Abstract:
Cyanoacrylate adhesive containing a pyrylium salt as a fluorescent dye are useful for bonding various substrates, particularly transparent plastics. The pyrylium salt may be present in relatively large quantities in the adhesive without any adverse effect on the storage stability and adhesive properties, yet shows very little inherent coloration in visible light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a cyanoacrylate adhesive containing a pyrylium salt as a fluorescent dye and to its use. 
     2. Discussion of Related Art 
     Adhesives of this type are already known. Thus, EP 0 105 062 B1 describes a cyanoacrylate adhesive containing at least one dye from the group consisting of C.I. Solvent Green 5, C.I. Acid Red 50 and C.I. Acid Red 52 in a quantity of 5 to 1000 ppm, based on the monomers. The disadvantage of these dyes lies in their bright, clearly visible color, particularly where the parts to be joined are transparent. In addition, the dyes are poorly soluble in the monomers. Accordingly, it is not possible to prepare a master batch of cyanoacrylates and the dye, i.e. a stock mixture of cyanoacrylates with a high concentration of dye which the user himself is then able to dilute to the extent required by the particular situation. 
     Against the background of this prior art, the problem addressed by the present invention was to provide a cyanoacrylate adhesive containing a fluorescent dye, the dye showing very little, if any, inherent coloration in visible light and lending itself to use in relatively large quantities without any adverse effect on the storage stability and adhesive properties of the adhesive. 
     SUMMARY OF THE INVENTION 
     The solution provided by the invention is defined in the claims and consists in a highly specific choice of the pyrylium salt. The cyanoacrylate adhesives according to the invention are characterized by an effective concentration of at least one pyrylium salt corresponding to the following general formula:                           
     in which R 1  to R 5  represent hydrogen, alkyl and aryl groups or a fused aromatic radical at two vicinal positions. The substituents may in turn bear alkyl, aryl, alkoxy, aryloxy groups, halogen or pseudohalogen atoms. The total number of carbon atoms in the substituents should be from 1 to 18 and, more particularly, from 6 to 10. The substituents may be the same or different. The counterion A −  may be selected from non-nucleophilic anions such as, for example, hexafluorophosphate, hexafluoroantimonate, tetrafluoroborate, perchlorate and tetrachloroferrate. Dyes used in accordance with the invention include 2,4,6-triphenyl pyrylium tetrafluoroborate, 2,4,6-triphenyl pyrylium trifluoromethane sulfonate, 2,4-ditert-butyl-6-(2-methoxyphenyl)-pyrylium tetrafluoroborate, 1,4-phenylene-4,4″-bis-(2,6-diphenyl-4-pyrylium tetrafluoroborate), 2,4-diphenyl-6-(4-methoxyphenyl)pyrylium perchlorate; 2,4-diphenyl-6-(p-tolyl)-pyrylium tetrachloroferrate. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The dyes are present in the adhesive in concentrations of 5 to 50,000 ppm and, more particularly, 20 to 10,000 ppm. 
     The adhesive is preferably free from solvents or solubilizers for the dyes and the monomers. Solvents are preferably used where a concentration of more than 0.3% by weight of dye is to be dissolved. 
     The following example illustrates the high solubility of the dyes used in accordance with the invention: 
     2,4,6-triphenyl pyrylium tetrafluoroborate                           
     can be introduced into ethyl cyanoacrylate at room temperature in a quantity of up to about 2.5%. By contrast, other dyes, such as for example Acid Red 50 and Acid Red 52, only dissolve in concentrations of up to a few hundred ppm at room temperature. 
     This high concentration enables the master batch process to be applied. If, for example, the stock solution has a concentration of 2.5% by weight, the user can easily dilute it to the required concentration without encountering dosage or homogeneity problems. The stock solution preferably has a concentration of 0.1 to 5% by weight of pyrylium salt (the upper limit being determined by the solubility factor). 
     Another unexpected advantage of the dyes used in accordance with the invention is their fastness to light. In the liquid state, a cyanoacrylate adhesive colored with the pyrylium salts mentioned above shows no reduction in fluorescence intensity, even after several minutes&#39; exposure to UV light (254 and 365 nm). 
     In addition, the relatively low inherent coloration of the dyes used in accordance with the invention in visible light enables them to be universally used because bonds established with the correspondingly colored adhesives do not show any visible inherent coloration in the adhesive joint, even where the joined parts are transparent. They fluoresce on exposure to short-wave light. The fluorescence signal reflected by the adhesive present on the substrate enables the application of the adhesive to be controlled with the assistance of a luminescence probe. 
     In the context of the invention, “α-cyanoacrylates” are understood to be both typical monocyanoacrylates and also biscyanoacrylates. 
     “Typical monocyanoacrylates” encompass compounds corresponding to the following general formula: 
     
       
         H 2 C═C(CN)—CO—O—R  (II)  
       
     
     in which R is an alkyl, alkenyl, cycloalkyl, aryl, alkoxyalkyl, aralkyl or haloalkyl group containing 1 to 24 carbon atoms and, more particularly, 1 to 12 carbon atoms, more especially a methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, pentyl, hexyl, allyl, methallyl, crotyl, propargyl, cyclohexyl, benzyl, phenyl, cresyl, 2-chloroethyl, 3-chloropropyl, 2-chlorobutyl, trifluoroethyl, 2-methoxyethyl, 3-methoxybutyl, 2-methoxyisopropyl and 2-ethoxyethyl group. The cyanoacrylates mentioned above are known to the expert on adhesives, cf. Ullmann&#39;s Encyclopaedia of Industrial Chemistry, Vol. A1, page 240, Verlag Chemie Weinheim (1985) and U.S. Pat. No. 3,254,111 and U.S. Pat. No. 3,654,340. Preferred monomers are the allyl, methoxyethyl, ethoxyethyl, methyl, ethyl, propyl, isopropyl or butyl esters of 2-cyanoacrylic acid. 
     “Bis(cyanoacrylates)” are understood to be compounds corresponding to the following general formula: 
     
       
         [H 2 C═C(CN)—CO—O] 2 R 6   (III)  
       
     
     in which R 6  is a branched or unbranched difunctional alkane radical containing 2 to 18 and, more particularly, 6 to 12 carbon atoms which may additionally contain hetero atoms, such as halogens and oxygen, or aliphatic or aromatic rings. However, R 6  is preferably a pure hydrocarbon. 
     It is important that the bis(cyanoacrylates) should be particularly pure. This requirement is satisfied, for example, by the following methods of production and purification. Essentially, monocyanoacrylates are transesterified with diols and the reaction mixtures are then worked up by fractional crystallization. Accordingly, a suitable method for the production of biscyanoacrylates comprises transesterifying 2-cyanoacrylic acid or alkyl esters thereof corresponding to the following general formula: 
     
       
         H 2 C═C(CN)—CO—O—R 7   (IV)  
       
     
     in which R 7  is a branched or unbranched radical containing 1 to 6 carbon atoms, with diols corresponding to the following general formula: 
     
       
         [HO] 2 R 1   (V)  
       
     
     in which R 1  is a branched or unbranched difunctional alkane radical containing 2 to 18 carbon atoms which may additionally contain hetero atoms, such as halogens and oxygen, or aliphatic or aromatic rings, to form bis(cyanoacrylates) corresponding to general formula III and then purifying the reaction mixture by fractional crystallization. Further particulars can be found in WO 95/33708. 
     Besides these two key components, the cyanoacrylate adhesive also contains typical stabilizers, for example hydroquinone, sulfur dioxide, boron trifluoride and methane sulfonic acid, in the usual quantities. 
     In addition, the adhesive according to the invention may contain other additives, for example plasticizers, activators or accelerators, thickeners and/or elasticizers, coupling agents and other dyes. 
     The adhesives according to the invention are prepared in the usual way by mixing the components. 
     The adhesives according to the invention may be used for bonding typical substrates, more particularly substrates of the same material or of different materials, for example metal, elastomers and plastics, and above all for bonding transparent substrates of polystyrene, polymethyl methacrylate and polycarbonate. 
     The invention is illustrated by the following Examples. 
    
    
     EXAMPLES 
     1. Reactivity and Stability in Storage 
     The substances 2,4,6-triphenyl pyrylium tetrafluoroborate and 2,4,6-triphenyl pyrylium trifluoromethane sulfonate were added in concentrations of 50 ppm to 1,000 ppm (see Table 1) to ethyl α-cyanoacrylate stabilized with 500 ppm of hydroquinone and 10 ppm of SO 2 . The additions readily dissolved in the ethyl cyanoacrylate by brief shaking. 
     The samples thus prepared were evaluated according to various criteria to determine whether the fluorescent additives had any effect on the reaction rate and/or storage stability of cyanoacrylate adhesives. Viscosity was measured both shortly after preparation of the samples and also after storage for 10 days at 70° C. (recirculating air heating cabinet) in polyethylene bottles. 
     The curing rate was determined as the setting time on a round EPDM foam rubber cord (diameter 15 mm) as follows: 
     The EPDM foam rubber cord was freshly cut. 1 to 2 drops of adhesive were applied to the surface and the ends of the rubber cord were immediately joined together. The setting time was determined as the time taken to establish flexing resistance. 
     The tests show that the adhesives according to the invention have virtually the same stability in storage and setting time as the unmodified adhesives. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Type and concentration [ppm] 
                 Setting time 
                 Viscosity [mPas] as measured by 
               
               
                   
                 of the compound added 
                 on EPDM 
                 the cone/plate method at 20° C. 
               
             
          
           
               
                 Test 
                 2,4,6-Triphenyl 
                 2,4,6-Triphenyl pyrylium 
                 [secs.] 
                   
                 After storage for 
               
               
                 No. 
                 pyrylium tetrafluoroborate 
                 trifluoromethane sulfonate 
                 After production 
                 After production 
                 10 days at 70° C. 
               
               
                   
               
               
                 1 
                  0 
                  0 
                 3 
                 2.4 
                 4.5 
               
               
                 2 
                  50 
                  0 
                 3 
                 2.3 
                 5.2 
               
               
                 3 
                 100 
                  0 
                 3 
                 2.6 
                 4.8 
               
               
                 4 
                 300 
                  0 
                 3 
                 2.5 
                 5.4 
               
               
                 5 
                 500 
                  0 
                 3 
                 2.4 
                 5.7 
               
               
                 6 
                 1000  
                  0 
                 4 
                 2.5 
                 6.2 
               
               
                 7 
                  0 
                  50 
                 3 
                 2.5 
                 4.7 
               
               
                 8 
                  0 
                 100 
                 3 
                 2.7 
                 5.2 
               
               
                 9 
                  0 
                 300 
                 3 
                 2.7 
                 5.2 
               
               
                 10  
                  0 
                 500 
                 3 
                 2.6 
                 5.7 
               
               
                 11  
                  0 
                 1000  
                 5 
                 2.8 
                 6.0 
               
               
                   
               
             
          
         
       
     
     II. Solubility 
     Various quantities of the following substances were dissolved in various cyanoacrylic acid esters each stabilized with 1,000 ppm of hydroquinone and 50 ppm of sulfur dioxide in order to evaluate dissolving behavior: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Type and concentration 
                   
               
               
                   
                 [% by weight] 
                   
               
               
                   
                 of the compound added 
                   
               
             
          
           
               
                   
                   
                 2,4,6-Triphenyl 
                   
               
               
                   
                 2,4,6-Triphenyl 
                 pyrylium 
                   
               
               
                 Test 
                 pyrylium 
                 trifluoromethane 
                 Monomer base 
               
               
                 No. 
                 tetrafluoroborate 
                 sulfonate 
                 α-Cyanoacrylate 
               
               
                   
               
               
                 12 
                 0.1 
                   
                 Ethyl cyanoacrylate 
               
               
                 13 
                 0.5 
                   
                 Ethyl cyanoacrylate 
               
               
                 14 
                 1.0 
                   
                 Ethyl cyanoacrylate 
               
               
                 15 
                 2.5 
                   
                 Ethyl cyanoacrylate 
               
               
                 16 
                   
                 0.1 
                 Ethyl cyanoacrylate 
               
               
                 17 
                   
                 0.5 
                 Ethyl cyanoacrylate 
               
               
                 18 
                   
                 1.0 
                 Ethyl cyanoacrylate 
               
               
                 19 
                   
                 2.5 
                 Ethyl cyanoacrylate 
               
               
                 20 
                 0.5 
                   
                 Methyl cyanoacrylate 
               
               
                 21 
                 0.5 
                   
                 n-Propyl cyanoacrylate 
               
               
                 22 
                 0.5 
                   
                 n-Butyl cyanoacrylate 
               
               
                 23 
                 0.5 
                   
                 Isopropyl cyanoacrylate 
               
               
                 24 
                 0.5 
                   
                 Methoxyethyl 
               
               
                   
                   
                   
                 cyanoacrylate 
               
               
                   
               
             
          
         
       
     
     The fluorescent additives were completely dissolved in concentrations of up to 2.5% by automatic shaking for 8 hours at room temperature. 
     For comparison, other fluorescent dyes (C.I. Acid Red 50 and C.I. Acid Red 52) were tested for their solubility in ethyl cyanoacrylate. Both fluorescent dyes—in concentrations of 0.1%—had dissolved only partly after shaking for 24 hours at room temperature. 
     III. Fluorescence Stability 
     The fluorescence property was determined using an Erwin Sick LUT 1-4 Lumineszenztaster (luminescence probe), lens 133. 
     The luminescence probe emits modulated ultraviolet light with a wave length of 365 nm and responds to light of the same modulation frequency in the wave length range from ca. 450 to 1000 nm. The distance from the lens to the adhesive was 40 mm. 10 mg drops of adhesive were applied to a glass slide; drop diameter on the surface ca. 5 mm. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Type and concentration 
                   
                 Measuring signal units 
               
               
                   
                 [% by weight] 
                   
                 as fluorescence intensity 2)   
               
             
          
           
               
                   
                 of the compound added 
                   
                 Immediately 
                   
               
             
          
           
               
                 Test 
                 2,4,6-Triphenyl 
                 2,4,6-Triphenyl pyrylium 
                 Monomer base 
                 after application 
                 After irradiation 
               
               
                 No. 
                 pyrylium tetrafluoroborate 
                 trifluoromethane sulfonate 
                 α-Cyanoacrylate 1)   
                 (beginning) 
                 for 5 minutes 
               
               
                   
               
               
                 25 
                 0.01 
                   
                 Ethyl cyanoacrylate 
                 55 
                 50 
               
               
                 26 
                 0.03 
                   
                 Ethyl cyanoacrylate 
                 &gt;99 3)   
                 &gt;99 3)   
               
               
                 27 
                 0.05 
                   
                 Ethyl cyanoacrylate 
                 &gt;99 3)   
                 &gt;99 3)   
               
               
                 28 
                   
                 0.01 
                 Ethyl cyanoacrylate 
                 80 
                 78 
               
               
                 29 
                   
                 0.03 
                 Ethyl cyanoacrylate 
                 &gt;99 3)   
                 &gt;99 3)   
               
               
                   
               
               
                   1) Stabilized with 500 ppm of hydroquinone and 10 ppm of SO 2   
               
               
                   2) Luminescence probe  
               
               
                   3) Input amplifier of the luminescence gauge overdriven  
               
             
          
         
       
     
     In addition to tests 25 to 29, other dyes were tested for their fluorescence: 
     Stabilization: 10 ppm SO 2  and 500 ppm hydroquinone; ester base: cyanoacrylate: 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Measuring signal 
                   
               
               
                   
                 units as 
                   
               
               
                   
                 fluorescence intensity 
                   
               
             
          
           
               
                   
                   
                 Immediately 
                 After 
               
               
                   
                   
                 after 
                 irradiation 
               
               
                   
                   
                 application 
                 for 
               
               
                 Dye 
                 Concentration 
                 (beginning) 
                 5 minutes 
               
               
                   
               
             
          
           
               
                 2,4-Di-t-butyl-6-(2- 
                 109 ppm 
                 9 
                 5 
               
               
                 methoxy-phenyl)- 
                 500 ppm 
                 33 
                 30 
               
               
                 pyrylium 
                   
                   
                   
               
               
                 tetrafluoroborate 
                   
                   
                   
               
               
                 1,4-phenylene4,4′- 
                 500 ppm 
                 19 
                 15 
               
               
                 bis(2,6-di-phenyl-4- 
                 750 ppm 
                 22 
                 19 
               
               
                 pyrylium 
                 1000 ppm  
                 15 
                 12 
               
               
                 tetrafluoroborate) 
                   
                   
                   
               
               
                 2,4-Diphenyl-6-(4- 
                 100 ppm 
                 97 
                 95 
               
               
                 methoxy-phenyl) 
                 300 ppm 
                 &gt;99 
                 &gt;99 
               
               
                 pyrylium perchlorate 
                   
                   
                   
               
               
                 2,4-Diphenyl-6- 
                 100 ppm 
                 90 
                 87 
               
               
                 (p-tolyl)pyrylium 
                 300 ppm 
                 &gt;99 
                 &gt;99 
               
               
                 tetrachloroferrate 
               
               
                   
               
             
          
         
       
     
     For comparison, the fluorescence dye C.I. Solvent Green 5 was dissolved in an ethyl cyanoacrylate (stabilized with 500 ppm hydroquinone and 10 ppm SO 2 ) in concentrations of 0.01 and 0.03%. Fluorescence measurement using the same test arrangement produced an intensity signal of 10 units (beginning) and 8 units (after 5 minutes) in the case of the sample containing 0.01% C.I. Solvent Green 5 and an intensity signal of 35 units (beginning) and 30 units (after 5 minutes) in the case of the sample containing 0.03% C.I. Solvent Green 5.