Patent Publication Number: US-2012040180-A1

Title: Pressure-Sensitive Adhesive Compound

Description:
The present invention relates to the use of a pressure-sensitive adhesive (PSA) for the adhesive bonding of optical components, more particularly optical films, in accordance with claim  1 . 
     The uses of PSAs are nowadays very diverse. In the industrial sector, accordingly, there exist a very wide variety of applications. Adhesive tapes based on PSAs are used in particularly high numbers in the electronics segment or in the consumer electronics segment. Owing to the high number of units, PSA tapes can be processed here very rapidly and easily, meaning that other operations, such as riveting or welding, for example, would be too costly and complicated. Besides their normal joining function, these PSA tapes may also be required to take on additional functions. Examples there might include thermal conductivity, electrical conductivity or else an optical function. In the latter case, for example, PSA tapes are used which have light-absorbing or light-reflecting functions. Another optical function, for example, is a suitable transmission of light. Here, PSA tapes and PSAs are used that are very transparent, have no intrinsic coloration, and also possess high light stability. 
     In many cases, a PSA for optical purposes, in addition to the joining function, has the function of excluding air, since air has a refractive index of one, and the optical films or glasses have a refractive index which is generally much greater. At the transition from air to the optical component, the difference in refractive indices leads to a reflection, which reduces the transmission. One way of solving this problem is provided by antireflection coatings, which facilitate the transition of the light into the optical component, and reduce reflection. An alternative or additional option is to use an optical PSA which has a refractive index similar to that of the optical component. As a result, the reflection at the optical component is significantly minimized and the transmission is increased. 
     Typical applications include, for example, the bonding of touch panels to the LCD or OLED display, and the bonding of ITO films (indium tin oxide) for capacitive touch panels. The bonding of ITO films in particular poses a special challenge. Here, for instance, particular requirements are imposed on the neutrality of the PSA formulation. The PSA must not contain any acid functions, which, for example, on contact with ITO films, could adversely affect the electrical conductivity over a prolonged time period. Another requirement is the flow-on behavior. For example, many ITO films have structuring, typically microstructuring in the surface, which is to be filled by the PSA. This ought to take place without the inclusion of bubbles, since formation of bubbles would lessen the transmission. 
     For transparent adhesive bonds there are a multiplicity of acrylate PSAs known that are used in the optical segment. In U.S. Pat. No. 6,703,463 B2, JP 2002-363523 A or US 2002/0098352 A1, for example, acrylate PSAs are described which have different refractive indices. These adhesives, however, have disadvantages in the context of electrical neutrality relative to ITO films. 
     Furthermore, however, silicone PSAs are used as well. Adhesives of this kind are described by EP 1 652 899 B1. The silicone PSAs used therein, however, have only a very low bond strength and are therefore not suitable for the permanent adhesive bonds above. Further silicone PSAs are also described in US 2006/008662 A1. The PSAs described here, as well, have only a low bond strength, and so are not suitable for permanent adhesive bonds. 
     Accordingly there continues to be a need for an improved optical PSA which does not have the disadvantages set out above. A suitable adhesive ought more particularly to have high optical transparency and also high UV stability. For the preferred application in the adhesive bonding of electrically conductive substrates, more particularly of ITO films, the adhesive ought to be inert in its behavior. 
     The present invention solves the problem described above, through the use of an adhesive in accordance with claim  1 . Preferred embodiments and developments are subject matter of the respective dependent claims. 
     In accordance with the invention it has been recognized that, contrary to the experience that PSAs based on silicone have only a low bond strength, there is nevertheless a suitable adhesive which provides sufficient bond strength and also, moreover, meets the necessary optical requirements. A silicone adhesive of this kind, more particularly a silicone PSA of this kind, is based on polysiloxane. 
     Silicone PSAs composed of poly(diorgano)siloxanes (e.g., PDMS—polydimethylsiloxane) and silicate resins (MQ resin) are diluted with toluene or xylene. It has been found that these silicone PSAs exhibit moderate to good bond strengths on low-energy substrates that are difficult to bond, especially also on silicones or siliconized substrates such as UV paints, for example. In view of the very stable Si—O—Si bond, silicone PSAs exhibit excellent resistance toward environmental influences such as UV light, ozone, chemicals, and high temperatures, and therefore have extremely high life cycles. The strength of the Si—O—Si bond is also the reason for the biocompatibility of silicones. The absence of C═C double bonds in combination with the absence of interaction between UV light and Si—O—Si polymer backbone results in excellent optical purity and in virtually complete transmission of light. Furthermore, silicone PSAs of this kind, on account of the low T g , of down to −120° C., can be processed even at temperatures of −50° C. and, moreover, exhibit excellent absorption properties with respect to shaking, vibration, noise, and temperature effects. 
     The invention accordingly relates to the use of a pressure-sensitive adhesive based on a condensation-crosslinking silicone, composed of the following components:
         a) a hydroxy-functionalized organopolysiloxane which has at least one diorganosiloxane unit,   b) an organopolysiloxane resin with the following formula: (R 1   3 SiO 1/2 ) x (SiO 4/2 ) 1 ,
           where R 1  is a substituted or unsubstituted monovalent hydrocarbon group, a hydrogen atom or a hydroxyl group and x is a number between 0.5 and 1.2,   
           c) optionally a stabilizer,   d) optionally an initiator
 
or on an addition-crosslinking silicone, composed of the following components:
   a) an organopolysiloxane which is composed of at least one diorganosiloxane unit and carries at least two silicon-bonded alkenyl groups in each molecule,   b) an organopolysiloxane resin with the following formula: (R 1   3 SiO 1/2 ) x (SiO 4/2 ) 1 ,
           where R 1  is a substituted or unsubstituted monovalent hydrocarbon group, a hydrogen atom or a hydroxyl group and x is a number between 0.5 and 1.2,   
           c) an organopolysiloxane which carries on average at least two silicon-bonded hydrogen atoms in each molecule,
           this organopolysiloxane being free from olefinic double bonds, and the amount of this organopolysiloxane being such that there are 0.01 to 10 mol of silicon-bonded hydrogen atoms per mole of the total alkenyl groups of components a), b), and, where present, e),   
           d) an organometallic catalyst from group 10 of the Periodic Table of the Elements,   e) optionally an inhibitor.       

     The adhesives, in accordance with ASTM D 1003, each have a light transmittance of more than 86% and a haze of less than 5%, and are therefore especially suitable for the adhesive bonding of optical components. On account of the good cohesion and adhesion properties, these adhesives are especially suitable as well for the adhesive bonding of flexible objects, examples being films. 
     Pressure-Sensitive Adhesive (PSA) 
     PSAs employed are more particularly those which are based on a condensation-crosslinking silicone composed of the components set out below:
         a) a hydroxy-functionalized organopolysiloxane which has at least one diorganosiloxane unit,   b) an organopolysiloxane resin with the following formula: (R 1   3 SiO 1/2 ) x (SiO 4/2 ) 1 ,
           where R 1  is a substituted or unsubstituted monovalent hydrocarbon group, a hydrogen atom or a hydroxyl group and x is a number between 0.5 and 1.2,   
           c) optionally a stabilizer,   d) optionally an initiator.       

     Silicone PSAs of this kind are freely available commercially. By way of example, mention may be made at this point of the following: DC 280, DC 282, Q2-7735, DC 7358, Q2-7406 from Dow Corning, PSA 750, PSA 518, PSA 910, PSA 6574 from Momentive Performance Materials, KRT 001, KRT 002, KRT 003 from ShinEtsu, PSA 45559 from Wacker Silicones, and PSA 400 and PSA 401 from BlueStar Silicones. 
     Alternatively, use is made as PSA of PSAs based on an addition-crosslinking silicone comprising the components set out below:
         a) an organopolysiloxane which is composed of at least one diorganosiloxane unit and carries at least two silicon-bonded alkenyl groups in each molecule,   b) an organopolysiloxane resin with the following formula: (R 1   3 SiO 1/2 ) x (SiO 4/2 ) 1 ,
           where R 1  is a substituted or unsubstituted monovalent hydrocarbon group, a hydrogen atom or a hydroxyl group and x is a number between 0.5 and 1.2,   
           c) an organopolysiloxane which carries on average at least two silicon-bonded hydrogen atoms in each molecule, this organopolysiloxane being free from olefinic double bonds, and the amount of this organopolysiloxane being such that there are 0.01 to 10 mol of silicon-bonded hydrogen atoms per mole of the total alkenyl groups of components a), b), and e),   d) an organometallic catalyst from group 10 of the Periodic Table of the Elements,   e) optionally an inhibitor.       

     Silicone PSAs of this kind are freely available commercially. By way of example, mention may be made here of the following: DC 7657 and DC 2013 from Dow Corning, and KR 3700, KR 3701 from ShinEtsu. 
     In order to achieve the necessary technical adhesive properties, the silicone formulations described are admixed with what are called MQ resins, with the formula (R 1   3 SiO 1/2 ) x (SiO 4/2 ) 1 . Referred to as the M unit therein are the (R 1   3 SiO 1/2 ) units, with the Q unit being the (SiO 412 ) units. Each R 1  independently of the others is a monovalent saturated hydrocarbon group, a monovalent unsaturated hydrocarbon group, a monovalent halogenated hydrocarbon group, a hydrogen atom or a hydroxyl group. The ratio of M units to Q units (M:Q) is preferably in the range from 0.5 to 1.2. 
     The MQ resins are advantageously resins having a weight-average molecular weight M w  of 500 g/mol≦M w ≦100 000 g/mol, preferably of 1000 g/mol≦M w ≦25 000 g/mol, the average molecular weight M w  data in this specification relating to the determination by gel permeation chromatography (see later on below; experimental section). 
     It has emerged as being favorable if adhesives are used in which the proportional ratio—based on percent by weight—of polydiorganosiloxane to MQ resin is in the range from 20:80 to 80:20, preferably in the range from 30:70 to 60:40. 
     MQ resins of this kind are freely available commercially. Mention may be made here, by way of example, of the following: SL 160, SL 200, DC 2-7066 from Dow Corning, SR 545, SR 1000, 6031 SL from Momentive Performance Materials, and CRA 17, CRA 42, and MQ Resin 803 from Wacker. 
     Besides the resin modification, further additives as well may be added to the PSA. These additives are selected more particularly such that they do not impair the optical properties. As further additives it is possible typically to utilize the following:
         in-process stabilizers, such as, for example, vinylsilanes or alkynols as inhibitors for the platinum catalyst   in-process accelerants, such as, for example, aminoorganyls   fillers, such as, for example, silicon dioxide, glass (ground or in the form of beads), aluminum oxides or zinc oxides, the fillers in particular being ground to such a small size that they are optically invisible   optionally, further polymers, preferably of elastomeric type; elastomers which can be utilized accordingly include, among others, those based on pure hydrocarbons, for example unsaturated polydienes, such as natural or synthetically produced polyisoprene or polybutadiene; chemically substantially saturated elastomers, such as, for example, saturated ethylene-propylene copolymers, α-olefin copolymers, polyisobutylene, butyl rubber, and ethylene-propylene rubber, and also chemically functionalized hydrocarbons, such as, for example, halogen-containing, acrylate-containing or vinyl ether-containing polyolefins, to name but a few   plasticizers, such as, for example, liquid resins, plasticizer oils or liquid polymers of low molecular mass, such as, for example, low molecular mass silicone oils having molar masses &lt;1500 g/mol (number average)       

     In order to obtain sufficient cohesion, the condensation-crosslinking silicone PSAs are preferably compounded with peroxo initiators. It is particularly preferred for this purpose to use benzoyl peroxide (BPO). The peroxo initiators are used more particularly in an amount of 0.2% to 5% by weight, based on the solids fraction of the silicone adhesive. In order to obtain a reasonable measure between cohesiveness and adhesiveness, a BPO content of 0.5% to 2% by weight is selected more particularly. In the case of the solventborne application of the adhesive, a temperature of 70-90° C. is selected initially for at least 2 minutes, in order to evaporate the solvents. Subsequently, for at least 2 minutes, a temperature of 170-180° C. is set, in order to initiate peroxide decomposition and hence the crosslinking procedure. 
     The achievement of sufficient cohesion for addition-crosslinking silicone adhesives is accomplished in particular by means of a platinum-catalyzed hydrosilylation reaction between the alkenyl-functionalized organopolysiloxanes and the corresponding SiH-functionalized organopolysiloxanes. In the case of application from solution, the solvent is first removed at a temperature of 70-90° C. in a residence time of at least 2 minutes. The temperature is subsequently raised to 100-120° C. and held constant for up to 2 minutes. 
     In addition to the conventional modes of crosslinking for silicone PSAs, by means of peroxides or transition-metal catalysis, these adhesives can also be crosslinked by actinic radiation, especially electron beams. If so, in the case of application from solution, the solvent is first removed at a temperature of 70-90° C. in a residence time of at least 2 minutes. This is followed by crosslinking with an electron beam dose of at least 10 kGy. This mode of crosslinking is particularly advantageous since it allows the cohesion to be adjusted almost infinitely, without adverse effect on the properties of tack and adhesion (cf. also DE 10 2007 053 432.0). 
    
    
     
       Further details, objectives, features, and advantages of the present invention will be elucidated in more detail below by reference to preferred exemplary embodiments. In the drawing, 
         FIG. 1  shows a single-sided pressure-sensitive adhesive tape, 
         FIG. 2  shows a double-sided pressure-sensitive adhesive tape, 
         FIG. 3  shows a carrier-free pressure-sensitive adhesive tape (adhesive transfer tape), 
         FIG. 4  shows the bonding of a rear reinforcement plate of a touch panel, 
         FIG. 5  shows the bonding of different layers of a touch panel. 
     
    
    
     PRODUCT CONSTRUCTION 
       FIG. 1  shows a single-sided pressure-sensitive adhesive tape  1  for use in the bonding of optical components, more particularly of optical films. The PSA tape  1  has an adhesive layer  2  produced by coating one of the above-described PSAs onto a carrier  3 . The PSA coatweight is preferably between 5 and 250 g/m 2 . Particularly in the visible region of the light, the PSA has a transmittance of at least 86%, so making it particularly suitable for optical application. 
     For application in the bonding of optical components, a transparent carrier  2  is also employed as carrier  2 . The carrier  2 , therefore, is likewise transparent in the region of visible light, thus preferably having a transmittance of likewise at least 86%. 
     In addition (not shown) it is possible to provide a release film as well, which covers and protects the adhesive layer  2  before the PSA tape  1  is used. The release film is then removed before the adhesive layer  2  is used. 
     The transparent PSA may preferably be protected with a release film. It is possible, furthermore, for the carrier film to be provided with one or more coatings. The PSA coatweight is preferably between 5 and 250 g/m 2 . 
     The product construction depicted in  FIG. 2  shows a PSA tape  1  having a transparent carrier  3  which is coated on both sides with a PSA and thus has two adhesive layers  2 . The PSA coatweight per side is again preferably between 5 and 250 g/m 2 . 
     In this embodiment as well it is preferred for at least one adhesive layer  2  to be lined with a release film. In the case of a rolled-up adhesive tape, this one release film may optionally also line the second adhesive layer  2 . It is also possible, however, for a plurality of release films to be provided. 
     A further possibility is for the carrier film to be provided with one or more coatings. Moreover, only one side of the PSA tape may be furnished with the inventive PSA, and another transparent PSA may be used on the other side. 
     The product construction depicted in  FIG. 3  shows a pressure-sensitive adhesive tape  1  in the form of an adhesive transfer tape, i.e., a carrier-free PSA tape  1 . For this purpose, the PSA is coated onto one side of a release film  4 , to form a pressure-sensitive adhesive layer  2 . The PSA coatweight here is typically between 5 and 250 g/m 2 , preference being given to a coatweight of 25 to 175 g/m 2 . This pressure-sensitive adhesive layer  2  is optionally also lined on its second side with a further release film. For the use of the PSA tape, the release films are then removed. 
     As an alternative to release films it is also possible, for example, to use release papers or the like. In such a case, however, the surface roughness of the release paper ought to be reduced, in order to produce a PSA side that is as smooth as possible. 
     Carrier Films 
     As carrier films it is possible to use a large number of highly transparent polymer films. Special highly transparent PET films can be used in particular. Suitability is thus possessed, for example, by films from Mitsubishi with the trade name Hostaphan™ or from Toray with the trade name Lumirror™. The haze, a measure of the clouding of a substance, ought in one preferred embodiment to have a value of less than 5% in accordance with ASTM D 1003. High haze denotes low visibility through the substance in question. The light transmittance at 550 nm is preferably greater than 86%, more preferably greater than 88%. A further very preferred species of the polyesters is represented by the polybutylene terephthalate films. 
     Besides polyester films it is also possible to use highly transparent PVC films. These films may include plasticizers in order to increase the flexibility. Moreover, PC, PMMA, and PS films can be used. Besides pure polystyrene, it is also possible to use other comonomers, such as butadiene, for example, in addition to styrene, for the purpose of reducing the propensity to crystallization. 
     Moreover, polyethersulfone films and polysulfone films can be used as carrier materials. These films are obtainable, for example, from BASF under the trade name Ultrason™ E and Ultrason™ S. It is also possible, furthermore, with particular preference, to use highly transparent TPU films. These films are available commercially, for example, from Elastogran GmbH. Use may also be made of highly transparent polyamide films and copolyamide films, and also of films based on polyvinyl alcohol and polyvinyl butyral. 
     Besides single-layer films it is also possible to use multilayer films, which are produced by coextrusion, for example. For this purpose it is possible to combine the aforementioned polymer materials with one another. 
     The films, further, may be treated. Thus, for example, vapor deposition may be performed, with zinc oxide, for example, or else varnishes or adhesion promoters may be applied. One further possible additization is represented by UV protectants, which may be present as additives in the film or may be applied as a protective layer. 
     The film thickness in one preferred embodiment is between 4 μm and 150 μm, more preferably between 12 μm and 100 μm. 
     The carrier film may, for example, also have an optical coating. Particularly suitable optical coatings are coatings which reduce the reflection. This is achieved, for example, through a reduction in the refractive index difference for the air/optical coating transition. 
     Release Film 
     To protect the open (pressure-sensitive) adhesive it is preferably lined with one or more release films. As well as the release films it is also possible—albeit not very preferably—to use release papers, such as glassine, HDPE or LDPE release papers, for example, which in one embodiment have siliconization as a release layer. 
     It is preferred, however, to use a release film. In one very preferred embodiment the release film possesses siliconization, more particularly fluoro-siliconization, as a release means. Furthermore, the film release liner ought to possess an extremely smooth surface, and so no structuring of the PSA is performed by the release liner. This is preferably achieved through the use of antiblocking-agent-free PET films in combination of silicone systems, more particularly fluoro-silicone systems, coated from solution. Besides fluoro-silicone systems, consideration is also preferably given to coatings of fluorinated hydrocarbons on release films. 
     Coating 
     The (pressure-sensitive) adhesives may be coated from solution. For coating from solution, the (pressure-sensitive) adhesive is dissolved in typical solvents, such as toluene, benzine, heptane, etc., and then coated via a coating nozzle or a doctor knife. Particular preference is given to manufacturing the (pressure-sensitive) adhesives from solution, in order to prevent premature crosslinking. However, it is also possible to use all other coating methods which allow solvent-containing coatings. 
     Use 
     The above-described (pressure-sensitive) adhesives and (pressure-sensitive) adhesive tapes are suitable particularly for use in optical applications, where preferably permanent bonds are performed with residence times of greater than one month. 
     One particularly preferred field of use encompasses the adhesive bonding of touch panels and also the production of touch panels.  FIG. 4  shows typical adhesive bonds in resistive touch panels. For this purpose it is preferred to use (pressure-sensitive) adhesive transfer tapes, i.e., tapes without carriers. Top film or reinforcement plate, however, may also be used and bonded in the form of a single-sided (pressure-sensitive) adhesive tape with the corresponding carrier. 
       FIG. 4  shows a touch panel  5  bonded by means of a first pressure-sensitive adhesive tape  1  to a substrate  6 , which is a plastic plate or a glass plate, for example. Applied to the touch panel  5  by means of a second pressure-sensitive adhesive tape  1  is then a top film  7 , which typically has an antiscratch coat. 
       FIG. 5  shows typical adhesive bonds for capacitive touch panels. For the bonding of structured ITO films  8 , in particular, pressure-sensitive adhesive layers  2  with adhesive coatweights of greater than 50 g/m 2  are used, to provide for effective wetting of the structuring. 
     The silicone PSAs are suitable particularly for bubble-free lamination to structured surfaces. In comparison to other PSAs, one particular advantage lies in the significantly lower layer thickness which is necessary in order to permit bubble-free lamination. Thus, for example, acrylate PSAs are used typically with layer thicknesses of 175 μm to 225 μm for this application. With the PSAs of the invention, the same properties can also be realized with layer thicknesses of 50 to 200 μm. 
     On the other hand, however, these silicone PSAs are also suitable for particularly thick layers, in other words, in particular, even for layer thicknesses of up to 250 μm. The production of layers with this kind of thickness may take place in particular in only one coating step, whereas, generally, two or more coating steps are needed for comparable layer thicknesses with acrylate compositions. 
       FIG. 5  additionally shows the bonding of a protective film or of a cell phone window  7 , of a substrate  6  as rear reinforcement plate of a capacitive touch panel, and also of a display  9 , with the PSA described. Both the PSA itself and the PSA in the form of an adhesive transfer tape may be used as a single-sided PSA tape or else as a double-sided PSA tape with carrier film. 
     EXAMPLES 
     Test Methods 
     A. Bond Strength 
     The peel strength (bond strength) was tested in accordance with PSTC-101. The adhesive tape is applied to a glass plate. A strip of the adhesive tape, 2 cm wide, is bonded by being rolled over back and forth three times with a 2 kg roller. The plate is clamped in, and the self-adhesive strip is peeled via its free end on a tensile testing machine at a peel angle of 180° and at a speed of 300 mm/min. The force is reported in N/cm. 
     In the case of measurement on an adhesive transfer tape, the release film is removed from the tape beforehand. This release film is subsequently replaced by a PET film 23 μm thick. 
     B. Transmittance 
     The transmittance at 550 nm is determined in accordance with ASTM D1003. The specimen measured was the assembly made up of optically transparent PSA and glass plate. 
     C. Haze 
     The haze is determined in accordance with ASTM D1003. 
     D. Light stability 
     The assembly made up of PSA and glass plate, with a size of 4×20 cm 2 , is irradiated for 250 hours using Osram Ultra Vitalux 300 W lamps at a distance of 50 cm. Following irradiation, the transmittance is determined by test method C. 
     E. Climatic Cycling Test 
     The PSA is adhered as a single-sided pressure-sensitive adhesive tape (50 g/m 2  coatweight, 50 μm PET film of type Mitsubishi RNK 50) to a glass plate, without air bubbles. The dimensions of the test strip are 2 cm width and 10 cm length. The bond strength to glass is determined by test method A. 
     In parallel, an adhesive assembly of this kind is placed in a climatic cycling cabinet and stored for 1000 cycles. One cycle includes:
         storage at −40° C. for 30 minutes   heating to 85° C. within 5 minutes   storage at 85° C. for 30 minutes   cooling to −40° C. within 5 minutes       

     After the climatic cycling test, the bond strength is determined again by test method A. 
     F. Electrical Conductivity Test 
     The PSA is adhered as a single-sided pressure-sensitive adhesive tape to an ITO film (Elecrysta®) from Nitto Denko. The dimensions of the ITO film are 12 cm×2 cm. The bond area is 10 cm×2 cm, and so 1 cm remains free for electrical measurements at each end. The assembly is subsequently stored in a climate cabinet for 500 hours at 85° C. and 20% humidity. The surface resistivity is then measured in accordance with DIN 53482. This is followed by a determination of the percentage drop in comparison to untreated ITO film. 
     G. Gel Permeation Chromatography (GPC) 
     By means of GPC it is possible to determine the average molar masses (M w ) of polymers. For the M w  determination of the MQ resins, the settings used were as follows: eluent toluene; measuring temperature 23° C.; preliminary column PSS-SDV, 5 μm, 10 2  Å (10 nm), ID 8.0 mm×50 mm; separation: PSS-SDV columns, 5 μm, 10 2  Å (10 nm), and also 10 3  Å (100 nm), and  10   6  Å (10 5  nm), each of ID 8.0 mm×300 mm; sample concentration 3 g/l, flow rate 1.0 ml per minute; polydimethylsiloxane standards). 
     Examples 
     Coating operations in the examples took place on a conventional laboratory coating unit for continuous coating. Coating was carried out in an ISO 5 clean room according to ISO standard 14644-1. The web width was 50 cm. The width of the coating gap was variably adjustable between 0 and 1 cm. The length of the heating tunnel was around 12 m. The temperature in the heating tunnel was divisible into four zones, and was freely selectable in each zone between room temperature and 180° C. 
     Production of the Specimens: 
     The PSAs were coated from solution onto release film. The adhesive sheet thus obtained was dried and crosslinked and then laminated with a further release film. For the measurement of the bond strength, a release film was replaced by a 23 μm thick PET film. 
     Preparation of Adhesives 
     a) Condensation-Crosslinking Adhesives 
     The condensation-crosslinking silicone PSAs were diluted with special-boiling-point spirit (b.p.=60-90° C.) to a solids content of 45%, blended with MQ resin and benzoyl peroxide (BPO), and stirred for 30 minutes. For the subsequent coating, coatweights (after drying) of 50, 100 and 150 g/m 2  were selected (corresponding to layer thicknesses of approximately 50, 100, and 150 μm). 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                 BPO 
                   
                 Bond 
               
               
                   
                   
                   
                 MQ resin 
                 concen- 
                 Coat- 
                 strength 
               
               
                 Ex- 
                 Silicone 
                 MQ 
                 addition 
                 tration 
                 weight 
                 (test A) 
               
               
                 ample 
                 PSA 
                 resin 
                 in % 
                 in % 
                 in g/m 2   
                 in N/cm 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 DC Q2- 
                 — 
                 — 
                 1.0 
                 50 
                 4.7 
               
               
                   
                 7735 
               
               
                 2 
                 DC Q2- 
                 — 
                 — 
                 1.0 
                 100 
                 5.8 
               
               
                   
                 7735 
               
               
                 3 
                 DC Q2- 
                 — 
                 — 
                 1.0 
                 150 
                 6.4 
               
               
                   
                 7735 
               
               
                 4 
                 DC Q2- 
                 DC 2- 
                 5 
                 1.0 
                 50 
                 5.2 
               
               
                   
                 7735 
                 7066 
               
               
                 5 
                 DC Q2- 
                 DC 2- 
                 5 
                 1.0 
                 100 
                 6.2 
               
               
                   
                 7735 
                 7066 
               
               
                 6 
                 DC Q2- 
                 DC 2- 
                 5 
                 1.0 
                 150 
                 6.9 
               
               
                   
                 7735 
                 7066 
               
               
                 7 
                 PSA 45559 
                 — 
                 — 
                 1.0 
                 50 
                 3.8 
               
               
                   
                 SVP 
               
               
                 8 
                 PSA 45559 
                 — 
                 — 
                 1.0 
                 100 
                 4.8 
               
               
                   
                 SVP 
               
               
                 9 
                 PSA 45559 
                 — 
                 — 
                 1.0 
                 150 
                 5.3 
               
               
                   
                 SVP 
               
               
                 10 
                 PSA 45559 
                 MQ 803 
                 5 
                 1.0 
                 50 
                 4.3 
               
               
                   
                 SVP 
               
               
                 11 
                 PSA 45559 
                 MQ 803 
                 5 
                 1.0 
                 100 
                 5.3 
               
               
                   
                 SVP 
               
               
                 12 
                 PSA 45559 
                 MQ 803 
                 5 
                 1.0 
                 150 
                 5.9 
               
               
                   
                 SVP 
               
               
                   
               
            
           
         
       
     
     b) Addition-Crosslinking Adhesives 
     The addition-crosslinking silicone PSAs were diluted with special-boiling-point spirit (b.p.=60-90° C.) to a solids content of 45%, blended with MQ resin and the platinum catalyst Syl-Off  4000  from Dow Corning, and stirred for 30 minutes. For the subsequent coating, coatweights (after drying) of 50, 100 and 150 g/m 2  were selected (corresponding to layer thicknesses of approximately 50, 100, and 150 μm). 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                 Syl-Off 
                   
                   
               
               
                   
                   
                   
                   
                 4000 
                   
                 Bond 
               
               
                   
                   
                   
                 MQ resin 
                 concen- 
                 Coat- 
                 strength 
               
               
                 Ex- 
                 Silicone 
                 MQ 
                 addition 
                 tration 
                 weight 
                 (test A) 
               
               
                 ample 
                 PSA 
                 resin 
                 in % 
                 in % 
                 in g/m 2   
                 in N/cm 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 13 
                 DC 7657 
                 — 
                 — 
                 1.0 
                 50 
                 3.8 
               
               
                 14 
                 DC 7657 
                 — 
                 — 
                 1.0 
                 100 
                 4.6 
               
               
                 15 
                 DC 7657 
                 — 
                 — 
                 1.0 
                 150 
                 5.0 
               
               
                 16 
                 DC 7657 
                 DC 2- 
                 3 
                 1.0 
                 50 
                 4.2 
               
               
                   
                   
                 7066 
               
               
                 17 
                 DC 7657 
                 DC 2- 
                 3 
                 1.0 
                 100 
                 5.1 
               
               
                   
                   
                 7066 
               
               
                 18 
                 DC 7657 
                 DC 2- 
                 3 
                 1.0 
                 150 
                 5.5 
               
               
                   
                   
                 7066 
               
               
                 19 
                 KR 3701 
                 — 
                 — 
                 1.0 
                 50 
                 4.5 
               
               
                 20 
                 KR 3701 
                 — 
                 — 
                 1.0 
                 100 
                 5.2 
               
               
                 21 
                 KR 3701 
                 — 
                 — 
                 1.0 
                 150 
                 5.5 
               
               
                 22 
                 KR 3701 
                 SR 545 
                 5 
                 1.0 
                 50 
                 4.9 
               
               
                 23 
                 KR 3701 
                 SR 545 
                 5 
                 1.0 
                 100 
                 5.6 
               
               
                 24 
                 KR 3701 
                 SR 545 
                 5 
                 1.0 
                 150 
                 5.9 
               
               
                   
               
            
           
         
       
     
     c) EBC-Crosslinked Adhesives 
     Both addition-crosslinking and condensation-crosslinking silicone PSAs were diluted with special-boiling-point spirit (b.p.=60-90° C.) to a solids content of 45%, blended with MQ resin, and stirred for 30 minutes. For the subsequent coating, coatweights (after drying) of 50, 100 and 150 g/m 2  were selected (corresponding to layer thicknesses of approximately 50, 100, and 150 μm). 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                 EBC dose 
                   
                   
               
               
                   
                   
                   
                   
                 in kGy 
               
               
                   
                   
                   
                   
                 (acceler- 
                   
                 Bond 
               
               
                   
                   
                   
                 MQ resin 
                 ation volt- 
                 Coat- 
                 strength 
               
               
                 Ex- 
                 Silicone 
                 MQ 
                 addition 
                 age = 
                 weight 
                 (test A) 
               
               
                 ample 
                 PSA 
                 resin 
                 in % 
                 100 V) 
                 in g/m 2   
                 in N/cm 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 25 
                 DC Q2- 
                 — 
                 — 
                 20 
                 50 
                 4.9 
               
               
                   
                 7735 
               
               
                 26 
                 DC Q2- 
                 — 
                 — 
                 20 
                 100 
                 5.5 
               
               
                   
                 7735 
               
               
                 27 
                 DC Q2- 
                 — 
                 — 
                 20 
                 150 
                 6.0 
               
               
                   
                 7735 
               
               
                 28 
                 DC Q2- 
                 DC 2- 
                 5 
                 20 
                 50 
                 5.3 
               
               
                   
                 7735 
                 7066 
               
               
                 29 
                 DC Q2- 
                 DC 2- 
                 5 
                 20 
                 100 
                 5.9 
               
               
                   
                 7735 
                 7066 
               
               
                 30 
                 DC Q2- 
                 DC 2- 
                 5 
                 20 
                 150 
                 6.3 
               
               
                   
                 7735 
                 7066 
               
               
                 31 
                 DC 7657 
                 — 
                 — 
                 20 
                 50 
                 4.3 
               
               
                 32 
                 DC 7657 
                 — 
                 — 
                 20 
                 100 
                 4.9 
               
               
                 33 
                 DC 7657 
                 — 
                 — 
                 20 
                 150 
                 5.4 
               
               
                 34 
                 DC 7657 
                 DC 2- 
                 3 
                 20 
                 50 
                 4.6 
               
               
                   
                   
                 7066 
               
               
                 35 
                 DC 7657 
                 DC 2- 
                 3 
                 20 
                 100 
                 5.4 
               
               
                   
                   
                 7066 
               
               
                 36 
                 DC 7657 
                 DC 2- 
                 3 
                 20 
                 150 
                 5.9 
               
               
                   
                   
                 7066 
               
               
                   
               
            
           
         
       
     
     Preparation of a Reference Example 1 
     The polymerization was carried out using monomers that had been purified of stabilizers. A 2 l glass reactor conventional for radical polymerizations was charged with 32 g of acrylic acid, 168 g of n-butyl acrylate, 200 g of 2-ethylhexyl acrylate, and 300 g of acetone/isopropanol (97:3 ratio). After nitrogen gas had been passed through the reactor for 45 minutes, with stirring, the reactor was heated to 58° C. and 0.2 g of Vazo67® (DuPont, 2,2′-azodi(2-methylbutyronitrile)) was added. The external heating bath was then heated to 75° C. and the reaction was carried out constantly at this external temperature. After a reaction time of 1 hour a further 0.2 g of Vazo 67® (DuPont, 2,2′-azodi(2-methylbutyronitrile)) was added. After 3 hours and after 6 hours, the batch was diluted with 150 g each time of acetone/isopropanol mixture. In order to reduce the residual initiators, 0.1 g each time of Perkadox 16® (from Akzo Nobel, di(4-tert-butylcyclohexyl)peroxydicarbonate) was added after 8 hours and after 10 hours. After a reaction time of 22 hours, the reaction was discontinued and the batch was cooled to room temperature. This was followed by dilution with isopropanol to a solids content of 30%, and by the addition, with vigorous stirring, of 0.3% by weight of aluminum acetylacetonate. The solution was then coated out onto a release film in the same way as in inventive examples 1-3, and dried at 120° C. for 10 minutes. The coatweight was 50 g/m 2 . The bond strength by test method A was 6.4 N/cm. 
     Results 
     Production of the test specimens showed that inventive examples 1-36 all have high bond strengths. The bond strengths are dependent on the coatweight. Reference specimen 1, based on polyacrylate PSA, shows similar bond strengths by test method A. Inventive examples 1-36 demonstrate that all of the examples are suitable for permanent bonds and not for temporary bonds. 
     For suitability in the optical sector, furthermore, optical measurements were carried out. The translucency is determined in the form of the transmittance by test method B. The clouding of the PSAs is determined by test method C and described by the haze value. The results are collated in table 1 below. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Example 
                 Transmittance (test B) in % 
                 Haze (test C) in % 
               
               
                   
               
             
            
               
                  1 
                 92 
                 0.3 
               
               
                  2 
                 92 
                 0.4 
               
               
                  3 
                 92 
                 0.6 
               
               
                  4 
                 92 
                 0.3 
               
               
                  5 
                 92 
                 0.5 
               
               
                  6 
                 92 
                 0.7 
               
               
                  7 
                 92 
                 0.3 
               
               
                  8 
                 92 
                 0.4 
               
               
                  9 
                 92 
                 0.6 
               
               
                 10 
                 92 
                 0.3 
               
               
                 11 
                 92 
                 0.5 
               
               
                 12 
                 92 
                 0.6 
               
               
                 13 
                 92 
                 0.3 
               
               
                 14 
                 92 
                 0.5 
               
               
                 15 
                 92 
                 0.6 
               
               
                 16 
                 92 
                 0.4 
               
               
                 17 
                 92 
                 0.6 
               
               
                 18 
                 92 
                 0.7 
               
               
                 19 
                 92 
                 0.3 
               
               
                 20 
                 92 
                 0.4 
               
               
                 21 
                 92 
                 0.6 
               
               
                 22 
                 92 
                 0.4 
               
               
                 23 
                 92 
                 0.6 
               
               
                 24 
                 92 
                 0.7 
               
               
                 25 
                 92 
                 0.4 
               
               
                 26 
                 92 
                 0.6 
               
               
                 27 
                 92 
                 0.7 
               
               
                 28 
                 92 
                 0.4 
               
               
                 29 
                 92 
                 0.5 
               
               
                 30 
                 92 
                 0.8 
               
               
                 31 
                 92 
                 0.4 
               
               
                 32 
                 92 
                 0.6 
               
               
                 33 
                 92 
                 0.7 
               
               
                 34 
                 92 
                 0.4 
               
               
                 35 
                 92 
                 0.5 
               
               
                 36 
                 92 
                 0.7 
               
               
                 Reference 1 
                 93 
                 0.4 
               
               
                   
               
            
           
         
       
     
     From the tests it is evident that all of the inventive examples have high transmittance values of 92%, which is reduced by the reflection of light. In the case of glass/glass bonds, with the subtraction of the reflection losses, transmittance values of greater than 99% can be measured. The polyacrylate-based reference example shows similar transmittance values. The inventive silicone PSAs are therefore suitable for high optical transmissions. The haze measurements according to test method C provide confirmation of this. The haze values measured are below 1% for all of the inventive silicone PSAs, and therefore meet extremely exacting requirements. 
     Subsequently, furthermore, various aging investigations were carried out. First, a light stability test was carried out by test method D. This test examines whether long sunlight irradiation causes a discoloration or yellowing. This is particularly important for optical applications which are subject to long-term irradiation, such as by a display, for example, or are used in the outdoor sector. The results are summarized in table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Light stability (test D) 
               
               
                   
                 Example 
                 transmittance in % 
               
               
                   
                   
               
             
            
               
                   
                  1 
                 92 
               
               
                   
                  2 
                 92 
               
               
                   
                  3 
                 92 
               
               
                   
                  4 
                 92 
               
               
                   
                  5 
                 92 
               
               
                   
                  6 
                 92 
               
               
                   
                  7 
                 92 
               
               
                   
                  8 
                 92 
               
               
                   
                  9 
                 92 
               
               
                   
                 10 
                 92 
               
               
                   
                 11 
                 92 
               
               
                   
                 12 
                 92 
               
               
                   
                 13 
                 92 
               
               
                   
                 14 
                 92 
               
               
                   
                 15 
                 92 
               
               
                   
                 16 
                 92 
               
               
                   
                 17 
                 92 
               
               
                   
                 18 
                 92 
               
               
                   
                 19 
                 92 
               
               
                   
                 20 
                 92 
               
               
                   
                 21 
                 92 
               
               
                   
                 22 
                 92 
               
               
                   
                 23 
                 92 
               
               
                   
                 24 
                 92 
               
               
                   
                 25 
                 92 
               
               
                   
                 26 
                 92 
               
               
                   
                 27 
                 92 
               
               
                   
                 28 
                 92 
               
               
                   
                 29 
                 92 
               
               
                   
                 30 
                 92 
               
               
                   
                 31 
                 92 
               
               
                   
                 32 
                 92 
               
               
                   
                 33 
                 92 
               
               
                   
                 34 
                 92 
               
               
                   
                 35 
                 92 
               
               
                   
                 36 
                 92 
               
               
                   
                 Reference 1 
                 91 
               
               
                   
                   
               
            
           
         
       
     
     From table 2 it is apparent that all examples, and reference example 1, have a stable transmittance, and there is no drop, or only a very small drop, in the case of reference example 1, in the transmittance. 
     A further aging test includes climatic cycling. Here, the exposure of the adhesive to very different climatic conditions is simulated, as may be the case, again, for end applications in the cell phone segment. The climatic cycling test was carried out by test method E. The results are set out in table 3. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Bond strength after 
               
               
                   
                 Bond strength (test A) 
                 climatic cycling test (test E) 
               
               
                 Example 
                 in N/cm 
                 in N/cm 
               
               
                   
               
             
            
               
                  1 
                 4.7 
                 4.9 
               
               
                  2 
                 5.8 
                 6.2 
               
               
                  3 
                 6.4 
                 6.8 
               
               
                  4 
                 5.2 
                 5.5 
               
               
                  5 
                 6.2 
                 6.7 
               
               
                  6 
                 6.9 
                 7.0 
               
               
                  7 
                 3.8 
                 4.1 
               
               
                  8 
                 4.8 
                 5.0 
               
               
                  9 
                 5.3 
                 5.6 
               
               
                 10 
                 4.3 
                 4.5 
               
               
                 11 
                 5.3 
                 5.6 
               
               
                 12 
                 5.9 
                 6.4 
               
               
                 13 
                 3.8 
                 3.9 
               
               
                 14 
                 4.6 
                 4.7 
               
               
                 15 
                 5.0 
                 5.3 
               
               
                 16 
                 4.2 
                 4.4 
               
               
                 17 
                 5.1 
                 5.4 
               
               
                 18 
                 5.5 
                 5.8 
               
               
                 19 
                 4.5 
                 4.9 
               
               
                 20 
                 5.2 
                 5.7 
               
               
                 21 
                 5.5 
                 5.9 
               
               
                 22 
                 4.9 
                 5.2 
               
               
                 23 
                 5.6 
                 5.8 
               
               
                 24 
                 5.9 
                 6.3 
               
               
                 25 
                 4.9 
                 5.3 
               
               
                 26 
                 5.5 
                 5.7 
               
               
                 27 
                 6.0 
                 6.2 
               
               
                 28 
                 5.3 
                 5.5 
               
               
                 29 
                 5.9 
                 6.3 
               
               
                 30 
                 6.3 
                 6.5 
               
               
                 31 
                 4.3 
                 4.6 
               
               
                 32 
                 4.9 
                 5.1 
               
               
                 33 
                 5.4 
                 5.7 
               
               
                 34 
                 4.6 
                 4.7 
               
               
                 35 
                 5.4 
                 5.6 
               
               
                 36 
                 5.9 
                 6.2 
               
               
                 Reference 1 
                 6.4 
                 6.9 
               
               
                   
               
            
           
         
       
     
     The measurements from table 3 show a very stable level of bonding for all inventive examples. The only marked feature, in the case of reference example 1, was the formation of bubbles that was found, probably caused by outgassing from the PET film. The inventive examples did not exhibit any such problem, in contrast. Nor in any of the example were there instances of lifting or the like. This is a further indicator of sufficiently high bonding strength. 
     As a last measurement, a bond on an ITO film was performed once again with all of the inventive and reference examples. ITO films are used very frequently for producing touch panels. The objective here is that the electrical conductivity of the ITO layer is not adversely affected even after bonding with a PSA. Test method F measures the bonding over a prolonged time period. It provides a simulation of whether there are aging effects which adversely affect the electrical conductivity over time. The loss in percent is determined by comparing the fresh measurement with the stored assembly. For touch panel applications, losses of less than 5% are necessary in order to possess high suitability. 
     The results for these investigations are summarized in table 4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Electrical conductivity test 
               
               
                   
                 Example 
                 (test F) loss in % 
               
               
                   
                   
               
             
            
               
                   
                  1 
                 &lt;1 
               
               
                   
                  2 
                 &lt;1 
               
               
                   
                  3 
                 &lt;1 
               
               
                   
                  4 
                 &lt;1 
               
               
                   
                  5 
                 &lt;1 
               
               
                   
                  6 
                 &lt;1 
               
               
                   
                  7 
                 &lt;1 
               
               
                   
                  8 
                 &lt;1 
               
               
                   
                  9 
                 &lt;1 
               
               
                   
                 10 
                 &lt;1 
               
               
                   
                 11 
                 &lt;1 
               
               
                   
                 12 
                 &lt;1 
               
               
                   
                 13 
                 &lt;1 
               
               
                   
                 14 
                 &lt;1 
               
               
                   
                 15 
                 &lt;1 
               
               
                   
                 16 
                 &lt;1 
               
               
                   
                 17 
                 &lt;1 
               
               
                   
                 18 
                 &lt;1 
               
               
                   
                 19 
                 &lt;1 
               
               
                   
                 20 
                 &lt;1 
               
               
                   
                 21 
                 &lt;1 
               
               
                   
                 22 
                 &lt;1 
               
               
                   
                 23 
                 &lt;1 
               
               
                   
                 24 
                 &lt;1 
               
               
                   
                 25 
                 &lt;1 
               
               
                   
                 26 
                 &lt;1 
               
               
                   
                 27 
                 &lt;1 
               
               
                   
                 28 
                 &lt;1 
               
               
                   
                 29 
                 &lt;1 
               
               
                   
                 30 
                 &lt;1 
               
               
                   
                 31 
                 &lt;1 
               
               
                   
                 32 
                 &lt;1 
               
               
                   
                 33 
                 &lt;1 
               
               
                   
                 34 
                 &lt;1 
               
               
                   
                 35 
                 &lt;1 
               
               
                   
                 36 
                 &lt;1 
               
               
                   
                 Reference 1 
                 15 
               
               
                   
                   
               
            
           
         
       
     
     The results of measurement make it clear that the comparative example, reference 1, causes a significant drop in electrical conductivity. Reference example 1 is based on a polyacrylate and contains acrylic acid groups. The inventive examples 1 to 36, in contrast, display virtually no loss in electrical conductivity of ITO. Examples 1-36 are therefore outstandingly suitable for the bonding of ITO films and have significant advantages over commercial acrylate PSAs which are likewise employed for ITO bonding in the optical segment. 
     In summary, the results of measurement demonstrate that the inventive PSAs and also their inventive use are extremely well suited to the bonding of touch panels or to the bonding of ITO films for capacitive touch panels. The PSAs have significant advantages over existing PSAs based, for example, on polyacrylate.