Two part self-indicating adhesive composition
This application is a continuation of Ser. No. 498,356 filed May 26, 1983 and now abandoned. 
    
    
     This invention relates to an adhesive composition of the type which exhibits fast curing through large gaps on inactive substrates (substrates not involved in the adhesive cure mechanism) following a controllable induction time. It relates more particularly to a two-component adhesive composition which is applied as a one-part adhesive, when the two components have been mixed. 
     A major disadvantage of prior art two-part mixed adhesive compositions that are room temperature-curing is that fast curing inevitably means a short pot life and problems with mixing equipment. This problem of a short pot life is conventionally solved by adding an inhibitor to the polymerization system. The inhibitor then reacts with the active species of the catalyst-activator system thus stopping polymerization until the inhibitor is consumed. The larger the amount of inhibitor the longer is the induction period and the smaller the amount of catalyst available at the end of the induction period. As the rate of polymerization is usually a function of the catalyst concentration a long induction period may result in slow and incomplete cure. 
     Another disadvantage of the said prior art compositions is that a mixture of the two components is difficult to distinguish from either of the components before mixing. It is known to incorporate a first dye in one component, and a second dye in the other component, so that the mixture of the components has a color other than that of either of the dyes. This however is an undesirable complication of the work of manufacturing such compositions. 
     A further disadvantage is that the mixture, once applied to a pair of substrates which are then placed or cramped in mutual contact, although visible at the edges of the joint, and more fully visible through a substrate if the latter is transparent, gives no visible inkling of its state of cure. 
     It is an object of the present invention to provide an adhesive composition in which a controllable induction period is followed by rapid polymerization giving complete cure through very large gaps. Another object of the invention is to provide a two-part adhesive composition wherein mixing of the parts produces a color change, enabling the mixture to be instantly recognized as such, and providing a visible remainder of its limited pot life. A further object is to provide an adhesive composition wherein cure is accompanied by a color change. 
     In accordance with the invention there is provided an adhesive composition of the type which exhibits fast curing through large gaps on inactive substrates following a controllable induction time, which composition comprises 
     (i) at least one addition polymerizable vinyl-type monomer as hereinafter defined; 
     (ii) a cure system comprising a free radical initiator and at least one accelerator therefor, namely a ferrocene type compound; and 
     (iii) at least one free radical stabilizer of the quenching type. 
     The composition may additionally comprise polymeric additives(s), dye(s), pigments(s) or filler(s). 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a series of exotherms plotted as temperature in °C. versus time in minutes showing the effect of varying stabilizer levels; 
     FIG. 2 is a series of exotherms plotted as temperature in °C. versus time in minutes showing the effect of varying initiator levels; and 
     FIG. 3 is a series of exotherms plotted in °C. versus time in minutes showing the effect of varying accelerometer levels. 
    
    
     The addition polymerizable monomers are preferably selected from the group that consists of monofunctional acrylate or methacrylate esters, diacrylate or dimethacrylate esters, urethane acrylate or urethane methacrylate compounds, and other unsaturated reactive comonomers. 
     Examples of these monomers (i) are Methyl methacrylate, Butyl methacrylate, Tetrahydrofurfuryl methacrylate, Cyclohexyl methacrylate and the reactive monomers described in U.S. Pat. Nos. 3,425,988 to Toback and Gorman and 4,018,851 to Baccei. 
     The cure system (ii) typically comprises a free radical initiator and a combination of free radical accelerators one of which is a ferrocene type compound. 
     Examples of these initiators are Benzoyl peroxide, p-Chlorobenzoyl peroxide, o-Methyl benzoyl peroxide and Lauroyl peroxide. 
     Examples of ferrocene type compounds are Ferrocene, t-Butyl ferrocene, t-Octyl ferrocene and Vinyl ferrocene. 
     Preferred accelerators other than ferrocene type compounds are tertiary aromatic amines, such as N,N-Dimethylaniline, N,N-Diethyl-p-toluidine and N,N-Dihydroxyethyl-p-toluidine. 
     Cure systems of the kind described are capable of producing a surface independent non-anaerobic cure. 
     The free radical stabilizer is of the quenching type. Examples of these stabilizers are Hydroquinone, 2,6-di-t-Butyl-4-methylphenol and 4-Methoxyphenol. 
     The invention will be understood in greater detail from the accompanying drawings and the following examples: 
     EXAMPLE 1 
     Adhesive compositions were prepared using the following ingredients: 
     (i) Monomers: 
     (1) Tetrahydrofurfuryl methacrylate 
     (2) Methacrylic acid 
     (3) 1,3-Butylene glycol dimethacrylate 
     (ii) Cure System: 
     (4) Benzoyl Peroxide (initiator) 
     (5) Ferrocene (accelerator) 
     (6) N,N-Dihydroxethyl-p-toluidine (accelerator) 
     (iii) Stabilizer: 
     (7) 2,6-di-t-Butyl-4-methylphenol 
     (iv) Polymeric Additives: 
     (8) Ethylene methyl acrylate copolymer. 
     In formulating useful two-part compositions from these ingredients, ingredient 4 must be kept apart from ingredients 5 and 6 by separate packaging or other means well known in the art. 
     A stock solution was made up according to the following recipe, in which the ingredients are designated by the respective numbers given above. The quantities are to be understood as parts by weight totalling 100. 
     
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Stock Solution            Quantities                                      
______________________________________                                    
(1)  Tetrahydrofurfuryl methacrylate (i)                                  
                              58.85                                       
(2)  Methacrylic acid         10.42                                       
(3)  1,3-Butylene glycol dimethacrylate (i)                               
                               2.08                                       
(8)  Ethylene methyl acrylate copolymer (iv)                              
                              28.65                                       
Total                     100.00                                          
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     Two-part adhesive compositions were then made up using this stock solution and ingredients 4-7 the proportions of which were as follows: 
     
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                      Quantities                                          
______________________________________                                    
Part 1.                                                                   
(4)   Benzoyl peroxide (ii) 1.0                                           
(7)   2,6-di-t-Butyl-4-methoxyphenol (iii)                                
                            0.1                                           
      Stock Solution sufficent to add up to                               
                            100.0                                         
Part 2.                                                                   
(5)   Ferrocene             0.2                                           
(6)   N,N--Dihydroxyethyl-p-toluidine                                     
                            1.0                                           
      Stock Solution sufficent to add up to                               
                            100.0                                         
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     On mixing Part 1, which was yellow, with Part 2, which was orange, a green solution was obtained and an induction time began. Following this induction period there was a period of rapid polymerization producing a hard yellow polymer. These color changes are of course of great utility to the user; the composition may be termed self-indicating by virtue of them. 
     EXAMPLE 2 
     Represented in FIGS. 1-3 are polymerization exotherms recorded with a thermocouple recorder unit and plotted as temperature in °C. versus time in minutes. 
     FIG. 1 shows how the induction time may be varied by changing the proportion of the 2,6-di-t-Butyl-4-methylphenol stabilizer in the composition of Example from 0.050 wt. % (line I), 0.075 wt. % (line II), to 0.100 wt. % (line III). 
     FIG. 2 shows that this effect also may be achieved by varying the proportion of the benzoyl peroxide initiator from 0.75 wt. % (line I), 1.00 wt. % (line II), to 1.50 wt. % (line III); and FIG. 3 shows that this effect also may be achieved by varying the proportions of the ferrocene and N,N-Dihydroxyethyl-p-toluidine in the accelerator system, respectively, in the composition, in accordance with the following (all parts are parts by weight): 
     
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          Accelerator System                                              
                      N,N--Dihydroxy-                                     
Line No.    Ferrocene ethyl-p-toluidine                                   
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I           0.40%     1.50%                                               
II          0.40%     1.00%                                               
III         0.20%     1.50%                                               
IV          0.20%     1.00%                                               
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     EXAMPLE 3 
     A two-part adhesive composition was made up according to the following recipe. The quantities are to be understood as parts by weight. Each part of this composition adds up to 100. 
     
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                      Quantities                                          
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Part 1.                                                                   
(1) Tetrahydrofurfuryl methacrylate (i)                                   
                            43.00                                         
(2) Methacrylic acid (i)    10.00                                         
(3) 1,3-Butylene glycol dimethacrylate (i)                                
                             2.00                                         
(4) Benzoyl peroxide (ii)    1.25                                         
(7) 2,6-di-t-Butyl-4-methyl phenol (iii)                                  
                             0.125                                        
(8) Ethylene methyl acrylate copolymer (iv)                               
                             22.125                                       
    Methyl methacryate butyl methacrylate                                 
                            20.00                                         
    copolymer (iv)                                                        
    Silicon dioxide (iv)     1.50                                         
Part 2.                                                                   
(1) Tetrahydrofurfuryl methacrylate (i)                                   
                            43.00                                         
(2) Methacrylic acid (i)    10.00                                         
(3) 1,3-Butylene glycol dimethacrylate (i)                                
                             2.00                                         
(5) Ferrocene (ii)           0.25                                         
(6) N,N--Dihydroxyethyl-p-toluidine (ii)                                  
                             1.00                                         
(7) 2,6-di-t-Butyl-4-methyl phenol (iii)                                  
                             0.15                                         
(8) Ethylene methyl acrylate copolymer (iv)                               
                            22.10                                         
    Methyl methacrylate butyl methacrylate                                
                            20.00                                         
    copolymer (iv)                                                        
    Silicon dioxide (iv)     1.50                                         
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     For use, equal weights of the two parts were intimately mixed and the mixture applied to a 2.54×1.27 cm area on each of a pair of 2.54 cm wide abraded gel coat glass-reinforced plastics strip substrates. The treated areas were placed in contact and clamped together. Uniform bond gaps of 4 mm were achieved with the use of glass spacers. The bonds were allowed to set. 
     A number of such bonds was allowed to stand at room temperature for 24 hours to develop full cure. They were then tested to destruction according to ASTM D1002-64 to determine their tensile shear strength in decaNewtons per square centimeter (daNcm -2 ). The mean result recorded was 23 daNcm -2  with substrate failure being observed in each case. 
     Thus the ultimate strengths recorded were determined by the cohesive strength of the substrate which was less than the forces of adhesion operating. 
     An inducation time of 30 minutes was recorded for the assembled adhesive using both the polymerization exotherm and color change as indicators.