Curable adhesive composition
This is a Continuation of Application No. 07/313,210 filed Feb. 21, 1989 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a curable adhesive composition, and more particularly to an adhesive composition which is curable by the action of heat or light and useful as an adhesive for surface mounting devices (hereinafter referred to as &#34;SMD&#34;). 
     BACKGROUND OF THE INVENTION 
     Mounting of SMD, such as chip resistors and chip capacitors, on printed circuit boards has been carried out by temporarily fixing the SMD to the circuit board with an adhesive and then soldering repeatedly. In the soldering step, the adhesion performance of the adhesive is reduced due to abrupt thermal shocks to cause release of the SMD. It has been, therefore, demanded to develop an adhesive whose adhesion performance is not subject to change between before and after the soldering step. 
     SUMMARY OF THE INVENTION 
     One object of this invention is to provide a curable adhesive composition suitable as an adhesive for mounting SMD on printed circuit boards. 
     The present invention relates to a curable adhesive composition comprising a polymerizable prepolymer, a reactive diluent, and a polymerization initiator, wherein said composition further comprises an inorganic filler and/or a (meth)acrylate having an isocyanurate ring. 
     The curable adhesive composition of this invention exhibits high adhesion strength at room temperature, which undergoes substantially no reduction even on repeated exposure to a soldering temperature, and hence effectively prevents release of SMD. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of the polymerizable prepolymer which can be used in this invention include unsaturated polyesters, epoxy (meth)acrylates, urethane (meth)acrylates, oligoester (meth)acrylates, etc. 
     The above-described unsaturated polyesters are polyesters having a molecular weight of from 1,000 to 10,000 and containing therein ethylenically unsaturated bonds and are commercially available under the trade names of, for example, Upica® 8554 (made by Nippon Upica Co., Ltd.) and New Track® 410 S (made by Kao Corporation). 
     The above-described epoxy (meth)acrylates are products obtained by esterifying epoxy resins having bisphenol-A, phenol or cresol novolak as a skeleton with (meth)acrylic acid and are commercially available under the trade names of, for example, Ripoxy®SP-1509 and SP-4010 (made by Showa Highpolymer Co., Ltd.). 
     The above-described urethane (meth)acrylates are generally obtained by reacting a diol such as aliphatic diols (e.g., ethylene glycol, etc.), ether diols having two hydroxyl groups at the ether chain terminal ends, and ester diols having two hydroxyl groups at the ester chain terminal ends with a stoichiometric excess of a diisocyanate such as toluene diisocyanate, isophorone diisocyanate, etc. to form a urethane compound having isocyanate groups at the terminal ends which is then further reacted with a hydroxyalkyl (meth)acrylate. Commercial products are available under the trade names of, for example, Uvithane® 893 (made by Thiochol Chemical Corp.) and U-6HA (made by Shin Nakamura Kagaku Kogyo Co., Ltd.). 
     The above-described oligoester (meth)acrylates are polyfunctional (meth)acrylates having, as a skeleton, an ester compound obtained from aromatic carboxylic acids and are commercially available under the trade names of, for example, Aronix®M-8060 and M-7100 (made by Toagosei Chemical Industry Co., Ltd.). 
     The reactive diluent which is used in this invention is added for the purpose of adjusting the viscosity of the curable composition or characteristics of cured products, and examples thereof include polyfunctional (meth)acrylates, monofunctional (meth)acrylates, etc. 
     Examples of the above-described polyfunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, tris[meth](acryloxyethyl) isocyanurate, 1,6-hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, neopentyl glycol hydroxypivalate di(meth)acrylate, (meth)acrylated compounds of bisphenol A having ethylene oxide added thereto, and diglycidyl ester di(meth)acrylates of dibasic fatty acids containing 4 or more carbon atoms. Commercial products are available under the trade names of, for example, Viscoat® #3700 and #700 (made by Osaka Yuki Co., Ltd.), MANDA® (made by Nippon Kayaku Co., Ltd.), Aronix®M6100, M6300 and TD-1600A (made by Dainippon Ink and Chemicals, Inc.), and ULB-20GEA (Okamura Seiyu Co., Ltd.). 
     Examples of the above-described monofunctional (meth)acrylates include hydroxyethyl (meth)acrylate tetrahydrofurfuryl (meth)acrylate, phenoxyethyl (meth)acrylate, dicyclopentenyl (meth)acrylate, etc. 
     The weight proportion of the polymerizable prepolymer to the reactive diluent can be arbitrarily varied but is usually in the range of from 10:90 to 80:20 and preferably in the range of from 20:80 to 70:30. 
     The polymerization initiator as referred to herein includes both a photopolymerization initiator and/or a heat polymerization initiator. 
     Examples of the photopolymerization initiator include benzoin ether-based compounds, benzophenonebased compounds, acetophenone-based compounds, thio-xanthone-based compounds, etc. Specific examples are benzoin isopropyl ether, benzoin isobutyl ether, benzophenone, 2,2-dimethoxy-2-phenylacetophenone-1-hydroxyisobutylphenohe, phenyl ketone, p-isopropyl-α-hydroxyisobutylphenone, α-hydroxy isobutyl ketone, 1,1-dichloroacetophenone, 2-chlorothioxanthone, 2-methylthioxanthone, methylbenzoyl formate, etc. These compounds can be used alone or in admixture. 
     As the heat polymerization initiator, usual ordinary organic peroxides can be used. Specific examples include ketone peroxides such as methyl ethyl ketone peroxide, cyclohexanone peroxide, etc.; diacyl peroxides such as acetyl peroxide, benzoyl peroxide, etc.; hydroperoxides such as t-butyl hydroperoxide, cumene hydroperoxide etc.; dialkyl peroxides such as dit-butyl peroxyde, dicumyl peroxide, etc.; alkyl peresters such as t-butyl peracetate, t-butyl perbenzoate, etc.; peroxycarbonates such as diisopropyl peroxydicarbonate, bis(4-t-butylcyclohexyl)peroxydicarbonate, etc.; and peroxyketals such as 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, etc. These compounds can be used alone or in admixture. 
     The amount of the initiator is usually in the range of from 0.2 to 20 parts by weight based on 100 parts by weight of the sum of the polymerizable prepolymer and-the reactive diluent. 
     The inorganic filler which can be used in the present invention preferably comprises a combination of a spherical inorganic filler and a flaky inorganic filler. The spherical inorganic filler includes spherical fused silica, glass, bead, alumina powder, silica balloon, and synthetic calcium silicate. Implicit in the flaky inorganic fillers are talc, calcium silicate, zeolite, kaolin, calcined clay, bentonite, sericite, and basic magnesium carbonate. Commercially available products of inorganic fillers include FB-44 (spherical fused silica produced by Denki Kagaku Kogyo K.K.), EGB-210 (glass bead produced by Toshiba-Ballotini Co., Ltd.), LMR-100 (flaky talc produced by Fuji Talc Co., Ltd.), and Iceberg® (flaky clay produced by Bergen Pigment Co., Ltd.). 
     In particular, a combination of spherical fused silica or glass bead as a spherical inorganic filler and talc as a flaky inorganic filler in a specific mixing ratio is effective. 
     The amount of the spherical inorganic filler to be used ranges from 10 to 50 parts by weight per 100 parts by weight of the total amount of the polymerizable prepolymer and the reactive diluent. The amount of the flaky inorganic filler to be used ranges from 50 to 150 parts by weight on the same basis. If the spherical inorganic filler and flaky inorganic filler are used in admixture, the total amount thereof ranges from 60 to 200 parts by weight per 100 parts by weight of the total amount of the polymerizable prepolymer and the reactive diluent, with the mixing ratio of the spherical inorganic filler to the flaky inorganic filler being arbitrarily variable within the above-recited ranges of the respective components. If the amount of either one or both of the spherical inorganic filler and flaky inorganic filler is less than the respective lower limit, the adhesion strength after soldering is considerably reduced. If it exceeds the respective upper limit, the initial adhesion strength before soldering is reduced. The amounts of these inorganic fillers can be appropriately controlled within the above ranges so as to obtain a viscosity suitable for end use. The (meth)acrylate having an isocyanurate ring which can be used in the present invention is represented by formula (I): ##STR1## wherein A&#39;s, which may be the same or different, each represents a hydrogen atom, ##STR2## wherein R represents a hydrogen atom or a methyl group. 
     The compounds represented by formula (I)are known per se and can be prepared by reacting a reactive derivative of isocyanuric acid and a reactive derivative of (meth)acrylic acid. 
     Specific examples of the compounds of formula (I) include FA-731A (a compound wherein all of A&#39;s are ##STR3## produced by Hitachi Chemical Co.,Ltd.), Aronix® M-215 (a compound wherein one of A&#39;s is a hydrogen atom, and others are CH 2  ═CHCO--, produced by Toa Gosei Chemical Industry Co., Ltd.), and Aronix® M-325 (a compound wherein one of A&#39;s is CH 2  ═CHCOO(CH 2 ) 5  CO--, and others are CH 2  ═CHCO--, produced by Toa Gosei Chemical Industry Co., Ltd.). 
     The (meth)acrylate having an isocyanurate ring is used in an amount usually ranging from 1 to 40 parts by weight, preferably from 5 to 30 parts by weight, per 100 parts by weight of the total amount of the polymerizable prepolymer and the reactive diluent. If the amount of this component is less than 1 part, sufficient adhesion strength at room temperature or high temperatures cannot be assured so that the SMD tends to fall down from the printed circuit board during soldering. If it exceeds 40 parts, curing reaction would drastically proceed to cause large strain during curing so that the cured adhesive tends to have brittleness and low adhesion. 
     The curable composition of this invention can, if desired, contain additives such as chelating agents, heat polymerization inhibitors, coloring agents, thixotropic agents, and curing accelerator, unless they hinder the object of this invention. 
     Specific examples of the chelating agent include iminodiacetic acid, N-methyliminodiacetic acid, nitrilotriacetic acid, ethylenediamine-N,N&#39;-diacetic acid, ethylenediamine-N,N,N&#39;,N&#39;-tetraacetic acid, N-hydroxyethylethylenediamine-N,N&#39;,N&#39;-triacetic acid, and sodium salts thereof as well as N,N,N&#39;,N&#39;-tetrakis-(2-hydroxypropyl)ethylenediamine, etc. 
     Examples of the heat polymerization inhibitor include hydroquinone, p-methoxyphenol, benzoquinone, phenothiazine, cupferron, etc. 
     Examples of the coloring agent include azo-based, phthalocyanine-based, isoindolinone-based and anthraquinone-based coloring agents, carbon black, etc. 
     Examples of the thixotropic agent include bentonite, ultrafine silica, etc. 
     Examples of the curing accelerator which is used in combination with cumene hydroperoxide include metal salts of long-chain organic acids, such as cobalt naphthenate, cobalt octenoate, etc.; amines such as dimethylaniline, N-phenylmorpholine, etc.; quaternary ammonium salts such as triethylbenzylammonium chloride, etc.; and aminophenols such as 2,4,6-tris(dimethylaminomethyl)phenol, etc. 
     The curable composition of this invention is prepared in the following manner. That is, the polymerizable prepolymer, reactive diluent, and optionally (meth)acrylate containing an isocyanurate ring as well as photopolymerization initiator, heat polymerization inhibitor, chelating agent, etc. are uniformly dissolved in advance, and thereafter, the thixotropic agent, coloring agent, etc. and optionally inorganic filler are mixed and dispersed. The heat polymerization initiator is then added and uniformly mixed. The mixing is usually carried out at not higher than 60° C. using a rotary agitator or a three-roll mill. 
     The present invention is now illustrated in greater detail by way of the following Examples and Comparative Examples, but it should be understood that the present invention is not deemed to be limited thereto. In these examples, all the parts are by weight unless otherwise specified. 
     Adhesion properties of the curable adhesive compositions obtained in these examples were measured according to the following methods. 
     1) Initial Adhesion Strength 
     The curable composition was applied in an amount of from about 0.3 to 0.5 mg onto a printed circuit board and, after a multilayer ceramic chip capacitor (3.2 mm × 1.6 mm) was mounted in the central part thereof, the assembly was irradiated with an ultraviolet light for about 20 seconds at a distance of 20 cm from a high-pressure mercury vapor lamp having a power input of 80 W/cm. Immediately thereafter, the assembly was heated for 10 minutes in an atmosphere at 120° C. Then, the shear adhesion strength (kg) per chip was measured at room temperature. 
     2) Adhesion Strength at 100° C. 
     The sample as prepared in 1) above was placed on an equipment by which the temperature of the printed circuit board was maintained at 100±2° C., and the shear adhesion strength (kg) per chip at 100° C. was measured. 
     3) Adhesion Strength Retention after Dipping in Soldering Bath 
     The sample as prepared in 1) above was dipped in a soldering bath kept at 260±2° C. for 10 seconds and cooled to room temperature. After this soldering operation was repeatedly conducted 6 times, the shear adhesion strength (kg) was measured. An adhesion strength retension after soldering (%) was obtained by dividing the shear adhesion strength after soldering by the initial adhesion strength as measured in 1) and multiplying the quotient by 100. 
     EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 TO 4 
     
         ______________________________________                                    
Unsaturated polyester (Upica ® 8554                                   
                       100 parts                                          
produced by Nippon Upica Co., Ltd.;                                       
molecular weight: ca. 2,500)                                              
Bifunctional acrylate (ULB-20 GEA                                         
                        70 parts                                          
produced by Okamura Seiyu Co., Ltd.)                                      
Tetrahydrofurfuryl methacrylate                                           
                       100 parts                                          
______________________________________                                    
 
    
     The above components were uniformly dissolved by thoroughly stirring. To 100 parts of the mixture were added inorganic fillers and other components as shown in Table 1 below, followed by stirring to prepare an adhesive composition. 
     Each of the adhesive compositions was evaluated according to the above-described methods, and the results obtained are shown in Table 1. 
     As is apparent from Table 1, the compositions according to the present invention prove superior to the comparative compositions. 
     
                                           TABLE 1                                 
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                             Comparative                                  
                                    Comparative                           
                                           Comparative                    
                                                  Comparative             
           Example 1                                                      
                 Example 2                                                
                       Example 3                                          
                             Example 1                                    
                                    Example 2                             
                                           Example 3                      
                                                  Example                 
__________________________________________________________________________
                                                  4                       
Components (part):                                                        
Mixture    100   100   100   100    100    100    100                     
Spherical silica*.sup.1                                                   
           20    --    10    --     70     10     --                      
Spherical glass*.sup.2                                                    
           --    50    --    --     --     --     --                      
Flaky talc*.sup.3                                                         
           90    --    120   90     50     --     90                      
Flaky clay*.sup.4                                                         
           --    50    --    --     --     --     --                      
Random-shaped                                                             
           --    --    --    --     --     120    20                      
Fused Silica*.sup.5                                                       
Photopolymerization                                                       
           3     3     3     3      3      3      3                       
initiator*.sup.6                                                          
Heat polymerization                                                       
           3     3     3     3      3      3      3                       
initiator*.sup.7                                                          
Heat polymerization                                                       
           0.1   0.1   0.1   0.1    0.1    0.1    0.1                     
inhibitor*.sup.8                                                          
Chelating agent*.sup.9                                                    
           0.1   0.1   0.1   0.1    0.1    0.1    0.1                     
Adhesion Properties:                                                      
Initial adhesion                                                          
           5.0   3.6   4.0   3.0    2.0    1.5    3.0                     
strength (kg)                                                             
Adhesion strength                                                         
           4.9   3.5   3.8   1.8    1.9    1.1    1.8                     
after soldering (kg)                                                      
Adhesion strength                                                         
           98    98    95    60     95     73     60                      
retention (%)                                                             
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 FB44 produced by Denki Kagaku Kogyo K.K.; avg. particle diameter:
 ca. 10.0 μm                                                           
 *.sup.2 EGB210 produced by ToshibaBallotini Co., Ltd.; avg. particle     
 diameter: ca 17.0 μm                                                  
 *.sup.3 LMR100 produced by Fuji Talc Co., Ltd.; avg. particle diameter:  
 ca. 3.0 μm                                                            
 *.sup.4 Iceberg ® produced by Bergen Pigment Co., Ltd., avg. particle
 diameter: ca. 1.4 μm                                                  
 *.sup.5 Fuselex ® RD8 produced by Tatsumori Ltd.; avg. particle      
 diameter: ca. 15.0 μm                                                 
 *.sup.6 Irgacure ® 651 produced by ChibaGeigy AG                     
 *.sup.7 Trigonox ® 22B75 produced by Kayaku Noury Corporation        
 *.sup.8 pMethoxyphenol                                                   
 *.sup.9 Tetrasodium N,N,N&#39;,Ntetraethylenediaminetetraacetate             
 
    
     EXAMPLES 4 TO 9 AND COMPARATIVE EXAMPLES 5 TO 6 
     
         ______________________________________                                    
Polymerizable prepolymer (Upica ® 8554)                               
                         35 parts                                         
Bifunctional acrylate (ULB-20 GEA)                                        
                         25 parts                                         
Reactive diluent (phenoxyethyl acrylate)                                  
                         40 parts                                         
Isocyanurate ring-containing (meth)acrylate                               
                         see Table 2                                      
Photopolymerization initiator                                             
                         3 parts                                          
(Darocur ® 1173 produced by                                           
Merk Co., Ltd.)                                                           
Heat polymerization initiator                                             
                         3 parts                                          
(Perhexa ® 3M produced by                                             
Nippon Oils &amp; Fats Co., Ltd.)                                             
Heat polymerization inhibitor                                             
                         see Table 2                                      
(cupferron)                                                               
Chelating agent (tetrasodium ethylene-                                    
                         0.05 part                                        
diaminetetraacetate)                                                      
Flaky talc (High Toron ® A produced                                   
                         100 parts                                        
by Takehara Kagaku Co., Ltd.;                                             
avg. particle diameter: 3.0 μm)                                        
______________________________________                                    
 
    
     The above components were mixed by stirring to prepare a curable adhesive composition, and each of the resulting compositions was evaluated as described above. The results obtained are shown in Table 2. 
     It can be seen from Table 2 that the compositions according to the present invention are superior to the comparative ones. 
     
                                           TABLE 2                                 
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              Example                                                     
                   Example                                                
                        Example                                           
                             Example                                      
                                  Example                                 
                                       Example                            
                                            Comparative                   
                                                   Comparative            
              4    5    6    7    8    9    Example 5                     
                                                   Example                
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                                                   6                      
Isocyanurate ring-containing                                              
(meth)acrylate (part):                                                    
FA-731A*.sup.10                                                           
              5    30   --   --   --   30   --     50                     
M-325*.sup.11 --   --   10   --   10   --   --     --                     
M-215*.sup.12 --   --   --   10   --   --   --     --                     
Inorganic filler (part):                                                  
High Toron ® A*.sup.13                                                
              100  100  100  100  90   50   100    100                    
FB-44*.sup.14 --   --   --   --   10   50   --     --                     
Adhesion performance:                                                     
Initial adhesion strength (kg)                                            
              5.0  5.8  5.0  5.0  5.8  4.8  3.0    3.0                    
Adhesion strength at                                                      
              1.5  2.0  1.8  1.8  2.1  1.8  0.5    1.0                    
100° C. (kg)                                                       
Adhesion strength                                                         
              90   92   96   94   97   98   60     65                     
retention (%)                                                             
__________________________________________________________________________
 Note:                                                                    
 *.sup.10 FA731A, a trade name of tris(acryloxyethyl) isocyanurate produce
 by Hitachi Chemical Co., Ltd.                                            
 *.sup.11 Aronix ® M325, a trade name of caprolactonemodified         
 tris(acryloxyethyl) isocyanurate produced by Toa Gosei Chemical Industry 
 Co., Ltd.                                                                
 *.sup.12 Aronix ® M215, a trade name of bis(acryloxyethyl)hydroxyethy
 isocyanurate produced by Toa Gosei Chemical Industry Co., Ltd.           
 *.sup.13 High Toron ® A, a trade name of flaky talc produced by      
 Takehara Kagaku Co., Ltd.                                                
 *.sup.14 FB44, a trade name of spherical fused silica produced by Denki  
 Kagaku Kogyo K.K.                                                        
 
    
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.