Abstract:
An solvent-based pressure sensitive adhesive (PSA) having improved shear adhesion is composed of an acrylic-based PSA resin, an ethylenically unsaturated polyester, an ultraviolet radiation (UV) photosensitizer, and fugative organic solvent for the acrylic-based PSA resin. The PSA is cured by evaporation of the fugative organic solvent coupled with heat and UV irradiation or UV irradiation alone. Such PSA has improved shear adhesion by dint of the addition of the ethylenically-unsaturated polyester and an ultraviolet radiation (UV) photosensitizer. A method for adhering two substrates using the PSA includes applying the PSA to one or both of substrates. The solvent in the PSA is evaporated from the applied PSA. Finally, the polyester component of the applied PSA is cured by exposure of the applied PSA to ultraviolet (UV) radiation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to adhesive compositions and more particularly to improving the shear adhesion and gasoline resistance of pressure sensitive adhesives (PSA&#39;s). The disclosed PSA&#39;s, then, will find wide utility for membrane switch applications, especially in rugged environments where, say, resistance to solvents (e.g., gasoline) is required. 
     The ASTM subcommittee on Membrane Switches defines a “membrane switch” as: “A momentary switch device in which at least one contact is on, or made of, a flexible substrate.” To be more specific, all of the following conditions or features often are said to be necessary in order to be truly classified as a membrane switch: 
     (1) direct ohmic contact—the poles of the switch must make physical contact, i.e., cannot be non-contact type like capacitive, ferrite core, or Hall effect; 
     (2) momentary action—upon release, the poles immediately separate as the flexing membrane returns to its original position; 
     (3) low voltage application—a membrane switch is designed to be used in low voltage, d.c. Logic-level-signal applications; and 
     (4) membrane layer—a think pliable layer that carries one pole, both poles, or that flexes during the switching operation used to short both switch poles together. 
     The Membrane Switch Manufacturers&#39; Association defines a “membrane switch” as having the following two essential characteristics: a switch, the actuator of which is a membrane, shall involve mechanical input via at least one component which is a membrane and an integral part of the switch; and its operation shall result in a change of electrical state between at least two terminals. See, for example, the Industry Standard for Membrane Switches, Issue No. 2, July 1991, published by The Membrane Switch Manufacturers&#39; Association, London, England. 
     A membrane switch typically is composed of up to three layers. The intermediate layer is a flexible screened layer with movable contacts. An insulator through which openings are cut at the aligned contact points separates it from the bottom layer. When pressure is applied to the top contact, the film or membrane flexes toward the stationary circuit contact until the circuit is closed. Removal of pressure breaks the contact and the circuit is opened by virtue of the flexible membrane returning to its normally open, separated position. Generally, a top layer is laminated to the intermediate layer. This top layer has graphics and/or alphanumeric characters that locate the switch push point for the user and may also describe the operation of the membrane switch. 
     In most cases the faceplate or top layer for the membrane switch is printed on polyester or polycarbonate material. Polyester is more resistant to chemicals found in industrial settings and is better suited for use with tactile feedback switches because it will not crack over time. Polycarbonate is optically clear at any thickness, provides a cleaner emboss, and will operate at higher temperatures. Thus, the designed use of the membrane switch ordinarily will dictate the materials of construction. 
     Laminate of membrane switch layers typically is accomplished with adhesives and often with PSA&#39;s. Not only must the PSA perform its intended function of laminating the flexible plies together, but it also must withstand the rigors of use of the membrane switches. When a membrane switch is to be used in an industrial environment or location (industrial or retail) whereat gasoline and similar solvating materials are being used and/or transferred, the PSA also must be able to display gasoline resistance to be considered a acceptable material for membrane switches. Too, the PSA of choice also must possess adequate tack, peel, and shear performance. In this regard shear adhesion is an important characteristic for membrane switch adhesives. 
     BRIEF SUMMARY OF THE INVENTION 
     A solvent-based pressure sensitive adhesive (PSA) having improved shear adhesion is composed of an acrylic-based PSA resin, an ethylenically unsaturated polyester, an ultraviolet radiation (UV) photosensitizer, and fugative organic solvent for the acrylic-based PSA resin. The PSA is cured by evaporation of the fugative organic solvent coupled with heat and UV irradiation or UV irradiation alone. Such PSA h as improved shear adhesion by dint of the addition of the ethylenically-unsaturated polyester and an ultraviolet radiation (UV) photosensitizer. A method for adhering two substrates using the PSA includes applying the PSA to one or both of substrates. The solvent in the PSA is evaporated from the applied PSA. Finally, the polyester component of the applied PSA is cured by exposure of the applied PSA to heat and/or ultraviolet (UV) radiation in the presence of an appropriate initiator(s). 
     DETAILED DESCRIPTION OF THE INVENTION 
     The first ingredient of the inventive adhesive is an ethylenically-unsaturated acrylic-based polymer or oligomer which can be cured in the presence of free radicals that are generated from the free-radical initiator in the adhesive. The major ingredient in the copolymer(s) broadly is a C 4 -C 18  alkyl (meth)acrylate monomer and advantageously a C 4 -C 12  alkyl (meth)acrylate monomer. n-Butyl acrylate and 2-ethylhexyl acrylate are two monomers that have proven advantageous in forming the copolymer(s) of the present invention, though other straight chain, alicyclic, and cyclic alkyl (meth)acrylate monomers may be used as is necessary, desirable, or convenient. By convention, the parentheticals used herein designate optional content, ie. (meth)acrylate means “acrylate” or “methacrylate”, and the same is true for the parenthetical plurals used herein. Also by convention, copolymer and interpolymer both mean a polymer of two or more monomers. The selection of the terms as used herein is for the clarity of understanding and not by way of limitation. 
     Referring now to suitable polymerizable monomers, broadly, such monomers include any ethylenically unsaturated monomer or oligomer which can be (co)polymerized in the presence of the initiator. In adhesives technology, acrylic or acrylate compounds find wide acceptance in industry. Another suitable class of ethylenically unsaturated compounds are vinyl compounds, while a third class are compounds containing backbone ethylenic unsaturation as typified by ethylenically unsaturated polyester oligomers. 
     Referring with more particularity to reactive acrylic or acrylate monomers or oligomers, a variety of monoacrylate monomers find use in accordance with the present invention. Monoacrylates include, for example, allyl (meth)acrylate, C 1 -C 12  alkyl and cycloalkyl (meth)acrylates, such as, for example, butyl acrylate, 2-ethylhexyl acrylate, isooctylacrylate, amyl acrylate, lauryl acrylate, iso-propyl acrylate, and the like, and corresponding monomethacrylates which include, for example, benzyl methacrylate, stearyl methacrylate, decyl methacrylate, cyclohexyl methacrylate, and the like, and mixtures thereof. The foregoing monomers are merely representative and not limiting of the list of acrylate and methacrylate monomers suitable for use in the present invention, as those skilled in the art will appreciate. 
     The organic solvent, or often mixture of solvents, must be effective in solubilizing the acrylic-based PSA resin. Such solvents broadly include alcohols, esters, ethers, or aromatic solvents, and mixtures thereof. Representatives of such solvents include, for example, ethyl acetate, butyl acetate, methyl ethyl ketone, cyclohexanone, acetone, toluene, xylene, benzene, cyclohexane, and the like, and mixtures thereof. Ethylacetate and toluene are presently preferred solvents. The proportion of solvent is adjusted to provide a non-volatile solids content of the reaction product of between about 25% and 75%. 
     Unsaturated polyesters additionally can be used in the present invention. These materials are synthesized, for example, by reacting maleic anhydride or a mixture of maleic anhydride and other ethylenically-unsaturated diacids with a diol or a mixture of diols. The mixture is heated in the presence of a catalyst to temperatures sufficient to remove the water formed in the condensation reaction. These materials have molecular weights of, for example, about 1,000-100,000 (Mn), and have the free-radically polymerizable unsaturated groups present in the polymer backbone. 
     Additional conventional additives may be incorporated into the adhesive composition, including, for example, wetting agents, pigments, antioxidants, ultraviolet absorbers, antistatic agents, lubricants, fillers, opacifying agents, anti-foam agents, rheology agents, and the like and mixtures thereof. The adhesives may be applied to one or both sides of the film face stock in a conventional manner, for example, by spraying, knife coating, roller coating, casting, drum coating, dipping, and the like. Indirect application using a transfer process with silicon release paper also can be used. After the adhesive has been applied, the coated film face stock is dried conventionally. 
     Ultraviolet photosensitizers or sensitizers are combined with the monomers or prepolymers used to form the polymeric networks for achieving cure in the presence of UV radiation. Useful UV sensitizers or photosensitizers include halogenated polynuclear ketones such as disclosed in U.S. Pat. No. 3,827,957; and organic carbonyl compounds selected from alkyl phenones, benzophenones, and tricyclic fused ring compounds as disclosed in U.S. Pat. No. 3,759,807. Further useful UV sensitizers include carbonylated phenol nuclear sulfonyl chlorides, such as set forth in U.S. Pat. No. 3,927,959. Additional useful photosensitizer combinations particularly suited for pigmented formulations are a combination of sensitizers comprising aromatic carbonyl compounds, aromatic aldehydes or aromatic ketones, and a synergistic sensitizer of about 0.05% to about 3% of 2,2′-dithiobis(benzothiazole), as set forth in U.S. Pat. No. 3,847,771. Typically, at least about 0.5% by weight of the UV sensitizer, and preferably about 1%-5% sensitizer, is added to the ingredients and thoroughly mixed or otherwise dispersed in the liquid carrier ingredients. 
     In practicing the present invention, the modified PSA is used to adhesively joint two substrates. Heat may be applied to assist is solvent expulsion from the PSA. Thereafter, UV irradiation, optionally with heat, can be used to post-cure the unsaturated polyester. By this technique, the resistance of the PSA to gasoline is improved as is the shear adhesion of the modified PSA. 
    
    
     While the invention has been described with reference to a preferred embodiment, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. In this application all units are in the metric system and all amounts and percentages are by weight, unless otherwise expressly indicated. Also, all citations referred herein are expressly incorporated herein by reference. 
     EXAMPLE 
     The following test procedures and reagents were used in the Example: 
     1. 180° Peel Test: PSTC-1 (November 1975), Pressure Sensitive Tape Council, Glenview, III. Results of this test are reported in pounds/inch for a 1 in strip. 
     2. 178° Shear Test: PSTC-7 (November 1975). Pressure Sensitive Tape Council. Results of this test are reported in hours/500 gm/0.25 in 2  at 22° C. 
     3. Polyken Tack Test: This test is conducted on a Polyken, Jr. Probe Tack Tester (Polyken is a trademark of the Kendall Company) supplied by Testing Machines, Inc. (Amityville, N.Y.) under the following conditions: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Probe: 
                 304 SS. 0.5 cm. diameter probe with a 280 grit 
               
               
                   
                 abrasive finish. 
               
               
                 Dwell Time: 
                 1 second 
               
               
                 Probe Contact Pressure: 
                 100 gm/cm 2   
               
               
                 Probe Retraction Rate: 
                 1 cm/sec. 
               
               
                 Annular Weight: 
                 20 gm.-100 gm/cm 2  pressure of a 0.5 cm. 
               
               
                   
                 diameter probe 
               
               
                 Procedure: 
                 A one-inch square of MYLAR ® polyester 
               
               
                   
                 film coated with the adhesive is placed on top 
               
               
                   
                 of the annular weight so that the hole is 
               
               
                   
                 completely covered by the adhesive area and 
               
               
                   
                 this assembly placed in the weight carrier well. 
               
               
                   
                 The machine is activated and the sequence of 
               
               
                   
                 probe pressure and probe retraction auto- 
               
               
                   
                 matically accomplished. The force required to 
               
               
                   
                 free the probe from adhesive coated film, 
               
               
                   
                 measured in grams/cm 2  is read from the 
               
               
                   
                 indicator dial on the machine. 
               
               
                   
               
             
          
         
       
     
     4. Adhesive Failure Code: 
     C=clean, adhesive failure 
     gh=ghosting 
     PT=partial transfer 
     TT=total transfer 
     5. Ingredients 
     Aroset 1860-A-45 acrylic PSA; typical properties 44-46% solids, 8250-13750 cps (Brookfield viscosity @ 77° F.), ethyl acetate/toluene/heptane/isopropanol solvents (5:33:7:6 ratio), 7.9±0.2 lb/gal density 20°-73° F. flash point range (SETA) (Ashland Chemical Company, Columbus, Ohio) 
     Aropol Q 6585 Unsaturated polyester resin; typical properties @ 25° C.: 65% nv solids, 900 cps (Brookfield viscosity), color 2 Gardner (max), AN 30, 0.01% water max (Karl Fisher), 9.3 lb/gal density (Ashland Chemical Company, Columbus, Ohio) 
     Sartomer CN-966J75 is an aliphatic polyester-based urethane diacrylate oligomer blended with 25% isobornyl acrylate; typical properties: clear liquid, 45 color alpha, 8.791 lb/gal density, 238% elasticity, functionality of 270 modulus (psi %), 25% monomer content, 1.4757 refractive index (25° C.), 428 psi tensile strength, Tg of −33° C., viscosity of 4240 cps @ 60° C. (Sartomer Company, Exton, Pa.) 
     Irgacure 500 is an equal mixture of 1-hydroxycyclohexylphenylketone (I) and benzophenone (II), as shown below:                           
     A PSA was formulated as follows: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Ingredient* 
                 Amount (g) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aroset 1860-A-45 PSA 
                 702.26 
               
               
                   
                 Aropol Q 6585 Unsaturated polyester 
                 105.33 
               
               
                   
                 Ethyl acetate solvent 
                 179.58 
               
               
                   
                 Sartomer CN-966J75 
                 11.85 
               
               
                   
                 Irgacure 500 
                 0.99 
               
               
                   
                   
               
             
          
         
       
     
     The adhesive formulation was coated to a 2 mil dry film onto a silicon release liner and transferred to clear PET (polyethylene terephthalate) face stock and air dried for 15 minutes followed by thermal treatment at 200° F. for 8 minutes.. One lot of the adhesive film then also was post-cured with about 1925 mJ/cm 2  UV radiation. The resulting film had good resistance to gasoline soak. The adhesive properties of the film are as follows: 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Thermal Cure Only 
                 Thermal Cure + 
               
               
                   
                 15 min Air Dry 
                 UV Cure 
               
               
                 Adhesive Properties 
                 8 min @ 200° F. 
                 ˜1925 mJ/cm 2   
               
               
                   
               
             
             
               
                 180° Peel Test 
                   
                   
               
               
                 (3 test avg., lb/in) 
               
               
                 15 min Dwell 
                 5.4 ± 0.4 (C) 
                 4.2 ± 0.3 (C) 
               
               
                 24 hr @ RT 
                 5.2 ± 0.3 (C) 
                 3.5 ± 0.4 (C) 
               
               
                 24 hr @ 160° F. 
                 5.8 ± 0.7 (C) 
                 6.2 ± 0.4 (gh) 
               
               
                 168 hr @ RT 
                 5.0 ± 0.3 (C) 
                 4.8 ± 0.3 (C) 
               
               
                 168 hr @ 160° F. 
                 7.6 ± 0.3 (TT) 
                 8.6 ± 0.4 (TT) 
               
               
                 Tack Properties 
               
               
                 Loop Tack 
                 2.4 (C) 
                 2.4 (C) 
               
               
                 (3 test avg., lb) 
               
               
                 Polyken Probe Tack 
                 672 
                 424 
               
               
                 (3 test avg., g) 
               
               
                 Shear Adhesion (hrs) 
               
               
                 1″ × 1″ × 4 lbs (no dwell) 
               
               
                 1 
                 19.1 (gh) 
                 496+ 
               
               
                 2 
                  9.6 (gh) 
                 496+ 
               
               
                 3 
                 84.3 (PT) 
                 496+ 
               
               
                   
               
             
          
         
       
     
     These results demonstrate that the inventive adhesive formulation maintained its adhesive properties (e.g., adhesion and tack) while greatly improving its shear adhesion. This improvement in shear adhesion coupled with its good gasoline resistance makes the inventive adhesive a prime candidate for use as a membrane switch in applications where solvent resistance (e.g., gasoline) is required.