January, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
11
Chemical Problems in Insulating Varnishes' By H. C. P. Weber WESTINGHOUSE ELECTRIC & MANUFACTURING Co., EASTPITTSBURGH, PA.
HE applications of varnishes in the electrical industry that electrical apparatus always. heats up and that its limit may be divided into three general groups with refer- at the present time is governed almost entirely by the inence to the effect desired. consideration of these ability of the insulation to withstand high temperatures, the groups will show that there are a number of requirements not importance of this item is readily understood. Practically, met with in the ordinary uses to which varnishes are applied. varnishes are tested in this respect by determining their life The manufacture of varnishes for electrical insulating pur- on metal strips at 100' to 110' C . Wide variations are enposes is really a highly complex and specialized part of the countered in making this test, results falling between 200 and general varnish industry. 1600 hours with various An insulating varnish is varnishes. Insulating varnishes are used for three general purThe characteristics enuused for one or more of the poses-surfacing, impregnating, and bonding. merated above are of imfollowing purposes : The finished coat is the result either of simple evaporaPortance for any of the three tion of the solvent or of evaporation accompanied by oxidagroups of insulating VarA-To produce an exterior surface film and finish. tion or the result of a condensation. nishes. For impregnating Specific varnishes must be employed for specific purand dipping varnishes the B-To obtain thorough imPenetration is an added facposes. The oil varnishes are most widely used and the pregnation of paper, cloth, Or Of an Of hlghly protective film is obtained by evaporation accompanied by tor of importance. This is fibrous material and metal oxidation either at normal or elevated temperatures. in part a question of the such as coils, windings, and During this process materials are formed which have low viscosity and body of the laminations. varnish, but more directly, electrical resistance. The nature of these products and (2-5'0 obtain cementing and perhaps, a question of the their source are shown in detail. They may amount to 3 bonding action in order to colloid nature of the varlend strength and rigidity per cent of the filmto an assembly. nish. It is somewhat d%cult to obtain a general test on the impregnating power of varnishes. The viscosity alone Requirements of Insulating Varnishes is not sufficient and even the physical properties of the varnish The requirements for finishing and surfacing varnishes are do not tell everything. The behavior varies a180 with the Perhaps the most obvious and approach most closely the character of the material to be impregnated. Usually, a test requirements of varnishes for other industrial purposes. is made by approaching practical conditions, impregnating Even here certain specifications must be met, specifications a small coil or other article similar to the work to be done. which also apply to all other insulating varnishes. To secure cementing or bonding action the varnish is really The varnish must be moisture-proof, 8 0 that when applied used in bulk, as a filler, and its strength and rigidity after to the article it will prevent the entrance of moisture into the reasonable treatment become important, In a surfacing fiber, Paper, 01 wood that has been treated with it. A Var- varnish or in a varnish for impregnation alone, oxidation pronish that is used for preservative or artistic effect alone may duces the hardening of the thin f i b s , One of the purposes well absorb a certain quantity of moisture without harm and of a filling varnish is to prevent access of materials to the may even give better life and service if kept slightly moist. interior of the structure. The ideal filling varnish is one that But a varnish film which, for example, turns white on irnmer- will enter the material to be impregnated with perfect freedom. sion in water is of no value for electrical purposes. Once in place it should remain there until by some simple Certain applications call for a material that Will also with- treatment it can be permanently congealed and hardened. stand acid fumes or liquors. This is true when installations The closest approach to such a filler is found in bakelite and are to be designed for acid plants, nitration processes, elec- resins of that type, Even here the requirements are not tro-chemical processes, etc. With such requirements it is completely met because there are a number of productsusually necessary to use varnishes of the asphaltic type, at water, ammonia, excess phenols-which must be eliminated the expense sometimes of other desirable properties. Very before a really good insulator is obtained. few of the natural oils and gums withstand corrosive acids or alkalies well. and one is comselled to assroach a carbon Production of Varnish Film film as closely as possible by -selecting asihaltic materials. Most of the information so far obtained concerning the Naturally, all insulating varnishes must have a high dielectric strength. Paper treated with varnish and then dried chemistry of insulating materials has been slowly accumuwill stand a voltage of 1200 to 2400 volts per mil before lated by trial and failure. Even yet most of the investigapuncturing, if an ordinarily good insulating varnish is used. tions are carried on by physicist and engineer, who have been Upon immersion in water for 24 hours the breakdown voltage compelled by the necessitiei of the work to acquire some drops 300 to 600 volts, even with fairly good varnishes. This chemistry to cope with the problems. However, the subject gives a very sensitive means for determining the moisture re- is worthy of greater participation by chemists who can sistance of the varnish, and at the same time indicates how systematize and coordinate the properties and characteristics of materials for the electrical industry. Approaching sensitive insulation values are to moisture. A further requirement for insulating varnishes is considera- closer to the chemical part of the subject we may review the ble resistance to temperature rise. When it is remembered process by which the film or bond is produced. The first method is by hardening the varnish at ordinary 'Received November 18, 1924. Presented before the meeting of temperatures. The hardening process is either one of simple the American Institute of Chemical Engineers, Pittsburgh, Pa., December evaporation or of evaporation accompanied by oxidation. 3 to 6, 1914.
INDUSTRXAL A N D ENGINEERING CHEMISTRY This would include all the spirit lacquers, solutions of shellac, copals, dammar, mastic, rosin, or other gums in Plcoholic solvents. Some manufacturers group all these under the term %hellacs.” It includes solutions of asphalts, coal-tar, and petroleum pitches, coumarone resins and rubber in benzine and aromatic hydrocarbon solvents, and finally, the airdrying oil varnishes made either with a small amount of oil or with a heavily blown oil or large quantity of drier, Usually one cannot expect so much from an air-drying lacquer or varnish as from a baking varnish, for the following reasons: In the case of alcoholic varnishes the solvent evaporates gradually, allowing the residual moisture in the solvent to accumulate in the film. The varnish gum is deposited considerably below its melting point and may separate in granules. The varnish film is softer and more subject to abrasion. Asphaltic lacquers are liable to shrink and crack or become brittle as they age and the last traces of solvent disappear. In the case of air-drying oil varnishes the very thing that causes their rapid hardening is ultimately the cause of their destruction. I n order t o produce hardening of the film at normal temperatures within a short time advantage of rapid oxidation is taken. Unfortunately, this does not stop when the film is hard, but proceeds until it becomes brittle and weak. Those varnishes that harden on baking at elevated temperatures are essentially oil varnishes. It is true that even spirit and “benzol” varnish films are improved by bakingthe shellac and the asphaltic material becoming harder and less fusible. On the whole, however, it is the conversion of the drying oil into the tough, insoluble linoxyn film that is desired. In most cases linseed or China wood oil is used. The oil may be either a blown oil or compounded with a drier. During the baking of such a varnish the oxidation is accelerated and brought near completion by the elevated temperature, excess solvent and residual moisture are driven out, and a more uniform and smooth surface is obtained since the oil and gum are at or near the fusion point. On comparing these facta with the conditions obtained in air-drying varnishes, the reason why baked varnishes give better and more uniform results as insulating materials becomes apparent. Ideal conditions would be obtained if, during the heating process, the oxidation could be brought to a complete and abrupt stop at a desired point. At present this is not possible and therefore oxidation proceeds and finally terminates the “life” of the varnish. At the same time various oxidation products accumulate. The third process for producing a permanent film or bond consists in causing a reaction or polymerization to occur after the insulating material is in place. AU the synthetic condensation products, such as the phenol formaldehyde resins, the furfural resins, and similar materials are hardened in this way. As a matter of fact, these are practically the only materials used in this way. For bulk filling, varnishes are not suitable as it is impossible to oxidize them when in considerable mass. Fusible gum and oil mixtures are used to a considerable extent, but these would be much more satisfactory if their melting point were far above the temperatures likely to be reached during operation of the machine-a condition which so far has been difficult to realize. Polymerization of the phenol resins is accompanied by splitting off either water or ammonia. These and excess of phenolic material must be eliminated before the synthetic resin shows its best electrical properties, and provisions for this removal must therefore be made. In the preparation of laminated and molded materials these resins have found an extensive field of application. For use as protective films, oil varnishes seem slightly more in favor. Phenol resins oxi-
Vol. 17, No. 1
dize fairly rapidly when in thin films, yielding a dark-colored brittle product. Chemical Changes in the Hardening of Baking Varnishes
During the operation and life of electrical apparatus troubles such as short circuits and over-heating of parts sometimes develop, although the original materials and even the complete and finished assembly have been submitted to very rigorous tests. Therefore, the question has been raised as to whether some process goes on in the insulation which becomes responsible for later trouble. This may be a process developing in the varnish itself or a reaction between the varnish and some of the other materials present. Under the influence of excessive temperature rise or of corona, all the organic insulating materials undergo disruption, with production of more or less conducting materials. This may bo regarded as accidental and need not be discussed now. Furthermore, it is customary to test insulating varnishes for acidity, and a varnish that gives green films with metallic copper is considered bad. However, no particular difficulty is experienced in obtaining an initially neutral varnish of high dielectric strength. The green color produced with copper metal is harmless as long as it is the result of high-molecular fatty acids, such as oleates, linoleates, or of resinates. These copper salts are themselves good insulators and not conducting materials. The particular point in question is whether, during perfectly normal conditions-with a varnish that is initially satisfactory-low-resistance materials develop which cause trouble when they accumulate. This can be shown t o be the case. I n a previous communication2 it was shown that the insulation resistance of a varnished fabric does decrease as a result of heating, but goes through a minimum and then increases again. This seems as though some material were accumulating which later was driven out. Furthermore, the material which accumulates can be shifted from one part of the apparatus to another under the influence of heating. Subsequently it was shown3 that the formation of these low-resistance materials follows the degree of oxidation and the proportion of oil in the varnish. The essential feature in the varnishing process is that we have a gum and a drying oil which on exposure to air and light goes over into the oxidized material, called linoxyn, which is tough and pliable. For the ordinary uses of varnishing this is the only important feature, and any intermediate or subsidiary reactions are of only secondary consideration; but in the use of varnishes for insulating purposes these subsidiary reactions may become of considerable importance. A more complete list of the transformations which do or may take place during the hardening, aside from evaporation of solvent, is as follows: Oxidation of resin and turpentine Oxidation of drying oils to hydroxy acids and lactones Oxidation and splitting of fatty acids to lower acids Oxidation of glycerol, subsequent to splitting Rearrangement of fatty acid and resin acids with glycerol, and possibly with cellulose base if present Polymerization with elimination of water Change in composition of drier
When a few strands of cotton-covered wire are taped together and then impregnated with varnish and allowed to air-dry, they will yield about a cubic centimeter of highly acid water when a slow current of air is passed over them at about a
Trans. Am. Eleclrochem. Soc., 42, 293 (1922). Ibid., 44, 63 (1923).
January, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
100’ C. The varnished fabric itself will itill be highly acid. That the acidity is not due to the fabric itself or to the varnish in its original condition can readily be shown by check tests on these materials alone. Distillates obtained in this way may have a resistance as low as 100ohms per cubic centimeter or less. It is not surprising that varnish materials yield low molecular acids and it has previously been noted; thus, Lewkowitsch4 noted an accumulation of volatile fatty acids equal to 8 per cent during 6-hour blowing of linseed oil; this on further blowing for 4 hours dropped to 0.9 per cent. While this is not unexpected its importance has not been sufficiently noted in insulation practice. The serious part is that these low-resistance materials are water-soluble and volatile.’ While distributed throughout the oily and resinous varnish phase they may cause little trouble or even escape attention entirely. If, by a series of temperature inequalities, they are distilled from one to another part of the apparatus, together with some moisture, they form a strong electrolyte within the electrical machinery, and then trouble is almost certain. It does not follow that the varnish oil itself is alone responsible for all the harmful products formed. Simple qualitative tests were made by soaking cotton tape with a solution of various substances, allowing the tape to air-dry for a day, and then keeping a t 100’ C . for about 8 hours. The materials were thus thinly distributed as a film over the cloth and oxidized rapidly. When tested with moist blue litmus paper some of the materials reacted acid, even after air-drying. After heating, the impregnated tapes reacted as follows with moist litmus paper: Copal gum Varnish No. 9 No. 1-A Linseed oil, tung oil, lime resinate Various other oil varnishes
ACID REACTION At once After 5 minutes Within 10 minutes After 10 minutes
Somewhat more instructive results can be obtained by extracting and titrating the quantity of water-soluble acids formed in the tapes. When this was done the following results were obtained: Expressed in percentage of acid (as acetic) (or other material) applied. Per cent Varnish A 3.06 Copal in alcohol 2.41 Varnish B 2.23 Varnish C 1.82 Calcium resinate 1.76 Varnish D 1.31 Varnish E 1 28
formed on the weight of varnish Per cent Linseed oil Varnish F Tiing oil Asphaltic varnishes
1.08 0.92 0.87 0.88
{:
I
“Chemical Technology and Analysis of Oils, Fats, and Waxes,” Vol.
111. p. 181. 8
Samples supplied by Sterling Varnish Company.
tained. At least thirty varnishes were examined in one series. Each varnish was treated by spreading on tape and airdrying until dry or practically dry (45 hours were allowed on all samples). One part of the tape was then examined and recorded as “air-dried;” the remainder was wound into a loose spool and baked for 45 hours at 90’ C . This gave increased oxidation accompanied by restricted volatilization, so that the result obtained was a balance between rate of formation and rate of removal of oxidation products. These results are recorded as “oven-dried.” For each sample the weight of the original tape, the weight plus wet varnish, weight plus air-dried varnish, and weight plus oven-dried varnish were determined, so that all the data to calculate body content, loss by evaporation, loss by heating, percentages of acidity formed, etc., were available. The weight of the wet varnish taken up was that which the tape would normally absorb and was usually about equal to the weight of the tape itself. I n a measure it was proportional to the viscosity of the varnish, but some striking deviations were found. The following combinations were used: 80 Per cent Oil Varnishes
A-Boiled linseed oil with drier. Gums: zinc resinate, calcium resinate, coumarone resin, and asphalt gum. B-Blown linseed oil, with four gums as above. C-Refined linseed oil. A blown oil but treated by manufacturers’ process after blowing. With four gums as above. D-Refined oil with dye added. Four gums. E-China wood oil varnish. Four gums. F-Miscellaneous, special linseed oil, X, China wood oil, X a 70 Per cent Od Varnishes A-Linseed oil, with black gum. B-Linseed and China wood oils. Zinc resinate. 60 Per cent Oil Varnishes A-Special refined linseed oil. Resinate gum. B-Linseed oil with dye added. 50 Per cent Oil Varnishes A-Boiled linseed oil. Gilsonite. B-Boiled linseed oil. Pitch. C-China wood oil. Gilsonite. Miscellaneous Raw linseed, blown linseed oil. Black gum, no oil.
Table I shows the effect of the oil on the low-resistance material formed. TARLEI
0.44
It is to be noted that other materials besides the oils develop the acidity. The relative values of linseed and tung oil are not accidental, nor are the low values for the asphaltic materials. These amounts are by no means inconsiderable. Imagine a 2-meter (7-foot) armature coil developing 3 per cent of acidity on the weight of the varnish contained. By repeated absorption of moisture and heating about 125 grams (l/d pound) of liquid of a resistance not much greater than 100 ohms could be obtained from this. I n a generator with one hundred such coils there would be 12.5 kg. (25 pounds) of dilute acid. This is perhaps an extreme case, but it shows the possibilities of these conditions. From the preceding paragraph it is clear that the resin, oil, and drier each plays a part in the oxidation of the varnish. From the evidence obtained so far the oil seems to be the most important factor. By determining the amount of low-resistance material produced by various varnishes of known composition6 some very suggestive figures were ob-
13
Boiled Blown 80 Per cent Linseed Oil Air-dried Per cent Per cent
Refined blown
Per cent
Per cent
5.00 1.70 4.30 2.10 3.27
4.5 2.4 3.3 3.75 3.48
2.9 2.4 2.4 2.4 2.6
3.7 3.9 2.2 2.9 2.75 3.7 0.7 1.4 2.34 2.98 80 Per cent China W o o d Oil
3.7 3.2 3.1 2.1 3.02
2.5 3.5 2.5 2.8 2.8
Zinc resinate Calcium resinate Coumarone Asphalt AVERAGE
4.1 2.5 2.5 0.5 2.4
..................Oven-dried
Zinc resinate Calcium resinate Coumarone Asphalt AVERAGS
Refined
..................
Air-dried Per cent
2.3 Zinc resinate Calcium resinate 1.4 1.4 Coumarone 1.3 Gilsonite AVERAGE . . . . . . . . . . . . . . . . . . 1.5 70 Per cent Oil Varnishes Linseed and black gum 0 .7. t Linseed and China wood oil and L N Res. 2.17 AYBRAGE.................. 1.43 60 Per cent Ozl Varnishes Linseed special refined 1.39 Linseed black dye 1.20 A V E R A G E . ............... .. 1.29
Oven-dried Per cent
1.9 2 0 2.1 1.3 1.8
1.12 2.47 1.79 3.10 2.84 2.97
INDUSTRIAL A N D ENGINEERING CHEMISTRY
14
TAELE I (Concluded) Air-dried Per cent 50 Per cent Oil Varnishes
Oven-dried Per cent
Certain inferences with regard to the making and use of' insulating varnishes may be made. Insulating varnishes should be made specifically for the purpose. The ideal insulating varnish is still to be found, and owing to the fundamental differences in application no one varnish can serve all purposes.
..................
0.88 1.29 1.43 1.50 2.40 2.50 3.27 3.48
China wood oil Boiled linseed Refined and blown Blown linseed Refined linseed
80
80
erated. In some varnishes this may run ag high as 3 per cent of the film,in some as low as 0.3 per cent of the film. Conclusion
Linseed gilsonite 0.85 1.13 Linseed pitch 1.01 2.5 China wood oil Gilsonite 0.77 1.4 AWRAGE 0.88 1.68 1.1 Raw linseed 3.4 Blown linseed 2.3 1.4 Black gum 2.1 0.7 Taking the averages, the following conditions are shown: Per Per cent Per cent cent
50 60 70 80 80 80
Vol. 17, No. 1
1.68 2.97 1.79 1.80 2.34 2.80 2.98 3.02
Prom the table it is obvious that linseed oil varnishes are the most acid, that the method of treatment influences the acidity, and that the increase in percentage of oil increases the acidity. On comparing the air-drying process with the baking process, the accelerating effect of heat is particularly noticeable with low oil Varnishes. The results of examination of the resins are shown in Table 11. TABLE IT Zinc resinate
Calcium resinate
Coumarone Gilsonite
Air-dried SamPEes Per cent 80
80 80 80 80 80 70
Per cent
Per cent
Boiled linseed
Blown
Refined Special Special blown Cbina wood Linseed and China AVERAD&.
Per cent
0.5 2.1 3.75 2.35
...
1.26 0.70 1.77
...
.................
3.29
Boiled Blown Refined Special Special blown China wood Linseed and China wood AVERAGE....
3.7 3.9 3.7 2.78 3.00 1.93 2.47 3.04
2.02
Per cent
2.5 4.3 3.3 2.36 1.43 2.78
...
Oven-dried Samples
Per cent
80 80 80 80 80 80 70
--.
..............
2.2 3.9 3.7 3.5 2.0
... ...
2.96
2.81
1.47
Here again it is plain that the resin compounds have a specific effect. The zinc resinate varnishes are a t one end and the asphalt varnishes are a t the other end of the series. The accompanying chart indicates that the low-resistance materials develop in the material parallel with the oxidation. The upper curve shows the average weights of twenty-four varnishes, including slow and fast drying ones, distributed over tape as previously described. These tapes originally took up, when immersed in the varnish, a weight of wet varnish equal to that of the tape. This makes 100 per cent additional, or a total weight of 200 per cent. After 48 hours the tapes weighed 178.5 per cent of their original weight. Reference to the lower curve, which gives the average acidity for the same samples at the time, shows that 0.56 per cent of volatile acids had developed. During the next 24 houri the average weight did not increase (individual varnishes fluctuated considerably). As this is an average of a considerable number of determinations, it is not an accident but indicates a balance between oxidation and evaporation. The corresponding point for acidity shows an increase to 0.67 per cent. After the fourth or fifth day the drying of the varnish at normal temperatures was pretty well completed and oxidation with accumulation of further acid products continued slowly. Finally, if the films of varnish are then heated to about 100' C., 8 to 15 per cent of the weight of varnish is driven off in a short time and with it about one-half of the water-soluble acids or low-resistance material is lib-
All insulating varnishes should have initial high dielectric strength and maintain the same during the process of setting. Rapid drying and good insulation are in a sense contradictory requirements, since the process responsible for the drying is at the same time responsible for electrical and mechanical deterioration. Where a perfectly neutral varnish is not obtainable, the best results can only be obtained by baking, This is particularly important in case of large apparatus which is expected to meet changes in humidity and temperature. Even with baking, pFeliminary slow drying is advantageous and desirable, for bakmg alone does not carry through the necessary chemical changes. Too rapid heating will simply produce a partially impervious film under which oxidation will subsequently take place. A succession of thin applications will always produce better results than the same amount in one coat. I n thin films evaporation is largely completed within 24 hours while oxidation proceeds to an appreciable degree for about 5 days. At the end of this time most of the undesirable accumulations can be discharged by baking. Make Cloth from War Waste-The Bureau of Standards has made a good grade of mixed silk and woolen cloth from silk yarn left over from the war. This yarn was made from low-grade waste, and was bought by the Government to weave into cloth for cartridge bags. It was a lower grade than any previously used in the normal silk industry of the country. The Bureau of Standards has woven this y a m into cloth, using the silk waste yarn as a single, unsized, and a wool yarn as filling. The resulting cloth is a good grade suiting, well balanced for wear.
