Ethylene-vinyl acetate silicone rubber adherent laminates and method of production
BACKGROUND OF THE INVENTION 
     Electrical ignition wires are essentially comprised of a central conductor such as copper wire surrounded by an insulating material. Crosslinked silicone rubber is desirable as the insulating material because it has appropriate electrical insulating properties and provides resistance to degradation by oil, water, heat and atmospheric conditions encountered surrounding an internal combustion engine. Unfortunately the silicone rubber is relatively expensive and the stiffness and tensile strength properties of the crosslinked material on the copper conductor are inadequate for this application. 
     Ethylene-vinyl acetate resins have appropriate electrical insulating properties for use in ignition wires and have satisfactory mechanical strength properties. Since the ethylene-vinyl acetate resins do not have the same oil, water, heat and oxidative resistance characteristics as the crosslinked silicone rubber, it is apparent that an ignition wire having a central copper conductor surrounded by a crosslinked ethylene-vinyl acetate resin which in turn is surrounded by crosslinked silicone rubber would combine the desirable characteristics of the two polymer materials and overcome the foregoing problems. Unfortunately, the adhesion of crosslinked ethylene-vinyl acetate resins to crosslinked silicone rubber is marginal at best. 
     Accordingly, it is the object of this invention to provide a method by which the bonding of crosslinked ethylene-vinyl acetate resins to crosslinked silicone rubber is improved so that superior electrical insulated products can be achieved. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method of improving the adhesion between crosslinked ethylene-vinyl acetate resins and crosslinked silicone rubbers and to the resulting electrical insulated product. More particularly, the invention relates to a method of improving the bonding of crosslinked ethylene-vinyl acetate to crosslinked silicone rubber by coating the surface of the ethylene-vinyl acetate with a solution of a silane or a member of the class of compounds known as polyfunctional monomers prior to adhering the ethylene-vinyl acetate to the silicone rubber and prior to the crosslinking of the two plastic materials. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The ethylene-vinyl acetate resins and silicone rubber resin and the crosslinking agents therefor which are used in the present invention are well known in the art. Many such materials are commercially available and the individual identity of these materials does not constitute a feature of the present invention. Accordingly, they need not be described any further here. 
     In general, the ethylene-vinyl acetate and silicone rubbers are separately compounded with the usual fillers such as carbon black, clay, etc., and an appropriate crosslinking agent such as a peroxide. Layers of the two compounded resins are thereafter placed together or extruded such that one resin compound concentrically surrounds the other resin compound. The composite laminate is then cured by heating, under pressure, to an appropriate temperature at which the crosslinking is initiated. 
     It has been found that in order to achieve a satisfactory adhesion between the ethylene-vinyl acetate copolymer and the silicone rubber after curing, the interface surface should be coated. This is accomplished by dissolving an adhesion promoting agent in an inert solvent, coating the surface of the ethylene-vinyl acetate or the silicone rubber or both, by painting, spraying, roller coating, dip coating or the like and then evaporating the inert solvent to leave a thin layer of the bonding agent on the surface of the plastic. 
     The silane adhesion promoting or bonding agents which have been found to be useful in the process of this invention are vinyl tris (beta methoxyethoxy) silane, gamma methacryloxypropyl trimethoxysilane, and the silanes manufactured by Dow Corning identified as Z-6032, Z-6020, Z-6040 and Z-6076 and having the formulas ##SPC1## 
     The polyfunctional monomer promoting or bonding agents which have been found useful in the process of this invention are triallyl cyanurate, triallyl isocyanurate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, 1,4-butylene glycol diacrylate, allyl methacrylate, diallyl fumarate, divinyl benzene, and triallyl phosphate. 
     The bonding agent is employed in an amount such that satisfactory adhesion between the silicone rubber and the ethylene-vinyl acetate copolymer is obtained with no effect or minimum effect on the two insulating materials. The amount of the adhesion promoting agent applied is related to the concentration of the agent in the solvent and type of coating process employed to coat the interface surface. Thus, the bonding agent is employed in an amount such that its concentration in the coating solution is about 1 to 80 weight percent, preferably about 1-50 weight percent. Any solvent which is inert with respect to the bonding agent, the ethylene-vinyl acetate resin and the silicone rubber can be employed. In order to facilitate evaporation of the solvent after the surface of the plastic has been coated, it is preferred to use a volatile material as the solvent, i.e., having a boiling point below about 200° C. and most preferably below about 100° C. Solvents useful in this process include alcohols, ketones, esters, hydrocarbon solvents, polyols, glycol ethers and the like. Typical examples of appropriate solvents are methanol, ethanol, acetone, methyl ethyl ketone, pentane hexane and tetrahydrofuran. 
    
    
     Various Examples are set forth below in order to further illustrate the present invention. In these Examples, all parts and percentages are by weight and all temperatures are in degrees Centigrade unless otherwise specified. 
     EXAMPLE 1 
     An ethylene-vinyl acetate resin containing 17% vinyl acetate and having a melt index of 1.5 was compounded with 2 parts of vinyl-tris(beta methoxyethoxy)silane and α,α&#39;-bis(t-butylperoxy)diisopropyl benzene. A silicone rubber was separately compounded with benzoyl peroxide as the crosslinking agent. Uncured plaques of each resin were then prepared, and the plaques pressed together and cured at 190° C. for 5 minutes. An attempt was made to measure the adhesive strength of one cured laminate on an Instron Peel Strength Tester but the adhesive strength was so poor that the values obtained were not meaningful. Thereafter, cured laminates were tested by hand and the adhesion rated as excellent, good, fair or poor. 
     EXAMPLE 2 
     The procedure of Example 1 was repeated except that before the plaques were pressed together, the surface of the ethylene-vinyl acetate plaque was coated with a 10% acetone solution of vinyl-tris(beta methoxyethoxy)silane and the acetone was allowed to evaporate. The cured laminate exhibited improved adhesion with respect to the laminate in which no surface treatment had been effected. 
     EXAMPLES 3-4 
     The procedure of Example 2 was repeated twice, once substituting a 10% solution of triallyl isocyanurate in acetone for the silane solution and once substituting a 10% solution of trimethylolpropane trimethacrylate in acetone for the silane solution. The cured laminates exhibited good adhesion between the plies. 
     EXAMPLE 5 
     The procedure of Example 2 was repeated substituting a 10% solution of gamma methacryloxypropyl trimethoxysilane in acetone for the solution of vinyl-tris(beta methoxyethoxy)silane. The resulting cured laminate exhibited excellent adhesion. 
     EXAMPLES 6-7 
     For comparative purposes, the procedure of Example 2 was repeated twice, once substituting a 10% solution of gamma aminopropyl triethoxysilane in acetone for the vinyl-tris(beta methoxyethoxy)silane solution and once substituting a 10% solution of dimethylsiloxane tetramer in acetone. The resulting cured laminates did not exhibit any improvement in adhesion with respect to the cured laminates compared without any surface treatment. 
     EXAMPLES 8-31 
     The procedure of Example 2 was repeated substituting a number of different bonding agents and solvents for the 10% acetone solution of vinyl-tris(beta methoxyethoxy)silane. The results are shown in Table I. 
     
                                           TABLE I                                 
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 Ex. No.                                                                  
      Bonding Agent  Solvent                                              
                          Amount                                          
                               Adhesion                                   
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 8    triallyl isocyanurate                                               
                     acetone                                              
                          25%  excellent                                  
 9    trimethylol propane                                                 
                     acetone                                              
                          25%  excellent                                  
      trimethacrylate                                                     
10    gamma methacryloxypropyl                                            
                     acetone                                              
                           5%  good                                       
      trimethoxysilane                                                    
11    Dow Corning Silane Z-6032                                           
                     methanol                                             
                           1%  excellent                                  
12    Dow Corning Silane Z-6032                                           
                     methanol                                             
                           5%  excellent                                  
13    Dow Corning Silane Z-6032                                           
                     methanol                                             
                          10%  excellent                                  
14    Dow Corning Silane Z-6032                                           
                     ethanol                                              
                           5%  excellent                                  
15    Dow Corning Silane Z-6032                                           
                     methanol                                             
                          50%  excellent                                  
16    trimethylol propane                                                 
                     acetone                                              
                          10%  excellent                                  
      triacrylate                                                         
17    triallyl cyanurate                                                  
                     acetone                                              
                          10%  excellent                                  
18    ethylene glycol dimeth-                                             
                     acetone                                              
                          10%  good                                       
      acrylate                                                            
19    diethylene glycol                                                   
                     acetone                                              
                          10%  good                                       
      dimethacrylate                                                      
20    triethylene glycol                                                  
                     acetone                                              
                          10%  good                                       
      dimethacrylate                                                      
21    tetraethylene glycol                                                
                     acetone                                              
                          10%  good                                       
      dimethacrylate                                                      
22    polyethylene glycol                                                 
                     acetone                                              
                          10%  good                                       
      dimethacrylate                                                      
23    1,4-butylene glycol                                                 
                     acetone                                              
                          10%  excellent                                  
      dimethacrylate                                                      
24    1,3-butylene glycol                                                 
                     acetone                                              
                          10%  excellent                                  
      dimethacrylate                                                      
25    diethylene glycol                                                   
                     acetone                                              
                          10%  good                                       
      diacrylate                                                          
26    triethylene glycol                                                  
                     acetone                                              
                          10%  good                                       
      diacrylate                                                          
27    1,4-butylene glycol                                                 
                     acetone                                              
                          10%  excellent                                  
      diacrylate                                                          
28    allyl methacrylate                                                  
                     acetone                                              
                          10%  good                                       
29    diallyl fumarate                                                    
                     acetone                                              
                          10%  good                                       
30    divinyl benzene                                                     
                     acetone                                              
                          10%  excellent                                  
31    triallyl phosphate                                                  
                     acetone                                              
                          10%  good                                       
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     Various changes and modifications can be made in the process and products of this invention without departing from the spirit and the scope thereof. The various embodiments set forth herein were for the purpose of further illustrating the invention but were not intended to limit it.