Abstract:
An adhesive is disclosed for use in flexible circuitry, laminates and electrical products. The adhesive has improved flexibility and adhesive properties. It may be applied in solution form or may be cast as a free-standing film which is later heated and compressed to give adhesive properties. The adhesive comprises a phenolaldehyde resin or other phenolic resin, an epoxy resin and an elastomer.

Description:
BACKGROUND OF THE INVENTION 
     Heretofore, thermosetting adhesives have exhibited serious deficiencies in bond strength and flexibility when used with certain substrates such as Kapton (polyimide) film, polysulfone film, copper foil, or nickel foil. Phenolic adhesives and epoxy-phenolic adhesives suffer from brittleness and poor adhesion making them unsuitable for use on flexible substrates. These deficiencies, coupled with adhesive failure when electrical components are soldered, preclude their usage in many applications. 
     The Watanabe U.S. Pat. No. 3,932,689 shows a composition containing a cresol formaldehyde resin, a bisphenol based epoxy compound, an acrylonitrilebutadiene copolymer and several other components. Watanabe U.S. Pat. No. 4,323,659 discloses several compositions containing a bisphenol type epoxy resin, an acrylic copolymer, an imidazole accelerator and a brominated novolak. These compositions however, do not include an ethylene acrylic elastomer. 
     An object of the invention is to provide an adhesive with the flexibility and adhesion necessary for use with the above mentioned substrates. 
     A further object of the invention is to provide a thermosetting adhesive with improved bond strength and improved ability to withstand the various manufacturing conditions necessary to produce flexible circuitry (e.g., high temperatures, soldering, or etching). 
     A further object of this invention is to provide an adhesive that may be applied to film in solution form, or may be cast as a free-standing film itself to be used as an adhesive in laminated form. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an adhesive suitable for use in flexible circuitry and electrical products. The adhesive withstands high temperatures, soldering and etching. It has improved flexibility and adhesive properties. The adhesive may be applied in solution form or may be cast as a free-standing film which is later heated and compressed to give adhesive properties. The adhesive comprises a phenol-aldehyde resin, an epoxy resin, and an ethylene-acrylic elastomer. 
     The adhesive may be applied in solution form since it is in solvent, or may be cast as an free-standing film to be used as an adhesive in film form. The liquid adhesive can be used to adhere copper to processed laminates, or Kapton (polyimide) film to copper foil. After a heat treatment to effect curing, the adhesive composition exhibits high bond strength, solder resistance, temperature resistance, flexibility and excellent electrical properties. The adhesive works well in circuit board manufacture where flexible films must be bonded together to make board substrates which subsequently are wired and soldered. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The adhesive composition of the present invention comprises or consists of a phenolic resin e.g. a phenol-aldehyde resin or cashew nut shell liquid apolymer, an epoxy resin, and an elastomer. A catalyst can be added to promote reaction between the epoxy resin and the phenol-aldehyde resin. A co-elastomer may be used where certain specific performance is desired. 
     The phenol-aldehyde resin is formed by the reaction of phenol and/or substituted phenols with aldehydes. The phenol can have more than one hydroxy group (e.g. resorcinol), and can have hydrocarbon and other substituents. More than one substituent can be present. The aldehyde for example can be furfural or preferably formaldehyde. These phenol-aldehyde resins include thermoplastic resins (novolaks, two-step resins), heat reactive resins, and thermosetting resins (resole, one-step resins). These are formed by reactions of aldehydes with phenols at aldehyde to phenol mole ratios of 0.5 to 0.89 for novolaks and 0.85 to 2.0 for resoles. There can also be employed modified phenol-aldehyde resins. 
     The phenolic resin types employed were heat reactive resins, resoles and novolaks. These resins may be based for example on phenol, nonylphenol, o-cresol, p-cresol, cresylic acids, cashew nutshell liquid, o-ethyl-phenol, para-tert-butylphenol, octylphenol (e.g. p-octylphenol) or the allylether formed by etherifying the phenolic hydroxyl group of trimethylol phenol. These resins may be modified with 5 to 40% of epoxies, nitrile rubber, cashew nutshell liquid, zinc, tall oil, acrylics, phenoxies, or polyvinyl butyral. 
     The epoxy resin may be a bisphenol A-epichlorohydrin resin, hydantoin epoxy resin or an aliphatic glycidyl ether such as butylglycidyl ether or other epoxy resins such as those shown in U.S. Pat. No. 2,500,600, the entire disclosure of which is hereby incorporated by reference. The epoxy groups in hydantoin-epoxy are connected to the nitrogen atoms, e.g. by reaction with epichlorohydrin. 
     The elastomer which has been most successful is an ethylene acrylic elastomer such as Dupont&#39;s Vamac. Vamac is made in accordance with Greene U.S. Pat. No. 3,904,588 the entire disclosure of which is hereby incorporated as reference and relied upon. The ethylene acrylic elastomer according to Greene is a copolymer (terpolymer) of ethylene and methyl acrylate plus a cure site monomer (--R--) Z  --COOH. The elastomers have the formula ##STR1## Thus there can be used a copolymer (terpolymer) of ethylene, methyl (or ethyl) acrylate, and about 0.5 to 10 weight percent of a mono alkyl ester of 1,4-butene-dioic acid in which the alkyl group of the ester has 1 to 6 carbon atoms, and the copolymer has about 20 to 35 grams of (--CO 2  --) units per 100 grams of copolymer; the copolymer having a melt index of 0.3 to 100 g/10 minutes. For example the copolymers can contain 40-62 weight percent of methyl acrylate units. Examples of mono alkyl esters are: propyl hydrogen maleate, ethyl hydrogen maleate, and methyl hydrogen maleate. 
     In place of Vamac in the working examples of this specification (see below) there can be used in the same amount the terpolymer of ethylene, methyl acrylate and ethyl hydrogen maleate, e.g., the terpolymer of Examples 2 or 5 of the Greene patent. 
     Acrylic elastomers such as Hycar 4051 made from various combinations acrylic acid, acrylate-esters and acrylamide, have also been used. 
     Nitrile rubber can be used as an elastomer, such as B. F. Goodrich&#39;s Hycar 1032 (33% acrylonitrile) and reactive liquid polymers such as carboxyl-terminated polybutadiene (Hycar CTB), or carboxyl-terminated polybutadiene containing acrylonitrile (Hycar CTBN with 10%, 18% or 26% bound acrylonitrile and Hycar CTBNX (contains 18% bound acrylonitrile and additional random carboxyl groups)). The properties of these compositions are as follows: 
     
         ______________________________________                             HycarPolymer      Hycar CTB Hycar CTBN CTBNXCharacterization        2000 × 156                  (1300 × 8)                             (1300 × 9)______________________________________Viscosity @ 27° C.        35,000    110,000    125,000BrookfieldRVT #7 20 rpm, cpsEPHR - Carboxyl        0.042     0.053      0.065Percent - Carboxyl        1.9       2.37       2.93Molecular Weight -        4000      3200       3400Number AverageCarboxyl     2.01      1.8        2.4FunctionalitySpecific Gravity at        0.9069    0.9483     0.955420°/25° C.Refractive   1.5145    1.5142     1.5141Index at 25°C.Heat Loss - 2 Hrs.        0.5       1.0        1.0@ 130° C. - %Unsaturation as:cis - %      22.1      23.8       16.3trans - %    53.4      64.2       68.7vinyl - %    24.5      12.0       15.0Bound        --        18-19      18-19Acrylonitrile,Ash - %      0.02      --         --______________________________________ 
    
     Vinyl-terminated polybutadiene containing acrylonitrile (Hycar VTBN (16% acrylonitrile and Hycar VTBNX (containing 16% bound acrylontrile and additional random vinyl groups)) having the structure ##STR2## where A is 1,4- or 1,2-butadiene and B is acrylonitrile, or hydroxy-terminated polybutadiene containing acrylonitrile (Hycar HTBN containing 10% or 17% bound acrylonitrile), may also be used as elastomers. The Hycar polymers can be substituted or unsubstituted. 
     Other elastomers tested were DuPont&#39;s Viton which is a fluoroelastomer based on the copolymer of vinylidene fluoride and hexafluoropropylene with the repeating structure possibly --CF 2  --CH 2  --CF 2  --CF(CF 3 )--; DuPont&#39;s Adiprene which is a polyurethane rubber, the reaction product of diisocyanate and polyalkylene etherglycol; DuPont&#39;s Hypalon, a chlorosulfonated polyethylene which is a synthetic rubber; Borg Warner&#39;s Blendex, a synthetic resinous product; and Arco&#39;s Poly B-D which is a low molecular weight liquid polymer based on butadiene containing controlled hydroxyl functionality (homo- and copolymers). 
     The Vamac and Hycar elastomers proved to be the most successful. The others failed to form unsupported films or to offer any advantage in adhesive properties over conventional adhesives. In applications where specific properties are desired, two or more elastomers may be used. The coelastomer ranges from 0 to 25% of the ethylene acrylic elastomer level, or from 0 to 20% of the final solids content of the composition. 
     A catalyst can be added to this system. This catalyst can be one which promotes the reaction of the epoxy resin and the phenol-aldehyde resin. A catalyst used with a nitrile-based elastomer can be one which promotes vulcanization of the elastomer. On occasion it can be desirable to use more than one catalyst. 
     The catalyst and curing agents include for example 2-methyl imidazole, tetraisopropyl titanate, trimellitic anhydride, pyromellitic dianhydride, Mondur S, Mondur SH, sulfur, mercaptobenzothiazole, tetramethyl thiuramdisulfide, di-ortho-tolyl-guanidine, N-oxydiethylene benzothiazole-2-sulfenamide, benzothiazyl disulfide. At present imidazole catalysts are preferred. Additional suitable imidazole catalysts are those mentioned in Watanabe U.S. Pat. No. 4,313,654, col. 4, lines 45-61. 
     The adhesive composition is prepared by combining the phenol-aldehyde resin (or other phenolic resin), epoxy resin and elastomer in solution. This may be done by putting each component in solvent and mixing the three solutions together, or by a general mixing of components and solvents until a solution is reached. 
     The ratio of epoxy resin to phenol-aldehyde resin may be from 20 parts epoxy to 80 parts phenol-aldehyde, to 80 parts epoxy to 20 parts phenol-aldehyde. The elastomer may be from 10% to 80% of the final solids material. Optimum results are obtained when the epoxy to phenol-aldehyde ratio is 30/70 to 50/50, respectively, and the elastomer is 40% to 60% of the final solids. 
     The adhesive of the present invention can also be used to produce an unsupported film. This film may be formed by coating the adhesive on a release medium, drying, and then rolling the film with or without the release medium. The adhesive may be coated on a substrate, dried, and then stored on the substrate for future use. The film is later heated and compressed to give adhesive properties. This permits use of this material in applications where a solvent should not be used. 
     The composition can comprise, consist essentially of or consist of the stated materials. Unless otherwise indicated all parts and percentages are by weight. 
    
    
     EXAMPLES 
     The following examples illustrate the invention in more detail. In each of the examples, the solids content of the composition was 35% in methyl ethyl ketone. The composition was coated on 200H Kapton film (polyimide film) at a dry film thickness of 0.001 inch. The coated samples were dried in an air circulating oven at 100° C. for 10 minutes. The samples were then laminated to 1 oz. RATK copper foil by pressing and roll lamination. The press procedure was as follows: begin with a press @ 350° F., place samples in at 400 psi, hold at temperature and pressure for one (1) hour, and then cool rapidly to ambient temperature. The samples for roll lamination were made at 4 FPM, 400 psi at 350° F. 
     These prepared samples were tested for bond strength at ambient temperature, bond strength after solder exposure of 550° F., and dielectric strength. 
     Example #1 
     Phenol was reacted with formaldehyde at an F/P mole ratio of 0.64 with oxalic acid as a catalyst. A 116°-120° (Ball &amp; Ring) meltpoint resin was produced. This resin was placed into solution with a brominated epichlorohydrin-bisphenol A epoxy (Epirez 5183). The weight ratio of phenolic to epoxy was 50/50. This material was then blended with an ethylene acrylic elastomer (Vamac) and Hycar CTBN-1300×13 (26% bound acrylonitrile). The final composition was 22.75% epoxy, 22.75% phenol-aldehyde, 45.5% elastomer (Vamac) and 9% coelastomer (Hycar) be weight. This material was reduced to 35% solids in solvent. The catalyst 2-methyl-imidizole was added (0.22% based on solids). 
     Example #2 
     Meta-cresol was reacted with formaldehyde at an F/P mole ratio of 0.887 with an oxalic acid catalyst. This resulted in a 190° C. (Ball &amp; Ring) meltpoint resin. This resin was blended with butyl glycidyl ether, Vamac and Hycar CTBN. The ratios were identical to Example #1. The solvent and catalyst levels were identical to Example #1. 
     Example #3 
     The resin of Example #2 was blended with Epirez 5183, Vamac and Hycar CTBN. The ratios were 18.2% phenol-aldehyde, 18.2% epoxy, 54.5% Vamac and 9.1% Hycar. The solids content was identical to Example #1 but the catalyst was omitted. 
     Example #4 
     The resin of Example #2 was blended with glycidyl methacrylate, Vamac and Hycar CTBN. The ratios, solvent levels and catalysts were identical to Example #1. 
     The Table 1 reports the test data on Examples 1 to 4. The values in the column labeled &#34;control&#34; were obtained using a standard adhesive system based on epoxy resin. The results were obtained by using IPC Method 2.4.9, and the SCI Method (described below). Table 2 shows bond strength data as an additional series of adhesives obtained using the SCI Method. The data in Table 2 is included to indicate the added scope of the invention in employing other types of resins. 
     SCI Method For Testing Fiber Adhesive Laminate Strips 
     Liquid adhesive was applied on 0.002-in. Kapton Type H polyimide film, by drawing 5×10-in. film strip containing excessive adhesive through stainless steel rolls. Roll separation was adjusted in order to give an adhesive thickness of 0.0015-0.0020 in. following exposure of the adhesive-coated film in an oven at 105° C. for 10 min. Approximately 1 to 2 in. of one of the 5-in. edges was left uncoated to provide a gripping surface for the pull testing. After removal from the oven, the adhesive-coated film was checked for tackiness by touch, and for bubbles by visual inspection. Bond strength was determined as follows: copper foil (1 oz.) was cut into 5×10-in. pieces approximating the size of the coated film strips. The adhesive-containing face of the film strip was mated to the copper foil. A temporary bond was achieved by touching two opposite edges of the film with a hot spatula sufficient to soften the adhesive and achieve bonding. 
     The foil-film laminates were permanently bonded in an EEMCO press (hydraulic with electric heat, 75 ton capacity, platen size 18×18 in.) as follows. Approximately 40 sheets of 14×14×0.0065-in. Kraft paper forming a bed approximately 1/4 in. thick, were placed on the polished side of a 14×14×1/8-in. stainless steel panel. On top of the Kraft paper two 5×10-in. foil-film laminates were placed side by side. On top of the laminates was placed another 1/4-in. layer of Kraft paper sheets, followed by the polished side of the second 14×14×1/8-in. stainless steel panel. This composite was placed in the pre-heated EEMCO press at 350° F. and held for 30 min. at 450 psi. 
     After removal from the press, and after cooling for 15-20 min., the 5×10-in. foil-film laminates were cut into four 1×8-in. strips for testing. The unbonded 1-2-in. margins at one end of the strips were coated with masking tape for gripping in an Instron tester (Model TT-C floor model with 10,000 lb. capacity and 20-lb., full-scale range). A &#34;T&#34; pull method was employed using a Model 3CA pneumatic-action clamp at a crosshead speed of 2 in./min. and a chart speed of 1/2 in./min. Results were recorded in lbs./linear in. Specimens were visually examined for adhesive and cohesive failure. 
     It is believed that the results obtained using the IPC Method are a truer reflection of the actual bond strength than the results obtained by the SCI Method. As shown in Table 1, the IPC Method results are approximately 160-165% of the values obtained using the SCI Method on the same composition. If the SCI bond strength values in Table 2 are multiplied by a factor of 1.6 then most of the values are approximately equal to the control value bond strength of 5-7 pounds/inch. 
     
         __________________________________________________________________________Test    Method Used            Units                 #1  #2  #3 #4   Control__________________________________________________________________________Bond Strength   IPC TM650-2.4.9.            Lbs./In.                 8-9 7-8 7-9                            4-6Roll Laminate   IPC Method A   550° F. Solder            Lbs./In.                  8-10                     10-12                         9-10                            Blistered   IPC Method CBond Strength   IPC TM650-2.4.9.            Lbs./In.                 10-11                      8-10                         9-10                            8-9  5-7Press Laminate   IPC Method A   SCI Method            Lbs./In.                 6.35                     4.73                         6.30                            6.25   550° F. Solder            Lbs./In.                  8-10                     10-12                         9-10                            Blistered                                 8-9   IPC Method CDielectric   ASTM-D-149            KV/Mil.                 3-4 3-5 3-2                            3-7Strength__________________________________________________________________________ 
    
     
         __________________________________________________________________________                 SCI Method        Melting Pt.                 Bond Strength                         Bond Strength × 1.6                                    IPC MethodResin Type   Physicals                 Lbs/In. Lbs./In.   Lbs./In.__________________________________________________________________________Para-tert-octylphenol         85° C.                 5.65    9.04Para-tert-butylphenol         83° C.                 5.53    8.85Phenol Novolak        116-120  6.66    10.66      10-11Cashew Nutshell Oil,         90-110° C.                 6.60    10.56Phenol NovolakNonylphenol Novolak         97-101° C.                 5.9     9.44Tall Oil,     90-100° C.                 5.2     8.32Phenol Novolako-Cresol Novolak         99-104° C.                 4.51    7.22M-cresol, P-Cresol,        O--Ethylphenol,                 4.78    7.65Novolak      150-165° C.Blendex, BTA,*        100-105° C.                 3.93    6.29Phenol NovolakCresylic Acid Resole         78-88° C.                 3.25    5.20Furfural-Phenol Novolak        100-105° C.                 3.43    5.49Acrylic-Modified        105-110° C.                 4.59    7.34Phenol NovolakPhenoxy-Modified        105-110° C.                 3.51    5.62Phenol NovolakCashew Nutshell        --       4.56    7.30Oil PolymerResorcinol Novolak        100-110° C.                 5.63    9.01M-Cresol Novolak        195°  C.                 6.20    9.92Methylon Resin        --       4.0     6.40Phenoxy PKHH --       5.10    8.16__________________________________________________________________________ *Blendex BTA is a thermoplastic copolymer of methacrylate/acrylonitrile/styrene For each of the resin types tested in Table 2 the following characteristics were constant: Resin Wt. Ratio: 22.75% Epoxy Type: Epirez 5183 Epoxy Wt. Ratio: 22.75% Elastomer: Vamac C Elastomer Wt. Ratio: 45.5% CoElastomer: Hycar CTBN CoElastomer Wt. Ratio: 9% Catalyst: 2methyl imidizole Catalyst Wt. Ratio: 0.22% The phenol  and substitutedphenol-containing resin in Table 2 contain solely a phenol reacted with formaldehyde except for the furfuralphenol novolak, the cashew nutshell oil copolymer, cashew nutshell oil polymer and the modified phenolic novolaks.