The invention relates to a pressure sensitive adhesive (PSA) and to a process for preparing it. The present invention also relates to a self-adhesive, removable article comprising the pressure sensitive adhesive and to a process for producing the self-adhesive, removable article.

Removable pressure sensitive adhesives are used on a variety of products including labels, tapes, films, and the like, to enable the product to adhere to a substrate and then to later be removed from the substrate without difficulty and without leaving a stain or residue. To be suitable for such applications, the pressure sensitive adhesive must have good adhesion with low peel strength and must not exhibit a significant increase in adhesion over time.

US2004/0143058A discloses a removable, water-whitening resistant pressure sensitive adhesive comprising a crosslinked aqueous emulsion polymer comprising:

(a) at least one hydrophobic monomer;

(b) greater than 1 wt % of at least one hydrophilic monomer;

(c) greater than 3 wt % of at least one mono-olefinically unsaturated monomer having an aldehyde or ketone group;

(d) optionally at least one partially hydrophilic monomer;

wherein said crosslinked aqueous emulsion polymer is crosslinked with at least one polyhydrazide crosslinker. In this application, a two-component (2K) system is provided and polyfunctional aziridine crosslinker is required to provide better removability and water whitening resistance.

W097/27231A1 discloses an emulsion composition useful in removable pressure sensitive adhesives and having a gel content greater than 90% comprising the polymerization product of a)

96 parts monofunctional acrylate monomer having one to twelve carbon atoms in the alkyl chain, and b) 4 parts multifunctional acrylate monomer. Example 2 of this application shows adhesive delamination after dwell one week at 72 °F, which indicates a bad anchorage on the paper.

Currently available removable pressure sensitive adhesives which demonstrate good anchorage to the film also exhibit a significant increase in adhesion over time. Accordingly, there is a need for a pressure sensitive adhesive which exhibits good anchorage to the paper or film and does not exhibit a significant increase in adhesion over time.

It was an object of the invention to provide a pressure sensitive adhesive which is a one component (1 K) adhesive and exhibits good anchorage to the paper or film and does not exhibit a significant increase in adhesion over time.

The object of the invention is achieved by a pressure sensitive adhesive comprising

A) a polymer synthesized from

a) 96 to 99.85% by weight, based on the total amount of monomers for the synthesis of polymer A), of two or more monoethylenically unsaturated monomers, b) 0.05 to 1 .0% by weight, based on the total amount of monomers for the synthesis of polymer A), of at least one monomer containing at least two nonconjugated polymerizable vinyl groups,

c) 0.1 to 3.0% by weight, based on the total amount of monomers for the synthesis of polymer A), of at least one monoethylenically unsaturated monomer having at least one keto or aldehyde group, wherein monomer c) is different from monomer a), B) at least one compound which comprises at least two functional groups reactive to the keto or aldehyde group,

wherein the synthesis of the polymer A) is carried out in the presence of 0.08 to 1 .5 mol%, based on a total amount of monomers for the synthesis of polymer A), of at least one chain transfer agent.

One aspect of the present invention is directed to a pressure sensitive adhesive comprising

A) a polymer synthesized from

a) 96 to 99.85% by weight, based on the total amount of monomers for the synthesis of polymer A), of two or more monoethylenically unsaturated monomers, b) 0.05 to 1 .0% by weight, based on the total amount of monomers for the synthesis of polymer A), of at least one monomer containing at least two nonconjugated polymerizable vinyl groups,

c) 0.1 to 3.0% by weight, based on the total amount of monomers for the synthesis of polymer A), of at least one monoethylenically unsaturated monomer having at least one keto or aldehyde group, wherein monomer c) is different from monomer a),

B) at least one compound which comprises at least two functional groups reactive to the keto or aldehyde group,

wherein the anchorage rate of the pressure sensitive adhesive is greater than 80% by weight.

The anchorage rate can be measured as follows:

applying the pressure sensitive adhesive on a paper in 17 g/m ² (solid content) and drying, then measuring the area percent of the residue adhesive on the paper after manually rubbing the adhesive layer one time by finger with constant force.

According to one preferred embodiment, the anchorage rate of the pressure sensitive adhesive is greater than 90% by weight. In a more preferred embodiment, the anchorage rate of the pressure sensitive adhesive is greater than 95% by weight. In the most preferred embodiment, the anchorage rate of the pressure sensitive adhesive is 100% by weight.

According to one preferred embodiment, the amount of the monoethylenically unsaturated monomer a) can be 97.5 to 98.5% by weight, based on the total amount of monomers for the synthesis of polymer A). The designation (meth)acrylate and similar designations are occasionally used below as an abbreviated notation for "acrylate or methacrylate". Usually, the monoethylenically unsaturated monomer a) can be selected from Ci to C10 alkyl (meth)acrylate, Ci to C10 hydroxyalkyl (meth)acrylate, preferably Ci to C6 hydroxyalkyl

(meth)acrylate, ethylenically unsaturated acid, vinyl esters of carboxylic acid containing 3 to 10 carbon atoms and vinylaromatics having up to 20 carbon atoms.

Examples of monomers of Ci to Cio alkyl (meth)acrylate include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, sec-butyl acrylate, pentyl acrylate, n-hexyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, sec-butyl methacrylate, pentyl methacrylate, n-hexyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate

The amount of Ci to Cio alkyl (meth)acrylate can be 70 to 95% by weight, preferably 80 to 90% by weight, based on the total amount of monomers for the synthesis of polymer A).

Examples of monomers of Ci to Cio hydroxyalkyl (meth)acrylate are 2-hydroxyethyl acrylate,

2- hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate,

3- hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 3-hydroxybutyl acrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 6-hydroxyhexyl acrylate, 6-hydroxyhexyl methacrylate, 3-hydroxy-2 -ethyl hexyl acrylate, 3-hydroxy-2 -ethyl hexyl methacrylate etc.

The amount of Ci to Cio hydroxyalkyl (meth)acrylate can be 0.5 to 8% by weight, preferably 1 to 5% by weight, based on the total amount of monomers for the synthesis of polymer A). The inventors of the present invention find that the Ci to Cio hydroxyalkyl (meth)acrylate can further improve the anchorage property of the pressure sensitive adhesive.

The ethylenically unsaturated acid can be a sulfonic acid, phosphoric acid or, preferably, carboxylic acid. In one preferred embodiment, the ethylenically unsaturated acid contains 10 carbon atoms or less, preferably 3 to 6 carbon atoms.

Suitable examples of the ethylenically unsaturated acid include (meth)acrylic acid, maleic acid, fumaric acid and itaconic acid, preferably, acrylic and/or methacrylic acid.

The amount of ethylenically unsaturated acid can be 0.1 to 5% by weight, preferably 0.2 to 2% by weight, based on the total amount of monomers for the synthesis of polymer A).

Examples of vinyl esters of carboxylic acid containing 1 to 10 carbon atoms, preferably 2 to 6 carbon atoms include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl neopentanoate, vinyl hexanoate, vinyl octanoate and vinyl 2-ethylhexanoate.

The amount of vinyl esters of carboxylic acid containing 1 to 10 carbon atoms can be 2 to 15% by weight, preferably 5 to 12% by weight, based on the total amount of monomers for the synthesis of polymer A).

Suitable vinylaromatic compounds include vinyltoluene, for example, a- and β-methylstyrene, obutylstyrene, 4-n-butylstyrene, 4-n-decylstyrene, and preferably, styrene. The amount of vinylaromatic compounds can be 0 to 5% by weight, preferably 0 to 2% by weight, based on the total amount of monomers for the synthesis of polymer A).

In one preferred embodiment, the following monomers are used as monomers a)

70 to 95% by weight, based on the total amount of monomers for the synthesis of polymer A), of Ci to Cio alkyl (meth)acrylate,

0.5 to 8% by weight, based on the total amount of monomers for the synthesis of polymer A), of Ci to Cio hydroxyalkyl (meth)acrylate,

0.1 to 5% by weight, based on the total amount of monomers for the synthesis of polymer A), of ethylenically unsaturated acid, and

2 to 15% by weight, based on the total amount of monomers for the synthesis of polymer A), of vinyl esters of carboxylic acid containing 1 to 10 carbon atoms.

In one more preferred embodiment, the following monomers are used as monomers a)

80 to 90% by weight, based on the total amount of monomers for the synthesis of polymer A), of Ci to Cio alkyl (meth)acrylate,

1 to 5% by weight, based on the total amount of monomers for the synthesis of polymer A), of Ci to Cio hydroxyalkyl (meth)acrylate,

0.2 to 2% by weight, based on the total amount of monomers for the synthesis of polymer A), of ethylenically unsaturated acid, and

5 to 12% by weight, based on the total amount of monomers for the synthesis of polymer A), of vinyl esters of carboxylic acid containing 1 to 10 carbon atoms.

Suitable examples of monomer b) include di(meth)acrylates of dihydric saturated alcohols containing 2 to 6 carbon atoms, (meth)acrylic and methacrylic esters of alcohols having more than 2 OH groups, di(meth)acrylates of the oligomers of ethylene oxide and/or propylene oxide, vinyl acrylate, vinyl methacrylate, vinyl itaconate, divinyl adipate, butanediol divinyl ether,

trimethylolpropane trivinyl ether, allyl acrylate, allyl methacrylate, pentaerythritol triallyl ether, methylenebis(meth)acrylamide, divinylbenzene. Di(meth)acrylates of dihydric saturated alcohols containing 2 to 6 carbon atoms can be selected from ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1 ,2-propylene glycol diacrylate, 1 ,2-propylene glycol dimethacrylate, butanediol di(meth)acrylates, for example butane-1 ,4-diol diacrylate, butane-1 ,4-diol dimethacrylate, hexanediol diacrylate, hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, 3-methylpentanediol diacrylate and 3-methylpentanediol dimethacrylate.

Examples of acrylic and methacrylic esters of alcohols having more than 2 OH groups include trimethylolpropane triacrylate and trimethylolpropane trimethacrylate.

Examples of di(meth)acrylates of the oligomers of ethylene oxide and/or propylene oxide are diethylene glycol diacrylate, diethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, tetraethylene glycol diacrylate and tetraethylene glycol dimethacrylate.

In one preferred embodiment, the amount of monomer b) is 0.1 to 0.7% by weight, preferably 0.1 to 0.3% by weight, based on the total amount of monomers for the synthesis of polymer A).

Monomer c) is a monoethylenically unsaturated compound having at least one keto or aldehyde group and is different from monomer a). Suitable examples of monomer c) are acrolein,

methacrolein, vinyl alkyl ketones having 1 to 20, preferably 1 to 10 carbon atoms in the alkyl radical, formylstyrene, (meth)acrylic acid C3 to C10 alkyl esters having one or two keto or aldehyde or one aldehyde and one keto group in the alkyl radical, preferably acetoacetyl (meth)acrylate,

acetoacetoxyethyl (meth)acrylate, and diacetone acrylamide.

In one preferred embodiment, the amount of monomer c) is 0.5 to 3% by weight, preferably 1 to 2% by weight, based on the total amount of monomers for the synthesis of polymer A).

In one preferred embodiment, polymer A) is synthesized in a single step reaction.

In one preferred embodiment, polymer A) is prepared by emulsion polymerization, and is therefore an emulsion polymer.

In the case of emulsion polymerization, use is made of ionic and/or nonionic emulsifiers and/or protective colloids and/or stabilizers as surface-active compounds. A detailed description of suitable protective colloids can be found in Houben-Weyl, Methoden der organischen Chemie, Volume XIV/1 , Makromolekulare Stoffe [Macromolecular compounds], Georg-Thieme-Verlag, Stuttgart, 1961 , pp. 41 1 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 normally below 2000 g/mol. Where mixtures of surface-active substances are used, the individual components must be compatible with one another. It is preferred to use anionic and nonionic emulsifiers as surface-active substances. Common accompanying emulsifiers are, for example, ethoxylated fatty alcohols (EO units: 3 to 50, alkyl: Cs to C36), ethoxylated mono-, di-, and tri-alkylphenols (EO units: 3 to 50, alkyl: C ₄ to C9), alkali metal salts of dialkyi esters of sulfosuccinic acid, and also alkali metal salts and ammonium salts of alkyl sulfates (alkyl: Cs to C12), of ethoxylated alkanols (EO units: 4 to 30, alkyl: C12 to Cie), of ethoxylated alkylphenols (EO units: 3 to 50, alkyl: C ₄ to C9), of alkylsulfonic acids (alkyl: C12 to Cis), and of alkylarylsulfonic acids (alkyl: C9 to

Further suitable emulsifiers are compounds of the general formula I

where R ⁵ and R ⁶ are hydrogen or C ₄ to Cie alkyl but are not simultaneously hydrogen, and X and Y can be alkali metal ions and/or ammonium ions. With preference, R ⁵ and R ⁶ are linear or branched alkyl radicals having from 6 to 18 carbon atoms or hydrogen and in particular having 6, 12 or 16 carbon atoms, with R ⁵ and R ⁶ not both simultaneously being hydrogen. X and Y are preferably sodium, potassium or ammonium ions, sodium being particularly preferred. Particularly

advantageous compounds I I are those in which X and Y are sodium, R ⁵ is a branched alkyl radical of 12 carbon atoms, and R ⁶ is hydrogen or R ⁵ . One example is Dowfax 2A1 (trade mark of the Dow Chemical Company).

Suitable emulsifiers can also be found in Houben-Weyl, Methoden der organischen Chemie, Volume 14/1 , Makromolekulare Stoffe, Georg Thieme Verlag, Stuttgart, 1961 , pages 192 to 208.

Examples of emulsifier trade names are Calfax- DB 45, Dowfax- · 2A1 , Emulan - · NP 50, Dextrol - OC 50, Emulgator- 825, Emulgator 825 S, Emulan - · OG, Texapon - · NSO,

Nekanil - 904 S, Lumiten - l-RA, Lumiten - E 3065, Disponil - FES 27, Disponil - FES 77, Disponil - BES 20, Disponil - LDBS, Disponil - SDS, Lutensol - AT 18, Steinapol VSL,

Hydropalat 885 and Emulphor NPS 25. For the present invention, ionic emulsifiers or protective colloids are preferred. With particular preference these are ionic emulsifiers, especially salts and acids, such as carboxylic acids, sulfonic acids and sulfates, sulfonates or carboxylates.

Preferred emulsifiers are those containing sulfate or sulfonate groups. Particular preference is given to fatty alcohol ether sulfates and to sulfosuccinic esters, and very particular preference is given to mixtures of these two. The surface-active substance is commonly used in amounts of 0.1 to 10 parts by weight, preferably 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 known as reduction-oxidation (redox) initiator systems.

The redox 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 emulsion polymerization initiators already mentioned above.

The reducing component comprises, for example, alkali metal salts of sulfurous acid, such as sodium sulfite, sodium hydrogen sulfite, alkali metal salts of disulfurous acid such as sodium disulfite, bisulfite addition compounds with aliphatic aldehydes and ketones, such as acetone bisulfite, or reducing agents such as hydroxymethanesulfinic acid and its salts, or ascorbic acid. The redox initiator systems may be used together with soluble metal compounds whose metallic component may exist in a plurality of valence states.

Examples of customary redox initiator systems include ascorbic acid/iron(ll) sulfate/sodium peroxodisulfate, tert-butyl hydroperoxide/sodium disulfite, and tert-butyl hydroperoxide/sodium hydroxymethanesulfinate. The individual components, the reducing component for example, may also be mixtures: for example, a mixture of the sodium salt of hydroxymethanesulfinic acid with sodium disulfite. These compounds are mostly used 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. The concentration of the aqueous solution is generally from 0.1 to 30% by weight, preferably from 0.5 to 20% by weight, with particular preference 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 for the emulsion polymerization.

According to the present invention, the synthesis of polymer A) is carried out in the presence of 0.08 to 1 .5 mol%, based on a total amount of monomers for the synthesis of polymer A), of a chain transfer agent. In one preferred embodiment, the synthesis of polymer A) is carried out in the presence of 0.2 to 0.8 mol%, preferably 0.4 to 0.8mol%, based on a total amount of monomers for the synthesis of polymer A), of a chain transfer agent. Suitable examples of the chain transfer agent include compounds containing a thiol group, for example mercaptans, such as without limitation, ethyl mercaptan, n-propyl mercaptan, n-butyl mercaptan, isobutyl mercaptan, t-butyl mercaptan, n-amyl mercaptan, isoamyl mercaptan, t-amyl mercaptan, n-hexyl mercaptan, cyclohexyl mercaptan, n-octyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan, mercapto carboxylic acids and their esters, such as without limitation, 2-ethylhexyl thioglycolate, methyl mercaptopropionate and 3-mercaptopropionic acid, alcohols, such as isopropanol, isobutanol, lauryl alcohol and t-octyl alcohol, halogenated compounds such as carbon tetrachloride, tetrachloroethylene, tricholoro-bromoethane, and combinations thereof.

The emulsion polymerization takes place in general at from 30 to 130 °C, preferably from 50 to 95 °C. The polymerization medium may be composed either of water alone or of mixtures of water and water-miscible liquids such as methanol. Preferably, only water is used.

The emulsion polymerization may be conducted 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 and heated to the polymerization temperature, the polymerization of this initial charge is begun, and then the remainder of the polymerization mixture is supplied to the polymerization zone, usually by way of two or more spatially separate feed streams, of which one or more comprise the monomers in straight or emulsified form, this addition being made continuously, in stages or under a

concentration gradient, and polymerization being maintained during said addition.

It is also possible, in order, for example, to set the particle size more effectively, to include a polymer seed in the initial charge to the polymerization. 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 the skilled worker. It may either be included in its entirety in the initial charge to the polymerization vessel or else introduced, continuously or in stages, at the rate at which it is consumed in the course of the free-radical aqueous emulsion polymerization. In each specific case this will depend both on the chemical nature of the initiator system and on the polymerization temperature. It is preferred to include one portion in the initial charge and to supply the remainder to the polymerization zone at the rate at which it is consumed.

In order to reduce the residual monomers, it is common to add initiator after the end of the actual emulsion polymerization as well, i.e., after a monomer conversion of at least 95%. With the feed process, the individual components can be added to the reactor from the top, through the side, or from below, through the reactor floor.

In the case of emulsion polymerization, aqueous polymer dispersions with solid content of generally from 15 to 75% by weight, preferably from 40 to 75% by weight, are obtained.

The polymer thus prepared is used preferably in the form of its aqueous dispersion.

The average particle size of the polymer particles dispersed in the aqueous dispersion is preferably smaller than 300 nm, in particular smaller than 200 nm. With particular preference the average particle size is situated between 140 and 200 nm.

By average particle size here is meant the dso value of the particle size distribution, i.e., 50% by weight of the total mass of all particles have a smaller particle diameter than the dso value. The particle size distribution can be determined conventionally using the analytical ultracentrifuge (W. Machtle, Makromolekulare Chemie 185 (1984), pages 1025 - 1039).

The pH of the polymer dispersion is preferably adjusted to a pH of more than 4.5, and in particular to a pH of between 5 and 8.

The glass transition temperature of the polymer is preferably from -60 to 0 °C, with particular preference from -60 to -10 °C, and with very particular preference from -50 to -20 °C.

The glass transition temperature are determined by Differential Scanning Calorimetrie (ASTM D 3418-08, "midpoint temperature" of second heating curve, heating rate 20 °C/min.).

One embodiment of the present invention relates to the polymer A) as defined above.

The pressure sensitive adhesive further comprises the compound B) defined at the outset.

Compound B) has greater than 2 functional groups, in particular 2 to 5 functional groups, more preferably 2 or 3 functional groups, very preferably 2 functional groups reactive to the keto or aldehyde group. Examples of suitable functional groups include hydrazide, hydroxylamine or oxime ether or amino groups. Hydrazide groups are particularly preferred.

Suitable compounds having hydrazide groups are, for example, polycarboxylic hydrazides having a molar weight of up to 500 g/mol.

Particularly preferred hydrazide compounds are dicarboxylic dihydrazides having preferably 2 to 10 carbon atoms.

Examples that may be mentioned include oxalic dihydrazide, malonic dihydrazide, succinic dihydrazide, glutaric dihydrazide, adipic dihydrazide, sebacic dihydrazide, maleic dihydrazide, fumaric dihydrazide, itaconic dihydrazide and/or isophthalic dihydrazide. The following are of particular interest: adipic dihydrazide, sebacic dihydrazide, and isophthalic dihydrazide.

Suitable compounds containing hydroxylamine groups or oxime ether groups are specified for example in WO 93/25588.

These are, for example, hydroxylamine derivatives of the general formula II (H ₂ N - 0) - ₂ A (II), in which A is a saturated or unsaturated aliphatic, linear or branched hydrocarbon radical of 2 to 12 carbon atoms, which may be interrupted by 1 to 3 nonadjacent oxygen atoms, or oxime ethers of the formula

in which A is as defined above, n is 2, 3 or 4, and R ¹ and R ² independently of one another are a Ci to Cio alkyl, Ci to Cio alkoxy or Cs to Cio aryl radical, which may also comprise 1 to 3 nonadjacent nitrogen, oxygen or sulfur atoms in the carbon chain or in the carbon ring and may be substituted by 1 to 3 Ci to C ₄ alkyl or alkoxy groups, R ¹ or R ² can stand for a hydrogen atom, or R ¹ or R ² together form a bridge of 2 to 14 carbon atoms, it also being possible for some of the carbon atoms to be part of an aromatic ring system.

The variable A in formulae II and III is preferably a hydrocarbon chain of 2 to 8 carbon atoms and n is preferably 2. The radicals R ¹ and R ² are each preferably a hydrogen atom or a Ci to C6 alkoxy group. In the case of the hydrogen atom it is possible for only one of the radicals, R ¹ or R ² , to be a hydrogen atom.

Examples of suitable compounds containing amino groups include ethylenediamine,

propylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, polyethyleneimines, partly hydrolyzed

polyvinylformamides, cyclohexanediamine, and xylylenediamine.

The compound B) may be added to the polymer A) or to the dispersion of the polymer at any point in time. In the aqueous dispersion there is as yet no crosslinking with the keto or aldehyde groups. Only in the course of drying does crosslinking occur on the coated substrate.

The amount of the compound B) is preferably such that the molar ratio of the functional groups of the compound B) to the keto and/or aldehyde groups of the polymer is 1 :10 to 10:1 , in particular 1 :5 to 5:1 , more preferably 1 :2 to 2:1 , and most preferably 1 :1 .5 to 1 .5:1.

The weight fraction of compound B) is preferably 0.5 to 3, in particular 1 to 2 parts by weight per 100 parts by weight of polymer A).

Polymer A), or the aqueous dispersion of the polymer, can be mixed in a simple way with compound B). The resulting mixture is stable on storage.

The pressure sensitive adhesives (PSAs) may comprise the aqueous dispersion of the polymer A) and compound B).

The PSAs may comprise at least one further additive selected from fillers, colorants, flow control agents, plasticizers, thickeners and tackifiers (tackifying resin), for example. Examples of tackifiers are natural resins, such as rosins and their derivatives formed by disproportionation or

isomerization, polymerization, dimerization and/or hydrogenation. They may be present in their salt form (with, for example, monovalent or polyvalent counterions (cations)) or, preferably, in their esterified form. Alcohols used for the esterification may be monohydric or polyhydric. Examples are methanol, ethanediol, diethylene glycol, triethylene glycol, 1 ,2,3-propanethiol, and pentaerythritol. Also used are hydrocarbon resins, e.g., 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, and vinyltoluene. Other compounds increasingly being used as tackifiers include polyacrylates which have a low molar weight. These polyacrylates preferably have a weight-average molecular weight M _w of less than 30 000. With preference the polyacrylates are composed of at least 60% by weight, in particular at least 80% by weight, of Ci to C& alkyl (meth)acrylates. Preferred tackifiers are natural or chemically modified rosins. Rosins are composed predominantly of abietic acid or its derivatives.

The amount by weight of tackifiers is preferably from 5 to 100 parts by weight, with particular preference from 10 to 50 parts by weight, per 100 parts by weight of polymer (solids/solids).

A further embodiment of the invention relates to a process for the preparation of the pressure sensitive adhesive of the present invention, which comprises

i) free-radically polymerizing

a) two or more monoethylenically unsaturated monomers,

b) at least one monomer containing at least two nonconjugated polymerizable vinyl groups, c) at least one monoethylenically unsaturated compound having at least one keto or aldehyde group, wherein monomer c) is different from monomer a),

in the presence of at least one chain transfer agent to obtain polymer A), and

ii) mixing the polymer A) obtained in step i) with at least one compound B) which comprises at least two functional groups reactive to the keto or aldehyde group.

In one preferred embodiment, polymer A) is prepared by emulsion polymerization as described above.

In a further preferred embodiment, step i) is carried out in a single step reaction.

The polymer obtained in step i) can be neutralized with a neutralizing agent before mixing with compound B. The neutralizing agent can be selected from hydroxide of alkali metal, for example, sodium hydroxide and potassium hydroxide, and ammonia. The pressure sensitive adhesive of the invention can be used for producing self-adhesive, removable article. One embodiment of the invention is self-adhesive, removable article which comprises a backing material and the pressure sensitive adhesive of the present invention, wherein at least part of the functional groups of compound B) has reacted with the keto or aldehyde group. The article is at least partly coated with the pressure sensitive adhesive. The self-adhesive article can preferably be removed again after bonding. The self-adhesive, removable article may be, for example, sheets, tapes or labels. Examples of suitable backing materials include paper (for example art paper) and polymeric films. In the case of self-adhesive tapes of the invention, the tapes may be single-sidedly or double-sidedly coated tapes comprising the pressure sensitive adhesive above. In the case of self-adhesive labels of the invention, the labels may, be of paper (for example art paper) or of a thermoplastic film. Suitable thermoplastic films include, for example, films of polyolefins (e.g., polyethylene, polypropylene), polyolefincopolymers, films of polyesters (e.g., polyethylene terephthalate) or polyacetate. The surfaces of the thermoplastic polymer films are preferably corona-treated. The labels are coated with adhesive on one side. Preferred substrates for the self-adhesive articles are paper and polymer films. Preferred self-adhesive articles are paper labels.

The articles are coated on at least one surface at least partly with a pressure sensitive adhesive of the invention. The adhesive may be applied to the articles by typical methods such as knife coating or spreading. The amount applied is preferably 0.1 to 20 g, more preferably 2 to 18 g, of solid per m ² . Application is generally followed by a drying step for removing the water and/or the solvents. The substrates to which the self-adhesive, removable article can advantageously be applied may be, for example, metal, wood, glass, paper or plastic. The self-adhesive articles are suitable more particularly for bonding to packaging surfaces, cartons, plastic packaging, books, windows, motor vehicle bodies or bodywork parts. The self-adhesive, removable article of preferred embodiments can be removed from the articles again by hand, without residue of adhesive on the article.

Adhesion to the articles is good, and yet the sheets, tapes, and labels are readily removable. This good removability is retained even after a relatively long time. Paper labels exhibit good

strikethrough behavior and good printability. In one preferred embodiment, the self-adhesive, removable article is a paper label. The paper label has a first surface and a second surface, the first surface being self-adhesive and being coated at least partly with a pressure-sensitive adhesive of the invention, and the second surface being printed, or the second surface or the label being at least partly colored. This coloration may have been produced, for example, by colored coating with pigments or dyes, by colored printing, or, in thermal papers, by exposure to heat.

A further embodiment of the invention relates to the use of a pressure-sensitive adhesive of the invention for producing self-adhesive, removable articles. The invention also relates to a process for producing self-adhesive, removable articles, which involves providing a pressure-sensitive adhesive of the invention and coating a backing material at least partly with the pressure-sensitive adhesive.

The pressure sensitive adhesive according to the present invention is a one component (1 K) adhesive and does not react in wet condition. The pressure sensitive adhesive of the invention has good performance properties, which exhibits good anchorage to the paper or film as backing material after coating on the paper or film, and does not exhibit a significant increase in adhesion over time. Examples

Raw material

SF-A: Sodium Lauryl Ether Sulfate

SF-B: Sodium Dodecyl Diphenyl Ether Disulfonate

Lumiten l-SC: 45-65% sodium di-ethylhexyl sulfosuccinate, 15-25% isotridecanol ethoxylate and 15-30% water.

Example 1

In a 1000 ml flask equipped with a stirrer, a nitrogen inlet/outlet, a condenser and feeding ports, 70 g water as an initial charge was added. Then the reactor was heated to 90 °C, and 5 g sodium persulfate aqueous solution (7%) was added into the flask. A feed emulsion was added into the flask together with 25 g sodium persulfate aqueous solution (7%) over 3.5 h. The feed emulsion comprises the following components:

9.9 g SF-A,

1 .4 g SF-B,

30 g water,

2.5 g sodium hydroxide,

251 g 2-ethylhexyl acrylate,

25 g methyl methacrylate,

26 g vinyl acetate,

6 g 2-hydroxypropyl acrylate,

1 .6 g acrylic acid,

2.3 g butanediol diacrylate (BDDA),

4.7 g diacetone acrylamide (DAAM),

0.4 g 2-ethylhexyl thioglycolate (EHTG)

Then, 6 g sodium acetonebisulfate (13% aqueous solution) and 6 g tert-butyl hydroperxide (10% aqueous solution) are added over 1 h to remove the residual monomers. The final product (Sample 2) was obtained with a solid content of about 60%, pH ~6.

By adjusting the amount of BDDA DAAM/EHTG and repeating the above mentioned procedure, sample 1 , samples 3 to 5 and comparative samples 1 to 4 were obtained. For comparative sample 5, the procedure of the preparation of sample 2 was repeated with the difference that 2-hydroxypropyl acrylate was not added. The amounts of BDDA DAAM/EHTG of each sample are shown in tablel .

Table 1

Example 2 Each sample obtained in example 1 was neutralized with NaOH solution (8 wt%) to PH 7-9, and then formulated with 0.4% (based on the total weight of the neutralized sample) Lumiten l-SC aqueous solution(58 wt%) and Adipic acid dihydrazide (15 wt % solution, DAAM: Adipic acid dihydrazide =1 : 1 (w/w)) to form the corresponding formulations.

Performance Tests

Each formulation was coated on a silicone paper (white Glassine paper with a basis weight of 58 g/m ² ) in 17 g/m ² (solid content) and transferred to a art paper with a basis weight of 80 g/m ² . Three passes in each direction with a 2 kg FI NAT test roller are applied. Then, the coated art paper was dried at 80 °C for 3 minutes to obtain a tape.

90° peel test: a stainless-steel test panel (Rocholl stainless panel, at least 5 cm * 25 cm) is used as the substrate on which the tape is applied. The tape is rolled twice with 2 kg roller to firmly bond it to test panel. All specimens are tested after laminated 20 minutes and after aging at 60 °C for 14 days, respectively. The peel speed is 300 mm/min at a peel angle of 90 degree.

Anchorage rate test: manually rubbing the adhesive layer of the tape 1 to 3 times by finger with constant force and recording the area percent of the residue adhesive on the art paper, wherein three individual measurements are carried out for each sample.

All test results are shown in the following table 2.

Table 2

A in table 2 means adhesion break, i.e. there are no residual glue or shadow on the stainless-steel panel after peel.

F in table 2 means heavy shadow, i.e. there exists shadow (not residual glue) on the stainless-steel panel after peel. f in table 2 means weak shadow, i.e. there exists weak shadow (not residual glue) on the stainless-steel panel after peel.

The results in table 2 demonstrate that the tape prepared by the adhesive of the present invention remains easily removable from the substrates, exhibits good anchorage to the paper and does not exhibit a significant increase in adhesion over time.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.