Patent Publication Number: US-2004049972-A1

Title: Method for producing adhesive polyacrylates using mercapto functional photoinitiators

Description:
[0001] The invention relates to a process for preparing pressure-sensitively adhesive polyacrylates by free-radical addition polymerization.  
       [0002] Pressure sensitive adhesives (PSAs) are increasingly being employed to produce PSA tapes, self-adhesive labels, adhering protective films or other self-adhesive products. The PSAs employed for these purposes must have certain properties, such as, for example, good surface tack, high cohesion, good tack at low and high temperatures, and good thermal load-bearing capacity.  
       [0003] DE 24 11 169 A1 describes UV-crosslinkable PSAs for which copolymers of (meth)acrylic esters, and, monoolefinically unsaturated ethers and (meth)acrylic ester derivatives of substituted benzophenone are used as copolymerized photoinitiators. The UV reactivity of the polymerizable benzophenones, however, is low, and after UV crosslinking the shear strength of the PSAs prepared from the polymers is too low.  
       [0004] U.S. Pat. No. 4,144,157 describes UV-crosslinkable PSAs which have been synthesized, for example, from acrylic esters and from (meth)acrylic acid 2-alkoxy-2-phenyl-2-benzoylethyl esters. Disadvantageous again is the low reactivity of these photoinitiators and the associated low cohesion of the PSAs prepared.  
       [0005] U.S. Pat. No. 4,737,559 discloses UV-crosslinkable acrylic PSAs containing copolymerizable (meth)acryloylbenzophenone derivatives in the polymer chain. These PSAs were specifically conceived for use in the medical sector, such as for plasters, for example.  
       [0006] The PSAs prepared in accordance with said patent application require a relatively long irradiation time, as a result of which bond strength and tack are adversely affected.  
       [0007] DE 38 44 445 A1 describes UV-crosslinkable PSAs based on (meth)acrylic ester polymers, which comprise a UV-reactive, copolymerized monomer in the form of an N-substituted (meth)acrylamide-benzophenone derivative; a (meth)acryloyloxy-benzophenone derivative or a styrene-benzophenone derivative.  
       [0008] UV-crosslinkable PSAs based on isoamyl (meth)acrylate copolymers are described in DE 38 36 968 A1. These PSAs are based on α-β-monoolefinically unsaturated acids whose homopolymers possess a glass transition temperature of below −30° C., on monoolefinically unsaturated acids and/or their anhydride, on further olefinically unsaturated monomers containing functional groups, and on a polymerizable (meth)acryloyloxybenzophenone or acetophenone derivative. Although the room temperature cohesion of the PSAs thus prepared; following brief UV irradiation, is acceptable, the thermal load-bearing capacity is inadequate.  
       [0009] The thermal load-bearing capacity was solved in DE 195 01 024 A1 through the use of a copolymerizable photoinitiator based on a diester of carbonic acid.  
       [0010] All of the methods set out and described above, however, have a key disadvantage. The copolymerized photoinitiators fulfill only the function of UV crosslinking after coating. Accordingly it is necessary, for example, for preparing acrylic PSAs in acetone, to add regulators, so that the polymerization can be conducted with only a low solvent fraction and gelling of the polymer is avoided. The reduction in the solvent fraction is of great interest particularly for polyacrylate-based hotmelt PSAs, since in this case, following polymerization, the solvent has to be removed, which is laborious, and it would therefore be desirable to minimize the amounts used, on economic and environmental grounds.  
       [0011] U.S. Pat. No. 5,942,555 used regulators likewise containing a photoinitiator to prepare telechelic polymers, and subsequently activated them with UV light.  
       [0012] Sulfur compounds in PSAs are known from the field of the rubber adhesives. There, they serve as stabilizers; see in this respect DE 198 26 103 A1, which describes the use of solid monothiols, soluble or dispersible in rubber, as stabilizers in melted PSAs based on natural rubber or synthetic rubbers, suitable tackifier resins, and promoters for the purpose of increasing the radiation crosslinking yield. Polyfunctional (meth)acrylates can be added here as crosslinking promoters.  
       [0013] It is an object of the present invention to provide a process for preparing UV-crosslinkable acrylic pressure sensitive adhesives, especially UV-crosslinkable acrylic hotmelt pressure sensitive adhesives, with the acrylic PSAs thus prepared no longer having the disadvantages mentioned of the prior art.  
       [0014] This object is achieved by means of a process as set out in the main claim. The subclaims relate to advantageous developments and embodiments of this process and also to the use of the adhesives thus prepared for producing PSA articles.  
       [0015] Claim 1 relates accordingly to a process for preparing pressure-sensitively adhesive polyacrylates by free-radical addition polymerization, in which mercapto-functionalized photoinitiators of the general formula (I) and/or (II)  
       H—S—R   (I)  
       R—S—S—R   (II)  
       [0016] are added to the monomer mixture or to the reaction mixture, R and R′ being chosen independently of one another from the following groups:  
       [0017] a) benzophenone, acetophenone, benzil, benzoin, hydroxyalkylphenone, phenylcyclohexyl ketone, anthraquinone, thioxanthone,.triazine, or fluorenone radicals, it being possible for each of these radicals to be substituted by one or more halogen atoms and/or one or more alkoxy groups and/or one or more amino groups or hydroxyl groups,  
       [0018] b) radicals which comprise one or more of the substituted or unsubstituted radicals specified under a).  
       [0019] In a first advantageous development of this process the fraction of the compounds (I) and/or (II) is from 0.01 to 5% by weight, based on the monomers employed.  
       [0020] In a further embodiment of the process, which is very favorable in the inventive sense, the polymerization takes place using a monomer mixture which comprises at least the following component:  
       [0021] a) acrylic and methacrylic acid monomers of the following structure  
                 
 
       [0022] where R 1 ═H or CH 3    
       [0023] and R 2 =an alkyl chain having 2-20 carbon atoms  
       [0024] with a fraction of from 65 to 100% by weight,  
       [0025] and which optionally comprises the following component:  
       [0026] b) vinyl compounds having functional groups  
       [0027] with a fraction of from 0 to 35% by weight,  
       [0028] it being possible optionally for further components to be present in the monomer mixture.  
       [0029] Examples of monomers from group a) are butyl, pentyl, hexyl, heptyl, octyl, isooctyl, 2-methylheptyl, 2-ethylhexyl, nonyl, decyl, dodecyl, lauryl or stearyl (meth)acrylate or (meth)acrylic acid. Examples that may be mentioned of group b) include maleic anhydride, styrene, styrene compounds, vinyl acetate, (meth)acrylamides, N-substituted (meth)acrylamides, β-pacryloyloxypropionic acid, vinylacetic acid, fumaric acid, crotonic acid, aconitic acid, dimethylacrylic acid, trichloroacrylic acid, itaconic acid, vinyl acetate, hydroxyalkyl (meth)acrylate, amino-containing (meth)acrylates, hydroxyl-containing (meth)acrylates, with particular preference 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and/or 4-hydroxybutyl (meth)acrylate, and double-bond-functionalized photoinitiators.  
       [0030] The composition of the corresponding monomers is preferably chosen such that the resultant adhesives possess pressure-sensitively adhesive properties in accordance with D. Satas [Handbook of Pressure Sensitive Adhesive Technology, 1989, VAN NOSTRAND REINHOLD, New York].  
       [0031] The free radical polymerization can be carried out in the presence of one or more organic solvents and/or in the presence of water or without solvent. It is preferred to use as little solvent as possible. Depending on conversion and temperature, the polymerization time is between 6 and 48 h.  
       [0032] In the case of solution polymerization, solvents used include preferably esters of saturated carboxylic acids (such as ethyl acetate), aliphatic hydrocarbons (such as n-hexane or n-heptane), ketones (such as acetone or methyl ethyl ketone), special boiling point spirit or mixtures of these solvents. For polymerization in aqueous media or in mixtures of organic and aqueous solvents, it is preferred to add stabilizers and emulsifiers which are familiar to the skilled worker for this purpose to the polymerization. Polymerization initiators used include customary radical-forming compounds such as peroxides, azo compounds, and peroxosulfates, for example. Initiator mixtures can also be used.  
       [0033] In the polymerization, in addition to the thio compounds modified with a UV photoinitiator, it is also possible to use further regulators for lowering the molecular weight and reducing the polydispersity. As so-called polymerization regulators it is possible, for example to use alcohols and ethers.  
       [0034] The polymerization may be carried out in polymerization reactors, which are generally provided with a stirrer, two or more feed vessels, reflux condenser, heating and cooling, and are equipped for operation under N 2  atmosphere and superatmospheric pressure.  
       [0035] Following polymerization in solvent, the polymerization medium can be rermoved under reduced pressure, this operation being carried out at elevated temperatures, for example, and advantageously in the range from 80 to 150° C. The polymers can then be used in solvent-free state, particularly as hotmelt PSAs. Depending on further processing and field of application it may also be of advantage to prepare the polymers of the invention without solvent.  
       [0036] To prepare the acrylic PSAs, the polymers of the invention can be modified in customary fashion. Added favorably, for example, are tackifying resins, such as terpene, terpenephenolic, C5, C9, C5/C9 hydrocarbon, pinene or indene resins or rosins, alone or in combination with one another. Depending on application it is further of advantage to admix plasticizers, various fillers (e.g., chalk, carbon black, glass microbeads, etc.) and aging inhibitors as additives. Optionally, furthermore, crosslinkers, and UV crosslinking promoters that are known to the skilled worker are admixed. Polyfunctional acrylate crosslinkers are particularly advantageous for the preparation of star polymers.  
       [0037] The polymers are advantageously applied conventionally by brushing, spraying, rolling, knifecoating, pouring or extruding, where appropriate at elevated temperature—usually in the temperature range from 20 to 150° C.—to substrates suitable as carrier materials for this purpose: for example, to paper, paperboard, wood, metals, and polymer films, made for example from plasticized PVC; polyethylene, polyamides, polyethylene glycol terephthalate or polypropylene. Where solvents are used, they can easily be evaporated from the coatings, where appropriate at room temperature or slightly elevated temperatures, generally at temperatures of 20-150° C., preferably of 50-100° C., using, customarily, radiant heaters or heated-air circulation apparatus.  
       [0038] In a way which is also very advantageous for the inventive process, the pressure-sensitively adhesive polyacrylates are crosslinked by ultraviolet radiation in a wavelength range from 200 to 400 nm. Crosslinking of the hotmelt PSAs of the invention takes place advantageously by brief UV irradiation with commercially customary high-pressure or medium-pressure mercury lamps having an output of, for example, from 80 to 160 W/cm. It may be appropriate to adapt the output of the lamp to the belt speed or, if the belt is running slowly, to shade off the belt partly in order to reduce the thermal load thereon. The irradiation time depends on the construction and output of the respective lamps.  
       [0039] The invention relates, moreover, to the use of the pressure-sensitively adhesive polyacrylates prepared by the inventive process to produce PSA articles, particularly for producing an adhesive tape comprising an acrylic PSA applied to one or both sides of a backing.  
       [0040] The UV-curing polyacrylates prepared by the inventive process are particularly suitable as melts or as solutions for preparing PSA materials having improved tack, improved bond strength, and high cohesion, such as PSA tapes, PSA sheets or PSA labels.  
       [0041] The PSA properties of the polyacrylates prepared by the inventive process, following UV irradiation, are determined by the test methods described below.  
       [0042] For the test, films of polyethylene glycol terephthalate were coated with an adhesive application of 50 g/m 2 .  
       [0043] Where dissolved polymers are used for adhesive performance testing, the solvents are evaporated in a drying oven at 120° C. for 10 minutes. The dry PSA films were irradiated using an Eltosch unit with medium-pressure mercury lamps. The distance of the UV lamps from the irradiated PSA films was 5 cm, the output of the UV lamps is 120 W/cm.  
       [0044] The invention is illustrated below by examples. Amounts, proportions, and percentages are based on the total amount of the monomers.  
       [0045] Preparation of the UV-Activatable Thior Gulators  
       [0046] Overview of the thio-functionalized photoinitiators prepared (see table 1)  
               TABLE 1                       Thio-functionalized photoinitiators                                                            Photoinitiator (I)                                                 Photoinitiator (II)                                                 Photoinitiator (III)                                                 Photoinitiator (IV)                  
 
     
    
    
     EXAMPLE 1  
     [0047] Preparation of 4-benzoylbenzoyl chloride  
     [0048] 1.0 kg (4.2 mol) of 4-benzoylbenzoic acid were filled into a 5 L reactor, equipped with a reflux condenser and a stirrer, and additionally 645 ml (8.84 mol) of thionyl chloride and 725 ml of toluene were added. Then 3.5 ml of DMF were added and the mixture was boiled under reflux for 4 h. After cooling, the solvent was removed under reduced pressure and excess thionyl chloride was removed by evaporating three times with 500 ml of toluene each time. The product was recrystallized from a 1:4 toluene/hexane mixture and recovered with 934 g (86% yield) after-drying in a vacuum oven.  
     [0049] 1 H-NMR at 300 MHz (CDCl 3 ) gave: 7.18-8.26 ppm (m, 9 H). The signals were consistent with the desired product. All chemical shifts are stated with respect to tetramethylsilane as internal standard.  
     EXAMPLE 2  
     [0050] Preparation of 4-bromomethylbenzophenone  
     [0051] 750 g (3.82 mol) of 4-methylbenzophenone were filled into a 5 L reactor, equipped with a reflux condenser and a stirrer, and additionally 2850 ml of benzene were added. The mixture was heated to reflux and 610 g (3.82 mol) of bromine in solution in 330 ml of benzene were added dropwise. The rate of addition was approximately 1.5 ml/min. The reactor was irradiated with a halogen lamp at 100 W to initiate the reaction, thereafter with different cycles of identical output.  
     [0052] In each of the cycles, the irradiation was carried out in alternation for a period of 5 s, followed by a period of 40 s without irradiation. After 1 h, this cycle was altered to a sequence of 10 s irradiation and 40 s without irradiation. After the end of the reaction (decoloration of the dark brown solution by consumptive reaction with bromine) the crude product was analyzed by means of GC. A mixture of monobromo and dibromomethyl-benzophenone and unreacted 4-methylbenzophenone was found. The reaction mixture was washed with 10 g of sodium thiosulfite in 100 g of water and 3 times with 200 g of water each time. The product was subsequently dried over sodium sulfate and recrystallized twice from 1:3 toluene/hexane. After drying under reduced pressure, 590 g of 4-bromomethylbenzophenone (56% yield) were isolated.  
     [0053] 1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 7.18-7.77 (m, 9 H), 4.49 (s, 2H). The signals were consistent with the desired product.  
     EXAMPLE 3  
     [0054] Preparation of 4-mercaptomethylbenzophenone (I)  
     [0055] 4.14 g (54.4 mmol) of thiourea were dissolved in 31.5 ml of ethanol (95%), 15.0 g (54.4 mmol) of 4-bromomethylbenzophenone were added with gentle heating and stirring, and the mixture was stirred overnight at room temperature. The solid product was isolated by filtration and washed several times with ethanol. After drying in a vacuum oven, 15.6 g were isolated (82% yield). No further purification was carried out.  
     [0056] 12.5 g (35.5 mmol) of this hydrobromide salt were dissolved in 250 ml of water with heating and then 5.7 g of sodium hydroxide (0.143 mol) in solution in 10 ml of water were added. After 45 minutes of refluxing the solution was cooled to room temperature, a pH of less than 2 was set using concentrated sulfuric acid, and the product was extracted 5 times with 60 ml of chloroform. The combined extracts were washed with 100 ml of water and dried over sodium sulfate. After the solvent had been removed, 7.9 g (97%) were isolated.  
     [0057] The melting point was 54° C.  1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 7.14-7.78 (m, 9 H), 3.70 (d, 2H) and 1.77 (t, 1 H). The signals were consistent with the desired product.  
     EXAMPLE 4  
     [0058] Preparation of N-(2-mercaptoethyl)-4-benzoylbenzamide (II)  
     [0059] 24.39 g (0.215 mol) of 2-aminoethanethiol hydrochloride in solution in 200 ml of chloroform were placed under argon in a 1 L three-necked flask. A solution of 50 g (0.204 mol) of 4-benzoylbenzoyl chloride and 250 ml of chloroform was subsequently added dropwise over a period of 45 minutes. The mixture was stirred at room temperature overnight. It was then washed with water and a 0.1 N HCl solution and dried over sodium sulfate. After the drying operation, twofold recrystallization from toluene gave 50 g of a white powder (86% yield).  
     [0060] The melting point was 112° C.  1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 7.18-7.82 (m, 9 H), 6.70-7.02 (m, 1H), 3.52 (q, 2H), 2.54-2.97 (m, 2H) and 1.37 (t, 1H). The signals were consistent with the desired product.  
     EXAMPLE 5  
     [0061] Preparation of N-(2-mercaptoethyl)-3,5-bis(4-benzoylbenzoyloxy)benzamide (III)  
     [0062] 46.2 g (0.30 mol) of 3,5-dihydroxybenzoic acid were introduced into a 250 ml flask with Soxlett extractor and reflux condenser. 48.6 ml of methanol and 0.8 ml of sulfuric acid were added and 50 g of molecular sieve (3Å) were placed in the Soxlett extractor. The extractor was filled with methanol and the entire mixture was refluxed overnight. The methylated crude product was subsequently isolated after removal of the solvent.  
     [0063] The entire product was placed in a 2 L reactor with reflux condenser and stirrer, and then 173.25 g (0.63 mol) of 4-bromomethylbenzophenone, 207 g (1.50 mol) of potassium carbonate and 1200 ml of acetone were added. During overnight reflux, the reaction was monitored by thin-layer chromatography. Following complete conversion the solid was isolated by filtration and acetone was removed under reduced pressure. The solid was dissolved in 1 L of water and then extracted with three times 1 L of chloroform. The extracts were combined with the acetone-soluble fraction and dried over sodium sulfate, and 177 g of the crude product were isolated. The crude product was recrystallized twice from acetonitrile, to give ultimately 145 g (87%).  
     [0064] The melting point was 130° C.  1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 7.22-7.78 (m, 18 H), 7.15 (d, 2H), 6.69 (t, 1H), 5.02 (s, 4H) and 3.84 (s, 3H). The signals were consistent with the desired product: methyl 3,5-bis(4-benzoylbenzoyloxy)benzoate.  
     [0065] 60.1 g (0.108 mol) of methyl 3,5-bis(4-benzoylbenzoyloxy)benzoate were introduced into a 2 L reactor and 120 ml of water, 480 ml of methanol and 6.48 g (0.162 mol) of sodium hydroxide were added. The reaction mixture was refluxed for 3 h. After hydrolysis of the ester, the mixture was cooled and methanol was removed under reduced pressure. The sodium salt which remained was dissolved in 2 400 ml of hot water and the free acid was subsequently precipitated using hydrochloric acid. Filtration, washing with water, and vacuum oven drying gave 54 g of a white powder (92%).  
     [0066] The melting point was 188° C. The melting point was 130° C.  1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 7.28-7.78 (m, 18 H), 7.15 (d, 2H), 5.86 (t, 1H) and 5.16 (s, 4H). The signals were consistent with the desired product: 3,5-bis(4-benzoylbenzoyloxy)benzoic acid.  
     [0067] 20 g (36.86 mmol) of 3,5-7bis(4-benzoylbenzoyloxy)benzoic acid were introduced together with 36 ml of toluene, 5.4 ml (74.0 mmol) of thionyl chloride and 28 μl of N,N-dimethylformamide into a 250 ml flask. The mixture was refluxed for 4 h. After the acid chloride had formed the mixture was cooled to room temperature, the solvent and excess thionyl chloride were removed under reduced pressure, and purification was continued by evaporating a further 4 times with 20 ml of chloroform each time. Recrystallization from toluene gave 18.5 g of the product (89% yield).  
     [0068] The melting point was 125° C.  1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 7.29-7.78 (m, 18 H), 7.20 (d, 2H), 6.79 (t, 1H), 5.08 (s, 4H). The signals were consistent with the desired product: 3,5-bis(4-benzoylbenzoyloxy)benzoyl chloride.  
     [0069] 4.19 g (36.7 mmol) of 2-aminoethanethiol hydrochloride were introduced into a 250 ml flask with reflux condenser and stirrer and then 15 ml of chloroform and 10.64 ml (76.5 mmol) of triethylamine were added. The reaction mixture was cooled to 0° C. using an ice bath and then 18.4 g (32.8 mmol) of 3,5-bis(4-benzoylbenzoyloxy)benzoyl chloride in solution in 50 ml of chloroform were added dropwise over a period of 50 minutes. After a further 30 minutes of ice cooling, the mixture was warmed to room temperature for 2 h. The product was diluted with 150 ml of chloroform, washed with 5 times 250 ml of 0.1 N hydrochloric acid, dried over sodium sulfate and finally recrystallized twice from 15:1 toluene/hexane. 12.9 g (65% yield) of the product were isolated.  
     [0070] The melting point was 114° C.  1 H-NMR at 300 MHz (DMSO-d6) gave [ppm]: 7.20-7.80 (m, 18 H), 7.00 (d, 2H), 6.66 (t, 1H), 6.52 (broad t, 1H), 5.08 (s, 4H), 3.50 (q, 2H), 2.74 (q, 2H) and 1.40 (t, 1 H). The signals were consistent with the desired product: N-(2-mercaptoethyl)-3,5-bis(4-benzoylbenzoyloxy)benzamide. (III).  
     EXAMPLE 6  
     [0071] Preparation of N-(2-mercaptoethyl)-2,6-bis(4-benzoylbenzamido)hexanamide (IV)  
     [0072] 3.65 g (20 mmol) of lysine monohydrochloride were dissolved in 8 ml of 2 N aqueous sodium hydroxide solution and the solution was cooled using an ice bath. A solution of 10.77 g (44 mmol) of 4-benzoylbenzoyl chloride in 17 ml of chloroform was added simultaneously with 4.48. of sodium hydroxide in 19 ml of water. The reaction mixture was stirred for 2 h with ice cooling and then for 3 h at room temperature. Hydrochloric acid was used to set a pH of less than 1, and then 60 ml of chloroform were added. The different phases were separated with a centrifuge and the aqueous phase was extracted with three times 50 ml of chloroform. The combined organic phases were dried over sodium sulfate. The mother liquor was diluted with water and the precipitated product was filtered off, dissolved again in chloroform and then washed with 10% strength aqueous sodium hydrogen carbonate solution, 1 N hydrochloric acid and water. The product was used without further purification.  
     [0073] 4.35 g (7.73 mmol) of the lysine derivative and 0.901 g (7.83 mmol) of N-hydroxy-succinimide were dissolved in 40 ml of 1,4-dioxane, and then 1.951 g (9.45 mmol) of 1,3-dycyclohexylcarbodiimide (DCC) in 10 ml of 1,4-dioxane were added. The ester was filtered off and dried (4.1 g, 81% yield). In a separate flask, 0.75 g (6.6 mmol) of 2-aminoethanethiol hydrochloride were dissolved in 15 ml of chloroform. 4.1 g (6.22 mmol) of the ester, following dissolution in 25 ml of chloroform, are added slowly dropwise to this solution at room temperature over a period of 30 minutes. After 4 h the reaction mixture was washed with water and 0.05 N hydrochloric acid, dried over sodium sulfate and then purified by column chromatography using a solvent mixture of 95% chloroform and 5% methanol. 2.2.g (yield 56%) of the product were isolated.  
     [0074] 1 H-NMR at 300 MHz (CDCl 3 ) gave [ppm]: 6.90-7.95 (m, 21 H), 4.42-4.87 (m, 1 H), 3.00-3.78 (m, 4H), 1.42 (t, 1 H) and 1.00-2.95 (m, 8H). The signals were consistent with the desired product: N-(2-mercaptoethyl)-2,6-bis(4-benzoylbenzamido)hexanamide (IV).  
     [0075] Polymerizations  
     [0076] 180° Bond Strength Test (Test Method A)  
     [0077] A strip 20 mm wide of an acrylic PSA laminated onto a polyester film was applied to steel plates washed twice with acetone and once with isopropanol. The PSA strip was pressed onto the substrate twice using a 2 kg weight. The adhesive tape was then immediately peeled from the substrate at a speed of 300 mm/min and at an angle of 180°. All. measurements were conducted at room temperature under standardized climatic conditions.  
     [0078] The measurement results are reported in N/cm and are averaged from three measurements.  
     [0079] Shear Strength (Test Method B)  
     [0080] A strip of the adhesive tape 13 mm wide was applied to a smooth steel surface cleaned three times with acetone and once with isopropanol. The area of application was 20 mm×13 mm (length×width). The adhesive tape was then pressed onto the steel substrate four times using a 2 kg weight. A 1 kg weight was fastened to the adhesive tape at room temperature and at 70° C.  
     [0081] The shear stability times measured are reported in minutes and correspond to the average of three measurements.  
     EXAMPLE 7  
     [0082] A conventional 2 L glass reactor equipped with a mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath was charged with 320 g of 2-ethylhexyl acrylate, 56 g of butyl acrylate, 20 g of acrylic acid, 200 g of acetone and 4 g of 4-mercaptomethylbenzophenone. After 30 minutes of nitrogen gas inertization, the reaction mixture was heated to an internal temperature of 58° C. with stirring and then 0.2 g of AIBN (azoisobutyronitrile) was added 0.2 g of AIBN was added after a reaction time of 1.5 h, and 100 g of acetone after 3 h. After 8 and 10 hours of reaction, in each case 0.2 g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl) peroxydicarbonate) was added, and after 10 h a further 100 g of acetone were added for dilution. The polymerization was terminated after 24 h by cooling. The product was a polymer having a molecular weight of 740 000 g/mol (M w  from gel permeation chromatography). The PSA thus prepared was diluted down to 35% (solids) with acetone and then applied at 50 g/m 2  (solids after drying) to a primed PET (polyethylene terephthalate) film 23 μm thick. The PSA tape was dried in a drying oven at 120° C. for 10 minutes. The PSA specimen was subsequently irradiated in a UV unit (from Eltosch) with a UV lamp (120 W/cm, 254 nm) in 2 passes at 20 m/min.  
     [0083] The UV-crosslinked PSA tapes were tested for bond strength on steel (test method A) and for cohesion by a shear test at room temperature and at 70° C. (test method B).  
     [0084] The results are compiled in table 2.  
                           TABLE 2                               SST, RT, 10 N   SST, 70° C., 10 N           BS steel [N/cm]   [min]   [min]                  Example 7   6.1   7 840   1 355                                  
 
     EXAMPLE 8 (COMPARATIVE EXAMPLE)  
     [0085] A conventional 2 L glass reactor equipped with a mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath was charged with 320 g of 2-ethylhexyl acrylate, 56 g of butyl acrylate, 20 g of acrylic acid, 200 g of acetone and 4 9 of Ebecryl© P 36 (acrylated benzophenone derivative from UCB). After 30 minutes of nitrogen gas inertization, the reaction mixture was heated to an internal temperature of 58° C. with stirring and then 0.2 g of AIBN (azoisobutyronitrile) was added. 0.2 g of AIBN was added after a reaction time of 1.5 h, 100 g of acetone after 3 h. After a reaction time of 8 h, 0.2 g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl) peroxydicarbonate) was added.  
     [0086] After 9 h, the polymerization had to be terminated, since the batch had completely gelled.  
     EXAMPLE 9 (COMPARATIVE EXAMPLE)  
     [0087] A conventional 2 L glass reactor equipped with a mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath was charged with 320 g of 2-ethylhexyl acrylate, 56 g of butyl acrylate, 20 g of acrylic acid, 200 g of acetone/isopropanol (97:3) and 4 g of Ebecryl© P 36 (acrylated benzophenone derivative from UCB). After 30 minutes of nitrogen gas inertization, the reaction mixture was heated to an internal temperature of 58° C with stirring and then 0.2 g of AIBN (azoisobutyronitrile) was added. 0.2 9 of AIBN was added after a reaction time of 1.5 h, and 100 g of acetone/isopropanol (97:3) after 3 h. After 8 and 10 hours of reaction, in each case 0.2 g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl) peroxydicarbonate) was added, and after 10 h a further 100 g of acetone/isopropanol (97:3) were added for dilution. The polymerization was terminated after 24 h by cooling. The product was a polymer having a molecular weight of 760 000 g/mol (M w  from gel permeation chromatography). The PSA thus prepared was diluted down to 35% (solids) with acetone and then applied at 50 g/m 2  (solids after drying) to a primed PET (polyethylene terephthalate) film 23 μm thick. The PSA tape was dried in a drying oven at 120° C. for 10 minutes. The PSA specimen was subsequently irradiated in a UV unit (from Eltosch) with a UV lamp (120 W/cm, 254 nm) in 2 passes at 20 m/min.  
     [0088] The UV-crosslinked PSA tapes were tested for bond strength on steel (test method A) and for cohesion by a shear test at room temperature and at 70° C. (test method B).  
     [0089] The results are compiled in table 3.  
                           TABLE 3                               SST, RT, 10 N   SST, 70° C., 10 N           BS steel [N/cm]   [min]   [min]                  Example 10   5.8   6 350   560                                  
 
     EXAMPLE 10  
     [0090] A conventional 2 L glass reactor equipped with a mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath was charged with 360 g of 2-ethylhexyl acrylate, 34 g of acrylic acid, 200 g of acetone and 6 g of N-(2-mercaptoethyl)-4-benzoylbenzamide. After 30 minutes of nitrogen gas inertization, the reaction mixture was heated to an internal temperature of 58° C. with stirring, and subsequently the procedure of example 7 was followed. The product is a polymer having a molecular weight of 710 000 g/mol (M w  from gel permeation chromatography). The PSA thus prepared was diluted down to 35% (solids) with acetone and then applied at 50 g/m 2  (solids after drying) to a primed PET (polyethylene terephthalate) film 23 μm thick. The PSA tape was dried in a drying oven at 120° C. for 10 minutes. The PSA specimen was subsequently irradiated in a UV unit (from Eltosch) with a UV lamp (120 W/cm, 254,nm) in 2 passes at 20 m/min.  
     [0091] The UV-crosslinked PSA tapes were tested for bond strength on steel (test method A) and for cohesion by a shear test at room temperature and at 70° C. (test method B).  
     [0092] The results are compiled in table 4.  
                           TABLE 4                               SST, RT, 10 N   SST, 70° C., 10 N           BS steel [N/cm]   [min]   [min]                  Example 10    5.2   +10 000   7 525                                  
 
     EXAMPLE 11 (COMPARATIVE EXAMPLE)  
     [0093] A conventional 2 L glass reactor equipped with a mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath was charged with 360 g of 2-ethylhexyl acrylate, 34 g of acrylic acid, 200 g of acetone and 6 g of benzoin acrylate (prepared by the method of Guse et al. in accordance with DE 27 43 979 A1). After 30 minutes of nitrogen gas inertization, the reaction mixture was heated to an internal temperature of 58° C. with stirring and then 0.2 g of AIBN (azoisobutyronitrile) was added 0.2 g of AIBN was added after a reaction time of 1.5 h, 100 g of acetone after 3 h. After a reaction time of 8 h, 0.2 g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl) peroxydicarbonate) was added.  
     [0094] After 9 h, the polymerization had to be terminated, since the batch had completely gelled.  
     EXAMPLE 12 (COMPARATIVE EXAMPLE)  
     [0095] A conventional 2 L glass reactor equipped with a mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath was charged with 360 g of 2-ethylhexyl acrylate, 34 g of acrylic acid, 200 g of acetone/isopropanol (97:3) and 6 g of benzoin acrylate (prepared by the method of Guse et al. in accordance with DE 27 43 979 A1). After 30 minutes of nitrogen gas inertization, the reaction mixture was heated to an internal temperature of 58° C. with stirring, and subsequently the procedure of example 7 was followed. The product was a polymer having a molecular weight of 750,000 g/mol (M w  from gel permeation chromatography). The PSA thus prepared was diluted down to 35% (solids) with acetone. and then applied at 50 g/m 2  (solids after drying) to a primed PET (polyethylene terephthalate) film 23 μm thick. The PSA tape was dried in a drying oven at 120° C. for 10 minutes. The PSA specimen was subsequently irradiated in a UV unit (from Eltosch) with a UV lamp (120 W/cm, 254 nm) in 2 passes at 20 m/min.  
     [0096] The UV-crosslinked PSA tapes were tested for bond strength on steel (test method A) and for cohesion by a shear test at room temperature and at 70° C. (test method B).  
     [0097] The results are compiled in table 5.  
                           TABLE 5                               SST, RT, 10N   SST, 70° C., 10 N           BS steel [N/cm]   [min]   [min]                  Example 9   5.3   +10 000   2 455                                  
 
     [0098] As apparent from the described examples 7 to 12, the mercapto-functionalized photo-initiator is very suitable for preparing acrylic PSAs by the inventive process. There is no need to supply any regulator to the polymerization (examples 7+10). The polymerizations conducted analogously (examples 8 and 11) gelled. Comparison with examples 9 and 12 shows that acrylated photoinitiators can be copolymerized by the addition of a regulator. Comparison of tables 2 and 3 and 4 and 5, respectively, indicates a significantly poorer thermal shear stability for this method, however.  
     EXAMPLES 13 TO 18  
     [0099] The polymerization of the following monomer mixtures (table 6: composition of the monomer mixtures used; amounts in % by weight) was conducted in a conventional 2 L glass reactor equipped with mechanical stirrer, reflux condenser, internal temperature sensor and a heating bath. The procedure adopted was analogous to that in example 7, retaining the amounts of solvent and initiator. There was no change either in the times at which each of the additions were made. The reaction time was 24 h.  
     [0100] The polymers prepared by the process of the invention were diluted down to 35% (solids) with acetone and then applied at 50 g/m 2  (solids after drying) to a primed PET (polyethylene terephthalate) film 23 μm thick. The PSA tapes were dried in a drying oven at 120° C. for 10 minutes. The PSA specimens were subsequently irradiated in a UV unit (from Eltosch) with a UV lamp (120 W/cm, 254 nm),in 2 passes at 20 m/min.  
     [0101] The UV-crosslinked PSA tapes were tested for bond strength on steel (test method A) and for cohesion by a shear test at room temperature and at 70° C. (test method B).  
     [0102] The results are compiled in table 7.  
                       TABLE 6                                      Mercapto-           functionalized           photoinitiator                                                 2-EHA   AA   2-HEA   IO   MA       Conc.       Example   [%]   [%]   [%]   [%]   [%]   Comp.   [%]                                                     13   0   5   0   75   19   I   1       14   46   1   5.5   46   0   II   1.5       15   0   5   0   75   18   III   2       16   88   10   0   0   0   IV   2       17   95   3   0   0   0   I   2       18   60   10   0   20   8   I   2                                                          
 
     [0103]                           TABLE 7                               SST, RT, 10 N   SST, 70° C., 10 N       Example   BS steel [N/cm]   [min]   [min]                                                13   4.8   +10 000   6 890       14   5.9      2 325      280       15   5.0   +10 000      7 275       16   4.6   +10 000   +10 000       17   5.3      5 640      1 065       18   4.4   +10 000   +10 000