Abstract:
Disclosed is an emulsion having a discontinuous phase which comprises at least two compounds of different structural formula selected from the group consisting of solvent-soluble polymers, oligomers, and monomers dissolved in an organic solvent, and a discontinuous phase which comprises an organic non-solvent for the compounds. A method of coating a conductor with a polymeric alloy is also disclosed. The conductor is immersed in the emulsion and a direct current is applied between the conductor and the emulsion sufficient to cause the discontinuous phase of the emulsion to migrate to the conductor and deposit on it. The conductor is removed from the emulsion and is dried and the deposit is cured.

Description:
BACKGROUND OF THE INVENTION 
     Graphite fibers are of increasing interest for the construction of structural composites because of their very high strength and high stiffness to weight rations. Performance characteristics of composites depend upon the properties of the materials comprising the composite and the process by which they are combined. Full utilization of the properties of these graphite composites has not been attained because of the weakness of the interphase bonding between the fiber and the matrix. This is due in part to the type of resin used, but also to the wetting of the graphite fiber by the resin. 
     Generally, graphite fiber or cloth is saturated with the appropriate resin by a dip coating process. Both thermoset and thermoplastic resins have been used to prepare graphite composites. Graphite fibers have also been coated electrophoretically in non-aqueous processes to improve the wetting and impregnation of the graphite by the resin (see U.S. patent application Ser. No. 717,046, filed Mar. 28, 1985, and now U.S. Pat. No. 4,664,768. Attempts to electrophoretically coat graphite fibers from aqueous media have not been very successful because very little resin (only about 1 to 2%) is electrodeposited, and the aqueous medium is deleterious to the fiber. 
     SUMMARY OF THE INVENTION 
     We have discovered that copolymers and polymeric alloys can be deposited on graphite and other conductors electrophoretically. (A polymeric alloy is a mixture of two polymers; there is no chemical bonding between the two polymers.) Graphite fibers coated by the process of this invention are wetted by the resin better than in prior processes, and there are fewer voids and pores in the resin. We have also been able to achieve resin content values on graphite cloth of between about 20 and about 50%, far in excess of that achieved by previous processes. Because of this high resin content, the graphite cloth can be stacked and pressed to form a laminate. Laminates prepared according to the process of this invention have a higher impact strength than do laminates prepared by conventional prepegging methods such as dip coating. 
     While polymeric alloys have been prepared by other methods such as physical blending, the polymeric alloys prepared according to this invention are structurally different from polymers prepared according to prior processes because the polymers are intimately mixed on a molecular basis. 
    
    
     DESCRIPTION OF THE INVENTION 
     The starting material in the process of this invention is an emulsion that contains a blend of two or more polymerized or polymerizable compounds in the discontinuous phase. That is, the emulsion consists of a discontinuous phase, which is a solvent containing two or more polymers, oligomers, or monomers. 
     Any two or more solvent-soluble polymers, oligomers, or monomers that are compatible can be used to form an emulsion according to this invention. Polymers are preferred to monomers or oligomers as they give better rheological properties on the conductor in that they are less prone to run off. Compounds are considered to be compatible if they are sufficiently miscible so that separate phases do not form. While the compounds can react after they are deposited on the conductor to form a higher molecular weight polymer or a copolymer, they do not react in the emulsion but exist as separate compounds. Any combination of polymers, oligomers, and monomers may be used, such as, for example, 2 polymers, or 2 monomers, or a polymer and a monomer. Also, the compounds may be of different types, such as an epoxy and a polyester, or they may be of the same type but have different structural formulas such as, for example, two diglycidyl ethers of bisphenol A having different molecular weights. It is often desirable to use two identical compounds of different molecular weights in order to produce a coating on the conductor which has the good properties of the higher molecular weight compound but which is easy to process due to the presence of the lower molecular weight compound. Suitable compounds that can be used in preparing the emulsion include polyesters, epoxies, polysulfones, polyethersulfones, polyimides, polyamide-imides, polyparabanic acids, polycarbonates, elastomer-modified epoxies, polyurethanes, polyethers, polyamide, polyethylene, polytetrafluoroethylene, polypropylene, bismaleimide, triazine, etc. The following pairs of compounds are preferred as they produce coatings with good toughness and high temperature mechanical properties: 
     polyimides and epoxies 
     polyamides and epoxies 
     polyimide elastomers and modified epoxies 
     polyamide imides and elastomer-modified epoxies 
     polysulfones and epoxies 
     polyethersulfones and epoxies 
     polysulfones and polyimides 
     polyethersulfones and polyimides 
     polysulfones and polyamide-imides 
     polyethersulfones and polyamide-imides 
     low molecular weight diglycidyl ethers of bisphenol A and high molecular weight diglycidyl ethers of bisphenol A 
     bismaleimide triazine and polyimides, polyamideimides, epoxies, elastomer-modified epoxies, polysulfones, polyether sulfones, or diglycidyl ethers of bisphenol A Two or more compounds may be mixed in any proportion such as, for example, 1 to 99% by weight (based on total solids weight) of one compound and 1 to 99% by weight of the remaining compounds, but it is preferable to use about 20 to about 80% by weight of one compound and about 20 to about 80% by weight of the remaining compounds in order to take advantage of the properties of both compounds in the resulting coating. If one of the compounds is a monomer or an oligomer, a catalyst is preferably included, as is known by those skilled in the art, so that the monomer or oligomer can polymerize once it has been deposited on the conductor. If one of the compounds is a thermosetting polymer, a catalyst is included, as is known to those skilled in the art, in order to cure or cross-link the polymer once it has been deposited on the conductor. 
     The solvent is an organic solvent, and it is a solvent for a sufficient amount of the compounds to result in the formation of the emulsion. Suitable solvents include methylene chloride, acetone, dimethylformamide, 2-methylpyrrolidone, 1,1,2-trichloroethane, dimethylsulfoxide, xylene, and decahydronaphthalene. Other suitable solvents will no doubt occur to those skilled in the art. 
     The non-solvent is also organic and is a non solvent for a sufficient amount of the polymers, oligomers, and monomers to result in the formation of the emulsion. The non-solvent can be miscible or immiscible with the solvent, but it is preferably miscible with the solvent as this tends to result in a coating that is more solvent-free. If the non-solvent is immiscible with the solvent, it may be necessary to include an emulsifier such as a tertiary amine in the emulsion to aid in its formation, as is known in the art. Non-solvents that can be used in forming an emulsion according to this invention include acetonitrile, dimethylformamide, acetone, and other ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Whether a particular organic liquid is a solvent or a non-solvent will depend upon the particular polymeric compounds used in forming the emulsion. 
     The weight ratio of non-solvent to solvent should be about 2 to about 10 as ratios outside this range tend to result in the deposition of less material or produce a less stable emulsion; the preferred weight ratio of non-solvent to solvent is about 3.5 to about 4.5. The emulsion should contain about 0.5 to about 1.5 wt.% solids based on the total emulsion weight. If less than about 0.5% solids are present, the electrodeposition process will still work but it will take a rather long time, and if more than 1.5% solids are present, the emulsion tends to be less stable; preferably, the percent solids is about 0.9 to about 1.1. 
     While an emulsifier is preferably not present, about 0.5 to about 1 wt.% (based on solids) of an emulsifier can be used when the solvent and non-solvent are immiscible or when the emulsion does not readily form. Preferably, the amount of emulsifier is about 0.1 to about 0.3% by weight. 
     In preparing the emulsion, the solid materials are dissolved in the solvent and the resulting solution is mixed with the non-solvent. If it is desirable to have the different polymeric materials in different droplets in the emulsion, they are dissolved in separate solvents and the separate solvents are added separately to the non-solvent. It may be necessary to heat and stir the mixture in order to emulsify the components. 
     The conductor can be of any shape including a flat surface, individual fibers, mat, roving, or woven cloth. It can also be of any material that conducts electricity including metals, graphite, boron, and silicon. Graphite fibers that are woven into cloth are the preferred conductor because they have a high specific modulus and a high specific strength which makes them very useful in forming laminates for aerospace applications. 
     The conductor is coated by immersing it in the emulsion and applying a direct current between the conductor and an electrode in contact with the emulsion. The conductor can be either the anode or the cathode, depending upon the particular materials used, but most commonly, the conductor will be the anode. A voltage of about 1 to about 500 volts is suitable. The current is applied until the desired thickness of coating on the conductor has been achieved or until additional material will not deposit on the conductor. The conductor may be coated in either a continuous or batch process. 
     After the conductor is coated, it is removed and dried. If the conductor is coated with a thermoplastic material, it is heated to fuse or melt the thermoplastic and form a smooth coating on the conductor. If the conductor has been coated with a monomer, oligomer, or reactive or cross-linkable polymers, it is heated to polymerize, react, or cross-link the polymeric material in the coating. 
     If a laminate is to be made, the conductor is preferably a woven cloth. The process of this invention will deposit sufficient resin on the cloth to permit the cloth to be used directly in forming the laminate. Sheets of the cloth are stacked, heated, and pressed to form the laminate. In addition to preparing laminates, the process of this invention can also be used to make coatings on metal surfaces, as insulation on metals, and as wire enamel on wires. It can also be used to coat graphite fibers for use in making articles by filament winding. 
    
    
     The following examples further illustrate this invention. 
     EXAMPLE 1 
     An emulsion was prepared by adding a solution of 1360 g N-methylpyrrolidone (NMP) and 61.48 g of a low molecular weight or a high molecular weight polyethersulfone sold by ICI Chemical Corporation under the trade designations &#34;Vitrex 100P&#34; or &#34;Vitrex 300P,&#34; respectively, from a separator funnel to 4788 g of stirred acetone. Graphite cloth 71/4&#34;×9&#34; was immersed into emulsion so that the immersed portion was 71/4&#34;×8&#34;. The cathode was an expanded nickel screen 9&#34; long ×43/4&#34; wide. Electrode separation was 3&#34;. A potential was applied for varying times and the coated cloth was dried in an oven using the following schedule: 5 min 75° C. +5 min 75°-100° C. +5 min 100°-125° C.+5 min 125°-150° C.+10 min 150°-180° C. The results of this electrocoating procedure are shown in Tables 1 and 2. 
     
                       TABLE 1______________________________________&#34;VICTREX 100P&#34; ON GRAPHITE CLOTH                                ResinGraphite  Time (Mins) Current           Content,Cloth  &amp; Voltage (dc)              mA        Coulombs                                wt. %______________________________________A      10 50-150   --        --      43B      8 150       160-155   92      39C      10 1/2  150 162-160   124     38D      10 150      156-150   116     31E      8 150       160-158   100     22F      9 150       165-160   109     33G      10 150      170-168   122     30H      12 150      180-178   141     26I      9 150       100-90    59      40J      11 150      98-90     65      40K      10 1/2 150  90-83     60      35L      14 150      85-75     72      35M      13 1/2 150  98-86     79      33N      14 150      100-87    93      24______________________________________ 
    
     
                       TABLE 2______________________________________&#34;VICTREX 300P&#34; ON GRAPHITE CLOTH                                ResinGraphite  Time (Mins) Current           Content,Cloth  &amp; Voltage (dc)              mA        Coulombs                                wt %______________________________________A      8 150       58-52     31      31B      9 150       55-49     32      30C      11 150      55-48     39      33D      9 1/2  200  73-60     45      37E      10 200      69-56     45      36F      8 200       82-73     46      34G      9 200       79-69     49      29H      11 200      78-64     58      28I      13 200      75-61     64      24J      11 250      92-72     66      24K      13 300       110-86   91      23L      7-200       72-66     33      47M      7 1/2 200   69-64     34      40N      6 250       86-79     34      41O      6 1/2 250   85-76     35      38P      7 250       81-74     36      32______________________________________ 
    
     EXAMPLE 2 
     In another experiment, the graphite cloth was stacked in the warp and fill direction such that the stacking symmetry was maintained about the center of the laminates. Specimens were cut in the warp and fill directions relative to the outside ply in all applicable cases. The coated graphite cloth described in Tables 1 and 2 was converted to a laminate by stacking the 6&#34;×6&#34; pieces of coated graphite cloth, 14 plies in all, alternately in a warp and fill direction and placing them in a cold press. A pressure of 1,000 psi was applied and the press was heated to 450° F. (about 1/2 hr). The laminates were held at 450° F. and 1,000 psi for 1 hr and were cooled to room temperature under pressure with cold water (approximately 1 hr). Impact tests were run on the laminates by machining specimens parallel to the mutually perpendicular edges of the laminate; the results are shown in Table 3. 
     
                       TABLE 3______________________________________Energy (in-lb/in.sup.2)      Initiation  Total        Fill     Warp     Fill   WarpComponents   Direction                 Direction                          Direction                                 Direction______________________________________Polysulfone - &#34;300P&#34;        180      100      900    580Commercial System        440      240      920    775Polysulfone - &#34;100P&#34;        180      265      1010   1140______________________________________ 
    
     As can be seen from Table 3, graphite-polyethersulfone &#34;100P&#34; composite was tougher than the Commercial System in both directions since the total energy absorbed is higher. (The Commercial System is sold by Hercules, and is believed to be type AS4 woven graphite fiber impregnated with N,N,N&#39;,N&#39;-tetraglycidyl ether of diamino diphenyl methane sold by Ciba-Gigy under the trade designation &#34;MY720&#34; cured with diamino diphenyl sulfone.) 
     EXAMPLE 3 
     In another experiment, weight loss as a function of exposure temperature was measured for the three systems. The following table gives temperatures that resulted in a 1% weight loss. Measurements were conducted on thermogravimetric analysis equipment. 
     
         ______________________________________Laminate Materials             Temperature______________________________________&#34;300P&#34; + graphite cloth             300° F.&#34;100P&#34; + graphite cloth             320° F.Commercial System 270° F.______________________________________ 
    
     From the above table, it is clear that laminates made from polyethersulfones (&#34;100P&#34; and &#34;300P&#34;) possess superior thermal stability over the Commercial System so widely used in the aerospace industry. 
     EXAMPLE 4 
     Dimethylformamide (540 g) was heated to 80° C. and 120 g of &#34;Victres 100P&#34; was slowly added (about 10 g/min) and stirred until it had all dissolved (about 10 minutes). At this point, 280 g of &#34;MY-720,&#34; heated to 80° C., was added, and stirring was continued at that temperature for 45 minutes. The dark amber solution was cooled to room temperature. A homogeneous solution was obtained. This composition contained a weight ratio of epoxy to polyethersulfone of (2:1), and had a solids content of 50%. 
     EXAMPLE 5 
     A second composition was prepared as described in Example 4 except that the weight ratio of epoxy to polyethersulfone was 1 to 2.33. The composition was as follows: 
     Dimethylformamide (DMF)=690 g 
     &#34;Victrex 100P&#34;=280 g 
     Diaminodiphenyl sulfone=60 g 
     &#34;MY-720&#34;=120 g 
     EXAMPLE 6 
     Milky colored emulsions were prepared by diluting the solutions in Examples 4 and 5 with DMF and adding them slowly from a separatory funnel to a stirred acetone solution. The compositions of the various emulsions are shown in Table 4. 
     
                       TABLE 4______________________________________                               Non-Solution  Solution             SolventFrom      From            Ace- toExperiment          Experiment                    DMF   tone Solvent                                      SolidsNo.  4(g)      5(g)      (g)   (g)  Ratio  Wt. %______________________________________1    5                   73    174.5                               2.31   0.992    5                   68    179.5                               2.54   0.993    5                   63    184.5                               2.81   0.994    5                   58    189.5                               3.13   0.995    5                   53    194.5                               3.50   0.996    5                   48    199.5                               3.95   0.997    5                   43    204.5                               4.49   0.998    5                   33    214.5                               6.04   0.999              5         58.5  135.1                               2.19   1.010             5         50.5  143.1                               2.70   1.011             5         42.5  151.1                               3.30   1.012             5         34.5  159.1                               4.24   1.013             5         26.5  167.1                               5.66   1.0______________________________________ 
    
     Electrodeposition was carried out at constant voltage in a Pyrex glass beaker. The anode was an aluminum rod (Type 6061T6) 0.250&#34; in diameter and 8&#34; long which was immersed in the emulsion to a depth of 2&#34;. A circular nickel screen designated (10Ni 12-2/0) served as the cathode and was placed around the anode such that the electrode separation was 1&#34;. 
     Aluminum rods were immersed in each of the emulsions and 300 Vdc were applied for 1 minute. The coated rods were dried 5 minutes at 100° C.+5 minutes 100°-150° C. and weighed. The results are given in Table 5. 
     
                       TABLE 5______________________________________     Weight of ElectrodepositedNo.*      Coating, mg______________________________________1         392         463         514         555         556         557         568         489         5210        5111        6312        5513        45______________________________________ *These numbers correspond to those in Table 4. 
    
     EXAMPLE 7 
     Emulsions A and B were used to coat the cloth. The compositions are as follows: 
     
         ______________________________________Ingredient     Emulsion A(g) Emulsion B(g)______________________________________Solution from Expt. 4          125           --Solution from Expt. 5          --             150DMF            1325          1275Acetone        3862.5        4533______________________________________ 
    
     Graphite cloth designated &#34;A370-5H&#34; from Hercul Corp., about 20 mils thick, was cut into 7&#34;×9&#34; pieces and immersed into the emulsions in a polyethylene container. The cathodes were of the same nickel screen material described in Example 5. The cathode was 5&#34; wide and 81/2&#34; long and the electrode separation was 3&#34;. The data for the electrocoating of the graphite cloth is shown in Table 6. 
     
                       TABLE 6______________________________________                                Resin Content         Time,   Current        of CoatedNo.  Voltage  (Mins)  (mA)   Coulombs                                Graphite (g)______________________________________1    300      10      160-95 104     242    300      13      160-80 108     223    300      18      110-55  97     154    300      36      80-50  144     155    300      10      165-120                        127     326    300      17      160-65 128     267    300      25      100-50 128     178    300      34      60-40  128     119    300      10      240-110                        122     3010   300      22      140-45 123     2611   300      32      80-40  122     1612   300      52      50-25  143      813   300      10      220-110                        113     3014   300      151/2   160-95 114     2315   300      241/2   110-40 114     1716   300      50      70-25  146     15______________________________________ 
    
     EXAMPLE 8 
     An emulsion was prepared which consisted of 150 g of a polyimide sold by Upjohn Chem. Co. as a 22% solution in NMP under the trade designation &#34;PI-2080,&#34; 130 g of N-methyl pyrrolidone, and 4500 g of acetone. Graphite cloth was electrocoated with this emulsion. The results are presented in Table 7. 
     
                       TABLE 7______________________________________                               Resin Content ofVolt-           Current        Coated GraphiteNo.  age    Time     (mA)   Coulombs                               (wt. %)______________________________________1    100    35 sec   170-145                       6.5     2.02    100    7 min    150-120                       77      243    100    10 min   140-110                       98      284    100    12 min   130-100                       118     315    100    141/2 min                130-100                       147     316    100    20 min   125-100                       156     307    100    201/2 min                125-100                       167     248    100    22       125-100                       188     219    100    35       110-80 214     1810   100    8        150-120                       88      2711   100    14       130-95 113     3912   100    14       130-95 115     3213   100    16       130-95 127     3114   100    17       125-90 133     2715   100    171/2    130-90 145     2316   100    211/2    110-80 164     18______________________________________ 
    
     The coated graphite was dried in an oven for 5 minutes at 70° C., 5 minutes at 70° to 100° C., 5 minutes at 100° to 150° C. and 5 minutes at 150°-200° C. 
     EXAMPLE 9 
     Polymer blends of different polymers were prepared by dissolving the polymers in a suitable solvent. Table 8 gives these compositions. 
     
                       TABLE 8______________________________________Ingredient   A        B        C      D______________________________________&#34;MY-720&#34;     10     g              10            g&#34;PI-2080&#34;    2.5    g     12   g   5    g   5    g&#34;Victrex 100P&#34;            3    g            5    gBismaleimide resin                          5    gDMF          34     g     35   g   23   g   85   g______________________________________ 
    
     Using the blends in Table 8 emulsions were prepared by adding the blends to acetone. The composition of these emulsions is given in Table 9. 
     
                       TABLE 9______________________________________                                EmulsionIngredient   Emulsion A Emulsion B                        Emulsion C                                D______________________________________Solution A   10 gSolution B         10 gSolution C                   8 gSolution D                           12 gDMF     55 g       25 g      58 g    26 gAcetone 181 g      82 g      191 g   85 g______________________________________ 
    
     Aluminum rods were electrocoated with these emulsions. The electrocoating procedure is described in Experiment 7. A voltage of 300 Vdc was applied for 15 to 30 seconds. In all cases a heavy coating was deposited on the anode. 
     EXAMPLE 10 
     Using the electrocoated graphite cloth prepared in Experiment 8, a laminate was prepared. The graphite cloth was stacked in the warp and fill direction such that the stacking symmetry was maintained about the center of the laminates. Specimens were cut in the warp and fill directions relative to the outside ply in all applicable cases. 
     The 6&#34;×6&#34; pieces of the coated cloth, 14 plies in all, were stacked alternately in a warp and fill direction and placed in a preheated press (590° F.). Pressure (4500 psi) was applied and the temperature was raised to 670° F. Upon reaching this temperature (about 10 minutes) the laminate was held at this temperature for another 5 minutes. The laminate was allowed to cool under pressure to a temperature of 590° F. (about 15 minutes) and further cooled to 300° F., at which point the cooling water was turned on and further cooled to room temperature. 
     EXAMPLE 11 
     Another laminate was prepared using the electrocoated graphite cloth prepared in Experiment 7. The graphite cloth was stacked in the warp and fill direction such that the stacking symmetry was maintained about the center of the laminates. Specimens were cut in the warp and fill directions relative to the outside ply in all applicable cases. Fourteen plies of 6&#34;×6&#34; cloth were stacked alternately in the warp and fill direction. The stack was placed in a cold press and pressure (1000 psi) was applied. The press was heated to 350° F. (30 minutes) and the laminate held for 2 hrs at temperature and pressure before cooling with water to room temperature. The laminates prepared in Experiments 10 and 11 were tested and the results shown in Table 10. 
     
                       TABLE 10______________________________________Energy (in-lb/in.sup.2)      Initiation  Total        Fill     Warp     Fill   WarpComponents   Direction                 Direction                          Direction                                 Direction______________________________________Upjohn &#34;2080&#34;         90      230      740    860&#34;100P&#34;0&#34;     160      185      690    590______________________________________ 
    
     As shown in Table 10, crack initiation energy for the two systems is similar to the Commercial System, but total energy to failure is lower than the Commercial System. This may be due to higher percentage of voids and lower resin contents found in these laminates. Table 11 lists volume percentages of reinforcement, matrix, and voids. 
     
                       TABLE 11______________________________________            Rein-            forcement Matrix   Void            Volume %  Volume   VolumeLaminate Type    (ml)      % (ml)   % (ml)______________________________________1.  Polyethersulfone,                58.0      38.0   4.0    &#34;100P&#34; and graphite cloth2.  Polyethersulfone,                64.5      32.5   3.0    &#34;300P&#34; and graphite cloth3.  Upjohn &#34;2080&#34; and                68.0      24.6   7.4    graphite cloth4.  &#34;MY-720&#34;,        70.0      21.0   9.0    Polyethersulfone    &#34;100P&#34; and cured with5.  Commercial       61.0      36.0   3.0    System______________________________________