WO2005054306A1

WO2005054306A1 - Process for preparing hydrogel skin adhesive under low oxygen conditions

Info

Publication number: WO2005054306A1
Application number: PCT/EP2004/013146
Authority: WO
Inventors: Felix Christian GÖRTH; Christian Hubert Weidl; Volker Frenz; Oskar Stephan
Original assignee: Basf Aktiengesellschaft
Priority date: 2003-11-28
Filing date: 2004-11-19
Publication date: 2005-06-16

Abstract

Abstract The present invention relates to a process of making polymerized hydrogels, in particular hydrogel adhesives which e.g. are capable of attaching to mammalian skin and can be used in various personal care products, such as waste-management articles, cosmetic and medical devices and a variety of functional articles to be worn by a human. The process described herein is characterized by influencing the adhesiveness by varying the oxygen content of the atmosphere present during the polymerization. Utilizing this method it is possible to influence one performance parameter (adhesiveness) independently from the others (e.g. rheology behaviour). This opens ways to a more easy and more flexible production of such hydrogels. In addition it is possible to obtain hydrogels with a combination of rheology and adhesiveness properties that were not or not so easily obtainable by conventional means. Lowering the oxygen content of the atmosphere at the polymerization zone also gives an additional benefit in lowering the amount of residual monomer(s).

Description

PROCESS FOR PREPARING HYDROGEL SKIN ADHESIVE UNDER LOW OXYGEN CONDITIONS

Description The present invention relates to a process of making polymerized hydrogels, in particular hydrogel adhesives which e.g. are capable of attaching to mammalian skin and can be used in various personal care products, such as waste-management articles, cosmetic and medical devices and a variety of functional articles to be worn by a human. The process described herein is characterized by influencing the adhesiveness by varying the oxygen content of the atmosphere present during the polymerization.

Adhesive hydrogels, in particular mammalian skin adhesives, their manufacture and use in consumer products such as medical and cosmetic devices, absorbent and other articles to be worn by a human have been described before in EP 1 025823, EP 1 025 866, WO 00 00235. Herein the need for secure attachment, stability of adhesion in presence of excess moisture, and painless removal are included. Other applications deal with the reduction of residual monomers by the use of scavengers and optimizing the material properties by the use of chain transfer agents (WO 04/003034). The material properties as described in the above mentioned patents are typically a- chieved by varying the type of monomers and/or their concentration, the solid content of the formulation, the type and/or amount of crosslinker and/or initiator.

By varying one of these parameters not only one material property, but all properties such as rheology behavior, adhesiveness and others are influenced at the same time. For example higher solid content or increasing the amount of crosslinker gives harder gels that are less sticky. Thus certain combinations of rheology behavior and stickiness are not or not easily obtainable by conventional means. For the above mentioned application it is however sometimes necessary to have such a combination of material properties.

The current invention relates to the possibility of varying one performance parameter (e.g. adhesiveness) independently of the other by varying the oxygen content of the atmosphere present during the polymerization. In addition the current invention relates to an alternative process for making hydrogels having a peel force on PET of between 0.3 to 5.0 N/cm².

The process described in this application deals with the manufacture of hydrogels, especially hydrogel adhesives that are capable of attaching to mammalian skin.

The current invention relates to a process for making a hydrogel comprising 10-90 wt% water, 10-60 wt% of cross-linked hydrophilic polymer made from at least one starting monomer type, and 10-80 wt% of at least one polyol. This hydrogel is preferably adhe- sive. It has more preferably a peel force on PET of between 0.3 to 5.0 N/cm². The process of making the hydrogel takes place in an atmosphere, which is different to air, preferred has a reduced oxygen concentration. This concentration is in the range of about 0 ppm to about 10.000 ppm.

This process comprises a first step consisting in preparing said monomer(s) solution from 10-90 wt% water, from 10-60 wt% of said starting monomer(s) and from 5-80 wt%, preferably 10-80 wt%, most preferably 30-80 wt% of said polyol(s).

In a second step the so formed reaction mixture is polymerized to thereby form a hydrogel. In preparing hydrogels in accordance with the present invention, the ingredients will usually be mixed to provide a reaction mixture in the form of an initial pre-gel aqueous based liquid formulation, which is then converted into a gel by a free radical polymerization reaction. This may be achieved for example using conventional thermal initiators, redox initiators and/or photoinitiators or by ionizing radiation. Such free- radical polymerization initiators are well known in the art and can be present in quantities less than 5% by weight, preferably less than 1%, more oreferably less than 0.5%, most preferably less than 0.4%. Preferred concentration ranges are from 0.02% to 2%, more preferably from 0.02% to 0.4%. Photoinitiation is a preferred method and will u- sually be applied by subjecting the pre-gel reaction mixture containing an appropriate photoinitiation agent to UV light after it has been spread or coated as a layer on silico- ne-coated release paper or other solid or porous substrate.

According to the present invention the second step of polymerizing the pre-gel mixture is carried out in an atmosphere different to air. This can be achieved by e.g. making the polymerization in a closed channel. The channel is equipped with inlets for gas and outlets connected to an exhaust system. Suitable gases are inert gases like Nitrogen, Argon, Helium, Carbondioxide, Steam and the like or lean air. Lean air is air with a reduced oxygen content. Nitrogen is especially preferred.

The oxygen content in the polymerization zone (reaction/crosslinking zone) is lower than the one of air, preferably between 0 and 10000 ppm, more preferably between 1 , 5, 10, 20 or 50 and 5000 ppm, more preferably between 100 and 2000, most preferably between 200 und 1000 ppm.

For use in forming the homopolymer or co-polymer component of the polymerized hydrogel, suitable monomers or co-monomers can be acidic, neutral, basic, or zwitteri- onic. Among acidic monomers, suitable strong-acid types include those selected from the group of olefinically unsaturated aliphatic or aromatic sulfonic acids such as 3- sulfopropyl (meth) acrylate, 2-sulfoethyl (meth) acrylate, vinylsulfonic acid, styrene sulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid, methacrylic sulfonic acid and the like and the respective salts. Particularly preferred strong-acid type monomer is 2- acrylamido-2-methylpropanesulfonic acid and its salts. Among acidic monomers, suitable weak-acid types include those selected from the group of olefinically unsaturated carboxylic acids and carboxylic acid anhydrides such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid, ethacrylic acid, citroconic acid, fumaric acid and the like and the respective salts. Particularly preferred weak-acid type monomer is acrylic acid and its salts.

Examples of neutral monomers include N,N-dimethylacrylamide, acrylamide, N- isopropyl acrylamide, hydroxyethyl (meth)acrylate, alkyl (meth)acrylates, N-vinyl pyr- rolidone and the like as well as differnet esters of the weak acid type monomers. Examples of cationic monomers include N,N-dimethylaminoethyl (meth)acrylate, N,N- dimethylaminoethyl (meth)acrylamide and the respective quaternary salts and the like. Most preferably, the hydrogel compositions of the invention are based upon acrylic acid monomers and its salts.

The cross-linking between polymer chains creates a 3-dimensional matrix for the polymer, also referred to as gel form or hydrogel. Physical cross-linking refers to polymers having crosslinks that are not chemical covalent bonds but are of a physical nature such that for example there are areas in the 3 dimensional matrix having high crystal- unity or areas having a high glass transition temperature or areas having hydrophobic interactions. Chemical cross linking refers to polymers which are linked by covalent chemical bonds, The polymer can be chemically cross linked by radiation techniques such as UV, E beam, gamma or micro-wave radiation or by co-polymerizing the monomers with a di/polyfunctional crosslinker via the use e. g., of UV, thermal and/or redox polymerization initiators. The polymer can also be ionically crosslinked.

Suitable polyfunctional monomer crosslinkers include polyethyleneoxide di(meth)acrylates with varying PEG molecular weights, IRR280 (a PEG diacrylate available from UCB Chemical), trimethylolpropane ethyoxylate tri(methacrylate with varying ethyleneoxide molecular weights, IRR210 (an alkoxylated triacrylate available from UCB Chemicals), trimethylolpropane tri(meth)acrylate, divinylbenzene, pentae- rythritol triallyl ether, triallylamine, N,N-methylene-bis-acrylamide and others polyfunctional monomer crosslinkers known to the art. Preferred monomer crosslinkers include the polyfunctional diacrylates and triacrylates.

Chemical crosslinking can also be effected after polymerization by use of polyfunctional reagents capable of reacting with polymer functional groups such as ethyleneglycol diglycidyl ether, polyols such as glycerol, and other polyfunctional reagents known to the art.

Crosslinking can also be effected all or in part by ionic crosslinking wherein groups of opposite charge interact via ionic interactions. Suitable ionic crosslinking agents in- elude those known to the art including polyvalent cations such as Al ³⁺ and Ca ²⁺, di/poly-amines, di/poly-quatemary ammonium compounds, including polymeric poly- amines and quaternary ammonium compounds known to the art.

The hydrogel compositions described herein can comprise a humectant, or mixture of humectants (also referred as a plastisizer), which is preferably a liquid at room temperature. The humectant is selected such that the monomer and polymer may be solu- bilized or dispersed within. For embodiments wherein irradiation crosslinking is to be carried out, the humectant is desirably irradiation crosslinking compatible such that it does not significantly inhibit the irradiation crosslinking process of the polymer. The components of the humectant mixture are preferably hydrophilic and miscible with water.

Suitable humectants include alcohols, polyhydric alcohols such as glycerol and sorbitol, and glycols and ether glycols such as mono- or diethers of polyalkylene glycol, mono- or diester polyalkylene glycols, polyethylene glycols, glycolates, glycerol, sorbitan esters, esters of citric and tartaric acid, imidazoline derived amphoteric surfactants. Particularly preferred are polyhydric alcohols such as glycerol and sorbitol, polyethylene glycol, and mixtures thereof. Glycerol or sorbitol is especially preferred. The humectant comprises 5-80 wt% of the hydrogel.

Other common additives known in the art such as polymerization inhibitors, scavengers, salts, surfactants, soluble or dispersible polymers, buffers, preservatives, antioxi- dants, pigments, mineral fillers, and the like and mixtures thereof may also be com- prised within the adhesive composition in quantities up to 10% by weight each respectively.

The term polyols refer to alcohol compounds having more than one hydroxyl group. Polyols include polyhydric alcohols and are also called polyalcohols. As it was men- tioned previously, polyols are well known in the art as common additives for making hydrogels. Therefore, a method for reducing by-products formed from these polyols during polymerization, is particularly useful.

The above mentioned polymerization process of said starting monomer(s) is mostly conducted at a pH 3.5 to 8, preferably 4 to 6.5, more preferably 4.5 to 6.

The resultant hydrogel comprises preferably 20-70 wt% water.

In addition the preferred hydrogel contains less than 200 ppm, preferably less than 100 ppm, more preferably less than 50 ppm, even more preferably less than 20 ppm, most preferably less than 10 ppm of residual starting monomer(s). In a preferred embodiment of the present invention, is provided a process where the polymerization is conducted at least partly by photoinitiation polymerization. Photoinitiation will usually be applied by subjecting the pre-gel reaction mixture of monomer(s) containing an appropriate photoinitiation agent to U V light after it has been spread, coated, or extruded as a layer on silicone-coated release paper or other solid or porous substrate. The incident UV intensity, typically at a wavelength in the range from about 240 to about 400 nm overlaps to at least some degree with the UV absorption band of the photoinitiator and is of sufficient intensity and exposure duration (e.g., 120-36000 mW/cm²) to complete the polymerization of the reaction mixture.

Preferably the polymerization is conducted by UV curing, and the integrated UV intensity at wavelengths less than 280 nm, preferably less than 300 nm, more preferably less than 320 nm is less than 10%, preferably less than 7%, even more preferably less than 4%, most preferably less than 1 % of the total integrated UV intensity with wavelengths less than 400 nm.

Also preferably the polymerization is carried out under a total UVA energy ranging from 0.1-30 J/cm², preferably from 0.1-25 J/cm² , more preferably from 1-20 J/cm².

Such free radical photoinitiation agents or photoinitiators are well known in the art and can be present in quantities up to 5% by weight, preferably less than 1 %, more preferably less than 0.5%, and most preferably less than 0.4%. Such photoinitiators include type α-hydroxy-ketones and benzilidimethyl-ketals. Suitable photoinitiators include di- methylbenzylphenone (available under the trade name or Irgacure 651 from Ciba Spe- ciality Chemicals), 2-hydroxy-2-methyI-propiophenone (available under the trade name Darocur 1173 from Ciba Speciality Chemicals), 1 -hydroxycyclohexyl-phenyl ketone (available under the trade name Irgacure 184 from Ciba Speciality Chemicals), dietho- xyacetophenone, and 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-methylpropyl) ketone (available under the trade name of Irgacure 2959 from Ciba Speciality Chemicals) and Igarcure 500. Darocure 1173, Irgacure 2959 and Irgacure 184 are preferred photoinitiators. Irgacure 2959 and Irgacure 184 are particularly preferred. In the hydrogel compositions described in the present invention, Irgacure 2959 is the most preferred photoinitiator. Combinations of photoinitiators can also be used. In addition, polymerization can be carried out by using thermal initiator(s) and/or redox initiator(s) well known to the art or one or more of these initiators in combination with the aforementioned photoinitiators. Suitable thermal initiators include potassium persulfate and VA044 (available from Wako). Suitable redox initiators include the combination of hydrogen peroxide and ascorbic acid and sodium persulfate and ascorbic acid.

In order to provide adhesives for secure initial and prolonged attachment and easy/painless removal the relation between the elastic modulus and the viscous modulus as well as their dynamic behavior is also of importance. The adhesive has an elastic modulus at a temperature of 25°C abbreviated G'₂₅ a viscous modulus at a temperature of 25°C of G"₂₅.

The adhesive according to the present invention preferably satisfies at least one of the following conditions, preferred at least two of the following conditions, most preferably all of the condtions:

G'₂₅ (1 rad/sec) is in the range 200 Pa to 30000 Pa, preferably 500 Pa to 20000 Pa, most preferably 1000 Pa to 10000 Pa.

G"₂₅ (1 rad/sec) is in the range 100 Pa to 30000 Pa, preferably 100 Pa to 10000 Pa, most preferably 300 Pa to 5000 Pa.

The ratio of G"₂₅ (1 rad/sec) / G'₂s (1 rad/sec) is in the range of 0.03 to 3.

The hydrogels described herein preferably have a 90^e peel force on dry skin of between 0.3 to 5 N/cm, more preferably 1.5 to 3 N/cm. Peel force can also be measured at 180^s on Polyethylene terephthalate (PET). The hydrogels herein preferably have a peel force on PET of between 0.3 to 5.0 N/cm, preferably between 0.5 to 3.0 N/cm and more preferably between 0.8 to 2.0 N/cm. The methods for measuring peel force on skin and PET are described hereinafter in the test methods section.

Additionally it is particularly important, that the hydrogel used must provide a very good safety profile (concerning residual monomers, impurities and by-procucts), especially for large scale production of consumer products. Lowering the oxygen content during the polymerization has been found to have an additional benefit in lowering the residual monomers of the polymerized hydrogel adhesive.

By varying the oxygen content during the polymerization step according to the present invention the material property of adhesiveness (expressed by the Peel on PET value) can be controlled independently from the Theological properties (expressed by the G' and G" values) of the polymerized values.

This is of great interest in the production of those gels, because it is now possible to produce hydrogel adhesives with different adhesiveness from one monomer mixture, without changing the composition. This is a very flexible, effective and cost saving method.

In addition it enables us to produce polymerized hydrogel adhesives with certain combination of rheological behavior and adhesiveness that are not or not easily accessible by conventional means (like changing solid content or amount of crosslinker). Test Methods

1. Rheology

The rheology of hydrogels is measured at 25°C using a HAAKE RHEOSTRESS 1 oscillatory rheometer or the equivalent. A sample of thickness of approximately 1 mm and diameter of 20mm is placed between two insulated Parallel Plates of 20mm diameter, controlled at a temperature of approximately 25°C using a Peltier system or equivalent. A Dynamic Frequency Sweep is performed on the hydrogel in either stress or strain mode at an applied strain within the linear elastic response of the hydrogel (e.g., up to a strain of about 10%), with measurements at discrete frequency values between 47,75 Hz (300rad/sec) and 0,143Hz ( 0,8992rad/sec). Results are quoted as G', G" and tan delta at frequency values of 1.0 and 100 rad/sec. The hydrogel is aged at least 24 hours before measurement. The average of at least three determinations are reported.

2. Peel Force on Dry Skin

The peel force to remove hydrogel from dry skin is measured using a suitable tensile tester, for example an Instron Model 6021 , equipped with a 10N load cell and an anvil rigid plate such as the Instron accessory model A50L2R-100. Samples are cut into strips of width 25.4mm and length between about 10 and 20 cm. A non-stretchable film of length longer than the hydrogel is applied to the reverse side of the hydrogel sample (e.g., the substrate side) using double sided adhesive. A suitable film is 23μm thick PET, available from Effegidi S.p.A, 43052, Colorno, Italy. For samples with release paper, the release paper is removed prior to applying the hydrogel to the forearm and then rolling it into place using a compression weight roller to prevent air entrapment between hydrogel and skin. The roller is 13cm in diameter, 4.5cm wide and has a mass of 5Kg. It is covered in rubber of 0.5mm thickness. The free end of the backing film is attached to the upper clamp of the tensile tester and the arm is placed below. The sample is peeled from the skin at an angle of 90 degrees and a rate of 10OOmm/min. The average peel value obtained during peeling of the whole sample is quoted as the peel value in N/cm. The average of triplicate measurements is reported.

3. Peel force on PET

Peel force to remove hydrogel from polyethylene teraphthalate) (PET) film is measured using a suitable tensile tester, for example a Zwick Z1.0/TH1 S, equipped with a 50N load cell and a pneumatic grip like Zwick Model: 8195.01.00 and attachment for a rigid lower plate, e.g. steel, oriented along the direction of cross-head movement.

Freshly produced hydrogel is stored in a closed aluminium bag or similar for at least 12 to 24 hours at room temperature before measuring. A defect free sample of at least 10cm in length is cut from the hydrogel sample. A piece of double sided adhesive, for example type Duplofol 020DIVB+L from Lohmann GmbH Postoffice box 145456504 Neuwied, at least 130mm long and 25.4mm wide is stuck to the front side of the lower plate. The hydrogel is punched out with a Zwick mechanical cutting press like Zwick model 7104 using a cutting tool 25,4 mm wide and 25,4cm long. The second liner is removed from the tape and it is stuck on the back side of the hydrogel sample. A strip of standard PET of 23μ thickness and no corona treatment, is cut to about 300mm x 28mm. Suitable material would include "Cavilen-Forex" from Effegidi S.p.A, Via Pro- vinciale per Sacca 55, 1-43052 Colorno, Italy. The release liner is removed from the hydrogel and the bottom end fixed to the rigid plate by regular tape. The standard substrate is then applied onto the body adhesive using a hand roller once forward and once backward at a speed of 1000 to 5000 mm/min. The roller is 13cm in diameter, 4.5cm wide and has a mass of 5Kg. It is covered in rubber of 0.5mm thickness. The measurement is preferably performed within 10 minutes of application of the substrate.

The free end of the standard substrate is doubled back at an angle of 180 degrees and the rigid plate is clamped in the lower clamp of the tensile tester. The free end of the standard substrate is fixed in the upper clamp of the tensile tester. The peel test is performed at a speed of 10OOmm/min. The initial 20mm of peel is disregarded and the average force over the remaining length is quoted as the peel force in N/cm. The average of triplicate measurements is reported.

4. Oxygen content of the atmosphere surrounding the polymerization (reaction /crosslinking) zone

The oxygen content of the atmosphere surrounding the reaction / crosslinking zone is measured with commercially available oxygen probes, such as XLT-11-15 Oxygen Sensor (Analytical Industries Inc.)

Examples

a) samples containing Na-AMPS

Approximately 22.4 parts of 50 wt% Na-AMPS solution, approx. 16.6 parts of acrylic acid and approx. 10.4 parts of deionized water are mixed together. To this solution approximately 5.5 parts 50 wt% NaOH is added slowly with constant stirring, while maintaining the temperature below 30°C. After addition of the NaOH approx. 44.8 parts of glycerol are added together with approx. 0.1 to 0.3 parts crosslinker (i.e IRR 210) and approx. 0.2 parts of photoinitiator (e.g Darocure 1173 or Irgacure 2959). The monomer mixture is extruded onto a substrate (e.g a nonwoven webbing) at a basis weight of approximately 1.0 kilograms per square meter. At this point the substrate with the monomer mixture extruded onto it is entering a closed channel that is connected to an exhaust system and flushed with nitrogen gas. Polymerization is carried out inside this channel by irradiating with UV light using 1 to 72000W Hόnle UV lamps or 1 to 12 high power 1ST UV lamps or a combination of both. The lamps can be equipped with glass filters that cut wavelength below 320nm. By this process the monomer solution is converted into an adhesive hydrogel. After passing the UV lamps this adhesive hydrogel is covered with a release liner (e.g siliconized paper or oriented polypropylene (OPP) foil), trimmed to the required width and wound up onto rolls.

b) samples not containing Na-AMPS

Approximately 57.8 parts of 50 wt% Na-Acrylate (70% neutralized) solution, approx. 41.9 parts of glycerol are added together with approx. 0.1 to 0.3 parts crosslinker (i.e IRR 210) and approx. 0.2 parts of photoinitiator (e.g Darocure 1173 or Irgacure 2959) The monomer mixture is extruded onto a substrate (e.g a nonwoven webbing) at a basis weight of approximately 1.0 kilograms per square meter. At this point the substrate with the monomer mixture extruded onto it is entering a closed channel that is connected to an exhaust system and flushed with nitrogen gas. Polymerization is carried out inside this channel by irradiating with UV light using 1 to 72000W Hόnle UV lamps or 1 to 12 high power 1ST UV lamps or a combination of both. The lamps can be equipped with glass filters that cut wavelength below 320nm. By this process the monomer solution is converted into an adhesive hydrogel. After passing the UV lamps this adhesive hydrogel is covered with a release liner (e.g siliconized paper or oriented polypropylene (OPP) foil), trimmed to the required width and wound up onto rolls.

Examples containing Na-AMPS

Examples not containing Na-AMPS

c) Samples containing sodium acrylate and sobitol

The samples are prepared as described in Example b, but 35.34 parts of 50 wt% Na- Acrylate (60% neutralized) solution, approximately 40.66 parts sorbitol, 2.55 parts wa- ter are added together with approximately 0.15 parts Igacure2959 and 0.08 parts IRR210.

Claims

1. A process for making a hydrogel having a peel force on PET of between 0.3 to 5.0 N/cm, comprising 10-90 wt% water, 10-60 wt% of cross-linked hydrophilic polymer made from at least one starting monomer type, and 10-80 wt% of at least one polyol, characterized in that the polymerization is conducted in an atmosphere with an oxygen concentration between 0 and 10000 ppm,

2. A process according to claim 1 , wherein the polymerization is conducted in a closed channel.

3. A process according to any of the claims 1 or 2, wherein that channel is flushed with a gaseous medium of inert gas or lean air.

4. A process according to any of the claims 1 to 3, wherein the gaseous medium is nitrogen.

5. A process according to any of the claims 1 to 4, wherein the oxygen concentration at the polymerization zone is between 50 and 5000 ppm, more preferably between 100 and 2000, most preferably between 200 und 1000 ppm.

6. A process according to any of the claims 1 to 5, wherein the polymerization of said starting monomer(s) is conducted at a pH 3.5 to 7, preferably 4 to 6.5, more preferably 4.5 to 6.

7. A process according to any of the claims 1 to 6, wherein said hydrogel comprises 20-70 wt% water.

8. A process according to any of the claims 1 to 7, wherein said starting mono- mer(s) comprise acrylic acid or its salt.

9. A process according to any of the claims 1 to 8, wherein said polyol comprises glycerol and/or sorbitol.

10. A process according to any of the claims 1 to 9, wherein the polymerization of said starting monomer(s) is conducted at least partly by UV irradiation.

11. A process according to any of the claims 1 to 10, wherein said reaction medium comprises a photoinitiator.

12. A process according to any of the claims 1 to 11 , wherein the hydrogel adhesives has an elastic modulus at a temperature of 25°C (G'₂₅) and a viscous modulus at a temperature of 25°C (G^), wherein said viscous modulus G'₂₅ (1 rad/sec) is in the range 200 Pa to 30000 Pa, or wherein said elastic modulus G"₂₅ (1 rad/sec) is in the range 100 Pa to 30000 Pa, or the ratio of G"₂5 (1 rad/sec) / G'₂5 (1 rad/sec) is in the range of 0.03 to 3.

13. A process according to any of the claims 1 to 12, wherein the hydrogel adhesives has a 90° peel force on dry skin of between 0.3 to 5 N/cm, more preferably 1.5 to 3 N/cm.

14. A process according to any of the claims 1 to 13, wherein the hydrogel adhesives has a peel force on PET of between 0.3 to 5.0 N/cm, preferably between 0.5 to 3.0 N/cm and more preferably between 0.8 to 2.0 N/cm.