it is possible to use molecular weight regulators (also called chain transfer agents). By this means it is possible to reduce the molar mass of the emulsion polymer through a chain termination reaction. The chain transfer agents are bonded to the polymer in this procedure, generally to the chain end. The amount of the chain transfer agents is preferably 0.05 to 4 parts by weight, more preferably 0.05 to 0.8 part by weight, and very preferably 0.1 to 0.6 part by weight, per 100 parts by weight of the monomers to be polymerized. Suitable chain transfer agents are, for example, compounds having a thiol group such as tert-butyl mercaptan, thioglycolic acid 2- ethylhexyl ester, mercaptoethanol, mercaptopropyl trimethoxysilane or tert-dodecyl mercaptan. The chain transfer agents are preferably compounds of low molecular mass, having a molar weight of less than 2000, more particularly less than 1000 g/mol. Preferred are 2-ethylhexyl thioglycolate (EHTG), isooctyl 3-mercaptopropionate (IOMPA) and tert-dodecyl mercaptan (tDMK).

The emulsion polymerization takes place in general at from 30 to 130°C, preferably from 50 to 90°C. The polymerization medium may be composed either of water alone or of mixtures of water and liquids miscible therewith such as methanol. Preferably just water is used. The emulsion polymerization can be carried out either as a batch operation or in the form of a feed process, including staged or gradient procedures. Preference is given to the feed process, in which a portion of the polymerization mixture is introduced as an initial charge, heated to the polymerization temperature, and subjected to partial polymerization and then the remainder of the polymerization mixture is supplied to the polymerization zone, usually by way of two or more spatially separate feeds, of which one or more comprise(s) the monomers in pure form or in emulsified form, continuously, in stages or subject to a concentration gradient, during which the polymerization is maintained. In the polymerization it is also possible for a polymer seed to be included in the initial charge in order, for example, to establish the particle size more effectively.

The manner in which the initiator is added to the polymerization vessel in the course of the free-radical aqueous emulsion polymerization is known to one of ordinary skill in the art. Either it can be included in its entirety in the initial charge to the polymerization vessel or else it can be inserted continuously or in stages at the rate at which it is consumed in the course of the free-radical aqueous emulsion polymerization. For any given case this will depend both on the chemical nature of the initiator system and on the polymerization temperature. Preferably some of it is included in the initial charge and the remainder is supplied to the polymerization zone at the rate at which it is consumed. To remove residual monomers, it is usual to add initiator after the end of the emulsion polymerization, i.e., after a monomer conversion of at least 95%. The individual components can be added to the reactor, in the case of the feed process, from above, in the side, or from below, through the reactor floor.

The polymerization may take place with seed control, i.e., in the presence of polymer seed (seed latex). Seed latex is an aqueous dispersion of finely divided polymer particles having an average particle diameter of preferably 20 to 40 nm. Seed latex is used in an amount of preferably 0.01 to 0.5 part by weight, more preferably of 0.03 to 0.3 part by weight, or of 0.03 to less than or equal to 0.1 part by weight, per 100 parts by weight of monomers. Suitable seed latex is for example a latex based on polystyrene or based on polymethyl methacrylate. A preferred seed latex is polystyrene seed.

In the preparation of the pressure-sensitive adhesive acrylate polymer (B) by free-radical ly initiated aqueous emulsion polymerization it is of course possible to use further, optional auxiliaries familiar to the skilled person, such as, for example, those known as thickeners, defoamers, neutralizing agents, buffer substances, preservatives, free- radical chain-transfer compounds and/or inorganic fillers.

Acid groups in the polymer may be neutralized by the feeding of a neutralizing agent during or after polymerization, with the acid groups being neutralized wholly or partly by the feeding of the base. The pH of the pressure-sensitive adhesive polymer dispersion is preferably adjusted to a pH greater than 5, more particularly to a pH of between 5 and 9 or between 5.5 and 8.

The aqueous pressure-sensitive adhesive composition preferably comprises 15 to 75 wt.%, more preferably from 40 to 60 wt.%, more particularly more than 50 and up to 60 wt.% of polymers (A) + (B), based on the total weight of the aqueous pressure-sensitive adhesive composition. The solids content may be adjusted, for example, by appropriate adjustment of the amounts of monomers and/or the amount of water used in or after the emulsion polymerization. Also preferred are aqueous pressure-sensitive adhesive compositions comprising 15 to 75 wt.%, more preferably from 40 to 60 wt.%, more particularly more than 50 and up to 60 wt.% of polymers (A) + (B) + (B1), based on the total weight of the aqueous pressure-sensitive adhesive composition.

The aqueous pressure-sensitive adhesive composition comprises the polymers (A) and (B) preferably in a weight ratio of polyisobutene (A) to pressure-sensitive adhesive acrylate polymer (B) from 1.5:1 to 1:19, more preferably from 1.5:1 to 1 :6, more preferably from 1 :1 to 1 :6, more preferably from 1 :1 to 1 :4, more preferably from 1 :1.5 to 1 :4, more preferably from 1 :2 to 1 :4 (based on solid content of (A) and (B), solid/solid). Also preferably, the aqueous pressure-sensitive adhesive composition comprises the polymers (A), (B) and optional (B1) in a weight ratio of polyisobutene (A) to sum of pressure-sensitive adhesive acrylate polymer(s) (B) and optional (B1) from 1.5:1 to 1:19, more preferably from 1.5:1 to 1 :6, more preferably from 1 :1 to 1 :6, more preferably 1 :2 to 1 :6 (based on solid content of (A) and (B), solid/solid).

The aqueous pressure-sensitive adhesive composition comprises the polymers (A) and (B) preferably in a weight ratio of polyisobutene (A) to pressure-sensitive adhesive acrylate polymer (B) from 1.5:1 to 1 :19, more preferably from 1.5:1 to 1:6, more preferably from 1 :1 to 1 :4, more preferably from 1 :2 to 1 :4 (based on the total amount of the aqueous dispersions of (A) and (B)).

Crosslinking agent (C)

The aqueous pressure-sensitive adhesive composition comprises at least one crosslinking agent (C) for the pressure-sensitive adhesive acrylate polymer (B). A crosslinking agent is particularly preferred for the use of the aqueous pressure-sensitive adhesive composition of the invention for making a removable protective film. Preferred amounts of crosslinking agents (preferably calculated based on the solid content of the crosslinker (C)) are from 0.01 to 10 parts by weight, more preferred from 0.05 to 5 parts by weight, more particularly from 0.1 to 3 parts by weight, and with particular preference from 0.2 to 2 parts by weight, more preferred from 0.5 to 2 parts by weight, based on 100 parts by weight of pressure-sensitive adhesive acrylate polymer (B).

Also preferably, the aqueous pressure-sensitive adhesive composition comprises the at least one crosslinking agent (C) in amounts from 0.01 to 10 parts by weight, more preferred from 0.05 to 8 parts by weight, more particularly from 0.1 to 5 parts by weight, and with particular preference from 0.1 to 3 parts by weight, based on 100 parts by weight of all polymers in the aqueous pressure-sensitive adhesive composition and calculated based on solid content of (C).

Preferred crosslinking agents are selected from the group consisting of aminotriazines, isocyanurates formed from diisocyanates and having at least two isocyanate groups, compounds having at least one carbodiimide group, chemically capped isocyanates, encapsulated isocyanates, encapsulated uretdiones, biurets, allophanates, aziridines, oxazolines, epoxides, and mixtures of the substances mentioned. Most preferred crosslinker are polyisocyanates, isocyanurates formed from diisocyanates and having at least two isocyanate groups, polyaziridines and carbodiimides. The crosslinkers may be used individually or as mixtures of two or more crosslinkers. Because of the crosslinking reaction that ensues, the addition of the crosslinker is not made until shortly before the subsequent use of the dispersion, i.e., shortly before the coating of the polymer film. Following addition of the crosslinker, there remains sufficient time, generally up to 8 hours, for processing.

Suitable polyisocyanates are, for example, aliphatic or cycloaliphatic or aromatic diisocyanates or polyisocyanates of higher functionality deriving from the diisocyanates. Examples of polyisocyanates contemplated include linear or branched C4-C14 alkylene diisocyanates, cycloaliphatic diisocyanates having in total 6 to 12 C atoms, aromatic diisocyanates having in total 8 to 14 C atoms, polyisocyanates containing isocyanurate groups, uretdione diisocyanates, polyisocyanates containing biuret groups, polyisocyanates containing urethane or allophanate groups, polyisocyanates comprising oxadiazine trione groups, uretonimine-modified polyisocyanates, or mixtures thereof. Examples of diisocyanates include aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (1,6-diisocyanatohexane), octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, tri methyl hexane diisocyanate or tetramethyl hexane diisocyanate; cycloaliphatic diisocyanates such as 1,4-, 1,3- or 1,2-diisocyanatocyclohexane, 4,4’-di(isocyanatocyclohexyl)- methane, 1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophorone diisocyanate), or 2,4- or 2,6- diisocyanato-1 -methylcyclohexane, or aromatic diisocyanates such as 2,4-diisocyanatotoluene, 2,6-diisocyanato- toluene, tetramethylxylylene diisocyanate, 1,4-diisocyanatobenzene, 4,4'- or 2,4-diisocyanatodiphenylmethane, p- xylylene diisocyanate, and also isopropenyldimethyltolylene diisocyanate. Polyisocyanates include, for example, polycyclic homologs of the aromatic diisocyanates stated above.

Mention may further be made of the following: a) Polyisocyanates containing isocyanurate groups and derived from aromatic, preferably aliphatic and/or cycloaliphatic diisocyanates. Particularly preferred in this context are the corresponding isocyanato-isocyanurates based on hexamethylene diisocyanate and isophorone diisocyanate. The present isocyanurates are, in particular, simple trisisocyanatoalkyl and/or trisisocyanatocycloalkyl isocyanurates, which represent cyclic trimers of the diisocyanates, or are mixtures with their higher homologs containing more than one isocyanurate ring. The isocyanato-isocyanurates generally have an NCO content of 10 to 30 wt%, more particularly 15 to 25 wt%, and an average NCO functionality of 3 to 4.5. b) Uretdione diisocyanates having aromatic, preferably aliphatically and/or cycloaliphatically bonded isocyanate groups, derived preferably from hexamethylene diisocyanate or isophorone diisocyanate. Uretdione diisocyanates are cyclic dimerization products of diisocyanates. c) Polyisocyanates containing biuret groups and having aromatic, preferably aliphatically bonded isocyanate groups, more particularly tris(6-isocy anatohexyl)biuret or its mixtures with its higher homologs. These polyisocyanates containing biuret groups generally have an NCO content of 18 to 22 wt% and an average NCO functionality of 3 to 3.5 or up to 4.5. d) Polyisocyanates containing urethane groups and/or allophanate groups and having aromatic, preferably aliphatically or cycloaliphatically bonded isocyanate groups, of the kind obtainable for example by reaction of excess amounts of hexamethylene diisocyanate or of isophorone diisocyanate with simple polyhydric alcohols such as trimethylolpropane, glycerol or 1 ,2-dihydroxypropane, for example, or mixtures thereof. These polyisocyanates containing urethane groups and/or allophanate groups generally have an NCO content of 12 to 20 wt% and an average NCO functionality of 2.5 to 3. e) Polyisocyanates comprising oxadiazinetrione groups, derived preferably from hexamethylene diisocyanate or from isophorone diisocyanate. Polyisocyanates of this kind comprising oxadiazinetrione groups are preparable from diisocyanate and carbon dioxide. f) Uretonimine-modified polyisocyanates. The polyisocyanates a) to f) may also be used in a mixture, including optionally a mixture with diisocyanates.

Preference is given to aliphatic and/or cycloaliphatic polyisocyanates, and/or diisocyanates.

Particularly preferred are hydrophilically modified polyisocyanates which are self-dispersible in water. For the preparation of the self-dispersible polyisocyanates, the polyisocyanates identified above are reacted with compounds containing at least one, preferably one, hydrophilic group, which may be ionic or nonionic, and at least one, preferably one, isocyanate-reactive group, such as a hydroxyl, mercapto or primary or secondary amino group (NH group for short), for example. The hydrophilic group may be, for example, an ionic group or a group which can be converted into an ionic group. Anionic groups or groups which can be converted into anionic groups are, for example, carboxylic or sulfonic acid groups. Examples of suitable compounds are hydroxycarboxylic acids, such as hydroxypivalic acid or dimethylolpropionic acid, or hydroxysulfonic acids or aminosulfonic acids. Cationic groups or groups which can be converted into cationic groups are, for example, quaternary ammonium groups and tertiary amino groups. Groups which can be converted into ionic groups are preferably converted into ionic groups before or during the dispersing of the mixture of the invention in water. For the conversion, for example, of carboxylic or sulfonic acid groups into anionic groups, use may be made of inorganic and/or organic bases such as sodium hydroxide, potassium hydroxide, potassium carbonate, sodium hydrogencarbonate, ammonia, or primary, secondary, and especially tertiary amines, e.g., triethylamine or dimethylaminopropanol. For the conversion of tertiary amino group into the corresponding cations, ammonium groups for example, neutralizing agents that are suitable are inorganic or organic acids, as for example hydrochloric acid, acetic acid, fumaric acid, maleic acid, lactic acid, tartaric acid, oxalic acid or phosphoric acid, or suitable quaternizing agents are, for example, methyl chloride, methyl iodide, dimethyl sulfate, benzyl chloride, ethyl chloroacetate or bromoacetamide. Further suitable neutralizing and quaternizing agents are described in US-PS 3479 310, column 6, for example. The amount of the ionic groups or groups which can be converted into ionic groups is preferably 0.1 to 3 mol per kg of the polyisocyanates that are self- dispersible in water. Nonionic hydrophilic groups are, for example, polyalkylene ether groups, especially those having 10 to 80 alkylene oxide units. Preference is given to polyethylene ether groups or polyalkylene ether groups which as well as other alkylene oxide units, propylene oxide for example, comprise at least 10 ethylene oxide units. Suitable compounds are, for example, polyalkylene ether alcohols. The amount of the hydrophilic nonionic groups, more particularly of the polyalkylene ether groups, is preferably 0.5 to 20, more preferably 1 to 15 wt%, based on the polyisocyanates that are self-dispersible in water. The preparation of the polyisocyanates that are self-dispersible in water is known from DE-A-3521 618, DE-A-40 01 783, and DE-A-42 03510. In the preparation of the polyisocyanates that are self-dispersible in water, the compounds having at least one hydrophilic group and at least one isocyanate-reactive group may be reacted with a portion of the polyisocyanate, and the resulting hydrophilically modified polyisocyanates may then be mixed with the remaining polyisocyanates. An alternative mode of the preparation is for the compounds to be added to the total amount of the polyisocyanates and then for the reaction to be carried out in situ. Preferred water-emulsifiable polyisocyanates are those having hydrophilic nonionic groups, more particularly polyalkylene ether groups. Here, preferably, the water-emulsifiability is achieved solely by virtue of the hydrophilic nonionic groups. As crosslinkers it is also possible to use polyaziridines. These are polyfunctional aziridine compounds having at least two aziridine groups. The aziridine groups may be substituted on the nitrogen atom, by an alkyl, alkenyl, aryl or aralkyl radical, for example. Suitability is possessed for example by aziridine crosslinkers based on polyethers or on substituted hydrocarbons, as for example 1 ,6-bis-N-aziridinohexane. The polyfunctional aziridine compound present in the composition of the invention may be selected preferably from the group consisting of the Michael addition products of optionally substituted ethyleneimine onto esters of polyhydric alcohols with alpha, beta-unsaturated carboxylic acids, and the addition products of optionally substituted ethyleneimine onto polyisocyanates. Examples of suitable polyhydric alcohol components are trimethylolpropane, neopentyl glycol, glycerol, pentaerythritol, 4,4'-iso- propylidenediphenol, and 4,4'-methylenediphenol. Suitable alpha, beta-unsaturated carboxylic acids include, for example, acrylic and methacrylic acid, crotonic acid, and cinnamic acid. With particular preference the composition of the invention comprises acrylic esters. The corresponding polyhydric alcohols of the alpha, beta-unsaturated carboxylic esters may optionally be alcohols which have been singly or multiply extended at their OH functions, partly or completely, with alkylene oxides. These may be the aforementioned alcohols singly or multiply extended with alkylene oxides, for example. Particularly suitable alkylene oxides are ethylene oxide and propylene oxide. Examples of suitable polyaziridines are trimethylolpropane tris(beta-aziridino)propionate, neopentyl glycol di(beta-aziridino)- propionate, glycerol tris(beta-aziridino)propionate, pentaerythritol tetra(beta-aziridino)propionate, 4,4'-isopropylidene- diphenol di(beta-aziridino)propionate, 4,4'-methylenediphenol di(beta-aziridino)propionate, 1,6-hexamethylenedi(N,N- ethyleneurea), 4,4'-methylenebis(phenyl-N,N-ethyleneurea), 1,3,5-tris(omega-hexamethylene-N,N-ethyleneurea)- biuret, and mixtures thereof. The polyfunctional aziridine compounds may optionally be substituted on their aziridine units.

Tackifier

The aqueous pressure-sensitive adhesive composition optionally comprises at least one tackifier. Tackifiers are chemical compounds used in formulating adhesives to increase the stickiness of the surface, also referred to as tack, of the adhesive. The amount of the tackifiers is preferably from 5 to 100 parts by weight, more preferably from 10 to 50 parts by weight, per 100 parts by weight of total polymers (solids/solids).

Tackifiers are, for example based on natural resins, such as rosin esters, rosins and their derivatives formed by disproportionation or isomerization, polymerization, dimerization, or hydrogenation. These natural resins can be present in their salt form (with monovalent or polyvalent counterions (cations), for example) or, preferably, in their esterified form. Alcohols used for the esterification of rosins can be monohydric or polyhydric. Examples are methanol, ethanediol, diethylene glycol, triethylene glycol, 1 ,2,3-propanetriol, pentaerythritol. Further tackifiers are terpenes, modified terpenes, aliphatic, cycloaliphatic and aromatic resins (C5 aliphatic resins, C9 aromatic resins, and C5/C9 aliphatic/aromatic resins), hydrogenated hydrocarbon resins, terpene-phenol resins, and their mixtures. Suitable tackifying hydrocarbon resins are for example coumarone-indene resins, polyterpene resins, hydrocarbon resins based on unsaturated CH compounds, such as butadiene, pentene, methylbutene, isoprene, piperylene, divinylmethane, pentadiene, cyclopentene, cyclopentadiene, cyclohexadiene, styrene, a-methylstyrene, vinyltoluene. Suitable tackifiers are also polyacrylates with a low molecular weight. Preferably these polyacrylates have a weightaverage molecular weight M_w below 30 000. The polyacrylate tackifiers are composed preferably at least 60%, in particular at least 80%, by weight of Ci-Cs alkyl (meth)acrylates.

Preferred tackifiers are natural or chemically modified rosins, rosin esters, terpene resins and their mixtures, such as for example Dermulsene® 222 or Snowtack® SE724G E. Rosins are composed predominantly of abietic acid or derivatives thereof.

The aqueous pressure-sensitive adhesive composition may optionally comprise further additives, such as at least one additive selected from the group consisting of wetting agents, fillers, dyes, flow agents, thickeners, light stabilizers, biocides, defoamers. The amount of the further additives is preferably 0.05 to 5 parts by weight, or 0.1 to 3 parts by weight of solids per 100 parts by weight of polymers (A) + (B) (based on solids).

The particle size D50 (measured by laser diffraction) of the adhesive polymer (B) dispersed in the aqueous dispersion is preferably less than 400 nm, more particularly less than 300 nm. With particular preference the average particle size is 50 to 250 nm, preferably from 100 to 250 nm, more preferably 140 and 250 nm. The adhesive polymer (B) may have a monomodal, or a bi-modal, or a multi-modal particle size distribution.

In one embodiment, the polyisobutene (A) and the pressure-sensitive adhesive acrylate polymer (B) both have a monomodal particle size distribution. In one embodiment, the polyisobutene (A) has monomodal particle size distribution and the pressure-sensitive adhesive acrylate polymer (B) has a bimodal or multi-modal (preferably bimodal) particle size distribution. In one embodiment, the polyisobutene (A) and the pressure-sensitive adhesive acrylate polymer (B) both have a bimodal or multi-modal (preferably bimodal) particle size distribution. Preferably, the polyisobutene (A) has a bimodal or multi-modal (preferably bimodal) particle size distribution and the pressuresensitive adhesive acrylate polymer (B) has a monomodal particle size distribution.

Preferably, the aqueous pressure-sensitive adhesive composition of the invention has a bimodal or a multi-modal particle size distribution. The pressure-sensitive adhesive acrylate polymer (B) or the polyisobutene (A) or both, i.e. the composition comprising the pressure-sensitive adhesive acrylate polymer (B) and the polyisobutene (A) may have a bimodal or a multimodal particle size distribution. "Multimodal” means more than two particle modes. The size distribution of the dispersion particles is monomodal when measurement of the particle size distribution contains only one single maximum. A bimodal particle size distribution is a particle size distribution defined by exhibiting two distinct maxima in the particle size measurement. A multimodal particle size distribution is a particle size distribution defined by exhibiting more than two distinct maxima in the particle size measurement. Preferably, a first particle mode has a weight average particle diameter in the range of 50 to 250 nm, preferably from 100 to 250 nm. A second particle mode has a weight average particle diameter preferably in the range of from above 250 nm and up to 25 m, preferably from above 300 nm and up to 2 m. Preferably, the average particle diameters of the first and second mode differ by at least 50 nm. Particle sizes can be measured by laser light scattering.

The viscosity of the inventive aqueous pressure-sensitive adhesive dispersion is preferably not more than 1500 mPa s and preferably at least 20 mPa s, for example from 50 to 800 mPa s measured in accordance with DIN EN ISO 3219 at 23°C and 250 sec¹.

The invention provides self-adhesive articles, preferably a self-adhesive label, a self-adhesive tape or a self-adhesive film including graphic films and protective films. The self-adhesive articles comprise a carrier substrate, preferably selected from paper, plastic films and metal foils, and at least a first pressure-sensitive adhesive layer made from an aqueous pressure-sensitive adhesive composition as described above. In one embodiment, the invention provides multi-layer self-adhesive articles, comprising a substrate coated with the first pressure-sensitive adhesive layer as described above and an additional layer selected from a primer layer (coated between the substrate and the first adhesive layer) and a second adhesive layer (different from the first adhesive layer and coated on top of the first adhesive layer) or comprising a substrate coated with a primer layer, the first pressure-sensitive adhesive layer and a second pressure-sensitive adhesive layer (different from the first adhesive layer) in this order.

The self-adhesive articles are at least partially coated with the pressure-sensitive adhesive. The inventive self- adhesive tapes may be tapes of the above-mentioned substances coated on one or both sides. The inventive self- adhesive labels may be labels made of paper or a thermoplastic film. Adhesive tapes made of thermoplastic film are particularly preferred. Suitable thermoplastic films include for example films made of polyolefins (for example polyethylene or polypropylene), polyolefin copolymers, films made of polyesters (for example polyethylene terephthalate), polyvinyl chloride or polyacetate. The surfaces of the thermoplastic polymer films have preferably been corona-treated. Foamed carriers are also possible. The labels have been coated with adhesive on one side. Preferred substrates for the self-adhesive articles are paper and polymer films. Polymeric film substrates are for example films of polyolefins (such as polyethylene or polypropylene), polyvinyl chloride (PVC), polyamide (PA), polyethylene terephthalate (PET), cellulose, cellophane, polyacetate, polyester (preferably biodegradable polylactates), polyolefin copolymers, for example cycloolefin copolymer (COG). Films of polyolefins are for example made of polyethylene or polypropylene (e.g., unoriented polypropylene (GPP) or oriented polypropylene (OPP)), preferably biaxially oriented polypropylene. The substrate material is preferably selected from polyolefins, wetstrength regenerated cellulose film and wet-strength paper, preferably biaxially oriented polypropylene. The polymeric films are preferably transparent. Transparent polymer films are for example polyolefins (such as polyethylene or polypropylene), PVC or polyethylene terephthalate (PET). PVC may be plasticized PVC. By plasticized PVC is meant polyvinyl chloride which includes plasticizers and has a reduced softening temperature. Examples of customary plasticizers are phthalates, epoxides, adipic esters. The amount of plasticizers in the plasticized PVC is preferably more than 10% and in particular more than 20% by weight. The thickness of the carrier substrate is preferably from 10 to 200 m or from 30 to 100 pm. The carrier substrate can be coated in customary fashion to produce the layer of adhesive on the carrier material. Typical coating methods are knife coating, spreading, roller coating, reverse roller coating, gravure roller coating, reverse gravure roller coating, brush coating, rod coating, spray coating, airbrush coating, meniscus coating, curtain coating or dip coating. The amount applied is preferably 5 to 30 g, more particularly 10 to 30 g, or 10 to 20 g, or 12 to 15 g of solid per m². The coating step is generally followed by a drying step for removal of water and solvents.

Surface treatment of polymeric film substrates ahead of coating with an adhesive polymer dispersion of the invention is not absolutely necessary, in particular not when a primer is used. However, better results can be obtained if the surface of the polymeric film substrates is modified prior to coating. In this case it is possible to employ customary surface treatments, such as corona treatment in order to boost the adhesion. The corona treatment or other surface treatments are carried out to the extent required for sufficient wettability with the coating composition. Customarily, corona treatment of approximately 10 watts per square meter per minute is sufficient for this purpose. Alternatively, or additionally, it is also possible, optionally, to use primers between film substrate and adhesive coating.

The self-adhesive articles have been at least partially coated with a pressure-sensitive adhesive on at least one surface. The adhesive may be applied to the articles by customary methods such as roller application, knife coating or spreading. The application rate is preferably 0.1 to 300 g, more preferably 2 to 150 g of solid per m². Application is generally followed by a drying step for removal of the water. The water may be removed by drying at 50°C to 150°C for example. The thus obtained coated substrates are used for example as self-adhesive articles, such as adhesive labels, adhesive tapes, or adhesive films. To this end the carriers may be cut into adhesive tapes, labels, or films before or after application of the adhesive. The side of the substrates coated with pressure-sensitive adhesive may be covered with a release liner or release paper, for example with a siliconized paper, for later use. The substrates to which the self-adhesive articles may advantageously be applied may be metal, wood, glass, paper or plastic. The self-adhesive articles are especially suitable for bonding to packaging surfaces, cardboard boxes, plastic packaging, books, windows, window frames, computer screens, vapor barriers, motor vehicle bodies, tires or vehicle body parts. Preferred self-adhesive articles are self-adhesive labels, self-adhesive tapes and self-adhesive films.

The invention also relates to a method of making a self-adhesive article as described above, wherein

(1) a substrate is provided which is optionally pre-coated with a primer, and

(2) an aqueous pressure-sensitive adhesive layer is applied to the substrate, wherein the pressure-sensitive adhesive layer is formed by coating the optionally pre-coated substrate with an aqueous pressure-sensitive adhesive composition as described herein.

In a preferred embodiment, the invention provides a protective film comprising a carrier substrate, preferably a polyolefin film, and at least a first pressure-sensitive adhesive layer made from an aqueous pressure-sensitive adhesive composition as described above. The protective films may have a multi-layer structure as described above. Preferred protective films are those in which detachment from a protected substrate occurs by adhesion breakage of the adhesive layer, whereby the adhesive layer detaches from the protected substrate and remains completely on the carrier substrate, preferably on the polyolefin carrier material.

Preferred protective films are characterized by the fact that the peel strength for removing the protective films from brushed steel is preferably no more than 8 N/25 mm, and preferably at least 3 N/25 mm, more preferably from 3 to 7 N/25 mm, after 24 hours storage at room temperature (23°C) and 50% relative humidity (see examples for measuring methods).

The application quantity of the pressure-sensitive adhesive is preferably 2 to 30 g/m², preferably 5 to 20 g/m². Layer thicknesses are e.g., 2 to 30 micrometers, preferably 5 to 20 micrometers. Polyolefin films, e.g., thermoplastic films, e.g., made of polyethylene or polypropylene, are suitable carriers. The carrier film thickness is preferably 10 to 250 micrometers or 30 to 100 micrometers, preferably 40 to 80 micrometers. The surfaces of the thermoplastic polymer carrier films are preferably pre-treated, e.g., by corona or plasma treatment. The protective films are coated with adhesive on one side.

The protective films are suitable for protecting a wide variety of surfaces. The surfaces of the articles to be protected can be made of metal (e.g., brushed steel), glass, plastics (e.g. PVC, PMMA, PS, polycarbonate, polyester, polyamide, polyolefin), wood, textiles or carpet, etc. and are suitable for protection against soiling or mechanical impact, particularly during the manufacture and transportation of the articles to be protected. The protective film is removed at the start of the intended final application or when the article remains in its intended location or use.

It is also possible that the inventive self-adhesive article, preferably the inventive protective film, remains on the surface of the article to be protected during one or more intermediate manufacturing steps, such as stamping or laser cutting e.g. of a metal sheet, and can be removed easily and without residue after the intermediate manufacturing steps. Further, the self-adhesive article, preferably the inventive protective film, preferably show sufficient adhesion during intermediate manufacturing steps.

The invention also provides the use of an aqueous pressure-sensitive adhesive composition as described herein for making a removable protective film comprising a substrate made of polyolefin plastic film, preferably selected from polyethylene and polypropylene, coated with said aqueous pressure-sensitive adhesive composition.

Particular advantages of the invention are

- good anchoring of the pressure-sensitive adhesive on polyolefin protective films and on challenging rough surfaces such as brushed steel

- a residue-free removal behavior from a wide variety of test surfaces, including rough steel and smooth steel

- a sufficiently constant peel strength after ageing - a peel strength for removing a PE film from brushed steel of preferably no more than 8 N/25 mm, and preferably at least 3 N/25 mm, more preferably from 3 to 7 N/25 mm after 24 hours storage at room temperature (23°C) and 50% r.h.

Examples

Ingredients and abbreviations:

PI B1 AquaPib® 60-25-150 (Emulco); polyisobutene, Mv = 85,000 g/mol, 60% solids

PIB2 AquaPib® 60-25-120 (Emulco); polyisobutene, Mv = 55,000 g/mol, 60% solids

PIB3 AquaPib® 60-25-100 (Emulco); polyisobutene, Mv = 40,000 g/mol, 60% solids

Crosslinker Basonat® HW1180 PC (BASF), isocyanurate based on hexamethylene diisocyanate (HDI), 80% solution in propylene carbonate

A 245 (B) Acronal® A 245 (BASF), aqueous dispersion of an acrylate copolymer containing carboxyl groups for pressure sensitive adhesive applications; based on > 70 wt.-% of alkyl acrylates with 2 to 8 carbon atoms in the alkyl group and > 0.4 wt.% (meth)acrylic acid; about 52% solids; Tg -45 °C

An 3633 (B) Acronal® 3633 (BASF); aqueous dispersion of an acrylate copolymer containing carboxyl groups for pressure sensitive adhesive applications; based on > 70 wt.-% of alkyl acrylates with 2 to 8 carbon atoms in the alkyl group and > 0.4 wt.% (meth)acrylic acid; about 60% solids; Tg -35 °C

An 3744 (B1) Acronal® 3744 (BASF); aqueous dispersion of an acrylate copolymer containing carboxyl groups, based on > 70 wt.-% of alkyl acrylates with 2 to 8 carbon atoms in the alkyl group and > 0.4 wt.% (meth)acrylic acid; about 50 % solids; Tg -7 °C

B_E1 Aqueous dispersion of an acrylate copolymer prepared based on EP 2697323 B1, Example D6

(prepared from 86.6 parts by weight n-butylacrylate (nBA), 4 parts by weight methylmethacrylate (MMA), 3.9 parts by weight ethylacrylate (EA), 1 .5 parts by weight AS, 1 parts by weight styrene, 2 parts by weight 2-hydroxypropylacrylate (HPA), 1 parts by weight ureidomethacrylate (UMA), all based on 100 parts by weight total monomer, the seed polymer was omitted); solids content = 59.8%, Tg = approx. -36°C

Determination of viscosity average molecular weight Mv

Mv (in g/mol) is calculated from the Staudinger Index Jo as follows:

Mv = (Jo x 100 / 3.06) ^{1/0 65}

The Staudinger Index Jo [cm³/g] is calculated from the flow time at 20°C through capillary I of an Ubbelohde viscometer via the Schulz-Blaschke equation:

Jo = q_sp / (c x (1 + 0.31 q_sp)) [cm³/g] with the specific viscosity q_sp being q_sp = (t / to) - 1 , where t = flow time of the solution with Hagebach-Courette correction to = flow time of the solvent with Hagebach-Courette correction c = concentration of the solution in g/cm³ Staudinger Index Jo is determined of solutions of polyisobutene samples in 2,2,4-trimethyl pentane (concentration 0.002 to 0.01 g/cm³) at 20 °C according to DIN 51652 in an Ubbelohde capillary microviscometer (AVS PRO III supplied by Schott Gerate GmbH), Capillary Ic, No. 53713.

The following aqueous pressure-sensitive adhesives compositions are prepared, wherein the amounts and weight ratios given refer to the total aqueous emulsions of components A and B and of solution of crosslinker C, based on the total weight of the aqueous pressure-sensitive adhesive composition:

Example C1

Aqueous polyisobutene emulsion PIB1

Example C2

Aqueous polyisobutene emulsion PI B1 , mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC.

Example C3

Acronal® A 245, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example C4

Blend of Acronal A 245, Acronal® 3633 and Acronal® 3744 in a weight ratio of 60:40: 15 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example E1

Blend of Acronal A 245 with aqueous polyisobutene emulsion PIB1 in a weight ratio of 115 : 37,5 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example E2

Blend of Acronal A 245, Acronal® 3633, Acronal® 3744 with aqueous polyisobutene emulsion PIB1 in a weight ratio of 60:40:15:37,5 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example E3

Blend of Acronal® A 245, Acronal® 3633, Acronal® 3744 with aqueous polyisobutene emulsion PIB2 in a weight ratio of 60:40:15:37.5 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example C5

Blend of Acronal® 3744 with aqueous polyisobutene emulsion PIB1 in a weight ratio of 100:37.5 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC Example E4

Blend of Acronal® A 245 and Acronal® 3744 with aqueous polyisobutene emulsion PIB1 in a weight ratio of 75:25:37.5 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example E5

Blend of Acronal® A 245 with aqueous polyisobutene emulsion PIB1 in a weight ratio of 100:20 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC

Example E6

Blend of aqueous dispersion of an acrylate copolymer B1 and Acronal® 3744 with aqueous polyisobutene emulsion PIB1 in a weight ratio of 60:40:37.5 parts by weight, mixed with 2.5% by weight of crosslinker Basonat® HW1180 PC.

Protective film preparation

The aqueous pressure-sensitive adhesive compositions are applied to a 50 pm polyethylene (PE) film with a coating weight of 20 g/m² and dried. The coated PE film is cut into 25 mm wide test strips.

Peel strength test

In the determination of peel strength 25 mm wide test strips are bonded to test surfaces made of brushed steel (Ra = 2.5 pm) and rolled once with a 1 kg roller. One end is then clamped in the upper jaws of a tensile strain tester. The adhesive strip is removed from the test surface at a speed of 300 mm/min and an angle of 180°, i.e., the adhesive strip is bent and removed parallel to the test specimen and the force required therefor is measured. The measure for peel strength is the force in N/25 mm obtained as the average value from at least two measurements. The test method corresponds substantially to the Finat test method FTM 1.

Peel strength is determined after 24 hours and after 7 days from bonding under defined storage conditions:

(a) 24 h in standard climate (23°C, 50% relative humidity)

(b) 7 days at elevated temperature (50°C, 50% r.h.) to simulate heat storage stability

(c) 7 days at elevated temperature and high humidity (50°C, 85% r.h.) to simulate heat and moisture storage stability

Determination of removability

Removability is assessed visually by examination of the residues and the failure pattern of the adhesive layer when the adhered film is removed from the brushed steel test surface is assessed. The rating of the failure mode is as follows: 1 Good Adhesion failure of the adhesive layer, adhesive residues remaining completely on the PE film, no residues on the steel test surface

2 Good Adhesive failure, optical imperfection

3 Acceptable Optical imperfection at border of adhesive area

4 Acceptable Light filmic residue or very light filmic residue

5 Acceptable/poor Filmic residue

6 Bad/inacceptable Cohesive failure of the adhesive layer, adhesive layer splits between steel test surface and PE carrier film, adhesive residues remain partly on the steel test surface and partly on the PE carrier film

The preferred target result is an ideally residue-free removal. Furthermore, peel strength for removing a PE film from brushed steel is ideally no more than 8 N/25 mm and at least 3 N/25 mm, more preferably from 3 to 7 N/25 mm after 24 hours storage at room temperature (23°C) and 50% r.h..

Sun test

50 mm wide test strips are bonded to test surface made of brushed steel (Ra = 2.5 m) by firmly rolling twice with a hand roller.

The test strips bonded to test surface are placed in a Suntest XLS+ apparatus (made by company Atlas) and exposed to intense light, including UV-A (300-800nm, 500 W/m²) for defined times:

(a) 1 day (1d) exposure of UV-A-light

(b) 3 days (3d) exposure of UV-A-light

After the storage, the condition of the adhered test strips is visually assessed, and the adhered test strip is removed from the test surface by hand. Removability is assessed in terms of ease of manual removal and failure mode when the adhered test film is removed from the brushed steel test surface.

The rating of the peel level is as follows:

0 Poor I acceptable Very easy/too low

1 Good Easy

2 Good Medium

3 Acceptable Difficult

4 Bad/inacceptable Very difficult

The rating of the failure mode is as follows:

1 Good Adhesion failure

2 Acceptable Light filmic residue or very light filmic residue

3 Acceptable/poor Filmic residue

4 Bad/inacceptable Cohesive failure The examples and results are summarized in Tables 1 and 2.

Table 1: Test results shear strength and removability F*: Failure mode; **CL: Crosslinker; *** without B1 (An3744)

The test data demonstrate that polyisobutylene emulsions with or without crosslinker (examples C1 and C2) are not well suited for protective film applications because of insufficient removability (cohesive failure of the adhesive layer). The test data demonstrate that polyacrylate adhesive dispersions (examples C3 and C4) lead to comparatively low adhesion with low peel strength on challenging rough surfaces such as brushed steel.

The test data show that blends of polyacrylate adhesive dispersions with polyisobutylene emulsions (E1 and E2) result in improved peel strength on rough surfaces combined with good removability (no remaining adhesives residues on the test surface after removal of the protective film). The test data demonstrate an additional benefit of excellent aging behavior in elevated temperature and high humidity conditions when an additional acrylate polymer of higher Tg is added (example E2). The test data also demonstrate that sufficient peel strength of at least 3 N/25 mm (e.g. after 24 hours storage at room temperature (23°C)) cannot be obtained using acrylate polymer of higher Tg (An3744) as sole acrylate polymer B.

Table 2: Test results sun test

**CL:Crosslinker

It is shown that the inventive examples show good to acceptable performance in 1 d and 3 d sun test. The test data also demonstrate that the peel level is too low (i.e. test strip can removed very easily) after 3 d sun test when acrylate polymer of higher Tg (An3744) is used as sole acrylate polymer B.

Claims

1 . Aqueous pressure-sensitive adhesive composition comprising

(A) an aqueous polyisobutene emulsion, said polyisobutene having a particle size D50 not greater than

100 m, preferably from 250 nm to 50 m, measured by laser diffraction and having a viscosity average molecular weight Mv of at least 45,000 g/mol, preferably at least 80,000 g/mol, measured as described in the examples;

(C) at least one crosslinking agent for the pressure-sensitive adhesive acrylate polymer; wherein glass transition temperatures are measured by differential scanning calorimetry according to ASTM D 3418-08 as the midpoint temperature when evaluating the second heating curve at a heating rate of 20°C/min.

2. Aqueous pressure-sensitive adhesive composition according to claim 1, wherein the aqueous polyisobutene emulsion (A) comprises at least one first polyisobutene with a viscosity average molecular weight Mv of from 45,000 g/mol to 90,000 g/mol and at least one second polyisobutene with a viscosity average molecular weight Mv of more than 90,000 g/mol.

3. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 2, wherein the pressuresensitive adhesive acrylate polymer (B) is made by emulsion polymerization of

4. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 3, wherein the pressuresensitive adhesive acrylate polymer (B) or the polyisobutene (A) or both have a bimodal or multi-modal particle size distribution, preferably the polyisobutene (A) has a bimodal or multi-modal particle size distribution and the pressure-sensitive adhesive acrylate polymer (B) has a monomodal particle size distribution

5. Aqueous pressure-sensitive adhesive composition according to any one of the preceding claims, wherein the acrylic acid alkyl ester monomers (a) are used in an amount of from 75 to 99 wt.%, preferably 80 to 98 wt.%, more preferably from 91 to 98 wt.%, based on the total amount of monomers and are one or more selected from ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, 2-octyl acrylate and isooctyl acrylate.

6. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 5, wherein the monomers (b) having at least one acid group are used in an amount of more than 1 and up to 4.5 wt.%, based on the total amount of monomers, and are one or more selected from acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, cratonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrenesulfonic acid, acrylamidoglycolic acid, acrylamidomethylpropane sulfonic acid, sulfopropyl acrylate, sulfopropyl methacrylate, and their respective anhydrides.

7. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 6, wherein the monomers (d) are used in amounts of from 0 to 15 wt.%, based on the total amount of monomers, and are methyl acrylate, methyl methacrylate or a mixture thereof.

8. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 7, wherein monomers (c2) are used in amounts of from 0 to 20 wt.%, preferably 0.1 to 10 wt.%, based on the total amount of monomers, and are selected from hydroxyalkyl (meth)acrylates having from 1 to 10 C atoms in the hydroxyalkyl group, vinyl esters of carboxylic acids comprising up to 20 C atoms, vinylaromatics having up to 20 C atoms, ethylenically unsaturated nitriles, amides of ethylenically unsaturated carboxylic acids, N-alkylolamides of ethylenically unsaturated carboxylic acids, phenyloxyethyl glycol mono(meth)acrylate, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 C atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, ethylenically unsaturated monomers containing amino groups, bifunctional monomers which as well as an ethylenically unsaturated double bond have at least one glycidyl group, oxazoline group, ureido group, ureido-analogous group or carbonyl group, preferably diacetone acrylamide, and crosslinking monomers which have more than one free-radically polymerizable ethylenically unsaturated group, or mixtures of these monomers.

9. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 8, wherein the weight ratio of polyisobutene (A) to pressure-sensitive adhesive acrylate polymer (B) is from 1.5:1 to 1 :19, preferably from 1.5:1 to 1 :6, more preferably from 1 :1 to 1:6, more preferably from 1 :1 to 1 :4, more preferably from 1 :1.5 to 1 :4, more preferably from 1 :2 to 1 :4, based on solid content of (A) and (B).

10. Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 9 additionally comprising at least one further acrylate polymer (B1) dispersed in water with a glass transition temperature of more than - 10 °C, preferably at least -7 °C.

11 . Aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 10 comprising the at least one crosslinking agent (C) in an amount of 0.01 to 5 parts by weight, based on 100 parts by weight of aqueous pressure-sensitive adhesive acrylate polymer, selected from one or more of aminotriazines, isocyanurates formed from diisocyanates and having at least two isocyanate groups, compounds having at least one carbodiimide group, chemically capped isocyanates, encapsulated isocyanates, encapsulated uretdiones, biurets, allophanates, aziridines, oxazolines, and epoxides.

12. Aqueous pressure-sensitive adhesive composition according to any of claims 1 to 11, additionally comprising at least one tackifier (D) in amounts of from 5 to 100 parts by weight per 100 parts by weight of pressure-sensitive adhesive acrylate polymer based on solids, preferably selected from rosins, rosin esters, terpene resins and their mixtures.

13. Aqueous pressure-sensitive adhesive composition according to any one of the preceding claims, wherein the pressure-sensitive adhesive acrylate polymer is partly or completely made of partly or fully bio-based monomers, preferably selected from partly bio-based esters of (meth)acrylic acid and bio-based alcohols, preferably bio-based ethanol, bio-based n-butanol, bio-based iso-butanol, bio-based iso-pentanol, bio-based 2- octanol or bio-based n-heptanol; and fully bio-based esters of bio-based acrylic acid and bio-based alcohols.

14. Self-adhesive article, preferably a self-adhesive label, a self-adhesive tape or a self-adhesive films including graphic films and protective films, comprising a substrate, preferably selected from paper, plastic films and metal foils, and a first pressure-sensitive adhesive layer made from an aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 13.

15. Self-adhesive article according to the preceding claim comprising the first pressure-sensitive adhesive layer and an additional layer selected from a primer layer and a second adhesive layer or comprising a primer layer, the first pressure-sensitive adhesive layer and a second pressure-sensitive adhesive layer.

16. Method of making a self-adhesive article according to any one of claims 14 to 15, wherein

(1) a substrate is provided which is optionally pre-coated with a primer, and its surface is optionally corona- treated, and (2) an aqueous pressure-sensitive adhesive layer is applied to the substrate, wherein the pressure-sensitive adhesive layer is formed by coating the optionally pre-coated substrate with an aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 12 and drying.

17. Use of an aqueous pressure-sensitive adhesive composition according to any one of claims 1 to 13 for making a removable protective film comprising a substrate made of polyolefin plastic film, preferably selected from polyethylene and polypropylene, coated with said aqueous pressure-sensitive adhesive composition.