OCTOBER, 1939 (32) (33) (34) (35) (36)
.-
137) I
(38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48)
INDUSTRIAL AND ENGINEERING CHEMISTRY
MoHargue, J. S., J . Am. SOC.Agron., 18, 1076 (1926). Markley, K. S., Ibid., 20, 1102 (1928). Miiller, O . , U.S. Patent 930,874 (1909). Muller, G. F., doctorate thesis, Rutgers Univ., 1930. Musser, D. M., J. Assoc. Agr. Chem., 22, 421 (1939). National Cottonseed Products Assoc.. “Cottonseed and Its Products,” 1937. Olcott, H. S., Soap, 14, 105 (1938). Peters, F. N., IND. EXQ.CHEM.,28, 755 (1936). Peters, F. N., Ibid., News Ed., 15, 269 (1937). Peterson, W. H.. Fred, E. B., and Schmidt, E. G . , J. Biol. Chem., 60, 627 (1924). Randolph, E. E., Grove, C. S., and Tucker, R. C., J . Elisha Mitchell Sci. SOC., 48, 26 (1932). Ritter, G. J., Seborg, R. M., and Mitchell, R. L., IND. ENQ. CEEM.,Anal. Ed., 4, 202 (1932). Robinson, G. C., J . Biol.Chem., 53, 125 (1922). Sadtler, 9. P., and Matos, L. J., “Industrial Organic Chemistry,” p. 89, Philadelphia, J. B. Lippincott Co., 1923. Schorger, A. W., IND. ENQ.CHEM.,9,556 (1917). Schreiber, W. T., Geib, N. V., Wingfield, B., and Acree, 9. F., Ibid., 22, 497 (1930). Sheets, E. W,, and Thompson, E. H., U. S. Dept. Agr., Farmers’ Bull. 1179 (1920).
1233
(49) Sherrard, E. C., and Blanco, G. W., IND. ENG.CHEM.,12, 1160 (1920). (50) Skinner, W. W., McCall, M. A., Post, R. E., Blanck, F. C., Veitch, F. P., and Herrick, H. T., “Preliminary Estimates of Farm Products, By-products and Wastes Available for Industrial Use.” D. 9. U. S. Dent. Am.. 1935. (51) Speakman; H. B., J . Biol. hhek; 58, 395 (1923). (52) Underkofler, L. A., Fulmer, E. I,, and Rayman, M. M., IND. ENG.CHEM.,29, 1290 (1937). (53) U. S. Dept. +4gr., Preliminary Statement of a Cotton Research Program, 1935. (54) Ward, A. L., Cotton and Cotton Oil Press, 39, No. 31, 3 (1938). (55) Weinstein, L., and Rettger, L. F., J.Bact., 25, 201 (1933). (56) Woolrich, W. R., and Carpenter, E. L., “Mechanical Processing of Cottonseed,” Knoxville, Univ. Tenn. Press, 1935. (57) Zakoshchikov, A. P. Ivanova, V. T., Korzheniovskil, G. A., and
Xurennova, A. M., Trest Khlopkochistitel. Prom., Collection of Papers, No. 1, 102 (1933). (58) Zakoshchikov, A. P., Ivanova, V. T., and Kurennova. A. M., Trest Khlopkochistitel. Prom., Collection of Papers, No. 1, 87 (1933).
CONTRIBUTION from the Multiple Industrial Fellowship of the Cotton Research Foundation at Mellon Institute.
Electrodeposition of Svnthetic Resins J
ANDREW GEMANT University of Wisconsin, Madison, Wis.
metals of synthetic resinous materials is described. The novel feature is the use of hydrophobic insulating liquids, chiefly mineral oils, as the dispersing phase of the necessary suspensions. Because of the absence of electrolytic products as well as traces of water in the deposit, the method seems to be especially suitable for making electrically insulating layers on metals.
The deposition from insulating liquids has the advantage of avoiding complications due to electrolytic products, since the conductivity of the system is negligibly small. The deposition proceeds without enclosure of water between the solid particles, which is highly desirable from the standpoint of the insulating qualities of the deposit. This paper presents some results of the research carried out in connection with the development of the process (4). Four synthetic resins were used: polymeric styrene (PS), supplied by the Bakelite Corporation; methyl methacrylate polymer (MM), butyl methacrylate polymer (BM), and an alkyd modified urea-formaldehyde resin (AU), all supplied by E. I. du Pont de Nemours & Company, Inc.
RECENT paper (3)described some quantitative results concerning the cataphoresis of resinous materials in insulating liquids, By measuring the cataphoretic mobility it has been possible to compute the electric potential difference across the boundary between the two phases. A microscopic method has been used for the measurements. The progress of cataphoresis can be followed also by visual observations of deposits of the resin on metals. In continuing research along this line, the author has found that the deposits have sufficiently high insulating qualities to warrant the development of a method of depositing insulating resins for industrial purposes. Deposition from aqueous suspensions of insulating materials is already known, especially for rubber ( I ) , but the corresponding method for hydrophobic insulating media has not previously been suggested. Indeed, very little scientific research in this special field has been done, and that has been merely qualitative (6).
Preparation of Suspensions The general principle of preparing the necessary suspensions was that of slow precipitation from real solutions. Synthetic resins can be dissolved in certain insulating organic solvents, and these systems present all the characteristics of real solutions, in spite of the fact that the unit molecules are extremely large so-called macromolecules. On the other hand, the solubility in mineral oils of these resins is rather limited; accordingly it is possible to obtain fairly stable suspensions if mineral oil is added to the solutions. The solutions themselves do not show any sign of cataphoresis, as explained previously ( 3 ) ; nor are mixtures containing about equal parts of solvent and mineral oil effective. The right proportion between solvent and oil is one of the essential points. The correct solvent-oil ratio is in the range 1:2 to 1:3. A larger proportion of oil is not advisable, since the suspensions become too labile. For the same reason it was not possible to increase the concentration of the resin beyond a comparatively low limit.
A new process of electrodeposition on
A
1234
INDUSTRIAL AND ENGINEERING CHEMISTRY
In certain cases the suspensions require stabilizers, and phenol has been found most suitable. Some of the suspensions have to be prepared a t room temperature, some a t higher temperatures, Each resin thus requires a fairly individual treatment, and no general method can be given. The method of preparing the suspension is described separately for each of the four resins: POLYMERIC STYRENE.PS is easily soluble in xylene, and a standard solution of medium viscosity containing 10 per cent of the resin is prepared. Then 0.8 cc. of standard solution is diluted with 7 cc. of xylene and heated to about 80" C., and 16 cc. of kerosene are added. When cooled, the solution becomes turbid at about 65" C. The suspension is allowed to cool to 50" C. when it is used for electrodeposition. This suspension is reversible with regard to tem erature. If heated above 70" C., it becomes clear again, and tiis cycle can be repeated any number of times, provided the xylene lost by evaporation is replaced. METHYL METHACRYLATE POLYMER. It is not so easy to obtain stable suspensions of MM resin. A combined addition of phenol and potassium phenolate gives satisfactory results. Phenol is added as one part phenol and one part benzene, and the salt is added as a saturated solution in a mixture of one part phenol and three parts benzene. The standard solution of resin is obtained by means of chloroform, since it is not readily soluble in xylene. The concentration is again 10 per cent. The mineral oil used for reci itation was a medium-viscosity paraffin oil [Merusol of t i e &andard Oil Company (Indiana)]. The following mixture is made at room temperature: 0.8 cc. of standard solution is diluted with 6 cc. of chloroform; then 1.8 cc. of phenol solution and 0.3 cc. of otassium henolate sohtion are added and finally 20 cc. of herusol. 8ontrary to PS this system is not therrnoreversible, since heating causes complete coagulation. BUTYLMETHACRYLATE POLYMER. BM appears to possess a comparatively low molecular weight; hence its solubility in mineral oil is high and the necessary solvent-oil ratio is low. The standard solution contains 10 per cent resin in chloroform. One cubic centimeter of this solution is diluted with 2 cc. of chloroform; then 0.5 cc. of phenol solution and finally 25 cc. of Merusol oil are added. UREAFORMALDEHYDE. The AU resin was supplied in the form of a highly viscous liquid containing 58 per cent solids, and a 10 per cent standard solution in chloroform was prepared. One and a half cubic centimeters of the standard solution are diluted with 6 cc. of chloroform and then preci itated by 16 cc. of low-viscosity minerap oil [Superla NO. 9 of the Standard Oil Company (Tndiana)]. This resin differs from the first three in that it is not thermoplastic but is thermosetting.
Electrodeposition Process The suspensions described remain stable for about a day and are thus suitable for deposition work. With the exception of PS, the process has been carried out a t room temperature. F~~~~~ 1. D ~ - The PS suspension, prepared at higher VICE FOR COATtemperatures, is cooled to 50" C. and INO WIRES then used for deposition. Sometimes the container is kept in a bath a t 50" C., but the suspension is generally allowed to cool further during electrophoresis. Two different types of electrodes have been used. Most of the experiments were carried out in a cylindrical vessel, A , 20 cm. in length and 1.3 cm. in internal diameter (Figure 1); the inner electrode is a copper wire, B, 1 mm. in diameter and kept in position by two insulating holders, C, D,a t the top and bottom. A number of experiments with PS resin consisted in coating small trimmer condenser plates (supplied by the SparksWithington Company, Jackson, Mich.). The outer electrode
VOL. 31, NO. 10
consisted of a flat container, A (Figure 2), into which the plate to be coated, B, was immersed in a horizontal position. Opposite and above plate B, a third electrode, C, was placed and electrically connected to container A . The charge of the particles has to be determined by preliminary experiments in each case. I n PS suspensions the particles were negative, in the other three positive. Accordingly the electrode to be coated was in the first case positive, in the other three negative.
FIGURE 2. DEVICE FOR COATING PLATES
Qualitative observations on rubber sols showed both charges simultaneously. I n our deposition experiments migration was distinctly unidirectional, a t least for a large majority of the particles. The electrodes of the "wrong" polarity either remain completely blank or are sometimes covered with a thin patchy layer. Some control experiments were also carried out with alternating voltages; the result was either negative (with PS) or very poor (for MM). The wires and plates to be coated were mostly copper but sometimes brass, tin, etc.; the nature of the electrode does not seem to make much difference. I n the rubber anode process the electrolytic ions liberated from the anode form a n essential agent in the process of coagulation. This is important here, too, although the possibility of depositing positively charged particles indicates that electrons emitted from the cathode can play the same role as ions. A thorough cleaning of the electrode surface with nitric acid solution and finally with benzene proved to be useful in obtaining uniform layers, especially with positive particles such as the MM resin. The effect upon the deposit of the voltage and the time of deposition is described in the next section. A rectifier set, providing voltages up to 2000 volts, was used. Because of the electrostatic character of the process, the generator unit can be quite small, since no power has to be delivered, apart from losses. To obtain good deposits is not easy, and a rigid observance of all details is necessary. After deposition is completed. the adhering oil has to be wiped off the layer of resin. The thermoplastic materials do not require any further treatment, but AU deposits are cured at a n elevated temperature (180" C.) to make them insoluble.
Voltage and Time Effects How deposition proceeds with time is important. From the rubber process we know that the progress is linear and continues so for considerable thicknesses of the deposit. There is, however, an essential difference between latex and the systems described here. The concentration of the solid in the latex mix is very high (35 per cent), whereas in our systems i t is rather low (0.5 per cent). The suspensions of the resins will finally be exhausted, and deposition will stop. The thickness obtainable for deposits will ultimately depend upon the size of the container and the total amount of liquid. I n our experiments the containers were comparatively small, and the maximum thicknesses about 4 mils. It could
INDUSTRIAL AND ENGINEERING CHEMISTRY
OCTOBER, 1939
be shown, however, that this is not the upper limit. Some wires were coated twice by renewing the suspension after the first layer had been produced, and deposits up to 8 mils thick ' were easily obtained. The lower limit is around 2 mils, and deposits below this limit are liable to become porous. The voltage most frequently used in our condensers with separations of around 0.25 inch was 200-250 volts. The time to obtain deposits of 3-4 mils was 20-25 minutes. Time periods less than 15 minutes (for 200 volts) yielded thinner A 10
075
o l u
Y
&05
2
025
5,
1
2 3 MINUTES
4
to find a suitable solvent of sufficiently low dielectric constant, instead of chloroform.
Continuity of the Process From a practical standpoint it is essential that the process be continuous. Although no direct experiments on this point have been made, some thought has been given to the possibility of regenerating exhausted mixes. Some experiments were carried out to show how the process would work on the reversible PS mixes. After the deposition had been made in the usual manner, the liquid was heated until clear, and then 0.6 cc. of standard solution was added to replace the loss. A few drops of xylene were required to compensate for the loss due to evaporation, and the suspension obtained a t 50" C. was used as in the previous work. The process was repeated twice more, SO that four deposits were obtained from the same basic suspension. It follows that a regeneration of the liquid is possible and will have to be considered if the process is to be continuous.
Quality of Deposits
5
FIGURE3. THICKNESS AND ELECTRIC STRENGTHOF POLYSTYRENE COATINGS PREPARED AT 1000 VOLTS
deposits; longer times, on the other hand, did not have much effect. It is known from the theory (3) that the cataphoretic velocity is proportional to the electric field. Thus i t would follow that for a given container-for example, the cylindrical one (Figure 1)-the time required for a given deposit should be inversely proportional to the voltage. This conclusion was checked for the resins used here and found to be fully valid for the PS resin. For the other three an additional effect interfered with the cataphoretic one and thus made the use of higher voltages impossible. The voltage across the cylindrical container with the PS resin wm raised from 200 to 1000 volts, and the time necessary to produce a 4-mil deposit decreased to 3 minutes. The effect upon thickness of time was then investigated for a voltage of 1000; the result is given by curve A , Figure 3. Up to 2 minutes the deposition is linear, the curve then suddenly bends downward, and the process stops because the mix is exhausted. Although with PS resin the time can be reduced considerably by increasing the voltage, this is not the case for the other resins tested. When the voltage is increased beyond 300 volts (in the cylindrical container), the thickness of the deposit decreases rapidly. The reason is probably the effect known as dielectric displacement, an accumulation of the component with higher dielectric constant a t points of higher field strength. In the PS mixes the solvent has a low dielectric constant and thus does not interfere with the cataphoresis of the resin. In the other cases the liquid phase consists partly of chloroform with a dielectric constant of 4.7; owing to the effect mentioned, the liquid phase probably tends to replace the solid near the wire and thus destroy the deposit. The effect is proportional to the square of the field; therefore, it is small for lower fields but becomes appreciable a t higher ones. Figure 4 shows the thickness of the deposits of resins MM and AU obtained for different voltages during a deposition time of 5 minutes. According to the theory, the increase is linear at the beginning, passes a maximum a t about 300 volts, and decreases rapidly. It follows that the voltage should not be increased beyond 250 volts, and that a reduction of time is not possible under these conditions. To overcome this difficulty with resins of dipolar character, it will be necessary
1235
In order to test the quality of the deposits, electrical breakdown tests and measurements of the dielectric loss angle were made. I n the puncture tests the wire was used as one of the electrodes, and water, in which the insulation was immersed, as the other. Most of the tests were made with continuous voltages, and a few also with alternating voltage. The following data are averages from several (six to eight) single measurements, with a maximum deviation of *20 per cent.
'
I 400
200
600 VOLTS
FIGURE 4. THICKNESS OF
I
I
800
I000
6-MINUTE
COATINff S
Loss angle measurements were carried out by means of a radio-frequency bridge of the General Radio Company, a t 106 cycles. Tin foil, pressed around the deposit by means of traces of vaseline, served as the outer electrode. From the measured capacity and the known thickness of the insulation (measured with a micrometer) the approximate value of the dielectric constant could be computed; the loss angle was measured directly. The following table gives a summary of the data: Resin
PS
MM BM
AU
* r. m. 8.
Breakdown Strength D.C. A. C. Kv./om. KB.(r. m. s.) cm.*
530
640 960 680 = root mean square.
Lose Angle
Dielectric Constant
...
... ...
0.0004 0.039
3.3
870
o:oig
2.7
...
...
Both the puncture values and the loss factor are of the same order as quoted by Dunton and Caress (2); thus the deposits are satisfactory from the standpoint of electrical insulation. Curve B, Figure 3, shows the results of puncture tests. The drop of the curve towards thinner deposits is probably due to faulty spots, whereas the slight drop towards thicker deposits is a phenomenon often observed with dielectric materials. Although it is likely that the deposits contain a small percentage of oil, their characteristic features are much the same as that of the pure resins. Thus PS layers are rather
INDUSTRIAL AND ENGINEERING CHEMISTRY
1236
brittle, whereas MM deposits are fairly flexible and BM deposits are somewhat sticky. The most satisfactory from a mechanical standpoint is the AU resin. After cure a t 180" C., its deposit is hard yet very flexible. The breakdown test before and after a wire is bent is a good check for flexibility. Thus the electric strength of AU deposits does not decrease after the wire is bent over a curvature radius of 0.4 cm. It is possible to reduce the brittleness of a resin by adding a plasticizer. The introduction of a plasticizer (dibutyl phthalate) by means of cataphoresis was therefore tried. The voltage used was usually 1000 volts, the time 2 minutes. Bending took place over a curvature radius of 3.5 cm. The following table shows the results: VOl. %
Dibutyl Phthalate in Mix 0 0.4 0.8 1.2
Breakdown Strength, Kv./Cm. Before After bending bending
Since R here is 0.65 cm. and r is 0.05 cm., Equation 4 becomes: d
m = 0.13 nE
n,
Vol. Solid/
Resin
E, Volts 1000
PS
MM
100 200
-15
d , Cm./Seo.
6.6 X 10-6 6 X 10-6
8.3 X 10-
Total Vol. of
Mix 3.2 X 10-8 2.2 X
5.5 X 10-8
r = - 4 7 qm €
The last column shows the extent to which the plasticizer increases the flexibility. The second and third columns indicate that the breakdown strength first increases, then decreases with increasing amounts of plasticizer. This is due to the fact that the deposit becomes too soft. Therefore a small amount (about 0.5 per cent in the mix) of the plasticizer appears to be best. It is not known to what extent the plasticizer is contained in the deposit, but it seems possible that accumulation takes place due to adsorption on the resin.
m,
Ratio of Cm./Sec. to Volt/Cm. 2.7 X 103.6 X 101 X 10-
f , Volt
0.066
If wanted, {, the electrokinetic potential a t the boundary solidliquid can be computed from m when using the following equation (3):
+E
1.6
(5)
?sow d can be found from the initial portion of the curves in Figures 3 and 4, whereas n can be calculated from the data given in the section on "Preparation of Suspensions." The following table shows the computation of mobility:
AU
yo Change in Strength Due to Bending -28 -12
VOL. 31, NO. 10
where? = E
viscosity = dielectric coilstant of liquid
(6)
This calculation was carried out for PS resin, and the result is given in the last column (with 17 = 0.05 and B = 2.3). In comparison with a { value of 66 mv. for PS, in the previous paper (3) { for PS resin was determined by means of a microscopic method, and values around 40 mv. were obtained. This is in fairly good agreement with the present determination.
Mobility of the Particles
Summary
The possibility arises of computing the cataphoretic mobility from data obtained in deposition experiments. It might first appear that conditions in a cylindrical container were not favorable for such a computation; closer consideration, however, shows that this is not necessarily true. Comparing different cylindrical cross sections within the liquid, it becomes evident that the total amounts of solid which pass during a given time are equal for all cross sections. The reason is that the velocity is proportional to the field strength and thus inversely proportional to the radius; the surface, on the other hand, is directly proportional to the radius. Hence velocity multiplied by surface is constant for each cross section. Thus the concentration of particles remains constant at every point, and the process is stationary. The deposition velocity is then constant, too, as long as particles are present. Diffusion has been left out of this consideration; therefore a stationary flow can be expected only for the initial period of the process. If m is the mobility of particles, n is their volume concentration, F is the field a t the wire, and r is the radius of the wire, we obtain for the volume, v, of the deposit per unit length and unit time:
The results of research into the electrodeposition of certain synthetic resins on metals are reported. Hydrophobic organic liquids, chiefly mineral oils, are used as the dispersing phase of the suspensions. The principle of making the suspensions is slow precipitation. Details of preparation for four resins and of the electrodeposition on plates and on wires are given. I n order to reduce the time of deposition, the voltage can be increased considerably in the case of polystyrene but, owing to a complicating effect of dielectric displacement, not for the other three resins. Regeneration of the exhausted mixes seems to be possible. The quality of deposits is checked by measurements of breakdown strength and dielectric loss factor; both appear to be satisfactory. The flexibility may be increased by adding a plasticizer to the mix-for example, dibutyl phthalate. The method can be used also for the computation of the electrokinetic mobility of the resin particles in the insulating media.
v = 2rrmnF
F is given by : l3
F =
r log ( R l r )
(2)
where E = total voltage R = radius of container
v, on the other hand, can be expressed by: v = 2rrd
(3)
if d is the thickness of deposit built during the time unit. From Equations 1, 2, and 3: dr log (R/r) m = (4) ?&E
Literature Cited (1) Beal, C. L., IND.ENQ.CHEM.,25, 609 (1933). (2) Dunton, A. R., and Caress, A., J . Inst. Elec. Engrs. (London), 79,463 (1936). (3) Gemant, A,, J . Phge. Chem., 43, 743 (1939). (4) Gemant, A,, U. S. Patent Amlication, filed Nov., 1937. ( 5 ) Lantl;, E. A., and Pickett,-0. A., IND.ENQ.CHEM.,22, 1309 (1930).
